Electromagnetic Methods2 in Applied Geophysics

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Series' Investigations in Geophysics

Volume 2, Parts A and B E. B. Neitzel, Series Editor


ELECTROMAGNETIC

APPLIED

METHODS

GEOPHYSICS•APPLICATIONS PART

A AND

PART

B

Edited by Misac N. Nabighian

Project Editor John D. Corbett

Society of Exploration Geophysicists P.O. Box 702740/Tulsa, Oklahoma 74170-2740

IN


Libraryof Congress Cataloging-in-Publication Data (Revised for volume 2)

Electromagnetic methodsin appliedgeophysics (Investigationsin geophysics;v. 3) Includesbibliographiesandindex. Contents:v. 1. Theory -- v. 2. Applications pts.A-B 1. Magneticprospecting.I. Nabighian,Misac N. II. Series:Investigationsin geophysics;no. 3. TN269.E387

1987

622' .153

87-63300

ISBN 978-0-931830-46-4 (Series) ISBN 978-0-931830-51-8 (v. 1) ISBN 978-1-56080-022-4 (v. 2)

ISBN 978-1-56080-061-3 (Paperback)

¸ 1991by Societyof ExplorationGeophysicists All rightsreserved.This bookor partshereofmay notbe reproduced in any form withoutpermission in writing from the publisher. Published1991, Secondprinting 1992 Paperback1993 Paperbackreprinted1996, 2001, 2008 Printed in the United States of America


To Frank C. Frischknecht,

outstandinggeophysicist and major contributor to these volumes.


This page has been intentionally left blank


CONTENTS

Part

Introduction

Chapter 1

Misac N. Nabighian

A

1

Physicsof the Electromagnetic InductionExplorationMethod G. F. West and J. C. Macnae

Introduction

5

Basic Principles 6 Magnetic Field of Currents The Electric Skin Effect

Field 12

8

9

Eddy Currents in an Ideal Circuit 13 Eddy Currents in Real Conductors 16 Dipole Sourcein a ConductiveMedium 19 Sources on a Half-Space 21 Modeling Induction in Large Isolated Conductors Effect of a Conductive

Overburden

Target Conductorin a ConductiveHost

36

Two-Dimensional

41

and Far-Field

Exploration Philosophy References

Chapter 2

28

32

Models

43

44

The MagnetometricResistivityMethod

47

R. N. Edwards and M. N. Nabighian Introduction

47

MMR Responseof a Layered Earth

50

Magneticfieldson the surfaceof a layeredearthexcitedby a pointsource 50 Magneticfieldswithin a layeredearth excitedby a point source 52 Electricalsounding beneatha conductivesurfacelayerusingdifferentialmagneticmeasurements Penetration of current beneath a conductive surface layer 56 Sourcesof the Magnetic Field MMR Anomaly 57 58 Presentation of Model Responses Mathematical

Methods

Stefanescu's algorithm Skeels-Watson

58

58

transformations

59 vii

55

54

viii

Contents

Vertical

Structures

59

The anisotropic earth The vertical

59

contact

62


The dipping contact The thick vertical

63

dike of infinite

vertical extent The thin conductive dike of infinite vertical extent Filled Sinks and Channels 69

Horizontal circular semicylindrical channel Outcropping hemisphericaldepression 69 The Current Dipole 72 The Small Target 74 The Alpha Center 75 Numerical

Methods

66

69

79

The conductive plate 79 Numerical results for a square plate Modification of 'Resistivity' software Case Histories

80 83

84

MMR surveys with surface current electrodes MMR surveys with buried current electrodes Offshore MMR surveys 93 Conclusions References

64

84 9O

94 97

Appendix•Field Examples of the Downhole MMR Method and Comparisonwith the TEM Method Michael W. Asten 99 Introduction 99 Field Measurement and Data Reduction

Field Examples Conclusion

104

Acknowledgments References

Chapter 3

lOO

100 104

104

Profiling Methods Using Small Sources

105

F. C. Frischknecht, V. F. Labson,

B. R. Spies, and W. L. Anderson Introduction

105

General Aspects of Small Loop Methods 106 Slingram Method 110 Equipment and procedures 111 Errors in slingram measurements 113 Processingand display of data 118 Model studies of slingram responseand interpretation methods Interpretation of slingram data 161 Ratio Methods 167 Wavetilt method 167

Polarization ellipse methods 168 Errors in wavetilt and polarization ellipse measurements Interpretation of ratio measurements 174 Direction Finding Methods 174 Tilt angle method 174 Errors in tilt angle measurements 175 Shootback tilt angle method 176 Responsefor the shootback tilt angle method 177 Frequency Differencing Methods 178 Time

Domain

Methods

181

Processingand interpretation

184

172

119


Contents

ix

Comparisonof time-domain and frequency-domainsystems Applications and Examples 191 Mineral resource exploration 192 Energy resource exploration 221 Ground-water exploration and development 223 Engineering investigations 225 Environmental investigations 227 Archaeological investigations 234 General geologic mapping 237 Summary and Future Directions 239 Acknowledgments 241 References

189

241

EM 303 253 Appendices--A. GEFINEX ao GENIE•IGS/EM-4 System 255 C. MaxMin I, MaxMin II, and MaxMin III Systems D. Slingram Portable ElectromagneticUnit 259 E.

EM-31, EM34-3, and EM-38 Instruments

F.

Cross-Ring system 260 GEM 5 and GEM 8 Systems 261 JEM and CEM systems 262

G. H. I.

Geonics PROTEM 47 and PROTEM 269

57

259

266

J. Crone PEM

Chapter 4

Large-Layout Harmonic Field Systems D.

Introduction

S. Parasnis

271

Field Operations 272 The Sundberg method The Turam method Elevation corrections

272 274 275

Galvano-inductiveversuspurely inductive excitation Effect of conductive

host medium

Depth penetration Appendix A 282 References

Chapter 5

277

278

282

283

ElectromagneticSounding

285

B. R. Spies and F. C. Frischknecht

Introduction

285

History of electromagneticsoundingmethods Principlesof ElectromagneticSounding 288 Sources Receivers

287

288 288

Quantities measured 290 Source-receiver geometries 290 Induction numbersand classificationof sounding Responseof Horizontally Layered Half-Space 293 Homogeneoushalf-space 293 Two-layer models 300 Resolution and equivalence 317 Depth of investigation 323

292

271

258


x

Contents

Sources of Errors in Soundings

333

Instrumental sources of error Geometrical sources of error

334 334

Geological noise 335 Electromagneticnoise 350 Field Procedures

353

Survey design 353 Calibration and instrument testing Good field practices 355 Data Processing 355

354

Frequencydomaindata reduction Time domain data reduction

355

357

Apparentconductivityandapparentresistivity Topographicand geometriccorrections 361

357

Plotting data 362 Inversion and Interpretation 363 Curve-matching techniques 364 Computer-assisted interpretation 365 Computer inversion 365 Depth Imaging 373 Published Case Histories

374

Deep crustal sounding Geothermal Groundwater Permafrost

375 376 376

Coal and petroleum General Discussion

376

376 376


374

378

378

Appendices•A. University of Utah 14-Frequencysystem S.H. Ward R. B. Pseudo-Random Binary Sequence(PRBS) Technique J. $. Holladay Co

UTEM

E.

USGS FrequencyDomain System F.C. Frischknecht 406 Colorado School of Mines TDEM Systems G.V. Keller 408 Soviet EM SoundingSystems B.R. Spies 411 412 IntegratedGeosciences, Inc. Deep TEM Field Systems C.H. Stoyer ZongeEngineering andResearchOrganization CSAMT andTEM Systems

I.

Y. Lamontagne and J. C. Macnae

D. Emer J.

414

Time DomainElectromagnetic Prospecting Methods 427

Basic Principlesof TEM Methods Theoretical

402

BRGM MELIS MultifrequencyEM Systems P. Valla 417 Maxiprobe System B.R. Spies 421 GermanDeep TransientEM Systems K-M Strack 422

M. N. Nabighian and J. C. Macnae Introduction

H.F. Morrison and

398

Do F. G. H.

Chapter6

and

388

LawrenceBerkeleyLaboratoryEM60 System N. E. Goldstein

386

N. Edwards

Basis of TEM

428

Methods

Uniform conducting medium 431 Conductinghalf-space 431 Conducting thin sheet 435 Horizontally layered earth 436

430

427

Contents

xi

Confined conductorsin free space

437


Confined conductor in conductive media Effect of conductive overburden 446 Two-dimensional conductors 447 IP effects 450

Magnetic permeability variations Superparamagneticeffects 452 TEM Configurations 453 TEM Survey Design 456 Field Techniques 457 Sources

of Errors

in TEM

452

Measurements

457

Data Processing 458 Interpretation of TEM Measurements Time

constants

459

467

Dip determinations 470 TDEM interpretation caveats Case Histories

470

472

Computer assistedinterpretation Acknowledgments 473 References Additional

442

473 References

472

475

M. N. Nabighian and J. C. Macnae 479 Appendices--A. TEM Systems B. EM37 Case History, Kolavi, Finland J.D. McNeill, M. Bosnar, and G. M. Levy 484 Co PEM Case Histories, Cigar and Winston Lakes, Canada J. Duncan Crone D. EMP Case History•Crater Deposit, Australia G. Boyd 495 E. UTEM Case Histories J. C. Macnae, Y. Lamontagne, and P. D. McGowan F.

SIROTEM

Case Histories

G. Buselli

G.

GEOTEM Case History

H

INPUT(}•)Applications andCaseHistories P.G. Lazenby 514

R. Pedersen and S. Thompson

Part

Chapter 7

506

B

Geological Mapping Using VLF Radio Fields J. D. McNeill

Introduction 521 VLF Transmitters

and V. Labson

523

VLF Signal and Noise Levels 524 Factors affecting radio wave propagationat VLF frequencies Atmospheric noise at VLF frequencies 535 539 Plane Wave Responsefrom a Horizontally Layered Earth Homogeneoushalf-space--Impedances, wave tilt 539 Homogeneous half-space--Subsurface fields 543 Layered earth•Surface impedance 549 Two-layered earth•Subsurface fields and currents 555 Plane Wave Responsefrom Two-Dimensional Targets 560 E and H polarization 560 Vertical contact•H polarization 560 564 Vertical contact•E polarization, tilt angle and ellipticity Dipping contact 570 Vertical

contact with conductive

Embedded prism Thick

conductive

521

overburden

570

574 and resistive

vertical

dikes

576

524

509

490 497


xii

Contents

Thin vertical dike•Variation of responsewith conductance Thin dike•Variation of responsewith dip 587 Vertical dike•Variation of responsewith depth 587 Vertical dike overlain by conductive overburden 592 Vertical dike•Limited depth extent 596 Multiple conductive dikes 599 Resistive vertical dike 599 Overburden/bedrock structures

599

Summary of responsesfrom two-dimensionaltargets Reversed polarity anomalies 606 Plane-Wave Responsefrom Three-Dimensional Targets Magnetic field response 606 Electric field response 609 Effects of Topography on VLF Magnetic Field Response Data Filtering Techniques 611 Case Histories 615 Use of Local Transmitters

599

606

611

626

Grounded electric bipole transmitter 628 Large loop transmitter 631 Summary 634 Appendix•Polarization Parameters 636 References

Chapter 8

637

The Magnetotelluric Method

641

K. Vozoff Summary

641

Introduction 641 Sources 642 Interaction with the Earth Uniform earth 648

648

Horizontal layers 653 Anisotropy 655 Inhomogeneity 656 Impedance tensor and tipper: 2-D and almost 2-D cases The general 3-D case 661 Statics, topographic, and regional effects 666 667 Alternative definitions of resistivity and impedance Source Sensors

effects and CSAMT 669 Field Procedures 675

668

Data Processing and Analysis General

considerations

676

676

Auto- and cross-spectra 679 Solutions to the impedance and tipper equations Remote reference Errors and noise

681 681

Tensor analyses for 3-D sites 683 Imaging and time domain processing Interpretation 689 An example 689 Discussion 693 Inversion 697 Effective strike direction

Statics compensation Case histories

707

703

704

688

680

658

577

Contents

xiii

Acknowledgments


References

Chapter 9

707

707

Controlled Source Audio-Frequency Magnetotellurics K. L. Zonge and L. J. Hughes

Introduction

713

Objectives 713 Description of the CSAMT technique Historical development of CSAMT Applications 715 CSAMT Theory 716 Introduction

714 714

716

Maxwell's equations 716 The wave equations for homogeneousearth Grounded horizontal electric dipole solution Vertical magnetic dipole solution 726 Nonhomogeneous earth solutions 729 Vector CSAMT apparent resistivity 733 Field Survey Planning and Logistics 734 Basic logistics 734 Instrumentation 735 Electrical noise 735 Cultural contamination

717 719

737

Tensor, vector, and scalar measurements

737

Optimum components for measurement 740 Plan-view coverage considerations 740 Depth of exploration 742 Resolution

considerations

743

Data density considerations 744 Topographic considerations 745 Geologic considerations 745 Loop-versus-dipole sources 745 Wire impedance 746 Exploration economics 747 When

to use CSAMT

748

Far-Field Data Interpretation 749 Types of CSAMT data 749 Data processing 751 Data display 752 Noise analysis 752 1-D interpretation 754 2-D interpretation 758 3-D interpretation 760 A general comment on modeling 760 Static effects

760

Topographic effects IP effects in CSAMT

766 data

768

Interpretation of cultural effects Source Effects

768

769

Nonplanewave effects 770 Source overprint 778 The shadow

effect

Comparison with MT Case Histories

781

783

784

Petroleum exploration (Structure mapping)

784

713


xiv

Contents

Geothermal mapping and monitoring 787 Massive sulfide exploration 795 Gold exploration: Mapping epithermal alteration 795 Gold exploration: Mapping silicified reefs 797 Gold exploration: Mapping structureunder alluvial cover 797 Uranium exploration 798 Comparisonof dipole-dipoleresistivity and CSAMT measurements Mapping alteration and silificationGolden Cross Mine, New Zealand Detecting subsurfacewater and structure 803 Mapping brine leaks from injection wells 804 Tracing injection fluids 806 Conclusions

806


Chapter 10

807

807

Airborne ElectromagneticMethods

811

G. J. Palacky and G. F. West Introduction

811

The story of airborne electromagnetics 811 Classification of AEM systems 814 Design Considerations 818 Desirable geophysical characteristics 818 Constraints in designingAEM systems 821 Example AEM Systems 825 Field Operations and Data Processing 828 Survey design and specifications 828 Data processing 832 Data Interpretation 850 Principles 850 AEM modeling 850 Interpretation of helicopter AEM surveys 855 Interpretation of time-domain, towed-bird AEM surveys Survey Examples 866 866 Prospecting for volcanic-associatedmassive sulfides Geologic mapping 870 Bathymetric charting and sea-ice measurements 875 Acknowledgments 877 References

Chapter 11

877

Drill-Hole Electromagnetic Methods A. V. Dyck

Introduction

881

Historical Development 882 Description of Methods 883 Dipole-dipole EM 883 Rotatable-transmitter

EM

884

Large-loop electromagneticsystems(LLEM) Borehole

EM

with remote

source

Wave propagation methods Tools of Interpretation 901 Models

861

901

Large loop EM

903

900

895

888

881

799

800


Contents

xv

Large loop responsesin conductive media 912 Approaches to Interpretation and Field Examples 914 Approaches to interpretation 914 Trillabelle--a comparisonof methods 915 Example of multiple-loop survey 916 South Bay Mine, Ontario 918 Ruttan Mine, Manitoba

918

Scale-modelexample 920 Gertrude (Sudbury), Canada 920 Survey in a conductiveenvironment(TasmaniaSIROTEM example) State of the Art


921

921

922

922

Appendices--A. Integral Equation Solution for EM Induction in a Thin Plate B. EM Induction in a Conducting,Permeable, Two-Layer Sphere C. Properties of Eigencurrents 929

Chapter 12

Electrical Exploration Methods for the Seafloor A.D.

Introduction

Theory

Chave, S.C.

Constable, and R. N. Edwards

931

932

The Oceanic

Environment

932

Magnetotellurics 935 Direct Current Resistivity Magnetometric Resistivity Self Potential 946 Controlled Source EM

Methods

Frequency domain EM Time domain

EM

938 941 947

949

951

Experimental CSEM 954 Experimental CSEM--time domain Acknowledgments 962 References

960

962

Appendix--Electromagnetic Induction Equations

Index

967

965

931

924 927




ELECTROMAGNETIC

APPLIED Part

METHODS

GEOPHYSICS--APPLICATIONS A

IN




INTRODUCTION

Misac N. Nabighian Electromagnetic Methods in Applied Geophysics, Volume I, Theory presented the mathematical and physi-

data interpretation and case histories. Where there were competing field instruments or techniques, a

cal

deliberate

foundations

common

to all

EM

methods.

The

purpose of Volume I was to help facilitate the understanding of the theory involved and to provide a limited amount of interpretational aids. Volume H, Applications is devoted to a method-by-method treatment of the principal EM techniques in common use. The first chapter gives a unified treatment of the physical basis of EM methods of exploration and is followed by a comprehensive chapter on the magnetometric resistivity method (MMR). The inclusion of MMR, essentially a dc technique, in an EM volume might seem puzzling at first. However, an increased understanding of current channeling (current gathering, galvanic) effects indicates that anomalies attributable to current channeling can be interpreted best using MMR concepts. In many instances, EM induction plays only a small part in the observed anomaly which is predominantly attributable to current channeling. A case in point is the VLF technique. Not long ago all interpretation was done using only EM induction concepts. Today, some geophysicists describe VLF as "MMR with phase", a tacit acknowledgment of the important role played by current channeling effects in observed

VLF

anomalies.

The following chapters give in depth treatments of: Profiling methods using small sources, Large-layout harmonic field systems, Electromagnetic soundings, Time domain electromagneticprospectingmethods, GeologicalmappingusingVLF radio fields, The magnetotelluricmethods, Controlled source audio-frequencymagnetotellurics, Airborne electromagneticmethods, Drillhole EM techniques,and Electrical explorationmethodsfor the seafloor. Each chapter describes in detail a given EM method, from instrumentationand field proceduresto

effort

was made to achieve

a balance

be-

tween sometimesdivergent claims. To this end, each chapter has an abundanceof case histories illustrating various viewpoints. Except for the common theoretical background which was outlined in the first volume, each chapter is self contained. Special attention was given to eliminating as much as possible duplication between various chapters and to obtaining a certain uniformity in presentation. The large number of topics covered necessitates dividing this second volume into two parts. The twelve chaptersare arbitrarily divided into Part A and Part B.

A large portion of the material has never before been publishedin a systematicway in the western scientific literature. Volume II presents an up-to-date treatment of EM methods that should prove invaluable both to exploration geophysicists(mining, petroleum, environmental, etc.) and to university students studying applied geophysicsor related sciences. Three years have passed since the publication of Volume I•1onger than was initially anticipated. With gratitude I acknowledgethe continuoussupport from Jack Corbett, Project Editor, from Dr. Stan Ward, and from the Society of Exploration Geophysicists. The authorscontributedgreatly by the additionalrewriting and updatingof their manuscriptsto provide state-ofthe-art information. Consideringthe rate at which new developmentsare being publishedeach year, this was no easy task and I am grateful for their efforts. Last, but definitely not least, my sincerestthanks go to the SEG Publicationstaff (Lynn Griffin, Jerry Henry, and JohnHyden) who patiently survivedmisseddeadlines, multiple updates of various chapters, and frequent changesin book format. Their professionalismwas indeedremarkable. Without their help this book would not have been possible.




CHAPTER

PHYSICS

OF

1

THE ELECTROMAGNETIC EXPLORATION METHOD

INDUCTION

G. F. West* and J. C. Macnae*

INTRODUCTION

The immediate objective of a geophysical survey is to obtain some information about the interior spatial distribution of one or more of the earth's physical properties from a limited set of measurements of a related physical field made on the earth's surface (or another accessible place). In the case of an electromagnetic (EM) induction survey, the most relevant physical property is the electrical conductivity, and it is sensedby means of a time-varying magnetic and/or electric field. The procedure of converting field measurementsto a physical property distribution is termed modeling or interpretation, and the formal corresponding mathematical process is termed inversion. Geophysical inversion is difficult in the best of circumstances because of numerous intrinsic ambiguities. For EM methods in particular there is an additional problem. The basic laws that relate the EM field to the physical property distribution are well known (Maxwell's equations) and a quantitative and calculable relationship between the physical measurements and the property structure can be established for certain idealized cases. However, we still lack practicable modeling capabilities that enable quantitative prediction of the EM field configuration produced by an arbitrary physical property distribution of even moderate complexity. Geologic scenarios are extremely varied, and few actual cases can be described accurately in terms of simple geometric forms like plane horizontal layers. Thus, only rarely can we feasibly turn geophysical observations directly into a reliable picture of earth structure simply by application of an automatic process. Generally, a human interpreter is still needed to

guide the interpretation process, and this human needs to have a good qualitative understanding of how physical earth structure can interact with EM fields. In addition the interpreter should be able to mentally extrapolate beyond calculable casesand to select more important features of the data from less important ones. Our objective in this tutorial paper is to assist readersin developingsuch an ability by discussingthe variousphysicalprocesseswhich arise in some simple situations.

Mental modeling of electromagnetic methods can be difficult. Unlike gravity and magnetic methods where most geophysicists can sketch with a fair degree of accuracy the potential field configuration around a given susceptibilityor density distribution, the survey results of EM

methods

seem often to resist visualiza-

tion in terms of the physical processesin the ground which produce them. One reason is the need to consider two vector fields simultaneously (electric and magnetic). Another is that the EM response is rate dependent. A third reason is that (as in seismic methods) the EM field is usually generated locally by a controlled source which is moved frequently during the survey, so often only one or a few measurements are made of the physical field before changing it. Furthermore, low-frequency diffusive EM field propagation is often more difficult to describe than highfrequency wave propagation where Snell's law ray theory offers a simple first approximation picture that anyone can understand. Although the mathematical theory of EM fields is an essential tool for EM modeling, focusing too intently on the mathematical

niceties

can distract

the inter-

preter from the essentialphysics. Also, description of

GeophysicsLaboratory, Departmentof Physics,University of Toronto, Toronto, Canada,M5S-1A7. Lamontagne GeophysicsLimited, 4A Whiting Street, Artarmon, New South Wales 2064, Australia.


6

West and Macnae

low-frequency EM induction by use of convenient mathematical analogies with wave theory can obscure the fact that eddy current induction at low frequency has more in common with potential field processes such as gravity and magnetics and diffusive processes like conduction of heat and with the widely understood properties of L, R, C electric circuits than it has with high-frequency wave processesepitomized by optical ray theory. Throughout this chapter we have chosen to draw out the electric engineering analogy of L, R, C circuit waveform analysis whose jargon is based on that usually used in the analysis of mathematical functions of complex variables. For readers without a backgroundin complex variables or circuit design,the descriptions of time-domain behavior may be more easily understood. Because this is a tutorial rather than a review paper, no attempt was made to cite all the significant contributions made during development of the field by very many scientists and engineers. The references given are to convenient

sources

of additional

information

and to the sources of specific data. Most examples shown are from work performed at the University of Toronto. The mathematical theory of EM induction is thoroughly reviewed in EM Volume 1, Ward and

TABLE a

dimension

A

inductive

S

limit

B magneticflux density c free constant D electric flux density e

emf

E electric field intensity G Greens function

t

To

y

H magneticfield intensity

z

electric dipole moment

J currentdensity wave

number

I horizontal scaleparameter L

inductance

L

inductance

matrix

m magnetic dipole moment M magnetization N depolarization factor

P electricpolarization q charge density r spherical radius R

resistance

R0 responseof overburden R

resistance matrix

s thickness

PRINCIPLES

The EM field has two directly measureable components, the electric and magnetic fields. In free space, the two fields can be described equally well either in terms of the field intensity vectors E (V/m) and H

(A/m) or the flux densityvectorsD (C/m2) and B (Wb/m2 - t½slas). Thesefieldsare, in general,functions of spatial position (r; x, y, z) and also of time t (seconds)or frequency f (hertz) (or angular frequency to = 2,rf). Inside a medium where the physical properties vary from point to point usually both the field intensities

and

the fluxes

must

be known.

Alterna-

tively, if all the magnetizations and electric polarizations created in the medium are known, only one electric and one magnetic vector is required for a full description. Since most geophysicistsare quite familiar with the force fields E and H of static potential theory, we describe the field mainly in terms of E and H.

At the most fundamental level, the EM field is a

OF SYMBOLS

diagonal matrix

magneticpermeability of free

time

overburden

space

transmission

cylindrical coordinate radius conductivity

filter

time constant

spherical coordinate flux

angular frequency

cartesian coordinates

Subscriptsand Superscripts

Z• impedance matrix

I current

k

BASIC

u(t) step function U distribution of magnetization U eigenvector matrix V volume x

h overburden thickness

j

Hohmann. To define terms, and to provide a brief revision of the basic physics, a short review of elementary principles is given in the first two sections.

current channeling number 13 half-space response parameter inductive response parameter skin depth b(t) delta function dielectric permittivity •0 dielectric permittivity of free space

admittivity 0 spherical coordinate dielectric susceptibility magnetic susceptibility relative ohmic susceptibility spatial wavelength A propagation wavelength magnetic permeability

Xa Xb X•. Xh

anomalous body conduction halfspace

xi}

Xj index Xo overburden (except e0, Xr receiver Xt transmitter Xr XE

total electric field

X • magnetic field Xj electricdipolesource X 'nmagnetic dipole source X p primary X s secondary X'

source

Physicsof ElectromagneticInduction Exploration Method

manifestation of the distribution of electric charge. The most direct expression of this electric charge is


P = K•EoE and

the Coulomb Law, which in differential form is

V-eoE = q,

7

M = K•H,

(1)

(5)

i.e., the electric field in free space diverges from any

where,• is calledthedielectricsusceptibility and,•

distribution of chargeq (C/m3).

the magnetic susceptibility of the medium. The defining relationships between the flux density and the intensity vectors are just

Charge generates another type of field when it moves. This type of field arises in isolation from the static electric field whenever

there is a differential

flow

of charges of opposite sign; a flow wherein the net charge density remains zero. Free electric charge can move at the macroscopic level in nature as free electrons (or electron deficienciesin a crystal lattice) or as mobile ions, and can also circulate widely inside a conductive medium without affecting the overall charge balance in the interior of the medium. Neutral current flow is described by the electric current flux

density vectorJ (A/m2)and,asshown inthefollowing discussion,generatesa magneticfield which circulates around J according to Ampere's law. Since charges cannot be created or destroyed, J must also satisfy a charge conservation constraint Oq

V.J=

-•.

ot

(3)

where the proportionality constant •r is a property of the material known as the electrical conductivity (S/m). Microscopic local imbalancesin charge distribution and local current

circulations

at the atomic

and

B = IX0(H + M).

level in a

material medium can create macroscopic EM fields. Electric and magnetic polarization vectors P and M describe these effects. (M is usually called just the magnetization.) In differential form the source relations are

D= e0E+P+P'

=eE+P',

where

e= s0(1 + •),

(7)

B = IX0(H + M + M') = IXH + DOM', where

tx= IXO(1+ •),

(8)

J = J•. + J' = o-E + J',

(9)

and where J•, is the ohmic conductioncurrent. Combining the above we obtain

POLE AND DIPOLE SOURCES OF THE EM FIELDS

(o)

V'eoE=

(6)

Separatingthose polarizations and currents which are induced directly in the medium by the EM field from other source distributions is often necessary. The terms "source distribution" (denoted by a primed symbol), refers to any spatial distribution of M, P, or J which is independent of the E and H fields that are under consideration. A "source" is usually supported by some external energy source. Collecting all the above relationships, we have

(2)

The density of current which flows in a material medium as the result of the electric field usually depends linearly on the electric field strength according to Ohm's law, J = o-E,

D=eoE+P

(b)

-V.P

and

V.H=

-V.M.

(4)

Whereas chargeappearsin the Coulomb law as a pole source, the polarization vectors act as source dipoles. The relationships are sketched in Figure 1. In many media, the polarizations are themselves created by the fields, according to linear proportionalities

Fig. 1. Configurationof the EM field near pole (a) and dipole (b) source distributions. Note that the M and P source dipoles are opposite in direction to the internal field.

8

West and Macnae

V'eoE = q-

V. (P + P'),

0B

VxE= Ot

Oq


V.j=

and

Ot

0D

and

VxH=•+J.

V.H:

-V(M+M').

(15)

Ot

(lO)

The first two of these equations are closely related. Writing both equationsin terms of the electric field, we

It is a useful shorthandto say that the magneticfield is generatedby a total currentJr which is the sumof the ohmic current flow and the so-called displacement

see that

current

V.(crE+ which

eOE/Ot)=

-V.(J'

+0P'/0t)

tion is suddenly establishedin a uniform medium, the

electricfieldwill equilibrate exponentially as e-*t/•. The time constant e/oris negligiblein most cases,being less than 1 •xs in a medium whose conductivity is greater than about 20 •xS/m (or resistivity less than 50 000 D-m). Thus, for low-frequency EM exploration problems, the only charge and polarization that is important in creating a sustained electric field is that which is maintained by a flux density of source current, i.e.,

-V-J'.

(12)

The divergence equations (10) have a simpler form when they are written in terms of the total flux density vectors, i.e.,

V.D=q, V.j=

Oq Ot

(13)

which in a conductive region becomes V.J=0,

(16)

This concept is most useful in the frequency domain when Jr can be related to the electric field by a generalized Ohm's law, Jr = (or +/toe)E,

(17)

where the complex quantity in brackets is known as the admittivity of the medium. In EM prospectingat low frequencies, any magnetic field generatedby the displacementcurrent term OD/Ot is usually (althoughnot always) negligible. Removal of OD/Otfrom equation (15) eliminatesthe wave nature of the EM field in free space so no propagationdelay will be predicted. This result is usually called the quasistatic approximation (or the EM field is said to be quasi-stationary). According to the quasi-static approximation, the primary magnetic field in free space generated by a local source loop of alternating current I is everywhere in-phase with I; and the primary electric field is everywhere in quadrature with I, being generated by the time derivative of the magnetic field. MAGNETIC

and

V.B=O,

V xH=Jr.

(11)

can be solved to show that if a source distribu-

V-rrE=

OD/Ot.

FIELD

OF CURRENTS

In many EM prospectingproblems, we can consider the EM field to be generated entirely from two electric current distributions; one being an externally supported current in a transmitter wire or coil J', the other being Jc the density of conductioncurrent impressed into the earth according to Ohm's law. The magnetic field of these currents

is derived

from

and

V-B=O.

(14)

+OD

Coupling between the E and H fields is describedby Ampere's and Faraday's laws. Each field generatesa component of the other in the form of a circulation

about its field lines, as sketched in Figure 2. An electric field is created circulating about any timevarying magnetic field, and a magnetic field is created circulating about any time-varying electric field or electric current density.

FARADAY' S LAW

AMPERE'S LAW (Generolized)

Fig. 2. The circulationrelationshipsbetween the electric and magnetic fields accordingto Faraday's and Ampere's laws.

Physicsof Electromagnetic Induction Exploration Method

VxH=J=Jc

+J'

m

(18) Hr =

2 cos 0

4xr

r

3

(21)

which has a general solution known as the Biot-Savart equation.


9

m

sin 0

Ho- 4•r r3

r013 1fv[Jc(r 0)+IrJ'-(r0 )]x(r-r0)

d3ro (19)

H(r) =•

the field is an azimuthal

(Note that Jc and J' must togetherform a continuous, divergence-free circulation for this equation to calculate a physically realizable field). A routine application of equation (18) is in calculating the free-space primary magnetic field of a known source current. Two specific cases are frequently needed, a small loop of radius a carrying current I which approximates a magnetic dipole and a straight current wire which is long compared to the distance at which its magnetic field is observed. These cases are

illustrated inFigure3. Forthecasea <•X,/p 2+Z2where the loop- can be considered small, and defining the

momentof theloopasm = ,ra2I, we have m

3pz

4•r (p2+ z2)5/2 (20)

Hz:

so the magnetic field strength drops off as the inverse cube of distance from the source. For the long wire,

m (2z2 - p2) 4,r (p2+ Z2)5/2ß

circulation I

H, = 2,rp and the fall-off

is as inverse

(22)

distance

from the wire.

In modeling induction of eddy currents in the earth, the source current and the earth's physical property distributions are known. Although the induced current system is unknown there are two basic approaches to finding the system. One approach is to divide the EM field into two partial fields, a primary component defined as the field which the externally maintained sourceswould generate in some reference earth model (often free space), and a secondarycomponent which is the remainder necessary to make up the correct total. The other approach is to consider only the total field. We shall use both approaches, but the primarysecondary separation often has heuristic and computational advantages because the relationship of the fields to the currents impressed in the earth is made clearer.

In spherical coordinates we can write equation (20) as

THE

ELECTRIC

FIELD

Before considering the external magnetic fields, we need to discuss the behavior

of the electric

fields and

associatedcurrent systemswithin the ground. In particular, we need to understand the effects of electrical inhomogeneity.The important conceptsof depolarization and current channelingnumber are introduced for later use.

At low frequency, an effective method for creating an electric field in a conductor

is to connect a source of

current to the conductor. A positive source current connected to a point in the interior of a uniform conductorinjectspositive chargeat the contact, which in turn creates a radial electric field. The electric field

createsa radial conductioncurrent accordingto Ohm's law. When the source current is first switched on,

charge will immediately collect at the contact point in an amount such that continuity will be achieved between

the source

and the conduction

current.

Note

that the configurationand strengthof the electric field in the uniform conductor depends On the location of the source current's

Fig. 3. Magnetic field about a current loop and a current line.

terminations

rather

than on the

path of the current in the insulated wire (Figure 4).


10

West and Macnae

In an insulatingregion, a static sourcedistributionof chargeor chargedipoles(suchas might be producedin a material which has strongly polar molecules) can create a continuing electric field. But in a conducting region, this field cannot be maintained without a continuing source of current because charge will immediately be transferred by the conductioncurrentsin an amount sufficient to neutralize the static charge distribution and nullify its electric field. Thus, only source

current

distributions

need

be considered

as

sourcesof a static or very low frequency electric field in any but the most insulating of regions. In practice, the source currents of EM exploration are usually lines of current in transmitter wires rather than continuous distributions of source current density J'. Continuous distributions of source current usually arise only in computational modeling as a mathematical artifice analogous to dielectric polarization and to magnetization in order to represent the extra conduction current which flows in a "target body" whose conductivity• is anomalousin comparisonwith its "host" surroundings(Figure 5). The density of conduction current in the anomalous region is considered to be partly a normal conduction current and partly a source current (usually denoted as J• and called an anomalous current or scattering current), with both considered to be flowing in the host material. J = Jc + J•, - o'hE + J•, where

Ja = •aE and

•a = •b -- Oh.

(23)

In this approach, the anomalous body is considered not as a region of different conductivity, but insteadas

a region within the host region wherein there is an anomalousdensity of source current. Spatial inhomogeneity of conductivity affects the EM field in two ways. First, the electric field will be directly distorted by the charge deposited at conductivity contrasts to ensure continuity of current flow (and if the frequency is sufficiently high, also at contrastsin dielectric polarizability). Second the electric field will be indirectly distorted by the induction of the electric field from the time-varying magnetic field of eddy currents. The former processis nearly instantaneous, i.e., essentially time invariant, whereas the latter is slow, i.e., rate-dependent.Overall, the induction process causes the total electric and magnetic fields to diffuse slowly into conductive regions. Both effects are important in EM exploration. They are discussedseparately because the physics of the two cases differ.

The direct distortion of the electric field by spatial variations in the conductivity of the earth is easiestto study at very low frequency where any complications due to rate-dependent EM induction vanish. As mentioned, the effect of a conductivity inhomogeneity in an otherwise uniform host space is determined by finding the anomalouscurrent distributionJa within the target zone that will make the total current density everywheredivergencefree. Ja is createdby the total electric field according to equations (23) and the total electric field, in turn, creates a secondary electric field inside and outside the inhomogeneity. In reality, the secondary electric field is generated by the concentration of charge which arises on the interface of the anomalous region when current flows throughthe field (where Ja is discontinuousand therefore V. Ja is non-zero). If the inhomogeneityis more conductive than the host medium, the charge creates a reverse secondary electric field inside the body which tends to cancel the primary field. If the homogeneityis

I

•INSULATED

(a)

WlRE

E I

4w-o-r2

(b) Fig. 4. Electric field created by injecting current into a conductive medium current line.

at the termination

of an insulated

source

Fig. 5. A local zone of anomalous conductivity (a) can be treated as a zone of anomalous (source) current flowing in the host medium (b).



less conductive, the field inside the body is enhanced. The effect is called depolarization in exact analogy with the corresponding effect in a dielectric medium (and demagnetization in a magnetically permeable medium). As an example, we analyze the case of a sphere in a uniform primary electric field. For this shape, both a uniform anomalous current density and a uniform secondaryelectric field are produced inside the sphere and a dipolar secondaryfield is produced outside. The

strengthof Ja dependson the conductivity contrast,

seen by studying the depolarization of an ellipsoidal body. Just as for a sphere we can show that if the primary field is uniform and along a principal axis, the anomalous current density must also be uniform and create a uniform internal secondary electric field in the same direction. The proportionality between the anomalous current density and the internal field is

writtenasEs = _NJatrh.Theproportionality factor N (called the depolarization factor) takes a value between 0 and 1 depending, respectively, on whether the inhomogeneity is highly elongated or highly oblate in the direction

i.e.,

11

of the current flow. The value is 1/3 for

a sphere.The expressionsfor Ja and total J are then

ES= _ Ja/3Crh

Ko.O' h

and

J,, =

J• = (try, - trh) (E p + E s)

(1 +NK•)

Ep

(26)

and

giving o- b

Ja =

3Orb(Orb -- O'h) crb + 2orh

J =

E e.

(24)

The expressionfor J• with •

Therefore the total amount of current flowing through the inhomogeneity is limited to something much less

than Orb Ep, (i.e., to 3Crh Ep) no matter how high tr• becomes. The effect is opposite in the case of a resistive inhomogeneity, with the total current density

then being forced to be larger than Orb Ep, i.e., (3orh EP/2). For modest conductivity contrasts, the strength of Ja is directly proportionalto the primary electricfield and to a measure of conductivity contrast which could be given the name "relative ohmic susceptibility" • in analogy with the magnetic and dielectric cases, i.e., Ko.--

E p.

(1 +N•)

(27)

positive is graphed in

Figure 6. Although the depolarization phenomenon may become more complicated if the anomalous body is not ellipsoidal or if the primary field is nonuniform, the basic effect is universal.

When a current

flux crosses a

conductivity interface, charge collects on the interface and adjusts to a level such that the secondary electric field generated is enough to keep the normal component of current density continuous. This effect can be represented as the creation of an anomalous current density inside the inhomogeneity whose strength will depend on the conductivity contrast. But, inside the anomalousregion, the secondary electric field of the

• ] , o.o/_

and

INDUCED MOMENT •

Ko.O' h

J. =

(1 + Ko-/3)

Ep

(per unit volume)••

(25)

and thus

- 1
<•c.

However, the proportionality no longer holds at strong conductivitycontrastswhere depolarizationcausesJa to saturate. The amount of depolarization is a strong function of the geometry of the anomalous body and the primary field, being minimal in a conductive inhomogeneity which is greatly elongated in the direction of the primary electric field, and extremely important if the along-the-fielddimensionis small in comparison to the across-the-fielddimensions.The oppositeis the case when the inhomogeneity is resistive, as can be

///

•

// ........

•

-•

at-t •x I

10 o

.....

x' •1

10•

•.

-•

x

INTERNAL

x'

,x., , ,,•,,I

• .... ,x•

•0 2

'O/

, ,,•,•l

•0 •

PROPERTY CONTRAST(K• m K.)

Fig. 6. The anomalouscurrent or magnetizationinducedin an ellipsoidalconductivityinhomogeneityas a function of shapeand conductivity contrast (from West and Edwards, 1985).


12

West

and Macnae

charge generally opposesthe primary electric field if the body is conductive or will be enhancedif the body is resistive. If the conductivity contrast is made strong, this field will reduce the amount of anomalous current that can flow. In an ellipsoidal body, conduc-

tive saturationbeginsto take placewhen • • 1/N and resistive saturation when • • -(1 - N). In the former case, the anomalous current density limits at (I/N) times the primary current density, in the latter at -1/(1 - N) times. Similar dimensionlessfactors (analogous to the relative ohmic susceptibilitynormalized by depolarization factor) can be constructedfor most geometries, and they are known as current channeling numbers.

In further

discussion

of conductive

inhomo-

geneities,we denote the current channelingnumberby • and assumethat the number is normalizedlike • N so that transition to saturation takes place at • • 1. Thus, in any small conductive zone of volume V, the anomalouscurrent Ja is approximatelyparallel to the primary electric field E •' and given approximatelyby

J• = l+a

.

(28)

The secondary electric field outside the body can be estimated roughly by considering the field to be produced by a source current dipole at the center of strength

jo- VJ,= 1+a

N

'

which

is common SKIN

E

(V2+k2)H=0 k2= -icolx(cr+icoe). (31) Equation (31) has one-dimensional solutions of the type

Ex(z)-

Eax exp _+ikz

Hy(z) = Hay exp +__ ikz where the exponential has a negative sign for propagation in the positive z direction and Eox

mix

Hay

k'

When cr >>toe, which is usual at audio frequencies and below, and if we consider the field to have its source in

the -z direction so that it propagatesand attenuatesin the +z direction, we can write the quasi-staticform of equation (31) as Ex(z) Eax • = exp (-z/S) exp (-iz/•)

Hy(Z)

Hay

(32)

where

(29)

Magnetic properties of the earth also affect the EM field. If the ground is magneticallypermeable(i.e., has a nonvanishingmagnetic susceptibility), spatial variations in permeability ix will have the same sort of time-invariant effect on the magnetic field as conductivity contrasts have on the electric field. However, permeability effects usually have only a minor influence on EM exploration. Except in a few highly magnetic formations, ix varies only slightly (<3%) in comparison to the several orders of magnitude of variation

or,if thetimevariation isa simple harmonic (eitøt) then

• --=

2 •/2 Eax • = -o'lxco 'Hay

ei•r/4

(33)

i.e., the E and H fields are perpendicular to one another, and they both attenuate by a factor e and shift

phaseby 1 radianin a distance g = X/(2/•lxco whichis called the skin depth. This is illustrated in Figure 7a. An interesting result known as the Cagniard relationship is obtained from the ratio of the electric and magnetic field components. The conductivity of the medium

can be found from

in •.

cr- Ixco

EFFECT

The simplestcase of eddy current inductionto solve mathematically is for an infinite uniform medium in which a laterally uniform field is created having only one vector component and a one-dimensionalspatial dependence(z), i.e., the classicalskin depth problem. Equations (15) are then easily simplified, combined, and solved for either E or H to give, in a uniform, sourcelessregion

.

(34)

The same results are easily found for the case of a uniform field steeply incident on the surface of an infinite conductingregion (half space). An alternating EM field is prevented from penetrating into the conductor by formation of an induced current system near the surface. The (secondary) field of the induced current cancels the incident (primary) field inside the conductor.

V2- trixOt

•lx

H= 0

(30)

The correspondingtime domain expressionsfor a step-like magnetic field created at t = 0 are



I-Iy(z) = • - err -•Ex(Z ) - •/2• r -•-

1/2z ]

2-55] (35)

exp

These expressionsare the normal distribution and normalfrequencyfunctionsof statisticaltheorywith a

13

thefieldsthenbecomes exp[i(h2 - k2)1/212 whichisto be compared with a variationof e-xz whenIklvanishes. The latter expression is just the (outward) continuationrelationshipfor potential fields. Thus the additional multiplicative attenuation and phase shift factor due to conductivity of the medium is

exp{[X - (X2 _ k2)•/2]z}

variance X/(2tkrl•). Thevariance factorX/(2t/•rl•) has units of length and is the time domain analogof skin depth, i.e., the factor is the distancethe field diffuses in time t into a medium of conductivity •r. Figure 7b showsthe spatial configurationof the fields at several successiveinstants.Penetrationis proportionalto the squareroot of delay time (time sincethe transient)or the inverse square root of frequency. These results are very revealing, but they must not be assumedto apply quantitatively to fields which have a more complicatedgeometry, becausethe diffusion rate is also a function of field geometry. This function can easily be shownby includinga harmonic

spatial dependence e-i(px+ qy)intheabove equations, i.e., a spatialwavelengthof 2,r/p and 2,r/q in thex and

y directions. WritingX2 = p2 + q2, thez variation of

which for large spatial extent when b <• 1/X becomes the usual skin effect attenuation

and for small spatial extent when b >> 1/X becomes

exp - (iz/Xb2).

(39)

When the spatial scale of the field is much larger than the skin depth, attenuation takes place accordingto the planewave skin depthformulas.But when the field is localized, (i.e., has mainly short wavelengthcomponents),it is progressively phase-shifted in passingto depthbut is not attenuatedin amplitudeany more than would

be the case if the field were static or in a free

space.

EDDY (o) FREQUENCY DOMAIN

(b) 1.0

œ(t)

TIME DOMAIN

(STEP-ON)

CURRENTS

IN AN IDEAL

CIRCUIT

An elementary case which is discussedin every introductoryexplanationof EM prospectingis induction in a simple loop circuit--i.e., induction in a conductiveloop of wire which is situatedin free space and subjectedto a time-variable magnetic field. Althoughthis model does not permit quantitativemodeling of induction in real, three-dimensional(3-D) conductors,the model does have great heuristicvalue and theoreticalimportance.The analysisis very simple becausethe currentinducedin a closedcircuit has a known, fixed geometry(that of the circuit), whereas the current system induced in a 3-D conductivity distribution could have a wide variety of configurations. In a circuit, only the intensity of the induced current

need be found.

If the circuit has a resistance R and inductance L,

which givesa time constant'r = L/R, and if the circuit is threadedby an alternatingmagneticfield from the

source sothattheprimary fluxis•?, thenthecurrent Fig. 7. Sketchesshowingthe diffusionof laterally uniform electric and magneticfields into a conductivemedium: (a) The attenuationof an alternatingmagneticfield. (b) The diffusionof a step changein magneticfield.

I inducedin the loop is (Grant and West, 1965)

I(•o)=l+i•0'r-•- = l+i•/ -•-' (40)

14

West and Macnae

If the frequency variable is taken as the dimensionless quantity •/ - to, (the inductive response parameter)


and

the

current

is measured

as a fraction

saturation level, all simple circuits respondidentically. Figure 8 shows the frequency dependence of I in several different plotting forms that are common in EM exploration. The key point is that the induced current grows linearly with frequency until a saturation effect sets in. The responseis a smooth transition between the so-called resistive (low frequency) and inductive (high frequency) limits of response. In the resistive limit, the resistance of the loop is large and the induced current which is in quadrature phase with the primary magnetic flux is so weak that the secondary magnetic flux which is produced in the loop is negligible. In the inductive limit, the secondary flux is so large that it becomes almost equal and opposite to the primary flux, giving a near vanishing total flux through the loop which, nevertheless, is sufficient to produce the eddy current. The transition

from

resistive

to inductive

limit

and resistance

of the circuit

which

e-t/*

(42)

where u(t) is the unit step function and •(t) is the unit impulse. These results are illustrated in Figures 9 and 10.

To relate the above discussion

to what is measured

in practical EM exploration, we may consider an EM system consistingof a transmitter and a receiver loop. A conductor in the ground is represented by a third

loop(Figure11).The mutualinductances Lij between each pair of loops can be calculated. They represent the magnetic flux cutting one loop (i) due to a unit current in another (j) and are symmetric in i, j. In a frequency domain EM system, the observed quantity

mightbe the secondaryvoltageinducedin the receiver measured in terms of the primary voltage, viz., es -

-itoL2312

ep -

-itoL•3I•

and

oc-

curs over about a decade of frequency centered about to -- 1/'r. The time constant 'r - L/R depends on the inductance

I(t)= -•- •(t)

of the

in turn

where

depend on its shape, size, and conductivity. For a conductive circuit of fixed shape having a scale size l,

- itoL1211

I2 R22+ itoL 22

we caneasilyshowthatL/R is proportional to o-•l2. The dimensionlessresponseparameter •/is then seen

giving

to be proportional to •r•tol2. Comparing thisresult with the skin depth relation from equation (33), we see

eS/e p -

that•/is thusalsoproportional to 12/•2. Theresponse parameter •/ may be thought of as a dimensionless frequency or conductivity. Sometimes working with the dimensionlessdistance l/• which is given the name induction number, is more convenient. The spectral responseof a loop circuit is a quotient of complex numbers in which the denominator has a root

at to -

iR/L

and the numerator

has a root at

to - 0. In the jargon of electrical engineering, the response is said to be characterized by a single pole and a zero, with the zero located at the origin of the complex frequency plane and the pole at R/L on the damping axis. The time-domain responseof any finite linear network due to transient excitation is always

made up of terms containinge -st where s is the complex frequency of each pole in the response.For a single loop with a single pole, there is only one such term which is a pure decay. We can easily show that the time-domain

result

of the induced

current

for a

positive step in the primary magneticflux u(t)dpPis (I)p

I(t)- -•- u(t)e -t/, and for an impulse excitation, the result is

(41)

_

L22L13 1 + ito'r''

(43)

where

'r - L 22/R22. The secondfactor in equation (43) isjust the frequency responseof the loop as previously discussed.The first factor is a real coefficient which expressesthe geometrical coupling between the coils of the EM system (loops 1 and 3) and the target body (loop 2). This factor is the only part of equation (43) that will changeif the EM systemis traversed on a profile over the body, and the factor will determine the strengthand profile form of the anomalous response. Understanding this factor is crucial to EM interpretation. The factor's significance is easier to visualize

if we think of an inductance

Lij as the amountof magneticflux that cutscircuiti due to a unit current in loop j. For loops in an insulating medium, this can be calculated using the Biot-Savart formula [equation (19)]. In summary, a study of frequency domain induction in a simpleloop reveals that strengthof the normalized response at saturation (inductive limit) depends only on loop geometry at the position of the EM system, whereas the time constant (L/R) involves both the



FREQUENCY DOMAIN

TIME DOMAIN

L.IN- LOG

bJ

15

!a.i

-

•'r,,•'• , ,, ,,,,I

• • •'•1

I/r

L

lot

r

2r

3r

b.i

I

O.I/r

I/r

IO/r

I.O

I

• , Jill [

O. Ir

•

r

I.O

!10G -LOG / Re [•]

!a.i

O.I

'=- ,I //

•x!?[4 I

:-;-' O.I/r

',,, I/r

RESISTIVE L IMI T

IO/r

INDUCTIVE L IMI T

I

O.

I I IIIII

lOT

EARL Y TIME L IMI T

CHARACTERISTIC FREOUENCY

L,4TE TIME L IMI T

CH,4R,4C TERIS TIC TIME

r'

OJC=I/T

Re or Im

Im

wSMAL•Gœ

Re

Fig. 8. Frequency responseof the current induced in a simple loop circuit by an alternating magnetic field, shown in several plotting formats.

16

West and Macnae

coil receiver and employing a sharp cut-off in trans-

geometry and physical properties(conductivity). As a first order approximation, any eddy current systemof restricted dimensions may be characterized by two parameters, its time constant and its strength at the


inductive

mitter current I as the source transient, the observed

primary and secondary signalswill be

eP/I = L 13•(t)

limit.

For a time domain system measuring voltage in a

(L12L23)(u(__•) exp (-t/x)(44)

eS/I = L22

where g(t) and u(t) are, respectively, the Dirac delta and Heaviside step function. The terms in these equations do not separate quite as neatly as in the frequency domain case. All have dimensions,so scale is important. In order to estimate the geometrical term from voltage measurementsand thus to interpret something of the geometry of the target conductor, we must first determine the time constant of the response. Because the induced current system flows in a closed loop, the system has a magnetic moment. To think of the induction process as creating a magnetic moment in the conductivebody which tends to oppose the primary field and which thus partially deflects the total field away from the conductor often is helpful. In many respects, the process is analogous to what happens when a relatively insulating body in a more conductive host medium is subjectedto a current field: An anomalouscurrent density is created opposingthe primary field whose intensity saturatesas conductivity contrast is increased at a value where the primary current density is almost cancelled. However, there is

STEP

(b)

PULSE

a fundamentaldifferencein phasecharacteristicsbecause eddy current induction is dependent on the rate of change of primary magnetic field whereas charge induction is dependenton the primary electric field.

LARGE r

SMALL

r

EDDY

....

i

FREQUENCY

IN REAL

CONDUCTORS

To find the eddy current pattern induced in a threedimensional conductor of finite-size in an insulating host environment is more complicated than to find the

Fig. 9. Transient current induced in a simple circuit by (a) a step or (b) an impulse in primary magnetic field. INPHASE

CURRENTS

OUAD

I

I

i

II11]

W

STEP t

IMPULSE

TIME CONSTANT;- LAt•GE(L) , MEDIUM(M), SMALL (S)

Fig. 10. Frequency and transient responseof a simple circuit, showingthe effect of changesin time constant.


compared to the single time-constant response of a simple circuit. Not only does overall geometry of the conductor affect the spectral form of the response, but structure

pattern in an ideal loop circuit, but the physics does not differ essentially. The time-varying magnetic field


causes a vortex

of electric

field to be induced

in the

conductive region subject to the condition that the field has no component normal to the boundary. Any possible normal component is immediately canceled by the depolarizing field of the surface charge which arises on the boundary because of the infinite conductivity contrast with the insulating surroundings.Then, any remaining electric field must be a divergence-free vortex generated entirely through Faraday's law. The main complication to finding the configuration of the induced current system is that the pattern is not fixed. In general, the pattern will changeif we alter the geometry or frequency of the primary field or the distribution of conductivity in the body. The effect of these complications on the conductor's spectral responseis usually to spreadthe transition from resistive to inductive limits over a wide range of frequency (response parameter). A comparison between the frequency domain response of a conductive sphere in a uniform field and that of a circuit is shown in Figure 12. Both are plotted as functions of a dimensionless

response paraemter (crlxtoa 2 in thecaseof thesphere and toL/R in the case of the circuit). The dimensionless responsefunction is identical for all circuits but differs for bodies of different shape and conductivity distribution and, as will be discussed in a later section, the function also depends on the configuration of the primary field and the position where the secondary field is sensed.Some of the differencemay correspond just to different scalingof the responseparameter (i.e., to a lateral shift of the responsecurve when the curve is plotted with a logarithmic scale of responseparameter). But there will also be, in general, some difference in functional form. Typically, the response of a 3-D conductor correspondsto the sum of several loop responses having a range of time constants. This response can be seen in Figure 12 where the slower variation is evident at large responseparameterswhen TRANSMITTER

17

of the conductor

at fine scale can also have an effect.

If a laboratory experiment is done with a loop made of a single turn of copper wire, the observed spectral responsenear the inductive limit will not likely be well representedby equation (40). This lack of representation is because

the inductance

and the resistance

of

any real wire loop dependsto some extent on the local distribution of current in the wire, whereas generally in theoretical

calculations

of inductance

we assume that the current distribution

and resistance

is uniform

over

the wife's cross-section (such as does occur at low

frequency). In designing electrical components, we usually account for the tendency of current to locate itself in the outer parts of the wire as frequency is raised (i.e., skin effect) by assigning a frequencydependent complex resistance to the wire instead of trying to solve the three-dimensional EM modeling problem of a wire of finite thickness bent in some specific way. The engineering approach is useful because it permits us to allow in a simple way for fine structure in the current system. For instance, in a real experiment with a singleturn wire loop, there will be a difference at high frequency if the wire is solid or if finely stranded, but the two cases are qualitatively similar and can be modeled in the same way using slightly different parameter values. The ac resistance per unit length R(to) of a long, straight uniform wire of radius a can be calculated quite easily (Smythe, 1968) and has a real component

I.O

w

0.1

RECEIVER •

n.-

O. Ol

0.001

o. oool

O.Ol

o.

IO

wl/R

CONDUCTOR L 22, R22

Fig. 11. Coupling of a simple circuit to the transmitter and receiver of a loop-loop EM system.

IO2

io 5

(LOOP)

o'/zwO 2 (SPHERE) Fig. 12. Comparison between the frequency responsesof a simple circuit and a spherical conductor in a uniform field.

18

West and Macnae

then we can find the volume

( la )

R= xra2cr 1+ ]-•8+'" 1 a


distribution

of U which

will nullify the primary magneticfield. This is a classic problem in static magnetismthat can easily be solved numerically if an analytical method is not available.

•,ra2cr• a>g.

(45)

However,

there is one crucial difference. Because

-dB/dt creates the electric field and the eddy currents, then

the

B field

rather

than

the

It

field

must

be

Since the skin depth 8 varies inversely as square root frequency, we see that the effectiveresistancerisesas the square root of frequency, once skin effect becomes significant.Also, a part of the self inductance(the part

annulled. To see how this works, we may recall the depolarizing properties of a sphere as previously dis-

known as the internal term and due to the magnetic field inside the wire) falls as 1/8, so that the total impedance of the wire (not including the part due to

In a sphere or along a principal axis of an ellipsoid, a uniform field is created by a uniform magnetization according to

cussed.

externalinductance) risesas(2ico) •/2at highfrequencies. The total effect is that the inductive

effect seen in a simple circuit is significantlymodified. The induced currents do not reach the inductive

the external

inductance

of the induced

-NM

and

limit

as quickly as the simple loop formula with constant (and real) L and R would suggest,and in extreme cases where

H =

saturation

current

systemis so low that the internal term dominates(such as can arise in one and two-dimensional modeling problems), the induced current may have an approximate 45 degreephaseangle with respectto the primary field over a broad range of conditions. In engineering jargon, this is equivalent to saying that the response can be representedas an infinite sum of singlepole and zero responseswherein there is a continuousdistribution of poles lying along the complex frequency axis over a very wide range of time constants. The effects of fine structure are not trivial. Many geologicformations (especially those in which conduction is by electrolyic fluid in pores) may conduct in a relatively uniform manner, but base metal sulphide deposits often have a coarse network structure in which there are different degreesof interconnectionat different scales. Induction in such bodies may exhibit a frequency responsethat differsdistinctly from that of a uniformly conductive formation.

Spectral responseis an important characteristicof a conductor; but equally important (if not more so) are the strength and geometry of the response. This strength and geometry must be predicted in order to interpret the position, shape, and size of a conductive feature from its EM response.Unfortunately, to solve completely for the eddy currents induced in a conductor of arbitrary shape at all frequencies is often difficult, but we can find the response at the inductive limit more easily. At the inductive limit, the eddy currents create a secondary magnetic field which exactly cancels the primary field everywhere in the body. Since the eddy currents form a closed vortex, they can be represented as a distribution of magnetization which we denote as U to distinguish it from a true magnetization M, and

B = Ix0(H + M) where N = 1/3 for a sphere. Thus for U to annihilate a primary field Bp at the inductivelimit, it must take the value

Ui•:

-BP/[p. 0(1 - N)] = -HP/(1 - N)

giving a sphere a total induced moment mr at the inductive

limit

mi• = (4*r/3)a3Ui•= -2*ra3Hp

(46)

which is exactly what is predicted by the full solution. The foregoing discussion leads to a convenient rough representationof eddy current induction in any body when the body is small enough that the primary field over the body is reasonably uniform. Approximating the spectral characteristicsas a pole and zero response,we may write (for one principal axis)

m= - 1+i3'[1-N]

(47)

where m and H p are componentsof the moment and field alongthe principalaxis, • • crt,ixcoaband a, b are appropriate dimensionsof the body akin to radius in the sphere case. Equation (47) is interesting when compared to equation (29), in which the anomalous current dipole produced in a conductive inhomogeneity by the electric field is given. To calculate the response of a conductor to an EM system, we must work out the primary field at the body due to the transmitter and the secondary field at the receiver due to the moment induced in the body. To the extent that the target body can be considered reasonably small in comparison with the distances from the body to the transmitter and receiver (and assuming as is common that the coils of the EM system are negligibly small also), it is possible to consider only the values for one representative point



19

This expression is very similar to the result for the circuit model except that all the terms in equation (49) can now be estimated numerically. Later we shall use the G notation to represent dipole

in the conductor. Then, we need only to make pointto-point calculations using dipole formulas. The field of a dipole sourceis a (secondrank) tensor quantity, since the source and the field are both vectors and in general each have three components.

field calculations of other kinds, so we introduce them

We could write

all here. There are four possible types.

Hj= G•mrn jk k

1

(48)

H _- GHm E = m,

where the j and k subscripts stand for one of three orthogonal coordinate directions and the Hm superscript indicates that G is the function that relates magnetic field to magnetic dipole moment. However, to keep the discussionssimple, we shall assumethat we know

the direction

of induction

in the conductor

and the orientation of the coils of the EM system, so we require only the field in a given direction from a dipole of known orientation. Then we can symbolize the body-to-receiver and transmitter-to-body couplings as

//r• =Grb/-/rn mb, H• = Gfftmmt

[ f2 Hm f2_Hm-

s

s

(50)

E = itotxG Em H = GHjj. m,

In these expressions,we have arranged that the quantity denoted by G is always real and constant at very

lowfrequency andhasdimensions of [length] -3 (upperline)or [length]-2 (lowerline)irrespective ofwhat sort of medium we assume the fields propagate through. However, we shall need only to make calculations for quasi-staticfields in free spaceor relatively insulating media, and then the G functions are independent of frequency or conductivity. DIPOLE

where G is now a scalar function and the subscripts now refer to the position of the field point and the dipole source. The directions of the components are assumed to be known. This knowledge immediately allows us to write an expressionfor the approximate normalized responseof an EM systemto a small target body as

I'-'rb •'Jbt

eS/e • -_HF/H • =[ •r•'•

GEJj ,

o' h

SOURCE

IN A CONDUCTIVE

MEDIUM

The most commonly used source for EM prospecting is a small current carrying loop, which is essen-

tially a magnetic dipoleof momentm = •ra2nI(a radius, n number of turns, I current). In the quasistatic approximation, the magnetic field produced by a magnetic dipole in free space has the same time variation as the dipole, and its strength falls off inversely as the cube of distance in expression(20). The field of an alternating magnetic dipole in an infinite uniformly conductive medium is more complicated. The expressionsfor that field are derived in most EM theory texts and are given below. A sketch of the field configurationnear the source is shown in Figure 13 for

z

Fig. 13. Geometry of the EM field of an alternatingmagneticdipole in an infinite uniform space.The directionof E• reflects the negative sign of equation (53).


20

West and Macnae

an axially directed magneticdipole at the origin of a spherical coordinate system and in Figure 14 for a similarcurrent dipole. Displacementcurrentshave not been neglected,but the equationsare given in two forms--one for an insulating(dielectric) and one for a conductivemedium. The results are most simply described in terms of a dimensionlessfrequency or

conductivity 13= Iklr2 (response parameter) or a dimensionless distanceI]dl/2r (inductionnumber), wherek2 = -itolx(cr+ itoe).The threezonesare

m sin0

4•

(itolx)(1 + ikr) exp- ikr

112

m

sinO

4•r

(itolx) 1+

112

x exp-(1

(1+

+ i)r/5

j

2 cos 0

4•1

r3

(53)

[ 1 + ik11]exp- ik11

defined as; near, intermediate, and far in which r<•, 2 cos 0

>>Ik1-1/2respectively In theinsulating dielectric case where the dominantphysicsis wave propagation,the

4 ,r'q

scaledistance Ikl-1/2istheradianwavelength A/2zr.In

i112

2(1 - i)r 2

1 r4

52

353

+ 254...

2 cos 0

H,.= 4,r r3 (1+ ikr)exp-ikr

(54) sin 0

J

=m2cos 0[1+(1+i)r 1 _m2cos 0[1- • + 353 g

r3

exp- (1+ i)r/5

ir 2 2(1 - i)r 3

4zr

r2

m

[ 254....

sin0

4zr

r

3

x exp-

sin 0

4zrxl r

3

x exp-(1

(1+ i)r 2ir2]

x exp-(1 (1 + i)r/5

(52)

Z

[1 + ikr- k2r2]exp-ikr 1+

(1+ i)r 2it2] +

+ i)r/5

(55)

H4'=4,r,q r2 'q[1+ ikr]exp-ikr

[1 + ikr- k2r2] exp-ikr

1+

3

j (0sin0) 45rrn r2 n 1+ (1+7)11 5 +i•-5 j (0sin0)[ r2]

sin0

m

Eo= 4zrxl j

114 (51)

Ho- 4• r3

exp - (1 + i)r/5

3

medium,i.e.,

4zr

(1 + i)r

2 cos 0

the conductive case where diffusion dominates, the radian wavelength is nearly the skin depth of the

m

1 +

3

+ i)r/8

where xl is the admittivity (or + itoe).

Z

H•x •-•'•'' Fig. 14. Geometryof the EM fieldof an alternating electriccurrentdipolein an infiniteuniformspace.

(56)


The points to note from the preceding are the


following' (1) In the near zone of an alternating magnetic

dipole, the magneticfield is everywhere in-phasewith the dipole current and its form and amplitude are the same as a static dipole. The electric field circulates around the dipole axis and is in quadraturephasewith the dipole current and proportional in amplitude to to. (2) The transition from near to intermediate zone takes place when the quadrature component of secondary magnetic field due to eddy currents begins to

21

source can be representedas a surface distribution of magnetic dipoles or as a closed lineal distribution of current line elements. However, the latter approach is the only one available if the sourcecurrent is grounded rather than closed. Treating a closed loop as a sum of current dipoles is straightforward for calculating the magneticfield, but may be confusingor inaccuratefor electric

field calculations.

Most of the electric

field of

quadrature component is modified and a negative in-phase secondarymagneticfield begins to appear as

a current dipole at low or moderate frequency is due to the current divergence at the ends of the dipole. If the current line closes on itself, all the divergence-caused parts will cancel. The remainder should be the small time-or frequency-dependent induced electric field but it may possiblybecome lost in the round-off error

(r/•)3' i e we canthinkof thecurrentinduced in the

of numerical

be noticeable, increasing as (r/b)2. Furtherout, the

earth as being negligible (in a fractional sense)out to a certain range of r. Beyond this, a quadraturecurrent is encountered first, and at even larger radius some in-phase induced current is found. (3) At large distances, the field has a completely different r dependence than near the source. In the dielectric case, the fall-off of amplitude eventually moderates as the radiation field in which amplitude varies

as 1/r becomes

dominant.

In the conductive

case, the field begins to attenuate rapidly in an exponential manner, becoming vanishingly small beyond a few skin depths. The equations for the electric current dipole have almost the same form as those of the magnetic dipole except the roles of E and It are interchanged. An electric dipole is a short linear current element which at its ends injects and removes conduction and/or displacement current from the surrounding medium. At the static limit (i.e., in the near zone) in a conductive medium, we have just the usual dc resistivity and magnetometric resistivity formulas for the fields of a buried source current. The given equations do not change if the current element is in a dielectric or conductive space. There certainly is a considerable practical difference between operating a current source in a conductor or in a dielectric, but the

difference lies in the electric potential difference required from the energy source to cause the specified current to flow. In a conductive space, a source of constant voltage will produce about the same current at any frequency, whereas a similar source in a dielectric space will only inject appreciable current (i.e., displacementcurrent) at very high frequency. In practice, to be able to inject displacementcurrents into an insulator, the current element must be made a resonant length in order to lower its impedance. In some EM exploration methods, the source has appreciable size. An extended source can be treated numerically, simply by consideringthe source to be a line or surface distribution of dipoles. A large loop

calculations.

SOURCES

ON A HALF

SPACE

The formal analytical solutionsfor the EM fields of a source on a uniform half-space or a layered earth in which the conductivity •(z) varies only in one dimension have been in the literature for the best part of a century (Ward and Hohmann, 1988, Volume 1). However, they involve integrals of oscillating Bessel functions which often must be integrated numerically. Only in the past two decades has it been relatively easy to calculate results for such cases. Even now, computationsof responseat high frequency where no quasi-staticapproximation can be made are laborious. Time domain solutions are also still laborious to obtain

(Goldman and Fitterman, 1987), although results can be synthesized from frequency domain solutions by Fourier

transformation

or from

s domain

solutions

usingthe Gaver-Stehfest algorithm. Although much of the physicsof the half-spaceproblem is common with the whole-space problem discussedpreviously, there are also important differences.They arise becausepart of the field is in free space where it suffers no ohmic attenuation, in sharp contrast to the part which diffuses through the conductive medium. Figures 15 and 16 show quasi-static results for an alternating vertical magnetic dipole (horizontal loop) on the surfaceof a uniform half space. Figure 15 shows amplitude response for a variety of geometries as a

function of response parameter [3= crpocor 2. Figure16 shows phase and amplitude of the field components generatedby a vertical magnetic dipole. From Figure 15 we see that the total magnetic field near the source ([3 < 1) differs little from the free space field of the dipole. Focusing on the responseof a vertical dipole (Figure 15, casesI and IV, and Figure 16) and considering how the field varies in comparison to free space values as the observationpoint moves outward on the surface along a radius, we see that the vertical component of the field begins to change amplitude and


22

West and Macnae

I

I

I

•/////////////////!

_(a)

_

.///-•/////////--•//// 1T

(N•ALIZATION

•mO0

•

I

FROM

I

4 I0 40 I00 400

,8=o',U.o•or•ImH•'H p (b)

40% /

-120%-80% -40% I

2 %HeH•H p -'-'1 ' ' ' ' • // / LAF'GE/6' "7 /

I

•½•,r •:

t/.•J•

-40 Yo

SMALL •

S/HzP PHASOR PLOT Hz Fig. 15. (a) Normalized amplitude of the magneticfield generated by an alternating vertical magnetic dipole on the surface of a conductive half space (after Wait, 1951b). (b) Phasor plot of case I, with the response of a thin conductive layer shown for comparison(Eadie, 1979).


Physicsof Electromagnetic InductionExplorationMethod phaseas 13>>1 and eventuallyis reducedto near zero, i.e., the secondary vertical field eventually becomes nearly equal and oppositeto the primary field. A surface vertical dipole source generates no primary (i.e., free space) horizontal componentof magnetic field at the surface, so this componentmust arise solely from currents induced in the ground. The componentis a roughmeasureof the relative strengthand phase of the induced current vortex flowing underneaththe observationpoint. Moving outward from the source, the horizontal component (normalized to the primary vertical magneticfield, caseIV) first becomes evident in the quadraturephase,becomesstrongerand shifts through in-phase, and then dies away at large distance as the phase moves to -45 ø. If the total magneticfield is observedas an ellipse of polarization, the field will be linearly polarized and vertical near the source, and then will become elliptically polarized at

23

largerdistance.At even largerdistance,the ellipsetilts and its principal axis becomeshorizontal. At largedistance,the amplitudesof both the vertical and horizontal magnetic field fall off much more rapidly than they would in free space.In the rapid fall-off region (r > 3g), the electric and horizontal magnetic fields on the surface begin to obey the Cagniard equation [equation (34)]. Rapid attenuation in the far field can be understood by realizing that the current cirulation induced in the earth forms essentially a diffuse,oppositelydirectedimageof the sourcewithin abouta skin depth of the sourceloop. The field of this circulation cancelsmost of the primary field, with the cancelation becoming more complete as the observation point is taken to large radius. In the near source zone where the secondary magnetic fields are weak, the electric field of an alternating magnetic dipole approximatesthe whole space equation [e.g., equation(53)]. E and thereforeJ fall off from

thesourceasr -2. Thustheyrisesteadilyin comparison to the primary magnetic field which falls off as I00

r -3 . Figure17shows howtheelectricfieldvariesfrom the near to far zone.

There are several important differencesin the EM field when an alternating magnetic dipole source is oriented with its axis along the surface of the half space.In the near-zone,the magneticfield isjust what would be found in free space.However, the near-zone electric field is entirely different from the free spaceor

I0

uniform medium case. In a uniform medium, the

electric field of a horizontal dipole would circulate about the horizontaldipole axis, with the field lines at the surface perpendicularto the earth-air interface. But becauseair is so highly insulatingin comparisonto almost all in-situ earth materials, the conductivity

<1:0.1

O. Ol RESISTIVE LIMIT

INDUCTIVE LIMIT

27o

1.0

0.8

Q 0.6

180

0.4

0.2

90

-0.2

O0

I

I0

I00

-0.4

FREQENCY (Hz) -0.6

Fig. 16. Phaseand amplitudeof the vertical and horizontal magneticfield componentsand parametersof the polarization ellipse as a function of distancefrom an alternating vertical magneticdipole sourceon an conductivehalf space.

Fig. 17. Normalizedin-phaseand quadraturecomponents of the electric field about an alternatingvertical magneticdipole on conductivehalf space(courtesy of B. Polzer).


24

West and Macnae

contrast at the earth's surface is virtually infinite; and even if displacement currents are taken into consideration, the admittivity contrast is huge for any EM prospecting frequency. Since the normal component of total current density (conduction + displacement) must be continuous through a boundary, a charge distribution and a correspondingjump in the normal component of E must occur there. The secondaryE field of the interface surfacecharge neutralizes

most of the E field that would

otherwise

exist within the earth, and it increases the E field in the

air. The only part of E which continuesto be found in the earth is a circulation about the primary magnetic field lines which penetrate into the earth. The situation is sketchedin Figure 18. A corollary of this reasoning is that an above-surfacetime varying magnetic source cannot produce an appreciable component of electric field

or current

in a stratified

earth

normal

to the

layering. The strong conductivity or admittivity contrast at the earth-air interface always nullifes any vertical componentof the E field as it passesinto the earth.

The secondary magnetic field of a horizontal alternating dipole on a half-space behaves qualitatively much like that of a vertical dipole. In the near zone, the secondary field is so weak that the total field is virtually unperturbed (in a fractional sense) from its primary (free-space) form. As the observation point moves outward, a significant secondary field is encountered which eventually becomes so large (relatively) that it doubles the horizontal field of the source (Figure 15, cases II, III). The currents induced in the earth behave approximately like a shallow image dipole source, but the image in this case is parallel with the primary dipole. At intermediate ranges,the image analogy is very inexact, and appreciablevertical magnetic field is produced by the induced current system (case VI).

The preceding discussion has been in terms of increasingdistanceof the observationpoint from the source.

Since distance

must be measured

in units of

the skin depth (g) which changeswith frequency, the same remarks can be applied to a fixed observation

point but with an increasingfrequency of excitation. A usefulterminologydescribesthe observationpoint for a particular excitation frequency as lying in either the near zone where the secondary field is relatively negligible, the intermediate zone or active region where the secondaryfield is comparablein strengthto the primary field and where the main eddy currentslie more or less underneath the observationpoint, or the far zone where the secondary field is comparable to and may nearly annihilate or double the primary field, and where in comparisonto r the total field penetrates relatively little (i.e., g) into the ground. For the far region, all of the main eddy current vortex lies between the observationpoint and the source. In simple cases such as a conductive half space where there is only one scaling distance, the zones correspond to r • g, r • g, r >>g. At even greater distances, it may be necessaryto consider propagation effects in the air (i.e., displacement currents). In the so-called wave zone, where r > • A, wave propagationbecomesthe significantphysics. Many featuresof the responsesshownin Figures 15 and 16 are also found in the responsecurve (Figure 8) for induction in a simple circuit. At a fixed distance, any secondary field component makes a transition with increasingfrequencyfrom a negligibleamplitude to a saturation level (inductive limit). However, the quantitative form of the responsemay be substantially

different from the single pole and zero responseof a circuit. As an example, Figure 15b shows a phasor display of the vertical secondary field from a vertical dipole sourceon a half-space and an infinite thin sheet. The responsenear the low- and high-frequencylimits is much more structured than the semicirculargraph for a simple circuit. The complications arise mainly becausethe induced currents are spreadthroughouta large region and they move relative to the observation point as frequency or conductivity is changed. The inductive coupling between the current system and a field detector of given orientation may thus change strength and sign as the current moves. There is also potential for complicated interactionsbetween various parts of the current system, particularly if the earth conductivity is strongly stratified. Diffusion of an EM field from a point source into a conductive

UNIFORM SPACE

IN HALF-SPACE

ABOVE HALF-SPACE

Fig. 18. Sketch of the effect of the air-earth interface on the electric field of a horizontal alternatingmagneticdipole.

earth

can

also

be studied

in the

time

domain. Just as in the one-dimensionalproblem described in a previous section there are strong similarities between frequency domain and time domain cases.For very short times after a sharp discontinuity in the source current, all observation points will be in the far zone. As time progresses, the active range sweeps out to increasing distance and eventually leaves all the observation points in the near zone of response.There are, however, some important com-

25



plications.The closestparallelbetweenfrequencyand time domain response is found when comparing the changingpattern of in-phasecurrent with decreasing frequency with the changingpattern of induced current following a step transient in source current. But, commonly in transient EM systemsthe magneticfield is monitored using the voltage induced in a coil receiver so dB/dt is measured. Another difference arises

from the conventional methods of viewing the data. In

the examples given in frequency-domain,we plotted the fractional strength of the primary and secondary magnetic fields. However in time-domain, it is absolute rather than relative field strength which usually is recorded, since the primary field is usually not available for measurement during the observation period. Comparisonsbetween frequency- and time-domain responsesmust take accountof differentnormalization practice. Nabighian (1979) has solved for the current system inducedby a stepchangein the strengthof a horizontal loop lying on a half-spaceand has aptly describedthe current system as an expanding and sinking smoke ring (Figure 19). For a step transient current in any kind of loop lying directly on a uniformly conductive

METERS

LOOP 4

ground, the inducedcurrent beginsas an image of the source current flowing immediately under the source wire. Essentially, the total current in and near the source wire remains constant, so when the source current turns off, a similar ground current starts up.

The current system in the ground then expands and diffuses at a time-varying rate which depends on groundconductivity.The diffusiondistanceincreases with the squareof the ratio of time to conductivity. The descriptionof the transient eddy current system as an expandingsmokering is apt, no matter whether absolute or relative field strengths are considered. However, the picturewill be alteredif the excitationis not step-like. For instance, if the source current is a pulse (or equivalently if induction by a step source current is monitored by observingthe rate of changeof secondarymagnetic field), the initial smoke ring will be seen to be followed immediately by a ring of oppositepolarity. The two will diffuseout togetherand eventually will merge and annihilate each other. Examples of transient fields on the surface of a conducting earth for the step response of a dipole transmitter are illustrated in Figure 20. Grounded wires are not so widely used as sources

METERS

x I00

8

12

LOOP 4

16

0--

8

x I00

12

16

, , , I

t/o- --O.01sm/S

'1 co,rous

LOOP

4

8

12

12• t/o= O. Ism /S CO/V TOU•e$ ß/d •a/m•

/,. 16

20

24

28

LOOP 4

8

12

16

20

24

28

i

24

28

2

t /o' =l • s rn/$

CO•ITOUt•S xIO-9A/rna

CO•ITOUt•S xIO-/•/ma

Fig. 19.Contoursof the currentdensityin the "smokering" of currentinducedin a uniformconductivehalf space by a step transientcurrentin a horizontaltransmitterloop. Four snapshots in time are provided.The induced current is flowing azimuthallyaroundthe vertical axis of the loop, i.e., normal to the plotted section(after Nabighian, 1979).

26

West and Macnae

for EM surveys as loops. However, EM effects are frequently seen in IP surveys, so this case is very important. We must also calculate the field of a buried current

source in a later section where the effect of a


conductive host region on the EM responseof a local conductor

formed from the source current line and vertical edges projecting deep into the earth from the grounding points. The magnetic field of an infinite line current is azimuthally directed and axially symmetric, i.e.,

is considered.

The magnetic field of a current-carrying grounded wire on a half-space is somewhat complicated in the active region. However, the field in the near zone is relatively easy to calculate using dc resistivity concepts. In either case, taking account of the diffuse current in the ground as well as the current in the source wire is required and this can be difficult. But, for the near zone (static) case of a current source in or on a horizontally layered half-space, Ampere's law provides a convenient trick to avoid finding the exact form of the earth currents. As is illustrated in Figure 21, the source current and the diverging earth currents at each end may be turned into continuous current systemsby adding cancelingpairs of vertical current lines from infinite depth to the system.Due to the axial symmetry and the fact the no current flows above the surface, Ampere's law shows that the earth currents with their attached current lines contribute nothing to the surfacemagneticfield. Put another way, the above surface magnetic field of the earth current from one end of the source

current

is identical

40

(57)

H0 2'rrp

where p is the distance from the current line. According to symmetry and the Biot Savart formula [equation (19)], the field of a half-infinite wire in the radial plane containingthe end point is half of this. The field producedby a horizontal sourcewire exclusiveof

I

CURRENT

BIPOLE ON A HALF SPACE

to that of a

vertical line current from the surface to infinite depth so, as far as concernsthe above surface magnetic field, the total current system is equivalent to an infinite buried vertical rectangular loop with an upper edge

5 I0 •0

I

=

tX•---8Orn

EQUIVALENT

LOOP + AXIAL

CURRENTS

/60

0.5

0.4

• ........ .......

•

0.3

,, Ampere's Nocurrent through law loop

0.?_ O.I

0

•

0

I

I O.5

I

I

I

I

I 1.0

krn

(a)

H

\

X

...%

I

t,,..____l•._.o. •?(Biot -Savart Formula)

,.o

•:" -o.6 -1.0

(b)

Fig. 20. (a) The normalized secondarymagneticfield at the surface of a uniform half spaceafter a current step in a small horizontal transmitter loop (a) radial component(b) vertical component (courtesy of B. Polzer).

Fig. 21. The magnetic field of a grounded source can be decomposed into three parts, the field of the current line along the surface and the fields of the earth currents at each end. At the earth's surface, the field of an axially symmetric earth current wire.

is identical

to that of a semi-infinite

vertical


Physicsof ElectromagneticInduction Exploration Method the ground currents is also obtainable from the Biot Savart equation. By summingterms for the horizontal segment and the source and sink ground currents, we obtain the total magnetic field of a static horizontal grounded wire system. Note that the ground currents contribute only to the horizontal magnetic field, whereas the wire (if it lies directly on the ground surface) contributes only to the vertical field on the surface. A somewhat surprisingcorollary of this derivation is that the static magnetic field in the air dependsonly on the radial symmetry of the sourceand sink ground currents and is completely insensitiveto horizontal stratification in the ground.

B

A

To give some idea of the induction effects which take place around a grounded source,' the transient step responseof the electric and magnetic fields on a half-space are illustrated in Figure 22. In a uniform half-space, there can be no charge distributions anywhere except at the ends of the source wire that might contribute a time-varying component of the electric field. The E transientis therefore producedonly by the magneticfield of the wire and thus is always parallel to the wire. Note that there are, however, anomalous E fields perpendicularto the wire with instantaneous(in the quasi-staticlimit) rather than transient changesin amplitude.

A

C

9 •

C

B

,o Hz I

t(s)

,t(s) i

i

_ i

27

Hx

i

_

i

i

i

-I

-I

D

F

o o E,y

t

D

9 Hz

.•• t(s) I

I

I

i

t(s)

Hx

ß

-.,l I ' '

F

E

i

i

i

-I

-I

I

G I

I

IO

i

t(s)

i

-I

-I

ELECTRIC

FIELD

TRANSIENTS-HALFSPACE

(a)

CASE

MAGNETIC

-I

FIELD TRANSIENTS-HALFSPACE

CASE

(b)

(c) Fig. 22. Transient electric and magnetic fields on the surface of a uniformly conductive half space at various positionsfollowing cutoff in a groundedbipole source (Eadie, 1981).

28

West and Macnae

MODELING

INDUCTION

IN LARGE

ISOLATED

I=

-(itolt )Z_- lLt .

(59)

CONDUCTORS


If we then consider the mutual coupling of each Whenever magnetic source EM is used to locate highly conductive zones in very resistive host rocks, only the eddy currentswithin the conductivematerials will be of appreciable amplitude. Then we can neglect any complicationscausedby the possiblycomplicated resistivity structure of the host medium and consider the conductive body as if it were in free space. We discussed such cases in previous sections and the basic physics is straightforward. However, for quantitative interpretation of field data, we must be able to model quantitatively a variety of conductor shapes. We must also take account of the coil configuration of the EM system. Unfortunately, there are few conductor geometries for which the induction equations can be solved analytically for a general sourcefield. Much of the past modeling done in support of practical EM interpretation has had to be carried out by means of laboratory scale model measurements. Recently, numerical modeling has been successfullyemployed to give some practical results. A conceptual framework for analyzing "conductor in free space" induction problemscan be obtainedby generalizing the circuit model given in the Eddy Currents in an Ideal Circuit section. We begin by noting that virtually any finite sized vortex of eddy currents may be decomposedinto a set of predetermined current patterns of fixed geometry (called a basis). The systemthen is analyzed as if the elementsof the basis were a set of simple loop currents whose mutual inductances are taken into consideration along with their self inductances

and resistances.

Interaction

does

not greatly change the algebraic form of induction equations from what was discussedpreviously, except to give them a matrix form, viz,

current

element

in the earth to a receiver

of an EM

systemvia mutualinductances Lrj of vectorLr, we can write the normalized system response in a form analogousto equation (43) as

eS/e • = - [LrT(imIt)Z_1Lt]/(mrtIt). (60) Equation (60) has a very interesting form if an eigenfunction decomposition is performed. We may then write

eS/e v = - {L•rUSUrL}/Lrt .........

t

(61)

where _Uis the eigenvector matrix of L_and S_is a diagonal matrix S •

•

'"

Lj' 1 + ito'r j'

,

wherethequantitites Lj-and'rj arethejth eigenvalues

of thematrices L_and(R_ -• L_),respectively. Thesig-

nificanceof equation (61) is perhapsbetter appreciated by writing it out as a summation

Lj'L 'rt] 1 + ito'rj'

(62)

where

L'•j =• Lrk uu , k=l

n

L'=•L tj

tk

ß

k=l

(_R+ io•L_)I= Z_I = - RoL_t I_i

(58)

where I is an n x 1 vector of the currentsIk in each loop, and the n x n matrices, L_, _R, and Z_ have elements

such that

Zjk = Rj•jk + itoLjk. The indicesj and k index the n elements of the current

basisso that Lj• is the mutualinductance between currentpathsj andk; Rj andLjj aretheresistance and self inductanceof the jth loop; Ljt is the mutual inductance between current path j and the transmitter

and is an element of the n x 1 vector Lt; and I t is the transmitter current. The matrix R_is real and diagonal, L_is real and symmetric, and Z_is complex and Hermitian symmetric. The vector L t is real and I is complex. I t is a scalar. The formal solution of equation (58) is just

Comparing equation (62) with equation (43) we see that the primed symbols act just like the mutual and self inductances and time constants of n isolated (non interacting) simple circuits. The foregoing showsthat we can, in general, find n sets of n basis current paths wherein the sets are totally independentof one another. Each set is collectively called an eigencurrent.Currents in the elements of the basis that have the same phase and have amplitudesthat are proportionalto the elementsof one eigenvectorhave no mutual coupling (mutual energy) with any other eigencurrent.The voltagesinducedby an eigencurrentflowing in the basis elementsare also in phase and proportional in amplitude to the eigenvector, so they serve only to sustainthe eigencurrent which generatesthem. Thus, an eigencurrentset behavescollectivelyas if it were a singlecurrent flowing

Physicsof ElectromagneticInduction Exploration Method in a single isolated circuit. Each eigencurrent has a distinct

time constant

and self inductance

associated


with it. The values of these will, in general, be different from those of any of the individual basis elements alone.

The idea of representing the current flow pattern induced in a conductor by an arbitrary source field as the sum of current systemshaving known geometry is a powerful one that can be applied both in analytical and numerical modeling. To understand how the current pattern evolves with changingfrequency (or time) if the basis of the decompositionis a set of eigencurrents, is particularly easy because of their simple circuit-like spectral or time response. If the induced current pattern at the inductive limit is decomposed into eigencurrents, we see, for instance, that the responsein the resistive limit is given by the same sum but with the terms reweighted by the inverse of their time constants. The time domain step current response is analogous.The induced current immediately following the step has the same form and eigenfunction decomposition as the inductive limit response in frequency domain. The induced current at a latter delay time is just the same sum but with the terms rescaled according to the time decay of each eigenfunction

29

Nevertheless, the modeling program is a most useful interpretational aid. The sphereis the only shapewhich has been widely used for analytical modeling purposes, and is a good basic model for compactly shaped conductors. Numerous

variants

have

been

treated:

various

source

field configurations, transient and spectral response, radially varying conductivity, changing permeability, etc. In all solutions, the induced eddy currents are decomposedinto componentswhich have related patterns. The primary classification is by the external magnetic field (according to the multipole moment from which it is generated). The secondary division is into current systems producing the same external multipole field, but configured inside the sphere so as not

to interact

with

each

other.

Each

is thus

an

induced currents in a continuous conductor, but rea-

eigencurrent and there is an infinite number for each multipole component, each having a slightly different time constant and amplitude coefficient. All eigencurrents of the same multipole order couple to the primary field in the same way, and thus for all geometric purposes,the currentsfor each multipole order can be treated as a group having a total spectral response governed by the pole (time constant) distribution of the eigencurrents. A characteristic of a finite body is that the time constantsof the eigencurrentsare distinct and there is an upper bound to the time constants in each multipole set (Nabighian, 1970, 1971). Figure 24 shows in (a) the spectral responsesof the lower order induced multipoles in a nonmagnetic sphere of uniform conductivity and in (b) the corresponding step responses. Each of the spectral responsesis a sum of many eigencurrents and thus is somewhat more smeared out than is the response of a single loop. The time domain decays have a visibly nonexponential form at early time. There is also a clear tendency for the range of time constants that contribute appreciably to each to shift systematically to smaller values as the multipole order increases. The high order terms become more important near the inductive limit (in frequency domain) or at early time (in time domain). If the sphere is magnetic (i.e., its relative permeabil-

sonably accurate results can often be obtained with a

ity •x/•xo is appreciablygreater than 1), there are two

small number

changesfrom the nonmagnetic case (Figure 25). First, the time constants of the eigencurrents increase with increasingpermeability. Second, at low frequency, an additional magnetic moment is created in the sphere which has an opposite sign to the induction moment. The magnetic moments strength is in proportion to the magnetic susceptibility and the internal field intensity and tends to increase the magnetic flux density inside the body. As frequency or conductivity is increased, the eddy currents flow so as to reduce the internal magnetic flux density until the density vanishes at the

(exp-t/xj). We seeimmediately thattheeddycurrent system eventually takes the form of the eigenfunction with the greatest time constant. To consider an example, a numerical algorithm was created by Annan (1974) for computing eddy current induction in a thin rectangular plate. First, a set of numerically described basis currents is set up using polynomials. The interaction matrix is then set up and eigencurrentdensity distributions are found (unique to a plate of given length-to-width ratio) along with the time constant and self inductance of each (from the eigenvalues). Then the mutual inductances between each of the eigencurrents and the source and receiver are calculated and combined according to the loop equations (62) to give the response of the plate to a given EM system. In theory, an infinitely large set of eigencurrents is required to represent exactly the

of terms in the summation.

In the 1981

version of the computer program implementing Annan's algorithm, only 15 eigencurrents can be used and these have only a comparatively small range of time constant (•5:1). Figure 23 shows a set of eigencurrents for a 2 x 1 plate. The small number of terms means

that cases where

the receiver

and transmitter

are close to the plate (relative to the plate dimensions) cannot be simulated accurately becausethe limited set of eigencurrents cannot represent a current system that is strongly localized in a small part of the plate.


30

West and Macnae

inductive limit. Thus the moment due to eddy currents can overpower the magnetic effect, so the same inductive limit is reached irrespective of magnetic permeability. The number of multipole terms neededfor accurate representationof inductionin a spheredependson the convergenceof the summation.The coefficientsof the terms depend strongly on the radial distance of the source and observation point in relation to the radius of the sphere. If the sourceand observationpoint are more than about one sphere radius a away from the spheresurface, the first (dipole) moment will strongly predominate. On the other hand, if both are close to the surface (say <0.1a), a very large number of moments will be required in the series, and the main part of the observed response will come from the higher order moments. In the former case, the contribution from the eigencurrentof longesttime constant (which is part of the dipole term) will be a large part of the total responsebut will not be in the latter case. The

POTENTIAL

observed response will then be characterized by a broad, almost continuousspectrumof time constants. Because of the spherical symmetry, induction in a sphere is triply degenerate.Therefore, the same pattern of eigencurrents and time constants can exist about any of the three cartesian axes (or in any orientation). They will not exist, however, if the conductoris less isotropic. At the other extreme from a sphere is a thin conductive plate in which eddy currentscan circulate only in the plane of the plate so there is only one set of eigencurrents.In an intermediate case like an oblate spheroid or slab, the eigencurrents which flow normal to the smallest principal axis will have both longer time constantsand (generally, becauseof their larger area) larger mutual inductanceswith the sourceand receiver than eigencurrents oriented in other directions. Observation techniques which reveal how the induction changes its strength and time constantwith orientationcan be very helpful in the interpretationof conductorgeometry.

POTENTIAL

I

CONTOUR INT = O.03AMP

CONTOUR

TC--.I.O

TC = 0.666

POTENTIAL

INT= 0.0,7, AMP

POTENTIAL

9

3

I0

CONTOUR INT=O. 02 AMP

CONTOUR INT= 0.02

TC =0.377

TC = 0.369

POTENTIAL

CONTOUR INT= 0.02 TC=0.312

POTENTIAL

12

AMP

CONTOUR

AMP

15

INT= 0.01 AMP

TC= 0.178

Fig. 23. Streamlinesof some of the principal eigencurrentsthat can be induced in a thin (2x 1) conductiveplate (from a set of 15) (after Annan, 1974).



Much EM interpretation has been based on a dipping, conductive, infinite, half-plane because only a few parameters are required to describe the model (depth-to-top, dip, surfaceconductivity). There is now a computing algorithm for calculating time domain induction by a dipole source in such a conductor (Weidelt, 1983), but most interpretational material in the literature has been obtained by scale modeling. It has long been possibleto compute the inductive limit responsefor an arbitrary source configuration. Figure 26 shows a comparison of spectral responsesfrom a half-plane model and a small plate. Becausethere is no fixed limit to the size of the eddy current vortex that

1.0

0.8

31

can be induced in a half plane, the low-frequency responsehas a slower frequency dependence (in the measurable range) than a loop or other confined conductor (Kaufman, 1978). Because of the half-plane's infinite extent, its eigenfunctions are neither discrete nor of bounded time constant. The spectrum of time constants is continuous and unbounded. Thus, the

transient response at late time never becomes exponential but follows an inverse power law instead. A related case that can be easily solved analytically, and one that is very useful for representing a conductive overburden, is the transient responseof an infinite thin sheet. The secondarymagneticfield responseto a local magnetic source undergoing a step change in intensity is exactly representedas the field of an image source created behind the sheet at the step instant, after which it moves away from the sheet at a constant velocity z/t = 2/(ix(rs) (s = thickness of sheet). This

leadsto a t -3 latetimefall-offin theverticalmagnetic n"o. 6

field correspondingto a continuousand unboundedset of time constants in the response as expressed in equations (62). The models discussedare often used to represent conductive mineralization. All models are of uniformly conductive bodies. However, we know that sulphide mineralization is often extremely heterogeneous.On a small scale, even relatively massive mineralization is

0.4 0.2

>.

0

z

• 0.2 --

an interconnected

0 1.0

I0

I02

I03

104

I05

RESPONSE PARAMETER o'p,o(O0• t 1,0

I

I

I B

z

o 13=

U.I

0.5 MUI TIPOI •'

OBD•'B

0

o.I

0.2

t/o'p. oa• Fig. 24. (a) Spectral responseand (b) transient responseof the multiple moments induced in a uniformly conductive sphereby a nonuniform primary magneticfield (after Nabighian, 1970, 1971).

that includes a substantial

percentage of insulating gangue minerals, and on a larger scale, we usually find that the tenor of a mineralized zone is anything but uniform. Often, a less highly mineralized halo will surround at least part of the core mineralization. Although we are far from modeling quantitatively the effects of such inhomogeneity, clearly there is a highly visible influence on spectral response,just as strandingor spacingof the wire in an inductor can have an important influence on the high-frequency impedance. The main effect is to broaden the responsespectrum, particularly near the inductive limit. Otherwise stated, the distribution of time constants among the eigenfunctionsof an inhomogeneous conductor will be extended and emphasized in the direction

n--I

0

network

of short time constants in com-

parison to that of a similarly shaped homogeneous conductor. If a model with uniform conductivity is usedto interpret data from an inhomogeneousconductor, we will find that the interpreted conductivity increasessystematicallywith decreasingfrequency (or increasing decay time). Figure 27 shows horizontal loop EM (Slingram)data over a sulphidebody which shows the effect very clearly. In resistive terrain where the inductive response of the subsurfaceconductor can be accurately determined to high frequency, this dispersionin interpreted conductivity is

32

West and Macnae

so regularly observed over sulphide mineralization that it has diagnostic value.

the target conductor and can therefore be modeled as a horizontally layered structure. We shall then determine the effect which the conductive


EFFECT

OF A CONDUCTIVE

OVERBURDEN

The overburden and host rock in some parts of the Baltic and Canadian Shields are often so poorly conductive that they can be neglected totally in EM modeling. But this is rarely the case elsewhere, so we must necessarily consider their effects. If they are sufficiently conductive, clearly the EM system will respond to them as was discussed in a previous section. However, the response of an anomalously conductive zone in a conductive earth is not just the sum of the responses of the host medium and the anomalous conductor taken separately. They respond jointly. Nevertheless, we may often be able to distinguish between the two on an EM profile or map, because the anomalous conductor will produce a local anomaly which is superimposed on a broader background responsefrom the host earth. In discussinghow a conductive overburden or host rock affects the local (stripped) response of a local target conductive zone, we shall assume that if the earth as a whole is appreciably conductive, it is laterally uniform and very extensive in comparisonto

overburden

host rock have on the residual response of the target, taking the free-space response of the target as a baseline for comparison. Often, overburden is much more conductive than

the bedrock in which the target conductor is likely to be situated. If the immediate surroundingsof the target are insulating, the effect of a conductive overburden interposed between the EM system and the target is relatively simple. The magnetic field of the source is somewhat altered from its free space form in passing downward through the overburden to the target, and the secondary field of the target will be similarly modified in passingupward to the receiver. The effect can be likened to the action of a low-pass frequency filter

or transfer

function.

Unless

the overburden

is

extremely conductive so there is a very strong attenuation, the direction of the field is little changed and the amount of filtering is similar everywhere in the target zone. The characteristics of the filter can be estimated roughly from equation (37). The primary field from the sourceand the secondary field generated by the target usually are localized fields such as were

1.o

0.5

g-o.s

-I.O

Fig. 25. Spectral responseof a permeable conductive sphere in a uniform alternating magnetic field (after Wait, 1951a).

and


Physicsof ElectromagneticInductionExplorationMethod

33

discussed in equation (37) and to which we may

sponse will be observed. On the other hand, the

ascribe a horizontal scaleparameter l (= •.-1) which

overburden will have no effect at very low frequency but then there may not be much response from the target either. In the middle range where the overbur-

characterizes the region encompassingthe source and the receiver point and the significantparts of the target conductor.For passageof the fieldsboth ways through an overburden of thickness h, the transfer function will

den skin depth is finite but substantiallygreater than l, the situation referred to in equation (39) arises, and the transfer function

take the form

exp -•-02 ] •-} =exp{[ 1- (1+i[30•-• 2

To •

(63)

where[30= •0 [xtoh2. As frequency is raised suchthat the skin depth in the overburden•0 becomescomparableor small relative

exp-

will become

(ico-olxtolh)= exp-

(ic[•ol/h). (65)

An unspecifiedfree constant c (• 1) has been inserted in this expressionto take accountof the arbitrariness of the scale factor l. This transfer function is a phase shift proportional to frequency which rotates the anomaly phase angle toward (or past) the free space inductive limit. Once the phase shift becomes large

to h, [equation (63)] becomesvanishingly small as

To --> exp -

(64)

[2(1 - i)h/go]

and the primary and secondary fields will hardly penetrate the overburden and negligible target reio3 .

i

! ! [ i iii

'

' ' ' ' ["l

- o,c•-,q'Ar/t• FREO. f

'

i , , , i,,.

PEAKANOMALY OF A DOUBLE DIPOLE

-'•,-•,

EM

SYSTEM. I

- '• W

!

""""• • •

,

i

i i 1111l

IO

SCAL. EMODEL.

H/L. =4.0 •YJ

I ! ,,,,I

I

i

i !

100

Iill

i

i

i ,i,,,l

1000

10000

FREQUENCY (Hz)

'

GERTRUDE

WEST

;

;

INPHASE QUADRATURE

--•-"-...•..---1777 Hz /

,/

(/3 Z

o

u)

I

.

t.I

-2O 4S i

103

i

i

i

i

i

i

i

i

i

i

104

i

i

i

i

444

Hz

2S

I

I

I

I

o

i

105

CONDUCTANCE. FREQUENCY (S Hz)

Fig. 26. Spectralresponseof a half plane comparedwith the spectral responseof a small plate, for a double dipole EM system over the edge of the conductor (after Lamontagne, 1975).

Hz

-I0

-I0

i0z

888

Fig. 27. EM responseobservedover a massivesulphidezone at four frequencies.When the observedspectralresponseis matched, frequency by frequency, with any simple uniformly conductivemodel, the fitted conductivity rises with decreasingfrequency.The effect is large unlessthe induced responseis in the resistivelimit, i.e., the ratio of in-phaseto quadrature responõeis well below one (HLEM data by courtesy of J. Betz and INCO Limited).


34

West and Macnae

(• 1 radian = 57ø), we must revert to the more accurate form [equation (63)] and we find that the amplitude attenuation begins to become very noticeable. Basically, the overburden acts as a low-pass frequency filter; which has a linear phase-frequencycharacteristic in the spectral region just below the cutoff fre-

e•: - To ([30, I/h)A t,1- i•! - Ro ([30, I/h)Ao

(66)

quency.

where[3o= •r0!•toh2, •/ = ito'rt,, Ao is a geometrical

The EM responseof the overburden itself (i.e., of an earth without the target conductor) can be thought of

factor describing the normalized high frequency (inductive limit) response of the overburden and, as in equation(49), Ao is the normalizedinductive limit free space response of the target body

as a reflection response,and its spectralform R0 will be approximately the complement of the transmission responsediscussed,i.e., R0 = (1 - To). The amplitude A0 (i.e., the inductive limit response)will depend on the position of the receiver and transmitter relative to the overburden and might be estimated using image concepts. In the case of a horizontal coil transmitter and receiver, A0 is just the negativeof the free space primary coupling. If we then approximate the free spaceresponseof the target conductor by a singlepole and zero response as in equation (47), we are in a positionto write an approximaterepresentationfor the total EM response of the target and the overburden together. PHASOR

DIAGRAM

AMPLITUDE

:--

FOR LOCAL

PLATE

UNDER

STRIPPED

At, = -(G tt, HmmbLG•m)/GtrHm , where m/•œ= V/(1 - N). Figure 28 illustratesthis by showingin a phasorplot the local (stripped) responseof a vertical plate target conductor

under

a conductive

overburden

as com-

puted by a full numerical modeling method. Figure 29 showsa field case where the horizontal loop response from a steeply dipping conductor in an insulating unweatheredPrecambrianbedrock is phase shiftedby the conductive

overburden.

The effect, in time domain, of a conductive overbur-

ANOMALY

OVERBURDEN

Q 2%

Ft?EE

SPACE

CONDUCTIVE PI_ATE

IO 50M

CONDUCTIVE PLATE UIVDEt? OVEA'BUt?DEIV

iP

!

bTHIN II :;.•:% "'.\J

1 DIKE

z

/

i ; \ '-.• •

'.

crd (S)

_ 2ø/o•

•

A B C

'....-..-..:.:_.= •. ß

OB

DIKE

2.5 2.5 0

15 50 50

N

/VEGAT/VE

TRANSMITTER

FIXED FREQUENCY (500 HZ) PLATE---

AND FIXED or OVERBURDEN

I000

x 500

DISTANCE

m

I/

500 x 250m

50 m THICK

PLATE DEPTH -- 15,Ore

A R

IOOOs 30

I 2

FIXED MODELS

C

10.5

$

0.02

O/B

Z)

7

4

0.045

S/m

E

3

5

O. lO

o. S/m O.Ol

(a)

II/X• •

0.3

= 250 m

•/

\_• t

.....

PLATE

S

A o 0.05

15

R o 0.05

50

%

ess

CO/1/DUC7'/I,•- PLATE

tl UIVDEt? OVEt•Ut?DEIV

o.I

o.I

I

IO

TIME (ms)

(b)

Fig. 28. (a) An exampleof the effectof a conductiveoverburdenon the strippedfrequency-domain responseof a target conductorbeneath it, obtainedby numericalmodeling(Lajoie and West, 1976) and (b) the strippedtime domainstep responseas transformedfrom the frequencydomainresults(Westet al., 1984).



den on changingthe local, residualresponseof a target conductorfrom the conductor'sfree spaceform is also quite simple. Transformed to time-domain, the first term of equation (66) is a convolution of the overburden's pulse transmissionresponsewith the free space target response. The secondary magnetic field responseof a target in free spaceto stepdiscontinuityin transmitter current is a simple exponential decay, as given in equation (44)

Ro•

(68)

The transformsof To and Ro then also consist of a simpleexponentialhavinga decayconstant'r0

To(t) • [1 - exp- (t/xo)]U(t)

[To(t) * Abu(t) exp-(t/xb)]

Ro(t) •

- [Ro(t)Ao].

1-itoxo,) 1•i•xo itoxo ,)'

To• '1+ icoxo

HS(t) Hv(O)

35

u(t) exp - (t/xo).

(69)

(67)

where To(t) andRo(t) are the Fourier transformsof To and R0. The overburdentransmissionfilter in equation (63) is a little difficult to transform,but for heuristicpurposes and when the overburden conductivity is not great, it too can be roughlymodeledas a circuit whose responseis the complementof a singlepole and zero loop responsewith time constant%

PROSSER

Convolution of the overburden filter with the target body's free space responsethen just blunts the onset of the transient and delays the overall responsean

amount 'r0, i.e., a finite rise time of order 'r0 is introduced on the target response. For example in Figure 28, 'r0 is about 1 ms, and the delay at times >2 ms is easily seen (curve B) when compared to the free-space response (curve C). The late-time difference between

SECTION

curves

B and C should

thus not be

20E

Fig. 29. A field exampleof a slingram(HLEM) surveyover a graphiticconductorin resistivebedrockshowing phaserotation by the overburden(courtesyof J. Betz).


36

West and Macnae

regarded as an amplitude enhancementat equal delay time. The more accurate filter function of equation (63) can be transformed, the only difference is that the transmitted pulse itself is predicted to have a blunt beginning. However, the effect on the overall response is little different from that of equation (69). Since the secondary magnetic field generated directly by the overburden layer [correspondingto the second term in equation (66)] has basically the same frequency responseas transmissionthrough the overburden, the time domain form of response to a step current in the transmitter will be essentially a decaying transient

with

about

the same time

constant

as the

transmissionresponse.At early times (relative to x0), the primary field is mainly reflected back upward, whereas at later time, it penetrates through the overburden sufferingonly a little delay and blunting. When it reaches the target body, a secondary response is excited, which then must be transmitted back to the surface which causes blunting and further delay. Obviously, if the time constant for double transmission through the overburden is long compared to that of the target body, the target will be invisible to a surface EM

CONDUCTOR

IN A CONDUCTIVE

depolarization factor of the body in the direction of JP). Its strengthis limited by the depolarizationphenomenon, i.e., by the charge which arises on the boundary of the conductor. From equations (26) and

HOST

A conductive host medium surrounding and in contact with the target body has a more complicatedeffect on EM response than does an isolated overburden, although there are some common features. This problem is difficult to analyze theoretically because of a lack of appropriate coordinate systems in which to describe the geometry of the conductors. Numerical methods of solving integral equations for the EM field have been used to make fairly comprehensive studies of the response of a vertical plate target in a conductive host to Turam and Slingram EM systems (Hohmann, 1988; Hanneson and West, 1984). A few cases where the target is a three-dimensionalblock have also been studied numerically. Analog model studies of other conductor geometries and other configurations of the EM system show characteristics similar to the numerical

Ja is related to the primary current densityJP in the host medium, the relative ohmic susceptibility• of the body, and to the shape of the body relative to the direction of current flow (represented by the shape

(28)

system.

TARGET

system, the source of galvanic current in the host medium is the regional scale induction, i.e., those induced currents that produce the host medium's background EM response, as discussedpreviously. In analyzing the galvanic current flow pattern due to the presence of a local target body, we consider the current density in the ground to be made up of two parts, a primary current density which is what would exist if no target body were present, and a secondary component due to the body's presence which is the aforementioned galvanic flow. Then, following the procedure discussedin an earlier section, the galvanic flow will be considered to be generated by an anomalous current flowing in the host material and confined within the target zone. The anomalous current density

PRIMARY

(a)

FIELDS

P Tx•

E •b)

SECONDARY FIELDS AND INDUCED CURRENTS

H$

models.

The basic novel element in the physics of EM induction when a conductive host is present is that a second type of current flow can produce a secondary magnetic field, in addition to the closed vortex type of current

circulation

that

is induced

in a conductive

body in free space. The situation is sketchedin Figure 30. The additional pattern is a current flow which passes through the boundary of the target. We shall refer to this flow as a galvanic current becauseit enters the target body by direct contact with the host medium. For a magnetically coupled (loop-loop) EM

(c)

GALVANIC VORTEX INDUCTION (d)

Fig. 30. Sketches of galvanic and vortex induction in a conductive zone. (a) primary magnetic field, (b) primary electric field, (c) induced galvanic current flow and its associated secondary magnetic field, (d) induced vortex current flow and its secondary magnetic field.

Physicsof ElectromagneticInduction Exploration Method •r•E

•

•


Ja-- (1+ N• )

of (distance) -2. However,thisis not so simpleto

J•

(1+ o•)N

(70)

where a = K•N is the normalized channelingnumber of the target conductor. The total electric current dipole moment of the body is then

Just as the toroidal induction vortex configuresitself so that its secondarymagnetic field tends to cancel the primary magnetic field in the target body, the primary and galvanic cu•ent circulations together produce a charge distribution on the surface of the target body which in turn creates a secondary electric field that tends to cancel most of the primary electric field inside the target body. The charge concentration arises nearly instantaneously, so the frequency dependence of the galvanic cu•ent systemis essentiallythat of the primary electric field in the target area. Skin effect in the galvanic cu•ent may modify this picture to some extent, but only if the target is very conductive and depolarization is minimal. We shall neglect this complication here, as it only arises in special cases. The nature of the primary electric field produced by in the host medium was discussed

in previous sections. For the moment, we shall consider only the case where the overburden and host rock are insufficiently conductive to attenuate the electric field much in comparisonto the resistive limit cases; i.e., the target zone is in the near zone of the transmitter. The near-zone electric field at the body due to an alternating magnetic dipole transmitter mt always has the form

E• = i•G•m

t

(71)

where G is a pure geometrical factor whose dimen-

sionsare (distance) -2. For example,the horizontal (azimuthal) electric field in the near zone of a vertical magnetic dipole transmitter of moment m at a cylindrical radius p and depth d will be - i•m

E, =

i

evaluate as is the surface magnetic field of a loop current (magneticdipole) becauseit is essentialto take account of the host space. The problem is familiar from analyzing the magnetometric resistivity method and was discussed in an earlier section. The surface

magnetic field of the total current system established by a source current in a horizontally stratified host

j• = VJ a .

the EM transmitter

medium

was shown

(72)

Thegalvanic cu•ent system setupbyE, cangenerate a secondary magnetic field at the surface. We may express the surface magnetic field at the receiver at low frequency, due to a short sourcecurrent dipole of strengthjo at the center of the target body in the interior of any horizontally layered earth, symbolically as

(73)

where, as in the case of the electric field of a magnetic dipole, G is a pure geometrical factor with dimensions

to be like that of an extended

rectangular buried loop of current where the source current dipole forms one side and the remainder is completedby two vertical segmentsto infinite depth. As an example, the components of the surface magneticfield along a traverse along the x axis due to a horizontal current dipole in the y direction at depth d under the origin are Jb

X

4,r (x2 + d2)3/2 (74) Jb

Z

HxS = •7rr (x2+ d2) 3/2' The form of this is generally similar to the field of an x directed magnetic dipole located at the same point (such as might be produced by vortex induction) except that this is a broader anomaly, because of the weaker distance dependence in these Green's functions (McNeill et al., 1984). At this point, with V,a, and N defined in equation (28) we can combine equations(71) and (72), include a transmission filter factor for the frequency filtering effect of conduction

in the overburden

and host rock

between the target body and the surface, and write an expressionfor the normalized anomalousresponseof an EM system due to the galvanic current system.

(es) =Tøh rt PP galv x(/{r•-•cøV) (1•a)} (75)

P

4x (p2+ a2)3/2.

rb Jo

37

wherethemiddle termcanberewritten asi •hV/Nl2in which•h=O'h lxo•l 2. To this result must be added the vortex

induction

response of the target body as it would occur in free space[equation(49)] and the responseof the host rock and overburden.

To roughly quantify the transmissionfilter in equation (75) we follow the procedure used in the last section where a simple expression was obtained for the filtering effect of the overburden on magneticfields passingthrough it, and was used to estimate the vortex induction responseof a target body under a conductive


38

West and Macnae

overburden. We can do the samefor the galvanic case. The magnetic field of an alternating galvanic source current systemwill, in general, sufferphasedelaysand attenuation in reachingthe surface,just as the primary electric field will suffer some phase delay and attenuation in reaching the target region from the source. The situationis completely analogousto the modification factor for the magnetic field given in equations (63-65). When the overburden and host rock are conductive but still relatively easily penetrated by the

er). The parametersa, 13,3' describein dimensionless terms the physicalpropertiesand property contrastsof the model; i.e., they are, respectively, the target body's normalized current channeling number, the overburden and host inductive response parameters, and the target body's normalized inductive response

fields, and the transmitter and receiver locations are

[3oh = O'ohp•told,

close to the target body, the filtering factor is just a phase shift

•h = O'hIxøø/2,

Toh = exp - (2cio'ohp•cold)

parameter.

ot = KrrNg,

(78)

(76) '¾ •

0.) Tb,

where

I = horizontal scale of the EM system and the target body, d = depth of the body, and h = thicknessof the

O'oh-- [o'oh + Crh(d- h)]/d and where d is the depth from surface to significant parts of the target body and h is the overburden thickness.Except for possibleminor differencesin the geometrical factor c, the same expressionwill apply to either the induction vortex or the galvanic currents. A more complicated formula could easily be written, based on equation (63), to include the attenuation effects which set in at higher frequencies. We can therefore write an approximate expression for the normalized responseof a small target conductor in a conductive

host rock and under a conductive

overburden to a loop-loop EM system which takes account of both galvanic and vortex inductioncurrents in the target body and the host responseas

Hs es

[GrbVGbt 1

HjEm H•,=e'-• =Toh([3oh) •'GrHtmNal2)

overburden.

The foregoingexpressionshould not to be expected to predict EM response accurately for a given model geometry,but the expressiondoesindicatethe form of the responseand how the responsewill changewith an adjustmentof model parameters. Note that the vortex inductive and galvanic terms in equation (77) have opposite signs. This is simply a consequenceof the sign conventionsfor the G functions. Usually, when the targetbody is well coupledto the EM system,both terms will contribute in the same polarity. Figure 31 shows some frequency domain response data for the Turam secondary field anomaly of a plate conductorin a conductive host, calculatedusinga full numerical model. The results show anomaly generation by both galvanic and vortex induction modes. West and Edwards (1985), show that this data can be almost perfectly reproduced by a simple formula like equation(77) if the geometricand other parametersof the target body are appropriately adjusted and the transmissionand reflection factors are computedusing layered earth modeling formulas. Certain parts of the responsesystem have been left out of the above analysis (specifically, the mutual coupling between the galvanic and vortex induced

•Or•-H• (1 St) ()ot i[3hJ-- IGrI•m V_G•ttm)

X 1nt-Ot x 1+i3,

q- AohRoh(•oh)

(77)

where the G functions are the low frequency normalized Green's functions expressing the geometrical relationship between a field component at one point

producedby a dipole moment at another point, Tohis the normalized low-pass frequency filter that expressesthe phase shift and attenuation sufferedby the primary and secondaryfields in beingtransmittedfrom surfaceto depth and back to the surfaceagain,Rob is the normalized spectral reflection response of the ground, and Aoh its inductivelimit amplitudenormalized to the free space primary response. V is the

volumeof thetargetbodyandN a andN i areitsshape depolarization factors along the axes of galvanic and vortex induction (usually perpendicular to one anoth-

currents and between them and the currents in the host

space and overburden). Also shown in West and Edwards (1985) is that in usual situations they have negligibleeffect on the observed response.This is an important observation, because accountingfor these effects is one of the main reasons that full numerical

solutionsare so computationallylaborious. It suggests that much interpretation could usefully be based on approximate solutionsthat are easier to compute. One advantageof a simple form like equation (77) is that it can be transformed to time domain quite easily because the frequency dependent parts are clearly identified. The step responsewill be


ns(t)


n(0)=

39

AohRoh(t) is the secondary(reflection)responsetran-

fGrb VGbt 1

sient of the host rock and overburden

hav-

ing the important part of its time constant spectrumcentered about *oh.

x l+c• (,•xl 2•(t)

Figure 32 shows some scale model time-domain magnetic field step responsesand also Fourier transformed versions of (Holladay, 1981) Lajoie's data from Figure

fGrl• m V

-- [Sr=•(1-N •)

31.

We obtain no information about responseamplitude just by looking at equation (79), but order of magnitude estimates are easily made by evaluating the G functions for a typical case. Consider a vertical magnetic dipole transmitter at a horizontal distance I from a vertical component magnetic field receiver and straddling a small target conductor whose depth to center is d (Figure 33). The free space inductive coupling

xu(t) exp (-t/,o) }+AohRoh(t) (79) where

Toh(t) is a delay and smoothingfilter for transmission from surface to the conductor depth and back, having an approximate time constant of about *oh. PHASOR DIAGRAMS-STRIPPED

A:•0% l

terms,Grl•m(thevertical magnetic fieldofa horizontal

PEAK TO PEAK LOCAL RESPONSE

B;,o%[

-IO%

-2o%

A

GEOMETRY L

f = 500 Hz L = 500m

B (dosheal f=250curve) Hz

crd(S)

DIKE

or(S/m)

A

I

o

crd 60

2

IxlO-4.

o-H

B C

I000 :30 10.5

D

7

HOST

:3

:3xlO

4 5

IxlO-3 :3xlO'3

6

5xlO'3

L

ß

500

0

250

m

$0

S

0

S/m

<> 0.002 0.008 e x +

0,006 0.02 0.06

0.024 0.08 0.24

TRANSMITTER LOOP

Fig. 31. Computedmodeldata showingfrequencydomaininductionin a conductiveplatein a conductivehalfspace. The data are shown as phasor diagramsof the amplitudeof the local (stripped)anomaly due to the plate. The anomaly from the half-spaceis not included (after Lajoie and West, 1976). (a) The effect of variations in the conductanceof the plate, (b) the effect of halfspace conductivity.


40

West and Macnae

4

6

8

I0

12

14

I

I

I

I

I

I

DIKE WITH

TRANSMITTER WIRE AT 0

3øø F

•jWITHCONTACT

6.4

3.2

100 _-• •DIKEONLY •eeeeeeee .... '

:---x - /

•

- !

tl.I

•

OVERBURDEN

0.8

_

I I

0.4

I

I

•NO CONTACT

I

I 0.1

I I

TIME (ms)

IOO[•

60m

200'

3

A FREE AIR

.•

SCALE

DIKE 80B

IN CONTACT

'"•..

x

.-F •-

30

0 -3 o 2xlO 6xlO-3 x 2x I0-2

5 + 6x10-2S/m

DIKE 80B .......... ....

NO CONTACT OB ONLY

MEASURING LEVEL

DEPTH=ISm T THIN OB SHEET I.IS DIKE •] 120S• 300x150m

'• o. I o.I

C

i

i

I

io

TIME (ms)

Fig. 32. (a) Time-domain step responsefrom a scale model experiment similar to the model in Figure 31 (b) plotted decay characteristicsand (c) decay plots of stripped responsefor a conductorin a half space obtained by Fourier transforming the curves in Figure 31 (after Lamontagne, 1975).


magnetic dipolein the body),Giftm (thehorizontal

dominant. However, if the overburden is much more

magnetic field in the body due to a vertical magnetic

conductive than the hostrock, all response from the target may be cutoff by the overburden transmission filter before that happens. The interrelationshipsare shown schematically in Figure 34.

dipoletransmitter), andGffm (theverticalmagnetic Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

41

field at the receiver due to the vertical magneticdipole transmitter)can be evaluatedfrom equation(21). The

lowfrequency galvanic coupling termsG•' (thevertical magnetic field at the receiver of the galvanic induced horizontal current dipole in the body) and

G•tm (thehorizontal electricfieldin thebodydueto the vertical magnetic dipole transmitter) can be ob-

TWO-DIMENSION•

AND

FAR-FIELD

MODELS

Most of the foregoing has dealt with controlled sourceEM where the sourceis near the target body. If

tained fromequations (72)and(74).Choosing l - 23/2

the zone of interest is in the far field of the source, or

d which gives maximum couplingfor vortex induction about a horizontal axis, and assuming a spherical conductor of radius a, we then obtain for the galvanic and vortex inductive terms of the secondary field anomaly

if the source field and the interesting conductivity

-

=

ep Hp Toh(•oh) [-•+ 27

i[3h

I +ot

1+i-y '

The result in equation (80) is just an example for one coil configuration, depth, and body shape. In this particular case, the inductive term dominatesas long as the inductive time constant is large enough. Al-

though N ø and(1 - N i) couldbe ratherdifferent in magnitudefor a stronglyellipsoidalbody, the obvious difference in the expressions is in their frequency dependence.The i[3h in the galvanic responseversus the i'y/(1 + i'y) in the vortex inductive responsegenerally ensures that, when the target body is a much better conductor than the host rock, frequencies can be found where the toroidal responsewill dominatethe galvanic response. If the frequency is raised sufficiently above that necessaryto reach the target body's inductive limit, the galvanic responseshould become

structure are two-dimensional, there are several im-

portant differences. A two-dimensional

case is one in which

the strike

length, both of the structureand of the sourcefield, is largecomparedboth to the skin depthof the field in the host medium and to the important dimensionsof the model in the principal cross-section.In such a case, the

manner

in which

the

actual

structure

and

its

exciting field terminate along strike has no effect on the observations, and this leads to great analytical simplifications.Unfortunately, it is rare in the interpretationof practicalEM datawhen thesecriteria hold throughoutthe observedfrequencyspectrum.Only for the highestfrequencydata is the host skin depthlikely to be sufficiently short. In a far field case, the exciting field comes to the regionof interestfrom a distantsourcethroughthe air. The rate of falloff in the far zone is rapid in terms of fractional distance between the source and receiver,

but because this distance is large compared to the dimensionsof the region of interest and to the skin depth in the host medium, the lateral falloff in the intensityof the surfacefield acrossthe zone of interest will be relatively small in comparisonto the vertical attenuation (skin depth). If the ground is uniform laterally, the primary field in the regionof interestwill be nearly uniformlaterally, at leastover distancesof a few skin depths in each direction. If the region considered in a model is a few skin depths in lateral and vertical dimensions,the primary field can be viewed as comingonly from the top. However, if skin depth in the host material becomes large compared to the regionof interest, the primary field mustbe considered as comingin from all sidesof the model. The environment of the modeled taken into account.

structure

will

then have to be

Two-dimensionalmodelingis attractive from a theoreticalpoint of view becausethe EM field can then be separatedrigorouslyinto two species,TE (transverse electric) and TM (transversemagnetic)modes. Contrary to what a geologistmight assume,"transverse" Fig. 33. Geometricalconfigurationfor the caseconsideredin equation(80).

here

indicates

that

the named

field

is in the strike

direction. The separationenablesus to write the EM


42

West and Macnae

field equations in terms of two scalar variables which each satisfycompletelyindependentequations.In the TE case, no charge distributions can form on conductivity contrasts because the electric field is always parallel to the interfaces. The distinctionbetwen galvanic and vortex induction then no longer exists. In a small conductivefeature, the principal componentof electromagnetic induction is a general unidirectional current flow along the body. Higher order terms involve flow in both directions. The principal term may be considered as a vortex induction closing at infinity or as a galvanic channelingcurrent which

CONDUCTIVE LAYERED EARTH + GALVANICALLY RESPONDING LOCAL FEATURE OVERBURDEN-HOST FILTER

enters the body at an infinite distance from the crosssectional plane. The strength of such a current is not

limited by external field self-inductionor by depolarization, and the casementionedin respectto equation (45) applies. The strengthof the anomalouscurrent in this case is limited only by skin effect in the current system.If the host mediumis relatively insulatingand the sourceis a nearby long wire, the secondaryfield has the usual resistive

limit where the anomalous

current is in quadrature phasewith the sourcecurrent and magneticfield and its strengthis proportionalto frequency. But as frequency is increased, the induc-

CONDUCTIVE LAYERED EARTH + INDUCTIVELY RESPONDING LOCAL FEATURE

Toh

OVERBURDEN-HOST FILTER Toh

ß GALVANIC

INDUCED

_

•

CURRENT \DIPOLE 0')

bJ

(log)

/

......

.VORTEX

INDUCTION E(m)

ym= I OVERBURDEN-

•

(log)

HOST FILTER

•

•oh =I

.VORTEX

•

/INDUCTION

(log) g ALVANIC

(log)

INDUCTION

VORTEX

b.I

INDUCTION

OVERBURDEN

OVERBURDEN + HOST

ANOMALY Ao•Ro, •

+ HOST

ANOMAL r Ao• Roh •

/

VORTEX INDUCT/ON•

ANOMALY(] )

•

ANOMAi_Y (j), • n,'

z o

VE••RESiS• •

CONDUCTI HOST

rIVEHOST (log)

H/GHL Y'• r/

TARGET

$MODERA TEL Y

(log)

coouc n TARGET

Fig. 34. Sketchesof the frequencyand time-domainresponsecharacteristics of galvanicand vortexinduction.



tive limit is reached very slowly, with the phaseangle remaining near 45 degrees, and the net amount of current flow along the target tending toward equality with the current in the source wire (but opposite in polarity). Unless the host medium is extremely resistive, its filtering effect on the primary and secondary fields will control the responsebefore a full inductive limit is reached.

In TM modes, the direction of current flow is in the principal plane of the model, so charge distributions are created on conductivity boundaries. The physics of TM mode induction

is therefore

more similar to the

three-dimensional, local source cases we have discussed. However, TM excitation of long strike con-

ductive bodies is generally impractical or unfavorable from an EM prospectingpoint of view, so suchmodels are of limited importance except for studying induction effectsin resistivity and IP surveys, or in CSAMT where electric field measurementis a dominatingfactor.

Modeling of far-field EM prospectingmethods such as VLF, E phase, AFMAG and magnetotelluricsgenerally needs a different approach from that which is appropriatefor near-field, controlled-sourceEM methods, and has not been considered here. The differences

are most marked for modeling the response of large structures, where the scale of the significant induced current system is controlled by the skin depth of the host medium rather than by the size and configuration of the prospecting system. Induction in small targets with strong conductivity contrast to a uniform host medium (where "small" means small in comparison with host skin depth) is basicallythe sameas described in the previous sectionexcept that the galvanicmechanism for generating anomalies is more important because the primary electric field is relatively strong. However, the backgroundresponsefrom the overburden and host medium has a different form. Only the secondaryfield returning to the surface can be considered a localized field as discussedpreviously. The incidentsourcefield will obey a one-dimensionalequation. It will generallybe impracticalto defineprimary field as the free spacefield of the source. A uniform or uniformly stratified half spaceexcited from a distance will be the usual reference

EXPLORATION

case.

PHILOSOPHY

It is usefulto considertwo paradigmsof EM exploration. The first has grown out of the search for massive sulphidebase metal ores and is the conductor search problem. The object is to discover highly conductive regionswithin the ground, and at the same time obtain as much geometrical, dimensional, and conductivity information about them as possible.The problem is to maximize sensitivity to all significant

43

conductors without obtaining too many false alarms from uninterestinggeologicalfeatures such as irregularities in conductive overburden, large moderately conductive bedrock formations such as graphitic metasediments, etc.

The secondparadigm is three-dimensionalmapping of ground structure. Here the objective is to map the three-dimensionalconductivity structureof the ground in as much detail as possible without making strong prior assumptions about what the ground structure may be. In practice, due to the diffusive nature of the EM field, a highly simplified picture is all that can be obtained, even in the best of circumstances. The

parameters of the EM measurement system will, in this case, be selected according to the depth range of interest and the anticipated range of earth conductivities in the investigatedarea. Any tuning of the system will be designedto maximize the information content of the EM system's data outputs over typical ground. If searchingfor conductorsis the objective, the logic developed in the conductive overburden and conductive host sections is applicable both in the design or selectionof the EM systemand in the interpretation of data. In choosinga system, we must first consider the ratio of target signal level to system noise level, and then devise means to combat the target to geologic noise problem. Basically, the transmitter moment, the physical scaleand geometricalconfigurationof the EM coils, and frequency range all affect the EM system's ratio of target signal to system noise, and both scale and moment must be maximized for deep penetration. Naturally, geologic noise will be much more apparent in the data from deep penetration systemsbecause of their very high sensitivity. Discrimination between prospective and nonprospective anomalies then becomes the key factor which limits performance. In the first instance, discriminationwill likely be basedon the observed time-constant(s)of the anomalousresponse. However, there may easily be a very significantoverlap between the response time constant spectra of desirable targets and those of local structures in the host medium and overburden. Indeed, if the target

respondsgalvanically, there will usually be little differencein spectralresponseof a bedrock target and an overburden structure. Various other means can then be used to effect discrimination but all are based in

some way on geometry. Witti a wide band EM system of fixed transmitter-movingreceiver type, the outward migration of the currents induced in the host-overburden structure can be tracked with time or frequency and separatedfrom the relatively stationary response of local structures. The mechanism for generation of each local anomaly componentcan then be identified by model fitting. Much the same can be done with a moving sourceEM system, althoughidentificationof the two componentsmay be somewhat more model

44

West

and Macnae

dependent. Ideally, of course, we would like to have broad band measurements


tion for several

different

made at each receiver

distances

and directions

sta-

to

the transmitter. In airborne systems, where the lateral separation of the receiver and transmitter is usually fixed and often small compared to flight height, differences in transmitter orientation can help with the discrimination problem. It is our view that in any difficult EM exploration problem, stripping of response into separate components will usually be essential to making an interpretation. This process is not enhanced by plotting data on logarithmic amplitude scales or by transforming data to apparent resistivities. Nevertheless, a component of the response will usually be due to host-plusoverburden conductivity, and any interpretation of other components will require an estimate of the host-overburden structure as a startingpoint. Thus, an apparent resistivity (or apparent conductivity) approach to interpreting the regional structure can be fully justified. However, the physics involved in producing the rest of the response is usually not akin to the host-overburden induction problem, so local anomaliesrequire a careful analysis. Free spaceinduction models may be very appropriate for interpreting the stripped late time or low frequency responseof a local conductive feature, even when strong galvanic response is likely in other parts of the responsespectrum.

When three-dimensional mapping of conductivity structure is the objective, there is little alternative but to use a stratified

earth model as the basic reference.

If

the EM survey is to provide a reasonably detailed, unambiguous interpretation, it must cover a wide enough frequency spectrum that skin depth in the important ground materials varies from much larger than the maximum depth of interest to a number that is small or at least comparable to the geometric mean of the EM system's scale size and the minimum depth of interest or required depth resolution.Whatever method is used, the soundings must be made with sufficient lateral density that the degree of lateral inhomogeneity in the ground can be determined in order that the suitability of a one-dimensional stratified earth interpretation model can be assessed. In a ground EM system with a separated transmitter and receiver, the spectral coverage should be sufficient that far-, active-, and near-zone response from the ground will be observed. The depth range explored by the induced currents

will

then

shift

from

a value

less than

the

receiver-transmitter coil separation (the minimum depth resolution) to one comparableto or substantially larger than the separation (the maximum depth of exploration). The high frequency far-zone data will reflect the conductivity of local surface structure near

the receiver whereas the low frequency near-zone data (if such can be accurately observed) will reflect not only the deepest but also the most general aspectsof the conductivity structure on a lateral scale larger than the receiver-transmitter separation. In a conductivity mapping scenario, rapid spatial variation in responseis much more likely to be due to the lateral heterogeneity of the typical conductors in the ground than due to zones of exceptionally high conductivity. Thus, it shouldbe expected that most of the local anomalies will be of galvanic rather than vortex inductive type. Interpretation methods must take this into account.

REFERENCES

Annan, A. P., 1974, The equivalent source method for electromagnetic scattering analysis and its geophysical application: Ph.D. thesis, Memorial University of Newfoundland.

Eadie, E. T., 1979, Stratified earth interpretation using standard horizontal loop electromagnetic data, Research in applied geophysics no. 9, Geophys. Lab., Univ. of Toronto.

Edie, E. T., 1981, Detection of hydrocarbon accumulations by surface electrical methods: a feasibility study, Researchin applied geophysicsno. 15, Geophys. Lab., Univ. of Toronto.

Goldman, M. M. and Fitterman, D. V., 1987, Direct timedomain calculation of the transient responsefor a rectangular loop over a two-layer medium; Geophysics, 52, 997-1006.

Grant, F. S. and West, G. F., 1965, Interpretation theory in applied geophysics:McGraw-Hill, New York. Hanneson, J. E., and West, G. F., 1984, The horizontal loop electromagnetic response of a thin plate in a conductive earth: Geophysics,49, 411-420, (Part I), 421-432 (Part II). Hohmann, G. W., 1988, Numerical modelingfor electromagnetic methods in geophysics, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics, Volume 1: Soc. Expl. Geophys., 313-363. Holladay, J. S., 1981, YVESFT and CHANNEL: A subroutine packagefor stable transformation of sparsefrequency domain electromagnetic data to the time domain: Research in applied geophysics, No. 17, Geophys. Lab., Dept. of Physics, Univ. of Toronto. Kaufman, A. A., 1978, Frequency and transientresponsesof electromagnetic fields created by currents in confined conductors: Geophysics, 43, 1002-1010. Lajoie, J. J. and West, G. F., 1976, Electromagnetic responseof a conductive inhomogeneity in a layered earth: Geophysics, 41, 1133-1156. Lamontagne, Y. L., 1975, Application of wideband, timedomain EM measurements in mineral exploration: Ph.D. thesis, Univ. of Toronto. McNeill, J. D., Edwards, R. N., and Levy, G. M., 1984, Approximate calculationsof the transient electromagnetic field from buried conductors in a conductive half-space: Geophysics, 49, 918-933. Nabighian, M. N., 1970, Quasi-static transient responseof a conducting sphere in a dipolar field: Geophysics, 35, 303-309.

Nabighian, M. N., 1971, Quasi-statictransient responseof a conductingpermeable two-layer sphere in a dipolar field: Geophysics, 36, 25-37. Nabighian, M. N., 1979, Quasi-static transient responseof a conducting half-space: An approximate representation: Geophysics, 44, 1700-1705.

Physicsof Electromagnetic Induction Exploration Method Smythe, W. R., 1968, Static and dynamic electricity, Mc-


Graw-Hill.

Wait, J. R., 1951a, A conducting sphere in a time varying magnetic field: Geophysics, 16, 666-672. Wait, J. R., 1951b, The magnetic dipole over the horizontally stratified earth: Canadian Journal of Physics, 29, 577-592. Ward, S. H. and Hohmann, G. W., 1988, Electromagnetic theory for geophysicalapplications, in Nabighian, M. N., Ed., Electromagnetic methodsin applied geophysics,Vol. 1: Soc. Expl. Geophys., 131-312.

45

Weidelt, P., 1983, The harmonic and transient electromagnetic response of a thin dipping dike: Geophysics, 48, 934-952.

West, G. F., Macnae, J. C., and Lamontagne, Y., 1984, A time-domain electromagnetic system measuring the step responseof the ground: Geophysics, 49, 1010-1021. West, G. F., and Edwards, R. N., 1985, A simple parametric model for the elctromagnetic response of an anomalous body in a host medium, Geophyscs, 50, 242-257.




CHAPTER

THE

MAGNETOMETRIC

2

RESISTIVITY

METHOD

R. N. Edwards* and M. N. Nabighian

The

INTRODUCTION

The traditional resistivity method maps the electrical properties of the earth by measuringdifferencesin potential at the earth's surface caused by galvanic current

flow

between

two

current

electrodes.

The

magnetometric resistivity (MMR) method differs from the traditional method in that the potential electrodes are replaced by a highly sensitive coil or magnetometer and one or more componentsof the magnetic field Magnetometric resistivity is an electrical explorabased

on the measurement

of the low-

current

level, low-frequency static magnetic fields associated with noninductive current flow in the ground. MMR has been used successfully to explore for massive sulfidesand for geothermal resources, to map regional geology, to study hard rock sites for nuclear waste disposal, to locate reef structures in sedimentary basins, and to obtain conductivity profiles of the sea floor with depth, both in shallow and deep waters. The active protagonists of the method are few. Consequently, we decided to present an overview of the history of the subject before describing in detail relevant theory, specificmethodology,and case histories. The MMR field technique dates back to a patent by

lateral

studies

lies

between

the

two

in the subsurface."

createdby his array in tesla is of the order of (Ix0/4xr) x [current]/[scalelength], where Ix0 is the magnetic

permeability of free space,4,r x 10-7 H/m. For typical current strengths of 50 A and scales of the order of 100 m, the field strength is of the order of 50 nanotesla or gamma (•/), which was well within the resolutionof his equipment. Yet, the method met with little success.Part of the problem was undoubtedly Jakosky'sapparentbelief that he could resolve layered structuresthrough measurementsmade on the surface of the earth. Stefanescu (1929), in one of the earliest theoreticalpapers on the magneticfields generatedby static current flow, had already shown that this was impossible. His result was later supportedin Maillet

"Current is applied to the ground between two electrodes connected with a suitable power supply. The separation of the two energizing electrodes may where

studied

By mappingthe field Bc, Jakoskyhopedto infer the

as an illustration and writes,

constant

be

distribution of current flow in the ground and hence identify conductivity anomalies. He developed both suspended-magnetand spinning-coil magnetometers, and claimed a sensitivity of the order of 1 •/per scale division for them. The size of the magnetic field

Jakosky (1933). In Jakosky'sclassictextbook on exploration geophysics(Jakosky, 1940) he uses Figure 1

be held

to

the essentiallyvertical magneticfield B a created by the flow of current through the energizing wire a, and (b) the complex field Bc created by flow of

are recorded.

tion method

area

grounded terminals or electrodes. The longer the legs of the 'U', that is, the further away the portion of the cable parallel to the line of the electrodes, the lessis the effect of the primary current flowing in the surface cable. The magnetometer is placed on line with the two energising electrodes 1 and 2. At this position, the field strength measuring apparatus is then subjected to two artificially created fields: (a)

are to be

(1947).

made, or it may be varied to increasingly greater separationsto obtain increasingly greater effective depth of penetration for vertical structural studies.

Stefanescu and his students persued for more than 40 years a continual, theoreticalprogram of study into

Departmentof Physics,Universityof Toronto,Toronto, Ontario,Canada,M5S 1A7.

Newmont Exploration Limited, One United Bank Center, 1700 Lincoln Street, Denver, Colorado, 80203. 47


48

Edwards and Nabighian

the magnetic field caused by current flow within conductivity anomalies excited by local current sources. In his 1929 paper, Stefanescu introduces the concept of current channeling and describes how to compute the anomalousmagnetic fields created by perturbation currents in the vicinity of a small target located within a half-space between two exciting current electrodes. He describes how free-charge is set up on the target and then sketches the total and the anomalous current

flow.

He shows how the electrical

electric

effect of a

small target reduces to that of a current dipole and then computes the surface magnetic fields of a buried, arbitrarily oriented dipole using image theory to account for the air-earth interface. Stefanescu (1958) and Stefanescu and Nabighian (1962) determined analyti-

cally the fields of a point sourcein the vicinity of some simple geologicstructuressuch as the vertical contact and thin and thick vertical, semi-infinite, outcropping dikes. They were also able to determine both components of the horizontal

field for the contact

and the

thin, perfectly conductive dike through analytic surface integration of the vertical component. The development of MMR as a viable experimental method stems from the interest shown by Scintrex Ltd. in extending the concept to measure induced polarization effects. The firm developed a very sensitive flux-gate magnetometer, the model MFM-3 (Sei-

gel, 1974).With an inherentnoiseof only4 x 10-• •//•, theinstrument canaccurately measure fields of the order of 10 m•/at a few hertz with little signal processing. Unlike SQUID devices, which were also introduced about the same time, the MFM-3 has proven a reliable, light, and robust field instrument.

/

Edwards (1974) and Edwards and Howell (1976) reported the first field tests of the MMR method using modern instrumentation. Current field procedures,the factors which influenceelectrodelocation and spacing, the reduction and normalization of the data, and the

definition and interpretation of"MMR

Edwards (1974) and Seigel (1974) point out that the MMR method may be superior to conventional resistivity methods in mapping inhomogeneities that are embeddedat depth beneath conductive, patchy overburden. Using an analog model, Edwards (1974) demonstrates the distortion that can be produced by an inhomogeneouslayer in the surface electric field and compares that distortion with the minimal distortion produced in the corresponding magnetic field. The magnetic field is an integral over a volume distribution of current, whereas the electric field is causedby local changes. The overburden has little effect on the measuredmagneticfield provided most of the total current flow is beneathit. Further supportfor this argumentis given in Howland-Rose et al. (1980a) who demonstrate the

effect

of intermediate

conductive

and resistive

layers on the surface magnetic and electric fields associatedwith a simple anomaly. Edwards et al. (1978) summarize some of the earlier theoretical

work

of Stefanescu

add some additional

results.

and his students

The characteristic

and

anom-

alies for an anisotropic earth, vertical and dipping contacts, thin and thick dikes, and semicylindricaland hemispherical depressions, as well as a-media are derived. They show that there are two factors which influence the MMR anomaly: the problem geometry, and the conductivity contrast between the target and the host medium. For many models, separatingthe two effects is possible. Type curves are presentedfor very large conductivity contrast to illustrate the effect of geometry alone. Ancillary curves enable finite conductivity contrasts to be deduced from field data. A modified

Fig. 1. Jakosky's concept of an MMR field survey (after Jakosky, 1940).

anomalies" are

described.

form

of the Biot-Savart

law has been

used extensively to compute numerical MMR responsesof two- and three-dimensional (2-D and 3-D) structures.The magneticfield of a distribution of static current in a conductive medium may be written as a volume integral over a function proportional to V•r x E where E and •r represent the variation of the electric field and the conductivity through the volume, respectively. For many simple problems, the gradient of the electrical conductivity vanisheseverywhere except on the surfacesdefining changesin conductivity. Consequently, the volume integral just given may often be reduced to a finite set of surface integrals, with a correspondingdecrease in computing effort. The integrand of each surface integral includes componentsof


MMR

the electric field tangential to the boundary. GomezTrevino and Edwards (1979) show that this field could be derived by a surface integral equation technique and could develop a rapid numerical algorithm for evaluating the three componentsof the magnetic field. Oppliger (1984) modified the surface integral equation method to model the effect of undulating topography while Nabighian et al. (1984) and Cheesmanand Edwards (1989) introduced yet another integral equation method to compute the MMR anomalies associated with multiple finite plates of arbitrary conductance. The plate is replaced by a distribution of Stefanescu's current dipoles in its plane. Having solved for the dipole strengths, the magnetic fields anywhere may be evaluated using Stefanescu's original theory. The numerical computation of MMR anomalies of more general 2-D and 3-D structures is also straightforward. Pai and Edwards (1983) describe a flexible finite-difference algorithm based on the 2-D resistivity program of Dey and Morrison (1976). Both the analytic and numeric methods, while developed for MMR, are used successfullyto interpret channeling anomalies excited by an inductive source. Macnae (1981) and Macnae and Irvine (1988) compute the low-frequency and late time, transient electric field near a contact excited by a large loop transmitter starting from the MMR response of an electric dipole near a contact, and invoking the reciprocity theorem. McNeill et al. (1984) and Flores and Edwards (1990) employ the plate program to compute low-frequency VLF and magnetotelluric channeling effects, respectively. There are several programs in existence to interpret approximately the electromagneticeffect of a single target which combine channelingand inductive effects separately, the channeling effects being computed with MMR software (e.g., McNeill et al. 1984). The availability of these computer algorithms has led to two significant subsequent numerical experiments. In the first study, the effect of an overburden layer on an MMR anomaly is shown to be predictable. There is a strong correlation between the percentage of current penetrating the overburden and the magnitude of the MMR anomaly. The second study, on the effect of measuring anomalies in drillholes, clearly demonstrated that lowering either the current electrodes or the magnetometer, or both, inside drillholes can lead to significant improvements in data quality and in ease of interpretation. Further, while lowering the electrodes in a borehole does improve the response,lowering the magneticdetector producesvery significantincreases in the observed anomaly. These observations prompted the development of drillhole MMR techniques by Newmont Exploration Ltd. (Nabighian and Oppliger, 1980).

49

Oppliger (1984) describesa drillhole survey in which both current electrodes are located in a single nearvertical borehole while the magnetic field is measured over the surface of the earth. The transmitter configuration yields no primary magnetic field at the earth's surface so the field observed is purely anomalous. Similar, mise-b.-la-massemagnetic field measurements at somewhathigher frequencies(78 Hz) are reported in Rodionov and Kormiltzev (1979).

A similar buried vertical bipole is used as the transmitter in the cross-boreholeMMR technique. The receiver measures the axial component of the total magnetic field in a secondhole. If both holes are nearly vertical, then the primary field in a uniform layered earth is transverse to the receiver. The presence of a lateral conductor channels the regional current flow causinga detectable anomalousvertical field. Surveys usingthis or very similar configurationsare reported in Acosta and Worthington (1983), Nabighian et al. (1984), and Lo and Edwards (1986). The latter reference includesa descriptionof the designand construction of an inexpensive sensor. The sensor used in conjunction with a standard resistivity unit, forms a reliable MMR survey equipment. We mentioned

earlier

that MMR

is used offshore.

The method, known as MOSES (Magnetometric OffShore Electrical Sounding) is introduced in Edwards et al. (1981) as a natural off-shoot of the crosshole technique. The transmitter is a vertical, long-wire bipole, extendingfrom the sea surface to the sea floor. A cornmutatedcurrent, generatedon the ship, is fed to two large electrodes: one at the sea surface, the other at the end of a long insulated wire. The current return path is through the sea and the subjacent rock. The receiver is a self-contained, micro-processor controlled magnetometer located on the sea floor. The total horizontal component of the static, cornmutated magnetic field generated by the current system is measured over a range of horizontal transmitter-receiver separations. The system geometry is carefully arranged to remove many of the adverse effects of the relatively conductiveseawater. In particular, accurateestimates of sea floor resistivity are possiblebecausethe data are proportional to the transmitted current from the source into the crustal material. Resistivity sounding at sea with any standard array does not generate data sensitive to sediment resistivity unless unreasonably large separations are used. At short array spacings, only the value of the resistivity reflection coefficientis obtained, an expressionwhich is close to unity for all crustal resistivities. In contrast, the MOSES method measures the resistivity transmission coefficient, which is inversely proportional to the crustal resistiv-

50


ity. The principle, methodology, and field examples

Magnetic Fields on the Surface of a Layered Earth Excited by a Point Source


are described in Chave et al., 1990, this volume.

An active and important area of researchnot closely followed by the geophysical community is biomagnetism. The study of magnetic fields originating in biological systems,particularly the human body, has important implicationsfor diverse areas of biological research, medicine, physiology, psychology, and occupationalhealth. Of interest is the use of the current dipole to representthe magneticfield associatedwith the movement of ions in body tissue. The theoretical work of Baule and McFee (1965) closely parallels earlier work of Stefanescu on layered structures. A current dipole in a conductive sphereis a model often invoked to describe localized activity in the brain and many of the mathematical theorems developed for layered structureshave spherical analogs(Grynszpan and Geselowitz, 1973; Cuffin and Cohen, 1977). Williamson and Kaufman (1981) have assembled an up-to-date review of the subject.

Supposea current electrode C is embeddedat depth d in a half-spaceof uniform conductivity and supplied with a constant current I through a cable A OC, as shownin Figure 2a. The total magneticfield measured at the surface is in two parts: the field due to current flow in the cable and the field due to current flow in the

ground from the point current source. In order to show how the total magnetic field can be evaluated, we replace the circuit shown in Figure 2a by the superpositionof two circuits shown in Figures 2b and 2c, respectively, in which the contributions from the two, vertical semi-infinite wires are arranged to cancel one another. In Figure 2b, the azimuthal field

B•• onthesurface oftheearth duetoupward current flow in the semi-infinite cable OB is given by the Biot-Savart

law as

(1)

B• = 4,rr' MMR

RESPONSE

OF A LAYERED

EARTH

where r is the radial distance from the origin. The

magnetic fieldBAøfromthesegment AO is essentially

The MMR method, with the exception of its marine adaptations described in Chave et al. (1990, this volume), has not been used systematically for vertical electrical sounding to determine the structure of a one-dimensional, layered earth. The reason is quite straightforward. The surficial magnetic field is not influenced by the distribution of undergroundelectrical properties of a horizontally stratified medium. MMR data collected with an array like that introduced by Jakosky cannot be interpreted in terms of a variation of resistivity with depth. The original, elegant, simple proof of this result, published in Stefanescu (1929), is worthy of repetition.

vertical and, if required, can also be computedexplicitly using the Biot-Savart law. The secondcurrent system shown in Figure 2c, now exhibits axial symmetry. It is easy to see that the magnetic field of this system cannot have a vertical component. The only horizontal component is in the

azimuthalq>direction.The magnitude of B, on the surface of the earth can be evaluated using Ampere's Circuital law. The circulation of the magnetic field around a circle of circumference

2,rr centered

at the

origin O must be proportional to the total current through the circle. We obtain

B • I I

A

I1//

(a)

I!

(b)

A+

/

/x,/

/

/

////,//

(c)

Fig. 2. The equivalent current circuits used in evaluatingthe magneticfield of a point source in a conducting half-space.

MMR

In other words, in sharp contrast to the resistivity

2'trrx B, = IxI,

method, surficial MMR measurements cannot be in-


or

B, - 2•r

(2)

whereB, is the combinedmagneticfield due to downward

current

flow in the wire BOC

51

and to the

current flow in the ground outward from the point source.

We may now obtain the total horizontal magnetic field of the system shown in Figure 2a by adding the horizontal fields of systems shown in Figures 2b and 2c, expressions(1) and (2), respectively. The resulting

terpreted as indicating horizontal layering. At first this property of the magnetic field seems strange because clearly the current flow in the earth is perturbed by layering, as in the example sketched in Figure 3a. However, the current flow in Figure 3a is the superposition of a normal half-space current flow and a perturbation flow shown in Figures 3b and 3c, respectively. The perturbation flow has the property of being poloidal (like a coil wound on the surface of a doughnut). Any poloidal current totally enclosesthe toroidal magnetic field generated by the current, and thus has no effect on measurements

fieldB•, is referredto as the normalhorizontal field over a conductive half-space and is given by

carried

out on the surface

of the earth. Presence of the current can only be detected by measuring the field beneath the earth's surface in a drillhole.

o•_

Bg,=B,+B, -4,rrr. If weinsertthenumerical valueofI• anddivideby 109, we can write lOOI

(3)

where B n is now in nanoteslaor gamma,for r and I in meters and amperes, respectively. Expression (3) is remarkable in two respects. First, its value is independent of the depth of burial of the current electrode. Consequently, the magnetic field due to two current electrodes of opposite polarity connected by a vertical wire vanishes at the surface of the earth. This fact has led to the development of innovative "pure-anomaly" MMR methods. Second, its magnitude does not depend on the conductivity of the half-space. Indeed, expression (3) is unchanged even if the uniform half-space is replaced by a horizontally layered earth. The derivation is identical to the one given previously because the axial symmetry of Figure 2c is not destroyed.

Any horizontal component of the normal field of a pair of buried electrodes can be obtained by the superposition of two solutions of the type given in equation (3). Let x, y, and z be a set of cartesian axes, where z is measured positively downward such that the plane z = 0 coincides with the earth's surface. Further, let there be a current source I and an equal but oppositecurrent sink at (0, L, z l) and (0, -L, z2) respectively,where z l and z2 are arbitrary. The horizontal cartesian component of the normal field orthogonal to a line joining two electrodes is then given by the expression

y/L+

•x0I

1

Bx(x,y)= 4•rL(x/L) 2 + (y/L y/L-

+ 1)2

1

(x/L) 2 + (y/L-

1)2

and is shown in Figure 4. The numberson the contours are values of the componentexpressedas a percentage of the value of the field at the center of the array,

•XoI/2•rL.If the electrodeseparationwere 4000 m, the

-t--

(o)

(4)

(b)

(c)

Fig. 3. An example of the total, the normal, and the anomalous current flow in a layered earth. The poloidal anomalous magnetic field is enclosed by the anomaloustoroidal current.

52


values would be in milligamma per ampere of current

of the current

flow.

hole surveys are designed to map confined lateral conductors through the local magnetic fields they produce, a knowledge of the background regional layered-earth response is required for the formal reduction of the data to an anomaly. The responseof a layered earth to a pair of electrodes buried at two different depths can, in general, be found by a processof superposition.We add the


For certain problems, we are interested in the magnetic field created by the current flow in the

ground fromapoint source, B•. Thecomponent may be evaluated by subtractingfrom expression(2) the field due to the completewire segmentBOC, which by the Biot-Savart law is equal to (txI/4,rr)[1 + cos 0]. There

results

sources. While most downhole

fields of the current

or cross-

flow from each of the electrodes

to

the field from the current in the cables joining the

B, - 4•r(1- cos 0)=•txI( 1- •/d2 ctXI dq-r2). (5) electrodes As far as surface measurements are concerned,

to the transmitter.

The fields of the current

in the cables are found through elementary applications of the Biot-Savart

law. The determination

of the

expression(5) can be thoughtof as the field due to a semi-infinite wire extending from C downward to infinity.

field from the current flow in the earth requires the solution of a boundary value problem which is illustrated in Figure 5.

Magnetic Fields Within a Layered Earth Excited by a

current

An infinite Point

Source

The problemof determiningmagneticfieldswithin a layered earth is not without relevance. Acosta and Worthington (1983), Nabighian et al. (1984), and Edwards (1988) have describeddownholeadaptationsof the MMR method in which the magneticfield is logged as a function of depth in a borehole. The fields in the hole, particularly the horizontal components,do vary with the vertical conductivity profile and the location

vertical

cable A O carries

I and terminates

in a current

the excitation

source

at the

originO of cylindricalcoordinates(r, •, z). The source is deliberatelylocatedat the interfaceof two layers, in order to simplify the mathematics.The resistivities and thicknessesof N layersbelow the sourcelayer and

,o[=3

Pi=2 Fi=2

/9i=2

['i-3

Fig. 4. The x compoundof the normal magneticfield due to current flow between a pair of electrodeslocated on the y-axis and separatedby a distanceL. The numbers on the contours are values of the componentexpressedas a percentage of the value at the center of the array. If L were 4000 m the values would be milligammasper ampere of

Fig. 5. Diagram for calculatingmagnetic fields within a

current

layered earth.

flow.

/9i=3


MMR

53

the M layers above it are denoted by subscriptsi and j on the symbols p and d, respectively. The indices i and j increase away from the source, starting from unity.

successiveapplicationsof expression(12) from a starting value

Thereare two typicalfieldpointsF i andFj alsolo-

A solution of the differential equation (9) in any upper layer j above the source is also

catedat layerinterfaces, between theithand(i + 1)th andthejth and(j + 1)thlayersbelowandabovethe source, respectively. The problem is clearly axi-symmetric. The one and only azimuthal magnetic component B satisfies the differential equation

02B

10B

Or2 -t r

Or

B

02B

r 2 t-0-•- = 0

(6)

at all source-free points within zones of constant resistivity. A Hankel transform pair relating any two functions A(r; z) and A(h; z), may be defined as

QN = 1/ONh.

U[cosh (hz) + V sinh (hz)].

(13)

(14)

However, expression (14) cannot be a complete representation of the magnetic field in this region no matter what the values of U and V. We have to add to

expression (14) a particular integral independent of z, the Hankel transform of the magnetic field in the immediate vicinity of the cable A O. We obtain

Bj(h; z) = U[cosh(hz) + V sinh(hz)] + IXoI/2½rh. (15)

The radial electric component retains the form

A(h; z)=f• rA(r; z)Ji (hr) dr, (7) and

•0Era(h; z)= -hp• U[sinh(hz) + V cosh(hz)]. (16)

To form a downward recursion relation, we define a

A(r; z)= f•

hA(h; z)Ji (hr) dh,

(8)

where J1 is a Besselfunction of the first kind of order 1. The Hankel transform of the partial differential equation (6) yields the simple secondorder equation d2B

dz 2

X2B = 0.

(9)

A solution of the differential equation (9) in any lower layer i, of constantresistivity Pi, thicknessdi is

ei(k; z)= U[cosh (hz) + V sinh (hz)],

(10)

where U and V are functions independent of z constrained by the boundary conditions. As the current flow is everywhere continuous, the radial electric field component is related to the magnetic component in equation (10) by Ampere's law, so that

[xEri(X; z) = -hpiU[sinh (hz) + V cosh (hz)]. (11)

Eliminating U and V from equations (10) and (11) and rearranginggeneratesan upward recursion relation

1 [?.!.x__Q!+_, +tanh (kdi)

Qi=o-•[OihQi+ • tanh (hdi) +1' (12) where the symbolQ denotesthe ratio B(X)/•Er(X) and Qi and Qi+• are the specificvaluesof Q at the top and

bottomoftheithlayer.ForanygivenX,thevalueof Q in the plane of the source Q• can be obtained by

new parameter P as the ratio [(•x0I/2•rk) - B(k)]/ •x0Er(k). Then, eliminating U and V from equations (15) and (16) gives

PJ =o•hLo•hP• 1•[ _p•hP{_+I +tanh (hdj) +ltanh (hd•) +1' (17) whereandPj andPj+I arethe specific valuesof P at thebottomandtopof thejth layer.For anygivenh, the value of P in the plane of the source P1 can be obtainedby successiveapplicationsof expression(17) from a starting value

PM = 1/OMh.

(18)

The processof upward and downward recursionto the plane of the source yields a pair of simultaneous equationsfor the fieldsB• and Erl there, namely B• IxoEr•

- Q•;

(19)

and

(}x0I/2'rrh) - B 1 IxoErl

= P1.

(20)

The equations have the solution

•0I[Q1.]

B1=2-• Q1+P1'

(21)

The fieldsat the typicalobservation pointsF i or Fj may now be computed by downward or upward recursion rules, respectively, which start with B1. The

54


downward recursion rulethrough theithlayermaybe


derived from equations(10) and (11) as

pikQi+! sech (kdi) ] (22)

Bi+1= pihQ i+1+ tanh ()tdi)' Bi.

The values of Q needed by the rule are saved values from the preceding upward recursion process. The algorithm avoids the evaluation of exponentialswith positive arguments.

Theupward recursion rulethrough thejthlayermay be derivedin a similarmannerfrom equations(15) and (16) as

pj XPj +1sech (Xdj )]

(Bs- Bj+i) = PJ XPj +1+tanh (Xdj ) (Bs - Bj),

(23)

where B s =

which is the inverseHankel transformof equation(21) with P• (X) set to zero, and B2 =B 1

f•[•2•.Pl sech (hdl) J1 (hr) d(hr). Xp1 + tanh (Xd 1)

(25)

If the magneticfield of the semi-infinitecurrent carrying cable shown in Figure 5 is removed, then the ratio of the measured

horizontal

fields above and below the

layer,B-(r; 0) andB +(r; d•), are

B+•

2Q 2xp 1 sech (Xd 1) Q 2k p1 + tanh (Xd 1)

J1 (kr) d(kr) (26)

where

dl

Thefinalevaluation ofthefields Bi andBs - Bj atFi andFj in spacedomainis throughtheinverseHankel transformgiven in equation(8). Electrical Sounding Beneath a Conductive Surface Layer Using Differential Magnetic Measurements

f•=1- •/dl 2+r2.

(27)

The ratio B+/B - must lie between _+1. Extreme values occur when the thickness d• is infinitesimal subject to the condition that the conductivity-thicknessproduct S• = d•/p• remainsfinite. Then expression (27) becomes

Edwards (1988) describes a special differential MMR technique the theory of which is based on the above analysis. The layered earth is excited by a current source at the earths surface. Horizontal magnetic field measurementsare made at the top and bottom of a conductivelayer of known propertiesfor a range of values of the horizontal separationbetween

If S • is relatively large or small comparedwith Q2 for values of x which contribute to the integral, (i.e.,

the hole and the electrode.

valuesof the orderof l/r), thenB +/B- is - 1 or + 1,

The data set thus obtained

B+/B-= I•[•2-S1]jl(Xr) d(Xr ). (28) 2q-S1

may be inverted for the resistivityof deeperzones. There appearto be severalpractical applicationsof the technique, includingthe mappingof resistive offshore permafrost which occurs beneath the shallow

respectively. Type curves for the method, plots of the ratio B +/B- as a function of the transmitter-receiverhorizontal separationr, may be generated from equation

Beaufort

(26). Consider first a basic model consistingof a conductive sea layer 20 m thick with a resistivity of 0.33 f•.m overlying a uniform resistive half-space. Curves for a set of realistic values of the half-space resistivity are displayed in Figure 6. The ratio is observedto be boundedby the extremevalues -1 and + 1. The curvesfor differinghalf-spaceresistivitiesare very differentand spana wide rangeof field ratios.The sensitivity, and consequently the usefulness of the method, deterioratesonly when the changein the field ratio for a given change in the resistivity of the half-spacedecreases.This occurswhen the conductivity-thickness product of the sea layer is relatively

Sea in the Canadian

Arctic.

Immobile

sea ice

covers the Beaufort until early April each year. The ice is easily drilled and providesa stableplatform for experimental work. Also, novel, sensitive horizontal magneticfield sensorswhich can passthroughsmall diameterholesin the ice andunfoldon theirway to the seafloorhavealreadybeendeveloped(Edwardset al., 1988). Let the known surfaceconductorbe boundedby the planesz = 0 andz =dl and have a resistivitypl. The regionsz < 0 and z > d l, representingair and the basement rock, have infinite resistivity and an unknown layered character,respectively. The magneticfields at points above and below the surfacelayer, B•(r; 0) andB2(r; d•), respectively,for the geometry shown in Figure 5 are

B1- 2,rr'

(24)

small, so that the value of the dimensionless number c•

is small comparedwith unity, which forces the field ratio to approachunity. The effect of includinga relatively resistiveintermediate zone representingpermafrost within the background model is shown in Figure 7. The zone is 50 m


MMR

thick and the depth to the top is 40 m. The background resistivity is selectedas 2 f•. m, the value for a shallow sedimenthaving a typical porosity of about 40 percent. The resistive (permafrost) layer clearly influencesthe field ratio substantially.The curves for different values of the resistivity of the intermediate zone are quite distinct and quite different from the family of halfspacecurves shown in Figure 6. Notice that data need

1.0

55

be collected

for

values

of the transmitter-receiver

separation r out to only about 300 rn to resolve the resistivity of the intermediate zone. If a Schlumberger soundingwere to be conducted in the same location, the range of Schlumbergerspacings(AB/2) required is apparently greater. The Schlumberger type curves are shown in Figure 8. The curves have a distinctive positive slope at small spacingsreflecting the contrast in resistivity between the sea water and the sea floor. The effectsof the resistive zone are clearly visible only as a variable peak in the curves at spacingsin excessof several hundred Penetration

meters.

of Current

Beneath

a Conductive

Surface Layer

Approx Exact

I

I

I

I

I I I I I

I

I

I

I

I I I I

I00

I000

RANGE (m)

Fig. 6. The ratio of the magneticfields above and below the sea layer plotted as a function of range for the half-space model

shown as the inset.

The MMR method is a preferred method of prospecting in terrains characterized by the presence of a thin, conductive, patchy overburden. Consider the caseof an electrode at the surfaceof the earth exciting a target beneath the overburden. The strength of the MMR response will be related to the strength of the current in the vicinity of the target. However, the conductive layer tends to channel the current flow and inhibit its progress to the depth of the target. How much current penetrates the overburden for a given geometry? The answer to this question is easily describedby a simple analytic function whose derivation is another application of the theory already presented. Let d•, p•, and 92 be the thicknessof the overburden

and

the resistivities

of the

overburden

and

a

uniform half-space, respectively. Further, for the ge-

I.O

IO

12 ?m

2•a,m 50m

P2 50m

t

I

I

I

I

I

I I I I

IOO

I

I

I

I

I

,• (•2m)= •0

I I I

IOOO

RANGE (rn)

0'110

I

I

I

I

I I I I I

I00

I

I

i

i

i i i i

I000

$P•CING ab/E (m}

Fig. 7. The ratio of the magneticfields above and below the sea layer plotted as a function of range for the permafrost

Fig. 8. The Schlumberger soundingcurves for the perma-

model shown as the inset.

frost model.

56



ometry shown in Figure 5, let the values of the magnetic field just above and just below the overburden at a horizontal distance r from the source be B1

and B 2. The current 12, a fraction of the total available current I, which penetrates into the lower halfspaceis, by Ampere's Circuital law, just IB2/B•. (Ampere's Circuital law is applied correctly. The current flow is everywheredivergencefree). The ratio 12/1is givenby expression(25), with Q2 set to the appropriatevalue given by equation (13). The result is

12/I =

'Plcosh (hdl)+ P2sinh (hd•) J1(Xr) d(hr).

the third and higher order terms are neglected, then there results

1-f(ot) r[6r+dl

(34)

12/I= sinh ([) [ where

f(ot) = (1/ot){(w/2)[H1 (1/or) - Y1(1/or)] - 1}. (35)

The functionsH1 and Y1 are the Struve function and the modified Bessel function of the second kind, respectively, each of order 1. The dimensionlessnumber otis given by

(29)

ot = d• /r•.

Let us supposethe thicknessd• of the upper layer is small compared with the range r, and that the resistivity p• of the overburden is small compared with the resistivity 1:)2of a lower uniform basementhalf-space. The approximations are not impractical. The object of a real survey would be to determine the electrical structure at depths at least of the order of d•, which implies that observations should be taken at ranges significantly greater than d•. The condition on the resistivities is an a priori assumption. Given these assumptions,we can find an analytic form for 12/1, a form which takes the limit of small 1:)1/92 and smalld•/r simultaneously. If we introduce the parameter [ defined by the equation tanh ([) = p•/92,

(30)

2• = log [(P2 + Pl )/(P2 -- Pl )],

(31)

SOURCES

OF THE

MAGNETIC

(36) FIELD

The Biot-Savart law which relates the magnetic field B(r) at a point external to a volume V containing a distribution of current elementsJ(r ')dv' is the integral relationship

B(r)= •

J(r')x V'•

dr'

(37)

with no restriction on the form of the current density J(r). The law is cumbersome in implementation as it involves a volume integral over the whole current flow. However, for static current flow, the volume integral may be converted to a set of surface integrals in the following manner. Using the vector identity

or

V' x (•A)=•V'

xA-Ax

V'•,

(38)

for any vector A and scalar (b, and Stokes' theorem, then expression (29) becomes

fvTM xAdv' =fs fixAds',

12/1sinh ([)f•cosech ([+hd•)J•(Xr) d(hr). (32)

The hyperbolic cosecantfunction may be expandedas a power series for small argumentswith the result

fool 1 (•+•d,)

12/1= sinh (•)

(•+ Xd•)- 6

+ O(i•+ kd•) 3 Jl(kr)d(kr).

(33)

The first term under the integral is evaluated by following the technique described in Edwards and Howell (1976). The second term is the sum of two well-known standard integrals of Bessel functions. If

(39)

where S is the bounding surface of the volume V, and fi(r') is the unit vector along the outward normal to S, the magnetic field may be rewritten in the form

Irx - J(r') r'l dv, - •--• r'l ds, txfvV' txfsfiIrX-J(r') '

B(r)= •

(40)

Consider the volume V to be the halfspace z > 0, on and beneath the surface of the earth, where the density J(r') is generated by flow from a current electrode embedded

in V.

The boundaries

of the surface S are selected to be a

planejust above the earth's surface and a hemisphere of large radius R which in the limit completely en-

MMR

57


closes V. The surface integral over S vanishes on the plane boundarywhere J = 0 and also vanisheson the hemisphericalsurfaceprovided that J(r) falls off at a rate greater than 1/R. Whence,

• = • (2- as ø)cos [s(4 - 4')] s=0

x

IxIvV'xJ(r') dv'. (41)

B(r)= •

Ir- r'l

Everywhere within the earth, for a static current, we may define an electric potential U(r') related to J(r') throughthe local conductivity•r(r') by

(42)

J(r') = -•r(r')V'U(r').

{b equal to zero. It is

• •1' 7)'3 /2 ixII•=I•(r75 r'cos4'

By= 8,rr 2

x }__•0(2 - as ø)cos (s•b')

Ir-x r'l IxIv •V'U(r') V'cr(r')

B(r)= •

(43)

This is a very useful expressionfor computingMMR anomaliesas it containsV'•r(r') which in many cases vanisheseverywhere except at the surfaceof the earth and on the boundariesof media of differingconductivities, reducingthe volume integralto a finite numberof surface integrals. Considera current electrodeembeddedat a depthd below

the surface

z =

0 of a flat

earth

OU(r')

Ox'-

I

xffJs(Xr)Js(Xr')dX (47)

The integral over {b' collapsesthe summation and yields

By= •

of uniform

conductivity•r and suppliedwith a constantcurrent I through a cable. We have already seen that the magnetic field of this systemmeasuredat the surfaceis in two parts' the field due to current flow in the cable and the field due to current flow in the ground. The field due to current flow in the ground is reevaluated independentlyhere as an example of the use of equation (43) which, we recall, is valid no matter what the form of J(r). The gradient of the conductivity is zero everywhere except on the planez = 0, where it has only a vertical componentgiven by &r/Oz' = 8(z')•r. The gradientof the electric potential in thex-direction on the planez = 0 due to the buried point source is x'

2xfer (r'2q_ d2)3/2 ' (44)

The y-componentof the magneticfield may be written as the surface integral

J1(kr)dX

x

d2)3/2 J1(kr')dr'. i•(r'2+pt2

If the cylindrical coordinatesof the vectors r and r' are (r, {b, 0) and (r', {b', 0), respectively, then the inversedistance1/R between two pointson the surface of the earth

is

(48)

The integrationover r' and X in order requiresthe use of two standardintegralsand gives first

By- 4,r

exp(-kd)J1 (Xr)dX, (49)

then

By- 4•rr 1

•/d2 +r2..

(50)

The cartesian magneticcomponent By at {b= 0 is the requiredazimuthal magnetic component Bq,,andis as quotedin expression(5). MMR

By- 8xr2 (r'2q_ d2)3/2 • ds. (45)

(46)

ThevalueofBy alongthex-axisis obtained by setting

Using the vector identity quoted earlier and observing that V' x V'q>= 0 we obtain a final expression

dv'.

Js(kr)Js(Xr')dX.

ANOMALY

We use a definition for the MMR anomaly which is consistentwith the descriptionin Edwards (1974)and Edwardsand Howell (1976). Supposea componentof the magneticfield dueto currentflow in the groundhas been measuredalongthe profileAA' distantI from the current electrode C shown in Figure 9. The x- and


58


y-axes are chosenparallel and at right anglesto AA', respectively, with the origin at C and the profile at y = -l. The angle [3 is positive for x > 0. The theoretical x-component of the normal magnetic field along AA' is •xI

l

B•(x,-l, 0)=4,r(/2+x2),

(51)

having its maximum value at the center of the profile of

B](0,-l, 0)= 4,rl'

(52)

Let any componentof the total magneticfield measured along AA' be B c(x, -l, 0). Let the theoretical normal valuesof this componentalongAA' be B•(x, -l, 0). We definethe anomalousfield B•' as

"(x ,

, 0) , (•3)

0).

(54)

The MMR anomaly therefore is basically the anomalous field which has been expressedas a percentage of a single value. Other normalization procedureshave been suggestedin which the anomalous field or even the total measured field is expressed as a ratio of the local horizontal field at the point of measurement.Our experience is that these methods lead to a gross distortion

of the theoretical

curves.

PIx,-•> A'

A

OF MODEL

RESPONSES

The collection of analytic and numeric model responsespresented here are of the MMR anomalies in the x- and z-componentsof the field. The direction of the x-axis is generally chosento be perpendicularto the strike of conductive structures. This may seem to limit the usefulnessof the curves because, in the field, profiles could cross structuresat any angle. However, in practice, the approximate strike of the target is often known from the prevailinggeology, and the MMR data can be collected in an optimal manner. The possible analytic models are obviously limited to the handful of simplegeometries.They include the horizontally and vertically layered earth, the anisotropic earth, the dippingcontact, the hemisphericalsink,

and the alphamedium.At the very least, they help our understandingof the MMR effects of various common geologic features like faults, dikes, or gravel filled channels. Additionally, precise analytic solutionsare important for evaluatingand checkingnumerical methods, before such methods are used to tackle more complex models.

and the MMR anomaly in the component as

aca(x, -l, 0)/a7(0,-l,

PRESENTATION

In nearly all the simple models studied, there appears to be two factors which influence the MMR anomaly; namely, basic geometry and conductivity

contrast.The shapeof the anomalycurve is principally an effect of geometry: the amplitude of the anomaly may be influencedboth by geometry and the conductivity contrast between the anomalous structure and

its environment. We have tried, wherever possible,to separatethesetwo effects.For example, anomaliesare computed for a very large conductivity contrast to illustrate the effect of geometry only; then an ancillary set of curves was preparedto enable the amplitudeof the anomaly to be deduced for an arbitrary, finite conductivity contrast. The numericalmodelsincludea plate of finite size, a layered earth with a variable surfacetopographyand certain two-dimensionalmodels, designedto illustrate the effect of layering in general and a thin, conductive overburdenin particular on MMR amplitudes. MATHEMATICAL

METHODS

Stefanescu'sAlgorithm

Stefanescu(1958) devised a method for computing the vertical component of the magnetic field of a current flow confined to the region z > 0 beneath the earth's surface. Consider a small volume dv about the

point P, as shown in Figure 10, and let the dv contain y

Fig. 9. The MMR anomaliesare calculatedalong the profile AA'. The current electrode I is at the origin of coordinates.

a current element dl whose componentsare Jxdv,

Jydv,andJzdv,respectively. Next replacethe air in the region z < 0 with a conductivity distribution which is the mirror image, in the plane, z = 0, of the

MMR

distribution in the regionz > 0. The current elements at the point P(x, y, z) and its imageQ(x, y, -z) change


andbecome1/2(Jxdv, Jydv,Jzdv)and1/2(Jxdv, Jydv, -Jzdv), respectively. Clearly this redistribution of current leaves the vertical componentof the magnetic field unchangedon z = 0. In certain cases(e.g., vertical boundaries),such a procedureleads to a solenoidalcurrent flow with axial symmetry about a horizontal axis in the plane z = 0. The magnetic field which is azimuthal is easily calculated and coincideswith the required vertical component on the plane z = 0.

59

and

y(X,y,z)=

oy [(X--X') 2 + (y _y,)2 + Z211/2 x

y', o)ax'

(56)

and are valid only in the regionz < = 0. VERTICAL

Skeels-Watson

ay',

STRUCTURES

Transformations

Whereas the vertical component of the magnetic field on the plane z = 0 due to Current flow in the region z > 0 can be obtained by the application of Stefanescu's algorithm, the horizontal components cannot be derived by this method. However, Stefanescuand Nabighian (1962) observedthat these componentscan be obtained analytically from the vertical componentby means of the convolution integrals of Skeels and Watson (1949). These componentsmay also be derived numerically using Fourier transform techniquesprovidedthe analyticvertical componentis describedin sufficientdetail over a large enougharea

of the planez = 0. The components Bx andBy are related to Bz as

ax(x,

[(X- X') 2 + (y _y,)2 + Z211/2 O) ax' ay',

(55)

••e(x,y,-c) Jdo/2 I

,, I I I

,, I I

,y,.c) J du/2

Fig. 10. An illustration of Stefanescu'salgorithm. The current element at P and its image at Q generate identical vertical magneticfields on the planez = 0.

The Anisotropic Earth

The resistivityof an anisotropicmediumdependson the direction

in which

the medium

is measured.

In the

simplestpossiblecase, the medium is definedby two values Pt and Pt which are the resistivities in the direction normal to the "bedding planes" and in the two directionsparallel to the bedding planes, respec-

tively,whereX = ¾/(Pt/Pl) isthecoefficient of anisotropy. If the bedding planes are horizontal, no MMR anomaly is generated in either the vertical or the horizontal fields. This is a consequenceof the general theorem first stated in Stefanescuet al. (1929) that structures of axial symmetry about a vertical axis throughthe current electrodegenerateno MMR anomaly at the surface of the earth. If the bedding planes are vertical such that the resistivity has componentsPt, Pl, Pl, respectively, in the x, y, an z direction, an MMR anomaly is generated at the earth's surface.Edwards et al. (1978) show that the anomalous vertical component of the magnetic field is given by txI

[

x

Xx

B](x,y, 0)=4--•yy (X2q-y2)1/2 --()k2X2 q-y2)1/2 ' (57)

The mathematicalprocedureusedby Edwards et al. is straightforwardbut cumbersome.We offer an alternative scheme which may be considered a general method for many similar problems. First, we invoke Stefanescu'salgorithmand replace the anisotropichalf-spaceby a whole spaceof similar properties, as shown in Figure 11. The point current source C at the origin of coordinatesis suppliedwith current I by a wire lying horizontally along the negative x-axis. Clearly, the divergencefree current flow producedhas axial symmetryaboutthex-axis. Define

a radialvectorr byr 2 = x 2 + z2, andanappropriate Hankel transformpair relating a functionA(r) and its transformA(p),

60

Edwards and Nabighian The difference of the fields (64) and (63) with r set

A(p) =/•rA(r)Jl(pr) dr; Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

(58)

A(r) -/•pA(p)J 1(pr) dp.

(59)

Edwards et al. (1984) show that the Hankel transform azimthal magnetic field B(p) obeys the differential equation d2B dx 2

h.2p2B= 0

(60)

equalto y is the requiredverticalfieldBz on the plane z - 0 and is equal to the vertical field given by expression (57). The MMR anomaly in the vertical field along the profile AA', at y -- -l shown in Figure 9 is, therefore,

hx

x

100()k2X2 + /2)1/2(X2+ /2)1/2 ß (65) In all casesof geophysical interest,k2 > 1 so the anomaly is antisymmetric, being positive for x > 0 and negative for x < 0. In the limiting case of kx very large for all x, the expression reduces to

for all x > 0.

By Ampere's circuital law and the symmetry of the current flow, the value of B in the plane x = 0, B(r; 0), is txI/4zrr.The transformedfield B(p; 0) is txI/4zrp.The solution of the differential equation for all x > 0 is just

B(p; x)= B(p; 0) exp (-Xpx) = (txI/4zrp) exp (-Xpx).

(61)

The inverse transform B(r; x) is given by

B(r;x)= •--d• exp(-Xpx)Jl(pr) dp, (62)

100sgn(x)1-(x2+/2)1/2. = 100 sgn (13)[1- sgn (13)sin [3],

(66)

which, clearly, has a maximum amplitude of 100. Physically, the correspondingcurrent flow in the earth is confinedto the halfplane (x = 0, z > 0)' The strongly anisotropic earth behaves like a thin, highly conductive, vertical dike. The anomaly is plotted for a range of values of k in Figure 12. The horizontal magnetic fields due to the anisotropic earth cannot be obtained using Stefanescu'salgorithm,

reducing to

F--(k2X2•-r) xx21/2 1 (63) ßx)=• L1 ]' B(r,

/4-SO -'øø

In orderto obtainthe fieldof currentflow in the whole

space alone,wemustremove theazimuthal fieldofthe

/

/1

wireBw(r; x).Theazimuthal field is

x ]'1

ß

B w(r, x) 1 221/2 ß

-

'

60

I00

Fig. 11. Geometry for calculating MMR anomaly over anisotropic earth. The anisotropic half-space is replaced by a whole space with similar properties.

Fig. 12. The vertical MMR anomalies for an anisotropic earth plotted for a range of values of the anisotropyfactor k 2'

MMR

but they may be obtained from the vertical field using the Skeels-Watson

transformations.

If we make

61

and

the

substitution x = r cos 0 and y = r sin O, then the expressionfor the anomalousvertical component be-

cos 0

By(r, O, O)= 4,rrxr sin2 0


comes

x [K(k)- cos2 0II(-k 2 sin2 0, k)]. (72) m

B•(r,O,O)= •

(1- k2sin 20)1/2cot0, (67)

The contour of constant total horizontal field, say c, joins points satisfying

(Bx 2+ By2)1/2= c.

where

h2-1

(73)

Theshape ofthecontour fora rangeofvaluesofk2but

k2=

for a constant value of c is shown in Figure 13. Notice

that as k2 increases from a value of 0, the circle is the anisotropy factor. Edwards et al. (1978) derive the anomalous horizontal fields from this expressionas

B•ixI sin 0{

correspondingwith the uniform earth deforms into an elliptical shape.

Usingdefinitions of K(k) andII(-k 2 sin2 0, k), the components Bx andBy and the total horizontalfield may be rewritten in the form

4,rr

Bx(r, 0, 0)= ,r sin2 0

2011 (-k2sin 20,k)]},

sin 0 G(k, 0),

(74)

[K(k) - cos

(68) and

By

4,rr

By(r, O,O)= •,rr cos0 G(k,0),

(75)

and

4,rr ixI cos 0{

=

4,rr

G(k

'

0)

(76)

'

where

,r sin 20[K(k)-cos 2011 (-k 2sin 20,k)] . (69)

G(k, 0) =--

(1- k 2 sin2 t)1/2 1-k

2 sin2 0 sin2 t

dt.

(77)

where K and II are elliptic integrals of the first and third kind. The anisotropicearth is unique in that the anomaliesgeneratedby the anisotropy are quite independent of the location of the current electrode. In this case, a useful way of looking at the MMR effect is to plot the shape of a contour of a constant total horizontal field. In the case of a uniform earth, this is a circle surrounding the electrode as •

x

(Bf2 +Bf2)1/2= •i/' (x 22q•i/' (70) 4,r (x +y2)1/2 y2)= 4,rr' which is constant if r is constant.

The total horizontal componentsfor the anisotropic earth are obtained by adding the normal field to the anomalousfield. They are •I

Bx(rO,O)

2

4,rr •rsin0 x [K(k) - cos2 0iI(_k 2 sin2 0, k)], (71)

Fig. 13. The shape of the contour of total horizontal magnetic field about a current electrode on an anisotropic earth

for a rangeof valuesof theanistrophy factork2.

62


computedby setting0 = 0 in the expression for By

for -b < x < 0 and x > 0, respectively. The values of the constantsF and G may be obtained by equatingthe tangential componentsof the magnetic and electric

which

fields on x = 0. There


As Bx is identicallyzero alongthe x-axis (0 = 0 or •r), the length b of the semiminoraxis of the figuremay be becomes

•I

2

, , IBy(b 0 0)1= 4•rb•rE(k)=c.

(0 = •r/2 or 3•r/2), the length a of the semimajoraxis of the figure may be computedby setting0 = •r/2 in the expressionfor Bx which is IBx(a, •r/2, 0) 1=

•I

2

4•ra

•r

F = G = -k21.

(78)

Similarly,as By is identicallyzeroalongthe y-axis

results

(84)

The values of the anomalousazimuthal magneticfield and equivalently, by Stefanescu'stheorem, the vertical component on z = 0 are obtained through the inverse Hankel transform of the parts of equations(82) and (83) which depend on k21. The result is equation (80).

K(k) = c.

(79)

In terms of the coordinates given in Figure 9, the MMR anomaly along the profile AA' (y = -l) is

The ratio of the semiaxes a, b is, therefore, the ratio of

two simple, completeelliptic integralsK(k)/E(k). The Vertical

{

b+,x-b, }

100k21 1 [(b+Ix- bl)2+/2]•/2 , (85)

Contact

The contact is modeled by two adjacent quarter spaces of differing electrical resistivity. The quarter spacesare bounded by the plane z = 0, representing the earth's surface, and by the halfplane (z > 0, x = 0). The resistivities of the spaces (x < 0, z > 0) and

(x > 0, z > 0) are p• and 92, respectively. If the electrode is located at the point (-b, 0, 0), then Stefanescuand Nabighian (1962) show that the expression for the vertical magnetic field is

2Ixl) q-y211/2 , (80) a p. Ik2• [1-[(bq-Ixl) (b q-

where the contact is located at x = b.

The anomaly is symmetric about the contact. The geometry of the anomaly is independentof the conductivity contrast, the latter only appearing through the multiplicative reflection coefficient k12. If the whole analysis described here is repeated with the electrodeon the oppositeside of the contact(in region 2) and separatedby a distance b, then the corresponding MMR anomalyhas exactly the sameform havinga maximum value again over the contact itself. This value is given by

Bz= 4•ry

where k21 = (92 - Pl)/(92 + Pl). The derivation of expression (80) follows a similar scheme to that given for the anisotropic earth. Suppose the electrode is suppliedby a current I in a cable running along the negative x axis to x = -b. The Hankel transform (58) of the azimuthal magnetic field Bw(x; r) of the cable for x > -b is

Bw(p; x)= 4-•pp exp [-p(x+ b)]. (81) The complete expressionsfor the azimuthal magnetic field in the "whole space" obtained by imaging the conductivity in the plane z = 0 are constructed by adding appropriate solutions of the differential equation (60) to equation (81). They are

B•(p;x)= 4-•pp exp(-pb) x [exp (-px) + F exp (px)];

b+,x+b, }

where the contact, in terms of the coordinates of

Figure 9, is at x = -b. If the electrode is on the contact, both expressionsreduce to

100k2• 1- (/2+x2)•/2 ,

(87)

100k2![ 1 - sgn 13sin [3],

(88)

or

and the maximumvalue of 100k2• is independentof I. The horizontal MMR anomaly cannot be obtained in closedform except for the useful, specialcase of b = 0. It is derived in Stefanescuand Nabighian (1962) as 1 + cos 13

-(100Ik21/•r) sin 13cos 13log

(82)

and

B2(p; x)= 4--•pp exp(-pb) x [exp (-px) + G exp (-px)].

{

100k21 1-[(b+ Ixq-bl)2q-/2]1/2' , (86)

(83)

COS•

ß (89)

The anomaly is antisymmetric about the contact, having its maximum gradient over the contact. The horizontal and vertical MMR anomalies, for this special case of the electrode in the contact, are plotted in Figure 14, where k2• has been assignedthe value -1.


MMR

63

The Dipping Contact

100C(8) sin 8 cos 13

The MMR anomalies of a dipping interface may also be calculated analytically for the special case of the electrode in the contact. The geometry of the dipping interface is shown in Figure 15 as a section perpendicular to the y-axis. The interface divides two regionsof resistivity Pl and P2 and dips at an angle 8 measured from the positivex-axis. The current electrodeis at the origin of coordinates.If • = •r/2, the geometryreduces

-rr(1- sin: 8 sin: [•)

to that of the vertical

contact

described

at the end of

x •rcos13-2sin•sin13cos13•(l+sgn13)-•

+cos8 sin13 log1-cosl3' where the factor C(•) is given by

the last section.

The computation of the MMR anomaliesin both the horizontal and vertical fields requires extensive algebra (Nabighian, 1976; Edwards et al., 1978). In the vertical field, the anomaly along the profile AA' is

(90)

C(8) =

P2 -- Pl

(28/-tr)(p2 - p•) + 2p•

.

If 8 = -tr/2,C reducesto k21 and the completeexpression reduces to

100k21(1 -sgn [• sin [•),

(92)

the anomaly of the vertical contact. The shapeof the anomaly is clearly influencedonly by the geometry, i.e., the dip of the contact. The amplitudeof the anomaly is governedby both the dip and the resistivity contrast through the multiplicative

o z

(91)

2O

factor

C.

The type curvesfor the vertical anomalyfor various values of • are plotted in Figure 16, where p• >> P2. These curves may be used with any conductivity contrast by multiplying them by the factor C. The latter is plotted for a rangeof values of • as a function of pl/132in Figure 17. Notice that as • becomessmall, an increasinglylarger conductivity contrastis required to producean MMR anomalycomparablein amplitude with that for the case p• >>02. In the horizontal field, the corresponding MMR anomaly is

-40

- 80

- 100C(8)cos2 [3 'n'(1- sin2 8 sin2 [3) Fig. 14. The symmetricvertical and antisymmetrichorizontal MMR

anomalies

due to an electrode

embedded

in a

x sin 28sin[•cos[•log 1-cos[•

vertical contact. The contrast in conductivity across the contact is very large.

+ (-rr- 28) (1 - sin2 8 sin2 [• + sin8 cos8 sin[•)

T

+,rsin•cos•

sgn13sin 13-'n'cos8

. (93)

If B = ,r/2, the expression reduces to

-(100k21/'rr) sin[•cos[•log 1- cos , (94) the anomaly of the vertical contact. The type curves for the horizontal anomaly are

plottedin Figure 18, assumingPl >>P2. As before, for arbitrary conductivitycontrasts,they shouldbe used in conjunctionwith the ancillary curves of the factor Fig. 15. The geometry of the dipping interface.

C.

64

Edwards and Nabighian The Thick

Vertical

Dike of Infinite

Vertical

Extent


The thick dike, of finite width d, is modeled as three

adjacentspacesof rcsistivitics,p•, 02, and 03, respectively. The plane z = 0 represents the surface of the earth, as usual, and the planes x = 0 and x = d divide region 1 from region 2, and region 2 from region 3, respectively. Clearly, there arc two independentsolutions to the problem, corresponding to an electrode outside the dike, in region 1 or 3, or an electrode inside the dike in region 2. The first solutionwas obtained by Stcfancscu and Nabighian (1962). The second was derived by Edwards (1975) using a similar method. The vertical field correspondingto the first case is

-180-

-160'

- 4xry Bz2 aIxIIk13q-k32 •

(kl3k32)n[2(n + 1)d- b- x]

.•o{y 2+[2(n +1)db-x]2} 1/2 •c (k12k32)"[2(n+ 1)d+ b - x]

+k12 • {y2 +[2(n +1)d +b- x]2}1/2' n=O

-20

•

-2

-i

0

i

(k12k32)n(2nd+ b + x)

+k12 n•O [y2 +(2nd +b+x)2] 1/2

x/?, 2

Fig. 16. The vertical MMR anomalies due to an electrode embeddedin a dipping interface plotted for a range of values of the dip angle. The contrast in conductivity across the interface is very large.

• (k12 k32) n[2( n+ 1)d- b+x]•

+k32 • {y2+[2(n+ 1)d-b+•c]•i72] ' n=O

(95)

1.0

0.8

0.4 0.2 I

•

•o

I

•o ep,/,o 2

Fig. 17. The correctionfactor for finite contrastsin resistivity p1/P2acrossthe dippinginterface plotted for a range of values of the dip angle 8. The curves in figures 16 and 18 are multiplied by the factor to obtain the corrected MMR anomalies.

MMR

The corresponding expressions for Bz in regions1 and 3 are

65

The MMR anomaly curves shown in Figure 19 are examples of the type curves that may be computed

fromthepreceding expressions for B•. The curvesare Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

a

for the profile AA' shown in Figure 9 where the edges of the dike are at x = _+d/2, so that the electrode is located symmetrically at the center of the dike. Regions 1 and 3, perhaps representing a host medium, have the same resistivity which is three times that of region 2, a conductive zone.

•

Bz• 4,ry k13 + (1 -- k12)k32

•

(k12k32)n[2(n + 1)d- b- x]

x ••]0 {y2+[2(n+

1)d-b-x]

2}1/2

A number

•

(k•2k32)n[2(n + 1)d+ b - x]

+k12 Z {y2 +[2(n +1)d +b- x]2} n=0

b-x

(96)

k12 [y2+ (b- x)2] 1/2 and

a

Bz3

..•

4,ry

k13 -- (1 -- k32)k12

(ki2k32) n (2nd + b + x)

[y2 + (2nd + b + x)2]1/2

•

(k12k32) hi2(n + 1)d- b + x]

+k32 Z {y2 +[2(n +1)d-b+x]2} 1/2 n=0

x-b ]

(97)

+ k32 [y2+ (x- b)2] 1/2 ' 8o

I

I

of the curves

are worth

noting. The vertical field is always a maximum at the edge of the dike. If d is less than l, it changes linearly through the dike which indicates that the current flow is uniform along the dike. The corresponding type curves of the horizontal MMR anomaly are shown in Figure 20. They were derived numerically using the techniques described earlier. The shape of these curves is characteristic of many MMR anomalies. The curve has a positive peak directly over the center of the conductive zone with smaller, negative side lobes over the host medium. Generally, the width of the anomaly is a complicated function of d and l. But if d/l is large, the anomaly width is related only to l. If the maximum anomalies in Figure 19 are compared for differing values of the ratio d/l, clearly an optimum ratio of d to l does exist which produces the overall maximum anomaly for the chosen model. Figure 21 plots the maximum anomalies as a function of d/l. Why does this curve peak? If d/l is large, the magnetic field observed along the profile differs little from that observed over a uniform earth. As d/l

decreases, the gathering ofcurrent into the dike is

• •o

,•

of the features

observed as an anomaly. Whend/l becomes very

•ss

I

I

x/l•

,o

o

Fig. 18. The horizontal MMR anomalies due to an electrode embeddedin a dipping interface plotted for a range of values of the dip angle 8. The contrast in conductivity across the interface is very large.

Fig. 19. The vertical MMR anomaliesdue to an outcropping dike of finite thickness plotted for a range of values of its normalized width d/•. The dike is three times more conductive than the host medium and the electrode is at the center of the dike.

66



small, the current no longer remains in the dike but leaks out into the host medium reducing the anomaly. We have plotted the curve for a conductivity contrast of 3 to 1 between the dike and the host medium. A more conductive dike is more effective at current

channeling and is characterized by a different curve from that shown in Figure 21. The curve of a more conductive dike peaks at a smaller value of d/l. In the extreme case of a perfectly conductive dike, the maximum

occurs at d/l = O.

The peak vertical MMR anomaly at the edge of a perfectly conductive dike of finite thickness may be shown to have a magnitude of 100

1-[/2 + (d/2)211/2 ß

(98)

This is the maximumanomalythat can be producedfor any dike of finite thickness and its decrease with increasingd/l is clearly a geometric effect. Let us now study the effect of the conductivity contrast between

the dike and its environment.

For a

given d/l, what must the conductivity contrast be to produce a maximum anomaly comparable with that generated by a perfectly conductive dike? In Figure

22, we plot the peak amplitudein B] for a rangeof valuesof d/l as a function of the ratio 91/92, the ratio of the resistivity of the host to the resistivity of the

dike, having first divided out the geometric factor. In other words, the curves are normalized to the maximum anomaly of the perfectly conductive dike. We can deduce, for example, that if d/l - 0.1, then a resistivityratio in excessof 100is requiredto produce a maximum anomaly of similar amplitudeto the maximum anomaly produced by the perfect conductor. For small values of d/l, the case of a relatively thin dike, many of the curvesin Figure 22 are parallel over a certain range of the ratio of p•/p2. Let us define a dimensionless parametera = dpi/2/p2anddraw on the figure lines of constant a. Notice, that for small values of d/l, these lines are parallel to each other and to the pl/P2 axis indicating that for this range of the ratio Pl/P2, all the curves reduce to a singlefunction of a. Clearly to determine this function by studying the electrically thin dike as a separateproblem is important.

The Thin Conductive

Dike of Infinite

Vertical

Extent

The thin dike has a conductivity-thicknessproductS and occupiesthe halfplane (x = 0, z > 0). The quarter spaces (x < 0, z > 0) and (x > 0, z > 0) have resistivitiesPl and P3, respectively.The anomalous vertical magnetic field is derived for a current source locatedin region 1 at (-b, 0, 0). The required expressions for an electrode in the center of the dike are then

obtained by setting b = 0. They are derived by Edwards et al. (1978) as

•30 l d=.st i

a

•I f•k31 --otq.j1 (q)

Bz'(X' Y)= 4xy 1+otq

x exp [(x-b)q/lyl] dq,

o=.25•

(99)

and

Bz3 a(x,y)=

(q) ixIf•k31 +otq.j•

4,ry

x exp [-

1 + otq

(x + b)q/lyl] dq, (100)

where a = S PlP3/(Pl + P3)IYI and the variable of integration has been changedby writing plyl = q. If a is much smallerthan k31, then, in both regions,

Fig. 20. The horizontalMMR anomaliesdue to an outcropping dike of finite thicknessplotted for a rangeof valuesof its normalized

width

d/e. The dike is three times more

conducting than the host medium and the electrode is at the center of the dike.

b)+ 2+b) y2]•/2' -Ixlk31 { [(Ixl+(Ixl }ß(101)

a(x y) = Bz ' 4,ry

1-

The function is symmetric in x and is identical in form to the field derived

for the contact earlier.

If a is much larger than 1, then, in both regions,


MMR

•.

67

2o

o z

DIKE

10

0 0.03

•

0.1

•

t

(33

1.0

• • _•._ or•d

3.0

Fig. 21. The maximumverticalMMR anomalyasa functionof thenormalizedhalf-widthof d/2l of the outcropping dike. The corresponding maximumverticalMMR anomalyas a functionof the normalizedhalf-widthr/l of the horizontalsemicylinder.Both structuresare respectivelythree timesmore conductivethan the host medium.

i.oL

O.8

n,. 0

0.2

I

Io

Iø•Pl/1ø2

Fig. 22. The ratioof the maximumverticalMMR anomalyfor a dikeof finiteconductivity to the maximumvertical MMR anomalyfor a dikeof perfectconductivityplottedagainstthe resistivitycontrast@1/@2 betweenthe dike and the hostmediumfor a rangeof valuesof the normalizedwidth d/l. The curvescorrespond to correctionfactorsfor the effects of finite conductivity.

68


a

IxI{

(Ixl-+-b) }


Bz(x' Y)= 4,ry1 [(Ixl+b)2+y211/2 sgn (x). (102)

The function is antisymmetric in x and is the field of a perfectly conductinghalfplane. The expressionshave the same form, showing that the pattern of the anomalouscurrent flow producing them also has the same form. In the case of the dike, the direction

of flow is inward

toward

,',

the dike from

both region 1 and region 3 and then the flow is along the dike. In the case of the fault, the direction of flow is from region 3 to region 1, assumingk3! is positive. For values of • less than 10, the field will generally have both a symmetric and an antisymmetriccomponent, dependingon the relative values of k31 and •. If we set 93 = Pl, b = 0, and x = 0 in the expression

z !

3o

2o

for Bz,, we obtainthe maximumverticalfieldfor the thin dike due to an electrode

a

Io

in the dike.

=ixIf• aqJl(q) dq.(103)

Bz, max 4,ry

1 + aq

The correspondingmaximum MMR anomaly on the profile AA' in Figure 9 is

-IO

-20

-30

f• aqJi(q) dq,(104)

f(a)= 100 1+ aq

Fig. 23. The horizontal MMR anomalies for a thin dike, characterized by a value for the dimensionlessparameter a

where a = p!S/21. Edwards and Howell (1976) evaluate this integral as

of 1, plotted for two valuesof the reflectioncoefficientk13.

f(a) = (100/a){[H! (l/a) - Y• (1/a)](-rr/2) - 1), (105) where the functionsH• and Y• are the Struvefunction

5O

and the Bessel function of the second kind of order 1,

respectively. Using the numerical Fourier transform technique, we have also computed the MMR anomalies in the horizontal field for the geometryof Figure 9 with

the electrode

in the dike.

These

4O

are shown in

Figures 23 and 24. They are for constant values of

the dimensionless parameter• but k3• is variedfrom 0

/:',,,',

to 1.

The symmetry of the horizontal anomaly is generally opposite to that of the corresponding vertical anomaly. The anomaly is also singularat x = 0. In Figure 23, • has the relatively large value of 1 and varying k3• has little effect. In contrast, • has a value of 0.2 in Figure 24 and the symmetry of the curve can be totally changedby varying k3•. It should be emphasized that • depends on l, the distance of the profile AA' for the electrode. If MMR profiles are measured at varying distances I from a single electrode, the value of • can change. The horizontal anomaly could start out symmetricat small l (large a) and turn antisymmetricat large l (small a),

Ki3=O.O -IO

-20

-30

Fig. 24. The horizontal MMR anomalies for a thin dike, characterizedby a value for the dimensionlessparametersa

of 0.2, plottedfor two valuesof the reflectioncoefficientk13.

MMR

provided k31 has a finite value even thoughthe twodimensional

structure


FILLED

retains a consistent

SINKS

AND

form.

CHANNELS

69

Notice that as l increases, the normalized anomaly also increases because of the leading (l/r) term. The perfectly conductingsemicylindercan, therefore, give very large MMR anomalies indeed because the whole current flow from the electrode

Horizontal Circular Semicylindrical Channel Our studies of structures of infinite vertical extent have shown that the MMR anomalies over them have

widths which are controled in part by the distance of the MMR profile from the current electrode. For structures of limited vertical extent, this is not the case; the width being controlled by the depth to the bottom

of the structure.

The structure

considered

is

the outcropping, horizontal half-cylinder of radius r and resistivity• which is embeddedin a hostmedium of resistivity P2. (The axis of the cylinder coincides with the y-axis while the current electrode is at the origin of coordinates.) Edwards (1975) derives the vertical

fields as

a (X y) Bz, ,

= -

2•-•sgn (x)

(106) and

=

With structuresof finite conductivity, this is not the case for what is gained through geometry is lost in current channelingability. We illustrate this in Figure 26 which is the equivalent of Figure 22 for the thick dike. If r/l = 0.05 (equivalent to d/l - 0.1), then a conductivity contrast in excessof 10 000 is required to produce an anomaly equal in amplitude to that obtained with a perfectly conducting semicylinder. The correspondinganomalies in the horizontal field are plotted in Figure 27, which should be compared with Figure 20. Again, they were determined by numerical Fourier transformations. The anomaly in the horizontal

field for the dike maintains

a width related

to l as d/l decreases. In contrast, the corresponding anomaly for the semicylinder decreases as r/l decreasesand may be shown to have a half-width related to r.

1- (pr)K1 J21Io (pr)K1 (pr)pr sin pydp, øj21Ko (pr)prI1 (plxl)

a (X y) Bz2 ,

is close to the surface.

-2•-• sgn (x)

1--(pr)I1 J21Io (pt)K1 (pr)pr sin pydp, fiø j21KO (pr)prK1 (plxl)

x

Outcropping Hemispherical Depression

In solving for the MMR anomaly due to the hemispheric depressions, we use a geometry consistent with that shown in Figure 9. The current electrode is located at the origin of coordinates. The center of the depressionis at the point (0, -b, 0) and the radius of the depressionis a. The resistivitiesof the depression and of the surroundingearth are p• and P0, respectively. Let (r, 0, •) be a set of sphericalpolar coordinates centeredat the center of the depression,where • is the

(107)

where J21 = 1 - (Pl/P2).

•3o o•

The MMR anomalies in the vertical component may be computed directly from the above expressionsas the geometry is consistentwith that of Figure 9. A set

:•0 0

of type curvesfor the caseJ21= 2/3, is shownin Figure 25. The curves may be compared directly with the equivalent curves for the thick dike previously presented in Figure 19. The anomaly again peaks at the edge of the structureand there is an optimum value of r/l for this conductivity contrast, at which the overall maximum anomaly is observed. This variation in the maximum anomaly for the semicylinder is plotted in Figure 21 and can be compared directly with the equivalent curve for the thick dike. The maximum anomaly at the edge of a perfectly conducting semicylinder is easily shown to be l

100(l/r) (l2+ r2)1/2'

(108)

:10 •

I

2

'

x//,

I0-

20-

Fig. 25. The vertical MMR anomaliesfor a horizontal semicylinders plotted for a range of values of its normalized half-width r/l. The semicylinderis three times more conductive than the host medium and the current electrode axis.

is on the

70


rotation angle measuredpositive in a right-handed senseaboutthe positivey-direction.Then, the vertical

B Za1(x • y) = -•

I•I sin 0 471'/'


fields as derived in Edwards et al. (1978) are

a(x y)= Bzl '

I•I sin 0

x • Cn(r/a)n +•(b/a)np• (COS 0),

4•rr

n=l

(111)

X E Cn(r/b)n+1p,(cos 0), n

(109)

and

n=l

and

Bza(x o , y)=

Bza(x o , y)-

i•I sin 0 471'/'

i•I sin 0 471'/'

X E Cn(b/r) nP•(cOS 0).

(112)

n=l

X E Cn(a/r)n(a/b) n+•P[t(cos 0),

The above expressionstake on simple forms when the host medium is either very resistive in comparisonwith the depression,P0>>Pl, or very conductive,

n=l

(110) where

P0<• Pl. The reflectioncoefficient Cn reducesin these two extremes to -1/n and 1/(n + 1), respectively, and

sin0 = x/r, cos0 = (y + b)/r,r 2 = x 2 q-(y + b)2, Pl

eachexpressionmay be writtenin oneof two standard forms (as shown in Table 1, Edwards et al., 1978). The magneticfields take on the form presentedin Table 1, where Ix = cos 0 and A = IxI/4xrsin 0. Two specialcasesof the expressionsin Table 1 shouldbe considered. Let us examine the MMR anomaly across the central profile of the hemisphere for a current electrode located at the rim, i.e., b = a = I. The MMR

-- P0

(n+ 1)pl +np0

and P•(cos 0) is the derivativeof the Legendrepolynomial of degree n with respect to its argument. The other case to be considered is for an electrode

located within the depression,b < a. Then,

1.0

0,8

06

&

04

(3'

0.2

0

I

Io

I

I

Io•

Io•

!

Io• P•/

Fig. 26. The ratioof themaximumverticalMMR anomalyfor a semicylinder of finiteconductivity to themaximum verticalMMR anomalyof a semicylinder of perfectconductivityplottedagainstthe resistivitycontrastP2/Pl betweenthe semicylinder andthe hostmediumfor a rangeof valuesof the normalizedhalf-widthr/l. The curves correspondto correctionfactorsfor the effectsof finite conductivity.

MMR

anomalyalongthis profile, the profileHA' of Figure 9,

71

for the conductive depression,and

is then

I 1

-100(l/x) 1- (/2+X2) 1/2'X--< l, (115)


x

100(/2+x2)1/2x _< l,

(113)

100(l/x) (12 +X2)1/2 X--> 1,

(114)

and

and

3o I

x ]

-1001- (x2+/2)1/2x _> l,

(116)

for the resistive depression.The functionsare plotted in Figure 28. The peak anomaly in each occurs at the rim of the hemisphereand the ratio of the conductiveanomalyto

theresistive anomaly at thispointis 1/(X/•- 1) or 2.42. This ratio does not differ substantiallyfrom the well known value of 2.00 when the hemisphere is excited by a uniform electric field. Indeed, at large distancesfrom the hemisphereand near the center of the hemisphere,the ratio is very close to 2.00. The geometries of the anomalous current flow for the resistive and conductive hemisphereexcited by a

r --.25 •

r =05•

r --0 125œ

uniform

current

field are also known

to be identical.

When the body is excited by the point source on its rim, the geometriesin the two casesare similar but not identical. To a certain extent, we can "see" these two

flows and compare them by plotting the stream function of the unique current distributionwhich, confined to a horizontal plane just below the earth's surface, generatesthe anomalousvertical field everywhere on the earth's

Fig. 27. The horizontal MMR anomalies for a horizontal semicylinderplotted for a range of values of its normalized half-width r/l. The semicylinderis three times more conductive than the host and the current

electrode

is on its axis.

surface.

The streamlines

are a two-dimen-

sional current flow. They are to the MMR method what an equivalent mass distributionis to the gravity method.

The streamlines

for the resistive

and conduc-

tive cases are shown in Figure 29. One important property of the plots is that the magnitudeand direc-

Table 1. Magnetic fields due to conductive or resistive hemispherical depressionfor arbitrary location of current source. (Edwards et al., 1978). Conductive depression Current

source

in host medium

B a =(A/b)

z•

B a = (/la/rb)

z0

Resistive depression

[1 - 2(rib)ix + (r/b)2]1/2

{

IX-

IX - (a2/rb)

[1 - 2(a2/rb)+ (a2/rb)2] 1/2

+ (r/b)2] 1/2 Ba=-(/l/r) {1 -[1 - 2(r/b)ix •- (r/b)v } Ba=-(A/a) {1 - 1-(a2/rb)ix } z0

[1 - 2(a2/rb)ix+ (a2/rb)2] 1/2

Current source in depression

Ba=(A/a) {Ix- IX --(rb/a 2) } Ba=(/l/r) {IX - • - (b/r) z1

[1 - 2(rb/a2) + (rb/a2)2]1/2

z0

[1 - a(b/r) + (b/r)2]1/2

I-(rb/a 2)ix

Ba = -(/la/rb) 1- [1 2(rb/a2)ix + (rb/a2)2] 1/2 B a = -(/l/b)

zø

1-

[1 - a(b/r)ix+ (b/r)2] 1/2

72


tion of the horizontal anomalousmagneticfield may be obtained directly from them. The direction of the field is locally orthogonal to a streamline and the magnitude is proportional to the local surface density of stream-


lines.

The MMR anomalies in the x-component correspondingwith the vertical anomalies of Figure 28 are shown in Figure 30. As usual, they were computed

8ø T 60-

40-

20\

/

lies have

a maximum

over

the center

of the hemi-

sphere, and they changesignand have maximum slope near the rim of the hemisphere. The peak anomaly for the conductive case is almost twice the peak anomaly for the resistive

case.

In practice, the contrast in resistivity between the hemisphere and its environment may not be large; neither may the current source be located at the rim of the hemisphere. We can show that the shapesof the profiles illustrated in Figure 28 and 30 do not change substantially when either of these parameters are varied. However, the peak MMR anomaly in the vertical field does change significantly. The variation as a function of Pl/P0 for a range of values of b/a is shown in Figure 31, which may be used to estimate anomaliesfor a variety of cases. The analysis presented here on the hemisphere is less than a brief summary of the work of Lee (1975). Lee shows that under certain circumstances

'x _20\

using the Skeels-Watson transformation. The anoma-

x/l• /

-40

--60

the MMR

anomaly of the hemisphere may be represented in terms of image sources. He reduces the anomaly to that of a contact when the radius of the sphereis large compared with all other distances. Finally, not only does he present many more profiles of the MMR anomaly across the hemisphere, but he also makes some progress in producing an inversion scheme for field data and in discussingthe detectability of depressions under a variety of conditions.

-80

THE

Fig. 28. The vertical MMR anomalies for a hemispherical depression. The solid and dashed lines correspond with perfectly conductive and perfectly resistive depressions, respectively. The current electrode is at the rim of the depressionor b/a = 1.

CURRENT

DIPOLE

The canonical problem in the theory of the magnetometric resistivity method is the determination of the magnetic fields of a buried current dipole. Stefanescu

Fig. 29. The current streamlinesfor left, the perfectly resistive depression,and right the perfectly conductive depression. The current electrode is at the rim of the depressionor b/a = 1.


MMR

(1929) defines a current dipole as a point current source and an equal but opposite point current sink embedded in a conductive medium and joined by a short, insulated length of cable. Current flows continuously from the current source through the medium to

ioo

8o.: •0'

4.0'

70'

\

73

the current sink, and from the current sink back to the current source through the cable element.

The current dipole is used as a teaching concept to explain the magnetic effects of currents channeled by a small inhomogeneity within the earth and is also a fundamental building block for numerical computations of the MMR anomaly associated with more complex structures. The dipole represents the anomalous, secondary, non-divergent current flow in the vicinity of the body. The magnetic fields of a dipole in a half-space may be evaluated by integrating other equivalent current filaments by the Biot-Savart law. An inclined current dipole in a conducting half-space and the equivalent current filaments which generate the same magnetic fields at the surface of the half-space are shown in Figure 32. The constructions are based on the theory given in the MMR Response of a Layered Earth section particularly expression (5). For a horizontal current dipole located at the point (0, O, z') and directed in the positive y-direction, the magnetic components at the point (x, y, z) due a current I flowing in the cable element length A are

/

-7o-

/

\

/

\

BxYa Ia'mY[ z--z'] = •

\ -40

I'3

,

yd= 0, By

-60

Fig. 30. The horizontal MMR anomaliesfor a hemispherical depression. The solid and dashed lines correspond with perfectly conductive and perfectly resistive depressions, respectively. The current electrode is on the rim of the depressionor b/a - 1.

(ll7)

(118)

and

Bz = 4,r

'

(119)

wherethelengthr isgivenbyr2 = x2 + y2 + (z - z')2 20

andthe dipolemomentmy by my = IA. The magnetic componentson or below the surface of the earth due to the current flow in the conductive

half-space are

b/a = O,25

b/a =0.5 b/a=4.0

b/u=2..00

-60

b/a =1,50

I

iO-3

iO-2

I

I

i0-•

I

iOi

I

iO2

I

iO3

P•/ Po Fig. 31. The maximum vertical MMR anomaly for the hemispherical depressionplotted againstthe ratio of the resistiv-

ity of the depressionto that of the host medium Pl/P0 for a range of locations for the current electrode b/a.

(a)

(b)

Fig. 32. The buried inclined current dipole and the equivalent line current representation which generates the same magnetic field on or above the surface of the earth (redrawn from Stefanescu, 1929).

74

Edwards and Nabighian Z+Z

•

ys_ IXmY { +z') } Ixy2]y2(z

Bx -

t2s 3

t4

,


(120)

ys_ IXmY

{[

Z+Z

simply as follows. Consider a small, thin, vertical plate dimensions 2a x 2b and conductance S, buried at a

mean depth d in a half space of resistivity p. Let the plate be energized by a current source +I and a current

•

sink -I

located

on the surface

of the earth a

wherethe lengthss andt are givenby s2 = t2 + (z + z')2 andt2 = x2 + y2 respectively

distance 2L apart, as shown in Figure 36. The impressed current flow in the vicinity of the plate may be described approximately as follows. A current per unit length Js enters the plate along its leftmost vertical edge and leaves it along its rightmost edge. The surface current in the plate is constant and equal to Js and is the total surfacecurrent becausethe strip has infinitesimal thickness so that the contribution to the total current from the primary current flow

The correspondingcomponents for a z-directed dipole are more simple, they are

within the strip is js/S.

By -

t2s 3 (121)

and

BzY•=0 ,

(122)

9

ø

(124)

.azza• 0•

(125)

An electromagnetic boundary condition requires this internal tangential electric field to be the same as the external tangential electric field at the surface of the plate. The latter is simply expressed as a sum of the external field of the plate and the image of the plate in the surface of the earth and the field of the exciting current sources.As there is only one unknown factor,

the surface current j• or equivalently the current

dipolemomentof theplatemy = 4abjs,theboundary

and

Bx - 4'rr

condition need be satisfied only at one point. The center of the plate P is the logical choice. The primary horizontal external electric field at P is just

'

4'rr

'

(128)

The vertical current dipole generates no external magnetic field. The surface magnetic fields of a horizontal current dipole do not vanish and are shown in Figure 33. The surface fields are as given even if the dipole is embedded in a layered medium. HowlandRose et al. (1980a) illustrate the latter point with an analog model experiment. They used a signalgenerator to energize a current dipole in a conductive halfspaceand made measurementsof the surface magnetic electric

fields

with

and

without

an additional

conductive layer. The results are shown in Figures 34 and 35. Notice the similarity of the horizontal magnetic field for the two cases and contrast

this result

with the effect on the horizontal electric field, which is greatly influenced by the additional conductor. THE

SMALL

plL

(127)

BzZS = 0.

and

is itself infinitesimal. Therefore, the total electric field

TARGET

The relative insensitivity of the MMR method to small conductive features is both the strengthand the weaknessof the technique. The order of magnitudeof the response of a small target may be obtained very

'rr(L 2+ d2)3/2'

(129)

The secondary external electric field at P due to the distributed line current sourceson the plate is

PJs f•o(p2+aa2)3/2dp= pmy 4'rra2( a2+b 12)1/2'

4'rr2

(130)

and the secondary external electric field due to the image of the plate in the earth's surface is

+•b(p +aa 4,r f2d a2a-

PJs 2

13' 2 2) 3/2 dp pmy 4'rr(2d) (131)

The boundary equation can now be formed by equating the internal and external tangential components of the electric field at P. The sum of expressions

(129),(130),and(131)mustequalmy/4abS. A simplified formfor myobtained by settinga equal to b and neglectingthe image interaction term (131) is 4a3IL

(L 2 + d2)3/21 +

(132)

MMR

75

where a is the current channelingnumber defined by


ot = pS/'n'a.

•my

4id 2

(133)

The magneticfieldsof the plate at the surfaceof the earth may be evaluated using expressions (117) through(121). Of particular interest is the horizontal magneticfield at the center of the array directly over the plate which is 8,rrd2'

100myL

100a3L 2

ot

.

d2(L2+ d2)3/21 + ot

(135)

Assuming that L and d are of the same order of

magnitude, thesizeoftheanomaly variesas(a/L)3.If d is of the order of a, variation is as a/L. Clearly, to resolve a small target at depth using the MMR technique is difficult. THE

(134)

and the correspondingMMR anomalyin percent is

=

ALPHA

CENTER

The alphacentermodelis an analyticrepresentation by a three-dimensionalconductivitydistribution.The

By

Bx

6( m

az

Fig. 33. The threecomponents of the magneticfielddueto a buriedcurrentdipole.The dipoleis at a depthof 10m, hasa lengthof 1m andis orientedin y direction.Thecontourvaluesarein milligammas perampereof current flow (after Edwards, 1974).

76


model, introduced by Stefanescu (1950, 1970) and developedby Stefanescuand Stefanescu(1974), dif-


fers from

conventional

models

V2(otU)= otV2U+ 2VotVU+ UV2ot, (139)

in that the boundaries

between media of differing conductivity are not sharply defined. Instead, the conductivityvaries continuously from point to point, which is often a better approximationof the real world (e.g., a halo of disseminatedsulfidessurroundinga massivecore.) The model may be introduced in the following manner. The differential equation which relates the electric potential U to the conductivity in a continuous, isotropic, inhomogeneousmedium is Vo-VU + o-V2U = O.

Substituteot = •

But

(136)

suchthat tr = ot2. Then

Vo' = Vot2= 2otVot,

(137)

so that

V2(o•U)- UV2o•= O.

(140)

Now, define a parapotentialq• = otU, and separatethe differential equation into two simultaneousequations in ot and q•which are

V2a

V2•

-f,

(141)

where f, the separationfunction, is some function of position. Stefanescu(1950)setthe separationfunctionto zero and investigatedthe simplestpossibleharmonic solu-

tionsof V2ot= 0 in a wholespace.He setot= C/R+ B, tr - (C/R+ B)2. HereR is thedistance of a field

and

otV2U + 2VotVU = O.

-lOOO

-

-800

-

-600

-

-400

-

-200

-

(38)

point P from the a-center S, where the conductivityis

HOMOGENEOUS

o • s•^cœ o

•,.-P:/--PJ--•.• •,

5"

o

200

o

o

--•X

400

%

2

•

-

Fig. 34. Effect of conductivelayer on magneticfield (MIP) results.(1) homogenous half-space;(2) conductivelayer at surface;(3) conductivelayer in the subsurface(after Howland-Roseet al., 1980a).

MMR

singular,and C and B are positive constants(Figure


37). The constant B cannot be set to zero for the

conductivityhas to be finite at large distancesfrom the center. A current I is introduced into the space at a point a distanced from the a-center. In a later paper, Stefanescu(1958) derived the vertical magnetic field on the plane z = 0 as txlC

(x•y - y•x)

B•(x,y,O) •TOt0 rRd(r +R+ d)' (142) The anomalouscurrent flow above the plane z = 0 is the image of the flow below. A consequenceof Stefanescu'salgorithm is that this field is preciselyequal to the vertical field that would be generated by current flow from the same sourcein the conductivity distribution occupyingthe halfspacez > 0 only. The current densitythere is doubledbut the vertical magneticfield on the plane z = 0 is unchanged. The anomalouscurrent flow is divergencefree and confined to a region in the vicinity of the current electrode and the a-center S. Consequently,we may

77

simplify the conductivity structure and set B to be a very small number in comparison with 2C/d. Then a0 = 2C/d, and

txI (x•y - y•x)

(143)

B•(x,y,O) 2•T rR(r+R+d)

which is independent of the choice of C. If we now refer to the geometry of Figure 8 and place the a-center at (0, -l, Zs), then the MMR anomaly in the vertical field along the profile AA' throughthe a-epicenter is

200x/2 (144)

rR(r + R + d)'

where r2 = x2 + 12R2 = x2 + zs 2 andd2 = 12= zs 2 9

ß

We plot the anomalyfor a range of valuesof z• in Figure 38. The correspondinganomalyin the horizontal field, derived through the Skeels and Watson transformation,is shownin Figure 39. The variation of conductivityin the halfspace,which has axial symme-

300

HOMOGENEOUS

250

HALF SPACE

O

o

(1)

•

2d

,=.

200

x ........

150

x A

d/2•

o

P2

o

1 oo O

5o

I

-4

-3

-2

-1

0

1

2

3

x/d Fig. 35. Effectof conductivelayer on electricfield(EIP) results.(1) homogenous half-space;(2) conductivelayer at surface;(3) conductivelayer in the subsurface(after Howland-Roseet al., 1980a).


78


try above a vertical axis through S, is shown in Figure 40. The unit contour in the latter diagram has been selected arbitrarily for the MMR anomaly and is a function of the gradient of the conductivity only and does not depend on absolute values of conductivity defined by selecting a value for the constant C. Stefanescu and Tang Muoi (1971) extended the theory and determined the magnetic field due to anomalous current flow in a conductivity distribution occupying the halfspace z > 0 which was of the form

where all the symbolshave their previous meaning. In the same paper, the authors derived the composite magneticfield due to any numberof pairs of symmetric or antisymmetric centers for an electrode located at any arbitrary depth in the lower halfspace. A further logical extension of the theory is to set the

separation functionto a nonzeroconstantK2 and investigate solutions of the Helmholtz equations

Z$--054

o•:•=C -•-•.

(145)

40-

This "antisymmetric" distribution differs substantially from the "symmetric" distribution considered previously: The conductivity is zero on the plane z - 0 rising to a maximum, singular value at S. The anomalous vertical magnetic field on the plane z = 0 due to a current electrode at the origin is

IxI (Xsy - ysx)(R + r + 2d)

B•(x,y,O) 2,r

rR(R +r +d)2

•

30-

o

z

20,

I

'

-2

I

•

•

2

x/l

- -I0

(146)

-20

-30

+I •x -40

Z •

Fig. 38. The vertical MMR anomaly of a buried symmetric alpha center plotted for a range of values of the normalized depth zs/l.

(equivalent short dipole)

Fig. 36. Geometry for calculation of MMR anomaly of small targets.

60i

S• 40' •

X

O'

20.

P

-I0

Fig. 37. The geometry of the alpha center model. S is the center and $* its image. P is a field point and the electrode is at the origin O of coordinates.

Fig. 39. The horizontal MMR anomaly of a buried symmetric alpha center plotted for a range of values of the normalized depth zs/l.

MMR

V2a - K2a = 0,

(147)

and


V2q•_ K 2q•= 0.

(148)

The simplest solutions for a are of the form

a=• =C• exp (-KR) +•-•exp (-KR

where the distancesare defined as in Figure 37. The anomalous vertical magnetic field for any number of these exponential, symmetric, or antisymmetrica-centers was derived by Tang Muoi (1972). His result for a single a-center was used to interpret field data by Edwards and Howell (1976). (Note, there is an error in the expression, for the anomalous magnetic field quoted in their paper and used by them: It is too large by a factor of two. Their a-center interpretation has to be modified slightly.) METHODS

Several general numeric techniques for evaluating the electromagnetic fields in a heterogeneousmedium excited by a finite source are previously describedin this volume and also in Volume I (Hohmann, 1987). The MMR response is essentially obtained by taking the limit of these solutions as the exciting frequency tends to zero. Of particular interest are two alternative techniques which are outlined here. The first is really the "Small Target" problem described in the Case Histories sectionextended to a plate of finite size. The software

written

Plate

A plate of finite size can be assembled from small rectangular elements in two distinct ways. The elementsmay be confinedto a singleplane or they may be arrangedin the fashionof a bottlebox, without top and base, as shown in Figures 41 and 42, respectively. The secondarycurrents within a fiat plate, confined for convenience to the vertical plane x = 0, are

edge and leaves along another, forming a horizontal (149)

second combines

The Conductive

describedapproximately as follows.A currentIyi entersthe ith rectangle uniformly alongonevertical

+ C2 exp(KR)+•-exp (KR*),

NUMERICAL

79

currentdipoleof momentmyi. Similarly,a current enters the same rectangle along one horizontal edge and leaves along another, forming a vertical current

dipoleof momentmzi. The unknowndipolemoments

{my}and{mz}arefoundby solving anintegral equation numerically. The integral equation is formed by equating the two components of the internal tangential electric field to the correspondingtwo componentsof the external electric field at test points at the center of each rectangular element. This is simply a generalization of the single plate equation given in the previous sectionand is describedfully in Nabighian et al. (1984) and in Cheesman and Edwards (1989). The basic form of the integral equation at a test point

onthejth element is thesimultaneous pair

myj/AjSj = E;j+ p/4xr Z myiF(i, j) i

+ p/4xr Z mzi G(i,j), i

and

mzd/AjSj = Ez•. + p/4?r Z myiG(i, j) i

to solve the static

"resistivity" problem with code exploitingthe special modified "static" form of the Biot-Savart Law, expression (43), to find static magnetic fields.

Fig. 40. The variation in conductivity in the space z > 0 for a buried, symmetric alpha center. The unit contour has been selected arbitrarily.

+ p/4xr Z mziF(i, j),

(150)

i

Fig. 41. Plane plate model to calculate effect of conductive plate.

80



whereAj andSj arethe areaandconductance of the jth rectangle andEp arethesource tangential electric fields, calculated from first principles. The dimensionless interaction functions F(i, fi and G(i, fi depend on the plate configuration, but may be assembledfrom standard integrals of the type

2-{-V2 5/2 f•dufo •dv(U2v2u-{2-½2) c2

to p = 4L, are examined.The dependenceof Bmax on plate depth is plotted with dashedlines in Figure 44 for each of the models. The half-plane profile from Edwards (1983) is also included. The profiles show that,

ab

(a 2+ b2+ c2)1/2(b2+c2)'

q-½2)5/2 f•dufo •dv(u2q-v2 3vc

1

1

= ac (b2+ c2)(a2+ b2+ c2)1/2 c2(a2+ c2)1/2 (52)

and

dv(U2-{-V2 -{-½2) 5/2 = (a2+ b2 f•dufo' 3uv 1+ ½2)1/2 1

1

1

(a2+ c2)1/2 (b2+ c2)1/2 +c

2L, the dip angle 0 (set to 90øfor the vertical case), and the product of the plate conductance S and the halfspace resistivity O. The number of independent linear parameters may be reduced from three to two by renderingp and h dimensionlessthrough division by L. In order to determine more precisely the relationship between the size and depth of a plate and the characterof the anomaliesproduced, a number of very conductive plate models, ranging in size from p = . 1L

(153)

For a plate in a half-space, there will be two contributions to each interaction function, the effect of the

ithelement andtheeffectof theimageof thatelement in the plane z = 0. The latter is needed to satisfy the boundary conditions at the surface of the half-space. For a multilayered earth, there are multiple imagesor an equivalent Hankel transform representation. The solution to the integral equation is stable for all values of S, even for the case of a perfectly conductive plate. Other integral equations often yield inaccurate

in general, the larger plates produce larger anomalies. This is not strictly correct for the curves labeledf and g. Curve f is for a plate whose side length is equal to the current electrode spacing. At shallow depths almost all the current enters the plate near one corner and is channeleddirectly to the opposite corner. In a larger plate current also enters and leaves the plate near the sources, but the current is more widely distributedwith depth within the plate so that the total anomaly is reduced. The anomaly of the largest plate (curve g) is in fact very close to the half-plane anomaly for depths less than h = L. The curvesfrom current dipolesof the samelengths, p, are plotted as dotted lines along with each of the four smallestplate profiles. The dipoles are located at depths correspondingto the depth of the top edgesof the plates which they represent. This location provides an opportunity to determine how accurately the plate anomalies may be modeled by the current dipole model. The shape of the curves from the plate models are reasonably well matched by their corresponding dipole representations. The amplitudes are less well matched becausethe method of calculating the current

answers at this limit.

For many purposes,a sufficientlyaccurate representation of the secondary magnetic field of the distrib-

/__./._./_./_.

utedcurrentdipoles {my}and{mz}maybeobtained by assumingeach distributed dipole is a point dipole at the center of the appropriate rectangle. We have already listed the fieldsof buried point dipoles,expressions (117) through (128). Numerical Results for a Square Plate

Cheesman and Edwards (1989) present numerical resultsfor a squareplate model located as illustrated in Figure 43. They deduced from a study of a "small target" (see The Small Target section)that the important model parameters are the side length p, the depth to the top of plate h, the current electrode separation

Fig. 42. Bottlebox model to calculate effect of conductive plate.


MMR

in the dipole model only approximates the amount of current which flows in a square plate. The behavior of the anomaly halfwidth w for the same range of plate sizes is illustrated in Figure 45 as a function of depth. The curves for the limiting cases, the small current dipole and the half-plane, are also included. Larger half-widths are found in larger plates because of current distribution within the plates. The curves for the largest plates, f and g, suggestgreater half-widths than for the half-plane model. However, the half-plane model, because of its infinite size, should yield the maximum anomaly width possible. The too-large values for platesf and g may be attributed to discretization errors in the forward modeling procedure. While it was not practical to increase the number of plate sections in the forward model to improve accuracy, decreasingthe number of sections was found to increase

the value of w obtained

for both

f and g. The halfwidths of the dipole models are not

surfece

o

o

Fig. 43. The square plate is situated at a depth h beneath and between the current sources.All profiles of the magnetic field are taken on the surfacetransverselyacrossthe top of the plate.

Depth (h/L)

Fig. 44. The maximumanomalyamplitudeBma x is plotted for a range of squareplate sizes. The dotted curves labelled with prime letters are the correspondingcurves for current dipoles of the same length p.

81

plotted, but in general they underestimate w because their current is concentratedat the depth corresponding to the top of the plates. A dipole positioned at a depth nearer the center of the plate sometimes provides a closer match of anomaly halfwidths. A general feature of electromagnetic methods is their ability to determine the dip angle of plate-like conductors. The vortex currents within the plate produce a skewed magnetic field profile which may be interpreted to determine the dip, independent of the size and depth of the conductor. In contrast, McNeill et al. (1984) point out that anomalies over confined conductors generated mainly by current channeling are relatively insensitive to dip. Yet Edwards (1983) demonstrates that MMR anomalies over half-planes are very sensitiveto the dip of the half-plane. Clearly, sensitivity to dip varies with the size of the conductor and its depth of burial. Cheesman and Edwards (1989) establishedcriteria for the interpretation of dip as an independent parameter. The current dipole produces anomalies which are symmetrical, and do not indicate dip. In the far field it may be shown that all finite conductors may be represented for the purposes of MMR by an equivalent current dipole of appropriate strength and orientation, with the result that effects of dip become indistinct. A very conductive dipping half-plane, however, produces asymmetric profiles strongly indicating dip. Moreover, the degree of skewnessis independent of the depth to the dike. This behavior is illustrated in Figure 46, where the responsesof half-planes dipping at intervals of 30 degrees were determined at a depth h = .5L. The form of the anomaly changes markedly with increasingdip. At a dip of 0 degreesthe anomaly is antisymmetric. The profile becomesless skewed as the dip increases; the vertical plate produces a per-

10-• 10 -•

........

half-plane

,

, , , , ,,,I 10 ø

,

, ......

10 •

Depth (z/L) Fig. 45. The anomaly half-widths for square plates. The dipole and half-plane curves represent limiting values for w.


82


fectly symmetric profile. The anomalies produced by real plates might be expected to possesscharacteristics intermediate to dipole and half-plane anomalies. The MMR anomaliesproducedby plates of sizep = L dipping at angles of 0, 15, 30, 45, 60, and 90 degrees are plotted in Figure 47. The first set, from plates located near the surface (h = .2L), shows the most

changein the character of the anomaly with changesin the dip angle. When the plate is vertical (curve f) the anomaly is symmetric and the maximum situated above the center of the plate. As the dip decreases toward 0ø the lower portion of the plate is displaced

4O

h -.5L

3O

--

from the profile center, and, to a lesserextent, so is the maximum of the profile. Concurrently, the profile becomes more skewed. The anomaly of a shallowly dipping plate is more complex. The peak amplitude is larger becausethe lower portion of the plate is closer to the profile line. The position of both ends of the horizontal plate is reflected in the anomaly by a sharp changein the slope. This effect is only clearly present for a near-horizontal plate at a shallow depth; there is little or no signof it in the profile for the plate dipping at 15 degrees. The profiles computed at h = .5L demonstrate a large decrease in the degree of skewness. Profiles from platesat greaterdepths(not plotted here) appearto the naked eye to be more or less symmetrical, and are indistinguishablefrom those produced by dipole-type sources.

•

20

lO

Curve Dip(ø)

-lO

a b c d

-20 -30

-40

'

-2

• -1

0 30 60 90 I

'

0

,

2

1

x/L

mines the amount

Fig. 46. The MMR response of perfectly conducting halfplanes at a depth h - 0.5L for various dip angles. 4O

h-•2L

Curve Dip a

-

3O

•,

We have already seen that there are two factors which determine the shape and amplitude of MMR anomalies. Geometrical factors include the size, shape,and depth of the conductorand the positionand type of current sources. These factors determine the amount of current which flows in the vicinity of a conductor and which is available for channeling. The conductivity contrast between the conductor and the host medium, or more properly the current channeling number • (see The Small Target section), then deter-

a

0ø

b

15 ø 30 ø 45 ø 600 90 ø

2o _

of the available

current

which will

actually be channeled through the conductor. To model dependence of anomaly size on •, a square vertical plate of side length p was placed in a uniform electric field oriented down strike. The peak anomaly was measured at the origin over a wide range of channelingnumbers. (The channelingnumber itself is determined by experiment to be 1.35pS/p) The results are plotted in Figure 48 as a fraction of the anomaly measured for a perfect conductor. The relative effect a/(1 + a) is plotted on the same graph. The correspondence between the two curves is remarkable.

i

-10

i

h:.SL

I

!

Curve Dip a b

a

-

•

! 30ø 45 ø 60 ø

f•• de -5

-1.0

0ø 15 ø

0

90ø 1.0

2.0

x/L

Fig. 47. The MMR response of perfectly conductingplates at depths h - 0.2L and h = 0.5L for various dip angles.

The dependence of anomaly character on plate conductance is more difficult to predict. At low conductancesthe current flow in a plate approachesthat expected in a homogenousmedium; surface current flows in the direction of the tangential electric field E impressedby the sourcesand is of strength SE. For high conductancesthe current flow within the plate is much greater and distorts the local electric field to a large degree. In such cases current flow in some portions of the plate may even be perpendicularto the source electric field, and might lead us to expect a large difference in anomaly shape from the poorly conducting case. In fact, when the anomalies of very poorly and very highly conductive plates are scaled and overlain for comparison,the difference in shapeis


MMR

so small that the two curves nearly plot as one; the reasonbeing that even thoughthe direction of current flow in a small portion of the plate may vary with conductivity, the general trend of the current remains relatively consistent•in the direction of the source fields.

The relative invariability of anomaly shape with plate conductance,combinedwith the ability to determine a channelingnumber and relative effect, provides a short-cut for calculating anomalies. The anomaly to be expectedfrom a plate of finite conductivitycan be closelyapproximatedby multiplyingthe responsesof perfectly conductiveplatesby the appropriaterelative

83

difference resistivity problem is described in Dey and Morrison (1976). If the conductivity is independentof the strike coordinatey, it is a simplificationto define a Fourier transform pair relating the real symmetric potentialfunction U(y) and its correspondingFourier transform U(q) as

U(q) =f•U(y) cos (qy) dy, (156) and

U(y) =(2/,r) f•

U(q) cos (qy) dq.

effect.

A correspondingsine transform may be defined for

Modification of 'Resistivity' Software

We have derived (Sources of the Magnetic Field section) a useful expression for computing the magnetic fields of a static current

(157)

flow in a medium

of

variable conductivity when the gradient of the electric potential is known a priori analytically, or may be computedwith establishedsoftware.The expressionis

antisymmetric functions. The finite-difference

form of the Fourier

transform

of the differential equation (155) is a system of linear coupledalgebraicequationsfor U(x, q, z) which may be solved using standard numerical matrix methods. The magnetic field in Fourier domain may be computedfrom equation(154). The componentsof the field are derived in Pai and Edwards (1983) as

B(r) =•-••fv V'U(r') xV'cr(r') dv'. (154) Bx(x,q, z)= •

Off

dx'

dz tl OZ•

In a medium where the conductivity •r varies con-

tinuouslywith position,the differentialequationsatisfied by the electric potential U is

•7cr•7U nt-ffV2U = VJs,

x U(x', q, z')2Ko(qs)(-q), (158)

(155)

whereJs is thesource current.TheSolution maybe

By(x,q, z)= •

found using standardfinite-elementor finite-difference techniques. The very useful two-dimensionalfinite-

dx' f dz' Ocr OU

Ocr O U] _

OX' OZ'

2Ko(qs), (159)

and

1.0

oBma x/Bma x(o=a3)•

Bz(x,q, z)=•---d•dx i

f dz'Ox' Ocr

x U(x', q, z')2Ko(qs)(q), (160)

0.5

wheres2 = (x - x')2 + (z - z')2. The x and z

0

10 -•'

10 -•

10 ø

10 •

10 z

alpha

Fig. 48. Bmax calculatedfor a squareplate is plotted as a fraction of the maximum anomaly of a perfectly conductive squareplate for various values of a, and is comparedto the relative effect a/(1 + a).

componentsare symmetric functions which are inverted to the y-domain usingthe cosineinverse transform given in equation (157). The y component is antisymmetricand its inversion is through the correspondingsine transform. The term V'•r(r') in expression(154) in the case of simple discrete models, vanisheseverywhere except at the surface of the earth and on the boundaries of

media of differing conductivitieswhere it is replaced by delta functions, reducing the volume integral to a


84


finite number of weighted surface integrals of the derivatives of the potential. In the two-dimensional problem, the double surface integrals in expressions (158), (159), and (160) reduce to contour integrals over boundariesdefiningcylindrical structures(see GomezTrevino and Edwards, 1979) and the whole processof computing MMR anomalies becomes very rapid indeed. CASE

its

effect

in the

The youngerformationsto the north of the Fault, on the down-thrown side, were expected to be more conductive than the dolomite to the south.

and Precambrian

measurement

area.

The

survey lines are run along profilesperpendicularto the line between the current electrodes (Figure 53). The

horizontal magneticfield (Hc), which is least affected by the mostly vertical magneticfield (Ha) of the cables joining the electrodes, is usually measured (Figure 1, Jakosky, 1940). After it became apparent that lowering either the current electrodes or the magnetometer (or both) inside drill holes can lead to significant improvements in data quality and interpretation (Nabighian and Oppliger, 1980) successfulMMR field surveys have been carried out, predominantly with the current bipole buried in a vertical drillhole ("pure-anomaly" MMR). This crosshole MMR technique was soon afterward adapted to offshore applications, culminating in the development of the MOSES technique (Edwards, et

The field apparatus consisted of a McPhar P654 constant-current induced polarization transmitter, a P654 receiver, which is essentially a sophisticated, Scintrex MFM-3 high-sensitivity flux gate magnetometer.

Two current electrodes were located 183 rn apart on a baselinewhich was establishedalong the strike of the fault, as shown in Figure 51. They were equispaced about the line labeled # 11E, along which all measurements were taken. The distance along the baseline between # 11E and the minor road is 336 m. The path taken by the cables, used to connect the electrodesto the transmitter, is indicated by the dashed line. The transmitted frequency was selected as 5 Hz, and a constantcurrent of approximately I A was driven into the ground. The horizontal component of the magnetic field normal to the baselinewas measured along # 11E. The amplitude of the current flowing into the ground was determined by measuringthe vertical component of the magnetic field at a fixed distance from a distant, straight section of one of the cables. This method of determin':ngthe current automaticallycompensatesfor any error in the calibration either of the magnetometer or of the receiver. The experiment, as outlined above, was repeatedfor two other electrode separations,305 and 549 m, respectively. The horizontal components measured along line # 11E are separatedinto normal and anomalousparts. The anomalousparts, recorded for the three electrode separationsof 183,305, and 549 m, respectively, are

plottedin Figure 5 l a. Their amplitudesare normalized ?,•o:•?w2'w

al., 1981).

B"

?.•ø'•,'w

4,.5ø20I/2' N

The various case histories presented here are grouped along the lines just outlined. MMR Surveys with Surface Current Electrodes Leitrim, Ontario (Edwards, 1974).•The test of the MMR

method was conducted

rocks

tuned ac voltmeter with a time constant of 20 s, and a

HISTORIES

Since the first successfulfield experiment with the MMR method reported in Edwards (1974) there have been a number of published case histories illustrating the applicability of the technique in various geologic environments. The earlier results were predominantly directed toward surfaceexploration, with both current electrodes and recording magnetometers located on the surface of the earth. Out of many possible MMR configurations,the one most commonly used in practice employs a longitudinalcurrent array oriented such that the primary current parallels the major axis or strike direction of the suspected target. The cable joining the current electrodes is laid in a U-shape to minimize

section BB' as shown in Figure 50, is from Wilson (1946).

first field

over a section

of the Gloucester Fault in the immediate vicinity of Leitrim, Ontario. This area has often been used, both by the Geological Survey of Canada and by private industry, for the testing of new geophysical methods and equipment. A plan of the area is given in Figure 49. The surface geology marked on the plan, and in

45ø19'N

B

I

3280 ft

i

IOOO m

Fig. 49. A map of the Leitrim area.

MMR

with respect to the normal field at the center of the array, and are plotted as a function of the electrode separation, to facilitate comparisonwith the theoreti-


cal curves.

The error

bars marked

on the curves

are

the scatter of repeated observations.

85

A set of MMR type curves for the theoretical model of two adjoiningquarter spacesis shownin Figure 5 lb. They are plotted for a range of values of the reflection coefficientK, which is (•r• - •r2)/(•r• + •r2), where •r• and •r2 are the conductivities of the two quarter -

200

rn

I -ioo '+++.

SEA LEVEL

PRECAMBRIAN dol, ls,Gn, Oz

I•

:.•.!

2 JJ]]]•SS NEPEAN FORMATION 3 ...... ":• SS, MARCH dol FORMATION

- -200

4• OXFORD FORMATION Is•dol 5 '"'/'"":"'"':'"• ROCKCLIFFE FORMATION Sh 6 J Sh,SS, ST. MARTIN Is,dol FORMATION 7• 8 :•

OTTAWA FORMATION Is Is EASTVIEW FORMATION

9• I0E•

BILLINGS FORMATION CARLSBAD FORMATION Sh• Sandy Sh Sh- Shale

Is - Limestone

•--300 3

-400

- -500

SS- Sandstone

dol- Dolomite

Gn-Gneiss

Qz- Quartzite

- -600

Fig. 50. The geological section BB' shown in Figure 49 (from Wilson, 1946). 40%

i-- 30% 40%

--

30%

--

Ll.I

r;-, 20%

20%--

--I <•

•X•x

•;

10% --

0 Z

K=-I K

IO%

K=-.67

K=-..•

• Z

K=-•33

O--

r•

DISTANCE

-10% --

FROM CONTACT

-IO%

03-20%-- x 0 ._1

'"'•-"=. ERRORBAR*_.5% -

<[-30%-x•__ 5 0 Z

L=305m

49m

-2o%

-40% -- X••x/ -5o%-

i - L

i

i

i

I

-.SL

-.6L

-.4L

-.2L

S

I

BS L

i

i

I

,4L

.6L

.SL

i .2L

POSITION ON LINE NO lIE (RELATIVE TO ELECTRODE SEPARATION)

I L

EXPERIMENTAL

N

-4o%

POINTS

L=183m

Fig. 51. (a) The anomaloushorizontal magnetic componentsrecorded at Leitrim along line #11E for electrode separationsof 183, 305, and 549 m, respectively. (b) Profile #11E plotted for a range of values of the reflection coefficient.


86


spaces. The effective conductivitiesare represented by •r• and •r2, to the south, and to the north of the fault, respectively. The experimentalcurve for the electrodeseparation of 183 rn comparesfavorably with the model curves when replottedin Figure 51b. An averageconductivity

A constant current of 2.5 A at a frequency of 3 Hz was generatedby a Heinrichs-Geoex Mk 4b inducedpolarizationtransmitter. Two number 14 gaugecopper

contrast across the fault of 9:1 (K = -0.8) may be deduced for material to a depth of the order of the electrode separation. This test showed conclusively

and the two current electrodes

that MMR

anomalies

can be both measured

and inter-

preted in real field conditions. Superior Area, Arizona (Edwards and Howell, 1976).•The field test was a joint project of the University of Toronto and Newmont Exploration Limited. The test area is the top of a plateau believed to contain a steep, faulted contact, between basement rocks of moderate conductivity contrast, which lies beneath approximately 500 rn of Tertiary volcanics and sediments. A simplifiedgeologicsectionis shownin Figure 52. The contact is exposedon one flank of the plateau, and its position under the test area may be inferred approximately from measurementsat the outcrop. The test area has been previously utilized by Newmont Exploration Limited for various experimental pur-

wires

the

L

between

current

electrodes

to the

electrodes.

was about 120 ohm. In

B coincides with the x axis, and lines A and C are at

A plan view of the experimental configuration is shown in Figure 53. The current electrodes, C1 and C2, were located along and immediately above the expected strike of the structure, directingcurrent flow parallel to the strike of the fault. distance

transmitter

any MMR survey the wire loop has to be located carefully. If the loop is too close to a measurementsite and if that site is above or below the plane of the loop, then the current in the loop will produce a horizontal magneticfield at the site and contaminate the data. In this region of rough topography, the loop was placed as far as possiblefrom a measurement site. The magnetic field was measured using a Scintrex MFM3 high-sensitivity fluxgate magnetometer. The magnetometerwas connecteddirectly into the input of a Heinrichs-Geoex Mk 4c induced-polarization receiver which measured the voltage and, hence, indirectly the magnetic field. Measurementsof the x component of the magnetic field Bx were madeat intervalsof at most .04L (122m) along the three lines A, B, and C shown in Figure 53. All three lines are at right angles to the baseline. Line y = .2L and y - -.2L,

poses.

The

connected

These wires followed the three sides of the square outlined in Figure 53. The total resistanceof the wire

respectively.

x

was

chosen 3048 rn (10 000 feet). A baseline, the y axis is defined as the line joining C1 and C2. The principal profile of the array is taken to be the x axis.

Baseline

C2 -y

-'1 :;;:.;4.:.:.:•;..."2>':.. ;•O.-:';'•!i •i :'•'.',o'--' •0 •h•':'.;.;-'./; :.'½,.' ..:'2' '.!':2:•.•:•"•.•:4 ':1 .!:.•.::!,;-'-./-;.,.-.'..':•:'..'..:-! .;:'.:': ..:-..'.¾:..'-.".: i.:. i-:. ¾-.':':.-'.--'-':!.: :'. :.:.:-.-:'•:'.:::•;-:-":-':"--

/-'-.!: ::-'...'.:-! :.:,; .... ..TERTiaRY CONGLOMERATE'..:-.". :.: ? ..- ',':. •;".". b'.v ø.I ß .-'..-. o:....•..:::: 2:::,' ..... -' ß',,...... ,,-.• '.:.'o-..z,v :•,..'::':•-.•'.':'• ..-..

05 '.'•; :.::o'-'.o •;'•-.;.--•.!,•.".".P-7511m'-' '-'•.'-;.':;";'"; -':•'. '"'•'.:-' ß'"'1 ßJ•.:.:.:'.e.-'.'.'•..:'.'-. :.::" ":'.*.-.' .':.".. *.'-.¾ :.5ø.-. ,,...:.'•:'.:.:". :.:.:o:.' ':',• .:.o.....: .....•o:

SEDIMENTS

P=50,2m

Coble

J

o I

Fig. 52. The geologic section along the x axis of Figure 53. The resistivities shown are estimates based on measure-

ments of the physical properties of drill cores.

.5

I

I

I

SCALE (Km)

Fig. 53. A plan view of the experimental configuration.The ticks on lines A, B, and C are 0.2L apart where L = 3048 m.


MMR

The anomalies measuredalong lines A, B, and C are plotted as single profiles in Figure 54a. The errors in the observationsare about 1 percent determined from the scatter of a few repeated observations. The short wavelength noise on the curves is not random but is strongly correlated with the topography.The noise is due in part to a horizontal field causedby current flow in the cables joining the electrodes to the transmitter and in part to the increased distancefrom the current flow in the ground. Both these effects have similar orders of magnitude and they have the same sign. The data were interpreted using three models; a contact, an outcropping thick dike of infinite vertical extent, and a single exponential "alpha" center. The model

of the contact

was chosen

because

it is the

simplest model that could possibly describe the geologic section of Figure 52. The contact proves to be a useful startingpoint enablingthe strike of the expected structure to be located, mainly by studying the horizontal gradient of the MMR anomaly (Figure 54b). The presence of a second contact is deduced. In addition, we can immediately infer, based on anomaly wavelength considerations, that the depth to the contact gets shallower as we go from line A to line C. The contact model leads naturally into the model of the thick dike. The two contacts delineated

earlier can

87

Spargoville Deposit, Western Australia (HowlandRose et al., 1980b).•Although this survey was originally a magnetic induced polarization (MIP) survey, the MMR anomaly was an integral part of the measurements and as such, can serve equally well as an MMR case history. The Spargoville deposit located in the Kambalda area, Western Australia, is a small subeconomic nickel sulfidebody about 245 rn (800 ft) in length. Its depth of burial (oxidation) is approximately 30 rn (100 ft), and the deposit is situated on the contact between ultrabasics and amphibolites (Figure 55). A current electrode separationof 366 rn (1200 ft) was employed, with the current electrodesat 82E on lines 294S and 306S. The central profile 300S was traversed and the MMR and MIP frequency-domainresultsfor a 1-Hz square waveform, together with the geologic section, are presented in Figure 55. As determined usinga 3000 ft. gradient array, the dc resistivity of the ultrabasics is about 20 tl.m. The polarizablenickel sulfidesare located on the boundary of the resistive amphibolites and the conductive ultrabasics which contain disseminated sulfides and mag-

netite(Pamphibolites > 10Pultrabasics)' Themeasured magnetic field was normalized by dividing the measured field with the magnetic field calculated at each measurement point, i.e.,

be simultaneouslyincluded in this model and a surprisingly good fit to the data was obtained (Figure 54c). Estimates of the resistivity contrasts across the contacts were

also determined.

The model

is limited

B

B/v BnX 100percent,

in

that it neglects the presence of the Tertiary overburden and also the limited depth extent of the contacts. The overburden produces no MMR anomaly, but does elevate the plane of measurementto 500 rn above the outcropping dike in the basement. This elevation must have the effect of reducing the contributions to the observed anomaly of the larger wavenumbers, of

the orderof .002m-•. The anomalyis thusbroader than the theoretical anomaly of an outcropping, thick dike.

One feature of the data is poorly explained by the dike model; the increase in amplitude of the MMR anomaly from line A through line B to line C cannot be representedin a model of linear symmetry. This field test was conducted in a region of severe topography where the overburden is quite thick, and where no other geophysicalmethod has given equivalent data. The interpretation was consistent with the geology, and the object of the survey, delineation of a deep contact, was achieved with the minimum of data processing.Additional modelingof the data revealed a second contact and a local region of high conductivity was mapped.

where B n is the magneticfield calculatedfor a uniform or a horizontally stratified earth, and B is the observed magneticfield. As mentionedpreviously, suchnormalization leads to distortion of the anomaly shape. Nevertheless, the main anomaly features are preserved.

The normalizedamplitudeB/v showsvalues greater than 100 percent over the ultrabasicsand less than 100 percent over the less conductiveamphibolites,which is indicative of vertical contrast. The B/v peak at 83E is located directly over the Spargoville deposit. Widgiemooltha Area•Australia (Howland-Rose et al., 1980b).•This MIP case history also is being used to illustrate the application of the MMR method in massive sulphide exploration. In the Widgiemooltha area of Western Australia, a zone of massive, nickeliferous sulfide mineralization was discovered by drilling under a gossanousoutcrop. The mineralization

lies in a fold on the contact

be-

tween Precambrianserpentinizedultrabasic rocks and amphibolites.The weatheringextendsto a depth of at least 45 rn (150 ft) and the resistivity of the weathered zone varies from 3 to 10 f•.m.

88


/

LINE


+2O

+

+•om-

+++++•+++• +++++-N+++

___+•+•++•

0 •+•.•

x/l

!

I

-0.2

.++

LINEA

o.'•

o:• -•.•--

++++•++• +

/

-h• ,,' • /

'.,

x/l

/ /

+++++++

.

+++ +

++++

lINE b

---'

+ %+

/ ++•+•

+•

+•+•+ LINEB +

\

++

+

++

\

++

\

++

+

+++++

+ -0.2

x/l

•)

+60

0:2 ++

++

LINE C

+

+40

+40 +

+

LINE C

+

+

+

+20

+20

++

++

+

b

o'.•

x/l

o'.4

+ +

-2O

-2O

+

(a)

(c) LINE A

' -.:• 0 • o Z

, /

•

x/L

.6

LINE B

>x/L 0 Z

•x/L

(b)

Fig. 54. (a) The experimental MMR anomalies in the horizontal field measured along the lines A, B, and C shown as Figure 53. (b) The horizontal gradient of the MMR anomalies measured along lines A, B, and C. The values on each line have been normalized independentlyto a maximum value of unity. (c) The measured MMR anomalies on lines A, B, and C compared with the theoretical anomalies due to the best fitting dike model. The boundary between region 1 and region 2 is at -0.16L, that between region 2 and 3 at 0.45L.

MMR


180

-

o-----.o O

O

x:

•x ABsP -AMPLITUDE

89

BN-NORMALIZED AMPLITUDE Bso-OUADRATURE COMPONENT

--10.0

-

10

-

7.5

CHANGE

160

-

140

-

- -5.0

120

I

- -2..5

-2.5_

ø

o

lOO

80-

-

2.5

60-

-

5.0

40-

-

7.5

I

78

I

E

80

I

E

82

I

E

84

I

E

86

I

E

88

--5

I

E

90

I

E

200r,. I

Surface

2IXI

Ft.

Fig. 55. Spargoville deposit, Western Australia. MMR and frequency domain MIP results (Howland-Rose et al., 1980b).


90


The MIP method was used to reduce the masking effect of the conducting surface weathered zone. A 365 rn (1200 ft.) longitudinal current electrode array was set out, with one current electrode placed 245 rn (800 ft.) north and the other 120 rn (400 ft.) south of station 2E. For MIP, a 2-s current on/off time was used.

Figure 56 shows the MMR and MIP results as well as the geologicsectionderived from the drilling on line 4N.

The

ore

zone

on this

section

breaks

into

two

easterly dipping lenseswhich would suboutcropat the base of the weathered zone, at about 1E and 2E. These

lenses grade between 1 to 3 percent nickel, with combined massive sulfidewidths of up to 15 rn (40 ft). The BN profile stronglyindicatesthe compositesulfide zone as a 168 percent conduction anomaly, and suggestsits dual character, showingtwo peaks at 1E and 2.25E.

Cork Tree Well, Australia.--Recently (HowlandRose, 1984), a number of MMR surveys have been conducted by Scintrex in the West Australian gold fields north of Laverton. The area chosen, Cork Tree Well, has a cover of Cainozoic sediments and rafted

ing the plate nearer the surface in the north will increase the anomaly above the plate, but may also provide a better current path to the southern section when that region is surveyed, since one electrode for the southern survey lies within the northern section, close to and above the plate. The improved current path producesa correspondingincrease in the southern anomaly. Increasingthe plate conductancehasthe same effect. A second compilation is the dependence of the anomaly amplitude on the direction of anomalous current flow. For a current flowing obliquely to the electrode line, only a componentof the associated magneticfield is measured,causingan apparentreduction in the anomaly amplitude. Producingan "island" in the modeled anomaly over the point where current flows directly north-south, was unavoidable even though the current flow in the plate is strongestat the southern edge of the northern survey area. Regardless of the goodness of fit, the host rock appears to provide a significant contribution to the anomaly only in the north. In the southern half, the current appears to be channeled toward the east, rather than continuing to the south and west inside the mineralized

zone. A conductor

more diffuse in nature

Permian tillites to a depth of at least 50 m. Gold mineralization occurs within a unit of rock comprised of graphitic schists (about 50 percent), interbedded cherts (5-60 percent) and pyrite (5-15 percent). The

than a plate, or several plates parallel to each other, might provide a closer match to the observedanomaly

unit measures

MMR Surveys with Buried Current Electrodes

12 rn to 30 rn in width and is surrounded

by basic lava and tuffs. Individual reconnaissance surveys consistedof a number of receiver lines run at 80 rn intervals perpendicular to 1.2 km dipoles. The MMR anomalies for two adjacent surveys are shown in Figure 57. The anomaly, measured as a MMR percentageof the normal field measuredat the center of the array (indicatedby open circles), is contouredin steps of 20 percent. Superimposed is the projected outcrop of the host rock. Dark circles indicate the position of boreholes. The open circles indicate the center of the survey, the electrodes being positioned 600 rn above and below

the center.

An attempt was made to model the buried conductor with a vertical, bending plate. Its basic shape was determined from the axis of the MMR anomaly. The depth to the top of the plate, and the plate conductance, was adjusted to fit the anomaly amplitude as closely as possible.The "best fit" anomaly is shownin Figure 58. Matching the anomaly amplitude on both survey sections simultaneouslyproved to be difficult. The maximum anomaly amplitude in both the survey areas is nearly the same, but nearly all plate models produceda northernmaximumamplitudeonly 80 to 90 percent of the southern maximum. Attempts to increase the northern amplitude without affecting the southernamplitude were only partly successful.Rais-

character.

Copper Creek, Arizona (Oppliger, 1984; Nabighian, et al., 1984).--The country rock in the vicinity of the test prospect consistsof an altered, fractured granodiorite with associateddacite porphyry plugs. Sulfide mineralization

occurs in both disseminated

and mas-

sive forms, the latter occurring occasionally as fracture fillings a few meters thick in near-vertical eastwest strikingfractures. The principal targetsare these massive

sulfide bodies which

have an electrical

con-

ductivity many times that of the country rock. In this environment any interpretation is likely to be complicated by barren fracture zones and highly altered country rock which act as moderately conductive current

channels.

The first test was carried out with an MMR survey in which both current electrodes were located in a single near-vertical borehole while the magnetic field was measured over the surface of the earth. As mentioned,

this transmitter configurationyields no primary magnetic field at the earth's surface so the field observed is

purely anomalous. The objective of the survey was to locate extensionsof a highly conductivesulphidebody intersectedby the drill-hole at a depth of 170to 230 m. The survey area topographyand drill-hole location are shown in Figure 59. Mimicking the mise h la masse


MMR

180

-

140

-

MIP

91

....

Bs3-CHANNEL 3 TRANSIENT

.....

,BN -NORMALIZEDAMPLITUDE

--4

-

•-•t

100

I

•. ,•---•

-

$0

-

;'"•. DIPOLE-DIPOLE

ARRAY

I

I

I

I

6W

4W

2W

0

t

i

'

'

I 2O0fl. I

, RESISTIV/TY (n-M)

,I 21

4E

6E

N=I

'-•o a'.a N=3

ß

,•

G.o

•:.

N=4

N=2 '

.'.

N=3 N=4

CHARGEABILITY i

i

4E

N=I

-•,.4 y -•.' (0:4_, N--1

N=2

N=3

) o.

N=4

SURFACE

Fig. 56. Widgiemooltha area, Western Australia. MMR and time domain MIP and EIP results (Howland-Rose et al., 1980b).

92

Edwards and Nabighian technique, one energizing current electrode was placed at the bottom of the hole, 1000 rn below surface. The magnetic field measured on the surface of


the

earth

corrected

for

the

effect

of a nonvertical

bipole is shown in Figure 60. The final true anomaly map corrected in addition for the effects of topography is shown in Figure 61. The inferred conductor extenFig. 57. MMR anomaly measured in the Cork Tree Well area in Western Australia. The dashed region is the projected outcrop of the rock unit containing the gold mineralization. (¸) indicates the center of the array, (0) are the position of boreholes.

sion was determined

to be westward

from

the drill-

hole.

Subsequently, crossholeMMR st/rveys were carried out using existing drillholes in the vicinity of the prospect. The plan and section view of the three drillholes is shown in Figures 62a and b, respectively. Hole DH-6 intersects a 65 rn interval of 15 percent total sulfides between

current

electrodes

C1 and C2.

Hole DH-2 intersects a 60 rn interval of 50 percent total sulfides between

the 1300 and 1400 rn levels. Hole

DH-5 is barren. The objective of the MMR survey is to trace the possible extensions of and connections beFig. 58. The MMR anomaly of the "best fit" vertical plate

tween the intersections.

The prototype field instrumentation used to collect

model.

SECTION

TOPOGRAPHY

A

D-9

A'

su.^c[

T 22O

METERS

SULFIDE ZONE

+I

300 •

CURRENT

ELECTRODE

DRILL HOLE •__

1000

ME

ERS

CURRENT ELECTRODE

METERS

,

I

100 200 (a)

30 METER

CONTOURS

,

METERS

0

,

I

100 200

-I

(b) Fig. 59. The plan (a) and section(b) views of the MMR survey in which measurementsof the surfacemagneticfield caused by a downhole nearly vertical bipole transmitter were made (after Oppliger, 1984).


MMR

the data for the case history, developed by Newmont Exploration Limited, consists of an Elliot 4 kW IP type transmitter modified to output a constantcurrent 3 Hz bipolar square wave, a downhole MMR sensor composedof a ferrite-cored coil tuned to 3 Hz coupled to a voltage-to-frequencyconverter, a MMR receiver which is basically a phase-lockeddetector, and a radio link from the current

transmitter

to the MMR

receiver

93

field generatedby the inclined transmitter bipole. The third is the difference between the first two, the anomalous field, which has been resolved into the vertical

director for interpretation purposes. The fourth is of the responseof a multiple bent plate model. Offshore MMR Surveys

which provides a current polarity reference. Typical currents delivered of 2A.

to the electrodes

were of the order

The axial magnetic field component due to the bipole current pairs C3, C1 and C2, C1 was measured downhole in DH-2, DH-5, respectively. The current injected was approximately 2 A, the frequency 3 Hz. The detector was stationary for all readings. The data are presentedas profiles in Figures 63a, b, c, and d, respectively. On each figure there are four graphs. The first shows the measurementsactually made. The second, labeled cable effect, is the primary

Bute Inlet, Canada (Edwards eta!., 1985).--The MOSES experiment was carried out in Bute Inlet, British Columbia, Canada. The experimental configuration is outlined in Figure 64. The inlet shown in Figure 65 is more than 50 km long, average 3 km in width, and is a vee shapedvalley containingsea water about 640 m deep. The sea water overlies sediments which were estimated at 600 m in thickness by extrapolating the shape of the adjacent topography downward beneath the sea. A section AA'

across the Inlet

in the vicinity of the electrical soundingis inset at the lower right of Figure 65.

MEASURED

MAGNETIC

FIELDS

TERRAIN MAGNETIC

CORRECTED FIELDS

MMR

HORIZONTAL VECTORS

MMR

HORIZONTAL

IN

VECTORS

MILLIGAMMAS/AMP

IN

MILLIGAMMAS/AMP METERS 0

100

VECTOR

SCALE'

100 -m•' / AMP

200

I

O ß

•-

,

DRILL HOLE COLLAR DOWN-HOLE

CURRENT

•':z:..:•.•::•,1%:.. INTERPRETED

METERS

ELECTRODE

Fig. 60. The measured surface magnetic fields (after Oppliger, 1984).

100

CONDUCTIVE

ZONE

VECTOR SCALE-

I

200

100'mY/AMP ,

Fig. 61. The measured field corrected for the effect of topography (after Oppliger, 1984).


94


The ship was systematicallymoved away from the sea floor receiver to a seriesof predeterminedtransmitter locations. These locations are marked by the solid circles labeled 2 to 9 and 11 to 16 on Figure 66. The transmittedconstantcurrent had an amplitudeof 25 A and reversed every 4 s. A plot of the measureddata, the azimuthal magnetic field per ampere of transmitted current as a function of transmitter receiver separationwith associatederrors in measurementmagnitudeand position is shown in Figure 67. Individual points on the plot are identified by transmitter location. Interpretation of the data provided reasonable estimates of the sediment thickness and resistivity. The sediment resistivity of 1.9 l•.m corresponds with a porosity of about 42 percent which is in the range of that measuredon core samples. The thickness of the sediments, estimated at 560 rn is lessthan the upper estimateof 600 rn obtained by extrapolatingthe shapeof the adjacenttopography downward beneath the sea. The interpreted range of basementresistivity doesnot includethe resistivityof a typical crystalline rock.

CONCLUSIONS

The application of analytic and numerical MMR responsesis not restrictedto MMR alone. By changing the source type or the quantity to be measured, the forward modelingprogrammight be usedfor a number of different techniques such as MT or TEM at the lower frequencies.The low frequencyapproximations made are subtle and deserve a brief elaboration.

in the earth). A complete mathematical treatment of the electromagnetic (EM) problem (Wannamaker et al., 1984a) indicates that the source field interacts with an arbi-

trary inhomogeneityin a hostmediumin a complicated manner.The primary electric field in the vicinity of the

5'œCT/OfiAA'

A

-6

eA'

-1600

DH- 2

-1400

PLAN -1200

-I000

T/• ...... •Z:e'

CURREN

d

cl (-)

d'

ELECTRODES I I

DH-2COLLAR

- 800

C'-O,•,.._...• HOLE BO'!-I'OM O'

.

C3

b

b'

DH-6 - 600 ioo I

A

I

200 ,

The

effect of any conductive 3-D inhomogeneityon the electromagneticfields set up in the earth by an external source is to modify the otherwise normal current flow, producing anomalous magnetic and electric fields. (We designateas "normal" or "primary" the fieldswithin a uniformor a uniformlayeredhalf-space. The terms "anomalous" or "secondary" are used for the fieldsproducedby the conductiveinhomogeneities

o

ioo

i

I

HORIZONTAL

2oo

I

SCALE

(METERS)

METERS C2(+) 400

Fig. 62. The plan (a) and section(b) views of the crossholeMMR surveys(after Nabighianet al., 1984).

MMR

(a)

95

(b) Bz (milligamma/Amp) Bz (rnilligamrna/Amp)


- 600

-•oo

........

-4oo

-3oo

-2oo

-Ioo

' ....

•oo

i ' '

c•,c,

•____L.•<_.... •_._•_

/

••

- moo

'

'

O

'

I•

]•'•

'

IOO

'

DH-2

'

[

----•... l,,oo •

/1•,•o'

.oo

ANOMA

(d)

(c)

Bz (milligammalAmp)

Bz (milligamme1Amp) -25O i

-200 i

-150 -I00 i

i

-50

0

50 i

I00 i

150 2•00

-IOO

O

IOO

200

i

D/-/- 5

•oo / EFFEC•I •1 '%

Of, C2

1200• •

12oo

•o.oo•u

.-•,

•) iooo

ANOMALOUS

8001

E EFFECT

ANOI•AœOUS ••, /•-

6OO-

Fig. 63. The vertical magneticfields in (a) boreholeDH-2, transmitterbipole, C1, C3; (b) boreholeDH-2, transmitterbipoleC1, C2; (c) boreholeDH-5, transmitterbipoleC1, C3; (d) boreholeDH-5, transmitterbipoleC1, C2 (after Nabighian et al., 1984).

96



125'00' W

I

I

Ikm

50'50'N

MOSES

Fig. 64. The principle of the MOSES method. The current flow is axisymmetricabout the bipote source.The relatively small amount of current entering the resistive crust is inverselyproportionalto the ratio of the crustalresistivityto the searesistivity. By Ampere's circuitattheorem,only this current contributesto the azimuthal magneticfield measured at a point OBM on the sea floor (after Edwards et at., 1984). as'

a4'

•'

Fig. 66. A plan view of the MOSES experimentshowingthe relative positions of the sea floor receiver (OBM) and the coded transmitting points (2-9; 11-16) (after Edwards et at., 1985).

125'00' i

i

i

i

i

i i I [

i

i

I

i

i i i

gl•lTISfl

2

3

.•-

5

125'

124'

IS3'

k

• IO -2•_

50ø$0 ,

la.I

-

½d')

_

0

-

½d')

-

,,4' 16s _ _

_

_

_

_

_

SECTION AA' _

_

SEA

m

600m

xx

10-4

I

I

I I I I II

I

I

I

I

I

I

II

iooo

SEPARATION (m)

Fig. 65. The location of the MOSES experiment, Bute Inlet, British Columbia (after Edwards et at., 1985).

Fig. 67. The measuredtotal magnetic field on the sea floor at a frequency of 0.125 Hz plotted as a function of OBMtransmitting point separation (after Edwards et al., 1985).


MMR

plate has a nonvanishing component parallel to the plate and a nonvanishing component of its curl or rotation perpendicular to the plate. The plate reacts to minimize in some average sense the values of both these components. The physics of the interaction of the primary fields with a plate-like inhomogeneity and the subsequentgeneration of secondary fields by the impressed anomalous currents can be simplified. To first order, the minimization of the curl is achieved through the induction of a rotational back EMF in the plate, while the reduction of the electric field parallel to the plate is achieved by setting up a free charge distribution near the ends of the plate. The associated perturbation vortex and channeled current flows we

shallcalljvandjg,respectively. Thesenseofjg issuch as to increase the total current flow through the plate.

The dependences of the currentsJv andjg on the electrical and geometrical parameters are given conceptually by

jco•xoSL

'1+ jco•xoSL and

s/crhL

Jg= Jg 1+ S/crhL '

where exp (jcot)definesthe time dependence,crh is the local host conductivity, and S and L are the conductance and characteristic linear dimension of the plate, respectively. The dimensionlessnumberscOlxoSL and S/crhLare termed inductionand channelingnumbers, respectively (see discussions in Berdichevskiy and Dmitriev, 1976; McNeill et al., 1984; West and Edwards, 1985). Our first low frequency approximation is to assume that the induction number of the plate, which physically represents the ratio of a self inductance to a resistance, is small compared with unity. The inductive interactions between current elements in the plate can then be removed from the theory, greatly simplifying the computations. The approximation neglects terms like the jco•xoSLterm in the denominatorof the expressionfor iv. At high frequencies,the approximation breaks down because the linear growth of the inductive response of the plate with frequency is not checked by back EMFs. The approximation is clearly not just a current channeling approximation, but at very low frequencies, the channelingresponseis likely to be larger than the inductive responsebecause of the latter's frequency dependence. LeMouel and Menvielle (1982) and Wannamaker et al. (1984b) have documented carefully the low frequency response of a 3-D structure. Secondary electric and magneticfields are observed to remain finite to

97

arbitrarily low frequencies, so they are attributed to current channelingby the inhomogeneity. Further, the frequency dependence of the secondary fields is just that of the primary electric field existing in the vicinity of the inhomogeneity. (This is actually the definition of current channeling given in Edwards et al., 1971). Since the channeling function displayed previously is itself independent of frequency, the interaction functions

which describe

the diffusion

of the second-

ary fields from the plate to the surface of the earth must also be frequency independent. The depth to the inhomogeneityfrom the surface is consequentlysmall compared with an electromagnetic skin depth in the layered medium. This is our second low frequency assumption.For a uniform half-space, the equivalent

assumption is that the response numbertOl•oo'h R2 shall be small compared with unity, where R is a typical distance from the observation point to the inhomogeneity. The two approximations we make are certainly not original. They are equivalent to those suggested in Berdichevskiy and Dmitriev (1976). The electromagnetic wavelength in the host medium should be long compared with the depth to the anomalous body (exterior condition), and the wavelength within the inhomogeneity should be long compared to the size of the inhomogeneity (interior condition). The exterior condition expressesa negligible mutual induction between body and host, while the interior condition expressesa negligible self-induction inside the inhomogeneity. Those wishing to apply the described numerical modeling for magnetotelluric interpretations are referred to Flores and Edwards (1990), who have developed an approximate algorithm to compute rapidly the 3-D MT response of a multiple plate-like structure embedded in a multilayered earth at low frequencies. REFERENCES

Acosta, J. E., and Worthington, M. H., 1983, A borehole magnetometric resistivity experiment: Geophys. Prosp., Baule, G. M., and McFee, R., 1965, Biomagnetism, J. Appl. Phys. 36, 2066. 31, 800-809. Berdichevskiy, M. N., and Dmitriev, V. I., 1976, Basic principles of interpretation of magnetotelluric curves, in Adam, A., Ed., Geoelectric and geothermal studies (EastCentral Europe, Soviet Asia): KAPG Geophys. Mono., 165-221.

Chave, A.D.,

Constable, S. C., and Edwards, R. N., 1990,

Electrical exploration methodsfor the sea floor, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics,Vol. II, Soc. Expl. Geophys., this volume. Cheesman, S. J., and Edwards, R. N., 1989, Current chan-

neling in square plates with application to the magnetometric resistivity method: Geophys. Prosp. 37, 553-581. Cuifin, B. N., and Cohen, D., 1977, Inst. Electr. Electron. Eng., Trans. Biomed. Eng., BME-24, 372. Dey, A., and Morrison, H. F., 1976, Resistivity modelingfor

arbitrarily shapedtwo dimensionalstructures,parts I and

98


II: Lawrence Berkeley Laboratory, Reports 4223 and 5284, NTIS. Edwards, R. N., 1974, The magnetometric resistivity


method and its applicationto the mappingof a fault: Can. J. Earth Sci., 11, 1136-1156. --, 1975. The magnetometric resistivity method: Presented at AIME Ann. Mtg, Reprint no. 75-L-82. --, 1983, An approximate model of the magnetometric

resistivity (MMR) and magnetic induced polarization (MIP) responsesof dipping dikes beneath a conductive overburden: Bull. Australian Soc. Expl. Geophys. 14, 30-35.

•,

1988, A downhole magnetometric resistivity technique for electrical soundingbeneath a conductivesurfacelayer: Geophysics, 53, 528-536. Edwards, R. N., and Howell, E. C., 1976, A field test of the magnetometric resistivity (MMR) method: Geophysics, 41, 1170-1183.

Edwards, R. N., Law, L. K., and DeLaurier, J. M., 1981, On measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method: J. Geophys. Res., 86, 11609-11615. Edwards, R. N., Law, L. K., and White, A., 1971,Geomag-

netic variationsin the British Isles and their relationshipto electrical

currents

in the ocean and shallow

seas: Phil.

Trans. Roy. Soc. (London), A270, 289-323.

Edwards, R. N., Law, L. K., Wolfgram, P. A., Nobes, D.C., Bone, M. N., Trigg, D. F., and DeLaurier, J. M., 1985, First results of the MOSES experiment: Sea sediment conductivity and thickness determination, Bute Inlet, British Columbia, by magnetometricoffshoreelectrical sounding:Geophysics, 50, 153-161. Edwards, R. N., Lee, H., and Nabighian, M. N., 1978, On the theory of magnetometricresistivity (MMR) methods: Geophysics, 43, 1176-1203. Edwards, R. N., Nobes, D.C., and Gomez-Trevino, E., 1984, Offshore electrical exploration of sedimentary basins: The effects of anisotropyin horizontally isotropic, layered media: Geophysics,49, 566-576. Edwards, R. N., Wolfgram, P. A., and Judge, A. S., 1988, The ICE-MOSES experiment: Mapping permafrost zones electricallybeneaththe BeaufortSea: Marine Geophysical Researches, 9, 265-290.

Egorov, M. N., 1967, Surveying in the vicinity of drillholes by measuringthe magnetic field of a vertical cable with low frequency current (in Russian), Vestnik LGU, Ser. Geol. i Geol., No. 12, 2. Flores, C., and Edwards, R. N., 1992, Approximate calculation of low-frequencythree-dimensional magnetotelluric responsesusing a multiple plate model: Geophysics,submitted.

Gomez-Trevino, E., and Edwards, R. N., 1979, Magnetometric resistivity (MMR) anomalies of two-dimensional structures: Geophysics, 41,947-958. Grynszpan,F., and Geselowitz,D. B., 1973,Biophys.J., 13, 911.

Hohmann,G. W., 1987,Numericalmodelingfor electromagnetic methodsof Geophysicsin Nabighian, M. N., Ed., Electromagneticmethodsin applied geophysics,Vol. 1, Soc. Expl. Geophys., 331-364. Howland-Rose, A. W.,

1984, The use of RRMIP as a

regional mapping tool with examples from the Eastern Gold Fieldsof WesternAustraliain GeophysicalExploration for PrecambrianGold Deposits, Doyle, H. A., Ed., Vol. 10, 139-164. Univ. of WA GeologyDept. & Extension Service.

Howland-Rose, A. W., Linford, G., Pitcher, D. H., and Seigel, H. O., 1980a, Some recent magnetic inducedpolarizationdevelopments--PartI: Theory, Geophysics, 45, 34-54.

•,

1980b, Some recent magneticinduced-polarizationdevelopments•Part II: Survey Results, Geophysics,45,

55-74.

Jakosky, J. J., 1933, Method and apparatusfor determining underground structure: U.S. patent, no. 1,906,271. •, 1940, Exploration Geophysics, Los Angeles TimesMirror

Press.

Lee, H., 1975, On the computationalaspectsof the magnetometric resistivity method and its application to the mappingof a sink: M.S. thesis, University of Toronto. LeMouel, J. L., and Menvielle, M., 1982. Geomagnetic variation

anomalies

and deflection

of telluric

currents:

Geophys. J. Roy. Astro. Soc., 68, 575-587. Lo, B. B. H., and Edwards, R. N., 1986, Design and field test of a sensorfor the cross-holemagnetometricresistiv-

ity technique; in Borehole Geophysics for Mining and Geotechnicalapplications,Ed. P. G. Killeen, Geological Survey of Canada, Paper 85-27,289-296. Macnae, J. C., 1981,Geophysicalprospectingwith electrical fields from an inductive source: Ph.D. Thesis, Dept. of Physics, Univ. of Toronto; available as Res. in Appl. Geophys., 18, Geophys. Lab., Univ. of Toronto. Macnae, J. C., and Irvine, R. J., 1988, Inductive source

resistivity: a tool for outlining silicificationin gold exploration: Exploration Geophysics,19, 471-480. Maillet, R., 1947, The fundamental equations of electrical prospecting:Geophysics, 12, 529-556. McNeill, J. D., Edwards, R. N., and Levy, G. M., 1984, Approximatecalculationsof the transientelectromagnetic response from buried conductors in a conductive halfspace, Geophysics, 49, 918-924.

Nabighian, M. N., 1976, The MMR anomaly of a dipping interface

for current

electrodes

located

on the contact:

In-house Newmont report. Nabighian, M. N., and Oppliger, G. L., 1980, Recent advances in the magnetometric resistivity methods: Presented at the 42nd meeting of the European Assoc. of Expl. Geophys., Istanbul. Nabighian, M. N., Oppliger, G. L., Edwards, R. N., Lo, B. B. H., and Cheesman,S. J., 1984, Cross-holemagnetometric resistivity (MMR), Geophysics, 49, 1313-1326. Oppliger, G. L., 1984, Three-dimensional terrain corrections for mise-a-la-masseand magnetometric resistivity surveys: Geophysics, 49, 1718-1729. Pai, D., and Edwards, R. N., 1983, ProgrammeMMR2DFD: Finite difference modelling of MMR anomalies. Res. in Applied Geophys., No. 25, Univ. of Toronto. Rodionov, P. F., and Kormiltzev, V. V., Eds., 1979, Electrical exploration with the mise-a-la-masse method with the measurementof the magnetic field: Academy of Science, USSR, Ural Division, Sverdlovsk.

Seigel, H. O., 1974, The magnetic induced polarization method: Geophysics, 39, 321-339. Skeels, D.C., and Watson, R. J., 1949, Derivation of magneticand gravitationalquantities by surfaceintegration: Geophysics, 14, 133-150. Stefanescu, S. S., 1929, Theoretical studies of electrical prospectingof the subsurface:Studii Technice si Economica, Academie de la Republique Populaire Roumaine, 14, no. 1 (in French). --, 1950, Modeles theoriquesde milieux heterogenespour les methodesde prospectionelectrique a courantsstationaires: Studii Technice si Economice, ser. D. n. 2, 52-71, Buchurest.

--, 1958, Uber die magnetischewirkung einiger heterogenes medien in des elektrischen bodenforschung:Ztschr. f. Geoph., 24, 175-183. •, 1970, Nouvelles applications de la theorie des milieux alpha harmoniquesa la prospectionelectrique en courant continu: Geophys. Prosp., suppl. 28,786-800. Stefanescu,S.S., and Nabighian, M. N., 1962, Uber magnetischestorfelderals forge senkrechtesschichtungenim gleischstrom:Rev. Geologieet Geographie,Acad. Repub. Pop. Roumaine, 6, 139-155. Stefanescu, S.S., and Stefanescu, D., 1974, Mathematical models of conductingore bodies for direct current electrical prospecting:Geophys. Prosp., 22, 246-261.

MMR

Stefanescu, S.S., and Tang, Moui, 1971, Sur le champ magnetiquedes courants electriquesstationnairesdans les milieux heterogenes alpha-harmoniques: Rev. Roum. Geol., Geophys. et Geogr., Serie de Geophys., 15, 159-


183.

Tang, Muoi, 1972, Sur le champ magnetique des courants electriquesstationairesdansles milieux heterogenesalpha exponentiels: Rev. Roum. Geol., Geophys. Et Geogr., Serie de Geophys., 16, 51-90. Wannamaker, P. E., Hohmann, G. W., and Sanfilipo, W. A., 1984a, Electromagnetic modeling of three-dimensional bodies in layered earths using integral equations: Geophysics, 49, 60-74. Wannamaker, P. E., Hohmann, G. W., and Ward, S. H., 1984b, Magnetotelluric responses of three-dimensional bodies in layered earths: Geophysics, 49, 1517-1533. West, G. F., and Edwards, R. N., 1985, A simple parametric model for the electromagnetic response of an anomalous body in a host medium: Geophysics,50, 2542-2557. Williamson, S. J., and Kaufman, L., 1981, Biomagnetism, Jour. of Magnetism and Magnetic Material., 22, 129-202. Wilson, A. E., 1946, Geology of the Ottawa-St. Lawrence Lowland, Ontario and Quebec, Geol. Surv. Can., Memoir 241.

REFERENCES

FOR

GENERAL

READING

Edwards, R. N., 1984, The cross-holemagnetometric resistivity (MMR) response of a disc conductor: Geophys. Prosp., 32, 955-969. Edwards, R. N., 1989, Controlled source electromagnetic mappingof the crust, in David James, Ed., Encyclopedia of Solid Earth Geophysics, Van Nostrand Reinhold, Stroudsburg, 126-139. Gurevitch, Yu.M., 1975, The magnetic field of spheroidal bodies and point current electrode source (in Russian), Electrical exploration in drillholes for massive sulphide deposits in the Urals region, Acad. Science USSR, Ural Division, Sverdlovsk.

Gurevitch, Yu.M., and Tzirulsky, A. V., 1975, The magnetic field of point electrodes in a homogeneous anisotropic halfspace(in Russian), Electrical exploration in drillholes for massive sulphide deposits in the Urals region, Acad. Science USSR, Ural Division, Sverdlovsk. Kormiltzev, V. V., Martyanov, V. V., and Shepeleva, I. M., 1975, The magnetic field of a grounded cable in the presence of a conducting layer, (in Russian), Electrical exploration in drillholes for massive sulphide deposits in

99

the Urals region, Acad. Science USSR, Ural Division, Sverdlovsk.

Kormiltzev, V. V., Semenov, V. D., Fedorov, I. M., and Shepeleva, I. M., 1975, The mise-a-la-massemethod mea-

suringthe magneticfield and its applicationon prospecting for non-ferrous deposits (in Russian), Electrical exploration in drillholesfor massivesulphidedepositsin the Urals region, Acad. Science USSR, Ural Division, Sverdlovsk. McNeill, J. D., Frischknecht, F. C., and Labson, V. C., 1991, VLF methods, in Nabighian, M. N., Ed., Electro-

magnetic Methods in Applied Geophysics, Vol. II, Soc. Expl. Geophys. Nabighian, M. N., and Edwards, R. N., 1987, Magnetometric Resistivity: A historical perspective: Revues Roumaine de Geologie, Geographie et Geophysique, 31, 39-70. Nobes, D.C.,

Law, L. K., and Edwards, R. N., 1986, The

determination of the resistivity and porosity of the sediment and fractured basalt layers near the Juan de Fuca Ridge: Geophys. J. R. Astr. Soc., 86, 289-317. Rogachev, B. V., 1965, Instruction on the mise-a-la-masse method with magnetic field measurements (in Russian), Nedra.

Stefanescu, S.S., 1953, Magnetic field lines of D.C. current (in Rumanian): Bull. Sci. Acad., Repub. Pop. Roumaine, 5, 199-207. Stefanescu, S.S., 1963, The magnetic field lines of a point electrode in a layered media (in Roumanian): Probleme de Geofizica, 2, 135-145, Bucharest. Stefanescu, S.S., and Nabighian, M. N., 1961, Magnetic field lines of the transmitter AB (in Roumanian), Probleme de Geofizica, 1, 181-186. Stefanescu, S. S., Schlumberger, C., and Schlumberger, M., 1929, Etudes sur la prospection electrique du sous-sol: Premier Serie, Inst. Geol. Roumaine, Imprimerie Nationale, Buchurest.

Surevitch, Yu.M., 1980, Methods of calculating the magnetic fields of currents flowing in volume conductors, (in Russian), Acad. Sci, USSR, Ural Division, Sverdlovsk. Szarka, L., 1983, Exploration of the high resistivity basement usingelectric and magneticfields of quasistaticpoint sources, Geophys. Prosp. 31,829-839. --, 1984, Analogue modeling of DC mapping methods over high resistivity basementstructures,Acta Geodaet., Geophys. et Montanist. Hung., 19 (3-4), 451-465. Wolfgram, P. A., Edwards, R. N., Law, L. K., and Bone, M. N., 1986, Polymetallic sulfideexploration on the deep seafloor: The feasibility of the MINI-MOSES experiment, Geophys, 51, 1808-1818.

APPENDIX

FIELD

EXAMPLES OF COMPARISON

THE DOWNHOLE MMR METHOD WITH THE TEM METHOD Michael

INTRODUCTION

The following examplesdemonstrateapplicationsof the MMR method using a gradient-array electric current source and a downhole receiver. The gradient

AND

W. Asten*

array sourceuseselectrodes,either on the surfaceor in boreholes,which straddlethe target zone of interest in the strike direction (Figure A-l). Among the advantages of making downhole MMR measurements are the resolution of direction to a conductor (a conse-

*Exploration Department, BHP-Utah Minerals International, P.O. Box 619, Hawthorn, Vic., 3122, Australia.


100


quence of measuring a vector magnetic field in the borehole rather than a scalar electric potential) and the noise reduction produced by a conductive overburden which, for a downhole MMR survey, becomesa shield against geomagnetic noise. A significant operational advantage of the technique compared with electric field surveys is that the magnetic field sensor is unaffected when operating within the insulating PVC pipe which is routinely placed in boreholes for the purpose of maintaining access.

The parametersB t, B w, B m, andB n refer to total magnetic induction, magnetic induction due to the transmitter wire, true magnetometric field, and "normal" magnetometric field, respectively. A right-handed coordinate system is defined as a system with the z-axis directed upward (equivalent to elevation or reduced level, RL) and the x and y axes directed toward the east and north, respectively.

FIELD

MEASUREMENT

AND

DATA

REDUCTION

The example surveys discussed were conducted either using a Huntec Mk 4 receiver and Phoenix IPT1-B 1.5 kW transmitter, or using a Zonge GDP-12 receiver

and 25 kW GGT-25

transmitter.

The transmit-

ted current was a square wave of amplitude 2.5 to 10 amps at a frequency of 4 Hz. In all examples discussedbelow, the borehole magnetic field detector

was a Sirotem

DHR-S1

borehole

probe attached to a four-conductor logging cable and winch, originally designed as a down-hole system for transient electromagnetic (TEM) surveys. The sensor contains a solenoidal coil 1.5 m long wound on a mumetal core, providing input to a broadband preamplifier adjusted to give an "effective area" of

10000 m2. While improvedsensitivityis possible using a tuned coil, broad-band coils designedfor TEM surveys have the obvious advantage of commercial availability. The receiving instruments described measure the time derivative of magnetic field and record it as a voltage amplitude and phase. After correction for the known instrument response, transmitter frequency and harmonic content, the magnetic-field total ampli-

tudeB t andphase•t are obtained.Suitablefieldunits are, respectively, picoteslas (pT) and milliradians (rnrad). FIELD

EXAMPLES

Dreghorn

Figure A-2 shows drilling of the Magpie prospect, Dreghorn Area, southeast of Charters Towers in northern Queensland. The prospect contains volcanogenic stratiform Cu-Pb-Zn mineralization dipping steeply south-southeast, with considerable faulting and dolerite intrusives breaking continuity between boreholes.

Holes DM001

low mineralization,

and DM004

intersected

but holes DM021

shal-

and DM007

failed to intersect ore-grade mineralization down-dip. Both downhole TEM (DHTEM) and MMR surveys were

used in holes

ascertain

whether

DM021

additional

and DM007

conductive

existed below the holes. Transmitter

in order

to

ore minerals

electrodes

for the

MMR survey were placed 500 m east and west along strike, and connected with a wire routed 300 m north of hole DM021.

Figures A-3 and A-4 show the MMR measurements for hole DM021 (the responsein hole DM007 is similar

but is not shown).The steepslopein B t at shallow depths is the signatureof overburden (a flat, conducting layer). The negative anomaly centered at 140 to 150 m depth is indicative of a conductor located toward-x, ie., west of and above the two boreholes.

Surface

EM37 anomaly

Fig. A-1. The relationship between a grounded dipole electric source, a conductor parallel to the y-axis, a borehole perpendicular to the y-axis, and magnetic fields in the magnetometric method.

Fig. A-2. Dreghorn-Magpie prospect: drillhole plan.

MMR

Modeling of the principal amplitude anomaliesfor holes DM007 (not shown) and DM021 (Figure A-4)


shows that the conductor lies above the boreholes, at

an angle of about 45 degreesto the boreholes(i.e., the conductor is sub-vertical) and has a nearest approach to the holes of 43 m and 37 m, respectively. In addition to the MMR data, downhole TEM (Sirotem) data was acquired usingthe transmitter loop 1 (marked on Figure A-2). The data clearly shows the existence of an anomaly arising from an off-hole conductor oblique to the borehole, which closely matchesthe computedresponsefor a conductingplate in free spaceof dimensions200 x 100 m and conductance 45S in free space(Irvine, personalcommunication). Conductors

inferred from MMR

and TEM

DDH DM021,

data are

DHMMR

101

shown on the geologiccross-sectionin Figure A-5, and a close similarity between the two independent interpretations is observed. Thalanga

The volcanogenic base metal (Zn-Pb-Cu-Ag) Thalanga deposit is 50 km west of Charters Towers, Queensland, and has been reviewed in Irvine et al. (1985) and Gregory et al. (1989). Mineralization occurs in near-vertical stratiform massive-sulphide bodies strikingapproximatelyeast-west. Downhole TEM and MMR measurements have been regularly used in on-going evaluation of this ore deposit (Irvine, 1987). Asten (1988) shows downhole MMR profiles for three holes where, in early experiments, data was acquired as amplitudes only. Figure A-6 showsMMR amplitudes and phase for a deep hole, TH112 which passesbelow the mineraliza-

tion. The similarityof B t andB TM and the consequent

900 ,

small residual B m show the importance of removing

aDD

/•

•

Bw

7oo./

the wire field from field data. The normal field B n was

computed for a uniform halfspace. The residual field B • [Figure A-7(a)] shows a broad positive centered at 400 m downhole, plus a lesser positive anomaly cen-

•

tered at about 200 m. These features

/

i

0

i

i

I

I00

i

i

200

DEPTH

i

•00

(m)

Fig. A-3. Measureddownholetotal fieldBt and computer

can be modeled

approximatelyby the conductorsshownsuperimposed on the geologic section of Figure A-7(b). These interpreted conductors are consistent with the known thicknessof 20 to 100 m of strong pyritic alteration on the northern footwall side of the orebody and its

wire field B w for DDH DM021, normalized to unit transmitter

current.

18t•00N

18800 N

APPROX. LOCATION OF

I

SURFACE SIROTEM ANOMALY

DM 02 I

DMOI4

7f•

/, ß

T

I

i

pui )d• •1 d

1200m

150

--'t)%,• • ulflde ,•MR

,

1150m

i

ß•o

'

•o

,

•o

'

o

0

Depth (m)

Fig. A-4. Fit of reduced MMR field data B m with model data for borehole

DM021.

Fig. A-5. Location of alternative interpreted conductors from DHMMR and DHTEM surveys on a cross-section through the Dreghorn-Magpie prospect.

102


downward extension(Gregory et al., 1989). It is interesting that the maximum phase anomaly occurs near the base of the borehole and may be indicative of

7OO

600

further


500

weak

mineralization

above

the base of the

borehole. It is clear that the MMR responsein this hole is not that of the highly conductive orebody

•-• 400 ;:1,

located at a distance of 150-200 rn off-hole, but is the

responseof footwall alteration.By contrast,downhole TEM surveysand modeling(Irvine, 1987)clearly show the responseof the orebody itself. A similar general

• :300 200

result is observed with MMR

data from hole TH62,

presentedin Veitch et al. (1990). Data from hole TH83 (Figure A-8) show interesting

100

100

200

300

400

500

600

Depth (m) 50

•

50

40

•

30 02o

•

-•5

lo

o'''

•dd'' •dd" •dd" •dd" •dd"

Depth (m) (b)

S

-lO

TEM

Massive $ulfides

Depth

v

v

v

v

v

DDH N

Plate

model

112 0

v

v

v

DEPTH

v

Pyroclastics •v

50-

(m) MMR

r _ v

v

v •

25-

v

v v

v

v v

v

v

Rhyolitic Pyroclastics

v

Peak anomaly position (TEM)

v v

v

v v

v

v

v v

--300

v v

v

v

-25-

-- 400

it

Peak anomaly position (MMR)

-50-

-- 500 I

I

-75

DepLh Fig. A-6. MMR fields observed and computed for hole TH112. The theoretical axial field B n is computedfor a half spaceof 300 ll.m. All fieldsare normalizedfor a transmitted

Fig. A-7. (a) Residual MMR field B r for hole TH112 after subtractionof the theoreticalhalfspacefield. The dashedline shows the theoretical field computer for the interpreted 2-D model. (b) Cross-sectionof hole TH112, the interpreted 2-D model from DHMMR, and the interpreted plate model from

current of 1A. Phase is shown for the total measured field B t.

DHTEM.


MMR

103

additional features due to the presenceof a conductive

reduced by subtracting the computed normal field

overburden.

from

This hole was drilled at the eastern end of

the mineralized horizon where the orebody is known to be discontinuous; the hole did not intersect oregrade mineralization. A known thickness of 45 rn of conductive Tertiary sediments overlies the host rocks in this eastern zone. Transmitting electrodes were placed on the surface along strike 1000 rn west and 600 rn east of the borehole.

The

normal

field

Data in the interval

field data.

The resultant

residual

must be added to the model data to achieve this match.

The plate-model interpretation of TEM data by Irvine (1987) is also shown.

was

computed for a three-layer earth, and as with the previous example, comparisonof the two curves B n and B m allows a genuineanomaly due to a conductive body to be differentiated from an artifact due to layering of the earth or variation in borehole geometry. The steep slopes in both observed and model curves for depths 0 to 45 rn are due to a high current density in the overburden.

the observed

curve is plotted in Figure A-9(a) together with the computedfield for the 2-D model interpretation shown in Figure A-9(b). A small linear gradient of 0.06 pT/m

135 to 200 rn are

Comparison of MMR and TEM Interpretations

The TEM interpretations shown on Figures A-7(b) and A-9(b) both place the edge of a conductor at 80

W 60

absent due to presenceof fragments of steel casingin the hole.

The positive anomaly between 200 and 400 rn is clearly due to a conductorlocated above the borehole. The apparentgradient superimposedon the anomaly is an artifact of borehole geometry and is significantly

&40 20

300 11o

400

500

Depth (m)

(a) • loo

s 0

TEM Plate

•o

7O

N TH 83

DEPTH (m)

9O

•

(b)

model 0 ....

160....

260....

1560 ....

460....

560

(m) 1oo •o

MMI

1(b)

model

•"> .....

m•o• 0• ,

-20 l -40

•

/

"'•6b ....

•6•

400•00

Depth(m)

Fig. A-8. (a) Phaseof totalmeasuredfieldB t for holeTH83. (b) MMR field B m observed in hole TH83, and (dashed line) the theoretical axial field B n for a three-layer earth, where Pl, 02, 03 = 20, 5,300 •l.m and depthsto top zl, z2, 7,3-- 0, 25, 45 m.

,/ (TEM positi•)n nOm'l' & MMR)_ ,lOOm,

Fig. A-9. (a) Residual MMR field B r for hole TH83 after subtraction of the theoretical layered-earth field B n. The dashed line shows the theoretical field computed for the interpreted 2-D model. (b) Cross-sectionof hole TH83, the interpreted 2-D model from DHMMR and the interpreted plate model from DHTEM.


104


greater distancesfrom the boreholesthan do the MMR interpretations.These TEM interpretationsillustrate a general observationobtained from comparisonof the two techniques elsewhere. The discrepanciesare attributable to the differing physics of the two techniques. TEM data are primarily a measurementof fields from induced (or vortex) currents flowing around a conductor, the strengthof which dependsonly on the absolute conductivity and thickness product and size of the conductor.

The MMR

data are a measurement

of fields due to galvanic (or current channeling) currents, the magnitude of which dependson the relative conductivity of the conductor compared to host rock as well as on the shape of the conductor. For a thin disc-shapedconductor, and square plate of the same size, the dimensionlessinduction response parameter [3 and current channelingresponsenumber f are, respectively, given by:

[3 = co•xSa(West and Edwards, 1985) and f = a/(1 + a) where

o• = S ph/(2a) (Nabighian et al., 1984) S = conductance of the disc or plate 2a = diameter (or side length) to -- frequency of applied field •x = permeability of disc

Oh = resistivity of the host half-space The parameter f takes in the range zero (no channeling of galvanic currents) to 1 (galvanic saturation). For conductive targets of relevance to mining geo-

physics,2a is of the order of 100 m, Ohof the order of 100 to 1000 f•.m,

and S of the order of 10 to 100 S.

Thus, o• >> 1 and f • 1 and it follows that galvanic saturation is the norm with such targets. Galvanic response is, therefore, to a first order a function only of the current flowing in the resistive host, while inductive responseremains proportional to target conductance and dimension, but is independent of host resistivity. Where a conductive target is surrounded by a less conductive halo (e.g., with conductance one or two orders of magnitude lower than that of the target center) it is evident that for the range of parameters considered above, galvanic saturation is likely to occur in the halo as well as the target center. The inductive response parameter will, however, be dominated by the size and conductanceof the target center and thus the composite target plus halo appears larger

(or nearer to the point of measurement) when galvanically energized than when inductively energized. CONCLUSION

These examples illustrated how the downhole MMR method is an excellent tool for locating direction to an off-hole conductor. Logistically MMR is fast and therefore cheap but its resolution of distance to and dip of the conductor is inferior to downhole TEM. The respective strengthsand weaknessesof the downhole MMR

and TEM

methods

are such that MMR

should

ideally be used as a complementary tool in TEM surveys over good conductors. Where targets are only moderately conductive (e.g., sphalerite-galena mineralization) and hence poor inductive targets, the MMR method is superior to TEM for downhole surveys. ACKNOWLEDGMENTS

I thank Professor

R. N. Edwards

for an introduction

to the MMR method in 1983, and for many fruitful discussionson the subject since that time. Thanks are also due to BHP-Utah Minerals International, in particular Mr. P. G. Harman, for encouragement and finance for the development of the technique. REFERENCES

Asten, M. W., 1988, The downhole magnetometric resistivity (DHMMR)method: Expl. Geophys. 18, 12-16. Gregory, P. W., Hartley, J. S., and Wills, K. J. A., 1989, The Thalanga volcanogenic zinc-lead-copper-silver deposit in Hughes, F. (Ed.), Geology of the mineral deposits of Australia and Papua New Guinea: Austral. Inst. Min. and Metallur., Melbourne. Irvine, R. J., Hartley, J. S., and Mourot, A., 1985, The geophysicalcharacteristics of the Thalanga volcanogenic sulphidedeposit, Queensland:Expl. Geophys. 16, 231-234. Irvine, R. J., 1987, Drillhole TEM surveys at Thalanga, Queensland: Expl. Geophys. 18, 285-294. Nabighian, M. N., Oppliger, G. L., Edwards, R. N., Lo, B. B. H., and Cheesman, S. J., 1984, Cross-hole magnetometric resistivity (MMR): Geophysics, 49, 1313-1326. Veitch, D. D., Van Leeuwen, E. H., and Asten, M. W., 1991, Electric and magnetometric fields within a layered earth containing buried electrodes: Geophysics, in press. West, G. F., and Edwards, R. N., 1985, A simple parameter model for the electromagnetic response of an anomalous body in a host medium: Geophysics, 50, 2542-2557. REFERENCES

FOR

GENERAL

READING

Lo, B. B. H., 1985, On the development of downhole magnetometric resistivity: research in Appl. Geophys. Rep. No. 32, University of Toronto. Szarka, L., 1987, Geophysical mapping by stationary electric and magnetic field components: a combination of potential gradient mapping and magnetometric resistivity (MMR) methods: Geophys. Prosp. 35, 42•. •.44.


CHAPTER

PROFILING

METHODS

3

USING

SMALL

SOURCES

FrankC. Frischknecht,* VictorF. Labson,*BrianR. Spies, õ andWalterL. Anderson INTRODUCTION

intermediate conductivity. For this model to be realistic, however, varying the composition and thickness of the overburden may be necessary. The use of dipolaf EM profiling methods for purposes other than mineral exploration such as ground water, environmental, or engineering investigations is rapidly increasing. In most of these applications, conductivity contrasts are much smaller than those encountered in prospecting for massive sulfides. Often, the most appropriate models are best described as a horizontally layered earth in which there are changes in someor all of the layers, as an interleaved set of thin sheets of finite dimensions, or as a set of prisms. In archaeological studies, variations in the response causedby changesin magnetic susceptibility are quite

Chapter 3 treats electromagnetic (EM) profiling methods using small dipolaf sources. Profiling techniques are designed to detect changes in electrical conductivity laterally, along a traverse, in contrast to sounding techniques which are designed to determine variations in the conductivity of the earth with depth (see Spies and Frischknecht, this volume). The distinction between the two approaches is blurred when broadband or multispacingmeasurementsare made at closely spaced stations. The source and receiver antennas are usually small loops which can be treated as magnetic dipoles, although, in practice, a grounded electric dipole could be used for either of the antennas. When dipolaf sourcesare used in fixed positions, the methods have much in common with large source methods, so there is overlap between Chapter 3 and Chapter 4 by Parasnis (this volume). Most of the profiling techniques described operate in the frequency domain (which reflects their historical development), but many of the interpretation procedures are applicable in the time domain. A detailed discussion of time-domain methods is given in Nabighian and Macnae (this volume). The first and most common use of dipolaf source EM profiling methods is detection and characterization of highly conductive bodies, and most instrumentation and interpretative techniques have been developed for this application. In direct exploration for conductive mineral deposits the geologic model is generally assumed to consist of a highly conductive target such as a massive sulfide ore body in a much less conductive host rock covered by an overburden of

useful.

This chapter first showsthe most widely used smallloop profilingtechnique, slingram,and then introduces several variants on the technique which have been developed to address slingram's sensitivity to loop position, topography, and geologicnoise. While not in as widespread use, the ratio, direction finding, and frequency differencing methods have advantages in specificsituations.A final variant is time-domain electromagnetic(TEM) profiling, which addressesmany of the difficulties encountered in the frequency domain by measuring the secondary field after the primary field has been shut off. Since TEM systems, in many instances, employ only a single loop, it is possible to efficiently profile with larger loop sizes than in frequency domain. Therefore, many of the examples used in the TEM sections are not strictly small loop profiles, having loop sizes as large as 50 to 100 m.

*Deceased.

*U.S. Geological Survey,MS 964,Box25046FederalCenter,Denver,CO 80225. õSchlumberger-Doll Research, OldQuarryRoad,Ridgefield, CN 06877-4108, formerlyARCOOilandGasCompany, 2300West Plano Parkway, Plano, TX 75075. 105


106

Frischknecht

Nevertheless, the profiles show many of the same features seen in the small-loopfrequency-domainprofiles. The final sectionsof the chapter are devoted to the applicationof profilingmethodsin a broad rangeof exploration and mapping problems.

et al.

with the abbreviations

this chapter. In the literature, the orientation of loops is describedin terms of both the planesand the axes of loops, whereasthe orientationof mathematicaldipoles is described in terms of their axes. The reader must be careful

GENERAL

ASPECTS

OF SMALL

LOOP

METHODS

The sensitivity of an electromagnetic system depends on its ability to accurately resolve small secondary fields. The design and evolution of frequencydomain dipolaf-loop profiling methods has been strongly influenced by the need to rapidly and accurately measure secondaryfields in the presenceof the primary field. The problem of measuringsmall secondary fields is complicated by operational difficulties related to topographicfeatures and vegetation in the survey area, and by the need to minimize costs such as brush cutting, surveying, and data reduction. Some methods are designed so that, to the first order, measurements are unaffected by topographic variations and surveying errors. In other methods, significant errors in surveying are not acceptable and the spacingand orientation of the loops must be carefully controlled to obtain high quality data. The time-domain methods, where measurementsare usually made after the primary field has been shut off, are relatively less sensitiveto topographicvariations and surveying

or names that will be used in

to determine

whether

the author

of an EM

paper refers to loops or to dipoles and, if loops, whether their axes or planes are described. Parasnis (1970) developed a comprehensivenomenclature for moving source systemsand described24 of the possible systems. In Parasnis' nomenclature, the directions of the axes of the transmittingand receiving loops, the direction

of the line between

transmitter

and receiver

and the angle between this line and the traversing direction are specified in terms of direction cosines. Unfortunately, this system has not been widely adopted, perhaps because remembering sets of numbers is more difficult than remembering acronyms or other short names.

Throughout this chapter the loops will be described by their plane of orientation. The Horizontal Coplanar (HCP), Vertical Coplanar (VCP), and Vertical CoAxial (VCA) configurations, shown in Figure 1, provide maximum coupling of the primary field between

T

R

HCP

errors.

Nearly all frequency-domaindipolaf-loop methods use one of four different types of measuringschemes: (1) the phaseof one or more spatialcomponentsof the field is measured

relative

2., •

to the current in the source

loop; (2) two spatialcomponentsof the field are sensed simultaneouslyand the results expressedas the ratio of the magnitudesof the componentsand the phase differencebetween the components,or as parameters of the polarization ellipse. Generally, such measurements are made without

• PERP

reference

to the current in the

e

source loop; (3) the dip and sometimes the strike direction of the field are measured by rotating the receiving loop to find positionsfor minimum signal; and (4) the difference between measurements of the same component at two or more frequencies is deter-

mined. All of thesemethodsare designedso that there is no need for calculationof the primary field or other quantities to obtain normalized results. Some of the methods employ null configurationsso that, in the absenceof surveyingor orientation errors, the secondary field is measured in a direction normal to the primary field. Although many loop configurationscan be usedwith eachof thesetypes of methods,in practiceonly one or two configurationsare preferred. The most commonly used configurationsare illustrated in Figure 1, along

• O

• VCA I 'l

5. //•54.7o e

, ,

8. •

•

NULL

PARALLEL

HWAVETILT

•== VWAVETILT

Fig. 1. Eight common dipolar loop configurations.



the loops; the perpendicular (PERP), NULL, and parallel (PAR) configurationsprovide no coupling of the primary field. Note that the PERP and NULL configurationsboth employ perpendicularloops. The PERP and PAR configurations are responsive to a layered earth whereas the NULL configurationis not. Sinha and Collett (1973) showed that to null the primary field in the PAR configuration, the axes of the loops must be inclined at an angle of 54.74 degrees with respect to the horizontal. The remaining configurations (H and V wavetilt) are two of many possible arrangementsin which two receiving loops are used. Note that by the theorem of reciprocity, the functions of the loops as source and receiver can be interchangedwithout changingthe response. For most methods, measurements are quite sensitive to the deviationsin orientationand spacingof the loops that may occur in uneven terrain. When the resistivity of all material within range of a system is very high, the dominanterrors are causedby improper measurementof the primary field. If the topographyis well known and if station locations and loop orientations are accuratelyrecorded, the in-phasecomponent can be correctedfor topography.Such correctionsare also useful when conductive material is present; but, in this case, errors in measurementof the secondary field which are more difficult to correct are incurred.

In

EM sounding,when the model can be assumedto be a layered earth with a flat surface, accurate corrections can be made for some types of orientation errors (Spies and Frischknecht, this volume). This assumption is often inadequate in profiling, since the results may be distorted by topographicfeatures in conductive overburden or host rocks, as well as topographic variations

in the measurement

surface.

107

wherer2 = x2 + y2 + (z - h)2 isthedistance fromthe transmitter to the receiver, and m = dipole moment (m = NIA, N = number of turns, I = transmitter current, A = transmitter area).

If the source is a horizontal, y-directed, magnetic dipole the primary field componentsare 3mxy

Hx= 4,rrrS, 3my2

(4)

m

Hy-- 4•rr5 4•rr3,

(5)

3my( z - h)

Hz =

4•rr• .

(6)

Rectangularcoordinatesare used becausemeasurements are usually made in these coordinates. However, for specific geometries, use of a mixture of rectangular and spherical coordinatesis often more convenient. From equation (3) or (5) we see that the magnitude of the coupling for coaxial systems

(m/2•rr 3) is twicethatfor coplanar systems (m/4•rr3). For any configuration in which the angular relationshipsare fixed, the primary field always varies as the inversecube of the separation.The field about a dipole that is neither horizontal nor vertical is readily computed by resolvingits moment into vertical and horizontal componentsand then summingthe results obtained from equations (1) to (6).

(0, o,h)

These distor-

tions require that rigoroustreatment of topographybe included in the models used in interpretation of the data. Formulas that can be used to develop first order topographiccorrectionsare listed here for the Cartesian coordinate system indicated in Figure 2 where the transmitter is located at a height h and the receiver at a height z. If the source is a vertical magnetic dipole, the primary field componentsare:

y

o'I ,

3mx( z - h)

Hx=

4,rrr 5 ,

(1)

0'2 ,

3my( z - h)

Hy= Hz

4,rrr 5 ,

3m(z-h) 2 4,rrr5

(2)

m

4,rrr3'

(3)

Fig. 2. Coordinatesystemfor dipolarconfigurations.


108

Frischknecht

As contrasted with large source methods, dipolar source methods are faster and easier to use, particularly for reconnaissancesurveys. However, the depth of investigation is usually less for dipolar source methods than for large source methods. Each class of methods has advantagesand disadvantagesin identifying and determining the characteristics of conductors. These differences result from many factors includingdifferencesin typical distancesbetween source and receiver and the distribution of the primary field. When the source is small and portable a variety of surveying techniques can be used. The distinction between the fixed source technique, which is always used with large sources,and the moving sourcetechnique, which is used with most dipolar sourcemethods, has an important influence on results and their interpretation (Frischknecht, 1959). With fixed source methods, the source remains in a fixed position while the receiver is moved about to explore the immediate area of interest (Figure 3). Thus, the couplingbetween the source and the earth is constant for each set of

measurements.Generally, fixed source measurements are plotted at the receiver position. With moving source methods, both source and receiver are moved

for each station (Figure 3), so that the coupling between the source and the earth changes at each station, and is usually plotted at the point midway between source and receiver. The plotting point is, of course, a matter of convention since each measure-

ment is influenced in a complex way by the conductivity structure throughout a large volume of earth. Notice that for a small target, such as that in Figure 3,

I I I [ I [ [

-1...

-1-

et al.

the moving source configurationproduces two separated

anomalies

whereas

the

fixed

source

method

producesa singleanomaly. For larger targets, a moving sourcemethod does not produce separateanomalies, but the shapeof the anomaly is still influencedby the changingposition of the source. In moving source methods, both the source and the receiver are usually located along a singletraverse line (in-line technique)(Figure 4). However, some equipment can be used with the "broadside" technique (Figure 4) in which the source and the receiver are placed on separate parallel traverse lines so that the line between the sourceand receiver is perpendicular to the traverse direction. In most cases, the broadside technique provides narrower anomalies and hence better spatial resolution than the in-line technique. Also, for some configurations, broadside anomalies have larger amplitudes than in-line anomalies. Since the receiver and transmitter are on separatelines, a larger area is explored by a single broadside traverse than by an in-line traversefor a given loop separation. Anomalies

obtained with fixed source methods are

highly dependent on the placement of the source relative to two-dimensional (2-D) and three-dimensional (3-D) conductive structures. Thus, with fixed source methods direct comparison of anomalies over different conductors may be difficult. For dipolar source methods that use symmetrical configurations,the positionsof the source and receiver can be interchanged without affecting the profile shape. Such configurationsproduce anomalies over conductors that are symmetric with respect to the configuration(Figure 5). On the other hand, asymmetric configurationsgenerally produce mirror symmetric anomaliesover symmetricconductorswhen the loops are interchanged(Figure 5). The change in response when loops are interchangedis not a contradiction to the theorem of reciprocity. The anomalies would be identical if the loops remain in their same relative

I]•.... F,XED SOURCE PLOT POI T

o

NG

SOURCE

PLOT

¬

T

POINT

PLOT

CONST*NT------I

POINT•

V

......... .•;............ •................. -75

-50

-25

0 DISTANCE

25

50

GEOLOGIC STRIKE

75

(m)

IN-LINE

Fig. 3. Methods of profiling using a fixed source and a moving source, and comparisonof results obtained over a small sphere.

x

PLOT POINT

BROADSIDE

Fig. 4. Plan view of in-line and broadside profiling techniques, and convention for plotting data.



positionsand their functionsinterchanged.The dependenceof profile shapeon the relative positionof loops having different orientations frequently complicates the interpretation of airborne data taken with asymmetric systems when alternate lines are flown in opposite directions. This problem can be avoided partially in ground profiling by maintaining the same relative positionof the sourceand receiver throughout an

area.

Anomalies obtained with symmetrical moving source methods are somewhat dependent on the traversing technique, traverse direction, and loop spacing. However, as contrasted with fixed source anomalies, moving source anomalies may be compared readily with each other if they are well separated. Often it is easier to recognizeand distinguishbetween responsesfrom variations in the overburden and those from deep conductors with moving source methods than with fixed source methods. On the other hand, the shapesof anomaliesobtained with moving source methods tend to be more complicated than those obtained with a fixed source, and moving source results are very complex when several shallow conductors occur within

a distance

on the order of the

loop spacingor less. Such closely spaced conductors may be most easily resolved with fixed source measurements. Another advantage of fixed source methods is that, for each position of the source, all of the measurementsof the magnetic field above the surface obey Laplace's equation and techniquesfrom potential field theory can be used in interpretation. These techniques are not appropriate when the source position changes for each measurement. Initially small loop profilingmethodswere generally used in exploration for massive sulfide bodies in

0.1

.'

...¾• ......... 'xx / \

0.0

.... X :

_

•

'.•.'

\

:

•

/

\

I',

I

'J,.'"........... _-'"-

I\

':

I

: I

'..I/ ",

I

\..'•-'

-

:

regionswhere the host rocks were highly resistiveand where the overburden was relatively thin and resistive, as is the case in large parts of the Precambrian shields in Scandinavia and Canada. Under these con-

ditions, the responseof the host rock and overburden is small and generallycan be neglectedin interpretation. Often a dipping conductive half-plane is an adequate model; in this case simple proceduresare

availableto determinethe conductance,dip, anddepth to the top of conductors.In recent years, small loop methodshave alsobeenappliedin mineralexploration in areas where the overburdenis relatively thick and conductive, and in areas where the host rocks are not always highly resistive. In these environments the induction numbers and responsesof the overburden and host rock are not negligible,partly becauseof the

use of large source-receiverseparationsto cope with thick overburden.

Variations

in the thickness of the

overburden, as well as in the resistivity of overburden and host rocks superimpose"geologic noise" on the responsefrom targets. This noise complicatesinterpretation of anomaliesand may completely mask the responsefrom small or deeply buried targets. However, the definition of geologicnoise and target depends on the survey objective; in many studies the overburden may be the target. Galvanic currentsare often important when conductive, elongated targets are in good electrical contact with

conductive

host rocks

or overburden.

These

galvanic currents, which are induced in the conductive

hostrock or overburden,may enhancethe responseof the target but they may also complicateinterpretation and make determiningparametersof the target more dif•cult. In prospectingfor highly conductivetargets, the influence of galvanic currents or current channeling is generally less for small loop methods than for large source methods when measurements are made outside the transmitting loop. However, when the conductivity contrast is small, galvanic currents through the target generally dominate over vortex currentscirculatingwithin the target, regardlessof the source.When the target is resistivecomparedwith the host rocks, the anomaly is primarily due to deflection

of galvanic currents in the host rock away from the target.

1-

•

109

-0.2

-0.3

_

_

HCP

_

//////////

I

I

-1.5

-1.0

I -0.5

_

JTARGET j , 0.0

0.5

j

j

1.0

1.5

x/r

Fig. 5. Comparison of profiles over a thin sheet obtained with the HCP configurationand two PERP configurations. The PERP A curve is for horizontal loop on the right and the PERP B curve is for the horizontal loop on the left side.

Methods of interpretingdipolar sourceprofile data depend on the objectives and the characteristicsof the data. For somepurposesthere is little need to do more than outline the extent of highly conductive units. In other cases,suchas in planninga drilling program, an effort must be made to derive as much information

as

possible about the geometry and conductivity of anomalousunits. Of course, the degreeof difficulty in interpretingprofile data dependson the characteristics of the data as well as the objectives.In simplecases,


110

Frischknecht

quantitative interpretation can be carried out rapidly, and the resultsare completelyadequatein terms of the objectives. In other cases,data are too complexto be interpreted quantitatively by existing methods, and qualitativeinterpretationmay not be adequateto meet the objectives of the work. Usually, the first stepin interpretationof profile data is to deduce the general characteristic of physical models that might fit the data. In selectingthe most suitable models, the interpreter needs a working knowledge of the responses of a wide variety of models. Once the appropriate type of model is selected, interpretative techniques designed for that model are applied. If, for instance, the model selected is a half-plane in a highly resistive earth, its parameters can be determined using nomogramsand simple empirical rules. If the data appearto representa horizontally layered earth, computer inversion can be applied (Spies and Frischknecht, this volume). In the general case, a well-defined inverse procedure does not exist. If the anomaliesare distinct and do not appear to be badly distorted by geologicnoise, a scale or computer model curve may be found that closely fits the data. Extensive albums of curves are required when this approach is used. If time and modeling facilities are available, supplementalmodeling may be carried out by the interpreter to find modelsthat adequatelyfit the field data.

For some models and ranges of parameters, profiles made with dipole sources have simple shapes that change gradually and systematicallyas frequency or other parameters are changed. However, in many casesthe shapesof profiles are complex and they may change drastically as parameters are varied. In one sense EM profile data are most complex and more ditficult to interpret than smoothergeophysicalprofile data such as gravity or magnetic results. On the other hand, the greater complexity of EM results often allows greater certainty and detail in interpretation. Until the past several years, interpretation of small loop profile data was based primarily on direct comparison with physical scale model curves or on application of nomogramsderived from scale model data. Early scale models consisted of thin sheets, prisms, and other simple structuresplaced in air. Later, horizontal sheets were sometimesplaced over targets to simulate a conductiveoverburden. Electrolytic tanks were used as early as the mid 1950s to study the interactions between targets and conductive host rocks. However, extensive results of tank modeling did not appear until the 1970s.For further information on physicalscalemodelingand referencesto published work, see Frischknecht (1987, Chapter 6, Volume I). Analytical and numerical models available for the interpretation of small loop data include spheres,

et al.

layered half-spaces,thin sheets, prisms and cylinders. Analytical solutionsfor a sphere in free space have been available for many years (March, 1953) but the sphere is not as useful a model as is a thin sheet or dike. The theory for a layered earth was also known for many years (Wait, 1951)but, due to computational ditficulties, extensive results were not available until the mid 1960's (Malmqvist, 1965; Frischknecht, 1967). A knowledgeof the responseof a layered earth is often

useful in interpretation, but a layered earth is not the appropriate primary model in most mineral exploration problems. Wesley (1958) and West (1960) developed general solutionsfor the responseof a perfectly conductive half-plane. This model, though seldom adequateby itself, is a useful limiting case, particularly becausenumericalresultscan be obtained rapidly with limited computerresources.Annan (1974) developeda method for computing the response of a finite thin plate. Lodha (1977), and Dyck et al. (1980) developed the frequently used computer program, PLATE, based on Annan's method. The latter authors also developed another versatile program, SPHERE, based on a solution by Nabighian (1971) for a two-layer spherein a dipolar field. Walker (1981) investigated the development of a program for two or more plates. Weidelt (1983) developedfrequency and time-domain solutions for a finite conductivity half-plane in free space. Hanneson and West (1984a) presented a method for calculating the slingramresponseof a uniform thin plate in a half-space overlain by an overburden. Rajala and Sarvas (1985) studiedthe same problem but permitted the conductanceof the plate to vary with depth. Wide dikes and prisms in a layered half-space are often the most appropriate models; numerical results for such models are beginningto become available. For further information on analytical methods and numerical methods relevant to the interpretation of small loop data, consult Volume I. The following sectionsof this chapter discussspecificprofiling methods. SLINGRAM

METHOD

The slingrammethod was originally developed and used in Sweden in the mid 1930s (Werner, 1947) and was first used extensively in North America in the 1950s (Byers, 1957; Bergmann, 1960; Ward and Gledhill, 1957). The Swedish term slingram is sometimes translatedinto English as loop-frame. In North America, slingram is often known as the horizontal loop electromagneticor HLEM method. Although the HCP configuration is most commonly used, the PERP, VCP, and VCA configurations are also used with the same equipment; thus, for simplicity, the Swedish term, slingram is preferred. The HLEM method is only one variant of the slingram method. Often the

111



trade name or the name of the equipment manufacturer such as Boliden, EM Gun, MaxMin, or Ronka EM is used synonymouslyfor slingram. Equipment and Procedures

Slingram loops may be of either aircore or ferrite core construction;in someequipmentthe sourceloop has an aircore and the receiver loop has a ferrite core. The transmitter is battery powered and generally consistsof a console, battery pack, and loop, which are carried and operated in the standingposition by one person. A secondpersonthen carries and operatesthe receiver, which generally consists of a console and loop, either separated or rigidly attached to each other. A reference signalis derived from the transmitter loop current or from a reference coil closely coupled to the transmitter loop and is transmitted to the receiver by a cable. Earlier designsdeterminedthe ratio of the signal from the receiving coil to the reference signal using a ratiometer or compensator. The ratiometer consists of a passive network with variable resistor-capacitor, or resistor-inductor elements and a null detector consistingof a tuned amplifier and a pair of headphones or an analog meter (Parasnis, 1966; Keller and Frischknecht, 1966). The in-phase and quadrature parts of the reference signal are adjustedby means of calibrated potentiometersor variometersuntil they are equal to the received signal as indicated by the null detector. Readings are displayed on dials attached to the potentiometers. In newer designs, readings are made electronically and displayed on meters. Ideally, slingram equipment is calibrated and operated so that the readings are the in-phase and quadrature parts of the secondaryfield normalizedby the primary field. In terms of the mutual coupling,Z, between transmitter and receiver loops, most equipment is designedto provide the following readings:

In-phase reading= (Re (Z/Zo ) - 1) x 100% = Re

x 100%

Quadrature reading = Im (Z/Zo) =Im

(7)

x 100% x 100%

where Z0 is the appropriate mutual coupling ratio between the transmitter and receiver loops in free

spaceandHp andHs aretheprimaryandsecondary magnetic fields, respectively. In some older instrumentsthe primary field was not suppressedso that the in-phasereading was Re (Z/Zo) x 100%. When the

PERP or PAR configurations are used, there is no primary field to suppressand the readingsare simply Re (Z/Zo) x 100%and Im (Z/Zo) x 100%.In this case Z0 is generally taken to be the same as for coplanar loops. There

are a number

of devices which

differ some-

what from "standard" slingraminstrumentsbut which are most easily classifiedand discussedas variants of slingram. In one type of instrument, the loops are placed at either end of a rigid boom carried by two men (Malmqvist, 1965; Parasnis, 1966; Westerberg, 1965); this instrumentsharesmany attributeswith rigid fixedloop airborne EM systems. A tracked vehicle and a sled were used to carry the boom for such a rigid system designedfor winter operations (Frischknecht, 1959). In this system an electromechanical ratiometer provided continuousrecording on a strip chart. For these rigid boom instrumentsto achieve a useful depth of investigationwith their small loop separationsthey are designed to make much more precise measurements than conventional profiling instruments. Because they can also be regarded as a variant of slingram, sensitive instruments operating at very low induction numbers (LIN), sometimes called "terrain conductivity meters" (McNeill, 1980a), are discussed here and in Spies and Frischknecht (this volume). One type of LIN instrument (see Appendix E, this chapter) uses a separated transmitter and receiver connected by a reference cable; either the HCP or the VCP configuration may be used. Other LIN conductivity meters use a rigid boom to hold the loops in a fixed position, most commonly the loops are coplanar so that either the HCP or VCP configurations can be used. However, other rigid boom instruments have been developed that use the PERP or PAR configurations (Howell, 1966; Parachas and Tabbagh, 1978). Tabbagh (1986b) did a comparative study of the HCP, PERP, VCP, VCA, and PAR configurationsfor measuring both conductivity and susceptibilityin archaeological studies. He suggestedthat the PERP configuration is most suitable;the major disadvantagesbeing that anomalies are asymmetric and are very complicated when the target is shallow. In measuring conductivity, only the quadrature component is measured with LIN instruments. The assumptionis made that the quadrature componentis proportional to conductivity and the output circuitry and display are designedand calibrated to read apparent conductivity, cra, rather than quadrature component. From the chapter on sounding methods (Spies and Frischknecht, this volume), the mutual coupling ratio for coplanar loops on a homogeneoushalf-space at low induction

numbers

is

112

Frischknecht 2

Z

= 1+


Z0

iP•ocO•rar

4

(8)

'

where •a and • for the half-spaceare the same. Expression (8) can be used to calculate normal quadrature slingram readings from apparent conductivity readings. The inverse relationship, -

Cra ix0cot 2

Im

(9)

et al.

conductiveboulders and very shallow conductivebedrock. These three prospectingdevices and some other metal detectors employ a single loop that functions as the inductor in a self-oscillating circuit. The frequency

of the oscillatorchangesin response to changes in the impedanceof the løop causedby closeproximity of conductive or permeable objects. Metal detectors that use this or similar operating principles will not be discussed further in this chapter. In locating large buried metal objects, loop-loop LIN instruments are sometimes

is used within LIN instruments to convert the quadra-

ture component to apparent conductivity. The inphase component can also be measured with some LIN instruments. Unless highly conductive materials are present, the in-phase component, as measured with LIN instruments, depends primarily on the magnetic permeability of the earth. For HCP loops on the surface of a nonconductingearth, the mutual coupling ratio depends on the permeability of the earth, according to the following expression (Keller and Frischknecht, 1966)'

Z Z0

tx- tx0

1 = •.

(10)

• + •0

Similarly, for VCP loops the mutual couplingratio is Z

Z0

1=

•0 - •

(11)

•0 + •'

These expressionscan be used to calculate "apparent permeability" from measurementsof Z/Zo (See sections on "Exploration for Iron Ores and Other Magnetic Bodies" and "Archaeological Investigations"). For operation in the low induction number range, the loop spacing must be very much smaller than a skin depth in any of the media within range of the system. When this is true, magnetic coupling between current systems in the earth can be neglected. However, it is not always recognized that galvanic currents can dominate

over local vortex

currents

in LIN

mea-

surements.

A large variety of electromagnetic (EM) metal detecting instruments are used by civil engineers to locate pipes, cables, drums, and other objects that are a part of buried utilities or other facilities or wastes. Archaeologists and treasure hunters use some of the same instruments

to locate

coins and various

metal

artifacts. Military engineers use similar devices to locate explosive mines. Hood (1980) describes an attachment for a prospector's boot and a "cane' , to be carried by the prospector, which provide audible signalswhen they are in close proximity to a conductive sulfide boulder. Seguin et al. (1984), describe a sled which is towed across the ground to locate

more effective

than metal detectors.

In exploration for conductive mineral deposits and geologic mapping, slingram traverses generally are made normal to the regional strike or to the probable strike of the deposits. Due to the connecting cable, almost always placing the transmitter and receiver in-line along a singletraverse line is necessary. Broadside traversing offers a number of advantages and could be employed on premarked lines if equipment usinga radio link or stable clocks for a phasereference were available. In heavily vegetated areas precutting traverse lines is generally necessary. Also, if the terrain is not flat, stations are often presurveyed and marked. In flat open terrain, surveys can be made without preliminary work. One crew member establishes the direction using a compassand the reference cable is marked

and used as a chain to establish

the

correct distances between stations. When the loop separation is small, communication between crew members is by direct voice. In some equipment a telephone circuit is provided through the reference cable. At great distance, small radio transceivers can be used, although communication can be unreliable in rough terrain. Loop spacingsemployed in standard slingram measurements vary from about 10 to 300-400 m. As a rule-of-thumb, when using the HCP configuration, the loop spacingshould be at least twice the depth to the upper edge of thin, steeply dipping targets; although with careful work large targets can be detected at depths well beyond half the loop separation. If broad or flat-lying, sheet-like bodies are the targets, a smaller spacing may be adequate. When large spacingsare used, resolution of individual conductorsmay be poor and

if more

than

one

conductor

occurs

within

a

horizontal distance on the order of the loop spacing, interpretation of the results becomes difficult. Frequently more than one spacing is employed to study particular anomalies; use of more than one configuration can also help. It is awkward, but not impossible, to employ two or more spacings while making the initial traverse; however, with some equipment, measurements with two configurations, or two or more frequencies can be made efficiently in a single traverse. For reconnaissancework, the station spacing



often can be as great as one-half the coil spacing without much risk of missing significant anomalies, particularly if the loop spacingis only $0 to 100 m. When large spacingsare usedor when good definition of anomaly shapes is needed, the station spacing is generallyone-quarterof the loop spacingor less. Also, a high station density is required when searchingfor small anomalies in areas where high-frequency geologic noise is prevalent. In detailed exploration, the spacingbetween adjacent lines must be small enough to avoid significant risk of missing short or equidimensionaltargets. The lateral range of detection is not very large so that, in extreme cases, it may be necessaryto spacetraverse lines at a distanceequal to or less than the loop spacing. Sometimes closely spacedtraverses are needed to carefully map variations in conductivity and attitude of a conductoralong its strike.

In conductivity mapping with LIN instruments, the specific instrument or spacingand the configuration are chosen to maximize the responsefrom the depth that is of greatest interest. Frequently measurements are made at two or more combinationsof spacingsor configurations. The range of station spacings employed in making LIN measurementsis much greater than the range used in making standard slingram measurements.In archaeologicalstudies,for instance, Frohlich and Lancaster (1986) used a station interval of 0.5 m with a LIN instrument that has a loop

separation of 3.66 m. On the other hand, in areas where lateral changesin conductivity are very gradual, LIN

measurements

are often made at station intervals

that are several times as large as the loop spacing. In conductivity mapping, where the features of interest are likely to have roughly equal horizontal dimensions or where the probable direction of elongated features is unknown, a square measurement grid is often employed. Frequencies used in standard, small source equipment range from about 40 to $6 000 Hz. Early instruments were single frequency, but, for the last two decadesor more, most equipment has been designed to operate at two or more frequencies. Often, reconnaissance surveys are carried out using only one or two of the available frequencies. Subsequently, specific anomalies may be studied using other available frequencies. If only a single frequency is used, the frequency shouldbe high enoughto obtain substantial response from the types of mineral deposits which may occur in the area of interest. However, geologic noise will be minimized by making the frequency as low as possible.In areaswhere the overburdenis thick and conductive, or where the overburden and host rocks are sources of serious geologic noise, use of all

113

available frequenciesin the initial survey may be most effective.

When earth conductivities are within the proper range for a LIN instrument the results are independent of frequency. Thus, these instrumentsare designedto operate at only one frequency for each spacing. Errors in Slingram Measurements Several kinds of measurement

errors and noise can

adversely affect slingrammeasurements.Instrumental errors result from improper adjustment, drift, or inherent flaws in equipment. Interfering fields from natural sources, primarily sferics, and from man-made sourcessuch as powerlines and radio transmitters can decrease the accuracy of measurementsor even preclude measurements.High winds cause difficulty in holding loops steady and may induce microphonic noise in receiving loops, most noticeably at low frequencies. Operational errors result from surveying errors or misalignmentof the loops. Man-made metallic structures such as fences and pipelines cause anomalies which mask anomalies from targets and complicate interpretation. It is important to recognize the various possible sources of measurement errors and, if possible, to reduce them to below the level of geologicand cultural noise. At least three types of errors due to improper adjustment and calibration of standard instruments shouldbe recognized.They are (1) failure of both the in-phase and quadrature readings to be zero over a non-conductive earth, (2) scale or normalization errors in which readingsover a conductiveearth are in error by a multiplicative constant, and (3) phase or mixing errors in which the in-phase reading is affected by quadrature signals and vice-versa. The exact cause and remedy for these problemsdependson the design of the instrument. Generally adjustments to correct some of these problems are available to the user and are describedin the operator's manual. Other misadjustments may require correction by the manufacturer. To check and adjust orthogonality between the in-phaseand quadraturechannelsmay be possiblein the laboratory, or in some instruments, in the field. The final checking and adjustment of standard slingram instrumentsis usually carried out in the field in a high resistivity area. Ideally, the induction number in the test area should be so small that free-space conditions can be assumed.Finding a suitable high resistivity test area may be difficult, dependingon the local geology and the frequency and spacingat which the instrumentwill be operated. For example, it may be difficult to find suitable test areas where the resistivity is much greater than 5000 f•.m. Yet the induction

number B [= (•r•to/2)1/2r] isabout0.5fora 5000f•.m

114

Frischknecht


half-space (tr = 2 x 10-4S/m),a loopspacing, r, of 300 m, frequency of 3600 Hz (to = 2•r3600 Hz), and ix = tx0. The correspondingin-phaseand quadrature readings for horizontal coplanar loops are 4 percent and 6.2 percent, respectively. To accurately check equipment operating at high frequencies and large spacings,it is usually necessaryto measurethe resistivity at the test site by resistivity soundingsor other independent means and to calculate the expected slingram response for comparison with the actual readings. The principal method of calibrating low induction number instruments that use a large spacing is by making measurementsover a well-characterized, conductive earth. Low induction

number instruments

that

use short loop spacingsmay be zeroed by raisingthem to a suitable height above the earth. As indicated by Figure 6, for heights equal to two or more loop spacingsthe HCP responseis almost exactly twice the VCP response. As noted in McNeill (1980b), this responseis true regardlessof the layering. Thus, the zero of a short spacing, low induction number instrument can be adjusted by raising the instrument to a height of twice the loop spacingand making the HCP reading twice the VCP reading. Sites for calibration of either standard or low induction number equipment 10'4

'

i

,

i

,

i

,

i

,

i

.

i

,

i

,

i

,

i

,

10's

,

0

I

i

I

,'

2

I

,

I

•

4

I

,

5

I

,

6

I

,

7

I

8

,

I

,

9

10

h/r

6 (a) 10-3

10'4

10'5

, 0

HCP

I 1

t

I 2

,

I 3

should read 95.3 percent. If the earth cannot be regardedas an insulator, the changein readingdue to a change in spacing can still be calculated if the resistivity is known. As an alternative to changing spacing,one of the loops may be misalignedby a known angle to cause a predictable change in the readings.To avoid errors when the responseof the earth is significant,the loop axis should be rotated and receiver.

be made in a similar manner if a conductive earth with

3

..

Z/Zo will be 1/(160/200) 3 _ 1.953andtheinstrument

Direct measurementof the quadraturescalefactor can

VCP

1

should,of course,be as homogeneous as possibleor at least be horizontally layered so that the slingram responseis uniform throughout the area and can be calculated. If the site has several layers with high resistivity contrasts, problems with equivalence in interpretingthe resistivity soundingmay make characterizingthe site for EM calibrationsimpossible.In checkingequipment,use of both the horizontal and the vertical coplanarconfigurationsis desirable.Discrepancies between calculated and observed readingsat normal spacingswill represent primarily zero errors, although, if the readings are large, sensitivity and phaseerrors may be a factor. The scale factor for the in-phase component of standardinstrumentscan be checkedby taking readings at spacingslarger or shorter than the nominal spacings.For a perfectly insulatingearth, the change in mutual couplingshouldvary as the inverse cube of the ratio of the actual to the correct spacing.If, for instance,measurementsare made at a spacingof 160 m when the equipmentis set for a spacingof 200 m,

normal to the line between transmitter

HCP 10'5

__

et al.

,

I 4

,

I 5

,

I 6

,

I 7

,

I 8

, "-I-- -- -9

h/r

6 (b) Fig. 6. HCP and VCP responsefor LIN instrumentsover a layered earth at varying heights. At heights greater than 2 loop spacingsHCP is almost exactly twice VCP response.

a very well known responseis available. Someequipment is designedin such a way that the quadrature scalefactor is the sameas the in-phasefactor. Phaseor mixing errors are readily recognizedwhile checking the in-phase scale factor. Over a resistive earth the quadraturecomponentshouldnot changewith spacing or loop rotation. Some equipmentprovidesadditional means of checkingfor phase errors. If possible, equipment should be tested often to reveal significanterrors in readings and should be readjusted to eliminate error. If the user cannot make the necessaryadjustments, errors can be noted and correctionscan be applied later to the data. The most appropriate procedure to follow if data are to be corrected dependson the specificequipment and the reasonsfor the discrepancies.For instance, in a rati-

ometer, improperadjustmentof the level and phaseof the reference signalwill cause zero error, scalefactor error, and mixing error. All errors can be correctedby convertingthe readings(IP and Q) to complexmutual couplingratios (Z/Zo), then dividing by the mutual couplingratio observedat the proper spacingover a


nonconductive earth and converting the corrected mutual coupling ratio to readings as follows'


(Z/Zo)' =

[(IP + iQ)+ 100%]/100% ß

[(IPo + iQo) + 100%]/100%'

IP' = 100% Re (Z/Zo ) ' - 100%;

Q' = 100% Im (Z/Zo)'. An example is:

115

the error exceeds 5 percent. At large values of the induction number, the quadrature component and the apparent conductivity for the HCP configuration becomes negative. Correction curves for high conductivity are generally provided by the manufacturer for specific instruments. If not, correction curves can be calculated readily by the user. Alternatively, the couplingratio, Z/Zo, may be calculatedfrom the measured apparent conductivity by use of equation (8) and then used in place of apparent conductivity in interpretation.

"Free

space" readings from test area

IP

Q

12%

7%

Survey Readings IP Q 12% 20% -30%

7% 1:5% -20%

Corrected Readings IP Q 0%

0%

7.6% -38.9%

6.7% -21.7%

When a direct reading electronic instrument is used the best procedure may be to first divide the readings by the scale factor correction and then add the zero correction

to the results.

In some equipment, provision is made for easy and rapid adjustment of the zero levels. Common practice once was to readjust the zero levels of suchequipment when the systemwas out-of-range of good conductors. In doing so, quantitative information about the conductivity of the overburden and host rock was lost as there was still some "background" response from conductive overburden or host rocks. In some equipment, the zero levels drift excessively with temperature or other factors. The drift, if not too rapid, can be monitored by making repeat readings when work is stoppedand started, and by looping back to reference stations periodically, just as is done in making magnetic or gravity measurements. In using LIN instruments in areas of very high conductivity, the responseis not proportional to conductivity and equation (8) does not hold. For high conductivities the complete expression given in the chapter on electromagnetic sounding (Spies and Frischknecht, this volume) must be used to determine the couplingratio. The conductivity at which the error becomes substantial depends on the spacing, frequency, and loop configuration. For example, if the HCP configuration is used, the error exceeds 5.3

percent if theinduction number B [=((•lx0to/2)1/2r] for a homogeneous earth exceeds 0.05. Thus, for an instrument operating in the HCP configurationwith a spacingof 10 m and a frequency of 5000 Hz, the error exceeds 5 percent when the conductivity is greater than 0.0013 S/m, (resistivity is less than 800 tl.m). For the VCP configuration B can be as large as 0.1 before

Some instruments are slightly sensitive to the electric as well as the magnetic fields at the signal frequency, and they yield a measurably different responsedependingon the height of either loop above the earth and the placementof the operator's handson critical elements such as the loops [see the discussion of problems with reference lines in the chapter on physical scale modeling (Frischknecht, 1987, Chapter 6, Volume I)]. The effects of electric fields or capacitive coupling are most likely to be noticeable when operating at high frequencies and at large spacingsin damp or swampy areas. When such effects are noticed, the crew should keep hands off critical components while taking readings and should be as consistent as possible in operating procedures. The operationsmanualsfor some instrumentsoutline specific proceduresfor limiting these effects. If the problem becomes severe, the cause may be a defective connection or component. Under these conditions there is no way of knowing exactly how much the spurious electric field response affects the results or how to correct

the results.

Abnormally high electromagnetic fields generated by nearby powerlines, radio transmitters, sferics, and other sources decrease the accuracy of readings and may even prevent making useful measurements. The effect of interfering fields on accuracy is easily estimated when older equipment using an audio signalfor nulling is employed. The effect of extraneousfields is somewhat less obvious in direct reading electronic instruments though it can be seen as erratic or "fluttering" readings in some instruments. Little can be done about the effects of sferics except to avoid making measurements when activity is high and to make the loop separation as short as possible. However, many present-day receivers have data acquisition units capable of averagingout much of the external noise. Generally, configurations in which the receiving loop is horizontal will be least susceptibleto interference from sferics and most susceptibleto interference from powerlines. Under some circumstances, man-made fields may cause bias as well as random

errors

in measurements.

The effects of man-

116

Frischknecht

made fields can be reduced by using short spacings, relocating traverses away from sources, using loop configurationswhich are least sensitive, and reversing the position of transmitter and receiver if this places


the receiver

further

from the source of interference.

Operators can steady loops againstmotion induced by high winds by use of walking sticks or by sitting down while making measurements.Little can be done about microphonicreceiving coils other than to shield them from high winds. Unless proper care is taken, errors in positioning and alignment of the loops are likely to be the largest sources of error in slingram measurements. Over a highly resistive earth, errors in loop spacing cause errors in the in-phase component which are proportional to the cube of the percentageerror in the spacing (equations 1 to 6). Thus, to keep errors in the in-phase component at less than 1 percent, errors in spacing must be no larger than about 0.3 percent. In flat terrain there is, of course, no problem in maintaining sufficiently accurate spacing provided measurementsare made along straighttraverses. In reconnaissancework the connecting cable is usually marked and used as a measuring tape to establish station positions. When making reconnaissancemeasurementsalong winding roads or trails in vegetated areas use of other methods to establish correct station spacingsin advance of the slingram survey may be necessary. When using conventional instruments in highly resistive flat terrain, misorientation of loops is seldom a problem. If one loop is misalignedthe error in reading dependson the cosine of the angular error for coplanar loops and the sine of the angular error for perpendicular loops. For example, the error in the in-phase component is 1 percent for an error of 8.1 degreesin alignment of one of the loops in a coplanar configuration, or an error of 0.57 degreesin alignment of a loop in the perpendicular configuration. However, misorientation can cause much greater errors in conductive regions where secondary fields are present. When measurements

are made over a conductive

overburden

or half-space using the HCP configuration, a large

horizontalfield component, Hy, is generated. If the receiving loop is tilted toward or away from the

transmittingloop by an angle, t9, the total field, Ht, sensedby the loops is

H t = H z cos t9 _+Hy sin t9.

(12)

At large induction numbers, errors of a few degrees in t9 cause substantial errors in both in-phase and quadrature components. At small induction numbers, the vertical and horizontal componentsof the quadra-

et al.

ture component of the secondary field are approximately equal for a half-space. Thus, maintaining proper orientation of the loops when LIN measurements are made with the HCP configuration is important. The PERP configurationis similarly sensitive to loop misorientationwhen secondaryfields are present. When the VCP configuration is used, the secondary field is normal to the plane of the loops so an error of t9 in the orientation of one of the loops causesan error proportionalonly to cos t9 in measurementof both the secondaryand primary fields. In resistive but uneven terrain, possible errors in both orientation and loop spacingmust be considered. If the terrain is gently rolling and open so that the operatorscan see each other, the proper station spacing can be establishedby use of the connectingcable and the proper angle of inclinationfor the loops can be determined by use of an inclinometer. Since slope rather than horizontal distances are used, there will be variations in the lengths of lines containing the same numberof stations.In rough terrain" secantchaining" is frequently used to establish stations at equal horizontal distancesby surveying lines with a measuring tape and inclinometer. The slopein degreesor percent grade and the slope distance are recorded and the mean slope between transmitter and receiver position is calculatedand entered in the operator's notebook so that when measurementsare made the loops may be oriented correctly. Thus, the measurementsare made with a loop separation that may be too long, but the loops are coplanar, coaxial, or perpendicular as desired and stationsfall on a regular grid when projected vertically onto a horizontal plane. It is also possibleto attempt to establish stations at the correct spacing alongthe slope. In this method, the stationswill not be equally spaced when projected onto a horizontal plane, and the actual loop separationwill be less than the nominal spacing.With appropriatecomputer software and a topographicprofile, a horizontal projection of the stationsand a set of in-phasecorrectionscan be readily generated from equally spaced stations in rough terrain. Thus, unlike the secant chaining method, the reference cable length need not be adjusted as the mean slope of the topography varies. Finally, to establishthe station spacingby one of the methodsjust describedbut to orient the loops vertically or horizontally rather than parallel to the mean slope is possible. Before interpretingthe data, correctionsshouldbe made if either the spacing or the orientation are incorrect. When the orientation for a coplanar or coaxial configurationis correct but the spacing is incorrect, the instrument reads



reading= 100

- 1 = 100

The result is, of course, a corrected measurement for the spacingr', rather than for r.

- 1

Hi, •7 + Hs•- iHsQ

= 100

Hi'

117

-1

(13)

If the spacingis correct and the loops are oriented horizontally but one loop is higher than the other, the mutual couplingratio for a nonconductingearth can be calculated from equation (3) as Z

•

where

Z0

Hv = primaryfieldfor correctspacing,

H• = Hv(r/r') 3 = primary fieldatactual spacing, Hs• = in-phasesecondaryfield at actual spacing,

HSQ= quadrature secondary fieldat actualspacing, H = total field at actual spacing, r = correct spacing, and r' = actual spacing.

where !9 is the angle in degrees between the line between the loops and the horizontal. For this case false in-phase anomalies may be avoided by adding

3 sin2 !9tothereadings. Thecorrection is 1percent for

Z i

(20)

an angle of only 3.3 degrees. If the spacing is correct but one loop is higher than the other in a nominal vertical coaxial configuration, the mutual coupling ratio over a nonconductingearth from equation (5) is

To correct the results the quantity 100

= 1- 3 sin2 !9,

(14)

1

2 !9Z0=• (3cos

1)= 1-3/2

sin20. (21)

can be added to obtain the intermediate result,

Hs• +iH sQ 1'

reading = 100 Hp

To avoid false in-phase anomalies, the quantity

3/2 sin2 !9

(15)

To normalize by the actual primary field, multiply equation (15) by the factor

should be added to the readings. For a nominal perpendicularconfigurationin which one loop is horizontal and the other vertical, use of equation (2) or (6) gives the coupling ratio Z

(16) to obtain

reading = 100

Hs• +iHsQ

(17)

When the orientation for a perpendicularconfiguration is correct but the spacing is incorrect the instrument reads

Hs• +iH sQ .

reading = 100 Hp

(18)

Corrected readings can be obtained simply by multiplying by the factor given as equation (16). When secant chaining is used to establish the correct horizontal distance between stationsworking with the percent grade, g, between stations may be more convenient than working with slope distance. In this case, r' -> r and the following expressioncan be used to substitute for r/r' in equations (14) and (16)

r'-CøS tan-•#

(19)

(22)

• = -3 cos 19 sin 19, Z0

(23)

where Z0 is for a coplanar rather than a coaxial configuration.Thus, the quantity 3 cos t9 sin !9 should be added to measurements made with a nominally

perpendicularconfigurationon a slope.In this casethe proper sign for the angle must be used; the sign dependson whether the receiver is above or below the transmitter.

Use of these correction

factors for misori-

ented loops avoids topographically caused false inphaseanomalies.When the loops are not in the proper coplanar, coaxial, or perpendicular configuration a correction should also be applied to the secondary fields, but, because the receiving loop is sensitive to both vertical and horizontal fields, the correction is not

known precisely unless the fields can be calculated from prior knowledge of the electrical section. Thus, when operatingon a slope, use of proper orientations with incorrect distances is preferable rather than use of proper spacingsand incorrect orientations. If one can be confident that secondary fields are negligible at the lowest frequency used, the in-phase readings at this frequency can be subtracted from those made at higher frequencies to approximately


118

Frischknecht

correct the primary field for terrain effects. If the purposeof the slingramsurveyis simplydelineationof poor conductors which always cause a quadrature response, the in-phase component may be neglected and allowances for terrain need not be made although the anomalies may be distorted if the loop positions and orientations are ignored. There is no problem with loop alignment when using the vertical coplanar configuration in uneven terrain; in some cases survey objectives can be met by use of this configuration. In principle, instruments that use a rigid boom between loops are unaffectedby errors in loop spacing and orientation. In practice, temperature changesor mechanical shocksmay slightly changethe spacingor orientation of the loops and thereby cause drift or errors in the in-phasereadings. Small instrumentswith a very short loop separationcan generally be rezeroed as necessary by raising them sufficiently high above the earth. With larger instruments to make absolute measurementsof the in-phase component may not be possible. Relative in-phase measurements can be achieved by returning to a base station to make repeat measurements. The quadrature component as measured with rigid boom instruments is generally very stable. Proper,orientation of the loops in small induction number instrumentsthat use a separatetransmitter and receiver

is a consideration

in uneven

terrain

when the HCP configurationis used. However, at the spacingsused with such equipment, the operatorscan generally see each other and maintain an approximate coplanar configuration. Some of the errors described, such as those due to

nonsystematicsurveyingerrors or poor signal-to-noise ratio, are approximately random. Random errors tend to mask the presence of small anomalies in the data and they distort recognizable anomalies thereby making their interpretation more difficult. Random errors can be reduced to almost any degreethroughprecision surveying, use of high-powered transmitters, averaging several sets of readings, and other means. However, to reduce the magnitude of random errors much below the geologicnoiselevel in the area being studied is not cost effective. Some errors, such as those causedby topographicfeatures when poor field or data processingprocedures are used, are not random. Such errors not only tend to mask anomalies from targets but they may be mistakenly identified as actual inphase anomalies. The absence of a corresponding quadrature anomaly in single frequency data is often indicative of a false in-phase anomaly. If measurements are made at more than one frequency, most false anomalies can be identified by abnormal variation of the anomaly with frequency. Consistent errors, such as those caused by misadjustmentof the equipment, can distort real anomalies leading to error in

et al.

their interpretation. Errors in the zero levels of the equipmentcan lead to wrong estimatesfor the parameters of the overburden and host rock. Mixing between the in-phase and quadrature channels can lead to topographically induced false anomalies in both the in-phase and quadrature profiles. Geologic noise and noise due to man-made or cultural considered in later sections.

features

will

be

Processingand Display of Data If measurements

are made over flat terrain

with a

properly adjusted instrument, processingis generally not required and the data can be directly plotted. If necessary,simple programscan be developedto make any of the corrections described in the preceding sections. An occasional error can be corrected by hand calculation. Sometimes,systematicprocessingis carried out to obtain derivative products that are useful in interpretation. One example is calculation and contouring of the phase angle from slingram data (Kahma and Puranen, 1958). A number of procedures are sometimes used to enhance the interpretation of LIN data. Greenhouse and Slaine (1983) contour the dimensionlessquantity 20 log•0 •ya/•y•,where •ra is the apparentconductivity at a station and •r• is the average or background conductivity. In some cases to divide the area into sub-areas and to calculate the average value for each sub-area may be desirable. The normalized LIN data, logarithmically contoured, can readily be compared with other data that are similarly normalized. McNeill (1985) suggestsdifferencing VCP LIN measurements, •ra^ and •ra}• , taken at two differentspacings,A and B, where A is the greater spacing.The resultant quantity, (A/B)•ya^ - •Ya}•,is less influencedby near surface variations in resistivity than either of the original measurements. It is fairly common to average LIN measurementstaken with the configurationat different azimuths, but at a fixed spacing, to reduce effects of geologic and cultural noise. Of course, the result of this procedure can be to mix poor data with better data. When the earth is approximately layered, estimating and removing the responseof one or more of the layers may be possible. For instance, if the conductivity and thickness of the upper layer are known or can be estimated from other data, the calculated responseof this layer can be subtractedfrom the total measured response using the linear expressionsfor LIN responsein the chapter on EM Sounding (Spies and Frischknecht, this volume). This enhances anomalies due to variations in the underlying layers. Duran (1984) applied this procedureto eliminate changesin response caused by variations in the thickness of coarsegrained material above the water table. Weber


ProfilingMethodsUsingSmall Sources et al. (1984) inverted multistation data to obtain a three-layer model and then subtractedthe responseof the upper and lower layer to obtain the conductivityof the middle layer. In this procedurestatisticalmethods were used to eliminate data having a low signal-tonoise ratio or insensitivity to the layer of interest. In applying these methods of removing layers, any differences from the assumed layering can give rise to false anomalies.

119

loops is greater than 1.0 at small induction numbers and becomesless than 1.0 and approaches0.0 as B is increased.Similarly, the quadraturepart of Z/Zo for HCP loops is positive at small values of B and becomesnegativeat large values of B. For VCP loops, the in-phasepart is always greater than 1.0 and approaches 2.0 at large induction numbers and the quadraturepart is always positive. The VCA response resemblesthe VCP response,and the PERP response

Resultsof large surveysare generally machineplotted. Most commonly, standardslingramdata are displayed as nestedprofileswith the in-phaseand quadrature componentson a singlehorizontalaxis. A variety of conventions

are used for the vertical

axis. Occa-

sionally,the in-phasepart of Z/Z o rather thanZ/Zo 1 is plotted. Generally, negative values of both components are plotted downward and positive values upward, but sometimes one of the components is plotted with the reverse senseto accentuatethe difference

between

1.5

N

the curves.

The results of large surveys are sometimes contoured or shown in plan view by shading (Werner, 1958). In mineral exploration, contour maps are a useful meansof tracing the areal extent of conductive units. They are a particularly useful meansof displaying apparent conductivity data obtained with LIN

0.5

instruments.

-0.5

,I,l,l,l,l,l,l,l,l,l,I,

0

Model Studies of Slingram Responseand Interpretation Methods

2

4

6

8

10

Induction Number

7 (a)

Numerical and scale model results provide a basis for the design of field surveys as well as for the interpretation of data. Model results for a layered earth, conductive overburden, half-planes and thin plates, spheres, prisms, and a number of miscella-

1.5

.i.i,i,i, i,i, i,i, i,i,I.t

neous models are discussed in this section. 0.5

Responseof a Layered Earth.--The responseof a layered earth is discussedin the context of electromagnetic sounding(Spies and Frischknecht, this volume). However, a few speciallayered earth models that are of particular importancein slingramprofilingare considered here. Regardlessof the application, the interpreter shouldhave a generalidea of the responseof a homogeneoushalf-space. Particularly in mineral exploration, a conductiveoverburdenover a much more resistive half-spaceis an important model. Also, the interpreter shouldunderstandthe responseof a large buried fiat-lying, horizontal conductor. In-phaseand quadratureparts of Z/Zo for five loop configurationson a half-spaceare plottedas a function

oftheinduction number [B- (,ix0to/2) 1/2 r] inFigure 7. Several characteristics of these curves are important,

suchas the fact that the in-phasepart of Z/Zo for HCP

-1.0

-1.5

0

• I • 2I . I , 4I . I , 6I , I , 8I , I ,110, I ,12 Induction Number

7 (b)

Fig. 7. In-phase(a) and quadrature(b) responseof a homogeneousearth for five loop configurations.


120

Frischknecht

et al.

is somewhat similar to the HCP responseexcept that the in-phasepart is 0.0 rather than 1.0 when B = 0. The total excursionin the in-phasecomponentis largestfor the PAR configuration and the total excursion in the quadrature component is largest for the PERP config-

two frequencies or spacingsfall on the appropriate curve, the correct model may be either a homogeneous earth or another model that, for particular spacings and frequencies, produces the same response as a homogeneoushalf-space. If data for several combinations of frequency and spacingfall on the theoretical curve, the probability is high that a homogeneous half-spaceis the only acceptable model. To determine the conductivity of the half-space the induction number correspondingto a data point is found by interpo-

uration.

Frequently slingramresults are plotted in phasor or Argand diagrams, such as Figure 8, so that both in-phase (horizontal axis) and quadrature (vertical axis) information can be shown on the same diagram. Phasor diagrams are not only a useful means of displaying results, but they can be used in interpretation. For instance, by plotting field results on Figure 8 one can immediately determine whether or not a homogeneous earth model

lation.

Then

2B 2

•r =

[Xo cot

2.

(24)

satisfies the field data. If the field

data do not fall on the appropriate curve, they cannot represent a homogeneous earth. If results for one or

In mineral exploration estimating the conductivity and

thickness

of conductive

overburdens

is often

3.2

4.0

PARALLEL

0.8

2.4

÷4.8

0.6

1.6

3.2

2.4

VCA

1.6

4.0

VCP

0.4

2.4 ,4.8

.8, 5.6

4.0

0.2

6.4

4.8

8.0

8.0

.8

-3.2

0.0'

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

12 8.

-1.6 -0.2

16 6.4 1

5.6 -0.4

6.4

PERP 5.6

-0.6

4

HCP

2.4

-0.8 3.2

Fig. 8. Phasor diagram for a homogeneousearth for five loop configurations.Numbered tick marks are values of induction number B along curves.

2.0


useful. Since ore bodies often subcrop at the base of the overburden, estimates of overburden thickness

supplementestimates of the depth to the top of finite conductors. Also, knowing the conductivity and thick-


ness of the overburden

is useful in order to assess its

affect on the responseof underlying finite conductors. Generally, it is feasible to display only two independent parameters on a phaser diagram. If the conductivity contrast between the overburden and host rock is known, then for someconfigurationsthe responseas a function of the ratio of thicknessto loop spacing(d/r) and induction number (B) can be shown readily on a phaser diagram. Phaser diagramsfor the HCP configuration for three conductivity contrasts are given in Figure 9. The range of induction numbers in these diagrams is limited so there is in most cases a unique correspondence between the response and thickness and the induction number. When the range of induction numbers is extended, uniquenessis lost and to portray the results is difficult, an effect which can be seenin Figure 9a. The phasordiagramfor a contrastof 100 differs little from diagrams for even larger contrasts. In practice when the bedrock has a resistivity of 5000-10

000 l•.m

or more and the overburden

has a

resistivity of 50-100 l•.m or less, Figure 9c can be used in interpretation. To use Figure 9 to determine the parametersof the overburden, the data are simply plotted on the figure and d/r and B are determined by interpolation. In principle a measurement at a single frequency and spacingare adequate. However, to have a high degree of confidence in the results, several combinations of frequency and spacing should be used. If each data point does not yield approximately the same interpreted parameters, then the data do not fit this model and the model is inappropriate. It is apparent from Figure 9 that resolution of the parameterswill be poor when the induction

number

is small and the overbur-

den is thin, an effect which is also found in layered earth computer inversion methods. However, when the overburden

is thin the conductance

121

of the useful rangesof d/r and B, there is not a unique correspondencebetween the response and these parameters. As a result, in comparison to the HCP configuration, to resolve thickness and conductivity separately with the PERP configuration is difficult. The response of a conductive half-space under a much more resistive surface layer is a useful model in the exploration for large flat-lying targets. HCP and VCP configuration phasor diagrams for this case are plotted in Figures 11 and 12 for three conductivity contrasts. When the induction number in the upper layer is small the response of the lower half-space predominates. Phasor diagrams such as these can be used for interpretation in exactly the same manner as phasor diagrams for a conductive overburden; however, the buried conductive half-space is less frequently encountered than the conductive overburden case.

Malmqvist (1958) provided tables and detailed phaser diagrams for the HCP, VCP, and VCA configurations for an insulating layer over a conductive half-space. Eadie (1979) plotted a number of phasor diagrams for the two-layer cases considered here and for a conductivelayer in a resistive half space. Phasor diagramscan be plotted from the tables for two-layers given in Frischknecht (1967) or the tables for two, three, and four-layers given in Verma (1980, 1982). Interpretation of data for a layered earth by use of phaser diagramsis generally very tedious unless there are no more than two unknown parameters. If more complicated models are required the interpretative techniques described in Spies and Frischknecht (this volume) are generally preferable. This reference also gives responsesfor a number of layered earth models that may be of some interest in profiling. In zeroing and calibrating small, low induction number instrumentsand interpretingthe data, the variation of response with height above the surface of a halfspace is useful to know. From McNeill (1980a), the variation in apparent conductivity with height for the HCP

and VCP

cases is

can be esti-

mated much more accurately than the conductivity and thickness separately. When the conductivity contrast between overburden and host rock is not high, a phasor diagram, such as Figure 9a or a model with the proper contrast should be used. Unless the contrast is known from other data, the contrast must be found by comparison with several diagrams and the one which provides the best match chosen. A phasor diagram for the VCP configurationis given in Figure 10. As compared with the HCP configuration, resolution of parameters is poor except at the largest induction numbers. A phaser diagram is not shown for the PERP configurationbecause,over much

•ra_ 4 O'a--4

+1 +1

(HCP), 2

(25)

(VCP) (26)

If the induction number is so small that the in-phase component due to eddy currents is negligible, the in-phasecomponentsdue to magnetic permeability for HCP and VCP loops are given by the following expressions (Keller and Frischknecht, 1966) (Text continued on page 126)

122

Frischknecht

et al.

HCP - Oo/Oh= 30

HCP - Oo/Oh= 10 2O

'

I

'

I

'

I

'

'

I

'

I

'

I

2O

'

1

10


I

B = 0.6

B=1.1

0.8 1.0

1.2

1.2

0 -

= d/r

-10

-10

'

1.4

-20

-20

1.6

-30

1.8

0.5 = d/r -30

2.0

-40

-40

2.0

2.2 -50

-50

-

2.4

-60

,

-5

I

o

,

I

I

5

10

,

I

ß

,

15

I

20

,

I

,

I

25

-60

,

30

35

-5

In-phase(Z/Zo- 1)%

0

5

10

15

20

25

30

35

In-phase(Z/Zo- 1)%

9 (a)

9 (b)

HCP - Oo/Oh = 100 2O

1

0.6 0.8 ß

1.2

1,4

.5=d/• -20

1.6

-30

.8

-40

2.0 -50

ß

-60

-5

0

5

10

15

20

25

In-phase(Z/Zo- 1)% 9 (c)

30

35

Fig. 9. Phasor diagramsfor HCP over a half-space under a conductive overburden. Responseas a function of the ratio of overburden thickness to loop spacing d/r and induction numberB for conductivityratios eo/eh of 10 (a), 30 (b), and 100 (c).


123

VCP - Oo/Oh= 30

VCP - Go/Oh = 10 6o

6O

'

I

'

I

'

I

'

I

'

I

'

1.6


B=I

1.4 5o

5O

0.2

:).2

B = 1.2

D.15

3.15

4o

4O

).1

).1

).05

:).05 3o

3O

0 =d/r

2o

2O

0 =d/r

lO

10

0

o

-lO

o

lO

20

30

40

50

60

70

-10

I

,

10

I

,

20

I

30

,

I

40

,

I

50

,

I

60

,

70

In-phase(Z/Zo- 1)%

In-phase(Z/Zo- 1)% 10 (a)

10 (b)

VCP - Oo/Oh = 100 6O 1.6

1.4

2.0 J

5O

0.5

1.2 B = 1.0

4O 0.1

3.05 3O

:) = d/r 2O

10

of

,

I

,

10

I

20

,

I

30

,

I

40

,

I

50


,

I

60

,

7O

Fig. 10. Phasor diagramsfor VCP over a half-space under a conductive overburden. Responseas a function of the ratio of overburden thickness to loop spacing d/r and induction numberB for conductivityratios•ro/•rh of 10 (a), 30 (b), and ]oo (c).

124

Frischknecht et al.

HCP - O'o/O'h = 1/10 20

HCP - Go/Oh= 1/30 2O

'1'1'1'1'1'1'1'1'1'1'1'


lO

o

-lO

d/r = 0 -20

0.56

d/r

-40

0.32

• -30 0.05 0.2 .

ß

-50

0.1

-60

-60

-70

-70

B=1.0• •

-80

.48

-80

-90

_ _• 0.56

/

-90

-IO0

-80

-60

-40

-20

0

20

40

-100 -120

In-phase(Z/Zo- 1)%

-100

-80

-60

•0

-20

0

20

40

In-phase(Z/Zo- 1)%

11 (a)

11 (b)

HCP - Oo/O'h= 1/100 2O

10

-10

0.15

0.2 = d/r

0.16

-2O

-7O

-80 B -- 0.32 -90

-100 -120

,

I -100

,

• -80

,

i -60

,

m , i -40 -20

,


• 0

,

i 20

,

40

Fig. 11. Phasordiagramsfor HCP over a half-spaceundera resistive overburden.Responseas a function of the ratio of overburden thickness to loop spacing d/r and induction

numberB for conductivityratiostro/trh of 1/10(a), 1/30(b), and 1/100 (c).


VCP- Go/Oh = 1/10

125

VCP ' Go/Oh= 1/30 5O

50t' •' I' •' •' •' i' i' •' •'

'

I

"

I

'

I

'

I

'

I

B = 0.64

'

I

'

I

'

I

'

I

'

I

'

0.4


0.8

0.48

40

a3

0.48

4O

2O

a3

o

0 = d/r

20

o

10

10

o

i0

20

30-

40

50

60

70

80

90 lOO

0

In-phase (Z/Zo- 1)%

10 20

30 40 -50 60

70 80 90 100 110

In-phase(z/zo - 1)%

12 (a) 12 (b)

VCP- O'o/(•h= 1/100 5O

' I ' I ' I ' I ' I ' I' I' I ' I' I 0.24

B =0.16 4O

::3 20• o 0.48

lO

0.4

0.56

I

0.5

0

10

20

0.64

30

40

50

60

70

80

In-phase(z/zo - 1)%

90

100

110

Fig. 12. Phasordiagramsfor VCP over a half-spaceunder a resistive overburden. Responseas a function of the ratio of overburden thickness to loop spacing d/r and induction

numberB for conductivityratios Cro/Cr h of 1/10 (a), 1/30(b), 12 (c)

and 1/100 (c).

126

Frischknecht -3/2


2

12

4

+1

and

1]=

x 4

+ 1

(VCP).

(28)

The quantities,era/Or, and Re (Z/Zo - 1)/U•, where U• = (• - •0)/(• + •0), are plottedas a functionof h/r in Figure 13. For large values of h/r, the HCP responseis almost exactly twice the VCP response. For

small and intermediate

values

of h/r the VCP

configurationprovides the largest responsefor a nonconductive magnetic earth whereas the HCP configuration always provides the largest response for a conductive

earth.

Response of a Half-plane and Thin Plates.--The perfectly conducting half-plane (Wesley, 1958; West, 1960), seldom an adequatemodel for use in interpretation, is a usefullimiting caseand can be employedto study several characteristics of slingram response. Generally, the upper edge is assumedto be below and

Fig. 13. Response of low induction number (LIN) instruments over a conductive, homogeneous earth (C) and a nonconductivemagnetic, homogeneousearth (M) as a function of the ratio of instrumentheight to loop spacingh/r for

the HCP and VCP configurations.Dashed lines represent negative values.

parallel to the plane of observation. In-line profiles calculatedfor several height to separationratios, z/r, above a vertical half-plane striking normally to the traverse are shown in Figure 14 for the HCP, VCP, VCA, and PERP configurations.There is, of course, no quadratureresponsesince the half-plane is a perfect conductor. Consideringfirst the HCP and VCP configurations,the responseis positive (Z/Zo > 1) when both loopsare on the samesideof the half-plane. In this case the direction of the secondaryfield due to the half-plane is an aid to the primary field. The responseis zero for all loop separationswhen either loop is directly over the plane. When the loops are on opposite sides of the half-plane, the secondaryfield directionopposesthe primary field and the responseis negative(Z/Zo < 1). Thus the width of the mainpart of the anomaly is independent of the depth to the halfplane. This result is in marked contrastto the behavior

-5/2} (HCP), (27)

Re[Z/Zo-

et al.

of fixed source EM

anomalies

which broaden

as the

depth increases. For values of z/r > 0.1, the HCP configuration produces a single minimum which is largestwhen the array is centeredover the half-plane. However, for smaller values of z/r, double minima are developed and the positive shoulders become very large. In fact, the responseis singular as one of the loops approachesthe half-plane (Ketola and Puranen, 1967). As is well known, the responsefor the in-line VCP configuration is rather small for a half-plane becausethe loops are not well coupled to the plane. For the in-line VCA configuration, the responseis positive when the half-plane is near one loop, and is negative when the half-plane is in a central region between loopsor at a substantialdistanceoutsideboth loops. Curves for different values of z/r do not all cross throughzero at the samehorizontal positionas they do for the HCP and VCP configurations.The maxima are large when z/r is small, but when z/r is large the VCA responseis actually less than the VCP response.This is not because the secondary fields are less for the VCA configurationbut becausethe results are normalized by the primary field which is twice as largefor the coaxial as for the coplanar configuration. Curves produced by the PERP configuration are highly asymmetric. The response is zero when the horizontal loop is directly over the half-plane. A positivepeak resultswhen the vertical loop is near the plane, and a smaller positive peak occurs when both loops are on the same side of the plane but the horizontal loop is closest. The magnitude of the response for the in-line PERP configuration is greater than the responsefor the other three configurationsin the range, 0.05 < z/r < 0.6. If the traverse is perpendicular to the strike of the half-plane, as was assumedpreviously, the only parameters needed to describe the plane are its horizon-


ProfilingMethodsUsingSmall Sources tal position and the depth to its top. The horizontal position of a vertical half-plane can be readily determined from the position of characteristicfeatures of the profilesfor any of the configurations.Similarly, the depth can be determinedfrom the magnitudeof the response.

Profiles for broadside configurations at various heightsabove a half-planeare shownin Figure 15. The broadsideresponsefor both the HCP and VCA configurationsis always positive (Z/Zo > 1), and is predominantly positive for the VCP configuration.The responsesof the broadsideHCP and VCP configurations for large z/r are smallerthan for the correspond-

HCP-

ing in-line configurations.When z/r is small, the responsefor the broadsideVCP configurationis much larger than for any other configuration.The anomaly for the broadside PERP configuration is asymmetric about the half-plane.The amplitudefor the broadside PERP configurationis aboutthe sameas for the in-line PERP configurationfor large values of z/r but the amplitudeis largerfor smallvaluesof z/r. As expected, anomaliesfor the various broadside configurationsare generally narrower than for in-line configurations. Broadsideprofilingis generallyimpracticalwith existinggroundslingramequipment.However, therewould be advantagesin usingthe VCP and PERP configura-

Inline

0.4

!

VCP-

!

!

i

!

'

!

'

i

.

!

127

Inline

0.20

....

ß

.....

0.3

0,2

0.10

0.1

0.05

'-•

0.3

0

0.4

0.6

0 -0.05

-0.10

$'

-0.3

-0.15

-0.4

-0.20

-0.5

-0.25

-0.6

12' -01.9 .5 ' -1. ' -01.6 ' -01.3 ' •) '-'01.3-'01.6 d.9' 11.2 ' 1.5

-0.30 -1.5

-1.2

-0.9

-0.6

-0.3

x/r

0

0.3

0.6

0.9

1.2

x/r

14 (b)

14 (a) VCA0.8

PERP

Inline ,

,

ß

B - Inline i

i

i

i

i

ß

!

ß

i

ß

i

ß

i

0.8

0.6

z/r=

0.6

O.

0.4

0.2 0.1

0.30ß4 0.6

0

0.2

-0.2

0.2

-0.4

0.1

-0.6 .

-0.8 -0.2

x/r

x/r

14 (c)

14 (d)

Fig. 14. In-line profilesat severalheight-to-separation ratios(z/r) over a perfectlyconducting verticalhalf-plane strikingnormallyto the traversefor HCP (a), VCP (b), VCA (c) and PERP (d) configurations.

1.5

128

Frischknecht

tions and possiblythe HCP configurationin the broad-


side mode.

The model which has been used most frequently in interpretationof slingramdata is the finitely conducting dipping half-plane in free-space.The parameters required to define this model are the conductance(crt, where t is the half-plane'sthickness),the depthto the upper edge, the dip, the strike, and the horizontal position of the projection of the upper edge on the measurementsurface. To be a good approximationto a half-plane, a steeply dipping thin sheet must have a depth extent of at least twice the loop-spacingand a strike extent of about four times the loop spacingfor traverses made over the center of the sheet (Ketola,

HCP-

et al.

1968; Nair et al., 1968). Consideringtypical sizes of conductive mineral deposits (Boldy, 1981), the halfplaneis often an accuratemodelwhenloop spacingsof 40-60 m are used but is much less likely to be an accurate model when the spacingis 200-400 m. Finite rectangular thin sheets may be used as models when infinite depth and strike extent are not appropriate. The additional parametersrequired to define a rectangular sheetwith its upper edge parallel to the surface are then the strike and depth extents.Negi et al. (1987) studiederrorsassociatedwith usinga half-planemodel for bodieswith a finite strike lengthand depth extent. They concluded from scale model studies that for bodieswith depth extents less than 2.5 loop spacings

VCP-

Broadside 6.00

i I i i i i i i i,

0.8

Broadside ,

,

,

,

5.25

4.50

3.75

•

";'

0.6

3.00

r = 0.05

N

'---'

2.25

.c:

1.50

--

0.75

0.1

-• 0.4

0.2

0.2

,3

0.3 0

0.4

0.2

0.4

0.6

-0.75 [

0

-1.50/

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

.

-1

x/r

x/r

15 (a)

15 (b) VCA-

0.12

PERP

Broadside i

,

,

2.0 i

i

i

i

,

B - Broadside i

i

i

i

i

i

1.6

z/r = 0.05 0.10

1.2

z/r = 0.05 0.8

0.08

0.4

0.06

0.1 0.2

0

-0.4

0.04

-0.8 0.02 -1.2 0

0.6 0.4 -0.02 _,

'-o'.•'-o'.•'-oh'-o'.•'

• 'oi• 'oh'o'.•'o'.•'

-1.6 -2.0

x/r

15 (c)

15 (d)

Fig. 15.Broadside profilesat severalheight-to-separation ratios(z/r)overa perfectlyconducting verticalhalf-plane strikingnormallyto the traversefor HCP (a), VCP (b), VCA (c) andPERP (d) configurations.


ProfilingMethodsUsingSmall Sources or strike extents less than 5 loop spacingsthe conductance is underestimated and the depth is overestimated. They go on to show the extent of the errors in interpretationfor severalsituations.Ward (1967)in his description of the electromagneticmethod provides charts which demonstrate the reduction of the peak responsefor the HCP configurationas the strike and depth extents are varied for a plate of fixed conductance.

If a half-plane or plate is not steeply dipping, inphaseprofilescan be considerablydifferentfrom those given in Figures 14 and 15. Also the quadratureprofile can be quite different from the in-phase profile, dependingon the conductance.Profilesfor two conductances represented by the dimensionlessinduction parametera (= •xto•tr)and two normalizeddepthsare given for five configurationsin Figures 16-20. The profileswere made over the center of plateshavinga strike extent, S = 4r, and a depth extent, D = 2r, dippingat various angles.Resultsfor dips lessthan 30 degreesare not included.When the dip is very flat, the effect of the bottom edge of finite plates has a large effect on the shapeof the profiles. Two setsof curves are required for the PERP configurationsince it is asymmetric. Consideringfirst the HCP configuration(Figure 16),

as the dip becomeslessverticalthe negativepart of the curvesbecomesmore asymmetricaland the magnitude and shapes of the positive peak differ more. The in-phasepositive peak is always largeston the downdip side, but the relative size of the quadraturepeaks dependson the dip and the conductance.The horizontal positionof the minimumvalue of the negativepart of both the in-phase and quadrature componentscan fall on either side of the vertical projectionof the upper edge of the plate dependingon the conductanceand depth. When the plate is steeply dipping, both the in-phase and quadrature componentscross through zero at x/r = +_0.5. When the dip is not steep and particularlywhen the depth to the upper edgeis large, the zero crossings are considerably displaced from x/r = +-0.5. However, the crossingsgenerally shift in the same direction so that their separationis approximately equal to a loop spacing or less. This is an importantpoint; the generalshapeof the profilesand the distance between the zero crossings are the most

important criteria in decidingwhether or not an anomaly should be interpreted using a half-plane or thin plate model. If the distancebetweenzero crossingsis much greaterthan a loop spacing,a thin plate or halfplaneis generallynot an appropriatemodel,and a thick dike or multiple plate model shouldbe considered. The VCA configuration (Figure 17) is much more sensitiveto changesin dip than the HCP configuration. If accurately determining the dip of a steeply dipping

129

sheet is important, making supplementary profiles with the VCA configuration may be worthwhile. The in-phaseand quadratureresponsesare quite similarin shapefor this configuration.As the responseis much more positivethan the HCP response,the locationof the zero crossingsare of little use in the interpretation. The criteria for deciding whether a half-plane or thin plate model shouldbe used, will be the general shape of the anomalyand the distancebetweenpositivepeaks, which should not greatly exceed the loop spacing. As noted in the discussion on vertical plates, the VCP configuration (Figure 18) is poorly coupled to steeply dipping half-planes and plates. The response increasesfor the more shallowdips. This configuration producesa much broader anomaly than the HCP or VCA and can be difficult to interpret. The two PERP configurations,PERP A in Figure 19 and PERP B in Figure 20, producedrasticallydifferent anomaliesover the samedippingplate. The anomalies can be quite broad, making interpretationdifficult. For both the PERP configurationsand the VCP configuration to judge if the dippinghalf-planeor plate model is appropriateis difficult. One of the first stepsin interpretationof an elongate conductor is to determine the strike length and strike direction. This is a simple procedure if the conductor is well defined and if a sufficient number of traverses

have been made across the conductor. Not only is this information

needed in overall evaluation

of the con-

ductor, but is needed before other parameters can be determined. If the strike length is too short, a half-

planeis not an appropriatemodel.The changein HCP anomaly amplitude near the end of a large plate is shown in Figure 21 for two heights above the plate. Interestingly, the in-phase component begins to decreasebefore the quadraturecomponentas the end is approached.If the conductorterminatesabruptly in the strike direction, Figure 21 or results given in Nair et al. (1968) can be used to accurately locate the end. If the ends are not well

defined

it should still be

possibleto make a sufficientlyaccurateestimateof the strike extent to decide whether a half-plane or a finite plate shouldbe used in further interpretation.Figure 22 showsan example of the variation of responseas a function of strike length for the HCP configuration. Changes in HCP profile shape with variation in traverse direction are shown for a vertical plate in Figure 23. As the angle between the traverse and the conductor becomes smaller, the magnitude of the negativetrough decreasesand the magnitudesof the positive peaks increase. The horizontal separation betweenpeaksincreasesbut the zero crossingsremain fixed.

Substantial

error will be introduced

if model

(Text continued on page 140)

130

Frischknecht

et al.

HCP - = = 40 z/r = 0.5

HCP - = = 40 z/r = 0.5


2.0

JI' I'I' I' I' I'I'I' I' I' I'I'I' I' It

1.5

__

30' 1.0

45' 60' 75'

0.5

90'

0

o--

-0.5 -1.0

T

-1.5

-2.0

-lO

I

-1.4

-1.0

•.6

•.2

0.2

0.6

I

1.4

-1.4

,

I

i

I

-1.0

I

I

i

I

-0.6

i

I

I

i

I

i

-0.2

x/r

,

,

0.2

,

!

,

I

,

[

I

i

0.6

i

I

I

I

1.4

x/r

16 (a)

16 (b) HCP - = = 2.5 z/r = 0.5

5[ I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I 2.5 4

2.0

1.5

Om

9- -o.s

-1.0

-1.5

-2.0

-1.4

-1.0

-0.6

-0.2

0.2

0.6

I

1.4

•

x/r

16 (c)

-1.4

-1.0

-0.6

-0.2

0.2

,II i i i 0.6

I

x/r

16 (d)

Fig. 16. In-line HCP profilesover the center of a plate in free-spacewith varying dips. The plate's strike length S is 4r, and depth extent D is 2r where r is the loop separation.(a) through(h) show in-phaseand quadraturefor inductionparameterstx of 40 and 2.5, and depthsz/r of 0.5 and 0.2.

1.4

131

ProfilingMethodsUsingSmall Sources


HCP- = = 40 z/r = 0.2

HCP- = = 40 z/r = 0.2

2

.

60'

15

'

11

$-90' 75.30• 0

.

-15

'•

õ:=o -25

-30

-35

"•

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

1.4

-12 I , I , I , I , I • I , I •I , I , I , I , I , I , I , I -1.4

-1.0

-0.6

-0.2

x/r

0.2

0.6

1

1.4

x/r

16 (e)

16 (f) HCP - •x = 2.5 z/r = 0.2

HCP - •x = 2.5 z/r = 0.2

8

6

16II 'I'I'I'I'I'I'I'I'I'I'I'I'i'I't

30'

12

-

ß

4

8

2

•

ø

' o

4•

0.90'30.• •_

-4

-12

-

-16

-10

I, -1.4

I •-1I0 i I ,-0.6 I , I ,-0.2 I J I , 0.2 I • I •0.6 I , I t II , I , 1.4 I

-1.4

-1.0

-0.6

-0.2

0.2 x/r

16 (g)

16 (h) Fig. 16, cont.

0.6

I

1.4

132

Frischknecht

VCA-

et al.

VCA-

•, = 40 z/r = 0.5

•, -- 40 z/r = 0.5


1.4

5.0 1.2 4.5

4.0 1.0

30' 3.5

0.8

3.0

2.5

2.0 1.5

1.0 0.2

0.5

0

-0.5

-1.0

-0.2 -1.5

-2.0

I • -' .4

I

•

I , -1.0

I

,

I , -0.6

I

a

I , -0.2

I

,

I • 0.2

I

,

I , 0.6

I

,

I I

,

I

•

I 1.4

-0.4

-1.4

-1.0

-0.6

-0.2

x/r

0.2

0.6

I

1.4

0.6

1

1.4

x/r

17 (a)

17 (b) VCA-

• -- 2.5

z/r = 0.5

VCA-

o• = 2.5

z/r = 0.5

0.8

0.7

1.6

0.6

1.4

0.5

0.4

0.8

o.1

0=9C

o

-o.1

30' -0.2

-0.2

-0.3

-0.4

-0.4

-0.6

-0.5

-0.8

-1.4

-1.0

-0.6

-0.2

0.2

0.6

I

1.4

-1.4

x/r

17 (c)

-1.0

-0.6

-0.2

0.2

x/r

17 (d)

Fig. 17. In-line VCA profiles over the center of a plate in free-spacewith varying dips. The plate's strike length S is 4r, and depth extent D is 2r where r is the loop separation. (a) through (h) show in-phase and quadrature for induction parameterso•of 40 and 2.5, and depthsz/r of 0.5 and 0.2.



VCA-

133

V CA- • = 40 z/r = 0.2

a = 40 z/r = 0.2

10tl ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' It 9

30'

•'

e=9o'

(•

45 ß 1

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

1.4

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

1.4

1

1.4

x/r

x/r

17 (e)

17 (f) VCA - (• = 2.5 z/r = 0.2

VCA-

(• = 2.5

z/r = 0.2

11

4.5 ß

lO

4.0

9

3.5 8

3.0 7

2.5

2.0 1.5 1.0

8

8 = 90'

3

75'

0.5

e =90'

60'

45' .

2

75' 0

1

-0.5 -

0

60' 45'

-1.0-1i'"'.'"'.'''''"""''"""'. 4 -10 -06 -0.2

0.2

0.6

1

I4

.4,4

x/r

17 (g)

17 (h) Fig. 17, cont.

-1.

-0 6

-0 2

.2

0

134

Frischknecht

et al.

VCP - o; = 40 z/r = 0.5

VCP - o; [] 40 z/r = 0.5


1.50

1.25

1.00

0.75

•

3 0.50

o.2s 0 = 90' 0 = 90'

45'

-0.25

-0.50 -1.4

-1.0

-0.6

-0.2

-1.4

0.6

0.2

-1.0

-0.6

-0.2

0.2

0.6

1.4

18 (b)

18 (a) VCP-

o; [] 2.5

z/r [] 0.5

VCP - e• = 2.5 z/r -- 0.5

1.2

1.0 2.0

30' 0.8

0.6 30'

1.0

0.4 0.5

0 = 90'

0 = 90'

-0.2

-1.4

-1.0

-0.6

-0.2

0.2

0.6

I

1.4

-0.5

x/r

18 (c)

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

x/r

18 (d)

Fig. 18. In-line VCP profilesover the centerof a plate in free-spacewith varyingdips.The plate's strikelengthS is 4r, and depth extent D is 2r where r is the loop separation.(a) through(h) showin-phaseand quadraturefor inductionparameters(x of 40 and 2.5, and depthsz/r of 0.5 and 0.2.

1.4



VCP - =; = 40 z/r = 0.2

135

VCP - • = 40 z/r = 0.2 5

24

30'

30'

o•

2

O = 90'

0

0 = 90'

o

75•

45'

30' 45:

-1.4

-1.0

-0.6

-0.2

0.2

0.6

-1.4

1

-1.0

-0.6

-0.2

0.2

1.4

0.6

x/r

•r

18 (f)

18 (e) VCP - =; = 2.5 z/r = 0.2

VCP - =; = 2.5

z/r = 0.2

8

30 ø

3.0

2.5

2.0

30'

45 ø

1.5

1.0

60 ø

75 ø

0.5

90 ø

O = 90' 60'

30 ø

30'

45'

-0.5

-1.0

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

-1.4

1.4

-0.6

-0.2

0.2

x/r

x/r

18 (g)

-1.0

18 (h) Fig. 18, cont.

0.6

I

1.4

136

Frischknecht

et al.

PERP A- e• = 40 z/r = 0.5

PERP A - e• = 40 z/r = 0.5 0.5


30'

30'

75' 90' -0.5

-1.0

-10

-12

'O

-2.0

o -2.5

-18

-3.0

-2o

-3.5

-22

-14

-1.0

-0.6

-0.2

0.2

0.6

I

-1.4

1.4

-1.0

-0.6

-0.2

x/r

0.2

0.6

I

1.4

x/r

19 (b)

19(a) PERP A - e• = 2.5 I

'

I

[

I

'

I

'

I

z/r = 0.5 '

I

'

I

'

I

PERP A- e• = 2.5 z/r = 0.5 '

I

'

I

'

I

'

[

'

I

'

I

'

I

__

30'

-1.4

x/r

19 (c)

-1.0

•.6

-0.2

0.2

0.6

1

x/r

19 (d)

Fig. 19. In-line PERP A profilesover the center of a plate in free-spacewith varying dips. The plate' s strike length S is 4 r, and depth extent D is 2 r where r is the loop separation.(a) through(h) show in-phaseand quadraturefor induction parameters ot of 40 and 2.5, and depths z/r of 0.5 and 0.2.

1.4



PERP A - a = 40 z/r = 0.2

137

PERP A - a = 40 z/r = 0.2

e

90'

60'

45'

90'

-10

-20

-30

-40

-50

-60

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

1.4

-1.4

-1.0

-0.6

-0.2

0.6

•r

19 (e)

19 (f) PERP A - ([ = 2.5

z/r = 0.2

PERP A- ([ = 2.5 z/r = 0.2 5

-10

-10

-15

-12

-20 -14

-16

-25

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

1.4

-1.4

x/r

19 (g)

19 (h) Fig. 19, cont.

-1.0

-0.6

-0.2

0.2

0.6

1.4


138

Frischknecht

et al.

PERP B -o[ = 40 z/r = 0.5

PERP B -o[ = 40 z/r = 0.5 I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

•

I

'

I

•

1.0

I

0.5

90 ø _

// ___

90 ø

-2.0

-10

-2.5

-12

.

I , -1.4

-14

I

,

I , -1.0

I

,

I , -0.6

I

,

I , -0.2

I

,

I , 0.2

I

,

I , 0.6

I

,

I , 1

I

,

I 1.4

-1.4

-1.0

-0.6

-0.2

0.2

0.6

1

1.4

•r

yJr

20 (b)

20 (a)

PERP B - o; = 2.5 z/r = 0.5 PERP B -o; = 2.5 z/r = 0.5

0.5

0.25

-0.5

-0.25 -1.0

-0.50 -1.5

1 -1.00 -2.5

•

135 ø

-1.25

-3.0 -1.50 -3.5

-1.75 -4.0

-2.00 -4.5 -2.25

-5.0

-1.4

:2.50

-1.4

-1.0

-0.6

-0.2

0.2

0.6

I

1.4

-1.0

-0.6

-0.2

0.2

0.6

I

x/r

x/r

20 (d) 20 (c)

Fig. 20. In-line PERP B profilesover the centerof a platein free-spacewith varyingdips.The plate's strikelength S is 4r, and depthextentD is 2r where r is the loop separation.(a) through(h) showin-phaseand quadraturefor induction parametersa of 40 and 2.5, and depthsz/r of 0.5 and 0.2.

1.4



PERP

B - e• = 40 z/r = 0.2

139

PERP B - e• = 40 z/r = 0.2 12.5

10.0

7.5

5.0

-7.5

-40 -10.0

-50

-12.5 -1.4

-1.0

-0.6

-0.2

0.2

0.6

1

1.4

20 (e)

-1.4

-1.0

•.6

-0.2

0.2

0.6

0.2

0.6

1.4

20 (f)

PERP B -e• = 2.5 z/r = 0.2

PERP B - e• = 2.5

3

z/r = 0.2

8

-lO

-12

-14

-16

-18

-1.4

-1.0

-0.6

-0.2

0.2

0.6

I

-1.4

1.4

x/r

20 (g)

-1.0

•.6

-0.2

x/r

20 (h) Fig. 20, cont.

I

1.4

140

Frischknecht

et al.

results for traverses

normal

to the strike are used for


traverses that are at an angle of less than about 60 degrees. Anomalies become much more complex when the plate is not steeply dipping and the angle between HCP-

z/r = 0.1 Plan View

20

Offset

18

.

.

16

4

•

,'5

•//•---•

12

/

8

6

4

2

10

15

20

25

30

35

40

45

In-phase (Z/Zo- 1)%

21 (a) -ICP-

and strike

of the conductor

is

small. Insufficient data have been published to carry out accurate interpretation when traverses are not approximately normal to strike. Strangway (1966b), Ketola and Puranen (1967), and Nair et al. (1968) published numerous phasor diagrams, nomograms, and procedures for interpreting HCP data using the half-plane model. Ketola (1968), Nair et al. (1968), Jones and Wong (1975), and Negi et al. (1987) published results for plates with finite strike and depth extents. Interpretation of HCP profiles using a half-plane model is particularly simple when the dip is 90 degrees. Unless there is interferencefrom nearby conductors, this case is easily recognized because the anomaly is symmetrical. The horizontal position of the sheet is marked by the center of the anomaly. This leaves only two unknown parameters, the depth to the upper edge of the sheet and its conductance. These two quantities are readily displayed on a single phasor diagram. The maximum values of the in-phaseand quadrature negative troughs are plotted as a function of normalized depth and induction parameter a in Figure 24 for a vertical plate. This plate, which has a strike extent of 4r and a depth extent of 2r, is a good approximationto a half-plane although it may be slightly more accurate to use results such as those given in Nair et al. (1968) which are for a larger plate. The depth to the top of the plate and its conductance are readily determined by plotting the field data point on the diagram and inter-

• '540 l•(center)' ß

5

the traverse

z/r = 0.3

8

.

0 Offset

'*//•/ /

/

/

•1.5 '\•(center) ß 40 HCP

16/, •, •, •, •, •, •, •, •, •,,,

5)

f

2

2.25,

2.5•

øo

2

4

6

8

10

12

14

16

18

20

22

5,, ',-

121-

24

-h ß

L /// '.. '"": '-

•

' /J///' / ' '

26

In-phase (Z/Zo-1)% Fig. 21. HCP in-line maximum anomaly amplitudes over a plate in free-spacefor profiles offset from the center of the plate at two depthsz/r = 0.1 (a), and z/r = 0.3 (b). The plate' s strike length is 4 r and depth extent is 2 r, where r is the loop separation.

• ,, , • , • ,

'•,

, -' '

' ' '

'

'''''

. .....• ,•A5o

\1

\lk•2•2.5

_

200

o

o

o

21 (b)

,'

•' ' ' '

2

•, •,,,

10

14

2

4

6

8

lO

12

14

16

18

20

22

24

26

28

30

32

34

36

In-phase(Z/Zo-1)%

Fig. 22. The effect of strike length on HCP in-line maximum anomaly amplitudes over the center of a square plate in free-space at a height of z/r = 0.2. Curves are .drawn for varying strike length, S/r, and induction parameter a.

Profiling Methods Using Small Sources 3O

'1'1'1'1'1'1'1'1'1'1'1'


2O

10

0

-10

-20

-3O

-4O Traverse ,

-50

I

,

I

,

I

i

I

-2

-3

,

I

,

I

-1

,

I

•

0

I

,

I

,

I

1

,

I•

2

3

x/r

23 (a) 10

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

'\

'

I

'

J '

I

'

/

/ I o •

I

1-,\

\

• II

45/ ,,/•?•

141

polating to find z/r and or. Knowing the frequency and spacing, z and (•t are readily calculated. If data are collected at more than one spacing and frequency, separate interpretations can be made for each data point. If the resultsare not consistent,the half-plane in free space is not an appropriate model. When HCP anomalies are narrow, as measured by the distance between zero crossings,and asymmetric, a half-plane dipping at an angle other than 90 degrees may be the correct model. The magnitude of the negativetrough as well as the shapeand magnitudesof the positive peaks are dependent on the dip. Thus phasor diagramsfor a vertical plate, such a Figure 24, cannot be used to determine the depth and conductance of plates which are not steeply dipping. One approach to interpretation is to first estimate the dip from the size of the positive peaks from a chart suchas found in Keller and Frischknecht (1966) and then to determine the depth and conductance from a set of phasor diagrams for different dips based on the magnitude of the negative trough. The half-plane or thin plate lying horizontally is another geometry of interest. This model might represent a discontinuousoverburden layer or a flat-lying graphitic layer. In the vicinity of the edge the techniques used for the dipping sheet become inappropriate. The responseover the edge of a flat-lying body is asymmetric and changesdramatically with conductivity and thickness, and with depth. Figure 25 from Keller and Frischknecht (1966) is an example of a scale model for the HCP configuration.The computationof the edge response is quite difficult, and most results shown in the literature

are from scale model studies.

Strangway(1966b) presentsseveral HCP profilesand a

•,• ' \

HCP

60'

2O

6

18•-// / /

/ '"•.1 =z/r

-10

,

-15

-3

I

,

I

-2

,

I

,

I

-1

•

I

,

I

0

•

I

•

I

1

,

I

,

I

2

,

I

3

x/r

23 (b)

Fig. 23. Changes in HCP profile shape with variation in traverse direction over a vertical plate for in-phase (a) and quadrature (b). Height, z/r is 0.2, induction parameter o• is 40, and plate dimensionsare $/r = 4 and D/r = 2.

•

0

5

10

15

20

25

30

35

40

45

•o-

50

55

In-phase(Z/Zo- 1)%

Fig. 24. HCP vertical half-plane solution approximated by a plate 4r x 2r. Maximum negative in-phase and quadrature valuesfrom a profile are used to determine induction parameter ot((XlXo•tr) and normalized depth (z/r).


142

Frischknecht

phasor diagram for interpretation. Frischknecht (1987) shows the effect of a flat-lying sheet as an overburden covering a deeper conductor. The most effective method for interpreting the results obtained over a horizontal plate whose dimensions are much greater than the loop separationis by use of the layered earth models as describedpreviously. Flat lying thin plates whose width is on the order of or less than the loop separationcannot be interpreted using the layered earth models. Such narrow ribbonlike

conductors

have

been

studied

in scale model

(Jones and Wong, 1975) and are relatively easy to model numerically. Anomalies of this type, while not common, will be encountered as overburden variations

and

streams.

The

ribbon-like

conductors

can

present a great range of responsesas the width of the conductor and the height of the transmitter and receiver are varied. The examples presented here are intended to display the characteristics of these responsesas an aid in identifying a ribbon conductor's presence. Jones and Wong (1975) provide phasor diagramsfor interpretation of several variants of this

et al.

The most distinctive responsedue to a ribbon-like conductor

occurs

when

the

conductor's

width

is

nearly equal to the loop separation. Most of the slingram transmitter and receiver configurationsare well coupled to these conductors and the responses are large. The responsevaries greatly with changesin transmitter and receiver height at a fixed induction parameter as can be seen in Figures 26-29. At a fixed height a similar range of responses are seen as the induction parameter varies (Figures 30-33). A profile at a single frequency and separation cannot separate the height and conductivity-thicknessproduct. The separation of these effects can only be accomplished by varying the configurationor frequency. (Text continued on page 146) HCP 25

'

i

'

i

'

I

'

I

'

i

'

i

20 15

o•

lO

ßT

5

0.4

model. .3

-10 0.2

d/r--

-15

(,3

- 20,

-20

d/r-- O. I, o•--

.

-25

-40,

.5 -1.25-1.0 -0.75-0.50-0.25

0.25

0.50

0.75

1.0

1.25

1.5

x/r

d/r= O. l,ot-- •.

26 (a) -8O

HCP

d/r=0.,., z,•=Z.?

50

......

4.0 30

• ß

60

20

lO

(,3

-m

0 0.4

<

40

--

0.3

(•

-20

0.2

I

•- ao

-30 -40

O--

-50

-1.5'-1'.25'-1'.0 '-0'.75'-0'.5 '-0'.25'•) ' 0'.25'0'.5' 0'.75'1'.0'1'.25' x/r

-•r)

-t.5

-t.o

-o.5

o

ct5

t.o

Fig. 25. HCP anomaly curves observedover a thin horizontal sheetfor depthsz/r of 0.1 and 0.3 and inductionparameter a of 1.0 and 2.7 (after Keller and Frischknecht, 1966).

26 (b) Fig. 26. HCP in-phase(a) and quadrature(b) profilesover a thin horizontal plate with a strike dimension of 2 r and a width of r at severalheightsz/r, and an inductionparameter aof4.



•CA

143

VCP !

]

.

!

.

]

.

]

,

]

,

i

,

i

,

[

,

[

!

35 4

3O _--

•

25

•-,

20

•

•5

a•

lO

_.:

5

.

.

1

o

-10

.....

i

.....

-•.•-•:•-•'.o-o'.•'-o'.•-o'.• o oi• o'.• o'.•' ;.o •:• •.•

0 -1.5

-1.25

-1.0

-0.75

-0.5

-0.25

.x_/r

27 (a)

0

0.25

0.5

i

!

0.75

1.0

1.25

x/r

28 (a)

vcP

VCA

!

80 f' ,

7O

z/r = 0.1

40 30

-o

20

0

10

20 !' ,.,

i

i

,

!

,

i

,

z/r = 0.1

14

5O

(•

!

16

60

•N

i

18

12

10 8 6-

4

2

0 -1 .,'

-1.25 -1.0

-0.75 -0.5

-0.25

0

0.25 0.5 0.75 1.0 1•.25'1.5

x/r

x/r

27 (b)

28 (b)

Fig. 27. VCA in-phase(a) and quadrature(b) profilesover a thin horizontal plate with a strike dimension of 2r and a width of r at severalheightsz/r, and an inductionparameter

Fig. 28. VCP in-phase(a) and quadrature(b) profilesover a thin horizontal plate with a strike dimension of 2r and a width of r at severalheightsz/r, and an inductionparameter

ot of 4.

ot of 4.


144

Frischknecht et al.

HCP

3ERP i

3O

B ,

5 I

,

i

,

i

'

,

i

i

i

,

i

,

i

,

i

i

i

,

i

,

i

i

,

i

O•

0.4, 0.25• .,..... 0.__6

•

20 10

0 06

-10

-15

0.4

0.3

-20 -30

0.2

-40

'•

-20

•

-25

•

-30

-50

-35

-60

-40

.5 '-1•.25 ' -11.0 '-01.75 ' -01.5 '-0:25 ' •) ' 0:25'0•.5 ' 0•.75 ' 1•.01.25

-;

-•.•-•'.•-•'.o

-0.75

-o'.•-o:• o

oi• o'.• o'.7••'.o •.• •.•

x/r

x/r

30 (a)

29 (a)

HCP

PERP

B

i , i , i [ i , i , I I , i , i ,j

40 o

20 -5

•

o

•N•-20 (•

•

-40

06 0.4

(•

0.3

-• -60 (•

0.2

.•

-15

-20

(:• -25

-80

-30

-100

-35

-120

-140

.

•

-40

,

-•.•'-•'.•'-•'.o'-o'.7•'-o'.•

-1.! -1'.25'-1'.0'-0'.75'-0'.5 '-0'.25'•) '

0.75

.0

x/r

x/r

29 (b)

30 (b)

Fig. 29. PERP B in-phase(a) andquadrature(b) profilesover a thin horizontal plate with a strike dimensionof 2r and a width of r at several heightsz/r, and an inductionparameter

Fig. 30. HCP in-phase(a) and quadrature(b) profilesover a thin horizontalplate with a strike dimensionof 2r and a width of r at a heightz/r of 0.2 for severalvaluesof induction

(x of 4.

parameter

145



VCP

VCA

80 f'

20 f,,,,,i ,,,,i,i i i ,,[.

70

6O

18

(z=10

cc=10

16

_

5O

%' 40

•

14

%-

12-

N

10

N

•

30

•_

20

•

--

10

c:

0

v

•

8

6 1

4

•06 04 025'••

0.6, 0.4, 0.25 2

-10

-20

....

•

,

-1.5 -1'.25'-110 -0'.75-0'.5 -0'.25'0

31 (a)

,

,

•

,

,

0'.250'.5 0'.75'1.0 1'.25

x/r

32 (a)

0 ! • -1.5 -1.25

-0.25

0

0.25

0.5

0.75

1.0

i 1.25

,

1.5

• J' i ' i ' i i i ' i ' i i i

60

50

16

j 10

40

•

14

(•

10

30

v

m

20

,-

lO

O

. -0.5

VCP

70L ' i ,,i,!,,6z,,i,i,, , i, N

-0.75

x/r

VCA

•

-1.0

2.5

o

'

1.6

'

I

.

0.25

4

-lO

2 .

0.6

-2o 0

-3o

.5

-1.25

-1.0

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1.0

1.25

.5

x/r

x/r

31 (b)

32 (b)

Fig.31.VCAin-phase (a)andquadrature (b)profiles overa thinhorizontal platewith a strikedimension of 2r anda widthofr at a heightz/rof0.2forseveral valuesofinduction

Fig.32.VCPin-phase (a)andquadrature (b)profiles overa thinhorizontal platewith a strikedimension of 2r anda widthofr ata heightz/rof0.2forseveral valuesof induction

parametera.

parametera.

146

Frischknecht


The responseto a ribbon-like conductordecreases as the width becomes smaller than the loop spacing. A conductorwith a width one-quarterof the loop spac-

ing has a greatly reduced anomaly (Figures 34-37). Thoughthe anomaliesdue to theseribbon-likeconductors can be confused with the anomaly due to two parallel vertical plates, they can be distinguishedby varying the configurationor frequency. Responseof a Sphere.--Induction methodsgenerally are not very sensitive to the exact shape of conductors, particularly when they are not near one of the loops. Thus a sphere is sometimesa useful approximation to a roughly equidimensionalmineral deposit. :•ERP '

4O

i

et al.

Solutions for a sphere in a uniform field and in a dipolar field are given in Ward and Hohmann, Volume I. To obtain the plots shown here, the program SPHERE (Dyck, 1981; Dyck et al., 1980) was used to calculate the response of a sphere in terms of the following parameters'

13= o'P,0toa 2 = response parameter z/a, z/r, x/r, y/r a r z x, y

normalized distances radius of sphere loop spacing depth to center of sphere horizontal coordinates with respect to center of sphere

HCP

B ,

= = = =

i

,

i

,

i

,

i

,

i

i

,

i

,

i

i

,

!

'

,

I

,

i

,

i

,

i

.,

!

,

[

,

[

,

i

,

i

,

3

3O

2

2O

10

o

1.6

:10 lO

-20 16 -30 -40

-50

-60

,

,

I

,

,

•

-0,75 -01.5 -0.25 -1.5 -11.25'-11.0

,

I

,

,

-6

,

0 01.25 01.50'.•.•. ' •'.•.

.5

-1.5 x/r

x/r

34 (a)

33 (a) PERP B 30

2

20

1

ø"'ø'-4o

• -•

1

o

•

1.6

•

-10

• -20

1.6 25

8

-30

-4

lO

-5

-40

16 _

-6

-50

-1.5 -1125'-1'.0 -0'.75-0'.5 -0'.250

0'.25' 0'.5 0'.75' 1.0 1'.25' 1.5

x/r

x/r

33 (b)

34 (b)

Fig. 33. PERP B in-phase(a) and quadrature(b) profilesover a thin horizontal plate with a strike dimensionof 2r and a width of r at a heightz/r of 0.2 for severalvaluesof induction

Fig. 34. HCP in-phase(a) and quadrature(b) profilesover a thin horizontal plate with a strike dimension of 2r and a width of 0.25r at a height z/r of 0.2 for several values of inductionparameter

parameter

147


ProfilingMethodsUsingSmallSources

VCP VCA

0.8

!

.

i

i

,

!

'

]

i

i

i

,

[

'

i

,

[

'

0.7

0.6

•-

1

•

0.5

•-

o.4'

•

0.3

•

0.2

__c

0.1

v

N

•

o _

0

2.5

4

1.6, I

-0.1 -0.2 -1.5

1:25'1.5

-1.5

' 1.5 ! , , -ot•5 , • ' ot•5'oi•' ot7•'•.10' 1•.25 '-•t•'-•:o'-o.7•-o'.•

36 (a)

x/r

35 (a)

vcp

VCA I

' i,i,i i i i.i,i i i

,

0.9

0.8

0.7

0.6

lO

o.5

0.4 0.3

• 625

0.2

2.5

0.1 0

i

-1.5

.5 -lt25-1. -0•.75-0•.5 '-0•.25 ' •) ' 0•.25 ' 0•.5'0•.75 ' 1•.0'1'.25 ' 1:5

-1.25 -1.0

-0.75 -0.5

-0.25

0

0.25

0.5

0.75

1.0

1.25

x/r

x/r

35 (b)

36 (b)

Fig. 35. VCA in-phase (a) andquadrature (b) profilesovera thin horizontalplate with a strikedimension of 2r and a width of 0.25r at a heightz/r of 0.2 for severalvaluesof

Fig. 36. VCP in-phase (a) andquadrature (b) profilesovera thin horizontalplate with a strikedimension of 2r and a width of 0.25r at a heightz/r of 0.2 for severalvaluesof

induction parameter a.

induction parameter a.


148

Frischknecht

The term "response parameter" is used here instead of "induction parameter" to distinguishthat this parameter defines the sphere only, and contains no information on source-receiver coupling. Profiles are shown in Figures 38-41 for the HCP, VCP, VCA, and PERP configurationsfor a sphere having a diameter only one-tenth of a loop spacingand a depth of burial (measuredfrom its center) equal to its diameter. As is characteristicof in-line moving source profiles over a small target, essentiallytwo anomalies are observed, one when the transmitting loop is near the target and one when the receiving loop is near. The separation between the two responsesis more com-

PERP 5

,

i

et al.

plete for the HCP than for the other three configurations. For the parameters used, the HCP responseis larger than the VCP and VCA responses, which are about equal. Note that the VCP anomaly is entirely positive for the sphere, whereas, for a vertical sheet the anomaly is predominantly negative. Because of this property of VCP anomalies, a single VCP profile can be used to distinguish between a roughly equidimensional body from a long cylinder with about the same cross section. If the responseis measured from trough to peak, the PERP configuration yields a greater response than the other configurations. The responsefor the PERP configurationis about twice as HCP

B .

6

i

[

,

i

,

]

,

i

i

,

i

,

!

I

,

i

,

,

oX?, 0

2

•

3.2

•25 1.6,1

lO

10 16

• ----w = 10(:1

-lO -12

-14 -1.25

)(Jr

x/r

38 (a)

37 (a)

0.5

0

-0.5

..•

'•

-2

-:>.5

(•

-3 -3.5

-4

-8

-4.5

-1.25

-10

.

-•.•-•:•'-•'.o

,

•

,

-0.75

,

•

-o'.• -o'.•' o

i

-1

,

x/r

x/r

38 (b) 37 (b) Fig. 37. PERP B in-phase(a) and quadrature(b) profilesover a thin horizontal plate with a strike dimension of 2r and a width of 0.25 r at a height z/r of 0.2 for several values of induction parameter a.

Fig. 38. HCP in-phase (a) and quadrature (b) profiles over the center of a conductive sphere in free-space at several

response parameters, 13= CrlX0toa 2ßThesphere's diameter 2a is 1/10the loop spacingr, and the depthof burial z (measured from the center) is equal to its diameter



VCP

149

VCA

5

6

5

--p= 100 32

10

•' 1.

10

ø.• -1.25

-2

-1

-0.75

-0.50

-0.25

0

0.25

0.50

0.75

1

.25

-3 -1.25

' -'1 '-0•.75'-0•.50' -0•.25 ' • ' 0•.25'0•.50 ' 01.75 ' • '

x/r

1.25

x/r

39 (a)

40 (a)

VCP 2.5

VCA ,

2.25

2.5 • o• 1.75

/}

32

• 1.5 •

-•

Ill

1.5

v

1.25

3.2

1.0

(• 0.75

o

0.5

-0

025 0 -1.25

i

i -1

-0.75

-0.50

-0.25

0

0.25

0.50

0.75

i I

1.25

-1.5/

-•.•

x/r

.....

-;

•

-o• -o'.•o-o'.• o

....

o•

•

o'.•o o'.• •

,

/

•.•

x/r

39 (b)

40 (b)

Fig. 39. VCP in-phase (a) and quadrature (b) profiles over the center of a conductive sphere in free-space at several re-

Fig. 40. VCA in-phase (a) and quadrature (b) profiles over the center of a conductive sphere in free-space at several

sponse parameters, [3= •qx0toa 2. Thesphere's diameter, 2a,

response parameters, [3= •qx0toa 2. The sphere's diameter

is 1/10the loop spacingr, and the depth of burial z (measured from the center) is equal to its diameter.

2a is 1/10 the loop spacing r, and the depth of burial z (measured from the center) is equal to its diameter.


150

Frischknecht

large when the horizontal loop is near the sphere as when the vertical loop is near. When the traverse line is offset in the y direction, the amplitude of the anomaliesdiminishesrapidly (Figures 42-44). When y/r = 0.05, HCP anomalies are only about half their value at y/r = 0.0 and at y/r = 0.1 the anomaliesare only about one-tenth of their amplitude at y/r = 0.0. Note that, in practice, the responseof a sphereas small as the one consideredabove is likely to be small due to dependenceof the responseparameter on a 2. As z/r is increased, the double-peaked response characteristicdiminishesuntil the maximum response PERP

B

et al.

for the symmetricconfigurationsis observedwhen the sphere is centered between the loops. Profiles at a responseparameter of 100 for all four configurations are shown for z/a = 2.0 and z/r = 1, and z/a = 1.8 and z/r = 0.9 (Figures 45-46). In these examples the PERP configurationprovides the largest and most complex

anomalies.The shapesof the PERP quadratureanomalies are significantlydifferent then the shapesof the in-phaseanomaliesdemonstratingthat the responseis not purely dipolar. The amplitude of the VCP and VCA anomalies are about the same, but the VCP

anomaly has the simplestshape. The HCP responseis substantially smaller than the responsefor the other configurationsfor z/a = 2.0 (Figure 45). This result is in contrast to the results for smaller spheres or for

10

HCP- y/r = 0.05 6 Plan View 2

4

1

r

2

Traverse

•

0

o

•

3.2 o

-2

N

10

•

_

v

10

32

-4

--100

•'

-6

32 =1•

-8

-10 , -1.25

• -1

, .25

-1.5

-1.2

-0.9

-0.6

-0.3

0

0 3

0 6

0.9

1.

1.5

x/r x/r

41 (a)

42 (a) PERP

B

HCP- y/r = 0.05

1.5 f' •' •' •' i' f' I' ,' i' •'.

--1•3= 10

1

2

.

0.5 32--

o

.

-0.5 .

-1 -1.5

-2

3.2

'

32

ß

o=1•

-2.5 I -3

-3.5

.25' -• '-01.75'-01.50' -0•.25 ' •) ' 0•.25 ' 0•.50 ' 0•.75 ' • ' 1.25

,

' o ' ....

' '

x/r x/r

42 (b) 41 (b)

Fig. 41. PERP B in-phase(a) and quadrature(b) profilesover the center of a conductive sphere in free-space at several

response parameters, 13= •qx0•oa 2.Thesphere's diameter 2a is 1/10the loop spacingr, and the depthof burialz (measured from the center) is equal to its diameter.

Fig. 42. HCP in-phase(a) and quadrature (b) profiles offset a distancey/r of 0.05 from the center of a conductivespherein

free-space atseveral response parameters, 13= •qx0•oa 2.The sphere'sdiameter 2a is 1/10the loop spacingr, and the depth of burial z (measured from the center) is equal to its diameter.



HCP- y/r = 0.15

HCP- y/r = 0.1 0.8

!

i

,

i

,

151

i

!

I

,

i

,

i

,

i

,

i

,

i

0.25

,

'

i

,

i

,

i

,

i

,

I

,

i

,

i

,

i

,

i

,

.

0.6 0.20

0.4

0.2 0.15

0

-0.2 0.10

-0.4

-0.6 0.05

-0.8

0

-1.25

-1

-0 5

-05

-0 5

0

0.25

0.5

0.75

-1.25

1.25

I

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

I

1.25

x/r x/r

44 (a)

43 (a)

HCP- y/r = 0.15

HCP- y/r = 0.1 0.2

'

I

'

I

'

0.10 I

'

I

'

I

'

I

'

I

'

I

'

I

,

i

'

i

'

i

'

i

'

•

'

i

'

•

'

•

'

i

'

'

0.08

0

•

-...-

0.06

10

• 0.04

-0.2

-0.3 0.02 -0.4

0 I -1.25 -1 -0.75_0i.5 ' -0.25 0 0.25 015, 0.75 1 1.25 -1.25

-1

-0

5

-0 5

-0.25

0

0.25

0.5

0. 5

I

1.25

x/r

x/r

43 (b)

44 (b)

Fig. 43. HCP in-phase(a) andquadrature(b) profilesoffseta distancey/r of 0.1 from the centerof a conductivespherein

Fig. 44. HCP in-phase(a) andquadrature(b) profilesoffseta distancey/r of 0.15 from the centerof a conductivespherein

free-space atseveral response parameters, •3- (rlX0o0a 2.The

free-space atseveral response parameters •3- (rlX0o0a 2.The

sphere'sdiameter2a is 1/10the loopspacingr, andthe depth of burial z (measuredfrom the center) is equal to its

sphere'sdiameter2a, is 1/10 the loop spacingr, and the depthof burial z (measuredfrom the center)is equal to its

diameter.

diameter.

8

12


PERP

PERP

10

B

B

ø"'9' 4

-2

-4

-1

-2t , I , I , •, I , I , I , I , I , !,t -1.5

-1.2-0.9

-0.6 -0.3

0

0.3

0.6

0.9

1.2

-6

-1.5-1.2-0.9-0.6-0.3

1.5

0

0.3

0.6

0.9

1.2 1.5

x/r

x/r

46 (a)

45 (a)

1.75

2.5

PERP B

PERPB

1.50

1.25

1.5

1

VCA

0.75 0.50

0.25

-0.25

-1.5

-0.50

-0.75

-2.5

-1

-1.5

-3

-1.2-o.9

-o.6 -o.3

o

0.3

0.6

0.9

1.2

1.5

,

i

,

-1.5-1.2-0.9

i

,

I

,

-0.6-0.3

I

,

i

0

,

i

0.3

,

i

0.6

,

i

0.9

,

I

1.2

,

1.5

x/r

x/r

45 (b)

46 (b)

Fig. 45. In-phase (a) and quadrature(b) responseof a large conductive sphere in free-space at a constant response parameter [5of 100 for HCP, VCP, VCA, and PERP configurations. Depth z/a = 2.0 and z/r = 1.0.

Fig. 46. In-phase(a) and quadrature(b) responseof a large conductive sphere in free-space at a constant response parameter [5of 100 for HCP, VCP, VCA, and PERP configurations. Depth z/a = 1.8 and z/r = 0.9.

Profiling MethodsUsing Small Sources

153

othermodelssuchas a horizontallylayeredearthor a


half-plane.

The HCP largestnegativeresponseof a sphereof fixedradiusis plottedas an Arganddiagramin Figure 47. Results for the smaller values of z/r may be

HCP- z/r-

1.6, B- 100

]'l'l'l'l'l'l'l'l'l'l'l'l'l'l

somewhatin error; accordingto the users' manualfor ß

SPHERE, errorsare lessthanonepercentonly when z/a -< 2.0. However, the resultsappearvery consistent, and the responseincreasesrapidly as z/a de-

.

o -2

ø"P' -4

creases.

The primary criteria in identifyingan equidimensional or vertical pipe-like conductorare the strike lengthand profile shape.The effect of changingthe size of the sphereon profile shapeis illustratedin Figures48-49. Unlessconditionsare ideal, a single profileis inadequateto distinguishbetweena sphere and a cylindricalshapedor dike-likebody; however,

•

-6

'•

-8

.•

-10

c--

-12 -14

to distinguish between these conductors and a thin -16

plate shouldbe possible.The best way to confirmthe presence of a sphere or vertical pipe is to run a

-18

traverse normal to the original traverse across the

-20

i ,-1 i -0.8-0.6-0.4-0.2 , i , i , i , i , 0i ,0.2 i ,0.4 i ,0.6 i ,0.8 i , 1I ,1.12, -1.5i •1.2

peak of the anomalyon the originaltraverse.Alternatively, parallel traverses can be run on either side of

x/r

the first traverse.Also, use of two or more loop spacingsis very helpful. Rai and Verma (1982) have developeda set of interpretationdiagramsfor a

48 (a)

sphere.

HCP- z/r = 1.6, B = 100

Responseof Dikes, Prisms,and Other 3-D Models.--In

0.5

manyinstances slingram profilescannotbe adequately attributedto the simplesourcesrepresented by halfplanes,thin plates,and spheres.To interpretsucha

ß

i,i,i,i,i,i,i,i,i,i,i,i,i,i,it

0 -0.5

profile a knowledgeof the responsesof three dimen-

-1

sional(3-D)bodiesis required.Additionally, thepresenceof a conductivehostrock canmarkedlyalter the

-1.5 -2

HCP!

r/a = 2.0 I

,

i

•

I

'

i

'

I

'

i

,

i

,

-2.5

i

.

18 32 ß 8 -

0.75 = z/r

-3.5

10

56

I• -4

5.6

100

-4.5

-5 -1.5, '-1. '2":0.'8 ......... -0.4

0

"'.04' "0'.8'"1 .

1.5

x/r

48 (b) ,

0

I

2.5

,

I

5

,

I

7.5

,

I

10

,

I

12.5

,

I

15

•

I

17.5

,

I

20

,

I

22.5

,

25

In-phase(Z/Zo- 1)%

Fig. 47. HCP maximumnegativeresponse of a sphereof fixed radius (r/a = 2.0) in free-spacein terms of z/r and response parameter 13.

Fig. 48. Changesin HCP in-phase(a) and quadrature(b) profileshapewith changesin sizeof a conductivespherein free-spaceat a constantresponseparameterof 100, and a fixed depth z/a of 1.6.

154

Frischknecht


HCP- z/r = 2, 13= 100 1

a/r =5•

o -1 -2

o•

-3 -4

!

•

-5 .

•

-7

I::

-8 .

-9

-lO -11

I,,,,,,,,,,, ......... ,,,., ....

-12 -1.5

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.5

x/r

49 (a)

HCP- z/r = 2, 13= 100 0.5 ß

.

0

-0.5

-1

-2

et al.

response. Slingram responses to dikes, prisms, and other 3-D models in half-space or layered-earth hosts are used in this section to demonstratethe problems which may be encounteredin interpretingprofilesover more complicated structures. Profiles that do not match the responsesexpected from the simple models shown in the precedingsections are inherently more difficult to interpret. The arbitrary nature of 3-D structure, the lack of sensitivity of small loop profiles to the geometry of finite targets, and the difficulty of arriving at either numerical or scalemodelresultsprevent the widespreadapplication of complex models to slingram interpretation. The responseof small loop systemsto 3-D targets can be obtained from either scale models (Frischknecht, 1987) or numerical solutions (Hohmann, 1987). The numerical solution presented by Newman et al. (1986) has been used here to demonstrate a few simple characteristicsof finite targets. Dike and prism-like bodies may exhibit responses quite distinct from the half-planes, thin plates, and spheres presented earlier. This can be partially explained by the differences in coupling of small loop systems to targets whose width in the traverse direction is not small compared to the loop separationas was the casefor the ribbon and spheremodels. Additionally, the effect of a conductive host can cause pronouncedchangesin responses,and this effect can be clearly seen in the dike and prism models. If the body is more resistive than the host, then galvanic currents can be deflected around the body. A more conductive body in a conductive host will produce strong galvanic currents channeled through the body at certain frequencies.Vortex eddy currents within a body dominatethe responsefor highly resistivehosts and conductive 3-D bodies. (The effect of conductive host is discussed further in the following section, Interpretation of Slingram Data.) To illustrate some of these effects, we consider the prisms in Figure 50. For each prism the profile's x-direction is orthogonalto the body's strike along the y-axis at the body's midpoint (y = 0). Each body is centered at x = 0, and the z-axis is oriented downward.

-2.5

I , i , i ,-0i.8, i •-0.4i , i , 0I , I i0.4I i i ,0.8i , i ,1.2 i , i1.5

ß5 -1.2

x/r

49 (b) Fig. 49. Changes in HCP in-phase (a) and quadrature (b) profile shapewith changesin size of a conductive spherein free-space at a constant response parameter of 100, and a fixed depth z/a of 2.0.

This coordinate system is used for all models in this section. In all profiles the HCP and PERP slingram configurationsat 100 m spacings are used, and the computed results are the normalized vertical and horizontal magnetic field components.Also note that the model responsesin this sectionare presentedin terms of frequency and distance rather than the dimensionless quantities used in the previous sections. Discussionson parameterizing3-D responsescan be found in Frischknecht (1987, Volume I) and West and Edwards (1985).


155


X:O

(•100m••) Tx

Rx

I

25m

(a)

Phost= oOor Phost = 100 o.m 25rn

X=O

, (•100m-• Tx

Rx

(b)

Phost=

100

o.m

Phost=

100

o.m

Phost=

100

o.m

•-• 100m.•• X=O

•100m-• Tx

Rx

(c)

X=O

(d)

Fig. 50. Three-dimensionalprism models.


156

Frischknecht et ai.

We first considera vertical thin prism in Figure 50a in bothfree spaceand a conductivehalf-space.Figures 51-52 showthe slingramprofilesfor sevenfrequencies over a free spacehost, and Figures53-54 give the sameprofilesover a conductivehost. Obviously,the platesolutionwouldbe a poortoolto interpretthedata in Figures53-54, becausethe galvaniccurrentsalter the responseat most of the frequenciesfor the conductive host. The free space and conductive host results give similarly shapedresponsesindicatinga vertical dike, but the profile'smagnitudesare changed and their base levels are shifted by the current channeling. However, the plate solutionin free-spaceis still a useful and quick device to determineapproximate body size and orientation,even for conductive hosts; subsequent3-D modelingwould be neededto further studybodydepthandotherparametersfor this

HCP 1.15

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0.90

0.85

0.80

case.

The effectsof body orientationand sizein a conductive half-spaceare illustratedin Figures55 through60,

0.75

with the 3-D body geometry and other parameters denoted in Figures 50b through 50d, respectively. Figure 50b showsa prism in horizontalpositionat a

0.70

depth of half the slingramspacing,where the prism depth extent is equal to the depth to the top of the body. The resultingprofilesare givenin Figures55-56. The horizontal prism is then rotated to a vertical positionshownin Figure50c at the samebody depth, with the corresponding profilesgivenin Figures57-58. If the 100m prismis reducedto a depthextent of 50 m (solid lines in Figure 50c), or even extended to a semi-infiniteprism, the resultingprofiles (not shown here) are identicalto Figures57-58 within a fractionof a percent.Thus, for this particularverticalprism,the depthextent would be difficultto determinebasedon the given slingramspacingand frequenciesused. Comparisonof the profiles in Figures 55 thru 58 indicatesa slightbroadeningof the responseand position of side lobes at all frequenciesfor the horizontal body (Figure 50b) compared to the vertical body (Figure 50c). In general,the broaderprofile responses would be expected at certain frequencies for the horizontalbody, and could be usedto determinebody orientationif enoughfrequencieswere usedfor a given slingram spacing. Otherwise the profiles are quite similar in shape and magnitude,where major differencesonly occurdue to lateral extent of the body, and not to the vertical extent or conductanceof the prisms. To show how variations in the prism's depth can effect the profiles,the prism in Figure 50d is the same size and orientationas in Figure 50c, but is placed 25 m closerto the surface.As expected,the resultingprofiles in Figures 59-60 indicatean increasein amplituderesponseascomparedto the deeperbodyin Figures57-58. To further examine a change in body width at 25 m (Text continued on page 161)

12970

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HCP 0.06

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Fig. 51. HCP in-phase(a) and quadrature(b) profilesat several frequenciesover the model in Figure 50a in a free-space host.

ProfilingMethodsUsingSmall Sources PERP


0.30

-'• N

(1)

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53 (b)

Fig. 52. PERP in-phase (a) and quadrature (b) profiles at several frequencies over the model in Figure 50a in a free-space host.

Fig. 53. HCP in-phase (a) and quadrature(b) profiles at several frequenciesover the model in Figure 50a in a 100 fZ.m

host.

158

Frischknecht et al.

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Fig. 54. PERP in-phase(a) and quadrature(b) profilesat several frequenciesover the model in Figure $0a in a 100 12. m host.

55 (b)

Fig. 55. HCP in-phase(a) and quadrature(b) profilesat severalfrequenciesover the modelin FigureSObin a 100 12. m host.

ProfilingMethodsUsingSmall Sources PERP

HCP

A 1.45

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56 (b)

57 (b)

Fig. 56. PERP in-phase (a) and quadrature (b) profiles at several frequenciesover the model in Figure 50b in a 100

Fig. 57. HCP in-phase (a) and quadrature(b) profiles at several frequenciesover the model in Figure 50c in a 100

•.

m host.

•. m host.

160

Frischknecht

PERP

et al.

A

HCP 1.7

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59 (b)

Fig. 58. PERP in-phase (a) and quadrature (b) profiles at several frequencies over the model in Figure 50c in a 100

Fig. 59. HCP in-phase (a) and quadrature (b) profiles at several frequencies over the model in Figure 50d in a i00

f•. m host.

11. m host.

Profiling Methods Using Small Sources PERP 0.4

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depth, comparethe profilesin Figures53-54 with 59-60. Here we observeonly a slightmagnitudechange(but no shapechange)in thesetwo cases.This observationonly reinforces the point that body size, depth, and depth extent may or may not show up in horizontal profiles. Thus careful and detailed analysisand further 3-D modeling may be necessaryto refine an interpretation. A 3-D model of some geologic significanceis a near surface lens structure in the upper layer as shown in Figure 61. Horizontal profiling over such a body may mask deeper target bodies and is a cause of geologic noise. For example, the resistivities selected in Figure 61 may be representative of a clay lens or prism in the overburden over a conductive half-space. The corresponding profiles for this model are given in Figures 62-63. The highest frequency responses in Figures 62-63 exhibit the largest variation in both the in-phase and quadrature profiles, and diminish quite rapidly to a half-space response as the frequency decreases. This effect, of course, would be expected for a near-surface body in a conductive layered host. If horizontal profiling with frequencies less than 1600 Hz were used for this model the lens structure would not be easily detectable. Note the shape reversal in the quadrature profiles at the highest frequencies for both HCP and PERP configurations.This reversal indicates the presence of a near-surface thin conductive body in a less conductive host; however, other structures and hosts could possibly produce similar effects at high frequencies as well.

'

0.1

,

161

A buried valley model is illustrated in Figure 64, with the correspondingprofiles given in Figures 65-66. The in-phase and quadrature profiles for both HCP and PERP configurationsshow a depressionover the body at high frequencies, but then reverses in shape as the frequency decreases. This result is different from measurementsobserved in scale-model experiments in free space reported in Villegas-Garcia and West (1983; their Figure 13) for a similar model, where all profiles have the same shape at all frequencies. The effect of a conductive host versus free space on the profiles is clearly evident in this comparison. Similarly, Newman et al. (1989) also report the effect of conductive and free-space hosts on transient soundingsin boreholes near 3-D bodies.

12970

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Interpretation of Slingram Data -0.7

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x (m) 60 (b) Fig. 60. PERP in-phase (a) and quadrature (b) profiles at several frequencies over the model in Figure SOdin a 100 12. m host.

Several techniques exist for the quantitative interpretation of slingram profiles. The simple methods, discussed in the preceding sections, for interpreting profiles where a layered earth, conducting half-plane, or spherein free-spaceare appropriate models, may be applied to a broad range of targets. Methods exist for interpreting profiles acquired over more complex ge-


162

Frischknecht

ologies, but these frequently require extensive numerical calculations. The recognition of where simple models may be applied and the limitations of the various interpretive procedures is necessary for the quantitative interpretation of slingram profiles. Before about 1960the necessarytools for semiquantitative interpretation of slingram data were generally not available (Brant et al., 1966). The horizontal positions of the axes of conductors were determined from examination of contour maps or nested profiles. Werner (1958) described a method for estimating the width and the direction of dip for dike-like bodies. Bergmann (1960) mentionsa method developedearlier by Sture Werner and David Malmquist for estimation of the depths to conductorsfrom measurementsmade at two spacings. Beginning in about 1960, phasor diagramsfor half-planes, such as those given in Hedstrom and Parasnis(1958), became available and began to be used for determination of the depth and conductance of anomalies where thin sheets or half-planes were appropriate models. With the publication of extensive setsof interpretativediagrams,similarto the diagram shown in Figure 24, by Strangway (1966a), Ketola and Puranen (1967), and Nair et al. (1968), semiquantitative interpretation of standard slingram data became

common.

Numerous examples have been published demonstratingthe use of thesediagramsto interpret slingram profiles. Bosschart (1961) used phasor diagrams to estimatethe conductanceof targetscausinganomalies that fit a steeply dipping half-plane model. He also carried out model studiesusingtwo or more sheetsto fit data that are not adequately represented by a half-plane. Brant et al. (1966) publishedphasor diagramsof HCP and VCA responsesover a vertical dike for estimation of the width, depth, and conductivity. Cratchley and Evans (1967) routinely made conductance estimatesfrom slingrammeasurementsfor many

et al.

conductorsthat were originally discoveredby airborne EM surveying. Strangway (1966b) calculated the dip, depth, and conductanceof a numberof steeplydipping and flat-lying conductorsusing his published phasor diagrams. Ketola (1968) made an extensive study of the response of multiple parallel half-planes, dikes, folds, and magnetic bodies. In particular, he gave a number of examples of complex profiles that were fit by scale modelsusingas many as three half-planesor dikes. Parasnis(1971)estimatedthe depth and conductance of a number of conductors using both multifrequency, single-spacingdata, and single-frequency, multi-spacingdata. The agreement between the estimates for individual conductors was rather poor. Parasnis suggestedthat two reasonsfor his inconsistent results were modification of the anomalies by overburden and the inadequacy of the half-plane model in interpreting thick conductors. MacNae and Walker (1981) used highly conductivehorizontal sheet and sphere models to interpret HCP and PERP slingram results. Since the quadrature response was extremely small, conductivity could not be estimated. Peltoniemi (1982) gave a number of examples of the estimationof conductanceor conductivity from slingram and airborne EM data using half-plane and dike models. In some cases, satisfactory agreement between the two sets of estimates was obtained only when dike, rather than thin sheet models were used. Peltoniemialso givesexamplesof interpretationusing sphere models. Other examples of estimation of conductanceusing half-plane models are given in Ketola and Puranen (1967), Malmquist and Parasnis (1972), and Haren and Whiteley (1981). Finite strike lengthand depth extent of a conductive target can modify the slingram response. Negi et al. (1987) offer methods determined from scale model studies for correcting estimates of conductance and depth of conductive targets in resistive hosts. The X =0

_(•100m-•

TxT Rx

2m

E

1131 = 100O.m

20m I 13 body =40o,m L. I-'

320m

-J

--I

132=20 o-m Fig. 61. Three-dimensionalconductivelens modelin a resistiveoverburdenlayer.

Profiling Methods Using Small Sources PERP A

HCP 1.45

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I

'

I

'

I

'

I

'

i

,

0.2

1.40


163

101Hz 1.34

405 I

\

/

o

\

I

1.30

N

811

/

1.25

/

\ i x.,

• / \\ •/ 3242Hz /

c

1.15

-0.4

•

-0.6

_c

-0.8

"-

..

"'

1621

/ "'

3242

\

/

1.20

No

..........

\

/

•

-0.2

\

\

1621

12970

-1.0

1.10 5

-1.2

1.05

101

1.00

0.95

,

I

,

I

•

-400

I

,

I

,

I

-200

,

I

,

0

I

,

-1.4

I

,

200

I

-1.6

,

,

I

,

I

,

-400

400

I

,

I

,

I

-200

,

i

,

0

I

,

i

,

200

400

x (m)

x (m) 63 (a)

62

PERP A

HCP 0.3

'

I

0 '

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I "

'

..........

-0.5

..--.-..- 101Hz

0.2

-0.10

811Hz 40-

0.1

-0.15

//

:• o

:•242

•

-0.20

:•

-0.25

:•

-0.30

..811

•,,/••?//1621

o

-0.3

-0.35

-0.4 -0.40 -0.5

3242 -0.45

-0.6

-0.50 -0.7

I

I

-400

,

I

,

I

-200

,

I

,

I

0

,

I

,

I

200

,

I

,

I

,

I

-400

,

•

I

,

I

-200

,

I

,

I

,

0

I

,

I

200

,

I

,

I

,

400

400

x (m) x(m)

62 (b)

63 (b)

Fig. 62. HCP in-phase (a) and quadrature (b) profiles at severalfrequenciesover the conductivelensmodelin Figure

Fig. 63. PERP in-phase(a) and quadrature(b) profilesat severalfrequencies overtheconductive lensmodelin Figure

61.

61.

164


model studies indicated

Frischknecht

that the conductance

et al.

conductance

estimate

estimates

which are smaller than the true

values.

for profiles at a given spacingincreasestoward the true value and the depth estimate decreasestoward the true value as frequency increases. Several authors have noted the effect of finite strike length and depth extent on interpretations of multifrequency slingram field data over thin plate-like targets when using the halfplane model. Lodha (1977) observed frequency dependent conductance and depth estimates for the Gertrude West deposit in the Sudbury Basin. Additionally, the estimated conductancedecreasedwith increased spacing, probably because the half-plane model was used in interpretation although the deposit has a very short strike length, an effect demonstratedin Figure 22. The estimated conductanceof the Montcalm copper-nickel deposit (Fraser, 1978) is also inversely proportional to frequency (Figure 67), due to the finite depth extent of the body. Telford and Becker (1979) made similar estimates of the conductance and depth of the Iso and New Insco orebodies using multispacing, multifrequency data. For the Iso orebody the depth estimates were quite consistent but the conductance estimates consistently decreased with increasing frequency. A similar variation in conductance with frequency was

Conductive overburdens may cause phase rotations of the anomaly resulting in estimates of conductivity thickness to decrease with increasing frequency. Lajoie and West (1977) give an example in which the overburden clearly causesa substantialphase rotation (Figure 68). Joshi et al. (1984) present scale model results for use in interpreting profiles over thin sheets which

are either

insulated

from

the conductive

over-

burden or electrically in contact. The matter of electrical contact is of significancesince the channelingof galvanic currents can substantially alter the anomaly. The EM responseof a conductor contained within a conductive

host rock or in contact

with a conductive

overburden can generally be described by a combination of "galvanic" and "vortex" currents. The basic model, from McNeill et al. (1984), is shown in Figure 69. The vortex current (current induced in a confined conductor) dominates when the host-rock is resistive, and free-space models are adequate to describe the observed response. As the resistivity of the host rock is reduced, the host starts to affect the total response, modifying and eventually obscuringthe responsefrom individual

observed for the New Insco conductor. However, the

conductors.

The strength of the galvanic component is determined by current flow in the host medium. The cur-

depth estimate increased with spacing, probably because the conductor has a very short strike length. Depth and conductivity-thickness estimates may also be affected by use of thin-sheet models when the target is not electrically thin. Joshi et al. (1988) demonstrate the role of thickness in interpretation by use of scale models. Their study shows that the half-plane or thin-sheet model is accurate only if the target has a

rents which flow in the host rock are either diverted

around a target if the target is resistive, or channeled toward the target, if the target is conductive, by the action of chargesimpressedon the edgesof the target. If these galvanic currents are large compared to the vortex current, valuable diagnositic information is lost, and only the location and depth of the target can thickness lessthanhalfa skindepth,• = (2/Ix0to•r)1/2. be determined. In addition, conductive ore bodies can concentratenatural telluric currents flowing in the host A target whose thickness is greater than half a skin rock and increase ambient noise levels (Lilley and depth will produce larger anomalies and a phase Woods, 1978). rotation, that is, a change in the phase angle relating The induction number and response parameter, as the in-phase and quadrature responses,comparedto a previously introduced, are convenient dimensionless thin conductor with the same response parameter. Thus, using half-plane solutions for bodies which are parameterswhich describe the asymptotic behavior of electromagnetic fields in isolated conductors or laynot electrically thin will result in apparent depth and

X=O

•100m-•)f t Tx

Rx

I

20m

I

/

20m

Pl =20ø'm

I

dy = 1000.m L•

160m

:00o.m

P3 = 10o.m •1

Fig. 64. Three-dimensional buried valley model.

Profiling Methods Using Small Sources PERP

HCP 1.6

ß

[

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

0.4

'

'

I

A

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

'

I

,

0.2

1.4


165

.

_

1621 Hz

-

203 Hz

1.2

---._•,,/.___ • .... 405 - '"--- • 3242 203

-0.2

-0.4

0.8

.,'

' '...1621

-0.6

0.6

-0.8

0.4

3242 -1.0

0.2

6485

-1.2

12970

-1.4

-0.2

-0.4

,

I

,

I

,

-400

I

,

I

:

I

-200

,

I

•

0

I

,

I

,

200

I

-1.6

,

,

I

,

I

,

-400

400

I

:

I

,

-200

I

•

I

,

0

I

,

I

,

200

400

x (m)

x (m) 66 (a)

65 (a)

PERP

HCP 0.6

0.4

,

i

,

i

,

i

,

i

,

,

'

I

'

I

'

I

'

I

1.0

'

A

: I : I '

I '

I I I '

I '

I '

I '

I '

0.8

-

0.6

12970 Hz

203 Hz 405

0.4

811 No

0.2

-0.2

-

_

.

1621

N v

'-

6485

0

-0.4

._.

......... (o

203

3242 -0.2

-o.6

.....

12970

o

....

•

/

6485

811

--•'.:="•'

......

405

..---' ;'"'--•42

x_ •__._..•..-......

1621

-0.6

-1.0

-0.8

-1.2

-1.4

•

•-•---•

-0.4

-0.8

I

I

-400

I

I

,

I

-200

,

I

,

I

,

0

I

I

I

200

'

I

,

I

-1,0

,

65 (b)

Fig. 65. HCP in-phase(a) and quadrature(b) profiles at severalfrequenciesover the buried valley model in Figure

,

I

-400

400

,

I

,

I

-200

,

I

,

I

,

0

I

,

I

200

,

I

,

I

,

400

x (m)

x (m)

64.

_

66 (b)

Fig. 66. PERP in-phase(a) and quadrature(b) profiles at several frequenciesover the buried valley model in Figure 64.


166

Frischknecht

ered earths at low and high frequencies. The concept of a "current channeling number" was introduced in West and Edwards (1985), based on earlier work reported in Edwards and Howell (1976), to describe the asymptotic behavior of current channeling. The specific effects of galvanic current channeling for a particular target and source-receivergeometry depend on their relative dimensionsand the pattern of current flow. Typically for slingram configurations over a vertical plate in a less conductive host, the effect of the current channelingis small when the induction number in the host is much less than one. Larger values of the host induction number lead to a strong clockwise phase rotation, as pictured in an argand diagram with the quadrature on the vertical axis, and enhance the

et al.

response of the plate. The anomaly enhancement continuesuntil the responseis about double the freespace response, reaching a maximum for host inductions numbers around three (Hanneson and West, 1984a and b; Kwan, 1989). The effect of current channeling can be seen in the comparison of the response profiles over the conductive prism in free space shown in Figures 51 and 52 and for the prism in a conductive host in Figures 53 and 54. An observation made in Kwan (1989) was that the responseof the plate is more sensitive to a change in the host induction number than to an equal changein the plate induction number in the channeling regime. The additional

effect of a conductive

overburden

is to

further attenuate the amplitude and rotate the phase clockwise.

3555

Hz

O't = 25S

quadrature

1777 Hz • '-•'--'•.---y--

/x ,/•• i •,'

•

--•_%• _=

•'• (:/t :n4'• phase

The relative importance of vortex and galvanic currents depends on the geometry of the target and source-receiver. Generally current channeling is minimized in a configuration in which the source and receiver are closely spaced, such as the in-loop TEM configuration(Spies and Parker, 1984). Negi et al. (1987) list errors in interpretation which are likely to result if free-space slingraminterpretation is carried out in conductive areas. Specifically, the conductorappearsto be more resistive and shallower than it really is. This was confirmed in PEM (Pulse Electro Magnetic, see Appendix J) field results of Poddar (1982). Gupta et al. (1980) describedifferences in the effect of a conducting host rock on in-line and broadside slingram systems, and suggestthat a comparison of the responseof the two configurationsmay have useful diagnostic value. Scale-model results of a variable overburden layer are given in Joshi et al. (1984). The offset loop configurationsused for profiling are sensitiveto conductivity anisotropy in steeply dipping conductors. Such anisotropy is frequently found in laminated sedimentary and metasedimentary rocks, and may be present on both microscopicand macroscopic scales (Spies and Frischknecht, this volume). The most common highly anisotropic conductorsencountered in exploration using the slingram method are carbonaceousor graphitic sedimentsand metasediments. The sensitivity of the method to these formational conductor'sanisotropy allows the separationof their

too

m

I

Fig. 67. HCP slingramprofiles over the Montcalm coppernickel deposit. Estimates of conductance(crt) are inversely proportional to frequency due to finite depth extent (after Fraser, 1978).

anomalies

from

those

due

to massive

sulfide

bodieswhich are rarely anisotropic.The applicationof slingramto recognizinganisotropic bodies is shownin the Association

of Economic

Mineralization

section of

this chapter. Few model results of the response of a steeply dipping anisotropic conductor can be found in the literature. There is no analytic solution for a finite anisotropicbody and numerical modelingis difficult or


Profiling Methods Using Small Sources RATIO

intractable. A simple scale model result (Frischknecht and Mangan, 1960) does demonstrate the sort of response observed by Frischknecht in the Sierra Nevada foothills of California (see Association of Economic Mineralization section and Figure 103). The scale model is made up of four parallel vertically dipping aluminum sheets. Profilesnormal to, and at 15 degrees to the strike of the target body are shown in Figures 70 and 71, respectively, and should be compared to the responsesshown in Figure 23 for a single isotropic sheet. The traverses perpendicular to both the isotropic and anisotropic conductors are quite similar in general character. The anisotropy of the scale model can be distinguished when the profile traverses 15 degrees from the strike. While the isotropic conductor's general character differs only slightly from the perpendicular profile, the response to the anisotropicconductor has a reversed senseof polarity. This "reversed" anomaly on traverses at an acute angle to the strike of a steeply dipping target is the basis for distinguishingisotropic and anisotropic con-

222

Hz

METHODS

Methods in which the ratio of two components of the electromagnetic field are measured have been used since the beginning of exploration with inductive methods. In some cases the ratio of the same component at different locations is determined.

In other cases

the ratio of different components at one location is measured. Generally, only the latter technique is used with moving sourceand small fixed transmittingloops. This section

addresses

methods

in which

either

calculated

from the other set. The two methods

3555

used to obtain the other set. Wavetilt

Method

Originating in communication engineering, the term "wavetilt" generally designatesthe ratio of the hori-

Hz

•- -22 < -20 DiP 90"

:3 -15 O

œ

--

Z

-10

-5 0

a.

0

-'5

-10

-15

-20

PEAK

NEGATIVE

CONDUCTOR I

have

many common characteristics, and equipment designed to measure only one set of parameters can be

MIGRATION to

the

complex ratio between the vertical and horizontal components, or the parameters of the polarization ellipse are used. Either set of ratio parameterscan be

ductor in field results.

FREQUENCY

167

-25

-30

-35

IN-PHASE,

]

-40

-45

-50-52

%

THEORETICAL

(FREE

SPACE)

Fig. 68. Phaserotation of an HCP anomalydue to a thin vertical conductorbeneatha conductiveoverburden.Field resultsand theoreticalresponseof a half-planein free-spaceare plotted on a phasordiagramof HCP peak negative responseof a half-plane in free-space after Nair et al. (1968) (after Lajoie and West, 1977).

168

Frischknecht

zontal electric to the vertical electric field of a propagating wave. The term has been used in exploration geophysics for the complex ratio of the horizontal


magneticfield H r to the vertical magneticfield Hz,

Hr -i•

W= •zze ,

(29)

where q>is the phase differencebetween the Hz and H r . Generally the primary field is vertical when this

definitionis used.The reciprocalof W, Hz/H r is also used in induction prospectingwhen the primary field is horizontal, and sometimeseven when the primary field is vertical, especially when working at large distances from the source. In the absence of conductive

material

or misalignmentof the loops, the ratio Hz/H r is infinity. For convenience, methods in which either ratio is measured

will be termed wavetilt

methods.

Generally, two receiving loops rigidly mounted together in orthogonal positions are used in making wavetilt measurements(Figure 1). Level bubbles or a sighting device may be provided for orienting the loops. The amplitude ratio and phase difference of the

et al.

voltages induced in the loops are determined by a ratiometer

or other electronic

circuits.

The source is a

portable horizontal or vertical loop transmitter. If the transmitter loop is vertical it is oriented in the "axial position" so that its axis is in the vertical plane containing the line between source and receiver (Figure 1). Although a phase reference between transmitter and receiver is not required, it is advantageousto control the transmitter with a stable crystal oscillator. Then a similar oscillator

in the receiver

can be used as

a local reference for synchronous detection of the received signals. Since only the ratio of the two signals is required, slightphasechangesbetween the transmitter and receiver oscillators are unimportant. Specific systems that can be used for wavetilt measurements include the cross-ring, GEM5, and GEM8 systems described in the Appendices, and the Maxi-probe and B.R.G.M. MELIS system described in Spies and Frischknecht (this volume). Variations

of the wavetilt

method

have been used

for both horizontal profiling and sounding;although, in recent years, the primary application has been in sounding. One of the chief advantages of wavetilt methods is that a reference

link between

the transmit-

ter and receiver is not required. Also, in moving source profiling, errors in source-receiver spacingdo

rib

not cause false anomalies or substantial errors in the results. In-line fixed source measurements can be

made, and either the in-line or broadside mode can be

used in moving source profiling. One of the main disadvantagesof the method is its sensitivity to misorientation

errors.

(a)

Polarization Ellipse Methods

(b)

Fig. 69. EM fields and currents about a conductive plate in a conductive half-space. Solid lines indicate magnetic field components, dashed lines indicate electric field components and currents. (a) Illustrates the generation of vortex currents

Jv by inductive coupling with the component of dB/dt normal to the plane of the plate. Bv is the secondary magnetic field due to the vortex currents. (b) Illustrates the

generation of channeled currentsJg by galvaniccoupling with the component of the primary electric field E in the

planeof theplate.Bg is the secondary magnetic fielddueto the galvanic current (after McNeill et al., 1984).

As definedin this section, polarization ellipse methods include those in which the major and minor axes of the polarization ellipse or their ratio (ellipticity) are determined and methods in which both ellipticity and the tilt angle of the axes of the ellipse are determined. In measuring the parameters of the polarization ellipse, the loops are generally placed initially in one of the wavetilt configurations (Figure 1). However the NULL configurationmay also be used. Those methods in which only the tilt angle is measured are discussedin a separate section. The superpositionof the harmonically varying fields that have the same frequency but different directions and different phase angles results in an elliptically polarized field (Stratton, 1941; Born and Wolf, 1980). That is, during every cycle the end point of the field vector traces out an ellipse in space. Consider the

superposition of a horizontal magnetic field,Hxeiø•t,

andavertical magnetic fieldHzei(•øt + 4,)toproduce the ellipse shown in Figure 72. The total field, h', at an


169

arbitraryangleor'with respectto the horizontalcan be written

HZ

1

as


1-

+iH z sin or' sin q>,

(30)

Ellipticity, which is the ratio of the minor axis, h2, to the major axis, h•, is a very usefulparameter.The fields along the minor and major axis are 90 degrees out-of-phasewith respectto eachother so ellipticityis

wherethetimefactoreitøtisimplied.To findthelength of the major axis of the ellipse,H• = Ih•l, we needto find the value of or'for which Ih'l is a maximum (Ward,

1967). To do so, first find the squareof the absolute

defined as follows:

value of h'

Ih'l2= Hx2 COS 2 or'nt-2HxHz sinor'cos

'

I

•

I

'

I

I

I

'

I

'

I

(33)

hi

(31)

An expressionfor the minoraxisof the ellipsein terms

The angleat which Ih'l is a maximumis the sameasthe angleat whichits squareis a maximum.Differentiating equation(31) with respectto or'and settingthe result equalto zerogivesthe followingexpression for the tilt angle (Smith and Ward, 1974)'

I

h2 •.

e = -i

x cosq>+ Hz2 sin2 of'.

'

Hz 2'

øt =•tan1 (•xx) (32)

h' = Hx cos of' + H z sin a' cos q>

I

2•xxcos q>

of the angle otis'

h2 = -Hx sin ot + H z cos of cos q> (34)

+iH z cos of sin q>.

'

I

I

I

'

I

'

I

'

I

'

I

'

I

'

I '

I z

o I

I0 0 I

I0

-0

•

_•/• 3crn • 2c•,

3cm•

90 I

Z O

m -10 U.I

Icm

-- -20

80 I

o

i -30 UJ 7o--

Icm •

60--

I

o I

50-o

4

Sheets

0 081

cmx

AI

20 cmx

64 I cm

Co•I separation = 10 crn

Frequency = 500 c. p.s.

I CM

40

• I • I • I • I I I • I • I t I I I I I I I I I I I I I • I I 20

10

0

10

20

30

40

40

30

20

10

0

10

20

30

I

40 CM

SLINGRAM COILS HORIZONTAL AND CO-PLANAR

Fig.70.HCPslingram response overa verticalanisotropic body.Scalemodeltraverse isperpendicular to fourthin aluminumsheets.Response cannotbe distinguished froma wideisotropicdike(fromFrischknecht andMangan, 1960).

170

Frischknecht

If the ratio, equation (34) divided by equation (30), is rationalized and equation (32) is substituted into the result, the ellipticity, •, is obtained (Smith and Ward,


1974):

sin 4)

sin 2a+2•xx sin acos acos 4)+cos 2a(35)

et al.

More generally, to determine the parametersof the ellipse measurementsneed not be made only in the vertical and horizontal directions;the parameterscan be derived from measurementsof the field in any two arbitrary orthogonal directions in the plane of the ellipse (Mizyuk and Podzharyi, 1963). Referring to Figure 73, the fields h3 and h4 can be expressedas: h 3 --h•

The sign in equation (35) correspondsto the assumption that positive rotation is in the clockwise direction. Using equation (32) and (35), first the tilt angle and then the ellipticity can be calculated from the wavetilt

[H• cos20 + H22sin2 O]•/2eiq•l , h4 = -h•

parameters,Hr/H z and 4), where Hr is the H• field component. An alternative set of equationsfor calculation of ellipticity and tilt angle from the wavetilt is given in Appendix H and in Born and Wolf (1980). An expression for the axes of the ellipse can also be developed that does not involve the tilt angle (Born and Wolf, 1980).

z

cos O + h2 sin O

(36)

sin O + h2 cos !•

= [H• sin20 + H22cos2 0] '/2eiq'•

(37)

where

H2

H2

tanq•-•tan!9,tanq•2 = H• cot!9.(38)

•4o

z

uJ

uJ

o

o

UJ

•50

•o

120

20

uJ

Z ß.

io '•uJ o

:•

•oo

•)

9o

o

-•o 4

Sheets

AI

0 081 cm x 20cm

x 64

uJ

I cm

,! d•pp•ng vertically

i

o i

Traverse

Traverse

Co•l

separation

o

= 10cm

Frequency ß 500 c.p.s

CM

40

30

20

10

0

I0

SLINGRAM

20

COILS

30

40

40

HORIZONTAL

30

20

AND

I0

0

10

20

;50

40

CM

CO-PLANAR

Fig. 71. HCP slingramresponseovera verticalanisotropicbody.Scalemodeltraverseis 15degreesfrom the strike of four thin aluminumsheets.Responseshowsthe distinctivereversedanomalyof an anisotropicbody (from Frischknechtand Mangan, 1960).



The angles•1 and •2 are the phaseanglesof h3 andh4 with respectto h 1, and H 1 and H2 are the magnitudes of h 1 and h2, respectively.The quantity that is measuredis % the differencebetween•1 andq•2-Using the trigonometricidentity for the differencebetween the tangentsof two angles,we obtain tan•-tan•2

=

sin (½1 -- ½2) cos ½• cos ½2

171

Thus, the determinationof ellipticity is independent of the attitude of the receiving loops, provided the axes of the loops are in the plane of the polarization ellipse. In particular, differencesin elevation between the source and receiver do not generate errors in ellipticity. This invariant property of ellipticity is the principal reason that the polarization ellipse parameters are often used in preference to the wavetilt parameters.

sin •

(39)

cos ½• cos ½2

Substitutingequation (39) in equation (38) gives the expression, sin • =

Tilt angle and ellipticity can be determineddirectly or indirectly in severaldifferentways. Tilt can always be determinedby means of a loop equippedwith an inclinometerfor readingthe angle and a null detector to indicatewhen a minimum signalis observed. When the positionfor minimum signalis found, the axis of the loop is parallelto the minor axisof the polarization ellipse. Having found the tilt angle, the ellipticity can be calculated

H•H2

[(H• 2cos 20 +H22sin20)(H22 cos 20 +H• sin20)]TM (40)

Next, sum the squaresof equations(36) and (37),

H32+ H42:H•2+ H•,

(41)

where H 3 and H 4 are the magnitudesof h3 and h4. Also note that the productof H 1 andH 2 can be written as

(42)

2H•H2 = 2H3H4 sin

Adding equations(41) and (42) and taking the square root,

H 1 + H 2 = [H32 + H42+ 2H3H4 sinq•]•/2.(43) Subtractingequation(42) from equation(41) and taking the squareroot,

H 1 - H 2 = [H32 + H42- 2H3H4 sin,]1/2. (44) If H 3 and H4 are measuredabsolutely, solutionsfor H 1 and H 2 are obtained by adding or subtracting equations(43) and (44). Normally, measurementsof H 3 and H 4 are only relative. However, the ellipticity can be readily determinedfrom the ratio H4/H 3 and the angle

from

voltmeter

measurements

of the

magnitudesof the voltageswhen the axis of the loop is parallelto the minor axisof the ellipseandthenrotated 90 degreesto be parallel to the major axis. Without moreelaboratesignalprocessing,sucha systemwould have very poor signal-to-noisecharacteristics.The equipmentusedfor the Bieler-Watsonmethod(Parasnis, 1966; Telford, et al., 1976; and Watson, 1931) measuresellipticity by comparingthe outputs of orthogonalreceiving loops, provided the loops are rotated into the proper plane. Paterson(1973) and Telford and Becker (1979) describe a fixed source profilingsystemin which tilt angle and ellipticity are measured.A monograph(Svetov et al., 1968) discusses theory, instrumentation,interpretation, and casehistoriesfor polarizationellipsemethods.Zietz et al., (1976) mentiona Soviet five-frequencyprofiling device in which ellipticity is measureddirectly using analogcircuitrythat basicallycomputesequation(45). To measure tilt angle and ellipticity with the University of Utah 14-frequencysystem (Spies and Frischknecht,this volume, Appendix A; Ward et al., 1974)the coilassemblyis rotatedto obtaina null. Then phasesensitivedetectors,gatedby a local reference signal,are usedto measurethevoltagesinducedin the two coils; their ratio is the ellipticity. Before each readingis taken, the frequencyof the referencesignal for the phasedetectorsis adjustedto be the same as

1/2

h2 1+

+2 H4 2

H4

sin q•

-

1/2

1+

e=-i •-1 =[1 +(•33)+ 2(•33) sinq• ] 1+

-2

sinq•

(45) - 2

sin•


172

Frischknecht

the transmitter frequency. The polarization ellipse parameters are sometimes calculated from wavetilt measurements made with other systems such as the Lawrence Berkeley Laboratory EM-60 systemand the USGS systems described in the Appendices of Spies and Frischknecht (this volume). The GEM5 and GEM8 systemsdescribedin Appendix G of this chapter were designed primarily for determination of the polarization ellipse parameters; however, they actually measure the ratio and the phase difference from which the ellipse parameters are calculated. Polarization ellipse parameters can be measured using all of the survey techniques that may be used with the wavetilt method. In addition, those parameters can be measured using the fixed location vertical loop source technique described in more detail in the section on direction finding methods.

Hx Fig. 72. Polarization ellipse describingthe superpositionof harmonically varying fields having the same frequency but different directions, amplitudes, and phase angles.

-loops

et al.

Errors in Wavetilt and Polarization Ellipse Measurements

As for the slingrammethod, instrumental errors that are likely to occur in wavetilt and polarization ellipse measurementsdependon the specificequipmentused. Possible

sources

gonal directions(H 3 and H4) in the plane of the ellipse.

in wavetilt

measurements

include misalignments between level bubbles, inclinometers, or sightingdevices and their loops, dc bias errors in either of the signal channels, and differences in the sensitivitiesof the two loops or differencesin the gains of the two channels. One very useful check is to take readingswith the receiver loop assemblyrotated 180 and 90, or 270 degrees. There should be no change when the rotation is 180 degrees, since the output voltages from both coils are simply shifted 180 degrees. When the rotation is 90 or 270 degrees, the reciprocal of the wavetilt at 0 degrees should be obtained. If only the wavetilt is measured this procedure is not adequate to identify the cause of the error. If the system provides separate measurements of the coil voltages as well as their ratio, bias errors could be identified from measurementsat 0 and 180 degrees, or 90 and 270 degrees. Differences in sensitivity can be identified from any pair of readings taken with the assembly rotated 90 degrees. Misalignment of level bubbles or inclinometers with respect to their loops can be identified by the techniques described in the section on the slingram method. Bias errors can be identified and quantified by measurements at short source-receiver spacingsin a high resistivity area. If there are no bias errors, the voltage induced in the loop that is not coupled to the transmitter should be zero, provided the source and receiver are properly oriented. Misalignment of the two receiving loops with respect to each other or electronic coupling or "crosstalk" between the two channels are other possible sourcesof errors that are probably best identified and corrected by the manufacturer or in a laboratory. Errors in measurement of the phase difference are difficult to quantify although the tests described may reveal their presence. The phase difference can be checked by making measurementsat various spacings in an area where the responseis well known or where the response can be calculated with certainty from independent data. Data can be corrected readily for bias errors and differences in sensitivity of the two channels, if these quantities are accurately measured and are stable with time. However, preferably equipment would be repaired and maintained so that instrument errors

Fig. 73. Determination of polarization parameters derived from measurementsof the field in any two arbitrary ortho-

of error

do not occur.

Possibleinstrumentalerrors in determiningpolarization ellipse parameters are basically the same as those in determining wavetilt; they are, of course, the same when tilt angle and ellipticity are determined from



wavetilt. Generally, the same procedures can be used for identifying and characterizing errors in both wavetilt and ellipse measurements regardless of the specificequipment used. Remember that measuredor calculated values of ellipticity should always be independent of rotation of the loop assemblyprovided the axis of the loops are kept in the proper vertical plane. The measurementsof wavetilt and tilt angle are very sensitiveto errors in the orientation of the loops. If the surface of the earth is horizontal and the resistivity is very high, the amplitude ratio, IWl, should be zero or infinity, depending on the definition used and the orientation of the transmitting loop. Unless necessary procedures can be employed, topographic variations will

cause false anomalies

equivalent to Hx, yields Hr

horizontal magnetic dipole having a moment rn sin O. Using equation (3) and (5) Hr • = cot a = -2 tan O.

"

Frequently the transmitter is parallel to the line between

the source and receiver

and thus tilted at an

angle O, and the receiver loops are horizontal and vertical (Figure 74c). Using the procedure used to obtain equation (48), we obtain

Hr

6 cos20-

1

H-•-cotot= 6 sin 2O- 1tanO. (49) Finally, in the most general case (Figure 74d) the source and receiver

are at different

elevations

but the

transmitter loop is not parallel to the slope. Let O' be the tilt of the transmitter loop and O the angle between the transmitter and receiver; then

Hr = cot a

3 sin O cos O

Hz=cotot= 3sin 2©- 1'

(46)

3 cos•' cos• sin• + sin•'(3 cos 2•- 1) (50) 3 sinO' cosO sinO + cos0'(3 sin2 O- 1)

and division of equation (5) by equation (6) for a

verticaltransmitterloop and H r equivalentto Hy, yields

Hr 3 cos20- 1 = cot ot = Hz 3 sin O cos O'

(47)

where a is the tilt angle and O the angle between the transmitter

(48)

Hz

or noise in I Wl. In most

situations one of the four cases indicated in Figure 74 will apply. For the case shown in Figure 74a, where the loops are oriented horizontally or vertically, and the earth is nonconducting, division of equation (1) by equation (3) for a horizontal transmitter loop and Hr

173

and receiver.

In the next case to be considered (Figure 74b), the transmitter and receiver are at the same elevation (i.e., z = 0) but the nominally horizontal transmitting loop is tilted by angle O by a local topographicfeature and the receiving loops are oriented horizontally and vertically. The primary field at the receiver can be considered to be the superpositionof fields from a vertical magnetic dipole having a moment rn cos O and a

T

T

Assuming that the angles O and O' have been measured, corrections can be made by evaluating the appropriateequation (46 to 50) and addingthe result to the field data. Corrections can of course be avoided by keeping the loops oriented so that the transmitter loop is either coplanar or coaxial with one of the receiver loops. If the terrain is suchthat this can not be done by sighting, the same procedures as described for the slingram method may be applied (see Errors in slingram measurements section). In cases of other sorts of misorientation, such as rotations

about vertical

ter and receiver, the corrections can be much more

difficult to apply. Errors can be expected for 3-D conductive targets, and when the overburden or country rocks are sufficiently conductive for the induction number in them to be large. Errors due to misorientations of the loops are generally larger than when the earth is conductive, and accurate corrections are more difficult to make. Unless the conductivity structure is known a priori, so that its effect can be calculated, the only way to account accurately for misorientation of the loops is to include the actual orientation at each station in the interpretation process. With present methods and computing resourcesthis is impractical, so extra care in the field is highly desirable to avoid

Fig. 74. Examples of topographic variations which cause

misorientation ods.

tilt measurements.

axes in the

vertical plane including the line between the transmit-

false anomalies

or noise in wave

axes or horizontal

errors or else use less sensitive

meth-

174

Frischknecht


Interpretation of Ratio Measurements

Wavetilt and polarization ellipse responsescan be obtained easily by combining the HCP or VCA responsewith the appropriate PERP responsegiven in the slingram section. Similar argand diagrams and families of responsecurves will be obtained. As ratio methods have not been widely applied in profiling, these curves are not included

here.

Over the range of induction numbers and response parameters frequently encountered in moving source profiling, the variation of the ratio quantities of wavetilt and phase, or tilt and ellipticity, may be very small. The errors due to misorientation may well be larger than many geologic responses. Of the ratio quantities measured, the ellipticity, is the least sensitive to misorientation errors. Hoversten (1981) demonstrated that the ellipticity is less sensitive to loop misorientation than any other measurementcommonly made in profiling, including time-domain measurements. However, in profiling, where the loop orientation is not that difficult to control, use of the mutual coupling measurementsdescribedin the slingramsection will generally be simpler. One important exception to the preference of mutual coupling measurements may be the application of profiling methods to extremely detailed mapping of near-surface features such as in ground-water and toxic waste studies. In these instances, resolution of

very small features, and sensitivityto dielectricproperties are desired. To obtain this resolution, frequencies in the megahertz range must be used. The problems of the receiver electronic'sdrift stability, and the difficulty of providing an accurate measure of the primary field at these frequencies make the ratio measurements very attractive. The ratio measurements are independent of the primary field, and the

fields sensed by the two receiving loops can be switched in sequence into the same electronics, thus eliminating problems of drift. DIRECTION

FINDING

METHODS

Tilt Angle Method

Methods in which only the direction of the field is determined were developed and applied in the 1920s (Jakosky, 1929; Mason, 1929). Interest in these methods was not very high during the late 1930sand early 1940s. However, following the development of improved equipmentand techniquesin the late 1940s,the tilt or dip angle method came to be used extensively, particularly in the evaluation of airborne EM anoma-

lies in North America. In recent years, the use of direction finding methods has declined. The tilt angle or dip angle method is often called the

et al.

vertical loop or VLEM method because the NULL configuration is nearly always used with a vertical source loop. Moving-source tilt angle techniques employ hand carried source loops driven by battery powered transmitters (Brubaker, 1957). Fixed-source methods use large collapsible aircore loops and transmitters powered by engine-driven generators or small rigid aircore or iron-core loops driven by battery powered transmitters. A tilt angle receiver consistsof a hand-held loop and inclinometer, an amplifier, and a null detector, which may be headphonesor a meter. To achieve adequate signal-to-noiseratios in tilt angle equipment, narrow-band amplifiers are used. When headphonesare used as null detectors, the ear often adds additional discrimination between signals and noises, such as powerline harmonics. Synchronous detection and averaginghave generally not been used in tilt angle equipment because a direct reference signalis not available at the receiver. However, by use of stable

oscillators

in both

the transmitter

and re-

ceiver, synchronousdetection and averaging can be used to improve the signal-to-noise ratio. The time constant of the system must be short enough to allow manual rotation

of the coil back and forth to find the

null. In early equipment, receiving loops were often attached to tripods (Jakosky, 1929; Mason, 1929). However, the greater precision that can be obtained is generally not worth the extra weight and trouble of using a tripod. Most tilt angle equipment operates at one or two frequencies ranging from about 300 Hz to 5000 Hz. The maximum useable separation ranges from about 200-300 m for moving sourceequipmentto as much as 1000m or more for fixed sourceequipment. The vertical loop tilt angle method is used in both fixed source, and broadsideand in-line moving source modes.

In

the

most

common

of the

fixed

source

modes, the transmitter is placed at the center of an area containing several lines (Figure 75). In making each new measurementthe transmitter loop must be rotated about a vertical axis so that its plane always passesthrough the receiver station. Unless the terrain is such that the transmitter operator can see the receiver stations, this is generally done by mounting an orienting board around the vertical mast of the loop. A map with lines drawn between the transmitter and each receiver is placed on the board and used to orient the loop. The receiver operator tilts the plane of the receiving loop back and forth around an axis in the plane of the transmitter loop until the position for a minimum signal, or null position, is determined. If the receiver operator cannot see the transmitter, the receivingloop may be orientedby use of a compassand a list of predetermined azimuths for each station. Alternatively, the operatorcan place the receivingcoil vertically and rotate it until a null is determined. The


Profiling Methods Using Small Sources axis of the receiving coil then points toward the transmitter provided there is no conductive material within range of the system.Reorientationof the source loop must be synchronized with movement of the receiver from station-to-station. This can be done by sight or voice, radio communication,or by use of a fixed

time

schedule

for

measurements.

The

fixed

source tilt angle method might more properly be described as a "fixed locationsrotating source" method since the source is fixed in position but not orientation. The couplingbetweensourceand target is somewhat

different

for each different

azimuth

of the

source loop. A similar technique is used in tracing conductors that have already been discovered. The transmitter is located directly over and parallel to the conductor. Short traverses, approximatelytangentialto the transmitter position, are made at increasingdistancesfrom the transmitter to map the axis of the conductor. Lines for in-line, moving source, tilt angle measurements are placed at an angle of 15-60 degrees with respectto the strike of expectedtargets. The transmitter is oriented vertically in the plane of the line and the transmitter and receiver are moved in tandem along the line with roughly constant spacing.This mode is generally used only for reconnaissancework. In passable terrain, lines are not premarked or brushed and the receiver operator establishesthe line by pace and compass.

When the broadside tilt angle method is used, traverselines are placed normal to the expectedstrike of targets. The transmitter loop is oriented normal to the transmitter

line and measurements

are made with

transmitter and receiver moving in tandem down separate lines. 6enerally, the separations are small enough to permit visual or voice communication.

Successfuloperation in the broadsidemode generally dependson use of premarked lines. There are a number of variations of the three operating modesjust described. In early work the strike angle as well as the tilt angle was often measured (Jakosky, 1929; Slichter, 1932). The strike angle is measuredin the horizontal plane by rotating a vertical receiving coil about a vertical axis. The strike angle providesinformationabout the attitude of conductors not available from the tilt angle. However, measurement of strike angle is somewhat more difficult than measurementof tilt angle, and the additional information generally does not warrant the extra expensefor collection. Another technique is to use a fixed source and to make measurementsby moving the receiver only along the line through the plane of the coil. Traverse lines are laid out at an angle roughly 45 degreesfrom the strike of expected conductors. In the absence of lateral variations in conductivity, the nulls obtained with tilt angle systems are very sharpif the signal-to-noiseratio is high. If the system is being used near its maximum separation,nulls may be rather broad; there is a regionaroundthe null where only noise and no signal is detected. The reading is taken as the angle half-way between the angleswhere a small signal is first detected. In the presence of conductors,the field is elliptically polarized and only a minimum, not a true null, in the signal is observed. At the positionfor minimumsignal,the planeof the coil is parallelwith the major axis of the polarizationellipse. Errors in Tilt Angle Measurements

There is little reason for systematic instrumental errorsin tilt anglemeasurements.Possiblesourcesof error are misalignmentbetweenthe receivingloop and the inclinometer,or stickingor other malfunctionsof the inclinometer. Some of the tests suggested for wavetilt equipmentcan be used to check tilt angle equipment. Failure in the receiver circuitry could cause the instrument

BASE

LINE

GEOLOGIC STRIKE

Fig. 75. Vertical loop tilt angle field layout for a fixed source and roving receiver. The transmitterand receiver are rotated to the NULL configurationfor each measurement.

175

to be sensitive to electric fields

imposed on the loop or amplifier by the operators body. If a differencein measurementis notedwhen the operatorwearsglovesor otherwisechangesthe degree to which the instrument is touched, the equipment should be repaired; there is no satisfactoryway of correcting the data in this case. Misorientation of the loops generally causeserrors in the results. If the earth is highly resistive and the surface is level, the only error that can occur is if the planeof the transmittingloop is not vertical. However, if the surface is not level, azimuthal error in orientation of the transmitting loop causes an error in the measurement. Ward (1967) showed that, in the absence of conductive material, rotation of the receiving


176

Frischknecht

et al.

loop axis in a plane perpendicular to the line between

measurementscan be made fairly efficiently by placing

transmitter

one receiver

and receiver

results in an error which

is

only about one-third of the error causedif the receiving loop axis is rotated in a vertical plane. In practice for the operator to make an accurate estimate of the preferred plane is difi%ult. Differences in elevation between

transmitter

and receiver

also lead to an error

if the axis about which the receiving loop is rotated is not in the plane containing the line between transmitter and receiver.

If a conductiveoverburdenis presentor the bedrock is somewhat conductive, an error in transmitter loop orientation causes an error in measurement, even when

the

surface

is level.

Errors

are further

com-

pounded when the surface is not level. The kinds of errors discussed in the preceding paragraphs may be viewed as measurement noise. If the measurement noise is large the ability to detect small anomalies is impaired. Furthermore, since measurement errors often depend on topography, they are not random and they can mistakenly be identified as anomalies.

Modern tilt angle surveys are rarely encountered, and quantitative interpretation is infrequently applied. For assistancein the interpretation of tilt angle data the reader should refer to the more complete description of field techniquesand responsecharacteristicsin Ward (1967). The remainder of this section will concentrate on a moving source variant to the tilt angle method, shootback, which is much less susceptibleto errors, and is more commonly encountered.

the traverse line. To make a measurement, one of the units is used as a transmitter

to select either

the receive

and the other is used as a

receiver to determine the null position by rotating the plane of the loop about an axis perpendicular to the traverse line (Figure 76a). Once the tilt angle at null is read and recorded, the functions of the two units are

reversed and a second reading is taken and recorded (Figure 76b). In the absenceof conductive material the two angles are equal. However, in practice, the readings are taken so that one angle is recorded as positive and the other is recorded as negative. The final result is obtained by adding the two angles together. In the absence of conductive material the resultant angle is always zero regardlessof the slope between transmitter and receiver. When conductive material is present the two angles are generally not equal in magnitude, and they may even have the same sign. Thus in adding the two angles together a nonzero result is obtained.

• T

The shootback tilt angle method was developed for use in rough terrain where the maintenance of proper loop orientation and the preparation of lines required for other methods is a serious problem (Crone, 1966). Equipment designed specifically for shootback measurements (Appendix H) consists of two identical transceivers; each unit includes a loop with inclinometer, a transmitter, a signal amplifier and null detector, and a switch

the transmit-

The shootback method is always applied in the in-line moving-sourcemode. Both loops are oriented so that their axes are in the vertical plane containing

FIRST

Shootback Tilt Angle Method

ahead and the other behind

ter.

READING

R

or transmit

model. Various models of shootback equipment operate at up to three frequencieswithin the range of 390 to 5010 Hz. Normal spacingsvary from 61 to 200 m. Actually, by reversing the positions of the transmitter and receiver at each station, almost any tilt angle equipment or slingram equipment operating in the PERP configuration can be used for making shootback measurements. When slingram equipment is used for this purpose, in-phase and quadrature rather than tilt angle measurements are obtained. This is very cumbersome if only a single receiver is available. If two receivers and three persons are available, shootback

SECOND

READING

Fig. 76. Shootbackfield measurementconfiguration.Both loops are oriented so that their axes are in the vertical plane containing the traverse line.



Symmetric anomalies are obtained over symmetric conductors. As originally conceived, the method was deployed with the axis of the transmitting loop at an angle of 15 degrees from the horizontal plane. With this configuration, the response of a moderately conductive overburden is small. Currently, the preferred practice is to orient the transmitting loop horizontally as indicated in Figure 76. When the transmitting loop is horizontal, the results are expressedin terms of the angle measuredfrom the vertical plane, whereas when the transmitting loop is nearly vertical the results are expressed in terms of the angle measured from the horizontal plane. Not only is the shootbackmethod unaffected by the slope between transmitter and receiver when no conductive material is present, the method is also relatively insensitive to misorientation of the loops. For instance, supposethe axis of the receiving loop is in a vertical plane which is at angle t9 with respect to the correct vertical plane containingthe traverse line and axis of the transmitting loop. When the transmitting loop is horizontal the error in the anglebeing measured will vary as cos t9, which is negligiblewhen t9 is small. Responsefor the Shootback Tilt Angle Method

Results for several models including a horizontally layered earth and spheres are included in the next sections. Lin (1969) has provided extensive sets of scale model curves for rectangular sheets, discs, a U-shaped conductor and hollow cylinders in air; most of his results are for the nearly vertical transmitting loop configuration. Other model results for both horizontal and vertical transmitting loops have been released as interpretative aids or parts of case histories by Crone GeophysicsLtd. Responseof Layered Earth.--The shootbackmethod is sensitive to a horizontally layered earth, thus, shootback

measurements

can be used to determine

the

conductivity of a homogeneousearth or the parameters of a conductive overburden, provided measurements are made at two or more frequencies or spacings. The responsefor different ratios of thicknessto loop spacing, d/r, is plotted as a function of the overburden's induction number, B, in Figure 77 for a conductivity contrast of 100 between the overburden and underlying rock. Assumingthis is a good approximation to the actual contrast, measurements made at a single spacing at two or more frequencies can be interpreted by curve matching. First, the field data are plotted on a transparent overlay using the same vertical scale as is employed for the master curves. The square root of frequency should be plotted on the abscissaon a log scaleof the samelength as that of the

177

master curves. The overlay is moved back and forth in the horizontal

direction until the best match to the master curves is determined. Values for d/r and B can

then be interpolated from the master curves and the thicknessand conductivity of the layer calculated. The same diagram can be used for interpretation of measurementsin which the spacingis changed, by plotting responsesversus spacing at the same horizontal log scale. In finding a match to the master curves, we must remember that changing the spacing changes d/r. Thus, if the spacingis changedby a factor of two, the overlay must be shifted so that the data points fall on master curves that differ by a factor of two. The procedure can be generalized to use data taken with any combinations of frequency and spacing. Response of Spheres.--As was the case for the slingram method, the shootback response to a sphere is a useful approximation to a roughly equidimensional conductive target. The profiles shown here were again calculated using the program SPHERE (Dyck et al., 1980; Dyck, 1981) in terms of the parameters:

13= o'lx0coa 2 = response parameter z/a, z/r, x/r, y/r a r z x, y

= = = =

normalized distances radius of sphere loop spacing depth to center of sphere horizontalcoordinateswith respect to center of sphere.

160

140

-•,

120

"o

1 O0

o

c)_

80

o•

60

d/r = 0.50 ß

o o

03

0.40

ß

0.30

40

20

0.2• 9.05 .

_

0.5

1.0

2.0

i

i

$'.0 4:0 5.0

Induction Number

Fig. 77. Shootback responsefor a half-space overlain by a conductiveoverburden(•rh/•ro -- 100) as the ratio of overburden thicknessto loop spacing(d/r) is varied.


178

Frischknecht

Profiles over a small sphere (a/r = 0.05) and a much larger sphere (a/r = 0.5) are shown in Figure 78 for various values of the response parameter. For the small sphere the positive peaks are very similar but slightly larger than the negative lows. The cross-over between negative and positive responseoccurs when either loop is almost directly over the sphere. In some respectsthe shootbackcurves are similar to the PERP slingram curves for the same small sphere (Figures 38-41). The anomaly over the large sphere consistsof a single broad negative response. The lack of a small positive anomaly at the center may help distinguish between curves for a sphere and a deeply buried vertical

dike.

FREQUENCY

DIFFERENCING

METHODS

In frequency differencing methods, measurements are made at two or more frequencies and then the difference

or ratio

of the results

is determined.

Fre-

et al.

quency differencing may be used (1) to reduce errors caused by misorientation of the loops or surveying errors, (2) to eliminate the need for a reference link between transmitter and receiver, and (3) to reduce geologic noise. At the frequencies used in induction prospecting, the free-spaceor primary field responseis independent of frequency. The secondary field depends on frequency but asymptotically approaches zero as the frequency becomes very low. Thus, if measurements are made simultaneouslyor consecutively at two frequencieswith the same loop orientation and spacing, false in-phase anomalies due to misorientation of the loops or surveyingerrors can be eliminated by differencing the measurements. Ideally, the low or reference frequency,fr, is sufficientlylow that the secondary field at this frequency is negligible.If this condition is met in a slingram system and the measurementsare differenced, then the result is

H•

[H•, +H• Hp H•,x100% (51)

x 100% =

8.0

6.4-

Hp

-

4.8-

-

3.2-

-

whereHp is the correctprimaryfieldfor the configurationusedandHp andH s are the actualprimary !

-1.6 --10 •'•

-

-4.8 -

-

-3.2

-

-6.4 -

-8.0 -1.!

,

--32 t

-- 100

I , I , I , I , -1.2 -0.9 -0.6 -0.3

I 0

,

I • I , I , ! , 0.3 0.6 0.9 1.2

[ H-•

-

-

.5

xJr

78 (a)

!

and secondaryfields measured at the orientation and spacing used. Of course, H[ may differ somewhat from the secondary field that would have been measured if there were no errors in orientation or spacing. In practice, use of a reference frequency that is low enoughfor the secondaryfield to always be negligible is not feasible. Thus, when measurements are differ-

encedtheresultis somewhat lessthanH[/Hp. In comparisonwith the other three basic types of dipolar loop profiling methods described in previous sections, there has not been much interest in fre-

1.0

several response parameters 13= •rlx0toa 2 anddepth(z/a)of

quency differencing techniques until fairly recently. Frequency differencingwas used in early airborne EM experiments to eliminate effects of the motion of towed birds (Keller and Frischknecht, 1966). Recently, ground profiling systems that use frequency differencing have been developed (Johnson and Doborzynski, 1986); descriptionsof these systems,the GENIE and the EM-4, and examples of results are given in Appendix B. These systems are designedto operate without a reference link as well as to be insensitive to errors in loop orientation and spacing. Since a phase reference is not used, the systems measurethe differencein the amplitudesof the fieldsat two frequencies and normalize the result by the amplitude at the reference frequency. As contrastedwith wavetilt systems, which also operate without a reference cable, the amplitudes of the transmitted signals must be highly stable to provide accurate measure-

2.

ments.

0 -0.9

•'

-1.8

•

-2.7

m -3.6

•

-4.5

•__ -5.4 -6.3 -7.2 -8.0

xJr

78 (b) Fig. 78. Shootback profile over the center of (a) a small sphere(a/r = 0.05), (b) a much larger sphere (a/r = 0.5), for



As noted in Johnson and Doborzynski (1986), the instrument reading is approximately the difference between the in-phase responses at the signal and referencefrequencies. If the conductivity of a target is sufficientlyhigh for the in-phaseresponseto be nearly the same at both frequencies, the anomaly will be small and the target may not be detected. The GENIE and EM-4 systems are operated with a nominal HCP configuration, thus, the shapesof GENIE and EM-4 profilesare nearly the sameas in-phaseHCP slingram profiles and many slingrammodel curves can be used qualitatively in the interpretationof GENIE and EM-4 data. Johnson and Doborzynski (1986) published nomographs and procedures specifically designed for semi-quantitative interpretation of GENIE and EM-4 data.

At sufficiently low frequencies the quadrature response of various conductive structures is proportional to frequency and the in-phase responseis nil. Kaufman (1978b) suggesteduse of the following differencing operation to reduce the geologicnoise from poorly conductingstructuresrelative to the response from more conductive targets:

AQ=Q1 -•-: Q:,

(52)

wherefl andf2 are frequenciesthat differ by a factor of roughly two and Q1 and Q: are the quadrature responsesat frequencyfl andf:, respectively.If the induction

numbers

for

all conductive

units

within

range of the system are sufficiently small, AQ will be zero. However, if a target is present that has a sufficientlylarge induction number that the responseis not proportional to frequency, an anomaly in AQ will

179

worked extremely well in suppressingthe responseof a synthetic inhomogeneousoverburden having a peak response as great as 15 percent. Olm's model, shown in Figure 79, consisted of an inhomogeneous,conductive, graphite-epoxy slab representingthe overburden, above a much more conductive, cylindrical target in a resistive host. This is a situation that might be encountered in massive sulfide exploration in a shield area covered by conductive glacial overburden. The HCP responseof the overburden alone (Figure 80) has the general appearanceof the thin horizontal sheet of Figure 25, with a significant amount of additive noise due to the inhomogeneity of the sheet. The responseof the target alone in Figure 81 is much smaller than the peak responsesof the overburden, and in the combined overburden and target responsein Figure 82 the target cannot be recognized in the quadrature profiles and is badly distorted in the in-phase profiles. The profiles obtained from both in-phase and quadrature differencing in Figure 83 are quite striking. The differencedprofiles reveal the character of the cylindrical target clearly; the overburden responsehas largely been removed. Frequency differencing has clearly removed the overburden effect more completely than would be predicted by a study of thin homogeneous sheets. Apparently, the frequency responseof the inhomogeneoussheetdiffersfrom the frequency responseof the uniform sheet. This behavior may be explained qualitatively if the inhomogeneitiesin the overburden are considered to be a set of thin sheets with varying horizontal

dimensions

and conductivities.

Sheets hav-

ing dimensions on the order of the loop spacing or smaller will behave as confined conductors.

The most

be observed.

Botha (1980) made an extensive study of the frequency differencingtechnique as applied to sounding. Olm (1981) studied the applicationof frequency differencing in profiling. At low induction numbers the in-phaseresponseof a confinedconductordependson the squareof the induction number. Olm experimented with differencing the in-phase component using the expression fl

AI=I1-•2212,

(53)

whereI• and 12 are the in-phaseresponsesat frequenciesf• andf2, respectively.As appliedto scalemodel measurements,the differencing technique was effective provided the induction number of the geologic noise source was sufficientlylow. They noted that the effect of a uniform overburdenwas removed only if the responseat the highestfrequency was no more than a few percent. However, Olm found that the technique

•a =2.6cm I = 60cm

h = 6cm

Fig. 79. Scale model representing an inhomogeneous conductive overburden above a much more conductive cylindrical target in a resistive host (after Olm, 1981). a = radius, 1 = length, h = depth.

180

Frischknecht

et al.


6

IN-PHASE •ook

•

kH•.

'12.5 kH•

,-•-•/:-,I'

,'

25 kH•

'/•,:-.•I

I-i-',.-I 0.0

t ! • t ' • 2.4 x/r

-2

-4

/ QUADRATURE

100 kH 25

kH

'12.5 k H•.

6.25 3.• 25 k .I. 56z5

k

0

-'-"--

-2.8

•ß•, ß

/.,---- ß

O0

• 3.125 .5625 kH• kH•

2.4 x/r

N

-4

-6

-8

-10

-12

-14

V -16

Fig. 80. HCP slingramresponseof the overburdenfrom the model shown in Figure 79 (after Olm, 1981).



pronounced feature in the overburden responseis the edge anomaly itself. Such edge anomalies are often a major component of geologic noise in HCP measurements. The frequency differencing method is quite effective at suppressingthis large response. This is not too surprising; the response near the edge of a semiinfinite sheet might be expected to resemble that of a confined body due to the restriction in current flow caused by the edge. The frequency differencing method can be an effective way of treating multifrequency profile data to suppress geologic noise. Olm (1981) found that the application of frequency differencing to field data improved the definition of good conductors. Further experimentation is needed to establish the kinds of environments in which the technique is effective in reducing noise.

•

181

TIME-DOMAIN

METHODS

The loop-loop profiling methods described in previous sectionsoperate in the frequency domain. Most of the early instruments and interpretive procedures were developed in Scandinavia and Canada, where background resistivities are usually very high. These instruments generally operate at high frequencies (hundreds to thousands of hertz) which are easily generated and measured. However, they are often ineffective in other countries which have highly weathered, variable, conductive overburden: In these areas

the time-domain EM methods have become popular (see Nabighian and Macnae, this volume). Timedomain or transient electromagnetic (TEM) methods usually employ a large fixed transmitter loop, but a number of manufacturers now offer lightweight,

INPHASE

0

2.4

-5

'fO0 kH•

-6 -7

3 2

QUADRATURE

•

..... -2.8

• -1 ' •

-2 -

-4 -

,

, •".

,•'•,

, , , , , ,.•.,,••."•...• .,:• •0.0. •,]'

,

•• L--•1.5625 kHa '

'•' '•

•'

x/r

•6.25

•00

kHa

kH•

-5 -6

Fig. 81. HCP slingramresponseof the conductive cylinder from the model shown in Figure 79 (after Olm, 1981).

Frischknecht


182

et al.

x/r -2

-4

QUADRATURE /' ß

'tO0kH•---.•

.

I/'-'\

•2.5 . \ \ 6.25 kH•.•?x._. • .5625

',

.

'• ß

'

'

H•

•

• x/r

s.•25 kHe

.

6.25 kH•

.•

•00

kHa

-8

-10

-12

-14

-16

Fig. 82. HCP slingramresponse of theoverburden andconductive cylinderfor themodelshownin Figure79 (after Olm, 1981).


low-power transmittersfor small loop, portable oper-


ation.

The distinguishingfeature of time-domain methods is that measurementsare usually made in the interval between transmitted current pulses, and hence are

183

made in the absence of a primary field. This absence eliminates many of the cumbersome procedures needed for correcting variations in the primary-field described earlier for frequency-domain instruments. Note that frequency-domain methods employing

2.4

x/r

0

_

-2.8

-2

Fig. 83. HCP in-phase and quadrature difference profiles of the combined overburden and conductive cylinder responseshown in Figure 82 (after Olm, 1981).

184

Frischknecht

et al.


wavetilt, polarizationellipse,and frequency-differ- body, the late-time decay is exponential with a time encing parameters are popular becauseof their inherent insensitivity to variations of the primary field. Loop configurations and measurement procedures used in time-domain systems are consequentlymuch simpler than their frequency-domain counterparts. Transmitting antennas are usually multi-turn loops, between 5 rn and 20 rn on a side, which in many cases can be treated mathematically as vertical magnetic dipoles (VMD's). Receiving antennas are generally ferrite-cored coils or air-coredrigid loopswhich can be oriented either horizontally or vertically, thus achieving a HCP or PERP configuration. Much smaller intercoil separations can be used. Generally timedomain systems can effectively explore to greater depths than frequency-domain systems, although recent advances in frequency-domain equipment and methodologyhave lessenedthe advantage.Currently, under optimum conditionsa frequency-domainprofile can detect targets at depths as great as twice the loop separation,and a time-domainprofile may be sensitive to depths three to four times the loop separation. However for steeply dipping conductorsneither time nor frequency domain methodswill penetrate to these depths. Many of the examplesof time-domainprofilingused in this sectionare drawn from publishedcasehistories and were acquired with relatively large transmitter loops. These loops, as large as 100 rn on a side, are certainly not small, but demonstrate the response which might be expected from profiles with much smaller loops. Several time-domain systems can be used with small loops, although only the Pulse EM (PEM) systems,describedin Appendix J of this chapter, use loops the size of slingramloops in a slingram configuration.Time-domainprofilingcan be efficiently applied in suitableterrain with loops as large as 25 to 50 rn on a side in coincident

or central

induction

configurations (see Spies and Frischknecht, and Nabighian and Macnae, in this volume, for descriptions of these configurations).

constant,

-r = trix0Q seconds.

(54)

The term Q dependson the geometry and size of the confinedbody. For some common shapesthe values are:

Q - a2/,r2 for a sphere (WaitandHill, 1972a), Q = 2a2/-rr 2 for a cylinder(Velikinand Bulgakov, 1971),

Q = ad/3 for a spherical shell (Wait and Hill, 1972b), Q - ad/5.5 for a thin circular disc (Kaufman, 1978a), Q- dL for a semi-infinite plate (Velikin and Bulgakov, 1971), Q = dw/8 for a long plate with d • w (Kaufman, 1989), where a = radius, d = thickness, and w = width. L has the dimensionsof length but is related to the size of the

plate along dip and its location with respect to the loop. Time constants for more complex bodies are tabulated in Kaufman (1989). Values of Q for a coincident-loop system, from Svetov (1960) are:

Q = 2dL/,r2 for a semi-finite plate, Q = bd/,r2 fora plateinfinitein strikeextentbutfinite in dip, and

Q = 2dr/,r2 for a plateinfinitein bothstrikeanddip extent, in a dipole field (plate far from loop),

where b is the down-dip dimensionof the plate and L is the loop size. The exponential decay rate of confined conductors is contrastedwith the transient decay observed over infinite conductors,which exhibit a power-law behavior. For a horizontal conductingsheet of conductivity thicknessproduct S, the late-time HCP responseas given in (Kamenetskii, 1976) is

V(t) = 3mlm2}xo4S3/16,rt 4.

(55)

For a homogeneousearth the late-time HCP response

decays ast -5/2 andthePERPresponse decays ast -3 9

Processingand Interpretation Results

are

normalized

for

transmitter

moment

(ml = nllA1, where nl = number of turns, I = current, and A• = area), and receiver moment(m2 = n2A2), and presentedas normalized voltage in conve-

nientunitssuchasvV/Am4ateachsuccessive sample time. Data are normallypresentedas profilesof V(t) at appropriate sample times t. TEM profiles can be qualitatively interpreted in similar ways to slingram quadratureprofiles (such as shown in Figures 16-20). The rate of transient decay dependson the conductivity and dimensionsof the conductor. For a confined

ß

The type of decay (exponential or power-law) dependson the behavior of the eddy currentsdiffusingin the conductor. In a confined conductor the eddy currents diffuse inward until, at late times they stabilize in position, creating a singleeigencurrentsystem which can be approximatedby a simple ring of current. This current systemdecaysin time with a simple exponential behavior. In an infinite conductive medium, by contrast, the induced currents continue to diffusein spaceas well as time, resultingin a powerlaw decay. In the general case of a confined conductor con-



tained in a conductive host, the response is more complex. The observed magnetic field dependson the magnitude of both induction in the confined conductor and current channeling through the conductor into the host rock, as well as the coupling between these current systems. Singh (1973) and Lee (1982) show that the late-time response is, in general, an inverse power law which is characteristic of the host rock response. There is often, however, a time window during which the responseof the conductive target is readily visible, as described later in this section.

TEM Responseof a Dipping P!ate.--The dipping plate or half-plane model is widely used in interpretation to simulate a mineralized contact or conductive

dike. Lacking analytic solutions, these results are based on either

scale models

or numerical

solutions

(Annan, 1974; Dyck et al., 1980; Weidelt, 1983).

Loop-loop Response.--A series of type-curves for the responseof the loop-loop TEM configurationover a thin dipping dike are given in Ogilvy (1986). In this study, parameters of dip, strike, transmitter-receiver separation, depth-of-burial and conductancewere varied. The effect of varying dip is shown in Figure 84. A remarkable feature of these curves is the similarity between the loop-loop TEM results and the slingram curves shown previously in the section entitled "Responseof a half-plane and thin plates". The asymmetry of the profile is directly related to the dip of the conductor. Various features of the profile shape such as the updip and downdip positive peak amplitudes, the downdip negative peak amplitude, and the zero cross-over and half-width distances, which can be used for interpretation are shown on Figure 84. The depth-of-burial can be determined from the width of the anomaly using the nomogram shown in Figure 85. Given the depth-of-burial, the dip and conductanceof the plate can be determined from the nomogram shown in Figure 86. Similar nomogramsspecificallytailored to the Crone PEM system (Appendix J) are given in Rai and Verma (1984). They also show a conductance-aperture diagram for the PEM system (Figure 87), similar to those used in airborne EM interpretation (e.g., Palacky and West, 1973). As in airborne TEM systems, peak responseshifts to progressivelylater time channelsas the plate conductanceincreases.In fact, for extremely high conductance sheets, the response may shift to times later than the last time channel.

Other type-curves and nomograms for the PEM responseof a thin dipping plate are given in Bartel and Hohmann (1985), who showed that the conductance of the plate can be reliably estimatedby plotting the ratio

185

of channel amplitudes (Figure 88). Further studies are described in Rao and Kabra (1983). In the studies previously described, the assumption is made that the responseof the plate depends only on its conductivity-thickness product, and neither the conductivity or thickness of the plate can be resolved separately. As mentioned previously, this is commonly known as the "thin-sheet" approximation, and is valid as long as the thicknessof the plate is less than one-half the skin depth in the frequency domain, or diffusion depth in the time domain (Joshi et al., 1988).

Coincident-loop Response.•While not a slingram configuration, the coincident-loop configuration is widely used for TEM profiling. The TEM responseof a coincident-loop system is simply the response of a loop-loop system as the source-receiver separation goes to zero. Interpretation procedures for the coincident-loop configuration are given in Velikin and Bulgakov (1971); Rao and Bhimasankaram (1973); Kamenetskii (1976); Spies (1980a, d); Ogilvy (1987); and many others. Typical profiles for a dipping plate are shown in Figure 89. For a vertical thin plate the responseis zero directly over the plate. The separation between the peaks is equal to the loop size for shallow depths, and increasesfor greater depths. An interpretation nomogramis shown in Figure 90. Weidelt (1983) describes methods for depth determination based on matching field results and theoretical decay curves. In practice, the use of different loop sizes is most diagnostic.

Responseof a Sphere.•Numerical model studiesof a spherical conductor for the Crone PEM system are described in Bartel and Hohmann (1985). Typical profiles obtained over a sphere as conductivity is varied are shown in Figure 91. The response of a sphere is somewhat similar to that of a vertical plate; however for a sphere the central peak decays more rapidly with time than the flanking troughs. For the vertical plate the peaks and troughs both decay at the same rate (Bartel and Hohmann, 1985). Current Filament Approximation.•An alternative interpretational technique to the plate and sphere models describedabove is the current filament approximation describedin Barnett (1984). In this technique a least-squares inversion procedure is used to fit a circular or rectangularcurrent filament to the observed eddy current distribution; the result can be used to estimate the position, size, and attitude of the target. Although designed specifically for fixed-transmitter TEM systems, this procedure could easily be adapted for loop-loop profiling configurations.A logical extension of the common scheme is used for analyzing

186

Frischknecht

et al.

.0050(MS) .8


.0025-

1.2 1.6 2.0

2.6 3.4 4.2 5.0 5,8 7.0

- .0025-

-.0050-1 -.0125 I

.........

I

•

.........

T

I

.........

I'

........

!

....

DISTANCE (M)

ß

DEPTH

OF

PLATE

25.

DIP 90.0 ø STRIKE 90.0 ø CONDUCTANCE

50.0

-lOO

84 (a)

(MS) .8

.0128-

1.2

1.6 2.0 2.6 3.'•

.006•-

.0000-

•t.2 5.0 5.8 7.0

.

.

.

.

.ee6•-

.

.

.

.

.Ol28• ,

T

DISTANCE (M)

ß

DEPTH iuJ

c•

OF

PLATE

DIP 45.0 ø STRIKE $0.0

ø

CONDUCTANCE

50.0

25.

-lOO

84 (b)

Fig. 84. Loop-loop TEM profilesover a conductingplate in free-spacefor dip anglesof (a) 90 degrees,(b) 45 degrees.


Crone PEM data in which the locus of migration of inducededdy currentsin a conductoris mappedby drawing vectors normal to the measured PEM reDownloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

sponse vector (Crone, 1977). Conductive Host or Overburden.--The

effect of con-

ductiveoverburdenor hostrock was briefly discussed earlier in this section. At early times the TEM responseis controlled solely by the thicknessand conductivityof the materialoverlyingthe finiteconductor. This phenomenais commonlyreferred to as "screening". The finite conductor is first observed at a time given by

t • 10-6•d 2 seconds,

(56)

187

which correspondsto approximately one diffusion depth, and is independentof source-receiverseparation (Eaton and Hohmann, 1987;Spies, 1989).At later times the responseis affectedby inductionin the finite conductor, as well as induction in the host or overburden, and interaction between these conductors. This

interactioncanbe in the form of inductivecouplingor galvanic current channeling (West and Edwards, 1985). At sufficientlylate times the responseis dominatedby the hostresponse.A detaileddescriptionof the layered-earth(host) responseto various sourcereceiver configurations is given in Spies and Frischknecht(thisvolume).There is then an optimum time window in which the responseof an isolated

DELAY

TIMES

(MS) .8 1.2 1.6 2.0 2.6

.025.

.

q.2 5.0 5.8 7.0

-. 025

DISTANCE(M)

I

'-

r•

'TX-RXSPACING 50 M -r' I--

DEPTH OF PLA•E 25. DIP

STRIKE

c•

15.0 ø

90.0

CONDUCTANCE

58.0

84 (c)

-I-

0

Fig. 84, cont. Loop-loopTEM profilesover a conducting plate in free-spacefor dip anglesof (c) 15 degrees.Parameters of profiles shown in (d). Pt, Updip positive peak

amplitude;P2, Downdip negativepeak amplitude;P3, 84 (d)

Downdip positive peak amplitude; W, Zero cross-overdistance;and x, Half-width (P2 - P3) distance.(After Ogilvy, 1986).

188

Frischknecht

et al.

•/r

==0 4

0 6


.ø08

..'

2.0

Z/r

-0

2

.'

[(•=90ø] 1.5

1.0

...

ß

0.50'.2

•3

.,

3' = t/Po Sr

' O' .5 '

'

••.0 p•/p•

'

'

' 5.0

Fig. 85. Nomogram for interpretingdepth and conductanceof dippingplate in free-space.The conductanceof the plate is S and the source-receiver separation is r; x is defined in Figure 84 (after Ogilvy, 1986).

•=75•90 •

40

60 •

45 .

30 ø 60

80

15 ø

....../0 3.0

2.0

/•'

.25

.,,.

0

,,.:

ß=o7•

3' = t Ip 0 S r

1.0

I 0.7 i , 1.0

2.'0

I 3:0 410 5'.06'.0 , 8'.0, 10

, 15

20

Fig. 86. Nomogram for interpreting dip and conductanceof dipping plate in free-space (after Ogilvy, 1986).



conductor can be observed. Examples are shown in the Elura case history in the Exploration Through Conductive Overburden section and in Nabighian and Macnae (this volume). The effect of conductive

overburden

on the Crone

PEM responseof a thin conductive plate is described in Bartel and Hohmann (1985) and Rai (1985a, 1985b). The loop-loop response of a conductive host and conductive overburden exhibits a sign reversal which often falls within the time windows of typical TEM systems. Typical responsecurves for a thin horizontal sheet are shown in Figure 92. Variations in the conductivity or thickness of the overburden can cause wild fluctuationsin the shapeof the loop-loop profile. For this reason the coincident-loop or in-loop configuration is often preferred in these areas (see also the Elura case history in the section entitled "Exploration Through Conductive Overburden"). Numerical model results for the effect of conductive

overburden

on an

airborne TEM system are described in Bartel and Becker (1988). The effect of a conductive host rock in slingram responseswas described earlier in the section entitled "Interpretation of Slingram Data". A specificstudy of the time-domain responseis given in Rai (1985a), who

189

describedthe screeningof the TEM responseof a plate at early times, followed by galvanic current channeling effects at later times for shallow conductors (Figure 93). The current channeling was not observed for deeper plates. Rai gives other nomograms useful for interpretation in conductive areas. Rai and Sarma (1986) give nomograms for interpreting the in-loop PEM response of a three-layered earth. Comparison of Time-Domain and Frequency-Domain Systems

With many different types of loop-loop profiling systems commercially available, it is logical for the reader to inquire about the relative merits of each system. In general, there can be no simple answer; the choice often dependsas much on budget and logistics as on theoretical considerations. However, a number

of comparisons(made under limited conditions) have been published. Lodha and West (1976) and Lodha (1977), for example, compared the response of five frequency-domain and time-domain EM systems to a dipping thin conductive plate in free-space, as parameters such as depth, dip, and conductance(conductiv-

ioo

Z/r ß 6 .5 .4

3o

.3

3

.2 iooo

-

i0 3

I0 4

.

4/5

z _

3

I'I

•oo

z z

ß1-

io

0.3

0.1

I 0z

I03

104

I05

crt r (Srn)

Fig. 87. Conductance aperture diagram for Crone PEM system for a vertical plate model (after Rai and Verma, 1984).

I 3

I 10

I 50

I 100

o't

Fig. 88. Conductance nomogram based on ratio of channel amplitudes for Crone PEM system (after Bartel and Hohmann, 1985).

190

Frischknechtet al.


e(t)

e(t)

pv

I

A 65O

$0•• t=0.57ms

10o /I1o--

1.1ms

I III!

,

•'"'""• ........

20m Loop

......

20m Loop

0o e(t)

pV

!

A

t

= 0.57ms

e(t)

pv

I

A 60

"0.57ms

0.79ms

4

0.79ms

I

rn••,•••... 2.3

4.1ms

d

20m

o

Loop

20m

Loop 'L

(20mLoop, o-1 = 90 S/re,o2 = 0 --,:o d2 ' 2m d I -- lorn, d 2 -- 2m) 0

&,.',•0,. 2

10 20 30 40 METERS

e(t)

T

PROFILES OVERA THINPLATEFORDIFFERENT DIPS

Fig.89.Coincident-loop profiles overa dipping platein free-space (afterSpies,1974).



191

(100-150 m) due to the frequencies and separations selected for this study. A full study of this type should consider further aspects such as instrumental noise, external interference, and the influence of a conducting host rock or overburden (often referred to as geologic noise). One study which assessedthe effect of geologic noise on certain systemsis given in Eaton and Hohmann (1987). They concludedthat time-domain systemswere inherently less sensitive to geologic noise than frequencydomain systems.However, Hoversten (1981) showed that frequency-domain systems measuring ellipticity

ity-thickness product) were varied. Instrument sensitivities were taken into account in estimating depth penetration and detectability. An example of profiles obtained with the five systemsover a vertical plate is shownin Figure 94. The most significantresultfrom an interpretational standpoint is that the time and frequency domain results for a given source-receiver configuration have similar shapes. Thus, the HCP configurations:PEM, slingram, and INPUT, produce symmetric anomalies, while the fixed source UTEM and turam configurationsproduce asymmetric anomalies. The slight asymmetry of the INPUT anomaly is due to the receiver being located lower than the

were

less sensitive

to errors

in coil

orientation

transmitter.

The sensitivity of the five systems to changes in conductanceof a vertical plate at 60 m depth (30 m for slingram) is illustrated in the conductance-aperture diagramsof Figure 95. All systemswere able to detect a plate with conductancebetween 2 S and 1000 S or above, with the exception of the Input system which is limited to about 400 S (this is due to the inability to measure to sufficiently late times on a movingplatform system). Additionally, Lodha concludedthat the amplitude for all systems was highest for a horizontal plate and lowest for a vertical plate. The depth of detectability was largestfor fixed-transmitter loop systems, due primarily to the larger source size (300 m), and smallest with the HCP slingram system

should be remembered that, methods such as fre-

quency differencingcan be applied to limit the effects of many types of geologic noise.

APPLICATIONS

AND

EXAMPLES

The range of application of small loop profiling methodshas greatly expandedsince the early days of geophysicalprospectingwhen profiling was used primarily in exploration for highly conductive sulfide

'•'- 9 55

iII/1•

," • i

,,

,,

1o0o.0

ii

,,•

,• P•

=o.l /

•

•

./•

•

/

• v

',

-

5

V

,

' .-'

./•

/

. '

.-"

255

.......................

10

[ =9• 1

75' 2

•

318

. -'''

_

92

.............30'

60'

45" 5

15'

10

P2/P,

Fig. 90. Dip and conductance nomogramfor coincident-loop TEM systemfor dippingplatemodel(after Ogilvy, 1987).

or

intercoil spacingthan were time-domain systems.McCracken et al. (1986) presenteda detailed comparison of frequency-domain, and time-domain step and impulseresponsesystemswhich can be usedin studiesof the noise sensitivity of various systems. Finally, it


192

Frischknecht

deposits. In this section applications are grouped according to the overall objectives such as mineral or ground water exploration. From a purely scientific point of view, it might be more logical to discuss applications in terms of geologic environments and geologic problems to be solved. However, the geophysicist must generally think first of the overall objective before considering how to approach the problem technically and scientifically. In this section, applications are divided into minerals exploration, energy exploration, groundwater exploration, engineering investigations, environmental investigations, archaeological investigations, and general geologic mapping. Obviously, some of these categoriesoverlap and their selection is rather arbitrary.

et al.

Mineral ResourceExploration

Profiling methods have been most commonly applied to mineral resourceexploration problems. Due to the long association of these profiling methods to mining geophysicalproblems the literature is rich with case histories describing exploration for a variety of mineral resources. The following subsectionson minerals exploration addressboth experience in exploration for specific mineralized targets, and considerations due to the host geology. Massive and nonmassivesulfides,vein deposits,iron, uranium, and kimberlite exploration are specifically dealt with. The geologic associationsof economic mineralization and examples of distinguishing economic from noneconomic geologicconductors are discussed.The sections

tO$

tO:'

1 S/m

3

ppk •o'

tO•

o

•

0

4

6

' 200

0

200

400

600

•

800

-200

d•stance

0

200

400

600

800

(m)

tO$

i0•

lO s/m

30 S/m

i0• I

0

6 7

8

h-•o•)m..I 50m

Fig. 91. Crone PEM responseof a conductingspherein free spacefor various values of conductivity (after Bartel and Hohmann, 1985).



on exploration through conductive overburden and exploration for conductive targets in conductive host rocks are presentedin this sectionbut are applicableto a much broader application. The section concludes with a brief sectioncomparingvarious dipolar profiling techniques,and comparisonto other geophysicalsurvey results. Exploration for Massive Sulfide Mineral Deposits.-Historically, the most common use of dipolar EM profiling methods is exploration for conductive massive sulfide deposits. Often, surface EM profiling is used to pinpoint the location and to help characterize conductors that have been detected by airborne EM surveys. In other cases, surface profiling is used as a primary exploration method, particularly when the area of interest is relatively small. Most of the massive sulfide depositsthat have been discovered

with

EM

methods

are

associated

with

submarine volcanic rocks and are of volcanogenic origin. A smaller number of deposits are associated ! 000

--

193

with other types of rocks including mafic and ultramafic intrusive rocks and sedimentary rocks and have been formed by a wide variety of processes. Most massive sulfide deposits are composed primarily of pyrite or pyrrhotite and their conductivity depends largely on these minerals. Copper and lead bearing minerals such as chalcopyrite and galena are conductive, and sometimes contribute significantly to the overall conductivity of a deposit. Sphalerite is generally a poor conductor and conductive minerals of nickel, silver, and other metals generally do not occur in sufficient quantities to have much influence on the conductivity of a deposit. Generally, pyrrhotite deposits are more conductivethan pyrite deposits;Gaucher (1983) observedthat thin seamsof pyrrhotite are often very good conductors whereas many thick pyrite lenses are poor conductors. In small samples pyrite, pyrrhotite, and many other sulfide minerals are highly conductive. However, the bulk conductivity of a deposit dependsmore on how well individual grains and the deposit as a whole are connected electrically. Siikarla (1964) observed that a small amount of sulfide in schistose rock can form good continuous conductors. Thus the metamorphic history of a deposit is likely to have more influence on its conductivity than the sulfide content and original conditions of deposi-

1oo

2

O'dr -'-

1o

•ooo

T H = aH/,Or

•oo

sm

T•,= 20-1 ms

8O

_J LU

z/r=

Z Z 200

t•

1.o

40

•n o•r host-

20

•:

20

spoce

Plate

Plate •n medium

i-

--

o.• Half-

0 -20

-4O

0.1

0.1

1'0

TIME

(ms}

Fig. 92. Crone PEM response of a thin horizontal sheet simulatingconductive overburden. Positive values shownby dashed lines (after Rai, 1985b).

1(5•

I0

i0-•

Fig. 93. Effect of conductinghost rock on the TEM looploop step responseof a thin conductingplate of thicknessd. z is the depth of burial of the plate, r is the source-receiver

separation, Tt,= %,dix0r,andTH = •rhlxO r2 (afterRai, 1985a).

194

Frischknecht et al.


UTEM

TURAM

SECONDARY

TURAM

FIELDS

REDUCED

_

& PHASE

10

CH. 4

•

1.20

0

RATIOS

(Ax=30

m

-

1.1o

u. -20 ß

H. 10

n -10

1.oo

)00Hz

Z

-4O

-20

.. 1ø F

, 4 !

20-

I

CH.

2

3

.-,

m

I

.•

CH 1

0

I

•

5.00

-

2.50

2

•=

I ••

-

10

CH. 4

,-,

o.9o

.,o.o,

o

½•-10

•,

•

•

50H•

800Hz

1

' • • • 0.00

-

ß

co

-1

-20

-4

;

-2

0

2

4x

: :: :;:: ::: ': • : : ::: : :;•' ,, x (m)

-5.00

10 2

-2

I

:':'

0

I

2

'I f-I--= X (m)

(b)

4 x 102

-• •

I

• :

-2 ',':

0 •:::','::;:;'4;;;•

2

4x

10;

x (m)

MODEL (c)

THIN

800 1000

SHEET

(600m x 300m)

600

400

INPUT

CPEM

200

H1

L=60m

122

m

100.

HLEM

80 •

L = 180m

20. 10

3 s• 10 -10

3

0

4

0

5

0

8

0

3

_

2

10

•

•

O

0

4

-20

•

2

1,•=•.

0

-10

-'/

3555Hz•

0

I

,

L

-2

-1

2 x 102

-4

-2

x (m)

(d) I

0

2

4 x 102

-O

x (m)

(e)

•

-3

0

3

5 X 102

x (m)

(f)

•

Fig.94. Response of UTEM, Turam,horizontal loop,CronePEM andINPUT (65m) systems to a verticalsheet at 60 m depth, o-d = 50 S (after Lodha, 1977).

195

Profiling MethodsUsing Small Sources


TURAM

SECONDARY

FIELD

UTEM SEC. FIELD ANOMALY QUADRATURE STEP RESPONSE

I0 •J f•oo

/•

•

•

-•o •ø•-• ]

'•f/09 8 _

O.I

• •

' '

•

IO0 •

•0ß •0

, CONDUCTIVITY

THICKNESS

....... , .......

•o

•oo

•ONDU•TIVITY

(S)

•ooo

THI•N•SS

(S)

95 (b)

95 (a)

CRONE

HORIZONTAL LOOP VERTICAL

QUADRATURE

PEM

LOOP MODEL

A SCALE

13555

r ,180m

S =60Ore, W ' 50Ore, D ,60m IOOO

B SCALE r ,90m

444

s,500m, r,45m

222

S,150m,

Hz

VERTICAL S•ET

w,150m,

D,30m

C SCALE -

W=75m,

D,15m

M•EL

S=600m,W-300m,D=•m, r ,leo

IN- PHASE

3555•

,,,%f / ,. //%( ,/

e ',

.

' ......

i

• CONDUCTIVITY

........

i

IOO THICKNESS

C '

.....

'1'•o (S)

95 (c)

,• i,

•ll

IO

........

CONDUCTIVITY

I

"',•o ......... IOOO '!

IOO

........

THICKNESS

I

IOOO (S)

95 (d)

Fig.95.Conductance aperture diagrams forvarious time-domain andfrequency-domain systems fora verticalplate model, (a) UTEM; (b) Turam; (c) HLEM; (d) Crone PEM.


196

Frischknecht

tion. Recent processessuchas weathering,permafrost action, and fracturing also have large influenceson the conductivity of sulfide deposits. According to usual definitions, massivesulfidedepositscontain at least 50 percent sulfidesby weight. In many case histories the actual sulfide content was not known or is not given. However, except as noted, most of the deposits mentioned in this section probably would be defined as massive sulfide deposits if the necessaryinformation were available.

Strangway (1966b) tabulated conductance estimates for a number of massive sulfide deposits and concluded that most deposits have a conductance in the range of 1 to 300 S. Conductances tabulated by Wellmer (1981) and Gaucher (1983) fall within this range. No doubt some deposits have conductances less than 1 S, but they would generally be poor EM targets. A few deposits have conductances much larger than 300 S. The conductance of the Burnt Nubble pyrrhotite (Labson et al., 1984) is estimated from slingram measurements to be several thousand siemens.

Boldy (1981) made a study of 115 significantPrecambrian volcanogenic massive sulfides discoveries in

,oopoo BARRINGER

VERTICAL

I0,000

SHEET

S=600m, W=300m,

INPUT

MODEL

D:65m

.-.

•

IOOO

a' i Ioo

I0 CONDUCTIVITY

I00 THICKNESS

'1000 (S)

et al.

Canada; he found the mean long and short axes to be 500 rn and 150 m, respectively, and the mean thickness to be 25 m. Wellmer (1981) gave similar information on the strike length and thicknessof a selectednumber of Archean deposits. Conductance estimates are frequently used to select targets which have the highest economic potential for drilling or other further investigation. Gaucher (1983) found that there is a correlation between

sulfide

content

and conductances

but

that the width is a better predictor of sulfide content. He found that wide medium-to-good conductors are only marginally better than wide poor conductors. Paterson(1968, 1969) noted the importance of width in determiningthe potential value of a conductor and also observed that Canadian deposits containing minerals tend to have a lower bulk conductivity than those composedof barren sulfides. Considering the results cited for CanadianPrecambrianvolcanogenicdeposits and the discoveriesof poorly conductingbut economic massive sulfides in many parts of the world (for example see Gunn and Chisholm, 1984), conductance shouldbe used with great caution in screeninganomalies for further investigations. With a few exceptions, such as Mason (1929), most early papers on electromagnetic methods that contained examplesof field data treat techniquesin which large horizontal loops or grounded wires were used. Starting in the latter part of the 1950's numerous case histories were published in which dipolar source profiling methodswere employed. Early examplesfrom Scandinavia include those reported in Werner (1947), Malmqvist (1958), Kahma and Puranen (1958), and Laurila (1963) in which the slingram method was used. Early examples from Canada in which the tilt angle method was used include those reported in Bergey et al. (1957), Fleming (1957), Seigel et al. (1957), Ward (1957), and Cheriton (1960). Paterson (1957) measured ellipticities as well as tilt angle. Although the slingram method was not used in Canada as early as the tilt angle method, case histories on its application in Canada were publishedat about the sametime (Bragg, 1959; Joklik, 1960). Pemberton (1989) includes a variety of slingramresponsesover Canadian deposits.The use of the shootback tilt angle method was described in Crone (1966) and Brock (1973). Applications of LIN instruments in mineral exploration are mentioned in Parasnis (1966), Peltoniemi (1982) and Jurick and McHattie (1982); the available case historiesprobably do not adequately represent the use of LIN instruments in mineral exploration. Even though profiling methods cannot distinguish

95 (e)

between

Fig. 95, cont. Conductanceaperture diagraqmsfor various time-domain and frequency-domainsystemsfor a vertical plate model, (e) INPUT. (after Lodha, 1977).

bodies, the field results can frequently reveal a great deal of information. Due to the very high conductivity of the deposits, the thin sheet models in a free-space

economic

and uneconomic

massive

sulfide



host are often applicable, thus the characteristicsof depth, conductance, dip, and strike length can be easily estimated. The ease with which loop configuration, separation, and frequency can be changedallow the targeting of particular sizes and conductances. The very high conductancesof massive sulfide deposits may require variation of configuration. The highly conductive Burnt Nubble pyrrhotite body in Maine (Labson et al., 1984) has a conductance of several thousand siemens. HCP profiles with a loop

LINE

3555

197

separation of 120 m (Figure 96a) show a saturated responsewhich is nearly independent of frequency. The profiles acquired at a 30 m separation(Figure 96b) are not saturated and may be quantitatively interpreted. Additionally, profiles at the shorter separation show distinctly separate anomalies for the massive sulfideand the adjacent graphitic hornfels conductors. The shorter separationprofiles were compiled to provide a map of the deposit (Figure 97). The case history of the Kallkyl•, Finland Cu-Zn

54

Hz

3555

Hz

1150

80 •oo -4__ _+_ _4--, I •.•-I. u• L••/•-----•-'--'•- -40 60• I••••._-1----••+--' QUADRAT E"' 20 , _._ ..•,,/IN-PHASE ' 80 loo •-• _ _+.- J. L •. I t I • -•---"--- -40 40

140

0

-80

1777

Hz

1777

180

40

140

0

-80

888 Hz

80

'•

•

-- -,,• I

•-

-- -- , --

0

60

20

• 444

•

Hz

444

180 I

20'E

.... 2

Hz

'•

'"',• •J

I ..-.,---. I

- .I-.

-

..

noo b:---=--t: .....,

"•.•

I

I

-8

-6

__• _.•__ •....•.,•., ._•-

•.,.-•Z.•_ •.

"'

S= _

n-

80

O

0

-40 80 -40 40

..

20

-80

-80

27..;• Hx

-

.•

I--

40 -

140

-40

Hz

1401•

180

,-

40

140

100 •'

•

888 Hz

-4

-2 DISTANCE

"'--"'-' -

- "-- •,,x'-" "-"

.,.• -10

I

-

-- '

..•..,le-"'

0

0

-80

2

4

6

8

NW

( X 100 ft.)

96 (a)

Fig. 96. HCP slingramprofiles at five frequenciesover the Burnt Nubble pyrrhotite body. 120 m loop spacing profiles (a) show a saturatedresponse.


198

Frischknecht

bearingpyrrhotite-pyritedeposit,amply demonstrates the application of HCP slingram to deposit scale mapping (Laurila, 1963). Maps of the in-phase and quadratureresponses(Figure 98) at 40 m spacingand a frequency of 500 Hz closely correspondto the mapped sulfideoccurrences(Figure 99). The detailsof

conductanceand dip are apparent in the individual profiles shown in Figure 100. The shootback configurationcan similarly provide highly interpretable responsesover massive sulfides. Examplesfrom Crone (1966) and Brock (1973) (Figure 101) show the responsesover bodies dipping near

3555

140 I

et al.

Hz

/• QUADRATURE •40

6•

IN-PHASE 1777

14•

Hz

•

4•

6•

-4•

888

z• 140 "• !00 •, ---,--j

Hz

•-, --

'-' T

- - ; • -40 •

LI,J

_

l

/

444

Hz

4•

!4•

SE

-4

-2 DISTANCE

0 ( x

2 100

½t

4

NW

)

96 (b)

Fig. 96, cont. 30 m spacing(b) is interpretableand the responseof the massivesulfideis distinguishable from the graphitic hornfels conductors.


ren sulfidedeposits, the location of any sulfide deposits is often regarded as a technical success.Although many more prospectsthan ore bodies are found, EM profiling has been instrumental in the location of many important ore bodies. Some of the important earlier finds were the Temegami Mine, Ontario (Bergey et al., 1957); the Poirier deposit, Quebeck (Boniwell and Dujardin, 1964); the Faro Deposit, Yukon (Brock,

vertical, 45 degrees,and near horizontal. These anomalies can be adequately represented by thin sheet solutions.

In most of the case histories cited in this section,


199

EM profiling was useful in finding or delineating conducting sulfides. Since existing EM methods, or, for that matter most other geophysical methods, are not capable of distinguishingbetween economic and bar-

44

46

Iz LU

Iz

48

o 12: LU Q.

Z

Q. 150 Z

•

50

Z

lOO

Z

z

z

•o

50

n

•

•2

o

•

•4

z

o 56

t

I

NW

400

I

I

I

200 DISTANCE

e--..e--...e

IN

PHASE

I

I 0

I 200

IN

COMPoNTENT

I

I

COMPONENT

SE

FEET

;*..***..• ß MA88 IVE SULFIDE

QUADRATURE

I

400

CONDUCTOR

:.:i•i..• HORNFEL8 CONDUCTOR

Fig. 97. HCP slingramdepositmap of the Burnt Nubble pyrrhotitebody. In-phaseand quadratureprofilesat 30 m spacingand 600 Hz frequency (after Labson et al., 1984).

200

Frischknecht

et al.


I

-x$5.0

IN-PHASE C).

0.1

C).2,

C).3

0./.•

0.5 k,m

(a) I

I

•b7.5

I

I

•68.0 •

W68.5

- x 55.0

- x5•.5

N

QUADRATURE O.

Od

0.2

O•

0/4

0.5 k m

(b)

Fig. 98. HCP slingramdepositmapof the KallkylfiCu-Zn, pyrrhotite-pyritebody.Contourmapsof in-phase(a) and quadrature(b) responseat 40 m spacingand 500 Hz frequency(after Laurila, 1963).



1973); the Caribou Deposit, New Brunswick (Cheriton, 1960); and the Kidd Creek Mine, Ontario (Donohoo et al., 1970).

bearing massive sulfide deposits within the carbonaceous units. Generally, slingram anomalies over the known massive sulfides are much smaller than those

due to the faults and carbonaceouszones (Figure 102) (Flanigan et al., 1981, Flanigan, et al., 1982, Flanigan

Associationof Economic Mineralization.--In many areas the explorationistmust considerthe possibility that sulfide orebodies

occur within

201

and Sadek, 1982).

more extensive

There is no one strategythat is optimal for locating economic deposits within otherwise barren sulfide or graphiticzones.In somecaseseconomicdepositsmay have a higher conductancethan surroundingparts of

conductive zones of noneconomic sulfides, or carbon-

aceousor graphiticmaterial. In the Thompsonarea of northern Manitoba, for instance, nickel ore bodies occur locally within conductive zones that are miles

the zone.

long (Dowsett, 1969). In the Muskeg Prospect in Quebec a massivepyrrhotite body grades into a graphitic zone containingstringersof pyrrhotite (White, 1966). Apparently the entire zone is delineatedby an

These

estimates

should be based on low

frequency measurements. Bazinet and Labrecque (1986) suggestedusing tilt angle and ellipticity measurementswith the fixed vertical loop configurationto locateregionsof high conductance.In their procedure the transmitterloop is placeddirectly over and parallel to the conductive zone which has already been mapped by previous surveys. To be most effective the ellipticity must be measuredat frequenciesas low as perhaps 1 Hz. Very commonly sulfide ore bodies are found in terrain that containsgraphitesor sulfidic "formational" conductors, although they are stratigraphically separate. When airborne surveys are employed, an attempt is generally made to screen out most of the

EM conductor which varies in width and conductance. In Saudi Arabia individual conductive zones in the

Wadi Bidah districtare about25 km long, and a nearly linear, discontinuous zone extends for at least 45 km

(Flaniganet al., 1981).The conductivezone is apparently causedmostly by carbonaceousmaterial within a metavolcanic sequence, but in some cases individual

conductors may be associated with parallel fault zones. Ancient mine workings and modern drilling indicate the presenceof a number of small copper

X_5 .(

x_.5/.4.5

_x5z4.0 O. •

•

j

•obbno

j Arnph•bol,t'•

I

0.9.

0.3

0/4

I

I

I

0.5 k m I

•

M•co-

• I

Gat. rio. f- cor.d,•r',kz-onfhoph¾ II,•- r'ock Or'• '[77•]D•se, o,nn,nofcd or'c•

Fig. 99. Geologic map of the Kallkylfi region (after Laurila, 1963).

202

Frischknecht

formational conductorsbefore doing detailed ground follow up. Short strike length and relatively high conductivity are two of the primary criteria that are

thicker. Thickness is easily determined if the conductor is steeply dipping but not if it is flat-lying. If the conductor is steeply dipping and thick, to make mea-

thick anisotropic zone differ in a fashion which is readily distinguishablefrom those taken acrossa massive isotropicconductor(Frischknecht, 1966). Figure 103 shows the slingram response of an anisotropic slate in the Sierra Nevada foothills, California. The profile normal to the strike of the slate showsa normal HCP responseto a steeply dipping body, while the profile at an acute angle is reversed. Not all formational conductorsare anisotropic so, unfortunately, there are no known criteria that can be applieduniversally and reliably in the analysis of EM data to distinguishpotentially economic sulfidicfrom noneco-

surements to determine

nomic formational

used in evaluation


et al.

of airborne

data to select conduc-

tors for further study. These same criteria are, of course, useful in evaluating ground EM data. Thickness is an important criteria; volcanogenic massive sulfidesare not likely to exceed 50 m in width (Boldy, 1981) whereas some formational conductors are much

whether

or not the conductor

is anisotropic may be useful. Graphitic or carbonaceousconductorsare much more likely to be highly anisotropicthan massive sulfide orebodiesdue to the laminated fabric of the rock where highly conductive carbonaceouslamina may alternate with resistivelamina. Offsetloop techniquessuchas slingramare highly sensitive to the anistropy of thick, steeply dipping beds. As shownin the Interpretationof SlingramData section,profilesnormal to, and at a sharpangleto a

conductors.

The most that can be

expected from interpretation of EM data is a good descriptionof the size, shape,attitude, and conductivity structureof the body. This information is then used in conjunctionwith all other available geophysical data, such as magnetics and gravity, and available geologicand geochemicaldata in evaluationof prospects and selectionof drill targets. Although formational conductorsare probably encountered in most exploration programs, few case

Pro½•lg E

•

DH

-•c

-- 2O

-2.c

- - 30

-3c

- -ao

5O

50

lOOm

DrofJl• •

m

%

Drof, I• D

• '• - ---

•

/•

'/'

-•0

-'2C

IN-PHASE --30 OH

-•0

DH..;-ao

50

50

lOOm

Fig. 100.HCP slingramprofilesacrossthe Kallkylfidepositalonglinesshownin Figure98b(afterLaurila, 1963).



histories

exist for which results over both formational

203

data. Peltoniemi (1982) gives a number of examples of airborne EM and slingramdata from the same terrain. Slingramand other geophysicalmethodsare useful in geologic mapping and in defining precise drilling targets but they are only indirectly useful in deciding which targets warrant drilling. Barbour and Thurlow (1982) used slingramand VLF in explorationin an area of Newfoundland where long graphitic zones are very common. Gravity measurementswere used to distinguishprobable sulfideoccur-

conductors and more interesting massive sulfide depositsare shown. Ketola (1979, 1982)describedexploration for copper and nickel-copper ore in Finland in areas where conductive graphitic phyllites or black schistsare almost ubiquitous. Often the black schists contain pyrrhotite and are magnetic. Ketola (1979, 1982) gives many examples from this terrain showing comparisonsbetween slingramand other geophysical data and geologic cross sections deduced from drill

30'

...-,,.

• ,•

!oo(

(c)

ZO

i•1

t/....... .,,,

•s

,s /

•.

,

•s

•s

,•

IOOC

E

•

•

'

•,

EOCHEMICAL ANALYSIS

:J

OF SOILS

?

,

,.•-

i

,.:: '''

I '

!1

o

i

0 ß

ß ßß ß ß eeßß ß ßß ß ßß ßßt ß ßß ß

-10

/

-•oo

S?•?l•

(a)

cu

DIPANGLE

-I•'

COIL

S•I•

A? alO-•N?

•s•O[ • I[?•[N

_3oL1T_.

•

v I

700

(survey not•

_

FIELD

500 m completed) ...... • -Iøø'

/'

-20'

/'

_

tn

,,

(o

I

N 20-E

S 20-W

............•;..i;•'-ßee"•

I

!••^•,o. •oo

wmmmmm mmm W •mm•mmm•mmmml• •m•

,•.

FaroNo.2 Deposit

EG E NO

•

• •SSdV• SkL'qm•tS

m,m.l•

•

G,I•PNITIC SCHIST • HIllCITE SCHIST•

raAmV! ew.•e•e•e.

LINE32 W

GE0tOGIC CROSS SECTION

0I

G•0Pt,I•ICAL RIOIrKES 1200 400

800

feet

(b)

Fig. 101.Shootbackprofilesover massivesulfideswith dips(a) nearvertical,AgnewLake propertyOntario(after Crone, 1966),(b) 45ø, Simon Option, Ontario (after Crone, 1966),and (c) near horizontal,Faro deposit,Yukon (after Brock, 1973).

I

204

Frischknecht

et al.

EXPLANATION


Qal cq$

Allt,vium, sandand gravel Calcareousquartz schist

cos

Calcareous carbonaceous schist

mb

Metabasalt

Fault zone, inferred where

shown with aquestion mark

4-

, Diamond drill hole

',

•

',•

,+,.,."

•

•/ :,,' /',,/

•", 39660'-160 l

/t

, ,• ,+-,

•t

•i•' ,•Tota•gnetic/ 'x+/•xl. ,e.... / 'V' •w / •,' fieldintensity •

•

(gamma) (mv)

39670 T '12ø

.,

• I

Magnetics SP

/+', /

\

•, •/'•X,,, , ,.,/

•

/•

/

"",396501-200 • /

•

l

• •39640 • 240

• •39630 ---280 So!lpolential• •39620

(Percent)

120+20

•/

---320

39610---360

.... 6o o

20 ---80

RB 9and 10• cqs Wa•

Qai

Qal mb section •/'CnS %•mb cross after

I:aoUnlte •. "•'?•'• ccs Kiilsgaard etel' (1978)

i • mmera,izedz•one i 0 t

40

20

0

20

100 rn I

40

Fig. 102. HCP slingram, total field magnetic, and self-potentialprofiles acrossconductive carbonaceousfaults and massive sulfide bodies in the Wadi Bidah district, Saudi Arabia (after Flanigan et al., 1981).



rences and massive

sulfides. At the Tulks

East de-

In some casesrelatively small amounts of sulfidesor

posit, black "graphitic" shale occurs within 50 m of the sulfides.In this particular case the two conductors are readily distinguishedon the basis of conductance from slingrammeasurementsmade at a frequency of 222 Hz, although the distinction is much less clear in measurementsmade at 3555 Hz (Figure 104).

other mineralsassociatedwith carbonaceousor graphitic conductorsmay representeconomicmineralization. In FinnishLapland, Puustinen(1977)usedslingram to map graphitebearingphyllitesthat containzinc mineralization.

150 140- T-1

10

I ' OFF SCALE f• I \• •

130-

/h

,•o_ , •..•, 100- ' • 902

The association of zinc mineralization

with carbonaceousdolomites, shales, argillites, and

AREA

0

205

/ •

•,•o •,•o•

6

•

I

I

•/•

• 4

I

50

-40

...

•••/•

,•.•

•o 0 • -10 •

•Conducting zone crossing,

atacute ;ngle •

8

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

I

I

z120•

I ••

"ø••/

I • I/ t•

••

,oo i,/V I/

•'••--I

I

•o•• •o

I

2o •

90

•

0

I

2

/ •

/

I / I

4

0

I

2

I • Conducting zone, partly •

/con'•c'•l ' nør•" •r"y •cu'••ng'•--

I • •

4

0

•

2

• • •

4

0

•

2

o

-10

80

/

,o

•

4

•

6

I

8

o

I

I

I

!

I

i

I

-20

I

-•0

I I

10 12 14 16 18 20 22

-40

lOOO

I,

I

meters

Fig. 103. HCP slingramprofilesacrossan anisotropicslatein the Sierra Nevada foothills, California. Profile T-4 is

normalto the strikeand T-1 is at an acuteangle.The anomalyon profileT-1 showsthe reversedpolaritytypical of profiles made at an acute angle to an anisotropicconductor.

206

Frischknecht

Exploration for NonmassiveSulfide and Vein Deposits.mElectromagneticprofiling has been used with varying degreesof successin exploration for sulfide depositsthat are not massive.The Missiondepositis a "porphyry" copper deposit located under 60 rn of

other rocks, while fairly common, does not appear to have been widely exploited in geophysicalexploration. EM methods are sometimesused in exploration for goldbearingcarbonaceousand sulfidicrocks(Gau-


et al.

cher, 1983).

30

222 •Z

10• ..... :•.. .

•

ß

.......... '

:,o

............ ..

MAX-MiN HORIZONTAL LOOP Eld

.... ß

COlt. SPACING •)001rl:

/

.

- 20

LEG•NO

BLACK GI•L=HI33C $HaI. E/MUOSTOI•

•[• FAUL? ZONE HOS? FEL$1C VOLC.•NIC$

I M&SSIVE $YNG[N•TIC StAJr•

IN-P

O

IOO

ZOO

300

400

feet

•,

BOUGU[R GRAVITY •

'.•..•// •'•

ß .J

/•

0 30 60 90 120

meters

620

'•,...

ß

VLFEM

ZINC CONTENT 0FBHORIZO• SO•L$

FRAS•'. RFILTRATION ß•.

ß/•

ß

.

o

--•",•.•

,

•

.

ß

.

.

ß

'--'"'""-;..•...

•' ' ' ;

' ß t .•-/'x •..o

E•v 1600

.:._..-.

1200

800 27 29 14

ß

ß

Z

ß

5•CTION W 50O0

ß

Z

ß

Fig. 104. HCP slingram,VLF, gravity, and zinc and coppersoil geochemistry profile acrossthe Tulks East Prospect,Newfoundland.The massivesulfidecanbe distinguished from "graphitic"shaleat 222Hz by the larger conductance of the sulfide (after Barbour and Thurlow, 1982).



alluvial cover in southern Arizona (Lacy and Morrison, 1966). In the early phases of exploration in this area, a number of conductor axes were mapped using the moving source tilt angle method. While these conductors did not outline the entire region of ore grade mineralization, the EM measurements were a critical part of the events that led to the discovery of the deposit. During subsequentdevelopmentwork the conductors, as identified by the surface EM measurements, were found to represent the aggregateoffset of many small conductors in a sulfide boxwork that had preferred directions. Conventional slingrammeasurementsat spacingsof 40 and 60 m, and a frequency of 19 kHz were employed in exploration for sulfidesin felsic volcanicsin the Vargistrfisk area of northern Sweden, but responsesof only a few percent were observed (Padget et al., 1969). An experimental three-component system operating at a spacingof 100 m and frequency of 18 kHz, and a VLF system provided only marginally better results. Induced polarization was useful in locating a drill hole that penetrated zones containing sphalerite with a little chalcocite and galena. The Swedish "two frame" and the slingram methods were two of several that were used in exploration of the Aitik copper deposit in northern Sweden (Malmqvist and Parasnis, 1972). The ore is low grade and the total sulfur content, containedmainly in pyrite and chalcopyrite, rarely exceeds a few percent. The EM profile results were useful in defining individual features

that

had

established

conductances

of the

order of 0.7 to 3 S/m. There is a tendency for these conductive zones to be richer than surroundingrocks in pyrite and pyrrhotite but not necessarily in chalcopyrite. Overall, ore reserveswere fairly well defined as the area inside the 1000 fl.m apparent resistivity contour determined from extensive resistivity measurements.

Many nonmassive orebodies are at best poor conductors. The Corridor orebody in the Mt. Lyell district of Tasmania has only about 10 percent total sulfides. Small anomalies were observed over the orebody using a fixed source system operating at 1000 Hz. More definitive results were obtained by a turam survey using a grounded wire source (Boniwell and McKenzie, 1961). Among the case histories cited in this chapter there are examples in which relatively small amounts of sulfidesformed good conductors. A good conductor was outlined at the Juniper Prospect using dual-frequency tilt angle measurements (Ward and Barker, 1958). From drill core and from analysisof gravity and magnetic data they concluded that 2 to 4 percent pyrrhotite in a stock work of veinlets in argillite was responsible for the EM response. Podolsky (1966)

207

describesan example from northwest Quebec where pyrrhotite and pyrite stringerstotaling less than 1 m in thickness represented a good conductor having an estimated conductance of approximately 175 S. Although not as conductive as most stratiform massive sulfidedeposits, veins sometimescontain enough connected sulfides to be EM targets. Sometimes the sulfidesin a vein may have economic value, but often they are associatedwith other valuable minerals such as gold. Also some veins that may contain valuable minerals are conductive because they contain clay mineralsor other alteration products, or becausethey contain water filled fractures.

Although fluorspar has a high resistivity, fluorspar veins in the Pennines

in the British

Isles are often

conductive because they contain clay and are more fissured than the host rock; also some veins contain disseminatedgalena which may lower the resistivity. Using a loop spacingof 60 m and a frequency of 3520 Hz, Coney and Myers (1977) observeddistinct quadrature slingram anomalies over fluorspar veins. The veins are also detected by dc resistivity and VLF profiles (Figure 105). The resistivities of the veins were estimated

to be on the order of 30 to 60 fl. m.

Ogilvy (1983) used a variety of methodsin a study of exploration problems in the United Kingdom. He observed substantial slingram, pulse EM, and VLF anomalies over a lead-zinc vein hosted by mudstone and rhyolite tuff (Figure 106). Tschanz and Frischknecht (1986) found small slingram anomalies associated with geochemical anomalies and quartz-calcite veins in Paleozoic sedimentary rocks in Idaho. Subrahmanyam and Jagannadham (1984) used the slingram method to map fracture zones within Precambrian phyllites and schistsin the vicinity of Kalori, India. In this area gold is associated with fractured zones; regions where disseminated sulfides, as indicated by IP surveys, coincide with fracture zones are particularly favorable. Exploration for Iron Ores and Other Magnetic Bodies.--Electromagnetic methods can provide information about iron ore depositsthat is not available from the application of magnetic or gravity methods. Zab1ocki (1966) made laboratory and borehole studies of the electrical properties of several types of iron ore in the Lake Superior region. In many unoxidized ores, bands or dendritic chains of magnetite or specular hematite form conductive paths that can reduce the bulk resistivity of the rock to less than 1 1/. m. At least in the Lake Superior region, cherty rocks tend to be more conductive than slaty rocks. Rocks that are high in iron silicates or carbonates tend to be highly resistive. Of course, rocks having high magnetite content are highly magnetic and can readily be located and


208

Frischknecht

traced with magnetic methods. However, due to variations in remanent magnetization, the magnetic anomalies causedby such rocks are often poorly correlated with magnetite content. Thus electromagnetic methods responding to magnetic susceptibility, which is well correlated with magnetite content, can be more useful than magnetic methods in estimating magnetite content (Ward, 1959; Seguin, 1978; Blokh et al., 1984). Also in taconite ore, conductivity is likely to be related to magnetite content. Separation of the offset of high susceptibility from the effect of high conductivity requires detailed analysis unlessmeasurementscan be made at a low enough frequency for eddy currents to be negligible or at high enough frequencies for them to dominate the response. Ward (1959) showed how this

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by electromagnetic methods. Note that oxidized ores are nonmagneticand generally cannot be located using magnetic methods. Ward et al. (1955) described use of the fixed source tilt angle method in iron ore exploration in several districts in the Lake Superior region. Conductive materials included massive bands of magnetite, graphitic or carbonaceous slates, massive pyrite and pyrrhotite, sulfides and magnetite together, porous hematite and soft goethite, and fault zones. Electromagnetic measurementswere useful primarily in tracing the iron formation and mapping other contacts. Frischknecht and Ekren (1961) made slingram and turam

/I .

can be accomplishedwhen the target can be modeled as a sphere. Ketola (1968) gave scale model results for thick vertical and horizontal magneticsheets.Tabbagh (1984) showed results for tabular magnetic bodies and in later papers (Tabbagh, 1985, 1986a, b) he gave low inductionnumber responsesfor tabular bodiesthat are both conductive and magnetic. However, adequate model results for the general problem do not exist. A geophysicaltechnique based on low-frequency measurementshas been developed in the Soviet Union for detailed mapping of large iron ore deposits (Spies, 1981). The relative proportions of remanent and inducedmagnetizationare estimatedby comparingmagnetic and in-phaseelectromagneticprofiles. Zablocki (1966) found that oxidized ore of hematite or goethite is porous and that in the Lake Superior region they have resistivities in the range of 20 to 200 f•.m due to porosity. Depending on the size of the deposit and the thickness and conductivity of the overburden, such areas could sometimes be delineated

Electrode intervol • J /,',/I

'. .... '•.. '•..........

et al.

160

200

2/.0 m

Fig. 105. HCP slingram, dc resistivity, and VLF profiles over a fluorsparvein in the Pennines,U.K. (after Coney and Myers, 1977).

measurements

over

iron

formations

on the

Cuyuna, Mesabi, and Gogobic Ranges in the Lake Superior region. In the Cuyuna Range, slingramanomalies were found over the hanging wall slates but not the hematite ore itself. Both magnetic and conductive responseswere noted in slingram profiles made over the Gogobic Range (Figure 107). The results suggested that changesin conductivity and susceptibilityidentified by slingram profile resolve lithologic units better than do magnetic measurements. Tilt and azimuth angles from a fixed vertical loop transmitter were measured at frequenciesrangingfrom 50 to 20 000 Hz over a tabular body of massive magnetite as reported in Ward (1961) and shown in Figure 108. The transition from magnetic responseat low frequencies to conductive response at high frequencies is clearly observed. Although the shape of the body was tabular, the results were fit fairly well with calculations based on a sphere having a relative permeability of 2.5. Measurements

of the vertical field from a small fixed

horizontal loop transmitter were made at frequencies



209

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210

Frischknecht

et al.

exploration are from the Athabasca Basin in northern Saskatchewan.The uranium depositsof this region are located in the immediate vicinity of an unconformity betweenmetamorphosedArchean to early Proterozoic basement rocks and overlying middle Proterozoic unmetamorphosed sandstone. Deposits along the east and southeastmarginsof the basin are associatedwith graphitic metapelites and semipelitesin the basement rocks. Airborne and ground EM methods have been used extensively to map graphitic conductors in the shallower parts of the basin. At Key Lake the basementis covered by only about 20 to 100 m of Proterozoic sandstone and glacial gravels. Tilt angle and slingramsurveyswere effective in tracing graphitic units which are associatedwith high grade uranium orebodies;Gatzweiler et al. (1981) show the HCP slingram response compared to dc resistivity, gravity, and magnetics over such an ore

rangingfrom 20 Hz to 4000 Hz alonga profile over the GogobicRangeas reportedin Frischknechtand Ekren (1963). One of the anomaliesobservedindicatesonly a magnetic response. One of the other anomalies changesfrom a primarily magnetic responseat the lowest frequenciesto a conductiveresponseat higher frequencies. Slingram profiles over the Brunswick sulfidebody (Brant et al., 1966) show a large magnetic response over an adjacentiron formationand a largeconductive responseover the sulfides.Podolsky(1966) discussed the problem of properly interpretingairborneEM and slingram results over sulfide and magnetite bearing strata when magnetic permeability modifies the responseof a conductor. Explorationfor Uranium.--Probably the most notable examples of the use of EM profiling in uranium MAGNETIC

CONDUCTIVE

RESPONSE [

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zone (Figure 109). Betz (1982) made an extensive set of scale model measurementsto study the responseof several types of conductive structuresthat are found in the Athabasca basin. He also gave several examples of HCP slingramand airborne INPUT resultsalong the same profiles, and discussed their interpretation in detail. In some cases anomalies ranging in magnitude

211

from two to several percent were observed over basement conductors at depths nearly equal to the loop spacings, which were 200 to 300 m. Galvanic currents were responsible for some enhancement of the anomalies.

Saracogluet al. (1983) gave a detailed history of the discovery of the McClean uranium depositsincluding

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212

Frischknecht

et al.

the role of geophysicsin locating drilling targets. The

the western U.S. Black et al. (1962) evaluated the use

sandstone

of slingramand several other methods in exploration for paleochannelsin Monument Valley, Arizona and

cover in the area is 150 to 180 rn thick.

A


somewhat conductive regolith that varies in thickness from 20 to 100 rn occursat the unconformity. Weathering is generally deepestin the graphiticunits. Crys-

Utah.

talline basement rocks contain some sulfides as well as

graphitic metapelite and semipelite. Uranium mineralization is associatedwith graphitic units and clay within or near the regolith. Slingramwas usedto verify and outline on the ground conductors which were found by an airborne INPUT survey (Saracogluet al., 1983; Jagodits et al., 1986). Slingram measurements also provided someinformationon the regolithand on fracture

The basement was subdivided into blocks having different resistivities. Drilling to intersect slingram conductors eventually resulted in discovery of the McClean deposits. Following the discovery of the depositsa variety of geophysicalmethodswere tested in the area with varying degreesof success. EM profiling has shown promise as a useful tool in exploration for sediment hosted uranium depositsin

A-A'

Z

o

e/ø tO

channels

are incised

in mudstone

and

Exploration for Kimberlite Pipes.--A variety of methodshave been used in exploration for kimberlite pipes that may contain diamonds. Fresh kimberlite is more dense than most rocks and generally contains considerablemagnetite. However, gravity and magnetic anomaliesare not observedover all pipes due to the differential weathering of kimberlite relative to other rocks. Weathered kimberlite is generally more conductive than the surrounding host rock so that resistivity and airborne and ground EM are useful in exploration for pipes. Additionally some kimberlites may be conductive because they contain metallic

zones in the sandstones and in the basement.

LINE 9+650

These

were later filled with conglomerate and other materials. Slingram,thoughnot very effective in mappingthe conglomeratelenses, was useful in mapping conductors that probably representmud bars depositedalong a stream that closely paralleled the original channel.

E

LINE 114.700 E_ Z

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Z

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•

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u

,

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z z8 • 8z

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minerals (Kamara, 1981). DaCosta (1989), in a case history of the Palmietfontein kimberlite pipe, South Africa, notes that for weathered kimberlites, airborne EM and magnetics, followed up by slingram, was the

most suitable exploration program. Macnae (1979) presented results obtained in two different areas in South Africa using a variety of methods. HCP slingramprofiles were very effective in locating and determiningthe edgesof the pipes (Figure 110). To aid interpretation of the field results, Macnae made scale model studies over the edge of a large flat-lying thin sheet and over a thick circular disc. A good understanding of the anomalies at the edges of such conductorsis essentialfor interpretation of field profiles over weathered pipes. Hausel et al. (1981) and Carlson et al. (1984) evaluated the use of a variety of geophysical methods, including LIN EM profiles, in exploration for kimberlites in the Colorado-Wyoming kimberlite province. They found distinctive resistivity anomalies over some pipes using an instrument having a 3.7 m spacing.The resistivity of weathered kimberlite in the ColoradoWyoming district is as low as 10 to 30 ll.m whereas surrounding near-surface granite has a resistivity of 180to 1000II. m. Geologicnoisedue to pocketsof clay

213

and small "potholes", thick alluvial cover, and lack of a weathered layer over some pipes sometimescaused difficulty in the application of EM profiling. Use of an instrument having a number of large loop spacings might provide better results in difficult areas. Interpretation of the results of an airborne INPUT survey made over Archean greenstones in western Kenya, showed a number of circular conductors (Barongo, 1983). Four of these anomalies were investigated using ground geophysical methods. Drilling indicated that three and perhaps all four anomalies were caused by kimberlite. Of the various ground methods employed, slingram profiling was most effective in outlining the near-surface extent of the elliptically shaped pipes. Anomalies as large as about 80 percent were observed using a loop spacing of 100 m and a frequency of 1777 Hz; generally the quadrature anomalies were a little larger than the in-phase anomalies.

Rai and Bhattacharya (1986) made loop-loop EM profiles in time-domain and frequency-domain and a resistivityprofile over a kimberlite pipe in the diamond district

of Andhra

Pradesh of southern

India.

Results

from all three methods indicated the presence of the pipe, but the time-domain results were most distinc-

400N

;

;

IN-PHASE

o- - ..e

QUADRATURE

200W

•

•ASSUMED OUTLINE

',•

•-

-o

•,

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- •-

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,

--ø_.•. I 200

\

/

-

•

,.

- ....

0%

/

Fig. 110. HCP slingram profiles over a weathered kimberlite pipe in South Africa. Loop separationis 75 m, frequency is 876 Hz (after Macnae, 1979).

214

Frischknecht

tive. The resistivity of the surroundingrocks is on the order of favorable

100 f•.m or less so conditions are less than in some of the cases mentioned in


preceding paragraphs. Exploration Through ConductiveOverburden.raThe term overburden

as used here includes

either trans-

ported material, or weathered rock and residual soil that overlies mineral deposits.Most early successwith EM profiling was in northern regions that were glaciated during the Pleistocene. In large portions of the glaciated regions, part or all of any weathered layers

that existedbefore the Pleistocenewere strippedaway and a layer of glacial material was deposited.When the glacial material is coarse-grainedgenerally there is little direct effect on EM profiles made with coil spacingsof 100 m or less, and the effect on measurements made with larger spacings is generally not severe. However, glacial materials may contain clay which substantiallylowers their resistivity. Most difficulty is encounteredwith glaciolacustrinedeposits that are primarily silt and clay. Accordingto Smeeand Sinha (1979), approximately 22 percent of the Canadian shield and substantialparts of other glaciated regions are overlain by glaciolacustrinedeposits.In some areas (Sinha, 1980), transgressionsof the sea have left saline marine clays. Smee and Sinha (1979) noted that clay horizons in the Timmins area of northern Ontario may have conductivitiesof the order of 30 to 100 f•.m whereas marine clays in the Ottawa Valley have resistivities as low as 2 to 4 f•.m. Pitcher (1985) found clays in the Black River-Matheson area of northern Ontario to have averageresistivitiesof about 27 f•.m. We have observed (F.C. Frischknecht, unpublisheddata) somewhatlower values of resistivity for clays in northern Minnesota and central Maine. In some areas of the Canadian Shield, glaciolacustrian deposits can be 100 m thick or more. Thus, the overburden in such areas is a severe impedimentto exploration by surface geoelectricalmethods. Although the resistivity of clay and silt layers is often relatively uniform, their thickness often varies abruptly due to the bedrock topography.Anomalies due to a narrow valley in the bedrock, such as the model shown in Figures 64, 65, and 66, can easily be mistaken for a bedrock conductor and other abrupt changes in overburden thickness can cause similar difficultiesin interpretation(Villegas-Garciaand West, 1983; Spies and Parker, 1984; Palacky, 1987). Scott and Fraser (1973) gave three case histories in which conductorsdiscovered by airborne EM surveys and delineatedon the groundby tilt angle, or combinedtilt angle and turam measurements,were drilled and found to be apparentlydue to the overburden.Subsequentto drilling, two of the anomalieswere studiedby resistiv-

et al.

ity, seismic, and gravity surveys. In one case EM anomalies coincided with narrow valleys in the bedrock. Resistivity soundingsindicated that the resistivity of the material in the valleys is about 15 12.m. In another case, the overburden is very deep and the anomaly was likely due to a clay lens in an old river channel. In the third case, the drill hole encountered water in a 15 m thick layer of limestone between the glacialoverburdenand underlyinggranitegneiss.Very likely the anomaly was causedby a narrow channel in the basement rocks that is filled with brine saturated

limestone (Figure 111). Examples of separating the effectsof bedrock conductorsand overburdenirregularities using multifrequency slingram are given in Villegas-Garcia(1979)and Betz (1975-1990, posters4, 4A, 4B, and 9A).

The influence of alluvium and other nonglacially transported material on EM measurements in mineral exploration is not well documented;EM profiling has not been employed extensively in prospecting areas where the overburden is mainly alluvial. Generally, alluvium is more conductivethan glacial depositsthat do not contain much clay. In desert regions where most precipitation evaporates, there is often a concentration of salt in alluvial deposits.However, clean dry sandsand gravels may have high resistivities. Extensive information on the electrical properties of alluvium is available from groundwaterand environmental studies.

Overburden composedof weathered rock and residual soil is a serious problem for EM exploration in large regions of the earth. Palacky and Kadekaru

I1•OO Hz •-'-•

•• •VERTICAL \ • • LOOP -".-..

IO*r480 Hz O e=

-IC) *[

•"•

.•••_•,OVER BURDE '•'"' LIMESTONE

•*' GRANITE GNEISS FEET

Fig. 111. Vertical loop tilt angleresponsedue to a salt-water lens in limestone confirmed by drilling (after Scott and Fraser, 1973).



(1979) made resistivity studiesof the weathered layer in severalregionsin Brazil. They found depthsto be as great as 80 m, although more typically depth is 10 to 40 m thick. Typically, weatheringis deepestin regions of greatestrainfall. In one area the weatheredlayer has a resistivity of only 8 g.m, but generallythe resistivity is considerably higher, sometimesas great as 100 to 200 g.m. In most instances, ultramafic rocks weather to produce the lowest resistivitieswhile graniteshave the highest resistivities. The weathered layered is much more conductive in the Santa Luz region, which receives0.75 m of rainfall per year, than in the Curaca Valley which receives less than 0.5 m per year. However, the alluvium in Curaca Valley contains salt and has a lower resistivity than alluvium in the Santa Luz region. Although the overburden was a handicap, Palacky and Kadekaru (1979), and Palacky and Sena (1979) found that EM methods could be used effectively in most of the areas they explored. Herrero and Norgaard (1982) and Ako (1982) discuss severe problems in the use of ground EM profiling in tropical forests in Venezuela and Nigeria. Herrero and Norgaard found the weathered layer to be 30 to 40 m thick and to have a resistivity of 10 to 20 g.m. Airborne EM was useful in their study area and there may be an optimal choice of frequency, spacing,and loop configurationthat would provide useful ground results in this area. In the area studied by Ako, the overburden resistivities

had a thickness of at least 20 m and as low as 120 g.m. The conductance of

this layer is not large enoughto precludeeffectiveuse of EM profiling; the lack of positive results in this

215

study is probably due more to the lack of good conductors in the area than to the presence of the overburden.

Weathered

overburden

is not confined

to humid

tropical regions. At the Murray Deposit in New Brunswick, Pennsylvanian age rocks preserved a fossil gossanthat is as much as 60 m thick (Fleming, 1961). A blanket of supergeneminerals extends to a depth of 30 to 60 m in the Flambeau Deposit in Wisconsin (Schwenk, 1976). According to Horton et al. (1985) the Hornet orebody in the West Shasta District in California is weatheredto a depth of at least 30 m, but locally the overburden is not very conductive. Peltoniemi (1982) found electromagneticanomaliesover a carbonatite mass which was conductive due to weathering. There are extensive areas in Canada (G. J. Palacky, pers. comm. 1987), the Lake Superior region of the United States, and Alaska where weathered rocks or a

regolith occurs at the surface or where the areas are covered by glacial deposits. Figure 112 shows a slingram profile in such a situation in Alaska. Thick layers of soil and saprolite occur over much of the southern Appalachian region of the United States and other similar areas. However, in humid regions, such as the Appalachians, salts are leached away and clay-poor weathered rocks are not highly conductive. The most seriousproblem with weathered overburden occurs in arid regionswhere salts accumulatedue to evaporation. Butt (1981), and Doyle and Lindeman (1985) described the weathering process in Australia which has developed some of the most conductive overburden found anywhere in the world. Resistivities

140

40

130

30

120

110

-

.o •u

100

----

•

2O

VERTICAL COPLANAR

•x• 2O TM

110

10 •

100 9O

0 --

••

•

•

••

• •

HORIZONTAL COPLANAR •

/

•

•O•

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•

•

o

8O

-2O

7O 6O

-10

-30

_

o• X

0 l

25O i

5OO i

•

-40

IN-PHASE

O QUADRATURE

METERS

•

FROZEN

ALLUVIUM

WEATHERED

DIORITE

-'

•

Fig. 112. HCP and VCP slingramoverburdenanomaliesdue to weathered diorite regolith at Hess Creek, Alaska. Frequency = 3600 Hz, spacing = 91 m.

TM o


216

Frischknecht

in the range from 1 to 10 f•.m and lower are found in salt lake areas (Doyle and Lindeman, 1985). As a consequenceof the thick conductive overburden over much of Australia, frequency-domain EM profiling is generally not used in most of Australia. However, in much of eastern and southeastern

Australia

overbur-

den does not pose a serious problem. For example, EM profiling was successfullyused at the Woodlawn orebody, New South Wales where the overburden is only 10 to 20 m thick and has a resistivity of 40 to 100 f•.m (Haren and Whiteley, 1981; Whiteley, 1981a, b). Webster and Skey (1979) found that tilt angle and slingramprofilingwere usefulin delineationof the Que River depositin Tasmania. The Elura deposit is a large, massive sulphide orebody in deeply weathered and conductive terrain near Cobar, New South Wales, Australia, which has been the subject of intense geophysical study (Emerson, 1980). The zinc-lead-silver orebody is cigarshaped, about 100 m by 200 m in cross-section,and has an average resistivity of about 0.3 f•.m. The orebody, covered by 100 m of highly weathered overburden with resistivity 10 f•.m (Hone, 1980), is a challenginggeophysical target. Due to the high conductivity of the overburden only time-domain techniques were successfulin detecting the deposit. Frequency-domainprofiles might possibly have detected the deposit if lower frequencies(100 Hz) and separations greater than 200 m had been applied, although this has not yet been demonstrated. Loop configurations for the TEM profiles described here are looploop (Crone PEM and Sirotem), coincident loop (Sirotem), and in-loop (PEM and Sirotem). Large fixed-transmitter loop TEM systems, although not discussed here, were also used and described in Emerson (1980). Typical loop-loop and coincident-loop profiles that run north-to-south over the deposit are shown in Figures 113 to 115. The coincident loop profiles (Figure 113) show a high-amplituderesponseat early times which is typical of the conductive overburden in the area. Not until later times are reached (t = 1 to 2 ms) is the response of the orebody observed. This is consistent with a rule-of-thumb

et al.

and at late times, when the loop-loop responsemirrors the coincident-loop response. Note that the profile shape of TEM systems with different loop configurations (e.g., loop-loop, in-loop, coincident-loop) are practically identical if the depth of diffusion of eddy currents is greater than the source-receiver separation (Spies, 1980b). A plan view of TEM responseobtained with 50 rn coincidentloops is shown in Figure 115. The data have been converted to apparent resistivity and are displayed at two sampletimes. At early times (0.8 ms) the response of the overburden dominates. The most prominentfeature is a narrow conductivezone striking north-northwest, contained within the overburden. At later times (4.2 ms) the orebody is delineated by a low-resistivity contour. The narrow conductive surficial feature was responsible for misinterpretation of a number of loop-loop profiles run in an east-west direction. In these cases anomalies were obtained directly over the orebody at early sampletimes, and were (falsely) attributed to the orebody. Profilesto the north, away from the orebody, would have produced identical anomalies. (Such "false" anomalieswere common in early days of EM applicationin Australia using slingramequipmentoperating at high frequencies.) These examples emphasize the importance of considering the time or frequency of measurement, as well as anomaly shape, when interpretingfield data in conductive regions.

__

: •t:O

8ms

--t:l;?ms

'"""••

• t:l6ms

'"'"•

t:2'Oms

•

t:2 6ms

t

5 8ms

mentioned earlier,

which states that a target covered by conductive

"""--•

t: I 1.8ms

overburdencannotbe detecteduntil a time of t • 10-6 õ0400N

crd2 seconds, whered isitsdepthofburialandcristhe average conductivity of the overburden. Loop-loop PEM profiles (Figure 114) are strongly affected by the conductive overburden. Sign changes occur at different times, which depend on the loop separation;this is the behavior expectedfrom a highly conductive surface layer (Spies and Frischknecht, this volume). The clearest anomaly from the orebody is obtained with the 200 m and 300 m loop separations,

A

50600

50800

A'

Fig. 113.Coincident100m loop SIROTEM profilesat Elura, traverse 2500E (after Spies, 1980b)location shownin Figure 115b.


Profiling Methods Using Small Sources Scale model studies of the Elura deposit were conducted (Spies, 1980b)to assessthe limits of detection of various TEM systems to the orebody buried at various depths and with varying host rock (and overburden) conductivity. One such study, for a 130 m coincident loop, is shown in Figure 116. The solid curves are the TEM responseat times rangingfrom 1.9 to 26.7 ms. Superimposed on the graph are dashed curves which represent the conductive half-space response. The lines join a locus of points where the responseof the Elura model is equal in magnitudeto the half-space response. The half-space responsecan be thought of as representing geologic noise. The half-space curves are concave to the left, which implies that for a given depth-of-burialand loop size the body can only be detected within a certain time window. At earlier or later times the response of the orebody will be less than the geologicnoise. Thus, if the resistivityof host rock were 6 l•.m (tr = 0.16 S/m) the body could not be detected any deeper than 75 m. At a depth-of-burial of 50 m the time window is 2 to 16 ms. In a resistive host (> 100 l•.m) the body could be detected at a depth of several hundred meters. Similar studies can be made for other loop configura-

217

A combination of variable but highly conductive alluvium

and weathered

material

often

hinders

EM

prospectingin the Middle East. A highly variable, but generally large, slingram response was observed throughoutthe length of slingram profiles at the Madhab prospect in the United Arab Emirates (Thomas, 1983). Anomalies due to conductors in fresh bedrock were distorted and masked by the overburden response.In the Wadi Bidah District in the Kingdom of Saudi Arabia slingram profiles made at a spacing of 200 m often show a large and variable response, although many bedrock conductors can be identified (Flanigan et al., 1982; Flanigan and Sadek, 1982).

tions and models.

20,000N

TEM APPARENT RESISTIVITY CONTOURS,,•.._, ( ohm - m)

•

L:50m t: 0 8ms 29 averages /

•'.•.,..)

(•_•2 œ1ura depos/!at 200m

2300 I

{

I

2600 I

I

i

29•0E

(a)

CHANNEL

•[200 •-r50

<1:•. 0

3

, --12 L=50m

L=lOOm CHANNEL

50700

•[200

,2,4Et5 38,6 7

<.t o 50500

( 0

L= 200m

o

300

L=300m

m

LINE 20,100E

Fig. 114. Crone PEM profiles at Elura, with varying sourcereceiver separations,location shown in Figure 115b (after Spies, 1980a).

,,,oo ,-LA

r

,

,,,oo

,

, •29,oo,

b)

Fig. 115. Contours of TEM apparent resistivity at Elura usingcoincident50 m loops, at (a) 0.8 ms, and (b) 4.2 ms (after Spies, 1980b).


218

Frischknecht

Eberle et al. (1985) employed slingram and other methods in evaluation of conductingzones located by an airborne EM survey in Botswana. Thin Kalahari sediments overlie pre-Kalahari metavolcanic and other rocks in this area. The slingram profiles indicated that the conductivity of the overburden is variable but generally high. A number of discrete anomalies were recognizedas being due to narrow flat-lying conductors and the edges of broad sheets. However, drilling indicatedthat many of the anomaliesthat were attributed

to basement conductors

were due to local

depressionsin the basement, or clay or salt water lenses within mudstones. Resistivity soundingsin the area showedthat somethin layers have a resistivity of less than

10 f•.m

and that resistivities

of less than

100 f•.m are common in near-surface layers.

In prospectingfor base metals in the region around Juniper India, Sharma et al. (1983) found conductive and sometimesdeep overburden. In areas where the overburden is not too deep, slingramprofiling using a spacingof only 90 m was effective in locatingwell-

10 000

Halfspace

O-0.16 S•m O=0.1

1 ooo

10

0=0.03 lOO

17

O=0.01

et al.

defined anomalies

due to sulfide mineralization

in the

bedrock.

Explorationin ConductiveHost Rocks.--The targets of electromagneticprofiling are frequently contained in rocks whose resistivity is low enough to have an effect on the response. In these cases of conductive host rock, the methods which assume a conductor in

free-space are often inappropriate. As mentioned in the sectionon Interpretation of slingramdata, depths and conductances may be incorrectly estimated, or anomalies obscured. The examples in this section point out the nature of a conductivehost rock's effect on electromagneticprofiles. Profiling methods can be used to identify targets in conductive hosts if care is taken in designingand interpretingthe survey. The use of shorterloop separationsand lower frequencieswill help limit the contribution from the host rock. In explorationof Precambrianshieldsthe conductivityof unweathered host rocks is often negligible, particularly when relatively short spacingsand low frequencies are used. In most case histories where EM profiling was used, the resistivity of the host rock was not determined. However, from available data the resistivity of most Precambrian host rocks is reasonably expected to be between 1000 and 10 000 lI.m within 100 to 200 m of the surface. At greater depths the resistivity of many granites and other rocks is considerably higher. Fracturing or minor weathering probably has some influence on resistivity within the range of explorationby conventionalEM profiling.Metavolcanics and metasediments generally have a lower resistivity than intrusive rocks; for example, in deep resistivity studiesof the Murchison GreenstoneBelt in South Africa, De Beer et al. (1984) found the resistivity of greenstonesto be a little over 1000 lI.m and the resistivity of granites and gneisses be over 10 000 As noted in the sectionon Interpretation of Slingram Data

27

1

the host rock has a substantial

effect on the

responseof a long sheet-likebody when the induction number in the host rock is about one or larger. However, in most cases when the finite resistivity of the host rock has an effect on the response, the overburden also has an effect and careful analysis is required to distinguishbetween these two effects. In

lO

10

,,I

100

, , ,I

5OO

Depth (m) Fig. 116. Modeled TEM responseof the Elura orebody at different depths, 130 m coincident loops. Geological noise (the responseof a homogeneousearth of varying conductivity) is shown superimposedon the graph as dashed lines (after Spies, 1980a).

the Athabasca

Basin of Saskatchewan

the overburden

is generallyvery resistive;but, locally the resistivityof fractured

basement

rocks

can be on the order

of

100012.m. From a comparisonof slingramanomalies over the graphitic zones at known depths in the basementand model results, a significantcontribution to the anomalies appears to be due to galvanic currents.


Profiling Methods Using Small Sources The Western World massive sulfide deposit in California is a stratiform horizon in variably altered andesite-dacite tuff that has a resistivity averaging about 100 •. m. The deposithas a shallow dip with the depth to the upper surface ranging from about 6 to 30 m. Pridmore et al. (1979) studied the deposit using the University of Utah 14-frequencyEM system (see Spies and Frischknecht, Appendix A, this volume). They measured tilt angle and ellipticity at frequencies from 10.5 to 86 000 Hz using fixed horizontal and vertical loops, and a vertical rotating source loop in the null configuration. The response of the sulfide deposit was clearly seen at frequencies below 600 to 1000Hz; at higher frequenciesthe responseof the host rock was dominant. The response of the sulfideswas larger for the horizontal than for the vertical loop source, but the anomalies obtained with the vertical loop source were more easily interpreted. The vertical rotating loop source provided the largest anomaly for the massive sulfides relative to the source. However,

this configuration does not provide adequate information on the host rock resistivity. Motter (1983) gave turam and slingram profiles for some of the

same lines

studied

in Pridmore

due to the influence

ings. Brant et al. (1966) made a series of multifrequency and multispacing measurementsover known conductors in the Bathhurst area, New Brunswick

using HCP and VCA slingram configurations. The object of the study was to obtain a variety of response curves over known conductors; among other results this study demonstrated that the HCP configuration generally produces larger anomalies than the VCP configuration, and that VCA anomalies tend to have simpler shapes than HCP anomalies. The dip of the Half Mile Lake sulfidezone (Figure 118) is much more apparent and interpretable in the HCP profiles, consistent with Figures 16-20 for very shallow dips. Note that for steeperdips the VCA responseis more sensitive to the dip. Paterson (1969) compared slingramand tilt angle results; he pointed out that more information can generally be derived from slingram than from tilt angle data. The additional use of ellipticity alleviates this problem. The slingram and turam methods, and the slingram and tilt angle shootback methods were compared in Gaucher and St-Amant (1970). In their study the maximum loop spacingsfor the slingramand tilt angle

et al.

(1979). As may be seen in Figure 117, low frequency slingram measurements made with a loop spacing of 122 m clearly define a flat lying highly conductive body. At frequencies of 1777 and 3555 Hz, anomalies due to the massive sulfide are evident but unusually large positive responseswere observed on either side of the conductor

219

83'/,

! l i¾ 7•*'

40 /

/ /

/•08}- .8,.

•

• / it.. ;5555 Hz

of the host rock.

Comparisonof Dipolar SourceProfiling Methods.--In many of the minerals case histories consideredpreviously an important objective was to compare several EM methods. In this section only studies which compare two or more dipolaf loop techniques are dis-

1777Hz -20

/

20 ff .•

cussed.

In one of the earliest comparative studies, Byers (1957) compared tilt angle systemsdesignedfor moving source measurements and fixed source measurements, a slingram system, and a systemfor measuring the intensity of the field or the tilt angle using a long groundedwire source. All of the methodswere readily capable of detecting most sulfide zones tested. The galvanic and loop-loop tilt angle methods were more effective than the slingram method in mapping the extension of one of the conductors, probably because the conductor was too deep for the slingram loop spacing used. Corbett (1961) compared the fixed source and moving source tilt angle, AFMAG, slingram, SP, and resistivity methods over the Caribou deposit in New Brunswick. Of perhaps greatest interest is the large difference in moving source tilt angle anomalies measured with 61 m and 122 m loop spac-

.............

888 Hz

-2O

o

444Hz

•v

-•0

2O

222Hz -2O

0

•

400E

x : 400'

HORIZ

•

LOOP

(MAXCOUPLED)

800E

1200E

1600E

: • : IN-PHASE ..--.--.e

OUT

OF

PHASE

(QUAD)

Fig. 117. HCP slingram profiles over the Western World

deposi t', Californiademonstrating the effectof a conductive host at high frequency (after Motter, 1983).


220

Frischknecht

et al.

shootback method were 61 and 91.4 m, not the larger

angle shootbackmethod in ground evaluationof air-

slingramloop spacingscurrently available. They con-

borne EM results in Brazil. They found that the

cluded that the turam method was more effective than

slingrammethod gave better results than the shootback method in this environment, perhaps in part because the tilt angle instrument used could not be

the slingram method for locating large conductorsat substantialdepths.For findingsmallnear-surfaceconductors, and most other purposes, Gaucher and StAmant concluded that slingram was more effective than turam. For thin steeply dipping conductorsthey found that slingramhas a much better signal-to-noise ratio than the tilt angle shootbackmethod. For relatively deep flat-lying or wide conductors,resultsfor the two methodsare comparable;however, the slingram anomaliesare positiveand their significanceis not always recognized.In actual operation,the shootback method is much less expensiveto use in hilly terrain. Palacky and Sena (1979) used the slingram and tilt HALF

MILE

LAKE

operatedat sufficientlylarge spacingsdueto signal-tonoise considerations.

Macnae (1980) presentedUTEM and slingramdata for the Cavendish

Test Site near Toronto

AFMAG, turam, VLF, LIN-EM and PEM data collected by various other investigators.Combinedwith all of the other resultspublishedby manufacturersand other investigators such as Ward et al. (1974) and Strangway and Koziar (1979), this area provides an extensive

set of results taken at the same site with

many different methods and instruments. Slingram,

SULPH!DEZONE HCP

150Hzr=50

•

as well as

2.5

•5•o Hz•= •oo

1

ß

_400

'400 Hz r=50

Hz

r=10

,

Q

1000

-•1000 Hz r=50

Hz

r

100'

i ,

,

Oq VCA IP

-+150Hz r-50

150

QJ IP

•

,,

•

I

d

i

,

--[

,

400'

1400 Hz r=50

IP•

Hz r=100

{

....

,

1000 HZr-•00

100•0 Hzr=50

LINE

LIN

0

100 ft.

8W

_ &t.D

1:3

•

.t•

_:•5

9N

_4ill

_3IN_

_?...N

IN

•

LEGEND

IP-IN-PHASE

Q-OUT

•

FIELD

OF PHASE

IN % OF

FIELD

PRIMARY

FIELD

IN % OF PRIMARY

FIELD

Fig. 118.HCP and VCA slingramprofilesover the Half Mile Lake sulfidezone, Bathhurstarea, New Brunswick at several spacingsand frequencies (after Brant et al., 1966).



fixed source tilt angle, turam, and time domain results for a conductor in Thomas Township Northern Ontario were described in Macnae and Walker (1981). Slingram, frequency differencing, and a variety of other large loop and time-domain results for the Night Hawk geophysicaltest range in Thomas Township are given in a seriesof papers (Barlow, 1981;Pitcher et al., 1982; Pitcher et al., 1983; Barlow, 1984, Pitcher, 1985). There are a number of graphitic conductorsin the area covered by about 90 m of glacial sand and gravel. Despite this large depth-of-burial, small but distinct anomalies were observed with moving source methods using a loop spacing of only 100 m. The presence of either wide or multiple conductorscausesan interesting problem in interpretation of the data. Haren and Whiteley (1981) and Whiteley (1981b) showed the results of slingram and tilt angle surveys over a massive sulfide orebody at Woodlawn, New South Wales. The Woodlawn volume (Whiteley, 1981a)contains results obtained over this deposit with many other methods. The top of the Woodlawn deposit lies at a depth of only 10-20 rn (Malone et al., 1981) so a strong responsewas obtained with most of the methods. Nevertheless, this set of case histories serves to illustrate several of the advantages and

disadvantagesof the many methodsthat were tested at

in this chapter, there are few published examples of the use of EM profiling in exploration for energy resources.

Osterkamp et al. (1983) and Kawasaki and Osterkamp (1984) studiedtwo near-surfacesourcesof hot ground water in Alaska using a LIN instrument and shallow temperature measurements made in pipes driven to depths of several meters. They found an excellent correlation between the apparent conductivity and the temperature measurements (Figure 119) and between zones of high conductivity and the positions of convection cells (Figure 120) interpreted from the temperature measurements.The hot springs studied are in permafrost terrain and the boundary between frozen and thawed ground outside the area of high temperatures was readily identified from the EM profiles. Because electromagnetic measurements can be made much quicker and more easily than temperature measurements, these authors recommended that

in future investigations of shallow hot water systems, EM profiling should be used for detailed studies in

20

ß

>-

18

•

16

CHENA HOT SPRINGS NAGNETIC

INDUCTION

ß•

Woodlawn.

z

After the discovery of the McClean uranium deposits in the Athabasca Basin, Saskatchewan, a variety of methods including slingram, fixed source tilt angle, turam, fixed source transient, VLF,

ß

•-E

o

øOooo ß ß

ß

and other meth-

o o

1986). The conclusion was that a combination of

a:

o

6 ß

4

ßo

ß

oo ß

o ßß

o

0

fixed-sourcetilt angle measurementsusing separations up to 670 rn and slingram measurements were consistently most useful tracing the graphitic conductorsand resolving complex basement conductivity, consequently providing well-defined drilling targets. EM methods using large sources generally appeared less effective, but placement of the source loops was not always optimal. VLF measurementsidentified fracture zones in the sandstonethat were not detected by other methods; there appears to be a relationship between and the intersections

NEASUREMENTS

E

ß

iz

•

mineralization

LINE

14

o

ods were tested over the deposits (Jagodits et al.,

the uranium

221

ßHCP

0

OOOoooOOO0¾op I

I

0000

6•0

SOUTH

•

80

0

,_

i00

NORTH

POSITION

(meters)

CHENA HOT SPRINGS 40

TEMPERATURE AT

THE LINE

MEASUREMENTS

Im

DEPTH

E

of

these fracture zones with the underlying graphitic conductors.

•

zo

IO

Energy ResourceExploration

Geoelectrical methods are used extensively in geothermal exploration and, to a lesser extent, in exploration for fossil fuel resources.In these applications, however, there is more need for deep soundingsthan for profiling. Consequently, excludinguranium, which is regardedas a mineral rather than an energy resource

40

20

0

2

40

SOUTH

6

I00 NORTH

POSITION

(meters)

Fig. 119. LIN conductivity profiles near a hot spring in Alaska comparedwith 1 m depth temperature measurements (after Kawasaki and Osterkamp, 1984).


222

Frischknecht

et al.

successfullyin delineation of coal resourcesand there are probably many regions in which EM profiling would be useful in mapping the thickness of coal seams and in mapping offsets and edges of layers. Examples of using EM soundingtechniquesfor coal exploration are given in Spies and Frischknecht (this

conjunction with a few measurementsof temperatures and water samples.Alteration causedby near-surface paleo-hydrothermal systems has been mapped using other geoelectrical methods to help guide exploration for existing deeper systems. In at least some cases, EM profiling could be used to map near surface alteration. As noted in the section, General Geologic Mapping, magma lakes and lava tubes can be mapped by EM profiling. As part of a multimethod study of a thick coal deposit in British Columbia, Kennedy and Woodbury (1983) made a fixed-sourcetilt angle profile along a test line. The largest tilt angles observed correlated with a low resistivity zone identified by an IP survey, and with near surface coal, but no discrete conductor axes were found. A VLF survey was adversely affected by conductive overburden, but indicated some conductive fault zones. The resistivity of coal varies widely depending on the grade and water content and can be higher or lower than that of the surrounding rocks. Resistivity and EM soundingmethods have been used

volume). In recent years there has been considerable interest

in geophysicalanomalies in the near-surface sedimentary rocks that may be caused by mineralogical changesassociatedwith leakage from petroleum reservoirs. Donovan et al. (1979) attribute magnetic anomalies over oil fields to near-surface diagenetic magnetite, and Sternberg and Oehler (1984) attribute resistivity and IP anomalies to secondary pyrite and carbonate minerals formed in a reducing environment above the reservoir. IP and resistivity anomaliesover oil fields have also been reported in the Soviet Union (Spies, 1983), and dipolar EM profiling methods possibly could be useful in mapping such near-surface anomalies.

PILGRIM MAGNETIC

SPRINGS

INDUCTION MEASUREMENTS ALONG

LINE 400

$

HOT

-

co CELL N•:C ß •ON

c ON CELL VmSCT•O.

175

150

z

o

u

I00

_

THAWED

75

GROUND

PERMAFROST

5O P 25

0

,

!

400

i

:500

WEST

i

200

i

!

!

,,,,i,

I00

0

I00

ZOO

POSITION

(meters)

,

500

,

400

500

EAST

Fig. 120. LIN conductivity profiles over a hot springin Alaska showingthe location of convectioncells and a hot stream (after Kawasaki and Osterkamp, 1984).



Ground-Water Exploration and Development

Production of large volumes of ground water is generally from unconsolidated materials or bedrock aquifers that are approximately horizontally layered. Normally, EM or resistivity sounding is much more useful than profiling to delineate possiblewater-bearing layers in such media. However, the EM profiling methods described in this chapter may be useful in mapping lateral variations in the lithology of aquifers or aquicludes. In some regions, nearly horizontal water-bearing strata do not exist but water may be produced from structures such as fracture zones in weathered, crystalline rocks (Palacky, 1987); fractures and karst openings in otherwise dense carbonate rocks; or narrow channels in bedrock filled with coarse-grainedmaterial. Unless suchfeatures are very wide, they are not easily detected or delineated by soundingsbut they may be mappedusingEM profiling methods.

In most circumstances, fracture zones in saturated rocks have a lower resistivity than the surrounding rocks due to the increase

in connected

voids in the

zone. In some cases alteration of the rock or deposition of clay in the fracture zone also contributes to a decrease in the resistivity. Above the water table, fracture zones may have a higher resistivity than less fractured rocks, particularly if the rocks inherently have a rather low resistivity. However, development of alteration products in the fracture zone may lower the resistivity of the zone, even above the water table. Thus, in exploration for fracture zones we generally look for zones of lower than background resistivity. However, if the water table is deep we might use shallowprofilingmethodsto look for either conductive or resistive

anomalies

in the

unsaturated

zone.

In

some respects exploration for conductive fracture zones is very similar to exploration for weakly conductive metallic mineral deposits.When standardslingram, wavetilt, or other equipment is used, normally the highest available frequencies should be employed to obtain a significantresponsefrom weak conductors. Experimentation may be required to select the most suitable spacing or spacings. If the water table is relatively deep, a large spacing may be required to sensethe saturatedpart of the fractures. On the other hand a small spacingmay be neededto resolve numerous small zones. If the zone has a large strike and depth extent the response is enhanced by using a spacing as large as is feasible. However, if the zone has a limited depth or strike extent there is little point usinga spacingmuch larger than the dimensionsof the zone as discussedin the section on the Interpretation of slingram data. Intrusive dikes, fault zones filled with clay, or other

223

similar steeply dipping features, sometimes serve as barriers to the lateral flow of groundwater. Suppliesof water are sometimeslocated up gradient from vertical barriers. In some areas, vertical barriers limit production from individual

wells.

Such vertical

features

can

often be mapped by EM profiling. The resistivity of saturated sands and gravels is generally intermediate between the resistivity of crystalline bedrock and many types of sedimentaryrocks. Thus, channels cut in bedrock and filled with sand or

gravel may have a lower or higher resistivity than the surroundings.Dry sand and gravel deposits have a high resistivity unless they contain some clay, and they cannot be easily mapped by EM induction techniques when the bedrock resistivity is also high. In some surficial deposits, confined sand or gravel lenses may exist within finer grained material. If such lenses contain fresh water they will be more resistive than the surrounding material and may be detected by EM profiling. Of course, detecting resistive bodies in conductive surroundingsis generally not as easy as detecting conductive bodies in resistive surroundings. In regions such as coastal areas, desert areas, and areas where salt depositsoccur, the primary problem in exploration may be in distinguishingbetween potable and brackish water. Qualitatively, at least, EM profiling is useful in distinguishingbetween potable water and highly salinewater. However the resistivity

contrastbetweenfine-grainedmaterialsor shaleand coarsegrained material filled with saline water may be small.

Published reports on the use of EM profiling in ground water exploration cover a variety of applications. Palacky et al. (1981) described using the slingram methodfor groundwater exploration in crystalline rocks in Upper Volta (Figure 121). In the area investigated, wells in fracture zones produce water for domestic use. Surface materials

consisted of a discon-

tinuous layer of alluvium, and layers of laterite and weathered bedrock overlying fresh bedrock at a depth of 15 to 40 m. The highestavailablefrequency, 3555 Hz, and a loop spacingof 50 m were used.A stationspacing of only 10 m was used to provide adequatedistinction between small anomaliesand short-wavelengthgeologic noise.The methodprovedvery effectivein locatingsites for productivewater wells. In dolomitic

areas of the Far West Rand in South

Africa, mafic dikes compartmentalize ground-water resources.Due to deep weatheringand variable remanent magnetization, these dikes cannot always be mapped using the magnetic methods. Beukes et al. (1984) tested the use of an LIN instrument in mapping two known dikes. Large, distinct conductivity anomalies were found

over a dike that is known

to be

generallymore than 20 m wide. Borehole logs showed


224

Frischknecht

that the cause of the anomaly is clay developed from weathering of the dike. Anomalies over another dike were very small, possibly because this dike is much narrower, or because its composition is different. Unweathered carbonaceous shale layers in the dolomite constitute a source of geologic noise in mapping dikes in this area.

Resistivity sounding and profiling, microgravity, and LIN loop-loop profiling were used as reported in Wood and Stewart (1985) to study fracture zones in limestoneaquifers in west-central Florida. A sequence of unconsolidated sands, silts, and clays overlie the bedrock in the study area. This layer was too thick to obtain useful results with the VCP configuration. Fracture zones were detailed using the HCP configuration with loop spacings of 20 and 40 m. Distinguishing between clay-filled and water-filled fracture zones was

Quadrature

et al.

a problem in the application of all of the geoelectrical methods.

Resistivity soundingand profiling and EM profiling, using a LIN instrument, were reported in studies of a chalk aquifer (Houston et al., 1986). EM profiling delineated a high resistivity anomaly that is associated with high yielding boreholes. Apparently this anomaly is a result of subvertical

fissures that drain water from

the vadose zone. The EM profiles were also useful in differentiating between some of the different lithostratigraphicunits in the area of investigation. Hoekstra and Standish (1984) used an LIN instrument in mapping channels filled with coarse-grained sediments that are cut into the surface of underlying sedimentary rocks in northern Alberta. The channels causelow conductivity anomalies even where they are covered with a blanket of glacial drift (Figure 122). In the Agat quadrangle,Territory of Guam, a highly resistive outcropping limestone serves as an aquifer and the underlying, low-resistivity clayey conglomerate is an aquiclude. Kauahikaua (1985) used slingram profiling/soundingto map the elevation of the contact between the limestone and the conglomerate; closed depressionsin the interface would be possiblesources of additional water supplies. Measurements were made with a loop spacingof 183 m. The data for each station were taken at an interval of 91.5 rn and inverted

2.5

-2.5

VLF 2O

---- -- Inl•ase

B

+ + -,

FUO

Ouadrature

using a two-layer model. The interpreted thicknessof the limestone ranged from about 0 to 70 m. Stewart (1982a, 1982b) found that profiling with an LIN instrumentwas very useful in mappingthe details of freshwater-saltwater interfaces in coastal aquifers in

10

EM 31

z

-10

o.

30

a.

2o =,

Resistivity

1000

A

B

8O0

\•

EM34.40 /

APPROX. GROUND SUR•:'•CE

600

'.:,-,-•-,-,- .-•-.-,-•-cL-l-•-•-d-•-•-•-,-•-•-, ß •-•' •' •'• •-•

500

• WELL(Q = lm3/h)

40O

k

oTM I -

I

E

•

L

k

J

L

.•

L

L

L

L

I

L i L •

u•

k

kb

[ L • [ [ [ I L I I•••'

'

- -_ _ lwealhered lawr-

.::::::::?.??,.. 10' .'.'.'......•::::::::::.:.:.:

to-

30•

L

_• • • • •

IO

T

FRACTURE

•- ' ZONE

30

i

0

Fig. 121. HCP slingram profiles at 3555 Hz and 1777 Hz, 50 m spacing over a water producing fracture in Upper Volta comparedwith VLF and dc resistivity profiles (after Palacky et al., 1981).

CLEARWATER ß;ß r•', * ß* * FORMATION i 200

DISTANCE

i 400

i 600

(meters)

Fig. 122.LIN conductivityprofilesmappingburied channels in northern Alberta (after Hoekstra and Standish, 1984).


Florida. He suggested making repeat EM measurements as a means of monitoring changesin the interface with time.


Windschauer (1986) used a combination of EM

profiling at two spacingsand resistivity sounding to map near-surface clay layers that would prevent leakage of water into a limestone aquifer from lakes that were to be developed in the area. Shallow clay layers were generally indicated when conductivities measured with a 10 rn loop spacingwere greater than those measured with a 20 rn spacing. Haeni (1986) tested the usefulness of LIN and VLF resistivity measurements in mapping lithologic variations in glacial-drift aquifers. A comparison of the geophysical results with borehole information indicated that the methods were successful in mapping lateral changes in the grain size of aquifer materials and in locating features such as an esker composedof coarse-grainedmaterial buried within fine-grainedmaterial. As expected, the LIN measurementswere more effective in mapping small variations in conductive layers than small variations in resistive layers. In areas where there were three or four electrical layers, additional data from resistivity or seismicsurveys, or other sources were necessary to distinguish the complex lateral and vertical

variations

in the section.

Engineering Investigations

There are a wide variety of problems in civil engineering where EM profiling can be used to reduce costs of preconstruction site investigations, or improve the reliability or safety of the structure being built. Fracture zones that may cause leakage from dams or leakage into or through mines, waste repository sites, and other underground structurescan often be located with EM profiling. Fault zones that might be reactivated can sometimesbe located by profiling. The resistivity of some rocks is stronglydependenton the degree to which they are weathered; EM profiling could be an inexpensive means of assessing their ripability in planning excavation. Also, the thickness of the weathered layer or the thickness of unconsolidated materials can be estimated from profiling and sounding data. Resistivity and EM profiling and soundingare useful in locating sand, gravel, and clay deposits that are needed in construction. Freezing generally causesa pronouncedincreasein the resistivity of earth materials. Olsson et al. (1984) used high frequency slingram measurements to search for fracture zones in crystalline rock in an area that was being considered as a potential repository site for disposal of high level nuclear waste. The geophysicalproblem in this case is the same as in ground-water exploration when the

225

targets are fracture zones, except that such zones are undesirablein a repository site. Olssonet al. were able to map small steeply dipping zones that intersect the surface. Flanigan (1981) used a standard slingram instrument in mapping faults or fracture zones in alluvium and tuff in the vicinity of Yucca Mountain, a potential repository site in southern Nevada. The problem of detecting small fracture zones at repository depths from surface measurementsremains unsolved. A number

of case histories

in which

LIN

instru-

ments were used in Alaska to locate sand and gravel for construction purposes are given in Kawasaki and Osterkamp (1984). While sand and gravel can generally be distinguished from silt, the effects of frozen ground complicate interpretation. EM profiling has been used extensively in mapping frozen and unfrozen ground in the Arctic. The variation of resistivity with temperature near 0øC depends on the nature of the material (Hoekstra and McNeil, 1973; Olhoeft, 1975, 1978). The resistivity transition with freezing is more abrupt for silt or for unweathered crystalline rocks than for clay or highly altered rock. The ratio of unfrozen-to-frozen conductivity is often greater in fine-grained than in coarse-grained soils (Sartorelli and French, 1982). Materials containing fresh water freeze at a higher temperature and more abruptly than the same materials containing saline waters. The resistivity of very dry sands or gravels does not change much with freezing, but the presence of large amounts of ground ice leads to very high resistivities. If the resistivity is extremely high the electrical permittivity can have an influence on high frequency EM induction measurements (Spies and Frischknecht, this volume). In a study of loop-loop methods that might be applied in investigation of permafrostterrains Sinha (1977) includedthe dielectric constant as an important parameter. Hoekstra (1978) compared VLF resistivity, LF resistivity, and LIN loop-loop measurements in areas where the permafrost is not more than about 20 m thick. Changes in VLF resistivity from frozen to unfrozen ground were small whereas large changes were seen in the LIN profile. Arcone, et al. (1978) compared VLF resistivity, LF resistivity, dc resistivity, and LIN loop-loop resistivity measurements at sites in the discontinuouspermafrost zone. The same features are seen on all profiles but the values for resistivity are generally quite different due to differing depths of exploration. In some cases differences in readingswere a result of poor resolution of very high resistivities by LIN instruments (Figure 123). Sartorelli and French (1982), and Kay et al. (1983) describe LIN profiling along pipeline corridors in Alaska and Canada. Generally measurements were made in the HCP and VCP configurationsusinga 3.7 rn


226

Frischknecht

loop separation, and in the VCP configurationusinga 10 or 20 rn separation. Along the Interprovincial Pipe Line route in the Mackenzie River Valley in Canada measurementswere taken at 10 rn intervals during the winter-spring seasons(Kay et al., 1983). A statistical analysis of this data shows that the percentage of frozen groundgenerallyincreasesin a northerly direction. Terrains in which the dominantmaterialis clay or organic material are more often found frozen than terrains in which till predominates. Sartorelli and French (1982) outlined procedures used for a preliminary interpretation of their data which was also used as a guide to drilling in critical areas. After drilling results were available, the boundary lines between frozen and unfrozen ground were determinedfrom the EM data (Figure 124). In addition to delineatingfrozen areas, the EM data were used to

et al.

estimate changesin thickness of distinct layers between boreholes. In doing so, the resistivities of the layers were generally assumed to remain constant. These resistivities

were determined

at borehole loca-

tionsby usingthe measureddepthsand solvingfor the resistivitiesfrom the apparentconductivitymeasurements. For a two layer earth, measurementswith two

spacingsor loop configurationsyield two equationsin two unknowns that can be solved to find the unknown

resistivities.When two or more boreholespenetrate the same layers, multiple linear regressionmethods were used to solve for more than two layers when necessary. Some of the difficulties that were encountered in application of the method were the small conductivity differences between frozen and unfrozen ground, such as when the material was dry sand or gravel, and near-surfacebedrock. Studies along the

4

'

!

'

I

'

I

'

I

'

I

'

I

'

_t

- WENNERRESISTIVITYAT

'

• •/rn'a' SPACING

-'" _

,d

c•

•ß

• •,

.

Noturol Cover

- RESISTIVITY ', ',, ¾'•-•-:•

LU n-

Cover

ß

•,./•'-',... •...•

-

I-z LU n'

I0•0 ,

I

I00

,

I

•:)0

•

I

,

300

I

400

,

I

500

,

I

600

%•

•

700 fl

A'

OrgonJc Loyer

• z ......

I

-

_

.....

•T• of•mofro• • •ofle••6yrs)

--

• O

I00

•

•00 i

i

i

ioo

400 l

500 150

•

700 ft 0 meters

DISTANCE

Fig. 123. LIN conductivityprofilesshowingthe approximatesubsurface permafrostprofilecomparedto dc resistivity,Fairbanks,Alaska (after Arcone et al., 1978).



Alaska Highway were difficult due to the effectsof salt used along the road which lowered the resistivity of frozen ground. An existing ten-inch pipeline caused difficulty in making measurementsalong much of a proposed route between Haines and Fairbanks in Alaska.

Material beneath deep lakes and rivers in the Arctic is generally unfrozen. To study the processof permafrost formation, a small lake in the Mackenzie Delta in Canada was drained. Sinha and Stephens (1983) made LIN measurementsover the drained lake using seven combinations of spacingsand frequencies. The interpreted thicknessesof permafrost were in good agreement with borehole data. However, the presence of several distinct layers complicated interpretation of results.

Boikov et al. (1984) described several methods employed in hydrogeologic investigations of permafrost terrains in the Soviet Union. An electromagnetic technique in which the ratio of the vertical to the horizontal

field

is measured

was

found

to be less

reliable than resistivity profiling, largely due to topographicallyinducedfalse anomalies.A high frequency techniqueusingelectric dipolescapacitatively coupled to the groundgenerally performed as well as resistivity profiling except during summer operations in brushy

227

or hummocky terrain where the dipoles could not be kept close enough to the ground. Kawasaki and Osterkamp (1984) made an extensive study of the use of a rigid boom LIN instrument for engineeringand other investigationsin Alaska. They found that the annual cold wave profoundly influences apparent resistivity. The largest contrastsin apparent resistivity between frozen and unfrozen terrains are observed in early spring. In some cases, however, other times of year may be more favorable for making measurements.

Measurements

over

an artificial

ice

mass and other features indicated that ground ice masses in frozen soil of otherwise uniform content can sometimes be detected.

moisture

Values of apparent resistivity for near-surface frozen ground reported in Sartorelli and French (1982), Kay et al. (1983), and Kawasaki and Osterkamp (1984) in the studiesjust mentioned are considerably lower than many of the values obtained by laboratory measurements (Olhoeft, 1975, 1978) or values obtained from field measurements over frozen silts (Frischknecht and Stanley, 1970;Spiesand Frischknecht, this volume). In some cases this may be a result of poor resolution of very low conductivities by LIN instruments.

Environmental Investigations fLP d t WS

I xAPI

fLP

md m W

d t WS

18 16

,.'.,_:"i-EM31 (HCP)

14

in!

,-,

!...".... ,: 71 r_._Ml iW,,X , , . . 8

...

E

12

o

10

E

E

,o

?,

,.' ........ \

,-•

E(M 234-3.,•J EU z

GROUND

SURFACE

10

900

1000

fLP xAP

dtWS mdm

W

1100

1200

1300

1400

1500

DISTANCE (meters) Fine-grained lacustrine plain Undifferentiated texture - alluvial floodplain Dense tall (>7m tall) White Spruce Moderately dense medium (3-7m tall)Willow

BOREHOLE

FROZEN (F) UNFROZEN

(UF)

Fig. 124. LIN conductivity profiles showing the location of deep permafrost in the Yukon Territory (after Sartorelli and French, 1982).

The study of environmental problems caused by poor wastepractices,irrigation,and other activitiesof man has become an important application of EM profilingin the past severalyears. Landfills, disposal lagoons,injection wells, mine tailingsand spoils, and abandonedmine workings are some of the principal sources of pollutants that may enter ground-water suppliesor otherwise adversely affect the environment. In somestudiesthe objective may be to directly detect the pollutantsand map the site; locatingthose contained and those leaking from the site. In other studies the primary objective may be to define the hydrologicconditionsthat governmigrationof pollutants throughand away from the site. Recordsfor old sanitarylandfillsand other disposal sites are often missingor inadequate. The resistivity structureof a landfill is generallyquite differentfrom that of the surrounding undisturbed media, partly because of the addition of waste material and partly

becausethe resistivityof earth materialsis altered by the process of excavation and refilling. Thus, EM profilingcan be very usefulin mappingtrenches,pits, and similarfeatures.Also EM profilingcanbe usefulin mappingthe subsurface configuration of surfacemines that have been covered

over and reclaimed.

Mine

disposalsitesare frequentlyof concernbecauseof the migrationof pollutantsaway from the site. If a lea-


228

Frischknecht

chate plume has a high content of salts, acids, or other inorganic chemicals, the conductivity of the plume will be greater than that of unpolluted ground water in the region and the most intense part of the plume can be detected and mapped by EM profiling. Plumes that contain only organic pollutants are generally difficult or impossible to detect and map. However, some crude oil contains enough salt to be conductive and heavy concentrationsof hydrocarbonsmay sometimes cause resistive anomalies. Acid water draining from old surface mine spoils, mine tailings, or underground mine workings can sometimesbe detected and mapped by EM methods. Another possible application of EM profiling is mapping brines or other pollutants driven into the near-surface by injection wells. Mapping of lithologic and structural features that influence the flow of ground water through and away from disposal sites or other sourcesof pollutants is an important indirect use of electromagnetic profiling. Coarse-grained, unconsolidated materials that may serve as aquifers have relatively high resistivities unless they are filled with brine or water containing other conductive pollutants. Clay layers that often form aquicludes in unconsolidated deposits have low resistivities. The resistivity of crystalline bedrock is generally high; by comparison, the resistivity of sedimentary bedrock is often low. Channels in bedrock filled with coarse-grainedsedimentsare often paths for migration of pollutants. Such channels may be resistivity highs, lows, or may cause no anomaly, depending on the bedrock and the conductivity of the water. Fracture zones that may be very important in the flow of ground water through consolidated media can often be detected as resistivity lows by electromagnetic profiling. In some cases the water table in coarse unconsolidatedmaterials can be detected and mapped through a combination of EM profiling and sounding. At some stage of most environmental investigations a number

of boreholes

are drilled.

To

enhance

the

amount of information obtained by drilling, boreholes shouldbe located usingthe results of prior geophysical surveys, and other information. In the event that drilling takes place first, geophysical surveys may still be useful

to detect

features

that have

not been test

drilled and to interpolate resultsbetween holes. Induction loggingof drill holes provides important resistivity information for interpretation of surface EM results. Often EM profiling is used in combination with dc resistivity or EM soundingto determine suitable locations for making the soundings and to interpolate between soundings. In hazardous waste site investigations, LIN instruments have been used almost exclusively for EM profiling. Standard slingram or other loop-loop instruments are of limited use in many investigations be-

et al.

cause the depths of interest are too shallow and resistivities are too high to obtain a significant response from the media of interest, although some newer slingraminstrumentsoperate at higher frequencies and shorter separations, and substantially overlap LIN instruments' capabilities. Conventional instruments could be useful in deeper studies where low resistivity materials such as clay layers or conductive bedrock may be encountered. Some of the anomalies encountered in environmental studies are large and distinctive but the investigator should be prepared to map subtle, small-amplitudeanomalies. Since hazardous waste sites are generally in or near urban areas, electrical interference from powerlines, radio stations, and other cultural sourcesis often a problem. Further, anomaliesfrom features suchas powerlines, pipelines, and fences often tend to mask the features

of interest.

Interpretation of profile results in environmental investigations is usually qualitative. The inhomogeneities of interest tend to have complex and ill-defined forms that are not easily modeled. The layered-earth model is of greatest use in quantitative interpretation of results in environmental investigation. LIN profiles taken at a number of spacings and configurations frequently are interpreted using the layered earth model. Some of the results given in the section on Responseof Dikes, Prisms, and Other 3-D Models are a useful guide in interpreting data over pits and trenches. Lenses of clay that can be regarded as plate models are sometimes encountered. In dealing with such targets the importance of galvanic currents on LIN

measurements

must be remembered.

Direct detection and mapping of leachate plumes from landfills and leaking contaminants from disposal lagoons is of great interest. Examples of studies in which LIN instruments were used to map plumes are given in Glaccum et al. (1982), Greenhouse and Slaine (1983) (Figure 125), Greenhouse and Harris (1983) (Figure 126), Grady and Haeni (1984), Rudy and Caoile (1984), Sweeney (1984), Weber et al. (1984), Barlow and Ryan (1985), Stephensand Graham (1985), Medlin and Knuth (1986), and Weber and Flatman (1986). In most of these examples, the anomaly over the highest concentrations of pollutants is large and distinctive. However, as pointed out in Rodrigues (1984), significant contamination cannot always be detected by EM surveys, particularly where there are changesin resistivity of the strata that are unrelated to pollutants. White et al. (1984) gave examples in which careful interpretation of small features was necessary for successfuluse of EM profiling. In investigationsof the leakage of contaminants from storage or disposal lagoons or ponds, EM profiling is used to map the plume migratingaway from the lagoon or pond. Barton (1985) used an LIN instrument to map areas at which



229

ELMIRA

o

BURIED WASTE LAGOONS

lOO

2oom

•

EM-34H 20m

ELMIRA

BURLEDWASTE LAGOONS

0 ß

100 *

ß

ß

ß

200m ß

ß

ß

ß

ß

*

ß

....

i

i

,

&

i

i

a

Fig. 125.LIN conductivity(a), and VLF conductivityandphase(b) over wastedisposallagoons,Elmira, Ontario

(afterGreenhouse andSlaine,1983).Conductivities arein db,20log]0(ø'app/ø'backgrøund)'

230

Frischknecht

mapped.Petersonet al. (1986)usedan LIN instrument to map a permeable channel and other hydrologic

a very conductivesolutionwas leakingthrougha dike around a pond.


et al.

In a studyof a landfill in glacialdrift in Farmington, Connecticut, Grady and Hanei (1984) were able to detect the plume when the apparent conductivity increasedby a factor of 1.5 to 2.0 over background conductivities, which were about 0.004 S/m or less. Conductivities as high as 0.033 S/m were noted in someareas. The specificconductanceof groundwater samplesfrom wells in the area rangedfrom 0.0115S/m to 0.182 S/m. McNeill and Bosnar (1986) found excellent correlation between LIN slingram profiles and total dissolved solids (TDS) in water samplestaken from wells at the Pittman site in Nevada (Figure 127). At the Pittman site the conductivitiesof the unpolluted media, and the TDS concentration and conductivities of the polluted media were both much larger than at the Farmington site studied by Grady and Haeni. Obviously the detectability of a plume dependson the absolute conductivity of the unpolluted media as well as the degreeof pollution and amount of geologicand cultural noise present at the site. In some of the studies cited in the preceding paragraphs, EM profiling provided information on hydrologic conditionsas well as contaminantplumes.Hoekstra and Standish(1984) give an example in which an organicplume was not detectedbut a probablepath for migration of pollutants in coarsegrained material was

features at a hazardous waste site.

Geophysicalmethodsare sometimesuseful in monitoring, as well as in the one-timedefinitionof hazardous waste sites. Model studies were used by Greenhouse and Monier-Williams (1985) to estimate the relative effectivenessof VLF, dipole-dipole and Wen-

ner resistivity, and LIN EM measurementsin monitoring changesin resistivity associatedwith a hypothetical landfill. All methods appeared to be useful; measurement noise was estimated to be lowest for the

resistivity method, but EM and VLF measurements can be made much more rapidly. Glaccum et al. (1982)

used EM profiling to monitor the migration of a leachate plume and a surface spill of contaminated water. Medlin and Knuth (1986) used repeated LIN measurementsto map changesin a conductive plume as a measure of the effectiveness of a waste recovery

system (Figure 128). Electromagneticprofiling is sometimesused to locate metal objects buried in landfills, or pipes and other objects related to hazardous waste sites. For compact steel objects such as drums, vortex currents

circulating within the drum are probably the chief source of EM response. Due to its relatively high conductivityand permeability, the responseof a drum is mainly in-phase. Nevertheless, the quadrature re-

300

APPARENT

•o

RESISTIVITIES

AND

CONDUCTANCE ALONG PROFILE A-A'

•

•0

•EM-16R

BASE BORDEN SANITARY LANDFILL SITE

CONDUCTIVITY •NTOURS IN MICROMHOS/CM ß

230

•

LAN•ILL

CONDUCTIVITY

SAMPLING

POINT

TH1 O SURFACE_

.•

•

0

. ..

1•

ß

•

- oo•••

Meters

Fig. 126. LIN conductivityprofile over a landfill, Ontario, comparedwith dc resistivity, VLF resistivity, and drill-hole conductivity samples(after Greenhouseand Harris, 1983).



231

dependingon the relative contributions to the overall responsefrom the pipe and the surroundingearth. A smaller positive response may be observed when a pipe or similar conductoris traversed with a broadside HCP configuration.The VCP in-line responseof a pipe is small; the VCP broadside response is positive and can be large. Koerner et al. (1982) evaluated two rigid boom EM instruments, a magnetometer, and a ground penetrating radar in locating buried steel and plastic containers. The EM instruments accurately located the steel containers but not the plastic ones. The plastic con-

sponse is large enough to be detected with LIN instruments. As contrasted with the response of an object such as a drum, the responseof a long horizontal pipe or cable in contact with the earth is probably due mostly to galvanic currents channeled into the pipe from the surrounding earth. In this case the quadrature component is likely to be large or to even dominate the in-phase component. The identification of a pipe or other long slender conductor is often fairly easy. When an HCP loop configuration straddles a pipe, the quadrature component and the apparent resistivity show a minimum and may be negative,

200I A •

•.

EM34-3

at 40m •ntercoll at 20 m

150

$Dacmg. I•or•zontal

d•Dole mode

"

.7m•nt'erco•l sDac•ng

100

50

(a) !

649

!

I

645

I

I

641

I

I

637

I

I

633

i

•

629

•

I

625

•

•

621

t

I

617

Well

No.

26 24 22

20

o• 18 E

,,.. 16 o

o• 14

o

12

'-'

10

•-

8 6

(b) i

0

661

i

657 653 649

645 641 637 633 629 625 621 617

WellNo.

Fig. 127. LIN conductivityprofiles(a) over the Pittman lateral, Henderson,NV, showingthe correlationof conductivity to total disolved solids(TDS) (b) (after McNeill and Bosnar, 1986).


232

Frischknecht

tainers, although empty, apparently were not large enough to constitute a detectable resistivity anomaly. Rudy and Warner (1986) used an LIN instrument and ground probing radar in locating abandoned gasoline storagetanks. Duran (1984) observed that buried steelreinforced concrete foundations caused large positive VCP responsesand negative HCP responses. Ladwig (1984) described the use of EM profiling to study the discharge of acidic waters from old reclaimed surface coal mines in Pennsylvania and West Virginia. The conductivity of mine spoil was generally quite variable although higher than that of unmined rock. The variability of the mine-spoil made identifying acidic plumes difficult. However, EM profilingwas very useful in locating potential sources of acid production and in defining boundaries of ground-water flow. Pockets of preparation plant refuse and coal cleaningwaste that have a high potential for producing acid were marked by high conductivity anomalies. Boundaries such as high-walls, side-walls, and outcrops, that influence ground-water flow were readily mapped, as were structural depressionsand structural

et al.

The detection of abandoned underground mine workings and other voids is of considerable interest. Although the voids themselvesare difficult targetsfor induction methods, networks of fractures that propagate upward from the voids may cause conductivity anomalies that can be detected by EM profiling. In a geophysicalstudy of an undergroundcoal mine, Rodriguez (1984) found that high conductivity regions mapped with an LIN instrument coincide with probable regions of active burning. The accumulation of salts in irrigated agricultural

lands has destroyed the productive capacity of large regions and threatens millions more acres in many parts of the world (Raloff, 1984a,b). The ability to rapidly measure soil salinity in situ is important in conductingresearch on methods for coping with salination and in identifying and mapping soils that are becoming excessively salty. The resistivity of soils dependson surface conduction in the clay fraction as well as the quantity and salinity of the water in the soil. However, through calibrations from saturated paste extractsfrom coresor from modelsbasedon soil type, soil salinity can be estimated from measurements of

rises.

SCALE

(a)

;,

,•o ,,.;....

Fig. 128.RepeatedLIN conductivitymeasurements (mS/m)usedto mapchangesin a conductiveplumeover time (after Medlin and Knuth, 1986). (a) 1984 conductivity.


soil resistivity (Rhoades, 1981). Resistivity methods using short-spacedsurfacearrays and probes that are


inserted

in the earth

sometimes

be a little less accurate than the multiple regression analysis, probably because the effects of magnetic susceptibility on the conductivity measurements are not considered in the new method, but are accounted

are used to measure

soil resistivity. For many purposes, short spacing, rigid boom LIN EM instruments are more convenient and rapid to use than resistivity instruments. De Jong et al. (1979) tested an instrument that has a loop separation of 3.7 m; they found that changes in soil salinity had a much larger effect on the resistivity than changesin soil moisture. They found a high degree of correlation between resistivity measured with the LIN instrument and the resistivity of saturated soil paste extract from cores (Figure 129). The variation of soil salinity with depth is an important factor in the growth of plants. Rhoades and Corwin (1981) developed an empirical method using multiple regression analysis for determining the vertical variation in resistivity of soil using measurements made at various heights above the surface with an LIN instrument having a separation of 1.0 m. Corwin and Rhoades (1982) developed a simpler method, based on the theoretical responsefunctions for LIN instruments, that requires only two measurements.The new method appears to

(b)

•

233

for in the statistical Reconnaissance

method. LIN

measurements

were

made

overan areaof about10000 km2 by Williamsand Baker (1982) to assess soil salinity hazards. They employed the VCP and HCP configurations at spacings of 10, 20, and 40 m. Stations were placed approximately on a 5 km grid, along with a smaller more detailed grid, and an 18 km long traverse at 500 m spacing.Interpretation was based largely on the VCP data since the HCP data showed much poorer correlations between results at different spacings, and because the HCP data were frequently affected by electrical interference from powerlines and by anomalies from fences. To help evaluate the EM results, salinity was measuredas a function of depth in 19 drill-holes in the area. Most of the variance

in conductivities

deter-

mined by EM measurements could be explained by salinity alone. As a result, some of the weathered

.

Fig. 128, cont. (b) 1986 conductivity.

234

Frischknecht et al.

rocks beneath alluvial and colluvial materials found to store substantial amounts of salts.

were

filing, EM methodsare generallymuchfaster and less expensive.Someof the earliestattemptsat measuring soilconductivitywith EM methodswere madeusinga transient system. The transient responsewas often found to be dominatedby lossesdue to the magnetic viscosityof soils (Tite and Mullins, 1969, 1970). Subsequently, experiments were carried out with a frequency-domainLIN instrumentusingloopsseparated one meter apart in the PERP configuration(Tite and Mullins, 1970). By measuring both in-phase and quadraturecomponentsit was, in principle,possibleto measureboth magneticsusceptibilityand resistivity. However, the in-phaseresponsewas generally much larger than the quadraturecomponentand the instrument was not very satisfactoryfor measuringresistivity. The instrument,thoughsuccessfullyusedto map shallow archaeologicalfeatures, was generally less useful than a magnetometerdue to the extremely limited depth range. Parchas and Tabbagh (1978), and Tabbagh (1984, 1986b)made measurementswith an instrumentoperating at a somewhat higher frequency with loops separated1.5 m in the PAR configuration.Generally,


ArchaeologicalInvestigations

Geophysical methods are used in archaeological investigationsto detectand map featuresor objectsin the subsurfacewhich indicate past human activities. Low induction number (LIN) slingraminstrumentsas well as metal detectors are used in investigationsof archaeologicalsites. Human activities often change the resistivity structure of the subsurface. In some casesresistivity changesare due to the emplacement of stone or brick structures

such as foundations

or

tombs.In other cases,changesare causedby replacement of the original soil with transportedsoils or by digging,compacting,irrigating, or other activitiesthat changethe texture or compositionof the originalsoil. Human activities also often changethe magneticsusceptibility of the soil through replacement of the original soil or through the action of fire which can convert hematite to maghemite(Tabbagh, 1984, 1986a) Direct current resistivity methods are employed extensivelyin archaeologicstudies.However, for pro-

(c)

• o

SCALE

.

zoo

40o

•[[T

Fig. 128, cont. (c) 1986-1984conductivitydifference.



satisfactory results were obtained in mapping both susceptibility and resistivity. Tabbagh (1984) did a computer model study of the responseof this instrument and of a magnetometerover small targets having low susceptibility contrasts with respect to their surroundings. The study was extended to include a system in which

the difference

was taken

between

ity" may causesubstantialerrors in in-phasemeasurements. Archaeologicalfeatures tend to have a higher viscosity-to-susceptibilityratio than do normal earth materials (Tite and Mullins, 1969). Since the response due to both in-phaseand quadraturesusceptibilityfalls off much more rapidly than the response due to conductivity as an LIN instrument is raised above the surface, the effect of quadrature susceptibilitycan be minimizedby makingconductivitymeasurementswith the instrument raised above the earth. Note, that for small heights, the ratio of the susceptibility response to the conductivity response is much greater for the VCP than for the HCP configuration. Since quadrature susceptibility is frequency dependent it should be possibleto recognize and eliminate its effect by making measurementsat two or more frequencies. However, additional studiesof the dependency of quadrature susceptibilityon frequency are needed (Tabbagh,

the

response of two separated receiver loops. Tabbagh concludedthat in many circumstances,an LIN instrument is more effective than a magnetometerin locating and delineatingthe target, particularly when the target is thin and flat-lying. Also, Tabbagh pointed out that a searchcan be made closer to a large steel object with an LIN instrument than with a magnetometer(Figure 130). This is particularly important in areas contaminated with large steel objects of recent origin. Tabbagh (1986b) suggestedthat the ideal EM instrument for archaeologicalstudiesshould be designedto measureboth conductivity and magneticsusceptibility and should

also be useful

effectiveness

as a metal

of such an instrument

detector.

1986a).

The

is enhanced

235

In archaeologicalstudies, significantanomalies often have very small horizontal dimensions and small amplitudes. Thus, a dense set of measurementsis generally required; Tabbagh (1986a) states that one point every one or two squaremetersis standardfor a survey intended to detect detailed features. Frohlich and Lancaster (1986) took readings at 0.5 m intervals with an instrumentthat has a loop spacingof 3.66 m. A

if

measurementscan be made at more than one spacing. The induction number must be extremely low; otherwise the smallin-phasecomponentdue to conductivity will cause errors in the susceptibilitymeasurements. When the conductivity of the earth is fairly low, magnetic loss mechanismsor "quadrature susceptibil-

20,

o

/\

18,

/

\

I

/

O

\

\

16-

_/

0

c- 14E

/

E 12108,

/

//\\

/I

//.c.

., •

/ •

Z'-. /

-o'

/ /

/

A.

I

-

-_

6'

4

2

METERS

Fig. 129. LIN conductivityprofile showingcomparisonto soil salinityderivedfrom saturatedpasteconductivities (after De Jong et al., 1979).


236

Frischknecht

relatively high degree of precision, but not absolute accuracy, is required in EM measurementsfor archaeological purposes. Frohlich and Lancaster studied (1) the effects of the configuration and traversing direction, (2) variations in repeat measurements by the sameperson, (3) variations in repeat measurementsby different persons, and (4) variations causedby weather and environmental conditions. They found repeat errors were significantly reduced when the instrument

SH3E.M.susceptibility in 10-5 USI

et al.

was oriented

where

0

5rn

Magnetic total field anomaly in Y

:':'

in the same direction

Roman

kilns were

once located.

Parchas

and

Tabbagh (1978) used a newly designed instrument to map a ditch at Garchy, France in which the original weathered limestone had been replaced by top soil having a higher susceptibility. The resistivity contrast was small or negligible depending on the time of year. Bevan (.1983) mapped a ditch in Illinois that was a conductive feature due to a high conductivity zone that occurred directly beneath the bottom of the ditch. The change of conductivity may have resulted from chemicals leaking through the ditch. Kawasaki and Osterkamp (!984) used an LIN instrument to search for caves in bedrock that were apparently used by ancient man and now are covered by talus. They found a number of small high-resistivity anomalies that may correspond to dry caves and some low-resistivity anomalies that may indicate the presence of water filled caves. Excavations

Fig. 130. LIN susceptibilitymappingdemonstratingthat EM devices are less affected by near surface sources than a magnetometer (after Tabbagh, 1984).

for all stations in

the entire grid. When all conditions were kept the same, differencesin repeat measurementsmade by the same person were small. Somewhat larger differences were observed when the measurementswere made by different persons, probably because of slightly different carrying heights and positioning of the instrument above specific points. Modeling and quantitative analysis of individual anomaliesis not generally usedin interpretation of EM data in archaeological studies. However, a variety of data processingand statistical techniques are used to analyze and display the large sets of data that are sometimesacquired. For instance, Scollar et al. (1986) used image processing techniques to enhance the display of magnetic data. Tite and Mullins (1970) used an LIN instrument in mapping susceptibilityanomalies associatedwith sites

to evaluate

their results were

not undertaken. Tabbagh (1986a) described mapping an ancient pottery workshop with an LIN susceptibility mappinginstrument (Figure 131), and usingan LIN instrument to detect Bronze Age metallic hoards in an area in Normandy, France, where there was no evidence of permanent settlement. Frohlich and Lancaster (1986) successfullyused conductivity mapping to locate undisturbed burial chambers in a 5000 year old cemetery in Jordan (Figure 132) and to locate early Bronze Age chambersin Bahrain. At one complex site in Bahrain, promising results were obtained by differencing the results obtained with the HCP and VCP configurations. At Duraz settlement in Bahrain, the highly irregular topography generated unacceptable reading errors. Frohlich and Lancaster made a detailed topographicsurvey so that the EM data could be corrected for variations in the mean height above the surface.

Profiling Methods Using Small Sources General Geologic Mapping


Thus far, we have discussedthe use of EM profiling

in specializedgeologicmappingfor specificand often narrowly definedobjectives.In theseapplications,EM profilingis generallyappliedin conjunctionwith other geophysical, geologic, geochemical, hydrologic, or geotechnicalinvestigations.Often geologicmappingis carried out for basic scientificpurposesand to provide a framework to supportfuture resource,environmental, or geotechnicalinvestigations.The use of regional potential field studies in conjunction with regional geologic studies is common and well accepted. In

0

237

many areas the use of geophysics in detailed general purpose geologic mapping can be well justified when resources

are available.

Electromagnetic profiling can contribute substantially to the quality and completeness of a bedrock geologicmap when outcropsare sparsedue to surficial cover and when there are large conductivity contrasts between

some of the bedrock

units. Even

when out-

crops are plentiful, EM measurementsmay be useful in mapping features, and near-surface lithologies or structures

that remain

hidden or do not extend to the

surface. Similarly, shallow EM profiling can be very

50m i

i

210 10" $[u 150

EM SUSCEPTIBILITY

SOIL

SAMPLE

SUSCEPTIBILITY

WEIGHT

PERGENT

BURNT

CLAY

Fig. 131.LIN susceptibility mapping(a) of an ancientpotteryworkshopin Francecomparedto susceptibility of soil samples(b) and weightpercentageof burnt clay in soil samples(c) (after Tabbagh,1986a).


238

Frischknecht

useful in mapping near-surfacefeatures such as variations in weatheringor changesin lithology of alluvium or glacial deposits.Conductivity contrastsin the nearsurface do not need to be very large to be detected. In metamorphic rocks, carbonaceous or sulfidic rock units often constitute

marker horizons that can be

traced by EM profiling. Frischknecht(1966) and Ekren and Frischknecht (1967) describe the use of the slingram method in tracing conductive carbonaceousslates in Maine where extensive glacial depositsmake geologic mapping very difficult. Many of the units that they mapped were weakly conductive and detectable only in the quadrature componentfrom the measurements that were made with a loop spacingof 61 m and a frequency of 2000 Hz. A few measurementsmade at 8000 Hz indicated that a higher frequency would have been desirablein at least part of the area (Figure 133). Gaa'l et al. (1975) used a 1:50 000 map of slingram data in delineating and studying structures in the vicinity of Outokumpu, Finland. Most of the anomalies on the map are causedby black schists,although someof the serpentinitebodiesin the area are conductive. Some of the individual

some cases distinct

anomalies

similar

to those

obtained in the vicinity of the edge of a conducting horizontal sheet were obtained (Figure 134). In other cases, differences in the overlying glacial drift on either side of the fault complicatedthe results.

HIGH

Sinha (1980) used an LIN instrument to prepare a resistivity map of an area underlainby freshwater clay and other glacial materials, and saline marine clays deposited on resistive bedrock. The results proved useful in mappingthe extent of the conductivemarine clay and in mapping areas where bedrock is shallow. Generally, the resistivity map prepared from the LIN data agreed well with a map produced from resistivity soundings. A

EM-31

+2O

8o

VERTICAL

COPLANAR, 8000

'

-

.-_

_..... •

I

ß--'

VERTICAL COPLANAR, 2000 CYCLES PER SECOND

140[ 120 • 100 I

-40 -20

0

+20

- +40

o. 12o

..

0

z

mmm8o

VERTICAL

Z

:•'••'::•.t::.'::? ;••.::•'•:•.• •.:
Z

+20 Z o

COAXIAL, 2000 CYCLES PER SECOND

o :• 16o o O 140

--60

o

- -40

U.I

U.I 120 -

- - 20

ß-

0

I

80-

-+20

Z --'

60-

- +40 'xJ

x-----x--x e----.•-----•

140

QUADRATURE IN-PHASE

COMPONENT

COMPONENT

- - 40

-

/ .... %

120 -

- - 20

-

•

- +20

60 -

- +40 HORIZONTAL

40

o

- + 60

HORIZONTAL COPLANAR, 8000CYCLES PERSECOND_ +80

8O -

- --

-20

(/) lOO

2O

•-

O

VERTICAL COAXIAL, 8000 CYCLES PER SECOND

q- lOO-

'

0

+20

80

100

•:t::-?..• ::?•f:•' '

PER SECOND

lOO

11, llllllllliHlllllllll:i!illIilIliiii,i,illill,11,liiillll .

CYCLES

-• -20

40

LOW

-20

lOO

black schists conductors

are more than 10 km long and are very straight; other conductorsindicate tight, complex folding. Slingram measurementsreported in Seaborne et al. (1979) were made over steeply dipping faults where rocks having different conductivities are juxtaposed. In

et al.

•

0

COPLANAR, 2000 CYCLES PER SECOND

•

500

•

1000

•

1500

+ 60

LENGTH OF TRAVERSE, IN FEET

Fig. 132. LIN conductivity profile over shaft Tomb at Bab edh-Dhra, Jordan. Shaft 1, and chamber 2, are filled with silt and are conductivity highs, chamber 3, is not filled and is a conductivity low (after Frohlich and Lancaster, 1986).

Fig. 133. VCP, VCA, and HCP slingramprofilesfor mapping conductive carbonaceousslates in Maine covered by extensive glacial deposits(after Ekren and Frischknecht, 1967).


Profiling Methods Using Small Sources SUMMARY

The use of EM profiling in geological mapping of sedimentary sequencesin East Anglia was described in Zalasiewicz et al. (1985) (Figure 135). In most of the areas studied, topography is a poor guide to geology and most of the surface is covered by surficial downwashed, solifluxion, or windblown deposits. In geologic mappingin theseareasthe bedrockis sampledby drilling many shallow holes with a hand-auger, but commonly the surficial deposits are too thick to be penetratedby this means. In evaluating the use of EM profiling, most measurementswere made at 10-50 m intervals using an LIN instrument with a loop spacing of 37 m. The data obtained were very useful in improving the quality of geologicmapsin areasof both fiat-lying and steeply dipping sediments;in the latter case contacts were particularly well defined. Of course, the results were dependenton resistivity contrasts between units such as sand and clay, or shale The conductivity contrast between molten and solidified lava is very large. Slingram profiling has been useful in mappingthe edgesof the lensesof magmain the Kilauea Iki lava lake, Hawaii (Hermance and Culp, 1982). Profiling with an LIN instrument is useful in locating and identifying lava tubes in active or very recent flows that contain molten rock (D. B. Jackson, personal communication, 1987).

FREQUENCY

AND

DIRECTIONS

60 M.

880 HZ.

120-0

x--x

IN- PHASE

o---o

QUADRATURE

IN-PHASE

.20-0

QUADRATURE 0-0

lOO.O

-20'0

80'0

COIL SPACING 120'0

FUTURE

Originally developed for applications in mining exploration, profiling methods using small sourceshave been applied to a broad range of ground water, archaeologic, and engineering problems. The applications presented in this chapter describejust a few of them. The future directions in equipment will most likely be the ability to acquire data over a broader span of frequenciesor times. This improvement will allow the applicationof more sophisticatedinversiontechniques to the results, and the acquisitionof higher frequencies and earlier times will provide greater resolution of very near-surface details to improve applications to environmental, ground water, archaeological,and engineering studies. A slingram device operating at frequenciesfrom 100 kHz to 30 MHz, and thus spanning a gap between the LIN devices discussed in this chapter and radar systems, is currently being developed by the U.S. Geological Survey. This high-frequency slingram is capable of providing extremely detailed profiles of the first few meters beneath the surface. Due to the high frequenciesused, the results are no longer attributed only to conduction currents, as has been the case in this chapter, but must also be attributed to displacementcurrents. Thus, the profiles can be interpreted in terms of subsurfaceelectrical

and sandstone.

COIL SPACING

239

60M.

FREQUENCY2640 HZ.

IN- PHASE

x•x/-'x--•x-"-• . .c)'

'/,

•

.20.0

QUADRATURE

x

%

100'0

tq•A•X-

__•.-....

,.

0'0

o

-2O-O

80 '0

o

1•

2•

METERS

BOULDER

CLAY

5 -12

METERS

45-80

OHM

METERS

LIMESTONE '•SHALE 20-&O METERS ))100 OHM METERS

45 OHM METERS

Fig. 134. HCP slingramprofilesfor mappingfaults beneathan overburdenin the EnglishPeak District, U.K., geologic section from outcrop and borehole (after Seaborne et al., 1979).


240

Frischknecht

conductivity and permittivity. The inclusion of permittivity will allow further determination of the subsurface geologic structure. The broadening of applications of small source profiling techniquesnecessitatesthe use of earth models more complicated than the plate and sphere in free-space used for the interpretation of mining targets in the Precambrian

shield.

The effects

The interpretation of the large data sets that will be acquired along profiles is impossible using the charts and curve matching techniques demonstrated in this chapter. The "imaging" of profiling results, that is, the conversionof the result from quantitiesof electromagnetic field versus frequency or time, and spacingto resistivity versusdepth, is required. Such methodsare being developed. Macnae and Lamontagne (1987) present a simple method for imagingTEM data which provides very informative profiles (Figure 136). Techniques for imaging FEM profiles have been shown in Sinha (1979), Wilt and Stark (1982), Won (1983), and Mundry and Blohm (1987). The techniquesdeveloped for helicopter EM data in Sengpiel (1988) and Bergeron et al. (1989) shouldbe applicable to FEM profiles

of conductive

host and overburden, and a multiplicity of conductive and resistive bodies, both wanted and unwanted, must

be dealt with. This complexity will be resolved, as mentioned above, by increasing the number of frequencies or times, and the spacings measured. The result

will

be to further

diminish

the distinction

et ai.

be-

tween profiling and soundingtechniques.

L'

%.

•

%.',s

/./1

r/RCo\ _%,•,.= ••'•'••'Xmapp Previousl¾• •'•-.4.• ed / Revised

boulder

clay boundary 255. e39

x

:%._•:•i;;•i;•!•i•i•i:';•i•i::::ii!::::!• •;,.,.•,., ,

254

d

,"•

'•'

•

•

\& Previously

clay bounda•

KEY

Ground conductiviW

Published geology

• 0-5mmhos/m -• Boulder clay '::•:•• 5-10 .. • Sand and Gravel F• Red Crag Sand •-• •

•

0

>20

..

Limit of area su•eyed 0.5

1 km

x

Sand and Gravel

observed

Boulder clay observed

Fig. 135. LIN conductivityprofilingin geologicmappingof sedimentarysequencesin two areasin East Anglia, U.K. (a) and (c) are LIN geologic maps, (b) and (d) are the preexistingand improved geologic maps (after Zalasiewicz et al., 1985).


241

, 400m , 022 1

022 2

022 3

022 4

022 5

022 6

022 7

022 8

022 9

023 0

023 !

DISTANCE 023 2

023 3

023 4


io

2O0

5O0

PTH ( 30

IMAGED

RESISTIVITY

Fig. 136. Image of apparent conductivity versus depth from UTEM profile (after Macnae and Lamontagne, 1987).

on the ground. Spies (1989), and Spies and Frischknecht (this volume) provide a more complete review of these imaging techniques. The ease with which profiling methods with small sources can be applied to a very broad range of problems will enhance the popularity of these techniques. The rise of innovative methods of converting the results into depth sections which can be easily communicatedto a geologist,engineer, hydrologist, or archaeologistwill prompt the use of electromagnetic profiles in situations where great detail of the subsurface structure is required. ACKNOWLEDGMENTS

This chapter was completed from an original draft, prepared by Frank Frischknecht before his death, and required the special assistance of many people. A special thanks is in order for Duff Stewart and Bill Leslie, who computed countlessnumerical model responsesfor this chapter. The thorough and thoughtful reviews by Jack Betz and Dave Fitterman were invaluable to the junior authors as they attempted to complete the chapter in a fashion which might have been acceptable to Frank. This chapter would not have been finished without the (near) constant urging of Misac Nabighian. REFERENCES

Aittoniemi, K., Hirvonen, M. T. Javanainen, R., Leinonen,

E., and Rossi, M., 1985a, Horizontal loop multicomponent EM measurement and interpretation system: 55th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 247-249.

Aittoniemi, K., Hirvonen, M. T., Rajala, J., Sarvas, J., and Soikkeli, J., 1985b, Broadband electromagnetic measurements over conductive orebodies and their interpretation with heterogeneous plate models: Acta Polytechnica Scandinavica, Appl. Phys. set. no. 151.

Results obtained with an experimental field source system were interpreted by computer using a variable conductance plate model. Ako, B. D., 1982, Geophysical and Geochemical Exploration in Oyo State, Nigeria, in Hidden wealth: Mineral exploration techniquesin tropical forest areas, Laming, D. J. C., and Gibbs, A. K., Eds., Association of Geoscientists for International Development, AGID Rept. no. 7, 141-144b.

Annan, A. P., 1974, The equivalent source method for electromagnetic scattering analysis and its geophysical application: Ph.D. Thesis, Memorial Univ. of Newfoundland.

Arcone, S. A., Sellman, P. V., and Delaney, A. J., 1978, Shallow electromagnetic geophysical investigations of permafrost, in Proceed. Third Intl. Conf. Permafrost, V. I.: Natl. Res. Council, Canada, 502-507. Barbour, D. M., and Thurlow, J. G., 1982, Case histories of two massive sulphide discoveries in Central Newfoundland, in Prospecting in areas of glaciated terrain--1982: Can. Inst. Min.

and Metal.

300-321.

Slingram, VLF, and gravity were used to locate and map sub-economicmassive sulfide deposits in Ordovician felsic volcanics that also contain many graphitic conductors. Barlow, R. B., 1981, Night Hawk geophysical test range results using two electromagnetic systems, District of Cochrane, in Summary of field work, 1981, Wood, J., White, O. L., Barlow, R. B., and Colvinem, A. C., Eds., Ontario Geol. Surv. Misc. Paper 100, 152-159. •1984, Night Hawk geophysicaltest range resultsusing two electromagnetic systems: District of Cochrane, in Wood, J., White, C. L., Barlow, R. B., and Colvina, A. C., Eds., Summary of field work; Ontario Geol. Surv. Misc. Paper 119, 139-146. Barlow, P.M., and Ryan, B. J., 1985, An electromagnetic method for delineating ground-water contamination, Wood River Junction, Rhode Island, in Subitzky, S., Ed., Selected papers in the hydrologic sciences.U.S. Geological Survey Water Supply Paper 2270, 35-49.

Includes comparisonof LIN measurementsand specific conductivity measurementsof water samples. Barnett, C. T., 1984, Simple inversion of time-domain electromagnetic data: Geophysics, 49, 925-933. Barongo,J. O., 1983, Geophysicalinvestigationsfor kimberlite pipes in the greenstonebelt of western Kenya: Jour. African Earth Sci., 1, 235-253. Gives examples of slingram profiles over four pipes.

242

Frischknecht

Bartel, D.C., 1984, Interpretation of Crone pulse EM data with model catalog:Dept. Geol. Geophys., Univ. of Utah, Salt Lake City. Bartel, D.C., and Becker, A., 1988, Time-domain electromagnetic detection of a hidden target: Geophysics,53,


537-545.

Bartel, D.C., and Hohmann, G. W., 1985, Interpretation of Crone pulse electromagneticdata: Geophysics,50, 14881499.

Barton, G., 1985, Determination of the integrity of a claylined dike using electromagnetics,pH, and estimated

porosity, in 55th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 145. Measurements made with a LIN instrument atop a dike indicated localities where an acidic ferric chloride solution

was leaking through the dike. Bazinet, R., and Labrecque, P., 1986, EM detection of localized conductancevariations in large regional conductors [abs.], in 56th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 165-166. Bergeron, C. J., Jr., Ioup, J. W., and Michel, G. A., II, 1989,

Interpretation of airborne electromagneticdata usingthe modified image method: Geophysics, 54, 1023-1030. Bergey, W. R., Clark, A. R., Frantz, J. C., Keevil, N. B., and Smith, F. G., 1957, Discovery of copper-nickel orebodies at the Temagami Mine, Ontario, in Methods and case histories in mining geophysics:Can. Inst. Min. Metallurg., 168-175.

Tilt angle measurementsdefined the massive parts of Cu-Ni pyrite ores. Bergmann,H. J., 1960, Horizontal loop equipmentin ground survey: Can. Mining J., 81, 57-61. Betz, J. E., 1982, The electromagnetic response of the different conductive phenomenaof the Athabasca Basin area of Saskatchewanto the slingram coil configuration: Researchin Applied Geophysics,24, GeophysicsLaboratory, Univ. of Toronto. Includes an extensive set of model curves and a discussion

of the interpretation of slingram results from several localities

in the Athabasca

Basin.

Beukes, J. H. T., Steenkamp, W. H. B., du Piessics,A., Keywegt, R. J., and Havgor, M. E., 1984, Experimental terrain conductivity surveysover dykes in dolomiticareas on the Far West Rand: Ann. Geol. Surv., S. Africa, 18, 1-17.

Bevan, B., 1983, Electromagneticsfor mappingburied earth features: J. Field Archaeol., 10, 47-54. Black, R. A., Frischknecht, F. C., Hazlewood, R. M., and Jackson, W. H., 1962, Geophysical methods of exploring for buried channelsin the Monument Valley area, Arizona and Utah: U.S. Geol. Surv. Bull. 1083-F, 161-228.

The use of a large variety of methodsincludingslingram for mappingburied uraniumbearingchannelsin sedimentary rocks was evaluated. Blokh, Yu.I., Garanskii, E. M., Dobrokhotova, I. A., Renard, I. V., and Yakubovskii, Yu.V., 1984, Compre-

hensive interpretation of data from ungrounded loop method and magnetic exploration for iron quartzites: Geologiya i Geofizika, 25, 104-109. Boikov, S. A., Afanasenko, V. E., and Timofeyev, V. M., 1984, Investigation of areas of icing (ruled) formation and subsurfacewater dischargeunder permafrost conditions using surface geophysical techniques, in Permafrost Fourth Intl. Conf. Final Proceedings: Natl. Acad. Sci., Wash., 211-216.

Boldy, J., 1981,Prospectingfor deep volcanogenicore: Can. Inst. Min. Metal. Bull., 74 (834), 55-65.

Boniwell, J. B., and Dujardin, R. A., 1964, Discovery and exploration of the Poirier ore deposit: Bull. Can. Inst. Min. Metallurg., 57(679), 945-952.

et al.

Broadsidetilt angle measurementswere used to outline a copper-zincdepositthat occursin a seriesof rhyolite and dacite flaws.

Boniwell, J. B., and McKenzie, A.M., 1961, Case history of the Corridor Orebody, Mount Lyell, Tasmania: Proc. Austr. Inst. Min. Metall., 198, 281-297.

Includes results of fixed source tilt angle, turam and other geophysical surveys over a Cu orebody that contains about 10 percent total sulfides. Born, M., and Wolf, E., 1980, Principlesof optics, Sixth ed.: Pergamon Press, Inc. Bosschart, R. H., 1961, On the occurrence of low resistivity geologicalconductors: Geophys. Prosp., 9, 203-212.

The conductivityof a numberof sulfidezones is estimated from slingram measurementsusing scale model measurements.

Botha, W. J., 1980, Basis of the theory and interpretation of the dual frequency method:Ph.D thesis, Colorado School of Mines.

Braekken, H., 1964, The cross-ring system, a multipurpose electromagneticvariant, with somemodel and field observations: Geoexploration, 2, 36-50.

Cross-ring or wavetilt results are compared with turam and drilling results. Bragg,J. G., 1959,Saturationgeophysicalexploration:Bull. Can. Inst. Min. Metallurg. 49-55. Includes severalexamplesof slingramprofiles over sulfide deposits. Brant, A. A., Dolan, W. M., and Elliot, C. L., 1966, Coplanarand coaxial EM interpretationtests in Bathhurst Area, New Brunswick, Canada, 1956, in Mining Geophysics, v. 1, Case Histories: Soc. Expl. Geophys., 130-141.

Resultsobtainedover sulfideand graphiticconductorsand a highpermeabilityiron formationwith HCP and slingram surveys are compared and parameters estimated using phasor diagrams. Brock, J. S., 1973, Geophysicalexploration leading to the discovery of the Faro deposit: Bull. Can. Inst. Min. Metallurg., 66, 738, 97-118. The shootbacktilt-anglemethodwas usedin the discovery of a large flat-lying lead-zinc-silverore body in Cambrian phyllites. Brubaker, D. G., 1957, Apparatus and procedure for electromagneticprospecting:Trans. Am. Inst. Min. Metallurg. Eng., 208, 777-780. Butt, C. R. M., 1981, The nature and origin of the lateritic weathering mantle with particular reference to Western Australia, in Doyle, H. A., Glover, J. E., and Graves, D. I., Eds., Geophysicalprospectingin deeply weathered terrains: Publication no. 6, University Extension, Univ. Western Austr., 11-29.

Byers, A. R., 1957,Comparisonof electromagneticgeophysical prospectingmethodsover known sulphidezones in the Flin Flon Area, Saskatchewan:Dept. Min. Resources, Prov. Sasketchewan Rept. No. 28.

A comparisonwas made of resultsobtained with moving and fixed sourcetilt angle, slingramand long wire methods over conductive

sulfide zones.

Carson, J. A., Johnson,R. B., McCallum, M. E., Campbell, D. L., and Padgett,J.P., 1984, Evaluation of geophysical techniquesfor diatreme delineationin the Colorado-Wyomingkimbedite province,in Kornprobst,J., Ed., Kimberlites., I.: Kimberlites and related rocks, Elsevier, 21-31.

Includes example of LIN EM and VLF profiles over a conductive pipe.

ProfilingMethodsUsingSmallSources Cheriton, C. G., 1960, Anaconda exploration in the Bathhurst District of New Brunswick, Canada: Trans. Am.

Inst. Min. Metallurg. Petr. Engrs., 217, 278-284.


Tilt anglesurveysoutlinedthe axis of the Cariboudeposit which occurs in metavolcanic and metasedimentary rocks of Ordovician age.

Coney, D. P., and Myers, J. O., 1977, Some comparative field results for resistivity, very-low frequency electromagnetic and horizontal-loopelectromagneticmethods over fluorspar veins: Trans. Inst. Min. Metall. (Sec. B: Appl. Earth Sci.), 86, B-l-B3.

Comparesslingram,VLF magneticfield and resistivity results over conductive

veins.

Corbett, J. D., 1961, An empirical demonstrationof geophysicalmethodsacrossthe Cariboudeposit,Bathhurst, N. B.: Trans. Can. Inst. Min. Metallurg. Eng., 64, 160162.

Fixed and movingsourcetilt angle, HCP slingram,resistivity and SP results are compared. Cotwin, D. L., and Rhoades, J. D., 1982, An improved technique for determining soil electrical conductivity-depth relationsfrom above groundelectromagneticmeasurements: Soil. Sci. Soc. of America J., 46, 517-520.

Cratchley, C. R., and Evans, R. B., 1967, Geophysical surveysfor mineral depositsin area C/D, Western Uganda: Geol. Surv. Uganda, Special Rept. No. 6.

Conductanceand depth estimateswere made from slingram data as part of the ground evaluation of many conductors located by an airborne survey. Crone, J. D., 1966, The developmentof a new ground EM method for use as a reconnaissancetool, in Mining Geophysics, v. 1., Case Histories: Soc. Expl. Geophys., 151-156.

Fixed source and broadside tilt angle and slingram measurements were used in the study. •1977, Ground pulseEM--Examples of surveyresults in the search for massive sulphidesand new equipment

developments:Bull. Austr. Soc. Expl. Geophys., 8, 3842.

da Costa, A. J. M., 1989, Palmietfontein kimberlite pipe, South Africa--A case history: Geophysics, 54, 689-700. De Beer, J. H., Stettler, E. H., Duvenhage, A. W. A., Joubert, S. J., and Raath, C. J. de W., 1984, Gravity and geoelectricalstudiesof the Murchison greenstonebelt, South Africa: Trans. Geol. Soc. S. Africa, 87, 347-359.

De Jong,E., Ballantyne,A. K., Cameron,D. R., and Read, D. W. L., 1979, Measurement of apparent electrical conductivity of soilsby an electromagneticinductionprobeto aid in salinity surveys: Soil Sci. Soc. Amer. J., 43, 810-812.

Donohoo, H. V., Podolsky, G., and Clayton, R. H., 1970,

Early geophysicalexplorationat Kidd Creek Mine: Mining Cong. J., 56(3), 44-53.

Fixed sourceand broadsidetilt angle measurementsand a few slingrammeasurementswere used in the study of a very large zinc-copper-lead-silverore body found in rhyolites and andesites.

Donovan, T. J., Forgey, R. J., and Robers, A. A., 1979, Aeromagneticdetectionof diageneticmagnetiteover oil fields: Bull. Am. Assn. Petr. Geol., 63, 245-248.

Dowsett, J. S., 1969, Geophysicalexplorationmethodsfor nickel, in Mining and Groundwater Geophysics, 1967: Can. Geol. Survey Economic Geology Rept. no. 26, 311-321.

Includes fixed sourcetilt angle results over the Thompson and Pipe mines in northern Manitoba. Doyle, H. A. and Lindeman, F. W., 1985,The effectof deep

243

weathering on geophysicalexploration in Australia--A review: Austr. J. Earth Sci., 32, 125-135.

Duran, P. B., 1984, The effects of cultural and natural interference on electromagnetic conductivity data, in Nielsen, D. M., Ed., Surface and borehole geophysical methodsin groundwater investigations,Natl. Water Well Assn., 509-530.

Includesexamplesof the effectsof powerlines,pipelines, and fences on LIN

measurements.

Dyck, A. V., 1981,A methodfor quantitativeinterpretation of wideband, drill-hole EM surveys in mineral exploration: Ph.D. thesis, Univ. of Toronto; available as Re-

searchin Applied Geophysics,no. 23, GeophysicsLaboratory, Univ. of Toronto. Dyck, A. V., Bloore, M., and Vallee, M. A., 1980, User

manualfor programsPLATE and SPHERE: Researchin Applied Geophysics, no. 14, Geophysics Laboratory, Univ.

of Toronto.

Eadie, T., 1979,Stratifiedearth interpretationusingstandard horizontalloop electromagneticdata: Researchin Applied Geophysics,no. 9, GeophysicsLaboratory, Univ. of Toronto.

Eaton, P. A., and Hohmann, G. W., 1987, An evaluationof electromagneticmethods in the presence of geologic noise: Geophysics,52, 1106-1126. Eberle, D., Bosum, W., and Damaske, 1985, A multimethodgeophysicalprocedurefor base-metalexploration as appliedin the sand-covereddesertof Botswana:GeologischesJahrbuch,Reihe, E., Heft 29, Bundesantaltfur Geowissonschaften und Rohstaffe, Hannover.

The slingramand shootbacktilt anglemethodswere used with other methods for mineral exploration in a very conductive

environment.

Edwards, R. N., and Howell, E. C., 1976, A field test of the

magnetometricresistivity (MMR) method: Geophysics, 41, 1170-1183.

Ekren, E. B., and Frischknecht, F. C., 1967, Geological--

geophysicalinvestigations of bedrockin the Island Falls quadrangle,Aroostock and PenobscotCounties, Maine: U.S. Geol. Surv. Prof. Paper 527.

Slingram was used to map conductive black slates of Silurian, Ordovician, and Cambrian age. Emerson, D. W. (Ed.), 1980, The geophysicsof the Elura Orebody, Cobar, New South Wales: Bull. Austr. Soc. Expl. Geophys., 11, 143-347.

Flanigan,V. J., 1981,A slingramsurveyat YuccaMountain on the Nevada Test Site: U.S. Geol. Survey Open-File Report 81-980.

Flanigan, V. J., and Sadek, H., 1982, Ground-followup studiesof the 1977airborne electromagneticsurvey in the Assifar and Mulhal areas, Wadi Bidah District, Kingdom of Saudi Arabia: U.S. Geol. Survey Open-File Rept. USGS-OF-03-09.

An extensive set of slingramand S.P. data are given. Flanigan,V. J., Sadek,H., and Smith, C. W., 1982,Phase3 geophysicalstudiesin the Wadi Bidah District, Kingdom of Saudi Arabia: U.S. Geol. Survey Open-File Rept. USGS-OF-02-45.

Slingram,S. P., and somedrillingresultsare given. Flanigan,V. J., Wynn, J. C., Worl, R. G., and Smith,C. W., 1981,Preliminaryreport on geophysicsgroundfollow-up of the 1977 airborne survey in the Wadi Bidah District, Kingdomof SaudiArabia: U.S. Geol. Surv. SaudiArabian Mission Tech. Record 9.

Fleming,H. W., 1957,Magneticand electromagnetic investigations in Paska Township, District of Thunder Bay, Ontario, in Methods and Case Histories in Mining Geophysics:Can. Inst. Min. Metallurg., 210-220.

244

Frischknecht et al.


The fixed sourcetilt angle method was used to delineate non-economicpyrite, pyrrhotite, and graphitezones. •1961, The Murray Deposit, Restigouche County, N. B., a geochemical-geophysical discovery:Trans. Can. Inst. Min. Metallurg., 64, 163-168.

Slingramwas used in locationof massivesulfidedeposit that is weathered and leached to depths of 15-60 m.

Fraser, D.C., 1978,Geophysicsof the MontcalmTownship copper-nickeldiscovery:Can. Inst. Min. Metallurg.Bull., 71,789, 99-104.

Slingrammeasurements were madeat severalfrequencies and spacingsover a Ni-Cu pyrrhotitebody in a mafic intrusion.

Frischknecht, F. C., 1959, Scandinavianelectromagnetic prospecting:Trans., Am. Inst. Min., Metallurg.Eng., 214, 932-937.

•1966, Application of electromagneticsurveyingto geologicmappingin northernMaine, in Mining Geophysics, v. 1, Case Histories: Soc. Expl. Geophys., 10-17.

anomaliesat SOQUEM: Bull. Can. Inst. Min. Metallurg., 63,697,567-572.

A comparisonof the slingram, shootback,and turam methodsusing scale model and field results. Glaccum, R. A., Benson, R. C., and Noel, M. R., 1982, Improvingaccuracyand cost-effectiveness of hazardous waste site investigations:Ground Water Monitoring Review, 36-40.

Includes examplesof time variations of LIN anomalies over a leachateplume and surface spill.

Grady, S. J., andHaeni, F. P., 1984,Applicationof electromagnetictechniquesin determiningdistributionand extent of groundwater contaminationat a sanitaryland fill Farmington,Connecticut,in Nielson, D. M., Ed., Surface and boreholegeophysicalmethodsin groundwater investigations:Natl. Water Well Assn., 338-367. Good correlation

was observed between

LIN

measure-

ments and specificconductancemeasurementsof water from wells in the survey area.

Illustratesuse of slingrammethodin mappingconductive black

slates.

•1967, Fields about an oscillatingmagneticdipoleover a two-layerearth, and applicationto groundand airborne electromagnetic surveys: Colorado School of Mines Quart., 62, 326.

•1987, Electromagneticphysical scale modeling, in Nabighian, M. N., ed., Electromagneticmethodsin applied geophysicsmTheory,volume 1, Chapter 6, Soc. Expl. Geophys., 365-441. Frischknecht, F. C., and Ekren, E. B., 1961, Electromagnetic studies of iron formations in the Lake Superior region: Mining Eng., 13, 1157-1162.

Slingram results over oxidized iron formation in the CuyunaRangeand unoxidizedtaconiteson the Geologic Range.

•1963, Evaluation of magnetic anomalies by electromagneticmeasurements:U.S. Geol. Survey. Prof. Paper 475-C, 117-120.

Shows multi-frequency (20--4000Hz) measurementsover conductive and nonconductive iron formation using a fixed source.

Frischknecht, F. C., and Mangan, G. B., 1960, Preliminary

reporton electromagnetic modelstudies:U.S. Geol. Surv. Open-File Rept. 60-53.

Frischknecht,F. C., and Stanley,W. D., 1970,Airborneand groundelectricalresistivitystudiesalongproposedTransAlaska Pipeline System (TAPS) route [abs.]: Am. Assn. Petr. Geol., 54, 2481.

Frohlich, B., and Lancaster, W. J., 1986, Electromagnetic

surveyingin current Middle Easternarchaeology:Application and evaluation: Geophysics, 51, 1414-1437. Gaa'l, G., Koistinen, T., and Mattita, E., 1975, Tectonics and stratigraphy of the vicinity of Outokumpu, North Karelia, Finland: Geol. Surv. Finland, Bull. 271. Gatzweiler, R., Schmeling, B., and Tan, B., 1981, Exploration of the Key Lake Uranium Deposits, Saskatchewan, Canada, in Uranium Exploration Case Histories:Internat. Atomic Energy Agency, Vienna, AG-250/1,221-242.

Includes slingramand other geophysicalprofiles over ore zone associatedwith a graphitic conductor. Gaucher, E., 1983, Estimation of sulfide content of a potential orebody from surface observationsand its role in optimizingexplorationprogrammes,in Fitch, A. A., Ed., Developments in geophysical exploration methods Applied Science Publ. Ltd., 1-37. Gaucher, E., and St-Amant, M., 1970, Evaluation of EM

Greenhouse,J.P., and Harris, R. D., 1983, Migration of contaminants in groundwater at a landfill:a casestudy,7: DC, VLF, and Inductive Resistivity Surveys: J. Hydrology, 63, 177-197. Describes results from a site where the upper layer is a

sandyaquiferthat is very favorablefor the applicationof geoelectrical methods.A contaminantplumewasmapped usingresistivity, VLF, and LIN measurements. Greenhouse,J.P., and Monier-Williams, M., 1985, Geo-

physical monitoring of ground water contamination aroundwaste disposalsites:Ground Water Monit. Rev. 5(4), 63-69.

The use of several methods, including LIN measurements,

for long term monitoringof groundwater contamination was considered.

Greenhouse,J.P., and Slaine, D. D., 1983, The use of reconnaissanceelectromagneticmethods to map contaminant migration:GroundWater Monit. Rev., 3(2), 47-59. LIN measurementswere used in studyingseveral industrial andmunicipalwastesites;the presenceof underlying shalesor clayslimited usefulnessof the methodin mapping contaminantsin upper layers. Gunn, P. J., and Chisholm,J., 1984,Non-conductivevolcanogenicmassivesulfidemineralizationin the Pilbaraarea of western Australia: Expl. Geophys., 15, 143-153.

SIROTEM, EM 37, PEM, and IP profilesover a massive sulfide.

Gupta, O. P., Joshi,M. S., and Negi, J. G., 1980,Scale model electromagnetic responseto inline and broadside systemsat skewtraversesof a dippinghalf planeembedded in a conductinghost rock: Geophys. Prosp., 28, 119-134.

Haeni, F. P., 1986,The use of electromagneticmethodsto delineatevertical and lateral lithologic changesin glacial

aquifers,in Surfaceand boreholegeophysicalmethods andgroundwaterinstrumentation conference andexposition: Natl. Water Well Assn., 259-282.

The LIN loop-loopand VLF methodswere usedto map lateral variationsin resistivitythat correspondwith lithologic changes.

Hanneson,J. E., and West, G. E., 1984a,The horizontal loopelectromagnetic response of a thinplatein a conductive earth: Part IraComputationalmethod: Geophysics, 49, 411-420.

Hanneson,J. E., and West, G. E., 1984b,The horizontal


Profiling Methods Using Small Sources loop electromagnetic responseof a thin plate in a conductive earth: Part II--Computational results and examples: Geophysics, 49, 421-432. Haren, R. J., and Whiteley, R. J., 1981, Slingram electromagnetic surveys of the Woodlawn orebody and the black shale, in Whiteley, R. J., Ed., Geophysical case study of the Woodlawn orebody, New South Wales, Australia: Pergamon Press, 193-203. Parameters of the Woodlawn orebody were estimated from slingram measurements. Hausel, W. D., Glahn, P. R., and Weeozick, T. L., 1981, Geological and geophysicalinvestigationsof kimberlite in the Laramie Range of southeasternWyoming: Geol. Surv. Wyoming, Prel. Rept. No. 18. Includes contour maps of LIN EM results over a kimberlite dike and a pipe. Hedstrom, E. H., and Parasnis, D. S., 1958, Some model experiments relating to electromagneticprospectingwith special reference to airborne work: Geophys. Prosp., 6, 322-341.

Hermance, J. F., and Culp, J. L., 1982, Kilauea Iki lava lake, geophysicalconstraintson its present (1980) physical state: J. Volcan. Geoth. Res., 13, 31-61. Herrero, E., and Norgaard, P., 1982, Ground geophysicsin tropical areas, in Laming, J. C. and Gibbs, A. K., Eds., Hidden Wealth: Mineral exploration techniquesin tropical forest

areas:

Assn.

of Geoscientists

for Internat.

Devel-

op., Rept. no. 7, 126-136. Hoekstra, P., 1978, Electromagnetic methods for mapping shallow permafrost: Geophysics, 43, 782-787. Hoekstra, P., and McNeill, D., 1973, Electromagnetic probing of permafrost, in Permafrost: North American Contribution, Second International Conference, Nat. Acad. Sci., Wash., 517-526. Hoekstra, P., and Standish, R., 1984, Applications of fixed

frequency conductivity profiling and transient soundings to ground water exploration, in Nielsen, D. M., Ed., Surface and borehole geophysical methods in ground water investigations, Natl. Water Well Assn., 150-173. Examples are given of the use of LIN instruments in mappingfilled channels, buried depositsof coarsegrained material, and unfrozen zones in Alaskan rivers in the winter.

Hohmann, G. W., 1987, Numerical modelingfor electromagnetic methods of geophysics, in Nabighian, M. N., ed., Electromagnetic methods in applied geophysics--Theory, volume 1: Soc. Expl. Geophys., 313-363. Hone, J. G., 1980, Geoelectric properties of the Elura Prospect, Cober N SW, in Emerson, D. W., Ed., The geophysics of the Elura Orebody, Cobar, New South Wales, Bull. Austr. Soc. Expl. Geophys. 2, 4, 178-183. Hood, P. J., 1980, Mineral exploration trends and developments: Can. Min. J., 101, 20-63. Horton, R. J., Smith, B. D., and Washburne, J. C., 1985,

Electrical geophysical investigations of massive sulfide deposits and their host rocks, West Shasta copper-zinc district: Economic Geology, 80, 2213-2229. Gives slingram, VLF, and single loop transient results over a pyrite-chalcopyrite-sphalerite orebody and over a probable graphitic conductor in Devonian rhyolites. Houston, J. F. T., Eastwood, J. C., and Cosgrove, T. K. P., 1986, Location potential borehole sites in a discordant flow regime in the Chalk Aquifer at Lulworth using integrated geophysical surveys: Quart. J. Eng. Geol., London, 19, 271-282.

Hoversten, G. M., 1981, A comparison of time and frequency domain E.M. sounding techniques: Ph.D. thesis, Univ. of California, Berkeley.

245

Howell, M. I., 1966, A soil conductivity meter: Archaeometry, 9, 3-19. Jagodits, F. L., Betz, J. E., Krause, B. R., Saracoglu, N., and Wallis, R. H., 1986, Ground geophysicalsurveys over the McClean uranium deposits, northern Saskatchewan: Bull. Can. Inst. Min. Metallurg., 79, 35-50.

Several methods including slingram and fixed source tilt angle were used to map deeply buried graphitic metapelite zones which serve to localize the Athabasca Basin.

uranium

mineralization

in

Jakosky, J. J., 1929, Operating principles of inductive geophysical processes,in Geophys. Prosp.: Trans. Am. Inst. Min., Metallurg. Eng., 81, 138-179. Johnson, I. M., and Doborzynski, Z. B., 1986, A novel ground electromagnetic system: Geophysics, 51,396-409. Joklik, G. F., 1960, The discovery of a copper-zinc deposit at Garon Lake, Quebec: Econ. Geol., 55, 338-353. Slingram was used to map a small pyrrhotite-pyrite-chalcopyrite-sphalerite deposit in metamorphosed Keewatin lavas, tuffs, and sediments. Results of a traverse parallel to the orebody as well as across strike are shown. Jones, B., and Wong, J., 1975, Some thin sheet responsesto a model dipole-dipole prospecting system: Geoexploration, 13, 187-196. Joshi, M. S., Gupta, O. P., and Negi, J. G., 1984, Scalemodel response of a thin vertical conductor below a conductive, inductive, or laterally inhomogeneous overburden layer: Geophysics, 49, 2159-2165. •1988, On the effects of thickness of the half-plane model in HLEM induction prospecting over sulphide dykes in a highly resistive medium: Geophys. Prosp., 36, 551-558.

Jurick, R., and McHattie, R., 1982, Mapping soil resistivity: Northern Engineer, 14 (4), 14-19. Kahma, A., and Puranen, M., 1958, Geophysical case history of the Vihanti zinc ore deposit in Western Finland, in Geophysical studiesin mining, hydrological, and engineering projects: European Assoc. Expl. Geophys., 84-96. The first slingram survey in Finland (1946) helped locate sphaleriteore bodies associatedwith pyrite and pyrrhotite mineralization

in Precambrian

schist.

Kamara, A. Y. S., 1981, Review Geophysical methods for kimberlite prospecting: Bull. Aust. Soc. Expl. Geophys., 12, 43-51.

Kamenetskii, F. M., 1976, Handbook for the application of the method of transient processes in ore geophysics: Nedra, Leningrad (in Russian). Kauahikaua, J., 1985, Mapping of a buried surface beneath limestone in Agat, Territory of Guam using electromagnetic profiling techniques: U.S. Geol. Surv. Open-File Rept. 85-124. Kaufman, A. A., 1978a, Frequency and transient responses of methods of electromagnetic fields created by currents in confined conductors: Geophysics, 43, 1002-1010. •1978b, Resolving capabilities of inductive methods of electroprospecting:Geophysics, 43, 1392-1398. •1989, A paradox in geoelectro magnetism, and it's resolution, demonstrating the equivalence of frequency and transient domain methods: Geoexpl., 25, 287-317. Kawasaki, K., and Osterkamp, T. E., 1984, Electromagnetic induction measurementsin permafrost terrain for detecting ground ice and ice-rich soils: Geophys. Inst., Univ. of Alaska, available NTIS PB86-14493 8.

Kay, A. E., Allison, A.M., Botha, W. J., and Scott, W. J., 1983, Continuous geophysicalinvestigationsfor mapping permafrost distribution, Mackenzie Valley, N.W.T., Canada, in Permafrost; 4th Intl. Conf. Proc.: Natl. Acad. Sci., Wash., 578-583.


246

Frischknecht

Keller, G. V., and Frischknecht, F. C., 1966, Electrical methodsin geophysicalprospecting:PergamonPress, Inc. Kennedy, L. A., and Woodbury, A.D., 1983, The Hat Creek coal deposit: a geophysical case history: Geoexploration, 21, 19-48. Ketola, M., 1968, The interpretation of slingram(horizontal loop) anomalies by small-scale model measurements: Geol. Surv. Finland, Rept. of Investigations 2.

An extensive set of examples of field data and its interpretation using multiple thin sheet and wide dike models. Scale model results for magnetic bodies and folded conductors

are included.

•1979, On the application of geophysicsin the indirect exploration for copper sulfide ores in Finland, in Hood, P. J., Ed., Geophysicsand geochemistryin the searchfor metallic ores: Geol. Surv. Can. Econ. Geol. Rept. 31, 665-684.

Includes many examples of slingram and other geophysical results comparedwith borehole lithology and borehole logs for areas in which black schistsare ubiquitous. •1982, On the applicationof geophysicsand geologyto exploration for nickel-copper ore deposits: Geol. Surv. Finland, Rept. of Investigations, 53. Ketola, M., and Puranen, M., 1967, Type curves for the interpretationof slingram(horizontalloop) anomaliesover tabular bodies: Geol. Surv. Finland, Rept. of Investigations

1.

Includes examples of interpretation of field data. Kiilsgaard, T. H., Greenwood, W. R., Puffett, W. P., Naqvi, Mohammed, Roberts, R. J., Worl, R. G., Merghelani, Habib, Flanigan, V. J., and Gazzaz, A. R., 1978, Mineral exploration in the Wadi Bidah District, 1971-1976 Kingdom of Saudi Arabia: U.S. Geol. Surv., Saudi Arabian Project Rept. 237. Koerner, R. M., Lord, A. E., Tyagi, S., and Brugger, J. E., 1982, Use of NDT methods to detect buried containers in saturated silty clay soil, in Proc., Nat. Conf. on Manag. of Uncontrolled Hazardous Waste Sites: HMCRI, Silver Spring, MD, 12-16. The effectiveness of a metal detector, a LIN instrument, a

magnetometer, and a ground probing radar in locating buried 30 gal steel and plastic containerswas determined. Kwan, K. C. H., 1989, A novel method of computing the EM response of a conductive plate in a conductive medium: M.S. thesis, Univ. of Toronto; available as Research in

Applied Geophysics, no. 45, Geophys. Lab., Univ. of Toronto.

Labson, V. F., Frischknecht, F. C., and Becker, A., 1984, A portable tensor magnetotelluric receiver design and field test: 54th Ann. Internat. Mtg., Expanded Abstracts, Soc. Expl. Geophys., 99-102. Lacy, R. J., and Morrison, B.C., 1966, Case history of integrated geophysical methods at the Mission deposit, Arizona, in Mining Geophysics,v. 1, Case Histories: Soc. Expl. Geophys. 321-325.

Tilt angle EM surveys showed a number of anomalies due to numerous small conductorsforming an anisotropicbox work. The results were important in the discovery of a large porphyry copper orebody. Ladwig, K. J., 1984, Use of surface geophysicsto determine flow patterns and acid-sourceareas in surface mine spoil, in Nielson, D. M., Ed., Surface and borehole geophysical methods in ground water investigations, Natl. Water Well Assn., 455-471.

LIN measurementswere useful in mapping the configuration of undisturbed rock beneath spoil at abandonedcoal mines and in locating possiblesourcesof acid production.

et al.

Lajoie, J. J., and West, G. F., 1977, Two selected field examples of EM anomalies in a conductive environment: Geophysics, 42, 655-660. Slingram data are interpreted for a case in which the overburden causessubstantialphase rotation of the anomaly from a bedrock conductor and another case where pronouncedcurrent gathering is observed. Laurila, M., 1963, The geophysical "case history" of the Kalliokylfi sulfide deposit: Geoexploration, 1, 16-24. Slingram was used to outline a number of Cu-Zn bearing pyrrhotite-pyrite deposits. Lee, T., 1982, Asymptotic expansionsfor transient electromagnetic data: Geophysics, 47, 38-46. Lilley, F. E. M., and Woods, D. V., 1978, The channelingof natural electric currents by orebodies: Bull., Austr. Soc. Expl. Geophys., 9, 62-63. Lin, W., 1969, A model study of the Crone Shootback EM method: Thesis, University of California, Berkeley. Lodha, G. S., 1977, Time domain and multifrequency electromagnetic responses in mineral prospecting: Res. in Appl. Geophys. no. 8, GeophysicsLaboratory, Univ. of Toronto.

Includes examplesof tilt angle data and interpreted slingram data compared with UTEM and other geophysical data.

Lodha, G. S., and West, G. F., 1976, A comparisonof the responseof some wideband EM prospectingsystemsto a deep conductingplate: EM Project, Res. Report 1, Dept. Phys. (Geophys.), Univ. Toronto. Macnae, J. C., 1979, Kimberlites and exploration geophysics: Geophysics, 44, 1395-1416. Includes HCP slingram results over kimberlite pipes and scale model studiesfor the edge of a thin sheet and for a thick circular

disk.

•1980, The Cavendish Test Site: A UTEM survey plus a compilation of other ground geophysicaldata: Res. in Appl. Geophys. No. 12, Geophys. Lab., Univ. of Toronto.

Slingram measurements at four frequencies and three spacingswere interpreted and the results compared with UTEM

and other results.

Macnae, J., and Lamontagne, Y., 1987, Imaging quasilayered conductive structures by simple processing of electromagnetic data: Geophysics, 52, 545-554. Macnae, J. D., and Walker, P., 1981, The Thomas Township test target: an example of EM interpretationusingsimple models: Res. in Appl. Geophys. No. 22, Geophys. Lab., Univ.

of Toronto.

Results obtained with several time- and frequency-domain methods including slingram and fixed source tilt angle were interpreted using the horizontal plate and sphere models.

Malmqvist, D., 1958, The geophysicalcase history of the Kunkberg ore deposit in the Skellefte District, Northern Sweden, in Geophysical studies in mining hydrological and engineering projects: European Assn. Expl. Geophys., 32-54. A variety of methods including slingram were used to delineatea smallpyrite-pyrrhotite zinc bearingore body in Precambrianquartz porphyries and tuffs. •1965, A numerical calculation of the electromagnetic field from a vertical and a horizontal magnetic dipole above a homogeneousearth: Geoexploration, 3, 175-227.

Malmqvist, D., and Parasnis,D. S., 1972, Aitik---geophysical documentationof a third-generationcopper deposit in north Sweden: Geoexploration, 10, 149-200.


Profiling Methods Using Small Sources Slingram and other electromagnetic methods were useful in delineation of the most conductive parts of a large low grade "disseminated copper" deposit in gneissicPrecambrian rocks. Depths and conductances were estimated using scale model data. Malone, E. J., Whiteley, R. J., Tyne, E. D., and Hawkins, L. V., 1981, Brief description of the Woodlawn orebody and summaryof geophysicalresponses,in Whiteley, R. J., Ed., Geophysical case study of the Woodlawn orebody New South Wales, Australia; Pergamon Press 3-11.

Includes a qualitative summary of responsesfor many different

methods

and instruments.

March, H. W., 1953, The field of a magnetic dipole in the presence of a conducting sphere: Geophysics, 18, 671684.

Mason, M., 1929, Geophysical exploration for ores, in Geophysical Prospecting: Trans. Am. Inst. Min., Metallurg. Eng., 81, 9-43. Tilt and strike angle results over an orebody near Sudbury, Ontario are compared with scale model data. McCracken, K. G., Oristaglo, M. L., and Hohmann, G. W., 1986, A comparison of electromagneticexploration systems: Geophysics, 51, 810-818. •McNeill, J. D., 1980a, Electromagnetic terrain conductivity measurementsat low induction numbers: Geonics Ltd., Technical Note TN-6. •1980b, Survey interpretation techniques for EM38: Geonics

Ltd.

Technical

Note

TN-9.

•1985, EM34-3 measurements at two inter-coil spacings to reduce sensitivity to near-surface material: Geonics Ltd. Technical

Note TN-19.

Note

TN-22.

Compares surface LIN measurementswith measurements of total dissolved

solids and borehole

tilted conductinghalf-planesto a horizontal-loop prospecting system: Geoexploration, 6, 207-244. Negi, J. G., Gupta, O. P., and Joshi, M. S., 1987, Corrections for conductivity estimates in induction prospecting of sulphide-dykes in a layered environment: Geophys. Prosp., 35, 718-734. Newman, G. A., Anderson, W. L., and Hohmann, G. W., 1989, Effect of conductive host rock on borehole transient EM responses:Geophysics, 54, 598-608. Newman, G. A., Hohmann, G. W., and Anderson, W. L., 1986, Transient electromagnetic response of a three-dimensionalbody in a layered earth: Geophysics, 51, 16081627.

Ogilvy, R. D., 1983, Transient electromagnetic prospecting technique applied to mineral exploration in the United Kingdom: Applied Earth Science, Trans. Inst. Min. Metallurg. (Sect. B. Appl. Earth Sci.), 92, 132-147. Compares HCP slingram results with HCP transient results over Pb-Zn veins in rhyolitic tuffs and mudstones. •1986, Theoretical EM response curves over a thin dipping dyke in free space--separated inline loop configuration: Geophys. Prosp., 34, 769-788. •1987, Interpretation of transient EM common-loop anomalies by response characteristics: Geophys. Prosp., 35, 454-473.

Olhoeft, G. R., 1975,The electrical propertiesof permafrost, Ph.D. thesis, University of Toronto. •1978, Electrical propertiesof permafrost,in Proc. 3rd Internat.

Conf.

Permafrost:

Natl.

Res. Council

Can. Ot-

tawa, 127-131.

Olm, M. C., 1981, Electromagnetic scale model study of the dual frequency differencingtechnique: M.Sc. thesis, Colorado School Mines.

McNeill, J. D., and Bosnar, M., 1986, Surface and borehole electromagnetic ground water contamination surveys, Pittman Lateral Transect, Nevada, U.S.A.: Geonics Ltd. Tech.

247

conductivities.

McNeill, J. D., Edwards, R. N., and Levy, G. M., 1984, Approximate calculationsof the transient electromagnetic response from buried conductors in a conductive halfspace: Geophysics, 49, 918-924. Medlin, E., and Knuth, M., 1986. Monitoring the effects of undergroundwater recovery system with EM, in Surface and borehole geophysical methods and ground water instrumentation conference and exposition: Natl. Water Well Assn., 368-378. The results of LIN measurementsmade about two years apart were differenced to evaluate the effect of a waste recovery system.

Mizyuh, L. Ya., and Podzharyi, V. M., 1963, Measurements of the parameters of an elliptically polarized field in inductive electric prospecting: Bull. (Izvestiya) U.S.S.R. Acad. Sci. Geophys. Ser., English edition, 7, 641-648. Motter, J. W., 1983, Broadband electromagneticmethods in developments in geophysical exploration methods 4, Fitch, A. A., Ed.: Appl. Sci. Publ. Ltd., 101-153.

Includesmulti-frequencypolarizationellipse slingramand turam results over the Western World deposit in Califor-

Olsson, O., Duran, O., Jfimtlid, A., and Stenberg, L., 1984, Geophysical investigationsin Sweden for the characterization of a site for radioactive waste disposal--an overview: Geoexploration 22, 187-201. Osterkamp, T. E., Kawasaki, K., and Gosink, J.P., 1983, Shallow magnetic induction measurementsfor delineating near surface hot ground water sourcesin Alaskan geothermal areas: J. Energy Res. Tech., 105, 156-161. Padget, P., Ek, J., and Eriksson, L., 1969, Vargistrb•k, a case-historyin ore-prospecting:Geoexploration, 7, 163175.

An experimentalthree componentslingramoperatingat 18 kHz was used in an effort to locate disseminatedsphalerite and chalcopyritemineralization in felsic volcanics. Palacky, G. J., 1987, Resistivity characteristicsof geologic targets, in Nabighian, M. N., Ed., Electromagneticmethods in applied geophysics--Theory, volume 1: Soc. Expl. Geophys., 53-129. Palacky, G. J., and Kadekaru, K., 1979, Effect of tropical weathering on electrical and electromagnetic measurements: Geophysics, 44, 69-88. Includes interpretation of slingram profiles over a Cu bearing conductor and examples of tilt angle shootback measurements

over weathered

volcanics.

Palacky, G. J., and Sena, F. O., 1979, Conductor identification in tropical terrain--Case histories from the Itapicuru greenstone belt, Bahia, Brazil: Geophysics, 44, 19411962.

nia.

Mundry, E., and Blohm, E. K., 1987, Frequency electromagnetic soundingusing a vertical magnetic dipole: Geophys. Prosp., 35, 110-123. Nabighian, M. N., 1971, Quasi-statictransientresponseof a conductingpermeable two layer sphere in a dipole field: Geophysics, 36, 25-37. Nair, M. R., Biswas, S. K., and Mazumadar, K., 1968, Experimental studies on the electromagneticresponseof

Includesseveralexamplesof slingramand tilt-angleshootback results over bedrock

conductors.

Palacky, G. J., and West, G. F., 1973, Quantitative interpretation of input AEM measurements: Geophysics, 38, 1145-1158.

Palacky, G. J., Ritsemo, I. L., and deJong, S. J., 1981, Electromagneticprospectingfor ground water in Precam-


248

Frischknecht

et al.

brian terrains in the Republic of Upper Volta: Geophys. Prosp. 29, 932-955. Parasnis, D. S., 1966, Electromagnetic prospecting--C. W. Techniques: Geoexploration 4, 177-208.

•1983, Night Hawk geophysical test range results, Night Hawk Lake, District of Cochrane, in Wood, J., White, O. L., Barlow, R. B., and Colvine, A. C., Eds., Summary of field work: Ontario Geol. Surv. Misc. Paper,

Includes comparison of results obtained with a "beam" and a conventional slingram system and a discussionof the effect of overburden on slingram anomalies. •1970, An elegant, universal nomenclaturefor electromagnetic moving source-receiver dipole configurations: Geophys. Prosp. 18, 88-102. •1971, Analysis of some multi-frequency, multi-separation electromagnetic surveys: Geophys. Prosp. 19, 163-

Poddar, M., 1982, Interpretation of Pulse EM anomalies over Gani conductors: Geophys. Prosp., 30, 86-100. Podolsky, G., 1966, An evaluation of an airborne electromagnetic anomaly in northwestern Quebec, in Mining Geophysics, v. 1, Case Histories: Soc. Expl. Geophys.,

179.

Interpretation of field results using half plane phasor diagrams and discussionof discrepanciesbetween results at different frequencies and spacings. Parchas, C., and Tabbagh, A., 1978, Simultaneous measurements of electrical conductivity and magnetic susceptibility of the ground in electromagneticprospecting:Archaeophysika, 10, 682-691. Paterson, N. R., 1957, A sulphidediscovery, Rubb-Jamieson area, Ontario, in Methods and Case Histories in Mining Geophysics: Can. Inst. Min. Metallug., 246-259. Fixed source tilt angle and ellipticity measurementswere used in the discovery of a pyrite-chalcopyrite-sphalerite orebody in Precambrian volcanics. •1968, Developments in ground E.M. prospecting: Can. Mining, 89, 106-110. Gives examples of interpretation of slingram data and compares the slingram and tilt angle methods. •1969, Exploration for massive sulphidesin the Canadian shield, in Morley, L. W., Ed., Mining and ground water geophysics:Geol. Surv. Can. Econ. Geol. Rept. no. 26, 275-289.

Includes examples of slingram, tilt angle and other data compared with drilling results. •1973, Extra low frequency (ELF) EM surveys with the EM-25 labs.I: Geophysics, 38, 188. Peltoniemi, M., 1982, Characteristics and results of an airborne electromagnetic method of geophysical surveying: Geol. Surv. Finland, Bull. 321.

Includes an extensive amount of interpreted slingram and other geophysical data that were collected to evaluate airborne

EM

results.

Pemberton, R. H., 1989, Geophysical response of some Canadian massive sulfide deposits, in Proceedings of Exploration '87: Ontario Geol. Surv., Special Volume 3, 517-531.

Peterson, R. C., Wagner, J. R., Hemphill-Haley, M. A., Weller, L. N., Kilbury, R., and Champlin, J. B. F., 1986, Resistivity mapping of a complex aquifer system at a hazardous waste site in the Salinas Valley, California, in Surface and borehole geophysical methods and ground water instrumentation conference and exposition: Natl. Water Well Assn., 187-202.

LIN measurementswere used to map a permeable channel and other hydrologic features. Pitcher, D. H., 1985, Night Hawk geophysical test range results, District of Cochrane, in Wood, J., White, O. L., Barlow, R. B., and Colvine, A. C., Ed.: Summary of Field Work

and Other

Activities:

Ontario

Geol.

Surv.

Misc.

Paper 126, 170-177. Pitcher, D. H., Barlow, R. B., and Wedge, D. R., 1982, Night Hawk geophysical test range results, Night Hawk Lake, District of Cochrane, in Wood, J., White, O. L., Barlow, R. B., and Colvine, A. C., Eds., Summary of Field Work: Ontario Geol. Surv. Misc. Paper 106, 152161.

116, 132-141.

197-205.

Interpretation of slingram and airborne EM anomalies over a non-economic conductor that has high magnetic permeability is discussed. Pridmore, D. F., Ward, S. H., and Motter, J. W., 1979, Broadband electromagnetic measurements over a massive sulfideprospect: Geophysics, 44, 1677-1699. Discussesinterpretation of tilt angle and ellipticity measurements at 14 frequencies made over a massive sulfide deposit (Western World, California) in conductive host rocks using fixed vertical and horizontal source loops. Puustinen, K., 1977, Exploration in the northwest region of the Koitelainen gabbro complex, Sodankyla, Finnish Lapland, in Prospecting in areas of glaciated terrain: Inst. Min. Metal., London, 6-13.

Slingram was effective in mapping Zn bearing graphitic phyllite. Rai, S.S., 1985a, Transient electromagneticresponsesof a thin conductingplate embedded in conductive host rock: Geophysics, 50, 1342-1349. •1985b, Crone pulse electromagnetic responses of a conductive thin horizontal sheet--theory and field application: Geophysics, 50, 1350-1354. Rai, S.S., and Bhattacharya, B. B., 1986, Quantitative interpretation of pulse EM measurements over a weathered kimberlite diatreme: Geophys. Prosp., 34, 220-231. Shows comparison of slingram and loop-loop TEM profiles over a kimberlite pipe. Rai, S. S., and Sarma, G. S., 1986, In-loop Pulse EM responseof a stratified earth: Geophys. Prosp., 34, 232239.

Rai, S.S., and Verma, S. K., 1982, Quantitative interpretation of horizontal-loop EM measurementsusing a permeable sphere model: Geophys. Prosp., 30, 486-500. •1984, Nomograms to interpret Crone PEM data using a dipping sheet model: Geophys. Prosp., 32, 740-749. Rajala, J., and Sarvas, J., 1985, Electromagnetic scattering from a platelike, non-uniform conductor: Acta Polytechnica Scandinavica, Ma 43. Raloff, J., 1984a, Salt of the earth: Science News, 126, 298-301.

•1984b, Surviving salt: Science News, 126, 314-317. Rao, I. B. R., and Bhimasankaram, V. L. S., 1973, Electromagnetic modeling of sheet-like bodies by the combinedloop version of the transient pulse induction method: Geoexploration, 11, 87-95. Rao, I. B. R., and Kabra, S., 1983, Model results for a dipole-dipole transient electromagnetic technique: Bull. Austr. Soc. Exp. Geophys., 14, 72-74. Rhoades, J. D., 1981, Predicting bulk soil electrical conductivity versus saturation paste extract electrical conductivity calibrationsfrom soil properties: Soil Sci. Soc. Am. J., 45, 42-44. Rhoades, J. D., and Corwin, D. L., 1981, Determining soil conductivity-depth relations using an induction electromagnetic soil conductivity meter: Soil Sci. Soc. Am. J., 45, 255-260. Rodrigues, E. B., 1984, A critical evaluation of the use of geophysicsin ground water contamination studies in On-

Profiling Methods Using Small Sources tario, in Nielson, D. M., Ed., Surface and borehole geophysicalmethods in ground water investigations:Natl. Water Well Assn., 603-617.


Three examples are given in which complexities in the geologyprevented mapping contaminantplumes with LIN methods.

Rodriguez, B. D., 1984, A self-potential investigation of a coal mine fire: in 54th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 165-168.

249

Discovery of the Mobrun Copper Ltd. sulphide deposit, Noranda Mining District, Quebec, in Methods and Case Histories in Mining Geophysics: Can. Inst. Min. Metallurg., 237-245. A pyrite-sphalerite-chalcopyrite deposit in metarhyolite was found by a mobile tilt angle system and outlined by fixed source tilt angle measurements. Sengpiel, K. P., 1988, Approximate inversion of airborne EM data from a multilayered ground: Geophys. Prosp., 26, 446-459.

High conductivity zones measured with a LIN instrument coincided well with SP anomalies that were thought to mark regions of active burning. Rudy, R. J., and Caoile, J. A., 1984, Utilization of shallow geophysical sensing at two abandoned municipal/industrial waste landfills on the Missouri River Floodplain: Ground Water Monit. Rev. 4, 57-65.

Sharma, J. K., Madhusudan, I. C., Sarker, B., Chakrararty, S., Singh, N. P., and Rao, B. K. V. S., 1983, Some aspects of geophysical exploration for base metals in major mineralized belts of Rojasthan and Gujarat, in Proceedingsof Symposium on Exploration Geophysics in India: 19451978, Calcutta: Geol. Surv. India Special Pub. Series no.

LIN profiles and resistivity soundingswere used to help site monitoring wells. Rudy, R. J., and Warner, J. B., 1986. Detection of abandoned undergroundstoragetanks in Marion County, Florida, in Surface and borehole geophysical methods and ground water instrumentation conference and exposition:

Slingram, turam, SP, and magnetic results over a number of conductors are compared with borehole information. Siikarla, T., 1964, The nickel occurrence of Hitura, Nivala commune, central Finnish Bothnia: Geoexploration, 2,

Natl. Water Well Assn., 674-688. A LIN

instrument

was used in location

of abandoned

Slingram was used to trace graphitic metapellite conducBasin.

Uranium

mineralization

is

associated with the subcrops of graphitic material. Sartorelli, A. N., and French, R. B., 1982, Electromagnetic induction methodsfor mapping permafrost along northern pipeline corridors, in French, H. M., Ed., Proc. 4th Canadian

Permafrost

Conf.:

Natl.

133-149.

Slingram and turam were used in exploration for Ni-Cu bearing sulfide deposits associatedwith serpentinite mas-

gasoline tanks. Saracoglu, N., Wallis, R. H., Brummer, J. J., and Golightly, J.P., 1983, The McClean Uranium Deposits, northern Saskatchewan--Discovery: Bull. Can. Inst. Min. Metallurg., 76, 852, 63-79. tors in the Athabasca

2, 173-196.

Res.

Council

of Can-

ada, Ottawa, 283-295. Schwenk, C. G., 1976, Discovery of the Flambeau deposit,

Rusk County, Wisconsin--a geophysical case history: Trans. Am. Inst. Min. Metallurg. Eng., 260, 208-214.

Slingram was used in the discovery of a steeply dipping Cu-Zn massive sulfide deposit in Precambrian felsic volcanics. Supergenechalcocite and bornite are prevalent in the upper part of the deposit. Scollar, I., Weidner, B., and Segeth, K., 1986, Display of archaeologicalmagnetic data: Geophysics, 51,623-633. Scott, W. J., and Fraser, D.C., 1973, Drilling of EM anomalies caused by overburden: Bull. Can. Inst. Min. Metallurg., 66, 735, 72-77. Shows tilt angle anomalies caused by inhomogeneousor irregular overburden and resistivity, gravity, and seismic data that can be used to properly identify the sourcesof

sifs.

Singh, S. K., 1973, Electromagnetic transient responseof a conductive sphere embedded in a conductive medium: Geophysics, 38, 864-893. Sinha, A. J., 1977, Dipole electromagnetic mapping of permafrost terrains: Theoretical developments and computer programs: Geol. Surv. Canada Paper 77-13. •1979, Maxiprobe EMR-16: A new wideband multifrequency EM system: Current research, Part B., Geol. Surv. Canada, Paper 79-1B, 23-26. •1980, Electromagnetic resistivity mapping of the area around Alfred, Ontario, with Geonics EM 34 system, in Current Research, part A, Geol. Surv. Can. paper 80-1A, 293-300.

A LIN survey was useful in mapping clay layers and areas where the glacial drift is thin. Sinha, A. K., and Collett, L. S., 1973, Electromagnetic fields of oscillating magnetic dipoles placed over a multilayer conducting earth: Geol. Surv. Can. Paper 73-25. Sinha, A. K., and Stephens, L. E., 1983, Permafrost mapping over a drained lake by electromagnetic induction methods, in Current Research, Part A, Geol. Surv. Can. Paper 83-1A, 213-220. Slichter, L. B., 1932, Observed and theoretical electromagnetic model responseof conducting spheres, in Geophys. Prosp.: Trans. Amer. Inst. Min., Metallurg. Eng., 443459.

Smee, B. W., and Sinha, A. K., 1979, Geological, geophysical, and geochemical considerations for exploration in clay-covered areas--a review: Bull. Can. Inst. Min. Metallurg., 72, 804, 67-82.

the anomalies.

Seaborne, T. R., Sirikci, D. H., and Sowerbutts, W. T. C., 1979, Examples of horizontal loop electromagneticanomalies controlled by geologicalfaulting: Geoexploration, 17, 77-87.

Seguin, M. K., 1978, Exploration of magnetic taconite and itabirite with integrated aeroelectromagneticand magnetic methods: Bollettino Di Geofisica Teorica Ed Applicata, 21, 68-90.

Seguin, M. K., Gaucher, E., and Desbian, R., 1984, Beep Mat 1--an automated miniaturized EM mineral detector, in Prospecting in Areas of Glaciated Terrain: Inst. Min. Metal., 193-200. Seigel, H. D., Winkler, H. A., and Boniwell, J. B., 1957,

Includes examples of slingram profiles by J. E. Betz over a good conductor buried by a thick clay overburden. Smith, B. D., and Ward, S. H., 1974, On the computation of polarization ellipse parameters: Geophysics, 39, 867-869. Spies, B. R., 1974, Transient electromagneticmodel studies, 1973: Bur. Min.

Resour.

Austral.

Record

1974/152.

•1980a, Interpretation and design of time domain EM surveys in areas of conductive overburden, in Emerson, D. W., Ed., The geophysicsof the Elura Orebody, Cobar, New South Wales: Bull. Austr. Soc. Expl. Geophys., 11, 272-281.

•1980b, One-loop and two-loop TEM responsesof the Elura deposit, Cobar, N SW, in Emerson, D. W., Ed., The

250

Frischknecht

geophysics of the Elura Orebody, Cobar, New South Wales: Bull. Austr. Soc. Expl. Geophys., 11,282-288. •1980d, The application of the transient electromagnetic method to Australian conditions:Field examplesand models studies:Ph.D. thesis, Macquarie University, Syd-


ney.

•1981,

Electrical geophysicsin the USSR: Bur. Min.

Resour. Austral. Record 1981/66, BMR Microform MF 181.

•1983, Recent developments in the use of surface electrical methodsfor oil and gasexplorationin the Soviet Union: Geophysics, 48, 1102-1112. •1989, Depth of investigation of electromagnetic soundingmethods: Geophysics, 54, 872-888. Spies, B. R., and Parker, P. D., 1984, Limitations of large-looptransientelectromagneticsurveysin conductive terrains: Geophysics, 49, 902-912. Stephens, L. E., and Graham, B. W., 1985, An electromagnetic survey over the Gloucester landfill site for the detection of contaminated ground water, in Current Research, Part A, Geol. Surv. Can. Paper 85-1A, 431-440. Areas of high conductivity that coincided with a land fill and with roads that may have been salted were fround by LIN

measurements.

Sternberg, B. K., and Oehler, D. Z., 1984, Electrical methods for hydrocarbon exploration: I. Induced polarization (INDEPTH) method, in Proc. from SMU SymposiumIII, Unconventionalmethodsin explorationfor petroleumand natural gas: Southern Methodist Univ. Press, 188-201. Stewart, M. T., 1982a, Evaluation of electromagneticterrain conductivity methods for rapid mapping of salt water interfacesin coastal aquifers: Ground Water, 20, 538-545.

Includes examples of use of LIN measurementsin mapping areas of salt water intrusion in Florida. •1982b, Evaluation of electromagneticterrain conductivity measurements for detection and mapping of salt water interfaces in coastal aquifers: Water Resour. Res. Center Publ. no. 60, Univ. South Florida.

Strangway, D. W., 1966a, Electromagneticscale modeling: in Runcorn, S. K., Ed., Methods and techniques in geophysics:Interscience Publishers, 1-31. •1966b, Electromagnetic parameters of some sulfide ore bodies, in Mining Geophysics, v. 1, Case Histories: Soc. Expl. Geophys., 227-242. Includes several examples of interpretation of slingram anomalies and a tabulation of conductances for many deposits as well as a set of phasor diagrams for a halfplane. Strangway, D. W., and Koziar, A., 1979, Audio-frequency magnetotelluric sounding--A case history at the Cavendish GeophysicalTest Range: Geophysics,44, 1429-1446. Stratton, J. A., 1941, ElectromagneticTheory: McGraw Hill Book Co., Inc. Subrahmanyam, B., and Jagannadham, M., 1984, A geophysical approachfor indirectly locating auriferousveins in Kolari Area, Maharashtra: J. Assn. Expl. Geophys., 5, 17-21.

Describes results obtained with slingram and other methods.

Svetov, B. S., 1960, Results of model studies by the inductive method: Trans. Acad. Sci. U.S.S.R., Ser. Geophys., No. 1 (in Russian). Svetov, B. S., Mizyuk, M. Ya., and Podzharyy, V. M., 1968, Rare electrical prospecting methods using the method of the elliptical polarizationof the magneticfield: Nedra, Moscow (in Russian). Sweeney, J. J., 1984, Comparison of electrical resistivity methods for investigation of ground water conditions at a land fill site: Ground Water Monitor. Review, 4, 52-59.

et al.

LIN and dipole-dipole resistivity data over a landfill are compared. Tabbagh, A., 1984, On the comparison between magnetic and electromagnetic prospection methods for magnetic features detection: Archaeometry, 26, 171-182. •1985, The response of a three-dimensionalmagnetic and conductive body in shallow depth electromagnetic prospecting:Geophys. Jour., 81,215-230. •1986a, Applications and advantagesof the Slingram electromagneticmethod for archaeologicalprospecting: Geophysics,51,576-584. •1986b, What is the best coil orientation in the slingram electromagneticprospectingmethod?:Archaeometry, 28, 185-196.

Telford, W. M., and Becker, A., 1979, Exploration case histories of the Iso and New Insco ore bodies, in Hood, P. J., Ed., Geophysicsand geochemistryin the searchfor metallic ores: Geol. Surv. Can. Econ. Geol. Rept. 31, 605-629.

Multi-frequency, multi-spacing slingram, tilt angle and ellipticity, and tilt angle as well as other geophysicaldata over the Cu-Zn massive sulfidedepositsin volcanic flows are interpreted and compared. Telford, W. M., and Geldart, L. P., Sheriff, R. E., and Keys, D. A., 1976,Applied Geophysics:CambridgeUniv. Press. Thomas, S.C., 1983, Ground geophysicalprospectingmethods: Somenoteson copper explorationin the United Arab Emirates: Trans. Inst. Min. Metallurg. (Sect. B. Appl. Earth Sci.), 92, B 191-B 199.

Includes slingramand other geophysicaldata over copper prospects.

Tite, M. S., and Mullins, C., 1969, Electromagnetic prospecting:a preliminary investigation:ProspezioniArcheologiche, 4, 95-102. •1970, Electromagneticprospectingon archaeological sites using a soil conductivity meter: Archaeometry, 12, 97-104.

Tschanz, C. M., and Frischknecht, F. C., 1986, Geochemical and geophysicalstudiesof selectedareasof the eastern part of the Sawtooth National Recreational Area, Idaho: U.S. Geol. Surv. Bull. 1545-D, 209-230.

Includes HCP and VCP slingramresults over conductive anisotropic"slates" and conductiveveins. Velikin, A. B., and Bulgakov, Yu. I., 1971, Transient methodof electricalprospecting(one loop version):Seminar on geophysicalmethods of prospectingfor ore minerals, UNO, Moscow, July 1967. Verma, R. K., 1980, Master tablesfor electromagneticdepth soundinginterpretation:IFI/Plenum Press, New York. •1982, Electromagnetic sounding interpretation data over a three-layer earth, 1 and 2: IFI/Plenum Press, New York.

Villegas-Garcia, C. J., 1979, On the electromagnetic response of nonuniform overburden layers: Scale model experiments:Researchin appliedgeophysicsno. 10, GeophysicsLab., Univ. of Toronto. Villegas-Garcia,C. J., and West, G. F., 1983,Recognitionof electromagnetic overburden anomalies with horizontal loop electromagneticsurvey data: Geophysics,48, 42-51. Wait, J. R., 1951, The magneticdipole over the horizontally stratified earth: Can. J. Phys., 29, 577-592. Wait, J. R., and Hill, D. A., 1972a, Electromagnetic surface fieldsproducedby a pulse-excitedloop buried in the earth: J. Appl. Phys., 43, 3988-3991. •1972b, The transient electromagneticresponse of a spherical shell of arbitrary thickness: Radio Science, 7, 911-935.

Walker, P., 1981,The inductive responseof thin platesin a stratified space:Research in Applied Geophysics,no. 21, GeophysicsLaboratory, Univ. of Toronto.


ProfilingMethodsUsingSmall Sources Ward, S. H., 1957,The role of geophysicsin explorationin New Brunswick, in Methods and Case Historiesin Mining Geophysics:Can. Inst. Min. Metallurg., 221-226. •1959, Unique determinationof conductivity,susceptibility, sizeand depthin multifrequency electromagnetic exploration:Geophysics,24, 531-546. •1961, The electromagneticresponseof a magnetic iron ore deposit:Geophys.Prosp., 9, 191-202.

Tilt angle and azimuth angles were measuredover a tabular magnetite body. •1967, The electromagneticmethod, in Mining Geo-

physics,v. 2, Theory: Soc. Expl. Geophys.,228-372. Ward, S. H., and Barker, 1958,Case History of the Juniper Prospect:Trans. Am. Inst. Min. Engr., 100-104. Includesinterpretationof fixed sourcetilt anglemeasure-

251

examplefrom basemetalexplorationactivitiesin the Savant Lake--Sturgeon Lake area, Providenceof Ontario Canada: Zeilschrift der Deutschen Geologischen Gesell Schoft, 132(1), 215-239.

Werner, S., 1947, Geophysicalinvestigationsin connection with prospecti..ng for manganeseores in the Parish of Jokkmokk, in Odman, O. H., Ed., Manganesemineralization in the Ultevis District Jokkmokk, north Sweden:

Sveriges Geologiska Undersokning, Ser. C., no. 487, 67-79.

Very smallquadratureslingramanomalieswere found to be associated with hollardite mineralization.

•1958, Geophysicalhistoryof a deep-seatedpyritic ore body in Northern Sweden, in Geophysicalstudies in mining, hydrologicaland engineeringprojects:European Assn. Expl. Geophys., 3-19.

ments over sulfide mineralization in slates and argillites.

Ward, S. H., and Gledhill, T., 1957, Electromagneticsurveying-groundmethods,in Methodsandcasehistoriesin mininggeophysics:Bull. Can. Inst. Min. Metallurg.,6370.

Ward, S. H., Anderson,G. J., Randolph,R. E., and Blake, R. L., 1955, The inductive electromagneticmethod applied to iron exploration:Trans. Am. Inst. Min. Engrs., 1121-1126.

The fixed sourcetilt angle method was used to map iron formations and other conductors in the Marquardt and Vermilion iron ranges. Ward, S. H., Pridmore, D. F., Rijo, L., and Glenn, W. E.,

1974, Multispectralelectromagnetic explorationfor sul-

Slingramwas usedto map near surfacepyrite mineralization that gradesinto a Zn-Cu orebodyat a depthof 150m. Wesley, J.P., 1958, Responseof a dyke to an oscillating dipole: Geophysics,23, 128-133. West, G. F., 1960,Quantitativeinterpretationof electromagnetic prospectingmeasurements: Ph.D. Thesis, University of Toronto.

West, G. F., and Edwards,R. N., 1985,A simpleparametric model for the electromagneticresponseof an anomalous body in a host medium:Geophysics,50, 2542-2557. Westerberg,1965,The "Boom-Slingram":A new portable EM-instrument for ore prospecting:Geoexploration, 3, 149-154.

White, P.S.,

1966, Airborne electromagneticsurvey and

fides: Geophysics, 39, 666-682.

ground follow-up in northwesternQuebec, in Mining Geophysics,v. 1, Case Histories: Soc. Expl. Geophys.,

Tilt angleand ellipticitywere measuredat the Cavendish test site using 14 frequenciesand the WAVETILT and

252-261.

NULL configurations. Watson, H. G.I., 1931, The Bieler-Watson method, in Studiesof geophysicalmethods1928and 1929:Can. Dept. Mines, Geol. Survey Memoir 165, 144-160. Weber, D. D., and Flatman, G. T., 1986, Statisticalap-

proachto groundwatercontamination mappingwith electromagneticinduction: a case study, in Surface and boreholegeophysicalmethodsand groundwater instrumentationconferenceand exposition:Natl. Water Well. Assn., 315-333.

Multi-spacingLIN profilesfrom the Pittmansite,Nevada were smoothed and the results were inverted by computer for each station.

Weber, D. D., Scholl, J. F., LaBrecque, D. J., Walther, E.G., and Evans, R. B., 1984, Spatial mappingof conductivegroundwatercontamination with electromagnetic induction: Ground Water Monit. Rev., 4, 70-77.

A methoddevelopedfor estimatingthe conductivityof the middlelayer (aquifer)of a three-layersectionusingLIN data sets for four spacingswas applied to study of a contaminantplume near HendersonNevada. Webster, S.S., and Skey, E. H., 1979, Geophysicaland geochemicalcasehistoryof the Que River deposit,Tasmania, Australia, in Hood, P. J., Ed., Geophysicsand geochemistry in the searchfor metallicores:Geol. Surv. Can. Econ. Geol. Rept. 31,697-720.

A Zn-Pb-Cu-Ag massive sulfide deposit in Cambrian dacite and andesite was traced using slingramand fixed source tilt angle measurements. Weidelt, P., 1983, The harmonic and transient electromag-

netic responseof a thin dippingdike: Geophysics,48, 934-952.

Wellmer, F. W., 1981,Geologicalinterpretationof geophysical data of a volcanic terrain in the Canadian Shield: an

A massive sulfide conductor, mainly pyrrhotite, that

gradesinto a graphiticconductorwas tracedusingslingram and other geophysicalmethods. White, R. M., Miller, D. G., Brandwein, S.S., and Benson, A. F., 1984,Pitfalls of electrical surveysfor groundwater contamination,in Nielson, D. M., Ed., Surface and bore-

holegeophysical methodsin groundwater investigations: Natl. Water Well Assn., 472-482.

Examplesare givenin whichgeologicnoiseand cultural noisecomplicatedinterpretationof LIN data. Whiteley, R. J., 1981a, Geophysicalcase study of the WoodlawnorebodyNew South Wales, Australia: Pergamon Press, Inc.

•1981b, Vertical loop, dip angle electromagnetictraversesat Woodlawn, in Whiteley, R. J., Ed., Geophysical casestudyof the WoodlawnOrebodyNew SouthWales, Australia: PergamonPress, Inc., 187-192.

The orebody was outlined with fixed sourceand shortspacedbroadsidetilt anglemeasurements. Williams, B. G., and Baker, G. C., 1982,An electromagnetic inductiontechniquefor reconnaissancesurveysof soil salinityhazards:Austr. J. Soil. Res., 20, 107-118. Wilt, M., and Stark, M., 1982,A simplemethodfor calculatingapparentresistivityfrom electromagnetic sounding data: Geophysics,47, 1100-1105. Windschauer,R. J., 1986, A surfacegeophysicalstudy of a borrow pit lake, in Surface and borehole geophysical methodsandgroundwaterinstrumentation conferenceand exposition:Natl. Water Well Assoc., 283-291. Won, I. J., 1983,A sweep-frequency electromagnetic exploration method, in Fitch, A. A., Ed., Developmentsin geophysicalexplorationmethods--4, Applied Science Publishers Ltd., 39-64.

Includesresultsobtainedwith a prototypetruck mounted

252

Frischknecht

loop-loop system and results of theoretical and scale model studies.


Wood, J. H., and Stewart, M. T., 1985, Geophysical and geologiccharacteristics of fracturezonesin the Carbonate Floridian aquifer: Fla. Wat. ResourceRes. Cntr., Pub. 88, Univ. Florida, available NTIS PB86-156999. Yamashita, M., 1983, A user-friendly electromagneticpros-

pectingsystem,in Pye, E.G., and Barlow, R. B., Eds., Exploration technology development program of the Board of Industrial Leadership and Development Summary of Research 1981-1983:Ontario Geol. Surv. Misc. Paper, 115, 115-124.

et al.

Zablocki, C. J., 1966, Electrical properties of some iron formations and adjacent rocks in the Lake Superior Region, in Mining Geophysics,v. 1, Case Histories: Soc. Expl. Geophys., 465-492. Zalasiewicz, J. A., Mathers, S. J., and Cornwell, J. D., 1985, The application of ground conductivity measurementsto geologicalmapping: Quart. J. Eng. Geol., London, 18, 139-148.

Zietz, I., Eaton, G. P., Frischknecht, F. C., Kane, M. F., and Moss, C. K., 1976, A western view of mining geophysicsin the USSR: Geophysics,41, 310-323.



253

APPENDIX

The following appendices describe several commercially available instruments designed for profiling with small loop sources in the time or frequency domain. The descriptions of several other instruments adaptable to profiling, which are primarily used

APPENDIX

A:

momentof 120Am2. Operating frequencies are approximately 220, 520, 1600, 3700, and 9500 Hz. A variety of loop spacingsbetween 40 and 200 m can be selected. A digital fiber optic cable link transmits communication, reference, and control data between and receiver.

Coder/decoder

case histories.

GEFINEX

The GEFINEX EM 303 employs an aircore transmitting loop and transmitter having a low frequency

the transmitter

for EM sounding, can be found in Spies and Frischknecht, this volume. The information about these instruments was obtained directly from the manufacturers, their technical literature, and published

circuits

also allow speech communication between operators. The multicomponentreceiver employsthree orthogonal elliptically shaped aircore loops that are nested together and carried on the operators chest. The loops have an inductive feedback loop so that their output voltage is independentof frequency. An N-path notch filter phase-locked to the powerline frequency is used for rejection of powerline noise. To eliminate sensitivity to harmonics of the operating frequencies, multistep approximationsto a sinusoidalwaveform are used for the reference signalsto the phase detectors. After the operator selects the loop spacing and maximum time allowed for a single measurement, the receiver automatically measures the in-phase and quadrature

EM

303

parts of the three orthogonalcomponentsof the field at all five frequencies. The variance of the in-phase part of the vertical component of the field is compared against a quality criterion to determine when an accurate measurement has been made; then the frequency is changed. All of the measurementsand the variance, time, and other desired header information are auto-

matically stored in nonvolatile memory. The results can also be displayed on an LCD readout in the field. Normally the transmitting loop is oriented horizontally so that slingramresultsare obtainedfor the HCP, PERP, and NULL configurations. Results for the PERP configuration are used in preference to results for the HCP configuration for a number of purposes including estimating the dip of steeply dipping thin sheets.The transverse horizontal component, as measured with the NULL configuration, is useful in resolving individual conductors and their strike when there are a number of closely spaced conductors. Also, the NULL configuration is less sensitive than the other two configurations to topographically induced in-phase noise. If desired, wavetilt or polariza-


254

Frischknecht

et al. 30

tion ellipse parameterscan be calculatedreadily from the HCP and PERP measurements.Although designed for horizontal use, the transmittingloop could be used vertically to obtain results for the VCP, VCA, and other less common configurations. A comparison of data obtained with the GEFINEX EM 303 systemfor the HCP and NULL configurations is shown in Figure A-1. The two sets of interpreted parameters agree very well. Noise due to position and orientation

errors is smaller in the NULL

20

10

-10 -2o

than in the

HCP results. Because of the absence of flanking maxima, contour mapsof NULL data may be simpler

-30

than contours of HCP data. Profiles for the HCP and

PERP configurationsand the theoretical profilesgenerated by computer interpretation are compared in Figures A-2 and A-3. Further information may be found in Aittoniemi et al. (1985a, b). Technicaldata and field exampleswere furnishedby Dr. Martti Hirvonen, Outokumpu Oy, Electronics Division, P.O. Box 85, SF-02201, Espoo, Finland.

Fig. A-2. Comparison of HCP field data (solid line) obtained over the Hallaperfi orebody, Finland and a computedmodel (dashedline). The parametersof the modelare ot= elXototr= 18, depthto upper edge = 10 m, dip = 128degreesand depth extent

= 150 m.

3O

20 -

2O

0

Y -t. 0

i

,20m ,

•

o

•--10 N

N

•

0

-30

-20

-•0

-t. 0

Fig. A-1. Comparisonof profilesfor the HCP (Z) and NULL (Y) configurationsacrossa tabular sulfidebody in Kalliokylfi in central

-2o

Finland.

Fig. A-3. Comparison of PERP field data (solid line) over the Hallaporfi orebody, and a computed model (dashed line). The parametersof the model are ot= 14, depth to upper edge = 7.6 m, dip = 122 degreesand depth extent = 120 m.



APPENDIX

B: GENIF_IGS/EM-4

The GENIE (GEometry Normalized In-phase EM) System was designedto measurethe ratio of magnetic field amplitudes at one or more pairs of frequencies (Johnsonand Doborzynki, 1986). The original SE-88 system is operated in a HCP configurationwithout a reference cable. Recently the IGS/EM-4 system was developed to make GENIE measurements without a reference cable or simultaneousGENIE and slingram measurements

The GENIE

with a reference

TM-2

transmitter

cable.

cored transmitting coils, crystal controlled drive circuits, and a battery mounted together in a single backpack unit. One coil transmits at a "reference" frequency and the other at a "signal" frequency.

Transmittermomentsare 136,91, 45, and 23 Am: correspondingto the frequencies, 112.5,337.5, 1012.5, and 3037.5 Hz. The five pairs of frequenciesthat can be used are 337.5/112.5, 1012.5/112.5, 3037.5/112.5, 1012.5/337.5,and 3037.5/337.5.Feedback loops in the transmitter keep the loops accurately tuned and maintain the relative and absolute outputs of the loops stable to within 0.1 percent. In the receiver, a singlecoil is used to measureboth frequencies. After amplification, the raw signal is filtered to remove noise at frequencies below the reference frequency and above the signal frequency. Notch

filters remove

the fundamental

and third har-

monics of the powerline frequency (50 or 60 Hz). After this preliminary filtering, the reference and signalare separatedinto two channelsby synchronousdetection and low passfiltering, and then integrated.The reading is calculated from the outputs of the reference and signalchannelsusing the expression,

As1)

reading = NsrA r

SYSTEM

The reading is displayed digitally with a resolution of 0.1 percent. The original SE-88 receiver uses an iron cored coil attached to the console. Magnetic field amplitudes at the reference and signalfrequencieswere displayed on analog meters. The amplitude ratio is read from a digital display. Signal averaging of 2, 4, 8, or 16 s is used.

The newer EM-4

consists of two metal

x 100%,

whereAs is the amplitudefrom the signalchannel,A r is the amplitudefrom the referencechannel,andNsr is a factor adjusting for different transmitter moments.

255

receiver uses an aircore coil which

is connected to the console by a short cable. The coil is designed to be mounted on a backpack or to be placed on the ground. Measurementsare continuously averaged. The operator monitors the magnetic field amplitudesor amplitude ratio and terminates the measuring cycle when satisfactory convergence is observed. Values for amplitudes, amplitude ratio, and the standard deviation of all one second samplesused in the amplitude measurements,as well as coordinate and time information, are retained in solid state memory. When a reference cable is used, in-phase and quadraturereadingsat two frequenciesare obtained as well as the GENIE-type amplitude ratio. The EM-4 is designed to output survey data to a printer or computer through a RS-232 interface. Sufficient output format control is provided to give either an organized listing or a printer-plot profile. The original SE-88 system was generally considered usable out to a coil separation of about 150 m. Improvements inherent in the EM-4 receiver design permit coil separationsin excess of 200 m. GENIE-type measurements can readily be made using either the in-line or the broadside technique as there is no connecting cable between the transmitter and receiver. Slingram measurementswith the EM-4 receiver are generally made only with the in-line technique, though, as with any slingram instrument, broadside

measurements can be made in areas free of brush. The EM-4 receiver is also used with the Scin-

trex TF-2 large loop multi-frequency transmitter in surface and borehole applications. GENIE results for both in-line and broadside survey

256

Frischknecht et al.


...... •: ....... ,.-...-..-.:.-.-,.-. .....,,,,

........ • ...... ---•.

_,• ........ ,•'•,•,.

,

. :-.•5'=-:7 ,L•5+oo, /,',,•,/• •,"•"'":"':;>':;•" •C,.//../•.•.•. ß ...... /'............ ' "'--••. •\ ./•..IZ .... ,----•:::.-: ,_-_:-;:---: ......................... ,i I. -ß._.._..::::Y::..-__-. \,, /'//\ o•o•o.._..._.o-----o

•.....-'

...o.. \

•"-o---....

/', \ \\"--• ....--_:_•:-•:.-_:.-. ....... ::L: \ ß.•+oo,

•.

\,, \

X•

II .... .,/ //,-..

•

'•-•//

'••.• ' /f.. ............. .::::.-.:.-..:..-._, . .,.•.__•_:•'

///

ß

/

ø/

/

'

.......................... •:_-_:::.•/ :::::::::::::::::::::::: /.____..-.:.-_:-'-.....:

0 0

0 0

0 0

0 0

0 0

0 0

0 0

•.

i,o

+

•

•

+

o

+

03

+

co

I

I

I

I

i

i

COIL

-•

SEPARATION

I00 m _

•'•T,.• 0 I

RATIO

ß

+

( % )

+ 20

I

+ I0

•O,.T I00 m I

./---.

3037.5/112 5

//•,/, .,.., .....1012.5 /112 5 /' ............ 3,57.5 /112.5

200 m I -20

Fig. B-1. GENIE in-line (top) and broadside(bottom)resultsare frequencypairs 337.5/112.5,1017.5/112.5and 3037.5/112.5 Hz. Coil separation and stationintervalare 100and 25 m, respectively. Plottingpointis midway between transmitter

and receiver.

Profiling Methods Using Small Sources +1o%

o

/-\

....... / '\\.../


IP

' .

3037.5

IP

........ß....-%, /

/

/-\

modes are shown in Figure B-1. The survey area is one of low-to-moderate relief with near vertical graphitic horizons

-lO%

\ 1012.5

:3:37.5

under

STANDARD

DEVIATION

3037. 5

1123.5

AMPLITUDE

3037.5

RATIO

0

lOOm I

400

IS

I

20

•D S

i

BiL

I

i

200 N

•

I

400N

Fig. B-2. EM-4 slingram and GENIE measurementsover line 100E of the Night Hawk Geophysical Test Range. Coil separation = 150 m. Station interval = 25 m.

Coil

4900W

and

5150W.

The

broadside

results

runs from

stations

150S to 50N.

The

in-

phase and quadrature measurements at four frequencies, the standarddeviations at two frequencies, and a single amplitude ratio are shown. Measuring times were typically 50 s per frequency pair. The coil separation is 150 m. The nearly equal in-phase slingram response at widely spaced frequencies shows that a target has very high conductance, resulting in a poor response in the GENIE measurements. Technical data and field examples were furnished by Dr. Ian Johnson, Scintrex Limited, 222 Snidercroft Road, Concord, Ontario, CANADA L4K lB5.

+10%

/ 112.5

10 rn of overburden.

shown in the lower half of Figure B-1 show a similar result. The broadside survey has, however, covered basically the same area with only two profiles, which required only half as much time to survey as the four in-line profiles. Slingram and GENIE measurements from an IGS/ EM-4 based survey over the Night Hawk geophysical test range are shown in Figure B-2. The target is a short, wide, graphitic conductor under 90 rn of overburden. On this survey line, the top surface of the conductor

112.5

less than

spacing for both surveys was 100 m. Readings were taken at frequency pairs 337.5/112.5, 1012.5/112.5, and 3037.5/112.5Hz. The in-line resultsshownin the upper half of Figure B-1 indicate conductors centered at station

\

257


258

Frischknecht

APPENDIX

C: MAXMIN

I, MAXMIN

et al.

II, AND MAXMIN

ceiver

console

are oriented

III

SYSTEMS

in the correct

azimuth.

The MaxMin II and III (Maximum coupled-Minimum coupled) systems were designedfor HCP and PERP slingrammeasurementsand broadsidetilt angle

Normally the HCP configurationis used, however, the receiver can easily be positionedhorizontally to make

measurements.

PERP

The

two

units

are

similar

but

the

MaxMin III has a heavier transmitter and operates at a lower range of frequencies than the MaxMin II. The MaxMin I is a completely redesigned system that operates at more frequencies, spacings,and configurations

than the MaxMin

The MaxMin

II

II and the MaxMin

transmitter

consists

III.

of an aircore

loop and battery powered transmitter carried by one person. Transmitter moments are 220, 200, 120, 60,

and30 Am2 corresponding to the fiveoperating frequencies 222, 444, 888, 1777, and 3555 Hz. In most units, available loop separationsare 25, $0, 100, 150, 200, and 250 m, or 100, 200, 400, 600, 800, 1000, and 1200 fl. The battery pack is separate from the transmitter.

Two ferromagnetic-cored receiving coils mounted on either

side of the receiver

console with their axes

vertical when used in the normal HCP configuration are used. A cable connecting the transmitter and receiver provides a reference signal which bucks out or compensatesthe primary part of the received field and also serves as a phase reference for synchronous detection of the secondary field. A switched-capacitor bandpass filter and noise clipping circuits before the synchronousdetectors and low-pass filters following the synchronous detectors provide narrow system bandwidth and relative immunity to sferics. Three scales provide full scale sensitivities of ___4percent, -+20 percent, and _ 100 percent for both the in-phase

and quadraturecomponentsof (Z/Zo - 1). Servingas a reference line, the interconnectingcable is part of an intercom system that also provides voice communication between operators. The intercom is not available when tilt angle measurements are made without the reference

cable. Both the transmitter

and receiver

are

equipped with electrical inclinometersthat read the tilt of the loops in the vertical plane containing the traverse line, provided the transmitting loop and re-

measurements.

PERP

measurements

are nor-

malized assumingthe primary field for a horizontal loop. With greater difficulty, vertical coplanar measurements can be made. The system can be used to make tilt angle measurementswith either a vertical or horizontal transmitter loop; normally the broadside mode is used for this purpose. Shootback measurements can be made in the tilt angle configuration,but for efficiency two receivers are required, one leading and the other trailing the transmitter. The MaxMin III loop and transmitter is normally carried by one person and a secondperson carriesthe battery packs. The MaxMin III receiver is basically the same as the MaxMin II receiver except for the difference in available frequencies and spacings. Transmitter moments are 600, 550, 300, 150, and 75

Am2 corresponding to thefrequencies 111,222,444, 888, and 1777 Hz. Loop separationsare $0, 100, 150, 200, 250, and 300 m, or 200, 400, 600, 800, 1000, and 1200 ft.

The MaxMin I system is similar in configurationto the MaxMin II and III and is designed to be operated by two people. Normal operatingfrequenciesare 110, 220, 440, 880, 1760, 3520, 7040, and 14,080 Hz; the correspondingtransmitter moments are 240, 235,230,

220, 180,90, 45, and22 Am2. Also,the receivercan measurethe tilt angle and amplitude of $0 or 60 Hz in order to use powerlines as sources. Three sets of 11 spacingseach are available: 100, 200, 300, 400, 500, 600,800, 1000, 1200, 1600, 2000 fi, and metric spacings in the sameproportionsin the ranges25 to $00 m, or 20 to 400 m. The systemcan be operated with HCP, VCP, and VCA configurations and two PERP configurations.

A new model, the MaxMin I-10, adds frequenciesof 28 160 and $6 320 Hz.

The transmitter

moments

are

slightly reduced, providing moments of 200, 190, 170,

140, 110, 80, 40, 20, 10, and 5 Am2 for the ten


frequencies. The longest separation in each of the three sets is replaced by a short separationof 50 ft, 12.5 m, or 10 m. The VCA and powerline configura-


tions are not available

in this model.

One MaxMin I PERP configurationis the same as for the MaxMin II, with the transmitterloop horizontal and the receiver loop vertical. In the other PERP configuration,the transmitter loop is vertical and the receiving loop is horizontal. When used in a PERP configuration,MaxMin I readingsare normalized by the primary field for a vertical rather than a horizontal loop. By making measurementsat each station using both PERP configurationsand summing the results, errors in the in-phase componentdue to topography are eliminated. The results are expressedas normalized in-phase and quadrature components, but the

APPENDIX

D: SLINGRAM

APPENDIX

The

Geonics

EM-31

shapes of profiles resemble those for the shootback method in which angles are measured. A small computer/digitaldata acquisition system is available for use with the MaxMin systems. This unit can be used in the field for variable-time stackingto improve the signal-to-noiseratio when long spacings are used and to calculate average transmitter-receiver slopes. Apparent resistivitiescan be displayed when operating as a low induction number slingram. Data stored in the field can be transferred to a printer, telephone modem, other computer, or to a cassette recorder.

Technical data were furnished by APEX Parametrics Limited, P.O. Box 818, Uxbridge, Ontario, CANADA

PORTABLE

The Sveriges Geologiska Slingram consistsof small square aircore loops for both the transmitter and receiver. Battery packs and a transmitter operatingat a single factory selected frequency of 32 000, 18 000, 3600, or 800 Hz are attached directly to the transmitter loop. Available loop spacingsare 40 and 60 m. The receiver electronics are attached directly to the receiver coil. An analog meter provides in-phase and quadraturereadingswith a sensitivityof +-10 percent

259

L0C

1K0.

ELECTROMAGNETIC

UNIT

and a resolution of 0.1 percent or a sensitivity of ---100 percent and a resolution of 1 percent. A small field computerfor acquisition, storage,and retrieval of data can be connected

to the receiver.

Measurementscan be made usingthe VCP and VCA configurationsas well as the standard HCP configuration.

Technical data were furnished by Sveriges Geolo-

giskaAB, BOX 801,S-95128 LULE•, SWEDEN.

E: EM-31, EM 34-3, AND EM-38 INSTRUMENTS

is a low

induction

number

(LIN) instrumentdesignedto be carried by one person.The loopsare placedin either end of a rigid boom;

the loop spacingis 3.66 m. The boom breaks down into three sections for convenience in transporting the instrument. The operating frequency is 9.8 kHz, al-


260

Frischknecht

though some early models operated at 39.2 kHz. As the instrument is normally carried, the loops are in the HCP configuration; however, the instrument can be held on its side to obtain readings with the VCP configuration. Normally the instrument senses the quadrature component, which is displayedon a meter as apparent conductivity at a full scale sensitivityof 3, 10, 30, 100, 300, or 1000 mS/m. The in-phase component can also be read; however, the in-phasezero may change due to changes in separation and orientation when the instrument

is disassembled

and reassembled.

The instrument is adjusted so that when placed at normal waist-high carrying height, it correctly reads the conductivity of a homogeneous earth. Rough soundingscan be made by taking readingsof various heights above the surface and by using both the HCP and VCP configurations.In most casesthe instrument is used for profiling at a constant height above the surface.

The EM 34-3 is a LIN instrument designed to be operated by two persons. Aircore loops, usually placed on the ground, are used for both transmitting and receiving. The system can be used with either the HCP or VCP configuration at three fixed spacingsof 10, 20, and 40 m. The corresponding operating frequencies are 6400, 1600, and 400 Hz; thus the induction number for a homogeneoushalf-space is independent of frequency. The transmitter and receiver are connected by a cable to provide a phase reference. The in-phase component of the received signal is displayed on an analog meter at a sensitive scale. To make a reading, the transmitter operator stands at approximately the correct position and the receiver operator adjusts the position of the receiving loop precisely until the in-phase reading is midscale. Then the quadrature component, displayedas apparentconductivity, is read from a second meter. In many instances

the induction

number

is so low that the

APPENDIX

F:

et al.

in-phase secondary field is very small, and negligible error is introducedby usingthe in-phasecomponentto establish the proper loop spacing. Five conductivity ranges are available 3, 10, 30, 100, and 300 mS/m. By making measurementsin both configurationsand all three spacings,six data points are obtained which can be interpreted as a sounding. Frequently the systemis used for profiling using a single spacingand configuration. The EM-38 is a small, hand-held, LIN instrument. The loops are attached to a rigid frame with a separation of 1.0 m. The operating frequency is 13.2 kHz. Readingsare taken with the instrumentplaced directly on the ground; either the HCP or the VCP configuration may be used. The quadrature component is read in terms of apparent resistivity with full-scale sensitivities of 30, 100, 300, or 1000 mS/m. The instrument may also be used as a magnetic susceptibilitymeter by readingthe in-phasecomponent.To accountfor errors in the instrument zero, the difference in in-phase readings with the instrument on the ground and at a height of about 2 rn is taken. The conversion between apparent conductivity and magnetic susceptibilityin rationalized

units is

K = 26 x 10-6 Acta, where Aria is the differencebetweenthe two readings taken with the HCP loop configuration. Depth soundingscan be made by making measurements at several heights and with both loop configurations. Generally the instruments are used for profiling with stations as close as a few meters, if necessary.

Technical data were furnished by Duncan McNeill, Geonics Limited, 1745 Meyerside Drive, Unit #8, Mississauga,Ontario, Canada L5T 1C6.

CROSS-RING

The cross-ringsystem is the first, or one of the first, systemsdesignedto measure wavetilt. The source for the systemconsistedof a horizontal loop and a battery

MKS

SYSTEM

powered transmitter operating at 3600 Hz (Frischknecht, 1959; Braekken, 1964). The receiver consisted of orthogonalaircore loops with a ratiometer and null



detector to measure the complex ratio of the voltages induced in the vertical loop to the horizontal loop. The complex ratio is simply the wavetilt in terms of inphase and quadrature components. The loops were constructed of a single turn of heavy aluminum tubing and were matched to the rest of the system through transformers. The equipment was usedfor in-line fixed source measurements at spacingsup to 200 m and for moving source profiling with spacings up to 100 m.

APPENDIX

G:

GEM

500 Am 2 at 41 Hz are used for measurementsat source-receiver separations up to 300 m. For greater separations, a backpack motor generator powered transmitter is used to drive large single conductor horizontal loops or a 19-turn loop made up of one or two 60 m long sections of flexible cable. The multiturn coil can be placed vertically by use of masts or the coil may be laid on the surface of the ground. When a 38 m diameter multiturn loop is used, the maximum sourcereceiver separation is 2000 m under good conditions. The receiving antenna is a pair of aircore loops mounted rigidly together in an orthogonal configuration. The receiver loop assembly is placed on the surfacewith the axis of the loop in the plane of the field to be measured. On level ground the loops are thus oriented nominally 45 degrees above and 45 degrees below

the horizontal.

No effort

exact orientation. However,

Generally, the in-line technique was used but broadside measurements

is made to establish

the actual tilt of the

assembly,q•0,is read from an inclinometerattachedto the coil assembly. The receiver measures the ratio of the magnitudes and the phase difference of the voltages induced in the two coils. Stacking is used for signal enhancement. In the original GEM 8 receiver, resolution is 0.1 percent for the ratio and 0.1 degrees

could be made also since there was

no connecting cable. A sighting fixture was used to keep the transmitting loop and the nominally horizontal receiving loop coplanar. Thus the system was difficult to use in hilly areasor on uncut lines in heavily vegetated areas. Braekken (1964) showed a number of scale model curves for the method and presented a number of examples of field results.

5 AND

The McPhar GEM 5 and GEM 8 systems are designed to obtain the polarization ellipse parameters. The GEM 8 (8 frequency Ground EM system), which preceded the GEM 5 system, operates at eight fixed frequencies of 41, 82, 164, 328, 656, 1312, 2624, and 5248 Hz. Large or small transmitting loops can be used. A 1 x 1 m loop mounted on a tripod and a battery powered transmitter providing a moment of

261

GEM

8 SYSTEMS

for the phase difference, which are displayed on digital panel meters. The ellipticity, e, and tilt angle, a, are computed with a hand calculator using the following formulas'

q•= 1/2tan-1 (tan2•/cos X = 1/2sin-1 (sin2•/sin a = q• + q•0 e = tan X

•/= tan- 1r r = magnitude ratio q• = phase difference

q•0= coil angle The GEM 5 is a five-frequency, lightweight system similar in many respects to the GEM 8. The source antenna is 0.9 x 1.05 m loop mounted on a swivel stand. A battery powered transmitter drives the loop at five fixed frequencies, 70, 210, 630, 1890, and 5670 Hz. The maximum range for coplanar loops is 200 m usinga 12 V battery pack and 300 m usinga 48 V pack. The coil angle q•0 is obtained automatically from an electrical inclinometer attached to the coil assembly. In the receiver, signals from the two coils are first amplified and then heterodynedto 8 Hz. They are then digitized and processedwith an internal microprocessor to obtain the amplitude ratio and phase angle. The tilt angle and ellipticity, and the standarddeviations of both the wavetilt and the ellipse parametersare calculated. All of these quantities are displayed with updates about every 2 s. When the operator is satisfied with the measurementsthey can be stored along with header information in memory for later retrieval.

262

Frischknecht et al. 15W

20W

10W

one A Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

• 0 -20

20

Zone B/

NZone A

5670 hz _

',.oøI•LLIPTICITY (%)

-20

....... - ...... .:. - -:,..•.

..*

0

-

20

-

-20

ß

-

-

. ..........

-

20

_

-

0

-

-20

-20

Zone B/

,

-

0

- Aß

-

.... "•', :,,,-•

630 Hz

..... ,,......... ..... !.O..H..z. ........... -

20

_

20

-

0

-

_

0

_

-20

_

20 ß'"

70

-20

I

loop on Line C, CavendishTest Range.

I •

10W

Fig. G-3. GEM 5 broadsideprofileswith horizontaltransmitting loop on Line C, CavendishTest Range.

5W

.-.-J

15W

20W

10W

5670

NZone A

Zone B/ 5670

Hz -

0

20 0

-

-20

-

-20

1890

Hz

--

_

-

20

-

0

-

.

0 _

_

20

-

630

-20

0 _

-20

20

0 -20

-_

...... ..•

210 Hz

20 0

-20 I

• :'"..• i

70 Hz i

I

.

1890

Hz

--

;. _

-

20

hz

0

Hz

-

ß

-20

5W

•

20

_

Hz

--

NZone A ZoneBy

20

70

-

I

Fig. G-1. GEM 5 in-lineprofileswith horizontaltransmitting

15W

Hz

0--20

-20

210

_

-

20

_

20W

.,,

Hz

0

i

5W

' ; 20

-20

0

10W

,"'•E LLIPTICITY(%)

,

0

20

15W

20W

ILTANGLE (DEGREES)

..•. !:;:/• ', -I"/

2O

5W

-20

-

630

Hz

_

-

20

-

0

-

.........,..•••__• 210 Hz ß

.

....

-20

-

20

-

0

-

70

-20

Hz

-

I

I

I

,

Fig. G-2. GEM 5 in-line profileswith vertical transmitting

Fig. G-4. GEM 5 broadsideprofileswith verticaltransmitting

loop on Line C, Cavendish Test Range.

loop on Line C, Cavendish Test Range.


The GEM systems can be used in several moving source and fixed source configurations.Moving source in-line profiles are made with the transmitting loop

throughthe traverse line for the receiver. The systems can also be used for making measurementsusing the normal fixed sourcetilt angle technique where ellipticity as well as tilt angle is then determined. By using

either horizontal, or vertical with its axis in the vertical


263

plane through the traverse line. The axis of the receiving coil are placed in the same plane. Moving-source broadsideprofiling is carried out with the transmitting loop horizontal or vertical; if vertical, the axis of the transmittingloop is in the vertical plane containingthe

receivers

traverse

surements. With the GEM 8 system, measurements can be made on a turam-type rectangular grid about a

line

for

the

transmitter.

The

axes

of the

receivingcoils are similarly placed in the vertical plane

5O

I

I

I

!

before

and behind

the transmitter

two or more receivers ter to make

I

//•ELLIPTICITY(%)

can be used with one transmit-

simultaneous

I

in-line

I

and broadside

I

•

4O

3O

Hz

2O

TILT

ANGLE

(DEGREES)

Hz

-lO lO

1312Hz -

>'

lO

I-

o-

I-

uJ

._:

-lo

656

lO-

•

Hz

0-

(5 Z

-10 10 -

•

o-

-

_

328Hz -

-lO lO

164 Hz

-lO lO

•

82 Hz

-lO lO 41

-lO

i 88

I 90

i 92

I 94

[ 96

STATION

NO. (100'S

0

in the

in-line mode, the tilt angles can be processed as shootback data to remove topographic effects. Also,

I 98

100

I 100

Hz

I 102

ft) m

Fig. G-5. Simultaneousprofiling and frequency soundingwith the GEM-8.

mea-


264

Frischknecht

large horizontal loop. Geometric or frequency depth soundingsor a combinationof both can be made using the in-line configuration. Examples of in-line and broadsideprofiles measured with the GEM 5 on line C, Cavendish Test Range, Ontario usingboth horizontal and vertical transmitting loops, are shown in Figures G-1 to G-4.. The results have been corrected for measuredtopographicvariations. When the in-line mode was used, significantly larger anomalies were obtained with a horizontal transmitter loop than with a vertical loop. When the broadside mode was employed, the anomalies were about the sameat high frequencies,but at low frequencies, the vertical transmittingloop produced the larg-

5O

25'--ft. 26 O' m

4O

i 260 f}' m 260.m

30

et al.

/

est anomalies.

I

20

o

I

ELLIPTIOITY o

10

© TILT ANGLE

0 41

An example of simultaneousprofiling and frequency soundingwith the GEM 8 is shown in Figure G-5. In this case,the tilt angleswere correctedfor topography by subtractingthe tilt angle measured at 41 Hz from the tilt angles at all other frequencies. From the small tilt anglesat 81 and 162 Hz this procedure appearsto be valid for this data set. Interpretation of the soundings at each station was carried out by iterative curve matching. Although there are obviously variations in conductivity along the profile, most of the soundings were matchedfairly well. One of the best examplesis shown in Figure G-6. Additional information can be found in Yamashita, 1983.

82

164

328

656

1312

2624

5248

FREQUENCY

Fig. G-6. Interpretation of GEM-8 sounding.

APPENDIX

H: JEM

The Crone JEM and CEM systems were designed primarily for making measurementsusing the shootback method (Crone, 1966). The Junior Electromagnetic Unit (JEM) system consists of identical transceivers employing aircore loops, and an audio null detector. The original system, manufactured in 1958, operated at a single frequency of 1800 Hz with spacings up to 61 m. A later model of the JEM operated at either 480 or 1800 Hz with spacingsup to 100 m. Both models are designedto be operated with the plane of the transmitter loop oriented 15 degrees from the vertical. The JEM equipment can also be used for

AND

CEM

SYSTEMS

conventional tilt angle measurements with the transmitter loop vertical. In the more recently available CEM (Crone EM) system, three operating frequenciesare available, typically, 390, 1830, and 5010 Hz. The maximum spacing is 200 m with an 18 V battery supply. For shootback measurements,the CEM system is normally operated with the transmitter loop horizontal. In areas having conductive overburden or country rock, the presence of quadrature signalsresults in broad audio nulls. For this reason a visual meter is also used as a null detector

with the CEM system, which makes accurate determi-



265

groundgeophysicalsurveys.At the Detour Lake deposit, gold is closelyassociatedwith pyrrhotiteand

nationof the tilt angleeasier.The CEM systemcan be used to measure the normal vertical loop tilt angle as well as to measurethe approximatemagnitudeof the in-phasecomponentof the fieldwith the HCP slingram configuration. The Detour Lake Gold depositin northeastOntario Canadawas found by drilling a conductordetectedby an airborne EM survey and was mappedin detail by

other sulfide mineralization in a cherty tuff horizon. Horizontal loop shootbackmeasurementswere made

with CEM equipmentalongtraverses122 m apart. A loopspacingof 91 m anda stationspacingof 15m were employed.Resultsare plottedin Figure H-1. The axes of the main conductor and of another minor conductor

2øI ILl

Z

2œ

20-

2W

-20

20

-

6W

20

-20

lOW

-20

14w

20

18w

-20

12N

10

8

-

6

4

STATION

2

NO

0

0

(100'S 100

2

4S

ft) m

Fig.H-1.Stacked shootback profiles overDetourLakeGolddeposit, northeastern Ontario, Canada atfrequencies of 1830Hz (solidline)and390Hz (thinline).Shown overa mapofconductor axes(heavysolidlines)andtheoutline of the main goldbearingzone shallowerthan 100m (pattern).


266

Frischknecht

et al.

are indicated as well as the principal zone of gold ore

lines L2W

within 100 m of the surface. The main conductive zone is not well defined between traverses L14W and

dips steeply to the north. Technical data and field example were furnished by Duncan Crone, Crone Geophysics Limited, 3607 Wolfedale Rd., Mississauga, Ontario, CANADA L$C

L 18W, probably becausethe conductorplungesto the west. Where determined by drilling, the dip of the main ore zone is approximately70øN. The profileson

APPENDIX

I:

GEONICS

cable.

The PROTEM 57 is a higher-powerversion, capable of transmittingup to 20 A usinga motor-generator,and has a wider time range to 80 ms. It can be used in either loop-loop or large fixed transmitter mode, with cable or crystal clock synchronization. The following field example shows a comparisonof Maxmin III+ and PROTEM 57 profile data over the Night Hawk Geophysical Test Range, Ontario. The target appears from geophysical measurement to resemble a long (700 m) horizontal, highly conductive cylinder approximately 100 m in diameter and at a depth to the top of 90 m. The unconsolidatedmaterial overlying the conductor is resistive. Conventional wisdom dictates that, when searching for thin, steeply dipping, massive sulphides with a horizontal loop EM system, the intercoil spacing should be about twice the anticipated depth-of-burial. For deep targets such a large spacingresults in substantial loss in the spatial resolution which is necessary for accurately locatingthe target. This behavior is well illustrated in Figure I-l,, which shows horizontal loop data over Night Hawk Lake at four different intercoil spacings. Obviously the largest separation

also indicate

that the conductor

1V8.

PROTEM

The Geonics PROTEM 47 and PROTEM 57 systems are portable, early-time versionsof the GeonicsEM 37 system [described in Appendix B of Nabighian and Macnae, 1991, (this volume)] and are designed for geotechnical applications and mineral exploration to moderate depths. The PROTEM 47 is capable of transmitting 3.5 A into a 40 x 40 m single-turn loop with a ramp time of 2.5 •xs(smalleror larger loop sizes can be used). The receiver has a time range of 6 •xsto 8 ms, and a solid-state memory for storing up to 2960 datasets. Synchronization is by means of a reference

and L2E

47 AND

PROTEM

57

providesmuch lessdetail than the shorter separations. That a large intercoil spacingis not necessaryfor the detectionof deep-wide or flat-lying targetsis shownby the fact that the Night Hawk Lake body is relatively easily identified from helicopter EM surveys, in which the intercoil spacingis of the order of 10 m. However, for frequency domain horizontal-loop ground surveys a major source of in-phase noise at short spacingsis causedby small errors in the spacing. For example at 100 m spacingthe correct spacingmust be maintained to within 1/3 percent or 30 cm in order to achieve an in-phasenoise-levelof 1 percent; suchaccuracycan be difficult to maintain, even in only moderately hilly terrain.

This

source

of noise

is absent

in

a well

designedtime-domain system, allowing a much better signal-to-noise ratio at short intercoil spacings, as illustrated in the time-domain survey data in Figure I-2 taken with a loop separationof 37.5 m. In this figure the different channels correspond to those of the EM 37-3 and extend from 80 •xsto 8 ms. Subtle changesin profile shapeduring the decay are evident; the overall width of the anomaly decreases with time as the configurationof the induced vortex currents stabilizes into the late stage. The small anomaly at station 1+ 50 N on the multifrequency data is completely absent, showingthat the anomaly is produced by an intercoil spacing that is comparable with the target depth. Conversely a small localized feature is evident at station 1+25 S on the time-domain data; the feature has a decay rate somewhatfaster than that of the main target (Figure I-3 showsthe time-constantsto be 2.4 and 3.3 ms, respectively). In summary, a highly conductive wide target which producesalmost constantin-phaseresponseand rela-



tively small quadrature phase response over the frequency range from 111 Hz to 1777 Hz is seen to produce an excellent response with high signal-tonoise ratio in the time domain, even with target depth of 2 1/2 times the intercoil spacing. Such a small intercoil spacingpermits accurate measurementof the

•INE

I

m

I

responseshape, as well as accurate localization of the target itself. Technical data and field example were furnished by Duncan McNeill, Geonics Limited, 1745 Meyerside Drive, Unit #8, Mississauga,Ontario, CANADA L5T 1C6.

LINE I+00E

I+00E

I

267

I

I

I

o

•

o

+

o

I

I

I

I

I

I

o

g

I

I

ß

o

4-

o

o

•o %

1777Hz !•0

1777Hz

888Hz

444Hz t•

444Hz

222 HZ t• IIIHz

$ = 150m

$ = lOOm

LINE

I o o

LINE

I+00E

!

I

I

I

I

I

I

I

I

o

o o

.1-

I+00E

I

-20 %

!

-•o %

4,

4'

Q

1777Hz

1777Hz

888Hz

888Hz

444Hz

444Hz

222Hz

222Hz

IIIHz

,•

s = 200m

2'b

s = 300m

Fig. I-1. HCP slingramprofilesat variousintercoilspacings overLine 1+00E, NighthawkLake testrange,showing change of anomaly shape and loss of anomaly localization with increasingseparation.

268

Frischknecht et al. (b)


•00 T /,.•87/•s 216/•s••

0 •

I00

/••----•277/• s

865/•s Is 11-15

15'

2.82ms

I0-

anneIs

7.04ms••-. •

0 0

I

+ e4 I

LINE I+00E

16-20

T

I

+ 0I

....

z

0 0

0

• ed I

I

•,

ß

I LINE I+00E

s = 37.5m

g

•

• ed I

f = 111Hz

I

0

• 0I

ß

Z

0

+o

I

7

s = lOOm __

Fig. I-2. Comparison of HCP slingram response alongLine 1+00E, Nighthawk Laketestrangeshowing (a) time domainresponseand (b) frequencydomainresponse. nV/m

z

I00o

ioo

io

••N••.•S•tatio n1+25S ( 2.4 ms

Fig. I-3. Transient decay curves at Stations 0+50S and 1+25S, Line 1+00E, Nighthawk Lake test range.



APPENDIX

J: CRONE

The Crone PEM (Pulse ElectroMagnetic) system, originally developed in the 1960s, is a time-domain system that can be used in a variety of loop configurations including slingramand in-loop profiling, and a large fixed transmitter loop. The large loop configuration is described in Appendix C of Nabighian and Macnae (this volume).

For portable operation, a battery-poweredtransmitter (24 V, 20 A) is used with a 7 turn, 13.7 m diameter flexible transmitting loop, suitable for slingram profiling. Alternatively, single turn loops up to 100 m square can be used for the in-loop configuration (a motor-generator can be used for larger loops). The transmitted waveform consistsof bipolar square current pulses terminated by a user definable ramp (the minimum ramp time is controlled by the inductance and resistanceof the loop). For the 13.7 m loop a ramp time of 0.5 ms is normally used. The on-off time base can be set at 8.33 ms, 16.67 ms or 33.33 ms in areas

with 60 Hz power, or 10.0 ms, 20.0 ms, or 40.0 ms in areas with 50 Hz power. The receiver

coil is ferrite

cored and mounted

on a

tripod for multicomponentmeasurements.The digital receiver can sample at up to 35 preset time windows between 70 Ixs and 31.6 ms after ramp termination.

Stacking canbe setfor 29 to 2•6 transients, andhas automatic spike rejection and VLF filtering. Results are stored in solid state memory (approximately 1000 20-channel readings), and can be transferred to a computer via an RS232 interface. Transient decay curves and profiles of selected time channels can be displayed on a 256 x 128 pixel graphics screen. Synchronization between receiver and transmitter is normally by radio for moving loop operation.

269

PEM

The following field example demonstratesthe PEM in the slingram configuration. The Lasail copper deposit is located in the Sultanate of Oman. This massive sulfide body occurs in basic pillow lavas that have been weathered to a depth of 30 to 40 m. The profile was first obtained with a loop separation of 100 m (Figure J-la) and exhibits a negative-positive-negative responsecaused by a steep easterly dipping body at 30 m depth. The width of the central portion of the anomaly is in excessof the loop separationindicating either

that

conductor

the conductor

is 60 m wide

or that

consists of two or more thin conductors

the in a

zone 60 m wide. The purpose of the second profile with a loop separation of 50 m (Figure J-lb) was to selectively explore the upper surface of the body to determine if the body is truly massive. The predominantely negative response typical of a flat surface confirms that the body is indeed massive. The sharp positive peaks are the responsesto the edges of the body. Multiple loop separationsare normally used to detail a target area. The spacingsare selected to start at a value approximately the expected depth of burial and are then expanded to two or three times this value. A catalog of model PEM responsecurves for interpretation were computed by D. Bartel at the University of Utah (Bartel, 1984) and nomograms for rectangular plate responses, were developed in Rai and Verma (1984).

Technical data and the field example were furnished by Duncan Crone, Crone Geophysics Limited, 3607 Wolfedale Rd., Mississauga, Ontario, Canada L5C 1V8.


270

Frischknecht

et al.

MOVING COIL PULSE EM a-

lOOM

LASAIL

'\

\:

'•'

/'

5

MASSIVE SULPHIDE COPPER OREBODY SULTANATE OF OMAN

ß

6

'•.•

.....

7 o

(a)

I l•W

I 2•W

I I•W

o_

i 0

1•E

I 2•E

I 3•E

A MOVl NG COIL PULSE EM a-

5OM

SECTION

O'

(b)

METERS

Fig. J-1. PEM profilesshowingthe selectiveenergizationof (a) the near vertical side surfaces(r = 100 m) and (b) the flat upper surface(r - 100 m) of the Lasail copper orebody, Sultanateof Oman.


CHAPTER

LARGE-LAYOUT

4

HARMONIC

FIELD

SYSTEMS

D. S. Parasnis*

INTRODUCTION

thousand times stronger. The dipole field at 200 m from the source is 64 times weaker

Harmonic field systems that use large, fixed layouts (rectangular or circular loops or long grounded cables as sources) are among the oldest electromagnetic prospecting methods. These methods were developed to a large extent in Sweden by Karl Sundberg and his group during the two decades following the end of World War I in 1918 and were used mainly for ore prospecting (Sundberg, 1931) but also for structural investigations in oil fields (Sundberg and Hedstr6m, 1934). They were still being used at the end of the 1940s but were gradually replaced by the small-loop (or dipole source) systems because the small loop systems were more flexible and, at the same time, adequate for the shallow exploration problems being encountered. The superiority of reflexion seismics over electromagnetics for oil prospecting was already manifest and the large-layout systems were no longer used in oil exploration. Large fixed source layouts were required to obtain sufficiently strong signals as inputs for the primitive electronic amplifiers. The present interest in these layouts stems from the necessity of deep prospecting. Becausemore power can be expended into fixed, large sources than into portable dipole sources, a stronger primary field can be created than with dipole sources. Moreover, this field decreases with the distance from the source at a slower rate than the field of dipole sources.The excitation of deep conductors, and hence their secondary field, is consequently expected to be stronger with a large-layout source. For example, the free-space primary field at a distance of 200 m on the axis of a typical, commercial, low-frequency, 2-3 W

than at 50 m while

the field of the large square loop is only about two and one-half

times weaker.

The large-layout harmonic field methods have gone through three distinct versions: (1) Two-frame measurements, (2) Sundberg or compensator method, and (3) Turam. Additional versions are expected now that interest in the systems is reviving. Sometimes all large-layout systems are grouped under the name Turam. This grouping is undesirable from several points of view and is at variance with the usage in Scandinavia where these methods were developed and where

the three

distinctions

have

been

well

estab-

lished for almost 50 years. One important operational feature of large-layout harmonic field methods is the relatively large maximum distance from the source (several hundred meters or more on poorly conductingground) at which measurements can be made. However, this distance also entails the second important feature of the methods, namely the relatively low excitation frequency that must be used in order that the normal field at large distances from the source be substantially the freespace field. For example, as shown later, if the field amplitude at 500 m from a long current-carrying cable on homogeneousground of 2500 • ß m resistivity is to be within 10 percent of the free-space value (0.4 nT per ampere cable current), the frequency must be less than 800 Hz. Frequently the near-surface layers are less resistive than 2500 •. m and the source frequency must be even lower.

The two-frame method mentioned is an ingenious and simple procedure enabling determination of the ratio of the amplitudes of the magnetic field at two points by measurementsof angles in space, that is,

dipolesourceis of theorderof 10-4 nT whileat the same distance on the axis of a square loop of side 400 m, carrying a current of 1 A, the field is about ten *University of Lulefi, Lulefi, Sweden. 271

272

Parasnis

without

the use of electronic

circuits.

Because

the

method is now largely of historical interest, and be-


cause the Turarn method

is its modern

and much more

accurate version, no further details of the two-frame method are given. For a fuller account of the twoframe method refer to Parasnis (1966).

mine the EM field completely and to construct the ellipse of polarization. This determination is seldom done in practice, however, and in most surveys only the vertical magnetic field is observed. The measured, that is, the resultant vertical field

(amplitudeBz, phasec•)in the receivercan be written as

FIELD

OPERATIONS

Bz cos (tot + ix) - KIo costot+ Bz cos (tot+ q•) (1)

The Sundberg Method

The Sundberg method, devised in 1925, is probably the first geophysical electromagnetic system measuring the real as well as the imaginary components picked up by a receiver coil in relation to a reference voltage. The layout is shown, in principle, in Figure 1. If the source is an insulated cable, it is laid in the geological strike direction and may be a few hundredmeters to several kilometers long. Cables of as much as 10 km length have been used. The alternating current generator may be placed anywhere along the cable which is grounded at both ends. Insulated rectangular loops are laid with the longer side in the geological strike direction. Typical loop dimensions for routine surveys may be 1200 m x 400 m. Circular loops are seldom used because they are inconvenient to lay. Source power is usually of the order of 200 W-1000 W and current strengths normally used are around

where KI o cos tot is the primary, free-space field

Bzt' costotof the sourcecurrent,c•is the phaseof the receivedfield,andB], q•are, respectively,the amplitude and phase of any secondary vertical field at the

measuringpoint. The primary sourcefield KI o costot, where K is a geometrical factor, can be calculated from

the Biot-Savart

zone, which will be the case if the frequency to/2,r is appropriately low. The calculation is quite easy for a cable or a rectangular loop. The free-space field of a

very long cable source is simply tXoIo cos tot/(2,rr) where

r is the distance

from

the cable.

The

vertical

field of a horizontally laid rectangular loop at an external point in the plane of the loop is (Parasnis, 1973)

4,r

1 A.

Measurements are made along profiles at right angles to the cable or the long side of the rectangular loop. In the latter case measurements can be made within as well as outside the loop. The phasereference is taken from the alternating voltage induced in a "feeding coil" placed on the measuring profile at a point very near the source cable or near the relevant branch of a rectangular loop.

law if we are in the near-field

-•A2 A3 A4

where the A's are the areas of the rectangles having the respective d's in Figure 2 as diagonals. All the terms in the bracket in equation (2) are of the same sign if the point is within the loop. Current A

source D

As shown in Hohmann and Ward, vol 1, (1988) the

magnetic vector of the electromagnetic field is, in general, elliptically polarized. By holding the receiver coil in the Sundberg method in three convenient, mutually perpendicular planes, it is possible to deter-

P

SOURCE

CURRENT

DETECTOR

I

I

CABLE

,I

AMPLIFIER

FEEDING

RECEIVING

COIL

COIL

B

Fig. 1. Layout of the Sundbergor compensatormethod.

C

Fig. 2. A large, rectangularprimary loop (A B C D).

Large-layout Systems The free-space field of a large circular loop is not easily calculated (except at the center) because it requireselliptic integrals, which is another reasonwhy circular loops are seldom used in large-layout meth-


ods.

From equation (1) we get the real (in-phase) and imaginary(out-of-phase)componentsof the secondary field as

real component $

Bz cos q•= Bz cos a - KIo (nT)

(3)

imaginary component

Bzs sinq•=Bz sina (nT).

(4)

The imaginary componentof the secondaryfield is the same as the imaginary component of the resultant field. The imaginary componentis obviously independent of the primary source field at the point of measurement. Its mere existence is indicative of a departure from free-space conditions, that is, the presence of a conductor. The phase relationships between the primary, secondary, and resultant fields are shown in Figure 3. Dividing by I o we get the componentsper ampere primary current amplitude: secondary real component

Bz io

cos ct- K (nT/A)

(5)

Io

sin ct (nT/A).

ring to Figure 4. Note that the resultant vertical field is greater on the source side than on the far side. In other words, the anomaly in the secondary vertical field is positive on the source side and negative on the far side. The sign of the secondary horizontal field is a matter

of

convention.

A

horizontal

field

instanta-

neously increasingin a direction away from the source is considered positive. Figure 4 also shows the responseof a horizontal conductor. (The conductors in Figure 4 are assumedto have a thickness smaller than the skin depth inside them.) The anomalousfields can be explained in their main features as being due to suitable linear currents and can be calculated from the Biot-Savart law. Thus, the secondary field in a long vertical conductor may, as a rule, be considered that of a concentrated linear cur-

rent at appropriate depth below the cross-over in the vertical-field anomaly or below the maximum in the horizontal-field anomaly. Complications arise, however, if the dips are flat or if there are two or more conductorsin close proximity. The secondary current concentration envisioned here is, of course, fictitious and evoked simply as an aid in quick routine interpretation. The induced current actually is a distributed current and the amplitude and phase of the current density vector are a function of position. For this reason the fictitious secondary current concentrationsindicated by the real and imaginary component anomalies need not be coincident. Again, as a rule, the distance between the two con-

centrations maybeconsidered as(-rr/2)(o•lXoCr/2) -1/2in standard notation, assuming a "non-magnetic" con-

secondaryimaginary component Bz

273

ductor.

(6)

The divisionBz/I o can be made automaticallyin the instrument. This way of expressingthe components obviates the need to hold the primary current amplitude constant and simplifiesinstrument design. The response of a vertical or steeply dipping long, tabular conductor, situated in nonconductingbedrock, to the low-frequency field of a large-loopor long-cable source can be easily visualized qualitatively by refer-

Figure 5 is an example of a measurement using the Sundberg method in northern Sweden. Only the real component is shown. The theoretical field of an infinitely long secondaryconcentration, normalized to the maximum in the measured secondary fields, fits the observed curve satisfactorily. However, a single current concentration may not be sufficientto explain the measurements. Werner (1958) reported a field case where four suitably placed linear currents were required. Figure 6 shows the results of measurements in a more complicated case. Four distinct linear current concentrations are indicated by the two vertical and the two horizontal componentsof the secondaryfield. Their depthswere estimatedfrom the following simple considerations.

The Bio-Savart law gives the vertical and horizontal secondary magnetic fields of a long conductor as p

Cx •

Fig. 3. General phase diagram.

B•= a2+ x'2

(7)

274

Parasnis

and

The distance between the real and imaginary current concentrations in Figure 6 is about 80 m so that the conductivity of the conductor can be estimated from

C


Bx• = a2+ x'2

(8)

therulegivenearlieras0.19Sm-1 (p = 5.3 f• ßm). Note that because the real and imaginary currents are not coincident in this case the dip can also be esti-

where C is a normalizing constant, a the depth, and x' the abscissaalong the profile from the epicenterof the current concentration as origin. These equationseasily

mated.

show that

The Turam

1

a=•e

An operational disadvantage of the Sundberg method is the necessity of a connecting link between the feeding coil and the measuring system. As a result,

(9)

and

the reference

cable has to be rewound

(10)

2

survey is unproductive. This disadvantagewas overcome in the Turam

method

which measures the ratio

of the resultant vertical-field amplitudesand the phase difference of the vertical fields at two neighboring points, by means of two identical receiver coils. The coils are usually held apart at a constant distanceof 10

wheree is thedistancebetweenthepointsat whichBz. s is maximum and minimum while w i/2 is the distance between the points at which Bxs is half its maximum value.

/Primary vertical field

Primor' yfield

L•

Secondary h eld--•k,,!.,•

J ertical

_

conductor

Secondary vertical field •••%•...•_Gradlent of secondary

-

01

vertical

4-

field

Primor9 field • •l• Secondary held-••

conductor Secondary

•

•..........___verticaI field Gradient

01

after it has been

laid out from the reel, so that almost half the time in a

1421/2

a =

Method

-

•

of

secondary

verticalfield

Fig. 4. Qualitative explanation of responsesin the Sundbergmethod.

Large-layoutSystems


or 20 m and moved along the profile so that the rear coil each time occupiesthe place previouslyoccupied by the forward one. A bridge arrangementto measure the ratio and the phasedifferenceis shownschemati-

275

From the phaserelationsin Figure 3 it is obvious that

+ 2z"z s cos,1,+

=

cally in Figure7. By convention,the reasonfor which will be apparentlater, the amplituderatio measuredis

s 2]1/2

=Bf1+2B--•cos,+ •B•] '

Bzl/Bz2whereBzl is theamplitude of theverticalfield in the coil nearer to the source. The phase difference

measuredis c•2- c• where a•, c•: are the phasesof the two fieldswith respectto the phaseof the primary current.

Dividing the measuredamplituderatio by the ratio

Bf•/Bf:of theprimaryvertical-field amplitudes at the two points we obtain the reducedratio (Bz•Bf:)/ (Bz:Bfl) which will be equalto one if there is no

(11)

While it is obviousthat (•: -a•)/c, where c is the smallcoil separation,measuresthe horizontalgradient of the phaseof the resultantverticalfield, the significance of the reducedratio (RR) needs to be examined moreclosely.LettingBz• = Bz andBz2= Bz• + dBz• we can write

secondaryfield. The phase differencec•2- c• will

RR=

normally be zero.

Bz (Bz •+dBz •) (Bz + dBz)B•

= [1 + (dBz•/Bz•)][1 -(dBz/Bz)] 0.0/,

(t•z dB dx z Bz 1• dB •x"/ •l

=1-c

0.02

s¾0m

/,8•00

S•,00m

' /,4•00 m

cable

•,

-

Gro. ph?'tic

Quartzite

shales

,

'

e

Equation (12) can be rewritten as

\ -0.02

Secondary current concertfra-

(13) (13a)

Thusthe departureof the reducedratio from unity is a measureof the logarithmicderivative of "the amplitude of the resultantvertical field at a point normalized

to thelocalfree-space field". For • = 0 andB] ,• Bf s •, = c times the • - c d/dx (Bz/Bz) negativegradientof B] if Bz• is approximately con-

517 Hz

we find that A

Vertical real

stant. The negativegradientof the secondaryfield in Figure 4 obviously shows a maximum above the

•c0'2J x•

= 1 + A (say).

hah assumedfor calculation

517 Hz).

0.3-

d

RR=1-c •xx[ln(Bz/Bz•)]

--0.0/,

Observedsecondar, y vertical component

Fig. 5. Sundbergmethodanomaliesand calculatedcurveon a graphiticshaleconductorin northernSweden(Frequency

0.4 t

neglectingquantitiesof higher order than the first.

Calculated secondary vertical component

---

(12)

conductor. This is the reasonfor adoptingthe measur-

Horizontal

•

ing conventionBzl/Bz2. The oppositeconvention would show a minimum in RR above a steeply dipping

'•0•-- Vertico[ •

ca -0.1 imaginary 200

400

I

I

•'•"•.•

R•

600 m

DETECTOR

I

Inferred secondary

Coil

Coil

-r•,•'"'"• •,•urrents Fig. 6. Sundbergmethod anomaliesand inferred current concentrationsacrossa thin sulphideconductorin Norway. (Frequency 517 Hz).

Fig. 7. Schematiccircuit for a Turam bridge.

276

Parasnis

plate conductor, a very common occurrence. It should be realized, however, that A is a measure of the gradient of the secondary field only in the special


situation• = 0, B] <• Bz•. The exactinterpretation of the reduced ratio is that implied by equation (13). From Figure 3 we also get

t•=tan -•B• Bz •sin 4• + Bz cos • S

RR-1 sin 4•cos 4•+BzVj dx' (15) In practice, the secondary current system in a thin plate-like conductor can often be approximated by a closed current flowing along the edges of the conductor or, as in the example of Figure 4, by long linear current filaments where the circuit must, of course, be closed ultimately by some "return" current at large distances.

Equation (15), which assumes 4• to be constant, shows that the reduced ratio is equal to one at exactly the same points at which the phase difference is zero. Such an observation in a survey, for example for the Turam anomaly in Figure 8, indicates that the secondary field is mainly due to a single closed circuit. The problem of interpretation is then to find the position, form, and orientation of this circuit.

from

the source

s

s

Bzj Bzj Bz• Bz• Bz•Bz•

(18)

it is only necessary to multiply the components obtained from equations (16) and (17) by the ratio of the primary-field amplitudes at the jth and the first point. The reasoning leading to equations (16) (17), and (18) is completely general and does not assume a constant 4•. When a constant 4• is indicated, according to the criterion already given, the position of a current concentration (or circuit) can be located accurately only after extracting the secondary field from the observationsof RR and (x2- (xl. In most routine work, however, the position of the current concentrations is identified

with that of the ratio-maximum

and

the phase difference minimum. While this procedure works fairly well for shallow and long linear conductors, like the one in Figure 8, it should not be relied upon in accurate interpretation. Figure 9 shows the secondary fields calculated from the measurement data in Figure 8. Note that the ratio-maximum is displaced some 4 rn compared to the true position of

R R ofverhcal

field

amphtudes at succesive points 1.50 -

The secondary field can be determined in a semiquantitative manner by supposingthat the indication is at such a distance

knownmagnitude BzP•.Since

(14)

'

Equations (10), (12) or (13), and (14), show that treating 4• as constant, which implies assuming the secondaryfield to be due to a single current filament,

obtained

It is more convenient to express the components in terms of a standard amplitude rather than the local amplitude. The most natural amplitudefor the purpose is that at the first point which, by hypothesis, has the

Raho

1./,-

that the

amplitude of the field at the first position of the rear

Turam coil, that is Bzl, may be consideredto be the primaryfieldBzPl,andthat the phase(Xl may be taken to be zero. Then the reciprocal of the first reduced

ratio, namelyBz2BzPl/BzlBzP2 becomessimplyBz2/BzP2 which will be the amplitude at point 2 normalized to the local free-space amplitude. Dividing this result by

thenextreducedratioBz2BzP3/Bz3BzP2 givesBz3/BzP3 and so on. The phasest•2, o[3... (xj, ... are obtained merely by the successive addition of the measured phase differences. From Figure 4 it is easy to see that the real and imaginary components of the secondary field at the jth point normalized to the local primary field will be given by

1.3-

1.2-

1'11c.....tcable

ß

,oo / , .......... '7,J /,50 m e

....

i 350m

**

0

300

0.9 •

Pyr•te ore

I

[•••atzite --10 ø - _20 e

(Bzi/B s zi) p cosq•= (B• /B$) cose•j- 1

(16)

(Bz/Bu) s p sin4•= (Bz•/Bz•)sin(x•ß

(17)

Succes•ve differences

Phase

and

Fig. 8. Turam profile acrossthe Kimheden orebody in North Sweden (Frequency 350 Hz).

Large-layout Systems

the current as indicated by the secondaryfields. While this discrepancy is of little consequencefor shallow drilling, as in this case, it could be serious in other


cases.

The depth of a long linear current concentration when the primary source is a long cable is given by d

-- 1]1/2

a=7 [(1+u2/d 2)1/2

(19)

current

concentration

from

the

ELEVATION

CORRECTIONS

If the measuring coil and source in the Sundberg method are not on the same horizontal level, the normal field in a coil held horizontally will not be that given by equations2 and 3. For a long cable-sourcethe

actualverticalfieldin thecoilwill be200(s 2 - h:) ]/:/ s: nTA-• wheres isthedistance alongtheprofileand h the elevation difference between cable and coil,

instead of 200/s. A spuriousreal-componentanomaly will arise if the latter value for the normal

where u is the distance between the points at which RR = 1 or or2- at] - 0 and d is the horizontaldistance of the

277

cable.

It

is

necessary to know d and this can only be obtained by extracting the secondary field. For preliminary estimates in shallow-depthproblemsd may be taken to be the distance (from a cable source) of the ratio-maximum or the phase-differenceminimum. The proof of equation (19) and a method of determining the phase and the relative strength of the secondary current, when the current is a long filament, are given in Appendix A to this chapter. If the real and imaginary componentsdetermined from equations (16) and (17) in a practical geologicsituation are plotted in a vector diagram, the phase of the secondarycurrent will vary from point to point along the profile, which is natural since a current concentration is only a fiction. The current in reality is a distributed one.

(s:/s•)[(1- h•/s•)/(1- h::/s::)] •/• insteadof s:/s•. No correction is needed for the phase difference. Errors due to slight misorientation of the coils from their nominal positions for measuring vertical fields are usually negligible. Rough topography may cause cables and loops to be laid in configurationsthat depart from the ideal ones we have assumed. Corrections

cosat( Relatwe tolocalB•) /.50 N

/.00 N

350 N

Position of maximum

ratio

0.5

B•cos •

(Relative to,•

300N

in such conditions

must

be estimatedfrom case to case and the free-spacefield calculated appropriately. No general rules can be given for the purpose.

GALVANO-INDUCTIVE INDUCTIVE

1.00

field is used

in equation (3). No correction is needed for the imaginary component since, as equation (4) shows, the primary field does not enter into its calculation. Similarly in the Turam method, the actual normal ratio of vertical fields in two coils at heights h•, he will be

VERSUS

PURELY

EXCITATION

A long cable as source necessarily involves grounding at the cable ends and a consequent galvanic injection of current into the earth while a loop is a purely inductive source. The difference in the anomalies obtained with these two source types is strikingly illustrated by the example in Figure 10 of a Turam profile in northern Sweden. The ratio anomaly with the purely inductive source is hardly discernible on this poor conductor. With a galvano-inductivesourcethere is a distinct ratio anomaly suggestive of the entire impregnation zone. Physically, it seems that the galvanically supplied alternating current in the ground flows preferentially into the sulphide zone adding to the current density present within it due to induction s

p

so that the term Bz/Bz in equation(15) is increased.

0.0

BzSsint•= Bzsinat

(Relativeto Bz pat/,50 N )

of •nflexionpoint

•nsecondary field

Apparently the phase of the secondary field in this example is displaced at the same time by exactly the amount necessary to keep the phase of the total vertical field, and hence da/dx, unchanged along the profile. Galvano-inductive supply as well as the channeling of induced

Fig. 9. Secondary fields calculated from Figure 8.

currents

in the host medium

can enhance

the EM response of electrical conductors. The effect discussedbriefly here has been known in Swedish ore

278

Parasnis

fields since about 1925, and in recent times has been

jargonistically renamed "negative screening".


EFFECT

OF

CONDUCTIVE

HOST

MEDIUM

The use of a long grounded cable as the sourceof an electromagnetic field as well as the channeling of currents referred to previously presupposesan earth of finite conductivity. Conduction currents will necessarily be induced in such an earth and their field might be expected to modify the anomalies we have been discussingso far which assumea very highly resistive earth (except for the target conductor). Here we mainly consider an infinitely long cable source laid on conductive half-space and carrying a uniform alternating current along its entire length. Such a current is not, strictly speaking, possiblebut at the low frequencies involved in large-layout systems we consider the current to be substantially uniform only over a sufficiently long part of the cable and assumethe effects of the current along the rest of the cable to be negligible.

540 c/s

Reduced Ratio

Phase

diff. per lorn

1.1

X_•

1.0

Ratio

0'--'-•

O-- 0'--•----'0•

Phase Insulated

seems to be little known

Loop

arenegligible andalsothat kolx

half-space conductivity •r• > 10-5 Sm-•. The values attached to points on the curves in

0o

-- -2 ø

Im(Bz) 0

-- -3 ø

10.0,

-- -t, o

7 2 ß

I

1 whereko 2 is the

propagation constant in air and x is the distance from the cable. These conditionsare satisfiedin practice in large-layout work if x < 1000 m, f < 1000 Hz and the

---1

-- --5 ø

850 m

in the west.

The field of a long cable on an isotropic, homogeneoushalf-space, normalized to the free-spacefield, is most vividly and succinctly visualized by Figure 11 (vertical field) and Figure 12 (horizontal field). It is assumed that displacement currents in the half-space

• +1 ø

O'•'ø'"•ø---O'--'--•Q•'"-

o.g

The detailed theory of this problem is treated in Hohmann and Ward, Volume I (1988) and, therefore, only a few theoretical aspects of immediate relevance to the planning and interpretation of large-layout surveys are dealt with here. The first rigorous and thorough treatment of the problem, analytically as well as numerically, was publishedin 1936 by the brilliant Russian geophysicist V.R. Bursian (1972). He treated a buried cable and a cable on the ground surface, the latter with explicit reference to the Sundberg method. He also outlined a complete solution to the problem of a long cable on a multilayered earth. Bursian's remarkable work (which also covers dipole sources),of more than 50 years ago,

0.2

0.4

0.15

0.8

1.0Re(Bz)

.

k,

_

-- _150 950 m S

Borehole 9010 m

I

Overburden

-0.4

,

,

Fig. 11. Real and imaginary components of the vertical field on a semiinfinite earth with a long cable on the surface. (From Bursian, 1972).

1.1

.

fo-o..•

5.0

60

Im(Bx) 0,2

_ +1 ø

1.0

'•----.,•----x----,x-• -"•'•x--x

x•

3.0

0o _ _1o

Grounded Cable x

• _•o

10.(2

I 2.

'

0 Re(B x)

• _/+o • _•o _ _15o

Fig. 10. Enhancement of Turam anomaly due to channeling of current.

-0.2

Fig. 12. Real and imaginary components of the horizontal field on a semiinfinite earth with a long cable on the surface. (From Bursian, 1972).



Figures 11and12arefortheparameter • = (o-••xIto)1/2

279

well suited to computer calculations since series developments are available for all the functions involved. For small values of •, equation (20), for example, can be simplified to

x. For moderately large values of • there is an appreciable horizontal field which is not in phase with the vertical field. The resultant field at any point is therefore elliptically polarized. Figure 13 showsthe inclinations of the major and minor axes of the ellipse as a function of the distance from the cable. On highly conductive ground the field is largely horizontal and standard type measurementswith the Sundbergor the Turam methods need to be modified to exploit fully the sensitivity of the measuring equipment.

'fl'

Bz= Bz p. 1- -•

[

+iln

2

+'Y/2x

tøx2}

4

The completeexpressions for theBz andBx fieldson the ground surface under the assumptions already

(22)

whereBzp is the free-spacefield and 'y = 0.5772... is

stated, are

Euler's constant. This equation can be used (instead of

Bzp) in calculatingthe "normal" field and the "nor-

•) _•XoI I(4 kei '(-2ker

mal" ratios in the Sundberg and Turam methods, respectively, on moderately conductive ground. The conductivity o-• can be obtained from the formula that follows from equation (22), namely

Bz- 2'rrx • +i(-2

kei•)-

4

4 ker '(

(2']

16

(20)

cr• -

2 Re [1 - (B•/B•)]

(23)

2xf• 4q2' (()

Bx=

(

2'rrx

-••+

2q•(()

by measurement ofBz/Bz p or by an auxiliaryresistivity

where ker, kei are the real and imaginary parts of the

measurement far away from any target conductor. Similarly equation (21) can be used to assess whether a small observed horizontal field in the operation of the Sundberg method can be reasonably ascribed to the effect of a conductive half-space or not. It is of some practical interest to calculate the

Besselfunction Ko(i•/2•)andql, q2 of thefunction Qo(i•/2•).Theseexpressions, givenby Bursian, are

induced currentdensityJy onthesurfaceandcompare it with the galvanicdensityJ} due to a directcurrent

4q l' (() + • + 2q2 (() (21)

f = 660 Hz

orl

%

%

I

I

I

I

o

0.1

0.2

0.:3

I

0

0.2

0

1

I

'

(axis inclination

'

I

I

I

I

I

0.4

0.5

0.6

0.?

0.8

I

04

I

0.6 2

I

0.8 3

I

1.0 4

1.2 5

I

1./, 6

km

33 •m

km

100 •rn

km

1200 •m

Fig. 13. The tilt of the major and minor axis of the ellipseof polarizationon a semiinfiniteearth with a long cable on the surface. (From Bezvoda, 1970).

280

Parasnis

from a long grounded cable. Bursian's calculations give


•--

-

q-

-i

x 10-12m -2

1 (24)

where I is the amplitude of the alternating current. The real and imaginary parts of the function in square brackets in this equation (24) are plotted in Figure 14, also reproduced from Bursian's work. There are no restrictionson t• (and hence crl) in this diagramexcept that displacementscurrents must be negligible.

A typicalvalueof crl= l0 -2 Sm-1 fora fairlywell conducting area (e.g. a sedimentary basin) and typical operational valuesf = 500 Hz, x = 500 m yield • = 3.1 and the amplitude of the bracketed function in equa-

tion(24)is seenfromFigure14to be0.3.HenceJy/I

= 7.9x 10-8 m-2. Thegalvanic current-density Jy'/l

at a distance x on the central profile at right angles to a cable of length 2L grounded at both ends is easily

shownby elementary calculation to be 2crL/•r(x 2+ L 2)3/2• 2o-/,n'L 2 if x <•L. For2L = 10km,say,andthe

The effect of half-space conductivity on measured

Turam ratiosBzi/Bz2and phasedifferences•2 - el, for a coil separationof 20 m and a frequency of 660 Hz, using a cable source can also be seen from Figure 15 (Bezvoda, 1970). Although a long cable has been considered in some detail, because it is the most commonly used type of source in large-layout work, similar considerations apply when a rectangular loop is used. Finally, the modification of the response of a vertical lamina due to the conductivity of the host-medium is considered. Studies of this problem were reported in Hohmann (1971) for the Sundberg long-cable method and in Swift (1971) for the Turam method. As an example, refer to Figures 16 and 17 from Hohmann's

work showingthe relativeamplitudeBz/Bz p and the phase% of the horizontal field. Neither of the quantities is the actually measured or calculated quantity in the Sundberg method described previously and to compare the response of the lamina when the host medium is nonconductingwith that in Figure 16 it is necessaryto considerthe correspondinggraph (Figure 18) of the function

above crl,thenJy'/I= 3.2x 10-•øm-2. -04

Re(Jy/l) -0.2

5

Bfl:[1- 2Ax(xr2 d) IBzl

(25)

where A is a constant and the meaning of the other symbols is shown in Figure 18. The calculation in equation (25) assumesone phase-reversedsecondary linear current -0.2

filament

and normalization

to the free-

space field since the external medium is now nonconducting. Such a distinction between primary and sec-

-0.4

-0.6

Bz1 Bz2

660 Hz

1.5 1/.,

-0.8

p (tim)

1.3

1.2 11

1.0

1.00

I

&

t

I

700

x(m)

-1.2

-1.4

Im (Jy/l) Fig. 14.Vectordiagramfor Bursian's functionJy/I (equation 24). (From Bursian, 1972).

ot2.-

(Degrees)'

Fig. 15. Turam ratios and phasedifferenceson a semiinfinite earth of different resistivities (After Bezvoda, 1970).



ondary currents is not generally possible when the latter currents are distributed. The responsein Figure 18 resembles that in Figure 16 and by an algebraic coincidencethe depth to the current concentrationis given by equation (19) provided u is now understoodto be the distance between the points at which the peaks in Figure 18 are obtained. Hohmann's principal resultsare now summarizedas

281

The ax anomalyis most interestingas it seemsto be almost insensitive to the depth of the lamina which would imply that it is better suited for detecting a deep-lying conductor in conductive environment than, for example, Bz/BzP. Paradoxically this also implies that the actual depth of a detected conductor in such a situationcannot be estimatedaccuratelyfrom ax.

follows.

--The maximumgradientin Bz/B• occursabovethe

-20

edge of the vertical lamina and becomes increasingly less sharp as the lamina depth increases.The peak-topeak anomaly in the amplitude also decreases markedly with the depth. --The peaks on the cable and the "far" sidesare not equally sharp when the host-medium is conducting. The far-side peak is flattened out considerably and disappears for relative depths >0.1. --The depth, if estimated using the same rule as for Figure 18, namely equation (19), is greater than the true depth. The conductor appears to be deeper. The estimate obtained from the cable-side peak only is, however, not far off from the true depth even when the host-medium is conducting.

I

I

I

i

I

I

•

100.1

D=0.5

O-

o.3

10-

20

½o0• '•

I

-

3{1•

--The phasea: of the verticalfield, not shownhere,

-

t.0

is little influenced by the lamina.

50

I

I

0

0.2

-

I

0.4

I

0.6

I

I

0.8

1.0

!

1.2

1.6

Fig. 17. Phase of the horizontal field across a lamina as in Figure 16. (After Hohmann, 1971).

1.4

IBzl

1.2

1.0

0.3

A = 0.15

1.0

]

!

!

[

x

!

d

0.4 -

r current filc•ment

_

0.2 .

0.5-

0.0

I

0.0

0.2

0.4

0.6

_

0.8

1.0

1.2 ,

Fig. 16. Vertical field across a lamina embedded in a conductive earth and in the presence of a long cable (After Hohmann,

ol

1971). The abscissa is the ratio of the lamina

distance to the skin depth in the host medium (D = lamina depth/skin depth).

Fig. 18. Graph of equation (25) for comparisonwith Figure 16.

282

Parasnis


DEPTH

PENETRATION

With the meaning of the symbols as in Figure 18, we

As mentioned at the outset, present-day interest in large-layout methods stemsfrom the necessityof deep prospecting and whether the methods live up to the expectation may be questioned. From a practical point of view the depth penetration of an exploration method is the maximum depth at which a target body may lie and yet be detectableby the method. The detectability depends on various factors suchas the size, shape,and physicalproperties of the target; the parameters of the measurement;the measurement accuracy; the platform of measurement (ground surface, aeroplane, ship, borehole etc); the geological setting; precision of topographicand other corrections; and so on. Practical depth penetration is, therefore, not a magnitude that can be stated once and for all for any exploration method, but must be related to the specifiedfactors, in particular to the geology of the target and its surroundings.

have

Ao

B•p = •,x

S __

A(x-

Bz -

d)

2

where A o is directly proportional to the primary current in the cable and A to the current in the postulated

long, secondarycurrentfilament.Bz/B• is givenby equation (11). Equation (A. 1) then leads to the condition

dx2 - 2(d2 + a2)x + d(a 2 + d 2) = 0.

(A.2)

(It turns out that the factor involving the phaseq•of the secondary current is automatically sorted out in obtaining this equation.) If x l, x2 are the roots of equation (A.2), then

2a(d2 + a 2)•/2

u-Ix, -x21-

d

'

In some recent accurate tests of the Turam method,

conducted by the author and his colleaguesin Sweden and Finland, ratio and phaseindicationswere obtained that could be related to proven, flat-lying conductors at depths of 100-400 m. The places for these indications coincided

with those for the indications

obtained

in the area with transient-field EM techniques. It would appear, therefore, that the practical depth penetration of large-layout harmonic-field methods is not less than that of TEM techniques, at least in the highly resistive

environment

in which

Solving for a yields the formula of equation (19). Determination of the Phase t• and the Relative

AmplitudeA/Ao of a Long, SecondaryCurrent Filament

With the notation as above and using equations (13a) and (14) it can be shown in a straightforward manner that

the above tests were

made. However, it is not now possibleto say definitely whether TEM and, for example, Turam would give equal practical depth penetrations in conductive environments if measurements were made with similar (or comparable) primary-source geometry, best possible accuracy of each technique, and so forth on exactly the same target. Although a number of theoretical calculations and model experiments can be, and have been, made in this respect, the question deserves a thorough practical study since such phenomenaas the channeling of currents, which also affect the depth penetration of an EM system, and other physical or geological complications cannot always be modeled adequately in theoretical work or laboratory experi-

x=d Aoa2sin • and

(doddx)x= d

(A/C)x= d

= --tan q•.

(A.4)

Equation (A.4) yields the phase q• and since a can be

estimatedfrom equation(18) we get A/A o from equation (A.3). For the profile shown in Figure 8 we have: u - 44m, d = 107m, a = 21.6m,

(do•/dx).,• = d : -- 19ø/10m - -0.0332rad/m,

ments.

(A/C)x: d = 0.50/10 = 0.05. APPENDIX

Hence• = 33.60A/Ao = 0.26

A

Derivation of Equation (19)

From equation (13) it is obvious that the condition for RR=

(A.3)

1 is d dx

In (Bz./BzP.) = 0.

(A.1)

It is interesting to note that the single fictitious current in this case is very nearly one-fourth of the primary current! In a more rigorous analysis of this profile, and acknowledgingthat the current is in reality a distributed one, Nissen (1986) has shown that the present ratio anomaly cannot be explained by means of induced

current

densities

alone and has estimated



that about 7 percent of the current density at any point in this orebody is due to the channeling of currents induced in the surroundingrock, even though it has a high resistivity of 5000 1• ß m. REFERENCES

Bezvoda, V., 1970, Fixed source systems in conductive environment: Geophys. Prosp., 18, 47-55. Bursian, V. R., 1972, Teoriya elektromagnitn'ich poleyey primenyem'ich v elektrorazvedke, 2nd edition: Nedra, Leningrad. Hohmann, G. W., 1971, Electromagnetic scattering by conductors

in the earth near a line source of current:

Geo-

physics, 36, 101-131. Hohmann, G. W. and Ward, S. H., 1988, Electromagnetic theory for geophysical applications in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics, Vol. 1, Soc. Expl. Geophys., 131-312.

283

Nissen, J., 1986, A versatile electromagnetic modelling program for 2D-structures: Geophys. Prosp., 34, 10991110.

Parasnis, D. S., 1966, Electromagnetic prospecting--C. W. Techniques: Geoexploration, 4, 177-208. Parasnis, D. S., 1973, Mining Geophysics, Elsevier. Sundberg, K., 1931, Principles of the Swedish geoelectrical methods: Gerlands Beitr. z. Geophys. Ergfinzungshefte, 1,298-361.

Sundberg, K. and Hedstr6m, H., 1934, Structural investigations by electromagnetic methods: World Petr. Cong. Proc. B (I), 107-118.

Swift, C. M., 1971, Theoretical magnetotelluric and Turam response from two-dimensional inhomogeneities: Geophysics, 36, 38-52. Werner, S., 1958, Geophysical history of a deep-seated orebody in northern Sweden, in Geophysical surveys in mining, hydrological and engineeringprojects: Eur. Asso. of Explor. Geophys., The Hague, 3-19.




CHAPTER

5

ELECTROMAGNETIC

SOUNDING

BrianR. Spies*andFrankC. Frischknecht * tains information on the variation of conductivity with depth. Of course, the two techniquescan be combined to yield soundings which contain more information than soundingsmade with a single technique. The first technique is sometimes called parametric sounding and the second is called geometric or distance sounding. Since both frequency or time and spacing are

INTRODUCTION

Electromagnetic soundings are made to determine variations in the electrical conductivity of the earth with depth. Electromagnetic (EM) soundingmethods include natural-field methods such as magnetotellurics (MT) as well as high-frequency radiation techniques such as radar probing. This chapter is concerned with controlled-sourceinduction methods of sounding, socalled because an artificial source is used to generate the EM field, and because the frequencies are low enough that the measurement distance is less than the free-space wavelength. This is the quasi-static range, where conduction currents rather than displacement currents predominate. In the following discussingthe general term EM sounding will be used for simplicity and to follow

factors

convention.

information

on the variation

number

we believe

it less

confusingto refer to the first technique as frequencyor time-domain soundingand the second as geometric sounding. The difference between EM soundingand horizontal profiling is not always distinct. A set of broadband or multi-spacing measurements spaced closely together along a traverse may be regarded as either a multifrequency, multi-time or multi-spacing profile or as a series of soundings, depending on the geologic structure and the way in which the data are interpreted. In this chapter we consider such measurements to be soundingsprovided the earth is approximately horizontally layered and provided the basic model used in interpretation is one-dimensional (l-D). Broadband measurements which are interpreted using dike or prism modelswill not be regardedas soundings.As the capability to quantitatively interpret results over twodimensional (2-D) and three-dimensional (3-D) structures improves, the definition of soundingmay change or the distinction between broadband profiling and sounding may cease to exist.

Most electromagnetic (EM) soundings consist of measurements at a number of frequencies or times using a fixed source and receiver. The distribution of currents induced in the earth depends on the product of electrical conductivity, magneticpermeability, and frequency. Since low-frequency currents diffuse to greater depths than high-frequency currents, measurements of the EM response at several frequencies or times contain

in the induction

of conduc-

tivity with depth. Alternatively, soundings can be made by measuring the response at several sourcereceiver separations at a single frequency or time. In practice, this technique is employed in the frequencydomainbut not in the time-domain. Frequency-domain measurementsmade at large source-receiver separations are influenced more by deep layers than are measurementsmade at small separations;hence, a set of such measurementsmade at several spacingscon-

This

discussion

is limited

to measurement

tech-

niques designedto obtain information on the variation of conductivity with depth and to interpretative methods which depend on 1-D models. There is overlap between this chapter and earlier chaptersdealing with controlled-source profiling methods (Frischknecht et

*Schlumberger-DollResearch, Old Quarry Road, Ridgefield, CT 06877

*U.S. Geological Survey:deceased

285


286

Spies and Frischknecht

al., this volume) because the same equipment and techniques are often used for both purposes. Some overlap occurs with the chapter on natural source methods (Vozoff, this volume) since many of the principals are the same and since natural fields influence measurements with controlled source systems. Generally, discussion in this chapter is limited to methods and applications in which displacement currents and nonlinear effects can be neglected. Most of the required theory was developed in Volume 1. A number of expressions without derivation and many theoretical curves are included here as necessary. EM soundingsare used in exploration for resources such as petroleum, groundwater, geothermal energy, and uranium which occur in horizontally layered strata. In some cases the objective is direct detection of a commodity such as fresh water. More often, soundingsare used to map structures and variations in lithology which may be guides to the location and delineation of resources. EM soundingsare sometimes used in direct exploration for flat-lying, tabular shaped, base-metal deposits but they are not used in direct exploration for steeply dipping mineral deposits. Sometimes EM soundings are used to measure the conductivity and thickness of surficial deposits or weathered rock overlying resistive basement which may contain mineral deposits. EM soundingsare often used in engineering studies to map structures and to help determine material properties of rock and in hazardous waste site investigations to map conductive plumes and hydrologic features. They can also be used in a wide variety of scientific studies where layered rocks are encountered, ranging from shallow investigations of weathering processesto delineation of deep crustal

structures.

There are many different EM soundingmethods and variations

of methods•each

with

their

own

advan-

tages and disadvantages. As a class, EM sounding methods have particular advantages and disadvantages compared with natural-source soundingmethods (AMT/MT) and direct current (dc) resistivity sounding methods. Some EM sounding methods provide better resolution and are less easily distorted by lateral variations in resistivity than soundingsobtained with natural-source methods. However, for reconnaissance studies and deep soundings, controlled-source EM soundingmay be more expensive than natural-source sounding. Due to power requirements EM sounding has generally been limited to depths of a few kilometers or less whereas

natural-field

methods

can be used

to sound through the crust and into the upper mantle. As contrasted with dc resistivity methods, most EM methods are effective in resolving the parameters of conductive layers but are less effective in determining

the resistivity of resistive layers. On the other hand, highly resistive layers do not screen deeper layers from being resolved by EM soundings, as is the case with dc soundings. Most EM measurements depend only on the longitudinal conductivity of a horizontally layered earth whereas resistivity measurements depend on both transverse and longitudinal resistivity. Thus, in principle, EM soundings can yield more accurate depth estimates than dc resistivity soundings over an anisotropic earth. Of course, EM soundings made with a grounded wire source and receiver give dc resistivity results at asymptotically low frequencies. The question of whether EM or resistivity soundings are least affected by lateral variations in resistivity depends on the specific techniques used in each case and on the scale of the inhomogeneity. Also the relative cost of EM and resistivity soundings depend on the technique and the application; either can be the least expensive for a particular purpose. This description of the practical aspects of EM sounding is designed for the field geophysicist who wishes to understand the concepts and to apply and interpret EM sounding techniques. Published papers are referred to where applicable, so the reader may follow-up on any particular area of interest. Because the EM sounding theory is developed in Volume 1 of this series, minimal mention is made here. An attempt has been made to avoid unnecessary overlap with texts which deal with this subject such as Grant and West (1965), Keller and Frischknecht (1966), Patra and Mallick (1980), Wait (1982), and Kaufman and Keller (1983), and the reader is referred to them for general reading. This chapter is divided into several sections. The first section gives a brief discussion of the history of EM sounding. The next section is a description of basic concepts and principles of EM soundingwhich includes a discussion of time-domain and frequencydomain

methods

and

source-receiver

combinations.

Results for the theoretical response of a layered ground are summarized and differences between various configurationsare discussed in the following section. A description of sources of errors in soundingis presented next and a discussion of survey design and field procedures follows. The following sections discuss data processingand inversion and interpretation. A list of published case histories is given and short descriptions of some frequency-domain and time-domain EM sounding systems are included as appendices. The appendices in the Time Domain Electromagnetic Prospecting Method chapter and in the Profiling Methods Using Small Sources chapter also describe systemswhich can be used for inductive soundings.

Electromagnetic Sounding

United States although a number of so-called "Flex" contractors still operated sporadically over the intervening years. In the late 1920s and early 1930s frequency-domain soundingswere carried out in Europe and the United States using the Sundberg compensatormethod (Sundberg and Hedstrom, 1934); theoretical results developed by Levi-Civita and Otto Mayr for the fields about

used in interpretation of results. The method was used in oil exploration but presumably fell into disuse because of its limited depth range and competition from other geophysical methods. While the early field experiments were being carried out, attempts were made to derive a strong theoretical basis for the methods. Stefanescu (1942) solved Maxwell's equationsspecificallyfor the geophysicalexploration problem. A comprehensive book (Sunde, 1949) summarizedrelevant work done at the Bell Telephone Laboratory. A series of papers appeared in the early 1950son the EM response of various types of sources on or above a layered earth (Wait, 1951a, b; 1952, 1953a, b, c, d, e, 1954, 1955, 1956a, b, 1958a, b; Wait and Campbell, 1953a, b; and Bhattacharyya, 1955, 1956, 1957a, b, 1959). Wait's developments were used in early geophysical publications which describe EM sounding theory (Grant and West, 1965; Keller and Frischknecht, 1966) and in calculation of numerical tables for use in compiling theoretical curves (Frischknecht, 1967). Further detailed studies of the solutions for Maxwell's equations in a layered earth were published in Wait (1960, 1961, 1962a, b, c, 1971, 1972), Barios (1966), Patrick and Bostick (1966), Bannister (1967, 1979), Dey and Ward (1970), Kraichman (1980), and Wait and Fuller (1972). Even with extensive development of the theory, EM sounding methods were not widely applied in the United States until the 1970s. This slow start can be partly attributed to the earlier disillusionment with the Eltran method, and also to the lack of suitable field equipment, lack of practical techniques for interpretation, and general disinterest within the geophysical community. EM sounding methods were developed independently in the Soviet Union. Keller gives an excellent review of early historical development in the introduction to Vanyan et al. (1967a). Layered earth solutions for harmonic sourceswere developed in Kraev (1941), Tikhonov and Shakhsuvarov (1956), Gil'fand (1955a, b; 1957). Molochnov (1955), Pavinskii and Kozulin (1956), Kozulin (1956, 1960), Gasanenko and Molochnov (1958), Sheinman and Frantov (1958), Gasanenko (1959a, b), with further papers in the 1960s and 1970s. Early Russian literature on transient or time domain methods was extensive and includes Kraev (1937), Tikhonov (1946, 1950b), Tikhonov and Mukhina (1950), Skugarevskaya (1951a, b), Tikhonov and Skugarevskaya (1957, 1958, 1959) and Chetaev (1956). Much Russian work was first brought to the notice of geophysicists in the West with the G. V. Keller's translation of a series of papers by Vanyan et al. (1967a) which included tables and curves of EM field behavior which can be used for interpretation of field surveys when large source-receiver separations are

a line source over a thin sheet and model studies were

used.

History of ElectromagneticSounding Methods


287

Electromagnetic techniques have been used in mineral exploration for over 60 years (Ward, 1980) with development of the Sundberg horizontal loop EM method (Sundberg et al., 1923) and the Beiler-Watson method of measuring the ellipse of magnetic-field polarization (Watson, 1931). These first EM methods were designed to detect the presence of a highly conductive orebody of limited extent rather than to give a quantitative measure of earth resistivity. Geoelectrical soundingswere first made with dc resistivity methods following the work of Schlumberger (1922) and Wenner (1912). The MT method has also gained wide popularity as a soundingmethod sinceits parallel development in France (Cagniard, 1950; 1953), the Soviet Union (Tikhonov, 1950a) and Japan (Kato and Kikuchi, 1950; Rikitake, 1950). Development of controlled-source EM sounding techniques has been relatively slow by comparison. Although the basic principles were understood, lack of adequate instrumentation and effective methods for interpretation greatly impeded the widespread use of EM soundingmethods until the last decade. One of the first attempts to use EM methods to study a layered earth in the USA employed the "Eltran" (Electromagnetic Transients) method, based on a patent filed by Blau (1933). In this method an EM field was generated with a grounded electric dipole and attempts were made to detect electrical from

boundaries

with

transient different

reflections electrical

reflected

conductivi-

ties using an electric dipole receiver located in-line with the source dipole. This method generated a considerable amount of interest within oil companies, and field trials were described in a series of papers, starting with Karcher and McDermott (1935) and continuing with a large number of papers which appeared in Geophysics (Statham, 1936; West, 1938; Hawley, 1938; Rust, 1938; White, 1939; Klipsch, 1939; and Evjen, 1948). Studies of the theory of the method were not reported until later years in Yost (1952), Yost et al. (1952), and Orsinger and Van Nostrand (1954), when it appeared that the required resolution to identify individual reflected events could not be obtained, given the resistivities in normal sedimentary basins. This led to

disillusionment

with

the

Eltran

method

in

the

288

Spiesand Frischknecht


A numberof early papersappearedin Europeon a

are less than five times the distance to the nearest

method of EM soundingin which measurementswere

receiver location.

made in the center of a large loop (Nunier, 1933; Stefanescu,1935, 1936; and Koenigsberger,1939). Thistechniquewaslaterdevelopedin Japanby Yoshi-

of wire laid on the surface and connected to the earth through low resistance electrodes at either end. The

zumi et al. (1959), but did not gain much attention in North Americauntil reportedin Patra(1967,1970)and

convenient location. Wires that are either much

Grounded-wire antennas areusuallystraightlengths transmittermay be connectedto the wire at any

Ryu et al. (1970).

shorteror much longerthan the distanceto the closest

Electromagneticsoundingmethodscameinto wider use in the early 1970's, with many paperson both theory and application appearing in the literature. Many of these papers are listed in Patra and Mallick (1980) and Kaufman and Keller (1983), and will be referencedwithin this chapterwhereapplicable.

receiverlocationmay be treatedmathematically as electricdipolesor line sources,respectively.The error

PRINCIPLES

OF ELECTROMAGNETIC SOUNDING

A large variety of possibleEM soundingmethods have been tried and some are used regularly. No universallyacceptedsetof terminologyfor identifying and classifying EM soundingmethods exists. This sectiondiscusses the elementsandparametersneeded to describethe functioningof almostany suchmethod. A simple, if not compact, schemefor classificationof methodsis provided alongwith a better framework for comparisonof methodsthan separatediscussionsof each method. Sources

in either approximationdependson the modeland the componentsthat are measured. Generally, if the length of wire is on the order of one to five times the

distanceto the receiver site, dipole or line source approximations are not valid and the actual length must be considered. In making EM soundings, groundedwires are generallysufficientlyshortand the frequency is low enough that the current distribution along the wire can be considered uniform. However, for very longgroundedwiresthisassumptionmay not be valid (Burrows, 1978). Becausethe primary field falls off less rapidly at

largedistances from groundedwiresthanfrom loops, grounded wires are often used in deep soundings where generation of adequate fields is difficult. For shallowsoundings,loops are generallyused because they can be deployed more quickly and easily than groundedwires. Small loops or groundedwires which can be consideredas magnetic or electric dipoles, respectively, or as line sources are often used to

Both insulatedloops and groundedwires are usedas transmittingantennasin makingEM soundings.Large loopshavingdimensionsof hundredsof metersgenerally consistof a singleturn of insulatedwire laid on the surface of the ground in the form of a square or rectangle. Loops of intermediate size are generally formed from a multi-conductor cable and are laid out

on the surfaceor are suspended in the verticalplaneby use of masts. Small loops generallyconsistof many turns of wire wound on a rigid form; a ferrite or metal core may also be used. In the analysisof systemsand interpretationof results, small loops can be treated mathematicallyas time-varyingmagneticdipoles.Of course, the moment (source strength)of the dipole dependson the currentaswell asturns-areaproductof the loop. When measurementsare made at distances from the loop which are roughlythe sameor lessthan the dimensionsof the loop it is generallynecessaryto take into accountthe actualconfiguration of the loop. However, if measurements are made at distances of

two or three times the loop dimensionsit is usually adequateand convenientto treat the actualloop as a fictitious circular loop having the same turns-area product. As a rule-of-thumb,intermediatesize loops are treated as magneticdipolesif the loop dimensions

minimize costsand difficultiesin interpretation.Generally, inversionof resultsfor a finite sourcerequires considerably more computer time than inversion of resultsfor a dipole source, unlessapproximate solutions (e.g., Stoyer, 1990)are used. The groundedwire is a more complex sourcethan a loop and, as will be shownin a subsequentsection, the responseof otherwise identical systemsusing the two types of sources can be quite different. This differencein behavior may be the most important factor in choosingthe type of source. Depending on which component is measured, the responsemay or may not depend substantiallyon the size or length of nondipolar loops and wires. In most frequency-domain (FEM) systems, a current having approximately a sinusoidal or a square waveform is driven through the antenna by an amplifier or switcher. If a square waveform is used, simultaneous

measurements

can be made at the fundamen-

tal frequency and some of its harmonics. In any case the frequency is usually changed in discrete steps to make a sounding, although experiments have been made with swept-frequency systems (Won, 1980, 1983). In time-domain (TEM) systems the most common waveform is a train of approximately square, bipolar pulses with an off-time between pulses. Other



repetitive waveforms such as triangles are sometimes used (West et al., 1984). In the past long, singlepulses were sometimes employed. The only transmitter parameters needed to describe the functioning of a system are the waveform, the amplitude of the current, and the frequencies or pulse repetition rates used. Of course, the amplitude of the current in the antenna is dependent on the impedance of the antenna as well as the output voltage, output impedance, and capacity of the transmitter. In the case of time-domain systems deviations of actual waveforms from their ideal form can be important. These issues are discussed in a later section.

sources are discussed in a later section, but it is of

interest to note here that dB/dt devices effectively prewhiten the noise, which may be of assistance in processing. Sensitive cryogenic SQUID magnetometers, which in theory have extremely low noise levels, in practice exhibit white noise behavior of relatively constant amplitude at frequencies above 1 Hz due to thermal noise in the external RF shield (Goubau et al., 1984). The preamplifier is a critical part of any sounding system since, in combination with the sensor, it governs the system noise. In some time-domain systems

the preamplifier and receiver loop determine the transient response of the system. Qian (1985) derives guidelines for the frequency bandwidth necessary for

Receivers

time-domain

Three types of sensorsare used for EM soundings: induction coils, magnetometers, and grounded wires. Small induction coils which measure a component of the time rate of changeof magneticflux density, dB/dt, are most commonly used. The time constant of the receiver coil (L/R) should be much less than the earliest time of measurement for TEM systemsand the self-resonant frequency of the coil should be higher than the frequencies of measurement for a FEM system. A variety of fluxgate, SQUID, feedback coil, and other types of magnetometersare sometimesused to measure

B rather

than

dB/dt.

Electric

fields are

sensed with a short grounded wire. The most important parameters of loop sensors are the sensitivity, which dependson the turns-areaproduct or equivalent quantity for ferromagnetic cored loops, the self-resonant frequency, and the noise level. For magnetometers, the sensitivity, frequency response, and noise level are the most important parameters. The length and sometimes the grounding impedance are the most important parameters for a grounded wire. Generally loops, magnetometers, and grounded wires used as sensorsare small enough or short enough that they are called dipoles, although strictly speakingtheir function is not that of a dipole. The amount of current they carry depends on the preamplifier as well as the parameters of the sensor and is not an important parameter. The usageof the term "dipole" for receivers stems from the reciprocity principle, that is, the responsewill be the same if the functions of the source and receiver antennas are interchanged. There are several practical and theoretical differences between

289

B and dB/dt measurements.

For detec-

tion of very good conductors, B measurements will often give superior results (Mallick, 1978; Gupta Sarma et al., 1976). West et al. (1984) point out that dB/dt measurements with a triangular transmitted waveform are equivalent to B measurements with a rectangular transmitted waveform. Natural EM noise

measurements.

If the lower

cut-off

fre-

quency of the receiving system is not low enough, the transient will exhibit sign reversals at late times, not dissimilar to those attributed to IP effects (see the Complex Conductivity section). Qian also examines the effect of the resonant frequency of the coil, which is to effectively add a time delay to the transient response, increasing its apparent amplitude at late times. The same effect is produced by the system's higher cut-off frequency (Spies, 1980c). Most frequency-domain systemsinclude a variable band-passfilter. Rejection notch filters for the power-line frequency and other man-made

noise are sometimes

used in both

frequency-domainand time-domain systems,but may significantly distort the transient response. In some frequency-domain instruments used for soundings,the amplitude of one or more components or the ratio of two components is measured without reference to the transmitter. More commonly, some type of phase reference or time reference scheme is used. In principle a cable or "hard wire" connectionis simplest. In practice, such a link is likely to introduce extraneous voltages into the receiver unless the line is properly isolated, or balanced, or both. Optical links using a length of fiber optic cable, which can transmit pulses or accurate analog signals, have become available in the past few years and their use avoids the problems found with metallic links. Radio telemetry can also be used to transmit a reference signal although the phase accuracy of such links may be inadequate due to bandwidth limitations. Accuracies of 100 ,•s can be obtainedusingVHF systemsbut they require near line-of-sight conditions for successful operation. Commercial WWVB receivers provide absolute accuracies to within 1.5 ms of NBS time; the relative accuracy between two receivers may be considerably better. Global Position System (GPS) satellites can be used to obtain absolute time to 200 ns; currently the receivers are too expensive for most users but the cost is expected to drop over the next


290


few years once full satellite coverage is achieved. Stable clock circuits running in synchronism in the transmitter and receiver are becomingmore popular in establishingphase or time references. Oven-stabilized S-C cut quartz crystal oscillators are stable to about 1

plane between source and receiver. Measurements of phase, ratios of components,or tilt angle and ellipticity are independent of the source moment. However, for amplitude measurements to be useful, the source

partin 108. Moreexpensive andcumbersome rubidium clocksare stableto about1 partin 10•ø corre-

and used to normalize the results. By careful calibration, frequency-domain results can be obtained in terms of V/I, B/I, or dB/dt/I. Frequency-domain measurementsof mutual impedance, Z = V/I, are often normalized by the free space mutualimpedanceZ0 or kZo to obtainthe ratio Z/Zo or kZ/Zo, where k is an arbitrary constant;normalization reducesthe range of soundingcurves and is helpful in interpretation. Slingram instrumentsare designedand

spondingto a drift of about 30 ns per hour. Some time domain receivers are triggered by the high amplitude pulse induced when the transmitter turns off but this is inaccurate since it dependson ground conductivity. When a phase reference is available, an analog or digital synchronousdetector may be used to determine phase and amplitude of the fundamental of the transmitted wave form. Low-pass filters are generally used to remove noise from the detector output. Alternatively, the reference signal may be processedusing a Fourier transform processor to establish the absolute phase between the source current and the received fields. In time-domain systems, the time reference signal controls the timing of data acquisition. Digital stacking is used in most modern frequency-domain and time-domain

receivers.

Quantities Measured

In the most general case, three orthogonal components of the magnetic field and two orthogonal components of the electric field parallel to the earth's surface can be measured.

At small distances above the

surface the vertical electrical field is usually too distorted by vegetation and topography to be useful, although the vertical field can be measured from aircraft or in boreholes. In many specific configurationsall of these componentsare not equally useful so generally no more than three componentsare measured. In the past it was not uncommon to measureonly the amplitude of one or more components. Most modern frequency-domain systems measure in-phase and quadrature parts of the received signal; commonly these quantitiesare converted to amplitude and phase as a first step in data processing. In some systems, two sensorsare used to measure the complex ratio of two components, such as the vertical and horizontal magneticfield or orthogonalcomponentsof the horizontal electric and magnetic field (see Zonge and Hughes, CSAMT, this volume). Alternatively, the parameters of the polarization ellipsoid may be measured directly with a coil system that can be rotated or they may be calculated from measurements of individual components. Often only the parameters of the vertical ellipse, tilt angle and ellipticity, are obtained. Ellipticity has the desirableproperty that its measurementis independentof the attitude of axes of the receiving loops provided they are in the vertical

current, I, must be monitored or set to known values

adjustedto directly measureZ/Zo or Z/Zo - 1, where the latter quantity is the normalized secondaryfield. A specialtype of slingraminstrumentused for geometric soundingsmeasuresonly the quadraturepart of Z/Zo and results are expressed as apparent conductivity. Measurementsof B/I are frequently normalized by the free-spaceprimary field to obtain B/Be (or H/He). Time-domain results are often expressedas impedances, V/I. To completely describe the results, the parameters of the receiving loop must be given when V/I is measured, and the units expressedas V/IM, where M is the moment (more precisely the turns-area product) of the receiving loop. Transformation of measuredquantities to apparent resistivity may also reduce the dynamic range of sounding curves and aid in interpretation. Apparent resistivity is defined as the resistivity of the homogeneous earth which would produce the measured response at each frequency or time, but sometimes asymptoticexpressionsare used to simplify the calculations; this is discussedin detail under the Apparent Conductivity and Apparent Resistivity section. Source-Receiver

Geometries

The geometry of most dipole-dipolesystemscan be specified by giving the orientation of the source and receiver antennasand the orientation and lengthof the line joining the center of the two antennas.If tilt angle and ellipticity are measured, the plane in which the angle is determined must be given. If the source is a loop with its axis vertical and the earth is a horizontally layered half-space,the azimuth of the line joining the antennasis immaterial. In describingthe orientation of small loops it is important to specify whether

the orientationis given in termsof the plane or the axis of the loop. Four commonloop-loopconfigurationsare frequently described as horizontal coplanar (HCP), perpendicular (PERP), vertical coplanar (VCP), and vertical coaxial (VCAX). In the PERP configuration used in sounding, one loop is oriented with its axis



vertical and the other loop is oriented with its axis horizontal and pointing toward the axis of the first loop. The description of the other configurationsis self explanatory. The more commonly used configurations are depicted in Figure 1. A geometric soundingmay be made by leaving the source fixed and moving the receiver, or, both source and receiver may be moved with respect to the center of the array. Also a set of frequency- or time-domain soundingsmay be made either by leaving the sourcein a fixed position and moving the receiver or by moving both source and receiver for each sounding. It is also possibleto perform a soundingusing a fixed frequency and separation but varying the orientation of the transmitter-receiver pair (Duckworth, 1975). Additional discussion of these fixed source and moving source techniques is given in a later chapter. Large loop or grounded wire sources are used primarily in the fixed-source mode. When several measurements are made on a regular grid the easiest way to specify the layout is to give the coordinates of

291

Measurements of the vertical magnetic field about grounded wires are best confined to a sector of a few tens of degreesabout the perpendicularbisector of the wire, in order to avoid low signal strengths and geologic noise arising from galvanic currents flowing through inhomogeneities. Similarly, electric fields are best measured

in this sector or off the end of the wire

in the direction parallel to wire. The latter scheme is commonly used for IP and dc dipole-dipole resistivity measurements.In measuringboth horizontal magnetic and electric fields for CSAMT, a sector between 30 degrees and 40 degrees from the direction of the wire

source should be avoided since this is the region of minimum electric field coupling (Sandberg and Hohmann, 1982).

The range of depths which can be explored with a frequency-domain system generally depends on the source-receiver separation as well as the frequencies used. In frequency-domain methods it is always necessary to somehow distinguish the secondary field from the primary field. The secondary field from a the source and the coordinates and orientation of the particular layer will be very small compared with the primary field unless the separationbetween the source receiver antenna at each site. Generally, large grids and receiver is on the order of the depth to the layer or are rectangular and measurementsof horizontal comgreater. Thus it is generally necessary to employ ponents are made along the grid directions. For large source-receiver separationsof the order of one or two loops it is practical to make measurements at any times the maximum depth to be sounded; at smaller location inside or outside the loop, excluding the separations than this it is not possible with most immediate vicinity of the wire. A specialcasein which systems to accurately measure the small secondary a vertical-axis receiving loop is used at the center of a field in the presence of the much larger primary field. large source loop ("central loop" or "in-loop") was a However, if a system is employed in which the pripopular configurationin early European studiesand is mary field is accurately compensated or other means now very commonly used in time-domain sounding. are used to accurately determine the secondary field (such as can be done with airborne systems), depths of investigation greater than the source-receiver separation can be achieved. If the spacingis too large there is difficulty resolvingthe parameters of thin layers unless (• Q Vertical Coplanar Loops they occur near the surface and the measurementsare extended to high frequencies. For measurements in the near zone, as described in the next section, the spacingshould not be larger than five times the thickness of any layers to be resolved. This latter criterion cannot always be satisfied although the use of two or ElectricDipole Source, • •----,or1• In-Line(Polar) Array more spacingswill help meet objectives. The depth range for time-domain measurements or <• or Broadside (Equatorial) Array ,,-'"'Ix/ ,/r --"'• -0- Electric Dipole Source, (made during the current off-time) depends on the sample time measured and signal-to-noise ratio, and • •-'"• or• or• Line Source not on the source-receiverseparation;in principle one can sound to any required depth using a single small • In-Loop orCentral Induction loop. However the maximum signal-to-noiseratio and • Single orCoincident Loop resolution will be obtained when the separationor loop size are of the same order as the depth to be sounded. The terminology "long offset", "intermediate offset", _-••_•// • or-0- Fixed Large Loop and "short offset" is sometimes used in describing Fig. 1. Typical source-receivergeometries. time-domain methodsand could, perhaps, be usedjust

<•

<•

Horizontal Coplanar Loops

-0-0-

-0•

Vertical Coaxial Loops Perpendicular Loops

292


aswell in describingfrequency-domain methods.Long


offset methods are those in which the source-receiver

separationis larger than the maximum depth to be explored and short offset techniques are those in which the receiver is inside the sourceloop or very near the source loop or wire. Further discussionof depth of investigationis provided in a later section. InductionNumbersand Classification of Soundings In the descriptionand analysisof EM soundings it is oftencumbersome to usefrequencyor time asparameters. Theoreticalresultsare usually describedusing inductionnumbersand other normalizedparameters.

coplanarloop systemover a two-layer earth, Howard (1983)concludedthat for the depthto be well-resolved the numberof measurementsshouldbe much greater than the depth dividedby the skin depth. In practice, the densityof measurementsneededdependson the electrical section and the scatter in the data. Consid-

ering the accuracy of current equipment, geologic noise, and current inversiontechniques,a densityof 10 measurementsper decade of frequency or time is more than adequate for most purposes and useful results can be obtained with as few as four measurements per decade.

Typicallytheinduction number B - (cr•x0co/2) •/2r is

A set of terminologyhas been developed,particularly by Soviet investigators,to describeparts of soundingcurvesor incompletesoundingcurves. Often

used in the frequency domain and the induction num-

this terminology is defined in terms of the mathemat-

berOrora = (cr•x0/4t) •/2r or [3= •

ical approximationsor dominantphysicalphenomenon which characterizepart of a soundingcurve. The

a isusedin the

time domain. Note that B(=r/gFD) is the sourcereceiver separationdivided by skin depth and that [3(=r/gTi)) is an equivalentquantity in time domain.

HeregFI)= (2/cr•x0co)1/2 is thefrequency-domain skin depthandgTI)= (2t/crUx0)1/2 isitstime-domain analog, often termed the diffusion depth. These induction numbers arise naturally in the solutions for the response of loop-loop systems over layered media.

terms "inductive zone" or "near zone" are sometimes used when the induction number is small. The term "wave zone" is sometimes used when the induc-

tion numberis large;magnetotelluric theory appliesto measurements made in the wave zone (see Vozoff,

The MagnetotelluricMethod chapter, this volume). The shapesof soundingcurves measuredin the wave

Slightly different induction numbers stem from solu-

zone are essentially independent of the source-re-

tions for other sources such as infinite lines. As

ceiver separation,whereassoundingsin the near zone dependon the spacing.The terms "late time" or "late stage" and "early time" or "early stage" are often usedin discussionof transientsoundings.Late time or early time are often definedin terms of asymptotic expressionsfor the responseof late or early times. If the electricalsectionis basicallytwo layers and if the depthto basementis Hi, three zonesmay be defined, the near zone when r/H i • 1, the far zone when r/H i >> 1, and the intermediatezone between these

described in Volume 1, models having identical shapes,conductivitycontrasts,and inductionnumbers

yield identical normalized responsesregardlessof their actual size or the frequencyor time measured. In the designof a soundingthe configurationand source-receiver separation should be selectedfirst and

then the appropriatefrequencyor time rangechosen. In the most general case, the upper end of the frequencyrange or the short-timeend of the time range should be selected so that the responseat these extremesreflectsprimarilythe resistivityof the upper layersof the sectionfor whichinformationis required. Similarly, sufficientlylow frequenciesor late times shouldbe selectedat the otherendof the rangesothat a significantresponseis obtainedfrom the deepest regionwhich can be explored,considering otherparameters.If the upperand lower end of the frequency or time range are selected using these criteria, a "complete" soundingcurve will be obtained. If the

objectivesfor the soundingare limited a complete curve may not be required, and in many casesinstrumentallimitationsmay precludeacquisitionof a complete sounding. Ideally a dense set of measurements should be

made;in fact by inversiontheory one of the conditions for obtaining unique results is exact measurements over a continuous,but finite, frequencyband (Parker, 1983).In a studyof the resolvingpower of a horizontal

extremes.

The "small inductionnumber" rangeof operationis defined when the induction number is so small that

mutualinteractionbetweencurrentsin the groundis negligible. By use of very low frequenciesthe skin depth in the earth is made much greater than the source-receiverseparation.In this range the quadrature part of the secondaryfield is muchlarger than the in-phasepart and, as indicatedin the next section,it is linearly dependenton the inductionnumber(McNeill, 1980, Kaufman and Keller, 1983, Wait, 1962a). Since changingthe frequencyin this regiondoesnot provide additionalinformation, the only way to make soundings at small induction numbersis to change the spacingor configuration.Such soundingtechniques are particularly important for shallow investigations since instrumentationand interpretativetechniques have not beendevelopedfor makingshallowfrequency-or time-domain soundings.Furthermore the nec-



essary frequencies might be so high or the times so short that displacement currents would be significant, thereby complicating interpretation (see Displacement Currents and Propagation Effects section). Of course, determination of the dielectric constant could represent valuable information in some applications. RESPONSE

OF HORIZONTALLY

LAYERED

This section is presented to give the reader a greater familiarity with the responsesfor specific configurations and to discussthe relative advantages and disadvantages of systems in terms of their responses for various earth models. A great deal of insight into the physics of soundingand the merits of various systems can be obtained by detailed analysis of the expressions for the response in various operating regions such as near-zone or far-zone. Kaufman and Keller (1983) give such a detailed analysis for vertical magnetic dipole and horizontal electric dipole sources. This section summarizes the formulas for the responseof a homogeneous half-space for various configurations and the

salient

characteristics

of these

con-

figurations by including a number of computed twoand three-layer response curves. In selecting a sounding method and planning a field study it is useful to consider the two following theoretical questions in addition to a host of other practical considerations:

(1) How effective are available methods in resolving the parameters of a layered earth? (2) What are the relative depths of investigation of the methods

glected (the effects of displacement currents and freespace propagation terms are discussed under Anisotropy in a later section). The expressions in Tables 1 and 2 are taken from Ward and Hohmann (Chapter 4, Volume I), and also from Wait (1951a, b, 1953a, b, 1955, 1956b, 1958b, 1960, 1961, 1962a, c, 1972), Wait

and Campbell (1953a, b), Sinha and Collett (1973), and Kaufman and Keller (1983), many of which have been summarized in Banos (1966), Bannister (1967), and

HALF-SPACE

illustrates

293

that are available?

The layered-earth models given here provide some insight into these questions;definitive answers depend on a detailed study of the individual problem under consideration.

Kraichman (1980). For frequency-domain configurations the mutual

coupling ratios, Z/Z o are also given. The mutual impedance, Z, is the ratio of the voltage induced in the receiver

to the current

The theory for deriving the response of a homogeneous half-space is given in Ward and Hohmann, Volume 1. We present two tables of formulas for transmitter-receiver configurations commonly used for sounding, located at the surface of a homogeneous isotropic half-space. Frequency-domain formulas are listed in Table 1, and time-domain formulas in Table 2. Where practical, upper-case symbols are used for frequency-domain field quantities and lower-case symbols for time domain. For EM sounding methods

described in this chapter, unless stated otherwise, the EM fields are assumed to be quasi-static; that is the measurement distance is much less than the free-space wavelength, and displacement currents can be ne-

in the source. For a

harmonic source, Z is given by V

Z-7=

-ilxotomH

I

(1)

where m is the moment, or turns-area product of the receiver and H is the field at the receiver loop. The ratio, Z/Zo, is the ratio of mutual couplingbetween a source and receiver in the presence of the earth to the mutual coupling between the same source and receiver in free space. This ratio is also equal to the ratio of the field H, in the presence of the earth to its value in free

spaceor primary field, He; Z/Z o = H/H e. Approximations for low induction numbers and large induction numbers are given; the low induction number formulas include two terms, as necessary, to represent the secondary field. Some of the approximations given in Kraichman (1980) include only the primary field and are not as useful for geophysical applications. We also include formulas for calculating apparent

resistivity Pa, derived from the expressionsfor the response of a homogeneous half-space. The most general formulas are valid over the complete range of induction numbers used in EM sounding, but may be dual-valued

HomogeneousHalf-space

transmitted

or undefined

for certain

combinations

of

induction number, configuration, and layering (Apparent Conductivity and Apparent Resistivity section). We have used asymptotic definitions of apparent resistivity correspondingto "early time .... late time", "wave zone", etc., in Tables 1 and 2 because they are relatively simple to calculate and are single-valued. Note that the apparent resistivity obtained with these formulas will match the true resistivity of a homogeneous half-space to a given accuracy only over a limited range of induction numbers. When used outside this range the asymptotic apparent resistivity will rapidly diverge from the true half-space resistivity, as discussedin the Apparent Conductivity and Apparent Resistivity section. (Text continued on page 300)

294


Table 1. Quasi-staticfields of various sourceson the surfaceof a homogeneoushalf-spacein the frequency domain. GeneralExpression

Free-SpaceValue

Mutual CouplingRatio


Vertical Magnetic Dipole Source -m

- i p•otom

E,=2,n. crr4 [3- (3+3ikr-k2r2)e -ikr ]

E•= 4,n. r2

(loop-wire) -m

Hz=2,n. k2r5 [(9+9ikr-4k2r 2- ik3r3)e -ikr-9] (horizontalcoplanarloops)

-m

HzP - 4•rr 3

Hr= 4•rr [Ii[-•-)Ki(-12[-•-)K2 ]

Z

2

Z

2

[(3+3ikr-k2r2)e -i•r-3] Z0 k2r2

Zo- k2r 2[(9+9ikr - 4k2r 2- ikr3r3)e -il•r - 9] where Z0 (perp) is set = Z0 (HCP)

(perpendicularloops)

Horizontal Magnetic Dipole Source (x-directed) Z -m

-m

H, =2,rrk2r5 [3+k2r 2- (3+3ikr-k2r2)e -ikr ]

H•- 4,n. r3

2 _

-

Zo k2r2 [3+k2r 2 (3+3ikrk2r2)e -iicr ] (vertical coplanarloops)

m

Hr- 2,n. k2r5 [12+2k2r 2- (12+12ikr5k2r 2

m

Hr p- 2,rrr 3

_ ik3r3)e-il•r]

Z

1

Z7=k2r 2[12 +2k2r 2- (12 +12ikr - 5k2r 2- ik3r3)e -ikr]

(vertical coaxial loops) Horizontal Electric Dipole Source (x-directed)

Ex=Er- 2,rrcrr P 3 [1+ (1+ikr)e -ikr ] (in-line electric dipole-dipole)

Ex= -E, =2,rrcrr -p 3 [2- (1+ikr)e -i•r] (equatorialelectric dipole-dipole)

[

ikr

- 11Ko) Hr= •2,n. r2 311K1 - • (IoK1 where 11, K1 have arguments(ikr/2) (wire-loop, equatorialconfiguration) 2,n-r2

[ikr\(ikr)

(wire-loop, in-line configuration)

-P [3- (3+3ikr-k2r2)e-i•r] Hz- 2,n. k2r4 (equatorialconfiguration)

P

l- ,fro. F3 (for homog.half-space) -p

Ex p- 2,rro. r3 (for homog.half-space) p

Hr p- 4,n.r 2

Z

1

Z0

2

[1 + (1 + ikr)e-ikr]

Z

--= Z0

2 - (1 + ikr)e

- 2 3ILK1

Zo

- ikr

-•-

(IoK1 - I1Ko)

where I1, K1 have argument(ikr/2)

-p

Hg- 4,n. r2 P

Hz p- 4,n.r 2

Z

-2

Zo-k2r 2[3- (3+3ikrk2r2)e -Jar]

Large Horizontal Loop -I

Hz=k-T•a3 [3- (3+3ika - k2a2)e -ika] (centralloop (in-loop)configuration)

I

Z

-2

[3- (3+3i&a - k2a2)e -i•] z0 k2a2

Uf: Line Source (x-directed)

Z -I

Ex=•r(rY • [1- ikyK1 (iky)] -I

Hz=,rl. k2.y 3[2ikyg 1(iky) - k•y2go(iky) - 2]

-½olxoI - • Ko(ikoy)

_

Z0

1 -- iky K 1(iky)

where Z0 is set = Zo (Ex, high-freqasymptote) -I

Z

2

Z0- k2y 2[2ikyK1 (iky) - k•y2Ko(iky) - 2] z_2y

Zo

+•

iky

ky

ElectromagneticSounding Table

295

1. Continued

High-FrequencyAsymptote(far zone, Ikrl >>1) Vertical Magnetic Dipole Source


-3m

Zh

E•- 2•rtrr 4

-6

h Ixøtør2

[3a =

Zo k2r2

9m

Zh

Hz h_2•rk2r 5

-18

h Ixøtør2

3m

Zh

-6

Zo

ikr

z

Pa= 18

Zo k2r2

Hr h=2xdkr 4

6

h

•L0 •0F 2

Im

36

Horizontal Magnetic Dipole Source (x-directed) I

m

H•=•2*rr 2m'r 3 2+

Zh

6

Zo

k2r2

Zh

12

--=2+•

Zo

h

- ixotor2

h

- ixotor2 12

k2r2

Horizontal Electric Dipole Source (x-directed)

Zh

1

Zo

2

271-/-3 Pa = •Ex

2•r•rr 3

P

hl •P

h

gh xdkr3

4

h •0tor2

[Im (Z•)] 2 h •0tor2 Jim (Z•)I 2

[Da =

ikr

-P 1+ H•=2•rikr 3 2-•r 2

(3)

2•-k2r4

Im./-3

Zh

2

Zo

ikr

Zh

-6

16

h Ixøtør2 Pa= 6

Zo k2r2

Im(z•)

Large Horizontal Loop -31

Zh

H• h- k2a 3

-6

h

•1,000a

Zo k2a2 Line Source (x-directed)

=•

7roy2

1-

e -iky

•

= 1-

Zh

2

Z 0 k2y2 I

Hy h=m. iky2

e -iky

Zo

Zh

-2

Zo

iky

4 im hg[0tøy2 (•oZ) (•)1/2(iky) 3/2e-i1ø'-2 Pa-

h Ixø tøY 2 Im 4

Pa=

296

Spiesand Frischknecht Table

1. Continued

Low-FrequencyAsymptote(near zone, Ikrl • 1)


Vertical Magnetic Dipole Source

in(k2r 2+k4r4)

E•=4,n. crr4

Ze

k2r2

Zo

4

Ze

k2r2

Zo

4

4

•4,rr 1- T

œ

-mk 2

Ze

k2r2

Zo

4

16-rrr

œ

Horizontal Magnetic Dipole Source (x-directed)

H•=•4,rr 1-

m

Ze

k2r2

Zo

4

Ze

ik3r3

Zo

15

œ

ik3r 5]

Hre=2,rr• 1- -i-•]

Pa --

}Xo cot 2

œ

15 Im

Horizontal Electric Dipole Source (x-directed) 2-rr{rr3

Ze

k2r2

Zo

4

Pa=

4

Im

,n-F 3 or pa =•Ex P -p

2-r(rr 3

Ze

k2r2

Zo

2

p•-

Im •oo - 2,n-r3

or

p

e 4-rrr2

1+•-

H•=&rrr -p 1- • --•

lnT+C+

In-7- +C- •

4,rrk2r4 k +

--=1+•

'n'lx ocot 2

In--+C+

Zo

8

4

32

Re---

1

Zo

Ze k2r 2( ikr

--=

1

e

'n'lx ocot 2

)

ln--

Zo

8

Ze

k2r2

Zo

4

4

321- ReZ--• Pa= •

Im

Large Horizontal Loop

Ze

•a-Sa 3 k2a2+4

k2a2

-1

---1+•

Zo

4

Line Source (x-directed) 2

gœ go

k2y2 In•2 +C- -2 2

iky 1)

œ

œ

ln•-+C-• k2y2( iky 3)

1-T

Hye =

,r•otoy

Pa --

Zo- 1-T

ln•-+C-

[ (z)

16Re 1-•oø œ

Iik

,r•otoy

Pa --

Z e -2iky Zo

3

4•xo coy 2



!

297

!

!


298 Spiesand Frischknecht

II II II II II


ElectromagneticSounding 299

! !

!

300



Quasi-static Expressionsfor the Responseof a HomogeneousHalfspace.--For quasi-static conditions the measurement distance is much less than the free space wavelength, and

kor <• 1,

(2)

where k0 is the propagationconstantof free spaceand is given by

k0 = (e0•x0co 2)1/2.

(3)

The quasi-staticassumptionin the time domainimplies that

t2 >> 2 e/rr-

elx0r2.

(4)

Asymptotic forms are given for the far zone (or wave zone) for which the additional restriction

>>

cartesian (x, y, z) coordinates. The magnetic dipole moment m, is the product of current I, and turns-area

andhasunitsof Am2. For mutualimpedance calculations the turns-area product of the transmitter and receiver coils are m• and m2, respectively. The electric dipole moment p, is the product of current and dipole length and has units of Am. For large loop configurations the loop radius is a, and for other configurationsthe source-receiver separation is r. The superscriptp identifies the primary field. Other quantities are:

Jn = In, K n = L• = H2 =

comments in Ward and Hohmann, Vol. I. C = .577216 (Eulers constant)

(5)

applies, and for the near zone for which

k r <• 1,

(6)

Besselfunctionsof order n modifiedBesselfunctionsof order n modified Struve function of first kind modified Struve function of second kind (Abramowitz and Stegun, 1972; see also

Time Domain Expressions.--The time domain formulas employ the dimensionlessinduction number Or or Oa where

where k is the propagation constant or wave number for the lower half-space as defined in Ward and Hohmann (Volume I) and is given by

k = (-irrtx0co)1/2.

(7)

Note that instead of k as defined here, a number of

other authors (for example, see the referenced papers by Wait or Kraichman) use •/(sometimes also written as k) where

•/= (io'[x0co)1/2.

(8)

(O'•0t 1/2

0=[ 4t] '

We summarizethe turn-off step function responseof the vertical magnetic dipole (VMD), horizontal electric dipole (HED), large horizontal loop, single loop, and long wire in Table 2. Expressionsfor the electric field, magneticfield and its time derivative and asymptotic apparent resistivities are given. The emf induced

in a coil receiver of turns-area product m2 can be derived from the expression

To convert to the results obtained by these authors,

Oh

the relations

•.

v =-[x0m 20t •/= ik

(13)

(14)

(9) The emf induced in a groundedelectric dipole of length dl is given by

and

•/2 = _k 2

(10)

should be used.

v = e,dl.

(15)

Other parameters are

The induction number B - r/• is given by B = Re(kr) =

o' •0•o) 1/2 r,

(11)

m p r a

= = = =

magnetic dipole moment = nlA electric dipole moment = Idl transmitter-receiver separation loop radius

erf = error function

or

B = Re(•/r),

(12)

where• = (2/½r•x0co)•/2 istheskindepth. Frequency Domain Expressiøns'--The expressions

givenin Table 1 are for eitøttime dependence and employ a combination of cylindrical (r, •, z) and

F(x) = Dawson's integral (Abramowitz and Stegun, 1972, p. 298) Two-layer Models

Results for a homogeneous earth and a suite of specific two-layer models are given in Figures 2 through 8 for several different frequency-domainand



time-domain configurations. The calculations were performed using programs developed by Anderson (1974; 1979a, b; 1982; 1984). A number of other results are also plotted to illustrate particular aspects of frequency-domainand time-domain systems. Frequency Domain Results.--Two-layer FEM curves for a variety of source-receiverconfigurations are presented in Figures 2 to 4. The induction number

B = (•r11x0e0/2) 1/2r isplotted onthetopaxisandranges from 0.1 to 10. The results are also scaledso that they apply to the specific model indicated in Figure 2-1, with frequency plotted on the bottom axis. Generally, magnetic fields are normalized by the primary magnetic fields and electric fields are normalized by the dc electric field of the same component. In some cases where the horizontal componentof the primary field is zero, the curves are normalized by the vertical component of the source field, and vice versa. For a vertical magneticdipole, tilt angle and ellipticity, and wavetilt and phase angle are also plotted; these quantities are inherently normalized. The responsesfor some systems,suchas a horizontal-axis or vertical-axis

source and horizontal

electric

dipole receiver are not given, sinceby reciprocity the mutual coupling ratios are identical to the responses for other systems described, but with the source and receiver interchanged. Also, for horizontal electric and magneticdipole sources,we have consideredonly cases when the receiver is along the axis (in-line) or normal to the axis of the dipole (equatorial or broadside). As contrasted with the responseto a dc resistivity system, the EM responseof a layered earth is gener-

ally rather complicated, particularly consideringthe many possiblecombinationsof sourcesand receivers. In examining the results, it is important to notice the behavior of the curve for the homogeneousearth (•r:/•r• = 1) and to use this as the reference in examining other curves. The differencesbetween the various curves are an important fundamentalmeasure of the degree to which the parameters of a layered earth can be resolved by soundings.In practice, the resolutionof layering dependson many factors including the accuracy, range, and density of the measurements and how well the actual situation is approximated by a one-dimensional model. Most of these other factors are dependent on specific instruments and field practices or on the characteristics of the specific sounding sites and they are discussed in subsequentsections. One means of classifying and comparing various frequency-domaintechniquesis to consider the lowfrequency and high-frequencyasymptotesof the response curves (Table 3). The vertical coplanar and

301

vertical coaxial loop configurationsand the electric dipole-dipolesystemshave amplitude asymptotes(1, 2) or (1, 0.5) and the phase asymptotesto 0ø at both low and high frequencies. Consequently, one might expect the resolving power of these configurationsto be inferior to that of systemsfor which the amplitude asymptote is (1, 0) or (0, 0). Whether or not this is true in practice depends somewhat on the instruments used. If, for instance,a slingramsystem(Frischknecht et al., Profiling methods using small sources, this volume) is usedin which the precisionand accuracyof measurementis a fixed percentageof the primary field, say 1 percent, then the vertical loop configurationsdo not appear to be much inferior to the horizontal coplanar configuration. On the other hand, if the soundinginstrumenthas a wide dynamic range capa-

ble of measuringIH/Hpl to three significantfigures over the range of induction numbers 0.01 to 1.5, then greater resolution can be achieved by using a configuration with amplitude asymptotes(1, 0) or (0, 0). This would be better illustrated by plotting the amplitude curves on a log-log scalerather than a linear-log scale, or by considering percentage changes between the various models. The polar or in-line electric dipoledipole configurationhas amplitude asymptotes(1, 0.5) suggestingthat this might be a relatively poor configuration for EM soundings and conversely a good configurationfor induced polarization (IP). However, the phase responsefor this configurationis relatively largeand, as shownin the followingsection,this configurationis, for someother models,theoreticallysuperior to other common configurationsfor EM soundings. In comparing responsecurves it is useful to notice the presence of features such as sharp peaks or troughs and inflection points that often aid in the interpretation of data. Phase curves generally have more distinct features and often they provide more information than amplitude curves. Of course relative accuracy of the phase and amplitude measurements must be taken into account in comparing the usefulness of amplitude and phase measurements. For the set of FEM modelsconsideredin Figures 2-1 through 2-15 where d/r = 0.25, there is generally little difference between curves in each family when B is larger than about 4. This similarity is becausethe fields are confinedmostly to the upper layer in this range of induction numbers; the top layer is one skin-depth thick for this model at B = 4. This result demonstrates

that, in operating in the wave zone where induction numbers are large, the thickness of the section to be resolved is not limited by the source-receiver separation, but rather by the skin depth. If d/r were smaller than 0.25, B would have to be still larger for the soundingto be in the wave-zone. (Text continued on page 307)

302

Spies and Frischknecht HORIZONAL

COPLANAR

LOOPS

HORIZONTAL

COPLANAR

LOOPS

B 0.1

B

1

lO

0.1

1.6 f i [ I ii;I[[ I I IIIIIIt


1.2 -

•ø-----:"•- '.- - •->-•--•"•-•

-

I

i

! i i i , ,11

,

i i i ! , i lO

.

1.4

•

!

40

1.8

o

, .3

-

'-

20

02/01- 100 ....

•-••,

c) - 40 LU

--

:•

.8

O.

.6

02/01 - 100 •-

_

_

c• - 60

SPECIFIC MODEL:

•r =1000m•

.4

'

-lOO

01 =0.235 S/m •d=250m 02

-12o

varies

_

0

-14o

i

i

i i i i ill

i

i

i i

i

lO-1

10-2

i

i i i i

FREQUENCY

•

-16o

lO1

lOo

lO2

i

i

, ii,,l

i

i

,

• ii,ll

i l•llll

i

i

i

i i i i

lO1

FREQUENCY

lo 2

(Hz)

2-1 (b)

VERTICAL

COPLANAR

LOOPS

VERTICAL

COPLANAR

LOOPS

B 0.1

B

1

10

o.1

1

40•

•

35

2.4

2.2

_

lO

, [ , , , T, I

' ' ' •''

30 _

25

2.0

(:3

N

•

,

lOo

(Hz)

2-1 (a)

..-.

,

lO-1

lO-2

,

-

_

I•

1.8

20

0

uJ 15

'•

1.2

1.0 .6

i

i , i IlilJ

10--2

I

, , ,,llll

10--1

I

I I I • ,ill

100 FREQUENCY

I

I I I f tl

101

10-1

100

101

FREQUENCY

102

(Hz)

2-2 (b)

2-2 (a) VERTICAL

10-2

102

(Hz)

COAXIAL

LOOPS

VERTICAL

COAXIAL

LOOPS

B 0.1

B

1

10

0.1

1

35

I

I

I

I

i

I

I

lO

I [

I

I

I

I

I

I

I I

_

2.4

3O

2.2

25

_

_

2.0

// 0

2O

.

1.8

N

15

UJ

10

-r-

5

'

1.6

1.4

O,•/O,= 100./-

-

' ' /

1.2

/'

1.0-•._•. "• /

0.8 f

0.6

10-2

_' ' / /

3o.' lO..•

. '

'''

•

/

/ /.Y

-I

3/,/.,?

-I

•'// / /

0

-5

-10

i ,•1,•1

•

10-1

• i i ,1•1• 100 FREQUENCY

i1,• i 101 .

i i i•1•

,

-15

102

10-2

,

i i iiiI

i

10--1

i

i i i ,ill

i

i

100

i

i i illl

i

i

i i i i i

10t

102

FREQUENCY (Hz)

(Hz)

2-3 (a)

,

2-3 (b)

Fig. 2. Two-layer FEM models for d/r = 0.25 with varying conductivity contrast. The induction number B =

(or1ix0to/2)1/2r onthetopaxisrefersto thegeneral model.Frequency, onthebottomaxis,refersto thespecific model, which is shown in the inset in Figure 2-1a.



PERPENDICULAR

303

PERPENDICULAR

LOOPS

LOOPS

B

0.1

I

1.8

• ' ' ' ' I

'

•

,

,

10

, , , •

0.1

1

120

100 I

1.6

1.4

60

:_:_•:._- - _ •_

•

1.2

60

_--_.• -... -.-. -._ --__ •. ,--•.•.•....

UJ

1.0

40

F-.

.8

20 -

:•

.6

0

.... o

• .....

lO

_______

N

ß

'•.

o,,/o, - •oo•.

30'- . .'--. '""-.,..,. -.. "'---_'"-,. '

-20

.4

-

. --

///

/// -

.

.0 -.2

10-2

10-1

100 FREQUENCY

101

-80

102

........

i

10-2

........

10-1

(Hz)

[

]

100

101

FREQUENCY

2-4 (a)

102

(Hz)

2-4 (b) LONGWIRE- Hz

LONGWIRE - Hz

B

B 0.1 1.0

I

F---::.• - - œ•---'-'----•

10

' ' •

o.1

........

11

lO

40 f i i [ i !iii

.9

i i i i ii[i

20

.8

o

•

-20 I:'"•-:'•'--

_"--...02/01

N -40 .5

,,, < -.o

-.

' :.'•

-100 .2

100

.

'---

..,....._.

-120

.1

-140 _

,

'010-2

,

......

i

•

•

• • •1

1• 1

i

lO0 FREQUENCY

lO1

-160

lO2

........

i

10-2

........

i

10-1

(Hz)

........

100 FREQUENCY

2-5 (a)

i

,

, , ,,4,

101

102

(Hz)

2-5 (b) LONGWlRE-Hy LONGWlRE-Hy

B o.1

B

1

10

! i,!iiIj , i iIiiiit _

o,,/o• =100 ..../-

30- - '

'

..-'"'"

..-•-'•-----•_"' - -

--''" 1 /

•

"-...

60

;,,

40

>.

20

UJ

o

--

0

.'

•-

' ---•"

-20

•-.•.

-40

-60 i

.1 .

_

,

-.1

10-2

,

,

, ,,,,I

I

10-1

I

100 FREQUENCY

2-6 (a)

_

-80

.

101

,

,

,

,,,,

-lOO

10-2

102

i

10-1

i

(Hz)

2-6 (b) Fig. 2, cont.

........

100 FREQUENCY

i

101 (Hz)

........

102

304


LONG WlRE-Ex LONG WlRE-Ex

B B

0.1

0.1

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120

10

i

i

i

i

i

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i

i

I

I

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10

I I I


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3.5 f i di[i]i['1i [i[ii!•t

8o

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3.0

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c• 6o _ :__

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.

1.5

•

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.

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10-1

,I

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100

, , ..... '

101

FREQUENCY

i

-80

102

i

i

, ilia!

10-2

i

i

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10-1

i

!

(Hz)

i

i

i i i ii

101

FREQUENCY

2-7 (a)

i i iiill

100

102

(Hz)

2-7 (b) ELECTRICDIPOLE(EQUATORIAL)-Hz

ELECTRIC DIPOLE (EQUATORIAL)-Hz

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B

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lO

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101

FREQUENCY

i lllll

-160

102

i

i

i

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10-1

10-2

i

i

i

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102

(Hz)

ELECTRIC DIPOLE (EQUATORIAL)-Hy

B

.9

i iiiii

2-8 (b)

0.1

1.0

i

101

FREQUENCY

ELECTRIC DIPOLE (EQUATORIAL)-Hy

1.1

i

100

(Hz)

2-8 (a)

1.2

i

_

-

B

1

,

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FREQUENCY

2-9 (a)

i

100

i i illll

i

101

i

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i

-70

ii

102

,

10-2

(Hz)

, , , ,,,,I

,

10-1

] , , ,,,ll

,

FREQUENCY

2-9 (b) Fig. 2, cont.

i , ,,,,•1

100

,

101 (Hz)

i • , ,,,

102


ELECTRIC DIPOLE(IN-LINE)-Hy

305

ELECTRIC DIPOLE(IN-LINE)-Hy B

0.1

1

1.1

' '

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'

......

2oO.1

10

I

......

'


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1

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.

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100

"

I

101

FREQUENCY

-80

I I Ill

,

10-2

lO2

10-1

100

101

102

FREQUENCY

(Hz)

2-10 (a)

2-10 (b) ELECTRIC DIPOLE (EQUATORIAL)-Ex

ELECTRIC DIPOLE (EQUATORIAL)-Ex

B

0.1 0.1

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100

(Hz)

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101

FREQUENCY

2-11 (a)

--

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102

(Hz)

2-11 (b)

ELECTRIC DIPOLE (IN-LINE)-E x

ELECTRIC DIPOLE (IN-LINE)-E x B

0.1

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FREQUENCY

2-12 (a)

I

100

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101

I

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-60

102

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(Hz)

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! I illll

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FREQUENCY

2-12 (b) Fig. 2, cont.

I

100

I I I 1111

101 (Hz)

I

I

I I I Ili

102

306

Spiesand Frischknecht HORIZONTAL

LOOP - ELLIPTICITY

HORIZONTAL

LOOP - TILT ANGLE B

0.1

I

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101

FREQUENCY

i

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102

i

i i i illl

i

i

lO0

(Hz)

i i i i ill

i

i

i

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2-13 (a) HORIZONAL

i

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2-13 (b) LOOP - WAVETILT HORIZONTAL

LOOP

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IN

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101

0

102

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10-2

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i

101

FREQUENCY

2-14 (a)

i i i i ,111

100

(Hz)

(Hz)

2-14 (b) PLANE PLANE

WAVE

90

WAVE

........

I

........

I

........

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.......

,

104 ........ I ........ I ........ I ' '' ''"

103 0

102 i,1 50

40 -

101

•

o• u.I n-

30

-

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10-1

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3.0.- // ..

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100 FREQUENCY

2-15 (a)

10

./(•2/Ol = 100

101

010-2 i0''

10

102

-1

(Hz)

10ø

FREQUENCY

2-15 (b) Fig. 2, cont.

101 (Hz)

102

i

i I i i ii

102



307

by the quantity,ro-y 2, wherey is the separation

For some configurations (e.g., vertical coaxial-Figure 2-3), the curves are fairly close to their lowfrequency asymptotes at B = 0.2. For others, such as perpendicular loops (Figure 2-4) this relationship is not at all true. The range of induction numbers over which the largest differences between curves occurs depends on the configuration. For instance, the largest differences for the perpendicular and vertical coplanar configurations (Figures 2-2 and 2-4) occur at low induction number (0.3 to 1), whereas the maximum differences for the horizontal coplanar and vertical coaxial configurations(Figures 2-1 and 2-3) occur at higher values of 1 to 3. This result shows that the optimum band of frequencies to use depends on the configuration. The normalized magnetic fields about a line source (long wire) and about an electric dipole are generally very similar, although differences between curves for various contrasts are slightly larger for the line source. There is a fundamental difference in the responsesfor these two sources when the receiver is an equatorial electric dipole because the electric field for a line sourcedependsonly on electromagneticinduction and it approaches zero as the induction number approaches zero. For the electric dipole source the results are normalized by the primary field. For the

figurationsfor o-2/o-1< 1, but poorer resolution for o-2/o1 > 1. For the set of models considered here, the greatest differences between the phase and amplitude curves for the two extreme casescr2/cr 1 = 0 and cr2/cr 1 -- 100 occur for five configurations' perpendicular loops,

line sourcethe Hy resultsare normalizedby the

equatorialand in-lineelectricdipole-dipole,and Hy

primaryfield for H z, and the Ex resultsare multiplied

and Ex measuredwith a line source.

between source and receiver. Use of this normalizing factor is based on the high-frequency asymptote for the electric field about a line source on a homogeneous earth (Table 1). The response for a finite horizontal grounded wire or bipole is intermediate between that for a dipole and a line source, except that an electric field exists at dc conditions.

Again neglecting all criteria but theoretical response curves, for the two-layer model considered here, the resolution of loop-loop and wire-loop FEM systemsis

generallypoor if the conductivitycontrasto-2/o-1 is less than one. The percentage difference between the curves for homogeneous earth and the curves for cr2/cr • - 0 is typically about the sameas the difference betweencurvesfor cr2/cr • = 3 and cr2/cr • - 10. A line source-loop configuration (Figure 2-5, 2-6) offers somewhat

better resolution

for this case. The electric

dipole-dipole configurations (Figure 2-11, 2-12) offer better resolution than loop-loop or wire loop con-

Table 3. Low-frequencyand high frequencyasymptotesfor frequency-domainsoundingconfigurationson the surface of a homogeneoushalf-space. Mutual Impedance Amplitude Source and configuration of receiver

low freq.

Mutual Impedance Phase

high freq.

low freq.

high freq. -90 ø -45 ø -90 ø

Vertical magnetic dipole Horizontal coplanar loops Perpendicular loops Tangential electric dipole

1 o 1

o o o

o 90 ø o

Horizontal magnetic dipole Vertical coplanar loops Vertical coaxial loops Tangential electric dipole (equatorial position)

1 1 1

2 2 0

0 0 0

Horizontal electric dipole Equatorial electric dipole Polar (in-line) electric dipole Polar horizontal magnetic field

1 1 1

2 0.5 0

0 0 0

-45 ø

1 o o

o o 1

o o 90 ø

-90 ø -45 ø 0o

1

o

o

-90 ø

Line

0 0

-45 ø

0 0

source

Vertical magnetic field Horizontal magnetic field Equatorial electric dipole

Large circular loop central loop (normalized curves

are the same as

vertical magnetic dipole and tangential electric dipole)


308


By comparison with all of the responsesof frequency-domain techniques using a finite source-receiver separation, response curves for a plane wave (MT or CSAMT; Figure 2-15) generally exhibit a greater separation for the conductivity contrastsconsideredhere. On the basis of these results alone, better resolution

might be expected with the CSAMT technique than with the other frequency-domain techniques. The resolution of the finite configurations is reduced by the relatively small source-receiver separation. If the separation is increased such that operation is in the wave zone, the differences between curves for the two layer model increases substantially. To illustrate this point, Figure 3 shows the results for a vertical coplanar loop configuration using the same parameters as for Figure 2-2 except that the spacing has been increased by a factor of four (d/r = 0.0625). The difference between curves for cr2/cr•= 1 and cr2/cr • = 0.3 is about the same as the difference between curves for cr2/cr • = 1 and cr2/cr • - 3 whereas for the smaller spacingthe soundingis less sensitiveto the resistivebasementmodel (cr2/cr • - .3). The differences between the curves are generally larger than for the smaller source-receiver separation. Of course, the use of large separations requires a much more powerful transmitter and has other disadvantages, such as susceptibilityto lateral variations in resistivity, factors which will be described in subsequent sections. Although the curves described in the preceding paragraphs extend to small induction numbers (B -< 0.1) they do not adequately illustrate sounding in the "low induction number" or "resistive limit" range. The low induction-number range for a layered earth could be defined as B n -< 0.02, where B n is the

VERTICAL

COPLANAR

induction number in any of the layers. For a homogeneous earth the actual response of vertical coplanar (VCP) loop is then 2 percent lessthan that given by the low frequency asymptotic expression in Table 1. For B = 0.045 the error is 5 percent. For horizontal coplanar (HCP) loops the same error is obtained at twice the induction

number.

For some purposesthis definition of the low induction number range may be too restrictive. McNeill (1980) has given a formulation for the response of a layered earth at small induction numbers in a very simple and practical form. For instrumental convenience, field measurementsand interpretation are carried out using conductivity and apparent conductivity rather than resistivity. For horizontal coplanar or vertical coplanar loops the near-zone formulas for apparentresistivity given in Table 1 may be inverted to provide a definitionfor apparent conductivity

O'a•

'-- 2

-

Ix0 00r

1 ,

(16)

.

(17)

or

O'a: McNeil

Ix0 00r

2 Im

defined other functions

called the cumulative

response which give the relative contribution to the secondary magnetic field or apparent conductivity from all material in a half-space below a depth Z. Thesefunctions,R v, for vertical axis loops (horizontal coplanar), or RH, for horizontal axis loops (vertical coplanar) are

LOOPS

VERTICAL

COPLANAR

LOOPS B

0.4

11

10

0.4

40

I

40j.] [ [ [ [I

2.4 2.6 f,

4O

10

[

35

[ [ ' [ [[' I

30

2.2

25

• ,.•[,,-•o ,,,' /./ •.•, ,,,•o ,- / / L•,'

-'•

/

."

0

•

1.0 .6• • , , ,,,,,I • • ••t•tl .8

10-1

2/,,1 •0

r- 4000m •

O1- 0.253 S/m• d- 250 m

100

FREQUENCY

3 (a)

•

/

.¾

••• 10-2

• 20

/

/

'

,OY•,•I • , '• 101

10 I 10-2

102

......

i•l 10-1

,

• i • I•il

• lO0

FREQUENCY

(Hz)

I • p .... I 101

i IiIIII•

(Hz)

3 (b) Fig. 3. Two-layer FEM models for the vertical coplanar configurationwith d/r reduced to 0.0625. Parameters for specific model are shown on Figure 3a.

102

Electromagnetic Sounding 1

Rv(Z) (4D2q-1)1/2,

(18)


Rn(Z) = (4D2 + 1)1/2- 2D,

(19)

where D is the actual depth z divided by the loop

spacing r. For a two-layerearththe contributions 0.a, and0.a= of theupperandlowerlayers,respectively, are 0.a1 = 0.1 [1 -- g(o)]

(20)

'

and

0.a2= 0'2 R(D).

(21)

The total response or actual instrument reading is the sum of contributions from each of the two layers,

0.a --- 0.1 I1 -- R(D)] q- 0.2 R(D).

(22)

To generalize for n layers, define the sequence of normalized depths D i, D 2, D3,..., D n _ 1, where D n- 1 is the normalized depth to the lower interface. Then O' a

= 0.1 [1 - R(D•)] + 0.2 JR(D1) - R(D2)] +'" (23)

'''

0.n-1 [R(Dn-2)

-- R(Dn-1)]

q- 0.n R(Dn-1).

Normalized two-layer sounding curves calculated from equation (22) are plotted in Figure 4 for the horizontal coplanar and vertical coplanar configurations. The response for horizontal coplanar loops (Figure 4a) is substantially larger than for vertical coplanarloops (Figure 4b), particularly for 0.2/0.] < 1.

HORIZONAL

•

COPLANAR

309

Just as for soundings made at higher induction numbers, both configurations are more sensitive to a conductive layer than to a resistive layer. However, regardless of the conductivity contrast, the sensitivity to layer resistivities and the depth of exploration, as measured by the separation between curves, is considerably greater than for frequency soundings at larger induction numbers. These characteristics of low induction number soundingsare related to the fact that the response of each layer is dependent only on the conductivity of that layer and a geometric "weighting factor" (Wait, 1962b), and also that skin effect is not the main factor limiting penetration of the fields. For practical purposes the maximum penetration in the small induction number range is about 0.4 skin depths. Esparza and G6mez-Trevifio (1987) show that sounding data in the low-induction number range can be analyzed using linear inverse theory, and interpreted as integrated conductance versus depth. There are some disadvantages in working in the small induction number range, one of which is that very small secondary (quadrature) fields must be measured in the presence of a strong primary field using low frequency. Also, sounding can only be carried out by varying the loop spacingand/or orientation (hence the term "geometric sounding") and not frequency.

Time Domain Resuits.--The transient response for loop and electric dipole receivers and step function (turn-off) excitation for a number of configurations is shown in Figure 5. The model is the same as for the FEM results discussed in the Frequency Domain

LOOPS

VERTICAL

102 ........ I .....;;•1•.,,..• I ........

102

lO1

lO 1

!

100

•

100

! i illill

:"

!

COPLANAR i i Iiiiij

i

LOOPS i i iiiii

. , ,,,,,

110 0" ,"• /

//

/

10•.....-'

ß_--.._•_;_.•" ."-'"•- """

..,.

1

'•••.,,•. 10-1

10-1

02/ øl = '01•x 10-2 ........ 10-2

I 10-1

ß

,

,,,

.•.1

,

100

i

• i • •,,I

101

•

,•

•,

ß

10__ 2 ........ 10-2

102

r/z

4 (a)

I i 10-1

i , illill

i

lOo

02/o7=.o•' 1

i , i,i,•1

,

lO1

i i ilill

lO2

r/z

4 (b) Fig. 4. Small induction number, two-layer FEM geometric sounding curves for the horizontal coplanar and vertical coplanarconfigurations,plotted as a functionof the ratio of loop spacingr to thicknessof the upper layer, z.

310



CENTRAL

''' ,, 10'3..110 .

LOOP

,

'

'

10-4 _

CENTRAL

II" .....

,

LOOP

10 '1 I"'''

•"•.•- •. '\•

x

.

\',•

10-5

'.

•

•

100

10'8•

'

30 \.

._. lO-6

N

•-

X

10-7

X'

?

\\\\3. \ \,,

I•

•

10-8

••.,,',,',,•,

10-9

,,,,,

10-10 10-3

10'2

10'1

1

10

10-14L ........ I

10•

10'3

, ....... I

10-2

...... •,1 •.... ;•,,1• ,x,

10'1

TIME (s)

I

10

102

TIME (s)

5-1 (a)

5-1 (b) HORIZONTAL

10 10-10- ,i

COPLANAR

LOOPS

1 ii

i

i

i

'

-ve

-

,•.

10-11

i

ill,!

HORIZONTAL

10'1 i

i

i •.-.-•-

i

i

1111 i i

10 10-14_: 1,1•

i

COPLANAR

LOOPS

1 i i

i

i

I I i i , i

10-1 i

I

i

_• •+ve

- •.--...

•,••,•', .. •./+ve-

10-15

10-12

10-16

O2/O1= 0

.... 10-17

• 10-13 N

ß 10-18

.c: 10-14

10-15;

10-19

10-16

--

10-20

titillill i iIlllid , ,IIIII•11 N•i%1 10_21i ,ii,,,,l

10_17, , ......I ........I ........I , ,,,,,,,I , .....,, 10'3

10'2

10'1

I

10

102

10'3

TIME (s)

5-2 (a)

10'2

10-I

I

10

102

TIME (s)

5-2 (b)

Fig. 5. Two-layerTEM modelsfor d/r-= 0.25 with varyingconductivity contrasts.Curvesfor both (a) magnetic-field response, and(b)voltage response areshown. Thetime-domain induction number [3- (tr•ix0/2t ) •/2r for thegeneralmodelis annotated onthetopaxis.Time,onthebottomaxis,refersto thespecific modelshownin theinset.r isthesource-receiver separation orloopradiusforcentral loop.Theresults forallconfigurations except central loop are normalizedfor unit dipole momentsof sourceand receiver.



Results section. Results are again given both for normalized parameters and for a specificnon-normalized model. Although most current systems are designed to measure db/dt or voltage, there is considerable interest in measurementof the magnetic field b or h (see Quantities Measured section and Apparent Conductivity and Apparent Resistivity section), so both voltage and the magnetic field response are VERTICAL

COPLANAR

10 10-10

311

plotted. The major differences to note are that the magnetic-field curves have a smaller dynamic range (due to the slower decay), and the initial undershoots and overshoots are no longer present. These features are further discussed in Raiche (1983a), Spies and Eggers (1986) and Newman et al. (1987). To aid in comparison of the various configurations,

it is useful to examine the behavior of v(t)/I o at early

LOOPS

VERTICAL

I

10'1

COPLANAR

10

10-14:

:

10-11

LOOPS

10-1

1

Ifil'

'

'

'

'

ifil,

i

i

i

,

Ill

I I I l:

10-15 _

ß

10-12

\

\\\,

\,,

30

,o',

10-16

10.13

10.17

10-14

10.18

\

',,

\ •,\ ', •o•, x,

10-15

10-19

10-16

10'20

10-17 10-3

10'2

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102

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1

• ,•.,,

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102

TIME (s)

5-3 (b)

5-3 (a) VERTICAL

COAXIAL

0

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5-4 (b) Fig. 5, cont.

•'•oo

I

10

lO2


312


and late times and note changesin signof the transient (Table 4). The late-time behavioris the mostimportant characteristic for sounding with short offsets (near zone sounding).Generally, configurationsin which the responsedependson a high order of •rn are the most

PERPENDICULAR

desirable because the difference

LOOPS

PERPENDICULAR LOOPS

1

10-1

1ø'1ø1• •-"•,-4•"'•'•'<•••-'........ r ..... • -

•

curves for

variousconductivitycontrastswill be largest. By this criterion the perpendicularloop configurationis most desirableand the electric dipole-dipoleconfigurations are leastdesirable.Note that the dependenceon O'n is

•

10

between

'...... •1

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1

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102


ElectromagneticSounding identical for the magnetic-field response. However, other criteria, such as the variation of the signal with distance from the source, are also of practical importance. The change in sign of the transient, which occurs for some configurations, is often a nuisance althoughit can be of diagnosticvalue in interpretation. Many TEM systems are not capable of making mea-

surements at a sufficiently early-time to observe the early-time asymptote, except when sounding over very conductive sediments with large transmitter loops. Generally, the percentage differences between curves for various conductivity contrasts are much larger for TEM than for FEM systems. The shape of

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I

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102

314


the curves is identical for central-loop, electric dipole--h z and vertical coplanar loops. The dynamic


rangeoverthe timeinterval10-3 s to 102 s is reasonably similar for all configurations(ignoringthe sign changes),but as mentionedearlier, is muchgreaterfor voltage measurementsthan for magnetic field mea-

surements. The vertical separation between the curves, and hence sensitivity to conductivity variations, at late time is greatestfor the perpendicular configuration,and least for the electric dipole-dipole configurations.

The choice of configurationdependson many fac-

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5-10 (b) Fig. 5, cont.



tors, including signal levels, instrumental considerations, and ease of operation. Practical considerations are often important, e.g., the vertical coaxial and vertical coplanar configurationsmay not be optimal for deep sounding, since it is easier to achieve a larger source moment with a horizontal transmitting loop than a vertical loop. The grounded electric dipole-dipole configurations (Figure 5-9) display a different early-time behavior than the other configurations.The early-time asymptote depends on the total section and not just the

conductivity of the top layer, and is given by Vctc-

(p/2xr•1 r3) for the in-lineconfiguration andVdc + (p/xrff 1 r3)fortheequatorial configuration. Theseconfigurations are likely to be very sensitive to conductivity variations in the vicinity of the electrodes, at least at early and intermediate times. Because of the large dynamic range of TEM transient decay curves, it is sometimesdesirable to transform the v(t)/I results to apparent resistivity or other normalization in order to remove the strong inverse dependence on time. When presented as apparent resistivity, results for the vertical-axis loop response about an electric dipole results bear a striking resemblance (Figure 5-10(b)) to plane wave FEM results which are reproduced here as Figure 5-10(a). The oscillations in the apparent resistivity curves are discussed in the Apparent Conductivity and Apparent Resistivity section. Perhaps the greatest advantage of TEM soundingis its ability to locate interfacesat depthswhich are large compared with the source-receiver separation. This advantage is illustrated in Figure 6, where results for d/a rangingfrom 0.2 to 4.0 for the central loop configuration are plotted. As noted in Lee (1977), placingthe interface at successivelygreater depths causescorre-

315

sponding delays in the earliest times at which the interface is sensed. The buried layer is first detected

when [3d/a• 1, i.e., the top layer is about 1 diffusiondepth thick. Magnetic field measurements at long offsets (far zone sounding)present a practical problem, as illustrated in Figures 6-1 and 6-2. For d/a •< 0.5 the separation between the magnetic-field curves is not visible until much later than the voltage curves. The early-time asymptote (Table 2) involves a 1 - t/• term which decays very slowly when displayed on a logarithmic plot; this is somewhat analogousto frequencydomain soundingin which the secondaryfield must be measuredin the presence of a much stronger primary field.

It is not until

intermediate

times

are reached

(2t/•lxa• • 0.2) thatthe separation of the curvesis detectable. Note that the curves would have diverged at earlier times had apparent resistivity been plotted, but the difference

would not have been detectable

with

field measurements. For these reasons, far-zone or

long-offset soundings can only be made using db/dt (voltage) measurements. For FEM methods the response from the lower layer of a two-layer earth generally increases as the depth to the interface decreases. However, for TEM methodsthere is an optimum spacingfor detecting an interface at a particular depth. Results for an equatorial wire-loop configuration are shown in Figure 7, where the depth to the interface is kept constant but the spacingis varied. For this example the top layer is one diffusion-depththick at a time of 25 ms. Earlier than this time the response is identical to that of a homogeneous half-space with conductivity equal to •; at later times the curves are influenced by the buried layer. Although the magnitude of the response varies greatly at early times for the different separa-

Table 4. Behavior of TEM responseat early and late time for a variety of loop configurations Sourceand configuration

v(t)/Io

v(t)/I

of receiver

(Early time)

(Late time)

Vertical magnetic dipole Horizontal coplanar loop Perpendicular loops Horizontal electrical dipole Equatorial vertical magnetic field (same as central loop) Equatorial horizontal magnetic field Polar (in-line) horizontal magnetic field Equatorial electric field Polar (in-line) electric field

Single or coincident loop

Sign

1/el 1/(elt) 1/2

•n3/2/t 5/2 •2 /t 3

alwayschanges alwayschanges

1/•1

O'n

_3/2/t5/2

constant

1/(e•t) •/2 1/(e•t) •/2 1/o' 1 1/•l 1/t

en/t2 en/t2 O'n 1/2/t3/2 •j/2/t 3/2 (•n TM /t 5/2

alwayschanges constant sometimes changes sometimes changes constant

conductivityof upper layer for a layered earth, (•n = conductivityof bottom layer, (• = o-n for homogeneous earth.


316


tions,by the time the buriedlayer is detected(25 ms) the responseamplitudesare reasonablysimilar.To study the effect of the buried layer the difference betweeneachcurvein Figure7 andtheresponse for a homogeneous earth is plotted in Figure 8. There is

initiallyan undershootin the voltagecurves,followed by a rapidrise as the secondlayer beginsto dominate the response.The percentagedifferencecurves are similarfor r/d •< 1 for both voltageand magneticfield results,sincetheseare "near zone" soundings. The

d/a = .2

d/a = .2

lO

I

lO-1

10-4 • x',,_x' ,0 , [

5',

X

x

x

',

100

lO

o• 10 '9 •o 1=•5

lO-1

J

' •

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10_7 [ 1ø'1•0'3 10 -2 10 -1 1

1

"5-%

'

10 102

1

10_14 ........ i ........ I .....1 10-3 10-2 10-1

T•ME (s)

10 1ø 2

T•ME (s)

6-1 (a)

6-1 (b) d/a = .5

lO

d/a = .5

I

lO-1

lO

10'3 .....•.._•

I

lO-1

10'7 ''' ' I"'' '

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10'2

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10

10-14

102

10'3

TIME(s)

6-2 (a)

• •'

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10-1

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10

02

TIME(s)

6-2 (b)

Fig.6. Two-layer TEMmodels forthecentral-loop configuration withvarying depth andinterface, d. Theloop radius isa. Theinduction number 13= (e]Ix0/2t) ]/2a onthetopaxisrefers tothegeneral model; time,onthebottom axis,refersto the specificmodelshownin the inset.The curvesare normalizedfor unit receivermoment.



greatestsensitivityat late times is obtainedfor values of r/d • 1. This is also true for in-loop sounding, but since signal levels are higher for large loop sizes the optimumspacingis r • d. Similar studiescan easilybe carried out for other configurations.

d/a

317

Resolution and Equivalence

The concepts of resolution and equivalence are closelyrelated. Two geologicalsectionsare said to be geophysically or geoelectrically equivalent if their

= I

d/a

lO

1

lO-1

= 1

lO

1

lO-1

10 '3,'''• I....... ' I..... 1

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10

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10-14

102

10-3

10'2

T•ME (s)

10'1

1

10

02

T•ME (s) 6-3 (b)

6-3 (a)

d/a

= 2

d/a=

10

1

10'1

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L':.:-;•..

•'

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lO'1 I .....

10'8

10 '4

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2

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•

10'9 I

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10-10•

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I-

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i:

02/01

lO -12

'•" ,•1o

10'13 10-10

10'3

10'2

10-1

1

10

10-14

10'3

02

........, • 10'2

..... . , •_•x,/. 10-1 TIME (s)

TIME (s)

6-4 (a)

.3--•\\ \:,'•--100 1

6-4 (b) Fig. 6, cont.

1

10

102

318



response, within certain error bounds, is identical. If one layer can be replaced by one or more different layers without changingthe response,as measuredin the field, the layers are said to be equivalent. Resolution may be defined as the ability to accurately quan-

tify the resistivity and thickness of the layers in the section. For dc resistivity methods it is well known that the response of a thin, highly resistive layer is controlled only by the product of its resistivity and thickness, the transverse resistance. Similarly, the

d/a=4

d/a=4

10-3

o

I

lO

lO-1

10-4

lO.8 . .

10-5

10'9

10 -6

.•'.•. :: .,:..'100. _

'3

10-9

'

10-10 10-3

10'1

,

1.....

lO-12

lO-13

02 10-2

lO'1

i .......

lO_11 '

a=1000 m

10-8

,

lo_1O• i

'•.X,x 10 '.

10.7

I

lO'7=1.... , ' '

02

I

10

lO-14 lO-3

102

3

........ i ........ i ..... ,•',l', ......,•'• 1,.... 10-2 10-1 I 10 102 TIME (s)

TIME (s)

6-5 (a)

6-5 (b) Fig. 6, cont. ' 100

--

-'-' 200•

t

O'.x• 10-6 - .oo•• 10-7

r

•

10'7L' '..... I........ I'....... I

-

1--0.25,•••00 •02=2'5 m.=

[;

10-9

m ..... '

10•

10-10

r= lOOm

.--200•.•

10'9

10-10

10-12

10-11

10-13

10-12 , i i i,,H , i !il-,I , , , ,,,,,I , , , ,,,. , , , ,,,, 10-4 10-3 10'2 10'1 I 10

10-14

10-4

TIME (s)

7 (a)

.....

...... •

•

10'3

10'2

.......

• ,X,",,¾,,,,• 10'1

1

TIME (s)

7 (b)

Fig. 7. TEM response of an equatorial wire-loop configurationover a two-layer model with source-receiver separationvaryingfrom 100m to 1600m (r/d = 0.25 to 4). The curvesare normalizedfor sourceandreceiverdipole moments

of 1.



responseof thin conductivelayers dependsonly on the conductivity-thicknessproduct or longitudinalor horizontal conductance (e.g., Madden, 1971). EM and dc resistivity methods respond to a layered earth in different fashions. The responsesare a direct

consequenceof the pattern of current flow within the earth. It is well known that the inductive responseof a thin conductive sheet depends only on the conductance of the sheet. Induction methods, however, tend to be sensitive to the thickness but not the resistivity

of a resistive layer. A large number of studies of equivalencefor EM techniqueshave been published. Sinha (1973), Mallick and Verma (1979), Verma and Mallick (1979, 1984) and Verma (1977, 1980a) have analyzed the response of various configurationsto both H-type sections(intermediate conductivelayer) and K-type sections(intermediateresistivelayer), and conclude that the horizontal coplanar configuration (VMD-VMD) provides the best detection of intermediate conductive or resistive layers. For the limited number of cases studied, it appears empirically that

theproduct crX/-• is theequivalence parameter for a 400

319

thinconductive layer,andthat ph2 is the equivalent parameter for a thin resistive layer. Several studies have indicated that systems using a groundedelectric dipole for both source and receiver offer superior resolutionfor detection of a thin resistive layer (Eadie, 1981, Passalacqua, 1983; Strack et al., 1989b). For grounded-dipolesystemsthe radial E field is stronglydependenton the transverseresistance of a resistive layer, while the same layer may be practically invisible to a vertical magnetic receiver. For other configurationsthough, the best resolutionof layering is obtained if conductivity increases with depth (Frischknecht, 1967; Ryu et al., 1970). In Figure 9 the differencebetween the responsesfor K-type section, in which the transverse resistance

p2T2 of the middle layer is fixed and its thickness varied, and the responseof a homogeneousearth are plotted. Similarresultsfor an H type section,in which the conductancecr2T2 of the middle layer is fixed are shown in Figure 10.

For the K-type section(Figure 9) the largestvalue of T2/r is 0.2, which is not smallenoughfor the intermediate layer to be considered mathematically a thin layer. The overall responsesare small for all of the configurationswhich use the magnetic fields. How-

350

ever, the relative differences between various curves

300

in a family are quite large. Conversely, the overall responsesfor the two electricdipoleconfigurationsare large, as expected from the two-layer results given previously,but the relative differencesbetweencurves are smaller than for configurations using magnetic fields. As expected the electric dipole-dipole response (Figure 9-8 and 9-9) tends to depend only on the

25O 200 150 .

lOO .

o ß

-50

I

-100

I

I

lO-2

lO-1

I i i,,,,I

10'3

10-4

j

transverse resistance of the intermediate layer. The

, , ,,,,,I

1

TIME (s)

8 (a) 4oo

,

.

,

I

' ' ,/,,",,•/

300 '

/iII i/

_

250

//11III

200

,

150

'

1...,•/ /

,oo

/ /

o.,,.½// /

.

Go

r,,•=0.2•.../,,? / /

.•_j•///

o

-

'•_

.•./

//

////

-50

-.100

j

• ......

'10'4

I

10'3

,

• , , ,,,,I

,

.......

10-2

I

10-'1

lO

TIME(s)

8 (b)

Fig. 8. Plots of percentagedifferencebetween the curves shownin Figure 7 and the responseof a homogeneous earth with cr = cry.

abilities of the various magneticfield configurationsto resolve the layering do not appear to differ substantially; however, the phase responsefor the vertical coplanar and vertical coaxial configurations(Figure 9-2 and 9-3) is notably smaller than for the other configurations. The overall responsesfor the H-type sections(Figure 10) are large. However, the differences between curves are quite small for the magnetic field configurations and for the equatorial dipole-dipole configuration. This is to be expected; essentiallythere would be no difference between curves if the intermediate layer were always thin comparedwith the loop spacing.On the basis of these results the polar (in-line) electric dipole-dipolearray is the best configurationfor resolving the parametersof the conductivelayer. Caution should be exercised in extending the results shown in Figures 9 and 10 to different models. The resultsdo, however, demonstratethat in some casesit may be advantageousto use configurationsin which (Text continued on page 323)

320

Spiesand Frischknecht HORIZONTAL COPLANAR LOOPS

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Fig. 9. Differences in responses betweenthree-layer K-typemodelanda homogeneous earth.The transverse

resistance p:T: ofthesecond layerisheldfixedwhileitsresistivity andthickness arevaried.Thedepthto center

of the secondlayerD• - 0.4 r. Normalized parameters are cr:/g• - 0.01,0.025,0.05,0.075and0.1 for curves labeledT:/r = 0.02,0.05,0.1, 0.15and0.2 respectively. Parameters for the specific modelareshownin theinset.

104


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104



magnetic fields are measured to resolve the thickness of the intermediate layer of a K-type section and conversely it may be most effective to use the polar electric dipole-dipole configurationfor measuringthe parameters of H-type sections. These conclusions differ somewhat from those reached by the authors cited and present a strong argument that more than one configuration should be used when trying to resolve the parameters of a thin layer.

323

gation is based on the detection of the basement in a two-layer earth. For example the maximum depth of investigationfor in-loop near-zone TEM soundingcan be calculated as follows. Let the average conductivity of the strata overlying the desired depth be or. The response at late times is given by the asymptotic expression

v(t) = 1.59x 10-17IA cr•/2t -5/2,

(24)

where A and I are the transmitter area and current,

Depth of Investigation

In assessinga methods theoretical capabilities, the maximum depth of investigation as well as resolution of parameters throughout the section is a concern. Depth of investigation is sometimesdifficult to quantify becauseit dependson many factors including the particular electrical section being considered. One approachfor estimatingthe practical depth of investiHORIZONTAL

LOOP

-

TILT

respectively. The latest time that can be measured depends on the threshold noise level detected by the

system •v, whichtypically reduces toabout0.5nV/m2 after 15 minutes of stacking. Thus, the latest measurable time occurs when v(t) = •v, i.e. when

tœ= 1.9 x 10-7 (IA)2/5•/5 ,B•2/5. (25) (Text continued on page 327)

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Fig. 10. Differences in response betweenthree-layerH-typemodelanda homogeneous earth.The conductance •2 T: of the secondlayeris keptfixedwhileits conductivity andthickness arevaried.The depthto the centerof the secondlayeris D• = 0.4 r. Normalizedparameters are •/• = 220,80, 40, 27, and20 for curveslabeled T:/r - 0.02,0.05,0.01, 0.15and0.2, respectively. Parameters for the specificmodelare shownin theinset.The conductanceof the secondlayer •2 T: = 10 S.


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326




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To resolve a buried layer at depth d underneaththe top layer with average conductivity (r• it is necessaryto measure to a time t, such that the top layer is about 1 diffusion-depthsthick, i.e.,

t • 6 x 10-7cr• d 2,

0.5

(IA)1/5

(27)

(Spies, 1989). Thus an increase in the source moment of 32 is required to double the depth of exploration.

Fora typicalfieldsituation, givenXlv= 0.5nV/m2and a 200 m-square transmitter loop with a current of 20 amps the maximum depth of investigation and latest sample time is given in Table 5. HORIZONTAL

LOOP

-

TILT

By a similar derivation it can be shown that if a magnetometer is used as a receiver, the analogousto equation (27) is

d 2.6 x10-3 (I•B) 1/3 --

(26)

as shown in Raiche and Spies (1981). Combining equations (25) and (26) and solving for d we find that the depth of investigation is given by d •

327

,

(28)

wheren• is thenoiselevel(typically 10-3 nT).Note that the depth of investigation is no longer a function of resistivity. Hence, magnetic-field measurements may be superior for sounding in conductive terrains. Similar

derivations

can be carried

out for other

con-

figurations. A summary for various TEM configurations and a comparison with magnetotelluricsis given in Table

6.

This type of analysis gives the maximum depth of investigation based on detection of a buried layer. It does not addressthe question of resolution, for which it may be necessary to measure to an extra decade or HORIZONTAL

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IN -20

UJ ::::)

-

•__ -.2

(/3

n

-30 _

_

-35

-.4 I

-.5

100

I

101

I

102 FREQUENCY

10-11 (a)

103

,

I

I

I

I

-40

I

104

100

,

, , ,•,,,I

101

,

, , ,,,,,I

102

FREQUENCY

(Hz)

10-11 (b) Fig. 10 cont.

,

, , ,,,,,1 (Hz)

103

•

, , ,,,,•

104


328


so later in time. Further discussionon this topic is given in Spies (1989). We now demonstrateanother approach, applied to a number of frequency-domain configurations, in which resolution is considered.The model consistsof a very thin conductive layer located at various depths in a less conductivehomogeneoushalf-space.The conductivity contrast is 100 and the thickness of the conductive layer is 1/50 of the source-receiver spacing. The effect of the layer can be studied by analyzing the difference between the responseof the model containing the conductive layer (5 S) and a homogeneous earth (p = 400 l•.m), as shown in Figure 11. Each

Table 5. Depth of investigationand latest sampletime for TEM sounding(assumes200 m square loop with a

currentof 20 amps,anda noiselevelof 0.5 nV/m2). Latest

Average resistivity of overlying strata

Depth of investigation

1 ohm-m 3 10 30 100 300 1000

sample time

600m 750 950 1200 1500 1900 2400

230ms 120 58 30 15 7.5 3.7

configuration has a range of induction numbers in which the amplitudeor phasedifferenceis positive and one in which they are negative, although, for any particular configuration,one range is likely to be much larger than the other. For most configurationsthe effect of the layer is substantialover most of the range of induction numbers shown. In fact, except for the vertical coplanar and vertical coaxial configurations (Figures 11-2, 11-3), the phase response of shallow layers is still large at B = 10.0. As in previous models the peak amplitude and phaseresponsefor the vertical coplanar and vertical coaxial configurationsoccurs at muchlower inductionnumbersthan the peak response for other configurations.Interestingly the largest amplitude differencesfor the in-line electric dipole-dipole configuration (Figure 11-9) occur at low induction numbers, where induction is not significant, whereas the largestdifferencesfor the equatorial dipole-dipole configuration (Figure 11-8) occur at higher induction numbers, where induction is dominant.

To study how the responsefrom the layer decreases as its depth is increased, the absolute value of the maximum excursion in amplitude and phase differences was determined and plotted as a function of d/r in Figure 12. All configurations studied would effectively locate the shallow layer. However, there are (Text continued on page 332)

Table 6. Depth of investigationof variousEM soundingmethods(from Spies, 1989) Near

zone

Far zone

(o.lxa2 2t->5) MT

Intermediate zone•rl•a2-•0.1;a-•3d

(minimize current)

N/A 1/5

TEM, in-loop, voltage

[5.7]

0.55

[2.8]

when a = 1.7d

TEM, wire-loop, voltage

0.48

fladl I ,/5 [5.7] ,•r•lv/

0.37 •

[4]

when a = 1.3d

TEM, in-loop, magnetic field

2.8 x10-3 (•/•B-AB)l/3[2.8 ] 1.4X10 -7I[1.4]

0.48

[2]

for a = 3d

0.28

[2]

for a = 3d can't

measure

can't

measure

•IB

when a -

TEM, wire-loop, magnetic field

2.4x 10 -3

[2.8]

2.9d

10_4 Idl•/2 [2] whena

= 1.7d

Note:•lv = voltagenoiselevelafterstacking (typically 0.5nV/m2),•IB = magnetic-field noiselevelafterstacking (typically 10-3 nT), a = loopradius(in-loop) or transmitter-receiver separation (wire-loop), A = looparea(in-loop), I - transmitter current,dl= groundeddipolelength(wire-loop).Theseexpressions are basedon detectionof a buriedfeatureat depth.It is usually advisableto use a larger sourcemoment (multiply Idl or IA by the factor shown in squarebrackets)to resolvethe resistivity of the lower layer.

Electromagnetic Sounding HORIZON1AL

COPLANAR

329

HORIZONTAL

LOOPS

1

0.1

COPLANAR

LOOPS

1

0.1

lO

100

,

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'

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'

lO

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'

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i i i ii•ii[ -r

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(/)

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<1::

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,'1

40

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0

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¾'x'-•--75.-

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-ø4

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o,=.oo2•s,•I%1 d, 02=.25

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•d2=20m

03: 01

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dl/r=.1

-80 ß

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(Hz)

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11-1 (b)

11-1 (a) VERTICAL

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COPLANAR

VERTICAL

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COPLANAR

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103

104

102 FREQUENCY

103

104

(Hz)

11-2 (b)

11-2 (a) VERTICAL

101

100

(Hz)

COAXIAL

VERTICAL

LOOPS

COAXIAL

LOOPS B

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10o

101

102 FREQUENCY

11-3 (a)

103

104

100

101

102 FREQUENCY

(Hz)

103 (Hz)

11-3 (b)

Fig. 11. Differencein response betweenthree-layerH-typemodelwith varyingdepthd• to thinconductivesecond layeranda homogeneous earth.Normalizedparameters areD•/r = 0.1, cr2/cr • = 100andcr• = or3.Parameters for the specificmodel, shownon the insetare dl = 100, 150,200, 300, 400, 500, 600, 800, 1000,and 1400m.

104

330


PERPENDICULAR

PERPENDICULAR

LOOPS

LOOPS

B o.1

I


1.2

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1:4 1:o

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FREQUENCY

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-70

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d1/r =1 ,

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100

101

102

103

104

(Hz) FREQUENCY

11-4 (a)

(Hz)

11-4 (b) LONG WIRE - Hz

LONG WIRE - Hz

B 0.1

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FREQUENCY

i

-100

103

104

100

i

i iiiiii

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101

(Hz)

i

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i i i i 111

i

i

102

FREQUENCY

11-5 (a)

j

-•1/r -.1

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i i i i 111

i

i

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103

104

(Hz)

11-5 (b)



B o.1

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i , , • i , 11

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=.1

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10o

-50

101

102 FREQUENCY

11-6 (a)

103

104

100

(Hz)

lO1

102 FREQUENCY

11-6 (b) Fig. 11, cont.

103 (Hz)

104


331

ELECTRIC DIPOLE(INLINE)-Hy

ELECTRICDIPOLE(INLINE)-Hy B

0.1

B

I

10

1 ,


I

• 0' (:3

N

1.4 1.0

.•---•,.-•_ ?••-

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........

I 101

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104

(Hz)

102 FREQUENCY

11-7 (a)

-

-30 -40

FREQUENCY

lO

j

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i.u -20

101

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,

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104

(Hz)

11-7 (b) ELECTRIC DIPOLE (EQUATORIAL)-Ex

ELECTRIC DIPOLE (EQUATORIAL)-E x B

o.1 U.I

(D

•n •

2.0

1

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.

]

80

o

•

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3:

'".q" --._•-/' ,,'.1•,,/ =.1

.

20

i

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-40

.•: '<

•ø

_ •.....3

-1.5

::3 •

1.4 4. _ _- . , - / /, .•'./<.. •-_----'1--..... '_.--..(_/ß / /-•---/ -/ .:-LLL•"_ :4' /I / // //

-

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.......

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,

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102

, ....

103

FREQUENCY

104

100

101

(Hz)

102 FREQUENCY

11-8 (a)

103

104

(Hz)

11-8 (b) ELECTRIC DIPOLE (INLINE)-E x

ELECTRIC DIPOLE (INLINE)-E x B

0.1

.2•_

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......

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'"" ''I

.11-

JoL ..................

--- "'-I

1.41.0.8 •

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0.1 120

....

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/

:2,

,. ,,' ,,/-'

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ILl

-I

100 f 80

/ fi

3:

60

-t

(.3

40

/

.,.,

?, 20

'

3:

.........

o

....

•_'-'•------------'• -,;h':,_ • - •_--

'----".... ' :.- -1

I

i.u -20 -1- -40

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///

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1½

102

FREQUENCY

11-9 (a)

....... ,

103

........

-80

104

100

(Hz)

101

102 FREQUENCY

11-9 (b) Fig. 11, cont.

103 (Hz)

104


332


importantdifferencesbetweenthe variousconfigurations when the layer is deep. The rate of decreaseof responseas depth increasesis an important factor as well as the magnitudeof the response.Considering amplitudeexcursionsfor the loop-loopconfigurations (Figure 12-1),the perpendicularand horizontalcoplanar configurationsprovide the largest responsesand the verticalcoaxialconfiguration providesthe poorest response at intermediate and large depths. The responseof the vertical coplanar configurationfalls off slowlywith depthandat greatdepthsit is greaterthan for the perpendicularconfiguration.The equatorial electric dipole-dipoleconfigurationproducesmuch larger amplitudeexcursionsthan any other combination of loopsor electricdipoles.For largedepthsthe amplituderesponsesof the polar electric dipole-dipole, electric dipole and equatorialvertical and hori-

HORIZONTAL

zontal loops, and electric dipole and in-line horizontal axisloopare all similarto that for the verticalcoplanar loop-loop configuration. Consideringmaximum phase excursions,the perpendicularloop-loop configurationprovides the best responseat intermediateand large depths;the phase angle associated with wavetilt measurements is com-

parable when the layer is at great depths. At large depthsthe verticalcoaxialloop responseis the poorest. At intermediateand large depths the equatorial dipole-dipoleconfigurationprovides a larger phase responsethan any configurationsother than perpendicularand wavetilt loop-loop.The equatorialelectric dipole-horizontalaxis loop configurationprovidesthe poorest response of all the configurations using an electric dipole. Except for the wavetilt phase angle, resultsfor tilt angleand ellipticity and for wavetilt are

LOOP - TILT ANGLE

HORIZONTAL

LOOP

-

ELLIPTICITY

B

1 •

!

lO

j

,

,

,

i

i

,

,

,

,

,

,

,

,

lO

j

,

.

20

LLI n-

-

•

_

10

LI,.I

_

1.4 1.o

0

•

.. • 77 --•----•-•_:

:

Z

O

I

o.1 .5

! , • _

u.i

-lO

• ,,.• '-•....

n- -20

j/

/

/

,,,,\

_ _

0

-30

i

"-••/Jdl/r =.1

LL I -40 •

-50

.

I

-J

• -60

LU

-.4

_

-70

'

-.5

101

100

105) FREQUENCY

103

104

'

' ' ''liJ

I

101

100

I

102

(Hz)

FREQUENCY

11-10 (a)

,

,

, ,

103

104

(Hz)

11-10 (b)

HORIZONTAL

LOOP

-

WAVETILT

HORIZONTAL

LOOP - WAVETILT B

0.1

30t_

1

,

, ,

0.1

10

j

3.5

........

_

i.u

"'

.<

lO

c/)

3.0

-

2.5

I

-I

ß

5).0

•

-lO

-

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1.5 _

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1.0

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-30

•

.5

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0

._1

-50

60

•

100

101

102 FREQUENCY

11-11 (a)

dl/r =.1 _

103

•:

.

-

T•• - • , 1.41.0.8 .6 .5

-

•/

-.,5

•

_

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-1.5

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104

(Hz)

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101

i

i

i iiiiiJ

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11-11 (b)

i

102 FREQUENCY

Fig. 11, cont.

/

-1.0

i i ,111J

103 (Hz)

i

i

i i , i i1

104


not given since the quantities are not directly comparable to coupling ratios. However, these quantities are relatively sensitive to the presence of a layer at depth.


Somewhat

different

results

are obtained

for other

models. For instance, Frischknecht (1967) plotted response curves for three loop-loop configurations raised above a homogeneous half space. For large heights the responsefor the perpendicular configuration is least and the responsefor the horizontal coplanar configuration is approximately twice that for the vertical coplanar configuration. Paul and Roy (1970) and McNeill (1980) showed that in the low induction number range the horizontal coplanar configuration is insensitive to near-surface materials, yet it yields twice the response of the vertical coplanar configuration when d/r > 1. For a simple model in which the loops are raised above the surface of a perfectly conductinghalf-space (Keller and Frischknecht, 1966)

LOOP-LOOP

333

the same conclusion

is reached

for d/r > •

4. Thus it

is apparent that the question of the relative depth of investigation of various configurationsdepends on the model.

SOURCES

Errors

OF ERRORS

which

arise from

IN SOUNDING

numerous

sources

can be

classified as instrumental (lack of calibration, drift), geometrical (misalignment, topographic), geological (lateral changes in conductivity, dipping layers, magnetic viscosity, IP effects) and those due to electromagnetic noise (including natural, man-made and wind noise) and cultural effects (fences and power lines). By identifying sources of error they may be reduced or eliminated by good field practices, or corrected by suitable data processingtechniques. We will deal with each source of error.

LOOP-LOOP

CONFIGURATIONS

103

101

'

I

CONFIGURATIONS

'

I

'

I

'

I

,

'

I

'

I

'

I

'

I

'

I

'

z z

03 n-

10

0

•

102

X X

I-

10-1

03

101

•

10-2

•

lO0

_

X

10-3 .1

10-1

i I , I , I , I , I i I , I • 19t i ' .2

.3

.4

.5

.6

.7

.8

ß

10

DEPTH/SEPARATION

DEPTH/SEPARATION

12-1 (a)

12-1 (b) ELECTRIC

DIPOLE

103

z

•

102

x

•

ELECTRIC

,

f_"""-"'<-'• 5•--- x ....

SOURCES

'

I

'

I ''

I

'

I

'

I

'

I

'

I

'

z

x 100

-"""""""""'• "" INLINE E 101

DIPOLE

101 - ' '1 ' ' I

,

i ' i ' i , i , i , i , i , i , i , i ,:

x

,•

SOURCES

...•__....•. •

.

'-... E_Q.UAT. Ex

---._

',N-L,N•' •y-".•.'.--.:_X...

.....

10-1

100

7,•,•3..•....-....

-:'

x

10-1 ß1

.2

.3

.4

.5

.6

.7

.8

.9

1.0

10-2 I ,1 • I •,,,I • I • I •, 17• I • I • I • ß1

12-2 (a)

.2

.3

.4

.5

.6

,

.8

.9

1.0

DEPTH/SEPARATION

DEPTH/SEPARATION

12-2 (b)

Fig. 12. Maximum difference in amplitude and phase for the model of Figure 11, for loop-loop configurations (Figure 12.1) and electric dipole transmitter and loop and electric dipole receivers (Figure 12.2).

1.1

334


Instrumental

Spiesand Frischknecht Sources of Error

Most problems with instrumental accuracy can be attributedto inadequateinstrumentdesign,resultingin high drift rates or lack of a calibrationfacility. Absolute calibration is not necessaryin many frequency-domain systemswhich measure ratios of fields or parameters of the polarization ellipse. The main criteria in such equipment is that the gains of the amplifiers and the Q of the coils for each component are matched to a sufficient accuracy, and that their relative drift is negligibleover a wide range of operating temperature and battery supply voltage. If the amplifiers and coils of a polarization ellipse systemare not well matched, it is possible to calibrate the total channelgainsunder field conditions,usinga procedure described in Ward et al. (1974). The reference coil is first aligned along the major axis of the ellipse of magnetic field polarization and the voltage recorded. The coil assemblyis then rotated 90 degreesto place the signalcoil along the major axis, and the voltage is again measured. The correction factor is the ratio of

the recorded voltages. Field experience will dictate how often this procedure needs to be followed. For absolute measurementsthe primary field must be corrected for by calculating the low-frequency asymptotic or free-space coupling at small sourcereceiver separations, comparing with measured lowfrequency values, and deriving the calibration factor. For time-domain measurements, the biggest source of instrumental

error often arises from clock drift and

resistance, and resonant frequency of the transmitter circuit. The phase and, for non-sinusoidalsignals,the waveform of the current, differ from that of the impressedvoltage if the load has appreciable inductance or capacitance. The phase or waveform of the current and primary magnetic field can be monitored by measuring the voltage across a calibration resistor connected in serieswith the antenna or by use of a current monitor or current transformer; use of the latter device is generally restricted to frequencies above 1 Hz. Generally, corrections should be made for finite transmitter turn-off time, which depends on loop size and other factors (Raiche, 1984; Fitterman and Anderson, 1987). Also, errors can be introduced into time-domain

measurementsif measurementsare regarded as step function results when the off-time between pulses is not much larger than the decay time (Asten, 1987). The

combined

effects

of all instrumental

factors

over a wide range of frequency or time can be combined into a "system response" and used to correct the field data. In the frequency domain the amplitude and phase responses of the total transmitter-receiver system over the complete frequency range must be measured. With knowledge of the transfer function it is possible to inverse filter the responseto correct for instrumental errors. The analogous technique in the time domain is deconvolution of the impulse-or stepfunction response.This processis describedfurther in the Data Processingsection. Unless the systemis carefully designedand properly used the potential for instrumental error is high if a

the inability of amplifiersto handle the large dynamic

metallic

range often encountered in field measurements. Clock

employed. The difficulty with reference lines is sometimes attributed to "capacitive" coupling. Occasionally, the effect can be noticed as a spuriousresponse when the receiver, if ungrounded, is touched. Grounding the receiver may make the problem less obvious but may also increase errors in the measurements. One problem is causedwhen the electric field of the surface induces a large common-mode voltage on the line and some of this common-mode voltage is converted to a differential-mode signal due to imperfections in the equipment. Another problem is that capacitive coupling between the ground and the line may allow significant common-mode currents to flow to the end of the line and the magnetic field of this current may be detected by the receiver coil. Further discussionsof reference lines and instrumental designare given in the chapters on instrumentation and scale model studiesin

drift can be checkedby measuringan auxiliary R-C or R-L calibration decay circuit at various times during the day, or repeating measurementsat a station near the transmitter loop. Overdriven amplifiers are also a common problem, especially when cultural or atmospheric noise is high, or when an overly sensitive receiver loop is used. When in doubt a set of readings at a number of gain settingscan be repeated to check for consistency. Timing techniques which rely on detecting the edge of the transmitted pulse also are prone to error due to pulse distortion and delay in a conductive earth. If the resonant frequency of the coil or cut-off frequency of the low-pass filter of the receiver system is too low, then the transient decay will exhibit a rapid rise, followed first by an overshoot and then by a decay paralleling the true transient response.If the lower cut-off frequency is too high, the transient will be pulled down at late times, and possibly even exhibit a sign reversal. There are usually fewer problems associated with the transmitter. The current, and therefore the trans-

mitted primary field, is a function of the inductance,

Volume

reference

line between

source and receiver

is

1.

Geometrical

Sources of Error

Geometrical sourcesof error include all departures from the assumed geometrical relationships of the



335

source and receiver, including loop shape, sourcereceiver spacing, and orientation of the source and

domain measurements

receiver.

relatively small, but the horizontal field exhibited

In an analysis of the field about a finite circular loop on the surface of a homogeneous conductive earth, Wait (1954) found that the error caused by assuming the loop to be a dipole does not exceed 1 percent if the distance to the point of measurementis greater than about 5 loop diameters. At frequencies approaching dc, the error is somewhat greater; in Frischknecht's chapter, Electromagnetic Physical Scale Modeling, Volume I, it is shown that in free-space the error is about 1.5 percent for a circular loop having a diameter one-fifth of the distance to a dipole source. Generally, a small, approximately constant, error in the absolute value of the moment of the transmitter or the apparent sensitivity of the receiver is of little importance. If extremely accurate measurements are made, a finite loop appears to have a small frequency-dependent error when it is treated as though it were a dipole. The behavior of a square loop is practically identical to that of a circular loop of equivalent moment as long as the

moderate

induction numbercrlxtoL 2 or crlxL2/T is lessthan 1 (Spies and Raiche, 1980); the differences depend on the earth model and may be negligible for larger induction numbers. Other loop shapesare examinedin Kauahikaua (1982). A grounded electric wire is equivalent to an electric dipole if the source-receiver sepa-

ration is greaterthan 5 timesthe dipolelength(Vanyan et al., 1967a). An error in the spacingof small dipolar loops or wires causes errors in the interpreted thicknessesand resistivities of the layers; errors in thicknessare proportional to the error in spacingbut errors in resistivity are proportional to the squareof the error in spacing. Misalignment errors have been studiedby a number of workers. Sinha (1980) discusseserrors in frequencydomain measurement for rotation toward and away from the transmitter, altitude differences, and uniform slope of ground. Fullagar (1981) and Fullagar and Oldenburg (1984) extend the treatment for 16 field quantities, which are given in a later section, Topographic and Geometric Corrections. Several generalizations can be made from these studies.For loop-loop systemsthe horizontal coplanar configurationis subject to larger misalignmenterrors than the vertical coplanar configuration. Measurements involving in-phase quantities are extremely sensitive to geometrical errors compared to quadrature or time-domain

measurements.

A comparison of the errors obtained with various frequency-domain and time-domain quantities over a layered earth is described in Hoversten (1981), who found that ellipticity was the component least susceptible to orientation errors (this study assumedthat the

two receiver coils were perfectly orthogonal). In time-

errors

errors in the vertical

field were

at late time.

To the first order, topographiceffects are similar to misalignmenterrors. Ward et al. (1974) point out that tilt angle errors are independentof frequency. Karous (1979) derives simple analytic expressionsfor calculating the 2-D topographic effects on EM induction with

distant

sources.

Further

discussions

are

con-

tained in Wilt et al. (1983) for the EM-60 system, Appendix G of Macnae (1981) for E measurements with the UTEM system, and Anderson et al. (1983b) for loop-loop FEM sounding. Verma (1973) considers the effect of tilting, height differences, and spacing errors on the horizontal coplanar, perpendicular, vertical coplanar and vertical coaxial configurations and presents the results as curves of mutual coupling ratios. GeologicalNoise

The term "geological noise" is used to include all geologicalphenomenawhich result in incorrectlayeredearth interpretations using quasi-static 1-D models. These phenomenainclude lateral changesin conductivity, anisotropy, dipping layers, magnetic viscosity, frequency-dependent conductivity(IP effects),displacement currents, and propagationeffects.

Near-surface Inhomogeneities.--Small-scale nearsurface changesin conductivity are much more likely to affect grounded-wire techniquesthan purely inductive loop-loop methods. The electric field is strongly controlled by the ground in the immediate vicinity of the electrodes, whereas the magnetic field is a weighted integral of all subsurface currents. This is borne out in field experiments reported in Keller et al. (1984) and UTEM E-measurements reported in Lamontagne et al. (1980) and in West et al. (1984). E-measurementsfrom a fixed source provide information about lateral conductivity contrastssimilar to the gradient-array resistivity method. Current channeling along conductive zones such as shears and faults is more likely to be substantialin E field grounded-wire techniques than in loop-loop techniques, particularly at low frequency, because a grounded wire always drives galvanic currents through the earth. These effects are often difficult to recognize unless multiple electric-field dipole locations are employed. As in the magnetotelluricmethods, the effect of a small inhomogeneity may, over most of the frequency range, be a displacement of the curve but not a change in its shape, an effect commonly termed "static shift". Further discussionson this are given in the Effects of Other Than

1-D Structures

section.


336


Olm (1981) made a scale model study of the "dual frequency method" (Kaufman and Keller, 1983)which is designed to discriminate against the effects of small inhomogeneitiesoOlm's results for a synthetic inhomogeneous overburden show that for a horizontal coplanar loop-loop system the inhomogeneitiesbehave as confined

bodies

rather

than

as infinite

sheets.

The

implications of this result are that (1) small inhomogeneities can change the shape of loop-loop sounding curves, and (2) for a limited range of frequencies, the dual frequency method is very effective in discriminating against some types of geologic noise. Small lens-shapedconductive bodies are one type of near-surface inhomogeneity that often distorts EM soundings.A thin horizontal conductiveplate is a good approximation to such an inhomogeneity. Profiles over a thin plate, embedded in a half-space, modeled using program "SHEET" (Weidelt, 1981), are illustrated in Figure 13. Only half the profile is shown because of symmetry. Both galvanic currents and vortex currents

2.0 .

'

i

,

i

,

i

,

i

'

i

,

I

,

I

,

i

,

i

are present in this example. Sounding curves were computedat various distancesout to 200 m along the profile. The largest excursions occur when the transmitter or receiver loop is located over an edge of the body (x = 25 or 75 m). Amplitude sounding curves, normalized by the responsefor a homogeneousearth, are plotted in Figure 14. Apparently the shape 6f a sounding curve is substantially modified by such a relatively small conductive lens, especially at high frequencies. The relative amplitudes can either increase or decrease depending on the geometry of the source, receiver, and inhomogeneity. Background values are reached only at distances greater than 120 m, i.e., when the closest loop is about one-half the loop spacingfrom the edge of the inhomogeneity.

Edges of Layers.--One of the simplest structures often encountered in sounding that is not 1-D is the edge of a horizontal layer. Lateral changesin conductivity outside the zone contributing appreciably to the

20

,

_

1.9 .

_

.

.

10

1.8 1.7

._.

1.0

N

1.5

/ • \\

-

_.N 1.4

/!

-

// •

1.3

::3

1.2

•

-;-w•______•__•-. 1.0 - -__i / .9

-

'"'•"--•- f = 13000 Hz

"'•.•/// ,,, __ ••-

-

3200

-

o

-10 -

"•

-

/

.5

'

I ß, 40I , 60I : 810: 100I • 1201. , 140I , 1• 0 , 180I

:

20

0

200

.•

•••5oo ///

• -...................

_ooo Hz - •-'••'/////•'-!13

-40 -50 -60

.-•...

-

_ •-•.•Edge ofbody

lOOO/

-

Ill -30

...

.6

.25 50 700 F200 400 :-_-•:--•-•__-•-_-_-•_-_-2_ &_-•z•_ =-q

-20

-

//

.7

A N

6500

...................... o_o_o____; 25' 54 100• 200' ,06 -

/

-

.8

-

......

'-- •--•_

• --•

•z½--••

-

?' -,,,,

-

i,u

'•

'

0

-

xx

-70

/

//

.'- • ",x•/•Edge of body I •, I • I ,

-80

0

20

40

60

I

•

00

I

,

I

100

,

I

120

,

I

140

•

160

I

100

, /

200

X(m)

X(m)

13 (a)

13 (b)

Fig. 13. Profiles of horizontal coplanar loops over a near-surface conductive inhomogeneity modeled by a thin horizontal sheet buried at a depth of 5 m in a uniform half-space. The sheet has dimensions 100 m (y-direction) by 50 m (x-direction) x 10 m thick, and a resistivity of 10 fl. m. The sheet is centered at x = 0, the host resistivity is 100 fl.m, and the source-receiver separationis 100 m. 4O

f

///:x\\

20t-

f2,•

10

0

•X

.....

-

/

z

72004•000 ,•s_00

_.._-./ /

= 80m

20

// ,,,x\\

//

lO

x• •..... .

• -•

o

•.•

.... ..,..•. ___ 100 ' .60

-..-----•.-•-=-=""•

. -lO

-20

x

-3o .

-60

0

,

20

.

40

|

00

00

100

120

140

160

180

-60101

X(m)

14 (a)

.2o

-so

o,

I ß

40 .

-40

-40 I•""-'-_ ////If =13 000 Hz F I

-:30

i

I i ,,11

i

102

i

,

i,

,ill

,

:

103 FREQUENCY

:

:

104 (Hz)

14 (b)

Fig. 14. Percentagedifference between soundingcurves and responseof homogeneousearth computed at various distancesalong the profile shown in Figure 13.

lO5


The problemof non-parallellayerscanonlybe solved numerically.However,for the specialcaseof a highly resistivesurfacethe magneticfield responseof underlying dippinglayers can be estimated.To do so we simplyassumethat the upperlayer is air andthat the loopsare tilted or placedat differentheightsover a layered earth. For a nominallyhorizontalcoplanar loop (HCP) configuration placedalongthe strikeand

detectedEM field are unimportantto the sounding. The volume of earth influencingeach soundingde-


337

pendson theinductionnumber;to sounddeeperis not possible withoutalsoincreasing the arealextentof the coverage.The TEM modelresultsfor the single-loop TEM configurationshownin Figure 15 illustratethat the stepdiscontinuityis observedat greaterdistances at progressivelylater times. At early times or high frequenciesthe edge of the

above a tilted earth the mutual couplingratio for the tilted configurationcan be computedfrom the responsesfor a vertical coplanar(VCP) and a HCP configurationby the expression,

layeris not detecteduntil the source-receiver arrayis quitecloseto the boundary.This distanceincreasesat late times or lower frequencies,and as the resistivity of the layer is increased.For a horizontalloop-loop systemthe boundaryis generallynot seenas long as

Z

z

z

both source and receiver are on the same side of the

Z0

boundaryandboth are one-halfcoil spacingor further from the boundary(Villegas-Garciaand West, 1983). Further examples are given in the literature. Kuznotsov(1982) discussesseveraldistortioneffects in EM sounding. Seaborne et al. (1979) presented frequencydomainloop-loopprofilesover geological fault models. Stoyer and Greenfield(1976) computed vertical polarizationellipseparametersover 2-D featureswith a loop-loopsystem.In a single-loopsounding, edgeeffectsmay be more difficultto detectthan 3-D effects, but can be easily recognizedby using an adequatedensityof stations.

COS 20+•'0sin2 0,

(29)

VCP

HCP

tilted

where0 is the anglebetweenthe planeof theloopsand the conductiveearth. This expressionwas used in Keller and Frischknecht(1961) in interpretationof sounding dataon a glacier.For a nominallyhorizontal coplanarconfiguration placedaboveand perpendicular to strike, the mutual couplingratio is Z =1+

(cos 0)3 To(A, B)

Z0

1

DippingLayers.--In interpretingsoundingdata layers are generallyassumedto be parallelto the surface.

(30)

BT2(A, B)sin 20 t - 1.0 ms

IOO

I0

nV

Am2

I

-300

I

I

.-ZOO

I

I

-I00

I

I

0

I

I

I00

I

I

ZOO

I

I

300

I

Fig. 15. Single-loop TEM profilesovera stepdiscontinuity model(afterSpiesandParker,1984).

I

400m

338


where TO and T2 are Hankel transforms using the


notation of Wait (1958a), Frischknecht (1967), and Wait (1982). For dips smaller than about 10 degreesthe error due to dip is rather small; the effect is slightly less when the

configuration is parallel to strike (Figure 16-1) than when it is perpendicular to strike (Figure 16-2), and is of opposite sign. The error due to dip can be reduced substantially by averaging two orthogonal soundings, as is often practiced in cross-spread Schlumberger resistivity soundings. Three-dimensional

Bodies.•Three-dimensional

fea-

tures usually generate EM components which would be nonexistent over a horizontally layered earth, and can thus be easily recognized by adequate measurements. Exceptions to this are axisymmetric soundings, such as positions directly over the top of a basement dome. Goldman (1977) has modeled the in-loop TEM response of a disc buried under a conductive slab and presents normalized apparent resistivity curves for a variety of loop radius/discradius ratios. Goldman and Stoyer (1983) present model results for the on-axis sounding of a cylinder in a conductive medium. They conclude that 1-D interpretation is valid for most cases in which the radius of the cylinder is greater than the source-receiver separation. For larger separationsthe interpretation will yield a stratified earth model with overestimated

resistivities

and smaller errors in deter-

mination of thickness. They also presentmodel results for on-axis basement highs and lows. Newman et al. (1986) describe in-loop model studies for structural interpretation of a two-layer earth with a basementrise and basement trough. They note that although many different 1-D models can be fit to the 3-D data, the 1-D interpretation generally follows the trend of the structure. Kauahikaua (1982) describes the effect of lateral inhomogeneities on FEM sounding in Hawaii. Newman (1989) modeled 3-D transmitter overprint effects for wire-loop TEM sounding. Many other studies, most made to guide interpretation of horizontal profiling data, provide insight into the effects of 3-D bodies (see other chapters in this volume). Recognitionof Non 1-D Earth.--To determine from a single sounding in which only one component is measured if the earth is horizontally layered and if the interpretation is valid is usually impossible, unless obvious contradictions to the sign rules in Table 4 occur. The optimum field procedure is to employ a dense array of sounding sites. If measurements at adjacent sites measured on a grid pattern are nearly identical, this is virtual proof that the earth is approximately horizontally layered.

A common field practice is to record only one component required by theory to study a 1-D earth. Often it is possible to fit a layered-earth model to 3-D geometries in these cases, as illustrated by the example in Figure 17. If nonredundant field components such as the tangential magnetic field of a vertical magnetic dipole source or horizontal component with a central-induction sounding are routinely measured, however, lateral effects can be readily recognized (Newman et al., 1987; Spies, 1988). This technique fails for soundingsin the center of symmetric 2-D or 3-D structures, for example central sounding in the geometric center of a circular uplift or depression. Further examplesof the effect of 3-D structureson EM soundingsare given in the Effect of Non 1-D Structures section.

Field experience suggeststhat the in-loop (central induction) configurationcan more readily be fitted to a layered-earth interpretation than most other configurations over a non 1-D earth. This could be a criticism

of the technique if only the vertical componentof the magneticfield is measured, but it is also apparentfrom results of model studies that this configuration has a superior depth-to-lateral investigation ratio. Koefoed and Biewinga (1976) discuss cases in which data for the horizontal coplanar configuration can be more easily fitted to a layered-groundresponsethan perpendicular coils, and interpreted this as an indication of decreased sensitivity to lateral changesin conductivity. In a number of different geologic environments we have noted that vertical coplanar measurementsare much less likely to be severely distorted by lateral changesthan horizontal coplanar measurements.This is particularly true where the earth contains lenticular shapedinhomogeneitieshaving horizontal dimensions on the order of the source-receiverseparation. Magnetic Susceptibility and Viscosity.--The magnetic susceptibility of earth materials in EM sounding is generally assumedto be so small that their magnetic permeability can be considered the same as for free space.Relative permeabilitiesixr, substantiallygreater than 1, are generally only encountered in prospecting for mineral depositscontaininghigh concentrationsof magnetiteor pyrrhotite, but may occur in flood basalts and other mafic rocks. However, magnetic susceptibility must sometimesbe consideredin high-resolution FEM measurementsand in transient soundings,par-

ticularly when the singleloop methodis used. If •r is frequency-independentand significantly greater than unity, the frequency-domain induction number becomes

to. r. B= •rlxr 2Ix0

(31)


PARALLEL

339

PARALLEL

TO STRIKE


-

I

TO STRIKE

I I IIIII I

i

i i iiiii I

i

i i iiiii

I

i

i i iii

1.4 f ,I.l,,,. I '•''"""1 """ ''"'1 1.3

1.2

5

1.1

0

.....

N

::)

1.0

.9

,

--

:•

.8

_

-15

01 =dl•o • 2

-20

.7 t2•...-03 .6

DiP =

-25

v•4 •

.5

-30

•, • .•,,,,,,,I

.4

100

I I III '"'1

101

. , ,,,,,.,1

102

. .

103

FREQUENCY

I

-35

104

I 1111111

ß I i IllIll

101

I

I I illAll

102

(Hz)

I

I

103

104

FREQUENCY (Hz)

16-1 (b)

16-1 (a) PERPENDICULAR

PERPENDICULAR

TO STRIKE

TO STRIKE

5

1.3

, i ! """""1

, w , """•"1

ß "ß """"Wl

""

! ""•""

0

1.2

-5

-10

1.0

-1.5

,o, :'

-20 -25

-30

.3 11 :,,1 : 100

101

102

FREQUENCY

16-2 (a)

103

-35

-40

V 20 ø -45

104

ß , ,,,,,,1

100

,

101

......

.i

,

FREQUENCY

(Hz)

, , ,,,,,1

102

i

103

I i i ill

104

(Hz)

16-2 (b)

Fig. 16. FEM horizontal coplanar loopresponse forthree-layer dipping modelwith:(Fig.16-1)strikeparallel,and (Fig.16-2)strikenormalto configuration. Normalized parameters aredl/r = 0.2, t:/r = 0.02,cr3/cr: = 0.01.The depthof thelayer,all, is measured fromcenterof the configuration perpendicular to thelayer.


340


The relative permeability •r appearsnot only in B but elsewhere in the equations for a layered earth. Keller and Frischknecht (1966) gave an expression for a horizontal loop-loop configurationraised above a nonconducting magnetic earth. When the loops are on the surface, the mutual coupling is Z

go

PLAN

21Xr

=

(32)

•r q- 1'

VIEW

Jt(Ition

0

5o0

10oom

,SCALE

I

i

i

I

CROSS-SECTION

c

100ll.,,,

1 000•*

o---50 m

m

I_

r-

_J

1 800m

In using instruments that measure both inphase and quadrature components at low induction numbers, the inphase component is frequently dominated by responsedue to variations in permeability so that reliance must be placed on the quadrature component in measuring conductivity. There has been little study of the effects of permeability on FEM measurementsat large induction numbers. Ignetik et al. (1985) studied the effect of permeability in the range 1.0 to 1.2 on TEM measurementsover a conductive half-space, and Singh and Lal (1982) computed the effect of magnetic permeability on wavetilt data. The phenomena of magnetic viscosity, although known for over half a century, has only recently been brought to the attention of most of the geophysical community. Magnetic viscosity, also known as superparamagnetism,refers to a particular type of magnetic instability, and is effectively a frequency- or timedependent susceptibility; in the frequency domain the susceptibility is complex having a quadrature component that is associated with dissipation of energy as well as an inphasecomponent (Mullins and Tite, 1973). Superparamagnetismarises from lowering energy barriers between stable magnetization directions in adjacent domains. The particles are thus magnetically unstable. Viscous magnetization is strongestin singledomain grains of the order of 500 nm in size, but can also be appreciable in larger grains 10 to 15 •m in size if they are elongated (Dunlop and Schutts, 1979). The phenomena has been observed in hematite (Creer, 1959), magnetite (Tropin, 1967), maghemite (Clark, 1980) as well as other minerals. Tropin (1967) showed that doubling grain size of magnetite particles can

change therelaxation timebya factorofupto 1016SO 2000m

500. ITI

that at any one temperature there will be a range of relaxation times corresponding to a range of grain sizes.

17 (a)

Clarke also discussed the geophysical aspects of magnetic viscosity. Grains with time constants in the

iooo

rangeof I to 10-4 s will exhibitthe strongest frequency dependent susceptibilitiesbetween I Hz and 10 kHz. Consider a grain with time constant ,. Upon removal of an applied field its magnetizationwill decay exponentially.

ioo

1000,Q,-m

1 447m

144.•,-m

$30m

I le

TIME

J = J0 exp (- t/x). I

I

lee

leoo

Thus the magnetization in the absence of an applied field obeys the differential equation

(ms)

17 (b)

Fig. 17. (a) 3-D model of Columbia River Basalt structural high. (b) Central-loop TEM data and 1-D interpretation at station 500 (after Newman et al., 1987).

(3 3)

,dJ/dt + J -- O.

(34)

If a sinusoidalfield H = H 0 exp(icot)is applied, we then have

,dJ/dt + J = koH

(35)


where k0 is the dc susceptibility.The steadystate


solution shows J will lag behind H. The complex susceptibilityk is given by

k = J/H = ko(1 - ito'r)/(1+ to2r2).

(36)

Thusthe real (in-phase)componentof susceptibilityis frequencydependentand is given by

kr = ko/(1 + to2'r2).

(37)

The quadraturecomponentis

ki = -kotoX/(1+ to2'r2).

(38)

Richter (1937) was the first to predict a logarithmic time decay of viscousmagnetization,which was later confirmedby workers suchas Nagata (1961)who also noted a logarithmic temperature dependenceand a linear responseto the appliedmagneticfield. Dunlop (1973), in an excellent discussion of theories and varietiesof superparamagnetism, reportsthat although for small fields the effect is proportionalto applied magneticfield, it may be independentand thusnonlinear at field strengthsgreater than 0.1 A/m. Until recently these effects were ignored in most applicationsof EM methods.Archeologists,on the other hand, often observedmagneticviscosity in soils (Colani and Aitken, 1966;Parchasand Tabbagh, 1978). The use of EM methodsin archaeologyis reviewed in the chapter, ProfilingMethods Using Small Sources (Frischknechtet al., this volume). The 1/t time decay reportedby archaeologists has been utilized in the designof metal detectors;for example Corbyn (1980) has designeda sum-of-exponentials eliminatorto cancel the viscousmagneticdecaywhich is a major source of geologicalnoisein explorationfor gold nuggetsin Western Australia. The 1/t decay arises from the fact that TEM receiversrespondto the time rate of change of the logarithmic decay. Magnetic viscosityis often observedin weathered environments, especially areas with lateritic soil cover. Jacksonet al. (1987) observedstrongviscosity

341

exposureto the magnetizingfield. In the frequency domain the effect manifests itself as an amplitude increase and phase shift which changes with frequency;generally,the effectsof viscosityare evident in frequency-domain measurementsonly when low induction numbers systemsare used. The field results in Figure 18 were recorded in the Murray Basin, Australia usingan MPPO-1 single-loop TEM instrument with a variety of loop sizes (Spies, 1978). The responsemeasuredwith small loops or at late times is dominated by the superparamagnetic component.The estimatedsuperparamagnetic component is shown by dashedlines, and upon subtraction from the observed field data and conversion to appar-

104

L=50m

103

25

m

102 RESPONSE

x•O

A

12.5m

\

10 \.

,2',,, \

\

x6.2 -

m

•

effects in fresh basalt on Kilauea volcano, Hawaii

when they attemptedto use the single-loopconfiguration. Effects on TEM geophysicalsurveys have been reported in Spies (1980b) and Buselli (1982). The effectsare most likely to be observedin caseswhere the receiver and transmitter loops are in close proximity and the appliedfield magnetizesmaterial in the vicinity of the transmitterloop. When the appliedfield is switched off, the induced magnetism in the superparamagneticminerals does not collapse instantaneously,but with a logarithmictime relationship.This transient magnetic decay is recorded by the sensor, and may dominatethe responseat late sampletimes. The decay time is approximatelyequal to the time of

\ \

\

10-1 %

\

10-2 o.1

I 1

t (ms)

Fig. 18. Single-loopTEM decay curves obtained in the Murray Basin, Australia, with several loop sizes. The dashed lines are the estimated superparamagneticcomponents (after Spies, 1978).


342


ent resistivity, yielded the corrected values shown in Figure 19. Field experience suggeststhat the magnitudeof the superparamagneticresponsefor a coincidentloop system can be expressed by V

L

• - k -t volts/amp

(39)

where L is the length of a loop side, t is the sample time, and k is a constant dependent on the content of superparamagneticmaterial in the soil. Typical values

of k are 4 x 10-•ø at Elura,New SouthWales,4 x 10-• in the MurrayBasin,and2 x 10-• in southeastern

Nevada.

Magnetic viscosity is much more pronouncedwith smaller loop sizes since the late time asymptotic half-spaceresponseis proportionalto loop size to the fourth power for a single loop system, whereas the magnetic viscosity is directly proportional to the loop perimeter. Buselli (1982) demonstratedthat separating the transmitter and receiver loops by 3 m was generally sufficientto reduce the viscous magnetic decay to negligible amounts for a 100 m loop in the Elura area, Australia, and advocated use of this "displaced loop" configuration. Lee (1984a, b) derived analytic expressions for the TEM response of a superparamagnetic thin sheet overlying a conductive ground, and also for a superparamagnetichalf-space. He showsthat there is a complex decay at early times which reduces to a 1/t behavior

at late times.

i

corrected

..............

,m•n •"'•''"•'".'•'.'.'•..?.:.'n'.

ß•'•::• E

A number of techniques can be used to detect the presence of superparamagneticmaterial in the field and should perhaps be used in a new area before routine soundingsare made, particularly if the single loop configurationis employed. For TEM systems, a multiturn small transmitter loop (less than 1 m) can be employed with a coincident receiver loop. Superparamagnetic effects are readily recognized as a 1/t decay since inductive effects will be very small by comparison. A specially built 100-turn, 5 cm diameter coincident transmitter and receiver loop can easily be built especially for this purpose. The same configuration can be used for frequency-domain systemsby observing changesin the quadrature componentas the loops are raised and lowered over the ground. This shouldbe repeated at a variety of sites in the field area.

Complex Conductivity.•Complex conductivity is frequency- or time-dependent conductivity. The concept is familiar in grounded electrode techniques employed in mineral exploration as induced polarization (IP) where it is also referred to as complex resistivity. The main cause of IP effects is the frequency-dependent barrier

to current

flow which forms at the inter-

face between ionic-conductinggroundwater and semiconducting ore minerals of mineralized rocks. Frequency-dependent conductivity may also be presentin sedimentaryrocks and in clay-bearing sands where the clay minerals bear a surface charge and impede the progress of ions through pore passages. Both of these processesinvolve diffusion of ions. IP is treated more fully in texts such as Keller and Frischknecht (1966) and Sumner (1976). In electromagnetic prospecting and sounding, conductivity is normally assumedfrequency-independent and IP effects are ignored. In some cases, however, this assumption may be inappropriate and it may be necessaryto revert to more complicated models. Consider the complete expression for the propagation constant or wave number,

k = [Ixe00 2 _ i•rlx00] 1/2

m

(40)

where •r is the conductivity and • = 808r is the dielectricpermittivity of the earth. •0 is the flee-space

dielectric permittivity, andequals 8.854x 10-•2 F/re. 8r is the relative dielectric permittivity, also known as the (complex) dielectric constant and often denoted as

6.2 m loop 0.2 0.5

I

I

I I

I

1

2

I

I

I

I

3

4

5

6

I

[

8

K m. Since, in general, K m is not "constant" but is a t •

10

t, ms

Fig. 19. TEM apparent resistivity curves from data shown in Figure 18. Corrected curves, obtained by subtractingthe superparamagnetic response, all lie within the shaded region.

function of frequency, the term relative dielectric permittivity is preferred (see also Ward and Hohmann (Vol. I) and Keller (Vol. I). The two parts of the wave number k given by equation (40) represent currents flowing i.n-phaseor quadrature with the electric field intensity. If •r >> e00, the currents are in-phase with the electric field inten-



sity and are traditionally referred to as conduction currents. If eto>>•r, the currents are in quadrature with the electric field intensity and are traditionally referred to as displacement currents. However, as is known, earth materials do not exhibit the classicalphysical behavior of ideal materials, and electrical properties of rocks must in general be considered complex functions of frequency (Olhoeft, 1985; Keller, 1988, Volume 1). These observations led to two approaches for modeling data. In the first approach, both terms of equation (40) are included, and experimental measurementsare modeledin terms of "effective" values of conductivity and dielectric permittivity:

O'eft(to ) = O" (to) 4- to E" (to)

(41)

and

(o,)

Eeff(to )=

4- e' (co),

(42)

where the singleand double primes representreal and imaginary quantities, respectively. Note that •r', •r", e', e" cannot be determined independently; it is only possibleto obtain the lumpedparameters•regand eeg. The effective complex dielectric constant, Keff(to)is given by

Keff(tO)=

Eeff(to)

=

--0'" (tO) + K m(tO),

(43)

where K m(tO)is the actual complexdielectricconstant. The in-phase and quadrature terms in equation (40) no longer represent purely conductionand displacement currents. In addition, •r" (to) need not be very large at low frequenciesin order to make the measuredKe•(to) drasticallylarger than the true Kin(tO). Examples of this approach are given in Fuller and Ward (1970) for both wet and dry rocks over the frequency range of 1 Hz to 1 MHz. For wet rocks over this frequency range, O'ef t decreasesby less than one order of magnitude.Keff(tO)is inverselyproportionalto frequency at low frequencies, and may reach values as

highas108below1Hz. Thesevaluesaremanyorders of magnitude greater than the maximum value predicted from classical molecular models, and were the

subjectof much early debate (Frische and von Buttlar, 1957; Wait, 1958c). Because there is ambiguity in dividing the real and imaginaryparts of any electricalmeasurementof rocks between real and imaginary conductivity and real and imaginarypermittivity, it can be arguedthat a distinction

should

be drawn

between

conduction

due to

movement of "free" charges which move distances larger than the molecular lattice, termed conductivity behavior, and cases where the chargesare bound and

343

moveverysmalldistances (or theorderof 10-•ø m), termed dielectric behavior. In this way the dielectric constantis restrictedto valuesapproximatelybetween 0.1 and 80 (Pelton et al., 1983).

The second approach, most often used in modeling EM and IP data, ignoresthe dielectric term in equation (40) and models the observed data by a frequencydependentconductivity or resistivity model. The form most commonly used is the Cole-Cole relaxation model (Cole and Cole, 1941; Pelton et al., 1978:

c' p(tO) =R0 {1-rn1- 1+ (itO'r)

(44)

where p(tO)= 1/•r(tO)is the complex resistivity, R0 is the dc resistivity value, rn is the chargeability, ,r is the time constant, and c is the frequency dependence. Typical ranges of these parameters for clay-bearing sands are rn = 0 to 0.1, depending on clay content;

,r= 10-s to 10,depending onparticlesize;andc - 0.1 to 0.5, depending on particle size distribution. If diffusionwere the main process, and all the particles were the same size, the frequency dependencewould be 0.5. However, a distribution of particle sizes tends to smear the relaxation over a broader frequency range, and the frequency dependence is decreased. The same model may be used for rocks containing semiconductingore minerals. Typical ranges for the parameters are then: rn = 0.01 to 0.09, dependingon

mineralcontent, ,r = 10-4 to 104 depending ongrain size, and c = 0.2 to 0.6, depending on grain size distribution.

The value of c can be increased

from the

theoretical 0.5 if double-layer capacitance plays an important role in conduction across the mineral-ionic interface. For capacitancephenomena, the frequency dependence is 1.0. Strictly speaking, this approach does not require that dielectric effects be ignored. If frequenciesand resistivities are high and displacement currents are likely to be significant it is possible to describe the ratio of total current density to electric field by

-•-=[' p(tO) + itOe(tO) JT where p(to) and e(to) are both complex functions of frequency. Field Observations.--For over 20 years, attempts have been made to measure IP effects inductively. In most casesEM data could be interpreted using simple models with real, frequency-independent conductivity. Early theoretical studies reported in Bhattacharyya (1964), Dias (1968, 1972), and Morrison et al. (1969) showed that EM responses are modified as polarization parameters are varied. Hohmann et al.

344



(1970) carried out further theoretical studies and field

(Figure 20). This work was extended in Wait and Debroux (1984). Theoretical studies of IP effects on

tests and concludedthat it was unlikely that IP could be measuredinductivelyin mostterrains,but may be

groundeddipole and vertical magneticdipole TEM

possible if the induction number was between O.1 and 10 and the loss tangent, •'/toe' was between 0.2 and 5

Hedison(1981)claimsuccess in qualitativeinterpreta-

systemsare reported by Rathor (1978a, b). Dias and

IO

0.1

I.O

IO

B

b)

-2.2

1.4

-21:)

-I.6

-I.8 TAN &ß

1.8

-I.6

-ZO

-I.4

Tan8 ß

-2.2

-I.Z

-2:.4

q-

:.6

-08

-2.8 -30

-o4

_•

0

.3&

02

•

-3.8

0.4 06 02

-4.0 1.0

•

•

B=(•oO)1/2 r 2

Fig. 20. Loop-loop FEM sounding curvesof a homogeneous earthwithvaryingdegrees ofpolarizability. Theloss angle,g, is theratioof realto imaginary conductivity. A losstangent,tan g, of o•represents a non-polarizable mediumanda valueof 5 corresponds to 40 PFE/decade (afterHohmannet al., 1970).


Australia (Figure 21a). Since sign reversals cannot theoretically occur with this configurationin a frequency-independentlinear medium (Gubatyenkoand Tikshayev, 1979; Weidelt, 1982; Guptasarma, 1984)

tion of inductive IP field data, but in most cases


345

attemptsto measureIP inductivelyhavefailed and EM data are generally interpreted using a noncomplex conductivity model. With some loop configurations,however, it is not possibleto interpret field data unless complex conductivity is incorporated.Spies(1980a)observedconsistent sign-reversalswith the single-loop MPPO-1 and coincident-loop SIROTEM TEM systems over a pyrrhotitic graphitic shale horizon in Queensland,

the anomalous

field

behavior

was attributed

to IP

effects. Care was taken to eliminate any source of spuriousinstrumentresponse,which can also causea similar behavior (e.g., Qian, 1985). Subsequentto the field experiment Lee (1981) and Raiche (1983b) modeled the TEM responseto a homogeneousearth with a

IOO

5

50÷

TIME taJ

(ms)

3

z o 13_

4

5

??N/4?2 E,Dae•e(• xcurve 50 mu.sing •oop C$1RO

equipment ond -50-•

21 (a) 10-1

10'2 10-3

ORE DEPOSIT RO r(ms) rn

'c. CU•!V_E LORNEX

126

COPPER CITIES 155

C

10-4

KIDD CREEK

16

c

0.1 0.46 0.16 6.9 0.42

0.28

31

0.31

0.81

C•

10-5 N

10-6

'10'7 A

10'8 I •A•B ß

10-1

ß

ß

i

i

ill|

1

10

DELAY TIME (ms)

21 (b)

lOO

Fig. 21. Comparisonof observedand modeledcoincidentloop TEM responses:(a) Field results measured over a pyrrhotitic graphiticshale near Cloncurry, Australia (from Spies, 1980). (b) Theoretical coincident-loopTEM response for a homogeneoushalf-space with Cole-Cole dispersion modelstypical of ore deposits(after Raiche, 1983b).

346


Cole-Cole dispersionbehavior and showedthat tran-

Smith and West (1988b) show that the effect of a

sients such as those observed in the field can result.

polarizablesurfacelayer at late times is to decrease the TEM responseinside a loop, and increaseit outside the 1oop•a phenomenareported in field re-

However, quite high and possiblyunrealisticpolarizDownloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

abilities were used in these models. Typical transient

decaysresultingfrom complexconductivitymodels are shown in Figure 2lb. Molchanov et al. (1984) observedcoincident-loopsignreversalsover kimberlite pipes in the Soviet Union, and modeledthe IP

response by a late-time negative t -2 decay.Walker and Kawasaki (1988) report sign reversal with the central-loop TEM configurationnear Kuparuk, in Northern Alaska, which they attributed to IP effects. They modeledthe responseof a 1000-mthick, 1000 fl-m polarizabletop layer overlyinga 50 fl-m basement, varying the frequency-dependence parameterc from 0 to 0.98 (Figure 22). The largestsignreversals are obtained for large values of c. Distinguishing betweenIP effectsand inductivere-

sponseswith mostEM field measurements is usually very difficult. Why sign reversalsare not observed with the single-loopTEM methodover many mineral depositswhich have a very high IP responsewhen measuredwith galvanicIP methodsis not clear, althougha possibleinductivelycoupledmechanismhas beenproposed(Smithand West, 1988a).GenerallyIP effectsare more readily observedwith configurations in which

both

source

and receivers

sults in Asten and Price (1985) and termed the "loop

effect". An exampleis shownin Figure 23. This effect maynotbe recognized with an in-loopsoundingunless the responseactually changessign at late times. For this reason, it is advisableto measureboth inside and outsideof the loop, and comparethe results (which should be similar at late times). Procedures for recog-

nizing, interpreting,and correctingfor IP effectsare currently the subjectof active research.

Displacement CurrentsandPropagation Effects.--As stated in Volume 1, and elsewhere in this chapter,

..6

consist of

groundedelectrodes(e.g., Johnston,1975), and are generallynot observedin purely inductiveloop-loop methods.Complicationsarise when mixed galvanicelectrodeconfigurations are employed,for examplea grounded-wiresourceand magneticsensor.IP effects are usually not consideredwhen carrying out an EM sounding, although a grounded wire-magnetometer configuration is usedin the magneticinducedpolarization (MIP) method (Siegel, 1974; Steemson,1982). IP effects have been reported with the UTEM method using a grounded wire field sensor in Lamontagne (1975), and are further discussedin Macnae and West (1982).By reciprocityconsiderations, the responses of loop-electrode and electrode-loop configurations should be identical when source and receiver functions

are interchanged,if the earth behaveslinearly. Recently, interestin the effectof IP on EM response has acceleratedwith the discovery that signreversals with the coincident-loopand in-loop TEM configurations can be attributed to the presence of a thin

polarizablesurfacelayer with quite modestpolarizability (Flis et al., 1989;Nabighianand Macnae, this volume). Those layers may not always be detectedin a conventionaldipole-dipoleelectrical IP survey becausetheir thicknessmay be muchlessthan the dipole length and their polarizationmay be strongerin the TEM frequency range.

½½

Fig. 22. Family of modeled central-inductionTEM responseswith varyingfirst layer Cole-Colefrequencyparam-

eterc. Thecurvesarefora two-layer earthwitha l•olarizable top layer. Here rn = 0.5, h• = 1000m, tr• = 10-' S/m, -r = 6.9 x 10-4 $, andtheconductivity of thebasement material

istr2 -- 2 x 10-2 S/m.Thehump-like featureto therightof the figurerepresentsnegativevoltage.The right-mostcurve is the responseof a homogeneous half-spacewith a simple

conductivity of 2 x 10-2 S/m(afterWalkerandKawasaki, 1988).



displacementcurrents can generally be neglected in inductionprofiling and sounding.However, when frequenciesabove the audio range or larger spacingsare employed, or the resistivity is very high (as in permafrost or granite), the validity of this assumptionshould be questioned. To gain insight into the use of high frequencies, the response of a horizontal coplanar loop-loop configurationon the surface of a homogeneousearth will be considered.The complete expression for the mutual couplingratio is Z

2

Zo- (k2_k•)r 2[(9+9ikr-4k2r 2-ik3r 3)e-ikr - (9 + 9ikor- 4k•r2- ik•r3)e-ikør]

(45)

(Wait, 1954), where Z0 is the free-spaceexpression given in Table 1.

k0 = (e0•x0)•/2to, and the other terms are as definedunder the Complex Conductivity subsection.In this section it is assumed that both • and er are independentof frequency, and are thus real quantities. Typical values of er are 4 for dry quartzite, 20 for rocks with a high water content, and 81 for fresh water.

In equation (45),e-ikoristhefree-space propagation

term, and the e-'tkr term includesboth conduction currentsand displacementcurrents, dependingon the nature of the individualterms in k (equation40). The free-space propagation term can generally be neglectedunlesslarge spacingsand high frequenciesare employed. The exact nature of the propagationterm easily can be understoodby consideringits behavior in the time

347

domain. Weir (1985) derived exact time-domain expressions for a VMD located on a homogeneous, nonpermeablehalf-space,includingdisplacementcurrents and propagationterms (the appropriate quasistatic solution is obtained by setting the relative dielectric permittivity to unity). The expressionsreveal two forerunners, the first traveling at the speedof light

infreespace (1/X/tx0e0) andthesecond atthespeed of lightin theconductor (1/X/•0}).A relaxation effect, dependenton the relative dielectric permittivity of the conductor, allows the first forerunner to travel along the surfaceeffectively undamped.The first forerunner is initially of very high amplitude (typically many orders of magnitude larger than the quasi-static asymptote),but decaysextremely rapidly with a time constante0(er - 1)/, to the initial value of the associated quasi-staticsolution. The initial high amplitude of the first forerunnerdecreasestypically as an inverse square function of distance. The secondforerunner is insignificant in comparison. Kaufman and Keller (1983) show that in a homogeneouswhole-space,the electromagneticsignal propagateswith a velocity of

1/•,

i.e., thesameasthesecond forerunner ob-

tained in Weir's half-space analysis, but taking magnetic permeability into account. These results suggestthat for constantconductivities and dielectricpermittivities, the initial quasi-static field values are achieved essentially instantaneously after the arrival of the first forerunner and persistuntil the beginningof the diffusionaltime scale. However as pointedout by Weir, it is conceivablethat allowingthe conductivityor dielectricpermittivity to be frequencydependentcould damp out the extremely large initial field values.

Propagation effects restrict the operation of TEM instruments operating in the microsecond range at large source-receiverseparations,since no signalcan

beobserved at thereceiver beforea timeof X/tx0e 0r (for example, 3.3 •xsat r = 1000 m). In addition, the finite propagation time of a current pulse traveling around a large transmitter loop may need to be taken into account.

It is convenient to define a complex induction number

B = kr = ([xeto 2 - i•rixto)l/2r Fig. 23. A vertical-componentTEM profile across a fixed transmitter loop on a thin polarizable surface layer with conductivity-thicknessproduct 6 Siemens and Cole-Cole

parameters c = 0.1, rn = 0.07 and'r = 4 x 10-3 s. The dashedline representsa non-polarizablelayer. The characteristicprofile shapeis sometimestermed the "loop effect" (after Smith and West, 1988).

(46)

for caseswhere quadrature or displacementcurrents are included, but propagation effects are neglected. This complex induction number obeys the electromagnetic scaling laws (Sinclair, 1948; Ward, 1967, p. 311) and reduces to the usual induction number

B = (,•x0to/2) 1/2r whendisplacement or quadrature currents are ignored.

348



The followingexamplesillustratethe importanceof includingall termsin equation(45) in sounding highresistivity rocks and in using high frequencies.As mentionedabove,both(r and e are assumedto be real, frequency-independent quantities.The magnitude and phaseresponse of a 10,000fl.m half-space areplotted in Figure24for variouscasesin whichthepropagation terms or dielectricterms are either includedor ignored. Real and imaginaryplots are shown,as well as customaryamplitudeand phase.Both frequencyand the realpart of the inductionnumberareplottedon the horizontalaxes,althoughstrictlyspeakingan "induction number"is meaningless whenpropagationterms are included.Curve 1 is the responsecalculatedfrom equation(45) ignoringthe free spacepropagationterm and settingœr= 0 to effectivelyeliminatethe dielectric term, althoughœrcan neverbe lessthan 1 in practice. In curve 2 œris, more correctly, set equal to 1 so that the dielectricpermittivity of free spaceis included. There is very little difference between the curves. Curves 3 and 4 are obtainedwhen the propagation termsare included(againeither settingœr= 0 or l).

FREQUENCY .01

10

20

1'81'1i, 51't

The propagationterms can only be ignoredin this exampleat frequenciesbelow about 60 kHz (B less than5), andare quitesignificant at higherfrequencies. When a moderatedielectricpermittivityis included (œr= 10), curves5 and 6 are obtained(propagation termsare includedin curve 5 and ignoredin curve6). The dielectrictermbecomessignificant at frequencies greaterthan about 7 kHz (B greater than 1.5). Note

thatat 7 kHz, eo•= 4 x 10-6, whereas (r = 10-4 , and so displacementcurrentscannotbe ignoredfor eo••> cr/25.For er = 10 the termseo•and (r are equalat a frequencyof 180kHz; displacement currentsarevery significant,but not more so than propagationeffects. The effectof changingthe relativedielectricpermittivity er in the samemodel when propagationterms areincludedis illustratedin Figure25 (notethechange in vertical scale). For œr> 5 the responseexhibits oscillationscharacteristicof both displacementcurrentsand propagation effects.For highvaluesof œr (=20, 40) the effect of the propagationterm is to amplify the oscillations.

Even when the resistivity and spacingare small,

(kHz)

50

FREQUENCY

100

150

200

250

.01

I

5

I

I

I

10

20

(kHz)

50

100

150

200

250

_

1.6

•o = 10 000

_

1.4

"•

•--•,..

N

_.N 1.0

%-•.•..""• ,,,.

.6

•

.4

•.Ourve r _,arm •.

i '

A 100

t......

:'1, '-.

,,

N

',,

i!

Propagation

Propagat,on "'-%,,•. ''-',. ,,,.

.8

_•

15o

"',.<-Sr =10

1.2

•

2oo

t3em

r =1000 m lude, d•//

--•{:...,

ß .r -50

: ',

:

-. Quasistat!c', I

-100 _

.2 .

.0

''

-.2 0

I

2

3

4

5

6

-200

Quas•stabc 7

8

9

-250

10

f

•

I

0

•

I

1

•

i

2

•

I

3

I 5

6

7

8

9

10

REAL (B)

24 (a)

24 (b) FREQUENCY

.01

•

4

REAL (B)

I

5

10

1.5 I ,I

20

(kHz)

50

, Ii

,

i

I,

FREQUENCY

100

•

,

i i,

150

I

,I

200

[

,

iI

250

.8'011 • , 5• 10 20

,

(kHz)

50

1010 1510 2100250

1.0 ,4

.2 •

Quasistatic -I '/

- .4[--

'•

,......-,.,.

,

.1

2..___,__/___ , ,'

,',••,.

/ Qua sista tic•

.6

.8

-1.0 -1.5

-1.2 0

1

2

3

4

5

REAL

24 (c)

6

7

8

9

10

0

1

2

(B)

3

4

5

6

7

8

9

10

REAL (B)

24 (d)

Fig. 24. Theeffectofdisplacement currents anddielectric termsontheHCPloop-loop response ofa homogeneous

half-spaceof 10000•.m resistivity.The source-receiver separation is 1000m.



displacementcurrents can be important at sufficiently high frequencies, as illustrated for a 300 •.m halfspacein Figure 26. Displacementcurrents can only be ignored in this example at frequencies less than about 30 kHz (B less than 2). At small spacingssuch as this, propagation effects are insignificant and can be ignored.

Other examples of the effect of displacement currents are given in Ward and Dey (1971), who calculated the responseof a horizontal magnetic dipole over a two-layer half-space and in Sinha (1977a, b) who carried out similar calculationsfor five loop-loop systems.

The properties assumed in calculating the results given in Figures 24 to 26 are not unrealistic. In some areas crystalline rocks often have resistivitiesas great as 10,000 •.m or more (Strangway and Redman, 1980) and a relative dielectric permittivity of between 5 and 7. The resistivity and relative dielectric permittivity of frozen materials is often even greater (Olhoeft, 1977).

FREQUENCY .01

I

5

10

20

349

The dielectric

constant

of water-saturated

Anisotropy.•Anisotropic materials are those in which conductivity varies with direction. In sedimentary rocks the conductivity parallel to bedding (1ongi-

FREQUENCY

(kHz)

50

100

150

200

unconsoli-

dated material can be relatively high, leadingto results such as those shown in Figure 26. Values of p = 300 •.m and er = 80 are typical of fresh water lakes. As mentioned earlier, the resistivity and dielectric constants of earth materials are often frequency-dependent, which adds a further complication. The results given here show that the quasi-staticassumptionwill be violated if complete soundingsfrom B = 0.1 to 10 are made over some classes of rocks. Propagation effectscould be included in interpretation if necessary. In principle, the relative dielectric permittivity could be included and determined in the inversion process. However, this problem has not been adequately studied, even for the case when the parameters are frequency-independent.

20

250

(kHz)

50

lOO

15o

200

250 I

250

4.5•_• ,• • •, ! , • , • •, • , • I,

_

4.0[--

200

P= 10000{')em -i

[-

r =1000 rn

150

'1

100

50

2 o 1.5 ' •_

'•,i-

o

iI •

••'__-_•'•_ _"'--------" '•

1.o

-50

- •;•-.20

-lOO

__.

-15o

cr =4o',,,• 26'

'

_

_';

-250

' 0

I

2

3

4

5

6

7

8

9

• 0

10

I

•

I

I 2

•

I 3

•

I

•

4

I 5

REAL

REAL (B)

25 (a)

•

[ 6

•

I 7

•

........ 1 • 8

I

•

9

(B)

25 (b) Fig. 25. Effect of varying dielectric constantin the same model as Figure 24. Propagationeffects are included. FREQUENCY

0.1• 151020

50

100

(kHz)

FREQUENCY

2100

500

1.8 • , II ,I I ,I I ,I I ' I ' I ' I ' Il ' I ' 1.6

•

1.4

•..

I 1020 200.1 I I 5

5,0

1010

(kHz)

200

500

.

0

P=300 •.rn

_ .--

750 I

U3

-20

n-

-40

r = 100 rn

1.2

N

,•N 1.0

•

•

N

-60 o

.8

-80

N

•

.6

•

,4

-

'•%;x,

.2

""

-100

<1:

-120 -140

.

.0

-160

-.2

• 0

I I

•

I 2

,

I 3

•

I 4

•

I

•

5

I 6

•

I 7

•

I 6

•

I

•

-180 0

9

REAL (B)

26 (a)

I

2

3

4

5

6

7

REAL (B)

26 (b)

Fig. 26. Effect of displacementcurrentsat relatively small spacing(r = 100m) and low resistivity (p = 300 f•.m). Propagation effects are insignificantat this spacing.

750 I

350


tudinal conductivity tre) is typically greater than the conductivity perpendicular to bedding (transverse conductivitytrt); the coefficientof anisotropyis de-


fined as

•k• •/0'œ/0' t.

(47)

On a petrographic scale, "microanisotropy" can be caused by preferential orientation of tabular minerals and mineralogic fabric and texture. Shales, for example, have coefficients of anisotropy between 1 and 5, and values of up to 10 or more have been found for graphitic slates.On a larger scale, "macroanisotropy" results from interbedding of lamina or thicker beds of sedimentary strata having different electrical resistivities, perhaps due to simple changes in porosity or permeability. A common example would be interbedded sandstones and shales which overall may have values of h up to about 6. In EM soundingapplications, anisotropy is generally present to some degree becauseof our inability to resolve layering with certain threshold parameters of thickness and conductivity contrast.

In the quasistatic range, loop-loop and wire-loop systems measure only longitudinal conductivity (Weaver, 1970). However, the response of wire-wire EM systems and dc resistivity systemsdo depend on the coefficient of anisotropy (Kaufman and Keller, 1983, Koefoed, 1979). Unless anisotropy is taken into account, the two classes of systems yield different results for the same earth. Loop-loop systems and wire-loop systems can provide correct answers for longitudinal conductivity and layer thicknesses.Wirewire systemsdo neither when the earth is anisotropic, unless the coefficient of anisotropy is determined, but they do provide information on transverse conductivity. In principle, a combination of measurementswith loop-loop or wire-loop and wire-wire configurations can be used to obtain both longitudinal and transverse conductivities as well as layer thicknesses (see for example, Strack et al., 1989b). It is, of course, possible for conductivity to vary with azmiuth. An example which may be encountered in making soundingsis a layer of younger sediments lying unconformably on a basement of folded metamorphic rocks, or vertical fissuresin karst terrains (Le Masne and Vasseur, 1981). From EM profiling we know that the response of loop-loop systemsis very sensitive

to the direction

of strike of rocks which are

azimuthally anisotropic (see Frischknecht et al., this volume). Most publishedstudiesof the effect of anisotropyon EM soundings have been for the MT method (Chetaev, 1960; Praus, 1966; O'Brien and Morrison, 1967; Sinha, 1969a; Negi and Saraf, 1972; Abramovici,

1977). Conclusionsfrom these studiescan generally be applied to far-field EM soundings.One problem with anisotropyis that it may not be possibleto differentiate between field results from a 1-D model with a dipping orientation of anisotropy and a 2-D isotropic model. The effect of anisotropy on the EM response of electric dipoles has been studied in Sinha and Bhattacharyya (1967), Chlamtac and Abramovici (1981), Wait (1982) and Kaufman and Keller (1983) for horizontal anisotropy. For near-zone and intermediatezone soundingthe magneticfield is only dependenton tre, but in the far zone, when displacementcurrents are important, crt also becomesimportant. The effect of anisotropy on the high frequency fields of magnetic dipoles have been reported in Sinha (1968, 1969b) and Wait (1966a, b). The relationship between petrofabric and dielectric anisotropy is described in Nowina and Strangway (1981). Cultural Effects.•Anomalous responses are often obtained from currents induced in metallic

conductors

such as fences, power lines, and buried pipes contained within the survey area. Due to the small crosssection,vortex currentswithin a pipeline are negligible but galvanic currents (current gathering) can be large. Similarly, large currents are often carried by wire fences, especially if steel posts are used. On the other hand, effects from wooden-post barbed-wire fences are generally small. Many power distribution systems include an extra wire, grounded at each pole, to protect the current carrying wires from lightning discharges; this ground wire generally carries large galvanic currents. Similarly, the metallic sheath on telephone cablescan causeproblems. At high frequencies, insulated wires which are buried or lying on the surface may carry currents even when they are not grounded; the capacitance between the wire and the earth is large enough to allow substantial current to flow. Often, the response of a wire or pipe is more dependent on the resistance and capacitance to earth and the resistivity of the earth than on the resistanceof the conductor. Since cultural conductors are generally very long, the field about them falls off as 1/r with distance, except as modified by the earth. Electromagnetic Noise

Electromagnetic noise in the context of controlledsource EM soundingcomprise all unwanted electrical or magnetic signals detected by the sensor. These include geomagneticsignals,power-line radiation, and wind noise. In frequency domain systems a very narrow band-passfilter is normally employed so that noise at other frequenciesis rejected. For TEM measurementsthe receiver system is generally broadband.



By averaging or stacking, the instrument frequencyresponseis reduced to a multitude of narrow spectral lines centered on the odd harmonics of the system's base frequency. Synchronous averaging in the time domain thus has a similar effect in reducing coherent noise as narrow-band filtering in the frequency domain.

GeomagneticNoise.--Geomagneticfields, which are classed as signal in field techniques such as MT, are one of the major sources of noise in EM sounding. Descriptions of geomagnetic fields are well documented

in texts

such

as Keller

and

Frischknecht

(1966), Patra and Mallick (1980), Kaufman and Keller (1981), Vozoff, this volume, and others. Specific references of interest include Bleil (1964), Matsushita and Campbell (1967), Llanwyn-Jones (1970), Strangway et al. (1973), Ginsberg (1974), Evans and Griffiths (1974), Keller (1971), Koziar (1976), CCIR (1978), Gamble et al. (1979b), Holland (1982), Macnae et al. (1984) and Labson et al. (1985). A typical spectral plot for geomagnetic variations averaged over a long period of time is shown in Figure 27. Below about one hertz the signal arises mainly from within and outside the ionosphereas a result of complex interaction between plasma emitted from the sun and the earth's permanent magnetic field. Below about 0.1 Hz, the amplitude of these signals, known collectively as micropulsations, is approximately inversely proportional to frequency. These signals are stronger during the morning and are also strong in equatorial regions (Lam, 1989). Micropulsations are the main contribution at MT frequencies but are relatively unimportant in EM soundingexcept at very low frequencies. Over the range of about 0.1 to 1.0 Hz the steepfalloff in the amplitude spectrumis causedby attenuation of the EM fields passing through the

10-

MICROPULSATIONS

-

• Pc2, 3 Pc I '1 I

I

X

10-1[ I

t

POWERLINE '

&RESONANCES .• •/11

I 10-1

I I

10-1

I

li,.

•\ // I 10-2

-

NOISE

\ SCHUMANN • /,, HARMONICS

10-4

10-5 10-3

'•'

ASSYNCHRONOUS

'•

I

ionospherewhich is conductive over a broad range of frequencies. The main source of geomagnetic noise in the frequency range above 1 Hz is atmospheric lightning discharges around the earth, referred to as "spherics". Over the lower part of this frequency range, the dominant mode of EM field propagation within the earth ionospherecavity is transverse magnetic (TM). The earth-ionospherecavity acts as a sphericalwaveguide which resonates at a number of frequencies (investigated in Schumann, 1957) referred to as Schumann resonances, at 8, 14, 20, 26, and 32 Hz. The waveguide has a strong absorption between 500 Hz and 2.5 kHz, resulting in a low spectral density of geomagneticnoisein this region. At higher frequencies the wave propagatesdominantly with tangential electric fields (TE mode) in the earth-ionosphere waveguide. The major thunderstorm centers are located in Brazil, central Africa, and Malaysia which average over 100 stormsper day per year. During the day the center of storm activity shifts westward with the sun, resulting in a fairly consistent level of spherics activity. Near storm centers the peak of activity occurs in the early afternoon, local time. There is a pronounced seasonal variation, with magnitudes one order lower duringwinter months, as shown in Figure 28. Spherics may be extremely intense during local summer storms. The level of sphericactivity also decreasesaway from the equator since the major sourcesare in the tropical regions. Jewell and Ward (1963) estimate that lightning dischargesoccur with a frequency of about 100 per second.The electromagneticnoise at a particular point is quasi-continuousbelow 300 Hz but predominantly impulsive in nature at higher frequencies. The impulsive nature of the noise is illustrated

in the time series

record shown in Figure 29.

10

f /.-,

POWER GRID

\

351

16z

ida 20 Hz

• ,z,,

4OHz

25OHz

10-3

500 Hz

10-4

I 10

I 102

• 103

104

10-5 105

FREQUENCY (Hz)

Fig. 27. Generalized geomagneticspectrum for horizontal magnetic field (H) and induced voltage (V) measurements.

AUG ISEp IOCT INOVI DEC IJAN I FEB IMAR IAPR IMAY IdUN I JUL 1979

1980

Fig. 28. Annual geomagneticspectralvariation in California (from Labson et al., 1985).


352


Over a plane-layered earth the magnetic field from distant sourceswill be nearly horizontal. In an average geological environment where EM soundings are made, the vertical magnetic field will usually be 6 to 10 times smaller than the horizontal field. An approximate value of the induced electric (telluric) field can be calculated from the magnetic field by the magnetotel-

roads in Europe are often a source of large-amplitude 16 2/3 Hz noise.

Communication signals are a major source of noise at high frequencies. Navy VLF communication transmissionsproduce strong signalsin the range of 15to 24 kHz (McNeill and Labson, this volume), which are roughly twice as large at night as in the day (Thiel, 1979). Omega transmissionsinvolve a sequentialtransmissionof three frequenciesin the range 10 to 13 kHz. Radio and radar stations broadcast at much higher frequencies, but may often overload induction EM systemsthat do not incorporate sufficientfiltering and shieldingand preclude the possibilityof making soundings nearby.

luricrelationship' E ---(tolx0p) 1/2H, wherep is the average resistivity of the earth to one skin depth. Electromagnetic noise is also discussedelsewhere in this volume by Vozoff, Zonge and Hughes, and by Nabighian and Macnae. Man-made

Noise Sources.--Man-made

noise arises

mainly from the power distribution grid and is ubiquitous in nature. Depending on the country the main frequency is the fundamental at either 50 or 60 Hz although the second, third, fifth, and sometimesother harmonics are often as much of a problem in some areas. Power line noises produce reasonably steady spectral lines which are usually within 0-1 Hz of their nominal value in developed countries but which may vary by ___5Hz in less developed countries or rural areas. They may also produce more damaging but weaker nonstationary components. Motor loads produce switching transients, sidebands,and subharmonics such as 30 Hz. Electronic cycle-by-cycle switching of rectifiers in many power systemproduce broadband transientsand high-frequencyharmonics. Electric rail-

-• .z•ao -

Motion-inducedNoise.--Spurious signalscan be induced in magneticsensorsby movement of the sensor in the earth's magnetic field. The earth's field is

approximately 106 timeslargerthan typicalfields measured in EM sounding, so vibrations induced by wind can often cause appreciable noise voltages known as "wind noise" or microphonics. Other mechanical noise can also be a problem. One example, cited in Buselli (1982) who termed it the "Ward effect", refers to the movement induced in a sus-

pended source loop causedby switching the transmitter current on and off. This is synchronousnoise which is not reduced by stacking, and can result in spurious signalsup to 1 IxV/A in a 100rn loop with a coincident-

. .......

J•Ji, i

c3-oooo •

'lnP*

'

•

_

<-m

I •

•o

I

I

I

I

I

I

I

I

! •.•

I

i

i

!

I

I

I

I

I

I

I

I

!

I

!

I

z.•

I

I

!

I •

I

I

!

!

i

I

I

I

I

I

!

I

I

I

I

!

I

I

4•

TIME {•}

(b)

Fig. 29. Typical wide-band natural electromagneticwaveforms: (a) median spectrumlevel, and (b) high-spectrum level, from Evans and Griffiths (1974).

I •

Electromagnetic Sounding loop TEM system. Nichols et al. (1985) have shown that ground motion makes a small, but significant,


noise contribution when making extremely accurate

magnetic field measurementswith remote reference cancellation, and that this can be corrected by the use of accurate tilt-meter

information.

Noise Cancellation.--A number of techniques, besides filtering and stacking mentioned above, have been proposedto reduce EM noise. These largely extend from remote-reference

noise cancellation tech-

niquesdescribedin Gamble et al. (1979a)for the MT method, and recently expanded in Goubau et al. (1984). San Filipo and Hohmann (1983) investigated TEM and FEM data acquisitionfor loop-loop systems

and concludedthat with a sourcemoment of 107

amp-m 2 the signal-to-noise (S/N)ratiowouldbe unsatisfactoryfor frequenciesbelow 0.1 Hz or below 0.3 Hz base frequency for TEM systems.They concluded that remote-reference

noise subtraction would

extend these low-frequency limits by one decade. Spies(1988)describesa predictivefilteringschemefor in-loopTEM soundings,in which simultaneous3-componentmagnetic-fieldmeasurementsare made in the center of a transmitter loop. Over a horizontallylayeredearth the TEM signalis containedsolelyin the vertical component and the horizontal components containonly noise. Using standardMT relationships, the noise in the vertical componentcan be predicted from the two horizontal components, and subsequently subtracted.Improvementsin S/N of about 10 are possiblewith this scheme.Other noiseprocessing techniquesfor TEM measurementsinvolvingpruning, tapered or random stacking, and pre-whitening,are discussed in Macnae et al. (1984), and selective recursive stackingby Strack et al. (1989a). Wilt et al. (1983)describea frequency-domainnoise-cancellation scheme for the EM60 using a remote reference magnetometer located several miles from the survey site. Two horizontal componentsare transmittedusing radio telemetryto the receiverwhere they are subtracted from the local signal. Noise levels are reduced by a factor of 10 usingthis scheme;the systemis described briefly in Appendix C. FIELD

353

choice of source, receiver and time- or frequencydomain, as well as the frequency or time range and source-receiverseparation.Many of these factors are interrelated.

The most important step is to estimate, from geologicalmodels,the rangesof layer depthsand resistivities which may be encountered.Forward modelingis then used to determine the frequency or time range, separation,and source strengthrequired to carry out

the sounding. These parameters are restricted by considerationof S/N ratios, field logistics, and available instrumentation.

Suitable instruments

or contrac-

tors are often not available, as the requirements are

outsidethe rangeof presenttechnology.In this caseit is necessaryto consideralternativesoundingmethods such as dc resistivity or natural-sourcemethods such as MT, AMT, and geomagneticinduction, or to redefine the geological model to be less ambitious. If possible,three or four soundingdesignsshouldbe generatedfor each model. Ideally, procedures incorporatinggeneralized inversetheory or linear statisticaltheory shouldbe used to determine optimum survey design parameters. Inverse theory is covered in the chapter on inversion (Hohmann and Raiche, Volume I) and, mentioned briefly later in this chapter. The information density matrix, parameter variances,and parameter correlations all contain useful data in planning a survey and interpretingthe results. Glenn and Ward (1976) used this approach with the University of Utah 14-frequency system,and concludedthat the phaseof Hx and Hz are the optimum parametersto employ if measurementaccuracyis better than 0.5 degreesbut in the presenceof noise it is preferableto measurethe polarizationellipse. Vozoff and Jupp (1977) further extendedthis approachin a studyof the resolutionof a buried layer. They describemethodsof assigninga tolerance or error bound, on the interpreted value of each parameterof the model based on its resolution and on the noise level of the survey. The range of frequenciesare thosewhich are capableof providinga stable inverse to a given set of data. A similar approach can be employed using catalogs or master curvesfor interpretationof EM soundingsif inversion programsare not available.

PROCEDURES

This section discusses the factors affecting the

proper designof an EM soundingsurvey, calibration and testingof equipment,and good field practices. Survey Design A number of factors should be considered in choos-

ing optimumsurveydesignparameters.Thesefactors include instrument selection, system geometry and

Rules'øf'thumb'--Frøm field experience and model studies certain rules-of-thumb

which aid in the initial

selectionof survey parameterscan be deduced. Frequency-domaintechniquesare generallysuperiorfor shallow soundingsor for use in high-noiseenvironments,althoughthey can alsobe usedfor deep sounding. Unless high-resolutiontechniquesare employed, the source-receiverseparation needs to be at least twice the depth of investigation.As noted in Keller

354


(1971), static coupling in a horizontal loop system varies inversely as the cube of distance, so for a given loop size and frequency the momentof the sourceloop


must be increased

as the cube of the source-receiver

separationto achieve a constant S/N ratio. It is also necessaryto decrease the frequency proportionally to the squareof the desiredincreasein depth penetration. As the frequency is lowered the voltage output of the receiver is lowered, and ambient noise increases in

inverse proportion to frequency. The net effect is that the moment of the source needs to be increased by the separationraised to the power of 4.5 to maintain the same signal at the receiver. This sets the practical upper level limit for source-receiverseparationand the lowest limit for frequency. Similar arguments can be applied to other configurations. TEM methods are suitable for most applications except for very shallow soundingssince the earliest sampletime measurablein presently available systems is about 10 •xs. Wire-loop configurations are more suitable for very deep soundingsthan loop-loop since a large source moment is more easily attained and instrumentation requirements are not as rigid. The practical aspects of wire-loop soundingshave been discussedin Keller (1971) and Stoyer (1980). As mentioned in the previous section, soundingresults from grounded wire transmitters may be susceptible to near-surface resistivity heterogeneities (Newman, 1989). The depth of investigation of the in-loop configuration was discussedin the Depth of Investigation section and is proportional to the one-fifth power of the transmitter moment for voltage measurementsand one-third power for magnetic-fieldmeasurements. The frequency or time range needed for a sounding can be estimated by consideration of the induction number. An essentially complete induction frequency sounding would always be obtained if the induction number for each layer that can be resolved were varied between 0.1 and 10.0. For many purposes this is an overly stringent requirement; furthermore, detailed conductivity information is not generally available before the sounding is made. Results presented in previous sectionsindicate that if a thick near-surface layer is present, it is generally sufficient to vary the induction number in this layer from 0.1 to 10.

Transmitter DesignParameters.--Some mention of calculating transmitter moment was given in the previous section and in the Depth of Investigation section. A complex description of choice of transmitter design parameters of a grounded-wire source is given in Keller (1981) and Keller et al. (1984), who consider factors such as wire resistance, sourcelength, grounding resistance as well as transmitter limitations of output power, voltage, and current. For any particular

area there is an optimum source length; at shorter lengths the circuit resistance is dominated by grounding resistance, and for longer lengths the resistance is dominated by the cable. Increasing the length past this lowers the current proportionally, while retaining the same moment. For a loop source Lamontagne et al. (1987) show that the optimum loop size is propor-

tionalto X/MV2/p,whereM isthewiremass,V isthe transmitter voltage and P is the total output power. For most applications there is a tradeoff between increasing the source moment and lowering the production rate. Since a great deal of time is often taken laying out the source, it often makes senseto employ methods in which measurements are made at a number of receiver locations about each transmitter site. The

optimum number of receiver sites depends on how rapidly the overall section changes laterally, near surface geologic noise, and the configuration being used.

Calibration and Instrument Testing

Calibration was discussed in a previous section, Instrumental Sourcesof Error. Calibration procedures vary widely according to the type of instrument being used, and manufacturer's directions should be fol-

lowed. A discussionof procedures for polarizationellipse systemsand checking for clock drift was given in the previous section. A discussionof calibration of slingram and other equipment which may be used in soundingis given in Frischknecht et al. (this volume). The

transfer

functions

of various

elements

of a

systemcan be measuredin the laboratory to obtain the overall system function. Some characteristics such as orthogonality of inphase and quadrature channels are best checked in the laboratory. However, it is good practice to measure the transfer function of the entire system under conditions which duplicate operating conditionsas nearly as possible.The transfer function of frequency-domainsystemsusinga large loop can be measured in the field by collapsing the transmitting loop and placing the receiving loop nearby so that the induction number is very small and the earth response is negligible. By reducing the size of the transmitting loop the receiver can be operated at normal signal levels and, at low frequencies the transmitter is operated at normal current levels. This technique is not satisfactory for calibrating electric field sensors because, for short spacings,the collapsedloop does not provide adequateelectric fields. A short electric bipole can

be used

to

calibrate

electric

field

sensors

or

calibration can be done by directly injecting a signal into the electric field pre-amplifier, provided a suitable reference signal is available. Receiving systemscan be checked and calibrated by



using them with miniature loops to make scale model measurementsover models having a responsewhich is known or which can be calculated (Becker, 1969). However, some specialproblems arise in direct testing of time-domain systemsat low signal levels. Although in principle a simple resistance-capacitance(RC) network can be used to test a single-loop time-domain instrument, in practice it is difficult to find components, particularly capacitors, which behave linearly over the large range of voltages encountered and, therefore, it may be impossible to test late time performance with an (RC) network. A set of scale model coils coupled to a copper ring, which gives an exponential decay, can be used to check for stability of a time-domain system. Good Field Practices

Good field practices are essentialto decreasenoise and to increase the accuracy and resolution of soundings. They encompassknowledge of geometrical, instrumental, and EM sourcesof error given previously. By burying magnetic sensorsunder a few inchesof soil or covering them with a wind shield wind noise is minimized.

Cables

should lie on the surface

of the

ground and should not be supported by branches or bushes. For low-frequency measurements,electrodes should be located in holes at least 6 inches deep and covered to prevent the sun and wind from drying the soil in which they are located. Multiple transmitter electrodes, separated by 3 to 10 m and connected in parallel, can be used to lower contact resistance. Both source and receiver

should be located

so that

they are not in closeproximity to cultural features.No definite rules on the necessaryseparationbetween the system and cultural features can be given. We have found that a high-frequency horizontal coplanar looploop system paralleling a power line carrying a ground wire should be at least one-half loop spacing away from the line. Obviously, the effects of cultural features can be minimized by arranging the source and receiver so that there is minimum coupling with the feature. In some cases it may be necessaryto experiment with various configurationsand spacingsto evaluate

the

effects

of

cultural

features.

If

the

exact

positions of buried pipes or cables is unknown they can be mapped readily by profiling with instruments such as those described in Frischknecht et al. (this volume). In usingprofilinginstruments,cultural anomalies can be recognized by their sharpness, profile shape, and strong dependenceon the azimuth of the instrument. Generally, the effects of most cultural features on soundingsare most pronounced at high frequencies or early times and these efforts may be recognized as distortion of the soundingcurves. Elec-

355

tric field sensors should ideally be placed in areas devoid of near-surface conductivity inhomogeneities, since the E field is highly vulnerable to distortion by any conductivity changes. Similar considerations shouldbe used for a groundedtransmitter dipole; the grounding points should not be near any known or suspected faults, shear zones, or other conductive zones since they will strongly channel the injected current.

DATA

PROCESSING

Field data should first be converted to appropriate units to facilitate presentation and interpretation of results. It may be necessaryto edit and correct data for systematic or known errors. We present below a number of common parameters obtained by standard data reduction for both frequency-domain and timedomain systems. Frequency Domain Data Reduction

In processing frequency-domain data it may be necessaryto correct for phase drift if crystal clocks are used for reference

between

transmitter

and receiver.

Generally this will require assumption of linear drift between

times when the transmitter

and receiver

are

resynchronized. Modern equipment is sufficiently stable that changesin gain with time are generally negligible. When individual field components rather than ratios

are measured

the results

must

sometimes

be

normalized by the calculatedprimary field. If accurate values

for the actual

normalized

field are to be ob-

tained, the calculation of the primary field must be done carefully taking topography into account. Depending on the equipment and field technique used, it may be necessary to also normalize the field data by the system transfer function to account for variations in gain and phase. Parameters of the polarization ellipse are often obtained using equipment in which the vertical and horizontal componentsof the field are measured separately. The first step in processing such data is to correct for any differences in sensitivity between the two channels. The radial and vertical magnetic field componentsof the polarization ellipse are

Hr ---IHxI eidpr directedalongthex axis,and (48)

Hz = IHzl e

directedalongthez axis

and the phase difference is

A•)-- •)z -- (Dr

where •z and •)r are the phasesof the magneticfield componentsH z and Hr, respectively.The equation

356


describingmagneticfield polarization is given in Stratton (1941, p. 279) as


X2

Z2

XZ cosA•

Hr 2+H775,2=sin 2A•, z 2 HrHz

(49)

(see Figure 30). The radial and vertical magneticfields

are H r and H z. The ellipseparametersare given in Dey and Ward (1970, p. 702) and Smith and Ward (1974) as follows: The wave

tilt is

HzHr sin A4

e= Hr -iA•

(5O)

W=•z-ze The tilt angle a is given by: tan 2a = +

2(Hz/Hr) cos A•

-

1 - (Hz/Hr) 2 '

(51)

The vertical ellipticity is H2

equation (49). The direction of rotation dependsupon the sign of the phase difference. With the phase difference defined as in Figure 30, right-hand rotation correspondsto positive A4 and left-hand rotation to negative A4. The tilt angle reflects the direction of rotation by being positive for right-hand rotation and negative for left-hand rotation. Smith and Ward (1974) offer an alternative definition for ellipticity:

H1

(52)

Here A4 is the phase difference as defined in Figure 30, and H i and H2 are the major and minor axesof the polarization ellipse, respectively. The magnetic field vector rotates as a function of time to trace out the polarization ellipse given by

which allows for sign variation. Amplitudes of the componentsof the magnetic field generated by a magnetic dipole source are commonly normalized with the primary magnetic field or, equivalently, the mutual couplingfactor between the source and receiver. These are taken to be the free space values given by

H(p 1)= 4,rrr3

and perpendicular coils and (54) M

H?) = 2,rrr3 forcoaxial coils,

T

SOURCE

Ur

[ / ////

>///////

/////

/

<--Hr--:•

HMD

(55)

where M is the dipole moment of the source and r is the transmitter-receiver separation. The VMD source

Hz

///

(53)

-M for horizontalcoplanarcoils

H zeia4,cosot- H r sino• Hzeia4'sin•x+ Hr cosot

VMD

H•2

H2

= -i •.

SOURCE

HO

/

/


includes the horizontal coplanar coil and perpendicu-


lar coil configurations givenas the H z andH r components, respectively. Since the normalization factor is the same for both components, the ratio of the components is the same whether they are normalized or not. The polarization parameters given above can be generalized for a horizontal magnetic dipole (HMD) source by replacing the tilt angle a by strike angle and the vertical ellipticity by the horizontal ellipticity,

and the vertical magneticfield H z by the tangential magneticfield H 0. Note that for the HMD configuration the perpendicular (H 0) and coaxial coil (Hr) configurationsinvolve different normalization factors. Thus, in order to compute ellipse parameters for the HMD source, the total rather than normalized fields should be used (Smith and Ward, 1974).

Time

Domain

Data

Reduction

Most TEM systemsrecord a voltage v(t) at a number of sample times. The data should be normalized by transmitter current, the amplifier gain, and moment of

the receivercoil, to yieldunitsof V/Am2 or more conveniently ixV/Am2.Forsystems employing a small (dipole) transmitter loop it is convenient to further normalize by the transmitter area to yield units of

V/Am4.A number of different procedures areusedto correct

for finite turn-off

time of the transmitter

or to

take turn-off time into account in interpretation (Raiche, 1984; Fitterman and Anderson, 1987). This correction is usually unnecessary for measurements at sample times greater than 30 times the ramp length. It is often useful to calculate other parameterswhich assistthe interpreter in deciding whether the earth is, in fact, horizontally layered in the vicinity of the sounding. For in-loop TEM soundings, for instance, the TEM tipper, T, defined by

rv(t) =

Vr(t)

Vz(t)

(56)

Apparent Conductivity and Apparent Resistivity

A further conversion can also be made to apparent conductivity or apparent resistivity. The traditional approach is to define apparent conductivity as the conductivity of the uniform half-space that would produce the observed data at each discrete time or frequency. This is comparable to the definition of apparent resistivity as used in dc resistivity sounding. Conversion to apparent conductivity is a normalization process that removes the data variation that normally occur with changes in transmitter-receiver separations, frequency, or time. The term apparent conductivity is most appropriate for electromagnetic sounding since conductivity rather than resistivity is generally employed in theoretical studies and in descriptions of EM phenomena. However, since most geophysicistsare more familiar with apparent resistivity (in ohm-meters) from its use in resistivity sounding rather than apparent conductivity (in S/m) we will use the former in this description. Becauseof the complexity of the theoretical expressions for EM field behavior, asymptotic expressions for apparent resistivity are often used. Many of these expressions, which correspond to near zone or late time and far zone or early time, were given earlier in the HomogeneousHalf-Space section. Apparent resistivity defined using the early-time or far-zone (wave stage) definition approachesthe true resistivity of the top layer at early times (or high frequencies) or very large spacings,and apparent resistivity defined using the late-time or near-zone definition approaches the true resistivity of the lower layer at late times (or low frequencies) or very small spacings. Outside these ranges, the apparent resistivity values depart widely from the true earth section. This is not a problem as long as the same formulations are used when constructingmaster curves for interpretation, but is often misleadingto an inexperiencedinterpreter and should be avoided if possible. These asymptotic forms continue to be used in the Soviet

Union

and some other

countries.

It is also possibleto calculate the "exact" apparent resistivity (to within a given degree of accuracy) over the complete range of induction numbers. For some

for voltage measurements, or

hr(t)

Th(t) =hz(t )

357

(57)

for magnetic-field measurements, give such information (Spies, 1988). The subscriptsr and z refer to the total horizontal and vertical fields, respectively. Over a horizontally layered earth, T will be zero, and 1-D interpretations will generally be valid if T is less than about 0.1. Examples of the use of TEM tipper are given in the Effect of Non-l-D Structures section.

cases formulas

are available

for direct calculation

of

apparentresistivity;in other casesiterative solutionof the appropriate transcendental equation is required (Raiche and Spies, 1981, Raab and Frischknecht, 1983). There are often difficulties in calculating "exact" apparent resistivity, since for certain layered models it may be undefined or multi-valued. With frequency-domain data the decision must be made whether to base the definition on amplitude, phase, inphase, quadrature, ellipticity or other components;


358


each will give a different value of apparent resistivity. Wilt and Stark (1982) describe a manual technique for determination of apparent resistivity from amplitude data and Kauahikaua (1982) describes a technique using ellipticity. Most frequency-domain response curves do not change monotonically as a function of frequency, and this causes difficulty in automatic computationof apparent resistivity; in addition there are regionswhere apparent resistivity is undefinedor poorly behaved. To demonstrate the difficulties and features of apparentresistivity, we will examineone configurationin detail: the central-loop or in-loop TEM configuration. Because of symmetry considerationsthe theoretical expressionsfor the response of a layered earth are similar to those for the wire-loop configuration of equivalent transmitter moment. This example is summarized from Spiesand Eggers(1986). We make use of the expressions for the magnetic field and voltage response of a homogeneousearth given earlier in Table

time values. The late-time asymptoticcurve is the one normally used with graphical interpretation techniques. Next, considerthe same approachused for a multilayer case. Three two-layer models, labelled A, B, and C, are superimposed on homogeneous half-space curves in Figure 33. Model A is a 10 11.m layer 25 rn thick overlying a 0.1 11.m basement. Model B is identical but with a basement resistivity of 1000 l•.m. For Model C, p• is 10 000 11.m, P2 is 10011.m and the

10a

1(9

2.

A suite of homogeneoushalf-space curves for the in-loop magnetic field TEM response is shown in Figure 31. The response asymptotically approaches the primary magnetic field at early times, and decays with time to the power of -3/2 at late times. Any of these curves can be transformed to apparent resistivity, as for instance, the 10 11.m half-space response shown in Figure 32. The solid curve is the "exact" apparent resistivity curve. The upper dotted curve is the late-time asymptotic apparent resistivity curve, and the lower dotted curve is the early-time asymptotic apparent resistivity. Notice that these curves diverge in the region between the "early" and "late"

10-1

10-3

10-2

10-1

1

10

t, ms

Fig. 32. Magnetic field apparent resistivity curves for a 10 11.m homogeneoushalf-space. The solid curve is the "exact" apparent resistivity, and the dotted curves are the early-time and late-time asymptoticapparent resistivities. 10-2

.-

........

i

........

i

........

!

........

i

........

!

i•.••lO, 10-3

0•,• X

........

•

10-4 A

hz, •

10-4 A

hz, •

10-5

10-5

10-6 10•

10-7

10-3 10-7 10-3

10-2

10-1

I

10

102

103

10-2

10-1

1

10

102-

103

t, ms

t, ms

Fig. 31. A suite of TEM in-loop magnetic field response curvesfor a homogeneoushalf-spacemodel. The loop radius is 100 m (Figures 31-41 from Spies and Eggers, 1986).

Fig. 33. TEM in-loop magnetic field response curves for three two-layer earth models superimposedon a suite of homogeneoushalf-space curves. The parameters for the two-layer models are shown on the figure.


layer thicknessis 250m. At early timesthe responseof all modelsis equalto that of a homogeneous half-space with the top layer resistivity. At about 0.03 ms for Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

Models A and B, and 0.003 ms for Model C the curves

divergefrom the half-spacecurve with the resistivity of the top layer and asymptoticallyapproach the responseof the lower layer. The apparentresistivity curves for Models A and B are shown in Figures 34 and 35. The "exact" apparent resistivity changes

smoothlyfrom the resistivityof the top layer to that of the lower layer. The apparent resistivity curve for Model C (not shown) is also well-behaved. The simple behavior of the magneticfield response

359

and apparent resistivity can be contrasted to the complex behavior when the time derivative of the magneticfield, or voltage, is measured. A suite of voltagetransientdecay curvesfor homogeneous halfspacemodelsis shownin Figure 36. Apparent resistivity curves for a 10 f/.m half-spaceare plotted in Figure 37. This time there are two "exact" valuesof apparentresistivitywhichcorrespondto eachvalueof the voltage response. Graphically, each half-space decay curve intersectsother half-spacecurves with different resistivities.For example it can be seenfrom Figure36 that the responseof the 10f/. m half-spaceis identical to that of a 1 f/.m half-spaceat 0.35 ms, a 100 f/.m half-spaceat 0.036 ms, and so on.

1(9

105

EXACTPa ASYMPTOTIC Pa 102

104

103

nV

Am 2 102

10-1

10-2

i

i i illill

10_3 ...... 1•)'_2

10-1

i

.......

i

........

i

1

10

........

i

%

102

10-3

103

10-2

10-1

1

10

102

103

t, ms

t, ms

Fig. 34. Magnetic field apparent resistivity curves for Model

Fig. 36. A suiteof TEM in-loopvoltageresponsecurvesfor a homogeneoushalf-spacemodel.

A. 1(9

1(9

MODEL B

••

•

ASYMPTOTIC Pa -

102 1(9

lO

1

10-1

10-1

10-2 10-3

10 -3

10 •-2 '

11

I

10

10 2

....

i

10-2

,

......

!

........

10-1

i

I

. % .......

i

10

........

i

102

.......

103

t, ms

lO3

t, ms

Fig. 35. Magnetic field apparent resistivity curves for

Fig. 37. Voltage apparentresistivity curves for a 10 •-m half-space.Two "exact" values of apparentresistivityare

Model

obtained at each sample time.

B.


360


When exact values of apparent resistivity are employed it is usual to choose the value that is more geologicallyreasonableand constructa singlecomposite curve. This procedureis often carried out automatically by a computer algorithm. For this example it is obvious which is the more appropriate curve at each particular sample time. The transition from early to late time occurs at t = 0.12 ms, when the slope of the curve is 45ø. Raab and Frischknecht (1983) describe a procedure to automatically select the appropriate curve for use in calculatingapparent resistivity, which effectively usesthe 45 degree point as its criterion for choosing the higher or lower value. Voltage decay curves for the two-layer modelsA, B, and C are shown superimposedon homogeneoushalfspace curves in Figure 38. These curves are the time derivative of the magnetic field curves, and as such exhibit more complex behavior. Note particularly the initial

overshoot

for

Model

B and

undershoot

for

Model C. Apparent resistivity obtained from voltage response is much more complex than for magnetic field. Voltage apparent resistivity curves for Models C, B, and A are shown in Figures 39 to 41. In practice analytic and numerical techniques are used to calculate the apparent resistivities, although they could easily be obtained from the appropriate intersections of the model curve with the homogeneoushalf-space curves in Figure 36. The apparent resistivity curvesfor Model C are shown in Figure 39. To choose the appropriate exact apparent resistivity curve or "branch", dependingon the sampletime is a relatively simple matter. In this case the changeoverpoint is at

1.2 x 10-4 ms,wherethecurvesundergo anabrupt change of slope. Model C is an example of a typical "far-zone" soundingsince the lower layer is observed only during the right-hand, descendingbranch of the homogeneoushalf-space curve for the top layer. The late-time asymptoticapparentresistivity is equal to the true apparentresistivity over most of the time rangeof interest.

The curve exhibits

the usual "overshoot'

at

about 0.01 ms before asymptotically approachingthe resistivity of the lower layer. In the literature, this behavior is often attributed to electromagnetic "interference effects" or "resonance" between layers. It can, however, be seen from the magnetic field curves 100

MODEL C

105

'

_

104

103

102

lO

lO-5

10-4

10-3

10-2

10-1

1

10

Fig. 39. Voltage apparent resistivity curves for Model C. 10,3

10-4

Pl A

P2

•

10

0.1

25

10

1000

25

104

100

250

_

B

10-5

C

•,%

102

MODEL A

lO-O

v

e(t),r•2 10-7

10-8

10-1

10-9

10-3

10-2

10-1

I

10

102

103

t, ms

10-2 10-3

....... i ...... i ...... I ........ i .•....... i i,,,,,,! 10-2

10-1

I

10

102

103

t, ms

Fig. 38. TEM in-loop voltage responsecurves for the same two-layer models as Figure 33 superimposedon a suite of homogeneoushalf-space curves. These curves are the timederivative of the magnetic field curves.

Fig. 40. Voltage apparent resistivity curves for Model A. It is not possible to obtain a smooth transition between the early- and late-time curves.

Electromagnetic Sounding in Figure 33 that the magneticfield decayssmoothly from the responseof the top layer to that of the lower layer. There are no oscillations,and thus the overDownloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

shoots and undershoots observed on the voltage re-

sponsedecaycurves(Figure38) andhencethe voltage apparentresistivity curves (Figure 39) are purely a resultof differentiatingthe magneticfield responseand applyingthe apparentresistivityalgorithm.So-called resonance, interference, or reflection phenomena are not observed in the received magnetic field for these models, nor can they be presentin any other response derived from it by processing. Other two-layer models result in much more com-

361

resistivitycomputedfrom the magneticfield displaysa much better behavior than that computed from the inducedvoltageresponse,and is superiorfor interpretation (Raiche, 1983a; Newman et al., 1987). This observationprobably appliesto other configurations as well. Algorithms for converting observed voltage responsemeasurements to magneticfield responseare givenin Eaton and Hohmann(1988).Partly becauseof the difficultiesassociatedwith apparentresistivity, the current trend is toward a direct transformation of the

field data to an imagedconductivity-depthsection,as describedin the Computer inversion section.

plex behavior. Consider the apparent resistivity

Topographicand GeometricCorrections

curvesfor Model A, shown in Figure 40. In this case the early and late-time curves do not converge;over much of the time range neither exact apparent resistivity value is intuitively appropriate.The transition from early- to late-time behavior takes place over a wide time range. In fact, the late-timebehaviorof the top layer appearsbeforethe early-timebehaviorof the lower layer. In cases such as this any automated schemeof choosingthe "correct" value of apparent resistivity, such as where the curve has a 45 degree slope, will be unsuccessful.The resultant apparent resistivity curve will exhibit a discontinuitybecause

Simple algebraiccorrectionsfor misalignmentand topographicerrors in frequency-domainsystemsare givenin Sinha(1980),for rotationtowardor awayfrom

the two curves do not intersect. The situation is even

worse for Model B, as seen in Figure 41. There is a regionbetween0.05 and 0.18 ms where exact apparent resistivity is undefinedsince the responseis greater than that of any homogeneoushalf-space. Thus, for the in-loop TEM configurationapparent

thetransmitter. If Hrr andHzr denote thetruemagneticcomponents, andH r andH z are the corresponding observed values, then

Hzr = Hz cos{b- nr sin{b,

(58)

Hrr = H z sin{b+ Hr cos{b,

(59)

and

where • is the misalignmentangle. Fullagar (1981) and Fullagar and Oldenburg(1984) suggesta more general statisticalapproach. In the derivation it is assumedthat {b is small and can be regardedas a normalrandomvariablewith zero mean

and (small)variance,v2. The resultsexpressed as variances

are'

For vertical magnetic field managements.• 103

102

Var [g Re{Hz}] •- (Re{Hr})2 v2

(60)

Var [gIm {Hz}l •- (Im {Hr})2 v2

(61)

Var [b IHzl] --

(62)

IHrl2 cos2 0w

Var [g 0z] -• v2[WI2 sin2 0w

(63)

For radial magnetic field measurements.•

10-1

Var[g Re {HF}] •- (Re {Hz})2 v2

(64)

Var [b Im {Hr}] •- (Ira {Hz})2 v2

(65)

Var [• [HF[] •- •2 iHzl2 cos20w

(66)

v2 sin2 0w 10-3

10-2

10-1

I

10

102

Var[b0r]•-

103

Iwl

(67)

t, ms

For wave-tilt

Fig. 41. Voltage apparentresistivitycurvesfor Model B. Exact apparentresistivityis undefinedin the region0.05 to 0.18 ms sincethe voltageresponseis greaterthan that of any homogeneoushalf-space.

measurements.•

Var[b [WI]•-Icos 0w + Wr - W/5 +2 WrWi sin0w[2V2

(68)

362


Var [a 0•]

0 = tilt of transmittingloop--positiveif edgeof

=

loop nearest receiver is above reference plane

[sin0•(1 + Wr2 - w,5 - 2 WrWi COSo•l 2 v2 Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

Iwl 2 (69)

q>= azimuth of transmittingloop--90 degrees whentotal horizontalcomponentof loopis directed toward the receiver.

where

It shouldbe noted that Z 0 (vertical coaxialloops)= -2Z0 (horizontal coplanar loops) (Frischknecht, 1967).Z0 (perpendicularloops)is actuallyzero but it is assumedto equal Z0 (horizontal coplanarloops) for

Wi = Im {W}, and

Wr = Re {W}

purposes of normalization. The effect of geometric errors on time-domain mea-

Vat [a Re{W}] -- I1 + Wr 2-- W/212 p2 Var[8Im{W}]

surements can be estimated from the theoretical

•- 21WrWil2 v2.

For soundingstaken on a hill with relatively constant slope it is often possible to transform the coor-

dinate axes so that conventionalinterpretationaids can be used; the sounding direction is taken to be normal to the plane of the slope. The orientation of a large transmitterloop can be estimatedby measurements at very low induction numbers (for FEM) (Anderson et al., 1983b) or during the transmitter on-time for TEM

measurements at the center of the

loop, since under those conditionsthe primary field will be much larger than the secondary.Andersonet al. give simple correction formulas for the mutual

coupling ratio of various loop configurationsfor soundingon a uniform slope: z

Zo

•y

cosO-

2

sinO sin•

II

IV

(70) z

Zo

cos 0

+

sin 0 sin 4), II

(71) where z

Zo

- mutual coupling ratio between radial com-

ponent of loop and radial receivingloop,

z

= mutual coupling ratio between vertical

component of loop and vertical receiving loop,

ex-

pressionsgiven earlier for the responseof a uniform ground. At late times, measurementsdepend only weakly on the transmitter-receiverseparation.Due to the absenceof the primary field, topographicand misalignmenteffects generally are much less severe for dB/dt TEM

measurements than for most FEM

measurements.

Plotting Data A number of conventions have been established for

plottingsoundingdata, and theseare normally stated by the instrumentmanufacturer.Frequency-domain soundings,for instance, can be plotted as phasor diagramsor amplitude-phaseplots. Time-domaindata are oftenplottedas a transientdecaycurve of voltage versustime. If data are convertedto apparentresistivity then soundingcurves of apparentresistivity versus time (or frequency) can be drawn. The questionof where data shouldbe plottedrelative to the sourceand receiver positionsoften arises. For an axisymmetricconfiguration (e.g., singleloopor in-loop) the answer is straightforward.For measurements of ratios of componentsin the wave-zone it is clear that the resultsshouldbe plotted at the receiver site since the ratios are independentof the source location. For loop-loop or wire-loop configurations and for large fixed transmitter loop/multi-receiver locationsmeasurements in the near-zone,the optimum plottingpositionis not as obvious.It is importantto rememberthat a large volume of ground is being sampledby each sounding,and the data are affected by ground in the vicinity of both receiver and transmitter, as well as in between. However studies re-

= mutual couplingratio for horizontal coplanar loops,

= mutual coupling ratio for perpendicular loops, = mutual coupling ratio for vertical coaxial loops,

ported in Kauahikaua (1982) suggestedthat vertical and horizontalsensorsare responsiveto changesin conductivityat differentdepths and lateral position. Kauahikauastudiedthe contributionof the secondary

fieldsof elementalcurrentsflowingin a conducting plate under the surface, inducedby a VMD source, extending an earlier study reported in Sidorov and Gubatenko (1974). The conclusions of these studies



were that the vertical component is most representative of the ground under the receiver and transmitter for shallow depths, and the region between the transmitter and receiver for greater depths, whereas the radial component is responsive mainly to ground under the receiver for all depths. Near-zone measurements combiningboth vertical and radial components suchas tilt angle and ellipticity are also relatively more sensitiveto the ground underneath the receiver. The convention usually followed for near-zone measurementsis that for a moving sourceconfigurationthe data are plotted midway between the transmitter and receiver, and for a fixed source system the data are plotted under the receiver. The plotting convention is not really important as long as the definition is taken into account in modeling and interpreting the data. If many soundingsare made in an area, pseudosectionscan be constructedby plotting apparent resistivity values underneatheach soundinglocation with a logarithmic vertical time or frequency scale. This method allows qualitative interpretation to be made of a large quantity of data. However, the disadvantages of usingapparent resistivity discussedin the Apparent conductivity and apparent resistivity section, should be taken into account in examiningthis type of data; it is preferable to use imaged conductivity as described in the next section.

INVERSION

AND

INTERPRETATION

In common with resistivity and other geophysical methods, the traditional technique for interpreting EM soundingsis to find a model which will closely reproduce the observed data. Models can be found by manual curve matching, by interactive trial-and-error matching using a computer and display device, or by automated computer programs. A number of techniques are available which depend on use of partial curve matching aided by nomogramsand simple mathematical relationships.Also, in principle, data may be interpreted using direct inverse proceduresdeveloped from inverse scattering theory. In practice, many direct inverse methods tend to be unstable and noisy. Techniquesfor direct transformation of EM sounding curves into approximate resistivity versus depth curves or "imaging" techniques are being developed (as describedlater in this sectionand in AppendicesD and K). Finally, qualitative interpretation can be carried out using pseudosectionsor stacked sounding curves.

The interpreter should have realistic expectations and objectives. Slichter (1951) proved that it was possibleuniquely to determine the analytic conductivity function with depth from noiselessmeasurements made at a single frequency at all distances from a

363

VMD source. Fullagar (1984) extended this to show that the same applied for measurementsmade over a rangeof high frequenciesat a singleseparation.These theoretical considerationstend to make us optimistic; but, as Fullagar also pointed out, the inadequacyand inaccuraciesof any realizable data set can often mean that the true groundconductivity may be totally unlike any model known to fit the data. The degree to which the physical model determinedby the interpreter corresponds to the actual model depends on the type, completeness,and quality of the data as well as on the actual geologic situation. There are many geologic situations where specific and sufficient information from EM soundingscan be obtained with a high degree of confidence. For instance, the depth to a water table in a thick section of clean sand and gravel or fresh basalt can be determined with a degree of accuracy sufficientfor most purposes. Similarly the depth and conductance

of a thin shale bed sandwiched

between

massive layers of unaltered carbonate rocks can be accurately determined. However, if the objective is to determine subtle layering within the gravels, the basalt, or the carbonates, the results are likely to be disappointing. Even taking a simplified view, it is known from electric logs that sedimentary rocks are generally composed of a great number of layers with widely varying thicknessesand resistivities. In inversion, models with many layers can be used to fit EM data taken over such rocks; but, without use of independent information, there is no way to chooseamong a large number of models which fit the data about equally well. In interpreting results from loop-loop or wire-wire configurations, any particular layer in a model can be representedby a group of thinner layers and the contrastsbetween these layers can be arbitrarily high if the thicknessesare appropriately adjusted. Multi-layer models in which the changesin resistivity and thickness between adjacent layers are generally small may be reasonable representationsof layered earths were resistivity varies gradually rather than abruptly, although in these cases it is probably better to use smoothly varying models as advocated in Constableet al. (1987). There is little justification for using many-layer models with large contrasts between many of the layers unlessthe objectives is to simulate an anisotropic section. In dealing with complex sections, a realistic objective in interpreting EM soundings is to find simplified models, having no more than perhaps4 or 5 layers, which portray average conductivities over large intervals and/or conductances and depths for a few arbitrarily thin layers. The total number of layers which one is justified in using dependson the type, range, number, and accuracy of the measurementsand the geologic noise.

364


Curve-Matching Techniques


A variety of techniquesfor manual interpretation of EM soundingcurves have been devised. To do direct matching of entire soundingcurves, an adequate album of families of calculated

curves must be available.

In order that the album be as compact as possible, responseshould be plotted as a function of the induction number (r/15)or its square using a logarithmic scale. Families of curves can be grouped either by resistivity contrasts or ratios of layer thickness to source-receiver separation. Field data are then plotted with time or frequency or the square root of these quantities on the same scale to correspond to the album curves. The interpreter then seeks the best possiblematch between the field curve and curves in the album; in doing so the time or frequency axis of the field curve may be shifted along the induction-number axis of the calculated curves. Interpolation between curves of a family and between families may be used. When a suitable match is found, the conductivity of the first layer is calculated from a value of the induction number on the calculated curve correspondingto any specific time or frequency on the field curve; all factors in the induction

number other than conductiv-

ity are already known. The conductivitiesor resistivities of the other layers are then calculated from the first layer conductivity and the conductivity contrasts of the curve which was selected. Similarly, all of the layer thicknesses are calculated from the source-receiver spacing and the ratios of layer thicknessesto spacingfor the curve which was selected. For frequency-domain soundings,amplitude and phase or inphase and quadrature curves must be fit separately;of course a compromise match should be estimated which fits two componentsequally well. Examples of complete curve matching are given in Frischknecht (1967, p. 13-19). Although it is a very simple technique, in practice complete curve matching is not very efficient when multi-layer models must be used due to the large album of curves required and the time needed to search for a fit. Also, it is more difficult to incorporate the instrumental system response than in computer schemes.

Frischknecht (1967) gives tables for the normalized mutual impedance for loop-loop and loop-wire systems over homogeneousand two-layered earth models. The two-layer models cover a range of first layer thickness to transmitter-receiver separationof 1/32 to 1 or 2. Conductivity contrasts are 0, 0.03, 0.1, 0.3, 3.0, 10, 30, 100, and o•. These tables are complimentary to four-layer master curves for the loop-wire configuration in Vanyan et al. (1967b) which apply to sourcereceiver separations which are generally large compared to layer thickness.

Dey and Ward (1970) and Ryu et al. (1970) present curves of tilt and ellipticity plotted versus induction number for layered models. Verma (1980b and 1982) has tabulated master tables and curves plotted as a function of period for interpretation of FEM mutual coupling curves. Dyck and Becker (1971) and Ketola and Puranen (1967) give results for FEM systemsover a thin conductive sheet. A wide variety of FEM master curves are given in Kaufman and Keller (1983) for loop-loop and wire-loop configurations. The curves are plotted as apparent resistivity versus normalized induction number, for up to three layers. Vanyan et al. (1967b) present a collection of graphical methodsfor interpreting TEM far-zone sounding data. Raiche and Spies (1981) present two-layer master curves for the coincident loop configuration. These curvescan also be usedfor the in-loop configurationat late times. Verma and Rai (1982) present curves for interpretationof homogeneousand two-layered earths for the Crone PEM system using the ratio of two channels.

Extensive

suites of TEM

master curves have been

produced in the USSR over the last twenty years for most configurations.These curves usually employ the late-time asymptoticformulationfor apparentresistivity. These are describedin Kaufman and Keller (1983) who also include many examples of such curves for two and three layers. Complete suitesof master curves have been published in the Russian literature by Kaufman et al. (1969a, b, 1970a, b, 1972), Rabinovich and Stepanova (1972), Goldman and Rabinovich (1972), Goldman et al. (1976), Rabinovich (1977) and others.

Phasor diagrams are rudimentary tools that can be useful in the interpretation of FEM data when extremely simple models are applicable. Frischknecht (1967) givesphasor diagramswhich can be used to find the conductivity and depth to a thick layer beneath a single highly resistive layer. Ketola and Puranen (1967) give similar diagramswhich can be used to find the conductanceand depth to a thin sheet buried in a highly resistive earth. In these two cases, measurement of the inphase and quadrature component at a single frequency are sufficient to obtain the two unknown parametersof the models.The degreeto which measurementsat other frequencies or spacingsyield similar values for the parameters, is a qualitative measure of the validity of the simple model. Eadie (1979) provided several phasor diagramsfor two- and three-layer models for use in interpretation of multifrequency slingram data. Other phasor diagramsfor a two layer earth are includedin Frischknechtet al. (this volume).

McNeill (1980) discussesgraphical techniquesfor interpretation of low-induction number geometric



365

and Stanley, 1970) is shown in Figure 42a. The solid curves are two of the best fits found; their difference provides a qualitative idea of the sensitivity of the EM soundingto the thickness of the second layer. In this example the EM results were used to determine the resistivity and thickness of the conductive layers whereas the resistivity of the resistive layers was determined from the resistivity sounding. The upper layer is unfrozen gravels, the secondis frozen gravels, and third is thought to be unfrozen silts and the fourth may be unconsolidatedmaterial filled with brackish

soundings.Interpretation of suchdata is, in one sense, very simple because the forward calculationsare almost trivial (see the Frequency Domain Results section). However, the number of measurementspossible with available equipment is small, precluding use of more than two-layer or three-layer models. Computer-AssistedInterpretation

Complete curve matchingcan be done by "trial and error" using a suitable computer and forward program. To expedite the work, forward calculations should be rapid and a display of the calculated curve and the field data should be provided (e.g., Sinha, 1976). Procedure efficiency dependson the skill of the interpreter in assessingthe results of previous trials and in guessingnew parameterswhich provide a better fit. This method is fairly effective for finding approximate matchesbut is generally very slow when trying to achieve a good match, particularly if the model has several layers. Although the interpreter gainsa qualitative appreciationof resolutionand equivalence, statistical information is not generated. An example of a FEM soundingfrom the Copper River Basin in Alaska which was matched by this technique (Frischknecht

water.

Computer Inversion

Since the advent of powerful, low-cost, digital computers, automated computer inversion has become widespreadand more popular than graphicalmethods, although curve-matching is often used to obtain a startingmodel. A number of computerinversion techniques are in common use, each having distinctive features, and all requiring experience to use successfully. An interpreter using graphical master curves soon developsan intuition for data quality, completeness,and the uniquenessand accuracyof the interpre-

(a)

<

qO ß

i

I

2.O

INDUCTION NUMBER 2P

(b)

IOO0 VES

C4

lO0

COMBINED

500 •,m 540 fi- m

VES-EMS

MODEL

116000 f•-ml

loo •,m

1<30 •-m

VES MODEL] I

5400 fi- m I

IO

i

i

i

i

I

I

i

ioo

< 30 fi- m I

I

I

i

I

i

i

I

iooo

Fig. 42. Interpretationof soundings fromCopperRiverBasin,Alaska;(a) FEM sounding (EMS) with two possible interpretations, (b) dc resistivity sounding(VES).


366


tation. When using computer inversion it is imperative to understand the use of correlation and parameter statistics; analysis of these statistics is needed to assessthe significanceof the final geoelectricalmodel. The purpose of this sectionis to describequalitatively some of the techniquesused and some of the problems and solutions that may be encountered in inverting field data. For a more completediscussionof inversion theory refer to Hohmann and Raiche, Volume 1. The Monte Carlo or Random Search.--In

the Monte

Carlo or random search method layered earth models are randomly generated (within certain bounds of parameters) and compared with the observed data. Examples of use of this method are given in Sternberg (1979) and in Silva and Hohmann (1983). The main advantage of this method is that all of parameter space can be searched. The more widely used inversion methods often converge to a local minimum and the global or "best" model is never found unless the interpreter uses a large set of starting models. Catalog look-up, or cataloginterpolation, makes use of a tabulated

suite of forward

models which are stored

in computer memory. The program interpolates between forward solutions to provide a relatively inexpensive interpretation (e.g. Rodriguez, 1978; Goldman, 1988).

Generalized Inversion.•Nonlinear least-squaresalgorithms (e.g., Marquardt, 1963) are widely used in the inversion of geophysical data. Generally the interpreter must provide a set of initial guessesfor the undeterminedparameters as well as the data file and known parameters such as loop spacing.The program then computes a forward solution, using the initial guessesas parameter, and a set of partial derivatives from which a set of new guesses are derived. The process continues until the computed curve fits the observationswithin specifiedtolerancesor until significant improvements are not achieved by further iterations. Such programs generally provide statistical information on the resolution of the parameters. However, the statistical information should be used with some caution because it is obtained by assuming that the problem is linear, and can often give misleading results (Bard, 1974; Inman, 1975). Although widely used, most nonlinear least-squares programs for inversion of EM soundings, at their present stageof development,must be regardedas no more than useful tools in inversion. Generally they do not provide satisfactory results without considerable intervention by the user because the inversion schemesconverge to local minima in the presence of noise. Some problems and questions which must be addressed

are:

Selection of number of layers in model Selection of a reasonable set of first guesses Nonuniform weighting of data Convergence to local minima rather than the global minimum Equivalence Distortion

due to 2-D and 3-D structures

"Tuning" the program for effective operation The appropriate number of layers may be known from independentgeologicor geophysicalinformation, or an informed guess may be made if the data are expressedin terms of apparent resistivity and if each layer can be seen to have a distinct effect on the curve. If the layers are thin and the resistivity contrasts are small, or if the data are expressedas mutual coupling ratios, ellipticity, or voltage decay curves, there may be no simplebasisfor estimatingthe number of layers. In this case it is generally advisable to obtain and compare solutions for different numbers of layers startingwith one- or two-layer models and progressing to more complex models. In the absence of independent information about the layering, it can be argued that there is little justification for continuing this processbeyond the point where the root-mean-square error or other measureof the fit stopsdecreasingas the number of layers is increased. Some interpreters prefer to arrive at the number of layers by increasingtheir number beyond the value where the fit improves until some adjacent layers have small conductivity contrasts or are so thin that they can as well be combined into single equivalent layers. At this stage it will be possibleto find a number of other distinctly different models with the same number of layers and which have about the sameerror. Again, lacking independent information about the layering, it is best to combine layers as much as possibleto avoid over-interpretation of the data.

Frequently, inversion programsallow the data to be weighted so that all points do not have equal influence on the selection of a model. If an error analysis of the data is available each point can be weighted inversely accordingto the estimate of its error. When the range of measured values is large and they vary approximately in accordance with an exponential or power function it is generally useful to weight the data by the inverse of their value or logarithm. Such weighting is not appropriate for normalized quantities such as phase angle or apparent resistivity. In some cases it may be useful to place extra weight on the ends of soundingcurves. In cases where the proper or optimum number of layers is known it may still be useful to start with one-layer or two-layer models, for which approximate fits can be rapidly found, to assist in making initial



guessesfor multi-layer models. If the initial guesses are very poor the processmay not convergeat all. The resistivity-depth transformations described in the Depth Imaging section, provide one of the best means of deriving a starting model. In fitting models with three or more layers it is generally necessaryto make several runs with different sets of starting parameters. Even though the process converges, it is likely to converge to a local minimum and not to the global minimum representing the best solution. In difficult cases it may be extremely time consuming or impossible to find the best possiblesolutionby simply trying different sets of starting parameters. In some cases computer time can be saved by approachinga good fit through trial-and-error forward modeling and then obtaining a final fit with the inversion program, using as starting guessesthe parameters from the best fitting model. Another useful approach is to fix one or more parameters during individual runs, especially if additional geologicalinformation is available. Once a close fit is obtained it may be possible to obtain a better fit through unconstrainedinversion by using the parameters for the close fit as initial guesses.However, we have found that this sometimes leads to poorer final results. Generally, sounding curves containing sharp peaks or troughs or abrupt changes in slope are the most difficult to fit by computer inversion since inversion algorithms tend to find models that produce smoother curves than the original data; weighting can sometimesbe used to improve the fit to a critical part of the curve.

Frequently, substantially different models are found which fit the observed data almost equally well. This may be due to equivalence problems as discussedin the Resolution and Equivalence section. Examination of the correlation matrix may reveal problems with equivalence that are not otherwise recognized. For instance, high negative correlation between the thickness and the conductivity of a layer indicates that neither parameter can be resolved very well separately, but that the conductance is probably well resolved. If questions of equivalence cannot be resolved, the interpreter should at least try to assessthe degree of uncertainty in the selection. Another type of problem with equivalent models arises due to truncation of data. Apparent resistivity curves, particularly when they are based on measurements of dB/dt, display overshootsand undershoots; as shown in the examples given under Apparent Conductivity and Apparent Resistivity section and also in the Computer Inversion section. If the data are truncated in the time range that contains an undershoot it may be possibleto fit the curve with a model in which the lower layer has a high resistivity rather than a low resistivity.

367

Generally, nonlinear least-squares inversion programs contain a number of operating parameters which can be changedby the user to make the program most effective for specificdata. Some programsallow the user to set upper and lower bounds on any of the parameters; without use of bounds, parameters may sometimes become so large or small that machine overflow

or underflow

results and the run is aborted.

However, upper and lower bounds must not be fixed so restrictively that a plausible solution, if any, cannot be found. Generally, the bounds on the parameters should be set outside realistic geologic limits to allow eventual convergence to a satisfactory solution. Tradeoff between speed and accuracy is another possible program parameter; the user can begin fitting a set of data with the parameter, adjusted for greatest speed in forward calculations and then use more accuracy in later calculations to obtain the final fit. Jupp and Vozoff (1975) present an overview of stable iterative methods of inversion. Many EM problems are ill-posed, that is, small changesin the data lead to large changes in the solution, and automated techniques must make allowance for this. It is often possible to combine parameters in such a way that they are nonunique, and thus reduce the complexity of the problem. Various parameters of the geoelectric section contribute

in different

amounts to the observed

response. Vozoff and Jupp (1975) distinguishbetween "important parameters", which have a large influence on model data and are well resolved, and "unimportant parameters" which have a small influence on model data. Large changes in these parameters result in a marginal improvement in fit, and thus they need to be constrained within certain limits during the inversion process. Layers outside the range of measurement, or fine microstructure within the section are examples of what they call "irrelevant parameters" and have no influence

on the model

data.

Most of the studies of equivalence mentioned in the Resolution and Equivalence section are limited to a restricted class of models and configurations. In the general case, the best insight into equivalence and resolution is obtained from the correlation and parameter statisticsobtained from computer inversion techniques. Use of the generalized derivative or the Frechet kernel, which analyzes the sensitivity of the measurement to small changes in the resistivity or thickness of the layer, is described in Gomez-Trevino and Edwards (1983), Edwards (1988), and GomezTrevino (1987). Analytic solutions for the Frechet kernel have been derived by Parker (1977) and Chave (1984) for 1-D EM induction, by Oldenburg (1979) for MT, and by Gomez-Trevino and Edwards (1983) for the HED-VMD system used in the pseudorandomEM system described in Appendix B. Gomez-Trevino


368


(1987) computesthe Frechet derivativesfor a uniform half-spaceto a plane-wavesourcein both the time and frequency domain. Correlations between parameterscan be assessed from the statistics obtained from generalized inverse theory, as describedin Vozoff and Jupp (1977), Glenn and Ward (1976), Gomez-Trevino and Edwards (1983) and Raiche et al. (1985), and mentioned earlier in the section.

Backus-Gilbert

Method.--Another

method of auto-

matic inversion being applied to EM soundingdata is based on the method given in Backus and Gilbert (1967, 1968, 1970). The basis of this method is that within the measurement error bounds there exists a finite number of models which fit the data. The

Backus-Gilberttechniqueaverages,or smoothes,conductivity at different depths to give the sameconduc-

F tl'

I I,•0.

It#.

134•.

tivity distribution for these models. Backus-Gilbert averageconductivitieswill often provide a valid characterization of the true ground conductivity over a depth range governed by survey parameters. Central to the Backus-Gilbert approach is the distinction between construction of particular models which fit certain data, and appraisal of one or more suchmodels in an attempt to identify features common to all models satisfyingthe data. For example, at any depth of interestthe user is free to choosean averaging function with large half-width in order to obtain a highly accurate average, or to accept an unreliable averagecorrespondingto a narrow averagingfunction if resolution is paramount. The method has been applied to interpreting coincident loop TEM data as reported in Fullagar (1983), from which the following case history is drawn. Figure 43 shows an example of the construction

1400.

,. ,s

e.

(b)

• ,Ii• (o) ST•TIN•

g io.to

_•.•

__

I TER =I 0

e. me_

)• O.el

e.le_

I

e.e%

[

•.•o

--

I '

.I me4'

-

i ß.

11oo.

.

l l•j'

134)0.

14QO.

1•4•.

13,o0.

1300.

1 4oo.

1see.

_0.$s

0.•5_

(c)

I e'

Io.

. i tee-

! ! oo.

I ttoo.

i )e,o.

i,ee.

I•oo.

I)oo .

14oo.

lice. I

_0.32,

(d)

g1NALHODEL

,.e.•e

i

I 'co'

I I•[•'i DEPTH

I Illf.

e.•e.

ITER-19

c0d•l)ucl IV I I!

e.,e.

ß' 1 IIe'

O.OS

e.es.

1l"e'

I 4ee'

I lee'

(M)

Iie'

I 'co'

ß I lee' DEPTH

I n'L

14ee'

(M)

Fig. 43. Model construction for theoreticaldata(a) Half-spacestartingmodel.(b) Estimateafter 10iterations.(c) Final constructedmodel,satisfyinga chi-squaredacceptabilitycriterion.(d) True conductivityfrom whichthe data were generated(Figures 43-46 from Fullagar, 1983).


phasein which a model satisfyingthe data is determined iteratively by successiveperturbation of a startingmodel. The rangeof resolution-accuracy com-

-.

11.0

I I .5

i 2.0

I 2.S

I 3.0

I 3.S

14.0

ß

0.0__

binations available at any depth is conveniently summarized in a trade-off diagram (Figure 44). Averaging functions for different depths are then calculated, consideringthe maximum tolerable level of uncer-

400


.__-0.S

._-I .0

25

._-• .S

369

....

tainty, as shownin Figure 45. Average conductivity values computedfor a particular constructedmodel will usually be in very good agreementwith the true average,as seenin Figure46. An exampleof applying

sd = 0.07. S/m

._-2.0

._-2.5

the method to horizontal loop FEM data is given by Fullagar and Oldenburg (1984).

_-3.0

_-3.5

Fig. 44. Trade-offcurvesat depths25, 100, 200, and 400 m _14-0

11.0

I •.s

i 2.o

I 2.s

! 3.o

I 3.s

for the constructed model in Figure 43c. SD denotes the standarddeviation of a BG average conductivity (in S/m), while SPREAD is a measure of the width (in meters) of the correspondingaveragingfunction.

•4.0

LOG l 0 (SPREAD)

_

,

Ii.

-

0.0•

I•0.

,.o

ZO - 25 N SO

IT# '

IIii.

ZO - I O0 In

%D -

S/l'l

0-03'

0.,

e. t

ß .m

'-t

_e.e

2e.,

i o-

it#-

im.

i j-

I,•.

,

It.

IT#.

fill.

' i'll '

lial.

'

I In'

•

I e'

I T#'

,u

'

ii

Im'

i n'

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,

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.

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'" ZO - 400 tl

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so,e.el st./

0.,

.l

.-I,I

-o. IL

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i e.

,

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in. DEPTH

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i •.

in.

i

DEPTH

Fig. 45. Averaging functions for reference depths (a)25m, (b) 100m, (c) 200m, (d)400m. As seenin Figure44

the standarddeviationof theconductivity averages associated withtheseaveraging functionsis relativelyhigh(0.02

S/m),sothefunctions depicted provideanindication of themaximum degreeof resolution supported by thedata.

370



Joint lnversion.--Sets

of data obtained with dif-

ferent sounding methods can be jointly inverted to obtain a superior solution. Parameters which are not resolved with one method may be resolvablewhen two methodsare employed. For example, Vozoff and Jupp (1975) describe a joint inversion of MT and dc resistivity sounding data that successfully resolved the resistivity and thicknessof a buried thin layer, which was not possible with each method used separately. Gomez-Trevino and Edwards (1983) invert pseudorandom EM and Schlumbergerresistivity soundingdata to better resolve a resistive layer. The joint inversion of coincident-loopTEM and Schlumbergerresistivity data is described in Raiche et al. (1985), who report superior resolution to H, K, A, and Q type sections. Effect of Other than 1-D Structures.--A necessary and usually sufficient condition for 1-D interpretation to be valid is that nearby soundingsproduce consistent models. Ideally, the changesin resistivity or thickness should vary gradually from site to site. When major structures or lateral variations are present, 1-D interpretations may still provide useful results, but the geophysicistshould have accessto 2-D or 3-D modeling data to interpret the anomaly patterns; a composite geological section reconstructed from individual soundings, at best, will not represent a true depth section and, at worst, may be very misleading. Lateral changes in conductivity can also be interpreted by computing the spatial spectra of the observed field at multiple receiver sites (Garg and Keller, 1986). If soundingsare not made in reasonableproximity, it may be necessary to try to identify distortions in sounding curves due to lateral variations. Sometimes this is easy because some diagnostic feature of the

sounding curve is beyond the range possible for a layered earth. For example, it is quite common for the amplitude of the mutual coupling measured with a horizontal coplanar loop configuration to exceed that possible for any layered earth model (Frischknecht and Raab, 1984). Similarly, ascending or descending branches of sounding curves may rise or fall more steeply than is possible for a layered earth. In the example shown in Figure 47, the last part of the central-loop TEM curve (Anderson et al., 1983a) rises faster than is possiblefor any layered earth. This is a common type of distortion seen in soundings that reach a high resistivity basement and could possibly represent a nearby lateral variation in the conductivity-thickness of the conductive layers, or an induced polarization response(see the Complex Conductivity section).

The use of more than one componentis very helpful in recognizing that there must be distortions in the curves. As an example, Figures 48-50 show separate interpretationsof the vertical and horizontal magnetic fields and the parallel electric field measured at the same location usinga finite groundedwire as a source. Although a good fit to both the amplitude and phase curves for all three fields was obtained, the three interpretations are very different. To verify that the solutionsare not equivalent, the parametersfrom the

ß- ßß

observed

predicted

•

lO2

z

ß

.-•

4

.

o.•o Iø'

o. to_

o.lo

o.lo.

• 101 ......10-3 10-4

lO-2

10-1

lO0

TIME (s)

modeled

o. ts_

103 •

.oo

ß

•o.

I •o4.

I •oo.

I 3.4.

1400.

•

102

m

101

_

I soo.

ZO

100 100

Fig. 46. Comparison of average conductivities computed for the true conductivity (Figure 43d) and for the constructed model (Figure 43c) using the same averagingfunctions. The averagesfrom the inverted data differ from the true averages by less than one standarddeviation everywhere.

.

j 200

.

j 300

, ! , ii , ! 400

500

600

.

! i i 700

800

, I 900

,

I 1000

, 1100

LAYER DEPTH (m)

Fig. 47. Example of distorted TEM curve obtained over a 3-D earth. The late-time part of the curve rises faster than allowed by any layered model (after Anderson et al., 1983a).



371

interpretation of the vertical magnetic field were used

ments of the horizontal

to calculate forward curves for electric field; there are

made to qualitatively identify the effects of lateral changes in resistivity on central loop measurements (Anderson et al., 1983a). The TEM tipper, defined as the ratio of horizontal-to-vertical response (equations 56, 57), is a useful quantity for evaluating these effects. We use as an example the 3-D TEM model study in Newman et al. (1987) shown in Figure 52. In this case

large discrepanciesbetween the forward curves and the observed data (Figure 51). The site is near the edge of an alluvium filled valley underlain at shallow depth by normally faulted volcanic, shale, and carbonate rocks. Although each data set can be fit with layeredearth models, the distortions causedby the 3-D structures are different for the two magnetic field components and the electric

field.

When soundingsare not closely spaced and when only one component is measured there is a high probability of not recognizing moderate distortions due to 2-D or 3-D effectsand the interpreted one-layer models may be misleading. Frischknecht and Raab (1984) and Spies and Parker (1984) suggestthat the central-loop configurationis probably more immune to the effects of lateral variations in resistivity than other commonly used configurations. Sometimes measure-

the model is rather extreme:

shoots and undershoots of the voltage measurements.

10-1 -

10

-

20

752fl-rn • 12

.7

11

101

slab embedded

data, which do not exhibit the characteristic over-

i

0

i

.8

a 10 l/.m

field are

in a 300 l/.m half-space and covered by a 1000 overburden. The TEM tippers vary from zero over the center of the body, and away from it, to around 0.5 near the edge of the body. Apparent resistivity curves and 1-D interpretations for magnetic-field measurementsare shown in Figure 53, and for voltage measurements in Figure 54. The sounding curves are smoother for the magnetic-field

Hz .9

as well as the vertical

i

i

i

i i •1 I

I

i

I I r ill I

,

I

[ [ ] [II

65

3050 f'/.m ; 33m - 60 33

171 _ 55

5.1

240

225

.6

-30•

14

- 50

25

I•1

.5

- 40 (.•

•

.4

50

w r'•

,•'•'•

I-

ß.• lO-2

•,•

ampiit ude•,.j,.,,• •

n'

-40

g

- 35

-60•:

•: .3

- 45

amplitu(

.2

30

7O

.1

-

.0

-90

-,1

25

80

20

10-3

-100

102

101

FREQUENCY

101

103

I

I I I I I I I[

•

102

• i • I i • ii

Fig. 48. EM sounding at Coal Valley, Nevada, using a grounded wire source. Comparison of observed data and best results from inversion for vertical magnetic field. Dis-

•

103

FREQUENCY

(Hz)

I

i

i

I

I

I

•

15

104

(Hz)

Fig. 50. Comparisonof observeddata and best resultsfrom inversionfor equatorialelectric field for samearea as Figure 48.

tance from wire is 975 m.

lO1

Hy i

'

'

'

'

' ' ' I

i

,

,

[

, i i i

10-1

15 -

I i ]] I

I

I

[

97200t3•..__.•.•t•.•

•',•

44

314

1.2

8

35

-

101

•

102 FREQUENCY

I

65

[

60

55

50

phase

w r'•

W

I--

45

10-2

-354

•-20 -25

'itude

\•, - ß

-30

10-3

103 (Hz)

Fig. 49. Comparisonof observeddata and best resultsfrom inversion for horizontal magnetic field for same area as Figure 48.

n-

-4o•

- -15

phase••--• • 10-1

I

- 0

mplitude

•,•.

I

• FORWARD MODELFROMHZ ==--FORWARD MODELFROMHZ

5

-5• .• lO0

I [ I if I

ß MEASUREDEx VALUES ß MEASUREDEX VALUES

10

101

• •11

I

I

I

I IIIll

102

I

103 FREQUENCY

I

I

I IIII

-

30

-

25

-

20

15

104

(Hz)

Fig. 51. Comparison of observed data and forward model for electric field using model from inversion of vertical magneticfield data for same area as Figures 48-50.

372


CROSS 1000

Qom


600

I 1o•'m

1200

DEPTH

In the inversion the resistivity of the top and bottom layers were held fixed to speed convergence, and it was necessaryto truncate the voltage data after 20 ms to improve the layered-earth fit. The depth and the resistivity of the conductive slab is quite well-resolved. The base of the slab is poorly resolved, and a transition layer is included in the interpretation. For other models in this study Newman et al. found that it was sometimesnecessaryto include an extra layer to accommodate the overshoot or undershoot present in voltage data, and that magnetic-field measurements often resulted in improved interpretation. The greatest distortions in the 1-D interpretation occur near the edge of the body, where the TEM tipper values are greatest. We have found that relatively reliable 1-D interpretations are obtained if the tipper values are less than about 0.1. Of course, low tipper

SECTION

300 •*m 1800

(m)

2400 3000

'

i

!

I

i

i

PLAN

i

I

VIEW

3000 2400

1800 1200

600

PROFILE I

Y(m)

-I

I

i I

-600

I o

i iaaa I

L .....

at the center of a 2-D or

3-D structure, as shown in this example. As mentioned above, a 2-D or 3-D structure will

i

I

-1200

values can also be obtained

I 2000 m

.J

result in overshoots

or undershoots

which

cannot be

-1800

104 -2400

-3000

L

I

I

I

-2400

I

!

-1200

I

I

0

I

i

1200

2400 Layer

102

X(m)

•

52 (a)

P(.•, m) H(m) D(m)

'1ooo538 538 566 1104

68 27665 '300

3 4

1964

3068

2 3

13 11971

4

'300

147 3886

663 4549

,%h(,•-m) I

!

i

,

lO3

30 ms

6

4

lO2

ß 1000 569 569 158 430

2

TEM

10'2

o

VOLTAGE TIPPER

774 1886

1343 3229

e300

10-1

100

101

102

10-2

10-1

'-parameter

100

101

held f,xed

102

103

TI ME (ms) -2

Fig. 53. Apparent resistivity curves and 1-D interpretation for in-loop (magnetic field) TEM soundingsover the model shown in Figure 52 (Figures 53, 54 after Newman et al.,

4

6

-3000

-2000

-1000

0

1000

2000

3000

1987). lO4

10 ms 4

10 3• TEM

MAGNETIC

0

FIELD

TIPPER

-2

•o•1I -6

-3000

_•

/ L,_oyer PJ•-m)"(m)D(m)

-4

i

-2000

i

-1000

i

0

i0

10 0

I

2000

3000

I

lO1 i

]0-2

52 (b) Fig. 52. (a) 3-D model used by Newman et al. (1987); (b) Profile of TEM in-loop tipper values for voltage and magnetic field measurements.

]

*]000

2

493

493

14

*-'parameter held f,xed

i

]0-]

•

'""'0, 0+ +=3-D observatton not used

i

]00

I

]0]

I

]02

}03

T• ME (ms)

Fig. 54. Apparent resistivity curves and 1-D interpretation for in-loop (voltage) TEM soundings at station 0 over the model shown in Figure 53.



fitted with a 1-D interpretation. This is illustrated by the central-loop TEM field example shown in Figure 55. The inversion program initially fit the data quite well with a highly conductive lower layer (Model A). However, if data for later times had been available,

this model would not have provided an acceptablefit. Because this model was geologically implausible, a more probable solution (Model B) was found by fixing the resistivity of the bottom layer. A completely satisfactory solution was not found, indicating severe

distortion

373

due to 2-D or 3-D structures.

The data were

then deliberately truncated to include only 11 points and inverted to obtain Model C, which is probably the best solution. Consideringthe first two layers in model B as one layer, the average resistivity of the first layer and the depth to the conductive layers are about the same for all three models. From rough lithologic logs in a nearby borehole the first layer (Model C) is conglomerate and sandstone, the conductive layers are mostly shale, and the bottom layer is dolomite with some shale.

102

,

,

MODEL

[

i

!

!

A

33•.m ; 241m 2.6

67

48

190

"static shift"). For this reason, it is often useful to

.029

z

101

i

10-3

i

I

I

I

TIME

i

10-1

10-2

The effect of a 2-D or 3-D structure, particularly on measurementsmade at large spacings,may be to cause an overall shift in the amplituderather than a changein the shapeof the curve (this is sometimesreferred to as

(s)

consider the amplitude curve to be multiplied by an arbitrary constant which is determined as one of the parameters in the inversion process (e.g. Newman, 1989). This procedure is also useful if an accurate absolute calibration is not available or if the configuration of the sourceor the source-receiverseparationare not accurately known. One technique which can be used to calculate

55 (a) lO2

'

'

'

'

'

'

'

'

I

MODEL

B

25C/ore; 95• 1430

144

2.0

the calibration

factor

involves

ex-

trapolation of the recorded data back in time to t = 0. The magneticfield at t = 0, or equivalently the integral of the time-derivative (voltage) responsefrom t = 0 to t = o,, should be equal to the steady state (primary) field, and for an ungroundedsourceis independentof the conductivity distribution of the earth.

37

10 000

Depth Imaging lO1

,

i

i

i

i

i

i

lO-3

i i I

10-1

10-2 TIME (s)

55 (b)

102 MODEL

C

35•.• 7.5

6.2

4.1

49

260

Low-cost techniques developed over the past few years provide an approximateimage of the resistivity section directly from observed data. These results are very useful for rapid initial analysis of the data, and also provide a starting model for more computerintensive iterative computer inversion schemes.These methods, althoughoriginally developed as a "processing" step, now appear very useful for interpretation. These techniques may be less biased by 3-D effects than are conventional 1-D inversion which parameterize the earth into a finite number of layers (Eaton and Hohmann, 1989).

101 10-3

i

i

i

i

i i I

10-2 TIME

0-1

(s)

55 (c) Fig. 55. 1-D inversions of a central-loop TEM data set from Midmont, resulting in three different interpretations. Loop size is 152 m.

Approximate interpretation schemeswere first developed for MT data (Bostick, 1977; Niblett and Sayn-Wittgenstein, 1960). These methods, based on asymptotic expressionsfor a two-layer earth, were later applied to FEM soundingdata for the Maxiprobe (Appendix K) and other frequency-domain systems (Mundry and Blohm, 1986; Wilt and Stark, 1982). Extensions of the concept to TEM data followed the concept of diffusing eddy currents or "smoke rings" described in Nabighian (1979). Macnae and Lamon-


374


tagne (1987) describe a technique, "conductivity imaging", whereby data obtained at different receiver stationsfrom a fixed loop is transformedto a distribution of 14 images centered at an equivalent depth at each sample time. The slownessof diffusion of this reference depth h, is directly proportional to a slightly smoothed

version of the cumulative

conductance

mea-

sured from the surface down to the reference depth. The imaged conductivity is obtained by differentiating the slowness with respect to depth. Comparisons of imaged conductivity for a number of three-layer models are shown in Figure 56. The imaged conductivity is a smoothly varying curve which approximatesthe true earth section. The smoothingis more pronouncedfor those caseswhere poor resolutionis expected, namely for a resistive layer. An example of depth image processingapplied to a 15 km-long profile of UTEM data in a sedimentary basin in Canada is shown in Figure 57. The data are displayed on a logarithmic depth scale to match the resolution of EM data with depth. The large dark blue zone is a resistive carbonate unit, the yellow zones are shalesand the dark red zone is thought to be brine saltwater porous sediments. The high resistivity surface layer (blue) is partially melted permafrost. This technique is described more fully in Appendix D. Similar schemes employing a single image of fixed size are described in Eaton and Hohmann (1989) and Nekut (1987). Their algorithms are simpler to implement and can be used for single receiver measure-

ments, but are not as accurate as the multi-image approach. More rigorous but computationally-intensiveimaging schemesfor 2-D and 3-D structuresare now being developed. James(1988) in a "source-oriented modeling" approach, describes a scheme for modeling the observed TEM components to determine the actual distribution

of currents in the earth. This scheme is at

present limited to the 2-D TEM case and has not yet been applied to field data. Lamontagne et al. (1987) describea related approach, "matched filter processing", which is based on transforming the measured data to conductivity cells within the earth. Although very computer intensive and still in the development stage, these imaging schemes are probably the first step toward the elusive goal of completely automatic interpretationof EM data in terms of 3-D conductivity structure.

Another area of research being actively pursued is EM migration. This technique determines the conductivity distribution of the earth by a downward-continuation

of the EM

field observed

at the surface.

The

downward-continuation is achieved in an analogous manner to seismic migration; in one technique the observation points are treated as sources, and the observeddata, reversed in time, are employed as the source functions. Forward modeling with the above conditions then yields an image of the conductivity variations within the earth. Studies to date are mainly theoretical and based on plane-wave sourcesand 2-D models (Zhdanov and Frenkel, 1982, 1983a, b; Lee et al., 1987), and have also been applied to deep crustal controlled-sourcesoundings(Velikhov et al., 1987).

102

PUBLISHED

A number 0

101

Z

I

10o

101

10o

lOo

102

........

i

........

lO 1

i

lO2

.....

lO3

DEPTH (h)

DEPTH (h)

102

102

•__101

101

O

100

10o .

z

10-1

,,• lO-1

100

101

102

DEPTH (h)

103

100

101

102

CASE

of case histories

HISTORIES

are referenced

in this

section, and several more are given in the Appendices. Numerous examples of the use of EM sounding for various applications have been published in recent years, and a representative sample are listed. Some published papers apply to mining applications (e.g., Duckworth, 1970); however, the majority show a strong correlation to the amount of government funding available for research projects in various disciplines. A recent three-volume publication on geotechnical and environmental geophysics (Ward, 1990) containsmany case histories, EM soundingapplied to engineering, groundwater and other technical problems, and the reader is referred to these for further examples.

DEPTH (h)

Deep Crustal Sounding Fig. 56. Comparison of imaged conductivity (dashedlines) with actual conductivity for a number of three-layer models (from Macnae and Lamontagne, 1987).

Lieneft and Bennet (1977) and Lieneft (1979) describe an EM sounding in the Western Basin and



Range province using a 1400 km groundedpower line as a source. A large grounded-sourceTEM sounding system designed to study the deep crust in southern Wisconsin, using both E and H receivers is described in Sternberg(1979); an analysisof the possibilityof IP effects in the data was made in Johnston(1975). Deep EM soundings in the eastern United States, using a 4.5 km diameter loop are describedin Connerneyet al. (1980). Heacock (1971) and Ward (1983) review a

number of deep EM soundingexperimentsfor probing the deep crust. Sinha (1983) describesa deep multifrequency soundingnear Bowmanville, Ontario. The use of a ultra-high-power magnetohydrodynamic(MHD) generator for deep crustal soundingin the USSR and Finland is described in Heikka et al. (1984), Halverson et al. (1987), and Velikhov et al. (1987). A recent MHD soundingexperiment in California is reported in The Leading Edge (1987) and is describedin more detail in Zollinger et al. (1987). Strack et al. (1990) apply a medium power (60-100 kVA) system to deep crustal studiesin West Germany.

375

Geothermal

Groundeddipole and loop-sourceTEM soundingsat Kilauea volcano, Hawaii, are described in Jackson and

Keller (1972) and Keller and Rapolla (1976). Other EM and dc soundingexperiments carried out at Kilauea are reported in Smith et al. (1977) and Kauahikaua (1982). Grounded-source TEM surveys over geothermal prospectsin Nevada are describedin Keller et al. (1978) and surveys in Washington and California are reported in Keller et al. (1984). A number of geothermal sounding surveys using the EM-60 system (see Appendix C) are describedin Wilt et al. (1979, 1980a, b, c, 1982, 1983). Tripp et al. (1978) report results of FEM and Schlumbergersoundingsurveys carried out at Roosevelt Hot Springs,Nevada. Reports of the use of FEM soundingsfor geothermalexploration at Bonneville, Utah, are given in Ward et al. (1976) and Ryu et al. (1972) reports results from the Santa Clara Valley, California. Results for both FEM and TEM soundingsof Medicine Lake California are described

_

20-

50100-

200-

500' 1000 -o

2000-

-lO -20

,

lOO

,

lO

RESISTIVITY (OHM-M)

Fig. 57. Color plot of imaged conductivity sectionfrom southernAlberta. The 15 km-long profile was surveyed with contiguous400-m transmitter loops. Individual profiles from each loop consisted of 30 stations at 40 m intervals. The color scaleis logarithmic,and rangesfrom 1 f•.m to 1000f•.m (courtesyLamontagneGeophysics, Ltd).


376


in Anderson et al. (1983a, b). Detailed TEM soundings of the Newberry Volcano in Oregon are described in Fitterman (1983), Fitterman and Neev (1985) and Fitterman et al. (1985), and are summarized and compared with MT and dc resistivity soundingsin Fitterman et al. (1988). Grounded-source TEM soundingsin the Urach Geothermal area in Germany are described by Strack et al. (1990). Groundwater

Koefoed and Biewinga (1976) describe FEM sounding for groundwater applications and evaluate various coil configurations. Palacky et al. (1981) applied FEM sounding for groundwater prospecting in the Upper Volta. The use of in-loop TEM soundingsfor groundwater applications is reviewed in Fitterman and Stewart (1986) and Fitterman (1987) and examples of mapping saltwater intrusions into acquifers in Florida and Massachusetts are given in Stewart and Gay (1983), and Fitterman and Hoekstra (1984). Model studies and field results for the HCP, VCP, and VCA configurations used to map resistive fresh-water channels in Brazil are described in Verma and Bischoff (1989). Further applications of EM techniques for mapping salinity are given in Cameron et al. (1981) and Corwin and Rhoades (1982).

ing usedto delineate a granitic overthrust in Wyoming. Further examples of structural mapping are given in G6mez-Trevino and Edwards (1983) using a pseudorandom systemand in Wilt et al. (1989) for a frequency domain sounding (Appendix C) in Washington state. Stratigraphic mapping of a conductive brine-filled high-porosity horizon is described in Spies (1983), Wightman et al. (1983), Vozoff et al. (1987) and Nekut and Spies (1989). Further examples of stratigraphic mappingare given in Macnae and Lamontagne (1987), and porosity mapping by Strack et al. (1989b). General

Frischknecht and Raab (1984) describe a comparison of coincident loop and central loop TEM with FEM and dc resistivity sounding at the Nevada Test Site. FEM soundingsin Tunisia using horizontal coplanar and perpendicularloops are describedin Biewinga (1977). A comparison of FEM soundingwith dc resistivity and MT is given in Patrick and Bostick (1966), and a comparison of FEM soundings with Schlumberger soundings is given in Mundry and Blohm (1986). Theoretical studies of EM probing of the moon are reported in Hohmann and Ward (1968) and Inouye et al. (1969). DISCUSSION

Permafrost

Daniels et al. (1976), Scott and Mackay (1977), Hoekstra (1978), and Sinha and Stephens (1983) describe a number of electromagnetic soundingmethods suitable for permafrost delineation. Walker et al. (1987) describe central loop TEM surveys used to map permafrost in Northern Alaska, and Walker and Kawasaki (1988) describe distortion of the soundingsby possible induced polarization effects. Coal and Petroleum

Asten and Price (1987) and King (1987) show examples of TEM sounding over moderately conductive coal measures and highly conductive cindered coal in Australia. Wideband loop-loop soundings of underground coal-burn cavities using a pseudo-randomEM system (see Appendix 2) are described in Quincy and Moore (1976) and Quincy et al. (1980). Poddar and Dhanasekaran (1986) describe field and model results for central induction and loop-electric dipole FEM soundingfor delineation of lignite depositsin India. EM soundingmethods have been used in petroleum exploration for mapping both geologicalstructure and stratigraphy. Shallow structural mapping using TEM in the Canning Basin of Australia and comparisonwith seismic reflection is described in Nekut and Spies (1989). Nekut (1987) shows an example of TEM sound-

Although the first controlled-source EM soundings were made over 50 years ago, EM sounding as a disciplinehas matured relatively slow. Rapid advances have been made in the past several years in instrumentation and interpretation methods and new applications for EM soundings are being discovered. Currently available instrumentation, field procedures,and interpretation methods are suitable for the solution of many exploration and earth-science problems by EM sounding. Some of the advantagesand disadvantagesof various EM soundingconfigurationsand techniqueshave been discussedin this chapter but no attempt was made to rank the various

methods

in order of usefulness.

The

effectivenessof any of the techniquesdependson the geologicproblem, the environment (includinggeologic, topographic,and man-made features), the available instmmentationand interpretative methods,and the experience and skill of the interpreter. Caution should be exercisedin comparingcurrently availablemethods,and predictionof future developmentsis much more hazardous. Nevertheless,we will speculateon possiblefuture trends.

At the present time, short-offset and long-offset transient

methods and the CSAMT

method

seem to be

of greatest interest for intermediate and deep controlled-source sounding. The present trend is that,



with the possible exception of the CSAMT method, transient methods are displacing frequency-domain methods in popularity. A previous section showed that, from a theoretical standpoint, transient soundingsgenerally appear to provide better resolutionthan frequency soundings. There are also a number of practical advantages of transient methods, some of which stem from measurement of the secondary field duringtimes when the primary field is absent. Through use of the Fourier transform, time-domain and frequency-domain results are sometimes considered equivalent. However, to successfullytransform data from one domain to the other the original data must be accurate and must extend over a very wide time or frequency range. Thus, although computer-generated results

can be transformed

from

one domain

to the

other, the results may be better understood and interpreted in one form than the other. In practice, timedomain and frequency-domain field data are not necessarily equivalent, due to limitations in accuracy and bandwidth

in the measurements.

Because of the ever-

increasing number of man-made cultural EM noise sourcesthere possibly may be greater use of frequency domain methods using very narrow-band signal processing to attain high signal-to-noiseratios. Conceivably, measurements might be made in the frequency domain and then transformed and interpreted in the time

domain

or a combination

of time-domain

and

frequency-domain methods may be used in a single sounding. Also, new methods for analysis and interpretation of frequency-domain data might possibly lead to greater use of data in this form. The merits of long-offset and short-offset time domain techniques are sometimes argued. Likely each will continue to be used. Long-offset measurements which permit making many soundingsfrom one source are generally less expensive than short-offsetmeasurements which require relocation of the sourcefor each sounding,and are generally more suitable for probing to great depths. On the other hand, short-offset measurementsare less likely to be distorted by other than 1-D effects.

Much of the cost of deep soundings,whether longoffset or short-offset, is in the deployment of the sourceloop or groundedwire. To increase the amount of information obtained from each position of the source the trend toward

broadband

measurements

of

more than one component at multiple receiver sites will probably continue, possibly combined with remote-reference

or local reference

noise cancellation.

Very likely the variety of configurations used for soundingto shallow and moderate depthswill increase both for theoretical and practical reasons.An example may be use of the in-line electric dipole•horizontal-

377

axis loop configuration along roads or trails where problems of accesspreclude use of large loop sources or equatorial configurations. The need for shallow soundingsin engineering and ground water applications is likely to increase. This need may lead to development of more versatile instruments for small induction number geometric soundings. In particular, there is a need for instruments that can make more measurements

over a wider

range of spacing.The need for shallow soundingmay also lead to the development of frequency-domain instruments which can operate at frequencies up to several hundred kHz or time domain impulsive source instrumentsthat can operate at equivalent short times. Such instruments offer the possibility for determination of dielectric constant as well as resistivity. Researchwill be required to determine how well the two parameters can be determinedjointly. Combined use of low induction-number geometric soundings and frequency or transient soundingsmay be one solution of this problem. In some applications determination of the anisotropy may be useful. Joint use of dc resistivity and loop-loop or wire-loop methodsis one means of determining the anisotropy, although combination of wirewire and wire-loop EM soundings may be a more effective method. Controlled-source sounding techniquesare being usedmore widely in the interpretation of data determined with other methods. An example is the use of in-loop TEM soundingresults in correcting for static shifts in MT data caused by near-surface inhomogeneities. The improvement of instrumentation and field techniques is challenging. Advances in interpretation methodsare probably much more ditficult to make but are of even greater importance. Rapid and economical methods for 2-D and 3-D modeling are needed to obtain a better understanding of the distortion of curves and ultimately could be used in quantitative interpretation. At this time it appearsthat methodsfor rapid, approximate, direct inversionof data may partly replace existing computer curve-fitting techniques. Continued research is needed to develop improved transformations for direct determination of resistivity as a function of depth. The need for such techniques may increase as soundingtechniques are applied to problems where a layered-earth model is a poor representation of the actual earth. As noted at the beginning of the chapter, the difference between EM sounding and broadband EM profiling is likely to become less distinct. However, very large strides in interpretation will have to be made before generalized quantitative 3-D interpretation can be carried out.

378



ACKNOWLEDGMENTS

Many people assisted with the preparation of this chapter and its appendices. A special thanks goes to Walt Anderson, who computedmost of the 1-D model curves using his plethora of programs. Greg Newman and Peter Weidelt provided the 3-D programs.Thanks also to Bryan James, Vic Labson, Jim Kauahikaua, Gary Olhoeft, and Ben Sternberg for their helpful reviews, to Pam Ketterer for her expert typing, and to Misac Nabighian for his endlessencouragement.

Bhattacharyya, B. K., 1955, Electromagnetic induction in a two-layer earth: J. Geophys. Res., 60, 279-288. 1956, Fields on earth's surface due to a transient disturbance: J. Technol., 1, 151-161. 1957a, Propagation of transient electromagnetic waves in a medium of finite conductivity: Geophysics, 22, 75-88.

1957b, Propagation of an electric pulse through a homogeneousand isotropic medium: Geophysics,22, 905921.

1959, Electromagnetic fields of a transient magnetic dipole on the earth's surface: Geophysics, 24, 89-108. 1964, Electromagneticfields of a small loop antenna on the surface of a polarizable medium: Geophysics, 29, 814-831.

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385

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Publ.

Co.

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1330.

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386


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APPENDIX

UNIVERSITY

OF UTAH

1983b, Electromagnetic migration, in S. E. Hjelt (Ed.), The developmentof the deep geoelectricmodel of the Baltic Shield (Part 2), Univ. of Oula, Finland, 37-58. Zollinger, R., Morrison, H. F., Maxwell, C., Demetriades, S. T., and Jackson, W. D., 1987, Application of an MHD generator to an electromagnetic sounding:25th Symposium Engineering Aspects of Magnetohydrodynamics (SEAM), Bethesda.

A

14-FREQUENCY

EM SYSTEM

Stanley H. Ward*

The University of Utah 14-frequency electromagnetic system was developed as a research tool at the University during 1972 through 1978 under a grant from the National Science Foundation. The system is designed to be broadband, multi-component, and capable of continuous sounding-profiling. Descriptions of the system are given in Ward et al. (1974) and Ward et al. (1976), and the following summary is drawn from these sources.

The system measures tilt angle and ellipticity in the

frequency range 10.5 Hz to 86,016 Hz in incrementsof two. The transmitter

coil is 9.4 m diameter

and can be

operated in either horizontal, vertical fixed, or vertical rotating mode. The receiver consistsof two orthogonal sensorcoils employing a phase-sensitivedetector. The output of the receiver is fed to three metered outputs;

the voltage of the reference coil (e•), the in-phase

signal coilvoltage (eJP)andthequadrature signal coil voltage (esQ). In the horizontal-coil configuration, the tilt angle

*Professor of Geophysics, University of Utah, Salt Lake City, Utah 84112.



387

'

.•o

90

-

•

-.io

8O -

-

•

-.o

.05

70

0 I

6o -

- o5

uJ 5o -io

2O

z

,•

40

• 3o {•

io

•

o

2o

FREQU,ENCY-Hz -IO ioI

Fig. A-1. VMD electromagnetic sounding at Delta Bonneville

site.

Circled

dots are observations

while

........

1 iot

solid and

........

io3

io4

io5

FREQUENCY-Hz

dashed curves are inverse fits to three-layer and four-layer models,respectively. • is the standarddeviationbetweenthe model and observed data (Figures AI-1 to A1-3 from Ward et al., 1976).

Fig. A-2. HMD electromagnetic sounding at Delta Bonneville site. Circled

dots are observations

while solid curve is

inverse fit to either a three-layer or a four-layer model.

RESISTIVITY ,Q,- M o

io

L)O

o 30

IO

l

2

2o

!

o) VES NS

b) VES

EW

3C) 0

o 3O ß

i c) HMD NS

Io

2o

t

i

,

d) VMO NS

•o o

2o

f) VMO NS

1 Fig. A-3. Resistivitymodelsobtainedwith Schlumbergerresistivity(VES) and electromagneticsoundings.


388


and ellipticity are measured in the vertical plane passing through the axis of a horizontal transmitting coil. In the vertical-axis configuration, tilt angle and ellipticity are measured in the vertical plane which passesthrough the axis of a vertical transmittingcoil. Finally, in the vertical rotating coil configuration, the transmitting coil is located along strike from the transverse and rotated to contain each point of observation in turn; measurements of tilt angle and ellipticity are made in a vertical plane normal to the plane of the transmitting coil. The time taken per soundingis about 1 hour.

The maximum transmitter-receiver separation is 1500 m with the horizontal coil configuration and 600 m with the vertical coil. The system is carefully calibrated by a procedure involving additional measurements with the coils rotated 90 degrees; this allows corrections to be made for mismatch of amplifier gain and the Q of the coils. Cross-talk and levelling errors are eliminated by repeat measurementswith the reference coil turned 180 degrees. For separations of less than 500 m (vertical coil), 400 m (vertical rotating) and 1100 m (horizontal coil), levels of precision estimated by the repeatability of measurements are 0.1 percent in ellipticity and 0.1 percent in tilt angle. Absolute accuracy is estimated to be about 1 percent in ellipticity and 0.24ø in tilt angle.

APPENDIX

PSEUDO-RANDOM

BINARY

The major source of noise at frequenciesless than 336 Hz is mainly microphonics; at higher frequencies the level of spherics noise increases, reaching the same level as microphonics at 5400 Hz. Soundingswith both the horizontal magnetic dipole (HMD) and vertical magnetic dipole (VMD) source were conducted

over unconsolidated

Lake Bonneville

sedimentsnear Delta, Utah. The data were interpreted usinga generalizedlinear inversion scheme(Glenn and Ward, 1976) to three- or four-layer sections. Typical soundingcurves for 61 m transmitter-receiver separation are shown in Figures A-1 and A-2, and a comparison of sounding techniques in Figure A-3. Detailed discussions on statistical

evaluation

of the inversion

data are given in Ward et al. (1976), where the importance of percent parameter standard deviations, parameter correlations, and least-square residuals are discussed.As expected, there are strong correlations between many of the parameters, e.g., the product of resistivity and thickness of the first layer, and the product of conductivity and thickness of the second layer. REFERENCES

See main reference list, page 378.

B

SEQUENCE (PRBS) TECHNIQUES

R. N. Edwards* and J. S. Holladay*

INTRODUCTION

Electromagnetic controlled-source systems for determining the electrical conductivity of the Earth are configuredin many different ways. One variable is the waveform of the transmitted signal. The optimum shape of this waveform dependson the componentof the EM field that is measured, the nature of the background noise, the characteristics of the transmitter and receiver instrumentation, and the frequency bandwidth required for the geological application at hand. The optimum experiment can sometimes be

shown to be the measurement of the response of the earth to an impulse in a component of the surface EM field, becauseinterpretation is simplified. An approximation to a true impulse•a high amplitude event lasting for a relatively brief interval of time--can be created using a magnetohydrodynamic source such as the Khibiny generator described by V. I. Pavlovsky and A. A. Zhamaletdinov (Heikka, 1983). Alternatively, a rather specialrepetitive sequenceof constantamplitude pulses having pseudo-random distributed widths•a PRBS•can be transmitted. An impulse may be synthesized from a PRBS signal in lag (time)

*Department of Physics, University of Toronto, Toronto, Ontario, M5S 1A7.



389

domain by forming its autocorrelogram,in much the same way as a short source wavelet may be synthesized from the Vibroseis repetitive swept-frequency

ple shown, n is 5 and the PRBS repeatsperiodically

controlled seismic source. The creation of an impulse in this manner is both elegant and cost effective. A switched waveform of constant power and relatively

computed in the manner shown in Figure B-2. The sequenceis shifteda time 'r next to an exact copy of the sequence. Suppose values of +1 and -1 are assignedto the areas of the smallestblack and white (hollow) rectangles, respectively. The two sequences may then be multipliedand the productsaddedto give the estimate of the autocorrelationat lag 'r. If the shift, or lag, is greater than 2/fc, the productsover a sequencelength add to -1 unit. At lag 'r equal to 0, the sum is 2 n - 1 units, 31 in this example. The final autocorrelation function, shown in Figure B-3, is a comb of trianglesseparatedin time by (2n - 1)/fc. The power spectrum of the PRBS, the Fourier

low amplitudecanbe generatedin a controlled,repeatable manner.

The receiver

instrumentation

can be

designedwith a prescribeddynamicrange, and crosscorrelation and signalaveragingtechniquesemployed to enhancethe signalin the presenceof uncorrelated noise. The scheme is obviously less hazardousto the operatorand the environmentthan the transmissionof the equivalent high power transient. The PRBS sounding technique has been studied continually at the University of Toronto for more than ten years (Duncan et al., 1980; Gomez-Trevino and Edwards, 1983; Boerner, 1987; Holladay, 1987). We

outline the philosophyof the method. We describein part the specializedapparatuswhich has been developed, and include some examplesof interpretedfield data.

THE PSEUDO-RANDOM

BINARY SEQUENCE

An example of a switched waveform following a pseudo-randombinary sequenceis shown in Figure B-1 immediatelybelow a regular squarewaveform, the clock, used to create it. The PRBS is characterized by two freely adjustableparameters•fc, the clock frequency, and n the sequencelength parameter. The sequencerepeatsafter 2 n_1 clock pulses.In the exam-

after 31 clock pulses. The autocorrelation of a PRBS of finite length is

transform of the autocorrelation function, is shown in

Figure B-4. The line spectrum is equispacedin frequencyby fc/(2 n - 1). Its envelopeis a sinc squared function whose first zero is at the clock frequencyfc. If n is set to a very large number, the power spectrum of the resulting PRBS contains many more lines at lower frequenciesthan those shownin Figure B-4. The spectrum is essentiallyflat, and the autocorrelation function, the Fourier transform of the power spectrum, is essentially an impulse at zero lag. THEORY

The objective of a field survey is to obtain the impulseresponseR(t) of the earth and subsequentlyits Fourier transform R(f) which we shall call the transfer

UUUUUUUUUULIIIUUUUUUUUUUUUUUUUUU

Fig. B-1. An exampleof a pseudo-random waveformgeneratedfrom the clockwaveformshownaboveit. The waveformrepeatsafter2n _ 1 or 31 clockpulses.The arrowsindicatethelengthof oneperiod(afterDuncanet al., 1980).


390


f

PRODUCTS

f

I+1-1-1+1+1+1-I-I-I-I-I+1+1-1-1+1-1+1+1-I+1+1

+1+1-1-1+1-1-1-1

SUM = (15+)+(16-)= I-

PRODUCTS

I+1+ I+1+1+1+1 +1+1+1+1 +1+1 +1 +1 +1+1+1 +1+1+1+1+ I+1+1 +1 +1+1+1 +1+1 SUM = $1 +

Fig. B-2. The method of computationof the autocorrelationfunction of the pseudo-randomwaveform shown in Figure 1 (after Duncan et al., 1980).

40

30'

2O

LAG (s)

(2/fc) -I0

($1/fc)

•

-20

Fig. B-3. The autocorrelationfunction of the pseudo-randomwaveform shown in Figure 1 (after Duncan et al., 1980).


391

function. Field data are collectedas crosscorrelograms of a measuredcomponent of the EM field component with

the

transmitted

PRBS

current

waveform.

A

crosscorrelogram c measured at the remote site is


[.0-

Cj= EOili_j, 0 <--j -< M, i

0.5,

0.03

o.i

0.3

FREQUENCY (f/fc)

Fig. B-4. The power spectrumof the pseudo-randomwaveform shown in Figure B-1 and the Fourier transform of the autocorrelationfunction shown in Figure B-3. The frequency

axis is scaledby the frequencyfc of the clock (after Duncan

where O is the sampled component of the EM field as seenat the correlator input and I is the simultaneously sampled current waveform. The sum over the index i may extend to many thousand samples;the larger the number of samples,the greater the improvement in the recovered ratio of signal to uncorrelated noise. The number of lagsM and their location in time dependson the geologicalproblem at hand. The autocorrelogram a of the current waveform is similarly defined

aj= Elili_j, 0 <--j --
et al., 1980).

0.8

The relationship between the impulse response S and the auto and crosscorrelogramsa and c is

0.6

Cj= ESiai_j,0 • j <-M. i

0.4

AUTOCORRELOGRAM (INPUT)

0.2 "',, CROSSCORREtOGRAM (OUTPUT) \ o v

•--'•5

•

IMPULSE

....... 5o• RESPONSE

parametersfc and 2n - 1, and the total number of

lag (ms) -..

samples takenwere 1 ms, 493 Hz, 127,and 2]7,

FREQUENCY RESPONSE

_1oo r i.o 9-60 •o.• u.i-40 • ß-r

-20

+2• c

FREQUENCY

The matrix a is Toeplitz, and special techniques (Levin, 1960) can be employed to recover $ in a least-squaressense. A typical example of a set of correlogramsis illustrated in.Figure B-5a. The vertical magnetic field at a site 350 m distant from a grounded current bipole of moment 150Am was measured. The cross-correlogram is an asymmetric function of the lag x, but only the positive lag side of it needs to be measured and only this side is shown. The sampling interval, the PRBS

(Hz)

Fig. B-5. The data reduction procedure. (a) The autocorrelogram of the current waveform in the transmitter bipole (input) and the crosscorrelogramof the measured vertical magneticcomponentand the current waveform (output); (b) the deconvolved impulse response of the Earth; (c) the transfer function or Fourier transform of the impulse response(after Duncan et al., 1980).

respectively. The corresponding autocorrelogram of the current waveform in the bipole is also shown in Figure B-5a. It is a symmetric function of lag x but differs from the theoretical triangular shape described earlier because it includes the effects of band-pass and 60 Hz notch filters located at the inputs to the correlator. The filters distort the correlograms. However, because they are present in both channelsand are matched, their characteristics are not required a priori, neither are they ever computed. The result of the deconvolution of the correlograms shown in Figure B-5a is the impulse responseshown in Figure B-5b. It representsthe temporal variation of the vertical magnetic field at the remote site with time


392


immediately following an impulsive current in the bipole which may be thought of as lasting for the sampling interval, in this example 1 ms. The frequency response, the Fourier transform of the impulse response, is plotted in Figure B-5c. Characteristically, the amplitude gradually decreasesfrom unity (no secondary vertical field) as the frequency increases, while the phase decreasesfrom a value of zero degrees as quadrature secondary fields are observed which lag the primary field. FIELD

The data at frequencies below about 400 Hz are relatively more noisy than those at higher frequencies, particularly at the two most distant stations 351 and 503 m. The sensitivity of the receiving coil used in the experiment decreases linearly with decreasing frequency. Also, there is the destructive influence of the power line harmonic at 180 Hz. An interpretation of the data yielded the simple layer over a half-space model shown in section in Figure B-7. The theoretical transfer function re-

EXAMPLES I.O

We have selected two examples of interpreted data collected with University of Toronto PRBS sounding systems over the Paleozoic sedimentary basin of southern Ontario. The geology of the basin is well known and well documented. Cores from many hundreds of drill holes reveal a stratigraphic sequence consisting of interbedded shales and limestones with some evaporites and occasional major reef structures which are known sourcesof hydrocarbons. The basin shelves to the northeast.

between

•

y (meters)

o6 o

bJ

N_ 0.4 /3? O

Z

02 229

Duncan et al. (1980) carried out a simple, preliminary soundingon the Niagara escarpmentsome 52 km west of Toronto. Here, the depth to basement is only about 710 m. There is only one major conductor in the section, a layer of shale about 450 m thick. The shale is sandwiched

0.8

more resistive

dolomite

at the

surface and limestone at depth which, in turn, rests on the very resistive Precambrian crystalline basement. The equipment was built around what was then an off-the-shelf standard laboratory Hewlett-Packard cotrelator. Measurements of the vertical magnetic component were made broadside to a grounded long wire transmitter bipole. The sensors for high (30 Hz-15 kHz) and low frequency (1-150 Hz) were a small

$•/ •05

I

0.0

IOO

iooo

IOOOO

FREQUENCY (Hz)

Fig. B-6. The amplitude of the H z magnetic component transfer function plotted as a function of frequency for a range of separationsbetween the transmitter bipole and the receiver which lies broadside to it. The superimposedsolid lines are the response of the simple conductivity model shown in Figure B-7, a resistive layer over a conductive half-space (after Duncan et al., 1980).

28 _+Sin

,900 2' 190 .tim

:1

I

I

I

/

t

air-cored coil, 100 turns on a 0.3 m diameter former,

followed by an electronic integrator and a Scintrex Model MFM3 magnetometer, respectively. The amplitude of the recovered transfer functions at the higher frequencies are displayed in Figure B-6 as a function of frequency. The length of the bipole and the root-mean-square (rms) current carried by it were 183 m and 2 A. The current

waveform

followed

-_-_-_'

T i T-• i

a 127

point PRBS based on 3.125 and 31.25 kHz clock frequencies. The separationsbetween the bipole and the receiver of were 76, 137, 229, 351, and 503 m,

respectively. Thedataat eachsite,218valuesof the magnetic field sampled at intervals Ax of 333 or 33.3 IXS,were crosscorrelatedwith the appropriate 127 point PRBS reference to give crosscorrelograms100 points long. The Fourier transforms of the latter were combined shown.

to obtain the broad-band

transfer

functions

.5-'

PO _• 5.ore

:.n

T•

i"1

LEG END

'5'5" DOLOMITE .....

' ''.

SHALE

LIMESTONE

!-'??!• P•' BASEMENT

Fig. B-7. The conductivity model which fits the data shown in Figure B-6 compared with a true section through the Paleozoic sediments (after Duncan et al., 1980).



sponsesof this model are superimposedon the data in Figure B-6. The resistivity of the upper resistive layer is the only poorly determined parameter. The known finite thickness of the second conductive shale layer cannot be resolved becausea skin depth in the shaleis only 160 m at 200 Hz, the lowest recovered frequency. Crosscorrelation techniques really do enable very small signalsto be extracted from a noisy uncorrelated background. It is instructive to calculate the primary rms voltage output from the receiving coil, described earlier, at the 503 m station. For a transmitted sinusoidal current of frequencyf and amplitude 2 Arms, it is approximately 6.4f nV. Now the power in the PRBS signal actually transmitted is distributed among about 50 useful frequenciesat the low end of the transmitter frequency spectrum. If the correlator sampling inter-

val Ax and the clock frequencyfc are adjustedso that each of these frequenciesis measured separately (this impliesthat fc/127 = 1/100Ax,which is only approximately true), then the amplitude of each frequency is

393

which are offset by distances of 50, 100,200,400, 840, and 1450 m from the eastern end of the set of bipoles. Figures B-10 and B-11 display the measured transfer functions

at each of these sites for the radial

electric

fieldEx andthehorizontal magnetic fieldHy perpendicular to the bipole. The componentE x varies slowly with frequency displayingonly weak inductive effects.

The component Hy reflectsthe presenceof a good conductor at moderate depth. Interpretation of these data using a layered-earth inverse modeling package yields the parameter values given in Table B-2. Each entry in the table is followed by its error level relative to the best-determined parameter, which had an estimated error on the order of 1 percent for Model 1 and 5 percent for Model 5. The

Model1fit wasobtained by invertingonlytheHy data. Including the Ex data in a joint inversion yielded Model 5, which illustrates the effects of improperly (Text continued on page 397)

6.4/X/•fnV or 0.9fnV rms! Additional

similar

measurements

of

the

vertical

magnetic field at lower frequency settingsenabled the electrical properties of the deeper structure to be resolved. The distance between the transmitter, a

bipole 1.2 km long carrying a current of 1.75 A, and the receiver, the MFM3 magnetometer, was increased to 1.75 km. The data reduced to amplitude and phase of the transfer function at frequenciesfrom 1 to 250 Hz are displayedin Figure B-8. The standarderrors on the data are not shown, but are uniformly about 2 percent

in amplitude and2 degrees in phase, obtained with2•s (=262 144) samples in the correlogram. The interpreted electrical section is inset in Figure B-8. All parametersin the model are well resolved except 93, the resistivity of the lower resistive half-space. The effect of varying 93 is illustrated. Again, it is instructive to compute the primary rms magnetic field measured by the magnetometer at the station. For a transmitted current of amplitude 1.75 A rms, it is approximately 0.69•. For each frequency in the PRBS, the correspondingamplitude is 97 m•/! The success of these simple PRBS experiments stimulated the systematic survey of the southern Ontario basin described

in Gomez-Trevino

and Edwards

(1983) and, later, the construction of more sophisticated hardware, the GESS-1 (Holladay, 1987), and the improvement of interpretation techniques (Boerner, 1987). The specificationsof the new equipment are given in Table B-1. The equipment was used to obtain field data in the Luther

Marsh Conservation

Area near

Orangeville, Ontario. The locations of the several groundedbipole transmitters and receivers are identified in Figure 9-B. We shall discussonly the soundings made using bipoles 5/7/8 and receiver stations 81-86

Fig. B-8. The amplitudeand phase of the H z magnetic component transfer function plotted as a function of frequency for a separation of 1750 m between the transmitter bipole and the receiver which lies broadside to it. The superimposedsolid lines are the responseof the conductivity model shown embedded. The parameters of the model are well determined except the resistivity 93 of the basement. The effect of varying 93 is illustrated (after Duncan et al., 1980).

394



Table B-I. Overall Configurationof GESS-I System Module

Name

Description

Signal Conditioning Modules (SCM's) Analog-Digital Conversion Module (ADM) Timebase Module (TBM)

Stacking Manager Module (SMM) Computer Interface Module (CIM) Power Supply Module (PSM) Receiver Controller (GFC- 1) Sensors-Electric

Field

Sensors-Magnetic Field

1 per channel, 3 presentlyincluded Can handle up to 7 SCM outputs Maximum sample rate 59 kHz 2 PRBS base frequencies 3 square-wavebase frequencies Ovenized quartz oscillator Tapered stackingcontroller Dedicated

interface

to Ports Board

Input: 24 -+ 4 VDC Output: +5, + 15 VDC logic supplies 4x _+ 18 VDC isolated analog supplies LamontagneGeophysicsField Computer 8 MHz 6809 ports board Stacking throughput 28 kHz (2 byte words) Data storage on 8" DSDD diskettes Copper Sulphatenonpolarizingelectrodeswith onboardpreamps(xl, x10) 2 per electric field channel, 4 total UTEM II coil (H, M band sensor) Scintrex MFM 3 high sensitivity magnetometer(L band sensor)

Transmitter Configuration Module

Name

Transmitter

Description

Controller

Contains timebase identical to that in receiver

Optically isolated logic waveform output 2 PRBS. 3 square wave switch-selectablebase frequencies Power supply-internalbatteries (supplementedexternally) U. of T. EMPULSE

Transmitter

transmitter

FET-bridge type 1.6 kW output (up to 4 A at 400 V) Isolated

Current

Monitor

Power supply•-Gasoline generator

Sensitivity 0..•-6i7 V/A

Powersuppl•r-•nternal batteries Detailed Specificationsfor SelectedModules Module

Name

Signal Conditioning Modules

Analog-Digital Conversion Module

Timebase

Module

Specification Instrumentation amplifier front end Switch-selectablegains 1 3 10 30 100 300 1000 Additional x 10 gain Logarithmic attenuator for tapered stacking --Attenuation range 0 to 60 dB Band-limiting filters (see table below) --Highpass-4 pole Butterworth --Lowpass-6 pole Butterworth --Notch-frequency Devices type 780 (60 Hz) Sample-holdamplifiersin SCM's, rather than in ADM System noise: --analog: --•0.1 mV pk (in 20 V window) @ 1 Hz. gain 100 --power supply: --•1 mV pk very high frequency noise Maximum Sampling rate 59 kHz Temperature coefficients: --gain 15 ppm/øC ---offset 10 ppm/øC Noise level 30 ppm (about -+1 1.s.b.) Input voltage range -+10 V Resolution: 16 bits (.3 mV/1.s.b.) For base and samplingfrequencies, see table below Erie ovenized crystal oscillator frequency standard Observed stability (aging)

--1 partin 109 over2 hours


395

Table B-I, continued.


Samplingand Filter Characteristics of ReceiverBands Band

Waveform

High High Mid Mid Low

PRBS Square PRBS Square Square

Record length

Sampling frequency

Base frequency

1016pts 256pts 1016pts 1024pts 2048pts

27.778kHz 27.778kHz 7812.5Hz 7812.5Hz 152.59Hz

27.340Hz 108.51Hz 7.6895Hz 7.6294Hz 0.14901Hz

Highpassfilter frequency

Lowpass filter frequency 8.00 kHz 8.00 kHz 1.69 kHz 1.69 kHz 19.84 Hz

120.00 Hz 120.00 Hz 1.00 Hz 1.00 Hz 0.04 Hz

I

.%

55

76

0

Ikm

TX4

/'

&

31-36

I

41- 46

ß

•

51- 54; 74-76;

81-86

Tx 3 (for example)

Fig.B-9.Thetransmitter andreceiverlocations fortheLuthersurveys. Threepowerlines,a buriedtelephone line, and a sectionof a groundedfenceare alsoindicated(after Holladay, 1987).


396


0.5

-0.5

10 -•

-1.0

10 -8

10-•

100

101 10: 103 Frequency(Hz)

10+

10-•

100

101

102

103

Frequency(Hz)

Fig.B-10.Theamplitude andphase oftheExelectric component transfer function measured atreceiver sites 81 through86 plottedasa functionof frequency (afterHolladay,1987).

10 -=

0,,• ßß__ ' -"...%I:)

10-3

:

ß

-II

,!., I-I-

'!'!'

• -0.5 '*":5

. .

ß ß

,, -1.0

. 10 -8

'

,,

ß ß

.•11 •

ß

ß

-1.5

II. I . 6

.

10 -8 10-•

i

i

,,111

i

1•

i i iiiii

10'

i

i 11111i

i

i i iiii/

102

Frequency(Hz)

i

103

i

111111

10+

10-•

1•

101

102

103

10'

Frequency(Hz)

Fig.B-11.Theamplitude andphase oftheHymagnetic component transfer function measured atreceiver sites 81 through 86plotted asa function offrequency. Thephase plotsforstations 84through 86havebeenoffset by-1.0 radiansfromthe othercurvesto improvetheclarityof thefigure(afterHolladay,1987).


397

Table B-2. Summary of inverted model parameters Model


Parameter

Expected

Parameter

Model

(S/mor m)

(S/m or m)

Model 1

0.•

0.01

0.024(1)

20-40 0.2

0.4

Model2

33 (600)

Model 5

0.035 (8)

0.033 (1) 0.033 (25)

4.2 (8)

5 (25)

.00125 (-)

0.000013 (20)

0.0073 (6) O.000001 (6)

93 (2)

87 (1)

18 (20)

160 (5)

0.05

O.12 (40)

0.07 (14)

315

200 (40)

380 (50)

0.01

0.0015 (-)

0.0073 (3)

bipole electrode,forced the inverted model far from

the resultwhichwas optimalfor the Hy inversion. Suchspatialaliasingin electricfield measurementscan be minimizedthroughthe use of antennadipoleswith lengthsof two or more stationspacings.Comparison of the inversion results to the geoelectric model expected on the basis of nearby drillhole information

indicates thattheHy inversion fitstheknowngeology quite well, althoughthe conductivityof the resistive Guelph-Lockport and of the relatively deep Trenton Group were essentiallyundeterminedby the data.

REFERENCES

Referencesgiven in the main Referencelist, page 378, are

not repeatedhere. Additionalreferencescalled out in this

0.07 (60) 240 (60)

O.O1 (300)

0.00014 (-)

sampledlateral inhomogeneityin the overburdenon the electric field measurement. The presence of a resistive zone near station 84,400 rn from the nearest

appendix follow.

Model 4

0.0125(2)

27 (15)

Errors

0.0001 (-)

0.002 90

0- 3

Values with Relative

Boerner, D., 1987, A generalisedapproach to the interpretation of controlled source electromagneticdata collected in sedimentary basins: PhD thesis, Univ. of Toronto. Duncan, P.M., Hwang, A., Edwards, R. N., Bailey, R. C., and Garland, G. D., 1980, The development and applications of a wide band electromagnetic sounding system usinga pseudo-noisesource:Geophysics,45, 1276-1296. Heikka, J., 1983, The MHD source and preliminary results of 5 componentregistrationsin northern Finland: in The

developmentof the deep geoelectricmodel of the Baltic shield, Part 2. Proceedingsof the 1st project symposium, Oulu, Hjelt, S.E., ed., Univ. of Oulu, Finland, 263-275. Holladay, J. S., 1987, The generalisedelectrosounding method for sedimentary basin exploration: PhD thesis, Univ.

of Toronto.

Levin, M. J., 1960, Optimum evaluation of impulse responsesin the presenceof noise: IRE Trans. Circuit Theory, CT7, 50-56.


398


APPENDIX

LAWRENCE

BERKELEY

C

LABORATORY

EM-60

SYSTEM

H. F. Morrison* and N. E. Goldstein

The EM-60 is a frequency-domain EM sounding system developed by Lawrence Berkeley Laboratory in conjunction with the University of California at Berkeley, primarily for geothermal exploration (Jain, 1978; Morrison et al., 1978). A number of modifications

have

been

made

since the initial

field

tests.

Appendix C gives a brief description of the system as it is presently configured (Figure C-l). The transmitter is powered by a 60 kW, 400 Hz alternator, and is theoretically capable of supplying 400 A peak-to-peak at 250 V into a large ungrounded loop. In practice, the current is usually limited to 150 A peak-to-peak. Loop sizes of up to 3 km have

provides spectral plots of all data. Source-receiver separations from 400 m to 30 km have been used. Depth of exploration is roughly one-half the sourcereceiver separation. One major innovation of the EM-60 system is the implementation of remote-reference geomagnetic noise cancellation, which enable measurements at lower frequenciesthan would otherwise be obtainable. Below I Hz the geomagneticfield increasesroughly as l/f, while the secondaryinducedEM field decreasesas l/f, resultingin a net signal-to-noise(S/N) ratio which

is proportional to 1/f2. The remote(reference) siteis

of the natural geomagnetic noise. The current waveform is squarewith a 100 percent duty cycle, enabling

located about 10 to 15 km from the transmitter loop so the observed field is almost entirely geomagneticsignal. This signal is transmitted by way of FM radio telemetry to the mobile receiver site, and subtracted from the signalmeasuredlocally. Figure C-2 showsan example of noise cancellation using this scheme. An improvement in S/N of about 20 dB is normally obtained, allowing reliable measurementsto be made

measurements

down to about 0.05 Hz.

beenused,with a transmitter momentof up to 108 Am2. Smaller,four-turn,200m squareloopswhich are easier to deploy are also used. The frequency

rangeis from10-3 to 103Hz, although it is usually difficult to obtain reliable data below 0.05 Hz because

of the odd harmonics

of the fundamen-

tal frequency to be made. At high frequencies the waveform shape tends to become quasisinusoidalbecause of the combined

effects of the inductance

and

resistance of the loop, so that the power of the odd harmonics is relatively smaller. The receiver system consistsof either magnetic- or electric-field sensors (or both) which, after conditioning, are multiplexed and input to a Nicolet oscilloscope used for digitizing and displayingthe measured signals. The data are then fed to a Hewlett Packard HP 9835 computer for processing, which results in an amplitude and phase estimate of all magnetic fields. Amplitudesare normalizedby the free-spaceprimary field from the transmitter. Phase referencingis maintained by highly accurate oven-controlled quartz clocks. The computer also calculates ellipticity, tilt angle, apparent resistivity, and confidence limits and

A field example from the Panther Canyon geothermal prospect, taken from Wilt et al. (1980c) and Wilt et al. (1983) is shown in Figures C-3, C-4, and C-5. Panther Canyon is located in Grass Valley, a northerly trending Basin-and-Rangevalley located in north-central Nevada; the dominant lithologies are a Paleozoic sequenceof cherts, argillites, and sandstones.Several

geophysical methods were used on two profiles, shown in Figure C-3. The EM-60 field survey consistedof eight soundingsarranged along two orthogonal lines crossing a central four-turn, 50 m radius horizontal loop. Transmitter-receiverseparationsvaried from 400 to 1600 m, and data at each site were recorded over at least two frequency decades in the frequency range 0.033 Hz to 500 Hz. An example of amplitude spectra at one site is shown in Figure C-4. (Text continued on page 402)

*Departmentof Materials Scienceand Engineering,University of California,Berkeley, California94720.

*EarthSciences Laboratory, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720.

Preomplifiers and

Horizontol loop, 14 ß10 6inks telemetry Control units F.

•

! I----J I "y SQUID

radio

5Component

•!

Transmitter truck


R

multiplexer

magnetometer

Phase reference

signal

Ilulti-chonnel

stacking spectrum analyzer

I.ire telemetry Discriminator

_

I

--

!Filters IZ Bucked signal electronics J• HHr

:

i

I

Bucking _

Hz

!' 5 Component SQUID

magnetometer

Fig. C-1. Schematic diagramof the EM-60horizontal-loop prospecting systemasusedin Nevadain 1979(Figures A3-1, 3, 4, 5, from Wilt et al., 1983). 120,

I

Sonon9o

Hx LOCAL

Range

Re•o

Field site "•

•ß T31N R$9E

Hx REMOTE Panther

Gross Vo//ey

Canyon

H

-Loop

t ransin itter

CANCELLED SIG•IAL

NATURAL MAGNETIC FIELD CANCELLATION

E'os/

Ronge x H x REMOTE

EM-60

Receiver

stations

T-T' Geophysical survey lines -7-..- HeQI

flow

contours (HFLO

10,50 , J

I

I I

2 I

3 ,

T'

4 •

Kilometefl

H x LOCAL

Fig. C-3. Location of the EM stationsin the PantherCanyon area, Grass Valley, Nevada, with respect to the heat-flow Hx(NOISE REMOVE D)

1•3,1

SAMPLE TRANSMITTER

FIELD RECORD FREQUENCY =O. 1 Hz

Fig. C-2. Example of data improvementusingthe telluric noise cancellation scheme. (A) Natural geomagneticsignal and initial cancelingat the receiver site with transmitteroff. (B) Same systemwith transmitter on.

anomaly.


400


COMPARISON

OF CALCULATED

AND MEASURED

DATA

10.00

z 1.00

z

0.10

j

0.01 1

z

!

0.01

0.10

1.00

'

,

I

100.00

10.00

,

1000.00

FREQUENCY (HZ) SODA

LAKE

CALCULATED

.72 KM NW TI

DATA

MEASURED DATA

LAYER

RESISTIVITY (9,m)

THICKNESS (N)

HR

HR

X

I

12.11,

305.4,

HZ

HZ

ß

2

1.77,

DATA VARIANCE

ESTIMATE

.00

.02

.1000E,11,

2.

0.

15.23

Fig. C-4. Normalizedmagneticfield amplitudespectranormaland radialfieldssoundingTTS PantherCanyon.


401


GrassValley, Line T-T' Dipole-Dipole Resistivily Model 0 -=

! 5.o

!, I I [ 150,0 o

I250

i

! I

z

s.o

I , 27.0

3,0

1

3

4

5

-

After •ey•r. 197'7

Di•'lonce in kilometers, Resi$livily in oltre-meters

EM-60 ResistivityProfile 0 -

2:50

-

Loop tronsmitt•

location

Recei•me Iaeatiaa

0 1250

i

2

3,

4

5

Distance

in kilometers

0 '=

•:•:i:•:•:•:i:•:i:•..•...•5.:•:•:i:•:•:i:i:i::.:????•.????.•.:.•:•:•.•:.::: :':':'•'=• •• :::::::::::::::::::::::::::::::::::

250

-

•?•i•?•i•!:•!:•!•i•!•!•!•!•!•!•i•!•!•!•?•!•!•.•...:•... ?'-"'. "•:•??:• Iiii:•i!'?.:•::i:*.::'•:i•iii:•i•'•'•'•!! i • ?.o' :':':': ...... '. '--%ii!i'•';'•:i:i:•:i:'•::: .'..... :::::::! ..... ." ..... :! _

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::.:.:.:.:.:.:. :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: =================================

0 1250

-

&,

,

i

2

&

& ,

3

4

5

I

I

I.

Fig. C-5. Resistivitycross-sectionover line in PantherCanyon' (a) two-dimensionaldipole-dipoleresistivitymodel; (b) profile of one-dimensionalEM-60 electromagneticsoundings;and (c) comparisonof (a) and (b).


402


The data were interpreted using a Marquardt leastsquares layered earth inversion scheme. Both amplitudes and phase data were used in the inversion over most of the frequency range, but polarization ellipse parameters were used at the highestfrequencieswhere the phase reference was less reliable. A comparison of EM-60 results and a 2-D dipoledipole resistivity interpretationon the north-southline is shown in Figure C-5. The EM-60 data are displayed as a composite of 1-D layered earth inversions. Both methods outline an irregularly shaped conductive body buried at a depth of about 100 rn associatedwith a region of high heat flow. The body is interpreted to

APPENDIX

be a warm water aquifer with fault-controlled recharge to the east. The time taken for the EM-60 survey was about half that taken for the dc resistivity survey. REFERENCES

Referencesgiven in the main Reference list, page 378, are not repeated here. Additional references called out in this paper follow. Jain, B., 1978, A low-frequency electromagneticprospecting system: Ph.D. dissertation, Univ. of California, Berkeley and Lawrence Berkeley Laboratory, LBL-7042. Morrison, H. F., Goldstein, N. E., Hoversten, G. M., Oppliger, G., and Riveros, C., 1978, Description, field test and data analysis of controlled source EM system (EM60): Lawrence Berkeley Laboratory, LBL-7088.

D

UTEM

Y. Lamontagne* and J. C. Macnae*

The UTEM system, originally developed at the University of Toronto, is a time-domain EM system designedto measure the step responseof the ground (Lamontagne, 1975; West et al., 1984). The system transmits an exactly controlled triangular waveform with high frequency pre-emphasis, which is deconvolved in the receiver to produce the exact squarewave responseof the ground. Measurementsof the full waveform are made because there is no system offtime. If the waveform repetition rate is set low enough, the square-waveresponseis equal to the stepresponse

(referred to as TEM magnetic-fieldresponseelsewhere in this section), in contrast to other TEM systems which measure the impulse or voltage response. Typical field setupsconsist of lightweight transmitter loops with dimensions of 300 rn to 3 km, and a roving synchronized receiver. Survey lines typically range both inside and outside the transmitter loop up to distancesof about 2 loop diametersaway. Either 10 or 20 time samplesof the waveform in constant time ratio are measured at each station. The UTEM system

was originally designed for mineral exploration of

*Lamontagne Geophysics Ltd., Suite 306, 49 Spadina Ave., Toronto, Ontario, MSV2JI.


confinedtargets, but has more recently been applied to sounding.

403

samplingconsistsof 10 channelswith successivetimes

in ratiosof 2, or 20 channelswith ratiosof dependingon the application.


DEPTH

SOUNDING

USING

THE

UTEM

SYSTEM

Use of the UTEM system in depth soundingapplications has evolved differently than for other TEM soundingsystems. The common practice with UTEM has been to use both time and geometrical soundings, i.e., to measurethe responseas a function of both time and distance

from the center of the source.

Originally, the measured data were inverted to a multi-layer model using a linearized inversion technique (Lamontagne, 1975; West et al., 1984). However, recently a "forward processing" method called "depth image processing" was devised to transform the measurementsto a near image of the conductivity structure when it is quasi-layered. An important feature of this processing is that in its first step it effectively removes the effect of transmitter-receiver geometryfrom the responseand transformsthe data to a function (apparent depth of the induced currents as a function of time) which is only dependent on the structure. The results obtained at this first step with different geometries can then be averaged with suitable weighting. FIELD

SURVEY

CONFIGURATION

Following this processingdevelopment, field practices were revised to optimize the resolution of processedresults by using various combinationsof sourcereceiver geometries. In the survey procedures which was developed, a number of square transmitter loops are laid out strad-

dlingthe surveyline. For eachloop, hz measurements are made at ten to forty stationsalong the survey line both inside and outside the loop. The portion of the survey thus covered with one loop is called a "loop traverse." In covering a survey line, the loop traverses normally overlap in such a way that each station is reoccupied several times to produce an n-fold coverage.

The operational procedureis adjustedin sucha way that with one of two receivers there is enough time to occupy all the stationsof a loop traverse while the next loop is being prepared. The loop survey times vary according to the loop size; one hour for small loops (300 m) and more than one day for very large loops (2 km). Station spacingshave rangedfrom 20 m to 250 m,

beingroughly 1/10to 1/4 the loop size. The loop size is chosenbetween 50 percent to 100 percent of the depth of interest, and the base frequency is adjusted to obtain suitable field penetration. The final frequency is often chosen after an initial trial to determine roughly the average conductivity of the section. The timing

EXAMPLE

OF SOUNDING

DATA

Figure D-1 showssomeof the hz data of 14 overlapping loop traverses surveyed using 10 channel sampling at a base frequency of 31 Hz. The data are plotted in continuously normalized format as described in West et al. (1984). Each transmitter loop in this case was 400 m in size, and the loop traverses 1200 m long to produce a 3-fold coverage. The individual loops are indicated by their numbering, and the wire positions by small circles. Adjacent loops have one common

side.

The responseis a monotonically decreasingpositive responseinside the loop, whereasthe responseoutside the loop starts as negative and turns positive at late delay times. The response range is roughly between -200 percent to +200 percent and data precision ranges from 1 percent at early channels to <0.1 percent on the later ones. The regularity of the data indicates that the ground is well layered, but there is evidence of minor lateral inhomogeneities in several locations, particularly on the four earliest channels displayed(largest anomaly amplitudes). The main feature seen in comparing these data is the relatively faster decay in the right part of the section, particularly later than the first three channels.Faster decay is an indication of higher resistivitiesover that part of the line.

APPARENT

DEPTH

DATA

PRESENTATION

The first step in the Depth Image processingis the transformation to apparent depths. Figure D-2 shows the stacked apparent depths obtained from the data of Figure D-1. Essentially, each data point is transformed to the mean depth at which the current appearsto have diffused. This mean depth is half the depth at which a distribution of loop images must be placed to fit the response.The depths plotted are the weighted averages of the three apparent depths available at each station. These depths are presented as a profile for each channel with a logarithmic depth scale. The reference line for each channel is the logarithmic average depth for the whole line (of which only a part is shown). In the apparentdepth sectionthe larger depths, such as in the right part of the section, are indicative of a higher resistivity starting at roughly 200 m depth. In this presentation, any lateral change within a well layered earth starts appearingat the proper depth, but it is difficult to resolve their depth extent.



I .... I .... I I .... I ' ' • ' I I .... I .... I



o o o o

o õ 405

o õ o g

406



SLOWNESS

CONDUCTIVITY

depth in the right of the section, and a conductive zone of •.m or less at a 500 m depth centered around

As discussed in Macnae and Lamontagne (1987), a very good approximation of the conductivity of a layered earth as a function of depth can be obtained by estimating the secondderivative of time t with respect to the apparent depth h'

station

1 d2t O-S ---

IXodh•'

The first derivative is the "slowness" of the apparent depth. Since this is a forward process, the numerical estimation could easily be constrained to be well behaved, in this case by using spline functions to obtain a minimum curvature estimate of the resistivity, i.e., one of the smoothest estimates compatible with the data. The results of this transformation are presented on vertical traces with a logarithmic scale. In Figure D-3, the base lines represent a resistivity of 30 •.m (shale), and the spacing between the lines, a factor of 10, with resistive anomalies plotting to the right (heavy hatching). Note that since the derivatives are, with respect to the data, dependent variable, this is a very nonlinear transformation and often gives results obviously not related to the apparent depth section. For example, in Figure D-3 the resistivities show evidence of a very good lateral continuity which is difficult to discern in the apparent depth data. The most prominent features evident in the section are a 300 •.m resistive zone startingmostly at a 200 m

Frank

is known

from

Our experience with this new EM soundingsurvey procedure and processing method shows that lateral changes and subtle targets buried within a layered earth can be resolved with much improved confidence over conventional

inversion

methods.

The main rea-

son for this success is that the processing is more tolerant of minor lateral changes, and that it tends to produce results that correlate well laterally where inversion methods would produce models which are equivalent but often visually difficult to correlate. ACKNOWLEDGMENTS

We thank BP Exploration Canada for permissionto show the data presented. REFERENCES See main Reference

list.

E

DOMAIN

SYSTEM

C. Frischknecht*

The United States Geological Survey has designed and assembled a number of systems for loop-loop or loop-wire frequency-domain electromagnetic FEM measurements. The system used to make the measurements described in Anderson (1977) employed com-

*Deceased.

feature

CONCLUSIONS

APPENDIX

USGS FREQUENCY

12 000. The resistive

drilling results to be a carbonate unit. The conductive zone is thought to be a brine-saturated zone within a porousrock unit. The remainder of the sectionappears to be uniform continuous shales with a thin partly melted permafrost layer at the surface as evidenced by the varying initial depths and very high resistivities at the top of the section.

mercially available lock-in amplifiers and bandpass filters. The drive signal for the power amplifier or switcher was derived from a high accuracy crystal oscillator with count down-circuits providing ten frequencies per decade. The phase reference for the



lock-in amplifier comes from an identical unit. In the field, phase drift of the oscillatorswas generally excessive for frequencies above about 100 Hz. A system designed more recently (Cooke et al., 1981) includes a radio frequency link operating at 3.35 MHz to phase lock the receiver oscillator to the transmitter oscillator. Signalsfrom the 10 MHz oscillators are counted down to provide 10 frequenciesper decade over the range 0.01-2,000 Hz. A transmitterswitchercapableof handlinga maximumcurrentof 20 ampereswas constructedas part of the system.In the receiver the signalis amplified,bandlimitedby a four pole variable frequencybandpassfilter, and then further amplified before being fed to square-wave synchronous detectors. The output of the synchronous detectorsis filtered by a two-pole bandpassfilter with variable time constant and then displayed on digital panel meters. Except for the transmitter,the systemis powered by rechargeable batteries. The current through the source is sensedby means of a current transformer at high frequenciesor by a small series resistor at low frequencies. The reference voltage is measured using circuits similar to those in the receiver.

Althoughthe receiver coil describedin Cooke et al. (1981) was a large horizontal loop made of flexible cable, two-component square frame ferrite or mumetal cored loops are generally used. The mu-metal cored loop has a self-resonantfrequency well above

2 kHzandhasa sensitivity of about2.4ixV/nT• which is adequatefor measurementsdown to frequencies of a few hertz. At lower frequencies a SQUID magnetometerhas been used as the sensor.When two or three componentsare measuredthey mustbe done one at a time since the receiver has only a single

407

channel. For most measurementsa lower power commercially available power amplifier has been substituted for the switcher that was part of the original system. Experiments with a battery powered data acquisitionsystemand computer indicate that use of samplingand arithmetic averagingwould be superior to the exponential averaging provided by the output analog filter. Due to problemswith the radio frequency link, the system has generally been operated without phaselocking the transmitterand receiver oscillators.Drift rates without phase-lockingare low enoughto permit accurate determination of tilt angle and ellipticity by successively measuring two spatial components in successionat the same frequency. Alternatively, a hard wire link can be used to periodically reset the counters which will eliminate any accumulated phase drift. To eliminate

the effects of common-mode

cur-

rents flowing along the cable the hard wire must be disconnected while measurements are being made. The results shown in Figures 48 to 50 in the main text were collectedusingthis technique. Examples of other results obtained with the system are described in Anderson et al. (1983b) and Frischknecht and Raab (1984). REFERENCES

Referencesgiven in the main Reference list are not repeated here. Additional references called out in this paper follow. Anderson, W. L., 1977, Interpretation of electromagnetic soundingsin the Raft River geothermalarea, Idaho: U.S. Geol. Surv. Open-File Rep. 77-557. Cooke, J., Bradley, J. A., Mitchell, C. M., Lescelius, R., 1981, A descriptionof an extremely low frequency looploop geophysicalsystem: U.S. Geol. Surv. Open-File Rep. 81-1130.


408


APPENDIX

COLORADO

SCHOOL

OF

F

MINES

TDEM

SYSTEMS

George V. Keller*

Research on the use of a transient electromagnetic method to perform resistivity soundingswas begun at the Colorado

School of Mines

in 1964 as an extension

of earlier work at the United States Geological Survey to develop a controlled source for use with magnetotelluric sounding. The purpose was to develop a method capable of sounding to considerable depths, ranging from a few kilometers to a few tens of kilometers. Such a method would be useful in exploration for deep-seated natural resources, such as oil and gas, or geothermal energy, or for providing information on the properties of the outer part of the earth's crust. In order to achieve deep penetration, use was made of high-moment sources, and longer windows for observing transient response than would normally be appropriate for minerals exploration. No single field system has been developed during this research; rather, equipment used in acquisition of field data has undergone a slow but continual change as new and better equipment became available for field operations. The field equipment used consists of two parts which are relatively independent of each other, a source system and a receiver system. The source system consists of a clock-actuated switch which forms a square wave of current to drive through the source cable. At present, two such switches are being used, one switch is capable of providing a square-wave with a peak-to-peak amplitude up to 250 A at a peak-to-peak voltage of 2000 volts, and the other switch is capable of providing currents of 2000 A peak-to-peak, at the same voltage. The smaller switch requires power from an AC generator of up to 50 kW size, while the larger switch requires a generator with 1000 kW capacity. Periods of 4 s or longer can be obtained with the smaller source, while periods of 80 s or longer can be obtained with the larger source. Rise time on each of the sourcesis about 2 ms, though with

the larger source, there is a dwell time of 60 ms as the current passesthrough zero. Digital recording has been used since the early field efforts (Jacobson, 1969; Kowalski, 1969). In the most recent version of a receiver system, a Digital Equipment Corp. MINC-23 digital system has been used. This provides for conversion of analog information to a 12-bit digital form. These data are then processedin various ways and stored on a flexible disk for mass storage. We feel that a programmable computer such as the DEC MINC-23 has been advantageousfor use in an educational role because data acquisition and processingprocedurescan be changedeasily by reprogramming. Various configurations of transmitter and receiver have been used over the years. The source has been used to energize both grounded wires and ungrounded loops (Keller and Rapolla, 1976). The source most commonly used is the grounded wire source. When combined with a distant receiver system, such a source provides the greatest depth of penetration of any of the EM sounding methods. At a receiving location, the EM field can be detected by measuringany of three componentsof the magnetic field or two components of the electric field. Most commonly, only the vertical component of the magnetic induction is observed, this being done by recording the voltage generated in a large loop of wire lying on the ground, or by differentiating the output of a vertical-axis Josephson-junction(SQUID) magnetometer. In some surveys, all three components of the magnetic induction have been observed, using either induction coils wound on high permeability wires, or a three-component Josephson-junction magnetometer (Souto, 1980; Keller, 1981). Large-scale time-domain electromagnetic surveys (TDEM) have been used on several occasions by

*Department of Geophysics, Colorado School of Mines, Golden, Colorado 80401.


409 RECEIVER

SOURCE


Offset

DCSupply Current tt.• '/'

ß•

Distance

R

,

-

J

'

•Recording E;lulprnent

Reversing SWitch

Electrodesto-'detectelectricfield

Fig. F-1. Principalcomponents of the ColoradoSchoolof Minestime-domain EM sounding system.

R lOW

Rgw

RSW

R7W

R6W

RSW

R4W

STATION

3106

DECONVOLVED

•t'SIQNAL

o

o

o

o

o

o o o o o

o

RAW SIGNAL

o

o

oo

o

o

oo

o.1

lO

TIME

o

o.1

lO

TIME

of Clear Lake. The source is located at the southwest end of

Fig. F-3. RecordedTDEM dataat variousstagesof processing. The trianglesto the left indicatestackeddata interpolatedat equalintervalsin log time, but not deconvolved.The squaresto the left indicate stackedand deconvolveddata. The data to the right have been smoothedby averagingover

Clear Lake.

a fixed interval in log time.

Fig. F-2. Layoutof megasource TDEM surveyin thevicinity

410


NORTH OF SOURCE 4-•;:1•SOUTH OF SOURCE STATION 2145 R=32• km

STArION

STArION

STATION

STArION

STArION

STABON

STArION

2137 R=20.1 km

3071 R=16.5 km

3002 R=10.2 km

3080 R=5.4 km

3103 R=6.0 km

3109 R=8.2 km

3105 R=11.7 km

lO

lO

lOO

'

I'

lOO E


ß

•_ lO EARLY

Z

1

<

o.1

1.0

TIME 1.0 S

INVERTED

0

10

1.0

RESISTIVITY 10

100 •

lOO

lO

100

I

lO

lOO

1

lO

lOO

lOO

,

' I'

1

2

1

lO

lOO

' I'

I

I

7

8 9

10

Fig. F-4. Profile of TDEM apparentresistivity soundingcurves(at top) and their inversionsto resistivity section(at bottom) for a north-south traverse through the source at Clear Lake. See Figure F-2 for locations of stations.

students doing graduate research, usually related to geothermal exploration, or exploration for oil and gas (Harthill, 1969; Skokan, 1974; Crewdson, 1976; Tulinius, 1980). Interpretation has been based largely on one-dimensional (l-D) inversion, inasmuch as the method has been applied in areas where structure is approximately 1-D (Daniels, 1974; Walker, 1979; Passalacqua, 1980). A case history from the Geysers area, California (Keller et al., 1984) is shown in Figures F-1 through F-5. The exploration objective was detection of a postulatedfractured geothermalreservoir at a depth of 2 to 4 km. The source used was a 1 km length of AWG-4 cable, grounded through multiple metal plates

Megaeoume TEM Soundlnge

INVERSIONS

OF TDEM

SOUNDINGS

NEAR

2 BOGGS of)

5oo

rr

I-

ß

SURVEY

750

**2

OF

BOGGS

buried at a depth of 1 m. A three-phase 60 Hz generator set with a capacity of 1000 kW was used as a primary power source. On rectification, a 2000 A peak-to-peak squarewave with 80 s period was driven through the source cable (see Figure F-1). The vertical component of magnetic field strength was detected at each of 245 station locations, as indicated in Figure F-2 using Josephson-junctionmagnetometers. The output of the magnetic sensor was analog differentiated to provide a signal comparable to that obtained with an induction loop. These signalswere converted to earlyand late-time asymptotic values of apparent resistivity, and subsequentlyprocessedat a central computer facility. Several stages of processing are shown in Figure F-3. The sounding curves were interpreted in terms of three to four layers, using a 1-D generalized linear inversion algorithm. A north-southprofile of apparent resistivity curves along with block diagrams of the inverted 1-D resistivity profile for each is shown on Figure F-4. Correlation between a well log and results of inversion of several soundings made within 1 km distance of the well are shown in Figure F-5.

Q. 1000

REFERENCES 1250

I

1500

I

] II

II II

1.0

Ifil

I 10

RESISTIVITY

I

I

I

I I II 100

OHM-METERS

References given in the main Reference list, page 378, are not repeated here. Additional references called out in this paper follow. Crewdson, R. A., 1976, Geophysical studies in the Black Rock Desert geothermal prospect: Thesis 1866, Colorado School of Mines.

Fig. F-5. Comparison of inverted TDEM soundingscollected near the Boggs 2 well with the induction electric log from that well.

Daniels, J. J., 1974, Interpretation of electromagneticsoundings using a layered earth model: Thesis 1627, Colorado School of Mines.

Electromagnetic Sounding Harthill, N., 1969, Deep electromagnetic sounding: Geological considerations: Ph.D. Thesis 1257, Colorado School


of Mines.

Jacobson,J. J., 1969, Deep electromagneticsoundingtechnique: Ph.D. Thesis 1252, Colorado School of Mines. Kowalski, M. A., 1969, Analysis of deep electromagnetic sounding system: M.S. Thesis 1261, Colorado School of Mines.

Passalacqua,H., 1980, Electromagnetic fields due to a thin resistive layer: Thesis 2390, Colorado School of Mines.

Skokan, C. K., 1974, A time-domain electromagnetic survey of the east rift zone, Kilauea volcano, Hawaii: Thesis 1700, Colorado School of Mines. Souto, J. M. G., 1980, Test of time-domain electromagnetic

exploration for oil and gas: Thesis 2278: Colorado School of Mines.

Tulinius, H., 1980, Time-domain electromagnetic survey in Krafla, Iceland: Thesis 2325, Colorado School of Mines. Walker, R. C., 1979, An S-layer model for electrical prospecting: Thesis 2171, Colorado School of Mines.

APPENDIX

SOVIET

EM

411

G

SOUNDING

SYSTEMS

Brian R. Spies*

A number of descriptionsof electromagnetic-sounding systemsbeing used in the USSR have been published in western literature in recent years. Transient electromagnetic(TEM) instrumentationis describedin Buselli (1980) and Spies(1980 c, Appendix A). Data on frequency-domain electromagnetic(FEM) instrumentation are more scarce, but some are mentioned in Spies (1981). The following summary is drawn from these sources.

TEM systemsare used much more widely than FEM systems for sounding. One portable FEM system, known as the EPP-2 is a loop-loopsystemoperatingat frequenciesof 78, 312, 1250,5000,and 20 000 Hz, with a transmitter-receiver separationof 120 m. The sensor consists of two orthogonal coils which are used to measure parameters of the polarization ellipse. Groundedwire or large loop sourcescan also be used. FEM sounding is used mainly for groundwater and engineeringapplications. Several TEM sounding systems have been developed in Novosibirsk. TSIKL (Cycle) is a high power system used for a variety of applications, including soundingin the near-zone for oil and ore prospecting. The transmitter provides up to 100 A at 500 V from a 50 kW motor generator to a loop sourceup to 1500 rn in size or a groundedbipole. For a 1000rn loop at 40 A the turnoff ramp time is 400 •s. The waveform is bipolar with an on-to-off time ratio of three. The

receiver samples between 0.1 ms and 44.8 s, at 20 samplesper decade. IMPULSE is a microprocessor-controlled systemdesignedfor measurementat earlier times usinga portable generatorsupplying20 A into a loop up to 200 rn in size. This system can be used in the coincident loop, in-loop, or loop-loop configuration. The time range is from 10 •s to 80 ms. Internal noise on these systemsis between 0.1 and 0.3 •V referred-to-input. A general purpose digital acquisitionsystem developed in Moscow, known as the Universal Digital Station, or TSES, is used as a generalpurposemagnetolluric or TEM receiver for deep sounding.TEM systems have transmitter-receiverseparationsof up to 15 km, and are suitable for sounding through a sedimentary cover of up to 100 S. In the last 10 years, Soviet geophysicists have pioneeredthe use of magnetohydrodynamicsourcesfor deep crustalsounding(Velikhov et al., 1983).These sourcesare capableof generatingup to 60 MW of power for short times (2-10 s), and can transmit

20 000 A into a low-resistance

load.

Further

descriptionsare given in Heikka et al. (1984) and Halverson et al. (1987). REFERENCES

References are given in the reference list, page 378.

*Schlumberger-DollResearch, Old Quarry Road, Ridgefield,CT 06877-4108.


412


APPENDIX

INTEGRATED

GEOSCIENCES,

ELECTROMAGNETIC

H

INC. DEEP TRANSIENT FIELD

SYSTEM

C. H. Stoyer*

The TEM sounding system offered by Integrated GeoSciences, Incorporated, is designed for deep structural exploration for oil, gas, and geothermal targets. Two transmitters are available: a mechanical

switch based system and a microprocessorcontrolled solid-stateunit. Both systemsare capableof delivering a squarewave to a grounded-wiresourceat power levels of up to 200 kW. The receiver is based on a minicomputer and is battery operated. Typical square wave periods are 40 s, and both long (LOTEM) and short (SHOTEM) offset capability are offered. Recording time at each stationis typically 20 minutes, so that 8 to 10 stationsper day can be achieved with a single receiverunit. The systemis capableof soundingto depths of 6 km or more, dependingon resistivity, and is capable of penetrating conductancesof more than 1000 S. The vertical componentof the time rate of changeof the magnetic field is measuredusing a large horizontal induction loop. The output of this loop is amplified using a multi-stage low-noise amplifier. Notch filters are integrated into the electronics in order to reject power line noise and its harmonics.Equivalent loop

areasof up to 5 x 10l0 m2 (including amplifier gain) have been successfullyused in field surveys, even in culturally noisy areas. The receiver

unit is based on the DEC

LSI

11/2

minicomputer,with DEC tape II cassettesand a 12-bit gain-ranginganalogI/0 board. Simple stackingof data is carried out in the field for quality control. The entire receiver unit weighs about 65 kg, and can easily be carried in a four-wheel drive vehicle or a helicopter.

Bidirectional selective stackingof the data is carried out in the field base quarters. The data are gathered into time-channels using Hanning windows and a selective averaging process. The present system is limited to 120 stacks, which take about 40 minutes to acquire. These are selectively averaged to form a

transientwaveform which is sampledin approximately logarithmically spaced-timeincrements. Error bars are calculatedduring the averagingprocess. The impulseresponseof the systemis accountedfor during ridge regression inversion of the transient soundingcurve. This is done by convolvingthe system responsewith each synthetic curve calculatedduring the inversion process. This has been found to be the most stable way of accounting for the system response. The error bars are used by the inversion software to reject excessively noisy points on the tail of the decay curve, and to bias the fit in favor of the statisticallymore significantpoints. Figure H-1 shows the results of inversion of a soundingtaken near the Kirkpatrick # 1 wildcat well about 2 mileswest of Condonin north central Oregon. The boxes on the plot indicate the data points, with error bars. The solid line is the synthetic curve from the best-fitmodel, shownat the right sideof the figure. Figure H-2 shows a comparison of the inversion resultsand the electric log from Kirkpatrick # 1. Note that the baseof the resistivebasaltis determinedquite well, as is the base of the conductive tuff section. These interfaces lie at about 2400 feet and 6300 feet in

the well log, respectively.

*Integrated Geosciences,Inc., P.O. Box 780, Golden, CO 80402-0780.

GO111BHZ

MODEL

ß

17.6

240.

H

476.

H

lO


9.m

lO

88.6 9om

lO

2.21

1

41.

M

9om

lO

13.1 9om

10

10

10

10

10

Y, ERROR: 5.34 CALTBRATTON: OFFSET= 7427.

IGS.

TIME (s)

1.1S M

INC

Fig. H-1. Inversionof TEM grounded-source soundingnearthe Kirkpatrick# 1 well, Condon,Oregon. RESISTIVITY

IN f•om lee

i

KIRKPATRICK

•1_

o

z

H

o

10 11

CO1 1 1BHZ

i2 i3 i4

i5 i6

i

I

I

Fig. H-2. Comparisonof TEM inversionresults(thick line) and electric log from Kirkpatrick # 1 well.


414


APPENDIX

ZONGE

ENGINEERING CSAMT

I

AND RESEARCH ORGANIZATION AND TEM SYSTEMS

Dudley Emer*

Zonge Engineering's GGT-25 transmitter and GDP-12 receiver system can be used for both TEM and CSAMT

measurements.

The

25 kW

transmitter

has a maximum current output of 40 A and output voltage in the range 50 to 1000 V, and can operate at frequencies from dc to 10 kHz. The GDP-12 is a field-programmableportable geophysicaldata acquisition system capable of time- and frequency-domain IP and EM, magnetolluric and electrotelluric measurements.

For

CSAMT

measurements

the

receiver

simulta-

neously measures the transmitted electric and magnetic fields with a sensitivity of 0.2 IxV in the frequency range of 0.5 Hz to 4096 Hz. The GDP-12 calculates both the Cagniard apparent resistivity and phase difference (Phase E-Phase H) and allows interpretation and modeling based on apparent resistivity, phase, and the magnetic field. In the TEM mode the system can measure two simultaneousinput channels in the time range 122 to 467 s in 12 to 32 time windows. The pulse repetition rate can be varied from 1/1024 to 32 Hz, with duty cycles from 12.5 to 100 percent. A comparison of field data of CSAMT and in-loop TEM

measurements

from

a test site located

in Avra

Valley, Arizona, is shown in Figures I-1 and I-2. The geology of this area consists of relatively uniform valley fill about 600 m thick overlying basement. The CSAMT data were obtained using a transmitter-receiver separationof 4.8 km. The transmitter consisted of a 1.2 km long groundedbipole with 22 A of current.

The receiver geometry used a 150 m long electric dipole oriented parallel to the transmitter bipole and a ferrite-cored coil measuringthe orthogonal horizontal magnetic field. Data were obtained from 0.5 Hz through 4096 Hz in binary increments. The CSAMT results, shown in Figure I-1, have a sharp transition zone between 4 and 32 Hz, indicating a resistive zone at depth. The far-field data, extending from 32 Hz to 4096 Hz are typical of a two-layer earth with a conductive lower layer. Figure I-1 shows a comparison with model data obtained with the U.S. Geological Survey's EMFIN program (Anderson, 1974). The data agree well, with the exception of the transition zone notch. The three-layer geoelectric interpretation is shown on the figure. A 26 l•.m, 34 m thick surface layer overlies an 8 l•.m layer 552 rn thick. The high resistivity basementextends to depth. In-loop TEM data were obtained with a 500 m square transmitter loop with a current of 14 A and a turnoff time of 175 Ixs. The receiver consisted of a two-turn 50 m loop of wire located at the center of the transmitter loop. The first two windows are adversely affected by the transmitter turn-off and have not been corrected for systemresponse;the last seven windows are within the background noise level. The TEM sounding results shown in Figure I-2 are very similar to the CSAMT interpretation, and show a relatively thin high resistivity surface layer overlying a more conductive layer. The data were inverted using the U.S. Geological Survey's NLSTCI program (Anderson, 1981). The interpreted surface layer of 26 l•.m is

*Zonge Engineering and Research Organization, 3322 East Fort Lowell Road, Tucson, Arizona 85716.



:

,'

,,

.

..

415

..

-

.

.

.

-

.

.

?

..

ß



AI I:AI: IS]•S3•

o

-•

,

I[,[[I[[ [

IN3[SN!•I

I[[[]l[[ •

•o•

• LaJ

"'o ¸

.,,_•

i

i


417 REFERENCES

32 rn thick and overlies an 8 l].m layer 549 rn thick. The high resistivity bottom layer evident in the mod-


eled CSAMT

is not reflected

in the modeled

TEM

data

which shows a 10 f•.rn layer to depth. The shallower penetration of the TEM data can be attributed to the noisy late time data produced when operating at low current in a high-noise environment.

Referencesgiven in the main Reference list are not repeated here. Additional references called out in this paper follow. Anderson, W. L., 1981, Calculation of transient soundings for a central induction loop system: U.S. Geological Survey Report USGS 81-1309.

APPENDIX

BRGM MELIS

J

MULTIFREQUENCY Pierre

EM SYSTEM

Valla*

The Bureau de RecherchesGeologiqueset Minieres (BRGM) has been working on multifrequency EM methods for more than 20 years. The first method

surements and computations indicated above, resulting in a commercially available system: the MELIS receiver unit with CMS magnetic sensors(Figure J-1)

named MELOS

and the FLUXTRA

was conceived

in the mid 60's when it

was proposed to compute near-field corrected apparent resistivities using only amplitude ratios between

vertical (H z) and radial (Hr) magneticfield and the tangential(Et) electric field for a small horizontalloop source laid on the ground (Duroux, 1967):

transmitter.

MELIS

RECEIVER

The MELIS receiver acts like a dual-channel spectrum analyzer; simultaneous measurements are made on both signalinputs, giving their amplitudes(range of

10-3 to 10-7 A/m for magneticfield)and relative Pa

The SYSCAL-EM equipment (1980) included inthe-field computation of this apparent resistivity, together with two other apparent resistivities using ei-

ther Hr/H z or io•lx'Hr/Et amplituderatios and the transmitter

receiver

distance.

Following new studies(Valla, 1982, 1984; Gasnier et al., 1985), funded in part by the European Community Commission, a new method has been proposed. This method, named MELIS,

is based on measurements of

the complex transfer function between two field components, from which apparent resistivities are computed using the imaginary (quadrature) part of either

Hr/H z or io•lx'Hr/Et. The main advantageof this technique is removal of topography or leveling effects at low frequencies, since the bias from these effects occurs on the real (in-phase) part. Equipment has been designed to perform the mea-

phase (with a precision of a few milliradian). One or three frequencies(fundamental plus first two harmonics) can be analyzed at a time, chosen from more than 100 frequencies per decade in the range 0.12 to 8000 Hz.

The analog part of the receiver uses hybrid circuits for power line-frequency rejection (50 or 60 Hz and harmonics) and signal amplification (10 to 640 gain). An overload test is included. A complete calibration of the system may be done in the field prior to every measurement. Data acquisition is performed at a 0.8 to 20 000 Hz sampling rate depending on the frequency studied. Anti-alias filtering is accomplished using seventh order switched-capacitor programmable filters. The digital processingis managed by an 8-bit microprocessorwhich also controls the analog and acquisition circuits, and drives the human interface (keyboard and LCD display, Figure J-2). The steps of the measuring process are as follows:

*Bureau de Recherches Geologique et Minieres, Avenue de Concyr, B.P. 6009-45060Orleans Cedex 2, France.


418


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. -,

:.... ..?...• ;....?..'.:-?.,• .?;-•".c',a,...:•,,?;:•i.;:,;',---•:•-•:.:;..•'L'•..:• •,::;., ::* ...... 7%-:--. ..... • ?-'.:.:, . ..., ..... ?-:'i-...:;?:.,..:.:...::..•....•"..•., ...........,..•..:i;:,.**

;--: ........ :::.:w/:•..;:...----•-.',.---.;,:-:,-.:v..-..-•::: ...:.:;:-. .... :.:?-o•.•,:-;..'::..: '....... ?'....•:"..:•:• ,..;";--" .';;,:,."'•-C•,. *-:;": '•!7-.*.,;;•'-".'":s ..::-;:::.:.,•/" '•.,. -....... ., .'o.'.... '"*•........................... :'::.::" .;':' .,:: ". '..... ::.•' . ...:.:.•::" :-::::..:," ::: .:.., ' .:.'i:: •:"-..,.*,'.?" "%,,';':*;::;:,:..':" ' ::•::•:-•4• '::"•:• :':":*':•.', ':.:*.: '.{.,.

:-.•::;'• ..,•?'....4, .... • ..............,..... .:.•.- ,.•...... ., • ,.: ....., ..•

•:*o..,...'• .,-...2-*..a '.-...,...•';.•---'..,. ; ..,..,:.:... ..... ::-. ,,,,..."t..r :...,.,•. ß½..... '. .....

-::•,..: ....

Fig. J-1. MELIS receiver with CMS magnetic sensors.

4T

Fig. J-2. Front panel of MELIS receiver.

....:.•:::..::...:, .;.,. ........ .,,.. ':..-.:



St.

I

I

I

11•-2

18-I

1

2

Fig. J-3. MELIS apparent resistivity soundingat Gi-

l/sqrt(F) ß BRGH/Gi:q-I ß FREHIS

419

R-

ß

65D

tonville (Eastern Paris Basin, France) with 1-D interpretation.

m

meters

I

lO

I

I

1Da

lO 3

RBx2

$chl umbe-ger

Fig. J-4. Schlumbergersoundingat Gironville (Eastern Paris Basin, France) with 1-D interpretation.

420


•Digital

acquisition of 256 samples for both input


channels, and Fast Fourier transform after uniform or Hamming windowing; ratio evaluation and weighted stacking implementation for mean values and standard

deviations of input amplitudes and transfer function; •Calibration correction and display of the results; •The operator controls the stacking duration using the continuously displayed results and may store the measurement

in internal RAM

for further data transfer

to a microcomputer through a RS-232 serial link. Digital processingallows storage of valuable information regardingeach measurementand provides high flexibility in the use of the receiving unit. Up to 256 measurement

records

can be saved in internal

RAM

continuousmemory; each record includes information on measurementparameters (station, frequency, number of stacks, etc), weighted averages of amplitude of the transfer fraction, and other statistical data.

Several array types are available using the MELIS receiver (and more could be added). These include the

two standardMELIS modes (Hr/H z and Hr/E t for small loop source)with in-the-field computationof the correspondingapparent resistivities, five absoluteref-

conductive layer. In this area the basin dips slowly westward (about 3 degrees) and its structure can be approximated as a horizontally stratified medium. The geological section is known from a nearby oil exploration drill-hole where an electric log was performed: --from

0 to 120 m: marls with 8 to 30 f•.m

resistiv-

ity, from 120 to 220 m: limestone

with 60 to 150 f•.m

resistivity, --from 220 to 600 m: clays and marls with 3 to 15 f•.m resistivity. A two-turn loop, 250 m by 150 m, was set up and connected

to the FLUXTRA

1 kW

transmitter.

The

receiving stations were 660 m away and consisted of two CMS surements

sensors and the MELIS were

made between

receiver

unit. Mea-

0.8 and 2000 Hz.

The

apparent resistivity results from the imaginary part of

Hr/H z ratio are shownin Figure J-3 (error bars indicate the standard deviation or 50 percent confidence interval), together with the simplest compatible 1-D geoelectrical interpretation. Computation and fitting were made with the FREMIS program on a HewlettPackard 9000 microcomputer.

erencemodes(Hx, Hy, H z, Ex, Ey) with a wire or radio link to the transmitter for phase and amplitude reference,and the standardCSAMT mode (Et/H r for grounded wire source).

Ig (l"),,.m) 0

10 I

CSM

MAGNETIC

100 ........

•

SENSOR

BRGM manufactures a modified version of a magnetic sensor designed by the Centre National de la Recherche Scientifique. This sensor, named CMS, has a 1 Hz to 10 kHz bandwidth and a sensitivity of 50 mV/nT. The CMS operates through flux feedback and its noise is in the order of the average natural AMT noise level. The CMNS weighs 1 kg and is 35 cm in length. FLUXTRA

:

200

400

TRANSMITTER

The FLUXTRA transmitter is a fairly lightweight unit (15 kg) which uses a 1.5 kW standard motorgenerator as power source. About 100 frequenciesare available in the range 0.1 Hz to 8000 Hz and a current of 13 A can be driven into a 4 to 8 t/resistance loop for the lower frequencies. Since this transmitter is easily transportable, the source array can be set up even in areas with rough topography.

600

depth (m) kIELIS

Schlumberger VES

FIELD

EXAMPLE

OF MELIS

SOUNDING

A field survey has been carried out with the MELIS system at Gironville, in the Eastern Paris Basin, to test the possibility of the method in detecting a deep

Laterolog

Fig. J-5. Comparisonof MELIS and Schlumbergersounding interpretations with laterolog data at Gironville (Eastern Paris Basin, France).



The experimental data, shows apparent resistivities of about 10 l].m for the highest frequencies, a downside peak at 600 Hz followed by an almost horizontal segment, which is typical of a resistive layer in the intermediate frequency zone, and a descendingbranch at lower frequencies, correspondingto low conductivities at depth. In the course of 1-D interpretation, a good fit with the data can be obtained only by assumingtwo conductive deep layers, the deepest being the most conductive. When compared with the simplified laterolog data (Figure J-5), a very good general agreement is seen.

A Schlumberger sounding(Figure J-4) has also been performed at this same site using an ELECTRA 1 kW transmitter to drive a 1 to 2.5 A into the groundfor AB spacing from 4 to 3000 m and ELREC receiver to measure the MN voltage down to 5 mV. Only the last data points show the start of a descending branch indicating conductive layers at depth. Therefore, the lower frequencies of the MELIS soundingsallow deep

APPENDIX MAXIPROBE

421

soundingsto be performed without the logistic costsof a dc sounding.

REFERENCES

Referencesgiven in the main Reference list are not repeated here. Additional referencescalled out in this paper follow. Duroux, J., 1967, Caracteres de l'onde electromagnetiquede surfaceengendreepar un dipole magnetique;applicationa l'investigation en profondeur de la resistivite electrique du sous-sol. Geophys. Pros., 15, 564-583. Gasnier, S., Pottecher, G., Valla, P., and Clerc, A., 1985, Mise au point et essai methodologiqued'ur. equipement geophysiqueelectromagnetiquemultifrequentiel a source controlee avec reference de phase dans la gamme du dixieme au millier de hertz (rapport final du contrat C.C.E. no. MSM-044-F): BRGM Open-File Rep. 85 DT 037 GPH-INS.

Valla, P., 1982, A test field survey using the MELOS electromagnetic method, 52nd Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 395-397. Valla, P., 1984, Broadband frequency electromagnetic sounding with absolute phase reference: 54th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 121-123.

K SYSTEM

Brian R. Spies*

The Maxiprobe system was developed and has been used primarily for soundings. However, it has been used also for broadband fixed-source profiling in the exploration for steeply dipping mineral deposits (Ghosh, 1982). The source consists of one, two, or three 11 m-diameter loops of flexible cable laid out on the surface and an engine generator powered transmitter (Sinha, 1979). The receiving loops are a pair of orthogonal ferrite-cored coils mounted inside a spherical shell to reduce wind noise. Inphase and quadrature parts of the vertical and horizontal fields are measured relative to a stable local reference operating at the same frequency as the transmitter. The ratio of the amplitudes of the vertical and horizontal fields and their phase difference is then computed. Measurements can be made at as many as 128 fixed frequencies between 1 Hz and 60 Hz. For some applications the parameters of the polarization ellipse are computed

(Ghosh, 1982). Separations up to 2 km can be used. Sinha (1980) discusses topographic and orientation errors that can be incurred in using the system. A method for interpretation of soundingsand examples of soundingsis given by Sinha (1982).

REFERENCES

References given in the main Reference list are not repeated here. Additional references called out in this paper follow. Ghosh, M., 1982, First five years experience with the Maxi-Probe EMR-16 System for deep mineral exploration (abstract): Geophysics, 47,432-433. Sinha, A. K., 1979, Maxi-Probe EM R-16: A new wide-band multifrequency ground EM system, in Current Research Part B: Geol. Surv. Canada Paper 79-1B, 23-26. Sinha, A. K., 1982, First five years experience with the Maxi-Probe EMR-16 system for deep mineral exploration labs.]: Geophysics, 47,432-433.

*Schlumberger-Doll Research, Old Quarry Road, Ridgefield, CT 06877-4108.


422


APPENDIX

GERMAN

DEEP

L

TRANSIENT

Kurt-Martin

The development of Deep Transient Electromagnetic Systems started in West Germany in 1984 as a continuation of work done at Group Seven Inc., Integrated GeoSciences Inc., USA, and Macquarie University, Australia. Since then several new generations of system hardware have been designed, built, and successfullyapplied around the world. The main objective in the system development is to improve the resolution of transient electromagnetic soundingsusing a groundedwire for shallow and larger depths. The emphasis during all the hardware development was given to improvements of the signal-to-noise ratios through data acquisition and processingin order to be able to use safe, medium size (up to 100 kVA) transmitters and small transmitter-to-receiver offsets (2-10 km). Presently, two different systems are being manufactured commercially in Germany: One system is based on a seismic acquisition system which allows the use of virtually an unlimited number of channels along a digital transmission line. The other system is designed to be a more integrated electromagnetic system by combining magnetotellurics and transient electromagnetics. Over the past five years several thousand soundings using about 100 transmitter sites during 20 surveys have been acquired in Western Europe, China, India, and South Africa. Applications range from coal exploration, oil and gas exploration, and geothermal exploration to deep crustal investigations (see references), The system concept is dictated by the extremely high cultural noise level in Western Europe. This requires the storage of all individual records to allow prestack data processing. The digital multichannel system allows the application of state-of-the-art seismic technology for electromagnetic applications. A typical survey setup is shown in Figure L-1. After acquiring all the individual records the field data are

EM

SYSTEMS

Strack*

'I'R•NSMITI'ER •

.•v[

( t ItltENl'

FOIt'•I

.

I I

"-...: '.... vV vvVV vvvVv vvvVVv vvvVv vv +v•••••E • •v vvV.vvvvv vVvvvVvV V N"

+ RECEIVER TRaNSIENT

ß

++++• '+• ++++•' +•+•

++

+ ++++++++ +•+•+• ß + + • ++++++++++ + + +$++ + +++T+•__++++-+•__ + ++ +++ + ++ +++++ + ++.+ + + +++• ++ + ++++

-• ....

•

lime

Fig. L-1. Typical survey setupfor a multichanneldeep TEM system system. The data are acquired and stored in the Remote Units (RU). Many of these units are connected by a digital connection which allows the sequentialtransmission of the data to the central recording unit.

generally processed using "true amplitude" digital filters and selective stackingalgorithmsto improve the signal-to-noiseratios (Strack et al., 1989). After stacking, the data can be transformed to apparent resistivities for imagingor directly interpreted usinginversion methods. Routinely at the University of Cologne a statistical 1-D inversion technique (Vozoff and Jupp, 1975) as shown in Figure L-2 is used. To improve and automate the interpretation, further variations of this inversion technique integrating a priori information from the well logs can be applied over complete profiles. A more realistic (but also more computer-

*Institut fur Geophysik u. Meterologie, Universitat zu Koln, Albertus Magnus Platz, 5000 Koln 41, FRG.



423

could be caused by a real structure or by a 3-D effect. More modeling would have to be done for this data to evaluate which interpretation is applicable. To combine the deep transient EM interpretation with the deeper parts of the section and to obtain less bias in the interpretation, the integration of magnetotelluric

intensive) inversion can be done using Occam's inversion (Constable et al., 1987) (see Figure L-3). This type of inversion results in a smooth resistivity versus depth distribution and allows the interpreter to directly recognize areas where anomalous structures and/or 3-D effects exist. In this figure the dipping conductor

RESISTIVITY

SECTION

HA8705

o

1

SW

2

3 KN

•5

05-

0 rrl

-1.0'

3O.20.

1915.

15. 14. 15. 11•'816.18' 18.20.20. 20.' - 1.5VERTICALEXAGGERATION 1'3

IGM

K• INVERSION RESULTS OF Hz-FIELD

Fig. L-2. Example of an interpretedresistivity cross sectionusing the individual 1-D inversion results (Stephan, 1989). kill 0

10

km

0

9._ 4

6 km

Profilelength 100

1000

RESISTIVrI•'

- DEPTH

CONTOURS

IGMK61n

Fig. L-3. Example of a resistivitydepth sectionderivedfrom smoothedl-D inversionsusingOccam's method.The top frame showsthe color sectionderived directly from the data and the samesectionafter applyinga horizontal low pass filter.

424 o

Spiesand Frischknecht o

measurements through joint inversion has been ex-

tremely helpful. Figure L-4 showsan example(H6rdt et al., 1990). Here MT and LOTEM

measurement

were carried out at the same site. The solid line shows


2.0_:

.

_.

,', 40_'---

O_

-

.

i !

6.0_

reduced well log

I AULE

I A08EY

8 0_: Joint inversion

-

AULE

. -

with

A08EY

-

IGiM Koh! $3490

10-1

100

101

102

103

Resistivity in •-m

Fig. L-4. Comparison of individual LOTEM and MT inver-

sionswith thejoint inversionresultsand the well log (H6rdt et al., 1990).

O

ma e

ectio

the reduced electrical log. The individual inversion of the LOTEM data (A08EY) doesnot resolveany of the resistivitystructurebelow 3 km depth. The MT inversion(AULE) seesa conductorof largerdepththan the well log and only an approximate structure in the upper part of the section. When carrying out a joint inversionof LOTEM and MT, the depth to the conductor and the upper part of sectionapproximatethe well log more closely than the individual inversions do. Anotherway to presentthe field data directly after processingis to display it as image sectionsas shown in Figure L-5. Here the apparentresistivitiesderived from the field data, and the measurementtimes, have been transformedto image-resistivitiesand pseudodepths respectively. Since 3-D modelingprogramsare extremely computer-timeintensive,they have only been applied to very selected field situations. Newman (1989) simulated some specific 3-D effects for the long offset transientelectromagnetic(LOTEM) system.His program is restrictedto simplegeologicmodelsas shown

1 Resistivit.,

Depth

ell 174

Scale 4

o

1

8 10

2 km

20

6O

8O 0

2

4

6

km

Fig. L-5. Simpleresistivityversusdepthimagesections for a LOTEM sounding line The imageresistivityis derivedfromtheapparent resistivities of thefielddataandtheimagedepthis obtained fromthesampletimes.The interpretedseismicreflectorsand the inductionlog are superimposed.

ElectromagneticSounding Plan

425

view

transmitter


3 - D

REVERSAL

Cross 2{)00

I

500toOl 10•'2m I

layer 1

I km Receiver

section

Air

m

600 m[

SIMULATION

'•-2000m

nlalion

200

1500m

'-•

•-m

layer 8500m

4000

I •om

SYNTHETIC

DATA

FIELD

: E

0

*

..........

& le3

i l-d !

' :'

{

•

• •

*.

k.......

lez

:

:

DATA

: ß

*

:

..k .........

o:

k .........

•o

.......3......... ,i..•i• '......... •. : ++• '".: [

;" i ,'......... • le• :"*'M*•: I

•z: 101

O-4

103

tO2

lO-I

•e-a lO8 10-1

•e-•

•e"

•e • IGMK6LN

TIME

(S)

TIME

75089

(S)

Fig. L-6. Example of using an integral equation method to calculate the 3-D responseof a conductive body. Here signal reversals are simulated (bottom left) and compared with field data (bottom right).

in Figure L-6. A larger class of models can be calculated usingthe programby Druskin and Knizhnerman (Druskin and Knizhnerman, 1988) which uses a finitedifference scheme and subsequenttransformation due to a smaller subspace.

statistical inversion techniques which give confidence intervals of the interpreted model parameters or with depth imaging methods which give a direct datarelated image of the subsurface structure. The deep transient electromagnetic technique itself is very robust even in areas with high cultural noise level. REFERENCES

CONCLUSION

Our experience with the LOTEM hardware, processingand interpretation software has been that 8 to 10 stations per day can be acquired when using single receiver systems. When using a multichannel system, the number of stations of amount of data acquired can be significantly larger (e.g., up to 192 channels per spread), at a small increase of capital equipment cost. The interpretation of field data can be done either with

References given in the main reference list are not repeated here. Additional referencescalled out in this paper follow. Druskin, V., Knizhnerman, L., 1988, A spectral semidiscrete

method

for numerical

solution

of three

dimen-

sional nonstationary problems in electrical prospecting: Physics of the Solid Earth, Izvest. Acad. Sc., USSR, 24 (in Russian). Stephan, A., 1989, Interpretation von transient elektromagnetischen Messungen (LOTEM) im Bereich der Halterner Sande und Entwicklung der lokalen Rauschkompensation, Diplom thesis--Geophysics, University of Cologne.




CHAPTER

TIME

DOMAIN

6

ELECTROMAGNETIC

PROSPECTING

METHODS

MisacN. Nabighian* andJamesC. Macnae* INTRODUCTION

Surface

In the traditional frequency-domain electromagnetic (FEM) methods of exploration the ground is energized by passing an alternating current (ac) through an ungrounded loop situated usually on or above the surface of the earth. The primary field of the loop will induce eddy currents in all conductorspresent in the earth. The secondaryelectromagnetic(EM) fields due to these induced currents, together with the primary EM field, are recorded with a suitable receiver at various points in space. In general, the secondaryEM field at the receiver, which contains all the information regardingthe undergroundconductors,may be several orders of magnitude smaller than the primary field. Under these conditions the separationof the measured total EM field into its primary and secondaryparts is

TEM

....

/-:,x::

__••_•.•_s_•., u_.•_• s

•'X• SURFACE COIL

•

•g•

......

RX

/

Y

•V NO

-.

WINCH

c=•..-"•

TX LOOP OVERBURDEN

Drillhole

TEll ORE BODY DRILLHOLE

difficult.

This fact led to the idea of using time-domain electromagnetic measurements(TEM), often referred to as transient EM techniques. In TEM measurements a strong direct current (dc) is usually passed through an ungrounded loop (Figure 1). At time t = 0 this current is abruptly interrupted. The secondaryfields due to the induced eddy currents in the ground can now

be measured

with

a suitable

receiver

Fig. 1. Common time domain EM survey configurations.

inductive TEM techniques have been used primarily for locating massive sulfide ore deposits at great depths, and their use is associatedalmost exclusively with the mining industry. More recently they have seen extensive use in the search for deep graphitic conductorsassociatedwith uranium deposits.

in the

absenceof the primary field (Figure 2). Historically, time-domain techniques have evolved along two parallel paths, transient EM sounding and transient EM prospecting, also known as inductive TEM. The first techniqueis usedprimarily to delineate layered structuresof interest in geologicas well as oil and groundwater exploration (Spies and Frischknecht, this volume). The technique is also used in delineating zones of high conductivity within the ground with the expectation that they might be associatedwith either geothermal or massive sulfide targets. By contrast,

The

earliest

disclosure

of an inductive

transient

technique to determine the parameters of massive sulfideore bodies was by Wait (195l a, b), followed by a patent application in 1953 on behalf of Newmont Exploration Ltd. (Wait, 1956). By 1962, McLaughlin and Dolan (as part of a joint Newmont-Cyprus Mines developmentprogram) had developed the first version

*Newmont Exploration Ltd., 1700 Lincoln Street, 26th Floor, Denver, CO 80203.

*Lamontagne Geophysics, Ltd., 4A WhitingStreet,Artarmon,NSW 2068,Australia. 427


428

Nabighian and Macnae

of the Newmont EMP system consisting of a large transmitter loop and roving receiver. The current source was composed of ninety 12 V automobile batteries in series which supplied a pulse of 700 A of about 100 ms duration. The receiving equipment consisted

of

a coil

with

an

effective

area

of

20 000

turns-m 2 anda receiverprogrammer. The transient signal was sampled for the first 50 ms after current ramp interruption. In 1964 this system was successfully tested in Cyprus (Dolan, 1970). A small-scale, lower-power, reconnaissanceversion and a drill hole version were field tested by McLaughlin in 1970 in South Africa.

In 1958, Barringer developed INPUT, the first airborne TEM system (Barringer, 1962). The horizontal •'^"* *•'• aircraft and was transmitter loop was "'• maximum coupled with the receiver loop towed in a bird behind the plane. The primary magnetic field was generated by periodic half-sine current pulses of 1.1 ms duration and alternate polarity and the transient decay was sampled from 0.15 to 2.7 ms after current turn-off.

In the Soviet

Union

research

on inductive

transient

EM techniques first began in 1958 at the Moscow Institute of Geological Prospecting and Exploration and subsequently, at various research institutes around the country. In 1960, their research led to the development of the single-loopversion of the transient techniques (MPPO-1) in which the same loop is used both as transmitter and receiver. The loop which can be of various dimensionsis usually between the limits of 5 x 5 rn and 200 x 200 m. The amplitude of current pulses varied between 0.5 to 2 A. The transient decay was sampled in the interval from 1 to 15 ms after current ramp interruption (Veilkin and Bulgakov, 1967). 50 -I00

CURRENT TRANSMITTED

AMPS PULSE-WIDTH 20-40MS

PERIOD 500 MS

VOLTS o IIIIIII

20 I I

VOLTAGE MEASURED

60

40 I

31

I

I

I

I

!

80 I

I00 I

MS I

In the 1970s,the advent of computer technology and the plethora of new electronic componentsled both to the proliferation of TEM equipment and to a real revolution in the way data are gathered and processed. In 1974, the first high-power, three-component, fully digital TEM system was introduced by Newmont Exploration Limited for in-house use, together with the required software package for data processing, presentation, and interpretation. In 1977, CSIRO introduced SIROTEM, the first microprocessor-controlled TEM unit operating on the single loop modification variant. Crone PEM was developed in 1972 initially as a small scale loop-loop system. The University of Toronto EM system, UTEM 1 was introduced in 1972 and became commercially available through Lamontagne Geophysics in 1981 as UTEM 3. The Geonics EM-37 was introduced in 1980, and a Russian modification (MPP4) became available in 1981. In addition, in 1978 the Russians introduced a

helicopter TEM systemwhile in 1982 Questor unveiled the helicopter INPUT system. In 1986, Geoterrex made available the GEOTEM digital airborne system while in 1988 A-cubed produced the first PROSPECT airborne EM system. Recently multipurpose systems such as the Zonge GDP-12 and the Phoenix V-5 system, originally designed for controlled-source audiofrequency magnetolluric surveying, have been reprogrammed for TEM data acquisition. As of 1988 all manufacturers were offering fully digital TEM systems.

More details on instrumentation are provided in Appendix A to this chapter.

BASIC

PRINCIPLES

OF TEM

METHODS

One of the most common configurationsin carrying out transient measurements is shown in Figure 1. A large ungroundedloop is energized by passinga strong dc current through it (Figure 2). After a finite time, during which all turn-on transients have practically vanished, this dc current is abruptly interrupted (step function excitation). According to Faraday's law of induction, the rapid changein transmitter primary field will induce eddy currents in a neighboring conductor. Under quasi-static approximation (Grant and West, 1965, p. 469-472), these induced currents will initially be confined only to the surface of the conductor (surface currents). The magnitude and direction of these currents will be such as to preserve the normal component of the pre-existing primary magnetic field at the surface S of the conductor (Weaver, 1970) i.e.,

CHANNELS

Fig. 2. Transmitter current and receiver waveforms for the Newmont EMP System.

b,(t)

= b,0 t=0

on S,

TEM ProspectingMethods


where bno is the normal componentof the primary (normal) magneticfield. This is tantamountto saying that at time t = 0 the magnetic field inside the conductorwill remain unchangedand there will be no interior induced current flow. At all exterior points, the magnetic field will change abruptly as a step function. In TEM terminology this is the early-time stage of the transient process. As a result of ohmic losses, the induced surface

currents will start dissipating(Figure 3). The region immediately inside the conductor will then see a decreasingmagneticfield and thus eddy currentswill start flowing through it. This processis repeated in time at successiveinterior points and can best be described as an inward diffusion of the current pattern,

even though the induced currents themselvesdo not usuallyflow in an inward direction.In TEM terminology this is the intermediate-timestageof the transient process.Once this processis completed,the configuration of the induced current distribution will be more or less invariant in time. The inductance and resis-

tance of each current filament has reached asymptotic valuesand we only have an overall amplitudedecrease

(Figure 3). In TEM terminologythis is the late-time stageof the transientprocess.The transientmagnetic field due to these currents or, more usually, its time derivative can be measured at a given station with a suitable receiver. For a current system fixed in geometrical location both the magnetic field and its time derivative have similarspatialvariation. In one variant of TEM methods(singleloop method) the transmitter

loop itself is usedas receiver after current turn-off. The description given is for an isolated, discrete conductor located at some distance from the transmit-

ter loop. For the casein which the surfacecurrentsare initially very localized(i.e., dipole sourcenear a large conductor) the configurationof induced currents may both spread out as well as diffuse inward (e.g., conducting half-space case treated later). The rate of change of these currents and of their accompanyingmagnetic field is dependent on the conductivity, size, and shape of the conductor. By contrast, the initial (t = 0) surfacecurrent distribution is independentof the conductivityof the body and is

429

only a functionof the size and shapeof the conductor. As a consequence,the early-time stageof the currents in the transient process is only weakly dependent on the conductivity of the body. As in the frequency-domaincase a good insightinto the physics of the transient effects can be obtained from simple circuit theory by comparinga TEM system with a transformer having three very loosely coupledwindings,with one of the windingsshortedto simulate the effect of a conductive body (Figure 4).

Let Mij be the mutualinductance betweenloopsi andj and R and L be the resistanceand inductanceof the conductor (shorted loop). For a step function excitation of the transmitter loop, it is easy to show that the current flowing in the conductorloop is given by (Grant and West, 1965, p. 542):

I(t)= Mo• Ioe -t/, t>•0, L

whereI0 is the transmittercurrentandx = L/R is the time constant of the underground circuit. In other words, a sharpdiscontinuityin the transmittercurrent will instantaneouslycreate a current in the underground circuit which will then decay exponentially with a time constant x. At time t = 0, the initial current

amplitude depends only on the primary field flux through the undergroundcircuit (IoMo•) and on the conductor size (L) and is independent of the conductance (l/R). By contrast, the time constantx depends on both conductance

and size.

The receiver coil output voltage es(t), which is proportionalto the time rate of changeof the secondary magneticfield created by currentsin the undergroundcircuit can then be written as

es(t)=M•2•=Io

L

( b ) INTERMEDIATETIME

t>•O, (2)

where •(,) is the unit impulseor delta function.

RECEIVER

TRANSMITTER

M02

I es(t),ep(t)

_L

( a ) EARLY TIME

(1)

•(t)

(c) LATE TIME

CONDUCTOR

Fig. 3. Sketch of the directionof currentflow in a spherical conductor at various times after transmitter current turn-off.

Fig. 4. A circuitrepresentation of a TEM system(Grantand

The amplitude decays with time.

West, 1965).


430


Expression (2) does include the main characteristics of transient fields (Figure 5a). For poor conductors (small -r), the initial voltages are large but the field decays rapidly. There is also a secondaryimpulse at time -r - 0 in addition to the exponential decay. For good conductors (large -r) the initial voltages are smaller but the field decays slower. We thus have a simple criterion of recognizingand differentiatingthe effects of various

V2 =o'tx• h '

conductors.

If the receiver is capable of measuringdirectly the magnetic field instead of its time derivative, no secondary impulse is present at t - 0 and the factor -rdoes not appear in the denominatorof expression(2). In this case, the initial amplitude of the magneticfield will be

independentof its conductancewhile its decay rate, as before, will be dependent on both conductanceand size (Figure 5b). One major drawback of EM methods in general is that many prospectiveusershave difficulty visualizing the physics of the phenomena. This difficulty is in sharp contrast to gravity and magnetics and to a certain extent dc resistivity where most experienced users can approximately sketch the shapeof expected anomalies. This differenceis partly due to the fact that we cannot as yet routinely calculate realistic threedimensional (3-D) EM induction models. We are thus forced to rely quite heavily on our own intuition whenever we attempt to interpret EM data. Although the present chapter contains extensive references to highly mathematical papers on TEM, we use the physicallyintuitive approachthroughoutwhen dealing with TEM

gation at the speed of light can be ignored, and the quasi-static approximation can be used (Grant and West, 1965, p. 469 to 472). In time domain, the EM field componentsin rectangularcoordinatessatisfy,in a homogeneousregion in the earth, the vector diffusion equation

methods.

A great help in this intuitive approachis that the EM induction process, at the low frequenciescommonly used in exploration, obeys primarily the diffusion equation so wave propagation effects can be neglected. This follows becausefor typical delay times of interest in TEM and for typical distances between transmitting and receiving circuits, we are well within the quasi-staticzone where time delays due to propa-

where cr and tx are the electrical conductivity and magnetic permeability respectively. In the air (or= 0) the diffusion equation above reduces to the vector Laplace's equation

which does not include any time derivative terms, althoughboth e and h are time dependent. An important consequenceof equation (4) is that if at any moment the magnetic field due to the then existingcurrent distributionin the groundis calculated directly by Biot-Savart law, the magnetic field measured at that moment at any point in the air is determined exactly. The past history of the induced current system or equivalently of its rate of change does not influencethe measuredmagneticfield. Thus, under quasi-staticapproximation,the magneticfield of the current systeminducedin the groundis a potential field. Additionally, as a direct consequenceof equation (4) each directional component of the transient magnetic field is calculable from a mapping of any other componentover a plane by using either convolution in spacedomain or multiplicationin frequency domain, similar to the MMR case (Edwards, et al., 1978; Macnae, 1984). This is a direct consequenceof the fact that for static magnetic fields the horizontal and vertical componentsare the Hilbert transform of each other (Nabighian, 1972). This pseudo-staticbehavior will prove extremely useful in understanding the results of transient THEORETICAL

,

(a) or

•

(b)

Conductor

t

t

Fig. 5. TEM receiver output voltage (a) and magneticfield detector output, (b) over good and poor conductorsafter a step current excitation.

(3)

field measurements.

BASIS

OF TEM

METHODS

A brief descriptionof the responsesfrom a number of simple geologic models follows. We assume the conductivity and magneticpermeability of the models to be frequency independent. The effect on TEM measurementsof a frequency-dependentconductivity (IP effects)and magneticpermeability (superparamagnetic effects) are treated separately. Two terms are commonly used in TEM methods to describe system responses.The more common term assumesa source waveform consisting of a step-off and refers to measurementseither as "the magnetic field", or as "time derivative", "emf", "voltage" or


TEM Prospecting Methods

"0b/0t" measurements.The other term usesengineering terminologywhere the effect of the sourcewaveform is explicit and describesthe systemin relation to the form of the primary field of the source as seen at the receiver. In engineeringterminologythe emf measurement from a step-off source is the "impulse response" and the magneticfield measurementwould be calledthe "step response".The engineeringterminology is more exact as can be seen if we consider the responseof a system such as UTEM 2 which measuredthe emf from a rampwith an abruptslopechange (West et al, 1984).With an appropriatelinear normalization this system simulatesthe measurementof the magneticfield from a currentstepexcitation,and thus can be classifiedas a step responsesystem. We use both terminologiesat differenttimes in this discussion.

431

ex(Z, t)-2H0 1 trV/•22•_ o-2• e_(•tt.z: T) and

hy(z, t) = Ho erfc

(8)

,

where erfc (z) is the complementaryerror function. For a fixed time t, the variation of ex(Z, t) and

hy(z,t) with depthz is shownin Figure6. They are similar to the normal density and normal distribution

functionwith zeromeanandvarianceV'2t/trlx. Keepingz fixed in equation(8) and equatingto zero the derivative of ex(Z, t) with respectto time, we find that

the transient

electric

field

reaches

a maximum

when

Uniform Conducting Medium

2

Althougha highly simplifiedmodel, the studyof the uniform conductingmedium leads to important concepts which greatly facilitate the understandingof TEM

t-

2

(9)

or equivalently

methods.

The electromagneticfield componentsin a homogeneous region and under quasi-static approximation satisfythe partial differentialequation,

H

H :ø'

(s)

For the simplestcase for which the fields are spatially one dimensional,the above equationreducesto

z =

=d.

(10)

The quantity d is the diffusion length* and represents the depth at which the local electric field (or current) reachesits maximum value for a given time (Figure 7). Immediately we see that this local maximum travels downwardwith a velocity given by 1

v- X/2trlxt. Oz 2

- trlx• H =0'

(6)

The similaritybetween the diffusiondepthd and the skin depth • is remarkable. In frequency domain

In frequency domain, the solutionof equation (6) leadsto the classicalskin-depthproblem. From Ward and Hohmann, (Volume 1) we obtain for the Ex and

the penetration is proportional to 1/• timedomain it isproportional to

Hy components of theEM field

Conducting Half-Space

Ex(z, t) = Exoe-iZ/ae-Z/aeiøøt

Hy (z,t)=Exo •'tre -i•r/4e-iz/ae-Z/aeiø•t,(7) whereg = V'2/trixto is theskindepth.As canbe seen from equation (7) at the depth z = g the EM field is reduced in amplitudeby a factor 1/e while its phase changesby 1 radian. The solutionof equation(6) in time domain can be found in Wait (1969), West and Macnae (1982), and Chapter 1, this volume. For a step function excitation of magnitudeH o establishedat time t = 0, the transient fields are given by

(11) whereas in

Although the theoretical solutionsfor the EM fields due to a conducting half-space have been known for quite some time (Ward and Hohmann, Volume 1), their numerical

evaluation

was often a difficult

under-

taking, especiallyfor nondipolarsourceconfigurations and

time

domain

calculations.

With

the

advent

of

digital computers, such calculations have become commonplacethrough the use of various techniques, e.g., fast Fourier and Hankel transforms,digital filters, etc., in addition to standardintegrationmethods. *Often, especiallyin transient soundings,one uses instead of equation(10) the so called diffusiondistance'r = 2•rd =

2•rX/2t/•rlx,in analogy withthefrequency domain relation betweenwavelengthand skin depth (X = 2•rg).

432


hy (t)

I

Ho

IO--• •ex(t) ß


2Ho

•

Late Time (d=1,000) i0-•

i0-I

,, Early Time (d = I00)

10~2

i0-4

With a step source and a rectangular transmitter loop, the calculatedX, Y, and Z componentmagnetic field decaysover a conductinghalf-spaceare shownin Figure 8a for a measuring point located outside the loop. The main features of the response are the sign changein the vertical componentand the sign invariance for the horizontal components.In addition, for dual loop configurations (Figure 8b) there is an outward migration of the zero-crossoverswith increasing transmitter-receiver separation. To understand the behavior of the transient magnetic field over a conductinghalf-spacewe start from a well known theorem (e.g., Weaver, 1970) that all above-surfacemagnetic sourceswill induce only horizontal

electric

fields and currents

in a uniform

or

horizontally layered ground. This result is a direct consequenceof the strong conductivity contrast at the earth-air interface which leads to the total cancellation i0-3

/"t

i0-5

of the vertical electric field by electric chargesinduced on it. In other words, no matter what the sourceloop shapein the air, the inducedcurrentsin the groundwill flow in horizontal planes.

10-6

loop is switched off, the induced surface currents, under the quasi-static approximation, will be distributed in sucha manner as to maintain the magneticfield everywhere at the value that existed before turn-off. These initial surfacecurrents are primarily localized in the vicinity of the transmitter loop. For a sourcelying on the ground they are an exact mirror image of the transmitter loop current. With the passageof time, the initial current distribution starts diffusing into the earth. As the current system diffuses and decays, it appears to move outward and down. At distancesgreater than those where diffusionis taking place, the local current distribution

--- hy(t) e. (t)

---

At time t = 0, when the current in the transmitter {o-4 0

I

li t

I

200

400

600

,

I

I

1

800

I000

izoo

1400

Fig. 6. Variationof ex(t)andhy(t)with depthz in a whole space conducting medium after a step current excitation.

ioo

maintains

io-I

10'2

0

0.5

1.0

1.5

Z.O

2.5

3.5 d

Fig. 7. Variation of ex(t) with depth at a fixed time t.

its initial

value.

A plot of the induced current density in the ground by a rectangular loop situated on the surface of the earth is given in Figure 9 for various normalized times. As can be seen, the downward and outward expanding current pattern can be aptly described as a system of smoke rings whose maximum, after a short initial period, movesdownward at an angleof approximately 30 degrees. Because the induced current system migrates with time, there is a significantdifferencein the responseof the surface magnetic field and its time derivative. From equation (4), we can obtain the magnetic field at the surface by spatial integration of the fields of the current system in the ground. The time derivative of the surface magnetic field can be derived by differentiation of the magneticfield or, more instructively, can be considered to be the spatial integration of the magneticfield of a conceptualcurrent systemwhich is



433

the time derivative of the true current system(Figure t0). As shownby Spies(1989)the dB/dt measurements have less sensitivity to conductive structure at depth

space can be easily understood.At time t = 0, after current interruption in the transmittingloop, the mag-

than do B measurements.

imagecurrent lying directly under the transmitterloop and which, at points exterior to the loop, creates a magneticfield with a negativevertical component.At later times, with the downward and outward migration of the equivalentcurrentfilament,the vertical fieldwill changesign. By contrast, the horizontal components

As shown, at no time is it necessary to have a downwardcomponentof current densityflowingin the earth in order to accomplishthis downward diffusion. It can be shown (Nabighian, 1979) that, on or above the surface of the earth, the combined effect of all

induced currents in the ground can be approximated by the effect of a single current filament of the same shape as the transmitter loop (Figure t t) and which moves downward with velocity 2

=

(12)

v •/•cr !xt

while increasing its horizontal dimensions propor-

tionalto the diffusion depthX/2t/•rlX. The downward movement of this equivalent current filament takes place at an angle of approximately 47 degrees, i.e., steeper than the 30 degrees for the actual induced currents. The current intensity of the equivalent current filament varies inversely proportionalwith time. With the equivalent current filament concept, the behavior of transient fields over a conducting half-

netic field at the surface

of the earth comes from

start from zero, reach a maximum, and then vanish

while keeping the same sign throughoutthe transient process.

The sign changein the vertical componentand the maximum in the horizontal components of the magnetic field take place when the equivalent current filamentis situatedapproximatelybelow the stationon the transmitter loop side: the more conducting the earth, or the more distant the station, the more time it takes the equivalent current filament to reach this position (Figure 12). It is easy to see that the zerocrossoverof the vertical componentmigratesoutward with

time.

At

late

times

the field

will

become

vertical

ßSTATION

5

-

'/111///I '/0'-- I S/m

L= 54

i0 4 -

IOOO

,._v

I0

-

'///////

L--86

....

(+ ve) (-- ve)

L= 120

IOO

z I0

-

POSITIVE: ....

NEGATIVE

i

i

IO -I

IO

I0

o

I0

i

I0

2

0.1

I

I0

t(ms)

MILLISECONDS

(a)

and

practically independentof position. This changeimplies that the rate of decay of the vertical field is

x

I0

an

,(b)

Fig. 8. Conductinghalf-space'(a)X, Y, Z magneticcomponenttransientdecaysat a stationoutsidea rectangular loop (Newmont EMP files), (b) transientdecaysfor a dual-loopconfigurationsystemfor various separationsL (after Spies 1980).

434



expected to be slower than that of the horizontal field. Indeed, it can be shown that at late times the asymptotic expansions of the transient fields over a halfspace can be written as (Wait and Ott, 1972)

and

(13)

where bz(t) and bh(t) are the vertical and horizontal

Meters

Meters x I00

?

x

components of the transient magnetic field. For the practical case in which we measure the voltage induced in a coil rather than the magnetic field, the above expressionsbecome:

I00

o_L..O.• P

4-

--

emfz (t) • O t•7. 5

Contours x I0'e A/ma

x

.,-

:•12

12-

le

• ß 0.01s/mho/m.

t

Meters

8

0 •-ø:

12 16 20 24 28.

0

emfh (t)• 0(tl-5),

• = 0.1s/mho/m.

Meters x I00

C>. _L.•)T .P 4

and

Contours x 10 -7A/m z

4

8

12

and the relation between the location of the equivalent

x I00 16

20

24

28

x I0-

A/m

3•

Contours x IO-IøA/m a

•. =0.4 s/m•/m.

current

filament

and the location

where

the vertical

component changessign or the horizontal component reaches maximum becomes more complex (Boyd, 1980a). A better approximation for time derivative measurements might be to consider two opposite current filaments. Silic (1987) shows that the time derivative of the current systemin the ground consists of two smoke rings of opposite sign diffusing down-

4.

012' 0

Contours

(14)

t_= 1.6 s/mho/m.

Fig. 9. Contour plots of normal component of current density in the earth for various values of t/•r. The loop, shown in the upper left hand corner has dimensionsof 400 x 800 m. Computations were carried out for a vertical plane passingthrough the center of the loop. Current densitiesare

ward as before. The main contribution

surface field is the shallower

to the measured

of the two.

As is intu-

itively obvious, the zero-crossover for the vertical component at a fixed station location occurs later in time for the impulse response (emf) than for the

expressed in A/m2 (afterNabighian,1979). t>O

DISTANCE (M) o¸

•oo

LO0 P

Iooo I

O'--- "0

.•'•X

t=t 500

t :t$

I000

Z

Fig. 10. The form of the time derivative of the smoke ring current system shown in Figure 9 (after Silic, 1987). Con2 4 6 tour units are dimensionless (•r a ixOE/Ot)xlO where a is loop radius.Plot at time t/•r•xa2=2.5. Horizontal and vertical axes extend over 10 loop radii.

Fig. 11. System of equivalent current filaments, at various times after current interruption in the transmitter loop, showing their downward and outward movement (after Nabighian, 1979).



step response,the crossoverbeing delayedby a factor of X/• in time. An inverse power law like expression(13) or (14) will plot as a straightline on a log-logplot (seeFigure 8). This is a very diagnosticfeature of the half-space responsein addition to the previouslymentionedoutward migration of the zero-crossoverfor the vertical component.

For the singleloop TEM system, a simpleexamination of the smokering patternsin the groundwill show that the measuredsignalwill not changesignin time. However, the late-time asymptotic expansions(13) and (14) remain the same. The early-time asymptoticexpressionsare of interest in certain applications,e.g., TEM soundings.A summaryof results for a vertical magneticdipole of moment m is given in Table 1 (See also Spies and Frischknecht,this volume). Here and in the remainderof this discussion,early-, intermediate-, and late-time TEM processesare described in a manner analogousto the near, intermediate, and far zones in the FEM techniques.Thus, we

435

such thin sheets it is thus assumed that there is no in current flow across the thickness of the sheet.

variation

Maxwell has shown (Grant and West, 1965, p. 500) that for an inductive sourcewhose current is abruptly interrupted at time t = 0, the decayingfield due to the eddy currentsin the thin sheetappear,from either side of the sheet, to come from an exact image on the opposite side which at t = 0 is at the mirror image location and then recedes with a uniform velocity v = 2/S• (Figure 13). Thus, even thoughthe inducedcurrentsare confined only in the plane of the thin sheet and they can only migrate outward (Figure 14), the equivalent current filament (the image)moves away from the locationof actual currents. In contrast to the half-spacecase, the size and intensity of the equivalent current filament remains unchanged, equal to the size and current intensity of the transmitter loop. As an example, for a circular transmitter loop of radius R, whose dc magnetic field on the loop axis is given by (Smythe, 1968, p. 291)

have:

•xI

where

'r = 2-rrd is the diffusion

1

Bz- 2R

Early stagefor 'r <• e Intermediate stagefor 'r = e Late stage for 'r >>e distance

and e is a

characteristicdimensionof the system.

2 3/2,

(15)

the transientfield on the loop axis due to the currents induced in a thin sheet and for a step function excitation can be immediately written as

Conducting Thin Sheet

Another model for which we can obtain a very simple solution is the thin, infinitely extensive conductingsheet. Such a sheetis one whose thicknessd is very smallyet whoseconductivity•r is solargethat the product S = •rd remainsfinite even as d vanishes. For

bz(t) =•-•[ 1+

vt

z+ 23/2 '

For late times,the aboveexpressionapproachesasymp-

totically 1It3 (or1It4 fortheemf).Asinthehalf-space h•>O

h•=O

h•
LOOP

locus of maximum electric field = 30 ø

locus of equivalent current

equivalent current

(16)

filament

= 47 ø

filament

Fig. 12. The use of equivalent current filament concept in understandingthe behavior of TEM fields over a conductinghalf-space.


436


case, the transient field behavior of various components can be fully explained from the movement of the equivalent current filament. They consist of an outwardly migratingpattern of crossoverssimilar to those obtained over the halfspace. The emf crossoversoccur a factor of three later in time than do the magnetic field

Horizontally Layered Earth

Transient field computations over a horizontally layered earth can now be relatively easily accomplished (Ward and Hohmann, Volume 1). A typical responseover a two-layer earth is shown in Figure 16. Not unexpectedly, these curves are similar in shapeto the half-space curves from Figure 8b. Hoversten and Morrison (1982) published computa-

crossovers.

Due to the inverse power law asymptoticexpansion, the late stage transients over a conducting thin sheet will plot again as straightlines on log-log plots (Figure

tion results for the induced

As can be seen from equation (16), the principle of equivalencefor S applies to the conductingthin sheet. In other words, the TEM responseof a thin conductor sheet depends on the conductivity-thickness product rather than on the conductivity tr and thickness d separately. This result can be generalized for most inductively thin conductors.

Table 1. Early and late time asymptotic forms for a vertical magnetic dipole.

hr(t)

hz(t)

Early time t---->0

3mt1/2

m[ 18t

4,rr3 1-•tr•r 2 -3m

e,(t)

2•rtrr4

Late time t---->o•

mrlx2tr2

•r3/2o -1/2[xl/2r4

field inside a few

two- and three-layered geoelectric sections (Figure 17). The computationswere carried out for a repetitive current pulse in the transmitter whose repetition rate was slow enough that it can be regarded as a step function excitation. If there is a conductinglayer in the section, the current will be trapped inside this layer and move mostly laterally, similar to the thin sheet case. If this is not the case, the induced current will move more or less like "smoke rings". By extrapolating the previous results, it is not difficult to see that an approximate solution to this problem can be found again by introducing an equivalent current filament moving downward and outward with varying velocities as it crossesvarious layer boundaries. As before, the late time behavior can be described by an inverse power law, i.e.,

15).

Component

electric

128•rt2

m(y3/2[x 3/2 30,r3/2t3/2

m(y3/2•5/2r

(17)

40•r3/2t 5/2

Transmitter Loop I

S=do'

/////////////•////////////////

d

Fig. 13. Equivalent current filaments (images)for a conductingthin sheet at various times after current interruption in the transmitter loop.



With such a simple physical interpretation, the curves from Figure 16 can be easily understood. The half-space and thin-sheet model represent particular cases of the layered-earth model. The transient response over a layered earth can be obtained by taking the inverse Laplace or Fourier transform of an expression of the form (Ward and Hohmann, Volume

R(X, ito) e-XZJo(Xp)dX,

wherep2 = x2 + y2 z is theheightabovethesurface and R(X, leo), the frequency-domain reflection coefficient of the layering, is a function of all resistivities and thicknesses

in the section.

It can be shown (Kaufman, 1978) that R(X, ito) has singularities which are continuously distributed from zero to infinity on the imaginary axis. Since R(X, ito) is a multivalued function of to, its inverse Laplace transform can be calculated by introducing a branch cut in the to plane (Figure 18). The integration along the branch cut can then be performed (Lee, 1982) and it can be shown that it has an asymptotic late-time expansion as an inverse power of time, i.e., O(1/t•). For each of the one-dimensionalmodelspresented,the transients will have a very diagnostic linear late-time asymptote when plotted on log-log paper.

1(56

I

I

I

I

I

I

0-7

437

Direct calculation of TEM layered earth responses can be performed using time stepping algorithms (Goldman and Fitterman, 1987). Using equation (12) or an equivalent, it is possible to derive a conductivity estimate by keeping track of the downward diffusionof one or more equivalent current sources. This concept has been used to create approximate conductivity• depth image sections of the ground by Macnae and Lamontagne (1987), Nekut (1987) and Eaton and Hohmann (1986). A detailed treatment of this subject can be found in Spies and Frischknecht (this volume). Confined Conductors in Free Space

The EM methods in general and transient techniques in particular were originally designedto explore for highly conducting bodies in very resistive host rocks. Unfortunately, even for this relatively simple case, the geometries that are amenable to analytic solutionsare very few and much use has been made of numerical and analog modeling. The TEM responseof a conducting sphere has been extensively studied in the literature and its results lead

5

8001 METERS STeATION 104

-

THIN

SHEET

/

CONDUCTANCE

=t S

I0 3

o-• TIMES

INDICATED

So=l

S

ARE FOR

tU i0z

FOR OTHER S. MULTIPLY TIME Sn

BY• TOGETNEW TIME

I0 • -IO

I0ø -II

400

800

1200

DISTANCE FROM DIPOLE(m)

Fig. 14. Current density induced by current turn off in a dipole over a horizontal thin sheetas a function of delay time (after McNeill, 1980).

I0-•

I0ø

I0 •

I0

MILLISECONDS

Fig. 15. X, Y, Z componenttransient decays over a conducting thin sheet with tcr = 1 S. The receiver is located outside a 400 x 800 m rectangular loop.


438

Nabighianand Macnae

l0'2

e(t) •a

--

to a good understanding of responses from various confinedconductors(Wait and Spies, 1969;Nabighian 1970, 1971).For a uniformfield excitationB0 oriented alongthe z axis, the stepfunctionTEM responseof a conductingsphere of radius a is given by (Ward and Hobmann, Volume 1)

L/a =:5.8•dßI. 5.•

,..,•,.,,., .•-halfspac .... ponse 0%

5.4

6B0 a 3

', ',

br(t)= 2 r3 cos0 S(t) b0(t) =

3B0 a 3 2 F 3 sin 0 S(t) 'IT

iO-5 positive

3Bo a 3

negative

e,(t) = T'rr 2 F 2 sin0 •:(t), i

(18)

i

i.e., it is equivalentto the field of a z orientedmagnetic dipole, situatedat the center of the sphereand whose

a•/• az

Fig. 16. Transient decaysfor a dual-loopconfigurationover a two-layer earth for various separationsL. The corresponding half-spacecurves are starred(after Spies 1980).

200

300

400

200

moment

decreases

with time.

In equation(18) r, 0, q>are sphericalcoordinates,the

functions S(t)and•(t) aregivenby

300

400

0

-ll.4• (d) t

!

MODEL

C

h1:50m• Pl =50 •- rn

hz=50rn,/;2 = I •.m

p$=50•-m (e) t = 30

(f) t =•

Fig. 17. Inducedelectricfieldin voltsper meterinsidea threelayergeoelectricsection.Computations carriedout for a repetitive currentpulsein the transmitter(after Hoverstenand Morrison, 1982).


Expression (19) has a noteworthy asymptotic property. Indeed for late times, provided that Ix is not much greater than Ix0, it reducesto a single exponential

-k 2t/'r

S(t) =

439


A• e -t/•'• -k2t/x

o•(t)= • e

which will plot as a straightline on semilogpaper. By contrast, at early times (t < 0) the decay obeys the

k=l

and•- = •r[•a2/rr 2 is thetimeconstant of thesphere. The functionsS(t) and•(t) are plottedin Figure19. Thus the responsefrom a spherical conductor can be represented as being due to an infinite number of eigencurrents each having a different time constant and amplitude:

n=l

•'n =

7I'

powerlawt -1/2(WaitandSpies,1969). These facts can be easily generalized for confined conductorsof arbitrary shapein free space. It can be shown that the secondaryfields generated by currents induced in the conducting body can be separated into componentsthat are functions either of time or spatial variations.

The time variations of the decaying induced current system J(r, t) set up after a step function excitation, can be expressedas an infinite sum of mathematically separatedeigencurrentsin the form (Annan, 1974)

F(t)= • An e •r•xa

(21)

2

2n 2

(19)

where •'n is the time constantof the nth eigencurrent ('r• > 'r2 > 'r3.... ) andA n is the correspondingamplitude factor which encompassesall geometric parameters of the problem. The spectral responsecan be obtained by taking the Fourier transform of equation (19). We immediately obtain

J(r,t)= • Jn(r)ane- t/xn.

(22)

n=l

Each eigencurrent has a fixed geometry given by Jn(r) and an exponentially decaying time variation with time constant•'n. The eigencurrentsdo not interact with each other. They can be thought of as uncoupled current filaments with a given resistance and inductance.

F(to )=

to-A. ico n' ton = 'rn ,

n=l

(20)

The eigencurrents and their time constants are functions only of the conductivity distribution and are

i.e., a spectral response with an infinite number of distinct poles situated on the imaginary axis. Imco

ioø

F

C

i0'l _

•o-z I0-• •

I I0-•

t

•-•a a

B

Fig. 18. Integration path and branch cut for calculatingTEM integrals over a layered earth (after Lee, 1981).

I I0-z

Fig.19.TheS(t)and•(t) transient functions fora conducting sphere.


440

Nabighianand Macnae

completely independent of the type of source field used. The effect of the sourcefield is accountedfor by the amplitudecoefficientsan. Once an eigencurrent system is determined, the transientsecondaryfield can be calculatedindividually for each eigencurrent by using the Biot-Savart law. Thus we obtain

hi(t)- E Cn,i

(23)

n=l

wherehi is eitherhx, hy or hz andthe Cn,i arenew coefficientsincorporatingthe a,'s and the eigencurrent-receiver geometrical coupling. For large t the responseis closely approximatedby the first term in equation(23) and will plot as a straight line on semilogpaper. The primary source which is not uniform can be expressedas a multiple expansionabout the principal axes of the body where each term creates its own system of eigencurrentssimilar to equation (19) (Figure 20). The longesttime constant (principal pole) for each multipole component will be progressively shorter as the multipole order increases(Nabighian, 1970).

It is evident from equation (19) that at early times the responseis nonexponentialwhereas at late time, the responsebecomespurely exponential.Also, due to the nonuniformity of the primary field, the excited time constantsare not only related to conductivityand dimensions of the conductor, but also to the location of the loop with respect to the conductor. As a result, time characteristicsof the induced fields may vary from one observationpoint to another, which greatly complicatesthe problem.

FI (t) 0-8

N:,

FOR VARIABLE K

The number of multipoles needed for an accurate representationof transient phenomena in a sphere increases with the proximity of either source or receiver to the sphere. For both source or receiver more than a few radii away from the sphere, the principal time constantis the predominantone and the response will be almost purely exponential. For source and/or receiver even moderately close to the sphere (i.e., shallow bodies) as much as 15 multipoles might be neededfor an accurate representationof the transient fields (Lodha and West, 1976). Along this line, it becomes evident that a large transmitter loop will generally induce fewer significant multipoles than a smaller (dipolar) transmitter loop. As a result, the chance of underestimatingthe geoelectricparameters of a given discrete target conductor from its time constantis greatly reduced with the larger loop systems. However, the large loop source will selectively tend to energizethe longer time constantcomponents of large features such as a conductive overburden.

Since the eigencurrentsin a sphereare confinedand thus cannot expand laterally, we would expect to find an equivalentcurrent filament which does not migrate downward. This is in sharp contrast to the layeredearth case in which the equivalent current filament does migrate downward to account for the lateral diffusion of induced currents.

Becauseof the sphericalsymmetry, the responseof a sphere is triply degenerate, i.e., each coordinate axes can support the same system of eigencurrents with the same time constants.

If the conductor

iI),• R

N--5 FOR

VARIABLE

K

0'6

0-4

0-2

•

oo ' '0

.05 -10 f/d

ß15 -20 .• • •

o.o

is not

spherical, each coordinate axis will support its own system of eigencurrentswith its own transient characteristics. As such, at early times the responsemight come predominately from the eigencurrentsaround a

.15

t/ d • b•

.20

'25

ß0

-10

t/d

'15

'20

l.t b2

Fig. 20. Normalizedtransientdecaysof variousmultipolesover a conductingsphere(after Nabighian,1970).



given axis whereas at late times the responsemight be dominated by the eigencurrents around another axis, leading to a characteristic rotation of the magnetic field with time. In all cases, the transient response can be represented as an infinite sum of exponentials or equivalently of distinct spectral poles (Kaufman, 1978).

An interesting example of possible rotation of the TEM field with time is provided by the case of an infinite elliptical cylinder under uniform transverse primary magnetic field excitation (McNeil, 1980;

441

the first 15 eigencurrentswhich have a comparatively small range of time constants. Increasing this default limit is not possible without complete respecification of the quadrature integration used in the program. These default

calculations

are not accurate

if either the

source or receiver are close to the plate or if the plate dimensionsare much larger than the dimensionsof the transmittingloop. However, despite its limitations, the PLATE program has proved invaluable in both understandingand interpreting observed transient fields.

Kaufman and Keller, 1985). As can be seen from Figure 21 the late time constants of the elliptical

cylinder for the two directions of the primary field excitation are significantly different from each other for large eccentricity values (a/b > 2). As a result, if eigencurrents in both directions are initially excited, the transient moment will eventually rotate in time and approach asymptotically the field due to the eigencurrents with the larger time constants. A most useful model in TEM interpretation is the finite thin conducting plate. Unfortunately, this problem does not have a simple analytical solution. Annan (1974) provided a numerical algorithm for computing the transient responseof a thin rectangularplate based on an approximate eigencurrent expansion similar to equations (22) and (23). Due to the nature of the problem the eigencurrents are confined to the plane of the thin plate. As a first step, Annan computes the induced eigencurrents which depend only on the geometry of the plate. Next, he calculates the coupling coefficients between each eigencurrent and the source and receiver. Finally, the transient response is obtained by summingover all eigencurrentsas outlined in equation

EIG ENPOTENTIAL

15

Fig. 22. Selected eigenpotentials in a rectangular thin plate (after Annan, 1974). IOO

i

i

i

(23). A few eigencurrents are shown in Figure 22 and a

calculatedtransient responsein Figure 23. The present version of the PLATE program written at the University of Toronto (Dyck et al., 1981) has as default only a = b

I

2

4

Q

2q(•) = I1.56 8.64 7.I

-• Hp

2q(•) = 11.5617.3 28.6 /xSo z- =•

8

16

6.5

5.9

50.7

•

hz RESPONSE

,,

97.3

,WHERE S=2crb

2q(•) Fig. 21. Table for calculation of the time constant of infinite elliptical cylinders (after McNeill, 1980).

0.1

I

I0

I00

DELAY TIME (ms)

Fig. 23. Decay of a voltage measured above a 100 m deep plate located in a resistive background.

442



All of the confined

models

discussed

were

of uni-

form conductivity. However, even relatively massive mineralization is often surrounded by a halo of less massive ore. Analytic treatment for nonhomogeneous conductors was carried out for a two layer sphere (Nabighian, 1971; Hjelt, 1971) and for a single sphere with a conductivity which varies radially. An analysis of the transient response from such inhomogeneousbodies shows that its pole distribution is extended and emphasized toward the shorter time constants of the respective homogeneousconductor. As a result, the target conductivity seems to increase with delay time, leading to a very diagnosticfeature for interpreting such targets. Figures 24 and 25 show the transient response for a two-layer sphere in a dipolaf field (Nabighian, 1971). ioø Fn (t)

F n (0)

i0-z

(a) i0'3

i0 -4

10"5 0

I00 .

.I

,2

!

I

.3

.4

i

.5

I

.6

•

.7

I

.8

I

.9

1.0

I

I

In spite of its simplicity, the spherical model has proved useful in the interpretation of TEM data. The SPHERE program (Dyck et al. 1981) written at the University of Toronto as a companion to the PLATE program, is now widely used in TEM interpretation. Confined

Conductor

in Conductive

Media

Most of our understanding of the influence of conducting host rock on EM measurements comes from the excellent work carried out at the University of Toronto (Lajoie and West, 1976; Lamontagne, 1975; Hanneson and West, 1984). When the target is located in a conductive host rock the inducing magnetic field, at the location of the target, is no longer a stepfunction but a slowly varying field due to the diffusion of the smoke ring. The:vortex currents in the target are now induced by the time derivative of this delayed and broadened field (Figure 26). In the case of confined conductorsin free space, the secondaryEM fields are created by toroidal vortex currents flowing in the target (Figure 27a). When the surroundinghost rock is conducting, the smoke ring electric field will instantaneouslybe opposedwithin the target by the field of an electric charge distribution created at the target boundaries (Figure 27b). The magnitudeof these chargesis determinedby the instantaneousvalues of the normal discontinuity in the electric field at the location of the charges. The process is sufficiently rapid that it can be described (Alpin, 1966; Kaufman, 1985) in terms of dc current flow in conducting media (the extension to timevarying fields is a straightforwardlinear process). The governing equations for dc current flow in a heterogeneous,source free conducting medium are:

I

div J = 0 and

divD=e I(fzL

XX

•

I0'$

FORVARIABLE O' z /O'$

-I

(24)

where p is the free electrical chargedensityand e is the dielectric permitivity. From equation (24) we can write divJ=div•E=E-

0

E=p

grad•+•divE=0, (25)

from which we immediately obtain io-4

div E = 0

.I

.2

,3

.4

.5

.6

.7

.8

.9

1.0

Fig. 24. Transient response for a two-layer sphere in a dipolar field for (a) variable multipole order n and (b) for variable cr2/cr3(after Nabighian, 1971).

1 o'

p E- grad • =-, E

(26)

and finally

E- grad • = - e E ß grad (log •). (27)



From equation (27) we see that electrical charges will appear whenever there is a nonvanishinggradient of conductivity and the electric field has a component parallel to it. For the simpler case in which the conductivity changesabruptly at the boundary between two media, relations (24) reduce to Jn2 -- Jnl -- ø'2En2 -- ø'lEnl -- 0

(28)

443

field intensity on both sides of the interface and •; is the surface charge density.

EliminatingEnl from equations(28) and (29) we can write

•2 En2 -- el or

and

O'1 •--En2 g2 En2 - •1 Enl= •,

(29)

En2 = •

(0'1œ 2 --O'2œ1).

(30)

Similarly, eliminatingEn2 we can write

where Jnl, Jn2, Enl and En2 are, respectively, the normal components of current density and electric

0'2 • =Enl (file2 - ø'2•1)'

(31)

Adding now equations(30) and (31) we finally obtain IOø

!

I

!

I

I

I

I

I

X;=

I

2 En, av[O'lœ2-- 0'2œ 1] o-1 +o- 2

,

(32)

whereEn,av = 1/2(Enl+ En2) is the averagenormal electric field intensity on the boundary. For the practical casewhen el = e2 = e, equation(32) reducesto o- 1 -- o- 2

•; = 2 . t ' En, av •.

o-1 +0- 2

(33)

As before, surface electric charges will appear whenever there is a conductivity contrast and the electric field has a component perpendicular to the boundary. The secondary electric field generated by these chargeswill cause a poloidal current flow which tends to cancel most of the primary electric field inside the conducting target (Figure 27b). This cancellation is more effective for short strike length bodies which, therefore, have less poloidal current flow than longer bodies. The strength of this poloidal current depends mainly on the conductivity of the host rock at the location of the target and it is only weakly dependent on the much higher conductivity of the target. As such the secondary magnetic field will decay at a rate governed chiefly by the host rock and its conductivity, beinglittle influencedby the conductivity of the target. The secondary magnetic field set up by this channeled current may be attenuated and distorted in reaching the observation points. In all cases, the general nature of these fields will be closely related to the magnetometric resistivity (MMR) response of the target (Edwards and Nabighian, this volume), the only difference being the nature of the electric field excitation.

Fig. 25. Transient response for a two-layer sphere in a dipolar field for variable b/a. Graph (a) is for n = 1 and graph (b) is for n = 2 multipole (after Nabighian, 1971).

The importance of current channeling (or current gathering) effects cannot be over-emphasized. The more elongated the target, the stronger the current channeling effects. Indeed, as seen in Figure 28, the

444


rents,whichin thelimit of smallbodiesvariesas 1/R3


current concentration (channeling) along the major axis of a prolate spheroid for large values of the

become sufficiently small to allow simple "superposition" as a viable working hypothesis, (Nabighian in Braham et al., 1978, McNeill et al., 1984). It is worth mentioning that for an equal depth to target, the spatial variation of current channeling anomalies will in general be slower than the spatial

(Figure 29). As in the case of vortex currents, the shapeof the anomaly due to current channelingvaries somewhat in time due to the variation of the spatial distribution of the primary electric field at the plate. Since the anomaliesdue to both poloidal and toroidal inductive effects are generally of the same sign at any point, the detectability of a given target is increased. At the same time, the combined anomaly due to both types of current flow will be spatially smeared toward longer wavelengths compared to that of vortex currents alone. The net effect is that the conducting body will appear deeper and a better conductor than the actual target if interpretation is carried out with free space models. Since the smoke ring from the transmitter loop travels outward and downward, current channeling effects will be relatively stronger exterior to the loop. Thus, time domain methods with separate transmitter and receiver loops are more likely to detect such anomalies than are the single loop methods in which measurements are made away from the region in which current channeling takes place. In addition, TEM systems with large transmitter loops have, in general, a larger primary electric field to primary magnetic field ratio than smaller transmitter loops, thus further emphasizing current channeling anoma-

variation

lies.

conductivitycontrast(r•/tr0 is limited only by the ratio a/b between the major and minor axis of the spheroid. Current concentrations in excess of 100 can easily be achieved. In addition, the interior current flow has a linear sensitivity to conductivity changesup to about the order of the a/b ratio (Seigel, 1974). It is not surprisingthat often TEM current channeling anomalies are larger than the anomalies due to vortex current flow, particularly if the channeling component has a shorter time constant than the inductive part. This is true, under certain conditions, even for the case of a conducting sphere in a conducting media (Singh, 1973). Walker and West (1987) have provided some simple rules to define a priori when inductive or channeled currents will dominate in any specific case. The interaction

between

the vortex

and channeled

currents is in general weak but of a complex nature. However, at intermediate times, the interaction can

of anomalies

due to toroidal

induction

cur-

2.5

Circular

Loop

2.0

Bx lOOm

fi

x

1.0

0.5

o

-0.5

0.05

O. I0

0.15

0.20

O.Z5

0.$0

j)t (tim-s) Fig. 26. Time derivative of subsurfacemagneticfield as a function of resistivity x time. B is normalizedwith respect to the transmitter dipole moment M and the half-spaceresistivity 9 (after McNeil et al., 1984).



Singh (1973) and Lee (1975b, 1981a) have shown that the transient responseof a confined conductor in a conducting medium can be calculated by evaluating the corresponding frequency domain expressions

O.

b.

T•

EP

around

445

a branch

cut similar

to the one-dimensional

case. The branch cut is taken around the imaginary axis where the poles of the confined conductor lie. It can be shown that the late time response can be represented as an inverse power law, i.e. O(1/t •) which is characteristic of the host rock response. This result is not unexpected if we take into account the fact that the late-time response of the host rock is given by an inverse power law whereas the late-time responseof an isolated confined conductor is given by an exponential expression and we have 1

e-at < •

tv

s

HS

TOROIDA L

H

J

POLOIDAL

(magnetic dipole)

J

(electric current dipole)

Fig. 27. EM fields and currentsabout a conductiveplate in a conductinghalf-space. (a) Illustrates the generationof vortex current Jv by inductive couplingwith transmitter primary field change OB/Ot. (b) Illustrates the generation of

channeled currents Jgby galvanic coupling withthecomponent of the primary electric field E in the plane of the plate (after West and Macnae, 1982). tooo

•/%:

PROLATE SPHEROID

iooo

for t large and v > O.

For a conducting body in a conducting host rock, the early time domain response is governed only by the response of the host rock since at t = 0 the induced currents will flow only on the surfaceof the conducting half-space. This is another way of expressingthe fact that the early time-domain response contains mainly high-frequency components with very small skin depths. It thus becomes evident that there is only a limited time window in which the response from the confined conductor can be recognized (Figure 30). In many practical cases of economic massive sulphide targets and conductive or overburden covered terrains, this window falls between a few milliseconds to a few tens of milliseconds, well within the measuring range of common time-domain systems. An excellent illustration of the above point is provided by the case history shown in Figure 31. As discussed in Silic (1987), McCracken et al. (1986a, 1986b), and Eaton and Hohrnann (1987) the time window for detection occurs earlier for the magnetic field measurementthan it does for the emf measurement.

•1oo

O'I /0' o =tOO

z

0'• /0' o : I0

t

I

I

tO

tOO

tOO0

a/b

Fig. 28. Current channeling by a prolate spheroid in a uniform field (after Siegel, 1974).

It is worth mentioning that for FEM techniques, since the main low-frequency responsesof confined conductors are quadrature components proportional to frequency, they cannot be separatedin exactly the sameway as exponentialdecays. To obtain equivalent information to TEM techniques, we need to measure sufficiently well the in-phase component or the frequency derivative of the quadrature component. This can be achieved, as for example, in the FEM system known as GENIE (Johnson and Doborzynski, 1988). As such, a FEM system measuringat a few improperly selected frequencies can conceivably fail to detect a given target, as would of course happen to a timedomain system measuring at inappropriate delay times. Gornez-Trevino (1987) discussed the relative sensitivity of time and frequency-domain measurements, which is a more complex subject than it might appear here. Additional insight regarding this problem can be

446

Nabighianand Macnae


found in papersby Singh(1973), Kuo and Cho (1980), Spies (1980), Oristaglioand Hohmann (1984), SanFilipo et al. (1985), Rai (1985), Newman and Hohmann (1988), Gupta et al. (1987), and Lee et al. (1989). Effect of Conductive

resistive host rock, is in conductive contact with the overburden.

In the first case, the presence of a conductive

overburdenwill delayandsmooththe targetresponse, similarto the caseof a conductinghostrock (Figure 32a). This delay is proportionalto the conductivity-

Overburden

thickness product of the overburden and the overall

In many parts of the world (Southwest U.S.A., Australia, Brazil, etc.), the weathered surface rocks or

dimensionof the measuringsystemand is simplythe delaycausedby diffusionof the transmitterfieldaway

overburdenare sufficientlyconductiveto significantly hamperEM measurementseven in the presenceof a relatively poorly conductinghost rock. From both model (Lamontagne, 1975; Lowtie and West, 1965) and numerical (Lajoie and West, 1976) studieswe now havea clearunderstanding of overbur-

from the source.

The initially induced surface currents will flow on

the surfaceof the overburden,completelyshielding the conductingtarget (overburdenblanking).At later times, the primary field penetrates the overburden layer, andthe responsefrom the targetcan be readily identified.The inducingfield seenby the conductor, which is the time derivative of the primary field, is

den effects both when:

(a) the underlyingtarget is located totally within a resistive host rock.

(b) the underlying target, although located in a

delayed, broadened, and attenuated. The model re-

0.8

I Vo rte x 0-7

/

x

0.6

Galvanic

Vortex 0.5

/

/

0-4

Galvanic

/ 0.3

/

/

/

I

/

I

/

0.2

I i

i

I

0.1

I !

i i / !

/ ! /

-0.1

-0.2

--

-0'3

--

- 600

Plate Location• -500

-400

-300

- :•00

- I00

o

• -i-IOO

Distance (metres)

Fig. 29.Profilesof vortexandgalvaniccomponents ofB for a platelocatedat a depthD = 200m. Plateconductance = 100S; half-spaceresistivity= 100f•.m; time = 3 ms (afterMcNeill et al., 1984).


sults shown in Figure 33a (Lamontagne, 1975) clearly illustrate this result. It is worth mentioning that, to a first order approximation, the late time responseof the


dike and overburden

alone are additive

if the two are

not in galvanic contact. In other words, superposition is a valid first-order approximation in this case. When the dike is in contact with the overburden

and

the geometry is favorable, currents from the overburden will channel through the more conductingdike. At early times this channelingeffect takes place mainly at the upper edge of the dike and can be approximated as being due to a single line current filament. At later times, once the overburden currents have migrated away, the responsefrom the dike can again be recognized (Figure 33b). Not unexpectedly, the anomaly is enhanced in initial amplitude and prolonged in time as a result of current channeling (Figure 32b). The above examples are for a step response system. The results will look rather different for an impulse response

447

Two-Dimensional

Conductors

The case of very elongated conductors, with a strike length much greater than their cross-section,is presented for completeness (see also Figure 21 for the case of the infinite elliptical cylinder). Kaufman (1978) has shown that for nonuniformprimary field excitation the spectral response of any two-dimensional (2-D) body has singular points continuously distributed on

the imaginaryaxis from a finite value too to infinity. The correspondinglate time response can then be shown to be of the form

i.e., the product of an exponential and an inverse power law.

system.

Not surprisingly, inhomogeneitieswithin the overburden itself may give rise to current channels whose effectscan easily be confusedwith those of conductors beneath the overburden. Smith and West (1987) have

200

E

I00

provided a computational method for calculating such data and accurately modeled a detailed field example described in Irvine and Staltari (1984). o

P-

106 -

•o

E

•Overburden

7.0 ms

•

8

ffi

6

z

4

<

2

• =O.IS/2

io3

o --75•

\\

i

2000 PLAN

SURVEY

I

I

Io-•

I

I½'•

I

i

Io-•

i

I

I

i

I

ZZO0 2400 E METERS

I

I

2600

I

I

2800

LINE

i

I

ELURA

DEPOSIT,

AUSTRALIA

TI IVl• ($• Fig. 30. Schematicpresentationof "detectability window" for a target under a conducting overburden (after McCracken, et al., 1986a).

Fig. 31. SIROTEM constant delay time profiles over the Elura deposit At 40 ms delay time (not shown) the anomaly vanishes. The detectability window is confined between 5-25 ms (after Spies, 1980).


448


A good insight into this problem is provided by Weidelt (1983) who gave an analytic solution for both the harmonic and transient responseof a thin dipping dike for a dipolar excitation. Leppin (1989) has recently presented results of the computer modeling of full 2-D structure with finite loop sources. For a horizontal circular cylinder and for a primary magnetic field that does not change along the axis of the cylinder, the transient response can again be represented as a sum of exponentials. Indeed, for a uniform field excitation B0 oriented perpendicularto the axis of a conducting circular cylinder of radius a, the TEM response is given by (Ward and Hohmann, Volume 1). 2

a

br(t)= 4B0•5 cos4)C(t),

•c

k=l

(35)

ilk

where•-k= (ytxa 2/nk2 andn• arethesuccessive roots of the equationJo(x) = 0 (i.e. n• = 2.40, n2 = 5.52 a.s.o.). The time constant of the cylinder is obtained at

latetimesas•- = •'l = (Ytxa2/n• 2 -• (Yt xa2/5.78. ThefunctionC(t) andits derivative •7(t),representing the induced emf in a coil, are plotted in Figure 34. The case of an infinite line current parallel to a 2-D conductivegeometry is called the 2-D case, that of 2-D conductivity and a three-dimensional (3-D) source is often called the two and one-half dimensional (2-1/2-D) case. 2-D models are only practically useful if they can be realistically used to model 3-D situations. The limitations of the approximation of a 3-D case by a 2-D model

and

-t/•'t

C(t)= • e 2

have

been

discussed

to some

extent

in the

literature. 2

a

b,(t)= 4B0•5 sin4>C(t),

(34)

where r, 4>,z are the cylindrical coordinates.As can be easily seen, expressions(34) are equivalent to the field of a linear magnetic dipole situated on the axis of the cylinder and whose moment decreases with time. In equations (34) the function C(t) is given by

BLANKING IOO

In 3-D cases, current channelingis a term often used to refer to current passingthrough a conductor whose source is a charge distribution at the conductor-host boundary. A common conception (e.g., Eaton and Hohmann, 1984) is that since local charge cannot accumulate on boundaries under the 2-D assumption, that current channeling effects are therefore not modeled in 2-D cases. San Filipo et al. (1985) reiterated this

$00

EFFECT

CURRENT CHANNELLING ; EFFEC T,

'•x•'FREE AIR

I00f•

•x••\•W ITHOB

TRAVERSE DIKE

3,00•I•x4M

w

3O

120 6

•d

DIKE ond OB IN CONTACT

IlS

DIKE

ONLY

t I

I I

.

I

I

I

I

m

m

m

m

m

8

7

6

5

4

3

2

I

CHANNEL

(o)

m

8

7

6

5

4

$

Z

CHANNEL

(b)

Fig. 32. Scale model of the decay pattern of a conductorunder overburden. The step responsewas measuredin 8 time windows numberedfrom ch 8 (100 s) to ch 1 (12.8 ms), and where appropriate, the overburden responsehas been stripped (after Lamontagne, 1975).



conception,while demonstrating numericallythat 2-D and 3-D modelsgaveequivalentresponsesonly in the casewhen current channelingeffectsdominatethe 3-D model. This inconsistencyhas not been discussedin the literature.

In fact, it is possiblethat effectsbestconsideredas current channelingmay be present in the 2-D case, even thoughno local current gatheringtakes place. Let us consider a test volume V within which currents

J are flowing(Figure35). We can separateJ into three components:closed loop (toroidal) currents T containedentirely within the volume, poloidalcurrentsP originating on chargeswithinthe volume,andresidual currentsL passingthroughthe volumewhichare both curl and divergencefree and thus obey Laplace's equation.ThesecurrentsL may arisefrom induction and/or chargesourcesoutsidevolume V. Figure 27 givesan exampleof T andP currentsfor a testvolume just largerthan a finite tabularconductor. In 2-D, the secondarycurrent modeled in tabular conductorsis unidirectional(e.g., Oristaglioand Hohmann, 1984). Undirectionalcurrent cannot be the resultof purelylocalinductionwhichrequiresclosed vorticesT (althoughasymmetriccurrentflow may be

4 I

6 I

-•--TRANSMITTER

8 I

I0 I

12 I

separatedinto a vortex componentT and a residual currentL). To createthe primary currentin the source wire, it is obviousthat a sourceof chargemustexist at +/- infinity, and that the electric fields of chargeat

infinitywill be parallelto local boundaries.The fact that the amplitudeof unidirectionalcurrentsmodeled in 2-D casesis identical to the amplitude modeledfor the unidirectionalchanneledcurrent in long 3-D models confirms that this 2-D current L is exactly the channeled

vertical fieldovera halfspace behaves as1/t2 rather than 1/t2'5 as observedand calculatedfor large loop systems.Thisdiscrepancy hasalsonot beenresolved in the literature.

The resolutionof the varying power-law falloff un-

doubtedlylies in the fact that the 2-D approximation is, in fact, incompatiblewith the quasi-staticapproxi-

14 I

<

WIRE AT 0

--

CH :5.

current.

Examplesof the use of two infinitewires carrying currentsof equaland oppositepolarity and 2-D geometry have been modeledin Kuo and Cho (1980), Goldmanet al. (1986) Oristaglioand Hohmann (1984) Adhidjajaet al. (1985) and Adhidjajaand Hohmann (1988). As discussedin Nabighian and Oristaglio (1984), the late time asymptoticexpansionfor the

4

6

8

I0

12

14

I

I

I

I

I

I

TRANSMITTER WIRE AT 0

CHI.

CHI.

CH2.

449

CH2.

•

-,

CH3. --

•

CH4. -

•

--•o

CH 5

•'/o

CH6. .•_ a-a' '•

40%

CH5 •f CH6.

- ..............

CH7.

CH 8. '......"'""'" ""•••'•'• ß

CH8.

60 m

DIKE E•

DIKE

_- UND MEASURING LEVEL

....

OB ONLY

.......

DYKE ONLY( MODEL A)

-- - INCONTACT

OB ONLY •

MEASURING

___

DEPTH=t2_m! THIN OB SHEET I.IS DIKE

12_0S , 300x 150m ii

LEVEL

DEPTH •. 15m'1• THIN OBSHEET I.IS DIKE [•

12.0S • •:)0 x 150m

(a) (b)

Fig.33.Scalemodelprofiles fromwhichthepeak-to-peak decays of Figure32werederived (afterLamontagne, 1975).

450



mation. The full Helmholtz equation for the electric field in the y direction is (Ward and Hohmann, Volume 1, p. 132)

02ey 02ey 02ey

0ey

02ev

Ox 2 + Oy 2 Oz 2

Ot

Ot2'

+ •

= !•r

+ !•

•

which is the equation solved numerically in 2-D algorithms. Subtracting the two last equations from each other, equation (37) becomes strictly true only if

02ey

(36)

02ey

OY 2 = }xeOt2 .

Under the 2-D and quasi-static approximation, equa-

Nowsince 02ey/0t 2isfinite, although small, 02ey/0y 2

tion (36) reduces to

must also be finite, and thus cannot be constant in the

02ey

02ey

Ox 2+

oz

02ey

2 = !x• ot 2,

(37)

I

(• (t)

limit as y tends to infinity under any circumstances.In fact, it can only be approximated by a constant for a reasonablefraction of a wavelength in the y direction, and when lengths become comparable to a wavelength charge distributions do in fact accumulate on boundaries parallel to the inducingfield (throughcapacitative effects), and current crossesthe boundary. The physics of this process is discussed in engineering texts such as Jordan and Balmain (1968) with respect to radio antennae. IP Effects

c (t)

iO -t

-2

I IO-$

IO-•

IO-i

t

Iøo

Fig. 34. DecayfunctionC(t) andits derivative •(t) overa horizontal circular cylinder.

TEST

VOLUME V

Fig. 35. The separation of currents in a test volume V into toroidal currents T locally induced and contained entirely within the volume and poloidal currents P originating on chargeswithin the volume, together with residual currentsL.

When interpreting most EM data, the conductivity can normally be assumed to be independent of frequency or delay time. In most casesthis assumptionis excellent for the bandwidth (approximately 10 to 10 000 Hz) of typical EM systems.Where variation in conductivity with frequency exists, the conductivity tends to increase with increasingfrequency, and such materials are termed polarizable. The effect is attributed to electrical polarization induced by current flow. Theoretical studies by Bhattacharyya (1964) and Dias (1968) indicated that polarization might be measurable

with

an inductive

sensor.

The inductive

EM

responseof a ground with a realistic small polarizability is generally indistinguishablefrom the response of another ground without polarization (Hohmann et al., 1970, Macnae, 1981). The current system induced in a polarizable body can be separated into two parts (Smith et al., 1988): one part, termed the fundamental inductive current is chosen to be exactly the current flowing in a similar nonpolarizable body; the other residual part, termed the polarization current, is generally much smaller than the fundamental inductive current and flows in the oppositedirection. The polarization current is the result of polarization generated originally by the fundamental inductive current. It has an actual reverse direction, and as it decays its measureddb/dt field is oppositeto the measureddb/dt EM decay. In coincident-loopmeasurements,the responseof a ground whose properties are linear and independentof frequency is purely positive (Gubatyenko and Tikshayev, 1972; Weidelt, 1982; Guptasarma, 1984). As a result, the very small negative transients occasionally observed in coincident-loop measurements (e.g.,



Spies, 1980) have been attributed to a polarization current. However, to model these observed negatives with either a half-space or with buried conductors is not possible unless the polarizability is unrealistically large (Morrison et al., 1969; Astrakhantsev et al., 1975; Lee, 1975a, 1981b; Raiche, 1983b; Lewis and Lee, 1984; Raiche et al., 1985; Flis, 1987; Thomas and Lee, 1988).

It is possible to generate negative transients with realistic small polarizabilities, but only when the following "favorable coupling" conditionsare satisfied: (1) The transmitter must couple well to the body so that the fundamental inductive current is large, and the resulting polarization current is significant; (2) The EM response associated with the fundamental inductive current must decay away quickly; and

(3) The polarization current must couple well to the receiver to enable its small response to be greater than the positive response of the induced current.

451

ular cases coincident loop negatives can be observed even when the bodies are only weakly polarizable. The example of an overburden edge provided by Smith (1988) is most illustrative. Figure 36 shows the responseof a nonpolarizable overburden edge calculated using the algorithm of Weidelt (1983). Over the overburden, say at Station 8840E away from the edge, the response is large at early time reflecting the good coupling of transmitter and receiver to the overburden. As the coincident-loop system is moved toward the edge of the overburden, the coupling remains large, but the responseexhibits a more rapid decay. For example, the response in Channel 7 is locally minimum

at station 9480E.

that these three favorable

cou-

pling conditions can be satisfied for the following geometries: a conductive overburden (Flis, 1987), two interacting conductors, an isolated conductor in a very resistive host, a dipping dike and an overburden edge (Smith and West, 1988a, 1988b, 1989). In these partic-

of minimum

response in each channel varies with position as a consequence of induced current migration near the edgesof the sheet. Since the responsehas effectively decayed by late delay times, the nonpolarizable overburden can be seen to satisfy both conditions (1) and (2).

When the overburden is polarizable, the strongest polarization current flows close to the transmitter where the fundamental

It has been shown

The location

current

was a maximum.

This

current does not migrate away from the transmitter and, for a coincident loop receiver, condition (3) is satisfied also. The response of a polarizable overburden is shown in Figure 37, calculated from the nonpolarizable response using the approximate convolution algorithm of Smith et al. (1988) which is similar to that Model: Polarizablehalf-plane

Model: Nonpolarizable half-plane 10 5 -

10 5 10 4 _ 10 •

-

102

-

10

-

lO

-lO

'

8840

I

I

I

I

I

,

I

,

I

,

I

o

-

8840

I

996O

9000 9160 93•2094•8096•4098•009960 Eosf,/rn

,

1 -1

I

,

I

o ,

I

,

I

,

I

,

I

,

E

Half-plone

Holf-plone

6t=4.75S,

R=90m

I

,

I

,

I

,

I

,

I

,

I

E

_c 160

m=O.03,

6t=4.75S, T=10ms,

R=90m c=1

_c 160

• 320 Fig. 36. Computed model of the coincident loop TEM responseover an overburden edge (after Smith and West, 1989).

32O

Fig. 37. The coincident loop TEM responseof a polarizable overburden (after Smith and West, 1989).

452


which the highest IP responsesof 7 mv/V were meaTeutonic Bore, Line 65440N,

160m loops

2900E

10 4 _

300 m and thickness

10 2 _

measurements. 8840

Fig. 38. Coincident loop field data exhibiting a response remarkably similar to the polarizable overburden edge shown in Figure 37.

of Sidorov and Yahkin, (1979). Negatives are seen only where the early-time response is large and the polarization current is closest to the receiver [i.e., conditions (1) and (3) satisfied], and these negatives show their largest and earliest time effects where condition (2) is best satisfied, i.e., at those stations close to the overburden edge where the individual exhibited

a local

minimum.

The

intrinsic

polarizability required to generate this negative is weak, a 2.7 percent variation in conductivity over the EM

50 m is located within

a 300 l•.m

half-space. A small polarization typical of clays was chosen for the body, modeled here as a Cole-Cole dispersion (Pelton et al., 1978) with parameters m = 0.1, c = 0.25, x = 1 ms. The model is successful in matching the reversal in sign at around 1 ms. This discussionhas concentrated on coincident-loop

10 a _

channels

and 3000E.

Figure 40 shows a computed 3-D model that matches many of the observed characteristics of the field data. A 30 l•.m body of horizontal size 300 m x

10 s -


sured between

bandwidth.

The field data that the model just discussed was designed to fit, was a profile collected along line 65440N at Teutonic Bore, Western Australia (Figure 38), which displays negative transients. The field data are remarkably similar to the model indicating that it can be attributed with some confidence to a weakly conductive overburden which comes to an edge. The decreasein amplitude of the negative transientsto the west compared to the model could either be attributed to a decrease in coupling of the transmitter to the overburden (as evidenced by the decrease in amplitude of the early time response), and/or a decreasein polarizability away from the edge of the overburden. A further example of field data from New South Wales, Australia (Figure 39) showing negative transients that can be modeled

with

IP effects has been

provided in Flis et al. (1989). The data shown were collected using 15 m displaced loops with 100 m sides. The 25 m station spacinghas detailed a negativeat late delay times between stations 2650E and 3100E. Dipole-dipole resistivity in this case indicated a surface conductive patch extends from 2200E to 3200E within

Similar

effects

can also be observed

with transmitter/receiver configurations which are similar to the coincident loop mode such as singleloop, central-loop or in-loop modes. In fixed transmitter, roving receiver surveys the polarization current will depress the vertical field inside the transmitter loop and enhance it outside. This distortion, termed the "loop effect" in Asten and Price (1985) can be accurately modeled with polarizable conductivity (Smith and West, 1988a). Raiche et al. (1985) have shownthat, even when the polarization current is very small, IP effects may seriously affect the results of conductivity modeling. Magnetic Permeability Variations

The main physical property affectingTEM measurements is conductivity where earth materials show many orders of magnitude variation. Local variations in magnetic permeability will also cause detectable TEM effects, however, except for some iron-rich minerals, the magnetic permeability varies by at most a few percent from that of free space. These permeability variations are commonly mapped using static field magnetometers such as proton precession devices. Where permeability variations in the near-surface cause, for example, a total 600 nT (gammas) or 1 percent positive anomaly in a background of say 60 000 nT due to the earth's magnetic field, any time varying magnetic field such as the total TEM response would also be slightly enhanced by about 1 percent. Such enhancement is usually only detected in measurements made during the transmitter on-time (Lamontagne, 1975) or correspondingly in the in-phase component of FEM surveys (Fraser, 1973) where a reference primary field amplitude is measured. SuperparamagneticEffects

Buselli (1981) reports anomalous transient recordings with the SIROTEM system characterized by a 1/t time dependence,leading to incorrect apparent resistivity determinationswith time. The main cause of this anomalous

transient

behavior

was

shown

to be the



presence of superparamagneticmaterial in the lateritic soil cover having a frequency-dependent magnetic permeability. The anomalous effects are generally localized to within 3 m of the transmitter loop and could only be detected with loop configurations where the receiver loop is in close proximity of the transmitter loop and the ground. Recent theoretical work by Lee (1984) also attributes the observed 1/t anomalousresponseto the presence of near-surface material with a frequency dependent magnetic permeability and suggests that under certain conditions superparamagnetic effects could also prove bothersome for large transmitter loop-roving receiver TEM systems. All these results are in agreement with Weidelt (1982) who showed that anomalous

results

could

be obtained

if either

the

conductivity or magnetic permeability of the ground vary with frequency.

453 TEM

CONFIGURATIONS

A common factor in all existing inductive timedomaintechniquesis the fact that they all utilize a more or less rectangular ungrounded loop as transmitter. Depending on the transmitter-receiver configuration the most common TEM arrays are: (1) Single loop transmitter and receiver (also known as one-loop). This configuration,used mainly in the Soviet Union, utilizes a single loop both as transmitter and receiver (Figure 41a). As long as the current is flowing in the loop it acts as a transmitter. Once the current is switched off, the loop terminals are connected to the receiver and the transient signal can be measured during the transmitter off time. The loops can be either square shaped or rectangular with sides of lengths varying between 5 m and 200 m.

m sec

I000

.25 32

42

Z

52 b.I

n,'

I0

I-Z

1.05 1.25

145 Z

0 -I

-t0

2400E

2:600 E

2800E

STATION

3000

E

32•E

3400

LOCATION

Fig. 39. Example of negative transientsfrom New South Wales, Australia (after Flis et al., 1989).

E


454


receiver array in that it usesa dipole multiturn receiver located at the center of the transmitter loop (Figure 4lb). (4) Separatedtransmitter-receiverloops. This array is similar to the familiar Slingram configuration with the transmitter and receiver loops separated by a fixed distance (say 100 m). The loops have

(2) Coincident transmitter-receiver loops. This array has the same geometry and responseas the singleloop configurationexcept that the transmitter and receiver are separate loops laid out spatially coincident. In practice this is achieved by using regular household electrical wiring with one wire each connectedto the transmitter and receiver respectively. (3) In-loop (or central loop) configuration. This array is a variant of the coincident transmitter-

dimensions

of a few

tens

of meters

on each

(Figure 41c). A variant of this technique consistsof a

1

l03-

ß10 ms•

.3 •

.3

l0 .5

5

l0ø•

/

/

/

//

\

\

\

I

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3

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10 '• _ •

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/

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-

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i

-700

-600

-500

-400 -300

-200

-I00

side

0

I00

200

300

400

500

600

700

DISTANCE (m)

Fig. 40. A 3-D model of a polarizableconductorfitted to the data of Figure 39 (after Flis et al. 1989).


TEM ProspectingMethods dipolereceiver kept a fixed distanceoutsidethe transmitter loop (Figure 4 l d). (5) Dual loop configuration. This array (Figure 41e) utilizes two adjacent loops connected in parallel to better couple with vertical conductors (Spies, 1975). Also, since the noise induced by remote sourcesof electrical interference is nearly oppositein the two loopsabove a homogeneous ground, a significantnoisereductioncan be achieved with this array. In areas with lateral conductivity variations the noise reduction is usually ineffective. (6) Large fixed transmitter, roving receiver. This array consistsof a large fixed transmitterloop and a smallroving receiver taking measurementsalong lines perpendicularto the loop (Figure 4If and Figure 1). The length of each loop side is of the order of a few

Tx

455

hundred meters up to a maximum of 1 to 2 km, althoughloops up to 5 km have been used. The transmitter loop is usually positioned for maximum magneticfield coupling with possibletargets. In certain cases(highly conductinghost rock or overburden) the transmitter loop can be positionedfor maximum or minimum current channeling. The receiver loop can be orientedin three orthogonaldirectionsto measurethe X, Y, and Z componentsof the transient magneticfield. One could also use a pair of grounded electrodes to measure the transient electric field in the

ground (e.g., UTEM, West et al., 1984). (7) Drill hole TEM. As an extensionof the large fixed transmitter system just described,the receiver dipole can be lowered in a drill hole (Figure 1). The componentof the transient

8•Rx

Sl NGLE

LOOP

or

LOOP

ONE

COINCIDENT TRANSMITTER(b} DIPOLE RECEIVER

(a)

RECEIVER

LOOP

Rx

d

(IN

Tx

LOOP METHOD)

d

(fixed)

SEPARATED

(c)

LOOPS

(d)

(SLINGRAM ARRAY) LARGE

-Tx

DUAL

LOOP

8• Rx

- SINGLE

WIRE

FORTRANSMITTER 8•

DUAL LOOP-SEPARATE

(e)

RECEIVER.

WIRES

FORTRANSMITTER 8• RECEIVER.

orVertical) ---•(Horizontal

Tx

(f)

DIPOLE RECEIVER

LOOP RECEIVER

LARGEFIXED TRANSMITTER LOOP WITH

ROVING

RECEIVER.

Fig. 41. TEM configurations.


456


magnetic field oriented along the axis of the drill-hole can then be measured at various depths along the hole (see Dyck, this volume). (8) Airborne TEM. Airborne TEM systems are described later in this Chapter. Additional material can be found in West and Palacky, this volume. TEM

SURVEY

DESIGN

(1) TEM loop size. A large transmitter loop generally offers a better capability for depth penetration if other factors are equal. Transmitter moment and accompanying primary field strength increase with increasing area of transmitter. In addition, it can be shown theoretically that the falloff rate of the magnetic field from a large loop approaches l/r, where r is the distancebetween the loop and the buried target. By contrast, the limiting

falloffratefor a smallloopapproaches 1/r3. A rough estimate of the comparative depth performance of various systemscan be accomplishedin the following way. Assume that both the conducting body and the source of geologic noise are small. The falloff rate of their respective secondary magnetic fields will ap-

proach1/r• , wherer isthedistance fromthereceiver. Since the signal from either source is proportional to the strength of the primary magnetic field we have: For large loop systems

bs"• 1/r bt -.•1/rt3 bn"• 1/rn•

For small loop systems

bs"• l/r3 bt .--.1/rt3 bn• 1/rn•

where r t and rn represent the distance from the receiver to the target and the sourceof geologicnoise, bs represents the strength of the primary magnetic field, and bt and bn represent the strength of the secondary fields due to target and source of geologic noise.

As a result, the signal-to-noise ratio (S/N) for this system can be approximated by:

(rn/rt)4 for largeloops

tenuation, the general conclusionspreviously reached are still applicable. By usinga large fixed transmitter loop, the systemof eddy currents induced in the ground is stationary: the loop does not move with each observation point as is the case with the single or coincident loop systems. Within the quasi-static approximation, the magnetic field of the current system induced in the ground is a potential field and thus the field can be meaningfully separated into various component parts which are each legitimate EM fields. With systemsusing a large fixed transmitter and a roving receiver, as a result of the large area coverage, it is thus possible, at least in principle, to separatethe observed responseinto various parts and estimate each individual system of induced eddy currents. By appropriate stripping, the geometry of the deep-seated conductors can be deciphered more easily. In addition, the TEM systems with a roving receiver have the added advantageof routinely measuringboth the horizontal and vertical componentsof the transient field. As such, anomalies due to overburden and host

rock can be easily recognized and accounted for. By contrast, a very conducting overburden/host rock or large formational conductors will yield stronger responseswith large loop-roving receiver systems and the response will vary with observation point. This is not the case for singleloop or in-loop systems. Also with large fixed loop TEM systems,one-half day each is lost for laying and picking up the loop if a minimal crew is used, although this is often more than compensatedfor by the fact that many traverses are made with a given loop location. Finally, if there are many conductorspresent in the survey area, a larger conductor with a larger time constant might screen a smaller behind

one with

(rn/rt) 6 for smallloops. Thus a large loop system with its large transmitter moment has an inherent advantage for exploration at greater depths as long as the source of noise is located at a shallower depth (i.e., overburdeninhomogeneity) than the depth of the conducting target. Although in conducting environments, many other important factors are involved besides geometric at-

time

constant

which

lies

For systemsusinga singleloop or coincidentloops, a good rule of thumb is to use loops of sides equal to the expected target depth. For large loop-roving receiver systems, the longer side of the loop is usually oriented parallel to the strike of the expected target and the loop is positioned to provide maximum coupling with the target. Some currently popular loop dimensions

and

a smaller

it.

are 800 rn x 400 rn and 600 rn x 300 m.

(2) Line and station spacing. The selectionof line and station spacingdependson the type of survey (reconnaissanceor detail). The choice

is based on the fact

that

the observed

half-

maximum half-width of an anomaly over a conducting target is approximately equal to the depth of burial of the target. The anomaly width is also related to the depth extent which implies that conductorscomingto surface are easily detected if they extend to a depth comparableto the station spacing.



For single or coincident loop systemsin reconnaissance mode, depending on the level of the geologic noise, the station separation is chosen to yield, at the very least, two anomalousreadingsover each conductor of interest. The corresponding line spacing is normally chosen to be commensurate with the expected strike length of the target. For large loop-roving receiver systems, the recommended station interval is usually smaller. In detailed surveys, from which quantitative estimates of target geometry and physical properties can be inferred, the station separationis chosento yield at least six to eight anomalous readings over each conductor of interest. The corresponding line spacing should be half or less of the expected strike length of the target. FIELD TECHNIQUES

(1) Single loop or in-loop configurations. At the beginningof the survey, experimentingwith a number of loop sizes to select the optimum dimensions is advisable. The most commonly encountered loop size is 100 m x 100 m. For such a loop a TEM profile is usually obtained by moving two 200 m wires along two parallel lines separated by 100 m, and joining them across with a few 100 m wires.

The loops may progress along the traverse (Figure 42) with no overlap (i.e., station interval equal to loop size) or for greater detail, with 50 percent overlap (i.e., station interval equal to half-loop size). If superparamagneticeffects are present, it may be necessary to displace the transmitting and receiving wires by 2 to 3 m (Figure 43). (2) Large fixed transmitter, roving receiver. The transmitter loop is laid in a roughly rectangularshape, with the longest side parallel to the regional geologic strike. Survey profiles are possible both within the loop as well as up to two or three loop widths outside the loop.

457

The chosen distance between profiles is commonly either 100 m or 200 m. In the searchfor large graphitic conductorsin uraniumexplorationbeneath400 m-600 m of sandstonecover, typical line separationsare 800 m or more. A station interval of 50 m is common for regional survey coveragewhereas for detail surveys, the station interval is reducedto approximately25 m. The transmitter loop is made of heavy gaugewire to reduce

electrical

resistance

and is often uninsulated.

The wire is usually wound on reels with appropriate lengths determined both by transmitter loop size and portability considerations. The motor generator and transmitter unit are usually located near an easily accessible area on the loop perimeter. The receiver coil is set-up at each station and its axis oriented either vertically or horizontally to measure the desired component(s). Care must be exercised in always orienting the coil in the same (positive) d:'rectionsthroughout the survey. SOURCES

OF ERRORS

IN TEM

MEASUREMENTS

(1) Geometric sources of error. Geometric sourcesof errors include all departures from the assumed geometric relationship between source and receiver. In sharp contrast with the frequency domain techniques, where it can be difficult to isolate the in-phase component of secondaryfield from the geometry dependent primary field, the geometric errors have a negligible direct effect on TEM data since measurements are taken during transmitter off time. An exception is the UTEM system in which the effect of taking measurements during transmitter on time is partially compensated for by normalizing the observed

transient

with

the

value

measured

at the

latest channel. However, this approach can require care when analyzing the fields of slowly decaying current systems.

Topographic effects are another form of geometric error. However, irrespective of topography, the smoke ring will soon behave as if the earth were flat with no residual anomalies left behind. To properly

Adjacent loops, no overlap

Coincident loop Loops

overlapped

Disploced loop

by 50%

Fig. 43. The displaced loop modification of coincident loop Fig. 42. Coincident loop profiling.

geometry.


458


correct for topography, when the surrounding host rock is relatively nonconducting, the relative position between each measuring station and the individual systemof eddy currents inducedin the groundmust be considered. This is in sharp contrast with the usual dc and EM techniquesfor which topographicerrors can be significantunder suchcircumstances.In conductive environments, topography can introduce severe coupling errors that can only be corrected for if a correct qualitative interpretation has been made so that the geometry of induced currents is known. (2) Cultural effects. Currents

induced

in metallic

conductors

such as

power and telephone lines, pipelines, fences etc. can produce anomalous TEM responses.Because of their small cross-section, the time constants of direct induction in these cultural features is usually negligible and, as such, their main contribution comes from their ability to channel currents induced in surrounding host-rocks. Their effect can be substantial and usually leads to strong elongated anomalies whose field falls off with distanceapproximately as 1/r. Their response can be minimized by laying out transmitter loops symmetrically distributed over the cultural feature. This ensures that little current is channeled into them.

(3) Electromagnetic noise. There TEM

are several

sources

of EM

noise that affect

measurements.

Geomagnetic signals below 1 Hz arise mainly from within and outside the ionosphere. Above 1 Hz the natural noise spectrum is primarily due to sferics, which are natural EM transientsgeneratedby lightning discharges. Man made noise comes mainly from the electric power distribution grid (50 or 60 Hz) while VLF radio stations create higher frequency noise (10 to 25 kHz). Motion induced noise or microphonics, due to the movement of the magnetic field sensorsin the earth's magnetic field, can be very significant because the earth's field is more than 100 000 times larger than the fields used in TEM

measurements.

This is often called

wind noise and is most significant in open areas and airborne EM systems. Virtually all TEM systems transmit a repetitive signal and use some form of synchronous detection and stacking (averaging) to enhance the S/N. The averaging procedures reduce the effective acceptance of the system of incoherent broad-band noise while still performing a broadband measurement of signal. The systemresponseto different types of stationary or nonstationary noise may be calculated for various processing techniques. By proper design, significant improvements in S/N can be achieved without any increase in transmitter power. As a result, contrary to common intuition, a broadband TEM system does not

necessarily have a S/N inferior to a narrow band frequency domain system. Additional details on EM noise and techniquesfor data processingcan be found in Macnae et al. (1984), Spies and Frischknecht (this volume), San Filipo and Hohmann (1983), McCracken (1979), West and Macnae (1982) and McCracken et al. (1986a, 1986b).

DATA

PROCESSING

In the field, virtually all TEM systems record the transient voltage at a number of sample times. In contrast with the seismic methods however, most signalprocessingis done in real time. Because8 to 30 numbers must be recorded each time a field component is measured, most current instruments employ some form of electronic data recording. In the office, field data is edited, checked for accuracy (repeatability), and normalized with respect to the transmitter current or calculated primary field and receiver

coil moment.

mitter turn-off

In certain cases the finite trans-

time is also corrected.

With most TEM systems, the measured fields are plotted, on a linear scale, without any additional normalization. This will preserve the shape of the anomalies but their amplitudes will decrease in direct proportion to the distance between the field station and the transmitter loop. Often (e.g., UTEM) such

data are normalizedwith respectto the anomaloushz value measured at a given station, usually at the anomaly peak. This "point normalization" preserves the shape of the anomalies while scaling them with respect to a reference value. A partial compensation of decreasing anomaly amplitudeswith distancecan be accomplishedby normalizing the observedfields at each point with the primary field value at the same point. This "continuous normalization" will preserve the amplitudes but will distort the shape of the anomalies. The primary field can be either calculated, if the system geometry is known, or it can be approximated with the measuredmagnetic field value at the latest time (e.g., UTEM). The latter approach, however, can lead to problems when analyzing fields of well localized current systems. In somecasesdata are plotted in a linear-log scalein which the upper part of the amplitude scale(i.e., large values) is logarithmic while the lower part (i.e., small values) is linear. This presentation works well for plotting anomalies in a resistive environment. In conductive environments,the resultingbackgroundanomaly distortion enormously complicatesinterpretation. To aid interpretation, various graphs are plotted. Some of the most popular presentationsare:



(1) Transient decay plots, in either log-log or semi-log paper, of voltage (in microvolts) versus time (in milliseconds). (2) Response profiles, i.e., graphs of measured voltage at a selectedtime along points of a profile. (3) Response contours, i.e., contours of measured voltage at a selectedtime at all stationsin the survey

area.

(4) Apparent resistivity plots, profiles or contours.

(5) Vector plots, either in a horizontal plane or in a given vertical section (XZ or YZ planes). INTERPRETATION

OF TEM

MEASUREMENTS

The interpretation of TEM data is usually accomplished in two stages. In the first stage, the location and possible shape of the conducting target is determined from profiles and contours of the field measurements at various times. In the second stage, the conductor "quality" is estimated from its time constant determined from decay plots of the field intensity at one or more key stations. The most useful tool in interpreting the observed transient decays is provided by the plotting of response or constant delay time profiles. Profiles of observed voltages at various times after current interruption yield diagnostic information regarding the location and depth of causative bodies. For large loop systems, host-rock responses(halfspace, layered earth, etc.) are characterized by bell shaped anomalies with zero crossoversin the vertical componentand with correspondingasymmetricanomalies in the horizontal components. Because of the migrating smoke ring, these anomalies will become steadily wider with the passageof time. For localized targets, the expected anomaly shape can readily be visualized from simple EM coupling considerations.For large loop systems, localized targets may yield symmetric bell shapedor antisymmetric anomalies depending on the measured component (Figure 44). For singleor coincident loop systems,the correspondinganomaly is doubly peaked (Figure 45). Some continuously normalized profiles for the UTEM system are shown in Figure 46. For the Crone PEM system used in the Slingram mode, the response to a conductive body is similar to the response obtained with conventional horizontal loop EM system (See Frischknecht et al., this volume), but with opposite sign. As such, the typical Crone anomaly will consist of a positive peak flanked by two smaller negative peaks. However, unlike the frequency domain Slingram systems, varying coil separation and elevation effects are not critical

with the time domain

methods.

The effect of conductor dip introduces asymmetries

459

in the above curves. The amount of asymmetry is a strong function of conductor size. Strike and plunge can be determined from the joint behavior of various components, particularly on adjacent lines. Field examples of responseprofiles for large loop systemsare shown in Figure 47 and for coincident loop systemsin Figure 31. More detailed case histories are shown in Appendices B through H. Various

rules-of-thumb

have been devised for deter-

mining depth from responseprofiles. For antisymmetric anomalies, the depth to a causative body of medium size is approximately equal to half the distance between maximum and minimum. For symmetric anomalies, the depth is approximately given by the half-maximum half-width of the anomaly. Information regardingconductor strike and shapeis provided by the response contours, as seen from Figures 47 and 48. When the conductor is located in a conducting host-rock, it is often possibleto separatethe individual effects by using the concept of superposition. This works

best when

the various

conductors

are not in

contact with each other and/or current channeling effects are minimal. In addition, the closer we are to

the window of detectability of the anomaly, the better the results. An illustrative example of applicability of superposition is shown in Figure 49. Apparent resistivity calculations are favored by a number of geophysicistsparticularly when a sounding type of interpretation is desirable. However, their information content is minimal in mineral exploration. In cases where the transient technique for the 3-D mappingof the ground is used, the apparent resistivity calculationscan yield a useful, although highly simplified, picture of the ground. An apparent resistivity presentation of SIROTEM data is shown in Figure 50. Calculations have been carried out with the algorithm described in Raiche and Spies (1981). To determine reliable apparent resistivity values from transient data, especially in the presence of strong current channeling effects, is not always possible. Also, for the large loop-roving receiver systems, the apparent resistivity is not necessarily a single valued function, which leads to additional complications. Further anomaly identification is provided by plotting observed decays on both semi-log and log-log paper. As shown, the late time host-rock responses will plot as a straight line on log-log plots, with a well defined rate of decay. By contrast, the late time response of confined conductors will plot as straight lines on a semi-log plot yielding directly the time constant of the causative body (Figure 51). For

drill

hole

measurements

all

discussed

tech-



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0.79

= 0.57

ms

ms

s

3.2ms Ires 5.2ms

i I 2om.

i --

i

i

i i 2om.

I LOOP

dI -20s/m d2-0 dI =IOta d2 =2m 0

k

I0

20

30

40

I

I

I

I

METRES

Fig. 45. Coincidentloopresponseas a functionof the dip of a half-planeconductor(After Spies,1974).

I LOOP


462


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•

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CHANNEL •

•

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CONDUCTOR (O.B. PATCH)

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D

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LATER

CHANNEl.

EARLY CHANNEL

E

EXTENSIVE

I/"'",•1

HORIZONTAL

CONDUCTOR

Fig. 46. The form of continuouslynormalized UTEM responsesover some simple conductors(West et al., 1984).



N

TIME 5.63

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POSITION OF # .

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2

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METERS 0

200

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0

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200

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EMP

SURVEY

Fig. 47. Exampleof fielddatapresentation in profile(b) andcontour(c) form from an EMP surveyin Australia (Newmont EMP files).

464

Nabighian and Macnae 1800E

2000E

I

51200N

2200E

I

i

2.•,

ø

2400E

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I

!

2000E

I

I

2200E

I

2400E

J

I

I

2600E

50200N

!

2800E

:5000 E

Fig. 48a. Contours of TEM responseat (a) late time over Elura orebody (after Rutter, 1981). 1800E

512OON

1900E

2000E

iI

I

2100E

I

2200E

2:•00

I

E

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2400E

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:•000 E

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51

lOON

ß

51000N-

51000

50900N-

50900N

5070(•1-

50600N

•

- 50700N

-

-50600N

50500N-

250 0 ß -50500N

50 40ON-

- 50400N

50:•4X)N50200N, 1800E

N

• 1900E

I 2000E

ß I

2100E

I 22(•

I E

2 :SO0 E

I 2400E

•PO•)O E't'00./ -50300N I

2500E

I 2600E

i 2700E

I 2800E

I 2900E

50200N :•000E

Fig. 48b. Contours of TEM responseat (b) early time over Elura orebody (after Rutter, 1981).


465


80

•

4o

DOWN

SCALE x2o

1.49

x 2o

I 62 I 82

.

•ZO• •

xlo

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x io

2 60

x5

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8.46

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14([0 E IJO E

I ø I

109ø

1,49

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2o

•X20 x 20

2.08 zs4•

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DOWN

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,,•,.

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--; 5

+

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20

109 ø

+5

•

OE

ß

•3OE

•

ß

1200E

LOOP

Fig. 49. Extraction of a residual component of a TEM profileby subtraction of a fittedbackground response (Newmont EMP files).


466

Nabighianand Macnae

ßt-

4,26

'1-

4,10

+

3,87

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•_/_--/-

-... -'-xT/---

/

Channel

2,5m of 2,5% Zn / 'x•••x,x

delay times

\

Instrument

Loop details

: Sirotern :Coincident lOOmloop Readinginterval50m

Scale : I: 5000 Date : $1-1-1984 Mop : SH50-4

Fig.50.Example of SIROTEMdatafromWestern Australia presented in apparent resistivity format(afterRutter, 1981).


TEM Prospecting Methods niquesapply. The main drawback is that only one component(downthe hole)is availablefor interpretation. Combiningresultsfrom various holes or using singlehole measurements for variouslocationsof the transmittingloop can remove ambiguitiesresulting from using a single componentfor interpretation (Dyck and West, 1984;Boyd and Wiles, 1984;Dyck, this volume). Some field results are shown in Figure

467

drill hole data is shown in Figure 53 (Boyd and Wiles, 1984). Time

Constants

As mentioned,the conductorquality is determined from its time constant.In general,the time constantof a conductingtarget can be expressedas (38)

52.

Automatic interpretationof TEM data may be accomplishedby fittingthe observeddata, at eachtime, with the magneticfieldof a circularor rectangularloop (equivalent current filament). The loop parameters (length,width, dip, plungeetc.) canbe determinedby usinga Marquardttype algorithm(Barnett, 1984)or a trial and error approach(McNeill 1982). When fitting equivalent current filaments to TEM data account must be taken of the fact that their size

encompasses only 50 percentof the total area of the conductor(Nabighian, 1978;Barnett, 1984).This filament tracesa path which is about 30 percentin from the conductoredgesand from early to late timestends to migrate toward the center of the conductor(Barnett, 1984). An illustrativeexample of loop fitting for

where K is a numerical coefficient and A is proportional to the effective cross-section of the conductor.

A tabulation of most commonly encountered time constants is shown in Table 2.

Extensive modeling results have shown that the transientdecaysobservedover various localized targetswill plot on top of eachother (on log-logpaper)if the time axis is normalized with respect to the time constantvalues given in Table 2. Expression (38) is thus experimentallyconfirmed. As shown before, the time constant of a conducting targetcanbe determinedfrom the straightline portion of the TEM decays plotted on semi-log paper. The cotangentof the slopeangle(in degrees)yieldsdirectly the time constant of the conductor. An alternate derivation of time constants can be

IOs

accomplished analytically,startingfrom the late time expression(21), which can be rewritten as

GERTRUDE

EMP

A(t) = Aoe-t/•.

SURVEY

(39)

Takingthe logarithmof both sidesof equation(39) one

iO4

obtains'

DECAY PLOT

STATION OON/ 15 W t

INTERPRETED

td

=

t•nA(t) tn Ao •,

: 650 MHOS

IO3

(40)

which is the equation of a straight line with the negativeslope of 1/,. By measuring the amplitudesat two timest l andt2, we immediatelyobtainfrom equation(40) the desired expressionfor the time constant

'""'• Co•,..

'r =

t2 -

t•

In A 1(t)/A 2(t)'

(41)

The time constantyieldsinformationregardingboth the size and conductivity (conductor quality) of the X

i

I0

I

20

target.Separationof thesetwo components is usually difficult, unlessindependentinformationis available

M ILLISECONDS"•'• •• i

I

30

40

I

50

I

60

70

Fig. 51. The decayof the TEM responsefrom an isolated conductorat late delay times is exponentialand plots as a straightline on a log amplitudeversustime graph(Newmont EMP files).

about either one of them. In principle, early time (t --•0) measurements couldprovideinformationabout conductor size which could then be used to determine

conductorquality from equation(38). However, this approachhasnot as yet founda satisfactorytreatment in the publishedliterature. The time constants encountered over various targets



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469

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STATION NO.

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220

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200

225

250

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sw

SCALE

TIME 1.22MS

x500

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200

225

250

275

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0

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xlO

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440

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Fig.53.A comparison ofdrillgeology andbestfitcurrent loopfordownhole EMPfielddata(afterBoydandWiles,


470


of interest vary between 0.5 and 20 ms. Very large time constants (a few tens of milliseconds) are often associatedwith pyrrhotitic bodies. Except in the case of nickel exploration, these are usually of little economic interest. Care must be exercisedwhen attaching geologic or economic significance to various targets based on their time constants

alone.

Dip Determinations

For thin tabular conductors, located in a resistive host rock, the dip is easily evaluated by fitting equivalent current filaments to data. The asymmetry in the measured components is more than adequate for a unique solutionto the problem, especiallywhen fitting is done simultaneouslyto more than one component. The only complication might arise when the tabular conductor is inductively thick, in which case loop fitting will yield an apparent dip rotated toward the direction of maximum couplingwith the primary field. The same rotation will also occur for approximately equidimensionalbodies (e.g., spheres,spheroids). When the target is located in a conductinghost rock or in the vicinity of another conductor, its response will be superimposedon the "regional" responsefrom the latter and this could result in the migration of anomaly crossover location. Unless this regional response is properly stripped, the target dip will be determined incorrectly. However, a check with the geologicallyacceptable dips in a given area can reliably validate or invalidate a given strippingtechnique. In addition, when the conductor is located outside the transmitter loop and has a reasonable strike length, strong channeling effects could be observed. These

effects can be modeled by a single line element of current flowing along the top edge of the conductor and as such the anomaly will display a more symmetric/antisymmetric shape. As a result, both conductor dip and conductivity-thicknessproduct will be difficult to determine and, as a general rule, they will be overestimated. It is worth mentioning that when the conductor is located within the transmitter loop or when single-loop or in-loop configurations are used, current channeling effects will be minimal. For single-loopconfigurationsyielding a characteristic double-peaked anomaly over a dipping plate (Figure 45), the conductor dip can be determinedfrom the relation (Velikin and Bulgakov, 1967) mml =

e

-2.76

Am2

where Aml/Am2 is the magnitudeof the ratio of the larger maximum to the smaller one and • is the angle (in radians)between the vertical and the plane of the thin/thick plate. Unless overlapping loops are used these maxima are not well enough sampledhowever. TDEM Interpretation Caveats

The guidelines shown in previous paragraphsare usually adequatefor carrying out routine TDEM interpretation. With a goodunderstandingof the principles governing current diffusion and induction, the interpreter shouldbe able to cope with practically any field data set. Nevertheless, a few caveatsare pointed out: (1) With large loop systemsand in areas of conducting overburden,currentchannelingeffectsat the edges

Table 2. Time constantsof various conductingbodies. Time constant

Conductor type

Reference

2

Sphere of radius a

1.71•txa

Cylinder of radius a

2

2 t•lxœ

2-D conducting plate of finite depth extent

2-D rectangular blockof sizeLx, Ly

2

--Zx o-txL 2[•2 1 71.2

Thin prism of thickness t and average dimensionL

Khomenyuk, 1963

2

2

c •txLt

Khomenyuk, 1963 Khomenyuk, 1963

Oristaglio and Hohmann, 1984 Lamontagne, 1975

10

IxSa

Infinite elliptic cylinder with semiaxes a, b and S = 2eb (see Figure 21)

McNeill, 1980


of the conducting units can cause large anomalous responses.Often such anomalies are not easily distinguished from the anomalies over a vertical plate,


especiallywhen only a singlecomponent(hz) is measured (Figure 54). By contrast, the single loop configuration readily distinguishesbetween the two cases (Figure 55). Additional situationsare treated in Spies

IO

(a) I

- I000

- 500

0

TXLOOP -

.-/,• .............

t

.:

•/"''"':C'• ....................... '............... T50m

I

(b)

Amt_l

471

and Parker (1984), Irvine and Staltari (1984), and Smith and West (1988a, 1988b). (2) When exploring large tabular conductors, of dimensions greater than the dimensions of the transmitting loop, the currents induced in the conductor have, at early times, approximately the same size as the transmitter loop. With passage of time, these currents will expand (Figure 56). However, for good conductingtargets, the outward diffusion can be slow enough to make the induced current system look stationary. (3) When strippingTDEM responsesin areas with pronouncedcurrent channeling,the stripped anomaly will decay at the same rate as the background anomaly. In such cases, it is difficult if not impossible to determine the actual time constant of the target. In addition, the obtained complex stripped decay patterns often complicate interpretation. (4) When interpreted dips do not agree with geology and seem to vary with time, the most probable cause can be variable background effects due to large conductors present in the area. The resulting cross-over migration will lead to incorrect stripping and erroneous dip determinations. (5) In areas with complicated multiconductor sections, and for fixed loop TDEM systems, screening

o -.I

I0 000

-I

I 000

.7

:

-IOO/

J J J -, I000J I I -5,00 I ' ' J I I (• I 1

IOO

,

Am z

,.5 IO

- I000

-500

500

0

I OOOm

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(c)

_ _

t=I,•,•$

IO

-I000

-500

0

500m

1.8•

Am"

Tx LO0• C

-

_

,

•

•Om

O'T =15 S

Fig. 54. Large loop TEM response over a horizontal slab boundedin one direction and over a vertical plate. (a) TEM loop located over slab, (b) TEM loop located outside slab, (c) TEM loop and vertical plate (after Spies and Parker, 1984).

-I000

-500

0

O-*'r=

500m

1'

Fig. 55. Coincident loop responseover a vertical conductor and an overburdenedge (after Spies and Parker, 1984).

472


/

I

'r =

10-o,5 xI


\

//

r : I0-•

r: I0'ø'5

---.___

AJ •0.0011

j

AJ: 0.011

AJ: 0.005 1 r : I0 ø'5

'r=l

\

x, / AJ:O.0001I

AJ: 0.0011

AJ:O. 0001I

a.

b.

Fig. 56. The transientcurrent flow in a horizontalhalf sheetfor a circularloop located(a) outside,and (b) over the half sheet.ß = t/tx Sa, S is the sheetconductanceand a is the radiusof the circularloop (after Weidelt, 1983).

effects, especially of poorer conductorsbehind large, good conductorsare expected. In suchcases,carrying out the survey with more than one transmitter location, and using shorter survey lines is recommended. CASE

HISTORIES

To accompanythe TEM chapter, we requestedfrom the various manufacturersand designersof equipment case history material we could use to illustrate both our text and the capacitiesof their instrumentation. To satisfy space requirements, we had to omit a number of the cases, and heavily prune those included. For clarity and consistency, we also edited the material. Any reference specific to a particular case history presentedis includedwithin its text, otherspertaining to EM may be found in the main reference list follow-

ing the TEM chapter. Each case history is presented as a separate Appendix (B through H). The

PEM

and

EM37

case histories

each

show

examples of the routine detection of a large target at depths of several hundred meters in a resistive environment.

The SIROTEM

case illustrates

that EM can

be successfulin very conductive environments using compact geometrical setup and by measuringto latedelay times to minimize the effects of extensive regional conductivity. The EMP case history illustrates the use of simple computer models in a case where extreme topographycausedsignificantanomaly shape changes from standard models. The fact that TEM systems can be successfully operated during the on-time is illustrated with the UTEM system case histories. The INPUT and GEOTEM

cases

illustrate

the

success

of airborne

TEM systemsin providingcomprehensiveexploration coverage for near-surface conductors. Computer AssistedInterpretation

The classicalinversion approach (West and Bailey, 1989), where data are fitted in a slow process using repetitive iterations of a forward modeling program has been applied to EM data, mostly in the case of 1-D (sounding)geometry (Spies and Frischnecht, this volume). Classical inversion programs are not easy to apply to the full TEM interpretation problem, due



chiefly to the difficult parameterization and inordinately long computer execution times for even the simplest of 3-D EM models. Eaton (1989) also found that EM inversion attempted in 3-D cases is quite adversely affected by model inadequacy in existing 3-D forward programs, and requires that assumptions made about background conductivity be correct. Recent research has investigated methods other than inversion where computers may aid the interpretation process.There are two related approachesthat have received attention. One approach is through anomaly recognition based on interpolation between a finite set of existing models, a method well suited to implementationon parallel computingdevices (Poulton et al., 1989). A second involves the calculation of "weighted arrays" from a large data set in a manner so as to correlate with conductors of a specifictype and location (Polzer et al., 1989). The correlation process may be designed to improve resolution of parallel conductorsfor example, or be used as a starting point for conventional interpretation. ACKNOWLEDGMENTS

We thank Richard Smith, Gerald Hohmann, and Marcus Flis who provided their most recent results to use in the induced polarization section. The assistance of the various equipment manufacturersin providing technical and case history material was greatly appre-

473

hole electromagnetic pulse system--Examples from eastern Australia: Geophysics, 49, 949. Braham, B., Haren, R., and Lappi, D., 1978, Lecture notes from the US-Australian electromagnetic workshop: Bull. Austral. Soc. Expl. Geophys., 9, 2-33. Buselli, G., 1981, The effect of near-surface superparamagnetic material on electromagnetic measurements: Geophysics, 47, 1315-1324. Dias, C. A., 1968, A non-grounded method for measuring induced electrical polarization and conductivity: Ph.D. thesis, University of California, Berkeley. Dolan, W. M., 1970, Geophysical detection of deeply buried sulfide bodies in weathered regions' GSC Economic Geology Rep., 26, 336-344. Dyck, A. V., Bloore, M., and Vallee, M. A., 1981, User manual for programsPLATE and SPHERE: Res. in Appl. Geophys. 14, Geophys. Lab., Univ. of Toronto. Dyck, A. V., and West, G. F., 1984, The role of simple computer models in interpretations of wide-band drillhole electromagneticsurveys in mineral exploration: Geophysics, 49, 957-980. Eaton, P. A., 1989, 3-D electromagnetic inversion using integral equations: Geophys. Prosp., 37,407-426. Eaton, P. A., and Hohmann, G. W., 1984, The influence of a conductive

host on two-dimensional

borehole

transient

electromagnetic responses'Geophysics, 49, 861-869. •, 1986, Technique for rapidly estimating resistivity of the earth from transient electromagnetic soundings:56th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 52-54.

•,

1987, An evaluation of electromagnetic methods in the presenceof geologicnoise: Geophysics,52, 1106-1126. Edwards, R. N., Lee, H., and Nabighian, M. N., 1978, On the theory of magnetometric resistivity (MMR) methods: Geophysics, 43, 1176-1203. Flis, M. F., 1987, IP effects in 3-D TEM data--theory and case histories: Presented at Austral. Soc. Expl. Geophys. Mtg., Perth, Abstract in Exploration Geophysics, 18, 55-58.

ciated. REFERENCES

Adhidjaja, J. I., and Hohmann, G. W., 1988, Step responses for two-dimensionalelectromagneticmodels:Geoexploration, 25, 13-35.

Adhidjaja, J. I., Hohmann, G. W., and Oristaglio, H. L., 1985,Two-dimensionaltransientelectromagneticresponses: Geophysics, 50, 2849-2861. Alpin, L. M., 1966, Field theory (in Russian):Nedra Moska. Annan, A. P., 1974, The equivalent source method for electromagnetic scattering analysis and its geophysical application: Ph.D. thesis, Memorial University of Newfoundland.

Asten, M. W., and Price, D. G., 1985,Transient soundingby the in/out-loop method: Presentedat Austral. Soc. Expl. Geophys. Mtg., Sydney, Abstract in Expl. Geophys., 16, 165-168.

Astrakhantsev, G. V., Gavratilova, I. E., Shuraleva, R. B.,

and Ulitin, R. V., 1975, Effect of the polarizingproperties of rocks on the build-up of the electromagneticfield: Izv. Acad. Sci. USSR Physicsof the Solid Earth, 11,330-333. Barnett, C. T., 1984, Simple inversion of time-domain electromagnetic data: Geophysics, 49, 925. Barringer, A. R., 1962,A new approachto explorationraThe INPUT airborneelectrical pulseprospectingsystem:Min. Congress Journal, 48, 49-52. Bhattacharyya, B. K., 1964, Electromagnetic fields of a small loop antenna on the surface of a polarizable medium: Geophysics, 29, 814-831. Boyd, G. W., 1980a, Smoke rings: Bull. Austral. Soc. of Expl. Geophys., 11,303-304. Boyd, G. W., and Wiles, C. J., 1984, The Newmont drill-

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Goldman, M. M., and Fitterman, D. V., 1987, Direct timedomain calculation of the transient responsefor a rectangular loop over a two-layer medium: Geophysics, 52, 997-1006.

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Mallick, K., 1972a, Conducting sphere in EM INPUT field: Geophys. Prosp., 20, 293-303. •1972b, INPUT response to single-turn conductive circuit: Geoexploration, 10, 255-259. •1973, Effect of magnetic permeability on INPUT responseof spherical orebody: Geoexploration, 11, 183186.

•1978, Note on decay pattern of magnetic field and voltageresponseof conductingbodiesin EM time domain system: Geoexploration, 16, 303-307. Mallick, K., and Jain, S.C., 1975, Conducting horizontal infinite sheet in EM INPUT field: Z. Fur. Geophys., 41, 127-133.

Mallick, K., and Verma, R. K., 1977a, Detectability of intermediate conducting and resistive layers by time domain EM sounding:47th Ann. Internat. Mtg., Soc. Expl. Geophys. •1977b, Equivalence in time domain EM soundingon 3-layer earth sections: Paper presented at Eur. Assn. Expl. Geophys. Mtg. at Zagreb, Yugoslavia. •1978, Time domain EM sounding with horizontal & vertical coplanar loops on a multilayered earth: Geoexploration, 16, 291-302. •1979, Time-domain EM computation of multilayered responseand the problem of equivalencein interpretation: Geophys. Prosp., 27, 137-155. McCracken, K. G., Hohmann, G. W., and Oristaglio, M. L.,

478


1980, Why time domain?: Bull., Austral. Soc. Expl. Geophys., 11,318-321. McCracken, K. G., Pik, J.P., and Harris, R. W., 1984, Noise in EM exploration systems: Expl. Geophys., 15,


169-174.

McManus, D.C., 1974, A model study of the effect of a conducting overburden on EM measurementsover a vertical thin dyke: B.Sc. thesis, Univ. of British Columbia. McNeill, J. D., 1985, The galvanic current component in electromagneticsurveys: Tech. rep. TN17, Geonics, Ltd. Mishra, D.C., Murphy, M. S. R., and Narain, H., 1978, Interpretation of time-domain airborne electromagnetic (INPUT) anomalies: Geoexploration, 16, 203-222. Morozova, G. M., and Kaufman, A. A., 1967, Transient electromagnetic field about magnetic dipoles on a homogeneoushalf-space: Geol. Geofiz., 11, 66-74. •1982, A review of time domain electromagnetic exploration: Proc. of Inter. Symp. of Appl. Geophys. In tropical regions:Belem, Brazil. Nabighian, M. N., and Davidson, M. J., 1969, The electromagnetic pulse (EMP) method: theory and interpretation: Presented at the 59th Ann. Internat. Mtg., Soc. Expl. Geophys. Nabighian, M. N., McLaughlin, G. H., and Dowling, F. L., 1971, Theory and application of time-domainEM methods in the search for buried conductors:

Inst. Electr.

Electron.

Eng. Internat. Symp., Univ. of California, L.A. Nabighian, M. N., 1982, A review of time domain electromagnetic exploration: Proc. of Inter. Symp. of Appl. Geophys. in tropical regions: Belem, Brazil. Nagendra, R., Ramaprasada Rao, I. B., and Bhimasankaram, V. L. S., 1980, Influence of a conductingshield in the one-loop version of transient pulse inductionmethod: Geophys. Prosp., 28, 269-282. Negi, J. G., and Verma, S. K., 1972, Time domain electromagnetic response of a shielded conductor: Geophys. Prosp., 20, 901-909. •1973, Effect of conducting surroundingsin time-domain EM projecting:Symposiumof EM explorationmethods, Univ. of Toronto. Nelson, P. H., 1973, Model results and field checks for a

time-domain, airborne EM system: Geophysics,38, 845853.

Nelson, P. H., and Morris, D. B., 1969, Theoretical re-

sponseof time-domain airborne EM system:Geophysics, 34, 729-738.

Ogilvy, R. D., 1983, Application of down-holepulseelectromagnetic surveysfor off-hole mineral exploration:Trans. Inst. Min. Metallurg. (Section B, Appl. Earth Sci.), 92, B148-B153.

•1986, Theoretical transient EM responsecurves over a thin dipping dyke in free space-separatedin line loop configuration:Geophys. Prosp., 34, 769-788. Olm, M. C., 1982, EM scale model study of dual frequency differencingtechnique: M.Sc. thesis, Colorado School of Mines.

Oristaglio, M. L., 1982, Diffusion of electromagneticfields into the earth from a line sourceof current: Geophysics, 47, 1588-1592.

Palacky, G. J., 1975, Interpretation of input AEM measurementsin areasof conductiveoverburden:Geophysics,40, 490-502.

•1976, Use of decay patterns for the classificationof anomaliesin time-domainAEM measurements:Geophysics, 41, 1031-1041.

Palacky, G. J., and Kadekaru, K., 1979, Effect of tropical weathering on electrical and electromagnetic measurements: Geophysics, 44, 69-88. Parasnis, D. S., 1979, Principles of applied geophysics: Chapman and Hall. Paterson, N. R., 1971, Airborne electromagneticmethodsas

applied to the search for sulphide deposits: Bull., Can. Inst. Min. Metallurg., 63 (705), 29-38. Patra, H. P., and Mallick, K., 1980, Transient soundings: Chapter 5 in GeosoundingPrinciples, 2, Elsevier Science Publ. Co., Inc. Pederson, R. N., 1980, INPUT

results over Woodlawn; in

Whiteley, R. J., Ed., Geophysical case study of Woodlawn Orebody, New South Wales, Australia: Pergamon Press, Inc.

Pemberton, R. H., 1962, Airborne electromagneticsin review: Geophysics, 27, 691-713. Preston, B., 1975, Review--Difficulties for the EM method in Australia: Geoexploration, 13, 29-43. Price, A. T., 1950, Electromagnetic induction in a semiinfinite conductor with a plane boundary: Quarterly J. Mech. Appl. Math., 3, 385-410. Rai, S.S., 1985, Crone pulse electromagneticresponseof a conducting thin horizontal sheet--Theory and applications: Geophysics, 50, 1350-1354. Raiche, A. P., 1978, The response of a coincident loop transient EM system above a uniform earth: Bull., Austral. Soc. Expl. Geophys., 9, 170-172. •1983a, Short note on comparisonof apparentresistivity functions for transient electromagneticmethods: Geophysics, 48, 787. •1984, The effect of ramp function turnoff on the TEM response of a layered earth: Bull. Austral. Soc. Expl. Geophys., 15, 37-42. Raiche, A. P., and Gallagher, R. G., 1985, Apparent resistivity and diffusion velocity: Geophysics, 50, 1628-1633. Reed, L. E., 1981, Airborne EM discovery of the Detour zinc-copper-silver deposit, Northwestern Quebec: Geophysics, 46, 1278-1290. Rutter, H., 1972, Use of EM transient processesas a ground method

in the search for sulfide ore bodies:

M.Sc./DIC

thesis, Imperial College, London. San Filipo, W. A., and Hohmann, G. W., 1985, Integral equation solution to the transient electromagnetic responseof a three-dimensionalbody in a conductivehalfspace: Geophysics, 50, 798-809. Sarma, D. G., and Maru, V. M., 1971, A study of some effects of a conducting host rock with a new modelling apparatus:Geophysics,36, 166-183. Spies, B. R., 1976, The transient EM method in Australia: BMR J. Austral. Geol. Geophys., 1, 23-32. •1977, Absolute EM scale modelling and its use in interpretation of TEM response: BMR J. Austral. Geol. Geophys., 2, 89-95. •1979a, TEM scale model investigations, Feb-March, 1979: BMR Record, 43.

•1979b, Interpretation and design of time domain EM surveys in areas of conductive overburden: Bull., Austral. Soc. Expl. Geophys., 10, 203-205. •1980a, One-loop and two-loop TEM responsesof the Elura deposit, Cobar, NSW: Bull., Austral. Soc. Expl. Geophys., 11, 282-288. •1980b, A field occurrence of sign reversals with the transient EM method: Geophys. Prosp., 28,620-632. •1980c, TEM model studies of the Elura deposit, Cobar, NSW: BMR J. Austral. Geol. Geophys., 5, 77-85. •1980d, Interpretation and design of time-domain EM surveys in areas of conductive overburden: Bull., Austral. Soc. Expl. Geophys., 11,272-281. •1980e, Resultsof experimental and test TEM surveys, Elura deposit, Cobar, N SW: Bull., Austral. Soc. Expl. Geophys., 11, 289-294. •1980f, Interpretation and design of time-domain EM surveys in areas of conductive overburden, in D. M. Emerson, Ed., The geophysics of the Elura orebody: Sydney, Austral. Soc. Expl. Geophys., 130-139. Spies, B. R., Hone, I. G., and Williams, J. W., 1980, Transient EM test surveys and scale model studies of

TEM Prospecting Methods Woodlawnorebodywith MPPO1 equipment,in Whiteley: R. J. (Editor), Geophysicalcase study of Woodlawn orebody,New SouthWales, Australia:PergamonPress, Oxford.


Stratton,J. A,. 1941,Electromagnetic Theory:McGraw Hill

115-125.

Thio Yong-Chu,and Gleeson,L. J., 1979,Electromagnetic fieldsin a half-spacescatteredby a buriedsphereby the method of transformation of local elements: Geophys. J. Roy. Astr. Soc., 57, 671-681.

•1970, EM fieldsof a pulseddipolein dissipativeand dispersivemedia:Radio Science,5, 733-735. •1971a, On the theoryof transientEM soundingover a

•1971b,

Toronto.

Vanyan,L. L., 1967,Electromagnetic depthsounding: (Translated by K. G. Keller), New York ConsultantsBureau. Verma, O. P., 1972, EM model experimentssimulating conditionsencounteredin geophysicalprospecting:Thesis, Univ. of Roorkee, India.

Verma, O. P., and Guar, V. K., 1975,Transformationof EM anomaliesbrought about by a conductinghost rock: Geophysics,40, 473-389. Verma, S. K., 1975a,Resolutionof responsesdue to conductiveoverburdenand orebodythroughtime-domainEM measurements--A field example: Geophys. Prosp., 23, 292-299.

•1976b, Transient EM responseof a permeablenonuniformlyconducting sphere:J. Appl.Phys.,46, 1124-1127. Verma, S. K., Bennett, L. A., Raiche, A. P., and Weidelt, P., 1984, Atlas of standardcurves for coincidentloop

TEM interpretation:Centerfor GeophysicalExploration Research, Australia.

Wait, J. R., 195lc, The magneticdipoleoverthe horizontally stratified earth: Can. J. Phys., 29, 577-592.

Transient EM propagationin a conducting

medium: Geophysics, 16, 213-221.

•1957, Diffraction of a sphericalwave pulse by a half-placescreen:Can. J. of Phys., 35, 693-697.

APPENDIX

EM pulsepropagationin a simpledispersive

medium: Electric Letters, 7, 285-286.

•1971c, Transientsignalsfrom a buried magneticdipole: J. Appl. Phys., 42, 3866-3869. •1972, Transientdipole radiationin a conductingmedium: Rev. Bras. Tech., 3, 29-37.

•1982,

Vallee, M. A., 1981,Scalemodelson transientresponseof multiple conductors:M.Sc. thesis, Univ. of Toronto; availableas Researchin AppliedGeophysics20, Univ. of

TEM

EM waves in stratified media: Pergamon-Mac-

Millian Company.

stratified earth: Can. J. Phys., 50, 1055-1061.

Book Co.

Svetov,B. S., 1960,Model resultsfor inductivemethodsof exploration:Izv. Akad. Nauk SSSR, Ser. Geophys.,1,

•1951d,

•1962,

479

Geo-electromagnetism:Academic Press.

Wait, J. R., and Spies,K. P., 1969,Quasi-statictransient response of a conducting permeablesphere:Geophysics, 34, 78%792.

Wait, J. R., andHill, D. A., 1972a,TransientEM fieldsof a finitecircularloop in the presenceof a conducting halfspace:J. Appl. Phys., 43, 4532-4534. •1972b, Transientmagneticfieldsproducedby a ste. p function excited loop buried in the earth: Electroinc Letters, 8, 294-295.

Ward, S. H., 1967,Electromagnetic theoryfor geophysical applications, in MiningGeophysics, 2: Societyof Exploration Geophysicists, 13-196.

Ward, S. H., Pridmore,D. F., Rijo, L., and Glenn, W. E., 1974,Multi-spectralEM explorationfor sulphides:Geophysics,39, 666-682.

Ward, S. H., Ryu, J., Glenn,W. E., Hohmann,G. W., Dey, A., and Smith, B. D., 1974,EM methodsin conductive terrains: Geoexploration, 12, 121-183.

Weir, G. J., 1980,Transientelectromagnetic fieldsaboutan infinite-simaly longgroundedhorizontalelectricdipoleon the surface of a uniform half-space: Geophys. J. Roy. Astr. Soc., 61, 41-56.

Woods, D. V., 1975,Model study of the Crone borehole

pulse EM (PEM) system:UnpublishedM.Sc. thesis,

Queen'sUniversity,Ontario,Canada. Woods,D. V., andCrone,J. D., 1980,Scalemodelstudyof a boreholepulse EM system:Bull., Can. Inst. Min. Metallurg., 96-104.

A

SYSTEMS

Misac N. Nabighian* and James C. Macnae

Ratherthanprovidea completesetof currenttechnical specificationsfor available TEM systems,we have chosento provide some details on the back-

WINDOWS

groundneededto evaluateindividualsystemspecifi-

To maximizesignal-to-noise ratios(S/N), all practical systemsmeasureaverage(or integrated)response

cations,and the compromises that needto be considered in achievingan actual measurement,and how thesecompromisesmay affectindividualsystems.

Volume 1. The nominaldelay time is usually taken to

over windows of finite width. The effects of averaging on S/N have been discussed in Becker and Cheng,

*NewmontExploration Ltd., 1700LincolnStreet,26thFloor,Denver,CO 80203. *Lamontagne Geophysics, Ltd., 4A WhitingStreet,Artarmon,NSW 2064,Australia.

480


which can differ significantly from the instantaneous


be the mid-point of the window. When a simple EM

decaysuchasanexponential e-t/tois windowed, the

amplitude e-k(t'+t2)/2 at the midpointof the time

amplitude A in a windowed average obtained by simple integration is'

window for delay times greater than to. Common system window widths increase with delay time, typ-

A = (e -kt' - e-kt2)/K(tl - t2)

ically with a width between . 1 and 1 times the mean delay. Long window widths compared to mean delay

whereK = 1/to,

Table A-1. Difference between point and windowed time samples. (t2-tl)/t

e -•t Decay amplitude

t/to

Channel

Relative

width

mean delay

1

0.368 0.368

1

.5

1 2 2 3 3

1 .5 1 .5

EM 47

300

250

30 25 I

I

J•

t

I EM 47

4 1 18 4 42 10

57

7.5

3 2.5

BASE FREQUENCY

.........

!

•10,

EM 37/47/57

20

RANGE

I

EARLY

STANDARD

TIME

I

IO

Iõ

O. I

ANALOG

l DIGITAL PEM

30

I

IO

POSSIBLE

SlROTEM

8

5

2o

32

32

4

I

0.01

Percentage difference

0.383 0.372 0.159 0.141 0.0706 0.0546

0.135 0.135 0.0498 0.0498

EM 37/EM

I

A Windowed decay amp.

I0 CHANNEL I

DELAY

I

TIME

20

CHANNEL

UTEM

RANGE

I0

I00

I000

DELAY TIME (ms) Fig. A-1. Window widths relative to near delay time for various commercial TEM systems.


time can affect measuredresponsesconsiderably. For mean delay time t we obtain the results given in Table A-1.


The

conclusion

derived

is not that

short window

widths are optimum, but that finite window effects need to be taken into account in quantitative modeling of field data. A constant window-width to delay-time ratio is helpful for scaling models. Figure A-1 shows on a logarithmic time scale that window widths relative to mean delay time of the various practical TEM systems vary somewhat, SIROTEM for example has very large window-width to delay-time ratio at its early channels. Generally, for a fixed number of channels, wider

windows

allow

for measurements

to cover

481

Some recent TEM systemshave programmabletime windows that can be adapted to virtually any number of time windows to cover any time range. These systems include the Crone digital PEM receiver, the Zonge GDP-12 and GDP-16 multipurpose receivers, and the Phoenix V-5 system. The Zonge and Phoenix systemshave primarily been used for magnetotelluric measurements, but can be reprogrammed for TEM operation. Since EM is a diffusionprocess,it is usually best to choose measured delays (and window widths) to vary logarithmically in time. TRANSMITTER

CHARACTERISTICS

a

wider range of delay time (bandwidth), narrow windows may provide more detail on specificportions of an electromagnetic (EM) decay, and will necessarily be noisier than wide windows.

There is considerabledifferencein practice between off-time TEM systems and on-time TEM systems (UTEM and the airborne Prospectsystems)in terms of factors affecting measurements. Figure A-2 presents the transmitter waveforms of step and impulse TEM systems,with equal samplingtime s. OFF-TIME

STEP

TEM

While the INPUT system has a unique waveform not discussedin detail here, ground systems measuring in the off-time have all adopted a "castle" waveform, as illustrated in Figure A-2. The transmitters tend to be voltage rather than current regulated and (compared to on-time systems) are high current, low voltage devices. Turn-on may be ramp or exponential, turn-off tends to be a ramp, where time extent is controlled approximately by available maximum voltage V and loop inductanceL with the slew rate dI/dt = V/L. The total time required to turn off peak current I0

(MEASURING TIME )

LINEAR RAMP-•5 IMPULSE

__/ PURE IMPULSE

is thus t = IoL/V. Increasing loop side L leads to

increased inductance asL2 andthuslong-time ramps. High power transmitterswhich increaseI 0 thus invariably tend to increase ramp times compared to their

Fig. A-2. Transmitter waveforms of step and impulse TEM systems.

Table A-2. Table of transmitter characteristics of off-time systems. Manufacturer

Crone (Canada)

Model PEM

DEEPEM

Turnoff

Quarter cycle

Turn off Time 0.5 to 1.5 ms

Zero Time Definition

End of ramp

Comments

Both timing of windows

cosine or

and individual

linear ramp

gains have varied in the

channel

past. Geonics

EM37

Ramp

40 •xsfor 30 A in 300 m x 600 m loop

End of ramp

EM47

Ramp

2.5 •xsfor 2 A in 40 m

End of ramp

(Canada)

x 40 m loop Geoinstruments

(Australia)

SIROTEM

Ramp

140 txsinto 100 m x 100 m loop

*Beginning of ramp

Early channels may be located in ramp time with high power transmitter.


482


lower power equivalents. Increases in available voltage are generally used to increase total current I0 rather than cause rapid shutoffs of a smaller current. Because of system bandwidth considerations (controlled by the output impedance to some extent) the ramp can never be perfectly linear. Table

A-2

summarizes

time transmitted

some characteristics

waveforms.

of off-

Note that the SIROTEM

system has a "time zero" definition at a different end of the ramp compared to the other off-time EM systems. This means on occasion that with the SIROTEM

high power transmitter and the usual synchronization procedure, early-time channels may have been measuredwithin the ramp itself. Finite ramp times may be taken into accountin the modelingprocessby either of two ways: (1) Using the approach illustrated in the case history of McNeill et al., (Appendix B) to make a first order correction to measured data, and (2) computing models for finite ramp times. The latter approach is useful in inversion programs. ON-TIME

TEM

(1980) and Spies amd Frischknecht, this volume] was operated to satisfy both conditions (3) and (1). UTEM can use either condition (2) or (3) for primary field correction (West et al., 1984) depending on which is most appropriate. Reasons why measurements made during on-time may be desirable are:

(A) Transmitter power/voltage/current limits are more effectively used, and (B) Less receiver idle time means more transients can be sampledin the same time (Figure A-2). The disadvantagesof on-time measurementsare the three required conditionsgiven. On-time systemtransmitters tend to be very difficult to design and expensive to build if condition (2) is to be satisfied. Alternatively, expensive correlators/array processors may need to be used if correlation is used in condition (3) (Duncan et al, 1980, Annan and Lobach, 1985). In fact, sinceit is impossiblein practice to get perfect control, i.e., perfect monitoring and geometry of Tx loop and Rx site, most on-time systemswork best when condition (1) is satisfied.

Several commercial and academic TEM systems make measurements in the on-time, when both sec-

ondary and primary fields are present. These systems are the UTEM (West et al., 1984), PROSPECT (Annan, 1986) and PRBS (a ground system described in Duncan et al. 1980 and Spies and Frischknecht, this volume, an airborne system describedin Zandee et al. 1985) systems. In addition, to common requirements such as adequate signal strength, an on-time TEM system(in an exact parallel to a frequency domain EM system) can function well if any one of three conditions are met.

(1) The secondary field is comparable in amplitude to the primary field. This situation usually occurs at distancesaway from the transmitter similar to the depth of the current system of interest in the ground, or if the receiver is oriented in a null-coupledposition with respectto the transmitter primary field. (2) The primary field is precisely controlled, which

involves

both

careful

transmitted

To allow for S/N improvement through stacking,all practical systems transmit a repetitive signal with a fixed period between successivecycles. To prevent dc bias effects, alternating waveforms are invariably used. The effect of the repetitive waveform can be obtained by analytic or numerical summation of the effects of previous (half) cycles. Significant effects occur when the EM transient decay is sutficiently slow that significantamplitude is present at the end of the half-cycle. Silic (1987) presentedformulas to determine the effect of periodicity

II

]•

PREEMPHASIZED

UTEM

STEP

current

control and accurate geometrical data to allow subtraction of the primary field from the total to obtain the secondary field. (3) The primary field is preciselymeasured.(Simple subtraction,or mathematically basedcorrelation or deconvolution techniques are used to extract the secondary field.)

Generally, the best results from an on-time system are obtained

PERIODICITY

if more than one of these conditions

LINEAR RAMP IMPULSE

T

I/A

=0 UNIT DELTA

PURE IMPULSE

,,"'•FUNC TION

is

met. The PRBS system, for example, [Duncan et al,

Fig. A-3. Received waveform correspondingto Figure A-2.



on measuredamplitudefor the systemfunctionwaveformsasshownin FigureA-3. For sampling a transient that hasa simpleexponentialdecayof the form Ce

where C is a constant,the sampledresponsecan be

found,for a givenramptimeA, asthelimitingsumof a geometricseries.We thusobtain Step:

483

mainpowerlinenoiseoccursat thefundamental (commonly60 or 50 Hz) and its oddharmonics.However, switchingtransientsand modern power-efficient, switch-mode generatorscancausesignificantamounts of energyat otherfrequencies. Moreinsidious thanthe noisefield of powerlinesis their groundingsystem, which can channel currents induced by the TEM

systemandproducesignificant anomalies. Isolatednoisespikes,commonlycausedby lightning strikes within a few hundred kilometers, are easily

4e -t/,S(t)=C

s(1 + e -s/,)

detectedby a threshold sensor,andareexcludedfrom thestacking process in all systems by keepingthedata in temporary storage beforeaddingto themainstack.

Linear ramp impulse:

C _t/x (1- e-a/,)(1 - e-s/,)

E(t)= X e

(1+ e-2S/*)

Pure impulse: I(t) =-

C e -t/•(1- e-S/•) 'r

(1 + e -2s/,.)

Theseequationspredictthat decayswith time constantslongerthan the samplingtime cannotbe resolvedby theimpulsesystems. SimpleS/N calculation of the ratio of stepto impulseresponseas sometimes quoted,is notvalidfor intersystem comparisons without full consideration of the actual noise and exact

signalconditions.For example,the UTEM system actuallyemploysa preemphasized transmitterwaveform which is exactly deconvolvedin the receiverto helpreducehigh-frequency sfericnoisewhichhasthe effectof makingthe systemappearasa mixtureof step andimpulseresponses (FigureA-3) for S/N analysis, but as a stepfor interpretation purposesafter deconvolution.

NOISE REJECTION

Practicalsystemscan minimizethe effectsof noise in a numberof waysasdiscussed in BeckerandCheng (Volume 1). Noise at powerlinefrequenciesis often minimizedby operatingat basefrequenciesthat are an even fraction of the powerlinefrequency.Although

powerlinevoltagesare usuallywell regulated,their currentsand resultingmagneticfieldsmay vary. The

OTHER

CONSIDERATIONS

In choosingan EM systemand the surveyinstrument for a particularexplorationtask, many other factors such as operatingtemperature,portability, transmitter loopwirerequirements, easeof protection,

price,andlocalavailability needto be considered. Suchconsiderations are important,but are beyondthe scopeof this discussion. REFERENCES

Annan, A. P., 1986, Developmentof the PROSPECTI airborneelectromagnetic system,in Palacky,G. J., Ed., Airborneresistivitymapping:Geol. Surv. of Can., Paper 86-22

Annan,A. P., andLobach,J., 1985,Experiments witha new generalpurposedigitalEM receiver:55thAnn. Internat. Mtg., Soc.Expl. Geophys.,ExpandedAbstracts,234236.

Becker,A., andCheng,G., 1988,Detectionof repetitive electromagnetic signals,in N abighian,M. N., Ed., Electromagnetic methods in appliedgeophysics, Vol. 1: Soc. Expl. Geophys.

Duncan,P.M., Hwang,A., Edwards,R. N., Bailey,R. C., andGarland,G. D., 1980,The development andapplicationof a widebandelectromagnetic sounding systemusing a pseudo-noise source:Geophysics, 45, 1276-1296. Silic, J., 1987,The natureof STEP and impulseTDEM systems: Expl. Geophys.,18, 204-207. West,G. F., Macnae,J. C., andLamontagne, Y., 1984,A time-domainelectromagnetic systemmeasuringthe step

response of the ground:Geophysics, 49, 1010. Zandee,A. P., Best,M. E., andBremner,T. G. T., 1985, Sweepem, a newairborneelectromagnetic system:55th Ann. Internat. Mtg., Soc. Expl. Geophys.,Expanded Abstracts, 236-239.


484


APPENDIX

EM37 CASE HISTORY,

B

KOLAVI,

FINLAND

J. D. McNeill,* M. Bosnar,* and G. M. Levy*

In the Spring of 1983, a demonstrationsurvey using the EM37 transient EM system was carried out in Finland.

Since the terrain was known beforehand

to be

resistive, the target, as described by the client, was simulated with a simple loop model and the response calculated as a function of transmitter loop size and position. This information was used to locate the actual transmitter loop for the survey. For the survey it was possible to accurately match the measured survey profiles of dB/dt (at late time) with the response from a loop model which in turn agreedwell with the known geology. The survey decay curve was also integrated numerically to produce profiles of the magnetic field B (rather than its time derivative) to illustrate the feasibility of this technique. SIMPLE

PLATE/LOOP

PROVED OF THE ....

sm Survey

r•oo

Line

// Kivivuopio i•

LIMIT OREBODY

INTERPRETED LIMIT OF THE OREBODY ß

DRILL

HOLES

/ /

(2km)I

/

/ I

•--

--.a_:x_

\

t

'\

ß ß / ß

;,/

oI

5?0

ioo? m

Fig. B-1. EM37 loop layout and horizontal projection of ore-bodies. Rautarukki Oy Exploration, Kolari Hannu-

PROGRAM

In many parts of the world base metal depositsare typically limited in thickness to a few meters and yet have depth and strike extent of several tens or hundreds of meters. A thin plate-like model is a useful representation of such targets (Annan, 1974). As discussed,important information about bounded

'r is determined by the product of S, the plate conductance (also called the conductivity-thickness) and a. Details of the algorithm are given in McNeill et al,

conductors

1984.

can be obtained

from measurements

made

at late time, when the decay has a single exponential time-constant (Nabighian, 1971, Kaufman, 1978). McNeill (1982) demonstrated that in a resistive environment the late stage responseis a simple function of plate parameters. Indeed if a and b are the minor and major dimensionsof the full plate, it can be replaced by a singlerectangularloop of wire of dimensions 0.7a and 0.7b centered on the plate, as long as the responseis not measured in close proximity to the plate. The wire loop carries a current describedby I0 exp (-t/x), where I0 is essentially a function of the product of the primary magneticfield H 0 averaged over the plate and over a. The late-stagetime constant

kainen.

TARGET

IMPULSE

AND

STEP

RESPONSES

The EM37-3 and most other TEM systemsgenerate a target response by causing an essentially static magnetic field at the target to be abruptly terminated. They are said to measure the target impulse response since the induced emf which drives the vortex eddy currents is, in resistive ground, proportional to the time derivative of the primary magnetic field (and the transmitter current) and is thus an impulse function. On the other hand, the UTEM system enjoys a triangular transmitter current waveform, the time de-

*Geonics Ltd., 1745 Meyeride Drive, Unit 8 Mississauga,Ontario, Canada L5T 1C5



EM37

LINE

Omtm:

Film

B80

KOLA.

485

X

X Componmnt

o o 00

ß1-

5

'0-

12

c

,

i

!

,

i

i

I

q_ o

x m

O!

: I

I

I

I

I

01 ß-•

. i

I

I

i

I

00'

Fig. B-2. (a) OBx/Otsurveyprofilescorrectedfor finite transmitterturn-off.

I

'19-

17

'18-

20



LINE DS0

EMS7 Doto:

i

Film KOLA. Z

Z Component

12

I:B-

I

Fig. B-2, cont. (b) samefor OBz/Ot. 17



487

rivative of which is a step function, and this systemis said to measure the step responseof a target. There are some significantdifferencesbetween the impulseand step response.For example, if the target is confinedit is easy to show that at the late stagethe impulse responseis proportional to (l/r) exp (-t/x) whereas the step response is proportional to exp (-t/x). Therefore, the impulse responsefrom targets with short time-constant will be relatively enhanced compared with the step response. This may be an advantageif we are searchingfor poor conductors,but a disadvantageif we are searchingfor goodconductors located in close proximity to poor ones. Apparently sometimes both the impulse and the step response

showsthat the target step responsewould be obtained if we measuredthe magneticfield itself rather than its time derivative. Conversely, if a well resolved transient decay curve of dB/dt is available, simple numerical integrationcan be usedto obtain an estimateof the step response.In order to obtain an accurate integral, particularly at early time, many narrow gatesmust be employedto get an accurate measurementof dB/dt. Note, however, that at late time the values of B may be in error by an unknown constant, since in order to integrate to t = o• we must extrapolate the transient,

should be obtained.

based on the last measured

Linear systems theory shows that the target step responsecan be obtained by integrating the impulse response.Since the receiver coil measuresdB(t)/dt, the integral of the transient response

EM37

Data:

File

$g•o•o

dt

TRANSMITTER

T'T'"'T''"•'"

gg•o

gg•o•o

if l?im B(t) = O t-->o•

values.

TURN-OFF

TIME

CORRECTION

The procedure of integrating the decay curve of dB/dt to obtain the step response assumesthat the

X Component gg•o•o

dt= B(t)

KOLA. X

LINE 680 g

••dB(t)

gg•o•o

gg•o•o

EM37

Data:

File

KOLA. Z

LINE 680

g

Z Component

•""•''"•'"'•' T'T''"T''"T'"

T'"'T'"'T""'T'

.=

E

....

I ....

(a) (b)

Fig. B-3. (a) Bx surveyobtainedby integratingthe OBx/Ot data in Figure B-2.

Fig. B-3, cont. (b) samefor Bz.

I ....

I •11- 20

488


•(mv) I0'


TRANSIENT DI•CAY CHARACTERISTICS

measured response is indeed the impulse response, which in turn implies that the transmitter current turn-off takes place infinitely quickly. In fact, however, it is impossibleto instantaneouslyswitch off high current in a large loop while maintaining a finite voltage across the loop terminals. This EM37 transmitter current turn-off exhibits a rapid linear-ramp decay with time which results in a target emf which is accurately a step-on step-off rather than an impulse function.

8z

The finite pulse length can cause appreciable distortion of the target response for short-time constant targets at all measurement times, and for homogeneous and layered-earth response at early time. For

Sin 940

MAL.,•

10z

both situations, removal of the distortion is desirable

to compare survey results with theoretical modeling based on an infinitely fast transmitter turn-off. A

450 L•rm I700

ß'= 2.3,5

m s

•z at $tn7660 HANNUKAINEN •' =3.66

I0 J --

ß' =3.5

HANNUKAINEN

ms

LOOP KIvivuoplo

ms

•

i0 o

X 7496.80

Lour,nolo

• S=22 mhos

Kuervooro

o

0

Fig. B-4. Best fit time decay curve with a singleexponential.

20

Fig. B-6. Best fit plate model in cross-sectionalview.

---1I r 1---! [ 1_. ,,,I.... '

r i I I r I--• r--i-•

•ZPlol, • HANNUKAINEN •x Channel

2.224

15

m s

12-

.I/

1 -

•.,.%.

-

', 64

•

68

70

72

74

76

78

80

a2

64

66

•

70

72

74

•

Fig. B-5. Best fit profile with plate model data (Dyck et al, 1980).

78

80

82



computer algorithm was developed which employs as input both the measuredvalues of dB/dt at each survey point along the profile and the known transmitter turn-off time. The program can calculate profiles of B and dB/dt, both corrected for finite turn-off time.

Figure B-1 shows a plan view of the survey area. The Kivivuopio orebody is a gently dipping, stratabound, skarn iron-ore deposit located at a depth of 300-700 m. It had been detected and outlined by magnetics and extensive AMT surveying as well as other geophysicalmethods (Pietila and Hattula, 1982), and confirmed by drilling as indicated. The bedrock is resistive: thickness of the deposits in this region ranges from a few meters to 40 m. An objective of the survey was to determine whether the body continued down-dip to the west. Using a standard EM37 system, survey profiles corrected

for finite

transmitter

turn-off

are shown

in

Figure B-2, and profiles of B obtained by numerical integration in Figure B-3. Fitting the time decay (Figure B-4) indicates that exponential decay commences at about 3 ms, with a time constant of 3.66 ms. Late

stage behavior beyond this time is confirmed by the profiles of Figure B-2 (the continuing change of the profile at the east end of the line is thought to be due to the adjacent conductor). The two "glitches" at the transmitter wire appear to have been caused by operator error. However, they offer an interesting example of the effect of correcting for finite transmitter turn-off. Since they are of lowtime constant (less than the transmitter turn-off time of 210 txs), correcting for the turn-off time substantially enhances their response (Figure B-2), whereas integrating to produce profiles of B reduces their response as well as reducing the overall dynamic range required to plot the data (Figure B-3). Figure B-5 illustrates the best-fit plate model; the location of the plate is shown in Figures B-1 and B-6. Note that the fit between the plate and target (at an average depth of 500 m) is good. Additional experimentation with the plate model confirmed the lack of down-dip extension to the target. Again both dimensionsof the target were approximately known beforehand and a single line survey could not determine the strike extent nor, with only two components, that the target also has a shallow plunge.

489

The survey data presented in this short note demonstrate the usefulness of a simple loop model for interpreting the results of transient EM surveys using large loop Turam-type transmitters, as long as neither the source nor the receiver is close to the plate, and the ground is reasonably resistive (in excess of a few hundred ohm-meters). Arguments and data are presented which indicate

the usefulnessof numerically integrating the transient curves so as to obtain an estimate of the magnetic field as well as its time derivative, and thus an estimate of

the target step response as well as the impulse response.

ACKNOWLEDGMENTS

The authors acknowledge the assistance given by Rautaruukki Oy in carrying out the survey and the permission to publish the result including geological data. The work described was partially funded by National TES

Research

Council

for Canada

IRAP

Grant

666.

REFERENCES

Annan, A. P., 1974, The equivalent source method for electromagnetic scattering analysis and to geophysical application: Ph.D thesis, Memorial Univ. of Newfoundland.

Dyck, A. V., Bloor, M., Vallee, M. A., 1980, User manual for programs plate and sphere; Research in Applied Geophysics no. 14, Geophysics Laboratory, University of Toronto.

Kaufman, A., 1978, Frequency and transient responses of electromagnetic fields created by currents in confined conductors: Geophysics, 43, 1002-1010. McNeill, J. D., 1982, Interpretation of large-loop transmitter transient electromagnetic surveys: 52nd Ann. Internat. Mtg., Soc. Explor. Geophys., Expanded Abstracts, (In the abstractthe functionsf• andf2 shouldbe interchanged and the "b" following 0.64 should be replaced by "a"). McNeill, J. D., Edwards, R. N., Levy, G. M., 1984, Approximate calculations of the transient EM responsefrom buried conductorsin a conductive half-space:Geophysics, 49, 918-924.

Nabighian, M. N., 1971, Quasi-static transient responseof a conducting permeable two-layer sphere in a dipolar field: Geophysics, 36, 25-37. Pietila, R., Hattula, A., 1982, An application of audiofrequency magnetotellurics and charge potential method in skarn iron ore prospecting in the Rautuvaara area, northern Finland: 44th Ann. Eur. Assn. Expl. Geophys. Mtg., Cannes.


490


APPENDIX

PEM CASE HISTORIES

CIGAR

AND WINSTON

J. Duncan

LARGE LOOP SURFACE PULSE EM (DEEPEM) CIGAR LAKE DEPOSIT, SASKATCHEWAN

The high grade Uranium Deposit in the Athabasca Basin, Saskatchewanwas discoveredby Cogema Canada Limited by utilizing, among other techniques, a surface DEEPEM survey undertaken by Crone GeophysicsLimited. The body occursat a depth of 450 m. The deposit is a typical Athabasca type uranium body consisting of a long horizontal "tube" of high grade uranium mineralization (cross sectional width usually less than 100 m) occurring at the base of the flat lying sandstone

sediments.

The mineralized

"tube"

is usu-

ally associatedwith the upper contact of a large, near vertical, graphitic formation occurringin the basement Archean rocks. EM surveys may be used to detect the near vertical, graphitic conductors.The exact location of the upper contact of the graphitic zone with the sandstoneis required from the survey since the follow up drill program is designedto test the area immediately above and at the discontinuity, rather than intersect the graphitic conductor. The drill pattern often

consists

of a "fence"

of three

vertical

C

LAKES,

Crone*

Profiles of both vertical and horizontal components are shown in Figure C-2. In this case, the vertical component is strongly influenced by currents generated in the extensive alteration layer. The field from the currents is nearly vertical across the length of the survey line•it shiftsthe vertical componentresponses stronglynegativeand the minus-pluscrossoverpattern usually expected from a vertical conductor (the PEM sign convention is opposite to all other TEM systems) becomes a more subtle inflection type response. The horizontal component response is not as strongly influenced by the currents in the alteration zone. The stackedhorizontal peak anomaliesclearly indicate the presenceof the basementgraphitic conductor. Fraser Filter (Fraser, 1969) treatment of the survey readings has proven an effective way of reducing the long wavelength effect of regional currents from this data. The filter selectively emphasizes the EM response from currents at up to a depth of five times the filter station interval. Using the filter with a 100 m station interval pinpoints the location of the basement conductor, (Figure C-3).

holes

spaced approximately 50 rn apart. The EM survey responsescan be complex due to the presenceof a flat lying, conductive clay alteration layer lying along the unconformity and extending into both the sandstone and basement

formations.

This alteration

CANADA

DEEPEM

zone which

can attain thicknessgreater than 100 m, is usually well developed in areas of uranium mineralization. The large loop, Pulse EM survey used transmitter loops of 400 rn x 800 rn powered by a 2000 watt waveform generator. Both vertical and horizontal component readingswere taken by the receiver coil at each survey station. Constant gain was used throughout the survey (Figure C- 1).

,•.-(W)..-• TM Fig. C-1. DEEPEM survey configuration.

*Crone Geophysics, 3607 Wolfdale Road, Mississauga,Ontario, Canada L5C 1V8.



""-

491

HORIZONTAL COMPONENT

,....

/I o

•1•1 m

W ,.',

•

IIlIli I w.... w _..-"w

--W

__W

,D

0

0

,D

,2,

700E

800E

900E

1000E

•i

',-?-- '"--,•, w

'"-•--__,,,

• .....

0

III•i w-----'•

"'•

._,_,:,

--•r-'--

DEPTH

(Meters)

300E

400E

500E

600E

0-

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••

100-

',¾]¾:¾]::¾:¾•¾]¾]¾i GRAP HITIBBONDUBTOI•.:i

200-

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 300

-

•••••••••••••••••••••••••••••••••i::::::::::::::::::::::::::::::::::::::::::::::::: :i:i:i:!:i:i:i:i:i:i:i:i:!:i:i:i:i

•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••-.•••••••••••••:•••:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:••

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::•iO•' •i•i•'6•.'L: I'f•,:r'l• •' ::::::::::::::::::::::::::::::: 400450', 500-

Fig. C-2. Profile of the vertical and horizontal secondaryfield measurementsusingthe PulseEM-DEEPEM surface survey method that assistedin the detection of the Cigar Lake uranium deposit Athabasca, Saskatchewan.


492


•

•---.

0

0

0

0

0

0

0

0

0

VERTICAL COMPONENT 100m FILTER

,.', o

0 0

,'., 0

I

HORIZONTAL 100m

t

0 0

I

COMPONENT FILTER

DEPTH

(Meters)

300E

400E

500E

600E

700E

800E

900E

1000E

0-

100-

:::::::::::::::::::::::::::::::: GRAPHITIC CONDUCTOR :::::::::

200-

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::: INTHEATHABASKA v.v.v.v.v.v.v.v.v.v. ................. :::::::::::::::::::::::::::::::: BASIN ATADEPTH OF

300

-

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ,:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.•50 METERS

::':::'

.................. ß............. ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ß ................ ..... ................ .....'

"'"'"'"""'"'"'"'""'""'""'"'"' .............. 400

-

450

ß

500-

.:¾:::¾ •/•,•l'l•J'l•l' 'I•I'I•IERALIZATION .'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'

....':...•.....':..'...':.:.':.:':•.!:..'. ................................. :.......... ,..., ............... •:::......•-'

ALT ERED CONTACT ZONE

BASEMENT

GRAPHITE

Fig. C-3. Fraser filteringof the DEEPEM results,using100m stations,as a meansto removethe longerwavelength contact zone responseand pinpoint the location of the graphitic conductor.



ISOLATED

493

BOREHOLE

Conductor

Fig. C-4. Borehole pulse EM survey configuration.

BOREHOLE

PULSE

EM

TRANSMITTERS--WINSTON

FROM

SURFACE

large "edge" type anomaly is obtained centered at 245 m down the hole. At this location

LAKE DEPOSIT,

ONTARIO

This survey procedure uses a pattern of five (usually) equi-sized transmitter loops laid out on the surface and the hole is surveyed from each loop by running a receiver probe down the hole, (Figure C-4). The transmitter loops are positioned to provide a wide variation of primary field directions in the possible target area. The changesin amplitude, shape, and sign of the anomalies

obtained

are used to determine

the

direction, distance, size, and to some extent the shape of the conductive body (Woods, 1975; Woods and Crone, 1980; Dyck and West, 1984). This case history is from a borehole Pulse EM survey carried out by Corporation Falconbridge Copper in 1982, at their Winston Lake property in northern Ontario. The property contained a small (180,000 tons), mined out, zinc deposit and a geologicalsetting that Falconbridge consideredhighly favorable for further mineralization (Simmons, 1983). Surface geophysics including Max-Min, very low frequency (VLF), and low-powered (500 watt) Pulse EM (DEEPEM) produced no significant anomalous targets. Detailed mapping and geochemicalsurveys outlined

a zone

of intense

alteration.

Four

drill

holes

explored this favorable area to a depth of over 200 m but intersected only 6 m of minor disseminatedmineralization

and no massive mineralization.

A borehole

survey was carried out down these holes prior to the planned continuationof the drill program. The borehole survey results from DDH Z0-4 are shown in Figure C-5. Five, 100 m x 100 m transmitter loops were used as shown in the plan map. A strong,

the hole inter-

sected a few millimeters of sulphide mineralization. The borehole survey profiles are all similar in shape indicating a sheet like body. The reversal in sign of the anomaly from the early to late time channels is a classic edge type response. The few millimeters of sulphidecut in the hole must lie at the very edge of a large conductive sheet. Since both north and south transmitter loop responses are similar in amplitude, the sheet must be equally continuous along strike in both these directions. The geological dip is known to be approximately 55 degreestoward the east. The key question is now whether the sheet continues up-dip toward the west or down-dip toward the east. The lack of surfacegeophysicalresponsessupportedthe downdip direction. This result is clearly confirmed by the borehole survey responsesfrom the east-west section of transmitter loops. The anomaly obtained from the west Tx loop is completely reversed in sign and much reduced in amplitude. Using Macnae's (1980) primary field direction curves, (Figure C-6), we see that the primary field from the west Tx loop is almost null coupled to a downward dipping conductor. This field at depth, would energize the body from its upper surface, not the bottom, reversing the sign of the anomalous response as obtained in the survey. An upward dipping conductor from the west Tx loop could only produce a strong amplitude anomaly of the opposite sign. For these and other geologicalreasons, the follow up hole Z0-5 was drilled down-dip of Z0-4 and intersected 1.1 percent Cu and 19.1 percent Zn over 2.1 m. A large mineralized sheet of massive sulphides(zinc, copper) has been drilled off and shaft sinking is presently in progress.

494

Nabighianand Macnae

-1 O0

-10

4.-10

4.-1O0 4.-1000

PLAN MAP OF TRANSMITTER


AND

DRILL

wloo. z• •

-100

-10

+10

w

+100

+1OOO

-1OO

-10

+10 +100+1OOO

c ,

-100

LOOPS

HOLE

c

100 m ---•

-10

+10

+1OO+1OOO

E

•

-1OO

s

-10

+10

+100

+1OOO

BOREHOLE P.E.M. SURVEY RESULTS USING DIRECTIONAL Tx LOOPS

WINSTON

LAKE DISCOVERY, ONTARIO

FOR CORPORATION

FALCONBRIDGE

COPPER

- 1982

Fig. C-5. Direction boreholePEM logs of the Winston Lake boreholeZ0-4 from 5 surfaceTx loops, the west Tx loop responsereversingin sign and much reducedamplitude.

TEM Prospecting Methods Tx LOOP TO E

COLLAR Tx LOOP Tx LOOP

495 REFERENCES

Tx LOOP TO W

•.- 10Om-•

Dyck, A. V., and West, G. F., 1984, The role of simple computer models of interpretations of wide band drill-hole EM surveys in mineral exploration: Geophysics, 49, 957-


980.

Fraser, D.C.,

1969, Contouring of VLF-EM data: Geophys-

ics, 34, 958-967.

Fig. C-6. Section showing primary field coupling to the Winston Lake deposit from the east, collar and west Tx loops placed on surface Borehole Z0-4. The Tx loop is taken as the frame

of reference.

Macnae, J. C., 1980, An atlas of primary fields due to fixed transmitter loop EM sources; Research in applied geophysics, Vol. 13: Univ. of Toronto. Simmons, B. D., 1983, Winston Lake Discovery by Corporation Falconbridge Copper: Presented at the Prospectors and Developers Association in Toronto. Woods, D. V., 1975, A model study of the Crone borehole pulse electromagnetic (PEM) system: M.Sc., thesis, Queens University. Woods, D. V., and Crone, J. D., 1980, Scale model study of a borehole pulse electromagnetic system: Bull., Can. Inst. Min. and Metallurg.

APPENDIX

EMP CASE HISTORY

D

CRATER

DEPOSIT,

AUSTRALIA

Graham Boyd*

During 1981, Newmont Holdings Pty. Ltd. conducted

an economic

evaluation

of the Baal Gammon

massive sulphide deposit (>4 mt @ 0.3 percent Sn, 1 conductor

evaluation.

Extensive exploration in the surrounding area located a number of smaller massive sulphide occurrences. The largest of these occurrences (named the Crater deposit) was discovered by drilling an EMP conductor

in an area

indications

of mineralization.

where

there

were

no surface

The Crater deposit consistsof pyrrhotite with accessory arsenopyrite, chalcopyrite, sphalerite, pyrite, magnetite, and cassiterite. The mineralization is thought to be related to the Elizabeth Creek granite which is the source of tin mineralization throughout the Herberton area. The mineralization is hosted by Silurian

sediments.

The topography in the survey area is extreme with

slopes up to 40 degrees over the area surveyed. The elevation difference between the highest and lowest point on the survey area is in excess of 200 m. The transmitter loop was placed at 45 degreesto the existing grid lines so as to align it with the geological strike. Eight lines (50 rn apart) were surveyed at 50 rn station spacings. Figure D-1 contains loop and line percent Cu, 70 g/t Ag) located 15 km northwest of Herberton, Queensland, Australia. The Newmont electromagnetic pulse (EMP) system was used for locations as well as contours of the voltage out of the EMP coil (in microvolts) at 9.7 ms after current switchoff.

Two anomalies are apparent in the contour plans appearing to indicate flat lying conductors. However, all known conductors in the area are steeply dipping. The extreme topography was considered to have an influence

*Geosolutions Pty. Ltd., 15 Grevillea Way, Belair, South Australia 5052.

on the results.

In

an effort

to take

into


496


I

Eli200

ELI400

ELI600

ELI800 I

NI0800

R •

,

i

NI0600

NI0400

•._•_ __..i__.LJ

; NI0200 IOO I

•oo I

I

SCALE

Fig. D-I. Loop location and X, Y, Z, R(esultant) voltage output at Crater Deposit.

TEM ProspectingMethods 10400

The modeling indicates that a conductor, approximately 100 m in strike length, dipping steeply in the west (45-75 degrees)and striking approximately north to south can explain the observed EMP response. A secondmuch smaller conductor (strike length approximately 20 m) at surface can successfullyexplain the other anomalous response. Decays over the larger

I

CR6 CR4


CRI (PROJECTED) GOSSANOUS

FRACTURE

497

ZONE

conductor

indicate

a time

of 19 ms. When

compared to the 10 ms time constant observed for decays from the main Baal Gammon deposit this conductor was recommended for drilling. Figure D-2 shows the recommended drill target. All holes on the section shown in Figure D-2 were drilled to test the geophysical target. Hole CR1 was

4

//

//

/

drilled

I

262.4

to intersect

the center

m

tion some 40 rn below

///

//

20

40m

320.7m

Fig. D-2. Boreholes recommended and drilled based on EMP results.

account topography and provide an accurate drill target, modeling was performed using station elevations. Fitting the data at the 9.7 ms sample time (Barnett, 1984) indicates that two circular loops of current, as projected in Figure D-I, can successfully explain the EMP responseat this delay time.

APPENDIX

UTEM

of the best fit current

loop (indicated by modeling) and 7 m of massive sulphide mineralization was intersected. Hole DR4 intersected 31 m of massive sulphide mineraliza-

205.5m

0

constant

CASE

the center of the fitted current

loop. Step out drilling confined the size of the Crater deposit to 100 000 metric tonnes of high grade mineralization. The strike length of the deposit is approximately 80 m which compares well with the 100 m estimated from the EMP modeling. The large time constant observed is the result of highly conductive and well connectedpyrrhotite, which is the dominant massive sulphide.

REFERENCES

Barnett, C. T., 1984, Simple inversion of time-domain electromagnetic data: Geophysics, 49, 925.

E

HISTORIES

J. C. Macnae*, Y. Lamontagne* and P. D. McGowan*

The UTEM 3 wideband, time-domain EM system is suitable for massive sulphide exploration in a wide variety of geological settings. The system has been described in detail in several publications (Lamontagne, 1975; West et. al., 1984). Data from two very massive sulphides in a relatively resistive rock with

little or no overburden. Only vertical magnetic field data are presented. The second site is located near Broken Hill, N.S.W., Australia. This well-documented deposit lies below a thick, highly conductive overburden. Both electric and magnetic field data are presented.

*Lamontagne GeophysicsLtd., 115 Grant Timmins Drive, Kingston, Ontario, K7L 4V4, Canada.

498


and is representative of a reconnaissancesurvey for

and depth, implying that it dips near the vertical. Due to the 50 m depth to top, it is difficult to estimate a thickness,but a comparisonof the crossoverlocations for the two loop positionssuggestsit may be 10-20 m. Conductor A has a very long time constant (> 15 ms) since there is a clear channel 1 (12.8 ms at 30 Hz base frequency) response. The decay curve (Figure E-4) indicates strong diffusion which is an indication of strong thickness effect or inhomogeneity in the conductance. By roughly fitting the decay curve to a finite dike model, the conductance of conductor A is esti-

15N is described.

mated to be 200 S.


UTEM SURVEY IN N.W.T.,

CANADA

The survey was run as a reconnaissance survey using the UTEM 3 equipment in 1983. The vertical magneticfield (Hz) was measuredover eleven survey lines spaced100m apart usingtwo transmitter1oops• one positioned on the east and the other on the west side of the grid (Figure E-l). Only the data from line different

environments

located

in the Canadian

are discussed.

The first site is

Shield near the Arctic

Ocean

Two prominent conductors,A and B, were found in the grid. The conductors are separatedby 650 m and run parallel for 800 m. Continuously normalized (channel 1) plots of the line 15N data from each of the two transmitter loops along with an interpreted section are presented in Figure E-2. In Figure E-3 the same data are plotted normalized to the primary field strengthat the crossover

of interest.

Below line 15N, the top of conductor A is interpreted to lie at a depth of 50 m (note the distinct top anomaly, Figure E-2). Data from the neighboringlines indicate that the body plungesto the south. The depth extent is determined

from the ratio of the width of the

negative anomaly to the width of the crossover and is estimated

to be about

150 m for conductor

A.

The

Conductor

B is similar in character

to conductor

A

but lies at a greater depth as the broad crossover suggests(Figure E-2a). It was barely detected in the original coverage (loop 2 was read first) and is only evident as incomplete long time constant anomalies (Figure E-2b). This body does not plunge, as did conductor A, and its depth to top is more or less constantat 130-175 m. The depth extent however does appear to vary between 150 m to the south and up to 300 m to the north. The body dips almost vertically. Using 250 m as an effective size and a 10 ms time constant, the conductance is estimated to be 120 S. Only one drill hole is located on the grid (Figure E-l). It appears that the main conductive zone was missed by about 60 m. Therefore, the showing in the

body appears to be poorly coupled with the field of loop 1, which would be nearly vertical at this distance


22

20N

) LOOP I

c'

k____., 15N

1

LOOP

2

ION

5OO m !

Fig. E-1. Survey grid showing loop position.

!

, -

•


499

! oox


B

A

ß

(a)

(b)

I

Io%

I

" :•

•-,•.

LOOP 2

50m

200s ,•o• conductor

A

Fig. E-2. (a) Line 15 N datareadusingloop1 (continuously normalized). (b) Line 15N datareadusingloop2 (continuously normalized).


500

Nabighianand Macnae

•o, ••.• •doto normolized to 4250œ /

(a)

i

(b)

/

LOOPI

,

,

;

,

,

•--,•.

140m /

LO0 P 2 •

IõOm

200

conductor

B

Os

•,conductor A

25O

Fig. E-3. (a) Line 15 N datareadusingloop1 (pointnormalized). (b) Line 15Ndatareadusingloop2 (point normalized).



o o o o

•

o

T

I

o

I

t

o

I I

I I

501

(%) apnl!ldl.uD •Ded o• •lDad

u• I I

O cxJ

O ro

o

o

o

o

I


502

Nabighian andMacnae

• I Flying Doctor .co,,,

{ /

w.,,•v•c'rø"•A • T•

Line 24.50

St'

ß

KILOMETERS

ß f'

•

•

!

I0 WI - i

I

I I'•'

'•

"-i'

'l,

,

900E L

,

ß

ß.

r-

.'

ß ß

ß

....Line

25.25

'n ß

ß

ß

ß

LOOP

2

LOOP

I

,

,

S _1

,':

i

.'

!

i

!

i

!

i

!

I

_

K)95E

1- ß

5-44

Line

26.00

S

S ß

,

..

..

,

ß

.

.

,

o

J ß

ß

...

Sheor Zone

IOO

, I

0

200

I

SCALE

300m

i

500

I000

f

1,6000

Fig.E-5.Flying Doctor testsite:looplayout andlocation map.(Notegridislaidoutinfeet).


original hole may only representa lessmassivesection of the mineralization.

503

hole EMP data has helped define the conductive characteristics of the mineralized zone (Boyd and Wiles, 1984). A UTEM 3 survey was carried out over the test site


UTEM

SURVEY

AT FLYING

BROKEN HILL,

DOCTOR

TEST

SITE

N.S.W., AUSTRALIA

The Flying Doctor test site is located near Broken Hill, N.S.W. (Figure E-5). The deposit is typical of those found in Australia in that the overlying weathered layer is thick (approx. 50 m) and very conductive. The mineralization occurs within siliceous gneissesof the Broken Hill mine sequence of the Proterozoic Willyama complex. The ore body dips steeply to the northwest and extends from near surface to a depth of more than 200 m (Figure E-6). The zone hosts an estimated 200 000 metric tons of 6 percent lead, 3 percent zinc, and 2 percent pyrrhotite (no pyrite is present). This deposit has been used as a test site for many geophysical systems, data from which are available from a number of sources. Recently acquired drill

on three lines using two transmitter loop positions as shown in Figure E-5 and both electric (E) and magnetic (H) data were collected. Data from both loops and all three lines is available but only the central line data from loop 1 is presented here. Figure E-7 shows the vertical Hz component, together with a geological section based on drill information. The data is plotted in both continuous and point normalized format. In addition to the generally conductive overburden, manifest as a pattern of migratingcrossovers,two conductoraxes are interpreted as shown by the dashed lines. Conductor A is evident as a sharp crossover superimposed on a positive background at earlier times, and as a much smoother crossover at later times. Conductor B is only evident at very early times (Ch 10-8, or .03 to .12 ms). Using standarddepth interpretation rules, the depth to secondarycurrents is about 10 m for early times on both zones A and B, but is about 60 m for the later time

response (CH 7-4, or .23 to 1.84 ms). Note that the interpreted late time current concentration shown agreeswith the location of the best fit late time current loop from publishedNewmont EMP drill hole data. The UTEM anomaly amplitude is too large to be caused by local induction (based on matching amplitudes to type curves) and indicates that current gathering is the major contributor to the measured response. Because of this, estimation of dip and depth extent of the conductor is not possible. The UTEM magnetic field data has however unambiguously detected a conductor at the base of the weathered layer, with a less conductive

extension

toward

surface.

The electric field data show quite complimentary facets of the geologicalstructure to the magnetic field data. Presentedin Figure E-8 are both E (across strike) and E (along strike) components normalized to the calculated primary field. If any nonhorizontal contacts are present between rock units with different conductivities, charge may accumulate on these boundaries leading to a change in

thelocalelectricfieldsirength. In thisdata,theshear

Fig. E-6. Flying Doctor section from drill data at line 25+25S. Inset: Best fitting EMP current loop (Boyd and Wiles, 1984).

zone boundarieshave shownup clearly, particularly in the E data. The E data exhibit a high over each zone, which indicates that the zones are relatively resistive compared to the surroundingrocks. It is important to note that the shear zones, being resistive, have no detectable expression in the magnetic field data (Figure E-7). Both conductorsA and B have little obvious electric field expression. This is


504


(a)

20z

ß

,/

(b)

Fig. E-7. Magnetic UTEM data for line 25+25S. (a) Continuouslynormalized.(b) Point normalizedto 600 W.



•60X

Ey

5•AR

ZOnE

G'•E,5S

•EAR ZONE

Fig.E-8.ElectricUTEMdataforline25+25S.(a)Crossstrikecomponent (Ex).(b)Alongstrikecomponent (Ey).

505


506


because their depth/size ratio is quite small (Macnae, 1981). The generally conductive weathered layer however shows up very well in both H (as a migrating crossoverand steady -200 percent limit at early times far from the transmitter loop) and E (as a level change from channel to channel) fields. If geological mapping of resistive features, or variations within them, is of interest as may be the case in precious metal exploration, then the E field data may usefully add to the survey information at little extra cost.

REFERENCES

Boyd, G. W. and Wiles, C. J., 1984, The Newmont drill-hole EMP system--Examples from eastern Australia: Geophysics, 49, 949-956. Lamontagne, Y., 1975, Applications of wideband, timedomain EM measurements in mineral exploration: Ph.D. thesis, Univ. of Toronto. Macnae, J. C., 1981, Geophysical prospectingwith electric fields from an inductive source: Ph.D. thesis, Univ. of Toronto.

West, G. F., Macnae, J. C. and Lamontagne, Y., 1984, A time-domain EM system measuring the step responseof the ground: Geophysics, 49, 1010-1026.

APPENDIX

SIROTEM

CASE

F

HISTORIES

G. Buselli*

SIROTEM (Buselli and O'Neill, 1977) has a square bipolar transmitter waveform, and measurementsare made after the loop current is switched off. The transmitter turn-off may be closely approximatedby a ramp, which is typically 140 I•s in duration for a 100-m transmitter loop. The latest delay time at which measurements are made after current switch-off may be varied from 30 ms to 180 ms; the earliest delay time is centered at 0.49 ms, and the measurement time has

now been extended one decade earlier to 49 I•s. Data collected with SIROTEM are interpreted with aids as described in Buselli et al. (1984). A number of computer programs have been developed for TEM data interpretation by the Mathematical Geophysics Group of the CSIRO Division of Mineral Physics. These include programs for one-dimensionalforward modeling (programs CLRTEM and RECTEM) and inversion (program GRENDL); three-dimensionalforward modeling of a dike in free-space (program OZPLTE, basedon programPLATE developedby the University of Toronto); and a sphere in a conducting medium (program SPASYM). The case studies presentedillustrate the applicationof these interpretation aids.

SIROTEM SURVEY--ELURA

DEPOSIT, NSW,

AUSTRALIA

Coincident and In-Loop Geometry

The Elura deposit located 43 km north-northwestof Cobar, NSW, Australia is a massive lead-zinc-silver pyrrhotite body covered by 100 rn of conductive overburden, and extending to a depth of 510 rn below

surface (Adams and Schmidt, 1980). The deposit occurs as a discretevertical pipe-like body in the core of an anticlinical or domal structure. The orebody is elongated north-south, with maximum dimensions 200 rn by 120 m. Originally apparent as an aeromagnetic anomaly, the body gives clear ground magnetic and gravity anomalies. SIROTEM surveysat Elura were initially conducted with the 100rn coincidentloop geometry. A current of approximately 7A was supplied to the transmitter loop. Figure F-1 shows profiles of the TEM response measured on line 50800N, a west-east traverse line that passed directly over the orebody (Buselli, 1980). A clearly defined anomaly in the time interval of approximately 2.6 to 50 ms is measured over the

*CSIRO Division of Mineral Physics, P.O. Box 136, North Ryde, NSW, Australia 2113



orebody. Mainly overburdenresponseis measuredat delay times earlier than 2.6 ms. The width of the body producingthe anomaly may be obtainedby using the theoretically calculatedgeometrical responsefunctionsof a sphericalbody given in Kamenetskii (1976). The half-width of the peak at a delay time of 5.8 ms is 200 m. Assumingthe depth of the top of the body is 100 m, the radiusr of the body is found from the best fitting theoretical response function

-_-CHANNEL

MEAN DELAY

- NO.

TIME (ms)

-

•

lO3

<[

w U3

10

- 3,-•-•-••

,,,•, 1.3

•

1.7

6,---"---,--•• 2.1

2

2.6

z

o

to be r = 75 m.

At the position where the maximum responsebeyond a delay time of 10.2 ms is measured,the time constant -r of the decay curve (with the background subtracted) is -r = 13.1 ms. The resistivity of the orebodymay be estimatedby assumingit is a prolate spheroidand applyingthe formula given in Kaufman (1978). Using the value of r = 75 rn for the dimension of the body, a resistivity of 0.16 l•.m, or a bulk conductivityof 6.4 S/m, is derived. When TEM decay curves measuredwith the coincident loop geometryat stationsaway from the orebody are transformed to apparent resistivities and

507

z

7

3'4

8

4.2

9

5.0 5'8 7'0 8'5 10'1 1'7 15'6 •'7

tO

lO

_-t2

z

1'0

t7

: tg •,•

25.0

- 21

.8 45.4 5'7.9

- 23 -25 _

I

I

1850

2050

I

I

2250

I

I

[

2/,50

I

•

2650

i

285OE

_SURFACE•i Om 100m

200 m

Fig. F-1. SIROTEM profilesalongthe west-easttraverseline 50800N over the Elura orebody. From Buselli (1980).

CHANNEL NO.

1850

DELAY TIME

I

(ms)

1950 2050 2150 2250 2350 2450 2550 I

I

20

I

I

I

i

3

1'3

4

1'?

5

2"1

I

2750 E I

20 11./,

o'g

2650

._•2•14..•• 10'6 11'211.3 •••l• -12,212,111'213'5 /•8 15' 8 15-517'6/22'1 13'0 12'6 12'1 14-4 / 16.8 16'9 16.7 19.723'5 20.--20am 13'9 13'6 13'1 15.3 j 17'/, 17'5

1

2

I

12,5

-

12,2

16'9 20(0 23.4,21.0

14.714.614.•/16.1 18.018.2

_

15'8

15'3

17'3

18'7

16.9

20'0

23.6

21.5

18'7

16.0

19-/,

23,?

21,?

20'3

15-4

19'2

24'8

23'1

6

2'6

15'5

?

3'4

17'6 16'5 17'6 19'9 21'•1

8

4.2 20-

9

10

5'0 5'8

21.8 • 27,523'0•5'2 2 1:•18'9!4.6 23.9,/' 25-725 27.9•2

11

7.0

27.1

22.? 23.4. 21.?

18'8

27'?

26'4 29'0

32.5

31.3

3012

13

8'5

3

10'140- •4! 11 '7

50-

15

13.2 70

16

15.6

j

-4O -50 '60 70

Fig. F-2. Apparentresistivitypseudo-section on line 50800Nover the Elura orebodyderivedfrom SIROTEM measurements usingin-loopgeometrywitha 100m by 100m transmitter loopanda 100-turn7 m by 7 m receiver loop. From Buselli (1980).

508



plotted as a function of delay time, the resulting sounding curves all exhibit a decrease of apparent resistivity with delay time. However, the geology and dc electrical soundingsindicate the basement is more resistive

than

the overburden.

The

behavior

of the

coincident loop soundingcurves has been found to be caused by a response from superparamagnetic(SPM) material in the soil. The presence of such a response may be inferred from plots of decay curves that have a

t -•'ø-+ø'•dependence at late delay times(Buselli, 1982).

In this environment, the SPM responseproducedby loop currents less than 10 A may be avoided by ensuring that the transmitter-receiver loop distance is at least 3 rn for a loop receiver and at least 15 rn for a dipole receiver. Figure F-2 presents a resistivity pseudo-section obtained with the in-loop geometry along the same traverse line for which TEM profiles are plotted in Figure F-1. For any off-orebody station (e.g., 1850E), apparent resistivity values increasewith delay time. Assuming a two-layer earth, one-dimensional inversion of sounding curves free of an SPM responseproducesa geoelectricsectionconsistingof a 70 to 140 rn thick conductive layer with a resistivity of 10 to 17 tl.m, overlying a resistive basement with a resistivity of 340 to 1200 l•.m.

a depth extent of 250 m, and a conductanceof more than 40 S producesa TEM responsethat fits the peaks of the vertical and horizontal componentprofilesplotted in Figure F-3. To the east of the body (beyond 4800E), the measured voltages are adequately modeled by the responseof a half-space with a resistivity of 600 ll.m added to the responseof the free-space dike with the dimensionsand depth derived from program PLATE. Similar modeling of the response to the west of the body indicatesthat the geoelectric section consistsof a thin conductive layer over a half-space with a resistivity greater than 600 tl. m. Further calculations with program PLATE demonstrate that the main lens of the Mt. Bulga deposit would be detectable with this loop geometry, even if

103 10a 8.5 5'8 /.,'2 ms

2.6

-101

ms

1'7

SIROTEM

SURVEY--MT.

BULGA DEPOSIT, NSW,

AUSTRALIA

-9

•' -10 :•

Vertical

• -lo* [

Fixed Transmitter Loop Geometry, Profiling

Mt. Bulga deposit, located 10 km NE of Orange, NSW, Australia, is a subeconomic copper-lead-zinc massive sulphide deposit (Harley, 1983). The main lensis near-verticaland has an averagethicknessof 3 m. The depth of weatheringis approximately60 m. TEM profiles measured along line 16300N passing directly over the main lens of the Mt. Bulga deposit are given in Figure F-3. For this survey, a SIROTEM high power transmitter was used to drive 50 A into a 300 rn by 200 rn loop, with the nearestedgeof the loop placed about 25 rn to the east of the vertical projection of the top of the lens. The longest side of the loop was parallel to the body, and on this line the loop sides were at 4675E

and 4875E.

,

[

I

•

'

I

I

• 102 -,,, 10* ß

0 -1

5,•m /,.2

-10•

2,6 1.7 0'9

_10:>

Horizontal

-10• _ I /,300E

The vertical and horizontal (across-strike) components of the TEM response measured with a dipole

•

I /,/,00

•

I •500

I

•

/,600

I /,700

i

I /,800

Component

i

I /,900

I 5000 E

60m

••Orebody

receiver withaneffective areaof 104 m2 areplottedin Figure F-3. The vertical component has a crossover directly over the body, while the horizontal component has a negative peak directly over the body and a small positive peak on either side. From modelingwith program PLATE, it is found that a free-spacevertical dike at a depth of 60 m, with a strike length of 140 m,

I

Component

Fig. F-3. Vertical and horizontal components of the TEM response measured on line 16300N over the Mt. Bulga deposit using the SIROTEM high power transmitter to drive a current of 50 A into a 300 m by 200 m loop, and a dipole receiver with an effective area of 104 m2. From Buselliet al. (1983).



the body were buried at a depth greater than 150 m. In particular, the horizontal componentresponseis more sensitive to the response of the body than to the host rock response, which is mainly vertically polarized. For example, for the body positioned at a depth of 100 m, the peak horizontal component response is a factor of 10 or more greater than the response of a 600 f•.m half-space. ACKNOWLEDGMENTS

The Electrolytic Zinc Company of Australia provided accessto the Elura site; accessto the Mt. Bulga site was provided by The Shell Company of Australia Ltd. (now Billiton) and Aquitaine Australia Minerals Pty. Ltd. Geological and geophysicaldata relevant to these sites were also provided by these companies. REFERENCES

Adams, R. L., and Schmidt, B. L., 1980, Geology of the Elura Zn-Pb-Ag deposit: Bull. Aust. Soc. Explor. Geophys., 11, 143-146. Buselli, G., 1980, Interpretation of SIROTEM data from Elura: Bull. Aust. Soc. Explor. Geophys., 11,264-271.

APPENDIX GEOTEM

CASE

509

Buselli, G., 1982, The effect of near-surface superparamagnetic material on electromagnetic measurements: Geophysics, 47, 1315-1324. Buselli, G., and O'Neill, B., 1977, SIROTEM: a new portable instrument for multichannel transient electromagnetic measurements. Bull. Aust. Soc. Explor. Geophys., 8, 82-87.

Buselli, G., McCracken, K. G., and Raiche, A. P., 1983, Transient electromagnetics in a highly weathered terrain: 53rd Ann. Internat. Mtg., Soc. Explor. Geophys. Expanded Abstracts 632-635. Buselli, G., McCracken, K. G., and Rutter, H., Eds., 1984, Manual for SIROTEM field procedures and data interpretation: CSIRO, Sydney. Harley, B. F., 1983, A geophysical case history of the Mt. Bulga prospect, with an emphasis on TEM: Abstracts of the Third Biennial Conference of the Aust. Soc. of Explor. Geophys., 110-115. Kamenetskii, F. M., Ed., 1976, Handbook of applied methods of transient processesin ore geophysics: Leningrad, Nedra.

Kaufman, A. A., 1978. Frequency and transient responsesof electromagnetic fields created by currents in confined conductors: Geophysics, 43, 1002-1010. Vozoff, K., Buselli, G., LeBrocq, K., and Moss, D., 1984, An Australian evaluation of some new electromagnetic methods for petroleum exploration, in 2nd Petro. Geophys. Sympos. of the Aust. Soc. Explor. Geophys., 179208.

G HISTORY

Rolf Pedersen* and S. Thompson*

In July 1986, a GEOTEM survey was flown by Geoterrex Limited over a portion of the BeardmoreGeraldton Gold Belt in the province of Ontario, Canada. Airborne electromagnetic surveys represent a typical step in a typical mineral exploration program. Figure G-1 showsa portion of the GEOTEM anomaly plan map from this survey. Zone R-8, located north of and outside the survey boundary, was designatedas a mandatory follow-up target. Figure G-2 displays the digitally processed, computer drawn profiles of the GEOTEM, magnetic, and terrain clearance values from line 126, which is where the best response is located.

Fitting a vertical plate model to this response indi*Geoterrex Ltd, 2060 Walkley Road, Ottawa K1G 3P5 Canada.

cates a source with conductance

of 7 S is located about

57 m below surface. The responseson the other two lines have weaker signatures.However, the enhanced signal-to-noiseratio afforded by digital acquisition and processing makes it possible to identify legitimate signalsin the middle channels with greater confidence. What certainly is conspicuous,however, is that the overall electromagnetic (EM) trend exhibits three criteria which are often distinctive of mineral deposits, namely: having a short strike length, being isolated from other similar conductors,and being closely associated with a magneticresponse.Figure G-3 showsthe interpretation results of airborne magnetic, electromagnetic, and radiometric (potassium) data and the


510

Nabighianand Macnae

SURVEY BOUNDARY

SCALE

0.5

0.25

0

1•31•10

0.5

I

I.SKm

,ABRIDGED GEOTEM ©ANOMALY LEGEND SYMBOL

(AT PEAK POSITION)

*• -• • • -•-•

DECAY

INTERVAL

CLASSIFICATION

I -2 Channels 3-4 Channels 5-6 Channels 7-8 Channels 9-10 Channels 11-12Channels

Fig. G-1. GEOTEM anomalymap plan.



-

511

MAGN ETOMETER

-

•

59 900nT

- 59 800nT -

RADAR ALTIMETER

59 700nT

- 500' 450'

"--L'-'""• • 650m('approx)•

GEOTEM CHANNELS

•50'

l

20(•ppm

I

5

5

6

6-

Fig. G-2. Digitallyprocessedand computerdrawnprofilesof GEOTEM, magnetic,and terrain clearancedata on line 126S. (Airborne geophysicalrecord).

512



relationshipof the bedrock conductorsto thesepostu-

the airborneconductor.A drill hole probed the north-

lated features.

ern limb

Due to a lack of outcrop in the immediate vicinity, detailed surface geology was unknown but the airborne anomaly was believed to lie within a large diorite-granodioritepluton surroundedby mafic volcanics. Duration Mines Limited commissioneda ground follow-up programto investigatethe airbornetarget in the geologicalenvironmentfavorable for gold miner-

Because it wasn't entirely clear whether the airborne target had been found and properly tested, further surveying using the MaxMin II-EM system was employed. A more conductivezone was outlinedapproximately 200 m southeastof the first drill site. Anomalous inducedpolarization (IP) readingsand a magnetic high coincided with the conductor. The second of two holes into this newer ground target intersected 30 m of sulphide mineralization graded 0.5 percent combined nickel-cooper per ton. The core also assayedanomaloussilver, platinum, and gold values. A new and larger ground grid was establishedwhich permitted additional. MaxMin II-EM, IP, and mag-

alization.

Anticipating a conductor striking approximately northwest-southeast, a ground grid with northeastsouthwestcrosslines was laid out. The initial very low frequency EM survey defined a subparallelpair of conductors, striking approximately east to west, which seemedto representthe groundmanifestationof

and encountered

a wet chloritized

SCALE

INTERPRETATION

LEGEND

Magnetic/Lithological contact Magnetic discontinuity(possiblefault) Keweenawan

EM

bedrock

dike

conductor

EMzone ofsurflcial conductivity Zoneof highpotosslum. Fig. G-3. Interpreted map of airborne magnetic,electromagnetic,and radioactive(potassium)data.

shear.



513

netic surveysto be run on east-westlines, as opposed

the conductor, which explains why it is the best of the

to the earlier northeast-southwest orientation. This work confirmed the north-south strike of the conduc-

GEOTEM

tor. Figure G-4 identifies the pertinent information from the groundwork, as well as the peak locationsof the three airborne responses. As shown, line 126 passed directly over and along

leaves uncertain the source of the line 124 and line 125

...... •o

responses.As of June 1988whether the mineralization will prove economic is unknown.

ß ...... zt

anomalies.

Likewise, lack of sufficient ground investigation

ß ß

14)

I lOOM

LEGEND Old Grid

•

New Grid

GEOTEM anomaly

VLF-EM trends

'•:•'• •

MaxMin I[- EM/IPanomaly Drill hole

Fig. G-4. Summary of ground and airborne results.

514


APPENDIX

H


INPUT (R) APPLICATIONS AND CASE HISTORIES P. G. Lazenby*

INPUT, since being introduced in 1960, has been used in the following applications in addition to its primary role in base metal exploration. (1) Diamondexplorationslocationofkimbefiitepipes (2) Uranium explorationsassociation with graphitic zones (3) Salt-water encroachment studies

(4) Gold explorationsindirectly by associationwith sulphidesand/or graphite (5) Surficial conductivity mapping (6) Geological mapping (7) Location of aquifers (8) Coal exploration (9) Airborne bathymetry Two examples of the application of INPUT in the detection of sulphide conductorsare describedin the following case histories. The first example illustrates the capabilities of the helicopter version in rugged terrain while the second example resulted in the discovery of the major gold orebody. THE

KUTCHO

CREEK

conductance

DEPOSIT

of British Columbia

of 35 S is returned.

Values for the down-

dip record are 25 m and 22 S.

The undevelopedKutcho Creek depositis locatedin the Cassiar Mountains

null in between can be clearly identified, particularly on Line 2 North. An additionalpeak, X, 250 m north of deposit is attributed to another conductor. The skew (marked by the arrowhead) of the late channelpeaks in the direction of dip is well displayed on both the up-dip and the down-dip records. The amount of skew also provides some indication of the depth extent of the conductor. Having recognized that the anomaly is probably causedby a dipping plate-like conductor, interpretation can be carried out by calculatingthe dip, measuring channel amplitudes and using a nomogram to determine depth and conductance.The dip is determinedby measuringthe ratio (A1/A2) of the leadingto trailingpeak amplitudesand usingthe dip chart shown in Figure H-2. The interpretation for the Kutcho anomaliesyields 50 degreesfor the up-dip record and 125degrees(which is equivalentto 180-125 degrees= 55 degrees)for the down-dipresponse.The depth and conductanceare then estimated by using the closest standard nomogram, which in this case is for 45 degrees.For the up-dip record, a depth of 15 m and a

at 58ø12'

North latitude and 128ø20 ' West longitude, approximately 100 km east of Dease Lake.

Sampling and diamond drilling have indicated that the part of the depositsurveyedby the helicopterhas reservesof 11 million tonnesgrading1.68 percentCu, 2.14 percentZn, 25.5 gramsAg per tonne, with a strike lengthof more than 500 rn and unknowndepthextent. Dip is in general 45 degreesto the north. The helicopterINPUT anomaliesshown in Figure H-1 were recordedover the Kutcho depositusingthe vertical-axis receiver coil and a 2 ms pulse width. Survey lines were 300 rn apart and they were flown in

These resultsare then used as entry parametersfor the PLATE programand the modelingresultsobtained are shown in Figure H-3. A comparison of these resultsand the surveyrecord showsthat in generalthe two sets of data match fairly well. Channel 1 amplitudeson the surveyrecordsare slightlylarger than the model, possiblythe result of enhancementby surficial conductivity, or surroundingdisseminatedsulphides. The null position (Point B) which is well defined in thin sheetmodelingis not seenon the field records because of the finite averagingtime of the airbornesystemand finiteconductorwidth. The absenceof any anomalyon a line that was flown west of Line 2 implies that the responseon Line 2 may be reduced in amplitude as it

opposite directions at a terrain clearance of 122 m.

is near the end of the conductor.

Line 3 South showsthe up-dip anomaly while Line 2 North was flown down-dip.

probably accountsfor the larger estimateddepth of

The characteristic

features common to anomalies

causedby dippingtargetsare evidentin theseprofiles. The leading and trailing peaks, A1 and A2, with the *Previously Questor Surveys Limited.

(R)Reg. TradeMarkBarringer Research Ltd.

This

reduction

25 m on this line.

The decrease in conductance

on Line 2 North

is

probablya real effect of diminishingsulphidecontent at the extremity of the deposit.



515

Line 3 S

North

X

A1

A2

South

Vertical

Axis Receiver

Line 2 N

Coil South

North

A1

ß 9o o120

o150

Channel

6

Channel

1

A2

X

125 ppm rn 10

Up-dip

nT

Down-dip

Flight direction Fig. H-1. Kutcho Creek helicopterINPUT test records. (Data courtesyof Noranda Exploration Company.)


516


From modeling, we know that the amplitude of an anomaly similar to Kutcho decreasesin proportion to the 3.7 power of the aircraft height so even at a 300 m survey altitude, a clearly defined response with a signal-to-noiseratio of 3'1 on channel 2 should still be recorded.

The modelingparametersfor Line 3 togetherwith a geological section are shown in Figure H-4. THE DETOUR

LAKE GOLD DEPOSIT

The Detour Lake Joint Venture gold property is located in northeastern

Ontario at 50ø00'N latitude and

79040' W longitude, approximately 725 km north of Toronto, Ontario.

The property was staked in May 1974 following a fixed-wing INPUT survey by Questor Surveys under contract

to Amoco

Canada

Petroleum

Co. Ltd.

This

survey, which was part of an exploration program for base metals, covered an area along the northwest rim of the Abitibi greenstonebelt. The latter is an Archean complex of volcanic centers hostingmafic, ultramafic,

Vertical

Axis Receiver

Coil

100.0

correlations

of 400-800

nT.

These

anomalies

...................

A•

the older footwall ultrabasic talc-carbonate rocks, and

within quartz veins in the hanging wall basalts. The zone strikes at 70-80 degrees, dips 60-80 degrees north, and plunges 40-45 degrees to the west. Surface diamond drilling has indicated that the zone extends down to 550 m and is still open at depth. It averages425 m in strike length and 9 m in horizontal overburden

Figure H-5 shows two of the INPUT anomalies recorded over the main zone using the standardtowed bird receiver and a 1 ms pulse width. The survey lines were 200 rn apart and they were flown in opposite directions at a nominal survey terrain clearance of 122 m. Line 66 South shows the up-dip anomaly while Line 67 North shows the adjacent down-dip response. Characteristic first and second peaks are fairly well defined on the up-dip record (A & B), while the down-dip record showsonly a singlepeak (J). Making the assumptionthat the anomalies are caused by a dipping plate-like conductor, the dip angle and the channel amplitude correction factor for vertical halfplane normalizationcan be estimatedby the use of the chart shown in Figure H-6. The ratio second peak

A2

lO.O

A1/A2

are

caused by sulphidesin the main zone of mineralization. The first hole drilled in October 1974 through the central part of the conductorassayed4 gramsgold per tonne over a 9 m intersection containing 10-15 percent pyrrhotite with up to 1 percent chalcopyrite. Ground geophysicsand subsequentdrilling has outlined an orebody with estimated reserves of 27.4 million tonnes grading 3.88 grams gold per tonne. The main zone of gold mineralizationis centeredon a chertytuff horizon along the north limb of an east-west striking anticline. Significant values are also found in

width down to the 275 rn level. Glacial varies in thickness from 10-15 m. Flight direction

12.5

and felsic tuffs, flows, and intrusions, along with a variable sequenceof sediments. The INPUT survey delineated a conductive zone 1800 m long including a section 600 m in length containing 5 and 6 channel anomalies with apparent conductivity-thicknessvalues of 8-13 S and magnetic

Down--dip

Fhght d•re 0.09 0.1

amplitude/firstpeak amplitude(B/A) for Line 66 South yields a dip of 80 degrees (up-dip record) and an amplitude correction factor of 1.2 as shown by the broken lines on the nomogram. 0.01

.

i

Up-dip

ß

0.00

,

ß

,

ß

ß

i

0

30

i

60

90

50

120

150

125

Dip - Degrees

Fig. H-2. Dip interpretation chart. Helicopter INPUT.

180

After applyingthe correctionfactor to the individual channelamplitudes,the depth and conductanceof the target can be estimated by fitting these values to the vertical half-plane nomogram.Without this amplitude correction for dip, the depth would be overestimated. The interpretation which is illustrated in Figure H-7 for anomaly J, indicates a conductanceof 13 S and depths of 10 m and 20 rn for the up-dip and down-dip records. Plate modeling confirmed this interpretation, and a drill section is shown in Figure H-8.

TEM ProspectingMethods Vertical

Axis

Receiver

517

Coil

Line 3 S

North

Line 2 N

Computer Modeling

South

South

ARC


North

CBA

2 50 ppm

15 m 35

Depth

S

800 x 800

Conductance

m

50 ø 250

25 m 22

Plate Size

S

800 x 800 m

Dip

130 ø

m

350

Up-Dip

m

Down-Dip

Fig. H-3. PLATE programmodelingresult. Helicopter INPUT.

Line 3

South

Geological Section

N

orth

•

.........

'"'--'"• /

•

MASSIVE ORE

SEMI-MASSIVE ORE

----• BANDED ORE • STRINGER/NETWORK ORE (IN

DOLOMITIC

/

/

/ "/

40

LIMESTONE)

'• ':"';:"?'"'. PALE GREEN SCHIST

80

Modeling Parameters Plate Size 800 Conductance

x 800

rn

Depth

15 m

35 S

Depth 15 rn

Fig. H-4. Kutcho Creek geological section and modeling parameters.

m


518


Line 66

S

Horizontal North

Axis

Receiver

Line 67

Coil

South

-----.

N

South Channel

6

Channel

1

90 ß 120

North

500 ppm m

.150

5O

Up-Dip

nT

Down-Dip Flight direction

Fig. H-$. Detour Lake survey records. Fixed Wing INPUT. Data courtesy of Amoco Canada Petroleum Co. Ltd.



•

0

•<

000

•. •. •. **•

,

e,i ,

C• 0

, ,

•

o •

, i ,

e• 00 •t0

/ /

!

, ,

,: , I'

o

,:66 6

o o

to •-

o o

519

E E

o

I

/,o

/

/ /-

/

l

i i i

:

i

i i i

i i i

:

,

o 6 o

-

_

520

Nabighianand Macnae

Fixed Wing INPUT' Horizontal Line 66


Coil

S

North

Line

Computer Modelling

South A

Axis Receiver

67

N

South

North

B

J

600 ppm

10m

Depth Conductance

13S 600 500

m

I

x 300

m

80 ø

,

...................

i

i

i

i

i

20

Plate

Size

Dip

m

13S 600

x 300

m

100 ø

L

500

m

Up-Dip

Down-Dip

Fig. H-7. Plate programmodelingresults.

Line 67

South

North

Generalized Geology

•m"• 1• *• OVERBURDEN ._?.• ..... , BASALTS

,o

I MAIN CHERT ZONE _•;• TALC CARBONATES 30

ß:""'" '• FELSlC AGGLOMERATE '":'"•MAFIC TUFFS • INDICATED ORE

60

Modelling Parameters Plate

I

i

Size

600

Conductance

x 300

m

Depth 20 m

13 S

Depth 20 m

Fig. H-8. Detourlakegeneralized geologyandmodelingparameters.

rn


ELECTROMAGNETIC

APPLIED Part

METHODS

GEOPHYSICS--APPLICATIONS B

IN

SEG_02REV.indd 20

9/25/09 2:02:59 PM



CHAPTER

GEOLOGICAL

MAPPING

7

USING

VLF

RADIO

FIELDS

J. D. McNeill* and V. F. Labson

INTRODUCTION

In 1964 Ronka (Paterson and Ronka, 1971) introduced the first commercially available ground VLF instrument, and within a few years similar instruments were available from other manufacturers. By 1969 several airborne VLF systems were also being flown commercially. All of these instruments, ground or air, basically measured either the magnetic field tilt-angle or the vertical or horizontal magnetic field strengthsso as to detect the presence of localized electrically conductivetargets. A different approach was taken by Collett and Becker (1967). Their Radiohm is basically a magnetotelluric type of instrument which uses VLF transmitters (rather than atmosphericnoise fields) as a source of signal. This instrument provides the advantage of a coherent source and allows the phase angle between the horizontal electric and magnetic fields to be accurately measured and used for interpretation. Unlike the earlier instruments which worked solely with the magnetic field components, the Radiohm directly measures the wave impedance, from which the terrain resistivity can be derived, for use in general geologic mapping even in highly resistive terrains. At about the same time Barringer (1973) commenced work on the airborne Radiophase and E-Phase VLF systems. These systems used the vertical electrical field as a phasereference and E-Phase was also unique in that it measured the quadrature component of the horizontal electric field, from which once again the terrain resistivity can be derived. Finally in 1974, Tilsley (1973) suggestedthe use of a portable VLF transmitter as a supplement to the regular VLF trans-

The fact that electrical properties of the ground affect the behavior

of radio waves has been known for

years. Indeed measurements of the conductivity and dielectric constant of the earth using "wave-tilt" techniques were first performed in the 1930's [Feldman (1933), Smith-Rose (1933), Barfield (1934)]. These early measurementswere, however, made at relatively high frequencies which resulted in a shallow depth of penetration. It was not until 1963 that Paal (1965) observed that radio waves at VLF frequencies (technically the 3-30 kHz band, but in fact limited to 15-25 kHz by the available high powered transmitters) could be used to prospect for electrically conductive orebodies. By surveying over known shallow orebodies in Sweden with a calibrated field-intensity meter tuned to VLF stations, Paal showed that the horizontal

VLF magneticfield was greatly enhancedover subsurface conductorsat exactly the samelocation where the modulus of the vertical magnetic field component became a minimum. A large rotation in the compass bearing of the horizontal magneticfield coincidedwith the location of the maximum in this component. Such behavior was recognized by Paal as being consistent with the responsefrom a current, inducedby the radio field, running along the top edge of the target. Furthermore, additional measurements of the field strength made in mines, although disturbed by machinery, cables, ore, etc., suggested that the horizontal field strengthat a depth of 275 m was still about 25 percent of the value at the surface, from which he concluded that

orebodies

•hould

be detectable

under

Swedish

mitters, which could shut down without notice. A

conditions to depths of about 100 m, and that prospecting could be carried out from the surface or the

portable transmitter is also useful when the electromagnetic field components from the available VLF

air.

*Geonics Limited, 1745 Meyerside Drive, Unit 8, Mississauga,Ontario, Canada L5T 1C6.

*U.S. Geological Survey,MS 964,Box25046FederalCenter,Denver,CO 80225. 521


522

McNeill

and Labson

transmitters are in poor coupling with the target. A summary of the various VLF instrumentsis given in

take this effect into account will have little relevance in

Table

There is still inadequate information about the interpretation of VLF surveys. When dealing with onedimensional(i.e., layered earth) geometriesthe mathematics is relatively straightforward and interpretive aids are easily devised. However when dealing with

1.

Although VLF instrumentation has been widely used to map geology for the past twenty years (in terms of instruments it is undoubtedly the most popular "EM" tool) there has been a lack of interpretation material. After an initial attempt to use the results of free-space modeling most geophysicists appear to have simply relied on field experience to perform limited survey interpretation. This situation might have changed when Madden and Vozoff (1971) published their encyclopedic volumes of computed VLF data and curves; however, apart from beingpurchased by large mining companies and government agencies these useful volumes appear to have had relatively small circulation. More recently a number of articles have appeared in the literature dealing with the responseof two-dimensional(2-D) conductingdikes in a conductinghalf-space(Olsson, 1978, 1980, 1981, 1983; Kaikkonen, 1979;Saydam, 1981;Poddar, 1982).These papers clearly demonstrate that, when dealing with plane waves at VLF frequencies, the effect of the half-space on the total response cannot be ignored. Calculations

or model

measurements

which

do not

the real world.

the more common 2-D and 3-D targets the mathemat-

ics becomescomplex, and we must resort to numerical modeling, even though these programs are still not widely available. For thesetargetsour emphasisin this paperis on the physicsrather than the mathematics,in an effort to enable the practicing geophysicist/geologist to better understandhis survey results, even if he is still not able to completely quantify them. Indeed it is possible that, with the limited VLF frequencies available (less than two to one frequency ratio) and the essentialcomplexity of the interaction of VLF plane waves with three-dimensional targets in a conductive earth, a qualitative understandingof the subsurface features is the most that can be expected. In view of the simplicityof VLF instrumentationand the low cost and speedwith which surveyscan be carried out, such qualitative information will continue to be well worth while on many occasions.

Table 1. VLF receiversin approximate chronologicalorder. Quantity Instrument

Measured

A/G

Phase Reference

No. of Comments

Coils

2 freq.

Geonics EM 16 Geonics EM 18 Crone Radem Geonics EM 16R

G A G G

T,E Hz(l , •) T, IHyl Ex(I, Q)

Barringer Radiophase

A

Hx(I, Q)

Hy Ez

Barringer E-Phase

A

H•(I, Q) Ex(Q)

Ez

Herz Totem 2A Scintrex VLF 3 Scintrex VLF 4

A G G

Hz(Q), IHyl H•(L Q) (i,

Hy Hy /-/y

3 •

3 freq. simultaneously 2 freq. simultaneously 3 freq.

Phoenix VLF ABEM Wadi BRGM VLF2

2

G G G

Hy Hy By

2 2 3

Continuously tunable Continuously tunable 2 freq. simultaneously

Omni Plus

G

Hy

3

Hy ny

2 2 2

Multi-freq. Collett and Becker Multi-freq.

3

Both Scintrex units can be used as VLF base stations

Ex(I, Q)

T,E Hz(I, Q) T,E Ex(I, Q)

EDA

T,Hz(I, Q), IHI Ex(I, Q)

3 freq. simultaneously Can be used as VLF

Notes:

Hx, Hy--horizontalmagnetic fieldcomponents (Hy in directionof primarymagnetic field). Hzmvertical magneticfield component.

Ex, Ey--horizontal electricfieldcomponents (Ex along radial direction to transmitter).

Ez--vertical electricfield component. T--tilt angle, defined in Appendix 1.

E--ellipticity, definedin Appendix 1. Iminphase component. Q--quadrature phase component. II--amplitude. A--airborne.

G--ground. Not all VLF receivers are commercially available.

base station

Geological Mapping Using VLF Radio Fields VLF

TRANSMITTERS


Watt (1967) has given an interesting account of the early days of VLF. Surprisingly the first large transmitters operating at these frequencieswere built between

1910 and

1912

in order

to achieve

reliable

long-distance communication. The requirement for such long-distance links was often related to colonial activities. For example, the early Dutch station at Kootwijk was linked with Malabar, Java; the French stations at Paris and Lyons with Saigon (FUO at Bordeaux, France, was built in 1920for the French by the U.S. Navy). British Marconi constructed GNC in Wales at the end of World War I; GBR at Rugby underwent major modifications in 1927, 1938, and 1958. In the early 1930s vacuum tube transmitters were being installed at VLF stationsat Balboa (NBA, Canal Zone), Pearl Harbor (NPM, Hawaii), Annapolis (NSS, USA) and Cavite (NPO, Philippines). VLF radio station Goliath was constructed at Kalbe, Germany in the early 1940s. These transmitters, used for naval communications, were radiating well over 100 kW during World War II. The demise of some VLF

transmitters

is also inter-

esting. NPO in the Philippines was destroyed by the Japanese in December 1941; Goliath was apparently removed from Germany in toto by the Russiansat the conclusion of World War II to teemerge as UMS, Moscow, and NBA was blown up by the United States Marines in the late 1970s. Sic transit gloria! Communicationat VLF frequenciescontinuesto be a priority with most of the world's major navies, the reasons being that at these frequencies (1) global communications are possible due to the very low attenuationrates in the earth-ionospherecavity and (2) the effective penetration depth into sea water, while small, is still large enough to permit submarines to receive VLF signalswhile submerged.

Wheeler

Insulator

523

Since the wavelength at VLF frequencies is of the order of 15 km the monopole transmitting antennas (even with actual antenna heights of 200-300 m) are electrically very small, thus requiring the addition of

"top-loading", whichconsists of enormous (• 106 m:) horizontal wire arrays located at (and connected to) the top of the antenna to increase the input capacitance. The total capacitanceof the antenna plus top hat is then resonatedwith a (physically very large, low loss) series inductor. For such a structure, when operated well below the self resonance of the antenna itself, it can be shown (Watt, 1967) that the radiated power in watts is approximately

P -- 7 X 10-13V2C2her 2 4

(1)

where V is the voltage across the antenna, ½ is the antennacapacitanceincludingthe top hat, he is the effectiveheight of the antenna(approximatelyequal to the real height h), andf is the operatingfrequency. In

viewof thef4 dependence thedesirability for largeC at VLF frequencies is immediately obvious; even so the largest values of C obtainable with the enormous structuresdescribedabove are of the order of 0.2 Ixf, which, with values for he of the order of 150 m, demonstratesthat in order to achieve radiated power of the order of 1 Mw at VLF frequencies, antenna voltagesof many hundredsof kilovolts are required. In short, VLF antennas tend to be large, complicated, and expensive structures (the VLF station at Oahu, Hawaii, was built acrossthe top of an extinct volcano crater, the antenna at NLK near Seattle, Washington, shown in Figure 1, spans the tops of two mountain ranges). Table 2 lists the characteristics of the most important VLF transmitters as of 1988. It should be noted that operatingfrequenciesare subjectto change. In the early days of VLF the transmitters were 100 percent amplitude modulated with occasional rather

Blue Mt.

ML

8700'

Tronsmdter

Building

MountainTop Tower

' Downlead

Counter weicjht Insulator

Fig. 1. Pictorial view of Jim Creek antenna (after Watt, 1967).


524

McNeill

and Labson

long off-times and/or shutdowns which were a problem, particularly in airborne surveying. More recently virtually all of the transmittershave gone to someform of frequency shift keying (FSK), a type of frequency modulation in which the transmitter continuously radiates a signal, greatly improving the performance of geophysicalinstruments. Shutdownsare also less frequent, and maintenance periods well publicized. The question often arises as to the future of VLF transmitters. Their principle use these days, as mentioned earlier, lies in communication with submerged submarines (using a long trailed electric field antenna which picks up the horizontal electric field component in the sea-water). Although new ELF communication systems are being evaluated (at frequencies of the order of 35-70 Hz) the data rates are so low at ELF (low even compared with VLF) that it seems likely VLF

transmitters

will

SIGNAL

distances

Ez

AND

NOISE

from

the transmitter.

Our interest

•1oloh 2'7I'

1 /)

/,2 •/,3

still be used for some time to

come. This likelihood is reinforced by the fact that the U.S. Navy, the owner of many of the stations, recently introduced the new VLF station in Puerto Rico. Should, however, ELF becomea practical exploration tool the concepts used at VLF frequencies will still apply. VLF

various

will lie in calculatingthe field strengthas a function of distancefrom the transmitter, and in learning how this field strength can be expected to vary with time. Assume initially that a transmitter antenna, a short vertical monopoleof height h (as describedearlier, the effective height is almost equal to the real height) is located on a flat, perfectly conducting earth. We can show that the electromagneticfield componentsat a radial distance r on the surface of the conductor (0 = ,r/2 in Figure 2) are given by (Watt, 1967)

X e-i13reitøt(volts per meter)

(2)

and

+ •-5 e /ø•t(ampere permeter) (3) Ioh(i• 1)_i[3re

H, = •

whereIoeicøt isthecurrent flowing inatthebaseofthe antenna. We assume that the time dependenceof all

LEVELS

time-varying quantities is represented by eitøt,where

Factors Affecting Radio Wave Propagation at VLF Frequencies

to, the angular frequency, is given by to = 2,rf. To find

The propagation of radio waves at VLF frequencies has been studied for many years, principally by Wait (1970). An excellent review is given by Watt (1967),

tities it will be suppressedexcept where we are explicitly interested in the time behavior.

from which the material

of this section is drawn. Radio

wave propagation is a complicated subject since different mechanisms govern the propagation loss at

theactualtimebehaviorwe takeRe (eitøt)= cos(tot). Sincethefactoreicøt appears in all timevaryingquanThe quantity •qois the intrinsic impedanceof freespace, and is given by

•0 = (Ix0/e0)•/2= 120,r ohms

Table 2. Major VLF transmitters.

Station

Freq. (kHz)

Location

NAA NLK NSS NAU NPM

24.0 24.8 21.4 28.5 23.4

Cutler, Maine Seattle, Washington Annapolis, Maryland

BGR UMS JXZ FUO

16.0 17.1 16.4 15.1

Rugby, England

NWC

22.3

North West Cape

Coordinates

Approximate Rad. power (kW)

U.S.A.

Puerto

Rico

Lualualei, Hawaii

67W17-44N39 121W55-48N12 76W27-38N57 67W 11-18N23 158W09-21N25

lOOO 125 400 lOO 600

01W 11-52N22 37E01-55N49 13E01-66N25 00W48-44W65

750 1ooo 350 500

114E09-21S47

lOOO

137E01-34N58

50

EUROPE

Moscow, Russia Helgeland, Norway Bordeaux, France AUSTRALIA JAPAN

NDT

17.4

Yosami

GeologicalMapping Using VLF Radio Fields

525

where

-5-n0

Ix0= 4x x 10-7 henrypermeter

which is independentof the radiusof the hemisphere since the surfacearea of the hemisphereincreasesas


and

r2whereas thevectorproduct E x H decreases asr -2

e0 = 8.85 x 10-12faradpermeter. The quantity [3 is the propagationphase constant, given by

•3= 2xX-•

(4)

where/t is the free-spacewavelengthin meters.

r-

(5)

inthefarfieldregion. Thus ther-1 terminEzandH4, is exactly that requiredto accountfor the "spreading loss" as the energyradiatesout into space. Since the groundis assumedto be perfectly conductive, there is no additionalpower loss to it. Equation(5) allowsus to rewrite equations(2) and (3) in a more useful form since usually the radiated power from a VLF transmitter is known rather than Ioh. Substitutingfor Ioh, assumingfar-fieldconditions, and evaluatingthe constants,gives

I3: (3)the terms proportional to requations -2, andr-(21)and arecalled the electrostatic, induc-

tion, and radiation terms, respectively. At a radial distancer which is much greaterthan one free space wavelengththe radiationterm dominatesfor both Ez

andH4,,andbothcomponents decrease asr -1. Thus

Ez = 9.5

in the far-field zone (r >> It) the field components radiatedfrom a monopolesituatedon a perfect conductor, measured at the surface, consist of a vertical electricfield and a horizontalmagneticfield which are

in phasewith eachother;theirratioEz/H4,= rio is independentof the radial distancer. These components are shown in Figure 2. Now it can be shown that when the outward

normal

componentof the vector productE x H (called the Poyntingvector) is integratedover a closed surface, the resultis the rate of energyflow acrossthat surface (Stratton, 1941).If this integrationis performedover the surfaceof a large (r >>It) hemispherecenteredon the monopoletransmitter,usingelectricand magnetic field componentssimilar to thosegiven by equations (2) and (3), but modified to take into account the correctbehaviorof thesecomponentswith 0, we learn that the total power flow acrossthe hemispherical surfaceis given by

(6)

r

H4, = Ez/ri o

(7)

whereP istheradiated powerinwattsandEz andH4, are the peak values of electric and magneticfields in volts per meter and amperesper meter, respectively. More normally used engineeringunits are E in millivolts per meter, P in kilowatts, and r in kilometers whence

qP(kw)

Ez(mV/m) = 300r(km) '

(8)

Finally in much of the literature on wave propagation the vertical electric field strength is often given in decibelsrelative to 1 mV/m, whereupon

Ez[db, 1mV/m] = 20log mV/m = 20 log Ez (mV/m)

= 20 log 300 + 10 log P[kw] - 20 log r[km] = 49.54 + 10 log P[kw]

- 20 log r[km].

(9)

2

_Y

Ez

r

H• I

In the real world several factors alter this simple relationship.The principlefactors are that the earth is sphericalrather than fiat, and is boundedby an electrically conductingionospherefrom which electromagnetic waves are reflected. Dealing first with the spherical earth (but without a conductive ionosphere) calculations

Fig. 2. Currentmonopolefield geometry(after Watt, 1967).

of the additional

losses taken

from Watt

(1967)are shownin Figure 3; the quantityplotted is 20


526

McNeill and Labson

loglWnl which is the loss term in decibelsto be added to equation (9) to account for the further signalloss due to the sphericalearth. At VLF frequenciesthis function is essentiallyindependentof terrain conductivity, but, especiallyat largedistances,variessignificantly with frequency. The effect of finite earthcurvatureis seento be very severe,adding50 db loss at a distanceof 5000 km (at a frequencyof 20 kHz). Such low-amplitude signalswould be undetectable. What permits VLF signals to be detected at distances of 10 Mm (=10 000 km) and beyond? The answer of courseis that the presenceof free electrons in the ionospherecausesit to act asa reflectivelayer at VLF frequencies.The dielectricconstantof this layer is essentiallythat of free space,whereasthe electrical conductivity is roughly proportional to the electron density, and inversely proportional to the collision frequencyof electronswith ions. Sincetheseparameters vary with height the ionosphereboundary does not exhibit an abrupt transition; indeed the manner in which the conductivity varies with height plays a major role in the propagation characteristicsof the earth/ionosphere cavity. Sincethis profilevariesdiurnally, seasonally,andwith sunspotactivity,we expect the VLF

transmission losses and thus the measured

wave componentreflected from the ionosphere;this componentmust be added to the groundwave previously described, to obtain the total electric field at the receiver. The reflected wave can interfere destruc-

tively with the ground wave as the distancefrom the transmitter is increased, causing a reduction in the signal strength. At larger distances from the transmitter the mode theorymustbe usedto correctlydescribethe behavior

of wavespropagating in the earth/ionosphere cavity;at VLF frequenciesthe TM (transversemagneticfield) modes dominate. Although these modes support a horizontalelectricfield componentin the directionof propagationat the top of the earth/ionospherewaveguide, this componentbecomeszero at a perfectly conductingearth surface whereupon the fields are again essentiallyTEM (transverseelectric/magnetic) with electricand magneticfield componentsshownin Figure 2.

The interference phenomenonpreviously referenced still occurs but is now attributed to interference between the different modes. Such interference is

evident in Figure 4 which shows calculated field

strengthusingthe complicatedfull modetheory. Fortunatelywe are usuallyinterestedin determining the electric field at large distances from the VLF

signal to vary. Furthermore the collidingelectrons interact with the earth's magneticfield to produce increasedionosphericreflectionlossfor propagation from east to west compared with that from west to east,with intermediatelossesfor northto southpaths.

longerdistancesthe oscillationsin field strengthtend to dampenout. The reasonfor this dampeningis that

Now at relatively short distancesfrom the transmitter, optical ray theory can be used to calculate the

at VLF frequenciesunder daytime conditionsand at great distancesfrom the transmitter, only the lowest

transmitter to assess the maximum

distance for a

successfulVLF survey. Figure 4 illustrates that at

order mode need be considered since the attenuation

rate for the secondand higherorder modesis substantially greaterthan for the lowest order mode; interference effectsbetweenthe modesbecomeless important. In this case the correct long distancebehavior is

shownby Watt (1967)to be approximatelygivenby

Ez[db,mV/m]= 44.3+ 10 logP[kw] - 10logf[kHz]

-2O

- 20 log hi[km] q- 20 log A

-10 log [a[Mm] sin (•)-at.

-40

(10)

-5O

For decibelsrelative to 1 i•V/m add 60 db (-20 log 1000) to equation (10). The correspondinghorizontal magneticfield strengthis given by

-6O

-70 2O0

500

I000

2000

5000

D•sfonce, d, Km

Fig. 3. Amplitude of the groundwave relative to the inverse

distance field,IWnl.Curvesvalidfor 10-3 < •r <4 S/m,and 2 < er <80.

H•[db, mA/m]-- E[db, mV/m]- $1.•.

(11)

In theseexpressionshi is the heightof the assumed reflectingboundaryat the ionosphere,A is a small factorwhichaccountsfor the efficiencywith whichthe lowestordermodeis launchedby the transmitter,a is

527

GeologicalMappingUsingVLF Radio Fields 8O

I

i

i

I

i

i i I I j

I

I

I

i

i

!

I

16.6 kc

....


7O

CalculatedfromPierce•s

DISTANCE (eqn 9)

Empirical Formula

-

Made Theory

(Daytime)

60

$0

40

50

3db/IWn N 20

•o

I

!

I ! I I !•J

I•

Z

3

4

$

6 1 8

,

. • ....... I !

1,000

2

DISTANCE

,T

!_1 5

4

6

I

I

!

8

I0,1)00

(k m)

Fig.4. A sketch of calculated fieldstrength basedonmodetheoryandanempirical formula of J.A.Pierce(after Wait, 1970).Alsoshownarecalculated fieldstrengths basedon thelowestordermode,asdescribed in thetext. the earth's radius (6.4 Mm), r is the distance from transmitterto the receiverin megameters,a sin(r/a) is

conductivities appreciably lessthan10-3 S/m.We

the spreading termin the earth/ionosphere cavity,and

uation rate is of the order of 3 db/Mm, rising sharply as the surface becomes markedly resistive. In general,

a is an attenuationfactor (given in decibelsper mega meter)which accountsfor power lossinto the finitely

conductinggroundand ionosphere.We will generally be interestedin day time propagation,when hi , the electricalheightof the ionospherecan be taken as 70

note that for the more conductive materials the atten-

the night-timeattenuationrate is lessthanthe daytime rate, with the result that at large distancesfrom the

km. Examination of the various factors affecting A show that its effect is small, and a reasonablevalue for

this term is 20 log A = -0.5 db. Figure 5 (Watt, 1967) showsa plot of 10 log [a sin (r/a)]. Finally, the last

•o

term contains the attenuation a, which is due to power

.

functionof many factorsincludingfrequency,direction of propagation,surfaceconductivityand roughness,time of day, seasonof the year, latitude, and

'-• -

solaractivity.Thesevariables havebeenanalyzed in

_o4

loss totheearth and ionosphere. This attenuation isa

detail by Wait and summarizedin Watt (1967). The significantfeatures are illustratedin Figure 6 (Watt, 1967)which showsthe total (groundplus ionosphere) daytimeattenuationrate for the first order modeas a functionof frequency,groundconductivity,anddirection. The conductivityof sea water is 4 S/m, conductivity of soils and many sedimentaryrocks is of the

orderof 10-2 S/m,andlow-porosity rockscanhave

o

2

4

6

8

I0

15

20

30

40

Distancet r t Mm

Fig.5. Lossin fieldstrength dueto spreading of energyin the earth-ionosphere waveguide(after Watt, 1967).


528

McNeill

and Labson

Frequency,

f, kc/s

Fig. 6. Directional effect on the total daytime attenuationrate for the first order mode (after Watt, 1967).

transmitter the field strength is generally greater at night than during the day, although this effect can be partly or wholly reversed by contemporaneous changes in modal interference. To demonstrate the effect of attenuation Figure 7 illustrates equation (10) evaluated as a function of r for P - 1 Mw, f = 20 kHz, h = 70 km, and various values of a. Since these curves are for the lowest-order

mode

and neglect intermodal interference, they will be inaccurate at shorter distances(lessthan 2 Mm). In general as a increases

from

zero

the

vertical

electric

field

decreasesmore rapidly with distance. Figure 8 (Wait, 1970) illustrates measuredcurves of field fall-off with distance; superimposedon this data are curves calculated using Equation (10) with a = 2 and 3 db/Mm. The fit is seen to be reasonably good. Indeed, on all measured data which we had available

(usually over sea water paths) the 3 db/Mm attenuation gave good results. Although 3 db/Mm is somewhat larger than predicted from Figure 6 it seemsadvisable to use this figure in order to provide a conservative estimate of field strength. Very complete electric field strength contour maps have been calculated in Hauser and Rhoads (1974) for those transmitters operated by the U.S. Navy, using an elaborate earth/ionospherecavity model. Unfortunately the calculations were done using earlier frequencies for many of the VLF transmitters but, as we have seen, this factor does not produce a large error. Results of their calculationsare shown in Figures 9a-f. Figures 9g-k show similar estimates of field strength by the authors for the remainder of the useful VLF transmitters. These calculationswere carried out using

the principlesoutlined in this chapter, combinedwith examination of Hauser and Rhoads' (1974) results (which have been shown to be in good agreementwith actual field strength measurements). In looking over the data of Figure 9, we see that most parts of the world have reasonable coverage (sometimes,however, with only one stationand therefore only one propagation direction). There are two major exceptions, central and southern South America and South Africa. Although VLF surveys have been carried out in South Africa usingNAA and GBR they (Text continued on page 535) 35

i

30

25

20

15

10

•o "•-5 -10 2

-15

-2O

-25

-3O

-350-1

I

DISTANCE

(Mm)

0

100

Fig. 7. Calculated signal strength as a function of radial distanceand attenuationfactor. Radiated power = 1 Mw, frequency = 20 kHz. For radiated power P(kw), db =

dbgraph -30 + 10logP(kw);for frequency = 16kHz, db = dbgraph + ldb,forfrequency = 24kHz, db= dbgraph - ldb.


529

$0 ' I I I I [ III I ' [ I I I I i I [ ] 70 • INVERSE DISTANCE (eqn 9) $onDie,.go.-e-Howoii; Dec. 19,54 16.6 kc


....

•

I -'•

• '•

60-

_

,50.......

•.••

Howoil----Woke-.--Tokio: Sept. 1954

--- -----Tok,o Woke: Aucj. 1955

,.•

(Doytime)

,,•

,._o

.•, C(. =2db/Mm

• 40 •

ß

:....:... -

•0-

o

.c:)

24)-

!

I 678

(a)

I,!300 DISTANCE

Z

J

I

I

I

1 I

4

5

I

7

6

I

G000

(k m)

8O

16.6 kc •

Woke--Howoii' 6 Aug.1955

TO

Howoil.---(I/2)SonDiego:IOAug.1955 Howoil-.-SonD,ego'II Aug.1955

.....

(Doytime) ...

-'

50

• •

4O

--

30

""

20

100 (b)

.cl= 2db/Mm

I

I

I

I I I, I I I

I

!

•

• • • • I

2

,T

4

S

2

3

4

5

6 7 8

1,000

DISTANCI=

6 '/ 6

10,000

(k m)

Fig. 8. A sketchof field strengthversusdistancedata obtainedby Bickel et al. (1957)in an aircraftfor differentpaths (a and b) (after Wait, 1970).Also shownare calculatedfield strengthsbasedon the lowest order mode, as described in the text.

530

McNeill


60*E

90*E

120'E

and Labson

150'E

180'

150'W

120'W

90*W

I

I

I

I

I

'

60*W

30*W

0*

30*E

I

I

I

I

60*E 90*N

60*N

30*N

0*

30'S

60'S

90'S

9 (a) 60*E

90*E

120'E

150'E

180'

150'W

120'W

90*W

60*W

30*W

0*

30*E

60*E 90*N

60*N

30*N

0*

30'S

60'S

I

I

I

I

I

I

I

90'S

9 (b)

Fig. 9. Signallevelcontours in db > llxV/m(a-f afterHauserandRhoads,1974).AreaswithEz > 0.5 mV/m(good signalstrengthlie withinthe 54 db contour,thosewith Ez > 0.25 mV/m (marginalsignalstrength)lie withinthe 48 db contour. (a) NAA, Cutler; (b) NWC, Northwest Cape.



60'E

531

90"E

120"E

150"E

180"

150"W

120"W

90"W

60"W

30"W

O"

30"E

I

I

I

I

I

I

I

I

I

I

I

60'E 90'N

60'N

30"N

Gø

30'S

60'S

60*E

i

I

I

90*E

120'E

150'E

180'

150'W

I

I

I

I

I

90'S

120'W

90*W

60*W

30*W

O*

30'E

i

I

I

!

!

60'E 90'N

,,,,,,,,,,,,,, ............

I

60'N

30"N

-

30øS

60'S

90'S

9 (d) Fig. 9, cont. (c) NLK, Jim Creek; (d) NDT, Yosami.

532

McNeili


60*E

90*E

120'E

150'E

180'

150'W

and Labson

120'W

90*W

60*W

30*W

0*

30*E

60*E 90*N

60*N

30*N

0*

30'S

60øS

90•S

9 (e) 60øE

90•E

120øE

150øE

180'

150øW

120øW

90•W

60•W

30•W

0ø

30øE

60•E 90øN

60•N

30øN

0o

300S

60•S

i

I

i

I

I

i

i

9 (f)

Fig. 9, cont. (e) NSS, Annapolis;(f) NPM, Laulualei.

90'S

533

GeologicalMapping UsingVLF Radio Fields

60OE

90*E

120'E

150'E

180'

150'W

120'W

90*W

60*W'

30*W

O*

30*E

60*E


90*N

60*N

30øN

O*

30'S

60'S

,

I

I

I

I

90*E

120'E

150'E

180'

I

I

I

I

........ 1,,,

120'W

90*W

60*W

30*W

I

90'S

9 (g) 60*E

150'W

I

O*

30*E

I

60*E 90*N

60*N

30*N

O*

30'S

60'S

I

I

I

,I

I

I

I

9 (h)

Fig. 9, cont.(g) GBR, Rugby;(h) NAU, PuertoRico.

I

90'S

534

McNeill and Labson


60=E

90=E

120•E

150•E

180 •

150•W

120•W

90=W

60=W

I

I

I

I

I

I

I

30=W

0•

30=E

60eE 90eN

60eN

30eN

0o

30øS

60øS

90eS

9 (i) 60OE

90øE

120•E

150•E

180•

150•W

120•W

90øW

60oW

30oW

0•

30OE

60øE 90øN

60øN

30øN

0o

30øS

60øS

90øS

9 (j)

Fig. 9, cont. (i) JXZ, Norway; (j) UMS, Moscow;



require patience and the willingnessto changestations when the signal strengthfalls to unacceptablelevels. Finally, it must be rememberedthat signaltransmission lossesvary with time for the reasonsgiven earlier and the data of Figure 9 must always be used with

earth's surfacefrom east to west following the passage of local afternoon or early evening. Since the interfering electromagnetic radiation is generated by vertical flow of current during lightning discharge, the interference radiation fields are essentially thosewhich were previouslydiscussed.Thus the field components are a vertical electric field and a horizontal magnetic field, and the same factors affect the transmissionlosses through the earth/ionosphere cavity as for those signalsarising from VLF transmit-

caution.

Atmospheric Noise at VLF Frequencies

Of course, the noise generated in the VLF receiver limits the range at which VLF signalscan be received. In a well designedreceiver the primary sourceof noise is external to the receiver and is generatedby electromagnetic fields radiated by atmosphericlightning discharges (both local and distant, since, as we have seen, the attenuation rate at VLF frequencies can be extremely low). Thus, since thunderstorm activity varies

both with

location

on the earth's

surface

ters.

Radiation from nearby storms tends to be very spiky, reflecting the influence of individual lightning discharges;radiation from distant storms includes the contributions from many storms and tends to have a

Gaussian amplitude distribution whenmeasured•with a narrow band receiver. However, radiation still retains

and

a strong spiky component when measured over a relatively broad bandwidth (100-1000 Hz).

with time, so do the received noise levels. Generally speakingthe world's thunderstormactivity is concentrated in three regions:

All

of these

factors

were

taken

into

(3) South-easternAsian archipelago.

1 kHz receiver

Superimposedon this pattern is a diurnal variation in which the thunderstorm activity moves across the

bandwidth.

Most VLF

receivers

90*E

120'E

150'E

180'

150'W

120'W

90*W

60*W

30*W

I

I

I

I

I

I

!

I

I

O*

30*E

60*E

90*N

30*N

30'S

60'S

I

I

I

I

in

have a


60*N

,

account

Hauser and Rhoads (1974) when generatingworldwide VLF noise maps, six of which are illustrated in Figure 10. The contours on these maps are of vertical electric field strength in decibels relative to 1 ixV/m within a

(1) Central America and central South America; (2) Central Africa; and

60*E

535

,

I

I

9 (k) Fig. 9, cont. (k) FUO, Bordeaux.

I

90'S

536

McNeill and Labson


60*E

90*E

120'E

150'E

180'

150'W

120'W

90*W

60*W

30*W

O*

I

I

I

I

I

I

I

I

30*E

60*E 90*N

60*N

30*N

O*

30'S

60'S

90'S

12

14

16

18

20

22

24 LOCAL

2

4

6

8

10

O*

30*E

12

TIME

10 (a) 60*E

90*E

120'E

150'E

180'

150'W

120'W

90*W

60*W

30*W

60*E 90*N

60*N

30*N

..........

ß=====================================

30'S

60'S

I 20

22

24

2

90'S 4

6

8 LOCAL

10

12

14

16

18

20

TIME

10 (b)

Fig.10.Atmospheric noise contours (db> 1 •V/m, 1kHzbandwidth) (afterHauser andRhoads, 1974). (a)July,

8:00 UT; (b) July, 16:00 UT.



537

60*E

90*E

120'E

150'E

180'

150'w

120'w

90*w

60*w

30*w

o*

30*E

I

I

I

I

I

I

I

I

I

I

I

I

60*E

I 90*N

60*N

30*N

0e ........

...................

.........

.................. ...................

........

:::::::::::::::::::::::::: ......................

!-

• 24

2

4

6

8

10

60'S

I

I

I

I

12

14

16

18

LOCAL

90'S 20

22

24

60*E

TIME

10 (c) 60*E

90*E

120'E

I

'

I

150'E

I

180'

I

150'W

120'W

90*W

60*W

30*W

0*

30*E

I

I

I

I

I

I

I

'190*N

60*N

30*N

0*

30'S

12

14

16

18

20

•

22

I

24

LOCAL

I

2

I

4

•_••.•• 60'S I I I 90'S

6

TIME

10 (d)

Fig. 10, cont. (c) July, 20:00 UT; (d) January,8:00 UT.

8

10

12

538

McNeill 60øE

90øE

120øE

I

I

and Labson

150øE

180 ø

150•W

120•W

I

I

I

I


,,

90•W

60'W

30eW

I

I

I

0o

30øE

60øE

,,,

I

I 90øN

60øN

30øN

0o

...............

.:.:.:.:.:.:'i:i:i:!:!:!:i'! .........

30øS

60øS

90øS

20

22

24

2

4

6

8

10

12

14

16

18

20

60*E

LOCAL

TIME

150•W

120•W

90*W

60*W

30*W

0•

30*E

I

I

I

I

I

I

I

10 (e) 60*E

90*E

120•E

150•E

180 •

90*N

60*N

30øN

0o

30øS

• 24

I

I

,I

2

4

6

8

10

60os

I

I

I

I

12

14

16

18

LOCAL

00os 20

TIME

10 (f) Fig. 10, cont. (e) January 16:00 UT; (f) January, 20:00 UT.

22

24

GeologicalMapping Using VLF Radio Fields bandwidth of about 1 Hz and since the noise voltage varies as the square root of receiver bandwidth, 20 log


(1000) 1/2= 30 db shouldbe subtracted from each contour value. Furthermore we wish to compare the noise values of vertical electric field with our previous field strengthcalculationsin millivolts per meter; since these contours are in ixV/m, a further 20 log 1000 = 60 db should be subtracted

from each contour.

The spatial and temporal variability of the VLF noise fields referred to is clearly evident on the figures, which were selected from a larger number of plots in order to show for (1) Australia and western Asia, (2) North and South America, and (3) Europe and South Africa the best (in the winter) and the worst (in mid-summer) noise behavior to be expected at the worst time of the daysi.e., late afternoon. Examination of all of the data shows that in general the noise decreases slowly during the night to a minimum which usually takes place at around 0800 local time, then rises fairly rapidly to a maximum at about 1600 local time before again decreasing. Thus, as is well known to geophysiciststhe world over, the best time to carry out electromagnetic surveys is usually from the early hours of the morning until noon. PLANE

WAVE

HORIZONTALLY

RESPONSE LAYERED

FROM

A

EARTH

HomogeneousHalf-Space•Impedances, Wave Tilt As mentioned earlier, the electromagnetic fields from a vertical electric dipole, situated on a flat, infinitely conductive earth, and measured near the surface, consisted of a vertical electric field compo-

nent Ez and a horizontalmagneticfield component

Hq,. Whenat a greatdistancefrom the transmitter (many free-space wavelengths) the curvature of the

j E'E'i •

a

ta

Incident wa•ve '•ted

539

earth, and of the field components, on a local scale is completely negligible, and the local electromagnetic field components can be viewed as being those of a plane wave impingingat grazing angle of impedanceon the earth's surface. We now examine the field components from such a plane wave at the surface of the earth which is assumedto have finite conductivity that is constant with depth (i.e., a homogeneous half space). Consider Figure 11 in which a plane wave is directed toward the earth's surface at an arbitrary angle of incidence 0i; the electric field lies in the plane of incidence. The air has zero conductivity and the permittivity of free spacee0. The groundhas conductivity rr• and permittivity e• = ere0, where er is the relative

dielectric

constant

which

varies from 3 to 10

for most terrain materials. When the primary electromagnetic field impinges on the surface it is both reflected back into the air and refracted

into the earth.

We are interested in the resulting field componentsin the air, which is where we make most of our measure-

ments, and in the ground, where the field components interact

with

subsurface

structures.

We first consider

the behavior of the fields above and inside a homogeneous half-space; later we extend the theory to a simple two-layered earth. Although extension to a multilayered earth is straightforward, the VLF band of frequencies is so narrow that not more than a twolayered earth can be resolved and this model is quite adequatefor our purposes. Our treatment follows that of Wait (1962).

Intrinsic Impedance, Wave Impedance.--Maxwell's equations govern the behavior of the electric and magnetic fields in the air and ground. They are

••.r m =0O 0--0

\H-r

PO•-PO

wave •0=•0 x

m=l

0t

01 =01 pl=PO

E1= E1

Fig. 11. Field componentsnear the surface of the earth.

540

McNeill

and Labson

2 =)k2 - km. 2 um

(26)

k0 = (to2IXoCo)1/2= to(ix0 Co)1/2

(27)

k1 = (to2lx0e 1 -- itolx0O'l) 1/2.

(28)

0B

V x E =

Ot

Faraday' s Law,

(12) From equation 24

OD


V x H = J+

Ampere's Law,

Ot

(13) and

V.D=p, (14) =0

in a homogeneoushalf-spaceor a horizontally layered

Now we can represent the magnetic field of the incident or primary wave as

earth, and

HPri O,)(e-ikox sin0•), Oy-- HOy(e -ikozcos

v.

(29)

from which, by comparison with equation (25)we recognize that

where

B=lxH,

D=eE,

J=trE.

Sincethetimevariationis givenby eitøt,OH/Ot= itoh etc. and the first two equationsbecome V x E-

-itolxH,

a0 = Hoy, the amplitudeof the primarymagnetic

(16)

field

b0 = amplitudeof the magneticfield of the reflected wave

and

(17)

X = k0 sin 0 i .

(30)

and

V x H = (tr + itoe)E.

(18)

Taking the curl of equation (18) and usingthe vector relationship

I7 x I7 x H = I7(I7 ßH) - V2H,

Now the boundary conditions at the earth/air interface are that the tangential componentsof the electric and magnetic fields must be continuous acrossthe surface. From equation (18)

(19)

E =

o' + itoe

V x H

(31)

we obtain or

V(V ßH) - •72H= (ff + itoe)V x E = cr + itoe(-itolxH),

(20)

Emx =

-1

OHmy

O'm q- ito• m

OZ

,

(32)

but

V ß I-I = 0,

and the boundary condition for the electric field be(21)

comes

and therefore the total magneticfield satisfies

V2H = (or+ ito•)itolxH,

1 OHoy (22)

or

itoeo Oz

1

OHly

o'1 + itoel

Oz

(33)

and for the magnetic field

V2H + k2H = 0,

(23)

Hoy = Hly,

(34)

where

where

k = (to2lxe- itolxtr)1/2.

Now as shown in Figure 11 the primary or incident (and therefore also the secondary)magneticfield has a y component only. The general solution for equation (23) is [as can be demonstratedby substitutionback in equation (23)]

Hmy= (ame-umz + bmeUmZ)e -ixx,

as before

(24)

(25)

where m = 0 in air, m = 1 in the half-space, X is a constant,and um is related to h and km by

Hoy= (aoe-"øz+ boe"øZ)e -ixx,

(35)

Hly = a1e-"•Ze-ihx

(36)

but now

sincethere can only be a downgoingwave in the earth, assumedto be of infinite depth extent. Applying the boundary conditions at z = 0 leads to a0 + b0 = a l,

(37)

GeologicalMapping Using VLF Radio Fields and which

and

541

can be measured

with

two

small electric

dipole antennas, one vertical, measuringthe reference Uo


ito•;o

-IXlal

(-ao + bo)=

, tr 1 + itoh;1

(38)

from which it can be shownthat bo/ao, the ratio of the reflected to the incident magnetic field components (i.e., the amplitude reflection coefficient) is given by

quantityEoz, and the other horizontal,measuringthe variable Eox, as was done with the Barringer E-phase system.

From equation (32) Eox =.

bo

Zo -- Z1

• = , ao Zo + Z1

uo t•oe0

=

(x2

OHoy

tO•o

0z

•,

tl o

i•oeo

2)

ik0(1 - sin2 0i)1/2

i•oe0

itoEo

= Xlo(1 - sin20i) 1/2,

(40)

(-ao e-uøz+ boeUøZ)e -ixx

=aoZo ( b•_oø ) -iXx 1 --

k0 toœo

= ([•0/œ 0) 1/2

377 fl,

(41)

and Z• is given by Z1=

(•k2__k•)1/2

(48)

dipole E0 = x10H0,

(49)

Eoz = HoyXlosin 0i .

(50)

and therefore

O'1 q- itoe 1

O'1 q- itoe 1

ikl

Hoy =ao( 1+

O'1 q- itoe 1

ko 2 )1/2

x 1- k-•sin2 0i

(42)

Xll1-k-•sin2 0i

(43)

ikl (o. ilxoto )1/2

0'1 + itoe1

=

' 1 + itoe1

.

(44)

Both •q0and •q• have the dimensionsof impedance; furthermore, •q• is related to the electrical properties of the earth (•r• and e•). It is an important parameter, often called the intrinsic impedance, which will occur frequently in the material that follows. Finally, the quantitiesZ0 and Z• also have dimensionsof impedance but are also functionsof 0i; they are called wave impedances. Field Wave Tilt.--Now

there are two other

derived quantities that are of great interest because they yield useful information about the properties of the earth and are also relatively easy to measure. The first is the electric field wave-tilt, which is defined as W •

Eox

(45) z=0

e

atz

= O.

(51)

Substituting equations (48), (49), (50), and (51), into equation (45) we obtain 1 ---

sin2 0

W= TI1 k•2 i x10 sin 0i

with

Electric

at z = 0.

From equation (35)

[•1

nl =

e

(47)

From our earlier discussion of the fields from a vertical

with

T[0 =

(46)

(39)

where Z0 is given by Zo=o

-1

,

(52)

where the functional dependenceon sin 0i is shown explicitly. Now when k0 and k• are evaluatedat VLF frequenciesfor typical values of terrain conductivity

•rl anddielectric permittivity el, 10-4< •r• < 10øS/m, and 3 < œr< 10, with el = œrœ0, and Ixl = Ix0, and f = 20 kHz we see that, in general

(53) and thus, regardlessof 0i W=

T•i

•.

1

x10sin 0i

(54)

Now for grazinganglesof incidencesin 0i • 1 with the result that TI1

W•

.

(55)

We see from equation (55) that even for grazing angles of incidence, there is still a finite horizontal electric field component.The answer to this strangebehavior

542

McNeill

and Labson


appears when we consider what happens to the refracted wave in the earth. Snell's Law (Ward and Hohmann, 1988) states that (with reference to Figure

11) the angle0t of the refractedor transmittedwave is related to the angleof incidence0i of the primary wave by k0 sin 0i : k• sin 0t

(56)

fromwhichwe seethat,sincek•2 >>k02,0t is always approximately zero regardless of the angle of incidence; the electrical properties of the earth at VLF frequencies are such that the relative index of refrac, tion is very high and thus the refracted wave propagates virtually vertically downward into the earth. This refracted or transmitted wave, as shown in Figure 11, supplies the horizontal electric field component which

we are able to measure

on the surface.

example if Er = 10 (which is a large value for typical

terrainmaterials), toe• • 10-5atf = 20kHz. Except in themostresistive rocks,• > 3 x 10-4S/m,•q•-• (ito•Xo/Cr•) •/2 and• canbe obtained directly.However, under exceptional circumstances it should be noted that more resistive material may be encountered (glacial ice, permafrost, extraordinarily compact bedrock) and under these conditions •q• will be affected somewhatby the value of e•. To examine this assumption a little further consider a simple parallel plate capacitor. Assume initially that the material filling the capacitor is infinitely resistive (• - 0). Then the current flow in the capacitor is caused solely by displacement currents as we polarize the dielectric. The current density is given by dD

Fortu-

nately at reasonablylarge distancefrom the transmitter the incoming plane wave is effectively at grazing angle (bear in mind that the effective height of the ionosphereis only about 70 km) and the approximation sin 0i • 1 is generally quite valid. Thus, since•q0is a constant (•377t/), measurement of the ratio W gives •q• (about which more will be said in the next section) directly.

dE

Jd= d-•-=e -•-= itoeE.

(59)

If now we let the material also have finite conductivity, an additional current component, that due to conduction (ionic or electronic, depending on the material) flows, and this component is given by

Jc = •E.

(60)

The total current density is the sum of these

Surface Impedance.raThe ratio of the tangential electric to magnetic field at the earth's surface is also simple to measure. Using equations (48), (51), and (39), it is easily shown that

=Z• HOy

=•ql 1---sin

k•2 2 Oi

(57)

and again for propagation at VLF frequencies the second term in the brackets is much less than unity

Jt -- (o- + itoe)E.

(61)

Thus the statement that • >> toe is equivalent to the statement that in typical earth materials conduction current flow greatly exceeds displacement current flow.

Assuming then that we can ignore displacement currents equation (44) for the intrinsic impedance simplifiesto

and

Eox ( ito•xo )1/2

Hoy

-- n• =

cr• + ito• •

'

•1 • (58)

to a high degree of accuracy, essentially independent of the angle of incidence of the primary field. The

quantity Eox/Hoy iscalledthesurface impedance ofthe ground. We seefrom equations(55) and (58) that the intrinsic

impedance• ratio

can be obtainedeither by measuringthe

of the horizontal

electric

field

itolx0

to the vertical

)1/2

(62)

and as stated above, measurement of these quantities

leadsdirectlyto •. We observe that,sincei 1/2= (1 + i)/21/2

(1+i) tolx0 21/2 o'•

)1/2

(63)

and the factor (1 + i) tells us that in

electric field (the electric field wave-tilt), or measuring the ratio of the horizontal

electric field to the horizon-

tal magnetic field. The latter measurement is, of course, the basis of the magnetotelluric method. In general • is seen from equation (58) to be a

relatively complicated function of to, e• and cry. At VLF frequencieshowever, the denominatorsimplifies since we can usually assume that •l >> toel. For

Eox_(l+i) (tOp. o) 1/2

Hoy 2•/2 \ cr•

(64)

the electrical phase angle of the horizontal electric field leads that of the horizontal magnetic field by 45ø, regardlessof the value of •, as shown in Figure 12.

SincealsoHoy= Eoz/Xlo


Eox _(1+i) tOlXo _ (1+i) toeo Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

Eoz- 2•/2x10tr•

(65)

2•/2 •,tr---•-/

543

uation with distance. The mechanismby which this is achieved is, of course, the horizontal electric field and

the power flow to the earth is proportional to another

and the phase angle of the horizontal electric field also leads that of the electric field by 45ø. Thus the phasor diagramfor all of these quantitiesis as shownin Figure 13; Figure 14 shows their spatial relationships. Previously we discussedthe conceptof the Poynting vector, E x H, which expressed the power flow per unit area in the direction of wave propagation.We also learned that the earth (and the ionosphere),becauseof their finite conductivity, causedthe propagatingwave to be slightly attenuated with distance; therefore a certain amount of power must be removed from the wave as it propagatesto accountfor power loss to the earth and ionosphere, and to cause the resulting atten-

PoyntingvectorE x It = EoxHoy.We notethat the attenuation decreasedwith increasingground conductivity (Figure 6) as also doesEox. That the attenuation is so small is an indication that the power loss to the ground is also very small. To those who are used to thinking in terms of electrical currents, the propagatingwave acts, in so far as the ground is concerned, as a "constant current" source of strength I. The losses in the ground behave as a variable resistor R, with a voltage drop given by IR, and drawing power from the current source at the

rateof I2R. WhetherR is largeor smallhasnoeffect on the magnitude of I. Thus whether the earth has a resistivity of 11•.m or 10001•.m the attenuation with distance

is so low that the vertical

electric

and hori-

zontalmagnetic fieldstrengths EozandHoyareessen-

H0y, E0z

tially constant over distances of the order of a free

spacewavelength.However, the value of Eox is constantly adjustingitself to meet the requirement of local power flow to the earth, and is thus constantly indicating the local value of earth resistivity, as indicated schematically in Figure 15. This concept is important to our understandingof the interaction of plane waves at VLF frequencies with the earth. HomogeneousHaif-Spac•Subsurface

•

t

Fields

We now need to obtain expressionsfor the subsurface magnetic and electric fields inside the half-space. These fields are very important because they are the source fields which, interacting with subsurface conductivity inhomogeneities(targets), produce the anomalous secondary fields that we measure at the surface in order to locate and interpret the anomalies. We

ted

transmitter

Fig. 12. Phaseangleof E0xleadsthat of Hoyor Eozby 45 degreesfor a homogeneoushalf-space. Im

E0x eiu•t

'45 ø

Re=_ H0y 'E0 z

Hoy= cos(rot)

•r E0x= (••1)1/2cos(mt+•')

Fig. 13. Phasor diagram for field componentsover a homogeneoushalf-space.

Fig. 14. Spatial relation of field componentsover a homogeneous half-space.

544

McNeill and Labson

must understand

the subsurface fields in order to

showingthat, since -q• ,• 'q0, the near subsurface magnetic field is approximately twice the primary magneticfield. This is in agreementwith continuityof RelationshipBetweenPrimary and SecondaryNearthe horizontalcomponentof the magneticfield across SurfaceField Components.--Welearned in the previthe interfacesinceabovethe surfacethetotalmagnetic oussection thatin general at VLF frequencies k•2 >> field (givenby equation(35) with z = 0) consistsof the k02; furthermore theincoming waveisclose tograzing primary componentplus the almostequal secondary incidence,Oi -• •T/2, whereuponequation(30) shows component[equation(39) gives b0 • a0 as long as thatX -• k0,andequation (26)(withk•2 >>k02) shows Z0 >>Z•] so that the total magneticfield is essentially thatu•2 -• -k•2. Thusequation (36)forthehorizontal 2a0, both above and just beneath the surface. magneticfield in the ground becomes We see, then, that when a VLF plane wave with electric field in the plane of incidence strikes the Hly = a•e-ik•ze-ikox. (66) surfaceof the earth at grazingincidence,the resultant At this point we are only interestedin the dependence horizontal magneticfield above and just below the on z of Hly , whichbecomes [usingequation (37)] earth's surfaceis essentiallytwice the incidentmagnetic field. It can be shownthat the amplitudeof the reflectedelectric field is, under these conditions,also e -il•z (67) Hly ( z) = ao 1 +


understandthe responsesthey produce.

= 2a0

•0

e -ik•z -• 2aoe -iktz

'q0 +'q•

essentially equal to the incident electric field so that the total amplitudeis again twice that of the incident wave. Thus the relationshipE = -q0H holdsfor both the individualand total field components.

(68)

I Remotely located transmitter E0z

H0y Conductive

ground -

E0z

moderate

... ...

H0y

E0x ...

•x

¾:: ß ....

....

.... Resistive

ground -

::::-.:,

•

E0z

large E0x

E0x ...

'" Very conductive

ground very small

Eox

Fig. 15. Wavepropagating overgroundof variableresistivity(assumed to changeslowlyandsmoothly fromone region to the next).

545

GeologicalMapping UsingVLF Radio Fields Behavior of Subsurface Field Components with Depth, Skin Depth.--Now the horizontal electric field at any depth is given by equation(32) (with cr1 >>00El)


as

E•x =

1 OHly or1

OZ

(69)

Of equal importance is the second term of equation (75) which gives the variation of the phase angle of both componentswith depth. In this chapter all electrical phase angleswill be measuredwith reference to the phase angle of the total horizontal magnetic field componentat the surface, which will act as the "phase reference". We first examine the phase behavior of

H•y(Z) with depth. Substituting equation(75) into

and, using equation (68)

equation (68) and reinserting the time dependence

E lx(Z) = 2a0•ikl e -ik•z

leads to

(70)

o- 1

Hly(z, t) = 2aoe- z/•'e - iz/8, eitot

= 2ao'rlle-ik•z

(71)

= 2aoe- z/•tei(tot - z/•t) (79)

= 'qlHly (Z) from which

and causes a horizontal current flow given by

Jlx(Z) = CrlElx(Z)= crlXllHly(Z).

(78)

(72)

Hly(Z, t) = Re[2aoe-Z/•'e i(ø't-z/•')]

(80)

= 2aoe-z/•' cos[tot- z/g•].

(81)

E•x(Z)andHly(Z) are the onlyelectricandmagnetic field components in the ground and the functional dependence with depth for both these field com-

At the surfaceHly = 2a0;as we proceeddownward the amplitude of the horizontal field strength de-

ponents isgivenbye-ihz.Nowthegeneral expression creases as e-z/•' and electricalphaseangleof the

for k• given in equation(28) simplifieswhen we ignore displacementcurrents compared with conductioncurrents (crl >•>tOE1), becoming

k1 -• (-iooi•OO'l) 1/2

(73)

00Ix0 o' 1

= (1- i)

2

(74)

whereupon the depth dependencebecomes

e-ik•z • e-(1 + i)(top. o'•/2)•/2z • e -(1 + i)z/8•

= e-z/•'e-iz/8,

horizontal magnetic field becomes linearly retarded with depth. We say that the field at depth lags the field at the surfaceby a phaseangleq>= z/g•. This behavior is indicated on a phasor diagram in Figure 17. Note that at a depthz = ,rg• the magneticfield is oppositein direction to that at the surface (i.e., lags by ,r radians or 180 degrees) and that at a depth z = 2,rg• the electrical phase angle will again become zero, the value at the surface, and we have, therefore, progresseddownward a distance of one wavelength in the ground. Thus

(75)

X 1 = 2'rr81

(82)

where 1/2

(76)

[note that hi is unrelatedto the constantof equation (26)]. It is useful to compare this wavelength with that

is the electrical skin depth of the ground, controlledby both the frequency and conductivity. We see from equation (75) that the z behavior is the product of two

terms.The first term, e-z/• showsthat bothfield

102

components decay exponentially with depth at a rate determined by the skin depth, since the components

become e-• timestheirsurface valueat a depthz g•. Typical values for skin depth as a function of conductivity and frequency are shown in Figure 16. A useful and quite accurate approximation for calculating skin depth is

al : 500(pl/f) 1/2

(77)

where p• is the ground resistivity in fi.m, f is the frequency in Hz, and g the skin depth in meters.

101

100

101

102

103

104

9 {Q.m)

100

10-1

10-2

10-3

10-4

olS/m)

Fig. 16. Skin depth as a function of resistivity or conductivity and frequency.

546

McNeill

and Labson

above the surface. In the air X0 = c/f where c is the

where, since •r• >>toe•

velocityof electromagnetic wavesin air (3 x 105

itolx0) ]/2 )1/2


km/s). Thus atf = 20 kHz we have X0 - 15km; at VLF frequencies the wavelength in air is very large. On the

otherhandif theground hasa conductivity •r• = 10-2 S/m, g• = 35m and we obtain X• = 222 m, a factor of nearly 70 less when comparedto X0. Now recall that we can picture the primary electromagnetic waves as traveling across the surface of the earth in the direction of propagation with a horizontal wavelength X0, and that at every point as the wave impinges on the ground it instantly propagates vertically downward, with wavelength X•, as shown in Figure 18. Beneath the surface, the horizontal variation of the magnetic field strengthwill be governedby X0 and the vertical variation by X• (<•X0). Since many interestinggeological structuresin the ground have horizontal length less than a few thousand meters (i.e., much less than X0) the primary subsurfacemagnetic (and electric) VLF fields will be essentially uniform over their length. Thus the structurescan be approximately analyzed as being in horizontally uniform electric and magnetic fields, as illustrated in Figure 19 (when their length becomesof the order of X0/4or greaterthis assumption is no longer valid). On the other hand if the structure has a depth extent of 200 m and X• = 222 m the amplitude and phase of the source field will vary substantially over the vertical dimension of the target and such variation

must be taken into account

in our

analysis.

Havingestablished the characteristics of Hly(Z, t) let us examinethe behaviorof E•x(Z, t). We recall from equation (71) that

E]x = •11Hly

(83)

Im

Re

e

i•r/4

o- 1

and therefore

E]x(Z, t) = Re

tølXO e i•r/42a oe --Z/•I e --iZ/•l e itot o- 1

-- 2aøx, costot+ ½r] / e-Z/a'

I

= 2a01xl] le-z/•lCOS tot+•- z/g]ß(85) We seethat, apart from the constantIxl•I, the behavior

of E•x(Z, t) is similarto H•y(Z, t) exceptthat the horizontal

electric

field at the surface leads the hori-

zontal magnetic field by ,r/4 or 45ø. However once againas we proceed down into the earth (increasingz) the phase retards linearly with z in exactly the same fashion as for the magnetic field. This behavior is illustrated in the phasor diagram in Figure 20. Behavior of Inphase and Quadratic PhaseField Componentswith Depth.--Now the customin geophysical instrumentation often is to use, rather than the phase angleof a field component, the inphaseand quadrature phasecomponentswith respect to the phasereference. As previously mentioned we will always employ the horizontal magnetic field at the surface as phase reference, so that in a phasor diagram this quantity is directed along the horizontal real axis. The inphase component of any quantity is then defined as the projectionof that quantity on the real axis; the quadrature phasecomponentis likewise the projection on the vertical imaginary axis. Thus the horizontal electric field at the surface, which leads the horizontal magnetic field by ,r/4 radians, has equal inphase and

z

Hly(=2ao)'•1=0

2.5

tO•o

(84)

quadraturecomponentsgiven by IE0xl cos (,r/4) and

2.0

0.5 I.¾...p 0¾>>1 0¾>>!,¾

Fig. 17.Phasordiagramshowing variationofHly withdepth.

!•

0¾

'-

PT

I--•,,I

loõJel L• '•1 T

•x

Fig. 18. Plane wave refracting into the ground.

Fig. 19. Field vectors frozen at one instant in time.


IE0xl sin ('rr/4), respectively. For z > 0 the inphase

rapidlythane-z/a'dueto thesin(xr/4- Z/•l) term,

component of Eoxvarieswithdepthas IEoxle -z/•' cos

crossingzero and changingpolarity at a depth of Z/gl

('rr/4 -

= 'rr/4 = 0.785.

z/•l)

and the quadrature component as

IEoxle -z/•' sin ('rr/4- z/81),respectively. With the Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

547

assumptionthat conduction currents prevail over displacementcurrentsthere is a horizontal current flow in the ground related to the electric field by J•x(Z) = o'•E•x(Z), which current flow thus has inphase and quadrature phase componentsgiven, at any depth, by the product of cry, and the inphase and quadrature phasecomponents,respectively, of Elx at that depth. The behavior

of the horizontal

electric

field and thus

also of thesecurrentswith depth is shownin Figure 21. We see that, for the homogeneous half-space, the inphase component of the electric field decreases

relativelyslowlywithdepth,lessquicklythane becauseof the influenceof the cos ('rr/4- z/8•) term, whereas the quadrature component decreases more

Im

Finally, Figure 22 schematically illustrates the amplitude (modulus) of the electric and magnetic field componentsbelow the surface of the earth. We have assumed that the highly refracted plane wave was propagatingfrom free spaceinto the ground which accounted for the presence of the horizontal electric field. There is another way of viewing the physics which helps to enhance our understanding. Let us again focus our attention on the horizontal magnetic field. We know that both above and within the earth there was a uniform horizontal primary (pri) pri

z

I I Z=

b-•0.5 Re 2.5

1.5

1.0

2.0

ß

ß

y or z, and which, sincethe wavelengthin free spaceis very large at VLF frequencies, can be consideredto be uniform with x as well. This primary magneticfield is time varying, and as a result of Faraday's Law [equation (12)] producesan electricfield E•x in the ground. The electric field causescurrent J•x to flow, which in turn generatesa secondary (sec) magnitude field both above and below the surface.

Eox,

ß

magnetic fieldgivenbyH0ywhichisnota function of

Now

at the surface we

measure the total magnetic field, which is the sum of the (essentially equal) primary and secondary fields. The primary magnetic field strength is virtually independent of the local ground conductivity, and as we saw earlier, the total magnetic field is also essentially independent of the local conductivity, yet we know that the secondary magnetic field is generated by currents which are dependent on the ground conductivity since [from equation (72)]

J•x(Z) = cr•E•x(Z)= cr•l•H•y(Z)

(86)

= (itolxoo'l)l/2H ly (Z) -

Fig. 20. Phasordiagramshowingvariationof E0xwith depth.

0e -i•,z ß (io•lxOCrl) 1/22a

_

(87)

itter

0

-.6

o

I

2

z

Fig. 21. Behavior of inphase (I) and quadrature phase (Q) componentsof horizontal electric field and current density (normalized with respect to the amplitude at the surface)as a function of depth.

Fig. 22. Schematicillustration of amplitude of field components beneath

the surface.


548

McNeill

Why is the secondarymagneticfield at the surfacenot a function of ground conductivity? To understandthe paradox we use Ampere's Law in its integral form to examinethe secondarymagneticfield •l•asec ly produced at the surface by the current distribution of equation (87), which is illustrated schematically in Figure 23a and 23b. We apply Ampere's Law around the rectangle indicated in Figure 23b which lies in the x = 0

and Labson

plane, is 1 rn in length alongy, and of depth z which we allow to go to infinity. Since the primary magneticfield

HPri notvarywithx' y, or z thecurrent Jlx 0y does cannot vary with x or y, but varies with z as indicated in equation (87) and Figure 23a. Furthermore, from symmetry the secondary magnetic field arising from this current must also have only a horizontal component, and can vary only with z.

o

y

b

a

Jlxl Note

that

everywhere

pri

Rs ly (in-ph

ri

Hly = HoPy At

I

surface

I

I

s (I) = Hly pri Hly At

I

great

depths

pri

H•y (13 = -Hly z

23 (a)

y

-

b

o a

ß

ß

ß

ß CurrentfJowlJlxl

ß

ß

ß

ß out

ß

ß

ß

ß

ß

ß

of page

$

Hly ß

23 (b)

Fig. 23. (a) Primary (pri) and secondary (sec) inphase magnetic field componentsbeneath the Earth's surface. (b) Path a-b-c-d for application of Ampere's law.


Applying Ampere's Law around the loop a - b -


c -

d we obtain

549

direction; deeper the current flow is in the opposite direction, so that the net magnetic field produced at the surface (which, if it existed, would be in quadrature phase with the primary magnetic field), is in fact zero. Clearly, anything which upsets the geometry of this quadrature phase current flow will upset the careful balancewhich produceszero quadraturephase secondarymagnetic field at the surface, with the result that an anomalous quadrature phase magnetic field componentcan be easily generatedby relatively insignificant conductivity inhomogeneities (in terms, for example, of conductivity contrast). In the case of the inphasecurrent the great majority of the current flows in one direction; only a smallfraction (at a depth below about 2.5g•) flows in the oppositedirection. Thus we would expect this component to be less sensitive to small changes in conductivity. Figure 24 illustrates schematically the flow of inphase and quadrature phase subsurface currents across ground of slowly varying conductivity.

_f• •4sec• dy+O+O+O -- (H0P• iq-'•0y! --- /4rpri '• 0y q-/4sec ß• 0y -'

J lx(z) dz

(88)

sincethe magneticfield has only y componentswhich

do notvarywithy, andalsoasz -• o•,H•y -• 0 from equation (68). Using equation (87)

/4pri /4rsec 1/2( '•0y + SX0y = (ic01X0tr1) 2a0) e-ik•Zdz

(89)

1

=(icoixotrl)l/2(2ao) i-•-• =2a0 (90) •r pri

t_r sec

whereupon weseethat,since,•0y= a0,,•0y = a0as well. Thus the numerical value of the total integrated current beneath the surface is controlled by the primary magneticfield strengthand is independentof the conductivity of the ground. The vertical distribution of this current is, however, strongly dependent on the conductivity sincethe distributionis controlledby the skin depth. For highly conductive ground the current densityis large near the surfacebut since• is small, it falls off rapidly with depth. In resistive ground the surface current density is small, but decreasesslowly with depth. In either casewhen we useAmpere's Law to calculate the secondarymagneticfield at the surface due to this current flow the secondaryfield is equal to and in the same direction as the primary field so that the total field is twice the primary field. Conversely at depthswhich are large compared to the skin depth the secondary field is still exactly equal to the primary field but is now oppositein direction, making the total field go to zero at great depth. In the limit of a perfectly conducting earth, an infinitely thin sheet of current

flows,of magnitude 2a0A m-•, sothatagain,beneath the current, the total magnetic field is zero.

It is important to point out that althoughJ lx is a complex function of z, the integral from zero to infinity of this quantity is purely real, as the integral must be to give real '•0y •jsec', conversely the integralof the quadrature componentwith respect to depth must be zero, as suggestedby Figure 21. Thus, although there is large quadraturephase current flow at most depths beneath the surface, at shallow depth this current flow is in one

Layered Earth•Surface

Impedance

Two Layered Earth.--The surface impedance, defined as the ratio of the tangential (horizontal) electric and magneticfield componentsmeasuredat the earth's surface, is widely used for magnetotelluric measurements at low frequencies. This ratio is also easily measuredto high accuracy at VLF frequencies, as first suggestedin Collett and Becker (1967). In magnetotelluric soundings the surface impedance is measured over a wide range of frequenciesto obtain information about the variation of resistivity with depth. Unfortunately at VLF frequencies only one, or at most two, closely spacedfrequencies are usually available. This lack is somewhat compensated for since, by using VLF transmitters, we can accurately measure the electrical phase angle between the horizontal and electric field components. We have previously illustrated that, in the case of a homogeneoushalf-space, this phase angle is 45 degrees regardless of the resistivity of the half-space. If the earth has two (or more) layers the phase angle departs from 45 degrees and gives useful information about layering. We will generally deal with, at most, a two layered earth (defined as one layer situated on a basement or substrate) since even this simple case presents three unknowns, i.e., the resistivity of the layer, the resistivity of the basement, and the thickness of the layer, whereas we can measure only two quantities, the magnitude and phase angle of the impedance at one frequency. We will, therefore, be especially interested in structures where there is a substantial

contrast

between

the two resis-

tivities so that one can be assumed to be negligible

550

McNeill

and Labson where


with respect to the other, resulting in only two unknowns. We show that these can generally be easily resolved with a surface impedance measurement. The surface impedancethen, is defined as

Zs -

ßll

•q12=

E0x

-•12

=

(91)

H0y

=

.

1+

(93)

+1

which, for a two-layered earth, becomes (Kaufman

andKeller, 1981.Note that theseauthorsuse½-itot.

The surfaceimpedanceis given by •1, the intrinsic impedance of the material of the upper layer, multiplied by a complex function of the real conductivity contrast function •q12 and the electrical thickness, h/•l, of the upper layer. The function •q12is plotted in Figure 25 where we see that it varies from -1 for a perfectly resistive basement to +1 for an infinitely

theirexpressions havebeenconverted to ½itot in this article).

(1 - •112 e -2ik•h)

Zs= Xll(l+ ,q12e_2ik•h) (1 - •q12 e-(1 +i)2h/•) (1 + •q12e -( 1+i)2h/B• )

Remote

conductive basement. Since•q12 variesas(o'2/o'1) 1/2a

(92)

gooddynamicrangeis achieved,that is, the functionis

Transmitter

Inphase current --zero •......••/•••

•

x

•

x

•-......

Note change of direction of current flow (a) INPHASE

CURRENT

(b) QUADPHASE

FLOW

CURRENT

FLOW

PlLARGEiiiii:?ii?ilil PlSMALL PlLARGE 8 LARGE •:•:•:•:•:8 SMALL

(C) EARTH

STRUCTURE

Diffuse

LARGE Boundaries

Fig. 24. Currentflow acrossearthwith laterallyvaryingresistivity.(a) Inphasecurrentflow; (b) quadraturephase currentflow for the earth structureshownin (c).



551

slow to saturate at either conductivity contrast ex-

given@a/@l and h/8• the figureimmediatelygives@a/@l

treme.

and4•. Conversely given@a/@l and4• we canobtain

At •r2/•r• = 1, •2 = 0 and Z s = •. Furthermorefor arbitrary •r2kr•, as h --> o• the two exponentialterms tend to zero and again Zs - •. As the electrical thicknessof the upper layer becomesvery large we no longer see any influenceon Zs from the basement. In the magnetotelluric technique an apparent resistivity may be defined by inverting the expressionfor the surface impedance of a uniform half-space. The resulting impedance determined from the measured

92/91and h/g• and thus, if p• is known, h. We seethat for any value of P2/Pl, Pa/Pl --> 1 (i.e., Pa --> Pl) and

4p --> 45 degreesas h/g• --> o•. Conversely, for any value of P2/Pl, as h/õ1 --> 0, Pa/Pl tends to the appropriatevalue of P2/Pl, i.e., Pa --> P2 as the upper

layer thickness approaches zero. Again4p --> 45 degrees. Note also that in the general case of an electrically thick upper layer over a relatively resistive

Eox/Hoy ' Zs= (itolxPl)1/2 canbeused inthedefinition for Pa by: 103

1

Pa=

1

Ixto

IZs12=

Ixto

Eo_•_x Hoy

h

(94) 0.02

The definition is equally valid at VLF frequenciesand additionally for both MT and VLF we define the phase angle of the apparent resistivity as =

3OO

0.03

IO2

IO0

RESISTIVE SUBSTRATE

(95)

3O

whereZQ and Z• are the quadrature and inphase componentsrespectivelyof Zs with respectto the total horizontal magnetic field measured at the surface. Using these definitions we obtain Pa

1 - 'rl]2e

iOI

IO

-(1 + i)2h/8•

(96)

1 q-T112 e-(• + i)2h/8•

0.80

/øI

Op= tan-•

I.C

1--•]2 e-(1 +i)2h/8•

0.60

ImTi1 1+ •2 e-(•+i)-•7'•[

0.30

1-•2e-(1 +i)2h/8• ' (97)

Re• 1+ •12e-(•+i)2h/8•

A graph illustrating the behavior of the complex apparent resistivity is shown in Figure 26 where the

Id I

0.10 CONDUCTIVE SUBSTRATE

amplitudePa/Pland phaseangleOp are plottedas

0.03

functions of the resistivity contrast 92/91 and the electrical thickness h/g1 of the upper layer. Thus, IC52

0,01 9.03

1.0 0.8

0.003

0.02

0.8 0.4

I h

0.2

•11t 0.0

& o,oo,

id•

-0.2

p,

-0.4

0

-0.8

iO

20

30

40

50

60

70

80

90

½• (deg)

-0.8 -1.0

1•'2

1•'1

100

101

102

Fig. 25. Conductivity contrast function.

103

Fig. 26. Amplitude and phase of complex resistivity normalized with respectto Pl for varying resistivitycontrastP2/Pl and upper layer thickness, h/8•.

552

McNeill

and Labson


substrate 0 < •p < 45 degrees; whenthebasement is relativelyconductive, 45 degrees< •p < 90 degrees. Finally, we observethat as the upper layer becomes electrically very thick the apparent resistivity undershootsor overshootsthe correctupperlayer resistivity by a small amount before settlingto the correctvalue. At h/81 = 1, Pa/Pl = 1 and this is the largest upper layer thicknessthat can be measuredusingthe surface impedance technique. At such large thickness all information about thickness lies in variation of phase angle with thicknessrather than apparent resistivity.

I

I

I

I

I

I

_

• 4•o

IOO

5 fi.m

to 160 m at 2000 fi.m.

Since there are only two measured quantities, Pa

and •p, we mustassumein generalthat one of the three two-layer parametersis known. If Pl is known, Figure 26 is useful for obtainingthe other two param-

etersas follows'givenPa, (•p, and Pl we calculate Pa/Pl-We enterthegraphat Pa/Pland•p to determine 92/91andh/•. •owing p• we calculateP2and also • so as to obtain h. In a second important class of problems P2 rather than p• is known or can be estimated.

I

,o•'o

I0 3

We note also that sincethe skin depth is a function of the upper layer resistivity so also is the actual exploration depth, varying for example from about 8m at

In this case it is convenient

1- 312 e-(1 +i)(2h/82)(P2/P•)•/21

P:- P:-IP: r

/

CONDUCTIVE SUBSTRATE

• -(1 + i)(2h/g2)(p2/p])•/2]•

//

The resultsof thesecalculationsare shownin Figure 27. This graph is used in exactly the same way as Figure 26. Finally, the expressionfor the surface impedance, which will be useful later, is often seen in a slightly different form and is derived as follows. From equation (92)

0.10

Zs •

RESISTIVE

SUBSTRATE 0.03

I0-•'

_

ß • -- "12•

(1 + i)(m•/2•l)1/2.

0.30

I0 -•

1

/

(99) where use has been made of the fact that 3• =

I••i•ø'5

p--••oø

(98)

,p=tan-'/•-•,•_•,,(•h,••,,•/ LReL (1+')l•l•-•i)(2h/a:•)•J]

,o 3•__•l.O /::'a

the

using thefactthatg•/g2= (Pl/P2) 1/2-Thus

Pa Pa 7

to reformulate

equations for Pa and •p in termsof Pa/P2andh/•2

1 - •12e-2ik•h 1 + •12e-2ik•h

(100)

whereupon,on substitutionof equation(93) for Xll2

Zs

0.01

('ql q-'q2)- ('ql - T]2)e-2iklh

Xll--(Xllq-X12) q-(Xll_ •12)e_2ik• h (101)

0.003

•12 1 q-e -2ik•h (102) I0-$

T]i(1-e-2ik•h)

0.001 0

I0

20

30

40

50

60

•x!2 q- 1 -e - 2ik•h 70

80

90

(l:)p (DEGREES)

1+ •

tanh (ik 1h)

•12

Fig. 27. Amplitude and phaseof complex resistivity normalized with respect to P2 for varying resistivity contrast 01/02 and upper layer thicknessh/82.

(103) -- + tanh (iklh) •12

Geological Mapping Using VLF Radio Fields

Limiting Cases.--The graphs of Figures 26 and 27


werecalculated for 10-3 < (p2/Pl)< 103 which represents a reasonable range for this ratio. Let us examine the behavior of the complex apparent resistivity in the limit where an electrically thin layer overlies either a perfectly resistive or perfectly conductive basement (we recall that if the layer is electrically very thick the complex resistivity reflects the actual layer resistivity for either case). (a) Electrically Thin Overburden Over a Perfectly Resistive Basement.--For a perfectly resistive base-

ment •2 = 0 and •12 = -1- Then

Zs

(104)

Representing the exponential by the first term in the power series expansion (since by definition for an electrically thin layer 12ik•hl• 1) we obtain

Z•

1 + (1-2ik•h)

'q•

1 - ( 1 - 2ik • h)

2-2ik•h

1

2ik • h

(105)

ik • h

or

Y[1

1

1

Zs- iklh- O'l h S'

(106)

The surfaceimpedance is purelyreal(ZQ = 0) andis determined by S, the conductivity thickness product of the surface layer. In this limit it is not possible to obtain • and h independently, only their product can be determined. The apparent resistivity is given by

1

1

1

The surfaceimpedanceis purely imaginary (Z• = 0) and is a function only of the thickness h of the upper layer. 1

1

1

82 1

Pa [xto Igsl2[xtoS 2 [xtoo'12h 2 2Pl•-• (107)

Pa Ixto IZs 12 (to,h) 2 2p•8• 2 P•-• =2 90ø.

(114)

We infer from Figure 26 that the equationsfor Pa under the limiting conditionsP2'-->0, ooare of limited values of p2/Pl before they are valid. Indeed phase angles of zero or 90 degrees are not normally seen in field data. However, these limits give useful information; they tell us that for the general case of a thin conductive layer over a relatively resistive basement to resolve both the thickness and the resistivity of the top layer will be difficult. Conversely, for the general case of a thin resistive layer over a good conductor, to obtain an accurate value for the resistivity of the top layer will be difficult. In each of these examples, despitehaving the facility to measuretwo parameters,

Paand0p, onlyonequantity(S for thefirstcase,h for the second) can be accurately determined. (c) Electrically Thin Overburden Over a Resistive Basement.--Fortunately, for the more general case of an electrically thin upper layer over a basement exhibiting large but finite conductivity contrast more information is easily and accurately obtained. Using the fact that for smallvaluesof ikl h we have tanh (ikl h) = iklh, equation (103) becomes

Zs •= 1

(112) (113)

pl

or

Pa

h2

usefulnesssince they require extremely large or small

1 + e -2ik'h

'q• 1- e-2ik, h'

....

553

1q-'q2 'q-•( ik•h .

(115)

W[1

1

•-•

'q2

ik•h

But

0p = O.

'131/W12 : k2/kl

(109) whereupon

(b) Electrically Thin Overburden Over Perfectly Conductive Basement.--Consider

Zs

now the case where the

basement is a perfect conductor, •1: = 1. Again expanding the exponent and retaining the first term,

ki + k2ikih

'ql k2+ik•2 h

(116)

or

we obtain

Z•

1 + ik2h

1 -- e-2ik'h 1 - (1 - 2ikih)

'ql 1q-e-2ik'h1+ (1- 2iklh) - iklh (110) Zs = i'qiklh = ito•h.

(111)

Zs= co•k2+ik•2h '

(117)

Consider now the case where the basement has high but finite resistivity, so that the skin depth in the

554

McNeill and Labson

basementmaterialis muchlargerthan the thicknessh of the upperlayer. Then Ik2hl'• 1 and


1

1

Zs-•top. k2+ik•h - 1 iklh

(118)

l+e

-2ik• h

gs='qll_e-2ik•h = gi + igQ whereZ• andZQ aretherealandimaginary partsof Z s, respectively.

Applyingour usualdefinitionfor Pa [equation(94)] we see that

1 + e -2ikl h

(119)

1

1-e

+S 1

(126)

-2ik• h

and

in agreement with equation(106)for large•q2.We see that for this case the surface impedancecontains informationaboutboththe conductance of the upper layer S1 and the conductivity•r2 of the lower layer, through•q2. Fortunatelyboth quantitiesare easily

obtained, for,bylettingY= (X/-•l•q2 I)- 1=

(2tolxp2)- 1/2

= tan. q>p •Z 1q-ZQ Z I - ZQ

(127)

Theseequationsare functionsof h/g• only; calcu-

latedvaluesof Pa/P•and4p asa function of h/g•are shownin Figure28. To usethe graphfind the valueof h/g•(=X) whichcorresponds to the measuredvalueof

it is easily shown that 1

Zs--(S1q-Y)- iY Z•+ iZQ (120)

4p, andat the sametimereadoff the correctvalueof pa/p•(= Y) from the ordinate. Then

whereupon

Pl = •-

1

(128)

and

y21+

1+ 1/2

(129)

h=X[•xtoy )

leading to Pa

Notethatforsmallvalues ofhill, {bp-• 0 andpa/Pl

2

is in goodagreementwith equation(108);for this case all of the usefulinformationlies in Pa/Pl, whereasat largevaluesof h/gl, pa/Pl is almostconstantat about

P2 1+ 1+

unityandtheuseful information is nowin {bp.

and 1

{bp= tan-l

S'

(123)

(e) Electrically Thin Overburden Over Conductive Basement.--Consider finally the case when the base-

ment is very conductive,so that in equation(117)

14--Y

Ik2I>•ik•2hl, then

Theseequationsare easilyinvertedto provide

92= -•-cosec 2{bp

1 + ik2h

Zs-•tolx k2

(124)

(130)

= n2(1 + ik2h)

(131)

= 'II2 q- ito•xh

(132)

and

=

cotan(bp- 1

Sl (}xtop a)1/2 cosec (b p

(125)

againin goodagreementwith equation(111)with very small. For this case we see that the surface impedance now contains information about the thick-

sothat,givenmeasured values of I%and4p,calcula- nessh of the upperlayer and, againthrough•q2,the

tion of P2 and S1 is easilyperformed.

conductivitycr2 of the basement.We can easilyshow that

(d) Thick ConductiveOverburdenOver Perfectly ResistiveBasement.mEquation(104) showsthat, for a perfectly resistive basement,

IZsl2- •tol92 1+ 1+ 2

(133)

GeologicalMapping Using VLF Radio Fields and

leading to


555

Pa

1

p:

2

1+

1+

=

(134)

and

•p =tan-• 1 +

.

(135)

Again these equations are easily inverted to provide

P2= 2paCOS2 q>p

(136)

(sin q•p-cos q•p).

(137)

As a result of the approximations used in this section p•, the upper layer resistivity, does not appear in any of the final expressions.In order to estimate the error causedby finite Pl, which will be a function of h and p•, the full two layered earth responseusing either Figure 26 or Figure 27 should be determined and compared with the approximate results. Multi-Layered Earth.--Although at most a twolayered earth can be resolved at VLF frequencies it is useful to be able to calculate the response from an earth having more than two layers to see how seriously the presence of the additional layers will affect calculated results interpreted on a "two-layer basis". Wait (1970) gives a simple iterative technique which allows calculation for any number of layers. The expression given here is for four layers (three on a basement); extension to any other number of layers is obvious.

IOO

0.10 =h__

Zsl = 'q1

0.20

Zs2 =

Zs3 = 0.40

Zs2 + •

tanh (ikihi)

xll + Zs2 tanh (ikl hi)

(138)

Zs3 q-'q2 tanh (ik2h2) 'q2 q- Zs3 tanh (ik2h2)

(139)

334q- 333tanh (ik3h3) 333q- 334tanh (ik3 h3)

(140)

Zs3 is calculatedfirst, followed by Zs2 and Zsl. Two Layered Earth--Subsurface Fields and Currents

0.50

For a two-layered earth our interest lies both in the field componentsin the upper layer, since inhomogeneities in this layer can give measurable anomalies at the surface, and in the field components in the substrate, which is where our targets are located. We will againmake the assumptionthat k• >>k0 and also that k2 >>k0, which means that in both media the waves propagate essentially vertically upward and down-

0.60

1.8 I

ward.

Our treatment will again follow that of Kaufman and Keller (1981) (taking note of the fact that these authors

usetimevariatione-itøt).Beforecommencing, let us review what we already know. In medium 2 (assumed to be of infinite depth) there can only be a downgoing

wave,propagating as e-•:z, and in this regionthe 0.1

o

IO

20

30

40

õ0

q)p(DEGREES)

electric and magneticfields will be related by x12,the intrinsic impedance. Thus, to find the field components at any point in the basement we need only know

Fig. 28. Two layered earth with perfectly resistivebasement,

the value of either E or H at the lower

Pa/Pland4•pasfunctions of firstlayerthickness h/•1.

since the fields have horizontal componentsonly, and

interface.

But

556

McNeill and Labson

these componentsare continuousacross the lower


interface,to knowthe fieldsin medium1just above the surfaceis sufficient.Furthermore,we will be measuring the field components (specifically the surface impedance)in the air just above the surface. However,bothhorizontalfieldcomponents that constitute the surfaceimpedanceare continuousacross the surface,andare thusonly necessary to obtainat

and, therefore, for 0 < z < h

Hly ( z) = Hly (0)

(e-ik•z+ ,q12e-2iklhe ik•z) 1 q-TI12 e-2iklh = Hly (0)

the top of medium 1.

(1 + 'q12e-2ik•(hz))eik•z 1 q-TI12 e-2iklh

(149)

(150)

In medium1wemustpostulate bothanupgoingand and similarly a downgoing-wave and in medium2 a downgoingwave, both satisfyingMaxwell'sequationsso that Elx(Z) = Hly(0)Xll

Hly(Z) = al e-ilc'z+ bl eilc•z

(141)

and

In medium

H2y(Z)= a2e-ilc2z.

(142)

e-2ik• (h -z) 1

(151)

H2y(z) = a2e-ik2z

(152)

-ik• z

x l+xl 2

but

But

b1ik• h)ik2 h

e -ikl h -Jr• e

a2 =al

Ex(Z)

=

e

al

1 OH1

(153)

where

c•10z

= Xll(ale-ik•z--bl eik•z)

Hly (0)

(143)

and

a1- 1+ x112 e-2i•1 h

(154)

and

E2x(Z)--

- 1 OH2 0'2

b1

- 'q2a2e-ilc2z.

•=

(144)

al

-2ikl h

Xl12e

(155)

and, therefore, for h < z

Applyingthe boundaryconditions of continuityof these componentsat z = h we obtain

b1= • 1-- TI2e -2ik• h,

•

al

•1 -t-•2

H2y( z) = Hly (0)

1q-• 12 -ikl he-ik2 (z- h)

1 + x112e-2ik• h e

(156) (145)

and

E2x(z) = 'q2H2y(z)

whereupon

(157)

and of course, in both media

Hly(g) = al (e-ik•z+ xl12e-2ik•heiklZ) (146)

Jx = crEx.

(158)

We focus our attention first on the electric field in the

and

upperlayer, givenby equation(151). We seethat the z

Elx(g) - al'rll(e-iklz- 'q12e-2iklheik•z). (147) We wish to expressthese fields in terms of the total

magnetic fieldat thesurface (weknowthatHoy=

2a0 = Hly(0), independent of the conductivity struc-

Hly(O)'ql e-itqzterm,multiplied bya complex factor whichisa function of xl•2(andthusof theconductivity contrastbetween the layer and the substrate)and termsof the form e -2iklh.Now as we have seen e -2iklh __e -(i + 1)(2h/gl)

ture of the earth).From equation(146)

Hly(0) = al(1 +'q12e-2iklh)

dependenceof the electric field consistsof the normal

and is a functionof h/8i. This quantityh/•l, the (148)

electricalthicknessof the layer, is crucialin determin-


GeologicalMapping UsingVLF Radio Fields ing whether the layer has a strongeffect on parameters measured at the surface, and also on the field components in the substrate. An electrically thick layer is defined by h/g• >> 1, an electrically thin layer by h/g• <• 1. Taking first an electricallythick layer (even a relatively resistive layer can be electrically thick since, in the VLF band the frequency, about 20 kHz,

557

thick conductiveoverburdenoverlying a resistivebedrock. We have, from equation (151)

Jlz(Z) = O'lElx -2ikl (h - z)

1 - 'ri12e

-ik• z

=HIy(0)ø'I'ril 1+ 'q12 e-2iklh e

(164)

isrelatively high),bothe-2iklhande-2ilq(h-z) •--0 and we have

Hoy(O) 1- •12 e

Elx(Z) -• Hly(O)'ql e-iklz.

of the substrate has little effect on the electric field in

the layer. Likewise if •r• = •r2, •i•2 = 0 and equation (151) againgivesthe correctbehaviorfor the field in a homogeneoushalf-space.

Supposeon the other handthe layer is electricallyso thin that we can replace the exponential terms with unity. Then at the surface 1 - •12

Elx(0) =Hly(0)Xil 1+ Xil 2 = H ly(0)'r]2

(160)

Now let the upper layer be electrically thin, and let the basementbe a perfect insulator so that Xil2 = --1. By replacingthe exponentialterms with the first term in their power series expansion equation (151) becomes

2- 2ikl(h - z) -ik z

Elx(Z) =Hly(0)'ril 2iklh e I (162) and

Elx (0) --

-ikl

Z

(165) since

Hly (0) = Hoy(0). Figure 29 illustratesthe behavior of the normalized

currentflow,Jlx(Z)x al/Hoy(O), asa function ofdepth for upper layers of various thicknessand a perfectly resistive basement. Dealing first with the inphase componentwe seethat, for h/•l < 1 (Figure29a-c) the inphasecurrent is virtually constantwith depth and that this componentis as given from equation(163) Jlx•l

(161)

and the electric field at the surface is controlled solely by the intrinsic impedance of the substrate, as we would expect.

e

= (1+ i) 81 1q-•12e-2iklh

(159)

We see that in this case regardlessof •i•2 the presence

-2ikl (h - z)

1

Hoy(O) (h) (166) or

Jlx

=

1

Hoy(O ) h'

(167)

This result is to be expected, since by holding the conductivityof the upper layer constantand reducing

h we are simplyfor.cingthe availableinphasecurrent

(determined byH•}•) to flowthrough a smaller region, thus increasingthe current density. Indeed from equation (167) we see that

Hly (0)

Jlx h = Hoy(O)

iklh

1

Hly (0)

=Hly(0) O'lh S

(163)

where S is the conductivity thicknessproduct of the electrically thin but conductive surface layer. On expandingthe remainingexponentialterm in equation (162) under the condition of electrically thin upper layer we observe that E•x is essentially constant throughout the upper layer, at the value given by equation (163), and is inphase with the surface magnetic field.

It is important to examine the current flow for the caseof the insulatingbasementwhen the upperlayer is of moderate but not large electrical thickness, since this is the case (at VLF frequencies)of a reasonably

as we would expect. As long as the upper layer is electrically moderatelythin there is no essentialdifference in the behavior of the inphasecurrent density at VLF comparedwith that at dc. This constancywith depthholdsup to h/g1 = 1, a surprisinglylargevalue; for larger h/g1 the inphase current starts to vary significantlywith depth, as shownfor h/g = 3. Considerthen, the geometrywhere a resistivebasementslowlycomescloseto the surfacein the direction of propagation.When the basementis deepcompared with a skin depth the current distributionis that of a homogeneous half-space.As the overburdenthickness decreasesand eventually becomes electrically thin, thenno significantvariationof Jlx with z takesplacein the overburden.

The current is constrained to flow

everywherein a sheetwherethe local currentdensity

558

McNeill

does not vary with depth and the total current density


is givenby Hoy(O)/h. The behavior of the quadrature component of the current is entirely different. For example, at small values of h/• the quadrature component becomes negligibly small, and at larger thicknesseswill exhibit a polarity reversal about halfway down the sheet. In Figure 29d we see that for h/• = 3, which is a good approximation to a homogeneoushalf-space, the quadrature component of the current reverses direction at z/• • 0.80 (in agreementwith Figure21) or z/h = 0.27. However, at h/•i = 1.0 (still an electrically thick sheet)the reversal occursat z/•i = 0.40(z/h = 0.4) and at h/•i = 0.3 the reversaloccursat z/•i = O.15(z/h-• 0.5). The presence of the resistive basement elevates the location

of the crossover

so that the crossover

and Labson

in primary field strength arising, for example, from diurnal variations in the state of the ionosphere, etc. From the above discussion the imaginary component of the secondary magnetic field above a layered earth always has to be zero at the surface. But in the general layered earth case there are certainly large quadrature-phase currents in the ground. The only way these currents can produce a zero horizontal magnetic field component at the surface is by having, regardless of the layering, exactly one-half of the quadraturecurrent flow in the oppositedirection to the surface quadrature current. Thus, there will always be at least one sign reversal in the quadrature-phase current

flow.

To return to our resistive basement and Figure 29 we observe that for intermediate values of upper-layer

thickness(h/• -• 1) the quadrature-phase components

always occurs about halfway through the overburden, ensuring that when we apply Ampere's Law to the quadrature phase current we get zero quadrature phase magnetic field at the surface, even in this layered earth. Indeed, Kaufman and Keller (1981) give a simple proof, valid for an arbitrary number of layers, that the quadrature phase componentof the secondary magnetic field is always zero at the surface, and the inphasecomponentis always equal to (and in the same direction as) the primary magneticfield. We can learn nothing about layering by measuring the horizontal magnetic field at the surface•all information about variation of resistivity with depth is contained in the

are indeed large and comparable to the inphase component. To obtain zero contribution to the surface quadrature-phasemagnetic field requires careful balance of the contribution from the plus and minus components; conversely, 2-D or 3-D conductivity inhomogeneitiesin the overburden that upset this balance will produce a quadrature-phase component of the magnetic field at the surface. Thus, in regions of thick conductiveoverburden we expect the quadrature component of the magnetic field to be active. At 20 kHz the skin depth in 50 tl.m material is 25 m, a

electricfield.The surfacemagnetic fieldHoy(O)= 2a0

the balance is still required, however the quadraturephase currents are much smaller so the effect of any

is thus a very convenient normalizing parameter with which to remove variations in Eox causedby changes

I

thickness

often encountered.

imbalance

For thinner

overburden

will be much smaller.

i

Fig.29.Normalized currentflow,J•x•l/HOy(O), in upperlayerasa function ofdepth,Z/•I, fora perfectly insulating

basementand variousupperlayer thickness.(a) h/• 1 = 0.10; (b) h/• 1 = 0.30; (c) h/• 1 = 1.0; (d) h/• 1 = 3.0.



The presence of a conductive layer produces other interesting effects. We show later that essentially the horizontal electric field in the basementproducesVLF anomalies from basement conductors (rather than the horizontal magnetic field) so we should concentrate our attention on the electric field. Again we assume that the overburden is considerably more conductive than the basement. We wish to examine the phase and the amplitude of the horizontal electric field component at the base of the conductinglayer as we vary its

A second feature

559 of the conductive

overburden

is

shown in Figure 31 which is a plot of the modulus of E•x(h), the exciting electric field at the top of the basement. We observe that, rather than decreasing as

e-h/a'theinitialdecayis muchfaster,sothat,instead of beinge-• or 0.368 of the surfacevalue at an overburdenthicknessof one skin depth, the amplitude has fallen to 0.090, down by a further factor of four. Only when the overburden thickness exceeds about one skin depth does the amplitude of Ex(h) start to

thickness,sinceoncewe know this quantity,Ex(h), we

decayas e -h/a'.The datain Figure31 was calculated

know that deeper in the substrate the horizontal elec-

for cr2/cr • = 0.01, a small ratio. Figure 32 shows a similar plot for various values of cr2/cr • . We see that the presenceof a small amount of conductive overburden profoundly modifies the horizontal electric field that will exist in the vicinity of conductive targets in the resistive basement, both altering its phase and

tricfieldwilldecayasEx(h)e -ik2(z-h). Now, if the thickness of the overburden is zero, the resistive basement will cause a large horizontal electric field at the surface,with equal inphaseand quadrature-phase components as given by equation (64). However

as soon as we allow

den to have finite thickness

a conductive

h the horizontal

overbur-

substantially reducing the amplitude of the electric

electric

field from the value that would exist in the absence of

field at the base of the overburden starts to drop rapidly as S(= • h) increases,as implied by equation (163). This equation also showsthat for a thin conductive overburden the surface electric field (which for electrically thin overburden is also essentially the electric field at the top of the basement) must rapidly becomeentirely inphase.These featuresare confirmed

by the complete calculation of Ex(h) using equation (151) with z = h, and shown in Figure 30. The figure illustrates how Ex(h) falls immediately as h/g• increases from zero, rapidly becoming essentially inphaseat smallvaluesof h/g•. Furthermore,this essentially inphase behavior is maintained to quite large valuesof h/g•, about 0.8. If then, as previouslystated, the electric field in the basement is primarily what excites basement conductors the presence of a conductive overburden will tend to make this component essentially inphase with the surface magnetic field.

the overburden. The fact that the horizontal

electric

morerapidlythane-h/•' is explained by reflection at the interfaces. When we assume there is an upgoing and a downgoingwave in medium 1, we glossover the fact that both of these waves are also made up of many waves, a result of multiple reflections. When the incoming wave from the air strikes the surface, much of the wave is reflected upward, a small fraction continues vertically downward into medium 1. When this wave strikes the interface at h, much of the wave is reflected back into medium 1, and a small fraction

continues on into the basement. Of the wave portion reflected upward, much is again reflected back down

lO 0

1

I

I

0.0

0.1

0.2

0.3

0.4

0.5

I (7:)I

_h__ •1 =o.o•

E•x(h/(•) / Eox(O) HHS -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

field at the base

of the overburden of thickness h initially drops much

0.6

0.7

0.8

0.9

1.0

IElx(h)I

Eox(O) 1(•1

Elx(h)

0

.4

.8

1•12

1.6

2.0

Fig. 30. Normalized horizontal electric field at base of

Fig. 31. Modulus of normalized horizontal electric field at

conducting overburden (cr2/crl = 0.01) as a function of

baseof conductingoverburden(•r2/•r1 = 0.01) as a function

overburden

of overburden

thickness.

thickness.


560

McNeill

and Labson

from the surface, and impingeson the interface at z = h, with a small fraction again continuing into the basement. Now, if the overburden is electrically thin, the waves traveling up and down do not suffer much attenuation per trip. The total wave that emergesinto the basement is thus made up of the sum of waves that have all encountered phase shift and attenuation as they were reflected and passed several times through medium 1. The phasor sum of these valuesis less than the original downgoingwave as a result of destructive interference

and this interference

is what causes the

electric field at z = h to decrease so rapidly with increasingh. Eventually, as h --• • the attenuationper trip increases so much that the wave that finally emergesinto the substrateconsistsof a "one-tripper"

only.Thereafter theamplitude decays ase-h/a,.When we make the casual assumption that the field in the

substrate willbedetermined bye-h/a,forsmallvalues of h/• we commit the error of ignoringthe multiple reflections

at the two interfaces.

PLANE

WAVE

RESPONSE

DIMENSIONAL

FROM

TWO

TARGETS

E and H Polarization

The first 2-D model we examine is the contact. This

model, which incorporates many features of the response from a variety of different targets and is fun-

damental to our understanding of VLF plane wave response,is discussedin detail. In our discussionof 2-D targets at VLF frequencies, we adopt the nomenclaturecommonlyused in magnetotelluric studies; we call E-polarization the geometry where the primary plane wave is propagating in a direction parallel to the strike of the contact or target (and thus the horizontal electric field in the ground is also parallel to strike). The geometry where the plane wave propagatesin a direction perpendicularto strike (so that the magnetic field is parallel to strike) will be called H-polarization. Plane waves propagatingat an arbitrary direction with respect to strike can be decomposedinto E and H polarization components.We see that the response to these components is very different, and the result, when the components are added vectorially, can be quite complex. For this reason choosinga VLF transmitter oriented (depending on the survey target type) either in the direction of geologicalstrike or perpendicular to strike is advisable. The number of transmitters producing useful signalstrengthat a given survey site is usually small so this option is not always possible;the resultingcomplexity must be kept in mind. In this and subsequentsectionssubscript 1 refers to field componentsmeasuredover medium 1, etc. Vertical

Contact--H

Polarization

Consider a plane wave propagating in a direction perpendicularto contract strike, as illustrated in Figure 33, where the transmitter is located at x = -o•. The

material at x < 0 has resistivity p• and that at x > 0 resistivity P2. Throughout we assumethat the magnetic permeabilityof all materialsis equal to ix0, the value for free space. In the previous section we noted that, for 1-D geometry, the responsefrom a layered earth could not be distinguishedfrom a homogeneoushalf spaceif the upper layer was electrically thick, specifically,greater than about 1V2skin depths. Obviously, the disturbance in the electromagnetic field componentsproduced by

Remote

Elz

ransmitter • i

01

02 (> 01)

Fig. 32. Modulus of normalized horizontal electric field at base of conductingoverburden as a function of overburden

thickness,h/•, and conductivitycontrast,•r2/•r•.

Fig. 33. Vertical contact - H polarization.



any type of resistivity inhomogeneity will effectively disappearat a distance of a few skin depths from the inhomogeneity. Thus, for the present examples we expect to find that for x/S1 < -2 the field components are thosenormal for medium 1 and for x/8: > 2 those normal for medium 2. We assumethat Pl < P: and thus 81 < 82. Let us review our knowledge of the horizontal

magnetic andelectricfieldcomponents, Hy andEx, on either side of the contact and at a large electrical distancefrom the contact. At x/• < -2 the normalized current density generatedby E•x is given by

Hly0

=

Hly(0 ) : 0-1•, 0-1 /

e -ik• z

1

(1+i)•-•e-z/ale --iZ/•l

(168)

and existsto a depth of the order of g•. For x/g2 > 2, the current density generatedby E2x is given by

(Z)=0-2 E2x ()Z)=0-2(iixto •J2x H2y(0 • 11/2 e-ik2z

H2y(0)

1

=(1+i)•22 e-Z/a2e-iZ/a2 (169)

X/gl = -2) we realize that the surfacemagneticfield at any locationis also equal to that at X/•l = -2. We are left with the important result that the magneticfield on the surface shows no variation

as we traverse

primary magneticfield. Conversely,sincefor x < 2gl,

Ex is givenby Elx = HlyZ1 andfor x > 292,E2x= H2yZ2,and Hly = H2y the horizontalelectricfield does vary acrossthe contact; measurement of this field component allows us to detect the contact.

Before examiningthe variation of Ex near the contact let us consider the parallel plate capacitor shown in Figure 34a. The separationd between the end plates is much smaller than their area A, so fringing can be ignored and one-dimensional(l-D) geometry results. The battery causes transfer of electric charges between the capacitor plates. These charges, in accord with Coulomb's Law, generate an electric field between the plates and the electric field in turn causes current flow, given at each point by J = erE. Consider first the case where the material within the capacitor has uniform conductivity cr or resistivity •. The total resistancebetweenthe platesis given by R = pd/A and thus the current flow, given by Ohm's Law, is V

0-AV

I = •- •--.

I

(170)

Applying Ampere's Law as in the previous section at x= -281 andx= +282 showsthat

0-V

Sx=• = d

(173)

and since

Jx = 0-Ex

Hly at• < -2

( x)

(172)

For the 1-D geometry of Figure 34a the current density J cannot be a function of y or z and must have an x component only, given by

Jlx(Z) dz at•-•<-2

= H2y at•-2> 2 .

over the

contact; measurements made of the magnetic field componentswill not detect a vertical or dipping contact (or, for that matter, dike) oriented parallel to the

and exists to a greater depth •2. Continuity of current requires, however, that

= J2x(Z) dz at• > 2 .

561

(174)

V

Ex=j.

(171)

The total magnetic fields are equal and since the primary field is the same at both locations so too is the secondary magnetic field. Within the vicinity of the contact the current must be deflected downward in passingfrom medium 1 to medium 2. However, at any location x, continuity of current requires that the total horizontal current flow

must equal that at X/gl = -2 (or at x/g2 = + 2). When we apply Ampere's Law to the x component of the current at any value of x (as we did previously at

(175)

Since continuity of current requires that Jx is not a functionof x, and cris constanteverywhere, Ex is also not a function

of x. Now let the material

in the left half

of the capacitorhave resistivity p• and that in the right half 92, where P2 > P•. Using the same argumentsas above Jx is still not a function of y or z and again continuityof current requiresthat Jx is not a function of x. In this case, however, p is a function of x. The result must be that

Jx = 0-1Elx = 0-2E2x

(176)

562

McNeill

and Labson

Application of Gauss' Law at the interface leads to

or

Elx

0'2

D1


E2x 0'1 P2

(177)

The electric field now changesdiscontinuouslyacross

e0(E2x - E•x) = S

(182)

where S is the induced surface charge density in coulomb per meter squared and thus

the interface so as to maintain continuous current flow. But for static conditions the source of all electric

Vp2 --Pl --

fields is electric chargesand we concludethat charges

(183)

must be induced at the interface in order to account for

S= 2e0d P2+ Pl

the discontinuityin E x. The inducedchargedensityis easily calculated knowing that the resistancebetween the plates is now

whencewe seethat S -7:0if pl -7:92. The electricfield and induced chargesare illustrated schematicallyin

d

132) 2A

(178)

Figure 34b.

This simpleexample illustratesthe generallaw that whenever the primary electric field has a component perpendicular to an interface between two materials of different resistivity, charges are induced at the inter-

and therefore

face so as to alter the total electric field in each of the

2 VA

V

(Pl + p2)d'

(179)

2V

(Pl + p2)d

= 0'•E•x = 0'2E2x, (180)

two media to maintain continuity of current acrossthe interface.

interface.

and, therefore, 2V

E1 x --

We encounter

this behavior

later in our

study of VLF. To return to our vertical contact of Figure 33 we realize that a sheet of induced chargeswill form at the In addition

there will also be a sheet of

chargesinducedon the surfacein the vicinity of the contact. Such chargesare presentonly at an interface between materials of different resistivities, such as at the contact and at the surface, and do not exist

elsewhere.With time-varyingfields,at any point in the

and

earth the total electric field is the vector sum from two 2V

E2x --

Pltd'

sources.The first sourceis the time-varying primary magneticfield, which producesEx, and the second source is the concentrationof induced chargesjust referred to and illustrated schematicallyin Figure 35

II

I

34 (a)

34 (b)

'

- x

Fig. 34. Parallelplate capacitoranalogy(a) for the verticalcontact- H polarization,and (b) Electricfield and induced charge density.



(the associated negative charges are located at great distance). It is clear that the induced charges on the surface will greatly effect the horizontal electric field measuredat the surface. Specifically, to the left of the interface

the total electric

field will be reduced

since

the field from the induced charges will oppose the primary horizontal electric field (which is directed to the right), whereas to the right of the contact the total field will be enhanced.

D'Erceville and Kunetz (1962) have calculated the horizontal

electric

field for a vertical

contact

563

excursions in IEx/Hylshownherewill undoubtedly be muted,yet thepositionof thediscontinuity in IEj/Hyl will give accurate location for the contact. Several important points about the responseshould be noted. The peak-to-peak responseat the contact is determined entirely by the resistivity contrast and is independent of the actual resistivities themselves. A 10 1•. m/100 l•.m contrastgives the samepeak-to-peak anomaly as 1000 l•.m/10 000 l•.m contrast. Conversely, the rate at which the response decays with

with H

polarization. The modulusIEx/HyliSshownin Figure 36a and the phase angle in Figure 36b. In these figures P2

P2 is varied to give the ratios indicated, and the horizontal distance is normalized with respect to •.

The curvesof IEx/Hylfor x < 0 showthat,at x/•l = -1, even for large resistivitycontrastIEj/Hyl has essentially achieved the value to be expected at great

distance fromthecontact,viz (pl/P2)1/2,since

•11 PHASE

_1.2

10

•1.0

:1•O. 8

-- _.__

_ •

0.6

Hy//[Hy/ = •2

•

'

(84)

:_

½b)

-o.s

For x/•i = 1, x/•2 is still < 1 so the curveshave not yet reached unity. The phase angle, which shows relatively small variation over the contact, requires somewhat larger values of x/•i before reachingthe undisturbed value of 0.79 radians (=45ø).

o

•45

o

x

o.s

1.0

6

4

The generalbehaviorof IEx/Hylis as we would expect from our simple capacitor model with the induced chargesillustrated schematicallyin Figure 35. D'Erceville and Kunetz also give a table of numerical

•"

valuesof IEx/Hylfromwhichthe maximumandmin-

•'• •

-0.31

'•'•

-o.14

imum values have been indicated on Figure 36a; we

seethat IE2x/Hylmax/IElx/Hylmi n -• p2/Pl,exactlyas

.6

for our capacitor [equation (177)]. In the limit of zero frequency the skin depth on either side of the contact becomes infinitely large, and this ratio would hold for

4

-0.031

all values of x.

In summary, for the case of H polarization, the vertical contact will be recognizedonly if the horizontal electric

field

is measured.

In the real world

2 -0.017

the 0.01

contact will not be perfectly sharp and the large

+

+

+++

+

+

x

,

-1.0

-0.5

0

0.5

1.0

•1

+

+ +

+',,•

Pl Current

Flow

P2

(a)

+

+

Fig. 36. Variation of amplitude (a) and phase (b) of horizonFig. 35. Induced charge density and current flow at contact.

tal electric field near a vertical contact (after d'Erceville and Kunetz, 1962).


564

McNeill

and Labson

distance away from the contact is influenced both by the resistivity contrast and by the actual resistivities, being essentially determined by the skin depths. The distance from the contact at which the anomaly disappears is skin depth controlled so that in conductive ground the anomalies decrease very rapidly with lateral distance whereas in resistive ground the anomalies will still be evident at large distances. This feature can be confusingwhen carrying out a survey interpretation since there is a natural tendency to associate spatially localized anomalies with shallow sources. This approach is correct when exploring for electrically conductive targets using closely spacedtransmitter/receiver systemsin resistiveterrain, for in this case the skin depth in the ground is everywhere (except in the target itself) much greater than the dimensionsof the exploration system. For plane wave systems operating at VLF frequencies the responseis a function of resistivity contrast, skin depth, and geometrical factors. Thus we must be prepared to alter our interpretation techniques which are based on "shortspread" electromagnetic systems. D'Erceville and Kunetz (1962) also give the theoretical anomaly when the contact does not extend downward to z = o•, but rather is underlain by a perfectly resistive or conducting basement at a depth comparable to a skin depth. The results are as would be expected in that the general behavior is similar to that shown in Figure 36 except that far from the contact the

phaseandamplitude ofEx/Hybecome theappropriate values for the relevant two-layered earth geometries. Vertical Contact--E Polarization, Tilt Angle and Ellipticity

We now consider the case where a plane wave propagateswith horizontal electric field parallel to the strike of the contact, as shown in Figure 37. Note that we have rotated both the coordinate system and direction of propagation from that used earlier; this rotation is in keeping with the coordinate system employed in Madden and Vozoff (1971). Many years ago these authors privately published a very complete set of calculations of the electric and magnetic field

anomalies produced by 2-D inhomogeneities; they have kindly allowed us to use some of their results in this article. Although of moderate accuracy (of the order of 10 percent) their calculations cover many examples of practical interest and are useful both for understandingthe VLF responseof 2-D targets and for survey interpretation. In addition to the behavior of the electric and magnetic field components, Madden and Vozoff

also calculate

field.

Since our aim is to unravel

other derived

field

the behavior

of the

anomalous fields we generally plot the components directly; the reader can refer to the Madden and Vozoff (1971) data for calculations of the derived ratios for the chosen

instrument.

Note

that

since Madden

and Vozoff (1971) employ the time variation of exp (-itot) the sign of their quadrature phase tabulated data should be reversed in every case. Their plotted data has had the sign reversed and is directly comparable with the results presented herein. To return to our contact, for the case of E polarization

the

horizontal

electric

field

does

not

have

a

component perpendicular to the contact so there are no induced charges. Furthermore, the horizontal electric field must change continuously across the interface. The surface horizontal magnetic field will vary near the contact but at large electrical distance from the contact the field will have the same value Hox on either side of the contact since, as we have seen earlier, the strength of the horizontal magnetic field over a homogeneous halfspace is unaffected by the resistivity. Since the majority of VLF measurements are made on the magnetic field componentsthe case of E polarization is particularly important. In order to explain the behavior of the magneticfield we must examine the behavior of the current density near the contact since the secondary magnetic field is directly related to current flow through the Biot-Savart Law. The current flow is, of course, controlled by the horizontal electric field. Several skin depths to the left of the contact

Fig. 37. Vertical contact - E polarization.

various

ratios commonly incorporated into commercially available VLF instruments (i.e., tilt angle and ellipticity of the polarization ellipse, modulus of the horizontal magnetic field, etc.). The behavior of the more useful of these ratios is quite similar to that of the correspondingfield components;for example, Appendix 1 shows that, for anomalies of moderate size, the tilt angle of the polarization ellipse is essentially the same as the inphase component of the vertical magnetic field, and the ellipticity is the same as the quadrature phase component of the vertical magnetic

the modulus

of the horizontal

electric

field at and beneath the surfaceis given by (since the z axis is now positive upward)


IE•y l - H•x


as shown schematically in Figure 38. A plot of the

oo•1•) 1/2

) 1/2eZ/a, e z/a,= Hlx(OOlxpl

= Hox(oolxp•)•/2eZ/a'

565

variationof thesurfacevalueof IEy/Hox I/•0 acrossthe interface is shown in Figure 39 for a 10/1 resistivity contrast. To the left of the contact where p• = 10 000 11.m (• = 353 m at 20 kHz) the electric field varies slowly with distance compared to the right of the contact, where P2 = 1000l•.m and g2 = 118 m. Now the current density is given everywhere by J =crE so that for x/8 ] < -2

(185)

so that if p• is large,8• is large,Ely is largeat the surface and decreases slowly with z. Several skin depths to the right side of the contact 1/2

IJlyl: CrlEly= Hox(tOlXCrl)l/2eZ/al (187)

ez/• = H2x(oolxp2) •/2eZ/•

and for x/82 > 2

= Hox(oolxp2) 1/2eZ/a2.

(186)

IJ2yl= cr2E2y = Hox(tOlxcr2)l/2eZ/a2. (188) Although the horizontal electric field varies continuously acrossthe interface the horizontal current density J =crE definitely does not, as is shown schematically in Figure 40. The surface current density across the contact is plotted in Figure 41; the factor of ten

If we assumeP2 < Pl the electric field to the right of the contact at the surface is small and decays rapidly with depth. As we progressfrom X/gl <• 0 to x/g2 >>0 the surface electric field decreases monotonically and the electric field also penetrates to shallower depths,

Fig. 38. Variationof modulus of the electricfield,Ey, shownby arrowsacrossa verticalcontact. .11

-

,øI =I0 000 s2-m

• =100011-m

•:,= 118m

.10

.O9 .O8

.o7

.05 .O4

.03 -1200

I -1000

I

I

-800

-600

I • -400

I -200

I 0

200

400

I 600

I 800

I

,

1000

1200

METERS

Fig. 39. Variation of horizontalsurfaceelectricfield acrossa contact(after Madden and Vozoff, 1971). Note that Hox is the value of the undisturbedsurfacemagneticfield far from the contact.


566

McNeill

and Labson

magneticfield, while the reduction in current density

contrast in resistivity causes the current density to change discontinuouslyby a factor of ten across the interface. Thus as we approach the contact from the low conductivity (high resistivity) side the decreasing electric field initially causes a proportionate decrease in current density. Immediately on passingthroughthe contact the current density increases directly by the conductivity ratio, but continues to decrease with further

distance

as the

electric

field

continues

on the left side will cause a reduction.

will, however, not completely follow the curve of Figure 41 since,althoughthe surfacecurrent densityin

medium 1 hasbeenreduced by (•r•/•r 2)•/2thecurrent density exists further downward by a factor of

(•r2/•r•) •/2duetotheincreased skindepthinthemore resistive material, as shown in Figure 40. To repeat an earlier statement, away from the contact over the more resistive ground the surface current density is small but the effective depth of the currents is large, so when we calculate the surface magnetic field using Ampere's Law we find that the total integrated current is the same on both sides of the contact; the increased depth correctly compensatesfor the reduced current density at the surface, and the horizontal magnetic field is the sameat large distanceson either side of the

to

decrease. Therefore, in the region adjacent to the contact on the high conductivity side the current density is considerably enhanced, whereas on the low conductivity side the density is significantlyreduced. Far from the contact the ratio of the current density on either side is related to the square root of the conductivity ratio, as indicated by equations (187) and (188). We next discussthe responseof the magnetic field. Obviously the enhanced current density on the right

contact.

side of the contact will cause an increase in the local

t ',

•t II

II II

h I]

t I i I

;

II

•l

!

Ii

II II I]•

]

II

ii

I'

The

behavior

of the normalized

horizontal

component of the magnetic field at the surface is

I

I I

II

This reduction

I

I

Fig. 40. Variationof modulusof the currentdensityJy, shownby arrows,acrossa contact.

P• =I0 000 r•n

P• =I000 tz-m

(•1 = 353m

(•Z= 118m

x

ration slow

o -1200

I

I

I

-1000

-800

-600

variation

I, -400

• -200

0

I 200

I 400

I 600

I 800

I

I

1000

1200

METERS

Fig. 41. Variation of horizontal surface current density acrossa contact with parameters same as Figure 39 (after Madden and Vozoff, 1971).


shown in Figure 42a. At large distanceson either side of the contact this quantity is unity. Near the contact on the conductive

side we observe

a substantial

rather differently with depth we also expect them to vary differently as we pass by the contact. Such differencesshouldbe reflected in the I and Q (inphase and quadrature phase) magnetic field components. That such is the case is illustrated in Figure 44a and b. In this figure the phase reference for the field components is the phase of the surface magnetic field at large distances from the contact (the reason for this distinc-

en-


hancement and on the resistive side a reduction,

corresponding to the enhanced and reduced current density. The anomaly extends to a distance somewhat greater than a skin depth on either side of the contact; the latter is located close to the point of inflection of the response. In Figure 42b the behavior of the vertical magnetic field exhibits a sharp peak directly over the contact and a relatively rapid decay on either side (falling more slowly on the more resistive side, in accord with skin depth considerations). The cause of the peak is that the depth to the average current flow decreasesas we progressfrom left to right over the contact, tilting both the subsurface and surface magnetic field as shown schematically in Figure 43. Now Figures 42 and 43 show the modulus of the magnetic field components. Many VLF instruments measure the inphase and quadrature phase components of the local magnetic field. Through the BiotSavart Law these components are related to the inphaseand quadraturephasecurrentdensitycomponents in the earth, and since these two current density components have previously been shown to vary

tion will be shown). We see that the behavior of the

inphasecomponentof H z and Hx is as expected,but that the quadrature phase components are reversed in sign and both have a perturbation near the contact.

In order to understand

hz

this behavior

we must recall

the behavior of both the inphase and quadrature phase componentsof current density as a function of depth. Figure 21 in the previous sectionshowsthe variation of both of these current componentswith depth for a homogeneoushalf-space.From Ampere's Law, knowing that the horizontal magnetic field at the surface is entirely inphase, we deduced that the net area under the quadrature phase current density plot of Figure 21 must be zero. Near the surface, quadrature phase currents flow in the positive y direction (using our current coordinate system), but at a depth of 0.8• the quadrature current density falls to zero and then

/o•=10000 ct-m

0.5

567

,øz= 1000•2.m

--

0.3 (b)

Hox 0.20.1

-

0.0

r

1.4

1.3 1.2

Hx

Hox

I;I

-

1.0 l

0.9

0.8 t

0.7

(a)

I

I

-1200

-I000

I

I

II

I

-800

-600

-400

-200

I 0

I

I

I

I

•

200

400

600

800

I000

1200

METERS

Fig. 42. (a) Variation of modulusof horizontalmagneticfield and (b) variationof modulusof vertical magneticfield across a contact (after Madden and Vozoff, 1971).


568

McNeill

reverses direction, maintaining this new direction until it eventually effectively falls to zero at great depth. The quadrature phase magnetic field at the surface causedby the quadraturephasecurrentsflowingin the +y direction is exactly cancelled out by that from the quadraturephase currents flowing in the -y direction. On the other hand the inphase current density changes sign only at great depth (2.3•) and can be effectively thought of as simply monotonically decreasingwith depth. Indeed as a very rough approximation we can consider the inphase density as being essentially uniform with depth to a depth of about a skin depth and of small value thereafter. This simple concept correctly predicts the overall behavior of the inphasemagneticfield componentsas shownin Figure 43 and is generally quite useful. The picture correspondingto Figure 43 but for the quadrature phase current density component might look like Figure 45, which suggeststhat the balance which produces zero net quadrature phase magnetic field away from the contact becomes disturbed in the vicinity of the contact. Apparently the result of the unbalance is that the vector quadrature phase magnetic field at the surface is dominated by the deeper quadrature phase currents, causing the reversed sign

for both H x and H z. The varyingnatureof the imbalance near the contact also causes the rapid perturbation in both components of the quadrature phase magnetic field near the contact. This strangebehavior appears on all calculationsof the magnetic field components and is not a reflection of the accuracy of those of Madden and Vozoff (1971). We can, therefore, surmise that without a detailed picture of the current flow in the earth in the vicinity of conductivity inhomogeneities to predict the behavior of the quadrature phase magneticfield componentswill be difficult, since they arise from the difference of two large and approximately equal current sourcesin opposite directions. We surmisefurther that the behavior of the quadrature phase magnetic field componentswill generally be far more complex than the inphase components. Most airborne and ground VLF systems measure the inphase and quadraturephase componentsof the vertical magneticfield, usingthe local horizontal magnetic field as the phase reference. Since the local horizontal field near the contact is phase shifted by a small amount with respectto the samefield component at large distance from the contact (as implied by the quadrature phase component in Figure 44a) some

differencecan be expectedwhen measuringH z with respect to the local reference. Figure 46 shows the

componentsof Hz/H x measuredwith respectto the local reference; comparison with Figure 44b indicates that for this model the difference is not large. Finally, at large distance from the contact the mag-

and Labson

netic field at the surface is, of course, linearly polarized. As we approach the contact the increasingvertical magnetic field, with its inphase and quadrature phasescomponents,resultsin an elliptically polarized magnetic field, as illustrated in Figure 47a and b and discussedin greater detail in Appendix 1. Some commercial VLF

receivers measure 0, the

angle of the semi-axisa with respect to the horizontal, and e, the ellipticity of the polarization ellipse, given by b/a. In the event that the vertical magnetic field is relatively small compared with the horizontal magnetic field, Appendix 1 shows that tan 0 is essentially

the same as Hz(I) (the inphasecomponentof the normalizedmagneticfield) and e the same as Hz(Q) (the quadrature phase component). For our vertical contact the variation of tan 0 and e is shown in Figure 48. Comparison with Figure 46 shows that they are closely similar. Note that in all of the examples given, the anomalies in the tilt angle are those which would be obtained with the operator facing in the +x direction with positive 0 in the direction shown in Figure 47. This convention

is discussed further in a later section.

If we now allow P2 to become 100 l•.m we would expect a larger anomaly due to the increased resistivity contrast of 100/1 and also expect that the anomaly will decay much more rapidly with distance on the low resistivity side of the contact, and this is indeed what happens, as shown in Figure 49. We note from Figure 49 that in spite of the increase to a 100/1 resistivity contrastthe peak value of the anomalieshas increased by only a factor of approximately two over the 10/1

contrast.The reasonsfor the small increasein Hz(I) are indicated by the current flow shown schematically in Figure 43. Fixing our attention on the left sideof the contact we assume that the bulk of the inphase currents, located above a depth of approximately one skin depth, start to become shallower at a lateral distance of about one skin depth to the left of the contact. On

the right sideof the contact,if we assumethat P2 the currents are already close to the surface near the contact (•2 m •l). Thus the maximum angle of the resultant magnetic field is of the order of 45 degrees

andwe expectHz(I) • H x, as shownin Figure49b. Clearly the most notable feature of the new response is the relative sharpnesson the low resistivity side of the contact, which the inexperienced geophysicist might easily confuse with the response from a structure at shallower depth. A feature of the horizontal magnetic field plots shown in Figures 42a, 44a, and 49a should be commented on. There is still a significant signal at x = 1000rn on all these plots. Earlier we remarked that the accuracy of the calculations was estimated to be of the order of 10 percent and this feature is a result of computational error; however, we have retained the


569


JHzl

ß

ß

ß

Direction of total magnetic field

CurrentflowInto page

Fig. 43. Magnitude of the vertical magnetic field and direction of the tilted total magnetic field across a contact.

0.5

•

0.4

0.3

Hz

(b)

Hox 0.2

0.1

0.0

QI

0.$

1.3

(a)

0.:;' I.:;'

Hx 0.1 Iol

Hox

o.o

I.O

-o. I 0.9

, -12:00

I , -I000

I

I

II

-800

-600

-400

I -ZOO

I 0

2:00

400

I 600

i, 800

I I000

,, 1200

METERS

Fig. 44. (a) Variation of horizontalmagneticfield (I and Q) and (b) variationof the verticalmagneticfield (I and Q) across a contact (remote reference) (after Madden and Vozoff, 1971).

570

McNeill

results for consistency until more accurate calculations are available.


Dipping Contact

The behavior of the magnetic field componentsfor a contact dipping at 45 degrees in either direction are shown in Figures 50 and 51 for a 10/1 resistivity contrast. These figures show that this degree of dip producesa relatively small effect as would be expected from galvanic current flow. Note that the data for Figure 51 has been plotted in the reversed direction from that shown in Madden and Vozoff (1971), with the result that the erroneously high Hx(I) that normally occurs at + 1200 rn now occurs at -1200 m. Vertical

We

Contact

now

With

Conductive

consider

the

case

Overburden

of a vertical

and Labson

the ratio of the inphase/quadraturephase responses has been altered, and (3) the small fluctuations that occurred in the quadrature phase response near the contact have disappeared. The reason for the reduction in the amplitude of the response is, of course, that the conductive overburden, although only 0.3 skin depths thick (• = 35 m at 100 l•.m), has substantially reduced the horizontal electric field in the basement material; much of the current flow is now in the overburden itself and is,

therefore, unaffected by the contact. Also the skin depth in the overburden evidently is controlling the rate of decay of the anomalies laterally away from the contact. Had there been no overburden we might have been able to estimate p• and P2 by measuringthe lateral rate of decay away from the anomaly, however

contact

(10 000 f•.m/1000 f•.m) covered with a thin layer of conductive overburden, specifically 10 rn of 100 f•. rn material. The results which are shown in Figure 52 should be compared with Figure 44. Three points are evident; (1) although the shape of the responsesis not greatly altered the amplitude of the responseshave all been substantially reduced and they occur over a much smaller range of distance from the contact, (2)

(a)

Currents into page

Currents out of page

Plane ofzero current ////•o density ©

O

T

Pl(>P2)

©

P,(.P•)

P•

(b)

Fig. 45. Schematic of the quadrature phase current distribution and resulting magnetic field.

Fig. 47. (a) Tilt angle, 0, and ellipticity, e = b/a, of the polarization ellipse and (b) variation of the magnetic field polarization ellipse across a contact for E-polarization.

•1 =I0000 11-m

0.5

P:,--I000 lz-m

0.4 0.3

Hz

0.2 0.1

I

0.0 -0.1

-0.2

• -1200

I -I000

I -800

I -600

I -400

I -200

0

I 200

I 400

I 600

I 800

I I000

'•x 1200

METERS

Fig. 46. Variation of vertical magneticfield (I and Q) acrossa contact (local reference) (after Madden and Vozoff, 1971).

GeologicalMapping Using VLF Radio Fields in the presence of overburden such a measurement would also reflect the resistivity and thicknessof the

peak responsesare shownin Figure 53, from which we see that the presence of the overburden has reduced the horizontalmagneticfield amplitudeto 29 percent andthe verticalmagneticfield amplitudeto 17percent


overburden.

It is interestingto comparethe behaviorof the phase and amplitude of the contact responsewith and without overburden. The phasors correspondingto the

o.s

571

of the values without overburden. These are substan-

tial reductions,especiallywhen comparedwith exp

ooo

,ooo

0.4

0.3

tan •

•

0.2

0.1

0.0 I I

I

-0.1

-0.2

I

-1200

-I000

-800

-600

-400

-200

0

200

400

I

i

600

800

I IOOO

I•x 12oo

METERS

Fig. 48. Variation of tilt angle and ellipticity acrossa contact. I,I

-

I.O 0.9 0.8 0.7 0.6 0.5

Hz

0.4

Hox

0.3

(b)

0.2 0.1 0.0 -0.1

-0.2

-0.;5 i

-0.4

0.9

I

i

i

1.9

0.8

1.8

0.7

1.7

0.6

1.6

0.5 _1.5 0.4

1.4

Hx 0.3 Hox 0.2

1.2

1.3

0.1

I,I

o.o

I.O

(a)

-0,1 0.9-

- 0.2

0.8

-0.3 0,7 -0.4

0.6 -1200

-I000

-800

-600

-400

-200

0

200

400

600

800

I000

1200

METERS

Fig. 49. (a) Variationof horizontalmagneticfield(I and Q) and(b) variationof verticalmagneticfield(I and Q) across a contact (remote reference) (after Madden and Vozoff, 1971).


572

McNeill

and Labson

example 10 m of 100 •2. m over 10 000 •2. m produces an apparentresistivityof 460 f•.m and a phaseangleof 12.5 degreeswhereasthe sameoverburdenover a 1000 •2.m basementproduces 270 •2.m and 26.8 degrees, easily detectable changes in apparent resistivity and phase angle. The electric field componentscalculatedin Madden and Vozoff (1971) confirm that for both E and H polarization for this geometry the electric field anomaly is relatively closely confined (within 50 m) to the region near the contact. Thus, although the presence of the conductive overburden reduces the magnitude of the apparent resistivity anomaly it does help to locate more accurately the contact when electric field

(-2t/g) = exp (-0.6) or 55 percent. The phase retardationsare 43 degreesand 16 degrees,respectively,to be compared with 2#15= 0.6 radians or 34 degrees. We are not too surprisedto see disappearanceof the rapid fluctuations in the quadrature phase response since they arise from small changes in the balance between two relatively large and opposingfield components.

Finally, 10 m of 100 12.m is not a lot of overburden nor is the overburden very conductive; for example, the value of surface conductance

S = •rt is 0.1 S which

would be regarded as quite small. As is well known and demonstrated here, at VLF frequencies the presence of a small amount of relatively resistive overburden can produce a very serious reduction in the magnetic field anomalies generated by basement tar-

measurements

are used.

To summarize our discussionof the responsesto a contact, for the case of H polarization we can detect and accurately locate the contact by means of the electric field component only. For E polarization we can detect the contact by means of either the electric or magnetic field components but for this case the

gets.

The best way to search for contacts under conductive overburden is by means of surface impedance measurements, which still produce quite measurable anomalousresponsesfor the geometry indicated. For

0.5

0.4 =

.

=

ß

0.3

0.2

Hz

Hox

0.1

0.0

_

-0.1

-0.2

Q

\

\

i

I

I

i

I

/

//

! ø21 I

i

I

0.3

1.3

0.2

1.2

0.0

1.0

I

(a)

Hox

-0.1

0.9

-0.2 0.8 -1200

I -I000

-800

-600

, I -400

• -200

a 0

a 200

•

•

•

,

400

600

800

I000

,-x 1200

METERS

Fig. 50. (a) Variation of horizontal magneticfield (I and Q) and (b) variation of vertical magnetic field (I and Q) acrossa contact dipping to the right (after Madden and Vozoff, 1971).


GeologicalMappingUsingVLF RadioFields

573

Hz

Hox

-.1

Q

I

0.3 1.3 I

0.2

1.2

0.1

1.1

0.0

1.0

-

Hx

Hox

-0.1

0.9

I -1200

-1000

I

I

I•

-800

-600

-400

I

I

-200

mZI

0

200

I

I

I

I

I

400

600

800

1000

1200

•X

METERS

Fig.51.(a)Variation ofhorizontal magnetic field(I andQ)and(b)variation ofvertical magnetic field(I andQ)

across a dipping contactdipping to theleft (afterMaddenandVozoff,1971).

/,/%=I00s•-m t=lOm PI =i0 000 •-m

P2=I000 •.m

0.2

O.I

Hz

(b)

Hox 0.0

-O.I

Q 0.2

Hx o.o Hox -o.I

I 1.2 -

I.O

(a)

0.9

0.2 0.8

-1200

I

-I000

I

-800

I

-600

i C•l -200

I I

-400

0

•2200i

i

400

i

600

i

800

i

i000

I•x

1200

METERS

Fig.52.(a)Variation ofhorizontal magnetic field(I andQ)and(b)variation ofvertical magnetic field(I andQ)

acrossa verticalcontactwith overburden (afterMaddenandVozoff,1971).


574

McNeill

and Labson

magnetic field gives the more accurate location. The most significant disadvantage of the magnetic field measurement is that information about the resistivity contrast is rather muted, and information about the resistivities themselvesis implicit in the rate of decay

errors caused by variations in H will be small. We have further justification for the remarks made in the Surface Impedance section that the horizontal magnetic field componentwas usually relatively constant.

of the fields with distance from the contact, which, as

Embedded

we see, is strongly affected by the presence of thin, conductive overburden. Undoubtedly the best interpretation would be based on measurement of both electric and magnetic field components. However, electric measurements are time consuming to make, and are inclined to be geologically noisy since the horizontal electric field in the ground reflects variations in subsurfaceresistivity due to both small nearsurface features and larger structures. Finally, it should be emphasized that quite measurablemagnetic field anomalies take place at contacts where both materials are very resistive, as long as a reasonable conductivity contrast exists. This is unlike the response of the usual low-frequency, closely spaced geophysical exploration systems which generally do not respond to material of resistivity greater than 1000 As shown earlier, the peak responsein the magnetic field components generally was not strongly dependent on resistivity contrast, even for a perfectly abrupt vertical contact. In the real world abrupt vertical contacts seldom occur and the more usual case is, of course, a graded contact. If sucha contact occursover a distance comparable with a skin depth in the more conductive material then the magnetic field anomalies will be further reduced. Thus, over ground where the resistivity is a relatively slow varying function of position, we would expect the magnetic field components to be relatively inactive, which is precisely what is seen in practice. Conversely the horizontal electric field componentis, as we have seen, directly related to the square root of the terrain resistivity. Therefore, since the horizontal magnetic field is inactive, measurementmade of the surfaceimpedanceby measuring the ratio IE/HI should be reasonably accurate since

Prism

We now investigate the response from a large, square prism embedded in a homogeneoushalf-space so that the prisms top lies just at the surface, as shown in Figure 54. The horizontal dimensionsof the prism, • x •, are much larger than a skin depth in either medium, and the depth extent is greater than a skin depth in the prism material, so is essentially infinite. Although this is not a 2-D model we will concentrate on the fields at the mid points of the sides where the 2-D response is still reasonably valid. The schematic responses shown in Figure 55a are the in-phase components for a prism which is relatively more resistive than the surroundinghost rock; for example, a granite intrusive. For this target to be large in terms of a skin depth it would have to be at least 1000 x 1000 rn if the resistivity was 2000 l•.m (• = 160m) so that responsesfrom each side would not appreciably interfere with the others. Note that if measurements are made only of the magnetic field components, the two boundaries A and B will not be directly sensedand must be inferred from the disappearance of the response from the two boundaries C

and D, as indicatedin Figure 55b for Hz. Such a disappearanceis often seen in surveys and can give difficulty in interpretation, particularly if the response from only one of the boundaries is included in the survey area (i.e., the survey includes only the region enclosed in the dotted line).

We see that the polarity of Hz is reversedon each side of the anomaly. The reason for this reversal is evident from the current flow shown schematicallyin Figure 55c (compare with Figure 43). Now, if the prism has lower resistivity than the surroundingarea, the responseis schematicallyillus-

z

PERCENT

0

iz

I0

20

30

40

50

0H,•••••. .. •, , , (b• Hz 20

Fig. 53. Phasorresponseof H z andHx with (a) andwithout (b) conductive overburden [peak responsestaken from Figure 52 for (a) and 44 for (b)].

Fig. 54. Square prism embedded in a homogeneous half-space.


tratedin Figure55d. Sincethe low resistivitymaterial is on the inside, the target can be smaller before


interference from the various sides occurs. For exam-

ple 150m of p = 50 •.m (• = 25 m) would be sufficient. Furthermorethe targetneednot extendto greatdepths to be effectivelyof infinite depth extent. Considerfor example, the geometry shown in cross section in Figure 55e where the vertical contact between material of resistivitiesp• and P2 has been truncatedso that region 1 has finite depth d. From our earlier resultsas

longas d is greaterthan a skindepth• the material beneathwill haveno effecton the fieldcomponents at the surface and we would expect that the survey

575

results would be exactly the same as for an infinite vertical contact. The calculations of Madden and Vozoff (1971) confirm this fact. Thus the results shown

in Figure 55d would also be obtainedover a square slab with a depth extent of 40 m (=1.5•2 at P2 = 50 •. m) so this figure gives an indication of the responsethat might be obtained over very thick lake bottomsediments.The remarksmade in conjunction

with Figure 55b still apply, so if the lake is roughly circular we can expect to get responsesonly from thoseedgesof the lake sedimentswhich are approximately parallel to a line in the direction to the transmitter.

Ey Hz

Ey Hz

Survey Direction

Hx

x

x

x

Survey Direction

Hx

x

x

x

•y r

•y

•y

To Transmitter

•y

T•Transmitter Hxl

•y

•y

x

x

Ey

x

x

55 (a)

Hz

Hx

x

x

55 (d) Survey Direction

Hz Survey direction

--..

m

w

'--I

Survey Area

ß

ß

ß

o91ß

ß

ß

55 (e)

55 (b)

ß

ß

ß Current Into page

.o2 (>.ø1)

H

Survey Direction Pl

P2(>Pl)

Pl

Fig. 55. Schematicresponses for a prism.(a) Resistiveprism (planview). (b) Responseof H z measurement madeonly on

onesideof prism.(c) Response of H z over a wideresistive 55 (c)

structure. (d) Conductive prism (plan view). (e) Vertical contact of limited depth extent.

576


Thick

McNeill Conductive

and Resistive Vertical

and Labson

Dikes

Since the vast majority of surveys made at VLF frequencies use magnetic field components, with transmitterschosen so that the excitation is E-polarization, the remainder of the models considered in this article will all be subjectedto E-polarization. Before considering the case of a conductive dike in a homogeneoushalf-space it is important to consider the relative role of the primary electric and magnetic field componentsin determiningthe responseat VLF frequencies.In the early daysof VLF surveyinterpretation the assumptionwas made that the responseof a conductive dike could be approximatedby assuming the target was located in free spaceand subjectedto an essentiallyuniform horizontal magneticfield. Thus the eddy current response would be controlled by an

concentratedor gathered in the vicinity of the conductive target. We call the responsefrom these gathered currents the "galvanic current" component thereby making a distinction from the "vortex current" response;the galvaniccurrentsflow throughthe conductive target,ratherthan flowingin closedloop or vortex paths within the target. Such different current flow paths will naturally result in a significantdifferencein the secondarymagnetic field. For example, at great distance from the target with respect to its width and thickness(but not length)the secondarymagneticfield from the galvanic current component can be roughly approximated as that arising from a linear current element, whereas at the same distance the field from the vortex current

quadrature phaseresponse decreases to zeroasR -]

component appears to arise from a line of magnetic dipoles. The galvanic current magnetic field components tend to decrease linearly with increasingdistance from the target, whereas the vortex current componentsdecrease with the square (or cube in the case of a 3-D target) of the distance. Another major difference in the responsefrom the two typesof current flow lies in the sensitivityto target orientation with respect to the primary field components. For example, the vertical plate of Figure 56a lies in maximum couplingwith the primary magnetic

and the inphase component saturatesto the so-called

field so that vortex current flow will be a maximum.

inductive limit. However

the plate is rotated 90 degrees about its long axis (Figure 56b) the vortex current flow will virtually disappearif the plate is thin, whereas the galvanic current flow will not be greatly differentfor this second case. Thus if vortex current flow predominates,a flat lying thin plate should produce essentially no VLF anomaly. Such, however, is not the case; tank modeling experimentscarried out by McNeill show that the amplitudeof the magneticfield anomalyfrom the plate is relatively insensitiveto whether the plate is vertical

induction number that was a function of characteristic

dimensions of the target expressed in terms of the target skin depth •. For example, for a sphereof radius

a theinduction numberisR = cr•o0a 2 wherecris the sphereconductivity and o0the systemoperatingangu-

lar frequency. ThisquantityR is equalto 2a2/••. At values of R • 1 the responseis essentiallyquadrature phase and proportional to R; at values of R >> 1 the

difficulties arose when VLF

surveys were interpreted on this basis and the next

step was to include the effect of the surroundinghost rock on the response. It was recognizedthat if the target was buried at a depth which was a significant fraction of a skin depth in the host rock the primary inducingmagneticfield would be phaseretardedat the target and the secondary magnetic field arising from eddy currentflow in the targetwould be further phase retarded in the return journey back to the surface. While this argument is correct, we see that, in fact, eddy currentsplay a minor role in the responsefrom virtually all targets observed in VLF surveys. We must be prepared to revise our concept as to which parameterscontrol the responsein VLF surveys, and indeed in any surveysusingplane wave excitation. Consider Figure 56a in which a vertical conductive plate is located in a less conductivehalf-space.An incident plane wave propagatingparallel to the strike of the plate generatesa primary magneticfield H • and a primary electricfield Ee in the vicinity of the target. Conveniently, the magneticfield can be consideredas generatingvortex or closed-loopeddy currentsin the conductive target. Such current flow would be induced even if the target were located in free space.Furthermore if the groundhas finite conductivitythe primary electric field will cause current flow in the homogeneous half-space and this current flow will become

If

or horizontal.

Furthermore, approximate calculations carried out by McNeill using University of Toronto computer

/••Galvanlc current

flow (a)

(b)

Fig. 56. Conductive thin plate in a conductive half-space (E-polarization), (a) plate vertical, and (b) plate horizontal.



programs described in Dyck et al. (1981) and Cheesman and Edwards (1988) have shown that for the typical long strike length targets identified by VLF surveys the vortex current component is responsible for only a small fraction of the target responseunless the depth extent of the target is large and the host rock resistivity is extremely high, of the order of 25 00050 000 l/.m. Thus except in the most resistive environment the vortex current flow can be ignored and we can assumeall the measured responsearises from the galvanic current component. Such a statement becomes increasingly true as the frequency decreases, for example, to those used in audio-magnetotelluric and magnetotelluric studies. The reason for this is that, as the frequency decreases, the electric field •/2 which excites galvanic current flow decreasesas to as shown by equations (85, 86); the anomalous magnetic field from this component will also decrease as

to•/2.Forthevortexcurrent component, however, the anomalousmagnetic field will decreaselinearly with to for low values of induction number (R <• 1) as described. Thus the ratio of galvanic to vortex anomalous magnetic fields will increase with decreasing

frequency as to•/2/to=to-•/2' at low frequencies the galvanic current response will become even more dominant

than at VLF.

Unfortunately several papers have appeared in the literature in the last decade presenting results of calculations and tank modeling which have assumed that the model was in free space.Resultsbasedon this assumptionhave little validity in the real world nor do they agree with calculationsor experimentswhere the finite conductivity of the host rock has been taken into account. As seen, the effect of varying target dip particularly will be in error. We turn then, to the thick dike shown in Figure 57, armed with the knowledge that we need consideronly

577

the galvanic current responsewhich is, of course, that current component with which we have been dealing in all previous sections. Furthermore from our earlier results we deduce that, if the thickness of the dike is

greater than a skin depth in the dike material there will be negligible interaction between current flow at either side of the dike and superposition of the separate responsesfrom each contact will give the correct thick dike response. Such superposition(using the data of Figures 44a, b) was used to generate the responses shown in Figure 58a, b for a 400 m wide dike of 1000

f•.m material (t•/• = 3.5 at 20 kHz) imbedded in 10 000 f•. m material, and also the responsesof Figures 59 and 60, where the dike progressively thins to 200 and 100 m, respectively.In the last case t•/• = 0.9, leading us to question the assumption that linear superpositionwill still produce a reasonably accurate solution. However Figure 61, from the complete Madden and Vozoff (1971) calculations shows that, even for an electrically moderately thin dike, superposition still applies with small error. Observefrom the above responsesthat, whereasthe

inphasecomponentof H z andHx behavesreasonably well acrossthe dike, the quadrature phase component exhibits peculiar variations. As described earlier, these variations are caused by small imbalances in the quadrature phase currents flowing in opposite directions beneath the surface in the vicinity of the two contacts; such variations are seen as a conspicuous feature of the quadrature phase responsefrom almost any type of subsurfacestructure. In the real world the presence of inevitable localized subsurfaceinhomogeneities also upsetsthe balance in the resultant quadrature phase magnetic field causedby the two opposing currents, with the result that quadrature phase magnetic field components often do not resemble those predicted from theory using simple structuresand, in fact, in many cases the quadrature phase response is difficult or impossible to interpret in actual VLF survey data. With our understanding of the general appropriateness of superposition, the overall features of the responsefor a relatively resistivedike shownin Figure 62 are easily understood. We note the expected reversal in sign for all field components. Thin Vertical Dike; Variation of ResponseWith Conductance

We saw in the previous section that the response from a thick vertical dike (thick in terms of its own

Fig. 57. Thick dike (E-polarization).

skin-depth;such a dike is called an electrically thick dike) could be approximated by adding the responses of two vertical contacts. However, as the electrical thicknessof the dike decreasesto less than t l/81 -• 1

578

McNeill

and Labson

this approach must fail due to mutual interaction (via the magnetic field) of the current flow at each contact.

shear zone imbedded

Now when the electrical

Consider first the case shown in Figure 63 which showsa vertical dike of thickness 10 rn and resistivity 100 12.m imbedded in a 1000 12.m half-space. Thus, S• = rr•t• = 0.1S and at 20 kHz (the frequencyusedin all our examples) L = 0.88, a small value. In spite of the relatively high resistivity and small thickness of the dike a reasonablylarge inphaseanomaly (15 percent in


horizontal electric field Ex acrossthe thicknessof the dike. When this condition is met Olsson (1983) states the

electrical

behavior

of the

vertical

dike

half-

space.

thickness of the dike becomes

very small there is, by definition, little variation in the

that

in an even more resistive

is

entirely defined in terms of a simple induction number L which is given by the conductivity thicknessproduct of the dike divided by the intrinsic admittance (reciprocal of the intrinsic impedance) of the material in which the dike is embedded.Thus, taking the real part

Hz, 40percentinHy) results.Thereasonfor thisresult is easily understood. As stated, when the dike is electrically thin the horizontal electric field everywhere acrossthe dike thicknessat depth z must be the same value as that at depth z in the adjacent host medium, being continuousacross each interface. Furthermore, since the dike is thin and also does not exhibit a large conductivity contrast with respect to the half-space, the presence of the thin dike will not greatly alter the electric field distribution that would

of the intrinsic admittance, the induction number is given by ffltl

L=(2o. 231/2 (189) where subscript 1 refers to the dike and subscript2 to the surroundingmedium. The only constraint is, once again, that the dike be electrically thin, a condition often encountered during VLF surveys where the anomaly is often due to a relatively resistive fault or

exist in the absence of the dike. But since the conduc-

tivity of the dike is ten times higher than that of the half-space the current density in the dike, under the action of the essentially undisturbed horizontal elec-

rc /ø2_ =I0 000 a-m

0.5

400m

--

I000 ll-m

0.4

P2=IOooo 11-m

3.5

0.3 0.2 0.1

Hz

0.0

Hox

-0.1 -0.2_

(b)

-0.3 -0.4. I

-0.5

Q 0.5

Hx Hox

I

i

I 1.5-

0.4

1.4-

0.3

1.3 -

0.2

1.2-

0.1

I.I-

0.0

1.0

-0.1

0,9-

-0.2

0.8

-0,3

0.7

-

- I?__oo

I -IOOO

• -800

• -600

• -400

• -?_.00

I ••• 0

200

(a) 400

600

800

I000

I700

METERS

Fig. 58. (a) Variation of horizontal magneticfield (I and Q) and (b) variation of vertical magnetic field (I and Q) across a thick (400 m) dike calculated from superpositionof contact responses.


tric field, will also be ten times higher, producing a large magnetic field anomaly even for a relatively resistive target. This high responseto relatively resistive targetsis an extremely important feature of plane-


wave

=

excitation.

An even more surprising effect occurs when we increase the resistivity of the half-space, keeping the parametersof the dike constant. Figure 64 showsthe same dike, but now embedded in a 10 000 11-m half-

space; we observe that the peak anomaly has increasedby a factor of about three. The reasonfor this increase

is that the electric

579

field in the 10 000

half-space is X/-i-6 timesthatin the 100011-mhalfspace, and so then, approximately,is the electric field

in thedike,toproduce x/ridtimesthecurrent density

(rrltl).

(190)

The first term in this expressionis, to within a constant, the electric field in the half-space(normalized with respect to the primary magnetic field), and the secondterm is a measure of the longitudinalconductance of the dike. The product of electric field and conductancegovernsthe magnitudeof current flow in the target, and in turn, the secondarymagneticfield. This dependenceon L holds, at VLF frequencies,for half-spaceresistivitiesup to of the order of 25 000 whereupon the galvanic current componentstarts to decreasewith further increasein half-spaceresistivity (becomingzero in the limit of infinite half-spaceresistivity) and vortex current flow starts to dominate. Now let us increasethe conductivityof the 100 10 rn thick vertical dike while maintaining the halfspace at 10 000 11-m. As the dike conductivity in-

in the dike over the 1000 11-m half-spacecase. Thus the anomaly dependsquite strongly both on the conductance of the dike and on the resistivity of the half-space, as indicated by equation (189) for L. Indeed the reasonfor dependenceof the responseon L becomesmore evident when we rewrite the equation

creases, the horizontal electric field in the dike will

in the form

start to decrease slightly as shown schematically in

Q

Hx U0x

•

I

0.6

1.6 -

0.5

t.5-

0.4

1.4

0.3

1.3-

0.2

1.2

0.1

I.I

0.0 -o.I

-

-

1.0 0.9-

-0.2

0,8-

- 0.3

0.7 -IOO0

-1200

-800

i

I

-600

-400

-200

0

200

400

600

800

I000

t200

METEItS

.,-200m-.I

0.6 0.5 0.4

-

/:>a: I0000 ,9,.m P1:1000 •2:I0 000 •.m /'Z.m .8

0.3 0.2 0.1

Hz

Hox

0.0 -0.1 -0.2

(b) -0.3 -0.4 -0.5

Fig. 59. (a) Variation of horizontalmagneticfield (I and Q) and (b) variation of vertical magneticfield (I and Q) acrossa thick (200 m) dike calculatedfrom superpositionof contactresponses.

Q


Hx Hox

!

0.6

1.6

0.5

1.5

0.4

1.4

0.3

1.3

0.2

1.2

0.1

1.1

0.0

1.0

-0.1

0.9

-0.2

0.8

(a)

-1200

I

I

-1000

-800

-600

-400

-200

0

200

400

I

I

I

I

600

800

1000

12oo

i

i

METERS

'-N

I•-- lOOm

•Z=I0000 •z-m

•z =I0000 •-m

0.5

--I000 •-m

0.4

1

-•= 0.9

0.3 0.2

HZ

Q

0.1

Hox

0.0

-0.1

(b)

-0.2

-0.3

I

-0.4

I

I

i

i

i

i

i

i

Fig.60.(a)Variation of horizontal magnetic field(I andQ) and(b)variation of vertical magnetic field(I andQ) acrossa thick (lee m) dike calculatedfrom superpositionof contactresponses. Q .6

Hx

! 1.6-

.5

1.5-

.4

1.4-

.3 1.3-

Hox .2

1.2-

.1

1.1 -

.0

1.0

-.1

0.9

-.2

0.8

-1200

I

-1000

-800

-600

-400

-200

0

200

400

600

800

1000

1200

METERS

'-H 0.5

Pz=•0000 fi.m

'"

0.30.2

-

0.1

-

Hox

Ij• t---pz=,oooo •-m 1= 0.9 •1 =1ooo•.m

0.4-

HZ

I•'- lOOm

Q

.0

-0.1

-

-0.2

-

-0.3

-

-0.4

(b) I

I

I

i

i

i

i

I

I

I

I

Fig.61.(a) Variation of horizontal magnetic field(I andQ) and(b) variation of verticalmagnetic field(I andQ) acrossa thick (lee m) dike (after Madden and Vozoff, 1971).

Q


Hx Hox

I

0.2

1.2

0.1

1.1

0.0

1.0

-0.1

0.9

(a)

I -1200

-1000

I

I

I

I

-800

-600

-400

-200

0

I

I

200

400

I

I

600

800

I

I

1000

1200

I

I

•x

METERS

--*

_

I*-- lOOm

P•:Ioo0 a.m •L P•=Ioo0 a-m

--

Hz

Hox

-

(b)

I

-.2

I

i

i

i

I

I

I

Fig. 62. (a) Variation of horizontal magnetic field (I and {2) and (b) variation of vertical magnetic field (I and {2) across a resistive dike (after Madden and Vozoff, 1971).

Hx

Hox

Q

I

.4

1.4

.3

1.3

.2

1.2

.1

1.1

(a) .0

-.1

1.0

0.9 -1200

I -1000

-800

-600

-400

-200

0

200

I 400

I 600

m

i

800

•X

1000

1200

i

i

METERS

P• - iooo •-m

/a2: IOO0•z-m lO meters of 100•.-m (S1= 0.1S)

0.2

•1 = 35m L = 0.88

0.1

Hz

Hox

Q 0.0

! (b)

-0.1

-0.2

I

I

I

i

i

i

i

i

Fig. 63. (a) Variation of horizontal magnetic field (I and {2) and (b) variation of vertical magnetic field (I and {2) acrossa thin conductive(S] = 0.1 S) dike in a 1000•.m host (after Madden and Vozoff, 1971).


582

McNeill

and Labson

Figure 65a, but the decrease will not occur as fast as the conductivity increases,and thus the net result will be an approximately linear increase in dike current and thus of anomalous magnetic field response with dike conductivity. The actual geometry of the electric field distribution within the vertical dike/half-spacewill not change greatly with relatively small increasesof dike conductivity (with respect to the half-space) and thus the phase angle of the anomaly (for example the ratio of the inphase to quadrature phase component of the vertical magnetic field) will also not vary greatly with increasing dike conductivity at low conductivity con-

forward, we expect no major surprisesin the shapeof the inphase magnetic field profile but the careful balance in the quadrature phase currents that produced zero quadrature phase magnetic field at the surface for the half-spacewill be seriously altered by the conductive

vertical

dike.

Finally, as the dike conductivity contrast becomes very large with respect to the half-space, we expect the situation of Figure 65c to prevail, in which further increasesin conductivity produce no further reduction in horizontal electric field in the dike, and no further

effective reduction in the depth z of current flow within the target itself. Beyond this high conductivity contrast we expect the magnitude and phase angle of the response to be invariant to further increase in dike conductivity. At this high contrast, where the anomalous current will be closely confined to the surface rather than having appreciabledepth extent in the dike (Figure 65c rather than Figure 65b) we expect that the inphase component of the secondary magnetic field can be approximated as that arising from a line current sourcelocatedjust beneath the surface. Such is indeed the case as indicated in Figure 66 which, when compared with Figure 64, shows that for a 1 11.m dike the

trasts. However, as we continue to increase the dike conductivity we eventually reach the point where the

skin depth in the dike material becomes substantially less than that in the half-space (Figure 65b); further increasesin dike conductivity will start to significantly alter the current distribution in the ground. Because the new distribution of the inphase and quadrature phase current componentswill be different, so will the new inphase and quadrature phase magnetic field componentsand we can now expect to get variation in the phase angle of the anomaly with increasing dike conductivity. Since the behavior of the inphase current flow with depth should still be reasonablystraightO I 1.4 2.4 1.2

2.2

1.0 2.0

Hx .8 1.8 Hox .6 1.6' .4

1.4

.2

1.2

.0 -.2

1.0 0.8

Q

I -1200

-1000

I

I

I

I

-800

-600

-400

-200

(a) I

o

200

I 400

I

I

600

800

I

I

1000

1200

•X

METERS

•Z: IOOOOrz-m

•z: IO0OOrzm 10 meters of 100rz-m (S1= 0.1S! L - 2.81

.8 .6 .4

Hz

Hox

.2

o

.0

I

m.4 --.6

i

(b)

i

Fig. 64. (a) Variation of horizontal magneticfield (I and Q) and (b) variation of vertical magneticfield (I and Q) acrossa thin conductive(S] -- 0.1 S) dike in a 10 000 11.m host (after Madden and Vozoff, 1971).

Geological MappingUsingVLF RadioFields

to understandthe publisheddata on the responseof a finitely conductinghalf-plane in a conductive halfspace.To date we have usedthe actualmagneticfield componentsto learn more about those terrain param-

secondarymagneticfields are closely confinedto the vicinity of the dike.


583

A corollaryof this behavioris that a conductivedike need have a depth extent only of the order of a skin depthin the dike materialfor it to behavetotally like a dike of infinite depth extent. This fact is discussed later in greater detail. In the limit as the dike becomesvery highly conductive the falling electric field in the dike eventually overcomesthe rising dike conductivityto produce an upper limit to the current flow. In fact, at suchhigh conductivitythe electrical skin depth in the dike will

eters which control the VLF target response. However, as mentioned earlier, many ground VLF instru-

ments measurethe tilt-angle • and ellipticity e of the polarizationellipsesincetheseparametersdo not vary with diurnal variations. Appendix A shows that for small values of the field components the tilt angle essentiallyequals the ratio of the inphase component of the vertical magneticfield with respectto the local horizontalmagneticfield, and the ellipticity the ratio of the quadraturephasecomponentof the vertical magneticfield with respectto the local horizontalmagnetic field. The published data on VLF responsesoften depictstilt angle and ellipticity. Before discussingsuch data one point should be made; throughoutthis article the conventionthat all

become much smaller than the dike thickness, where-

uponcurrentflow in the dike will occurat the top near the contact with the half-space.At suchhigh conductivity contrastthe dike again appearsas two closely spaced contacts, and using the same argument presented earlier for a contact with high conductivity contrast, we can predict that the maximum inphase anomalyfor a dike whosetop is near the surfacewill

complex phasors varyaseitøthasbeenadopted; thus, haveHz -• Hy or 0 -• 45 degrees. A veryconductive if the primary magnetic field is used as the phase reference,the horizontal electric field on the surfaceof a homogeneoushalf-space, which leads the refer-

dike causesthe electric field to drape over the top of the dike. Of course, if the depth to the top of the dike is increasedthe amount of drapingis reduced, as is the maximum tilt angle which, therefore, decreaseswith increasing dike depth. We note that, unlike the casefor vortex current flow where an infinitely conductive target produces zero quadraturephase magneticfield response,the VLF planewave response,primarilycausedby the galvanic current component, still has large quadraturephase

enceby ,r/4,is givenby ei•r/4asillustrated in Figure 67a. Had e -icøtbeen used for the time behavior the sameelectricalfield componentwould be describedby

e-i•r/4asshownin Figure67b.Thus,a polarityambiguitycan appearto ariseandthe readershouldalways

ascertain whethere-icøtisbeingemployed in thepaper under consideration, and, if so, should reverse the

polarityof the ellipticityprofilesto comparewith those shownhere. The problemdoesnot usuallyarise with tilt angle since most authors (and manufacturers)

response.

With this backgroundwe are now in a goodposition

LOW RESISTIVITY CONTRAST MODERATERESISTIVITY CONTRAST HIGH RESISTIVITY CONTRAST

TRAVERSE ß

LIJ ß

ß

ß

ß

ß

ß ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß ß

ß

ß ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß ß ß :

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

Current

ß

ß

ß

out

of

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß

ß ß

ß

ß

ß

ß ß ß ß

ß

Z

ß

ß

ß

ß

ß

page

Conductive ,(a)

Dike (b)

(c)

,

Fig.65. Verticaldike:schematic electricfielddistribution asa functionof dikeconductance for low (a), moderate (b), and high (c) resistivity contrasts.


584

McNeill

and Labson

adopt the convention that, in traversing over a conductive vertical dike, the tilt angle initially becomes positive, becomes zero right over the dike, and negative as one passesbeyond the dike. We always assume in our diagramsthat the observerpassesfrom left to right. Where data taken from the literature does not conform to these conventions for tilt angle and ellipticity such data has been redrafted for consistency. Our first set of data for a vertical dike, kindly supplied by Saydam (1984, unpublisheddata) shows how the tilt angle and ellipticity vary as the conductance of the dike is varied with respect to the halfspace. For this data the vertical dike is immersed at a depth to top of 10 m in an 8000 •.m homogeneous half-space, and the frequency is 17 kHz. We see from Figure 68a-g that the inphaseanomaly (tilt angle) increasesmonotonicallywith L, startingto saturate as L approaches about $0. The quadrature phaseresponse(ellipticity) exhibits decidedlydifferent behavior. At low values of L ellipticity too increases with L (and has the same polarity near the dike, in agreement with Figure 64) but further increase in L above about five causes this anomaly to effectively reverse polarity as L increases above about ten, in agreement with Figure 66. For galvanic current flow,

we see that, like vortex current flow, there is phasor rotation of the response as dike conductanceis increased, but for the quite different reason discussed earlier (i.e., the result of residual imbalance between

the large and opposingquadrature phase current flows within the earth near the dike). We

ime Increasing Ex (a) e i(•t

P2=•oooon-m 10 metersof 10•n (S1 = •(l•m)

2

L = 281

1

0

(b) e-i(•t

Fig. 67. Phasorbehaviorfor (a) eitøtand(b) e -itøt.

3

Hz

of the

k Hy

4

Hox

half-width

Increasing

/02 =IOooo m-m

5

note also that the effective

inphase anomaly (260 m for L - 0.23, 190 m for L = 3475) is insensitive to the target conductance,being determined essentially by the skin depth in the surrounding medium. The effect of varying the half-spaceconductivityfor a dike of fixed conductanceis shown (Saydam, 1981) in Figure 69, in which we see that the modulus of the

lOS)

= 3.5m

Q

-1

(b)

-2 -3 I

-4

Q

I

I

I

I

I

i

i

i

i

I

6

7

5

6

-

-

4

5

-

Hx 3

4 -

HOX 2

3 -

I

2

0

1

-

-1

0

-

-2

-1

-

(a)

I -1200

-1000

I

i

-800

-600

I , -400

I -200

I 0

200

II 400

I 600

I 800

I

I

1000

1200

METERS

Fig. 66. (a) Variationof horizontalmagneticfield (I and Q) and (b) variationof verticalmagneticfield(I and {2) across a thin conductive dike (after Madden and Vozoff, 1971).

0.4

_ _

_

•..,o•

2O

0.2 I

_

I I

_

_

_

_

G

_

_

%

. _

_


_

_ _

Anomalyhall--width •--(defmed

--

by amplitude

- -0.2

=•.max,mum value)

_

_ _

_

_

-ii -40 -:500

I iiillJl

iiiiiiii

- 200

Jillill

iii

- I00

i iii iii II ill III IIIIJllll 0

DISTANCE

I00

IIIII-

-0.4

200

-500

;500

-IO0

0 DISTANCE

I00

200

500

IN METERS

68 (b)

68 (a) _llll

200

IN METERS

I [IllJill

IIII

IlJlllll

,_ 0.4

IllllllllJlllllllllJll[l[111

IIII

40:11111 IllIJl IllIIIIIII1• 'III11 IIIllIIIIJl IIII III[J IIllI1' 1110'4

o.2 I

I

bJ

"

0.0 •_

>L ...... --"

-0.2

-40 IIIIIIIII -.'.'.'500

Jl'''l• -200

fflIIl,l,Jf,l,lll,,J,i,,lll,

lllJl'•llllll

IOO

-•00 DISTANCE

-0.4 200

I

I\

J

-40 11111111JllllIIIIIJl•l• -300

300

- 200

- I00

0

DISTANCE

IN METERS

I

/1 I00

I•l•J•ll•l 200

]ø:i 300

IN METERS

68 (d)

68 (c)

......-'-• 0.2 I

/f•'• ..... •j 0.2 jI •

_

r

/

/o.o•

• I:: ,,, ol_

•

I:

/

I/ /-y

:t g _,o.o •

I

•-•20

'""•..._. _._• • .__...... •'

-0.2

_

_

_

_

_

-40 -ii iiii -500

iii

Jill

-200

i iii

ilJll

iiiiii 0

-I00 DISlANCE

I00

200

-40 - .'500

500

illl Ill IlJlllll

I00

200

-0,4 .'500

IIIIJllllll

III 0.4

/• "-' •"--'"'"""'"'"'-'

•

//

_

• 0

0.0 •

- 200

Fig. 68. Vertical dike: tilt-angleand ellipticityas a function of dike conductance(after Saydam, 1984). For all modelst =

20//•

-100

ii iIiiiii OlSTANCE

68 (g)

0 IN METERS

68 (f)

40 ]Jill [111IllIt Ill IlJl IIIlllll

-500

- I0 0 DISTANCE

68 (e)

-40

- 200

IN MEIERS

0 IN METERS

I00

i i i Jli i i i i i i 200

500

1 m, h = 10 m, P2 = 8000l-l-m, •2 = 343 m, h/•2 = 0.029. (a) p• = 100•.m, S• = 0.01 S, L = 0.23, (b) pi = 13 •.m, S• = 0.077S, L = 1.78, (c) p• = 5 •.m, S• = 0.2 S, L = 4.63, (d) p• = 1 •.m, Si = 1 S, L = 23.2, (e) p• = 0.2 •.m, Si = 5 S, L = 115,(f) p• = 0.1 •.m, S• = 10S, L = 232,(g) p• = 0.0066•.m, S• = 150, S, L = 3475.

586

McNeill

and Labson

where the subscript p refers to "same polarity";

responseincreasesapproximately as the square-root of the half-spaceresistivity until about 25 000 l•.m. This figure showsclearly that the VLF method is most


sensitive

in resistive

valuesof Sp for differentP2arelistedin Table3. Figure 68 clearly showsthat by L = 50 the anomaly responsehas been effectively saturatedand we can no longerresolvethe value of Ss (the subscripts standing for saturation)from the phasor response.Ss is thus given by

bedrock.

Two other important points arise from the vertical dike response. From Figure 68 we observe that the quadrature phase responsehas the same polarity as the inphaseresponseup to a value for L of approximately three. The statement is often heard that, for poor conductorsthe polarity of the two componentsis the same, but just how poor the conductorshave to be is not generally appreciated, nor is the fact that this conductancealso dependson the half-space resistivity. Inverting equation(189) for L - 3, resultsin

Sp --m

2 )1/2 10.7 P•2 --

!'gO ø•P2

L1

180

Ss- P•2 and

this

value

of

conductance

is also

(192) listed

as a

function of 02 in Table 3. From Table 3a it is apparent that responsewith the same polarity will be exhibited by only the poorestconductors(even with host rock 100l•.m) andthat, furthermore,the anomalyresponse also saturatesat low value of dike conductanceexcept in relativelyconductiverocks (resistivitylessthan 300

(191)

20

1.83

2o

•2 (m)

o.o

27

.2o 20

4.09

o.o

61

7.32

108

-'

-20 20

12.95

o.o

0

-'

191

-20

40.95

2O

2O

0

o.o

-20

-2O

606

-40

-300

.200

.lOO DISTANCE

o

lOO

200

IN METER8

Fig. 69. Tilt angle(solid)andellipticity(dashed)profilesover a 1 S verticalconductorat 20 m depthasthe resistivity of the hostrock (02) is varied (after Saydam,1981).



1•. m). This feature, which has often been commented on, is generally ascribed to the effect of the high frequencies used at VLF on the vortex current response. We see from equation (189) that, even though

the anomalies are in fact produced by the galvanic current component, the high frequency is still the culprit; the situation would be substantially improved at lower frequencies. Finally Table 3b illustrates that VLF measurementsare generally poor for distinguishing between even moderately conductive geological structures and much more conductive ore bodies,

since saturation is almost invariably reached at low values of S•. Conversely, sincethe responsesaturates quickly with dike conductance particularly in relatively resistive host rock we will often be justified in using responses calculated for infinitely conductive dikes, a useful feature in view of all of the other

variables that come into play. Thin Dike•Variation

of ResponseWith Dip

The response from a dipping dike of varying conductivity, calculated by Olsson (1983), is shown in Figure 70 (which shows both tangent of the tilt angle and ellipticity in percent). The thickness of the 1000 l•.m overburden is 20 m, a value which will cause an

appreciable reduction of the amplitude of the responsesbut a small effect on the anomaly shape. We observe from Figure 70 that the effect of varying dip is not pronouncedfor dip greater than 60 degrees. For smaller values the responsebecomes asymmetrical, with both the inphase and quadrature components showingincreasedpeak responseon the up-dip side of the conductor

and increased

distance

to the reduced

peak on the down-dip side. The greatestvariation with dip occurs for the quadrature phase component of the poorest conductors. We note that for the better conductors the peak-to-peak amplitudesof both tilt angle and ellipticity are essentially unchanged by varying dip; a good conductor with a shallow dip of 30 degrees still produces a large responsedue to galvanic current flow. Indeed, unpublished calculations by Nabighian (pets. comm., 1972) in Figure 71 show that large peak to peak amplitude is still maintained for a horizontal infinitely thin conducting sheet. The shape of the

Table3. Valuesof Spfor differentP2. P2 10000 3000 1000 300 100 30

(a) St, (Siemens) O. 11 0.20 0.33 0.62 1.1 2.0

(b) Ss(Siemens) 1.8 3.3 5.7 10 18 33

587

responseat dip of zero degreesresembles that from a vertical contact (see Figure 44b) as we would expect from galvanic current flow considerations. Figure 72 shows additional unpublished calculations by Nabighian (pets. comm., 1972) for the same horizontal sheet, now as a function of depth. We observe that the asymmetry of the response decreases with depth inferring that accurate dip information can only be obtained from relatively shallow targets. Vertical Dike•Variation

of ResponseWith Depth

The effect on tilt angle and ellipticity from increasing the depth of burial of a moderately conductive vertical dike (L = 4.6) is shown in Figure 73a-f. From our discussionto date we realize that the depth to the top

of the dike in terms of the half-spaceskin-depthh/•2, is what controls the response; this parameter is indicated for each figure. Dikes with different conductance show similar behavior. We see that both tilt angle and ellipticity decrease with depth of burial; tilt angle decreasesmore rapidly. The location of the maximum value of tilt angle moves progressively outward with depth (we will later see that, in the absenceof conductive overburden, this distance can be used to calculate the depth) whereas the shape of the ellipticity is a complex function of depth. Since the ratio of ellipticity to tilt-angle changes with depth there is phasor rotation as well as attenuation of the response,both due to the fact that the horizontal electric field which generates the response is progressively phase retarded and attenuated with increasing depth. We note that at shallow depth (h/g, << 1) the anomaly half-width [as defined in Figure (68a)] is independent of depth, being determined by the skin depth in the half space. At larger depth, however, the half width is also influenced by the depth, and we conjecture that, in the general

case,theanomaly shape iscontrolled by h2 + x2/•2. As an interpretive aid Saydam (1981) has calculated a series of graphs in which he presents peak-to-peak tilt angle (0) and ellipticity (e) as a function of vertical dike conductivity and depth of burial for a variety of half-space resistivities. Peak-to-peak values are used for two reasons. (1) The vertical magnetic field component on which 0 and e are based extends for a large distance on either side of the target. In the case of multiple anomalies any given anomaly is usually sitting on the wings of a distant target which gives an (often spatially slowly varying) vertical offset to the anomaly response,as shown in Figure 74. Measurement of the peak-to-peak anomaly removes the problem of such offsets. (2) We saw above that a dipping dike distorted the anomalies in 0 and e but left the peak-to-peak anomaly values relatively unchanged.In fact, one-half of the peak-to-peak value is sometimesused since this


588

McNeill and Labson

Cl

TILT

75

ELLIPTIC ITY

S0

Re(•)-O

DIP0

25

= 90 ø

Re(•/) = 0.00

i-

33.3

0.03

w

10.0 3.33

0.10

o rv" W n

0.30

1.0

1.00 -25

1

Re(Q) -0

Re(•/) -S0

•;2 = 353m (a)

-7S

I

-288

0 X IN

288

488

METERS

TILT

75

ELLIPTICITY

S0

DIPO=

25

z _

w w tl

ß,',' .,"' [_'_' -"

•

-

• ......

-2S

---0.03 L= 33.3 """-""=•Re(•/) =0.00

'" •" //•'

.......

60 ø

0 ß30 1.00

",,

3 ß33 1.0

.. _

Re{•)-0

-S0

-7S

(b)

,

I

-488

!

-288

I

0 X IN

!

288

488

METERS

Fig. 70. Computed VLF curvesfor the dipping,conducting half-plane model.Frequency20 kHz, resistivityof overburden1000•.m, thicknessof overburden20 m. Dip = 90ø(a), 60ø(b), 45ø(c), and30ø(d) (afterOlsson,1983).



589

TILT

7S

ELLIPTICITY

50

DIP 0 = 45 •'

25

0.00 0.03

33.3

10.0

0.10

3.33

0.30 1.00

1.0

-25

-50

(c)

-75

X IN

METERS

d

TILT

75

ELLIPTICITY

50

DIP 0 = 30 ø

2S

Ret'r/) = 0.00 33.3 10.0

0.03 0.10

0.30

3.33 1.0

1.00 -25

-50

(d)

-75

-4BB

-2BB

0 X IN

2110

METERS

Fig. 70, cont.

400

590

McNeill

and Labson

value is usually numerically comparablewith the vertical magnetic field component. An example of one of Saydam's graphs is shown in Figure 75. To use the figure, given the half-spaceresistivity, simply enter the


measured

values of 0 and e to obtain the conductance

and depth of burial of the dike. These graphs, which were calculated for a VLF frequency of 17 kHz, are slightly frequency dependent. However, with the knowledge that the governing parameter is the induction number L all of Saydam's plots for different half-space resistivities can be combined into one frequency independent plot, shown in Figure 76. The figure is used in the sameway as Figure 75 exceptthat now L and h/• 2 are read from the graph; knowing the VLF frequencyand 92, equation(190) is used to calculate the plate conductance;next the skin depth is calculated, with which h, the actual dike depth, is obtained. A problem arises in that the half-space resistivity must be known in order to interpret the VLF anomaly since, as we have seen, this resistivity has a very significantinfluence on the response. One way to solve this problem is to make a VLF measurement of the surface impedance in an area adjacent to the VLF anomaly; from this quantity, as we have seen earlier, to derive the terrain resistivity is easy, indeed most

VLF impedance instruments are calibrated to read half-spaceresistivity directly. Making sucha measurement, while much faster than conventional resistivity, is slower than tilt angle and ellipticity measurements and will thus reduce survey productivity. However, in order to carry out the interpretation, the half-space resistivity must be known or estimated. Another way to estimate the resistivity is from the rate of spatial decay of the anomaly away from the peak values. Examination of Figures 68 and 73 shows that, very roughly, the anomaly has fallen to one-half the peak value at a distance of from 0.55 to 0.85 skin depths away from the dike, depending on conductanceand depth. Taking 0.7 as an average value (and realizing that it could be in error by ___20percent) we can calculate 92 to within ---40percent and thus both the conductanceand depth to -20 percent. The lateral fall off of tilt-angle rather than ellipticity was chosen because examination of Figures 68 and 73 shows that the wings of the ellipticity are much larger than those of the tilt-angle, with the result that adjacent anomaly interference is much more severe for this component. Furthermore since, as we have seen, the ellipticity (quadrature phase) anomaly is the result of a balance between two large and opposing current flows, a

1.0

I

I

I

I

I

'

i

0.9

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i

i

D•p:O ø

_ 0.8

0.8

0.5 0.4

0

-

0.6

_

I'•D'P _

0.5 0.4 0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 -0.1

Hz Hz

_

0,7

0.7 0.6

I

0.9

-0,2

-0.1

-

-0.7_

-

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Hox-0,3

-OA

-0.4

-

0.4-

0.4

0.3-

0.3

0.2-

0.2

0.1

0.1

-

0.0 r

0.0

-0.1

.

-

-0.2

-0.2

-0.3

-0.3

-0.4

-0.4

-0.5

-0.5

-0.6

,

-0.1

-0.6 I -I.6

I -I.2

I

I

-0.8

-0.4

X./I 0.0

I

I

I

I

0.4

0.8

1.2

1.6

Fig. 71. Vertical magnetic field (I and Q) over a perfectly conductingdike as a function of dip (after Nabighian, 1972).

-I.6

- 1.2

-0.8

-0.4

0.0

0.4

0,8

1.2

1.6

Fig. 72. Vertical magnetic field (I and Q) over a perfectly conducting horizontal dike as a function of depth (after Nabighian, 1972).


GeologicalMappingUsingVLF Radio Fields

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Fig.73.Vertical dike: tilt-angle andellipticity asafunction ofdikedepth (after Saydam, 1984) t = 1m,p]= 5•.m, Pz= 8000•.m, 8z= 343m,Sl = 0.2S,L = 4.63,(a)h - 0 m,h/Sz= 0.00,(b)h = 10m,h/82= 0.029,(c)h = 20m,h/82= 0.058,(d)h = 50m,h/8z = 0.15,(e)h = 80m,h/82= 0.23,(f)h - 130m,h/82= 0.38.

-0.2

-0.4 300


592

McNeill

and Labson

balance which is easily upset by "geological noise", i.e., localized (and less localized) resistivity inhomogeneities, to use the tilt angle (inphase)anomaly for interpretationis always advisableif possible. Using the tilt angle we see from Figure 76 that the maximum depth to which we can expect to detect and interpret a conducting dike is, in the absence of overburden, about 40 percent of the skin depth in the

and can probablybe usedwith reasonableaccuracyfor a dipping dike as well. Unfortunately, as we shall see in the next section, the presenceof a smallamountof not very conductive overburden can greatly affect the shape and size of both components of the anomaly profile, seriously impairingthe utility of both of these approaches.

host rock.

There is an alternateway to obtain the depth of the target which uses the fact that the position of the tilt-angle maximum moves out with increasingdepth. The procedure, illustrated in Figure 77, is taken from Poddar (1982); knowing Ap (the distance between the positive and negative excursions of the tilt angle expressedin percent) and Ay/2 (in terms of •:), the graphgivesplate dip and depth (alsoin terms of •:) directly. Although the figure applies to an infinitely conductive dike, checking against previously unpublished results of Saydam shows that the graph is essentially correct for a vertical dike of arbitrary conductance,

Vertical Dike Overlain by Conductive Overburden The serious affect of even a shallow overburden

the amplitudeand shapeof VLF anomalieshasalready been demonstrated in Figure 52 for the vertical contact. In our discussionof the field componentsin a two-layeredearth we realizedthat, becauseof multiple passesthrough the relatively conductive overburden, the horizontal electric field at the base of the overburden was much less than would be calculated

on the

basis of single-passattenuation. This horizontal electric field component (further attenuated and phase retardedif the targetlies at an appreciabledepthbelow the overburden) is that which causes the anomalous VLF responsefrom the dike, and any reduction in this exciting field will, of course, reduce the target response. Reduction of response cause by conductive overburden is a serious problem with plane wave systemsat VLF frequencies. As stated earlier, in the absence of conductive overburden, the parameters

Fig. 74. Responsefrom a shallow anomaly vertically displaced by the responsefrom a distant, deep anomaly.

Os0

•..o•

o•"'"• lo

15o

10

20

3o

40

50

60

70

80

90

100

I

I

I

I

i

I

i

I

i

i

0 m•

PEAK-TO-PEAK TILT ANGLE (DEGREES)

Fig. 75. Response from a verticaldikein a 2500•.m hostrock, withoutconductive overburden (S3 - 0) (after Saydam, 1981).

on


x•p2

L =S•

593

(193) 25

and

CX--60 e


h

H• = x/h 2+ x2/•2

(194)

completelydefinedthe responseof an electricallythin

2O

semi-infinite vertical sheet of conductance S• im-

io

mersed at a depth to top h in a homogeneoushalfspaceof resistivity02- The quantityunderthe squareroot in equation (193) effectively introduced the horizontal electric field exciting the target, as discussedin connectionwith equation (190). In the most generalcase,the additionof a conductiveoverburden (of conductivitycr3 and thicknesst3) considerably complicatesthe response.If, however,the overburden is electrically thin, i.e., t3/• 2 < 0.3, the response becomesmuch simpler. From equation (119)we see that the surface impedanceof a two-layered earth, with the upperlayer electricallythin, canbe written as Zs =

Elx(O)

Hly(0 )

•-

•2

;

P2

• o.15

15

io I

o'2

I

.

i q-S3•q2

(195)

10.3

But since the overburden is electrically thin, the electric field in the overburden is essentiallyconstant with depthandequation(195)givesus the electricfield at the base of the overburden, normalizedwith respect to the primary magneticfield at the surface. Now in the case of no overburden our dike response

I

I

0.2

0.3

I • 0.4

Fig. 77. Characteristicdiagramfor VLF-EM interpretation in areaswithout conductingoverburden./Xyis normalized with respectto •2 (after Poddar,1982).

50

- o'"'"'"•130

x/_...........•©••

1950

.2

2.89

1.30

30

40

50

60

70

80

90

100

I

i

I

I

i

I

&

I

max

PEAK-TO•EAK

TILT ANGLE (DEGREES)

Fig.76.Generalized response froma vertricaldikein theabsence of conductive overburden (S3= 0). Derivedfrom Saydam, 1981.

594

McNeill and Labson

was controlledby Six!2 [althoughwe usedL = S1 x Re (x!2)].The x!2was a measureof the electricfield at the surface; thus in the more general case of an overlyingconductivebut electricallythin overburden,

81 illustratethat, for a largevalue of L3, the response is no longer a function of L• and L•, but rather of S•/S3 in accordwith equation(200). Not only the anomaly amplitudebut also the anomaly shapeand width dependon the parameterL 3. It was suggested by Kaufman(personalcommunication)


where the electric field at the base of the overburden is

given by equation (195) we conjecturethat the dike responsewill now be controlledby L =

that this width

is a function

of

(1 + }..[0c0S3a2) r/82

S1•2

1 q-S3Xl2

(196)

where

implying that L3 = S3 x Re (x!2) is the correct

r = x/h 2+ x 2

(202)

ß

response parameter to account for the influence of the

electrically thin, conductiveoverburden.

Letting L i = S 1Xl2

(201)

It is a simplematter to show that the quantity in equation(201) is equal to

(197)

(1 + 2L3) r/g2

and

(203) t3

m• =53• 2

(198)

the parameterL' is given by P2

Li

L' = 1+L•

(199)

50

and we see that the "drive" responseparameteris

40

simple.

30

Severalimportantfactorsemerge:(1) if IL•l << 1 the overburdenshouldhavevirtually no effecton the dike response,regardlessof the value of S3. We will see shortly,however,that anotherfeatureof the response will renderthis statementuntrue.(2) if IL•l >> 1

20

Li

L'

•-

L• S3

I

I

I

lO 0

-20

S1

=

I

-30

(200)

-40 -50

I

-lOO and the responseparameterbecomesindependentof L• andL•, and alsoof the hostrock resistivity,being determinedby the ratio of the targetand overburden 50 conductances.Increasingthe conductanceof the over40 burden proportionatelyreducesthe excitingelectric 30 field,whichis directlycompensated for by increasing 20 the conductanceof the target. (3) in the generalcase where lEVI ñ 1 bothL• andL• (or L• andL3) control •' lO

the response. To summarize, in the case of a vertical dike in a

conductivehalf-space,the responseparameterswere

givenbyL• = S• x Re(x!2)and¾/h2 q-X2/•2. When we add an electricallythin conductiveoverburdenwe

mustaddtheparameterL 3 = S3 x Re (x!2).Figures78 and 79 illustratethe dependenceof the responseon theseparametersfor a low value of L 3. Despitethe differentvaluesof S1, P2,andS3, the amplitudeof the peak wave-tilt and ellipticity is the same for both cases. The anomaly widths, however, are different,

aboutwhich morewill be saidbelow. Figures80 and

,-

I

I

-60

I -20

20

60

1O0

X(meters)

0

• -10 co -20 -30 -40 -50 -100

I

I

-60

I

I

I

-20

20

I

I

60

I

100

X(meters) Fig. 78. Responsefrom a vertical dike with low overburden

conductance.S1 = 1.13 S, 02 = 1000•.m, S3 = 0.0183S, L• = 10, L 3 -- 0.16.



from which we see that when L 3 << 1, the anomaly width is determinedby r/•2 as we would expect, but that when L3 >> 1 the anomaly width is governedby S3 and is independentof P2. A parametric analysisof many model responsescalculated usingthe program of Madden and Vozoff (1971) confirmedthat when L 3 < 0.3, P2determinedthe anomalywidth, when L3 > 1.4, S3 determined the anomaly width, and for L3 lying between these values, both P2 and S3 determinedthe width.

Figures 82a-d from Madden and Vozoff (1971) illustrate the effect of overburden having various values of L 3 on the responseof a relatively conductive (S1 = 10 S, L - 281) vertical dike with its top just beneath the overburden. In all figuresfrom Madden and Vozoff

(1971) the inphase componentof Hz/H x (equal to 100 x tan 0) is plotted rather than 0 itself, thus

Hz/H x = 40 percentis approximatelyequalto 0 = 20 degrees. Also shown on these figures is the relative amplitude of the horizontal magnetic field. We see that 50

I

I

I

the inphase component of the vertical magnetic field is seriously affected which is unfortunate since, for the reasons given earlier, the inphase component is usually the preferred component with which to work. Apparent is the substantial reduction in total anomaly width caused by more conductive overburden as well as the variation in horizontal distance betwen peak values of ellipticity and, unfortunately, tilt angle, as the overburden conductanceincreases. Clearly almost any amount of overburden will prevent determination of anomaly depth usingthe technique by Poddar (1982) referred to in the previous section. Figure 82e shows the same dike and overburden as Figure 82d, but this time with a 1000 12.m half-space; note again that the responseshape is completely independent of the halfspace resistivity. The overall behavior of the VLF response as a function of overburden conductance is shown in Figure 83, which was calculated using the program of Madden and Vozoff (1971). From Figure 83 we see 50

I

I

40

40

30

30

20

I

10

.o

o

e-

-10

• -20

I

I

-20

20

I

I

I

10

E

o

.•.

-'10

• -20

-30

-30

-40

-40

-50

-50

O0

..

-60

-20

20

60

1O0

O0

-60

X(meters)

40

30

30

20

60

100

20

e-

10

e-

10

,..-

0

,--

0

-'10

•

co -20

-'10

• -20

-30

-30 -40

-40

-100

1 O0

50

40

-50

60

X(meters)

50

•

I

20

e-

'•'

595

I

I -60

I

I

I

-20

20

I

I 60

I

-50

100

X(meters) Fig. 79. Responsefrom a vertical dike with low conductance overburden. S1 = 2.05 S, 02 = 300 fl.m, S3 = 0.033 S, L1 = 10, L 3 = 0.16.

-100

-60

-20

20

X(meters) Fig. 80. Responsefrom a vertical dike with higher conductanceoverburden.S1 = 1.11 S, 02 = 1000fl.m, S3 = 0.36 S, L] = 10, L 3 = 3.16.


596

McNeill

that, contrary to the behavior predicted by equation (199), valuesof L• only slightlygreaterthan zero have a substantial effect on the dike response. The reason for this effect can be understood by referring to Figure 65c, where we observe that, just beneath the surface, the horizontal electrical field in the half-spacenear the dike is anomalouslyhigh compared with that at remote distance. If we now place a thin, conductive overburden on the half-space, although in general the electric field will be everywhere reduced as shown by equation (195), an anomalously high electric field (which is continuous across the overburden/half-spacecontact) will now exist within the overburden in the vicinity of the dike; the anomalous overburden current produced by the product of this electric field and the overburden conductance will produce an anomaly that enhances the anomaly arising from the conductive dike itself. We surmize that, as the depth to the top of the dike is increased, the anomalous electric field in the halfspace illustrated in Figure 65c will decrease, as will 50

I

I

I

i

i

I

I

i

i

i

-20

20

I

I

i

i

20-

10-

o

-10

-20

-30 • -40

i

-100

-60

60

100

X(meters)

50 40-

30 20 e-

.o •

also the enhanced dike response at low overburden conductance.

At larger values of overburden conductance or induction parameter L 3 the effect of increasingL3 is similar to that of burying the dike deeper, shown in Figure 76. A more conductive overburden attenuates and retards the phase of the response;the rather rapid decreaseof the inphaseresponseis actually due to the phasor rotation. Finally, we note that presenceof an overburden induction parameter greater than 1.0 results in a large reduction of the inphase response. In order to perform an interpretation, we must know both 92, the resistivity of the half-space,and L3, the induction parameter of the overburden. Again the use of a VLF surface impedance measurement offers a solution to the problem. As shown earlier, in the case of an electrically thin overburden, the surface impedance can be inverted to yield both the conductanceof the overburden and the resistivity of the underlying host rock. Furthermore, to construct a series of diagrams of the type of Figure 83 for selected values of

Hi is possible.Given L 3 from the surfaceimpedance I

30

-50

and Labson

10

o -10

• -20 -

measurement (made sufficiently far from the anomaly so as to be unaffected) and 0 and e, the appropriate diagram is selected, and the interpretation procedure is exactly as describedearlier. Two such diagramsare given in Figures 84a and b. To summarize our discussion of the conducting dike, apparentlythere are many variableswhich influencethe VLF response.Some authorsignorethe finite conductivity of the dike; others the presence of conducting overburden. The latter is an especially serious limitation. The procedure given here takes into account all of the significant variables. Unfortunately, this procedure requires the use of two different types of VLF measurement for the interpretation. The presence of conductive overburden offers yet another problem. We have seen earlier that significant VLF anomalies, generated by the galvanic current component,can arise from relatively small resistivity contrasts at relatively high values of resistivity. Such anomalies are easily generated by changesin conductivity or thickness of a conductive overburden, often preventingthe use of VLF techniquesin arid environments where overburden anomalies can easily completely mask the responsefrom bedrock conductors.

-30

Vertical DikeLimited

-40 -50 -100

I

I -60

I

I -20

,

I 20

I

I 60

I 100

X(meters) Fig. 81. Responsefrom a vertical dike with higher conductance overburden.S• = 2.05 S, P2 = 10 000 f•.m, S3 = 0.36 S, L• = 31.6, L 3 = 10.

Depth Extent

Detailed analysisof the calculationsby Madden and Vozoff (1971) show that, as long as the depth extent of the dike, with or without overburden, vertical or

steeplydipping, is greaterthan a skin depth in terms of the dike material, the dike can be considered to have

infinite depth extent. Since 1 •.m material has a skin



160

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•

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40 •////..... •_................... •••....._...... 3.5 2.5•. h5

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-40

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2.5 w

-80

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-0.20

!

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0.40

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82 (b)

82 (a) 160

!

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160

80

8O 40

20 IZ

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0 •

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'3.,5 _

-•o,5 -80

tu >

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tu

-IZO

-0.5

-160 -I.40

I

I

I

I

I

-hZO

-hO0

-0.80

-0.60

-0.40

I -O.ZO

I

I

0.0

O, ZO

I 0.40

I 0.60

I 0.80

I

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1.00

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- 0.5

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-0.60

-0.40

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1.00

X KFT

82 (c)

40

20

-zo -40

1.5

tu n-

-I.40 -,6o

,

-I.20

-•o

,

-hO0

,

-0.80

•

-0.60

,

-0,40

,

-0.20

[

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x•

82 (e)

,

0.20

,

0.40

,

0.60

,

0.80

,

1.00

, -•o

1.20

Fig. 82. Responsefrom a vertical dike, S• = 10 S, L• = 281, (after Madden and Vozoff, 1971). (a) Without overburden,

P2 = 10 000 fl.m, S3 = 0, L3 = 0. (b) With overburden, P2 = 10 000 fl.m, S3 = 0.0335 S, L3 = 0.94. (c) With overburden,P2 = 10 000 fl.m, S3 = 0.10 S, L 3 = 2.8. (d) With overburden,02 -- 10 000f•.m, S3 = 0.33 S, L 3 = 9.3. (e) With overburden,P2 = 1000f•.m, S• = 0.33 S, L• = 2.9, L• = 88.

598

McNeill


1=L3 3.16 ............. I

•

depth of 3.5 m at 20 kHz a dike with quite small depth extent can still exhibit the responseof an infinite dike. When the dike depth extent becomes significantly less than a skin depth, tank modeling experiments made by McNeill show that, while the response can still be moderately large, the quadrature phase response now has the same polarity as the inphase response, even for a quite conductive target. The reason for this response is that a target of shallow depth extent gathers current from a relatively shallow depth (in terms of the half-space skin depth) and such currents lead the horizontal magnetic field by a phase angle of 45 degrees, i.e., exhibit, using our convention, a positive quadrature component. As the dike depth extent increases, deeper currents, which are retarded in phase, are caused to flow in the target and the overall responsebecomesphase retarded; the quadrature phase component falls to zero and then becomes negative as depth extent further increases. In summary, dike-like targets with positive quadrature phase responseoften have small depth extent.

/ / •0.3161 •

i 0.0

•' 4o

"•30

'"' ...... • 10:L• ............... 3.16

0

and Labson

20

40

60

80

100

120

140

160 180

O[p-p(percent)

Fig. 83. Vertical dike response with a thin overburden. Peak-to-peaktilt angle and ellipticity as a function of L] and L3 for h/B2 = 0. For the reasons discussedin connection with equation(200), responsefor valuesof L3 > 3.16 are not plotted.

5O

0.316=L•

2=0.2

..

4O

0.316=L3

•'

0.0

................ '............. '" 100 I .... '":: ...... ........... '........... "31.6

3O 3.16

10=L1 1

,•

20

,•

20

3.16 lO

lO 1.0

1.0 o

0

20

40

60

80

o

(p-p (percent) 84 (a)

20

40

60

8O

(p-p (percent) 84 (b)

Fig. 84. Vertical dike responsewith a thin overburden'peak to peak tilt angleand ellipticity as a function of L] and L 3. Depth of burial (h/•2) is 0.1 (a) and 0.2 (b).

GeologicalMapping Using VLF Radio Fields Multiple Conductive Dikes

The response from two closely spaced vertical thin dikes is, as one might expect, not greatly different


from that of one thick dike. In the absence of conduc-

tive overburden as the interdike spacingincreasesto about 0.1 skin depth (in the half-space) the dikes become well resolved, as shown in Figure 85a, b. The presence of overburden of conductance 0.1 S makes the dikes more difficult to resolve, as shownin Figure 86; sucha responsewhich is often seenduring surveys can be caused by multiple targets. Resistive

Vertical

Dike

As would be expected the response from a thin resistive

vertical

dike

is undetectable.

As the dike

thicknessincreasesto about 0.1•l (in terms of the resistive dike material) a reverse crossover of a few percent starts to appear if the resistivity contrast is greater than about 10/1 and there is no overburden. This response increases with dike thickness but stays relatively small, as shown in Figure 87, even for a dike of thickness0.3 skin-depths.The responsefrom resistive dikes will be easily obscuredby the presenceof a small amount

599

that the responseof Figure 93 is very similar to that for Figure 91 since for both overburden resistivities the skin depth is lessthan the depth to basementand again the boundaries are seen as simple contacts. The presenceof overburden on top of these features suppressesthe response.

Summary of ResponsesFrom Two-Dimensional Targets

Figure 94 summarizes the responsesfrom the various types of conductive and resistive 2-D targets discussed to date. In addition to wave tilt (0) and

ellipticity (e) the vertical (H z) and horizontal (Hn) magnetic field and the horizontal electric (En) field componentsare also given. In the case of the horizontal magnetic field component the primary magnetic field componentis usually substantiallylarger than the secondary component and thus the responseis essentially the inphase component since the total component is given by

H• = [(Hx p 4-nxS(]) 2 + nxS(Q)2] 1/2

(204)

of overburden.

if Hx (I), Hx (Q) '• Hxp. Overburden/Bedrock

(205)

Structures

The anomaly produced by a depression filled with conductive overburden is shown in Figure 88. The overburden has resistivity of 100 •.m and skin depth of 35 m so that the depth extent of the anomaly is of the order of a skin depth. Thus on the left side the depression is effectively a vertical contact and on the right a dipping contact, as indicated by the response which, in view of the narrow target width (in terms of a skin depth) consists of the superimposedresponse from both. Note the sharp peak on the negative anomaly which resembles that for the dipping dike. The depression of Figure 89, identical except wider, shows the separate anomalies somewhat better resolved, and in the case of Figure 90 where the infilling material is 30 •.m with a skin depth of 20 m, the resolution is even more complete. These figures clearly show that the inphase responsefrom a narrow (in terms of its own skin depth) overburden-filled channel will be difficult to distinguish from the responsedue to a vertical or steeply dipping conductive dike; the lack of significantquadraturephaseresponse may be more diagnostic. The bedrock ridge of Figure 91 shows a marked reverse crossover which is even larger in Figure 92 since the wider

structure

allows

each of the contact

responsesto become more fully developed before they encounter the response from the opposite side. Note

The other point to be observed from the responsesis that the horizontal electric field response shows best target spatial resolution for the case of H-polarization, i.e., when the electric field is perpendicular to target strike. As discussedearlier this enhancedsensitivity to target boundaries arises from changes induced on the contacts from the perpendicular electric field. Also as discussed earlier, a 2-D conductor or resis-

tor that is oriented with its strike parallel to the primary magnetic field (H polarization) does not exhibit a magnetic field anomaly. Thus in the general case where 2-D targets have strike at arbitrary orientation • with respectto the direction to the transmitter (i.e., 90 - • with respectto the primary magneticfield) the responsewill in theory be proportional to cos •. In fact since large structures are often semi-sinuousby nature, with perhaps a portion of the structure in good coupling with the electric field at some not very great distance from the measurement site, rigorous dependence on cos • is often not observed. This fact notwithstanding, every effort should be made to use a VLF

transmitter

whose

electric

field

is as close as

possible to the direction of geologic strike when making magnetic field component measurements. (Text continued on page 606)


600

McNeill

160

I

'

I

I

I

I

and Labson

'1

I

' I

I

I

'1'

I

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80

'1

60

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,

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,

I -0.80

I -0.60

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I 0.80

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-8O

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I

8O

KFT

85 (a) 160

•

120

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-

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80

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2O

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0

rr

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x

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0,0 X

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,

I 0,40

I 0.60

I 0.80

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1.00

1.20

-8O

KFT

85 (b)

Fig. 85. (a) Two verticaldikesseparatedby 0.06 •2, Sl -- 10 S, L• = 281, P2 = 10 000 fl.m, S3 = 0. (b) Vertical dikes separatedby 0.11 g2 (after Madden and Vozoff, 1971).

601

GeologicalMappingUsingVLF Radio Fields


160

i

i

i

i

i

i

$0

i

120

-

60

80

-

40

40

-

20

0

rr

3.5 -40

-2.5

-

-20

-

-40

w xI--

-80

- •.5

rr -GO

-120

- 0,5

-160

I

-I.00

-0.80

I

-O.GO

I

-0.40

I

-0.20

I

0.0

I

0.20

I

0.40

I

0.60

I

0.80

-80

1.00

X KFT

Fig.86.Twovertical dikesseparated by0.06•: butunderneath conductive overburden, Sl = 10S,L• = 281,p: = 10000•.m, S3 = 0.10 S (afterMaddenandVozoff, 1971). 80

160

60

120

40

80

ZO

4O

bJ w Li.I

3.5

-20

-40

-2.5 -80

_

W -40

XI--

1.5

cr -60

-120

- 0.5 -160

-i.00

I

I

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I

-0.80

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-0.40

-0.20

I

I

I,

0.0

0.20

0.40

x KFT

-80

0.60

0.80

1.00

Fig.87.Thickresistive dike,p] = 10000fl.m, • = 353m,tl/• 1 = 0.28,@2= 1000fl.m, S3 - 0 (afterMadden and Vozoff, 1971).


602

McNeill and Labson

160,

80

120

6O

801-

40

4O

20 w w

o

o

n• w

-40

-20 LLI

-80

-40

1.5

-120

-60

0.5

-160 -1.40

-I,20

-I.00

-0,80

-0.60

-0,40

-0,20

-80

0.0 X KFT

0.20

0.40

0.60

0.80

1.00

1.20

Fig.88.Overburden filleddepression, Pl = 100f•.m, Bl = 35m, d1 = 50m, dl/Bl = 1.4,w1 = 80m, Wl/Bl= 2.3, 92 -- 10 000 •.m, S3 = 0 (after Madden and Vozoff, 1971).

160

8O

h-

wl

-4

120

6O

8O

4O

40

2O w w

o

m

-2o iii

2.5

>

xi-

-8O I.,5

-4o LU

-120

-6o

0.5

-160 -I,40

I

I

I

I

I

I

-I.20

-I.00

-0.80

-0.60

-0.40

-0.20

I 0.0

I 0,20

I 0.40

I 0.60

I 0.80

I

I

1.00

1.20

-80

X KFT

Fig.89.Overburden filleddepression, Pl = 100f•.m, gi = 35m, d1 = 50m, dl/81 = 1.4,w1 = 150m, w1/81=

4.3, P2 = 10 000 ll.m, S3 = 0 (after Madden and Vozoff, 1971).

6O3



160

I

I

I

I

I

I

I •

I

I

w•

I

•

I

I

80

I

60

120

40

8O

40

-20

-40

-40

-80

-60

-I 20 -0.5

I

-160

-I.40

I

-I.20

I

-I.00

I

-0.80

I

-0.60

I

-0.40

-0.20

i

I

0.0

0.20

I 0.40

I

I

I

I

0.60

0.80

1.00

1.20

-80

X KFT

Fig.90.Overburden filleddepression, p] = 30fl.m, 81 = 20m, d] = 50m, d]/8] = 1.4,w] = 150m, w]/8] = 7.5, 192- 10000•.m, S3 - 0 (afterMaddenandVozoff, 1971).

160

I

I

I

I

120

I

I

I

I

I

I

I

I

80

I

60

resistivity

•,•w•

•

8O

40

40

20

-5.5

-2O

-8O

_

-I..5

xl--

-40 ,,,

-60

-120

-160 -I.40

-I.20

I

I

I

I

-I.00

-0.80

-0.60

-0.40

I -O.ZO

I

I

0.0

O. ZO

I 0.40

I 0.60

I 0.80

I

I

1.00

I. ZO

-80

X KFT

Fig. 91. Buriedbedrockridge,19]= 100fl-m, 8] = 35 m, d] = 50 m, d]/8] = 1.4,w] = 80 m, w]/8] = 2.3, 192= 10000•'m, S3 not defined(afterMaddenandVozoff, 1971).


604

McNeill

and Labson

160

80

120

60

8O

4O

4O I.iJ

-3,5

-20

-8O

-4O

-IZO

-6O 0,5

-160 -I,40

I

I

I

I

I

-I.20

-I,00

-0,80

-0,60

-0.40

I -0.:;:'0 X

I

I

0,0

0,:;:'0

I 0,40

I 0.60

I 0.80

I

I

1.00

I,:;:'0

-80

KFT

Fig. 92. Buried bedrockridge, p• = 100 12.m, g• = 35 m, d• = 50 m, di/g• = 1.4, wi = 150 m, w•/g• 92 = 10 000 •'m, S3 not defined(after Madden and Vozoff, 1971).

160

I

I

I

I

I

I

I

I

I

I

I

I

80

I

120

-I 60

8O

4O

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-

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-

-40

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I

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I

I

0,0

0,:::'0

I 0,40

I 0.60

I 0.80

I

I

1,00

I,:::'0

-80

X KFT

Fig. 93. Buried bedrockridge, Pi = 30 f•.m, w• = 20 m, d• = 50 m, dl/g• = 2.5, w• = 80 m, wi/g I = 4, P2 = 10 000 f•.m, S3 not defined(after Madden and Vozoff, 1971).



•-,

.'•. •\.

• N

z

605

o

O'(3 O.C

Eo o •

• ,

•

cO '•0 z

606

McNeill


Reversed Polarity Anomalies

Reversed polarity anomalies are occasionally encounteredduring VLF surveys. We will see in a later section that a topographic depression (valley) can produce such an anomaly. Less obviousis the buried ridge of resistivematerial intrudingupward into more conductive overburden as shown previously. Another target which producesa reversed anomaly is the thick resistive dike, and finally, at least in theory, a resistive but magnetically permeable dike can produce a reversed polarity inphase anomaly, since the induced magnetic dipoles will align themselves in the same direction as the primary magnetic field. If the dike is also conductive, the galvanic current responsemay overcomethis reversedpolarity

and Labson

current flow within the conductive spheroid, Jt = criEi, is larger than the remotecurrent flow away from

the spheroid,Je = cr2Ee p' Furthermorein the vicinity of the spheroidthe secondaryelectric field causedby the induced chargesat the ends modifies the amplitude and direction of the local electric field so as to produce a resultant electric field causingcurrent flow as shown in Figure 96. The combination of enhanced current flow within the conductive spheroid and reduced current flow alongsidethe spheroidproduces the anomalous magnetic field outside the spheroid. Kaufman and Keller (1981) show that the ratio of current density inside and outside (remote from) the spheroidis given by

behavior.

•r 1

PLANE

WAVE

RESPONSE

DIMENSIONAL

FROM

Ji

THREE-

o'2

•ee1+ -1D

TARGETS

(206)

Magnetic Field Response

All published VLF modeling studies deal with the simple case of 2-D targets. We shall use some approx-

where D, the depolarization factor for the spheroid, is given in terms of the spheroidellipticity e by

imate methods to obtain a crude idea of the difference

in magnetic field response between 2-D and 3-D structures, using a horizontal conductive prolate spheroid

D- •e5 loge _

-2e

(207)

as our target.

The base of our approachwill be the knowledgethat most of the magnetic field responseis due to galvanic current flow. We then assumethat the dc responseis adequateto explain the basic features of 3-D response. We thus implicitly assume that both the depth of the target and its cross-sectional dimensions and length are much smaller than a skin depth. The target is embedded in a uniform whole space (we ignore the air/earth interface) and subjectto an axial electric field as shown in Figure 95. Our interest lies in the behavior

of the azimuthalmagneticfieldcomponent H,• as a function of the target semi-axes a and b and conduc-

_ø2

Ep

,-

tivity contrast•r•/•r:, a problem which is discussedin detail in Kaufman and Keller (1981).

Whena primaryelectricfieldEe• is orientedperpendicular to a contact between two materials of differing resistivities, surface charges are induced at the interface, as described previously, and also as shown in Figure 95. These surface charges induced at the ends of the conductive spheroid cause a secondaryelectric field both inside and outside of the spheroid. Within

Fig. 95. Magnetic and electric fields in vicinity of conductive prolate spheroidin a uniform whole space.

the spheroidthe uniform secondaryfield Es is directed in oppositionto Ee• sothat the resultantfieldwithinthe spheroid,E i, is lessthanEe• as indicatedin the figure. In a relatively conductive spheroid, however, the reduction

in the internal

electric

field is less than the

conductivity contrast so that as expected, the axial

Conductive Spheroid

Fig. 96. Current flow in and near a conductive spheroid.

GeologicalMapping Using VLF Radio Fields and where


e=

1-

.

(208)

We see that, in general, the current enhancementis a

rather complicatedfunction of the spheroidellipticity and conductivity contrast. For our first case we fix b and allow the spheroidto degenerateinto a sphere, a --• b, e --• 0, D --• 1/3 and o- 1

Ji

607

a 2-D cylinder)D is much lessthan 1 and even for large valuesof •/•2 the current still linearly increaseswith •. At some point, however, as •/•2 becomeslarge

enough, Ji/Je againsaturates tothevalueD-1. Forthe very prolate spheroid a high value of • before the internal

electric

is needed

field starts to decrease as

-• andsaturation commences , as shown in Figure

O'1

97.

In Figure 98 the samefunction is plotted but now for fixed valuesof •/•2. We seethat, for smallvaluesof •1/•2, reducingthe length of a very long (essentially 2-D) target to a rather short target produces a relatively small reduction in Ji/J e (and thus, of course,

0'2

targetresponse H,) whereasfor largevaluesof •/•2

1+• From this expressionwe seethat, as the ratio of increasesto about 10 the ratio Ji/Je rapidly saturates to a value of 3. This behavior is shown in Figure 97. The explanation for this responseis as follows: for the sphere geometry, at low values of conductivity contrast, as the contrast increases the induced charge

reducingthe target length can produce a large reduction in response.The difference in responsebetween a 2-D and a 3-D target is dependenton the conductivity of the target itself and for a highly conductive target the 3-D response will be significantly less than that predicted by 2-D theoretical modeling, whereas for

density also increasesbut at a slightly slower rate than the conductivity is increasing;the net result is that E i falls more slowly than • is increasing,and the internal current densityJi = o'1Ei increaseswith increasing At a relatively smallvalue of •, however, the point is

lO4 •.1 i

i I

I

I

i

I

! i i i

I

i

i

i

i i i i

I

I

I

! o

ol

reached whereEi startsto fallessentially as•f-• and

lO3

further increasesin • do not result in an increaseof Ji; the responsesaturatesat a relatively low value of O'l/O' 2. If we now examine the responseof the oppositecase of a very prolate spheroid with a >>b (much more like

ol

0'•

(•'•2 ) FIXED,

IOOO

(-•-) VARIES

3OO

102 ..

Ji

IOO

30

lO3

. lO1 (•) FIXED, (•)VARIES

lO2

-

io

.

.

IO

.

.

Ji

3

•ee 101

••

1(SPHERE) 100 .

I

.

.

10o

0.3 .

i

1•'1

O.I

.•'• ..................... 100

101

102

103

104

103

•.,-....,-"'"'•

102

101

100

a

b

Fig. 97. Current concentration in a prolate spheroid as a function of •r•/•r2.

Fig. 98. Current concentration in a prolate spheroid as a function of a/b.

608

McNeill

and Labson


poorly conductive targets the response will not be greatly smaller as long as they are of reasonablelength (i.e., a/b > 3). Equation (206) can be rewritten as

Kaufman and Keller (1981) also discussthe proce-

durefor calculating the ratioH,/Ef for the prelate spheroidin a whole space. Using the surface impedance (equation 58) to relate Ef to the primary magnetic field strengthHe• Figure99 shows(againfor dc) the approximategalvaniccurrent componentresponse of a prelate spheroidas a function of half length a for various •r•. The dashed line gives a detection limit (causedby geologicalnoise) of about 2 percent of the primary magnetic field strength. The response from the spheroidis enormous, especially when compared with the vortex current response from a horizontal

As o-2 --->0, Ei/E e --->0 as well, and at high values of half-space resistivity the galvanic current component will go to zero as statedearlier, leaving only the vortex component which finally starts to dominate the magnetic field response.

lO2

1 I

I

I

!

i

!

i

!

!

!

cylinder having the same value of b and the indicated

!

.

IOImho/mß 01 02 ß O.001mho/m'l lO

1

I0 ø

100

i0'l

Detection Llml[-Y-.7/-Oi : IO'mho/m

Vortex Current Response

1•1 i

103

i

I

I

I

I

I

I

I

I

I

I

I

i

101

•o

I

I

I

I

I

100

a (meters)

Fig. 99. Prolate spheroid--calculatedsecondarymagneticfield anomalyfor VLF.


values of conductivity. It is apparent that long, shallow, moderately conductive targets oriented parallel to the primary electric field can (and do) give very large


VLF

anomalies.

There is another important difference between the responsesof very long and short targets. For very long targets (D ,• 1) of intermediate conductivity contrast the internal electric field in the spheroid is approximately equal to the external field (equation 210) and the internal current density is approximately

Ji • 0-1Ee p'

(211)

ButsinceEe • increases asp•/2SOalsodoesJi andso will the resultant magnetic field anomaly, exhibiting similar behavior to the vertical dike responseshown in Figure 69. To examine the other extreme let a short target be represented by the sphere shown in Figure 100. When placed in a static uniform electric field the induced dipole moment is given by (Ward and Hohmann, Vol. i, •9gg•.

p =

0-1--0-2a 3EeP

0-1 +2o'2

(212)

and the resultant azimuthal magnetic field strength (in a homogeneouswhole space) by 0-2P

H, =

2ß

(213)

UsingthefactthatIEePl = (to•0-2) 1/2/4p .he, we

con-

609

anomaly in such a region as a conductive sphere of radius3 m with •r• = 0.3 S/m buried to a depth of 3 m in terrain with •r2 = 0.2 S/m; such conductivity contrast can be easily realized in arid overburden conditions. Assumingthat we are interested in the magnetic field at the surface (z = a) and working at 20 kHz we calculate, using equation (206), that the resultant magnetic field anomaly can be of the order of 13 percent, large enough to be a serious problem. We see that in areas

of

variable

conductive

overburden

we

must

expect significant magnetic field anomalies from the overburden itself. Conversely, in more resistive overburden, for example •r2 = 50 m S/m, small variations in conductivity such as an increase to 75 m S/m will produce anomalies of the order of a few percent and will not generally constitute an important source of noise.

Electric Field Response

In the Layered Earth-Surface Impedance sectionwe stated that, as a VLF plane wave propagated over ground of varying resistivity the variations in the horizontal magnetic field were small enough so that this component acted as a good amplitude and phase reference against which to measure the larger variations in the horizontal

electric field. In the Plane Wave

Responsesfrom 2-D Targets section we justified that argument by showing that the maximum variation in the magnetic field was of the order of 100 percent, certainly not negligible, but much smaller than variations in the horizontal electric field which, being

proportional to p•/2,canvarybynearlytwoorders of

clude that

IH•bl_ a3

(0-1-- 0'2)

0-1 +20-2 He p z2(0911,02)1/2

magnitudeas p• variesfrom one to many thousandsof

(214)

which shows that for a "blobby" target the VLF

magnetic fieldresponse canbeexpected to increase

essentially as cr2•/2, a quitedifferent fromthe long target. Such behavior can be important when working in regions of thick conductive overburden where the conductivity can be expected to vary significantly from point to point. Suppose that we represent an

ohm meters.

The question arises as to how large a target area must be to allow us to use 1-D theory in our survey interpretation; the results of the preceding sections suggestthat for a circular anomaly such as in Figure 101 the diameter d must be at least several skin depths

(in terms of p•) in order to escape the influence of "edge effects" in electric and magnetic field components measured at the center of the anomaly. What about small target detectionusingelectric field measurement?Figure 100 illustrated a sphereof radius a, assumedto be small comparedwith a skin depth; to

L o2

(

d

Pl

,

_1

)

P2 Fig. tOO.Smallspherical anomalous region.H, is measured above sphere center.

Fig. tot. Cross sectional view of a large circular anomalous region with diameter d.


610

McNeill and Labson

be detectablethe spheremustalsobe locatedat depth z which is also shallowcomparedwith a skin depth. With these assumptionssimpledc theory will again give an approximationto the electricfield response. Ward and Hohmann, Vol. I, (1988)showthat the total, horizontal electric field along a profile at vertical distancez from the spherecenteris givenby

Ex_

contrast but quickly saturatingas the contrast approaches 0.10 or 10. Electric field measurements are

poorat resolvingthe conductivityof highlycontrasting targets.

The peak anomalyfor a saturated(crl/tr2 > 10) conductivesphereis given by Esat

0-1-- 0'2 3 2X2-- z2

1 (216)

Ee p- 1+0'1 + 20'2 a (x 2+ z 2)5/2.(215) Examplesof calculatedprofilesfor a sphereburiedat depth to center of 1.5a are given in Figure 102. The

which means that, if an anomaly with a value of Esat/Ee p of 0.75 is consideredmarginallydetectablethe maximumdepthto centerwill bejust under 1.5a. Less

peak anomaly is, of course, much smaller than the

conductivitycontrast,initiallyincreasinglinearlywith 1oS

1,4

m

1.3

-

1.2

-

01

1.1

Peak anomalies for•22<1

1.0 1.5

0.9

0

E( x )

01 0,7

m

0.6

-

0.5

-

0.4

-

0.$

-

0.2

-

0.1

-

ø'•22

0.0 -5

-4

-3

-2

-1

0

I

2

3

4

5

X

a

Fig.102.Electric fieldprofiles overa smallburied sphere. Depthtosphere center = 1.5a. ForO'l/O' 2 < 1onlypeak

valueis shown(profiles areinvertedversions of thosefor O'•/O'2 > 1).



contrasting targets will, of course, be detectable only at shallower depths. Finally, remember that these calculationsare at best crude approximations; furthermore, in the general case of targets of dimensions comparable to a skin depth the responsewill be a complicated function of target size, shape, depth, and conductivity contrast. EFFECTS

OF TOPOGRAPHY FIELD

ON VLF

MAGNETIC

RESPONSE

The VLF responsefrom terrain topography is basically a function of the amount of topographic relief in terms of a skin depth and thus there is no simple way to correct for topography without knowledge of the terrain resistivity. When the strike of the relief is in the direction of propagation (i.e., of the direction to the transmitter, Figure 103a) and where the skin depth is much smaller than the relief, it is evident from our earlier argumentsthat the slope of the total magnetic field must tend to parallel that of the topography, as will thus also the tilt angle. This effect is shown in Figures 104, 105, and 106 where the average terrain slope is approximately 25 degreesand the corresponding tilt angle 23 degrees. In each case a smaller quadrature phase anomaly is generated by the imbalance between the opposingquadrature phase currents that exists in the vicinity of the slopes. In the other limit, where the skin depth is much larger than the topographicrelief, the anomaly due to topography can be small. However, we must realize that the presence of patches of conductive overburden or weathered rock, either localized or extended on otherwise resistive slopes,will producean appropriate localized or extended anomaly. Thus, in the general case where the direction of propagation follows the strike, the responsefrom the topography will be a more or less subduedversion of the local slope angle. When the direction of structure strike is perpendicular to the direction of propagation as in Figure 103b, application of Ampere's Law (as in the Vertical Contact•H Polarization section) shows that there is no magnetic field anomaly. Several papers have been written on the treatment of topographic effects on VLF surveys. In view of the complexity of the response, depending as it does on local resistivity, the best approach is simply to be on guard for normal polarity anomalies that are located close to topographic peaks and reversed polarity anomalies located near valley bottoms. In the event of difficulties Wittles (1969) suggeststhat to sketch out the topographicprofile along the survey line is useful, and also to calculate and plot out local slope in either dip angle or percent (dependingon which units were used for the VLF survey) for direct comparison with

611

the VLF data. Anomalies which are not obviously related to topographic features are then selected for further investigation. Unfortunately, there will always be cases where bona fide conductive targets are also associatedwith topography and these may get left out in this simple analysis. If long slopesare present, large and slowly varying VLF anomalies may also occur, with the result that smaller anomalies

due to local conductors

will often be

riding somewhereon the flanks of regionaltopographic anomalies and a local crossover will not occur; the location of the anomaly is now marked by the location of the point of maximum inflection, as shown in Figure 107. Use of a differencing filter, such as the Fraser filter (described in the next section), is often effective in removing the affect of topographic anomalies, particularly when their spatial wavelength is substantially greater than that of the desired conductors, since such filtering effectively removes the responsefrom slowly varying features by a high-passfiltering action. DATA FILTERING

TECHNIQUES

Partly to avoid problemsarisingfrom slow temporal variations in primary field strength, most VLF receivers in one way or another measure the ratio of the vertical to horizontal magnetic field strength which, as we have seen, produces a cross-over response from vertical dikes. Sucha responsecannotbe conveniently contoured. Fraser (1969) overcame this problem by devising a simple numerical filter which converts cross-oversof the correct polarity into peak responses by differencingsuccessivevalues of tilt-angle alongthe survey profile. His filter was to satisfy four criteria: (1) to shift the dip angle data by 90 degrees to convert cross-oversand inflections into peaks, (2) to attenuate the long spatial wavelengths, (3) to not increase random noise in the data, and (4) to be simple to use.

Taking successivedifferencesof tilt angle or vertical field strength data (typically obtained at 15 rn station intervals),i.e., calculatingM 2 - M1, achievesthe first two objectives;numericallyaveragingweightedvalues of three adjacent sets of such differences, i.e., taking (M 2 - M1)/4 + (M 3 - M2)/2 + (M 4 - M3)/4 reduces noise; finally, eliminating the constant multiplier producesthe extremely simple and effective filter (M 3 + M4) - (M 1 + M2). This filter is widely used in the reduction of VLF (and other geophysical) data today. An example of both original and filtered data from Fraser (1969) is shown in Figures 108 and 109. The raw dip angle data in Figure 108 clearly indicates several conductors but the overall structure remains quite obscure. The ill-

612

McNeill and Labson

(a)

E-Polarization

VLF

Magnetic

Field


VL

/

Survey Line•,//••urrent Flow

(b)

H-Polarization

VLF

///•

VLF Magnetic Fiel --,

Current

Flow

.-,

Fig. 103.Effectof topographyon VLF currentflow andmagneticfield(a) E-polarization,(b) H-polarization.

160

I

IZO

-

80

-

40

-

I

I

I

I

I

I

I

I

I

I

I

I

80

60

slope distance dl•.->,• j• •'--

40

w•

ZO

z

bJ

•o

0

uJ -40

-3ø5 -

-zo

-80

-40

-IZO

-60

-0,5 -160 -I,40

I

-I.2O

I

-I.OO

I

-0.80

I

-0.60

I

-0.40

I

I

-o.2o

o.o x

I

o,2o

I

0.40

I

0.60

I

0.80

I

I.OO

I

1.2o

-80

KFT

Fig. 104.Topographic high,p• = 100l•.m, • = 35m, dI = 250m, dl/•a1 = 7.1, h• = 100m, hl/• 1 = 2.9, w• = 400 m, w•/•

= 11.4 (after Madden and Vozoff, 1971).


614

McNeill and Labson

tered data in Figure 109 has been enormously enhancedand now evidentis the relationshipbetweena zone of nearly massivepyrite and two brecciatedfault zones.


For easy visualization the filtered data are to be

preferredyet many geophysicistsprefer to also work with the raw data for several reasons'

(1) The actionof the filter can displacethe anomaly peak slightlyalongthe line (as shownby comparisonof the two figures)andfor accurateanomaly location the raw data are the more reliable.

e

Anomaly dueto conducting body

ß .,.,

.

graphic anomaly

(2) When the anomalyis of complex shape,for examplearisingfrom multiple structures,the detail in the raw profilecan be substantiallyalteredin the filtered

version.

(3) The act of filtering makes the responsefrom contacts look like that from linear conductors.

J

FACING :

I I

I I

I

GROUND

ß SURFACE -

(4) Examinationof Figure69 showsthat the slope of the cross-overdirectly above the anomaly,and thus the amplitudeof the filtered anomaly(which is proportionalto this slope),are relatively insensitive to the conductivity-thickness product of a vertical conductingdike.

Fig. 107. Schematicresponseof a conductorsuperimposed

There are thusgoodargumentsfor usingboth the raw and the filtereddata in surveyinterpretation. A somewhat different approach has been taken in Karous and Hjelt (1983). Startingwith the Biot-Savart Law to describe the magnetic field arising from a subsurface2-D current distribution, these authorsuse linearfilter theoryto solvethe integralequationfor the

on topographic effects (after Whittles, 1969).

current distribution, assumed to be located in a thin

onducting mineral body

5S 2S

17N

25N

2ON

54N

39 N

35 N

35N 3ON

DIPANGLE DEGREES ? 2,0.p

••'-

FEET o

Fig. 108.Dip-angledatain the vicinityof the Temagami mine.The arrowdefinesthe VLF-EM primaryfield

directionfrom the transmitterat Seattle,Washington (afterFraser,1969).



horizontal sheet of varying current density, situated everywhere at a depth equal to the distancebetween the measurementstations. By selectingdata points at progressivelygreater distanceapart, the behavior of the current distribution in the assumed sheet, now at

progressivelygreater depths, can be inferred. These authors

determined

that the shortest filter that cor-

rectly inverts the field of a singlecurrent line element with error less than 8 percent has the simple form

each of the models, and also the current data contoured in cross-section.

+ 0.205H3

article. CASE

(217)

where /Xz is the assumed thickness of the current

sheet,/Xx is the distance between the data points and also the depth to the current sheet, location of the calculatedcurrent density is beneath the center point of the six data points, and the values of H are the normalized vertical magneticfield anomaly at each of the six data points. Figures 110 and 111 show the results of using this filter on responsescalculated for 2-D vertical and dippingdikes in a relatively resistivehalf-space.Illustrated are the current distribution for four depths for

It is observed that for the case

of the shallow dippingdike the dip is indeed indicated by the current density contours. These authors also attempt to apply the same filter to the quadraturecomponentof the vertical magnetic field strengthfrom these models. The results are rather meaninglessfor the reasons outlined earlier in this

Ia(AX/2) = - 0.205H_2 + 0.323H_• - 1.446H0 + 1.446H• -0.323H2

615

HISTORIES

In this section we present some case histories describing use of the VLF method (both magnetic and electric field measurements)for mineral exploration, groundwater exploration,and generalgeologicalmapping. Case histories illustrating application of data filteringhave alreadybeenpresentedand use of a local transmitter will be described in the following section. Most

of the case histories

will

be taken

The first case history is selected to illustrate the relationshipbetweenthe VLF vertical magneticfield strength and terrain resistivity. The first panel in

98

107' /

-6

-3

-18

-27

-18,

,0

?

the

Case History 1 (Prakla Seismos•location not known)

-26

•T

from

publishedliterature so that the interested reader can easily find further details.

,7o

Fig. 109. Filtered data computedfrom the map in Figure 108 (after Fraser, 1969).


616

McNeill

and Labson

Figure 112 showsHz(I) and Hz(Q) at 10 rn intervals

map given as Figure 114. VLF transmitter NAA was

down an 850 rn survey line over the geologyillustrated in the bottom panel. The secondpanel in the figure is the simple horizontal difference of the inphase data (extracted from the data in the first panel). The third panel is the result of Schlumbergerresistivity profiling with AB/2 = 205 rn and MN/2 = 5 m. The generally good correlation between laws in the differenced inphase vertical magnetic field data and the Schlumberger apparent resistivity laws is immediately obvious. The filtered inphase data appear as a vertically compressedversion of the resistivity profile. The good correlation between the differenced inphase data and the apparent resistivity in this example is due in part to the fact that the low resistivity anomalous areas are dimensionally of the order of a skin depth or less in horizontal extent; there are apparently no thick dike

used.

anomalies, where the VLF present only at the contacts.

Case History 2•(Telford,

anomalies would be

et al., 1977)

The second case history provides an excellent example of VLF responseover a vertical contact, in this

To the southwest

of the contact

the surficial

rocks are resistive (1000s of 12.m) dolomite, limestone and sandstone; to the northeast conductive (85 12.m) shale. VLF survey data of tilt angle and ellipticity are

shown in Figure 115 (the operator was facing northeast). Both setsof data clearly indicate the presenceof the Gloucester

fault

as well

as two

other

contacts

further to the northeast. The positive dip-angle anomaly at the Gloucester fault shows a gentle rise on the southwest and a steep descent to the northeast, correctly indicating conductive material on the northeast side of the contact. On line 45 + 00 S a second contact

with negative peak is seen to have a shallow ascent to the northeast, and thus resistive material on the north-

east side of this contact. A third anomaly, positive on line 37 + 00 S, again has steep descent to the northeast, indicating conductive material to the northeast. Confirmation of the correctness of this interpretation is shown by the profiles of apparent resistivity obtained from VLF electric-field measurementsalong three survey lines, shown in Figure 116. The existence of the block

of resistive

material

to the northeast

is

clearly evident.

case the Gloucester Fault near Ottawa, Canada. The

geologic section is outlined in Figure 113 and a plan

Eegend

Case History 3•(Palacky et al., 1981)

Case history 3 describesuse of VLF magnetic field measurements for detecting water-bearing fracture

lOO

/-•'-N

lO m

//' '",\

C 20m

/,"/-'x',,x Re

z9.

•'

o

-lOO I

-'

k' ',•N ,

oi

o__, o, ] _oo 40 m- ---2o

lOOm

½0' 10 000 •.m

½1' 10•.m

m

lOOm

-lOO

o

1

90'10 000 f/.rn ••-01 10

40

6O

lOOm

:

m

b)

'øø t Re t

ß

.•,'// \

'x %.

/,,'// \ \ 'x '•'•

I•.' I It'll •, .' /

/,//

N

•

'••.•

....-100 ,_.._._.=_.:E=•,• 0 0

-lOO "" .• • "., • -' •.• •

o

maximum

lOO m

' •.

:• •

20 -- -- ---- ---o

o-

/--

100 m

100 m

- lOO o

m90 ' 10 000 Q. m,'••_•91 ", -10 Q. m

i

o

lOO m

I

I

20

40

PO =10000f/.m•Pl =10f/.m .

60

Fig. 110. Computed equivalent current density profiles (real part) at various depthsfor the plate models shown, depth = 5 m, depth extent = 50 m, width = 20 m, Re = real part (Az Ia/2,rHo) (after Karous and Hjelt, 1983).

Fig. 111. The equivalentcurrent-densitycross-sections(real part) for the plate models shown. Parameterssameas Figure 110 (after Karous and Hjelt, 1983).


617

•NE

so.-(%) SSW .

25'


.

VLF-measurement o

Transmitter: GBR(Rugby-England) Frequency-16 kHz

ß

..

L__

'::::::::.• Ht Hx

Surveydirection Linepoints(spacing 10m) Verticalcomponent Horizontalcomponent Inphasecomponent(correctedvalues) ................... Outof phasecomponent (corrected value•)

Resistivity

-25

'

t

o......... ............ •ooo•

profiling

u

(L/27- (a/2)•

I

a

• '

I

,

..

40

I

I

50

60

70

550• (m)m•

and Geology

= Serpentinite = Calcium Silicate rock

,

I

500:]: ks

•l!

m ,¾, I

ooo)

a/2 = 5 m (halfpotential electrodespacing) L/2 = 205 m (halfcurrentelectrodespacing) .................. L/2 = 105 m (halfcurrentelectrodespacing)

S

......:

(t•n)

Pa =

Topography

ß

10 Horizontally differenced data

' '•:

•o•

•

ß

Gg = younger medium grainGranite gnfi = Biotite-Plagioclase-schist Gneiss gnz = Biotite-Plagioclase-slip bandGneiss

400

"'

•-:..•

90

,

-

120

•m mm•m

,I

-I

•

•'

i1,/•1I

.• II

-

.

••

110

100

I

e

•. •

80

sol

"1 I•1•'• ..I

I

•

I I;.•

•1

350

40

50

60

70

80

90

100

llO

Fig. 112. CombinedVLF and resistivitysurvey(after Prakla-Seismos GMBH, 1981). 400' -

• oo'

' = -'- _-. ß:. :-.'•-_. :. :- - ' _....

SEA LEVEL

-200'

-400'

I

-600" -800' -I000'

-1200" I---.

ICARLSBAD

--

: ----4---'--

.1'-•'•'1EAST VIEW

•OXFORD

I-'1o •IFoRMAT•oN •sT SS, MARTIN i '-'7::lSh, SandySh i::•::iF.o.•T,o. •',.•"lSh, Is,dom i: - _ils,dom •-:x.•-'--• FORMATION

•OTTAWA Y,9/JFORMATION •?• •sORMATIO N •r.•5• Sh

I/?/ISh

.!.'•NEPEAN

I,

1:4-_-IFORMATION

•

FORMATION SS, dol

IPREcAMBRIAN

FORMATION I ' idol, Is,Gn, Qz

SS

Sh-Shale

__

Is-Limestone

SS-Sondstone

doI-Dolomite

Gn-Gneiss

Qz-Quartzite

Fig. 113.Geologicalcross-section of Gloucesterfault, Leitrim area, Ontario(after Telfordet al., 1977).

120

•

•


618

McNeill

and Labson

zones in Burkina Faso (formerly Upper Volta)just south of the Sahel region in West Africa. Precambrian basement rock in the surveyed areas consisted of either a volcano-sedimentary sequence (quartzite, schist) or granite. In either case a relatively thick weathered zone (saprolite) has formed over the basement complex, from which sampleweatheringprofiles are shown in Figure 117a and b. Typical resistivities for the variouscomponentsof this sequenceare shown in Table 4. The exploration objective in this environment was the detection

of those fractured

factory over this geology,a fact confirmedby Palacky et al. (1981). The skin depth at 20 kHz in 40 •.m material

is 22 m so that at least the shallower

water-

soakedfracture zonesshouldbe detectable.Figure 119 shows results of a survey profile over three photolineaments using both FUO in France (which was in poor coupling) and NAA in the United States (which was in better coupling but remote) as transmitters. Neither VLF dip angle profile confirms photo-lineament L1 but both profiles detect two nearby conductors, A and B. The profile using NAA also clearly showsthejunctionof photo-lineaments L2 and L3, for which FUO is in poor coupling.The cross-oversin the dip-angleprofilesare relativelyclear-cut;the ellipticity profiles do not appear to add useful information. Horizontal loop EM (HLEM) was also successfully

zones in the

bedrock which could function as permanentaquifers, illustrated schematicallyin Figure 118. The amount of water that can be extracted from these small aquifers

is notlarge(1-5 m3/h)butis adequate for ruralwater suppliesused for drinking and domesticpurposes. As suggestedby Figure 117and Table 4 the saprolite is generally thinner and more resistiveover the granite so that we expect VLF techniques to be more satis-

used to locate water

soaked fracture

zones.

The re-

sults using HLEM, VLF, and Schlumbergerprofiling are shown in Figure 120. Excellent agreement is

LEGEND

'• Carlsbad Shale • Ottawa Limestone • Oxford Limestone • March Sandstone !.ø•Nepean .Sandstone

I t

I

/ / /

t

/

t

t

o

\ !

o

i

o

i

iI i

x.x.x.x FaultTrace VLF Survey

\

'•.,'•,'•, FaultTraceGeological Survey AeromagneticContours•3rrrrms

!

l

o øo ß

ß

.

o

--

_

_

_

_

_

--

_

ttor/zonlalScale 0

1500 feel

Fig. 114. Geology and aeromagneticcontours,Leitrim area (after Telford et al., 1977).



w

w

._l

I-

I

619

N '•. 0

• > • t"' t"-

620

McNeill

shown with all three techniquesin locating conductors A and B. Drilling in this case showeddepth to bedrock of 12 m. Again the quadrature phase VLF is uninfor-


mative.

An examplewhere saprolitethickness(from drilling) is 20 m, is shown in Figure 121. A weak anomaly again shows up with all three techniques (we now have a reversed polarity quadrature phase VLF anomaly). The original hole, drilled on the basis of resistivity

only,produced a yieldof0.7m•/h.A subsequent well 5 rn to the southwest was dry, whereas a third well, drilled after the HLEM and VLF measurements, and therefore located 5 rn northeast of the first hole,

yielded1 m•/h illustrating the usefulness of the two latter techniquesin providing accurate target location.

and Labson

Case History 4 (Langron, 1972)

While there are many instancesof the use of VLF techniquesin very resistive environments, where the theoretical depth of exploration in excessof 100 rn can probably be achieved, the more common case resembles the relatively conductive weathered zone described in the previous case history. Shortly after introduction of the first VLF instrumentation by Ronka, a series of tests with the device were carried out in Australia by Langron in areas where geological, geophysical,and drilling informationwere availableto assessthe techniquein areas of steep topographyand conductiveregolith. Australian VLF transmitter NWC was employed. Figure 122 shows dip-angle and ellip-

Horizontal Scale

Vertical Scale o I I

....... • M/I'/'/MHO'$/M'O'••'

Fig. 116. VLF apparentconductivityand phaseprofileson three lines, Leitrim area (after Telford et al., 1977).



ticity data taken in an area of ruggedtopographyin Tasmania. Both the responseto the shallow mineralization, showing a short spatial wavelength response, and the more slowly varying topographic responses are evident.

The latter has caused the mineralization

cross-over to be substantially displaced vertically, as

621

gram and SP anomalies,which trenchingrevealed to be causedby graphiteand carbonaceousbedsbeneath the peak cover, and the second,that appearedto be overburden

related.

Results of another VLF profile in Tasmania, along with geophysicaland drillingdata, are shownin Figure

is often the case.

125. Both VLF

Figure 123 shows a nearby line in the same area. Again the topographicresponseand offsetare substantial but the mineralization, at a depth in excessof 30 rn was felt to have been detected. The entire survey data were Fraser filtered; the results are shown in Figure 124. The central anomaly correlates well with drilling and an out-croppinggossan,and the westernzone also correlateswith mappedgossanand old workings.The eastern anomaly is associatedwith graphitic and pyritic slates in a very steep part of the survey area. Conversely a flat swampy region showed two VLF anomalies, one that coincided with conventional Slin-

tivity lows as seen from the IP survey results. The anomaly at 300 S coincideswith the Turam anomaly over a mineralized zone; the anomaly at 750 N is probablyrelated to clay which was intersectedin the

anomalies are related to surface resis-

drill hole.

Several VLF profiles were carried out in the Kalgoorlie area of Western Australia, the most striking feature of which was the predictablevariability caused by changes in the weathering and salinity of the overburden.

The overall conclusionreached by Langron was that the depth of explorationusing VLF varied from 30 rn in general, to probably less than 15 rn in Western Australia, and less in the Northern Territories.

LATERITE

LATERITE

Case History 5 (Poddar and Rathor, 1983) WEATHERED LAYER

WEATHERED

{SANDY CLAY)

LAYER (CLAY)

This case history illustrates use of electric field measurements for mappingoverburdencover in southern India. In the surveyarea the bedrock, Precambrian granite and gneiss, is overlain by surficial reddish sandy soil of several tens of centimetersthickness, which in turn overlies unconsolidated materials, the

TRANSITION ZONE

productof chemicalweatheringof the bedrock.Previousexperiencewith VLF had shownthat the saprolite was sufficientlyconductive to prevent detection of underlyingbedrockconductorsdue to anomaliesfrom the overburden.Sincegroundwater in this area occurs within the intergranularpore spacesin the weathered layer (as well as in joints and fracturesin the bedrock) the decision was made to attempt to map saprolite thicknessand resistivity using the VLF electric field and layered earth theory. Survey data taken with

ITRANSITION ITRANSITION ZONE

{QUARTZITE)

PARENT

RCX•

(C•ANITE)

ZONE

(SOlST}

PARENT

Australian transmitter NWC is shown in Figure 126,

(QUARTZITE,

a)

schisT}

b)

Fig. 117. Weatheringprofilestypical of the PrecambrianC volcano-sedimentarysequence(a) and Precambrian C gran-

ite (b). The compositionand thicknessof the weathered layer dependuponunderlyinglithology(after Palackyet al., 1981).

Table 4. Resistivities (11.m) obtained in West Africa (Palacky et al., 1981). BasementRock

Granite

Schist

Weathered layer

25-50

10-30

Transition

40-200

250-400

zone

Fresh parent rock

> 1500

> 1000

Amphibolite 5-15 10-80

>500

where the first two panelsshowtilt angleand ellipticity, respectively.The tilt angleexhibitsa high degree of variability (note that the actual responseis rather small, generally of the order of ___5percent); the ellipticity is somewhatsmootherand generallyshows anticorrelation with the ellipticity profile. In both tracesthe rapidfluctuationsare consistentwith a small skin depthand thus conductivesurficialmaterial.The next two panelsshow the phaseangleand magnitude of the VLF resistivity,respectively.The phaseangleis consistently less than 45 degrees, indicative of a conductivelayer over a resistive substrate, and the apparentresistivityvaries from 40 to 100 f•. m. Comparisonof the dip angledata with apparentresistivity


"

FRACTURE ZONES

r•TTT•LATERITE CRUST

•

TRANSITION ZONE

•

•

GRANITE

ALLUVIUM

• CLAY tWEATHERED • SCHIST :*.:• SAND LAYER

.'.F•?'• OUARTZITE

Fig. 118. Schematicsectionof Precambriangeologyand hydrologicalconditionsin Upper Volta (now Borkina Faso). Perched water table sinks and rises dependingon alteration of wet and dry season.The true water table is in excessof 15 m. Economicaquifersare associatedwith fracturedzonesin granitesand more easily weathered lithologies,suchas schists,in the volcano-sedimentarysequences.Wells can be dug (W) in soft materials,but must be drilled (D) in hard rock (after Palacky et al., 1981). HLEM

A,,,,'"•,,, /,,,, ; --,

- .%,,-

•..

4.

.-e-e Inphase . .-.

,_

2.5

•,

-2.5

Quadrature

VLF 2O

B

* * *

FUO

Ouackature

10

:=

0

-10

Resistivity

•8o0 t ß 600

• FRACTURE

500

c• 400

ZONES

200m

.•. WELL(O = lm3/h)

øTM

E

E•

= 20

Quadrature

• 30 t

FRACTURE

A•Profile Fig. 119. VLF results obtained in a granitic area near Rapadama. Three conductors (A, B, C) can be identified at both frequencies (NAA and FUO). The anomalies are associated with fractured zones in granite (after Palaky et al., 1981).

lo6•

t

ZONES

Fig. 120. Results of three geophysical measurementsobtained in a target area near Rapadama. From the top, horizontal-loopEM (HLEM) at two frequencies,VLF (FUO transmitter) and Schlumbergerresistivity profiling (AB = 200 m, MN

-- 40 m). Two conductors, A and B, were

identified by the three methods. Drilling of conductor B revealed a productive aquifer (after Palacky et al., 1981).



showsthat in generala cross-overor inflectionpoint of the correct polarity (the operator was facing to the northeast) is usually associatedwith an apparent resistivity low (see for example station 600 S) and vice-versa, as we would expect. Poddar and Rathot (1983) assume that the ground can be everywhere modeled by a three-layered earth consistingof thin soil, unconsolidatedweathered material, and basement. Such a model has, unfortunately, five parameterswhereas only two (apparent resistivity and phase angle) can be measured. Wennet soundings carried out at stations 490 S, 10 N and 500 N showed, however, that the soil layer uniformly had a resistivity of 150 •.m and thickness of 0.4 m, and furthermore that the basement resistivity was about 1500 •.m. Their approach was, therefore, to ignore the presence of the thin resistive soil layer (whose conductancewas only 0.0027 S and to assume a uniform bedrock of resistivity 1500 •.m, thus reducing the number of variables to two, vis weathered zone resistivity and

----

Inphase

623

thickness. With these assumptionsthe simple phasor diagram shown in Figure 127 (and essentiallythe same as Figure 28) was constructed for interpretation. Examination of the diagram shows that over the range of interest(40 < Pa < 100•.m) the variation of apparent resistivity mainly reflects changesin Pl, and that the phase angle responds principally to changes in thicknesshi. At the extreme values of hi = 0 the apparent resistivity tends to 1500 fl.m with phase angle 45 degrees,and at large hi, the apparentresistivity tends to the overburden resistivity and the phase angle again to 45 degrees.Variation of P2 about 1500 fl.m makes little difference in the interpretation as long as the overburden is reasonably thick. Results of the interpretations are shown in the bottom of Figure 126, which shows that the overburden thickness changes relatively slowly but that there are large variations in overburden resistivity (17 to 69 fl.m) and that these variations are indeed causing the response in the magnetic field components. To further confirm this interpretation the survey profile was measuredusing a Wennet array with small a spacing of 5 m, the results of which are shown in the fourth panel of Figure 126, and which in general agree well with the interpreted values of Pl. We see that to carry out a quantitative interpretation of electric field VLF data in areas when the geometry is simple and straightforwardis possible.

ß ,-o Quadralure

lO

•'

Case History 6--(Scott, 1975)

c

'

Our last case history deals with mineral exploration for massive sulphides, using electric field measurements. The location is Agricola Lake in the Canadian

f-1777Hz

North West Territories; VLF transmitter NAA in the east coast of the USA

was the source.

Unlike

the

previous surveys, this example takes place in a highly resistive environment, indeed the area is within the Resistivity

-lO

c

30o

2o0 15o

•••,,•'•

oSW

_ •

a0;

ß 4O

•'WELL (elm3/h)

ti

-•kWELL {Q-O.7m3/h) •

•

::.

ß

NE

D•Y HOLE

l i:i:!:i:i:

FRACTURE

ZONE

continuous permafrost zone where the ground is perennially frozen. Despite this fact, there is a wide variation of resistivity, both within and outside the mineralized

zone.

The Yava Syndicate deposit, containing pyrite, chalcopyrite, galena, and sphalerite, is approximately 30 m thick, dips nearly vertically, and has a strike length of 200 m. The deposit lies in a zone of hydrothermal alteration in a sequence of metavolcanics, which in turn lie between granites to the south and metasedimentsto the north. The results of the survey, which consisted of about 900 measurements

Fig. 121. HLEM, VLF and resistivity profiling results obtained in a target area near Rapadama. Although the conductor C was detected by all three methods, its center was determined more accurately by the EM methods. The water yield depends critically upon the location of the drill holes (after Palacky et al., 1981).

taken on a

30 m x 15 m grid in four and one-half field days, are shown in Figure 128. Considerable detail is exhibited by the resistivity contours; the lowest value of apparent resistivity was 1 •.m and the highest 64 000 g.m. Phaseangle measurementswere difficult to interpret

624

McNeill and Labson


40

30

%

C3

20

O

I--

IN-PHAS

10

-10 I

I

t

I

I

I

t

1400

DDH

3

ng mlnerallzatlon

1300 Oxidize, Ore

iii

2,

1200

e Shales,

, /, Un½onformlty• Mudstones,.,, ' \

z

ß Galena,/?

z

elsrite

o

iii

iii

• Shales, Dolomites, tuffs

/i

Dolomites

1000 900

p• Quartzites,

Mlcaceous Shale,

Ineraliza /•? and Slates ....... •

800

700

,//

Sph ,;'

Micaceous Shales, Slate, etc

Unconformlty œ

600'

• 20W

i

i

i

&

i

I

16

16

14

12

10

6

50 ß ß

ß

'

I

0I

2•)0' ' ' •)

50 {

100

2•)0

150 meters

4'•)0 feet

Fig. 122.VLF anddrillingresults,traverse18S,Zeehan,Tasmania (afterLangron,1972).

I

625



40

i

i

'

,

-10

'

1400

-

t

I

ß

i

t

I

•

30

20

10

I

I

I

1300

DDH

4

DDH

•_ 1200 iii iii

Z

I

Weathered Dolomite and shales

pE Mlcaceous Shales and

1100

Slates

Z

O 1000 Shales, iii

.•

Dolomites

Discontinuous

and tuffs

weak

mineralizatlon

900

iii

8OO

700

600

.

!

:,OW

I

I

I

I

&

18

16

14

12

10

50

200

0

50

•)

100

2•)0

150 meter8

4•)0feet

Fig. 123.VLF anddrillingresults,traverse28S,Zeehan,Tasmania(afterLangron,1972).


626

McNeill and Labson

but the apparentresistivitywas consistentlyrelatedto knowngeology.The clearlydefinedcentralresistivity low of 40 I2.m agreeswell with the surfaceprojection of the limits of the orebody as outlined by diamond drilling. Surroundingthis low lies a regionof lessthan 100012.m, the outline of which enclosesthe regionof hydrothermalalteration. Within this region the area enclosed by the 1000 I2.m contour of the southwest of

the sulphidesoccurswhere shalefragmentshave been observedin frost boils, and the area is tentatively interpreted as being underlain by shale. The outer regions to the north and south coincide with areas mapped as metavolcanic. Finally the traces of two

the VLF resistivitytechniqueis an excellentmapping tool aslongas it is not limitedin depthof operationby conductive

surficial material.

USE OF LOCAL

TRANSMITTERS

As will be seen shortly, local transmitters,either of the groundedbipoleor largeloop type, give anomalies that are often virtually identical to those obtained using remote VLF transmitters(particularlythe inphasecomponent). This is clear when one realizes that VLF anomaliesare essentiallyelectric field induced

faults, inferredfrom distortionsin the resistivitycon-

andthat the surveyrequirementis, therefore,onlythat the electricfield be reasonablywell duplicatedby the

tours, are shown in Figure 129. Scott concludesthat

local transmitter.

e

i

4

ZW

0 I4S

ZOW: tO II . t4S

c•

c•

$zs

8

&

4

ZW

•) 5O

200

0

50

•) CONTOUR

100

150

i

I

200 INTERVAL

meters

400feet 10%

Fig. 124.Contoursof filteredVLF data,Zeehan,Tasmania(afterLangron,1972).

o


627


0

-30 1.3

1.1

TURAM

1.o

\

,"• ,,"

•.

'

'

-S0ø

RESULTS

660 HZ

-

, PHASE

[ I•'"' ",' DIFFERENCE •, I

8

1000S

L'100ø

6

4

2

0

2

4

6

8

1000N

pcI

IP RESULTS F.E. % DIPOLE-DIPOLE CONFIGURATION 5.0/0.3 HZ

1000S i

8

6

i

4

,

2

i

R.L. 1600•

•4oo• •

',

1200 OWEN

0

I

•

2

i

•

4 l

6

8

1000N

0

_•. •:
I.

<•

--

• O •,•

•

•

'.

,:

•

....

- .....

% 215'8 '•

,

lOOO PROJECTED BORNITE •,SCHIST •LITTLE ALTERED LODE

'•-'"

••

• VOLCANICS*

%• ',,

INFERRED

GEOLOGICAL SECTION

VOLCANIC BRECC•A/

•••••SERICITIC_ MINERALIZATION SCHIST / CONGLOMERATE

Fig. 125. Comparison of geophysicalresults and geologicalsectionsin the Comstock area, Queenstown (after Langron, 1972).

628

McNeill o

moos


I

,oo •o•

eoo I

and Labson

I

L•ne

$oo I

!

400 I

I

•oo I

I

o I

I

2oo I

I

4oo I

6oo

I

I

I

0

6oo I

I

,ooo N I

I

,•oF

00

,50

•00 •0

o

-20 -&o

1500

BEDROCK 1500

'

1500

Fig. 126. VLF-EM, VLF-EMR, and Wenner resistivitymeasurementsin a graniticarea. From the top: VLF-EM real component,imaginarycomponent,EMR phaseangle, amplitude,Wenner resistivityand EMR interpretation. Numbers indicate the interpretedresistivitiesin 11.m and dashesthe base of weathering.Elevation referenceis arbitrary (after Poddar and Rathor, 1983).

Grounded Electric Bipole Transmitter

the azimuth of the magneticfield at intervals of 150 rn alongthe survey profile, particularly when nearingthe

Use of a grounded electric bipole at VLF frequencies was describedfirst in Tilsley (1976). The transmitter, which is basicallya low-power VLF signalgenerator operatingat 16.55 kHz, is fed into a long thin wire of total length 1000to 1500m. The ends of the wire are connectedto the earth galvanically, using steel stakes, when the ground is relatively conductive, or capaci-

ends of the wire or when near man-made

conductors

such as grounded fences, to establish that undue field curvature or large scale changesof direction were not occurring. Although the VLF anomalies are produced by the primary electric field we are measuringthe magnetic field components and, therefore, must also be con-

tively, usingabout6 m2 of screenat eachend,for resistive ground. The output stageof the generatoris tunable and maximum current into the long line is slightly over 1 A. Power for the signal generator is suppliedfrom a 500 W gasoline-drivengenerator. In the field the long wire is oriented parallel to the directionof anticipatedgeologicalstrike and is located about one-half the wire length away from the survey area. Profile lines, which are laid out at the normal spacing, are oriented perpendicularto the long wire and of course can be on either side of the wire.

A plot of the measured field strength from such a long wire is shown in Figure 130. Measurementof the standardVLF componentscan generally be made as near as a few hundred meters to the wire, and out to a distanceof from 1000to 3000rn at low and highterrain resistivities, respectively. Indeed Tilsley states that underfavorableconditionsa null of -2 percentcan be achieved at 4.5 km from the wire. He also measured

•a(.f)..m) Ioo

lo

IOOO

= 1500fl- rn

•)p 20

25

•o

•

Fig. 127. A phasor diagramfor two-layer interpretation of EMR data. A knowledgeof P2is necessaryfor the use of this technique (after Poddar and Rathor, 1983).

uJ


o

o

•)

•

I

o

o

•)

I000

526' I I I I I

I I I

I

I

---- J 790N

,

0

•0

t

,I

N

VLF APPARENT RESISTIVITY (.ri.. m)

I00 m

Tx NAA (CUTLER)

I

17.8

Khz

Fig. 128.Contourmapof VLF apparent resistivity, AgricolaLakemassive sulfideprospect, N.W.T. (afterScott, 1975). Ld

1,165 N

.......JAXIS OF RESISTIVITY

LOW

/

//

-..

INFERRED FAULT "x

,SHALE BOUNDARY .•••----,,.• •

...... 0

50

.........

I

I, 000 N

790

I00 m

Fig. 129.Geological interpretation of VLF apparent resistivitydata,AgricolaLake massivesulphide prospect, N.W.T.

(after Scott, 1975).


630

McNeill and Labson

cernedwith the behaviorof the primarymagneticfield. Sincethe returncurrentflowsbackto the wire through the ground at an effective depth of the order of one-half skin depth the primary magneticfield can be thoughtof asarisingfrom a verticalloopof current,as shown schematicallyin Figure 131. Figure 131 also illustratesthat, while the primarymagneticfieldwill be essentiallyhorizontal at large distancefrom the wire (in termsof a skin depth)there will be an appreciable vertical component at nearer distances. This feature

dictateshow close to the wire the survey may be carried out. Various filteringprocedures,suchas the Fraserfilter, are effectiveat removingspatiallyslowly varying features from the VLF profile and are thus useful for removing this progressive wave tilt that occurs as the wire is approached. Survey results made in Tilsley (1973) over the Cavendishtest range using both a portable electric bipoletransmitterwith a 1 km longwire and a regular VLF station are shown in Figure 132. The wire was locatedslightlyover 1 km from the targetarea, and at

this site measurements couldbe madeto -+3 percent 0%

:)%

70%

Grounded

Ion,

PORTABLE VLF SOURCE -I$ KHz -LINE B ,,50 %

/

'% ....

I

0%

i

• INPHASE/

s - 50%

'""... / ,1, •

'

•

%,.z"'x•,% ....

+

NBA - BALBOA, CANAL ZONE PANAMA - 24 KHz

Fig. 130. Inclination and dip of magneticcomponentof electromagneticfield (shown by arrows) and contoursof relativefield strengthfor a longgroundedwire source(after Tilsley, 1976).

Fig. 132.VLF surveydataat Cavendish,Ontario,usingboth groundedlong-wire source and NBA, Canal Zone (after Tilsley, 1976).

\x,x.Magnetic component arising from grounded wire

transmitter

';'"",'Nearfield'dueto primary current inlongwire 131 (a)

131 (b)

Fig. 131.Grounded long-wire transmitter. (a) schematic current flow,(b)magnetic component of electromagnetic

fieldin planeat 90 degreesto axisof grounded wire (afterTilsley, 1973).

631


both for inphase and quadraturephase components.

in extremely resistiveground. The field components from a horizontal loop are shown in Figure 133; at distanceswhich are near the loop (in terms of a skin

Large Loop Transmitter

Hr; at largedistances theconverse is trueandthefield

The use of a large horizontal loop transmitter was describedin Sinha and Hayles (1988). Their objective wasthe designof a transmitterthat couldbe employed

components eventuallyasymptoteto thoseof a propagatingplanewave. The surveydesignproblemis that neartheloopthe verticalcomponentH z dominatesthe measurementwhereas at large distances the radial

precisionat distancesin excessof 3 km from the wire. Agreementbetweenthe two transmittersis excellent,


depth).Figure134showsthatH z is muchgreaterthan

component, whichdecreases as r-4 becomes too Structure

small to measure with satisfactory precision. Figure

134bshowsoptimumsurveydistances for a 500 x 500m singleturn loop of No. 10 wire carryinga currentof 5 A at a frequencyof 16.55kHz. The loop, resonated with high-voltage,high-powercapacitors,was driven by a poweramplifierwhichwasin turnfed by a trailer mounted diesel generator.

measurements) I

The objectiveof the surveywas (1) to ascertainthe degreeto which the loop could duplicatesurvey results obtained with a remote transmitter in good cou-

pling with the target and (2) to establishthe survey deteriorationresultingfrom poor couplingby deliberately locatingthe loop transmitterso that it was in LOOP ANTENNArelativelypoor couplingwith structurethat had been A=500m x500m mappedusinglocal and remotetransmittersin good coupling.Locationof the surveyarea,loopsites,and

Fig. 133. Field componentsfrom a large loop source.

\•\ •\

10-.• -

ßM

10_ 6 -

(a)

77" 27.4' 4 6" 5.6'

Ph" Hz

3x•0• • f•

I

10'7 -

•h"

77' 21.6' 4 6" 5.8'

Hr

• •

• ,o'•.m• lO-.

102

10 ] DISTANCE

104

\

U. BA88

(m)

U.VE AREA 50-

,Oh= 104•.m

30-

(b)

-

\

')h'

\ \

.

\ \.

OPTIMUM / /

----\ •

/ /

3x103• ßm, 10 3

/

/ / /

i

10 4 0

DISTANCE

1.0

2.0kin

(m)

Fig. 134.(a) Variationof thehorizontalandverticalmagnetic fieldstrengthswith distancefrom the centerof a loop source

overresistive half-spaces (Ph= 3 x 103 and104 l•'m. (b) Plot of tilt angleversusdistancefrom the centerof the loop source(after Sinha and Hayles, 1988).

4e" 00' ??" 27.4'

71'

4e" 00' 21.8'

Fig. 135. Generallayout of the Chalk River researcharea showingthe locationof the surveyareaandpositionsof the local loops(after Sinhaand Hayles, 1988).

632

McNeill

and Labson

survey lines is shown in Figure 135. The survey area was generally highly resistive with deep and irregular bedrock troughs filled with moderately resistive (200 11.m) overburden.


Measurements

taken

of the azimuth

offsetdue to residualH z arisingfrom the loop transmitter. Interestingly, the results from loop 3 located the various conductors seen from loops 1 and 2 in virtually the same position as for the case of essentially perfect coupling. Furthermore the magnitudeof the tilt angle and ellipticity was not significantlydifferent for coupling up to 20 to 30 degreesaway from

of the total

horizontal magnetic field at each site showed good agreement with theory. Typical results for tilt angle and ellipticity are shown in Figures 136 and 137. Agreement using the two sourcesis quite satisfactory after taking into account that the tilt angle (and, to a lesser extent, the ellipticity) exhibit a slowly varying

the maximum

and these authors

114N

115N

116N

It? N

lION

119N

120N

114 N

II•N

116N

II?N

11ON

119N

120N

I

i

I

I

I

I

confirmed

that two

VLF transmitters,in directions at right anglesto each other, would map all the structure that could be located using VLF techniques.

I

I

I

I

(a) TILT

ANGLE

(RAW DATA)

I 40

(b)

%

FACING

I

I

I

DIFIECTION

•o[ -40

I

•' .... •'•'""" ,,

ß"ø-''"

......... '-' •.•'•'

/

IO4 E

,[oo, ,,

,.... •,-'NAA

/

o

200

[, i i

F •,•o

METERS

DIRE[:

TO

!oNl L

HAA

LIMITS

FOil

SIMULATED

DIRECTION

TO

NAA.

Fig. 136. Comparisonof the tilt angle and quadraturesurvey results at Chalk River usingNAA and the local loop (after Sinha and Hayles, 1988).



The minimum survey distance from the source is usually a few hundred meters and the maximum distance, dependingon the transmitter current, is of the

A horizontal loop or long groundedwire transmitter can also be used with VLF resistivity instruments.The groundedwire should be about 1000 m long and the singleturn loop of the order of $00 m in diameter. The orientation of the field components and preferred surveyline directionsare shownin Figure 138a,b. For both transmittertypes the vertical magneticfield com-

order of 1500 to 2000 m for 1 A.

In general, portable VLF transmitters have three areas of application. These are (1) in regions where there are no useful VLF transmitters (i.e., southcentral Africa) or the available transmitter is intermittent to the point of being unusable; (2) where the available VLF transmitter is in poor coupling with known strike; and (3) where there are two sets of structure, requiring transmittersin two different directions. In any of these situations the advantages of a portable transmitter can definitely make-up for the

ponentH z decaysrelativelyrapidly with radial distance from the source so that, at distances of the order

of one skin depth the field componentsare essentially

E•, andHr. Asforplanewaveexcitation themagnetic field receiver is used first to determine

the direction of

the horizontal magnetic field component so that the electric field electrodes can be laid out at right angles.

)DOE I

(a)

i0i E I

10Zœ !

10$E I

104œ I

iO5E I

,,,,,,.

I0

633

IO6E I

/

I

"--'"

NORTH

-I' •,/..•'•/ '/•'/ • CONOUClON 4 ,.o•, " CONDUCTOR

./ ., /

./•

..---" -"

---

NSS---•"

io1[

.4

(b) QUADRATUNœ (RAW

DATA)

•0

i

%

/

//tL/ ,' !

'•"

,

,, ,

TO

'•.•_ ....... /.3',.: ....... .......

//

/./'/

".... IOO[

/

/'• /'

;.•_./ LOOP •/'

,"

.

NSS.

o,

i

IOZ[

IO3œ

I

io4[

I

IO5[

I

i

IO&[ I

IO7[ i

FACING DINœCTION

,o -10

-•0

LOCAL

..<.•. ..... --..,• ....... ............................. ..

'--.•.'-"-'-' .. _

.......

LOOP

• NSS _""..,E.:• -.... •-*._-'-'-.-....-•,•"•

._ :•,,•.. 112N

o I

200 ,,

I

METERS

Fig. 137.Comparisonof the tilt angleand quadraturesurveyresultsat ChalkRiver usingNSS andthe localloop (after Sinha and Hayles, 1988).

634

McNeill

and Labson

disadvantagesof reduced survey area and the extra time and effort involvedin layingout the loop or long wire.


SUMMARY

straightforwardbut that the quadraturephasecomponent could give difficulties. Finally, we learned that for the case of a conductive overburden overlying a resistive bedrock, the amplitude of the horizontal

In this article we have attempted to give some physical insight into the factors that control the responseof various types of subsurfacetarget to plane wave excitation at VLF frequencies.A summarization

electric

field at the base of the

overburden decreasedmuch more rapidly with overburden thickness than we might expect from simple skin depthconsiderationsas a resultof multiple reflections in the overburden.

of these factors follows. Contact--H

Horizontally Layered Earth

We learned that in the absenceof abrupt boundaries the surfacehorizontalelectricfield continuallyreflects the local surface impedance, from which local resistivity can be derived. From the complex surface impedance, layering can be detected and, in the case of a (electrically) moderately thin conductiveoverburden overlyinga resistivebedrock, both resistivityand thickness of the overburden can be obtained. We saw

that, in general,the inphasecomponentof the current density was well behaved with depth but that the quadrature component underwent a reversal in direction, and that the absenceof a quadraturemagnetic field componentat the surfacewas due to balancingof the contribution

from current flow in both directions.

This behaviorled us to suspectthat interpretationof inphasemagneticfield anomaliesmightbe reasonably

Tx

Polarization

We saw that there was no magnetic field response and that the electric field response(which accurately located the contact) could be explained in terms of induced charges at the contact and at the earth's surface. The overall electric field responsewas controlled by the conductivitycontrastand the skindepth on each side of the contact.

Contact--E

Polarization

We learned that the behavior of the inphasemagnetic field responsecould be easily explainedin terms of the current flow on either side of the contact but

that, as predicted, the quadrature phase response exhibitedstrangebehavior. Once again, the response was controlled by the conductivity contrast and the skindepthon either sideof the contact.A dippingdike was recognizablefrom magnetic field measurements. A conductive overburden altered the phase of the responseand greatly reduced the amplitude. Finally, we learned that, for small secondarymagneticfields, the tilt angle and ellipticity of the polarizationellipse gave good approximationsto the inphase and quadrature phasecomponentsof the vertical magneticfield.

(a)

Embedded

Prism

We learned that we would have to be on the alert for

different responsesas the orientation of the prism boundarieschangedwith respect to the primary field components.

Dike•E-Polarization

HzV E• /•

•Hr

(b)

Fig. 138. Preferred survey line orientation for electric field measurements, (a) horizontal loop transmitter and (b) grounded wire transmitter.

The most important factor learned here was that, for plane wave excitation at VLF frequenciesand for the long targets typically excited by these fields, the galvanic current response substantiallyexceededthe vortex current response, except at very high values of terrain resistivity. An electrically thick dike behaved essentiallyas two closely spacedcontacts. The magnetic field responsefrom an electrically thin dike was controlled by three parameters,



L• =S•

2 ' •/h •[x0tøp2 2q_ x2/•2

and the dip angle. Lateral fall-off of the responsewas determinedby h and •2 and was essentiallyindependent of dike conductance.Since the responseis electrically driven, the dike magnetic field responseincreased with increasing 92- To carry out an interpretation,knowledgeof 92 (which could be obtained from measurementof the adjacentsurfaceimpedance)was required.Dike responsesaturatedat low values of S1, particularly in a resistive host rock. A highly conductive dike need have only very small depth extent to behave as a dike of infinite depth extent.

The presence of an electrically conductive overburden altered the phase and greatly reduced the amplitudeof the magneticfield responsefrom the dike. For electrically thin overburdenS3 the parameter

L3 =S3

2 X0 cop2

also controlledanomalyamplitude,phase,and width. Once again, an adjacent measurementof surface impedancecould be used, at least in theory, to interpret the magneticfield response. Finally, for a relatively conductive dike of finite length we expect that the magnetic field response would be substantiallyless than for a dike of infinite length. Other

Factors

We learned

that we could

obtain

measurable

field

responsesfrom short "blobby" changesin conductivity in a thick conductiveoverburden. We also learned

635

that topographicresponsecould be significant,particularly in conductiveground. Finally, we saw that use of local transmitterswas justified since they had only to duplicatethe subsurface electric field arisingfrom a remote VLF transmitter; as a consequencemeasuredresponseswith either sourcewere quite similar. In summary,severaladvantagesand disadvantages of the VLF techniquehave emergedfrom our study. The disadvantagesare principally (1) the relatively shallow depth of exploration in all but the most resistive terrain; (2) the large number of variables that controlthe responseof 2-D and 3-D targets,combined with the fact that we generally measure only two variables,makingany but the most rudimentaryinterpretationdifficultor impossible;(3) the relativelypoor ability to resolvebetter conductors,and (4) the existence of significanttopographicresponse. These are seriousdisadvantages.However the advantagesare also extremely important, sufficientlyso to have made the VLF technique by far the most popular electromagnetictechnique in current use. In the magnetic measurement mode these lightweight, relatively inexpensive tools allow the geologist or geophysicistto surveylargeareasrapidly and inexpensively, to locate and roughlydefine subsurfaceelectrical conductors,be they mineralization or geological structure (generally the latter), at small values of resistivity contrast and at high values of resistivity. When used in the electric field mode the techniqueis, in simple environments,capable of quantitative interpretation and once againthe speedand relative cheapness of these

devices

makes

them

a natural

"first

electromagnetictool" to use in reconnaissancemapping.


636

McNeill

and Labson

APPENDIX

POLARIZATION

As stated throughout this article, the tilt angle was approximatelyequal to the inphasecomponentof the ratio of Hz/H x and the ellipticityapproximatelyequal to the quadrature phase component, as long as the vertical field was small compared to the horizontal field. Let the horizontalfield be givenby Hx and the

A.

PARAMETERS

and

Hz

Hz(Q)=•xxsin q>

(A-4)

we obtain

vertical fieldbyHzei•'. Thenit isshown in Smithand Ward (1974) that tilt angle 0 is given by

tan(20)=2Hz(I) (H•_xZ) 2 (A-5) 1-

2(Hz/Hx) cos q>

tan(20)= 1- (Hz/Hx) 2

(A-l) and

andthe ellipticityof the polarizationellipse(seeFigure A-l) is

b

Hz Hx sin q>

a

IHzeiø•sin0 + Hx cos012'

• = H z(Q)

I[Hz(I) + iHz(Q)] sin0 + cos012 (A-6)

(A-2)

Rewriting these equationsin terms of the normalized

or

• = Hz(Q)

inphaseand quadrature phasecomponent Hz(I) and

(Hz(I) sin0 + cos0)2+ (Hz(Q) sin0)2. (A-7)

Hz(Q), where Hz

Hz(I)=•xxcos q>

(A-3)

Equations (A-5) and (A-7) give 0 and e in terms of the relevant components of the normalized vertical field component,multiplied by a correction factor which is

a functionof Hz/H x andq>.Giventhesetwo quantities it is a simple matter to obtain the correction factors. Conversely,when the secondary magnetic field components are relatively small compared with the pri-

marymagneticfieldtan (20) = 2Hz(/) and• = Hz(Q). ACKNOWLEDGMENTS

Grateful acknowledgement is extended to T.R. Madden and K. Vozoff for allowingthe use of selected results from

their extensive

calculations

of the re-

sponseof 2-D bodies, the study of which contributed greatly to understandingof the physics.A.S. Saydam kindly suppliedunpublisheddata. McNeill also thanks A.R. Barringer for the initial opportunity, many years ago, to become involved in Fig. A-1. Polarizationellipseparameters.

the applicationof VLF radio waves to geophysical



prospecting, to thank A. Kaufman for many stimulating discussionsover the years on the electromagnetic methods and, finally, to thank M. Nabighian without whose gentle but irresistable pressure (also over the years) this manuscriptwould never have seen a printing press. REFERENCES

Barfield, R. H., 1934, Some measurements of the electrical constantsof the ground at short wavelengthsby the wave tilt method: Proc. Inst. Electr. Electron. Eng. 75,214-220. Barringer, A. R., 1973, Radiophase: United States Patent 3763419.

Bickel, J. E., Heritage, J. L., and Weisbrod, S., 1957, An experimental measurement of v.l.f. field strength as a function of distance using an aircraft, Report 767, U.S. Navy Electronics Laboratory, San Diego, California. Cheeseman, S., and Edwards, R. N., 1989, Current channeling in square plates with application to magnetometric resistivity: Geophys. Prosp. 37, 553-581. Collett, L. S., and Becker, A., 1967, Radiohm method for earth resistivity surveying: Canadian Patent 795919. D'Erceville, I., and Kunetz, G., 1962, The effect of a fault on the earth's natural electromagneticfield. Geophysics,27, 651.

Dyck, A. V., Bloore, M., and Val16e, M. A., 1981, User manual for programs Plate and Sphere: Research in Appl. Geophys. no. 14, Univ. of Toronto. Feldman, C. B., 1933, The optical behavior of the groundfor short radio waves. Proc. Inst. Electr. Electron. Eng., 21, 764-801.

Fraser, D.C., 1969, Contouring of VLF-EM data: Geophysics, 34, 958-967. Hauser, J.P., and Rhoads, F. J., 1974, Coverage predictions for the Navy's fixed VLF transmitters. NRL Memorandum Report 2884. Kaikkonen, P., 1979, Numerical VLF modelling: Geophys. Prosp., 27, 815-834. Karous, M., and Hjelt, S. E., 1983, Linear filtering of VLF dip-angle measurements:Geophys. Prosp., 31,782-794. Kaufman, A. A., and Keller, G. V., 1981, The magnetotelluric soundingmethod: Elsevier, N.Y. Langron, W. J., 1972, A study of results of the VLF-EM method of prospectingin Australia and Papua: Proc. Aust. Min. Metall., 241, 27-38. Madden, T. R., and Vozoff, K., 1971, VLF model suite (2nd Edition) 17 Winthrop Road., Lexington, Mass. 02173. Nabighian, M. N., 1972, Private communication. Olsson, O., 1978, Scattering of electromagneticwaves by a perfectly conductinghalf plane below a stratified overburden: Radio Science, 13, 391-397. •1980, VLF anomaliesfrom a perfectly conductinghalf plane below an overburden: Geophys. Prosp., 28, 415434.

•1981, Reply to comments on "Numerical VLF modelling" and "VLF anomalies from a perfectly conducting half plane below an overburden": Geophys. Prosp., 29, 303-307.

•1983, Computation of VLF response over half-plane and wedge models: Geophys. Prosp., 31, 171-191. Paal, G., 1965, Ore prospectingbased on VLF-radio signals: Geoexploration, 3, 139-147. Palacky, G. J., Ritsema, I. L., and de Jong, G. J., 1981, Electromagnetic prospecting for groundwater in Precambrian terrains in the Republic of Upper Volta. Geophys. Prosp., 29, 932-55. Paterson, N. R., and Ronka, V., 1971, Five years of surveying with the very low frequency-electromagneticmethod: Geoexploration, 9, 7-26. Poddar, M., 1982, Very low frequency response of a per-

637

fectly conducting half-plane in a layered half-space: Geophysics, 47, 1059-1067. Poddar, M., and Rathor, B. S., 1983, VLF survey of the weathered layer in southern India: Geophys. Prospect., 31,524-537.

Prakla-Seismos, GMBH, 1981, Hannover, West Germany. Technical report on Geolectrics. Saydam, A. S., 1981, Very low frequency electromagnetic interpretation using tilt angle and ellipticity measurements: Geophysics, 46, 1594-1605. •1984, Private communication. Scott, W. J., 1975, VLF resistivity (Radiohm) survey, Agricola Lake area, district of Mackenzie: Geol. Surv. Can. Paper 75-1, Part A, 223-225. Sinha, A. K., and Hayles, J. G., 1988, Experiences with a local loop VLF transmitter for geological studies in the Canadian nuclear fuel waste management program: Geoexploration, 25, 37-60. Smith, B. R., and Ward, S. H., 1974, On the computation of polarization ellipse parameters:Geophysics,39, 867-869. Smith-Rose, R. L., 1933, The electrical properties of soil for alternating currents at radio frequencies:Proc. Roy. Soc. London, 140, 359-377. Stratton, J. A., 1941. Electromagnetic theory: McGraw-Hill, New

York.

Telford, W. M., King, W. F., and Becker, A., 1977, VLF mapping of geologic structure; Geol. Surv. Can. Paper 76-25.

Tilsley, J. E., 1973, A portable VLF-EM source for use in geological mapping of veins and fault structures and conventional prospecting. Report: David S. Robertson and Associates, Toronto, Canada. •1976, Very low frequency electromagnetic surveying for geologicalstructuresusing a portable signal generator: Appl. Earth Sci., Inst. Min. Metal. Trans., Sec. B, 85, 74-77.

Wait, J. R., 1962, Electromagnetic waves in stratified media, Pergamon Press. 1970, Electromagneticwaves in stratified media, 2nd Edition: Pergamon Press, New York. Ward, S. H., and Hohmann, G. W., 1988, Electromagnetic theory for geophysicalapplications, Vol. I, in Nabighian, M. N. and Corbett, J. D., Eds., Soc. Expl. Geophys., 131-307.

Watt, A.D., New

1967, VLF radio engineering:Pergamon Press,

York.

Witties, A. B. L., 1969, Prospecting with radio frequency EM16 in mountainous regions: Western Miner, 51-56. ADDITIONAL

COMPILED F.C.

VLF

AND

REFERENCES

ANNOTATED

BY

FRISCHKNECHT

Arcone, S. A., 1978, Investigation of a VLF airborne resistivity survey conducted in northern Maine: Geophysics, 43, 1399-1417. •1979, Resolution studies in airborne resistivity surveying at VLF: Geophysics, 44, 937-946. Barbour, D. M., and Thurlow, J. G., 1982, Case histories of two massive sulphide discoveries in Central Newfoundland, in Prospectingin Areas of Glaciated Terrainm1982: Can. Inst.

Min.

and Metal.

300-321.

Slingram, VLF, and gravity were used to locate and map subeconomic massive sulphide deposits in Ordovician felsic volcanics that also contain many graphitic conductors.

Bezvoda, V., and Segeth, K., 1982, Mathematical modelling in electromagneticprospectingmethods: Univerzita Karlova, Prague.

638

McNeill

•1982, On the resolving power of the VLF method: Pure Appl. Geophys., 120, 348-364. Coates, J. S., Pease, S. F., and Gallagher, M. J., 1984,

Explorationof the ScottishDaalradian,in Prospectingin areas of glaciated terrain: Inst. Min Metall. London,

and Labson waves over certain

mountains

and shorelines:

J. of Atm.

and Terres. Phys., 33, 101-110. Hattula, A., 1977, Geophysicalmodels in thick overburden, in Prospectingin areas of glaciatedterrain, The Inst. Min. Metal., London, 120-127.


21-34.

Includes a comparisonof VLF, IP and magneticmeasurements over a number of conductive mineralized zones.

Coney, D. P., and Myers, J. O., 1977, Some comparative field results for resistivity, very-low frequency electromagnetic and horizontal-loop electromagneticmethods over fluorospar veins: Trans Inst. Min. Metall. (Sec. B: Appl. Earth Sci.) 86, B-l-B3. Comparesslingram,VLF magneticfield and resistivity results over conductive

veins.

Crossley, D. J., 1982, The theory of EM surface wave impedancemeasurementsin GeophysicalApplicationsof Surface Wave Impedance Measurements, Collett, L. S. and Jensen, O. G. (eds.), Geol. Survey Canada paper 81-15, 1-17.

Culley, R. W., 1973, Use of airborne resistivity surveysfor gravel location: Can. Inst. Min. Metall. Bull. 66, 733, 70-74.

Dhanasekaran, P. C., and Poddar, 1985, A program to

compute EM scatteringof a plane wave by a perfectly conducting half-plane in a finitely conducting layered half-space: Computers and Geosciences,11(1), 1-17. Eberle, D., 1982, A method of reducing terrain relief effects from VLF-EM data: Geoexploration, 19, 103-114. Includesterrain connectedresultsover part of the Rammelsberg Zn-Pb deposit. Fischer, G., Le Quang, B. V., and Muller, I., 1983, VLF ground surveys, a powerful tool for the study of shallow two-dimensional structures: Geoph. Prosp. 31, 977-991.

Includes comparison of VLF resistivity data over a fault zone with model calculations.

Flanigan, V. J., Long, C. L., Rohret, D. H., and Mohr, P. J., 1986, Apparent resistivity map of the rift systems of Kilauea and Mauna Loa Volcanoes, Island of Hawaii, Hawaii: U.S. Geol. Survey Map MF 1845B. Fraser, D.C., 1971, VLF-EM data processing:Can. Min. Metall.

Bull.

39-41.

•1981, A review of some useful algorithmsin geophysics: Can. Min. Metall. Bull. 74, 828, 76-83. Frischknecht, F. C., and Stanley, W. D., 1970, Airborne and ground electrical resistivity studies along proposed transAlaska Pipeline System (TAPS) route: [abs.] Amer. Inst. Pet. Geol. Bull. 54, 2481. Greenhouse, J.P., and Harris, R. D., 1983, Migration of contaminants in ground water at a landfill: a case study, Jour. Hydrology, 63, 177-197.

Describesresultsfrom a site where the upper layer is a sandy aquifer that is very favorable for the application of geoelectrical methods. A contaminant plume was mapped using resistivity, VLF, and Low Induction Number (LIN) measurements.

Haeni, F. P., 1986, The use of electromagnetic methods to delineate vertical and lateral lithologic changesin glacial aquifers, in Surface and Borehole Geophysical Methods and Ground

Water

Instrumentation

Conference

VLF surveyswere used to map conductorsbeneath 25-60 m of glacial overburden. Hayles, J. G., and Sinha, A. K., 1986, A portable local loop VLF transmitter for geologicalfracture mapping: Geoph. Prosp. 34, 873-896. Hjelt, S. E., Kaikkonen, P., and Pietila, R., 1985, On the interpretation of VLF resistivity measurements:Geoexploration, 23, 171-181. Hoekstra, P., and McNeill, J. D., 1973, Electromagnetic probing of permafrost, in Permafrost: North American Contribution, 2nd Internat. Conf. Nat. Acad. Sci., Wash., 517-526.

Hoeskstra, P., Sellman, P. V., and Delaney, 1975, Effective wave tilt and surface impedanceover a laterally inhomogeneoustwo-layer Earth: Radio Sci., 10, 1001-1008. Jones, D., and Telford, W. M., 1982, Mapping bedrock terrain with the EM16R- VLF unit, in Collet, L. S., and Jensen, O. G., Eds., Geophysical applicationsof surface wave impedance measurements: Geol. Survey Canada paper 81-15, 35--48. Karous, M. R., 1979, Effect of relief in EM methods with a very distant source: Geoexploration, 17, 33-42. Karous, M. R., and Hjelt, S. E., 1977, Determination of apparent current density from VLF measurements:Contribution no. 89, Dept. Geophysics Univ. Oulu. Kennedy, E.G., and Prior, J. W., 1969, A combined geological-geophysical survey, northern Saskatchewan: Can. Min. Jour. 90, 113-118.

The VLF method was used to map fault and fracture zones, some of which are mineralized.

Katelaar, A. C. R., Giberti, I., and Monne, B., 1986. On the

use of the VLF signaturein geologicalmapping:Geologie on Minjnbouw, 63, 109-112. King, R. J., 1977, On airborne wavetilt measurements: Radio Sci., 12, 405-414. Ku, C. C., Hsieh, M. S., and Lim, S. H.,

1973, The

topographiceffect in electromagneticfields: Can. J. Earth Sci., 10, 645-656.

LaFleche, P., and Jensen, O. G., 1982, Wave impedance measurement at 60 kHz, in Collet., L. S., and Jensen, O. G., Ed., Geophysicalapplicationsof surfacewave impedance measurements: Geol. Survey Canada paper 81-15, 67-78.

Lord, A. E., Koerner, R. N., and Freestone, F. J., 1982, The identification

and location of buried containers via nonde-

structive testing methods: Jour. Hazardous Materials, 5, 221-233.

Mares, S., 1984, Introduction to applied geophysics:Reidel. Mathieson, C., and Crossley, D. J., 1982, Interpretation of singlefrequency VLF data, in Collet, L. S. and Jensen,O. G., Eds., Geophysical applicationsof surface wave impedance measurements:Geol. Survey Canada, paper 8115, 49-65.

McGrath, P. H., and Henderson, J. B., 1985, Reconnaissancegroundmagneticand VLF profile data in the vicinity of the Thelon Front, Artillery Lake map area, District of Mackenzie, in Current Research, Part A., Geol. Surv. Can. paper 85-1A, 455-462.

and Ex-

position: Nat'l Water Well Assoc., Dublin, 259-282. The LIN loop-loop and VLF methods were used to map lateral variations in resistivity that correspondwith lithologic changes. Harrison, R. P., Hocksher, J. C., and Lewis, E. A., 1971, Helicopter observations of very low frequency radio-

VLF profileswere made acrossgranatoidmigmatites,metavolcanics and metasediments; the latter contain highly conductive

units.

McNeill, J. D., and Hoekstra, P., 1973, In-situ measurements on the conductivity and surface impedance of sea-ice at VLF: Radio Science 8, 23-30. Middleton, R. S., 1977, Ground and airborne geophysical


GeologicalMapping Using VLF Radio Fields studiesof sand and gravel in the Toronto Region: Ontario 6eol. Surv. Study, 6S 18. Middleton, R. S., and Campbell, E. E., 1979, 6eophysical and geochemicalmethodsfor mappinggold-bearingstructures in Nicaragua, in Hood, P. J., Ed., 6eophysics and 6eochemistry in the Search for Metallic Ores: 6eol. Survey Canada Econ. 6eol. Rept. 31,779-798. VLF surveys were used to map conductive quartz-goldbearing veins. Mitchell, 6., 1982, Mapping structured stratigraphy with VLF-EM and VLF resistivity (Radiohm) surveys: A case history from the Canadian Arctic: 52nd Ann. Internat. Mtg., Soc. Expl. Geophys. Expanded Abstracts, 388-390. Paal, 6., 1968, Very low frequency measurementsin northern Sweden: 6eoexploration, 6, 141-149.

Describes experimental measurement of inphase and quadrature phase parts of three spatial componentsover good conductors. Palacky, G. J., and Jagodits, F. L., 1975, Computer data processingand quantitative interpretation of airborne resistivity surveys: Geophysics, 40, 810-830. Parasnis, D. S., 1986, Principles of geophysics,fourth edition: Chapman and Hall. Includes elementary description of the VLF method. Parker, M. E., 1980, VLF-electromagnetic mapping of strata-bound mineralization near Aberfeldy, Scotland: Trans. Inst. Min. Metall. (Sect. B: Appl. Earth Sci.) 89, B123129.

ments.

Peltoniemi, M., 1981, Comments on "Numerical VLF mod-

elling" and "VLF anomaliesfrom a perfectly conducting half plane below an overburden": Geoph. Prosp., 29, 298-302.

Phillips, W. J., and Richards, W. E., 1975, A study of the of the

VLF

method

for

the

location

of

narrow mineralized fault zones: Geoexploration, 13,215226.

Includes a comparison of VLF, mineralized

A slingram survey outlined a Zn-Cu-Ag deposit having a rather low conductance

estimated

to be 4S.

Reynolds, 6. A., 1980, Influence of glacial overburden on geoelectrical prospecting in Ireland: Trans. Inst. Metall. (Sect. B: Appl. Earth Sci.) 89, B44-B49. Includes example of combined use of LIN and VLF resistivity to map parameters of overburden and resistivity of bedrock.

Sedel'nikov, E. S., 1983, The effects of terrain relief on the field of a remote sourceof a variable electromagneticfield: Iszvestiya, Earth Physics, 19, 575-579. Seguin, M. K., 1978, Exploration of magnetic taconite and itabirite and integrated aeromagnetic and magnetic meth-

ods: Bulletino di 6eofisica Teorica ed Applicata, 21, 68-90.

Singh, R. P., Kumar, B. U.S., and Lal, T., 1984, Wave-tilt characteristics of EM waves over a two-layered earth model: J. Geomag. Geoelect. 36, 139-147. Singh, R. P., and Lal, T., 1981, Wave-tilt characteristicsof TE mode waves: Can. J. Earth Sci., 18, 382-385. Singh, R. P., and Tarkeshwar, L., 1980, Wavetilt characteristics of EM waves over a homogeneousearth model: Inst. Elect. Electron. Eng. Trans. Geosci. Rem. Sens., GE-18, 285-288.

Sinha, A. K., 1975, Altitude dependenceof plane wave EM fields, in Rept. of activities, Part A, Geol. Surv. Can. Paper 75-1A: Geol. Surv. Can., 147-148. •1976, A technique for obtaining correct ground resistivity from airborne wave tilt measuring systems, in Report of activities, Part B, Geol. Surv. Can. Paper 76-1B: Geol. Surv. Can. 281-283.

Resistive barite bodies within conductive graphite schists were mapped using VLF tilt angle and ellipticity measure-

effectiveness

639

IP, and SP results over

•1976, Determination of ground constants of permafrost terrains by an electromagneticmethod: Can. J. Earth Sci., 13,429. •1977, Charts for the correction of airborne E-phase data, in Report of activities, Part C, Geol. Surv. Can. Paper 77-16: Geol. Surv. Can. 63-66. •1977, Influence of altitude and displacement currents on plane-wave EM fields: Geophysics, 42, 77-91. Slaine, D. D., Lee, P. K., and Phimister, J.P., 1984, A comparisonof a geophysicallyand geochemicallymapped contaminant plume, in Nielsen, D. M., and Curl, M., Eds., Surface and borehole geophysical methods in ground water investigations: Natl. Water Well Assoc., Worthington, OH, 383-402.

fault zones.

Poddar, M., and Rather, B. S., 1983, A study of the effect of tropical weathering on electromagnetic measurements: 6eoph. Res. Bull., 21,326-337. VLF and transient loop-loop measurementswere made over bedrock conductors in an area where there are large changes in the conductivity of the overburden. Podolsky, 6., and Slankis, J., 1979, Izok Lake deposit, Northwest Territories, Canada: a geophysical case history, in P. J. Hood, Ed., 6eophysics and geochemistryin the search for metallic ores, 6eol. Surv. Can. Econ. 6eol. Rept. 31,641-652. VLF, slingram and a variety of other geophysical measurements were used to outline a large flat-lying Zn-Cu-Pb-Ag sulfide deposit. Powell, B. W., and Jensen, O. G., 1982, Radiohm mapping of permafrost, in Collet, L. S. and Jensen, O. G., Eds., Geophysical applicationsof surface wave impedancemeasurements: Geol. Surv. Canada paper 81-15, 19-33. Rathor, B. S., and Poddar, M., 1983, VLF-EM survey of geological structures near Anantapur, Andra Pradesh: Geoph. Res. Bull., 21,244-250. Reed, L. E., 1981, The airborne electromagneticdiscovery of the Detour zinc-copper-silver deposit, northwestern Quebec: Geophysics, 46, 1278-1290.

After comparison with the resistivity and the low induction number loop-loop methods, VLF was selectedfor study of a sanitary landfill in Ontario. Smith, C. G., Gallagher, M. J., Coats, J. S., and Parker, M. E., 1984, Detection and general characteristics of stratabound

mineralization

in

the

Daldradian

of

Scotland:

Trans. Inst. Min. Metall. (Sect. B., Appl. Earth Sci.) 93, 125-133.

Soonawala, N.M., and Dence, M. R., 1982, Geophysics in the Canadian Nuclear Waste Management Program--a case history from the Lac De Bonnet Batholith in Southwestern Manitoba [abs.]: Geophysics, 47,415. Stewart, M., and Bretnall, R., 1984, Interpretation of VLF resistivity data for ground water contamination surveys, in Nielsen, D. M., and Curl, N., Eds., Surface and borehole geophysicalmethods in ground water investigations:Natl. Water Well Assoc., 368-382.

VLF resistivity and tilt angle measurementswere useful in mapping a plume from a landfill and the boundaries of another

landfill

in Florida.

Stewart, M., and Bretnell, R., 1986, Interpretation of VLF resistivity data for ground water contamination surveys: Ground Wat. Monit. Rev., winter, 71-75. Stone, P., and Gallagher, M. J., 1984, Mineral exploration in lower Paleozoic turbidites of south Scotland, in Prospect-

640

McNeill

ing in areas of glaciated terrain, 1984: Inst. Min. Metall., London, 201-211.


VLF tilt angle measurements were used to delineate a thick arsenopyrite-pyrite zone in graywacke. Taylor, R., 1985,

and Labson

Thiel, D. V., and Chant, I. J., 1982, Ionospheric induced very low-frequency electric field wavetilt changes: Geophysics, 47, 60-62. Tod, J., 1986, VLF-EM investigationsin insular Newfoundland, in Current research (1986): Newfoundland Department of Mines and Energy Min Dev. Div. rept. 86-1, 239-250.

Airborne surveys can cut exploration costs: Can. Min. Jour. 106, 4, 41-43. An airborne VLF survey mapped 3 long conductorsat the Hemlo gold deposit. Telford, W. M., and Becker, A., 1979, Exploration case histories of the Iso and New Insco ore bodies, in P. J. Hood, Ed., Geophysicsand geochemistryin the searchfor metallic ores: Geol. Surv. Can. Econ. Geol. Rept. 31, 605-629.

VLF, slingram, tilt angle and ellipticity, and tilt angle as well as other geophysical data over Cu-Zn massive sulfide deposits in volcanic flows are interpreted and compared. Teemull, F., and Crossley, D. J., 1982, Inversion of VLF data for simple lateral inhomogeneities, in Collet, L. S. and Jensen, O. G., Eds., Geophysical Applications of Surface Wave Impedance Measurements: Geol. Survey Canada paper 81-15, 79-86. Thiel, D. V., 1973, Relative wavetilt measurements at VLF: Geoexploration, 17, 285-292. •1985, VLF surface impedance measurements at Zeehan, Tasmania: Exploration Geophysics, 16, 387-390. Independent measurements of the horizontal electric and magnetic VLF fields were made in the vicinity of mineralized veins.

Towle, J. N., 1983, VLF electromagnetic investigations of the crater and central dome of Mount St. Helens, Washington: Jour. Volcan. Geoth. Res., 19, 113-120. Vozoff, K., 1971, The effect of overburden on vertical componentanomaliesin AFMAG and VLF exploration: a computer model study: Geophysics,36, 53-57. Watts, R. D., 1978, Electromagnetic scattering from buried wires: Geophysics, 43, 767-781. Whiteley, R. J., 1980, Very low frequency electromagnetic profiles at Woodlawn, in Whiteley, R. J., Ed., Geophysical Case Study of the Woodlawn Orebody New South Wales, Australia, 151-164, Pergamon, Oxford.

Distinctive VLF tilt angle and ellipticity anomalies were measured over the Woodlawn orebody and the nearby black shale.

Witherly, K. E., 1979, Geophysical and geochemical methods used in the discovery of the Island Copper Deposit, Vancouver Island, British Columbia, in Hood, P. J., Ed.; Geophysics and Geochemistry in the Search for Metallic Ores: Geol. Surv. Can. Econ. Geol. Rept. 31,685-696. VLF was useful in mapping major structural features. Zablocki, C. J., 1978, Applications of the VLF induction method for studying some volcanic processesof Kilauea Volcano, Hawaii: Jour. Volc. Geoth. Res., 3, 155-195.


CHAPTER

THE

8

MAGNETOTELLURIC

METHOD

K. Vozof•

SUMMARY

systemsis now done in real time, while the signalsare being acquired. These computed impedancesare next interpreted in terms of electrical conductivity versus position and depth. Numerical models for one-dimensional, two-

In the magnetotelluric (MT) method, natural electromagnetic fields are used to investigate the electrical conductivity structure of the earth. Natural sourcesof MT fields above about 1 Hz are thunderstorms

world-

dimensional, and three-dimensional

wide, from which lightning radiates fields which propagate to great distances. At frequencies below 1 Hz, the bulk of the signal is due to current systemsin the magnetosphereset up by solar activity. In both cases the electromagnetic(EM) fields at the surface of the earth behave almost like plane waves, with most of their energy reflected but with a small amount propagating vertically downward into the earth. The amplitude, phase, and directional relationshipsbetween electric (E) and magnetic (H or B) fields on the surface depend on the distribution of electrical conductivity in the subsurface. By use of computed models, field measurement programs can be designed to study regions of interest within the earth from depths of a few tens of meters to the upper mantle. Equipment to carry out the measurements consists of magnetometers for the frequency range of interest; pairs of electrodes separated by suitable spacingsto sense the electric field variations; plus amplifiers, filters, and suitable digital recording and processing systems to permit the signals to be captured and analyzed. The magnetometersin particular must have very low noise and great stability because those sig-

structures are

used for this last step. Interpretation is the most difficult part of the method because information is seldom complete and the models are never complex enough to represent a real earth. For that reason, and to use the MT data to best advantage, other available external data such as well logs, seismic, and any other electrical data are commonly utilized to help with interpretation. The main shortcomingof the method is the difficulty of obtaining data in electrically noisy areas or where the surface is unstable.

The strengthsof the method are its unique capability for exploration from very shallow depths to very great depths without artificial power sourcesand with little or no environmental impact. At high frequencies, audio frequency MT (AMT) has been used to map groundwater and major base metal deposits at depths from 50-100 rn to several kilometers. However,

the

major applicationof MT is to petroleum exploration in areas where reflection seismologyis very expensive or ineffective, such as in extreme terrain and beneath volcanics. Another major successfulapplication has been to geothermal exploration. Major research problems at the moment concern interpretation in areas of complex three-dimensional structure, and improvements in production rate.

nals are so weak.

Once the signals have been recorded they must be processedand analyzed. Processingis normally done in the frequency domain becausethe theory is simpler than in time domain. Hence processing begins with Fourier transformation, from which earth impedances to the incident waves as functions of frequency, direction, and position are computed. Processingin many

INTRODUCTION

This material provides seismologistsand geologists with a review aimed at increasing understanding and

*Centre for GeophysicalExploration Research,Macquarie University, Sydney, N.S.W. 2109, Australia. 641


642

Vozoff

effectiveness of MT procedures and with a list of sourceswhere detailed information may be found. The review summarizes the past seven or eight years' research for the field geophysicist who has not had time to keep up, or does not see the more academic journals. The enormous growth in the use of the MT method since the late 1960s has been a pleasant surprise. MT seemsto have offered help of the right kind at the right time for many commercial purposes. As exploration was forced into more difficult areas, and with the

volatility in resource prices, the importance of MT in conjunction with other techniques has tended to grow continuously. Its usefulnessin poor seismicareas and its negligible environmental impact are integral parts of effective exploration at minimum cost. With all the new data, problems emergedwhich had barely been anticipated. Fortunately some qualified research people became interested in solving these problems at about the same time. As a result, the literature must now be one to two orders of magnitude larger than in 1970. Included are new applications, new field data, new instruments and field computers, remote referencing and robust estimation, 3-D modeling and inversion, and a much broader understanding of both modeling and inversion (and their limitations) generally. Our knowledge of factors that control resistivity at depth improved, and we now better understand the complementary roles of natural and applied field methods. To cover these factors in one chapter is difficult; several books on the subject are now in the planning stagesor in print. My 1972 review (Vozoff, 1972) reflected the situation in 1970. The present update proved a much larger project because many of the most important developments are unpublished, and thus are not generally available. The Society of Exploration Geophysicists Magnetotellurics Reprint volume (Vozoff, 1986) was one attempt to help. The following sections summarize the sources, the interaction of their electric and magnetic fields with the complex conducting earth, the equipment with which the measurements are made, field procedures, and data processingand interpretation. A list of references, by no means comprehensive, follows. Some important questions remain to be answered, particularly the proper and practical use of impedancesin 3-D structural

environments.

In

such cases the current

state of development is indicated. SOURCES

The dependence of MT on natural fields is both its major attraction and its greatest weakness. Other EM methods require power supplies and current control

systems. For low frequency and deep penetration these methods can involve large truck-mounted installations and thus restrict access in many circumstances and at times lead to undesirable

environmental

dam-

age. The geometry of their source fields also complicates interpretation as compared with MT. These sourcesdo not easily provide the kind of tensor data for structural

definition

that MT

does. The other

side

of this coin can be the cost and frustration of attempting to collect MT data in the presence of artificial EM noise or natural interference such as ground motion. Because the amplitude and polarization of source fields is obviously important to the successof MT and AMT surveys, a general description is presented. Familiarity with the normal appearance of pulsations will often make recognition of data contaminated by noise possible. With the present generation of MT equipment, loss of survey time while waiting for signal to exceed system noise in the frequency ranges .001-

ß1Hz and10-104Hz isunusual. However,togetgood data near 1 Hz can take a great deal of patience because of a persistent spectral energy low there. Another signal notch at 1-2 kHz can usually be ignored because it is so narrow. Likewise, since the introduction of digital acquisition, magnetic field polarization has not been a problem for tensor surveys below 1 Hz. These surveys, however, can still require specialcare at audio frequencies. This sectiongives an overview of signal sources and the factors that affect their intensity and polarization. Aside from engineering considerations, a curiosity about the nature and sources of pulsations is developed after observing them for many long hours in the field. Examples of pulsationsare shown in Figures 3, 8 and 12. Table 1 shows the classificationsystem developed to describethe phenomenon. Natural EM signals come from an enormousvariety of processesand from sources ranging from the core of the earth to distant galaxies. Within the frequency range of interest in

exploration, say.001-104 Hz, onlytwosource regions are important. These are the atmosphere and the magnetosphere.Electrical storms in the lower atmosphere are the dominant cause of fields between 1 Hz and 10 kHz, whereas below 1 Hz the fields originate primarily in hydromagnetic waves in the magnetosphere. The magnetosphere, the region around the earth in which the main magnetic field is trapped by the solar wind (Figure 1), includes the ionosphere and the atmosphere (Rostoker, 1979) and contains gases, especially oxygen and nitrogen, decreasing in density with elevation. These gasesare ionized by ultraviolet and other solar radiation, but below about 100 km the high pressure forces the ions to recombine rapidly so there are few of them. Above 100 km, chargedparticle



density increasesrapidly up to about 250 km and then starts to decline again with decreasingpressure and particle density (Figure 2). This region of relatively high conductivity is called the ionosphere. The ionosphere is an anisotropic electrical conductor, because the magnetic field exerts a force on any moving charge perpendicular to both the direction of charge motion and the magnetic field itself. For that reason an electric field in one direction may set up currents in the other directions (Parkinson, 1983). One additional property that plays an important part is the existence of EM hydromagnetic waves, or plasma waves, which can only occur in conductive fluids containing magnetic fields. These waves travel at relatively low velocities and can set up resonances along magnetic field lines in the magnetospherewith frequenciesin our range of interest (Rostoker, 1979). This very complex system is constantly buffeted by a solar wind of energetic ions and weak magnetic fields, ejected from the sun. From the impact zone, where a permanent shock wave is maintained, to the far tail, the conductive solar wind sweeps turbulently past the earth, bottling up the magnetic field and injecting erratic bursts of its own ions and fields. Consequentlythe time behavior of magneticpulsation fields is essentially chaotic but includes features localized in frequency and/or spaceas a result of particular local conditions. As well, the earth's main dipole field imposes a global pattern on the behavior. Figure 3 showstypical examples of ultra low frequency (ULF) signalsin the electric and magneticfields. Long time averaging gives smooth spectra decreasing monotonically with increasingfrequency (Figure 4). The level of these spectrawill be greatestat times of high solar activity, with typical differencesof 20 dB between quiet and active times, depending on frequency.

To reach the earth's surface from the magneto-

643

sphere, fields must travel through the ionosphereand then throughthe insulatingatmosphere.Vertical E and H fields do not penetrate the ionosphereand thus the horizontal componentsare stronglymodified(Hughes and Southwood, 1976a, b; Orr, 1984). Additional horizontal ionosphericcurrents are set up in this interaction and it appears that the pulsations seen at the surface are largely due directly to those currents. Later sectionsshow that the vertical component of the natural field is important to the interpretation of MT data. Vertical magnetic fields are normally very small, both because horizontal gradients of current density are small, and because induction in the earth itself tends to suppressthem. Any vertical components reaching the surface are a result of restricted current systems associated with oscillations localized

Fig. 1. Major features of the magnetospheric current systems. The earth and atmosphere are the small spherical features in the center. (From Potemra, 1984).

Table 1. Classificationsystemused to describepulsations. Name

Frequency

Name

Time

Scale

Period (s) SHF UHF VHF HF MF LF

3-30 GHz 0.3-3 GHz 30-300 MHz 3-30 MHz 0.3-3 MHz 30-300 kHz

Pc 1 Pc2 Pc3 Pc4 Pc5

2,r/to 2,r/to 2,r/to 2,r/to 2,r/to

VLF ELF ULF

3-30 kHz 3-3000 Hz

Pil Pi2

<3 Hz

sc,si

'I'r '- 1-40 'I'r '- 40-150 'rr • 300-

= = = = =

0.2-5 5-10 10-45 45-150 150-600

Rise Time (s)

1 GHz = 109 Hz

1 MHZ = 106 Hz

From Lanzerotti and Southwood (1979)

1 kHz = 103 Hz

644

Vozoff


on particular field lines, as in Figure 5 (Samson and Rostoker, 1972; Southwood and Hughes, 1978). These vertical componentsare largely cancelledby induction in the earth, but model calculations for 50-60 s periods

indicatethat the primaryverticalcomponentof H, H z can be as large as 20 percent of horizontal H at the surface under such conditions. (This fact can confuse interpretation which depends on quantities related to

H z, and which assumesthat H z is zero exceptnear a fault or similarfeature.) A large vertical electric(Ez) pulsation field is also predicted at the surfacein these conditions, which are most common on the auroral

tribute significantly throughout the spectrum. These signals can be heard in audio outputs taken from antennas, and have been given descriptive names such as "dawn chorus," "hiss," "whistlers," "saucers," and "lions roar." Their source regions are shown in Figure 7 (Shawan, 1979). At other times the division becomes a gap, especially at lower latitudes, as discussed later.

Polarization in the horizontal plane varies in an organized way with time of day and latitude near the auroral zone (Samson et al. 1971). If polarization is very strong, as is sometimes observed at audio fre-

zones, from 65 to 70 degrees magnetic latitude (Figure 6). It is also expected but not yet reported near the magnetic equator. The division of natural EM signal sourcesnear 1 Hz is not rigid. At times, in higher latitudes and remote from storm centers, magnetospheric processes con-

8OO

,•- 600 ua 400 F

REGION

2OO

I

i0fo

iOff

iOf:•

--

0.5Hz Timing Signal

-3

ELECTRON

DENSITY

m

2 (a) Fig. 3. Typical 5-channel ULF signal set. Sinusoidal time signal (bottom trace) has a period of 2 s.

180

160

' "'"'•"'•,.• -

'•._

....... Pedersen

----

Cowling

•.....,•,

_

••

• • --- Direct -I

120

I00

3xI• 5

I0-4

CONDUCTIVITY SM-I

I• 3

4xI0-3 oo

2 (b) 2

Fig. 2. (a) Ionospheric electron density at winter noon, midlatitude. (b) Electrical conductivities in the anisotropic ionosphere, mid-day. (After Parkinson, 1983).

I

o

-I FREQUENCY

-2

-3

(Hz)

Fig. 4. Typical observedmagneticfield amplitude spectrum.


The Magnetotel!uricMethod

quencies, it can in principle cause problems for MT tensor surveys. However there are no known reports of difficulties on this account at lower frequencies despite the extensive MT work carried out in western Canada, Alaska, and western Siberia. This may be because of the large dynamic range obtained in digital recording. The Finnish Meteorological Institute has taken a special interest in effects of the electrojet on natural electromagnetic fields of the region. Hfikkinen and Pirjola (1986), Pirjola and Hfikkinen (1990), and Hfikkinen et al. (1990) have shown theoretically that impedancesnear the electrojet differ significantlyfrom those of plane waves. For their models this difference becomes important at periods greater than 60 s. The fields must be treated as if they are due to a line

Magnetospheric fields x 0.05

Amplitude

!

source. The group are now applying their theory to data from the EISCAT magnetometerarray in northern Scandanavia.

Much knowledge of pulsationshas come from studies dealing with high latitudes in and near the auroral zone, yet many areas in which MT surveys are done are tropical or subtropical.Fields weaken very rapidly southwardfrom the auroral zone. Typical results show changes of 20-40 dB in a few hundred kilometers, beyond which the decrease is much more gradual to near the magnetic equator. The few systematic midto-low

latitude

studies also indicate

that field varia-

tions there have much less spatial structure. It is

knownthat primaryH z is so smallthat it is difficultto distinguishit from the H z inducedby lateral conductivity changes. The zone within 5 degreeseach side of the magnetic equator is known to have specialbehavior becauseof the equatorial electrojet, a very intense eastward current on the day sideof the earth. This current amplifies the northward component of slower magnetic variations. Hutton (1965) suggestedand Carlo et al. (1982), Sarma et al. (1982), and Sastry et al. (1983) confirmed

that pulsationamplitudeswithin the ULF band of MT signalwould alsobe affected.In any case,H z should be treated with caution when interpreting MT results from the geomagneticauroral and equatorial regions. Above 1 Hz the radiation and propagation of EM signalsfrom lightning dischargesexplains the character of signal over most of the globe. Lightning gener-

Fields at the

ground

645

\

!

I

\ bx

~250

o'' øo

km

66 64

62

180 ø_

• ß

6O

z

58

:::bz

•

Phase

ß

................ bx

56 E

-•;'............ -.

o

54 õ

o

52

by

.." 20O LONGITUDINAL

-

"•,bz 4OO DISTANCE

,

60O km

0930

0945

UNIVERSAL

Fig. 5. Theoretical profiles of magnetic field componentson the earth's surface from the magnetosphericfields shown at top. The surface fields include a vertical component. (After Southwood and Hughes, 1978.)

0930

(b)

44

0945

TIME

Fig. 6. Latitude-time variations of the Hx componentof a 55 s. Pc4 event observed at ten observatories. (After Orr, 1984.)


646

Vozoff

ally consistsof sequencesof dischargescalled strokes (Golde, 1977; Pierce, 1977; Ogawa, 1982; Volland, 1982). The sequences, called flashes, include 3-4 strokes on average and last 200-300 ms. The first dischargeof each stroke is the cloud-to-groundleader, which ionizes a channel for subsequentcurrents. This is followed by the ground return stroke which is the major radiator of EM energy. Typical ground return currents are 20 000 A lasting about 40 Ixsand extending from the surface into the lower part of the responsible thundercloud (Ogawa, 1982). Estimates of the occurrence frequency of lightning flashes around the world vary between 100 and 1000 per second. The field seen by the AMT system depends on the strengths, path lengths (cloud heights), occurrence frequencies, and distances of the discharges which take place during recording. Signals (called atmosphericsor spherics),are therefore largest on summer afternoonsin the tropics. The fields die off with distance, very much like those from vertical radio antennas. The vertical electric and transverse magnetic fields are

dM/dt

M

Ez(t) =

4•rEor

3+

d2M/dt 2 +

4,rreo cr2 4,rreoc2r

(la)

and

B,(t) =

Ixo(dM/dt) 4,rr 2

Ixo(d2M/dt2)

+

(lb)

4,rcr

where

c= = ix0 = =

velocity of light 3 x 108m/s permeabilityof free space 4,r x 10-7 H/m

e0 = permittivity of free space = 8.87 x 10-12 F/m. r is distancefrom the dischargein meters, dM/dt is the electric dipole moment in ampere-meters, and M is thus the net moment of electric charge in coulombmeters. The vertical electric field E is in volts per meter, and the transverse magnetic induction B is in webers per meter. M is a rapidly time-varying function. The expressionsfor E and B are those of a vertical electric dipole,

JMagnetosheath J Trapped continuum

I Magnetotail I VLF hiss

ß.. ß' .' o

ß .

,

ß

,

.

ß

ß

". '. ß ß ß

'

' ' Tailele,cffolslqtic npisf•, '

ß .

ßß

aurormne•a/•neTurDu•ence

magnetic noisebursts,VLFhiss

n cyclotron wavesß::..: '.

VLF hiss Chorus

LF hiss

.';.'';:::.

,

:...

,

emissions '.'.'•'.".'.:.•.'3 •;;. ;' ' :": :":'"' Neutral sheet Electrostatic electron'ßt...... ...... .'...

cyclotron emissions... •: .. ....... • Ioncyclotron whistlers ".'.'.IPIgstoasheetJ

ELF'hiss

Bow s hock

.::.'...2•:•

LHRnoise '.'..'.'."•

Upstream waves

plasma waves turbulence

Electron whistlers ß

ß

ßß '

•

ß'

ß ' .' UHRnoise . ß ßßßßFieldqligned

Sdimple emissions iscrete emissions

plasmaoscillations

....

'.

ß

icropL

' "' : ' ß ' .',a,• cur.ren. ts.. '

'l Boundary layer J

ELF h

,

ß

Micropulsations

Ioncyclotron waves

IBowshockJ

Auroral field line turbulence Auroral kilometric radiation

MagnetopauseJ PS = Plasmasphere PP = Plasmapause

Fig. 7. Sourceregionsin the magnetosphere for the varioustypesof observedatmospherics.(After Shawan, 1979.)



valid beyond a few kilometers out to about 50 km. Beyond this the spatial decay rate depends on frequency. That is, the field behaves approximately as a linear system

A(f)

= S(f) x P(r, f)

(2)

where f is frequency, S(f) represents the spectrum of the source, and P(r, f) is the effect of propagation to distance r. The general behavior of the signal in various frequency bands at 50 km from the dischargeis shown in Figure 8. Figure 9 illustrates how the broadband fields change with distance (note the expanding vertical scale with distance). These fields can have strong linear polarization if they arrive from a singledominant storm center. Since this polarization can make the MT tensor impedance calculations unstable, special care is sometimes necessary to ensure that signalsare captured from more than one source.

Some very large spherics are observed to travel more than once around the earth which suggeststhat the energy is somehow guided by the earth's surface, since EM waves would normally be expected to travel in straight lines. In fact the insulating shell of air between conductive earth and conductiveionosphere forms a waveguide, partially trapping the fields. Like most waveguides, its responsedependson frequency in a way determined by its size, shape, and the nature

extensive shield and permafrost areas. At the ionospherethe resistivity gradually decreaseswith increasing height over a range of 30 km to a low of about 500 11.m, depending on local time and solar activity. The base is 20 km higher on the day side than on the night side of the earth. Thus the cavity is not an ideal spherical shell, and its response characteristics are irregular in time and space. In the majority of arrivals there is little energy around 2 kHz. The exceptions are those whose sources are within

a few hundred

kilometers.

This is

clear from the shape of P(r, f) in Figure 10 which shows very high attenuation (>30 dB/1000 km) in this part of the spectrum, especially in daytime. The reason is that the dominant mode of propagation changes, with the lowest order mode dying out and the next mode having a lower frequency limit near 2 kHz. Phase velocity (Figure 11) is greater for the higher mode, and atmosphericsfrom distant sources show clear dispersion (Figure 12). The spectral gap also shows very obviously in the Fourier transforms of such arrivals.

The earth-ionospherecavity also can resonate. The fundamentalfrequency of these "Schumann resonances" is near 8 Hz, with higher modes near 14, 20, and 26 MAGNETIC

of its boundaries.

The efficiencywith which energy is trapped depends on the sharpnessand regularity of the boundaries. In the earth-ionosphere guide the inner surface is a spherewhose surfaceresistivitiesrangefrom lessthan

647

FLUX

DENSITY

Hump

2x I(•6 Wb/rn2

Wb/m'

0.25ll.m in the oceansto greaterthan104 ll.m in 2km

Ikm

Initial peak

Wb/m2

t

2x•O -7\..............

Wb/m 2

o

50

IOO

I

I

I

I0 km

ps 5 km

_L

••j0m-28 • Zero cross

Wb/mz

T 50

km

;

ps 200

•

I00

m$

Fig. 8. Atmospherics from a near source as seen through four different band-passfilters. (From Volland, 1982.)

km

Fig. 9. Spatial variation of the magnetic transients from a discharge. The solid line is from the first discharge; the dotted line is from subsequentdischarges. (After Volland, 1982.)

648

Vozoff

Hz (Polk, 1982). Their Q is only about 3, but they define the upper limit of the region of weak signal


centered

around

1Hz.

Few plots of true spectral density at audio frequencies appear in the literature. Labson et al. (1985) includeone collectednear Berkeley, California(magnetic latitude 45 degrees)in mid-summer(Figure 13). This is consistentwith the lower frequency spectra shownpreviously.Labsonet al. alsoprovidesynoptic spectra consisting of two minute averages taken hourly over a 24-hour period in summer(Figure 14) and spot values at 20, 40, 250, and 500 Hz taken at monthly intervals (Figure 15). The latter show a winter minimum which becomes more pronouncedas frequencyincreases.This is compatiblewith the rangein average values between summer and winter for north-

ATTENUATION dB/Mm

/

/

\\

ern California at 10 kHz shown in CCIR Report 322 (CCIR, 1964).Figure 16, from Watt (1967), givesthree monthlythunderstormoccurrencefrequencyfor September-November. Figures 17 and 18 (alsofrom Watt, 1967), show two extremesof expected 10 kHz atmos-

phericnoisein Ez from0400-0800GMT in SeptemberNovember and from 0800-1200GMT in June-August. Envelopesof spectralamplitudedensityfor the MT frequencyrangeare presentedin Figure 19. They were compiled from many sources, including Clerc and Gilbert (1964), Labson et al. (1985), and numerous unpublishedreports and brochures.They are "samples of opportunity" whose statistics are unknown. Except around 3 kHz, the range is in quite good agreementwith a similar,after normalization,drawing given in Spaulding(1982, Figure 3). For prediction purposes,an envelopeof the electric field spectracan be derivedfrom thesespectrafor any desiredapparent resistivity curve using equation (5). A full descriptionof sphericsamplitudesrequires their statistical amplitude distribution, in addition to the averages provided. CCIR (1964), Watt (1967), Volland (1982), and Spaulding(1982) present those statistics and discuss their use for radio communica-

tion engineering. INTERACTION -- ---

Uniform

i01

i02

i03

FREQUENCY

10 4

Hz

Fig. 10. Attenuation of sphericsversus frequency, in dB/ 1000km. Peak attenuationis responsiblefor the weak signal in the 1-2 kHz band. (After Volland, 1982.)

-

•-

DAY

--

_

o

_J

0-9--

--

i

i

i

0.1 3

I0

I

II

I0 2

I0 3

FREQUENCY

WITH

THE

EARTH

Day Night

I IO4 Hz

Fig. l l. Normalized phase velocity associated with the attenuationof Figure lO. This describesthe frequencydispersion in the spherics.(After Volland, 1982.)

Earth

To see why the MT fields carry information about the structure of electrical resistivity in the earth we have to look at the way the fields and the earth interact. Here I refer to resistivity structure in the sameway as one refers to geologicstructure:it is "the generaldisposition,attitude, arrangement,or relative position" of the conductivityzonesof an area (Sheriff, 1984). The relationshipsbetween resistivity and petrography are described in Volume I, and in another recent review by Parkhomenko (1982). Keller (1982) providesextensivetablesof the electricalpropertiesof rocks and mineralsunder a wide variety of conditions. Special attention should be drawn to the recent developmentsin the fundamentaltheory relating porosity and permeability (Katz and Thompson, 1987). These developmentsbegin to explain Archie's well-known empiricalrelationshipsbetweenporosity and conductivity as well as the apparentabsenceof permeability from those relationships. This sectiondescribesqualitatively the interaction of the fields with the earth, in order to develop an intuitive appreciationof the processesinvolved. This descriptionis followedby a more detaileddescription for

some basic

structural

models.

It

is assumed

throughoutthat the magneticpermeabilityof the earth


649

81.40


61.05

40.70

20.35

0

•

-20.35

<1:-40.70

-61.05

-81.40

-I01.75

-122.10

i

I

I

17.06

34.13

I

I

I

i

i

I

I

I

68.66 85.33 102.40 119.46 136.53 153.60 170.66 187.73 204.80 221.86 238.93 256.09 273.06

51.20

TIME

Fig. 12. An observedatmosphericshowingdispersion.

isI• = I•0= 4xrx 10-7 H/mandthattimevariation is as eitøtwhereangularfrequencyto = 2xrf.It is also assumedthat displacementcurrentscan be neglected everywhere in the earth. The configurationof the EM fieldsabovethe surface is slightly different for sphericsthan for ULF pulsations: the former propagate horizontally while the 162

latter are assumedto propagatevertically. Becauseof the very large resistivitycontrastbetween the air and the earth, both signalsbehave in the same way at the surfaceand beneathit. They diffusedownward within (at most) 1-2 degreesof vertical, the changinghorizontal magneticfield inducinga changinghorizontal electric field at right angles, through Faraday's law. The electric field in the conducting earth drives the telluric currents. Excellent general theoretic treatmentsof the processare found in Stratton (1941) and

,oooo •

--800

"' o

• t • I ,,,,I

I0

, , , I ,,,,I

• , • I, ,,,I

102

, , , I .... I

103

, , , I, ,,

104

105

FREQUENCY (Hz)

Fig. 13. An observedspectraldensity, 1Hz to 30 kHz. (After Labson et al., 1985.)

II , , , I , , 12.00

16.00

I

20.00

I , , ,

24.00

, , , I , , , I , , J

4O0

8.00

12

TIME

Fig. 14. Variation over a day in audio frequencymagnetic field spectraldensity. (After Labson et al., 1985.)


6•0

Vozoff

in Jackson (1975), but some aspectsare described here. The fieldsare assumeduniformenoughthat they behave like plane waves above and within the earth. Spherics fields are trapped within the earth-ionospherewaveguidebecauseat eachboundarythey are almost completelyreflectedback into it. However, a

and this energyis refractedaccordingto Snell'slaw, in the sameway light is refractedat a velocityboundary. The velocity of EM waves in the conductingearth is far lowerthanin the near-dielectric air, sothat (Figure 20)

sin 01/sin 02 = Vl/V2 >•>1.

little energy is lost into the interface on each reflection

(3)

The situationdiffers from the familiar optics case in that the earth rapidly absorbs the energy and the

io-2

telluriccurrents convertit to heatthrough ohmic(I2R) losses.

•.. :-

"'l..,

-

40 Hz

250Hz 500Hz

The structureof the sphericsfield is interesting.Just abovethe surface(Figure 21) it is a transversemagnetic (TM) wave. E is nearly vertical and B is horizontal, transverse to the propagation direction, as describedin the previoussection.The leakageof energy into the surfacemeansthe Poyntingvector S = E x B must have a small real componentvertically down-

...,,m"" '"" ..... ...•...-•'

• • •'

-5

_

_

--'7"

Aug

i Sept i Oct i Nov i 1979

Dec

ward which forces E to have a small horizontal com-

I Jan i Feb i Mar i Apr I May i

Jun

I Jul

1980

Fig. 15. Variation over a year in magneticfield spectral densityat four frequencies.(After Labson et al., 1985.)

ponent in the propagationdirection. Hence the total E vector at the surface has a slight forward tilt in the direction of travel. The size of the horizontal E com-

ponent (more correctly, the ratio E/H = Z), is deter-

mined by resistivity structureand frequency. Z is

60

3O

6O

9O

Fig. 16.Frequency of occurrence of thunderstorms, September-October. (AfterWatt, 1967.)



called impedance.Fields from very low frequency (VLF) transmittershave this sameconfiguration.At least two other, slightly different, impedancedefinitions, E/B and c = Z/iixoO•,are used by various authors.Their use really makesno differenceas long as usageis consistentthroughout.

8 is the skin depth in meters, k is the propagation constant,and n is the unit vector pointingvertically downward.

The ratioE•:/Hyat thesurface(thesubscripts indicate horizontalcartesiancoordinates)is particularly important viz

To extract the resistivity information from the fields it is necessaryto look at the relations at the surface

Ex/Hy =

betweenthe vectorsE and H and the resistivity.This requires the assumptionthat the fields are plane waves,which is very nearlyalwaysin agreementwith observation.At eachangularfrequencyo•,in (or at the surface of) a uniform half-spaceof conductivity•r (Stratton, 1941, p. 490).

= (1 + i)(tolx/2•r) 1/2.

n x E,

(5)

Frequency will be accurately known since relative

time must be preciselymaintainedwhile acquiring data. Therefore equation(5) showsdirectly the rela-

k H=

651

tionshipsbetween the measuredfieldsand the conduc-

(4)

tivity.In particular, theratioof Ex to Hy is propor-

where

tional toV•p, where p= lkr.Setting Ex/Hy = Zxywe a=

get

-i)a

Zxy= •-= (1+ i)

= 1/8,

9O

180

150

120

90

60

30

0

30

60

90

©

(6) 150

180

6O

3O

6O

9O

Fig.17.Expected atmospheric vertical electric fieldnoiseat 10kHz,in dBrelative tol Vm-1/X/•, 0400-0800 GMT, September-November. (After Watt, 1967.)

652

Vozoff

or, solving for p,

px• =

ß

Zx•Zx5

(7)


// ß

where Z* is the complexconjugateof Z. The phase,•, of Z is the differencebetween the phasesof E and It.

/

//

e

e• ß

/

e/

/

/•e ß e// ß

iI

ß

Fromtheequation forZxywe seethatEx mustleadHy in phase by 45 degreesin this (uniform half-space)

• i0_1

model.

La •o-•

rr

Ex 2

px• - •o••

'

(8)

. .

=Pxy-

10 4

(9)

IZO i

,? . ,,;•

90 i

60 i

30 i

I05

I

102

5/86

I

I0•

•62

I Hz

Fig. 19. Envelope of observed horizontal component magnetic field spectraldensities.

0 i

I

FREQUENCY

IEyI/IHjI in thismodel,butin thatcasewe haveto use 150

e/ ß

CGER I

Results will be the same at all frequencies and for

i

S

., .-"

ß

/

•"..:.$.'

1 Ex 2

180

©e

i0-•

tude, then

90

ee ea'i

i0-=

commonunits(millivoltsper kilometer and gammasor nanoteslas,respectively), and looking only at ampli-

• •

I

:

.

•

and0 isgiventhephaseofZ. Converting Ex andHy to

i

,,

/'• I

Equation (7) is usuallywritten

1

I

/

30 i

60 !

90 I

120 i

150 i

180 i

60

-. 30

o

$o

__AOZ 60

- -104 •.

--

90

I

I

-

I

I

I

I

I

I

I

I

I

Fig.18.AsFigure17butfor0800-1200 GMT,June-August. (AfterWatt,1967.)

_ I

The Magnetotel!uric Method


(-Hx)

in order to keep S = E x H in the right

653

with the plussign,•xy is in the firstquadrant(0ø to

direction. ThusthephaseofZyx differsby 180degrees fromthephaseof Zxyin thismodel.Strictlyspeaking, there shouldbe a By rather than the Hy in the

+90ø), and with the minussign(as in Figure 50) it will be in the fourth quadrant (-90 ø to 0ø). In each case,

denominatorof equation(9), but the equationis rarely written that way in practice. As long as the numbers used are in the correct units (nanoteslas,which are units of B), the correct values of resistivity will be

used in this chapter. In practice, both phasesin field resultsare sometimespresentedin the range(0øto 90ø). In order to satisfy Maxwell's equations,within the earth E and B vary as

•yx istwoquadrants away.Theplussignconvention is

obtained.

A = Aoe-i(kz-tot)

Two different phase ranges are commonly used,

depending uponwhether thetimevariation ise+itot or e -ttøt. The choiceis arbitrary, and can make no difference to thephysics. If q>ij isthephaseofZij, then

= A oe itøte -iaZe-az where A0 is the surfacevalue. That is, the fields vary as the product of four terms'

1. eitøta sinusoidal timevariation,

2. e-i•z, a sinusoidal depthvariation, VI =C

AIR

v2<
EARTH

3. e -m, an exponentialdecaywith depth, and 4. A0.

Figure 22 showsthe z-dependencesof the E fields near the interface. From the third term, the amplitudes at z = 8 = 1/a are 1/e of their surface values, which is

why 8 is called the skin depth. A plot of skin depth versus frequency and resistivity can be found in the Zonge and Hughes CSAMT chapter (this volume). A useful approximationis given by

DIRECTION

• = 5000x/•f meters. z

Horizontal Layers

Fig. 20. Refraction of a plane EM wave incident on the earth's

In more complicatedmodels, e.g., horizontal layers or 2-D or 3-D structures, the relations between the E

surface.

and H fields also become

Ezl

more involved.

In horizontal

layers some energy is reflected at each interface, and internal reflection occurs within each layer. The expressionsfor E and H includetwo terms in each layer,

E1

of the form

propagation direction

Exl +x

/

2

Fig. 21. Fields of a sphericpropagatingin the +x direction. The earth's surface is z = 0. The total electric field above the

surfaceis nearly vertical but has a small tilt in the direction of propagation.

•/•E

Einc.

i

refl••Etrans Fig. 22. A "snapshot" of the E field of a plane EM wave vertically incidenton the earth's surface.Most of the energy is reflected. The small part that is transmitted attenuates exponentiallywith depth. S representsthe propagatingenergy (Poyntingvector) into the earth.

654

Vozoff


Ae +ikzq_Be-ikz

one for up-goingand the other for down-goingenergy. The fields remain horizontal and at right anglesto one another unless there is anisotropy in the horizontal

plane.We canstillgetEx andHy (orEy andHx) onthe surfaceand do the resistivity calculation,but the result depends on frequency and is now an apparent resistivity Pa(•' As a specificindication of the intuitive behavior of

Pa, consider two 2-layer cases (Figure 23). The top layer is the samein both models,but P2 = 10pl in the one case and P2 = (1/10)pl in the other. At high frequenciessuchthat the skin depthin the first layer is much lessthan its thickness,Pa = Pl in both cases.As frequency decreasesand skin depth increasesto the point that it is much greater than d, Pa goesto 0.1 in

(Figure 24), then frequency has to go to a lower value in the thicker layer model for the skin depth to exceed thickness and the second layer to become important. In a three-layermodel, Pa is asymptoticto p• at high

frequency and to P3 at low frequency. In between it approaches02. How close it gets depends on the thicknesses and resistivities of both layers 1 and 2 (Figure 25). Obviously our ability to resolve several layers depends on their resistivities and thicknesses, on the range of frequencies we record, and on the scatter in the points on the curve.

If Pa is plotted on a log-log scale, its phase is proportional to the slope of the curve but from a baseline at -45 degrees. The relationship is (e.g., Parker, 1983)

one case and to 10 in the other, and stays there as

frequency goes to zero. Note the small undershoot/ overshootin Pa and q) as frequency decreases.Also note the phase responseoccurs at higher frequencies than the apparentresistivity response.Phaseis asymptotic to 45 degreesat both high and low frequenciesfor a finite number of uniform layers. If thickness varies but Pl and P2 are the same

q)(f) =•- 'rr

Again, p• is the high-frequencyasymptoticvalue of Pa(f)' Weidelt (1972), Kunetz (1972), and othersshow that, as a crude approximation,

1+

0(f) •7 MODEL A

71=1ohm-rn

•, Pl f2_f2'

o log Pa(f)

(10)

0 log f

MODEL B

71=I ohm-rn

IOOOm

?2=10ohm-rn

/•1= I00-m

IOOOm

?• = I00 0 - m

72= '1ohm-rn 2 LAYER

MODELS TWO .= ' [ .... "1

' ''"'"l

LAYER ' ''"'"1

CURVES ' ''"'"1

' ''""'l

' ''"'•

_

_

.

_

_

•

i04

75

_

• phase B

o

104

-

-•

.

-

.

_

60

>.. I03

o

>.. I03

•

i02

45

Z

I0 •

30

LU

•10••

300m •

• I0• <1:I0ø

<1:I0ø I0" 10-5

I0-4

I0-3

I0-2

FREQUENCY

I0 -•

I0 0

0 I0 •

Hz

I0 -5

I0 -4

I0 -3

I0 -•

FREQUENCY

Fig. 23. MT apparent resistivity and phase responses of two-layer models. Model A--resistive basement. Model B--conductive

basement.

I0 -•

I0 0

I0 •

Hz

Fig. 24. Changesin MT responsewith layer thickness T for a two-layer model.



These are called dispersion relationships. Figure 26, from Weidelt (1972), shows simple examples where the approximation is in error by 10 to 15 degrees. The net result is that, as frequency decreases,phase anticipates the behavior of Pa' Within a finite frequency range, the phase shows features beyond the reach of the resistivity, so there is an advantage to interpreting them together. For the same reason, features which are too shallow for 4 may still appear in the highest frequency Pa. The other facet of this behavior is that, as long as Pa is asymptoticto some constant value at low frequency, the phase always returns to •r/4. This is different from the apparent resistivity, which can be affected over a very broad frequency range by a superficial irregularity. The importance of this property of the phase is discussed later in this section and in the Interpretation Section. The terms "penetration depth" and "investigation depth," instead of skin depth, are sometimesused in depth-variable media. These terms have a variety of definitions, as discussedin Spies (1989).

655

almost any bedded sediment is easier along the bedding planes than across them: currents can follow the most conductive bands without crossing the resistive ones, and the rock is thus anisotropic. Rocks under stress are also known to become anisotropic. If E is applied along the beddingone value of current density j (and therefore of •r = j/E) will result, but if E is applied acrossthe bedding a smaller value of •r will be

indicated. (Unitsof currentdensity j areA/m2).If E is applied in a direction which is neither parallel nor perpendicular to bedding, then j will generally have componentsin both directions and will not be parallel to the applied E. To some extent all rocks are anisotropic, as can be seen from well logs. Then in three dimensions j = ere

(11)

or

Jx

tYxx tYxy tYx z Ex

Jy

tYy x tYyy tYy z gy

J•

(12)

tYzx tYzy O'zz g z

Anisotropy

In anisotropic media, conductivity varies with the direction of the applied electric field E. Conduction in

Pat ß

ooot oooh

IOOOI-

•1 =I0fi.mIT•

?•: I000 •-m

....

500L

IO lOOO fi.m

I

ø1' ' '! I-' iool-ZOOl-•

T•

f3 = I0 fi.m

50

300

,,'L/I-' •01 z • THREE

LAYER

I00

CURVES

•P

•

I•01 I?

?3 10 4, --•T•o

105 ' ' ''""1

-[:50ø T1

I km

104 _ E

71

I00-M

? 2

IOOO

73=

IO

_

Pat I

I0 lOOO fl.m

'i

!-

_

103 -

fi.m5o•

I0 •

IO

T2 -- IO km

I0o I0• :

I0 •

I02

IO•

104s .--• T

0-3

_ _

i0o =--

10"5

10-4

10-3

10-2

FREQUENCY

I0 -•

I0 o

I0 •

Hz

Fig. 25. Changes in MT responsewith second layer thickness in a three-layer model.

i/I

true phase ß approx. phase

Fig. 26. Phase errors in the approximation to the Hilbert transform (After Weidelt, 1972).

6•6

Vozoff

is the general situation. If, for example, we apply a

fieldE = Ex (withH = Hy), andEy = Ez = 0, then


Jx = {rxxEx

jy = O'y xEx Jz = O'zxEx.

(13)

The current (and therefore the electric field) will have componentsalong x, y, and z although the applied electric field is only in the x direction. Since our assumed incident plane EM wave had its magnetic field in the y direction here, j is no longer perpendicular to H.

The effect on Pa of anisotropywithin a horizontal layer is shown in Figure 27, from Vozoff (1972). Apparent resistivity then depends on measurement direction, just as if there were lateral variations. However unlike lateral variations,in this casePais the same everywhere on the surface, and no vertical magneticfield is induced. The theory was publishedin Abramovici (1974). When a geological unit has a differentresistivityin the horizontaland vertical directions, which is normal in sediments,MT respondsonly to the horizontal

value because the E field is horizon-

tal. An independentmeasurementin which E has some

verticalcomponentis necessaryto determine{rz (Jupp and Vozoff, 1977). This has been used to predict the character of sedimentsfor hydrologicalpurposes(Vozoff et al., 1982).

rotated I

tensor resistivities I

Inhomogeneity

Anisotropy, one of two possiblecomplications,exists to some extent in all rocks. The other major complication is inhomogeneity, whose description is, after all, the main point of most geophysical surveys. In a 2-D or 3-D structure, the conductivity, currents, and fields vary in x, y, and z (x and z in the 2-D case), and so E and H are again usually not at right anglesto one another. What effect do the lateral resistivity boundaries

have on currents

and field? When current

encountersa region of discrete or gradationalresistivity change,it setsup and maintains a chargedistribution in the region (Figure 28). These charges, which are available because of very small differences in integrated current from the surface to z = •, produce their own electric fields that are exactly those needed to satisfythe boundary conditions,and they modify E and j in the vicinity. As an example consider the simplest2-D model, a vertical contact striking in the y direction at x = 0, between uniform quarter spacesof resistivitiesp• and 92, with 92 > Pl. A correctguess might be that, for a given magnetic field intensity, the induced current will be different accordingto whether it has to flow across the fault, or parallel to the fault stayingon one side. That is, we expect different values of j/H and therefore E/H according to whether H is parallel or perpendicular to strike. If the two impedancesare different, then the apparent resistivitiesmust also differ in the two directions.

The boundary conditions dictate that the current density normal to the boundary must be continuous across the boundaries. The E field of the resulting charge distribution adds vectorially to the fields in-

I

I0 000

MODEL ,D = 10 fi.m -

\

•,,,•,•100 a.m"•Ikm

I 000

•.•,"•10 mm•

N, 2km

100D ß rn

E

_

i

I

Es

J

+

I

+

I00

•..••

I0

-

I

.ool

Pa for E .L strike.

•2

to strike,all o."" •

I

.Ol

I

I

I

I.

IO.

frequency (Hz)

Fig. 27. Effect on apparent resistivity of anisotropy in the second layer of a 3-layer model. Theta is the dip of the smaller (10 11.m) principal resistivity axis.

Fig. 28. Charge accumulation due to current crossing a contact, cr2 < cr1. The E field of the charges(Es) opposesthe applied field on the left of the contact and augmentsit on the right, so E2 >



duced by H in each medium. This reduces the current density on the conductive side over what it would be if

there were no contrast. SinceJx = crEx, the electric field and the impedance are also reduced, making the conductorappear even more conductive. On the resistive side the field adds to the initial field, making the region appear even more resistive. These induced chargesmake it possibleto accurately locate contacts and are responsible for many of the characteristic responsefeatures of 2-D and 3-D bodies. When we are far enough away from the contact we expect its effect to vanish. That is, Pa takes on the value appropriate to a uniform half-space. Far enough away is about one skin depth, so that the distance is greater at low frequencies than high frequencies. Figure 29 shows the apparent resistivity, at one frequency, for E1 to strike. This resistivity can be computed for many frequencies and presented as a pseudosection, as in Figure 30a, where the vertical axis is frequency increasingupward. The model is the same as for Figure 29. The effect of the contact is seen at all frequencies, from lowest to highest. When H is perpendicular to strike then E and j are parallel to strike and no charge is built up on the

IO3

-24-22-20-18

-16 -14 -12 -I0 -8 -6 -4 -2

657

contact. The boundary condition just requires that E be continuous across the boundary and adjust to the different skin depths on the two sides. Figure 30b showsa pseudosectionof Pafor Ell to strike. Distances over which the adjustment occurs are again related to the skin depths on the two sides. [This general behavior must be distinguishedfrom the specific behavior for which the term "adjustment distance" is used (in the General 3-D Section) to describe large scale effects due to effective anisotropy of the lower crust-upper mantle.)

There is another major difference between the two polarizations: currents are squeezed closer to the surface on the conductive side because the skin depth is smaller there. From the vector equation

I7 x E = -ixOH/Ot we see that a vertical component of H is set up when V x E has a vertical component, that is, when there is a lateral changein horizontal current density such that

eitherOjy/OX or Ojx/Oy (butnotOjx/OX) is nonzero. This H z componentmust decay with distancefrom the

contact.In the 2-D casethisarisesforjy but notJx, 0

2

4

6

8

I0

12 14 16 18 20 22 24 km

102

E

iJJ

I-

10 •.m

400

•.m

O-I

Fig. 29. Apparent resistivity versus position acrossthe contact shown at bottom, at 0.1 Hz.

658

Vozoff


sinceO/Oy- 0. When H z is normalizedby the horizontal magnetic field it is variously called the tipper, induction vector, Parkinson vector, or Weise vector. The magnitude of the tipper in our fault model (in percent) is also shownin pseudosection,sinceit varies with frequency and position (Figure 30c). This explains why care must be taken interpreting data from high and equatorial latitudes. If the contact does not come to the surface, but is buried beneath an overburden (Figure 31), then its effects at the surface are smoothed to some extent. In this case the overburden consists of 1.0 g•.m material

0.5 km thick. The amount of smoothingdependson the thickness and conductivity of the overburden and on the frequency. At high enough frequencies, when the skin depth in the overburden is much less than its

quency limit, when skin depth is much greater than overburden thickness, the overburden will be nearly invisible and the contact will appear to outcrop. Impedance Tensor and Tipper: 2-D and Almost 2-D Cases

The relationshipsamong the field componentsat a single site are systematicallycontained in the impedance and the tipper. They are the quantities from which conductivity structureis interpreted. In general,

Hx hasan associated Ey andsomeEx, bothof which areproportional toHx. Likewise,Hy causes anExand someEy, sothatat eachfrequency we wouldexpecta linear system to behave as

thickness, the contact will not be detected. We will see

Ex = ZxyHy + ZxxHx

only the overburden, and the responsewill be independent of position and direction. In the low fre-

Ey = ZyxHx + ZyyHy ,

10

i i

5

0

i

5

10

I0

5

0

5

I0

i

i

i

i

i

i

NIII

,o

,,z, 16

Z

I

/øø t

O

_

-

IO

i(• 6 • '

•

'1

I

30 (b)

30 (a) 10 i

10I

5

0

5

10

i

i

i

i

_

I

zi62 Fig. 30. Pseudosectionsacross the vertical contact. Horizon-

i 30 (c)

i 60

i

I

i

tal axis is distancein kilometersfrom the contact.(a) @afor E perpendicularto strike. (b) @afor E parallel to strike. (c) tipper in percent.



where each term is frequency dependent. This is commonly written

659

the vector E field through angle +0 (clockwise as seen from above) to be E', then cos{)

Zyy) (HH•) (14) or

sin

-sin 0 cos

Ex

Ey

(15)

or

E=

ZH.

E'

In a uniformor horizontally layeredearth,ZxxandZyy arezero,Zyx = -Zxy, andthe equations reduceto

= RE.

In the same way,

H'=

Ex - ZxyHy

_RH

(16)

_Z'= _R_Z _R r

(17)

and

Ey = ZyxH x -- -ZxyHx ' In a 2-D case, if the x or y axis is along strike then

where_13rthetranspose of R is

Zxx : Zyy -- 0,

cos0 _R r=•,sin0 (cos 0-sin 0)'

but

(18)

Startingfrom a tensorimpedanceZ_which is derived

Zxy •-Zyx.

from measurements, and assuming 2-D conditions, several different ways have been used to find the

If neither axis is along strike then

gxx : -Zyy • O.

rotation angle 00 between measurementdirection and

Equation (14), proposed in Cantwell (1960) and in Rokityanski (1961), assumedthat the system is linear so that the electric fields are due only to magnetic fields, and the contributions of noise were ignored. The problems of unraveling these relationships in noisy data are discussednext. As a practical matter we often wish to look at the fields or the tensor elements as if they had been

degrees), plot them on a polar diagram, and pick an optimum angle from the plots. An optimum angle

measured

strike.Oneof theseis to rotatetheZij in steps(say5

in some other set of coordinate

maximizes or minimizes somecombination of theZij. These interesting diagrams, called polar figures or impedance polar diagrams, are usually plotted at many frequencies,becausein practice the strike direction often changeswith depth. Figure 32 shows such

plotsfor a 2-D model(Zxyonly),andFigure33 shows a realexamplewithbothZxyandZxx.

directions.

For instance, the strike direction is seldom known

Another way to find 00 is to useone of Swift' s (1967)

very precisely at the time of a field survey. If we rotate

solutions,in whichthe expressions for Zxy(0)and

1.0 contact

0.8

0'6

xX•}•XX-' x-X •x•=x

--

--x

x•x• x

x x --.

I

0 15

ß

x

0.01Hz

X"X"X Xx

I0

ii

overburden(500m, no overburden

I

5

i xx

0

5

0.01 Hz

x I,OHz I0

15 km

1.0

Fig. 31. Effect of overburdenon tipper, at two frequencies.No overburden(x' s) is for the same contact model as Figures 29 and 30. The overburden curves (dots) are for a layer 0.5 km thick of 1 fl.m material.

660

Vozoff

Zyx(O)are differentiated with respectto 0 to givean angle 00 which optimizes Apparent resistivity response throughout periodrange

IZ•cy(00)l 2+ IZ•x(00)l 2


apparent resistivity P•

Pl

P2

P2

(•)'

P•

Pz

(19)

at each frequency. His solution

P•

'

400 = tan-•

I

[(Zxx--Zyy)(Zxy q-Zyx)*q-(Zxx--Zyy)*(Zxy q-Zyx)] IZxx- z.l 2- IZx + Zxl2

TEI

_

(20)

Impedance polardiagrams at givenperiod To

alsomaximizes IZxylandminimizes •

IZxx12q-IZyyl2.

x

There is no solution in the 1-D case, whereas in a clearly 2-D caseit usually has a definitevalue. In the 3-D caseits meaningis usuallyquestionableand there

:...'. -'

is considerableresearchin progressinto ways to

presentand interpretZij in structuralterms.The Fig. 32. Impedancepolar diagrams(at one frequency),and apparentresistivities,at four siteson a simple2-D contact model.TE andTM referto TransverseElectric(EII to strike) and TransverseMagnetic(It II to strike)field polarizations, respectively.(After PhoenixGeophysics, Inc., pers.comm.)

SITE

matter will be discussed further. Of the four values

between 0 and 180 degrees, the "choice" of strike directionis startedby evaluatingequation(19) at two adjacent values, one a minimum and the other a

maximum. This leaves four possiblesolutionsat 90 degreeintervals,or two possiblestrikedirections.The choicebetweenthesesolutionscanonly be madefrom

A

SITE ,

...

:.

/

\

""%1

ß ß

'.""........ •ß.."........ '.;36 ß "..oI

:•Oooo .... •o• 9'

t..."'.,., ,......

"" ...... '"•'5

.0•5 ,

I

...* •, .0088

...'" i

..:(

.

...'

.0044 ..... .002: > ßRotation Direction

Zxy........

Zxx

Zxx Normalized

Fig.33.Impedance polardiagrams at twosites(A andB) for 16frequenciesß Dottedcurvesareforoff-diagonal (Zxy),solidcurves arefor(Zxx).Asterisk indicates Swiftrotation direction. (AfterPhoenix Geophysics, Inc.,pers.

comm.)



independent information, usually the relations between vertical and horizontal H components or from geological constraints. Still another way of showing the angular variation in impedance elements is by means of impedance rotation ellipses, plotted in the complex plane, with properties that are explained in Word et al. (1971) and Eggers (1982). These ellipses should also be examined for many frequencies, and are used mostly to examine the general behavior of the tensor elementsas rotation angle varies. Several useful propertiesof _Zare mentionedhere. When the coordinates are rotated, certain combina-

tions of terms are constant even though the individual terms vary. These are

Zxx -JrZyy - Cl,

(21)

Z•y - Zy• = c2,

(22)

ZxxZyy - ZxyZyx - c3,

(23)

and

where

rarely as great as 1, with .1 to .5 being the common range. The lower part of the range is often blurred by

noise,sinceH z is soweak. The requiredrotationangle q>to x' can be estimatedby the field data by findingthe horizontal direction y' in which H(q>) is most highly

coherentwith H z. Thereis generallya definitesolution in the "almost" 2-D situation. In that case the phases

of Tx and Ty are the same,the ratio Ty/Txis a real number, and

4) = arctan(ry/rx). Another use of the tipper, besides helping resolve ambiguity in strike, is to show which side of a contact is more conductive.

Near

a conductor-resistor

bound-

ary, the near-surface current density parallel to strike is larger on the conductive side. If it is looked at as a simple problem in dc magnetic fields of the excess currents, the magnetic field in the vertical plane perpendicularto the contact will "curve around" the edge

of the conductor,by Faraday'slaw. Thus H z will be the absolute

value

is the determinant

of the

impedancetensor. The ratio c•/c2 is the impedance skew, a. c• will be zero in (noise free) 1-D and 2-D models, so the skew is used as a measure of three dimensionality. It does not change with rotation of coordinates.

directed downward when the horizontal component is outward, and vice versa, depending on current direc-

tion. In a real situation,the phaseof H z dependson the conductivities, frequency, and distance from the contact. However in practice the relations can often be used to indicate the direction to a conductive region. The General

A quantity which does vary with setup direction is impedance ellipticity,

Zxx(0) -- Zyy(0)

[3(0) =gxy(O) +gyx(O)'

(24)

This is zero (for noise free data) in the 1-D case, and in the 2-D case when the x or y axis is along strike. Impedance ellipticity, like impedance skew, is used to indicate whether response at a site is 3-D.

It can usuallybe assumedthat H z -- 0 except near lateral conductivity changes, where V x E has a vertical component (but see discussionin the Sources

Section).There, the relationshipbetweenH z and the horizontal magneticfield componentsat any frequency can be written

H z = T•Hx + TyHy

(25)

where the elements Ti are complex since they may include phase shifts. Given a 2-D structure with strike in the x' direction, in those coordinatesequation (25) simplifies to

H z = Ty'Hy'.

(26)

Here, T', since it represents a tipping of the H vector out of the horizontal plane, is called the tipper. T' is of course

661

zero for the 1-D case.

The

modulus

of T' is

3-D Case

In practice 3-D problems commonly range from the effects of small isolated conductivity anomalies to those of large scale structures such as curved coastlines and complex mountain ranges. There is at the moment a particular interest in understanding the effects of superficial 3-D bodies, on account of the important static (vertical) shifts they can produce on Pa curves.

Our ability to interpret results from areas of 3-D structure is far less developed than for 2-D areas. The mode separationthat helps so much to simplify the 2-D case does not occur in 3-D, where there are surfaces exposedto charge accumulationregardlessof current direction. The vertical magnetic field, which is enhanced by long current paths in structures with a definite strike direction, is generally smaller and more difficult to interpret in the absence of this condition. Numerical model programs and results for 2-D have been in use since the middle 1960s, whereas those for

3-D have been available only since the early 1980sand are still considered costly and lacking in generality (Reddy et al., 1977; Jonesand Vozoff, 1978; Ting and Hohmann, 1981; Ranganayaki and Madden, 1980; Wannamaker et al., 1984a; Pellerin and Hohmann, 1990). Berdichevski and Dmitriev (1976) and Larsen (1977) model special cases. In addition, a few analyt-


662

Vozoff

ical or semi-analyticalsolutionsexist for specialcases (e.g., Bailey, 1977; Fischer et al., 1978). Hohmann's (1988) paper is a very good review of the situation. Examination of a few specific models is helpful. Figure 34, from Ting and Hohmann (1981), showsthe distribution of apparent resistivity at a single frequency around a simple 3-D body, a conductive block in a more resistive half space. At the frequency used the skin depth in the half spaceis about 15 km, so the block is effectively quite shallow. The principal elements show the effectsof currentsbeing attracted into

the conductive block. In Pyxthey-directedelectric

West and Edwards (1985) set out general rules of responseto an EM dipolar source of a small, simple conductive 3-D object in a conductinglayered halfspace. They show the factors affecting the induced sourcestrengthwithin the scatterer, current gathering from the host, and induction from the field. Inductive interaction with the host is shown to be relatively weak. The argumentsare useful in the MT casewhen the "target" is compact, althoughthey apply strictly only to systemswith dipolar source and receiver. The discussionis primarily a list of caveats. Similarly, Park et al. (1983) and Park (1985) classi-

field and current density are enhanced where the current is "collected" by the block. They are reduced on the sides, where the current density has been reduced. This can also be interpreted as the effect of inducedchargeson the +y edgeof the block augmenting the normal electric field. When the normal electric

fied the first order

fieldis in thex-direction,in Pxythe samebehavioris

mode for 2-D models (Madden, 1971), and a look in passingis instructive. Figure 36 shows the effect of width in a 2-D body at the surface of an otherwise uniform half-space. Apparent resistivity versus frequencyis plotted for various body widths and for the 1-D asymptote when width reaches infinity. The observation point is in the center of the body. The decreasein low-frequency apparent resistivity as the body becomes narrower is remarkable and indicates how small superficial conductors can produce such important static shifts. Figure 37 shows pseudosections for a very similar model. Horizontal current gatheringcan only occur in 3-D bodies.The effect in 3-D slabswas studiedin Ting and Hohmann (1981). They showedthat the E field of the chargebuilt up on the edges,and therefore the statics effect, persiststo surprisinglylarge distances. When the model is complicatedby adding layers beneath the inlier, those layers will also affect its ability to attract current. For example, a more conductive half-space beneath the body makes it easier to gather current from below. This has been found to be an important factor in regional studies.In the case of the resistive deep crust-conductiveupper mantle system, the combinationforms an effectively anisotropic layer (Ranganayakiand Madden, 1980). Park et al. (1983) show that, in such a situation, a surface conductor must be very wide in order to gather current from the conductive mantle beneath. The length in-

seen, rotated through 90 degrees. The two diagonal elementsare seen to be near zero except around the corners of the block, where the induced charges distort current flow to set up "anomalous" magnetic fields.

In another specificcase, Berdichevsky and Dmitriev (1976)modeledthe low-frequencyasymptotic(statics) effects on the principal axes of resistive and conductive (vertical) ellipsoidal cylinders in a thin surface layer. Both cylinder and layer were underlain by an infinitely resistive secondlayer, which in turn lay on an infinitely conductivebasement, so their effectswill be extreme. For a given contrast, they showed that resistive

features

create

smaller

disturbances

than

conductiveones, and in the opposite sense. Groom and Bailey (1989) devised a composite2-D and 3-D model, to study the behavior of various processing and interpretation techniques in such cases.In particular they used a numerical2-D solution to representa "regional" 2-D structure, and superimposed a small conductivehemisphereon the surface, to represent a body giving rise to statics effects. The hemispherewas assumedto affect the fields only in its immediate vicinity, and the effects of local E and H fields were computednumerically. Lines through the center of the hemispherein the principal directionsof the 2-D model are the principal axes of the fields around the hemisphere. The authors studied impedances and directions just outside the hemisphere, findingmarked differenceswith positionrelative to the principal axes. Along those lines the impedancescontained only off-diagonalterms, and those were staticshiftedversions of the 2-D impedancesat those positions. At sitesoff the principal axesof the hemisphere, all four impedanceelementswere mixtures of the two 2-D

terms.

effects of 3-D bodies on MT

fields

into three categories:vertical current distortion, horizontal current distortion, and local induction (Figure 35). ("Current distortion" is also called "current gathering"or "current channeling".) Vertical

current

distortion

also occurs

in the TM

volved,calledthe"adjustment distance," ist = V'SR, where S = •rAz and R = pAz are the integrated conductivity and integratedresistivity, respectively, over the zone of thicknessAz and typically is hundreds of kilometers. The effect introduces a strong downward bias on E at low frequenciesin measurements near continentalmarginsand on islandsbut is lessimportant in many problemsof explorationscale.


663


x

o o

x

x

m •

0

m_

.o

o

I o

o

I •

0

.

m

0•0 ¸

0 0

_0

ß

X

m• l=

¸

•oo

¸

:' :::::::::::::::::::::::::: :':!::::::::1 •:::::::::::::::::::

¸

"'"'"'"'"'""'"'""'• E'.'.'.'.'.'.'.'.'.'.'.'.''"'"'"'"• .'.'.:.:.:.:.L•;'""'"'""'"" ...................]

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

•N

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

o


664

Vozoff

As shown later, at low enough frequencies, current gathering effects become frequency independent, but local induction effects remain proportional to frequency, and are thus fundamentally different from current gathering (Park, 1985; West and Edwards, 1985). Adequate data can provide a way of distinguishing the two phenomena in particular cases. These conclusions regarding the domains of induction and distortion are borne out by other studies of

low-frequency asymptotic behavior of 3-D models. The studies demonstrated that the secondary electric field becomes a fixed percentage of the primary, so that the surface charge effect on E and thus on Pa is present down to dc. However as a consequence of Equation (10) the effect on the phase vanishes as was shown for specificmodels in Ranganayaki (1984, their Figures 8 and 9), and analytically in Wannamaker et al. (1984b). The latter, in a fundamental contribution, show that at the surface of a layered medium containing a 3-D body

E h = [•I + _PIEi

(27)

and

Hh = [I_+ Q_Z_e]Hi,

(28)

where Z_eis the impedance of the layered medium

without the body, _Pand Q_are scatteringtensors, I_is a 2 x 2 identity matrix, and the subscriptsh and i indicate horizontal components and incident fields, respectively. In this case,

Z_= [I_+ _P]Z_e[I_ + Q_Z_e]•, Local

Horizontal Current Gathering ...................

Vertical

Current

Gathering

Fig. 35. Patterns of current flow due to an isolated 3-D conductor in a more resistive host. (After Park, 1985.)

2-D CURRENT

GATHERING

where Z_is the complete impedance tensor. When the 3-D body is relatively conductive and frequencies are low enough that skin depth within it is larger than the

body, then they show that _Pand Q_becomereal constants,independentof frequency, and I•Z_evanishes (as _Ze)as frequency goes to zero. In that case,

Z_= [I_+ _P]Z_e

-

10 4 E

>- lO 3

I ,0..m J•-i km

k-W

come

400

Pxx= PeP2 xy 1-D

{ w:o:))

--_

Pxy= pe(1+ Pxx)2

_

W:4 7.5

km

and

1¸ •

pyy= pepyx 2

(31)

where Pe is the apparent resistivity at that frequency for the unperturbed layered medium. The coefficients of Pe are always positive so that the apparent resistiv-

< 10o n 10 -i 10 -s

Pyx= pe(1+ pyy)2

-==

i7o5

• z

(30)

so that the four (unrotated) apparent resistivities be-

-

_

• 1¸•

(29)

induction

10 -4

10 -3

10 -2

FREQUENCY

10 -t

10ø

10 i

Hz

Fig. 36. Effect of vertical current gathering on apparent resistivity for E across strike in a 2-D model. W is the width of the rectangular conductive cylinder. The observation point is in the center of the body on the surface.

itiesare alwayspositive.Pxyand Pyxcanbe greater than, less than, or equal to Pe, dependingon the signs

of PxxandPyy.Theywill dependontheresistivity and position of the inlier relative to the measurement point. The phasesare those of the layered medium in this approximation.Hence, simply using 1-D inversion

givesmodelresistivities toolargeby Pij/Peanddepths

The MagnetotelluricMethod

toogreatbyX/Pij/Pe. Whenthesuperficial feature is

665

considersan apparentresistivityand phasedefinedby the determinant of Pa

2-D with strike in the x direction, then no surface


chargedistributionis createdandPx• is zero. Then a

1

1-Dinversion of Pxy(Ellto strike)will giveresultsless distorted thanthePyx(E.L)component, at leastat low

=

frequencies.

IZxxZ -

and its phase

Wannamaker et al.'s (1984b) conclusionsspecifically extend and supportthe earlier semi-analytical resultsof Berdichevskyand Dmitriev (1976) and are confirmed in the model computationof Groom and Bailey (1990). Two other characteristicsof 3-D responseswhich are observedin practiceare their enhancementat low frequencywhen resistivityincreaseswith depth, and systematically reducedtippersascomparedwith those of 2-D features. This behavior, which is anticipated intuitively, is also explainedby the resultsof Wannamakeret al. (1984b)and Ting and Hohmann(1981). The numberandvariety of responsefunctionswhich are used for interpretation pose a problem in data presentation.In a combinedcase history and model study, Ranganayaki(1984) experimentswith ways of presentingthese and related data. In particular, she 3'0

Oa = phaseof (ZxxZyy- ZyxZxy). The recovery from the effects of superficialfeatures with decreasingfrequencyof the individualphasesand of the phaseof the determinant,and the persistenceof those effectsin Pa and Pd, are illustratedgraphically. Clearly, phase is easier to interpret than apparent resistivityin these situations,as shownby Figure 37 from Ranaganayaki(1984). While the ultimate goal of 3-D researchmight be to produceinexpensive,accurate,inversiontools, at the presentstageit would be highlydesirablejust to have simplified,if lessaccurate,ways to do 3-D modeling. Flores and Edwards (1985) use the simplifiedbehavioral guidelinesof West and Edwards(1985), applied to plates, to interpret the results from a complex geothermalarea.Their modelcouldbe usedasinputto

I

•20 0

Pdet =1Zxy Zyx I ohm-meters

•

-0'5

-

_o

-3.0

-

-•.½

_

3'0--

/

•5••0 •,•

45

•45

o -3'0

-

-4'0

-

I Im(ZxyZyx)

-{)det =•. arctan ' Re ( ZxyZyx )

degrees

•

4300 IOmO0 500o, 500i0004300distance (m) MODEL

'•

I 5.,mJ 501AV, I from center -

m

50

Fig. 37.Theeffectsof a shallowbodyon apparent resistivity andphase.Model,shownat bottom,isverysimilar to that of Figure 36. (After Ranganayaki,1984.)

666

Vozoff

a more accurate forward program if desired, but it could equally well be used to start a 3-D inversion.


Statics, Topographic, and Regional Effects

The results of the previous section are, of course, fundamental to an understanding of statics effects, which became increasingly noticeable as station spacing in surveys decreased. In this section I summarize the definition and appearance of statics effects and their relationship to the 3-D theory of Wannamaker et al. (1984b). I also point out the existence and cause of regional effects, the importance of which Madden and Mackie (1989) emphasized. Topographic effects are also important in some locations and have some of the characteristics of statics effects. Certain aspects are discussed

at the

end

of this

section

and

are

ing layer overlying an insulating layer on a perfectly conducting basement. In the 3-D case the observed effects are anticipated from Equation (30). By definition, the frequency is low enoughthat _Pis a real constant.Intuitively the magnitude of the statics effect of a 3-D feature smaller than that of a 2-D feature

would be

with the same cross

section, becausecurrent can bypass the former. However, there will be a TM-like response in the 3-D case regardlessof field polarization.

also

described qualitatively in the Zonge and Hughes CSAMT chapter (this volume), where references are given to more quantitative work in the literature. Statics effects are defined as vertical displacements of apparent resistivity curves, between adjacent sites or between the two curves at one site, without other

differencesin either the shapesof the curves or of the phases. An illustration is shown in Figure 38, from Sternberg et al. (1985). The implication is that the offset is a result of a conductivity feature at surface that is so thin that its effect on phase has disappeared above our highest frequency. Hence by definition, frequency is so low that induction effects in the causitive body have vanished. If we went to still higher frequencies, then differences would appear first in the

shapesof the Pa curves, and then in the phases.If we also recorded at more closely spaced sites we would expect to be able to map and interpret the responsible feature. In a sense, statics are due to undersamplingin both time and space. The effects are most evident in CSAMT data because of the close station spacingsand short dipoles normally used: if the sites were further apart the differences might be mistakenly attributed to structural complications. Likewise, longer dipoles will hide the effects of small statics features by averaging the field across the entire length of the dipole. Static

shifts

can

have

either

2-D

or 3-D

period (s)---increosing

source

regions. In the 2-D case, the charge buildup in the TM mode will shift that apparent resistivity by a constant factor when the thickness of the anomalous body is much less than the skin depth in either the host or the body. Then, the TM apparent resistivity will depend

on the width of the body and on position,but a shift would be predicted even for very narrow bodies. Much lesser shifts are expected for the TE mode, and if the body is narrow enough then no shift should be seen. Berdichevsky and Dmitriev (1976) include type curves of the statics effects of 2-D bodies in a conduct-

92 > 91 Fig. 38. Sketch of model used to explain static effects. The middle apparent resistivity curve is that which would be observed if the inlier had the same resistivity as the layer containing it. (After Sternberg et al., 1985)



Two special 3-D cases have been described in the literature. Zhang et al. (1987) are concerned with a Shield area having a 2-D regional "grain" in one direction overlain by a very thin cover with a 2-D grain in a different direction. They show that _Ztakes on particularly simple forms if the measurements are made along either of the two directions. In particular, if a measurement axis lies along the superficial strike direction, then the two diagonal terms are linearly related. If one of the measurement axes is along

regionalstrike,thenZxxis a real multipleof Z•x and Z• is a (different)realmultipleof Zx•. The other specialcase is that of a small 3-D body in a regional 1-D or 2-D environment (Berdichevsky and Dmitriev, 1976; Groom and Bailey, 1989, 1990). That problem is treated using specific models. Berdichevsky and Dmitriev (1976) use a vertical elliptic cylinder in a thin conductive layer over an insulating middle layer underlain by a perfectly conductinghalf-space. At points along the axes of the ellipse they examine its distortioneffect on the impedance,comparedwith that of the unperturbed layered model. Results are described in terms of distortion coefficients, which are

different from 1.0 according to whether the inclusion causes a "flow

around"

or "concentration"

of the

667

tice. Topographic effects have been modeled with the same computer programs used to model 2-D and 3-D effects of features buried in a flat earth, e.g., Wannamaker et al. (1984b), Wannamaker et al. (1986), etc. In addition, Jiracek and his students (Jiracek, 1973; Reddig, 1984; Kojima, 1985) have used their general Rayleigh-FFT technique to model topographic effects, and Holcombe (1982) developed special purpose 3-D finite element code for the purpose, in responseto the problems of interpreting MT surveys in mountainous geothermalareas of the Western U.S. (Mozley, 1982). I calculated the 2-D effects that might be anticipated in a proposedtraverse into the Himalayas (Vozoff, 1984), and Jiracek and I modeled the (more likely) 3-D situations of measurements along the roads, which follow river valleys transecting the linear ranges at right angles. In the 2-D case there is a marked difference between the TE and TM cases, as is usual in other

2-D

models.

TM

effects

can be described

as

primarily galvanic, slope dependent, and weakly frequency dependentas comparedwith TE effects, which behave inductively and are generally smaller and smoother. In the general 3-D situation, it is recognized that every polarization gives galvanic effects, although their magnitude will be smaller than in the 2-D case.

current at the observation point. The distortion coef-

Inductive

ficients

dimensions of the conductors. Considering that the thrust zones of interest in the Himalayas are locally expected to be nearly 2-D conducting planes, they should be detectable in the tipper at least. Two good recent reviews (Jiracek, 1990; Singer, 1990) deal with topographic effects and statics effects.

are real scalars.

The Groom-Bailey model consists of a very small conductive hemisphere near a vertical fault in a thin layer. The layer is underlain by a more conductive half-space.The purposeof the fault is to provide a 2-D "regional" environment for the hemisphere, and the authorsexamine the impedancevariation with azimuth from the center of the hemisphere. They find that, on azimuths along the regional principal axes, impedances are shifted by real scalars, in accordancewith the results in Berdichevsky and Dmitriev (1976). However, on azimuths intermediate to the two principal directions, the effect of the hemisphere must be described by a real tensor rather than a scalar. The analysis and decompositionof these tensors are de-

effects

be small because

of the finite

Alternative Definitions of Resistivity and Impedance

As previouslymentionedother definitionsof impedance are sometimes used. Although this is usually done for theoretical reasons, there are some practical reasons also. With one of these alternative definitions,

impedancecan be measuredwithout using electrodes. Schmucker and Weidelt (1975) show that

•Z= -poH•r/Hx

scribed in two later sections.

In someapplications,as in the study of sedimentary basins, the field within the hemisphere is of primary importance. The regional fields impose a systematic bias on both E and H at low frequencies, which means that measurements within it, as well as interpretive models, must account for this condition (Madden and Mackie, 1989) as was discussedpreviously. Topographic features force currents to flow in patterns different than what they would be if the surface were flat, and hence they affect both magnetic and (especially) electric fields at the surface. Impedances are, therefore, affected as well. This influence is predicted theoretically and is often observed in prac-

will

where

H[ = OHx/OZ

and P0is the resistivityover the depthrangewhere the derivative is measured. This makes it possible to do MT

in locations

where

E cannot

be measured

on

account of surface conditions, or in an oil well, by differencing the outputs of a pair of vertically offset magnetometers or by using a gradiometer (Vozoff, 1982). The local resistivity must then be determined by other means, such as a well log or a surface electrical measurement. Edwards et al. (1988) used this ap-


668

Vozoff

proach to make measurementsin shallow portions of the Arctic Ocean. They obtained the gradient by placinginductioncoilsjust beneaththe ice and on the seafloor. Seawater resistivity was estimated from its temperature and salinity, and the resultsprovided the first MT soundingsin the Arctic Ocean. Spies and Eggers (1986) point out that many definitions of apparent resistivity, including that of Cagniard, are somewhat arbitrary, and that others can be devised that have more intuitively appealingbehavior.

IntheMTcase, instead ofIgijl2itisequally legitimate touseRe[(Zij)2], orIm[(Zij)2], asexamples. Figure 39 comparesthree-layer @acurves @a,Izl obtainedin the conventional way, using the real part of Z @a, Re (z), and in the time domain Pa, T.D. The second and third are better behaved than the first in that they do not show the well known overshoot

and undershoot

which are characteristicof the Cagniard definition, and their variations are confined to a smaller range of frequency.

Since the introduction of the impedance (Cantwell, 1960), mathematical difficulties have been noted in

dealingwith the _Ztensor in which the diagonalelements are commonly near zero and the major elements are off-diagonal(Swift, 1967; Eggers, 1982). Cevallos (1986) has overcome this difficulty by showingthat it is due to an improper physical formulation of the problem. Using classical EM theory, he shows it arises from the fact that the magnetic field H is a pseudovector or axial vector, rather than a true vector. This is not an obvious property, nor a question that arises very frequently in geophysics.It is only apparent in the behavior of H under reflection, and is related to the fact that H arisesfrom the curl operation. On the other hand, E is a true vector. The result is that, as defined,

_Zis a pseudotensor which doesnot transformas a true tensor (Jackson, 1975). The proper definition of the MT impedancetensor _Zis E = •(H x n).

(32)

Similarly, a proper admittance tensor can be defined by IOO

H = 9J(E x n).

Thereis a simplerelationship betweenthe Zij, the elementsof _Zand •,

the elementsof •, namely• • =

Zxy,[12= -Zxx, [21 = Zyyand[22 = -Zyx' Thus io-• .....

gymZy x.

TIMEDOMAIN /Oo,T.D. FREOUENCYDOMAINjao, Re(z)

---- -- FREQUENCY DOMAIN jao,I z l

I 2'

i0-•

I

I0

I02

(33)

The major terms are now on the diagonaland combinations of • and its complex conjugatetransposeare hermitian, overcoming the objections pointed out in Swift (1967) and Eggers (1982). In particular the hermitian forms (•) and (• + •) are shown to have fundamentalphysical significance,their principal directionsare real, and (•/to•x) is an apparentresistiv-

I03

t or /,s

(a) IOO

p•lp, =0.01

ity.

h•= h•/2

Source Effects and CSAMT

(b) Fig. 39. Alternative apparent resistivities for three-layer models. Dashed curve is for usual definition, solid curve is

from real part of impedance. Dot-dash curve is from time domain. (a) Resistive second layer. (b) Conductive second layer. (After Spies and Eggers, 1986.)

We started by assumingthat the source fields are effectivelyplanewaves. It is observedin both practice and theory that the impedancestaken from fields due to nearby sourcesare different from those obtainedor expected with plane waves. The farther away the (natural or artificial) source, the closer the impedance approachesthat for a plane wave. The justificationfor the planewave assumptionin MT was debatedin early literature but was finally accepted on the basis of a report by Madden and Nelson written in 1964 but unpublished until recently (Madden and Nelson, 1986).

At audio frequenciesthe use of an artificial sourceis

The Magnetotelluric Method now well established

as the Controlled

Source

Audio

Frequency MT (CSAMT) method, and thus appearsin this volume.

The E/H ratios from a CSAMT

transmit-


ter or from a visible thunderstorm are often very different

from those obtained

from remote

storms.

In

such cases the impedance depends on the kind of source (loop, grounded wire, horizontal or vertical lightning stroke), its orientation, and its distance in skin depths. Among other complications there is no single obvious plotting position for an interpretive model since both transmitter and receiver positions are involved in the result. Because skin depth decreases with increasing conductivity and frequency, the impedance may well depend on transmitter location at low frequencies but approach the plane wave value at high frequencies. In CSAMT surveysan effort is made to place the transmitter as far from the survey positions as possible, consistent with adequate field strength, so as to take advantage of the simplicity of interpretation for plane wave source fields. Figure 40 shows apparent resistivities at a site from AMT and CSAMT measurements at the same dipole position. The CSAMT transmitter was parallel to the receiver dipole and about 10 km away along a line at right anglesto the dipoles. Agreement is very good down to a transitionfrequency of 10 Hz (0a) or 30 Hz (phase). How

far

does

the

receiver

have

to be from

the

source?Literature estimates range from seven to three skin depths (Bannister and Williams, 1974; Zonge and Hughes, this volume). Plots in the CSAMT chapter showing the fields from a grounded wire source indicate that the answer depends on azimuth relative to the source, as well as on distance. Dmitriev and Berdichevsky (1979) show that the fields can vary linearly across a region without affecting impedance, but this applies only to the 1-D (horizontal layers) case. There are several papers dealing with the TM approximation and discrete sources, including Law and Fannin (1961) and Quon et al. (1979). Yamashita et al. (1985) describe one approximate technique of correcting CSAMT data for sourceeffects at low frequencies. This and a few other approximate methods are discussedin the Zonge and Hughes CSAMT chapter, this volume.

No methods proposed to date are very satisfactory: the only solution with CSAMT is to use a more powerful sourceat greater distances.The real problem arises when conductive

near-surface

669

rocks are under-

lain by even a thin resistive zone. Without other, independent information it is then difficult to tell whether the increasing apparent resistivity at low frequencies is a source effect or is due to increasing resistivity at depth.

SENSORS

Proper choice of sensorsis critical to the collection of good field data. We will confine this discussionto onshore measurements, although marine MT is done both for deep crustal studies(e.g., Filloux et al., 1985) and for petroleum exploration (Hoehn and Warner, 1983).

The electric field is measured by recording the voltage variation V versus time between a pair of groundedelectrodes, distance • apart, and settingE = V/e in V/m. This assumes that E is constant

over the

line and givesthe averagevalue. To obtainE x the line joining the electrodesmust be in the x direction, and e must be big enough that V is much greater than the noise generated in the electrodes themselves. The major problems associated with the E measurement are electrode-generatednoise and noise induced in the wire by wind motion. An electric field E m = vm x B0 is induced in a wire moving with velocity Vm in the earth's main field B0. Motion-induced noise is also a major problem when large loops are usedto measureH

components,as is often done with H z. Wires and electrodes must be fixed so that they do not move during measurement. At high frequencies the wire joining the electrodesmust be straightor an additional voltage will be induced in the wire by time variations

in H z (Swift, 1967). An electrode

in contact

with the earth is an electro-

chemical cell. A voltage exists at the interface which depends on the chemical natures of the materials involved, on their interaction, and on the temperature. This voltage is of no interest to us, but it can be a major noise source if our electrodes are not chosen properly and treated carefully. Several types of electrodes are now in wide

use. In addition

to calomel

standard cells, the common electrodes are nonpolar-

izing Cu - CuS04, Cd - CdCl4, andAg - AgCl cells. (Cadmium cells are not recommended because of their toxicity.) Petiau and Dupis (1980) describe a Pb PbCl2 systemin Plasterof Paris or clay which can be very stable if properly made (Pelton, pers. comm.). Figure 41 shows the noise spectra of a variety of electrodes. However, different electrodes made of the same material can have vastly different noise, and each should be tested periodically. In the field it is commonpractice to install electrodesa day before use if possible, to give the system a chance to come to equilibrium. The effect of stabilization is also shown in Figure 41. No such data are available for frequencies above 10 Hz. Petiau and Dupis suggestthat electrode noise is near signal at 1 Hz for a 100 rn electrode spacing,and that signal/noiseimproves at both higher and lower frequencies.This improvement would obviously depend on resistivity and signallevel. Karmann


670

¾ozoff

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(1986, pers. comm.) attempted to measurebroad band noise in a commercial

electrode

but found that it was

less than the noise of the amplifiers he used.


The electric

field measurements

are often affected

by very rapid, large changes in earth currents due to power systems and lightning. These changescan appear as spuriousoutput signalunlessthe preamplifiers have very high common mode noise rejection and rapid response. In exploration practice it is usually possible to increase the electric signaljust by increasingt•, the electrode spacing.This luxury doesn't exist for the H measurement.If the magneticfield strengthis lessthan the equivalent magnetic noise in the sensorsthen the situationis seriousand better technique,better conditions, or better sensors are needed for satisfactory results. A completely satisfactory magnetometer has yet to be devised. Two varieties of magnetometer are in general use, induction coils and SQUIDS, and both are commercially available. At its simplest the induction coil is a loop of wire which produces a voltage proportional to its area

Noise I001

671

multiplied by the time derivative of B across the area. Given a sinusoidalB, the output voltage increasesin proportion to frequency. The phenomenon is described in first year university physics textbooks. Sincethe fieldsof interest are very small, various steps must be taken to increase the loop voltage even before it can be electronically amplified. Its area can be increased substantially if it is to be laid out flat on the

surface,to senseBz. In additionuseof manyturnsin series results in output proportional to the number of turns. Large, multiturn loops are, in fact, used for many surveys. However, such a large area air-core loop is obviously impractical for sensingthe horizontal componentsof the field. Instead, effective loop area is increased by winding turns around a core of high magnetic permeability. Materials such as mumetal or moly-permalloy, alloys of molybdenum and nickel in steel, are used for lower frequencies. Nonconductive ceramics such as MN-60 (Ceramic Magnetics Inc.) are used for higher frequencies. As can be seen in Figure 42, the effective magneticpermeability dependson the material and its length/diameterratio (Bozorth, 1964). These cores and the coils wound on them are long and narrow, so as to obtain the smallest demagnetization factor and thus the highest effective permeability of the core.

•IVcc

Real coils made up of many turns of wire on a permeable rod thus constitute circuits including distributed resistance, inductance, and capacitance, as well as core losses. A simplified equivalent circuit is

10 6 8 6

Amplifier noise 0-1

0-001

0-01

0.1

I

I0

I00

Frequency(Hz)

IO

$

8

I00 Noise

4

•Vcc C

Dt•E.

4

i0z: 6 4

Amplifier doi io

0'• i i i 111111 i i i iiiiii i i i iiiiii i i i ii170 0 0'001 i i i iii111 0'01 0'1 I I0 Frequency (Hz)

Fig. 41. Electrode noise spectra for several kinds of electrodes. Upper curvestal/2-1 hour after setup. Lower curves--after 24 hours. The two quietest are Pb - PbC•2 and Ag - AgC•. (After Petiau and Dupis, 1980.)

Fig. 42. Apparent magneticpermeability versus true permeability and length, for permeable rods. m is length/diameter ratio. (After Bozorth, 1964.)


672

Vozoff

shown in Figure 43. This circuit will have a resonant frequency, above which the voltage output decreases with increasing frequency. Because of the distributed nature of the circuit components there will be other resonancesat higher frequencies as well. It is desirable to avoid operating at frequencies much above the first resonance because the response becomes difficult to compensate and is often temperature sensitive. Good coil design is an engineeringproblem requiring matching to a particular amplifier circuit and optimization for a particular frequency range and application. The basic references on coil design are the theses by Clerc (1971) and Karmann (1975), but Karmann (1977) is more easily available. In addition, useful literature is

The intrinsic dynamic range of an induction coil is very great. At the large signal end the range is limited by the range over which the incremental permeability of the core material is linear, whereas at the small signal end it is limited by quantum effects in the core material and thermal effects in core and windings. Thus as long as the signalis much greater than thermal noise, and lessthan the Tesla level required to saturate the core material, coil output at a given frequency will be proportional to the input field. In a well designed systemthe limitations will lie in the amplifier. Typical noise in a good instrumentationpreamplifieris 1/2 •xV rms. With a typical supply voltage of 10 V this gives a

available

The other magnetometer commonly used in MT is the SQUID, based on the Josephsonjunction effect in superconductors.These should combine a high dynamic range with the lowest noise theoreticallypossible. The operatingprincipal of the SQUID magnetometer depends on a quantum effect discovered in 1964 (Jaklevic et al., 1964). The principle is not as easily describedas the induction coil, nor does it yet appear in first year physicstextbooks. Clarke (1976), Zimmerman and Campbell (1975), and Falco and Schuller (1981) discuss the principle. A special Society of Exploration Geophysicists publication is devoted to the SQUID and its applications (Weinstock and Overton, 1981). Other descriptionsof the SQUID are found

from

time

to

time

from

universities

and

manufacturers. Stanley and Tinkler (1982) give complete construction details and performance curves for a magnetometer designed for subaudio frequencies. The frequency dependenceof inductioncoil response below resonance is ideally suited to the measurement of natural fields below 10-100 Hz, where it very nearly compensatesfor the 1/frequencyfalloff of the average natural spectrum (Figure 19). This prewhitens the sensoroutput, effectively extending the dynamic range of the entire system. For higher frequencies,where the spectrum flattens and then increases, a frequencyindependentmagnetometerresponseis desirable.This frequency independence can be achieved by feeding back into the coil a magnetic field proportional to the current.

The scheme is described

in Clerc and Gilbert

(1964), MacIntyre (1980), and Labson et al. (1985). The technique can be carried a stagefurther, as shown in the responsecurve of the Metronix KIM802 (Figure 44). However this approach may not be worth the effort because no two data sets are alike and spectral behavior is only regular on average. On the other hand, if the interest is in applied fields, then it would seem reasonableto tailor system responseto a highly predictable spectrum.

rangeof 2 x 106 or 126dB.

in literature

obtainable

from manufacturers.

Two varieties of SQUID magnetometers, dc and RF, have been built, but only the RF SQUID is available in geophysical magnetometers. The central element is a very thin ring of material which becomes superconducting (resistance = 0) at liquid helium temperatures, around 4 degrees Kelvin. The material most commonly used is the metal niobium, although many others are available. The niobium is deposited on an insulating rod .(Figure 45), and the circuit is nearly broken at one point by a notch, the Josephson

L

101 / I .i i i I I I Vs

I

IC[4

(GAIN SEL=I0)

IC•3

I0-2

I0-•

I0 0

I01

102

103

04

Hz

Fig. 43. Simplifiedequivalent of inductioncoil magnetometers. In practice the circuit elements are frequency dependent.

Fig. 44. Responseof one commercial magnetometersystem designedto prewhiten the natural spectrum.



junction, forcing the superconductingcurrent through a tiny area. When it is superconducting, and in the absence of the notch, whatever magnetic flux threads the loop cannot be changed,since additional supercurrents will be induced which exactly cancel any new applied field. With the notch, both the current and the flux will change with the applied field, but not in a simple linear way. During part of the change the flux throughthe loop (• = B, x loop area) increasesmore

slowly than the applied field q>e'As t•e is increased further (Figure 46) • jumps discontinuouslyby amount q>0(1- q>c/q>ec), where q>cand q>cedepend on the

SUPERCONDUCTING

FILM

BRIDGE

QUARTZ

TUBE

Fig. 45. Sketch of an RF SQUID element consistingof a thin film of superconductingalloy on an insulatingcylinder. The weak link (bridge) in the alloy circuit enablesthe Josephson effect to occur. (After Clarke, 1976.)

21•0

-2½o I

673

geometry of the loop and notch and q>0is the flux quantum,

•o = h/2e • 2 x 10-]sWb. If t•e is further increasedthe same happensagain, but if t•e decreasesthen q>initially decreasesmore slowly and then drops discontinuously.Thus during parts of this sequencethe ring appears diamagnetic and in the other parts it appearsparamagnetic. In the RF SQUID this responseis sensedby an inductance bridge consisting of a tuned circuit whose inductor is magnetically coupled to the ring (Figure 47). The voltage in the circuit then depends on the state of the ring. There have been numerousimplementationsof magnetometers using this effect. In most, a detector is used to look at the RF voltage (as in Figure 47), and the current is fed back into the bridge inductor to keep (•e at a minimum, the distancebetween minima correspondingto +_q>0. The feedback signalis then proportional to the magnetic field change. If the signal becomeslarge enoughto indicatethat (•e has changed by q>0then the feedback current is reduced by an amount equivalent to q>0and the system shifts to the next minimum. A counter keeps track of the number of such transitions to add to the output. In the recent generation of SQUIDS the counting and output are digital. Then the A/D conversion is arranged so the least significantbit correspondsapproximately to the

rmsnoise.Thisarrangement variesfrom10-4 to 10-5 nT in different equipment. [Very recent developments

appearto have achievednoiselevelsof 10-7 nT (Henderson, R.J., pers. comm., 1987)]. To keep the sensingelement in liquid helium, special containers (dewar flasks) are necessary. Unlike other SQUID applications, the container for a magnetometer must be nonmagnetic.They are made of fiberglass, with double walls separatedby a vacuum. Dewars lose helium through evaporation as heat slowly leaks through the walls. In addition, vibration in transport is

_•2'½0•e '•, J•ec oc

-2•o

AMPLIFIER

Fig. 46. Flux q>passingthroughthe superconductingloop in an RF SQUID, versus applied flux q>ee.q>changesdiscontinuously (arrows) as q>eeincreases(decreases)beyond a local maximum (minimum) in the smooth curve, thus changing the effective inductance of the circuit. (After Clarke,

Fig. 47. Simplified circuit of an RF SQUID system. The tuned circuit (lower left) is driven by the RF oscillator and senseschanges in the effective inductance of the SQUID

1976.)

element. (After Clarke, 1976.)

--


674

Vozoff

converted to heat in the helium, causingfurther evaporation. Hence containers must be refilled periodically. To avoid too frequent refilling the containers must be large enough to hold at least enough helium for a day, and preferably for several days. Capacities of 5-10 liters are made, but 30-50 liter sizes are more

common (Figure 48). Larger containers of 100-300 liter capacity are used for bulk storage. Liquid helium is sometimes

difficult

to obtain and awkward

to trans-

port, which has tended to discourage the use of SQUID magnetometers. A persistent problem with SQUID magnetometers has been the effect of spherics. Their high frequency content shifts •e faster than the responsetime of the electronic circuitry. The system shifts to a new minimum, but the amount of shift is uncertain and the

Finally, it is important to look at the noise levels of various sensors,with respect to signal levels and to one another. Some but not all of this data is provided by the manufacturer. Figure 49 shows many of the available noise data in terms of equivalent magnetic field spectral density. It is particularly interesting to look at the SQUIDS data. On the basis of laboratory measurements and theoretical predictions SQUIDS

should havea noiselevelof about10-SnT/(Hz)•/2, yet in the field the level is consistently about 10 times larger, based on remote reference analysis (see the Data Processing and Analysis Section). Numerous explanations have been offered, especially that the noise level is a result of microseismic

motion.

A series

of experimentsbegun in 1983 has shown that much of the anomalous SQUID noise is seismic in origin (Goubau et al. 1984; Morrison et al. 1984; Nichols et al. 1988). By using one SQUID to cancel the field at another several kilometers away, they showed that much of the residue is highly coherent with a tiltmeter. They then used the tiltmeter output to cancel the

system is said to have lost synchronization. To overcome this behavior it is now common to low-pass filter the field by building a conductive shield into the dewar or by wrapping a shield around the element. The frequency responsecan be selectedvery precisely. In addition, the faster response of newer circuitry also helps overcome this situation. The dynamic range of SQUIDS must be larger than that of induction coils because SQUID responsedoes not prewhiten the natural pink spectrum. One manufacturer quotes a range of 193 dB (32 bits) with a

70 dB in parts of the spectrum. At present, without such cancellation, induction coils have lower noise than SQUIDS above 100 Hz, but it remainsto be determinedwhich is superiorin the

resolution to 10-5 nT overtheentirerange.

critical

residue. In all, the cancellation amounted to over

•o4

0.1-10.

,

•o3

Hz band.

,

•oa

,

•o'

,

•

, icGE, :5/06

•½'

•o-a

FREQUENCY Hz

Fig. 48. Commercial 3-component SQUID magnetometer. (Courtesy Biomagnetic Technologies, Inc.)

Fig. 49. Noise figures reported for various magnetometers. S 1 and S2 are, respectively, typical observed and theoretical figures for SQUIDS.



FIELD

PROCEDURES

675

width and the same error bars. MT has an even greater advantage if tensor measurementsare desired. On the

certain gnawing animals pose more or less severe problems to cables and other equipment. It is prudent to pack up and remove everything possible when leaving an area overnight, the exception being electrodes intended for use the following day. Even in the most remote regions, care must be taken to obtain good data. Magnetometers, which must be shieldedfrom wind and sun, are commonly buried in trenches or holes covered by dirt or a wooden sheet. Depending on soil conditions, this may or may not be easy for horizontal, permeable core induction coils which require only shallow trenches. It is more difficult for SQUIDS becauseof the size and shapeof the dewars, so a combination of partial burial and a streamlined shield are commonly used. Induction coils

other hand, once the TEM

for H z requiredrillinga vertical hole. Given an ade-

Careful field procedure is probably more crucial to successful results in MT and AMT than in any other EM technique, simply becausethe fields are so small. Site selectionand sensorinstallation are the two major factors governing data quality. The cost of MT surveys depends upon the length of time which must be spent at each site. From experience (Vozoff et al., 1985), given good recording conditions and considering the time to install the TEM transmitter, MT measurements at a single site should take less time than transient EM measurements

for

the

same

transfer

function

band-

transmitter has been in-

stalled, subsequent TEM measurements can be done more quickly. Hence TEM has a distinct advantage over MT for intensive scalar surveys over an area which is accessible from a single transmitter. Conversely, MT has an advantage where tensor information is required over a large area. As a general statement, the better the installation, the better the data and the less time required at the site. However there are many variablesover which the crew has no control. Many crew-years of experience have taught us to avoid powerlines, pipelines, electric railways, electric fences, welding activities, smelters (especially aluminum smelters), irrigation pumps, and radio and radar transmitters. Towns, vehicular traffic,

and even pedestrian traffic can act as local field sourcesfor which the impedance differs from the plane wave value. Metal structures such as billboards, flagpoles, (passive) metal fences, and metal strands on wooden fenceposts can locally distort the magnetic field, or cause sensor motion when the wind blows. Large grounded conductors such as culverts, rails, and telephone and power groundingcircuits will distort the E field locally. The amount of flexibility in locating sites differs in every case, and the range of interference differs for every source. If possible, avoid large, active sources such as powerlines, railways, and distributed, wellgrounded conductors such as pipelines and metal fences by at least 1 km; poorly grounded passive conductors and vehicular traffic by 100 m; and small, passive, static objects by 50 m. With experience in a given area some of these rules can be relaxed but others may be inadequate. Wherever practicable avoid natural features which might affect the fields, especially the electric field. These objects include abrupt topographicfeatures such as sinkholesand pinnacles, small salt pools, outcrop in areas that are otherwise soil covered, etc.

In common with other geophysical field methods,

quate thickness of soil the hole can usually be dug quickly by power auger. Where soil conditions are more difficult, a small drill can put down enough holes for many days' operation in a few hours. In some situationsit is more convenient to lay an open loop on the surface, anchored against wind motion by clumps of dirt. As noted earlier, coils should be placed as far as possible from the roots of trees, because the trees will move when the wind blows.

Electrodes should be installed, at least a few hours

and preferably a day before they are to be used, in shallow holes which have been water saturated, and covered to retard evaporation and temperature change. Resistance of the electrode circuit must be checked before recording begins, as is done automatically by some systems.If the resistanceis too high for the input impedance of the preamplifier, breaking up the surrounding soil and adding more brine can overcome the problem. This procedure is not always effective enough. Where resistance is unacceptably high, an alternative is to install several electrodes in parallel. These electrodes must be as far apart as possible to be effective, but their maximum spacing has to be a small fraction of the end-to-end spacing. The consequencesof high contact resistance can be severe, in terms of noise and lost signal. The consequences are illustrated and discussed at length in the Zonge and Hughes CSAMT chapter this volume, (Zonge and Hughes, 1989), where it is suggestedthat impedance-matching amplifiers be used at the electrodes. The problem is most important at high frequencies and large electrode spacings. The wire between electrodesshouldbe as near a straightline as possible. Otherwise the wire forms part of a horizontal loop

whichrespondsto changesin H z, the effectincreasing with frequency (Swift, 1967). When remote reference acquisition is carried out [Clarke et al. (1978)] then consideration must be given to the location

'of the reference

site and to means of


676

Vozoff

collating the data. Data collection is done by radio telemetry, by hard wire link, or by synchronized recording. The first is preferred if the reference is used for a number of recording sites and when the distances are more than 1-2 km. Hard wire is simplest in principle, but synchronized recording is a practical option with the crystal clocks which are now available. The spacingbetween the reference and the data sites depends on the nature of the major noise source anticipated: if it is instrumental then they can be quite close together, as discussedin the next section. Major determinants of survey cost, aside from accessibility of the area and the benevolence of the environment, are the spacing between sites and the recording time required. Whereas recording time is increased by adverse noise conditions, as noted above, the minimum is determined by the depth of interest and the resistivities of the section. The greater the depths and the lower the resistivities, the lower are the frequencies required and, therefore the greater the recording time. Nominal safe recording times for various geologicalenvironments in the absenceof serious noise contamination are shown in Vozoff (1972, Figure

position, with a spatial filter applied to the array of E measurements.For reasonsof practicability, only one local and one remote magnetometer are used in EMAP. Hence only partial tensor information is obtained from the EMAP measurement, although full

36). The best indication of minimum time is obtained

The object of data processingis to extract from the noiselikesignalsa set of smooth, repeatablefunctions representingthe earth's response, which can be used to interpret conductivity structure. In this area and in interpretation our capabilities are changingmost rapidly as a result of new developmentsin robust estimation, 3-D modeling, and impedance tensor analysis. The responsefunctions which we seek correspond to those which can be computed for models. Basically they consist of only two functions, impedances and tippers, and their attributes phases, special directions, skew, and ellipticity. Apparent resistivities are calculated from the impedances.Various estimatesof noise can also be computed from repeated field measurements, for use in estimating confidencelimits on the parameters of the models which are fitted to the responsefunctions. These estimatesinclude the coherenciesamongfield components,samplevariances, and signal/noiseratios. With the incursion of ever greater computing capability into the field, much work is now done in real time or almost real time. Figure 50 is a summaryplot of conventionalresponsefunctionsfrom a single site, as delivered by one contractor. It illustrates the need in MT for rapid and flexible graphics output even in the field. The emphasisin the sectionis on the processesrather than on specific implementa-

from inversion

on assumed models. This inversion

can

show directly, for particular noise assumptions, the frequency range and record length necessary to resolve a chosen target. For petroleum exploration the recording times range from 1/2 day to 2 days. In geothermal problems, 1/2 to 1 day is typical and in AMT surveys we have spent 2-6 hours at a site, dependingon signal strength. Minimum site spacingalso dependson the conductivity structure and the degree of lateral resolution needed. In (conventional) tensor surveys, spacingsof $-10 km might be used in reconnaissance,and 1/2-1 km or less where greater detail is wanted. As experience with the method increasedand closer spacings were used for better lateral resolution, it became clear that MT, like the reflection seismic method, sometimes suffered from what are now called

"statics" problems (Sternberg et al., 1982; Warner et al., 1983; Andrieux and Wightman, 1984; Sternberget al., 1985). That is, entire apparent resistivity curves can be vertically shifted on account of large but superficiallateral conductivity contrasts.The problem is discussedlater in this chapter. Special procedures have been devised to cope with this situation. The most elaborate of these is the EMAP technique, which requires contiguous E field measurements along a profile (Bostick, 1985, 1986a and b). This is obtained by using a multiconductor seismiccable with takeouts to electrodes spacedthe order of $00 m apart. Lateral changes in the electric field are smoothed, over distances equivalent to skin depth at that frequency and

tensor

data could

be collected

at the same time if

enough equipment and data channels were available. Processingand presentation of EMAP data are discussed in a later section.

Finally, there are now (late 1990) appearing general multichannel receivers permitting many tens of channels to be digitally recorded simultaneously. These advancesare bound to affect field procedures, making it feasible to collect data at more closely spaced stations at reasonable

cost. The same devices

can in

principle be used for measurements with applied fields, reducing the cost of statics compensationprocedures.

DATA

General

PROCESSING

AND

ANALYSIS

Considerations

tions.

The bulk of data processingis done in the frequency domain. There have been proposalsfor time domain processing(Wait, 1954; Kunetz, 1972), but these proposals have never been implemented for routine surveys, as far as I know. The following discussiondeals

The Magnetotelluric Method APPARENT I00


I

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RESISTIVITY

FREQUENCY I

11111 I i

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677

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COHERENCY(E-EPRED)

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INVERSION

'TRUE' RESISTIVITY

102

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STRIKE

I

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i

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ii1!

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Fig. 50. Summaryof the processing resultsat one station.(After Z-Axis Exploration,Inc.)

104 i

i

I

iiiii

678

Vozoff

with frequency domain processing,but the time do-


main ideas are described

at the end of this section.

In a homogeneousor horizontallylayered earth, and in the absenceof noise, apparent resistivity could be determined simply by simultaneouslymeasuring the

amplitudes of Ex andHy at a specifictime andfrequency (using band-passfilters, say), squaring the ratio, and multiplying by the appropriate constant. This could be done for a sequenceof frequenciesto define as much of the curve as desired.

Much

MT was

done this way in the early years, and it is even the basisof one new system.However, in practicethere is always noise in the data and there are usually lateral resistivity changes occurring close enough to a site that apparent resistivity depends on measurement direction. Hence almost universal practice is to measure both horizontal componentsof E and It in order to find •Z, instead of trying to use just one perpendicular E-It pair. I first

describe

the conventional

methods

used to

extractsmoothestimates of theZij(f), etc., fromthe noisysignals Ei(t) andHi(t), wherei andj canbex or y. The equationsthat must be solved are [Equations (14)1

Ex(f) = Zxx(f)Hx(f) + Zxy(f)Hy(f)

Ey(f) = Zyx(f)Hx (f) + Zyy(f)Hy (f), i.e., two complex equations in four complex un-

knowns,the Zij(f ). The equations are complexbecause all quantitieshave both magnitudeand phase. They can be solved because we can obtain many

independent estimates of the El(f) andHi(f). Solutions can be obtained in either of two ways: by

averaging repeatedtimemeasurements sincethe Zij will not change with time, or by averaging over a number of closely spaced frequencies since they changeonly very graduallywith frequency.If the data have a large signal/noiseratio the results are the same either way. Assumethat the signalsare in digital form: virtually all MT and AMT data now collected are amplified, filtered,multiplexed,and digitized,typicallyby (10 to 16 bit) analog-digitalconverters(ADC). Frequency ranges and conversion rates are chosen to suit the geophysicalproblems and available hardware. Some signal conditioningmust be done before digitization and further signal preparation may be desirable or essential before spectral analysis. Signals must be low-passfiltered before the ADC. The low-passfilter output must conform to the requirement that there be no significantenergy above the Nyquist frequency entering the ADC (Bendat and Piersol, 1971). In addition,high-passfilteringis sometimesdoneto overcome instrumental

drift

and reduce

the demands

on

dynamicrangeof the low frequencypart of the signal spectrum. Repetitive signal such as powerline noise must be notch filtered. These are most important for the 10-12 bit ADC's and when long data sets are desired. (One of the SQUIDS describedin the previous sectionhas a dynamic range of 32 bits in order to dealwith its dc responseto the I/f spectrum.)Removal of the mean, detrending(subtractionof a linear trend) or low-passfiltering,andwindowing(taperingthe ends by a cosineor similarfunction)are most conveniently done after digitalconversion(Young, 1981). Conversionfrom time to frequency domain is usually doneby Fast Fourier Transform(FFT) becauseof its speed. After Fourier transforming, a correction is made to the resultingspectrumof each channelfor the absolute complex frequency responseof that channel's instrumentation,resultingin true spectraof the E

fieldsin V/m X/-•zz,andin nT/X/•zzfor theH fields. Most algorithmsrequire that M, the number of data

points analyzed,be M = 2n, where n is an integer. Typical values of n are 5 to 10 (M = 32 - 1024) and rarely exceed 12. Values of the complex spectrumare produced at N = M/2 frequencies, linearly spaced from fl to fN, where fN = 1/(2At) is the Nyquist frequency and At is the samplingperiod of the ADC. Each spectrumcan be expressedeither in terms of its real and imaginary parts, or in the equivalent magnitudeand phase.From the physicsinvolved, this phase and (the logarithm of) impedance must be smooth functionswhen plotted on a logarithmicfrequency scale. The curves can easily be definedby a half dozen points per frequency decade. Hence the FFT generallyyields many more frequenciesthan can be used for interpretation, especially at higher frequencies. In one approach, impedance values are averaged over adjoining groups of frequencies, or windows (Figure 51) to solve the underdetermined equations,in order to provide smootherresultsand to reduce the number

of data to be dealt with.

To obtain

n windowsper decadeof frequency,the ratio of center

frequencies of adjacent windows is log•.+ 1/fj)= 1/n.

3

2

3

4

5

•

6

7

8

9

I I'

512

19 WINDOWS

Fig. 51. Windows (1-19) used for logarithmicaveragingof linearly spacedharmonics(1-512).


The MagnetotelluricMethod

Another approachis to digitally filter many short data sets at only the desiredfrequencies,thus obtaining multiple solutionsof the impedanceequationsat a few frequencies.These solutionscan then be stacked or otherwise averaged,as discussedin the Errors and Noise Section. Avoiding the full FFT computationcan save computingtime. An ingeniousschemefor continuousMT acquisition and processing,calledcascadedecimation,is given in Wight and Bostick (1981). In their implementation, shortdata sets(32 points)are collectedin dualbuffers, one of which is being filled while the other is being processed.The latter is sine- and cosine-transformed at a few mid-band frequencies and those values are saved. The data are then low-passfiltered to half the original Nyquist frequency and alternate points are discarded.

data

ratio at thosefrequenciesin our inductioncoils. Interestingly,in a simplesedimentarybasin,we were able to obtain smoothand repeatableapparentresistivities directly from the fourier transformsof signalsfrom an orthogonalsensorpair, without averaging(Holliday, 1981). Auto- and Cross-Spectra

Once we have a data set in the frequency domain, the basic tools of the band averagingproceduresare the auto-spectraand cross-spectra.Say the corrected

spectralvalueof channel A at frequency fj is Aj =

aj + ibj,(i = %/-1).Then anestimate oftheaverage auto-spectral density,or spectrum, in theband•'-m, fj+m)around J} is

The same is then done to the data in the

other buffer while the first is being refilled. The two decimated data sets are then joined to form a new 32 point data set at half the original samplingfrequency and the processis repeated.This could be continued indefinitely. The few transformvalues which are kept from each data set can be used to obtain autopower and crosspower in the same way as the averages describednext, but they must be averagedover several

679

sets rather

than

over

a band

of several

adjacentfrequenciesfrom a singledata set. In our AMT

A•A•

(A(fi)) = m•-1k=j-m = (AjA•}•/2,

(34)

which is real in this case. The square of this is the

autopower spectral density,or autopower, atfj. In the sameway we can calculatethe crosspowerdensityat

fj of twochannels, sayA andB, by taking

work we found that useful results were

generallyobtainableonly when the data set included one or more obviousspherics,at least for frequencies from 8 Hz to 10 kHz. Presumablythis was becausethe signal/noiseratio was too small otherwise. For that reason the data acquisitionsystem was designedto accept data in a flow-throughbuffer until a selected "trigger" level was detected. The buffer memory retained 100-200 pointsbefore the onsetof the trigger signal and continued so as to make up a 1024-point data set for eachchannel.The 5 (or 7) channelsof 1024 pointswere then automaticallytransferredto another buffer and simultaneouslydisplayed, via a digital/ analog converter, on the screen of an oscilloscope. The operatorhad the option of savingand processing that data set for possiblestacking,or of discardingit. The triggerwas reset as soon as a data set had been transferred, so very little signalwas lost while processing.The resultswere availableon a monitor screen within a minute or so of capture, and it soon became clear which kinds of sphericsgave useful results.This procedurealso allowed selectionof sphericson the basisof their sourcedirection, by switchingthe trigger circuit to any desired component. The approach worked very well for frequenciesabove 5-8 Hz, the waiting time for triggersvarying from nil to a few minutes. It was not successfulfor frequenciesbetween 0.3 Hz and 8 Hz, partly becauseof poor signal/noise

1

j+m

(A(fj), B(fj)) =2m+1 • AkB•, =

(35)

which will generallybe complex. The squareroot is calledthe crossspectraldensity,and is alsocomplex. The coherence or coherency of the two channels at

frequency fj is definedas

1

j+m

A•B•,

coh (Aj, Bj) 2m +1 •=j-m = coh(A, B)j

(36)

which is complexwith modulusbetween0 and 1. It will have a phasegivenby arctan[Im (coh)/Re(coh)]. The coherency and crosspowerare very powerful tools in the extraction of information from the pulsation signals.A simplecalculationwill showthat coherencyis largewhen the ratiosof imaginaryto real parts

of theAj andBj areaboutthesameoverthesummation band. That is, high coherencyimplies that any variationsin the modulior phasesof the two functions over the band occur together.

680

Vozoff calculated

Solutionsto the Impedance and Tipper Equations

from

The conventional way of solving the impedance

H=

YE.


equations assumes that the Z•j are constantoveran averaging band (window), which is physically reasonable if the bands are narrow enough. In each band,

calculated.

eachequation hascrosspower takenwithHx andHy in

channelswith the least noise were used to predict the

turn, giving pairs of equations such as

From this, an H p and a coh (HH p) could also be

(ExH• >-' Xxx(HxH• >q-Xxy(HyH• >

(37)

each Z are found by solving the pair of simultaneous equations, viz

(H zH*•) = Tx(HxH*•) + Ty(HyH*x)

From additional data records one can obviously com-

pute additionalestimatesof eachZij whichwill be between

(40)

As with the horizontalcomponents,an H z predictabil-

TxHx + TyHyandthe r i fromequation (40).Hzp will (38)

giving a least-squaresestimate of Zxx over the band.

of noise variation

(HzH}) = Tj(HjH}) + Ty(HyH}).

ity canbe calculated from coh(Hzp, H z) usingHzp =

The same is done for the other three tensor elements.

because

best when

crosspowers of H z are takenwith H x andHy to give two equations in Tx andTy,

knownsZxxandZxy.The realandimaginary partsof

different

were

The tipper is found by solving Equation (25) in the same way as used to find the impedance. That is, the

or two complex equations in the two complex un-

Zxx -

that results

other channels.

(ExH*x)-- Xxx(HxH*x} q-Xxy(HyH*x}

(ExH*x)(HyH*x) (ExH*y) (HyHy) * {HxH*x){HyH*x) {HxH} ) {HyH} )

Sims found

records.

Havingestimated valuesfor theZij thereis nowthe interestingpossibility of usingEquation (14) to predict

whatEx andEy shouldbe if theyweredueentirelyto

also be real.

Tipper direction and tipper strike were explained in a previous section. Their derivation in the 2-D case is straightforward. In the more general case, the ratio

Ty/Txis complexandthereare severalpossible ways to define the rotation, none of them completely satisfactory (Vozoff, 1972; Jupp and Vozoff 1976; Gamble et al., 1982). This is to be expected because the concept is basically derived from 2-D models. One commonly used rotation, which maximizes the cross power between horizontal and vertical components,is

the observed H components, that'is, if they were noise-free. This is done by writing

(a 2 + c2) arctan(c/a) + (b2 + d 2) arctan(d/b)

Ex p -- ZxxHx+ ZxyHy

r2 (41)

Ey p = gyxHx q-gyyHy

(39)

where the superscript indicates a predicted value.

Differences between like componentsof E and E p

whereT x--(a+ib),Ty-(c+id),andT2--(a2 +b2

+ c2 + d2).Anotherdefinition, fromSimsandBostick (1969),maximizes Ty'in Equation(26).Thatgives

arise only on account of the part of the noise which is not common to E and H. It is then a straightforward matter to compute the coherencies between the ob-

servedandpredictedcomponents,coh(Ei, E?). These have become a very useful measure of data quality. They are called multiple coherencies or predictabilities, and are real numbers in the range (0, 1).

In estimating theZij as in Equations (37)and(38), crosspowers couldof coursebe takenwithEx andEy insteadof Hx andHy. In fact six separateestimates are available. Sims et al. (1971) examined the possibility of using some combination of the six possiblekinds of estimates to get smoother impedances. They also studied the admittances, _Y, y=z

-1

q)2 = (1/2) arctan

2(ac + bd)

(a 2+ b2)- (c 2+ d2)'

In each case, 90 degreesshouldbe added or subtracted for strike direction. When these points are badly scattered, plots of several of the varieties of • can sometimes be helpful. Each tipper definition gives a different phase. Phase •T correspondingto definition (41) is (I)T

(a 2 + b2) arctan(b/a) + (c2 + d2) arctan(d/c) r2

and is the one usually plotted in routine surveys with ( -900 --<•T --< +900)ß Like ITI, •T is invariant under


rotation. In the2-Dcase•r is thephase of T• in equation (26). A third attribute of the tipper is its skew, defined by


Sr = 2(ad- bc)/T2. Sr iszeroin the(noisefree) 1-D and 2-D casesand is invariant under rotation. Jupp and Vozoff (1976) give a more detailed descriptionof tipper analysis. (Note that the first equation in that paper is incorrect. Equation (20) here is correct.) Remote

Reference

The well known problem with the above procedure is the biasing effect of noise through the autopowers. That is, if A = Sa + Na, and B = St, + Nt,, where S and N are signalsand noises, respectively, then

AA* = SaS; "{-2SaS*a + S aS*a and

AB* = SaS; + SaN; + SbN*a + man; . If it is assumed that signals are coherent but that signalsand noise are incoherent (coherency • 0), then

AA* • SaSa* but AB* • SaSt,*. Thus the autopower estimates will be biased upward whereas the crosspowers will be unbiased. Every equation (such as equation 38) which contains autopower will suffer a

biaserror.If weletHx beA andHy beB, thesolutions to the four simultaneousequations can be written

(ExA*)(HyB*) - (ExB*)(HyA*) (HxA*)(HyB*) - (HxB*)(HyA*) (ExA*)(HxB*)

681

are mostcommonlysignalsfromHx andHy magnetometers

a few hundred

meters

to a few

kilometers

away. Sometimesit is more convenientto use a pair of E field receivers. Recent field experiments showed that the reference sensorsneed only be a few tens of meters from the central array to be effective (Goubau et al., 1984), but this must depend on the origins and spatial distribution of the noises. Situations do arise when it is impractical to separate the two sites sufficiently, and the reference site is subject to the same noise as the main site. No benefit can be expected from RR measurements

in such cases.

One fascinating aspect of the RR approach is that the noise spectrum itself can also be calculated for each channel (Goubau et al., 1978; Gamble et al., 1979b). It is not unusual for noise to exceed signal in some parts of the spectrum of some data sets, particularly near 1 Hz (Figure 53). The RR method has made it possible to routinely extract usable results even under those conditions (Figure 54). As a byproduct, the ability to estimate a noise spectrumfor each signal channel provides a useful means of identifying noise sourcesin equipment and in an environment. The present assessmentof RR is that it is definitely cost-effectiveunder "average" conditions, but may be unnecessaryin very quiet locations and is unable to help in extremely noisy locations. RR is widely used in routine surveys. Errors

and Noise

Despite the RR method, data acquired in noisy

- (ExB*)(HxA*)

ZxY =(HyA*)(HxB*) - (HyB*)(HyA*) (EyA*)(HyB*) - (EyB*)(HyA*)

gYx =(HxA*)(HyB*) - (HxB*)(HyA*)

environments

does show

scatter.

For

both

RR

and

conventional data, a variety of processeshave been

adopted to dealwithscatterin theestimates ofZij, and results by and large have been good. To begin, it

(EyA*)(HxB*) - (EyB*)(HxA*)

I00

Zyy =(HyA*)(HxB*) - (HyB*)(HxA*)

,

I

I

I

I

I

I

I

I

I

I

(42)

Each of these equations is subject to bias error since each contains autopower terms. Much of the scatter observed in many MT results can be explained this way. Each data set has different

I._. O•

_

noiseand henceeachZij estimateis different.A developmenthaving major impact on the effectiveness of MT has been the remote reference (RR) method (Clarke et al., 1978; Gamble et al., 1979a), in which two independent signal channelsare recorded for use

asA andB, in placeof H x andHy in Equation(42).By providing a kind of synchronousdetection, the method helps compensatefor noise, both internal and external to the equipment. Figure 52 comparesapparent resistivities calculatedin both ways from the samedata set.

Theremotechannels, usuallydesignated Rx andRy,

0'1 0'01

I

I

I

I

I

0'1

I

'

I Period

I

,

I

I0 (s)

Fig. 52. Comparison of apparent resistivities obtained by conventionalprocessing(open circles) and by Remote Reference (dashed lines). (After Gamble et al., 1979a.)


682

Vozoff

should be pointed out that the kinds of summation stacking of signalswhich are carried out on repetitive signals (radar, reflection seismic, transient EM) will not work with MT: random signalswill merely stack to their long term mean value of zero. The same is true for their Fourier transforms. Instead, stackingmust be done with the autopowersand crosspowers,which are nonnegativeand will not sum to zero. In-field comput-

ers have made this a practical part of commercial surveys.

Simply stacking powers produces arithmetic averages, and the contribution of each data set is, therefore, weighted according to its signal strength. There are many situations in which this is not appropriate, e.g. when the largest signals are due to occasional bursts of noise (Egbert and Booker, 1986). Hence it is desirable to provide for some other weighting using a

measure suchaspredictability, ortheproduct (ExPEyP). •o

I

I

The same weight must be applied to every autopower and crosspower from that data set. When in-field computingpower is not adequate, the approach has sometimes been to obtain many estimates of apparentresistivity in each band and take the mean, or better, the geometric mean of those values, and the mean of the phases. The median is still better than the geometric mean and gives more stable results (Moore, 1977) although it requires that the estimates be sorted. This is sometimes refined by discarding

I

Signal Noise X axis

ß

o

¾ axis

*

v

.m

I1•

-

g

outliers

o• I

which

are more

than

one or two

standard

deviations from the median, and computing a new

-

median.

o ils6_ 16?i

I

I

Period {s )

Fig. 53. Spectral densitiesof magneticsignal(solid lines) and noise (dashed lines) obtained by Remote Reference processing. Noise exceedssignalin the period range 3-20 s in these data sets. (After Gamble et al., 1979b.)

Still more improvements are emerging from the statistical research specialty called robust estimation, which deals with such problems in the presence of large outliers and nonGaussian distributions (e.g., Huber, 1981). The median filter is one type of robust estimator. From repeated measurements many estimates are obtained of each impedance element at each frequency, and a 'best' estimate is determined from them. Conventional least squares processingassumes that the estimates

have a Gaussian

distribution.

Each

point is weighted accordingto the square of its difference from the best estimate, which has the well known

result of giving greatestimportance to extreme values. This is optimal when values have a Gaussiandistribution, but it was shown in Egbert and Booker (1986) and Sutarno and Vozoff (1989, 1991) that this is often far from true.

2

I

LOG

0

-I

FREQUENCY

P;xy =[

-2

-3

(Hertz)

Pyx=]

[, ] • 50%

Fig. 54. Apparent resistivities with bars showing 50 percent sample confidence limits. Solid curves are the result of smoothing. (After Phoenix Geophysics, Inc.)

The absolutevalue (L• norm) method weightseach estimate according to its absolute difference from the best estimate, minimizing the sum of the differences. This method still gives undue influence to outliers. In the Robust-M (maximum entropy) estimation being applied to geomagneticproblems, estimates beyond a specific distance from the current best estimate are given weights which become less as the difference increases. Different kinds of weighting are still possible, and are a topic of ongoing research. Robust methods require much more computation than least squares. However, the methods now seem capable of reducing the time which must be spent in the field and possibly doing away with the need for a remote reference, easily offsettingincreased computingcosts.



Use of robust-M approaches was stimulated by Egbert and Booker (1986) and Chave et al. (1987). More recent comparative papers are Joneset al. (1989) and Sutarno and Vozoff (1989). The latter work was done with an RR data set in which both survey and reference sites were highly contaminated with noise from a large smelter. Robust estimation improved the apparent resistivities remarkably. However, it appeared to improve apparent resistivities much more than phases. We also found that analyses without the RR were superior in parts of the spectrum where the reference signal was highly contaminated. Thus there are benefits to be had by analyzing data from noisy environments

both

with

and

without

the

reference

data. However, one characteristic of these results is

that they generally do not give smooth curves when the data are noisy. Most recently Sutarno and Vozoff (1991) showed how to get still greater improvement by using the physical constraintswhich apply. At each step in the robust estimation process Hilbert transforms are applied to the real and imaginary parts of the impedances, forcing them to vary smoothly with frequency. One strong measure of the effectivenessof this process is the agreementbetween RR data processedboth with and without RR processing.The results obtained in the examples done to date appear very satisfactory (Figure 55). The strict mathematical validity of the procedure remains

to be demonstrated.

In presenting apparent resistivities and other results, sample variances 2 i

•(X i -- Xavg) 2 (n-

1)

(for n samples), sample standard deviations s, or the range of some proportion of the estimates, (say, 50 percent) are often computed in real time and plotted at the same time (see error bars in Figure 54). Recording can be discontinued

when these statistics have become

satisfactorily small. To compute population variance, standard deviation, or confidence limits (usually 95 percent), it is usually assumed that the scatter has a normal (Gaussian) distribution. Among its other uses, the variance is often used in inversion, to define numerical confidence limits in the model parameters. Gamble et al. (1979, a and b) derived the commonly used expressionsfor population variance and confidence limits of the impedance elements, apparent resistivities, rotation angle, skew, etc., in single site and RR data, assuming Gaussian distributions. However, confidence limits based on an assumed population should be used with nonGaussian.

caution

because

distributions

are often

683

Tensor Analyses for 3-D Sites

It is now accepted, on the basis of both field results and 3-D modeling, that the impedance does not always degenerate to an off-diagonal (or diagonal) tensor in some "optimum" coordinate system, and that the failure to do so is not simply due to noise. Instead, in the general 3-D case it is known that there are no special directions along which modes decouple, in the sensethey do in 2-D structures. A linearly polarized H field will generally induce an elliptically polarized E in this case. The special directions are replaced by polarization states. Each state consists of a pair of ellipses, one each for E and H, which are related in a relatively simple way. The ellipses are characterized by spatial directions of their principal axes, ellipticities, phases, and a ratio of the lengths of the major axes. The H ellipses induce the E ellipses, so that the ratio of the major axis of an E ellipse to that of the correspondingH ellipse is an impedance. The polarization states are special in that those two pair of ellipses have the largest and the smallest ratios possible at the site, for that frequency. Much of the work so far on 3-D interpretation has focussed

on the

statics

effects

of

shallow

bodies.

These effects are simpler than the general case because they are frequency independent, but are still very important. In areas of slightly disturbed 2-D structure, rotation angles derived from the conventional Swift 2-D tensor analysis can still provide useful structural information (Jones and Vozoff, 1978; Ting and Hohmann 1981). While the resulting arctangents are complex, their imaginary parts may be small enough to be ignored. From the spatial patterns of impedance and tipper rotation directions it is possible in this "almost 2-D" situation to locate and map simple anomalousregions. Accurate determination of depth, thickness, and conductivity of these bodies can be done by modeling and/or inversion. In a sensethis very simple 3-D case is like the 2-D case except that more data are required. However the more usual 3-D case involves regions with complicated boundary shapes, and it is still very difficult to obtain more than a descriptive interpretation, using techniques like that used in Berdichevsky et al. (1980) where a survey area is divided into regions in which the apparent resistivity curves have similar appearance. The directions obtained from the Swift analysis in such cases can be highly variable with frequency and position, and simply reflect local current directions.

Two tensor analysis methods more general than that of Swift are being developed. They are eigenanalysis and explicit tensor decomposition. The first includes the work of Eggers (1982), LaTorraca et al. (1986),


684

Vozoff

REF.

---x--

STD.

METHOD

STD.

ROBUST

_-

•

•

(a)

-- -- -e - - I:OdT.

, RR ROBUST

io I

ioo

io-I

!

iO-Z

tO-!

tO0

w

!

i

Illw

I0!

FREQUENCY (He r t z ) 135 ..

,.

o

io-$

---©--

RMT.

REF.

---x--

STD.

METHOD

_.-

STD.

ROBUST

m

RR ROBUST

I[

(b)

1% I

%

,..,-1

\

I IIIIIIIJI IIIIIIIJ I IIIIIIIJ ! IIIIlllJ I IIIIIII 1 •o'z

•o'•

•oo

•o•

•o•

FREQUENCY (Hz)

Fig.55.A comparison ofconventional androbust estimation oftheimpedance where boththesurvey siteandthe reference sitewerecontaminated bylarge scale industrial noise. Therobust estimates using Hilbert transforms are

verynearlythesame withandwithout theRR.Botharesubstantially betterthanthestandard 4-channel estimates andthenormal RRestimates. (FromSutarno andVozoff,1991)



Counil et al. (1986) and Cevallos (1986). The secondis representedby Spitz (1985), Yee and Paulson (1987), and Groom and Bailey, (1989, 1990). It should be emphasized that these general tensor analyses are a difficult topic of active research and debate, and the usefulnessof many of the results is only now (late 1990) being demonstratedon real data (Groom, personal comm.). The eigenanalyses are based on the fact that the tensor equations (14) are simply a pair of complex linear equations, that the elements of E, H, and Z change systematically as the coordinate system rotates, and that the elements of Z are simplestin some sensefor specificdirections.The object is to find those directions, since they should be related to structural features. Eigenanalysis of the impedance was discussedin Swift (1967), but the first implementation was in Eggers(1982). Eggerslooksfor a tensor_Aand vectors E i and H i such that

E i= A•iHi

(43)

E i ßH i= 0,

(44)

assumingthat

with i = 1, 2. The solutionsare pairs of eigenstates, with

~ -- _ i 0 ,

(45)

andthetwo eigenvalues •.i arefoundfrom

•.i = C2/2q-iC22/4 - det_Z, whereC2 = (Zxy

(46)

Zyx). The corresponding pair of

eigenvectors are

Z q_•.i

685

Figure 56 showsthe two pairs of polarizationellipses for the tensor

((0.097,0.208)(1.140,

Z_= (_ 274,-.457)(0.297,-.138)'

which has been rotated into its principal coordinates.

Thetwoeigenvalues areX• = (1.060,0.945)andX: = (0.354, 0.469). The advantagesgiven for this approach are that it is not intrinsically 2-D, and that it uses all of the information in the impedance. LaTorraca et al. (1986) use a different formulation of the eigenvalue problem, comparing it with that of Eggers for the same example impedance, and then showingthe results for a simple model. They use the singularvalue decomposition(SVD) or "shifted eigenvalue analysis" of Golub and Van Loan (1983) and Lanczos (1961), respectively. Instead of the two pairs of ellipsesand two real eigenvaluesobtained in Eggers, LaTorraca et al. obtain the E ellipsesfor a pair of orthogonal, linearly polarized H fields, and two complex eigenvalues.The major axesof their E ellipsesare not perpendicular to their respective H vectors, the deviation being a redefined skew angle. These new eigenstatescorrespondto the direction of maximum and minimum IEI/IHI. Figure 57 shows the two sets of eigenvectorsfor the example used in Eggers; clearly the directions are very similar, as are the magnitudes of the eigenvalues.Figure 58 shows the pattern of the two sets of E vectors at four frequencies around the model. Superficially at least these look very much like the patterns obtained in Jones and Vozoff (1978). Eigenanalysis of Cevallos' (1986) new impedance givesprecisely the sameresults as Eggers, without the orthogonalityassumption(44). However Cevallos then avoids the problem of complex eigenvectorsby using the Hermitian forms • and • + •. The directions

(47)

and

Ei=[Zy x+•.iß

(48)

All theparameters arecomplex.Theeigenvectors Ei andHi arerelatedto eachotherby a 90degreespace rotation,scalingby Ikil, anda phaseshiftof arctan [Im(Xi)/Re (•.i)].Thenthe@i= lki12/•Ol x.Thesolutions represent two situations ("states") in which an elliptically polarized H field will induce an E field which has the same ellipticity and a major axis perpendicular to that of H, as required by Equation (44). Thus they

are relatedby a real multiplication (by hi) anda 90 degree rotation. Eggers shows that the results reduce to the familiar

ones in the 1-D and 2-D

situations.

Fig. 56. Eggers' (1982) eigenstatesfor his impedance example. The X are his two eigenvalues.


686

Vozoff

for the first correspondto one of LaTorraca' s principal directions while the latter are different from any of Eggers' or LaTorraca. In the absence of noise, Cevallos' two directions are virtually identical. The most general, and certainly the most elegant analysisto date is that of Yee and Paulson(1987). Two different canonical tensor decompositionsare derived, one in polarization parameters and the other in more familiar elliptic parameters, and the results are related to those of Eggers (1982), Spitz (1985), Counil et al. (1986) and LaTorraca et al. (1986). They show that their polarization parameter decompositionrepresents a physical system in which a magnetic field having the polarization characteristics of one of the H eigenstates induces an electric field with the corresponding E eigenstate by simple (scalar) multiplication and phase shift. In that case the scalars are just the singular

valuesof Z (or of Z'Z, whichare the same.The ? indicates hermitian conjugation). The analysis deals with relations at a discrete frequency, and concludes that there are 8 degrees of freedom (independent parameters) at each frequency. While some results of

x

the decomposition applied to real data were provided by Paulson (personal comm., 1988), none have thus far been published. The least general but most successful analysis to date is that of Groom and Bailey (1989, 1990). These authors treat the problems of strike determination, statics, and information content of the tensor impedance in a model consistingof a 2-D structure perturbed by a small hemisphere (Figure 59). The 2-D structure provides a regional impedance background in which local 3-D channeling takes place. In analyzing this model they also answer some of the questions of information content which were previously noted.

Writing E = •CEr,where E is the "measured" electric field, Er is the "regional" field and C_is a channeling tensor, E can be written E = •ZmH, where Zm is the measured impedance. Then if _Z2 is the assumed regionalimpedance,Z•mcan be decomposedas •Zm :

R__C_Z2_R r, whereR_is the rotationmatrixfromthe measurementaxes to regional coordinates. Groom and

Bailey then show that C_can be written as the product of three tensorsand a scalar, i.e., C_= gTS__A. The tensors are responsible for "Twist", "Shear", and "Anisotropy", effects, respectively, and g is necessary for normalization. The three operations are illustrated with a simple model (Figure 60). Then with

x

Z•m: gR_ T_S _A _Z 2_R r, theydemonstrate thatit is not

H

I+1

Y

x

Y

possibleto tell whether gA_is differentfrom the identity matrix[on the basisof a singlelocal measurement, so the terms gA•Z2 are lumped together. Then _Zm =

R•T_S_Z•_Rr, wherenow_Z•= gA•Z 2. Thisrepresentation of •Zm contains seven real parameters, three angles, and two complex principal impedances. They are found by least squaresor other fitting procedure to the data, giving a misfit as the eighth parameter. This eighth parameter could be reinterpreted as an imaginary component of the Twist. In very good, noisefree data it would measure the suitability of the channeling assumptions.Otherwise it would simply indicate sig-

x

E

nal/noise levels.

p.2= 0'542e i54'Iø

y•+=1.420e14)'7ø ),._=0'588e i52'9ø

7E•= 2.58ø

11t+=12.0 o

¾HI= 81.82 ø

lit- :78 ø

œE•= -0'217

F.,+:-0-285

œH•=-0-0141

œ_:-0'

p.t =

1'538ei40'0ø

I$O

Fig. 57. Comparison of the eigenstatesof LaTorraca, et al, 1986, (left) and Eggers, 1982, (right). (After LaTorraca et al., 1986.)

This decompositionwas tested on real data from a complex geological area, and dramatically improved the smoothnessand credibility of the apparent resistivities compared with those obtained with conventional processing (Cavaliere et al, 1986). Although smoothnessis desirable simply on the basisthat model responsesare smooth, it does not yet guarantee interpretability. Groom and Bailey (1990) and Bahr and Groom (1990) show that perturbations in both electric and magnetic fields could be included by means of distortion tensors, the magnetic tensor being complex, frequency dependentand goingto unity at low frequency. Comparing the computed results with the parameters of their 2-D regional model, they found that impedance


The Magnetotel!uric Method was in generala mixture of the two regionalimpedances, except on lines along one of the regional directions through the center of the hemisphere.In somepositionsthe minorprincipalimpedancewasdue entirelyto contaminationof that electricfield by the major regional electric field, so the two apparent resistivitycurves were parallel. The magneticfield perturbationresultedin smallphasechangesat high frequencies. Those authors went on to do a thorough comparison

of the analysesof Eggers(1982), Spitz (1985), LaTorraca et al. (1986) and Counil et al. (1986) for points immediatelyoutsidethe smallhemispherical perturbation. They concludedthat, exceptfor stabilityin the presenceof noise,the methodsof Eggers,LaTorraca et al., andSpitz(Q parameter)givethe sameresultsas those of Swift. These are controlled by the local current direction which, in turn, is dominated by the

hemisphere. As regardsabilityto extractthe regional directionsand impedances,only theirs and the other Spitz' orientation(U) were successful. Groom (pers. comm., 1989)extendedhis comparisons to include the Yee-Paulson analysis. He found some difficultieswith numerical stability, particularly in the 1-D and 2-D cases. When stable results were

obtained,they were very similarto thoseof the other

687

four methods. This does not necessarily detract from

the validity of the Yee-Paulsonresults, but indicates additional effort will be required before they can be used.

The related questionof the number of degreesof freedom(independentparameters)givenby an impedance is still not fully resolved. An impedancemeasures the amount of new information which a site provides. There are limits to the effort expended in acquiring data which tells nothing new, so if there were no

degreesof freedomthenthe datawouldhaveno value. An impedanceZ containseight parametersat each frequency,correspondingto the real and imaginary

partsof the four Zij, but thesedegenerate to one parameter in any horizontally layered structure throughthe dispersionrelationsand azimuthalindependence(Weidelt,1972).In the principalcoordinates of the 2-D casetheseparametersgive us two complex impedanceelements (four parameters),two directions•strike and perpendicularto strike (two parameters)•and two combinationsof tensor elements• usuallyskewand ellipticity•which vanish.However, of these, only one of the two directionscan be independent,and the phasesare again related to the apparentresistivitiesthroughthe Hilbert transform (FischerandSchnegg,1980).Hencethereare onlytwo

plan v•ew

2o km• Cross-Section

•

ix, I$

3'33s

2 Km [_.t•-•

2Km

1000tl.m

50 Km

100fl.m

150 Km

50 tl.m

20Km 10000 fl.m

Y

Fig.58.Thedistribution ofLaTorraca E fieldeigenstates atfourfrequencies around the3-Dmodelshown. (After LaTorraca et al., 1986).


688

Vozoff

degrees of freedom in 2-D. Yee and Paulson (1988) proved from fundamental principles that the same dispersion relations hold in the general 3-D case, at least for the principal terms, so that there are at most six (and perhaps fewer) independentfrequency functions in any (four channel) MT data set. (There is still debate about this result.) Also possible relationships between the real and imaginary parts of the diagonal terms of Z in the general 3-D case remain to be discovered, so further work has to be done in this area (Yee, pers. comm., 1989). The major challenge that remains is to devise practical proceduresfor quantitativeinterpretationwithout the expenseand inconvenienceof computingeight (or even only four) parameters for 3-D models at many sites, over a broad spectrumfor comparisonwith field data.

Imaging and Time Domain Processing

We saw in a previous section that the solution to Maxwell's equations in a layered earth consists of downgoing and upgoing waves in each layer, the upgoingwaves having been reflectedfrom the bottom of the layer. The picture holds equally well in time domain if we consider sourcefields that are impulsive. The waveforms will change rapidly on account of dispersion, but this can be modeled with little difficulty. Kunetz (1972) explored some characteristics of modeling and processingin time domain, using con-

cepts from reflection seismology.He consideredonly the 1-D case, showed how an impedance could be derived without Fourier transformation, computed model curves (Kunetz's Figure 4-14) and outlined several alternative inversion procedures. Spies and Eggers (1986) discussedthe behavior of time domain responsesin comparison with alternative frequency domain definitions of apparent resistivity. They conclude that the time domain Pa has much more intuitively appealingcharacter in its lack of oscillationand rapid convergence to its asymptotic values. The method has not received much use with real data, as

far as I know, but appears to even be applicable to tensor measurements. The question raised in both papers about the transient approach is the practical difficulty anticipated in accurately measuring transients of significant length. Recalling that MT data acquisition now rarely includes less than four frequency decades, and that TEM systems obtain three time decades only with difficulty, it is unlikely that a natural field transient system would reliably record waveforms

more than one decade in duration.

Hemisphere

-y

radius= lOOm

resistivity =10n'-m •,, +x

•

300.rz-m

5km

Thus

time domain seems an unrewarding approach to MT data acquisition. On the other hand, time domain could be a useful way of presentingand interpretingdata and might also have some benefits in processing.Whittall and Oldenburg (1986) actually invert on impulse responses,but they obtain them from the frequency-domainimpedance, assuming several different models of noise.

40000•-m 3000n.-m

10km

40km 10.o.-m

frequency < 3000Hz

Fig. 59. The perturbed2-D modelfor which impedanceswere computedat pointsw aroundthe hemisphere.(Groom and Bailey, 1991)

The Magnetotelluric Method Their impulseresponsesare plottedin the sameway as deconvolvedseismicreflectivity time sections,which has a certainpsychologicalappeal. Levy et al. (1986)

689

mon data sets, particularlywith regard to their suitability for 3-D interpretation. INTERPRETATION


discuss thepossibility of usingeithertheTE or TM modes, or both, to produce such sectionsfrom 2-D data and showa syntheticsectionfor a 2-D model,and Stinson et al. (1986) model near-surface effects on imagingand apply the method to Kurtz's Vancouver Islandfield data. Wanget al. (1985, 1986)and Liu et al. (1987)alsoapply time-domaintechniquesto 2-D models and demonstrate their use on real data.

Lee et al. (1987) extended Claerbout's (1976) 2-D numericalmodelingtechniqueto the MT case,for both the forward and inverse problems. The latter is the same as Claerbout's migration, and produces good imagesof EM reflectivity.Vertical resolutionis found to dependon the range of frequenciesand station

spacingused,andtheimagingmustbe doneseparately for each frequency. Conductivity determinationappearsto be semiquantitative.Ten to twenty frequencies were used for the results shown. Lateral resolution is said to be on the order of half the station

spacing.The method,referredto as "phase-fieldimaging", is evidently fairly computer-intensive.It would be of considerable interest to evaluate this

An Example

Interpretation, the processof drawing geological conclusionsfrom the geophysicaldata, is most effectively done with referenceto conceptualgeological models.In addition, interpretationrequiresa thorough understandingof the physicsof the method and the technical details of the measurement, so that data

artifacts are not attributed to geological causes. As

with most geophysicalmethods, MT surveys raise difficult questions which the practicing interpreter must do his best to answer every day. This section describes the tools available and the way they have been brought to bear on these problems. To illustrate the nature of MT interpretation problems it is easiestto begin with a real example, since

they rarely conformto the simpletheoreticalpictures. I selectedan AMT survey acrossthe Torrens Hinge Zone of SouthAustralia (Figure 61). Ten sites(Figures 62 and 63) were surveyedacrossthe feature, thought

method and the impedanceseries inversion on com-

i

NORTHERN TERRITORY T

i- ...........

-]

:, _

i

QUEENSLAND

,

I

/,'

.___ ',

/-

•

--

\

"'"x

-..

AMT_%, AI

Traverse•

•1

I

IA,, / I

I

Fig. 60. Model incorporatingthe three types of distortion underlyingthe Groom-Baileydecomposition.Measurement location is at the center of the very conductive (black) region.White regionis resistiveand the spottedregionhas intermediateconductivity.The regionalE field is at angle0t from the vertical, and currentsreach the center by twistingT

through0t. The centralellipsoidalconductoris responsible for anisotropyA_.(From Groom and Bailey, 1989.)

Fig. 61. Location map, TorrensHinge Zone AMT traverse, South Australia.

5G

57

5845'

59

•

6o

FleeIs

'e4 IVo•7h


Paradise

',

/

'13 ._

IS'

'01 o Bore

Bank

Dam

99

Port Augusta

Rear'IonT•nk .L•..-

97

5

IO

I

kms

Fig. 62. Site locations,Torrens Hinge Zone traverse.



691

to be a major geosuture (Figure 64), during March

fault-bounded

1983.

the western edge of this geosyncline, a contemporaneous, gently dipping shelf facies was deposited over the irregular, undulating basement surface.

A brief geological description of Figure 64 from an unpublishedreport by Nelson and Greenhalghfor the South Australian Department of Mines and Energy

the continental

crust. On

This is known as the Stuart Shelf, and the zone of

follows.

intensive faulting and deep fracturing that separates it from the geosyncline is known as the Torrens Hinge Zone, part of a northerly trending, ancient lineament of continental proportions. Northwest of Port Augusta, the Adelaidean sediments are known to overlie unconformably the Gawler Range Volcanics, but little is known about the preAdelaidean basement on the Stuart Shelf in the area immediately north of Port Augusta, and virtually nothing about basement beneath the Adelaide Geosyncline."

The "Northern Eyre Peninsula, which lies in the western part of the transect is structurally part of the Gawler Craton, a basement complex of rocks, highly folded and intruded by granites, and ranging in age from Archeaen to Middle Proterozoic. The oldest known rocks are highly metamorphosedbasic rocks and metasedimentsof probable late Archeaen age. These are overlain by granitoid rocks of the Hutchison Group. Mildly metamorphosedrocks of Carpenterian age overlie Hutchison Group rocks (e.g. metaquartzites of the Moonabie Formation). The final phase of consolidationof crystalline basement was the extrusion of a vast sheetof porphyritic rhyolitic lavas•the Gawler Range Volcanics•extending from the Gawler Craton eastward, possibly across the future site of the Adelaide Geosyncline. The thick sedimentary sequence that forms the Adelaide Geosyncline accumulated during Late Proterozoic in a subsiding trough east of the Gawler Craton. It has been recognized as an aulocogene, or

In fact, neither is much known about the superficial unconsolidated sediments of the region. The survey area is sandy and dry most of the time, relatively flat, with low scrub, and isolated homesteads tens of kilometers apart. Lakes and streams shown on the maps are often empty for years at a time. Torrens, Gairdner, Frome and Eyre are salt lakes whose levels are also widely variable. The major noise source in the region is an (50 Hz) earth return power grid. Fortunately, current densities are fairly low.

SECTION

C

basin within

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20 km

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CROSS SECTION

TORRENS

HINGE

SHOWING

ZONE

AMT

SITES

AMT

Fig. 63. Geological cross section, Torrens Hinge Zone. Numbers across the top show AMT sites.


692

Vozoff

SECTION ALONG 31ø30 ' TORRENS LINEAMENT

MT. PERNATTY ISLAND PwtGUNSONLAGOON LAGOON ß ' ß

'

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KILOMETERS

KILOMETERS

VERTICAL

z

LEGEND ß

•h ! Hawker Group

ß

ß!

ßøe.Pø,OlJ Pandurra Formation i ^ o /•1 "C, arpentarian" volcanics

_uI ^cc^1 eg= Corunna Conglomerate o

Tent Hill Formation Willochra

Formation

(Whyal la Sandstone )

N I v V VI m i o oI

o

Gawler Range Volcanics Moonabie Formation

"'!ø•:1 Cultone granophyre

• o

o

Topley Hill Formation

--v--v--v-

Sturtian

Unconformity

glacials

Burro Group

:- v Vl Callanna Beds

+ 3+.l Syntectonic granite

•

Hutchinson Group (B.I.F)

v acbv Bed Vol ics v

v

a

can

i

Fig. 64. Largerscalecrosssectionshowing interpreted geology,TorrensHingeZoneAMT. Note depthscale changeat the Lineamentat thewestedgeof theAdelaideGeosyncline. (AfterNelsonet al., pers.comm.)



Between

sites 5 and 7 the traverse

crosses the Torrens

lineament, extending north from Port Augusta, which is thought to be the trace of the geosuture. Pairs of scalar (Cagniard) apparent resistivities with their phases are shown in Figure 65, while the coherencies, rotation angles, tippers, impedanceskews and H predictabilities appear in Figure 66. Coherenciesfor

(Ex,Hy) and(Ey,H x) in therange(0.7-1.0)areplotted at the top. Predictabilities Hxp andHyp for the same range are immediately below. The third box shows tensor rotation anglesfrom equation (20) (theta 1) and from Eggers (1982) E polarization (theta 2). Impedance skew is plotted in the range (0-0.5). Beneath that is the tipper modulus ITI (range 0-1.0). The last pair of plots are tipper rotation angles,from equation (41) (angle A) and another (angle B) suggested by D. Kerr (pers. comm.). We did not plot tipper skew, tipper phase, or

Hzp. Data quality is poor to fair by comparisonwith much of the current generation of commercial MT output. The data were acquired with an AMT system designedand built at Macquarie University in 1980and described in summary in Zile et al. (1983). Scatter in these plots can arise for any of several reasons. If the site is approximately 1-D over part of its frequency range, then the two apparent resistivities will of course

have the same value.

In that case the

computed rotation angles are undefined and the points will scatter. If the situation is truly 1-D we should also

find that the Pa and q>plots are smoothbecausethey will be unaffected by rotation. In those conditions, large scatter is also expected in tipper rotation angles becauseHz will be smalland the tipperrotationangles will be poorly defined. Of course, when data are noisy then coherencies and predictabilities will be low, in which case all of the responsefunctionsincludingPa and phase are expected to scatter. The band of low signallevel due to poor propagation near 2 kHz is seen in the apparently enhanced tipper and in the scatter in I•a, phase, skew, and rotation angles. The decreasingS/N ratio below 5 Hz in these data causes similar artifacts. (A rapid change in phase of some of these data at frequenciesabove 3 kHz was caused by the electrodes. This undesirable characteristic was not evident until late in the field program.) Discussion

The first step in interpretation, after discardingthe worst data points, was to assessthe results for internal and external consistency. This assessmentwas done in order to establish the range of resistivities and depths represented. Layered models were fitted to each of the apparent resistivity-phasecurve pairs. With the rotation anglesand the tippers, these were then compared with the geological models to see whether they were

693

compatible. In particular, there should be a general agreement between strike directions, at least for shallow depths, and the resistivity values should be in the broad range expected for the geological environment, the prevailing hydrological regime and any independent information.

In the present survey, dc resistivity depth soundings were carried out at each site by the South Australian Department of Minerals and Energy (Figure 67). These soundingswere used to confirm the near-surface resistivities and for joint inversion with the MT (Jupp and Vozoff, 1977). Both MT

and dc results showed

near-surface

resis-

tivities of less than 10 l•.m and often less than 1 1•. m,

which was consistentwith the known high salinity of the region. The values at each site were in excellent agreement.Tippers were generally 0.1 or lessabove 10 Hz, increasing toward lower frequencies, and largest at site 6. This fact, that tippers are largest at suture locations, as predicted by theory, agrees with the expected suture location. Impedance skews were scattered, on account of noise. Where they form smooth plots the values are small (•0.1), increasing toward lower frequency. Skews at sites 4 and 9 were uniformly larger (0.1-0.2), and at site 5 they were off-scale (>0.5). This indicated that local structure is most complex at site 5, although the apparent resistivity and phase curves were well behaved and the impedance rotation angles were the best defined of the survey.

Closecomparisonof the Pa curvesshowedthat sites 7, 8, 9, and 10 were almost l-D, from highest frequencies to between 10 and 100 Hz, with a resistive basement becoming shallower to the west. Sites 1, 3, and possibly 2 were also nearly 1-D from 10 kHz to 10-100 Hz. Site 3 appeared to have a static shift (discussedbelow) of about 0.5 relative to sites 1 and 2. Sites 4, 5, and especially 6 showed the greatest departures from l-D, but even these are not very marked. At sites 4 and 5 the departure is apparent only in the mid-band. Thus the tensor rotation angles are fairly indefinite and the tippers are small, as was expected. Under these circumstances, and in light of the large spacing between sites relative to the skin depths achieved, there seemed little point in going beyond a 1-D interpretation, possibly after some static shift compensation at site 3. There was a question regarding the tipper rotation angles (they were not tipper strike angles), which we would have expected to be east or west, for geological reasons. Instead, where they did cluster they were consistently northward, except at the lowest frequencies at sites 1-3 where they were easterly. No explanation is available, but the evidence suggestsan eastwest cross structure, such as a fault very near the traverse, superposed on the major geosyncline. An-

694

Vozoff SITE

1

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695

other alternative was suggested by the results of Groom and Bailey (1989), where poorly conditioned matrices led to numerical instability and physically meaninglessresults. Figure 68 is a compositeof 1-D inversionson the TE (xy) curves. This simple presentation confirms the view from Figure 65 that statics were not a serious problem on this line. The zone of shallow high resistivities in the center suggestsa low porosity zone in mid-line, between sites 5 and 7. If the traverse line grazed one end of such a block it would help explain the tipper rotations. The TE inversions were usually usedfor this purpose becausethat mode changesmore graduallybetween sites,which makescorrelationeasier. In this case we had a conceptual model of the structure, including a definite idea of strike direction. This model was used to lay out the traverse, and site

reason it is strongly advisable to get areal coverage instead of single traverse lines. In practice, there are many situationsin which there is no clear strike direction, because an area is virtually layered, or 3-D, or because of noise. A great deal of effort has gone into devising ways of extracting values of effective strike and impedances from the data in some of these cases (Jones and Vozoff, 1978; Gamble et al. 1982; Wannamaker et al. 1984b; Zhang et al.

SITE 7

SITE 9

spacingwas taken to be some convenient value (2 km) with the idea that additional sites might be added later. The situation was fairly typical, as were the results. Interpretation started with the classificationof curves as TE or TM at each site. It is quite usual to find that, with a single line traverse, the choice will sometimes be difficult or arbitrary. This difficulty arises because of 3-D conditions

at one or more of the sites. For that

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696

Vozoff

1987; Groom and Bailey, 1989). These situations are

The paper of Zhang et al. (1987a) and especially the Groom and Bailey papers (1989, 1991) deal with both problems. Another departure from the simple picture of MT field behavior, discussedbriefly in a previoussection, arisesin the presenceof topographicrelief, which is a major problem where slopesand resistivitiesare large.


discussed later in this section.

The sourcesof statics effects were described previously, and their treatment is discussedin the Statics Compensation Section. Ultimately the problems of statics and strike determination are closely intermingled, since both usually result from 3-D phenomena.

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697

Inversion

to the observations at each site by trial and error,

MT responsesare usuallyinterpretedthroughmodels. Occasionallyit may be enoughto find the location

adjustinglayer thicknessesand resistivitiesuntil the modelresponseshoweda reasonablygoodvisualfit to

of some anomalous feature such as a minimum in

the field data. However because this procedure was time consumingfor the geophysicist,it soon became

apparentresistivitiesor a directionchange,but usually the result sought is in terms of the distribution of resistivities.In the early years, 1-D modelswere fitted

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sion is not the same as interpretation:it is only one step in the process. There are now a number of general approachesto

uniformlayerswhich might representa sedimentary section. In others, the variation of resistivity with depth is continuous,as might be expectedin a geothermal section,for example. Inversionmethodsalso


1-D inversion in common use, in addition to several

2-D inversionschemes,and still othersfor joint (l-D) inversionof MT with other types of data. In spite of (or perhapsbecauseof) the extensiveliterature, aspects of the topic remain sources of confusion and subjectsof philosophicaldiscourse.Menke (1984) is probablythe mostusefulpracticaltextbookfor general geophysicalapplications,while the Parker (1983)paper providesa superboverview of the subject,free of the jargon responsible for much of the confusion.

differ in their ease of use, in the decisionsthat must be

made alongthe way, and in the nature and amountof information they yield. Even the data used differs: most 1-D inversionchoosesone of the modesof Pa,

sayPxy,andinvertsto thatdatasetwithor withoutits phase. Others use the real and imaginaryparts of the

corresponding impedance element ZxyorofCxy,where Cxy= Zxy/ico•x. Realandimaginary partsof Z andc are thoughtto be better behavedstatisticallythan apparent resistivity and phase. Berdichevskyand Dmitriev

Parker deals with the existence of solutions, their uniquenessand construction,and the errors and mean-

(1976),andRanganayaki (1984)used(Pxy+ Pyx)/2,

ing of the results,by reviewingboth his own work and the literature generally.Much of the followingdiscussionis basedon that paper. However, Parker assumed that little or no other information

was available

699

avoidingsomeof the difficultiesof scatter,particularly due to noisy rotation angles. However the major differencesseemto be in what is

about

asked of the inversion and in the existence and use of

the section,whereaswe often know a great deal about it from other sourcesThis knowledgecan changeboth the approachto the problemand the significanceof the

prior information. Exploration problems sometimes require a simple "yes" or "no" answerto questions

results, as noted.

the conductorshallowerat point A than at point B?", in order to establisha site for drilling. In other applications we would like to map stratigraphic facies variation and require quantitative estimatesof resistivity in a particularhorizon, completewith confidence intervals for those estimates.In many situationswe may know the depths to particular interfacesvery accurately, as from a reflection seismicsurvey or a well. We may also know resistivityfrom well logs or

such as "does a buried conductor exist here?", or "is

At the other extreme to Parker's approach are papers giving practical approximate construction methodologies,without regardto accuracyor dependability (Niblett and Sayn Wittgenstein1960; Bostick, 1977;Goldbergand Rotstein, 1982). They reflect the rangeof needswhich existsamongusers.Parker asks whethera resultis possible,whetherit is unique,what rangeof solutionsis allowedwith the prevailingnoise, andhowwell eachfeatureis resolved.However, many practitioners are content just to have a model that roughly fits the data. Inversion algorithmsdiffer in the types of models they use. In some, the earth is made up of a stack of

TORRENS i0

9

8

7

core measurements. In other cases, of course, we have

no prior information. Where information is available, some existingcodespermit incorporatingthe data as constraintson inversion so that other, unknown parameters

HINGE

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determined.

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52


700

¾ozoff

Except for direct curve matching, the earliest and the simplestof the 1-D MT inversionmethodswas first describedin Niblett and SaynWittgenstein(1960),and was later independentlyput forward in Bostick(1977) and in Vanyan et al. (1980). Jones(1983)showedthat the three formulationsare the same.The pa(T) curve (whereT is the period)is transformedto pB(h)(where h is a depth) by 9B(h) = 9a(T)

1 + m(r) 1 - m(r)

whereh = V'Tpa(T)/2'n'lx andm(T)is theslopeof the log (pa) -- log (T) curve, d[logpa(T)]

m(T)= d[log (T)] Jones'paperincludesa brief historyof this and related methods, including modificationsby Weidelt and the KLM Group (Weidelt et al., 1980) and by Goldberg and Rotstein (1982). The main modificationis to use equation (10) to replace the slope m(T) by a phase term, i.e.

pB(T) = Pa(r)24;iT)1 , where 4(T) is the phase.Resultsare not quitethe same in the two cases because of the approximation in equation (10). The important attractionsof this Niblett-Bostick (N-B) inversionare that the computation involved is trivial, and that assumingan initial model is not necessary.Becausecomputationcan be donewith pencil and paper if necessary,it is widely used in the field, even thoughresultsare qualitative. That is, if the input is a Pa curve calculated for some particular model, the N-B inversionwill not generally return the same model. However, common practice uses N-B output both for data presentationand as input to more quantitative inversion programs. An example of a result is included in Figure 50. Quantitative inversion requires considerablymore computationbecausethe relationshipbetween Pa(f) and p(z)is not linear. That is, doublingthevalue of p(z) at some depth can increase, decrease, or have no detectable affect at all on Pa(f) at a particular frequency. The characteristicof quantitative inversion that distinguishesit from N-B inversionis that, given a theoreticalPa curve for some model, the quantitative algorithm will return that input model as closely as desired, subject to computer roundoff. Parker (!983) discussesthe existenceand uniquenessof (l-D) solutionsfor a given data set. Obviously therewill be data setswhich are not sensible'they may be irregularor discontinuous(possiblyon accountof

faulty equipmentor incorrectprocessing),whereaswe know that every forward modelcurve mustbe smooth and continuous.He showsthat every valid 1-D apparent resistivity curve can be generatedby a particular integral, but that not all such curves have an inverse solution. In the case of numerically sampled and truncated data, Parker showed that, if any solution exists,the best fitting onewill consistof a set of highly conductivethin sheetsseparatedby large air gaps,and is extremely unlikely geologically. No geologically acceptablemodel will give a better fit to the data. He also shows how to decide, on a statistical basis,

whether a particularnoisy data set is compatiblewith any 1-D model. This serves as a guide when it is clearly necessaryto discardsome data points. As regards the uniquenessof a solution, it was shown earlier (Tikhonov, 1965; Bailey, 1970) that a solutionis unique only if it fits perfectly to a continuous data set at all frequencies.Thus in the practical case there are no mathematically unique solutions, although it may be that, in a geological sense, all allowable solutionsare equivalent. It is now generallyacceptedthat inversioninvolves two steps, finding a model to fit the data and then assessingits significance.Assessmenthas both a statistical and a geological part: the statistician and theoretician are concerned with the range of other models that could fit the data equally well, without regard for geologicalrealities. The practitioner, who may be concernedwith explaining the subtleties of basingrowthor may have to plan a multi-milliondollar well, must worry about both. As noted, two general kinds of models are used, discrete and continuous,and in some algorithmsthey can be combined. The discrete models are piles of layers, each of which is uniform. Some algorithms allow the layersto be anisotropic(Abramovici,1974). A majorjustificationfor usinglayered modelsin many casesis their correspondence to geologicalconditions. In the usualcase(e.g., Weidelt, 1972;Juppand Vozoff 1975)the layers can take on any finite positivethicknessesand resistivities. In fact the parametersused are log (9i) and log (ti) , which are alwayspositive. In most inversion algorithms, it is necessary to assumesome startingmodel, includingthe number of layers.(A brief discussion of exceptionsfollows.)The algorithmsassumethat the parametersof the answer are closeto thoseof the startingmodel. For that model they calculatePa(fi) and 4(j•) at each frequencyj• where there are data. The calculatedvalues will generally be somewhatdifferent from the measuredvalues, and the differencesare taken to be fitting errors ei- The partial derivatives of each calculatedvalue

with respectto eachparameterpj and tj are also calculated, either analytically or by differencing, at


each frequency. Using the notation of Raiche et al. (1985)


e = ,[$P,

where e is the vector of errors, ,[ is the matrix of partial derivatives, P is the vector of (logarithms of) the parameters, and we want to find the parameter changes gP. When they have been found, these changes are added to the initial parameter values to give a modifiedmodel. The processis repeateduntil no further sensiblechangesare being made, asjudged by the size of the fitting errors relative to measurement errors, or by the relative changes being made in the parameters.

Many different methods have been used for solving the equations. I have used the singular value decomposition (SVD) (Golub and Reinsch 1970), which is fast, allows satisfactory control over the inversion process, and provides the information necessary to assess the resulting model. In particular, we have found that (a) the results are relatively insensitive to the startingmodel (b) if the final model is inappropriate

it is very obviousin the plot of observedand modelPa, at which point a different starting model can be used, (c) the fitting error diminishesas the number of layers is increased, rapidly at first and then slowly, but the parameter values become unstable as the number of parameters increases. Fischer and Le Quang (1982) describethis behavior, and Raiche et al. (1985) suggest the use of the APRE statistic to determine the optimum number of layers. In this procedure it is clear that some of the parameters of the model may have very little effect on the apparent resistivities. For example, a layer may be too deep to affect the results for the range of frequencies under consideration, or it may be too thin. It is also well known that, for a sufficiently resistive layer in a conductive section, changing its resistivity will have no effect on Pa or phase. This is an unimportant parameter. When inverting to noisy data it is essential that such unimportant parameters are not allowed to change during inversion. This destabilizing effect of unimportant parameters is one of the fundamental difficultiesof EM inversion. The problem is said to be poorly posed. Both the SVD and Marquardt schemes incorporatefeatures to control the changesin unimportant parameters.

Another practical difficulty is properly accounting for data noise in inversion and, in particular, in attributing confidence limits to the final parameter values which take the actual noise conditions

into consider-

ation. Clearly there can be more confidence in the output value of any parameter when the noise levels are low than when they are high. The method, as

701

described in Jupp and Vozoff (1975), allows for a variance estimate to be input with each data point. Output confidence limits thus directly reflect the size and location of noise in the data. The sample standard deviation measured in the field is used routinely for this purpose. However in some circumstances it is necessary to arbitrarily reject data points which are clearly outliers, because of their distorting effect when usingleast squaresminimization. Parker (1983, pg. 11) provides a rational objective criterion for rejecting outliers.

Several published methods describe procedures to transform directly from the observed data to a model without minimization, essentially by 'layer-stripping' from the top. (Consideringthat each data point yields a thin layer, the N-B schemecan be said to operate in the same way.) Parker (1983) shows his results for a COPROD (Jones, A. G., pers. comm.) data set. The method is said to be computationally simple and flexible, and results can be made to fit the observations

as closely as desired. The fitting of smooth resistivity-depth functions is described in Oldenburg (1979), Parker (1983), Coen et al. (1983), and Constable et al. (1987). In the last reference, model smoothness(specified in terms of average slope or average second derivative over the full depth of the section) is minimized along with fitting error and the balance between them is determined by an arbitrary weighting coefficient. The smooth resistivity-depth function is approximated by a large number of thin layers. Becauseof this, the model response can be made to fit the observed data as closely as desired, but of course the closer the required fit, the more irregular the model. By varying the weighting coefficient, it is possible to obtain any fitting error desired, but with a corresponding change in model roughness. Model roughnesswas measured either by

R•=f (dm/dz) 2dz or

R2 f (d2m/dz2)2dz. Their objective function to be minimized had the form

u=

+

2-

where e0 is the desired sum squaredfitting error, e is the error with the current model, and X is the arbitrary weighting coefficient. The procedure used by Constable et al. (1987) was to sweep through a range of coefficientsuntil they obtained the desired fit. Because of the nonlinearity it was necessary to iterate from a starting model (which could take on just a constant value).


702

Vozoff

Without

Parker's (1983) approachto the continuousfunction problem is somewhatmore elegant, and involvesfunctions which are truly continuousrather than consisting of many thin layers. He choosesto use an analytical scheme, which requires that the noisy and incomplete field data be converted to a smooth, continuous im-

pedance function. His contribution was to find a smoothing scheme which is both well suited to the kind of data and convenient to manipulate. To compare the results of five very different approachesapplied to a commondata set, which Parker presents in his paper is thought provoking. These results are summarized in Figure 69. In this connection, some effort has gone into techniques to find all acceptable models which fit the data by randomly changingthe parametersof layered modelsand keeping those which fit the measurementsto within some chosen sum squared error (Monte Carlo search). The conclusions

were that the results were unbiased

but

very expensive on account of the vast number of forward models which had to be generated, especially if the model had more than a few layers. In the end, some external guidance was required to choose between

such information

we have no basis to choose

between these various pictures, or still others which might be produced in the future. Thus some other data (seismic, gravity, magnetic, or borehole) is necessary. However given such information, the MT data are clearly capable of describing the electrical conductivity structure in useful detail. It is an empirical fact that computer inversion is routinely used for many practical purposes. Considering the pragmatic nature of explorationists, this can only be so because it works. Hence in these cases, at least, an appropriate model family has been found. Having established that an output model has the right form, what can be said about the specific parameter values which were returned by a particular program for a specificdata set? Backus and Gilbert (1970) showed that, for continuousconductivity-depth variation, it is possible to state with confidence a kind of average conductivity over a range of depth. If the conductivity-depth function we get is •r(z), then at depth z0 we can confidently state the average value

•ravg(Z0) =••o'(z)A(z, Zo) dz.

results.

We are then left with the questionof the significance of the data, as noted earlier. A wide choice of possible interpretations is available for the same data set, and we must apply some outside constraints if we are to make senseof them. It is necessaryto know something about the geologicalhistory and style of the location.

If A(z) were a delta function A(z, z0) -- •(z, z0), then

•ravg(z0) = •r(z0)andwe wouldhaveperfectresolution. A(z, Zo)is a window throughwhich the modelhas to be viewed. It is a smoothfunction with its peak near (z = z0) and a width which dependson the model and

lO0

_• E lO_I

o

.- 10'2

::3

O

o

lO'3

-4

0

100

200

300

400

500

600

700

800

900

Depth (km)

Fig. 69. Five of the inversion results obtained by Parker from a COPROD data set and his smoothedversion of it. Circles indicate the conductancesof delta function sheets(in kS). Open circles gave the best fit to the data. (After Parker, 1983.)


on the frequency range of the measurements. The width of A generally increaseswith depth, which is recognized when we present our results on a log depth


scale. Error

in the measurements

introduces

error in

O'avg. As mentioned

the results

of inversion

can some-

times be improved by fixing parameters known from other measurement which often improves the resolution of some of the other parameters. Also different electrical and EM methods differ in their ability to resolve certain kinds of parameters. For example, inductivity coupled EM methodscan resolve the thickness and conductivity of thin conductive layers much better than dc methods can. Conversely, dc methods can resolve the parameters of thin resistive layers much better than is possibleby, say, MT. This behavior has been used to advantageto improve the resolution of a complete section by jointly inverting dc and MT data (Jupp and Vozoff 1977; Constable et al. 1987), dc and transient EM data (Raiche et al. 1985) and MT and large scale TEM (LOTEM) data (H6rdt, pers. comm., 1990). The improvements obtained by these means can be dramatic.

The same inversion techniques used for layered models can be applied to 2-D models, although the processesare somewhat more time-consuming. Weidelt (1975), and Jupp and Vozoff (1977) used models consistingof rectangular blocks whose resistivitiesare the variable parameters. Using the TE apparent resis-

tivitiesandH z (Weidelt)or bothTE andTM apparent resistivities (Jupp and Vozoff), the algorithms developed for 1-D inversion were used to find best values for block resistivities. A major limitation of these methods is the need to fix block geometry at the start. Pek (1987) modified the Jupp-Vozoff (1977) algorithm to include

block

boundaries

as variables.

Travis

and

Chave (1986) described a program in which the finite element mesh moves in response to high current density gradients. This program should provide better eificiency as well as more accurate results, since current density would be expected to rearrange itself as frequency changes. [A recent book by Crank (1987) deals entirely with the problem of moving boundaries in numerical solutionsof partial differential equations.] Another approach is that of Jiracek et al. (1987a), in which a smooth 2-D conductivity section made up of continuous functions is produced. DeGroot-Hedlin and Constable (1990) report on the extension of their Occam inversion to the 2-D geometry. The arguments for smooth or blocked cross sections are directly analogous to those in the 1-D case. Smooth sections seem better suited to problems such as the temperature and fluid distribution in hydrothermal gold and geothermal prospects, whereas the blocks seem a better representation of many other environments.

703

A major stumblingblock to more widespread use of 2-D and 3-D inversion has been the large amount of computing required. Smith (1988) and Oldenburg and Ellis (1990) found that it is possible to use short-cuts which allow major reductions in the computing load. Smith (1988) did approximate inversion one site at a time along a traverse, but using partial derivatives that depend on both vertical and lateral variations in a 2-D model. The resulting single site models are strung together with interpolation between, to make up a new 2-D model. The response of this model is then computed and compared with the observations. The residuals are then used to modify the parameters in the single site inversions. Very satisfactory results are obtained with a few percent of the computing cost of straightforward 2-D inversion. In the second reference, Oldenburg and Ellis (1990) also use approximate inversion and accurate forward modelingto reduce the computation. Two approaches have been developed. In one, the approximate inversion is applied to both the observationsand the model response. The two resulting models are expected to differ, even if the observationsand model responseare identical, because the model response•is computed exactly. The model is then altered on the basis of physical ideas, its responseis recomputedexactly, the approximate inversion is repeated, and the models are comparedagain. This procedure continuesiteratively until the differences

stabilize.

The difficulties with this approach are that the models become increasingly complicated as iterations proceed, and that there is no obvious objective function to be minimized since one generally does not have a true model for comparison. To enable comparisons to be done on responseswhile still using approximate inversion, a different procedure was devised in which the data are modified, the modified data are inverted

approximately, and the response of the resulting model is computed exactly. These responsesare then compared with the observed data. The new difference is then added to the observed data and the process repeated. The sequence was found to converge very rapidly, although regularization was still required in order to keep the models from taking on an oscillatory character.

There seemslittle doubt that practical 3-D inversion is within reach, given these developmentsin approximate inversion coupled with simplified forward modeling. The major problem in the future will be collecting enough data, in the right locations. Effective

Strike

Direction

Defining regional trends is an important objective of many geophysicalreconnaissancesurveys.Estimation


704

Vozoff

of strike direction is straightforward in many MT surveys through the combination of tensor and tipper rotation angles. However the estimation becomes a problem as soon as any significantthree-dimensionality enters. Certainly many casescan be found in which it has not been possibleto uniquely specify strike, as illustrated in our example at the start of this section. For example, as discussed further, 3-D statics in (otherwise) 2-D conditions cause offsets which alter

the relative valuesof Pxyand Pyxand causethe impedance rotation to choose the wrong curve. This can be rectified

if a reasonable

statics correction

can

be made. When the three-dimensionalityis more profound, the meaning of strike must be reconsidered. There have been two approaches to solving the problem. Conventional definitions can be applied to 3-D

models

and their

usefulness

can be examined

graphically. Jones and Vozoff (1978) examined tensor and tipper strike directions for a suite of 3-D models and concluded that, for the limited set of models studied, tensor directions were marginally more stable

than tipper directions. However Wannamaker et al. (1984b), for a different model, concluded just the reverse. They also show that tipper directions recover with decreasingfrequency in the same way as impedance phase, and so they are expected to be a better interpretive tool. Gamble et al. (1982) discussthe general problem of defining strike in 3-D geology. They then test the definitions

in a set of field data which

includes

exten-

sive areal coverage. The area is known to be strongly 3-D in placesbut is expectedfor geologicalreasonsto have an underlying linear regional grain. By averaging the tipper and impedance strike directions in all frequency bands from all sites, they obtained a direction which was within a few degrees of the expected regional strike. They also concluded that the impedance directions were the more stable of the two. Zhang et al. (1986) use the formalism of Wannamaker et al. (1984a, b) to study data from an area in which there are two strike directions, one superficial and one regional. They show that the local (superficial)can be found by rotating the impedance tensor until the two terms on the diagonal are linearly related (i.e., have the same phase). Similarly, the regional strike direction is defined when the terms in each column have the

same phase. Least-squares procedures are given for computing the two directions, which are frequencydependent. Like other rotation anglesderived from Z, these have a 90 degree ambiguity. These results are in agreement with those of Groom and Bailey (1989) for the same model.

More experience with these new analyses must be acquired, but the best way to fit strike direction, if one exists, may be near a satisfactory solution.

Statics Compensation

Causes of the statics problem were described in a previous section. Their compensationis an interpretive matter since, by definition, the data themselve are inadequate. The awareness of static shifts has emerged as experience accumulated with closely spaced sites. The shifts are analogousto the static time delays in reflection seismology.Apparent resistivity curves at closely spacedsitesare sometimesfound to look very similar, but are displacedvertically alongthe Pa axis. Alternatively, the TE and TM curves at a single site may look the same except for a vertical displacement. In each case the phases may be indistinguishable.This condition is attributed to superficial features that multiply apparent resistivity curves by a frequency-independent constant. Some attempt must be made to identify and compensate for these or the interpretation of deeper features will be adverseley affected. An analytic demonstration of the effect was developed in Wannamaker et al. (1984b) and was discussed previously. Simply using 1-D inversion in such cases

givesmodelresistivities toolargeby Pij/Planddepths

toogreatbyV'Pij/Pl. (RefertotheGeneral 3-DCase Section.) When the superficial feature is 2-D with strike in the x direction, then no surface charge distri-

butionis createdandPxxis zero. Then a 1-D inversion

of Pxy(EII to strike)willgiveresultslessdistorted than the Pyx(E _1_) component, at leastat low frequencies. For example, if Pa is shifted vertically by a factor of two, then the interpreted resistivities will be too large by a factor of two and the thicknesseswill be too great

byafactorofV•. Statics inMT arediscussed inAdam (1976), Sternberg et al. (1985), Berdichevsky and Dmitriev (1976), Ranganayaki and Madden (1980), Larsen (1977), Jones (1988), and Orange (1989), among others.

A number of solutionsto the statics problem have been suggestedto date, but none have proven completely satisfactory. For a time, it was common practice simply to shift curves to a common high frequency asymptote based on any defensible assumption. Larsen (1975) developed a rational argument for this approach, using it to correct MT soundingsin Hawaii for the effect of the surrounding ocean, and as noted previously, Berdichevsky and Dmitriev (1976) produced semi-analyticaldc models to provide guidelines for shifting the curves.

The quantity Pa (or Pdet)of Ranganayaki (1984), referred to in the General 3-D Case Section, was

introduced by Berdichevsky and Dmitriev (1976) to simplify data presentation. Since the quantity is independent of rotation it avoids data shifts arising from noisy rotation angles,and henceis a kind of smoothing



operator. However, it does not remove statics effects, as can be seen in Figure 37.

Several papers on MT statics correctionsgiven at the Society of Exploration Geophysics meetings in recent years, particularly in connection with CSAMT surveys, tend to follow those approaches used for seismic data. Statics problems will be more severe in CSAMT than in MT because, among other reasons, they can be set up by irregularities at either the transmitteror the receiver. Zonge and Hughes (1991) in the CSAMT chapter in this volume describe several empirical techniques for spatially smoothing those data. Warner et al. (1983), using data from many closelyspacedsites,calculatedan averageof log @aat each site and then adjusted each curve so that these averages varied smoothly between sites. The adjustment was justified on the basis that sites were only a few hundred meters apart in a relatively undeformed sedimentarybasin. Their results,presentedin the form of their N-B inversions, are said to accurately depict known subsurfaceconditions. Andrieux and Wightman (1984) describe the use of independent electrical information

to fix the near

surface

resistivities.

In

particular, they suggestthe use of shallow resistivity logs, transient EM, or dc resistivity. Sternberg et al. (1982, 1985)discussboth spatialsmoothingand the use of independentinformation. They show an example in which a transient EM central loop soundingwas used to shift the MT @acurve. The inversionof the shifted curve fitted the resistivity log much more closely than that of the original curve. Bloomquist et al. (pers. comm.) used an approach very similar to the one described in Sternberg et al. (1985) involving joint inversion with dc resistivity measurements.This approach can be quite satisfactoryprovided the region of overlap is isotropic, otherwise the two setsof data will be incompatible and no sensible result can be expected. Kurtz (pers. comm.) in interpreting a regional surveyfrom Vancouver Island, forces depth to a major horizon to vary smoothly and accommodatesthe data with a smooth variation in the resistivity of that horizon.

705

improvementswere made in the appearanceof resistivity depth sections (Bostick, 1986b). Torres-Verdin (1985) demonstratesthe rationale of the approach in the case of 3-D resistivity perturbations with very small contrast. Figure 70 is an example, kindly provided by Prof. Bostick, of apparent resistivity-frequency sections in N-B format, before and after EMAP filtering. The data are then interpreted by conventional modeling. Further examples of EMAP results can be found in Word et al. (1986) and Shoemaker et al. (1986, 1989). Some major assumptionsmade in the development of EMAP include the assumptions that the traverse line is perpendicularto strike and that adequateinterpretation can be done with only one component(TM) of the E field, so that its quantitative validity remains an open question. Another limitation of the method is its inability to deal with depths greater than a fraction to the total length of the traverse, so that difficulty would be anticipated with smoothingwhen the upper part of the section is highly resistive. Investigations of the deep crust and upper mantle, one of the most productive applications of MT, would seem to be out of the question. In its defense, EMAP has in some places greatly improved the quality of the results obtained.

Finally, Bahr (1985, 1987) studies several ways of decoupling local (i.e., upper crustal) and regional (deep crust upper mantle) effects in impedances.One way involves determining magnetic transfer functions

at Sq periodsby meansof a Geomagnetic Depth Sounding profile between the site of interest and another

site which is free of local distortion.

If a 2-D

conductivity model can then be found which satisfies both the MT impedances and the GDS profile, the problem can be said to be solved. Bahr also suggestsa tensor distortion technique based on Wannamaker et al. (1984 a and b) similar to those used in Larsen (1977) and others, in Zhang et al. (1986), and in Groom and Bailey (1989) as discussed above. It is notable that Gamble et al. (1982), Bahr (1985, 1987), Bostick (1986b) and Park and Torres-Verdin (1988) invoke external data (i.e., data other than impedance from a single location) to help solve the problems of statics

The most elaborate of the methods thus far put forward to overcome the problem of static shift is EMAP. Bostick (1985, 1986a, 1986b) and TorresVerdin (1985) contend that E field fluctuations due to shallow structurecan be filtered out with proper data acquisition and processing.To achieve this filtering, data are collectedfrom contiguouscolinear dipoleson lines tens of kilometers long, usually with only one pair of reference orthogonal magnetometers in the neighborhood.Impedances are smoothedspatially at eachfrequencywith low passfilters whosewavelength is of the same order as the skin depth. In examples

enoughdata that interpretation in critical areas can be

usingboth modelsand real field data, very impressive

made with confidence.

and strike

determination.

This

is in accord

with

the

conclusion of Groom and Bailey (1989), that there is no internal solutionto the staticsproblem from the MT data at a single location. Thus at present there are several ways to remove the effects of statics from impedance tensors, and it will be necessaryto gain some experience with them in a variety of environments to see what each has to contribute.

However

the best

solution

is to collect


706

Vozoff

100-

10

.01

10

20

3O

DISTANCE

40

5O

4O

5O

(Miles)

100

10

>(..3 z LU

O

.01

10

20

30 DISTANCE

(Miles)

Fig. 70. EMAP data, with raw section above and spatially-filtered section below (Bostick, pers. comm.)

The MagnetotelluricMethod Case Histories

Unlike the situation a few years ago, there are now


a reasonable number of instructive case histories in the

literature, and othersin preparation,in additionto the one at the start of this section. The Society of Exploration GeophysicistsReprint volume, Magnetotellurics (Vozoff, 1986) includesa case historiessection. Still others are found in Geophysics, Geophysical Prospecting,ExplorationGeophysics,the Geophysical Journal, the American Association of Petroleum GeologistsBulletin, the CanadianJournalof Earth Sciences,in the extendedabstractsfrom the SEG and ASEG meetings, in the geothermal literature, and elsewhere in this volume. As well, numerous nonpro-

prietarysurveyshavebeencarriedout and are available commercially.

707

areas with local and regional conductivity anomalies: Ph.D thesis, Univ. of Gottingen. •, 1987, Interpretation of the magnetotelluricimpedance tensor: Regional induction and local telluric distortion: J. Geophys., 62, 119-127. Bahr, K., and Groom, R. W., 1990, Corrections for near surface effects: Tutorial paper, 10th Workshop on EM Induction in the Earth, IAGA WG-I2.

Bailey, R. C., 1970,Inversionof the geomagneticinduction problem:Proc. Roy. Soc., 315, 185-194. •, 1977, Electromagneticinduction over the edge of a perfectly conducting ocean: The H-polarization case: Geophys. J. Roy. Astr. Soc., 48, 385-.392. Bannister, P. R., and Williams, F. J., 1974, Results of the August 1972WisconsinTest Facility effective earth conductivity measurements:J. Geophys. Res., 79, 725-732. Bendat, J. S., and Piersol, A. G., 1971, Random data: Analysis and measurementprocedures:John Wiley & Sons.

Berdichevsky, M. N., and Dmitriev, V. I., 1976, Basic principlesof interpretationof magnetotelluriccurves, in Geoelectricand geothermalstudies:Adam, A., Ed., Akademini Kiado, 165-221.

ACKNOWLEDGMENTS

For their help, of severalkindsand often in considerableamounts,I am indebtedto Bob Anderson,Dick Bailey, Rod Bashford,Francis Bostick, Ted Bowen,

Nigel Edwards,CarlosFlores,Ron Goss,Phil Hallof, Hu Wenbeg, George Jiracek, Alan Jones, Khader Khan, Ron Kurtz, Lou Lanzerotti, Jerry LaTorraca,

Liu Song, Frank Morrison, Ed Nichols, Steve Park, Bob Parker, Ken Paulson, Bill Pelton, Risto Pirjola, Dave Redman, Dave Rostoker, Ken Roussel, Joe

Sevenhuysen,Doddy Sutarno,Charley Swift, Carlos Torres-Verdin,J. Ver½,WangJia-ying,GordonWest, Peter Wolfgang,Darrell Word, EugeneYee and Ken Zonge. The AMT data of the InterpretationSection werecollectedaspart of an AMIRA-sponsoredproject at MacquarieUniversity.The hospitalityprovidedby the GeophysicsDivision, PhysicsDepartment,Universityof Toronto madepossiblea pleasantand productive stay. That of the Geophysics Department, China University of Geosciences,Wuhan, forced me to read and clean up what I'd written in Toronto. Finally, I do sincerelyappreciateMisac Nabighian's patience. REFERENCES

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14201-14213.



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•,

1977, Resolving anisotropy in layered media by joint inversion: Geophys. Prosp., 25, 460-470. Karmann, R., 1975, Optimization of the signal/noiseratio of inductancecoil magnetometersfor magnetotellurics.Thesis, Doctor of Engineering, Brauschweig Technical University. •, 1977, Search-coil magnetometerswith optimum signal-to-noise ratio. Acta. Geodaet. Geophys. Mont., 12, 353-357.

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REFERENCES

FOR

GENERAL

READING

Allan, W., and Poulter, E. M., 1984, The spatial structure of different pulsation types: A review of STARE radar results: Rev. Geophys. Space Phys., 22, 85-97. Bloomquist, M. G., and Warner, B. N., 1983, Magnetotelluric survey of the Pescadito Dome near Laredo, Texas: 53rd Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded abstract RW3.3,643-646. Campbell, W. H., 1965, Equatorial studiesof rapid fluctuations in the earth's magneticfield: in de Mendoqa, F., Ed., Proc. 2nd Int'l. Symp. on Equatorial Aeronomy, Internat. Union Geodesy and Geophys., 495-511. Cerv, V., and Pek, J., 1981, Numerical solution of the two-dimensional inverse geomagneticinduction problem: Studia geoph. et geod., 25, 69-80. Clarke, J., Gamble, T. D., and Goubau, W. M., 1978, Squids and magnetotelluricswith a remote reference: in Symposium on applicationsof superconductingquantum interference devices, Univ. of Virginia. Clerc, G., 1970, Shallow magnetotelluric prospecting using electromagnetic variations at frequencies over the range between 8 Hz and 16 kHz: C. R. Acad. Sc., Paris, 270, 645-648 (in French). Fischer, G., Schnegg, P.-A., Peguiron, M., and LeQuang, B. V., 1981, An analytic one-dimensional magnetotelluric inversionscheme:Geophys.J. Roy. Astr. Soc., 67,257-278. Grillot, L. R., 1975, Calculation of the magnetotelluric tensor impedance: Analysis of band-limited MT signal pairs: Geophysics, 40, 790-797. Jacobs,J. A., 1970, Geomagneticmicropulsations:Springer Verlag. Levy, B.C., 1985, Layer by layer reconstruction methods for the earth resistivity from direct current measurements: Inst. Elect. and Electron. Eng. Trans. Geosci. Remote Sensing, GE-23, 841-850. Matsushita, S., and Campbell, W. H., 1967, Physics of geomagnetic phenomena: Academic Press. Parker, R. L., 1977, Understanding inverse theory: Ann. Rev. Earth Planet. Sci., 5, 35-64. Parker, R. L., 1980, The inverse problem of electromagnetic induction:

existence

and construction

of solutions

based

on incomplete data: J. Geophys. Res., 85B, 4421-4428. Parker, R. L. and Whaler, K. A., 1981, Numerical methods

for establishingsolutionsto the inverse problem of magnetic induction: J. Geophys. Res., 86B, 9574-9584. Rokityansky, I. I., 1982, Geoelectromagnetic investigation of the earth's crust and mantle: Springer-Verlag. Schmucker, U., 1980, Induktion in geschichten Halbrfiumen durch inhomogene Felder, in Haak, V. and Homilius, J., Eds.: Protokoll fiber das Kolloquium Elektromagnetische Tiefenforschung, Berlin, (unpublished). Weidelt, P., 1985, Construction of conductance bounds from magnetotelluric impedances:J. Geophys., 57, 191-206.




CHAPTER

CONTROLLED

9

SOURCE AUDIO-FREQUENCY

MAGNETOTELLURICS

Kenneth L. Zonge*andLarryJ. Hughes g/X/•m,whentheseparation between thetransmitter

INTRODUCTION

Controlledsourceaudio-frequencymagnetotellurics (CSAMT) is a frequency-domainelectromagnetic soundingtechniquewhich uses a fixed groundeddipole or horizontal loop as an artificial signal source. CSAMT is similar to the natural-sourcemagnetotellurics (MT) and audio-frequency magnetotellurics (AMT) techniques;the chief differencescenteraround the useof the artificialCSAMT signalsourceat a finite distance. The source provides a stable, dependable signal,resultingin higher-precisionand more economical measurementsthan are usually obtainablewith natural-source measurements in the same spectral bands. However, the controlled source can also com-

plicate interpretationby addingsourceeffects,and by placingcertain logisticalrestrictionson the survey. In most practical field situationsthesedrawbacksare not serious,and the methodhas proven particularlyeffective in mapping'thetop 2 to 3 km of the earth's crust. The CSAMT sourceusually consistsof a grounded electric dipole about ! to 2 km in length, ideally

dipole and the receiver station is greater than 4g, althoughwe have found that maximumdepth of investigation is limited in many casesto about 3 km due to the practical constraintsof the measurements.Lateral resolution is controlled by the electric field dipole length, which normally is between 10 and 200 m. Vertical resolutionis .5 percent to 20 percent of the depth of penetration, dependingupon resistivity contrasts.

Since its introduction in the mid-1970s, CSAMT has

been used in exploration for petroleum, geothermal resources,massivesulfides,base and preciousmetals, structure, lithology, and sourcesof groundwatercontamination.Much of this work hasbeen quite successful due to the inherent capabilitiesof the technique. However, much remains to be learned about CSAMT

interpretation, particularly in regard to static and sourceeffects.By exploringthe presentunderstanding of the theory and practice of CSAMT we hope to provide a basis for further development.

located atleastfourskindepths (4g= 2012X/O/o/f; p= resistivity,f = signalfrequency)from the area where soundingsare to be made. Measurements are made within the 0.1 Hz to 10 kHz frequencyband. Magnitude and phase are measured for two to five electric

Objectives

andmagnetic fieldcomponents (Ex,Ey, Hx, Hy, Hz), using either one or two sources. Grounded dipoles detect the electric field and magnetic antennassense the magneticfield. The ratio of orthogonal,horizontal electric and magneticfield magnitudesyields the apparentresistivity. The differencebetweenthe phaseof the electric and magneticfieldsyields the phaseof the impedance.In tensor measurements,these quantities may be treatedby standardMT processingtechniques. CSAMT depth of investigationis roughly equal to

In recent years the controlled source audio-frequency magnetotellurics(CSAMT) sounding technique has gained acceptanceas a viable geophysical exploration tool. Its high data quality, good lateral resolution, cost-effectiveness, and relative ease of

interpretation have made CSAMT an effective technique for a variety of energy-explorationand geotechnical investigations.Unfortunately, development of the techniquehas been limited by a shortageof information on its practical application. Because most interpretational skills and case histories are still held

*Zonge Engineering& ResearchOrganization,Inc., 3322 East Fort Lowell Road, Tucson, Arizona 85716.

*TerraGraphics, 1016CrestViewCourt,Kingston Spring, TN 37082: Formerly Zonge Engineering. 713


714

Zonge and Hughes

as proprietary by CSAMT contractorsand exploration companies, a single comprehensive review of the method has not been published in the literature. Our primary objective is to present a broad, practical review of the CSAMT technique. Specifically, we seek to establish an understandingof the phenomena being measured, and to provide a practical guide for making the measurementsand interpretingthe results. We have included a number of field examplesto show how the CSAMT technology is applied to common exploration problems.

Description of the CSAMT Technique

CSAMT is a frequency-domain electromagnetic (EM) sounding technique used to establish a threedimensional (3-D) apparent resistivity map of the earth. The technique uses a fixed, groundeddipole or horizontal loop as the energy source. Orthogonal electric and magnetic field componentsare measured, ideally in the plane-wave portion of the field far from the source (Figure 1). Apparent resistivities are obtained by taking the ratio of the perpendicular,horizontal electric, and magnetic field magnitudes. The difference between the electric and magnetic field phase angles provides impedance phase. Measurements are usually made in the frequency range of 0.1 Hz to 10 kHz.

CSAMT is most effective at detecting and mapping resistivity contrastsin the top 2 to 3 km of the earth's surface. The data can provide information on twodimensional (2-D) and three-dimensional (3-D) structure, lithology, mineralization, electrochemical alteration, ground water changes, and permeability.•

SOURCE Transmitter

•

(i-3 km long)

SOUNDING R•e'i•e'r Digital • •(>4S) 5-10km (10 to 300rn long) Hxl

Eyl

Hxg

Ey2• Measured

•

(Ma9nlfude• & Phase)

Hzl

•xl 2 yl

Hy2

Hz2

Fig. 1. Field set-up for a tensor CSAMT survey, which measuresfive electric and magnetic field componentsfrom two source polarizations. A common variation is scalar

CSAMT,whichmeasures ExandHy froma singlesource. In

some circumstancesloop sourcesmay be used instead of a grounded dipole.

Historical Developmentof CSAMT

Electromagnetic sounding techniques have been used in geophysicalexploration since the early 1900s. As summarized in Spies and Frischknecht (this volume), these techniques evolved slowly during the 1930s and 1940s both in the West and in the Soviet

Union. Theoretical effortsby Wait and others, coupled with improvementsin instrumentation, brought about a resurgenceof EM methodsin the 1970s.Today, time and frequency domain EM techniques account for about 70 percent of worldwide expenditures on electrical geophysics(Montgomery, 1986). One of the milestonesin EM work was the independent developmentof the magnetotelluric(MT) method in the 1950s by Cagniard (1953), Kato and Kikuchi (1950), Rikitake (1950, 1951), and Tikhonov (1950). The low frequencies (down to 0.001 Hz) used in MT work advanced

the MT method as a means of achiev-

ing penetration depths to hundreds of kilometers. Spurred by major advances in instrumentation and data processingcapabilities, MT has developed into an important tool for evaluating deep structure in petroleum and geothermal exploration. Audio-frequency magnetotellurics (AMT), an extension of MT to the audio-frequencyrange (10 Hz to 10 kHz), developed during the 1960sas a shallower-penetratingtool. The primary drawback of MT and AMT is the erratic signal strength. Both techniquesmeasure naturally occurring earth tellurics induced by solar-related ionospheric activity and worldwide thunderstorms. The telluric signals are weak and often vary considerably in strength and source direction. In the caseof MT, weak and variable signalsresult in typical stacking times of 5 to 10 hours per site, making soundingsquite expensive. AMT signalsare acquired faster, due to the higher frequencies measured, but variability of local thunderstorm sources and signal attenuation around 1 Hz and 2 kHz can compromise data quality. In an effort to solve the signal strengthproblem, an artificial signal source was proposed by David Strangway and Myron Goldstein at the University of Toronto (Goldstein, 1971; Goldstein and Strangway, 1975). Their work formed the theoretical and practical basis of the CSAMT

method.

CSAMT

was tried intermit-

tently by severalexploration companiesand by at least one contracting company (Geotronics Corporation) in the mid-1970s, primarily to map massive sulfides. In 1978 a full-scale development of CSAMT instrumentation and interpretation, begun by Zonge Engineering and Research Organization, Inc., (Zonge et al., 1980, 1985; Ostrander et al., 1983; Hughes, 1984; Hughes et al., 1984, 1986; Hughes and Carlson, 1987; Zonge and Hughes, 1985; Carlson et al., 1985, 1986;


CSAMT

Hughes and Maas, 1987) involved the acquisition and interpretation of several thousand line-kilometers of data. Much practical information presented here is based upon this work. A number of other contracting service companies, exploration companies,and governmentagencieshave also introduced CSAMT capabilities. These include:

Agencies in Finland (Lakanen, 1981, 1986; Hjelt et al., 1986),

Southwest Research Institute (Duff and Lepper, 1980),

Sandia National Laboratories (Bartel, 1982; Bartel and Dobecki, 1982; Wayland et al., 1984; Wayland and Leighton, 1985; Leighton and Wayland, 1987; Bartel and Jacobson, 1987),

University of Utah (Sandberg and Hohmann, 1982; West and Ward, 1988),

Bureau de Recherches Geologiqueset Minieres or BRGM (Valla, 1982; Valla and Gasnier, 1984), Phoenix Geophysics, Ltd. (Yamashita and Hallof, 1985; Yamashita et al., 1985), Japanese government (Yokokawa, 1984; Uchida et al., 1987, Ikeda, 1987), Aberfoyle Resources Ltd. (Eadie et al., 1987), and Cominco American (Van Blaricom and O'Connor, 1987).

Applications

Although CSAMT has been applied extensively to mineralsexploration, few resultshave been published. Some published mining-related natural-source AMT work includes the paper by Strangway et al. (1973), and work by Scandinavian researchers reported in Lakanen (1986). Massive sulfides are a common AMT and CSAMT target due to the conductivenature of the mineralization.

Limited

CSAMT

results

have

been

shown over the Crandon orebody in Wisconsin (Zonge et al., 1986), over hydrothermally altered and sulfide targets (Ostrander, 1981), and over a massive sulfide body in Japan (Yamashita and Hallof, 1985). Van Blaricom and O'Connor (1987) present results of CSAMT surveys over the lead/zinc/silver Red Dog deposit in Alaska. A uranium and a sulfide/graphite target was examined in Yamashita and Hallof (1985). Lakanen (1981, 1986) and Hjelt et al. (1986) present both AMT

and CSAMT

data over various base metal

targets in Finland and Lakanen (1986) gives a good review of this work. A comparison of AMT and CSAMT is also available in Strangwayet al. (1983) and in Yamashita and Hallof (1985). Uchida et al. (1987)

715

obtained tensor CSAMT data over a copper/lead/zinc mine in Japan, and Lawler and Vadis (1986) used CSAMT to map mineral-rich geology. CSAMT has been used successfully in precious metals exploration, mostly for alteration and structure mapping. Yokokawa (1984) describes some of the earliest hydrothermal gold exploration carried out in Japan over the Hishikari mine. Austpac Gold (1987) announced success in mapping silicified epithermal gold zones in New Zealand. We have obtained considerable data over prospects and existing mines, but unfortunately very little data has been released or published in the general literature. Geothermal development has been another major objective of AMT and CSAMT work. Numerous natural-source AMT studies have been conducted by the U.S. Geological Survey and others (see Wright et al., 1985, for an excellent review and bibliography). Controlled source surveys also have become increasingly common. Sandberg and Hohmann (1982) describe CSAMT

measurements

over

the

Roosevelt

Hot

Springs geothermal field in Utah, and Hughes and Maas (1987) present the results of CSAMT mapping and reservoir monitoring over that same field. Yamashita et al. (1985) and Yamashita and Hallof (1985) describe

work

done

over

the

Hatchobaru

field

in

Japan. Bartel and Jacobson (1987) use CSAMT for determining the source and geometry of the Puhimau field in Hawaii. The approaches used in geothermal studies include mapping reservoir fluids, mapping thermally-caused resistivity anomalies, evaluating ground water mixing, mapping subsurface structure, delineating hydrothermal alteration, and monitoring reservoir changes. In petroleum exploration, CSAMT has been used for mapping structure related to oil entrapment (Hughes et al., 1984; Yamashita and Hallof, 1985; Hughes and Carlson, 1987). In general, the technique is most useful for reconnaissancemapping prior to final detailing with reflection seismics, but it is also useful in areas where seismic is not effective, such as over basalts. Some attempts have been made to use CSAMT for mapping petroleum-driven electrochemical alteration of shallow sediments (Ostrander et al., 1983; Hughes et al., 1984; Ostrander et al., 1984; Carlson et al., 1985, 1986). CSAMT has proven effective in investigatingground water quality. Published examples include mapping brine leakage from improperly plugged injection wells (Fryberger and Tinlin, 1984; Syed et al., 1985; Zonge et al., 1985; Hughes et al., 1986; Tinlin et al., 1988). We have also used the method for mapping brine plumes from leaking tanks, pipelines, and waste-containment ponds. These data have been used effectively


716

Zonge and Hughes

in litigation concerning groundwater contamination problems. Geotechnical applications have abounded in recent years. Some typical examples include structural analysis in mine planning (Bartel and Dobecki, 1982), detecting voids in underground mines (Duff and Lepper, 1980), mapping burn fronts in underground coal gasificationprojects and coal mine fires (Bartel, 1982; Bartel and Dobecki, 1982), and monitoring enhanced oil recovery operations (Bartel, 1981; Wayland et al., 1984; Wayland and Leighton, 1985; Leighton and Wayland, 1987). We have used CSAMT for mapping spilled petroleum products at refinery sites and for monitoring leachate solutionin in-situ copper recovery projects. CSAMT is a relatively new and undeveloped technique and the applications listed will surely be expanded in the near future. One obvious extension of the technique is to use CSAMT in a surface-to-underground or surface-to-borehole configuration. A recent theoretical paper, West and Ward (1988), indicates that surface-to-borehole

CSAMT

measurements

could

prove useful in mappingsubsurfaceconductivezones. Surface-to-undergroundmeasurementscould possibly be used for monitoring underground in-situ leaching operations in existing mine operations. Development of many geophysical and geotechnicalapplicationsis anticipated in the coming years. CSAMT

respect to the source. Finally, we present the basic formulation for inhomogeneousearth situations. The developmentassumesa finite sourcewith a time

harmonic oscillation eitøt , in whichtois the angular frequency. The coordinate systems (Figure 2) are selectedsuch that the vertical z-component is positive downward. Note that the cylindrical coordinate vector normally written as "p" is labeled "r" in this paper, in order to avoid confusion with the symbol for resistivity. Table 1 defines the main symbolsand SI units used in this discussion.

Maxwell's Equations

An electromagnetic(EM) field can be described in terms of four field parameters: E

Electric field intensity (V/m)

D Electricfluxdensity(C/m2) It

Magnetic field intensity (A/m)

B Magnetic fluxdensity(Wb/m2). The fundamental, empirical equations governing EM field behavior in a source-free region are the well known Maxwell's equations: 0B

I7 x E =

(1)

VD

THEORY

I7 x H = J +

Introduction

Basic theoretical principles for natural-sourcemagnetotelluric (MT) soundingsare now well developed and understood (see Vozoff, 1986, for an excellent compilation of key papers on MT theory and practice and Vozoff, this volume). The theory for the MT case is conveniently formulated due to the assumedsource independenceof the EM field. But the introduction of a distinctly polarized 3-D source at a finite distance from the soundingpoint complicatesthe system. Because the finite source problem has been addressed by a number of authors (e.g., Ward and Hohmann, Chapter 4, Volume 1, 1988; Wait, 1961, 1962a, b, 1982; Bannister, 1966; Kaufman and Keller, 1983) the mathematical development is not duplicated here. Instead, we state the main results as they relate to CSAMT and derive a good physical understanding of the phenomena observed in the field. The following discussion presents the homogeneous-earth solutions to the vector wave equation for horizontal electric and vertical magnetic dipole sources. We examine at some length the CSAMT response as a function of sounding location with

(Faraday's Law)

Ot

V. D=qv I7 ß B = 0

Ot

(Ampere's Law)

(Coulomb'sLaw) (Continuous flux law).

(2) (3)

(4)

In these and the following equations, •r is the medium's conductivity (Siemens), e is its electric permittivity (Farads per meter), Ix is its magnetic permeability (Henrys per meter), J is the electric current Cartesian Coordinates

Cylindrical Coordinates

z

Fig. 2. Coordinate system nomenclature. The symbol for the radius vector in cylindrical coordinates, which is normally "p", has been changedto "r" in this paper to avoid confusion with resistivity.

CSAMT

717

density, and qv is the electric charge density. The


charge density is related to current density by the continuity equation: V. J+

Oqv ot

=0.

(5)

The field quantities are related by the constitutive relations, valid for a homogeneous, isotropic earth (scalar e, Ix, and cO' D = eE

(6)

B = •H

(7)

J = ere

(Ohm's law).

(8)

Resistivity, expressed in units of ohm-meters, is the reciprocal of conductivity:

p = 1/cr.

(9)

The Wave Equations for HomogeneousEarth As shown in Hohmann and Ward (Volume 1), the fields E and H satisfy the differential equations'

V2E + k2E = 0

(10)

V2H + k2H = 0.

(11)

and

The propagationconstant or wave number k is defined by

k2= Ixeoo 2- ilxcroo = Ixoo(eoo - icr)Re (k) > 0 (12a)

Table 1. Summary of Field Quantities and Units Units

Unit ohm ohm-meter

Ampere Siemen Volt Weber Coulomb

Henry Farad

Symbol

Definition Electrical

f•'m A S V Wb C H F

Equivalent Units resistance

Electrical resistivity Electrical

current

Electrical conductivity Voltage Magnetic flux Electric charge Magnetic inductance Capacitance

V/A V.m/A V/fl A/V.m W/A V's A.s l•.s

c/v

Field Quantities E D H B

Electric field intensity(V/m) Electric displacement(C/mQ Magnetic field intensity (A/m) Magnetic induction (Wb/m)

J

Electriccurrentdensity(A/•2)

qv V I Z

Electric charge density (C/m') Voltage drop (V) Electric current (A) Complex impedance (I•)

Media Properties P

Electrical conductivity (S) Electrical resistivity (I•.m) Electric permittivity (F/m)

k

Magneticpermeability(H/m) Propagationconstant(m TM) Phase constant (m -')

Attenuation constant (m- l) v

Skin depth (m) Phase velocity (m/s) Wavelength (m)

Other Quantities t

f t'

Time (s)

Angular frequency (rad/s) Linear frequency (Hz or cycles/s) Distance (m)

Constants E0 c

Free space electric permittivity Free space magnetic permeability Speed of light in vacuum

8.854 x 10-•2 F/m 4,r x 10-7 H/m 3.00 x 108 m/s

718

Zonge and Hughes

k2= _•/2= -itxco(cr+ icoe)Im (k) < 0.

E

Z •


(12b)

The first term (•o) in the parenthesesof equation(12a) is the displacementterm, which dominatesthe relation at high frequenciesand in a nonconductivemedium. The second term (•r) is the conductionterm, which dominates when frequencies are low and when the medium is relatively conductive. The dominance of the conductionterm over the displacementterm holds for most earth materials at CSAMT frequencies. Following the conventionof Ward and Hohmann (1988, Chapter4, Volume 1), the propagationconstantcanbe written in complex form as

k = o•- i• = [-i•m(• + im8)]1/2

(13)

in which the phas• constant• is gNen by

• = •

1+

+ 1

(22)

This definitionwill be usedin the following sectionsin solvingfor resistivity. Quasi-static Limit ({r >>eto).•It

is useful to examine

two extreme limits of the wave solutions' the quasistatic limit and the dielectric

limit.

We look first at the

quasi-staticlimit, which is of most interestto geophysical applications. Earth materials usually have a conductivity cr >

10-4 S (p< 104 D-m)anda permittivity ofe < 10-ll F/m. For frequenciesbelow 100 kHz, cr >>e•o,and can be neglected,which is the quasi-staticapproximation. In this casewe have o• = [3and the propagation constant simplifiesto

k=(1-i)•tx• tø

(14)

ß

(23)

For the horizontal electric field componentEx, with

and the attenuationconstant • is gNen by

harmonic timedependency ei•øtimplicitlyassumed, •=m

1+

-1

.

(15)

we have for a horizontal plane wave propagating downward along the z axis

E• =Eo•e -iaze - •z .

Skin depth • is definedby

• = 1/•.

(16)

Equation (24) can be written usingo• = [3as Ex = E oxe -i v'•mø/2Ze - v/•mø/2z .

The wavelength of the signal is

k = 2•,

(17)

and the propagationvelociW is given by

•gular by:

(19)

For a horizontal plane wave propagatingdownward alongth• z •s in a homogeneous•arth, th• solutions to wav• •quations(1) and (2) ar•:

R = Roe-iaZe i•t = Roe-i•e-•Zei•t

(20)

• = Hoe-ikze i•t= Hoe-i•e-gZei•t.

(21)

Eo and Ho ar• th• m•imum •l•ctric and magnetic fi•ld strengths,r•sp•ctiv•ly. In th•s• •quations, not• that the amplitudedecaysin conductiw media according to th• attenuation constant •, whil• th• phas• reference of the fields depends upon the phase constant

rewritten

as

(26)

At a depth equal to the skin depth (z = •), we obtain

frequency m is r•lat•d to signal frequencyf

m = 2•f.

(25)

With the definitionof g = 1/[3, equation (25) can be

E• = Eo•e-i •/ae-:•/•.

Vp

(24)

•.

Wave impedance is defined as the ratio of the orthogonal componentsof the E- and H-fields:

E•: = Eo•:e-ie-1

(27)

and the real part of the electric field is attenuatedby 1/e or 63 percent of the original field strength. The magneticfield has an identical behavior. In the quasistatic approximation,equation(16) reducesto

a=

•rto'

(28)

Withix= tx0= 1.256x 10-6 H/m, andconverting to frequencyin Hertz (•o = 2,rf), equation(28) can be written in terms of resistivity as

• 503V• =

m.

(29)

The equivalent depth of investigationD of a plane wave can be derived from asymptoticrelationsbased on a uniformly layered half-space(Bostick, 1977):

CSAMT

D •/•/-•356½ Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

=

=

m.

719

dielectricmedium occurswhen displacementcurrents

(30)

Note that penetrationis dependentupon two parameters' the resistivityof the earth, and the frequencyof the signal being used. Penetration is shallower with decreasing resistivities and increasing frequencies. Conversely, penetrationis deeper with increasingresistivitiesand decreasingfrequencies.By varying the signalfrequency,continuousvertical soundingscan be

dominate conduction currents. In this case

the propagationconstantbecomes

k = toJlxe,

and the skin depth approachesinfinity. The propagation velocity is then 1

Vp - V•'

obtained.

Figure 3 shows the skin depth and the maximum effective depth of investigationfor typical earth resistivities at frequenciesnormally used in CSAMT. A literal interpretationof this plot suggeststhat CSAMT is capableof penetrationas deepas tensof kilometers. In practice, this is not the case. The maximum penetrationis usuallylimitedby sourceeffects(dueto finite separationbetween source and receiver), or lack of usable signal (due to extended separationbetween sourceand receiver). As a rule, CSAMT is mostuseful for exploring within 2 to 3 km of the earth's surface. Under quasi-static conditions, for normal ground resistivities and typical CSAMT frequencies, signal wavelengthswithin the earth are quite large. This is an important fact since direct, reflection-typeresolution is limitedby the wavelengthof the investigatingsignal. Equations(1_7)and (30) showsthat the wavelengthis

always2'rrX/2 largerthantheeffective depthof investigation D. For example, in 10 •.m ground at 4096 Hz, the wavelengthis 156 m and increasesto 10 km at 1 Hz. The large wavelength to depth of penetration ratio demonstratesclearly the impossibilityof achieving direct, reflection-typeresolutionof most subsurface features with CSAMT or any other electrical soundingtechnique. Instead, features are detectedby changesin electricalimpedance.Measurementof electrical impedancevariationsprovide a smoothedimage of subsurfacefeatures when compared to reflection

(32)

(33)

The propagationvelocity equalsthe speedof light for

•x= •xo ande = So(c = 3.0 x 108m/s).Dielectric effectscan be observedin very resistivegroundand at very high signal frequencies,but not generally at the frequenciesusedin CSAMT (f < 10 kHz). Hence the quasi-staticapproximationis good for nearly all earth materials.

Grounded Horizontal Electric Dipole Solution (HomogeneousEarth)

Since CSAMT measurements employ either a grounded electric dipole or a horizontal loop as a signal source, we examine the solutionsto the wave equationsfor both types of sources. The most common

CSAMT

source

is an insulated

wire groundedat each end, which in EM terminology is referred to as a horizontal electric dipole or bipole. We examine the case of a dipole of lengthdl, directed along the x-axis, and grounded to the surface of a homogeneoushalf-space.The quasi-staticapproximation (•r >>• and r ,• X0 where X0 is the free-space wavelength)is made throughoutthe discussion.The electric and magneticfield components,expressedin cylindricalcoordinates(with r as the cylindrical-coordinate radius vector and z+ directed upward), are givenin Wait (1961),Bannister(1966),andin Kaufman and Keller (1983) as:

seismics.

I dl cos q>

Er= 2•rr3 [l +e-ikr(1 + ikr)] (34)

The propagationvelocity under quasi-staticconditions is

I dl sin q>

v, =

E, = 2•rr3 [2- e-ikr(1 q-ikr)] (35)

=a.,.

For CSAMT-range frequencies, the propagationve-

Ez

locityranges fromabout103 to 107 m/s.For 10•.m

itxotoI dlcos q>I1

(36)

ground at4096 Hz,forexample, Vp= 6.4x 105m/s, decreasing to 104 m/sat 1 Hz.

Dielectric Limit ({r <• eto).--Most

HI'

2•rr2

311•- K1 •

+2

earth materials

may be consideredconductorsat the frequenciesused in CSAMT. Very resistive materials can behave like dielectrics.The case of a material appearingto be a

(37)

720

Zongeand Hughes

H• = Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

Hz•

I dl cos 2•rr 2 4)I•

31 dl sin 4) 2•rk2r 4

K••

(38)

The I m and K m symbols represent modified Bessel

functionsof the mthorder.Ez is calculatedat z = 0-. z is positive upward in these formulations. It is evident that the relative strengths of these components are dependent upon r, the separation

[1- e-i•'r(1 + ikr- •1k2r2)].

betweenthe sourceand the soundingpoint, and upon

(39)

(a) Skin Depth

•ooo •_ øø , j/

oo,O oooOO [- / øOo% o,•••5//,/ •/-

• •

•

/ /

/

////////

/o.•o•%o•oo%,/// / • •oøooo • •/

////////

/

5o

/

oqo%

•/////

/

/ -

-

/••o•o•o•• / /

/

/////•

g.l•5 0.•5 0.51.0 Signal Freauency (Hz)

(b) Effective Penetration Depth I000

I/

•

-f

• •oø ooO• oS

• Ioo

•o

ß•

• •o o•

•

•oO•• •V

0.125

////////

0.25

0.5

1.0

/

2

4

8

///•

16

32

64

-

// / •o•o• /: / /

128

256

•o•

512

_

1024 2048 4096

Signal FreQuency (Hz)

Fig. 3. (a) Skindepthasa functionof groundresistivityandsignalfrequency. (b) Effectivepenetration depthas a function of resistivity and frequency.

CSAMT


media propertiesand frequency. Media propertiesand frequency determinethe skin depth g. We can visualize the functionaldependenceof equations(34) to (39) as the ratio of source-soundingseparation, r, to skin depth, g. From equation(23) we have

721

skin effect. SoundingsA, B, and C are made at various distances from the source, with the sounding at C

penetrating according to D = g/V/•for a givenfrequency range. Sounding A is made in the near-field zone, where the source field is strongly curved. The

electric fielddecays at a rateof 1/r3 andthemagnetic fieldas 1/r2. The sounding depthandtheresistivity

(40) measured there are a function of array geometry.

SoundingB is made in the transitionzone, where there is moderatewave curvature. The electric field decays

and therefore

Ikrl= (rvt•)/g• r/g,

(41)

where Ikrl is commonlyreferredto as the induction number.

The region electrically near the transmittingdipole

is characterized by smallinductionnumbers(Ikrl4 1, r ,• g) and is known as the "near-field" zone. The region electrically far from the dipole is characterized

ata rateof 1/r3 andthemagnetic fielddecays at a rate between1/r2 and 1/r3. Depthof penetration is a complex function of array geometry and frequency. Sounding C is made in the far-field zone, where the impingingsourcefield approximatesa plane wave, as assumedin magnetotellurictheory. Here the electric

andmagnetic fieldsdecayat thesamerate(l/r3), and depth of penetrationis independentof array geometry.

by large inductionnumbers(Ikrl>>1, r >>g) and is

However, do not confuse these near-field/far-field

known as the "far-field" zone or "plane-wave" zone. The region between these zones is called the "nearfield/far-field transition zone", or simply the "transi-

discussionswith the near-field/far-field usage in antenna theory. In antennatheory, when discussingEM propagationby radiation, far-field is used to indicate the realm where the separation between transmitter antenna and receiver antenna is much larger than the free-space wavelength or r >>Xo. Conversely, the near-field zone is where r ,• Xo. For the quasi-static assumptionsused for CSAMT calculations,near-field and far-field notations take on an analogousmeaning with the substitutionof the wavelength(or skin depth) in conductive earth for the free-spacewavelength.

tion"

zone.

These three zones are illustrated in Figure 4. The source antenna emits a current establishingan EM field which propagatesthrough the ground. At the soundingpoint, E and H are measured. The drawing

showsa scalarEx/Hy measurement, and the signal decay with increasing depth which results from the

D

Near-fieldResponse (r <• •, IkrI <• 1).raThelimitsof equations (34)to (39)for Ikrl 1 canbe determined by using the appropriate approximationsfor the complex Bessel functions and exponentials(Wait, 1961, Bannister, 1966). The resultsare:

gr •

I dl cos • 71-O-1-3

(42)

I dl sin •

E, = 2,r0-r 3 E decaysas I/r •, H as I/rZta I/r 3

EZ

D depends an geametry, frequency and resistivity

i•xotoI dl cos • 471'1.

(43) (44)

I•] FARFIELD (rc>>•)

E, Hdecay asI/r3 D dependson frequency

J• z

I dl sin q>

and resistivity

Fig. 4. Diagrammaticsketchof the electromagneticfield in a homogeneous half-space.Three zonesin the p, ropagating EM 3 2 field: (a) Near field (Cn '• 8), E decaysas 1/r , H as 1/r , D dependson geometry, independentof frequency; (b) Tran-

sitionzone(rB"• 15)E decays as1/r3, H as1/?'to 1/r3, D dependson geometry,frequency,and resistivity;(c) farfield (rc >>•), E, H decayas 1/r3, D dependson frequencyand resistivity.

Hr =

4,rrr 2

(45)

I dl cos •

H, =

4,rr2

(46)

I dl sin •

Hz =

4,rrr 2

(47)

722

Zonge and Hughes

For the near-field case, we can write the wave imped-


ance

I dl cosq> -i•r/4

H,•--2,rr•/lxcrcor3 e (54) dlsin 4)e-i•/2 (55) Hz=- 31 2,rrlxtrcor4

as:

IE, I o'r 2

Z= •

(48)

Note that the phasedifferencebetween E and H in the near field is zero. Equation (48) is usuallywritten in terms of resistivity as /-

E4,

(49)

Equations(42) to (47) showthat for a homogeneous half-space, E is directly proportional to ground resistivity in the near field, and is independent of frequency. In contrast, H is independentof both resistivity and frequency in the near field. Under such conditions, H is said to be "saturated," as it is nonresponsiveto what we wish to measure(variations in groundresistivity)and to how we wish to measureit (by changing the frequency). Since E and H are independent of frequency, so is the impedance, demonstratingthat the data no longerrepresenta sounding in the parametric sense of equation (30). Hence, ground resistivity sounding information cannot be recovered from near-field data by performing "nearfield corrections"

to make

the data look like far-field

data, since the frequency soundinginformationis not in the data.

Note that the near-field apparent resistivity is a

function of r, sinceE andH vary as 1/r3 and1/r2, respectively. This showsthat near-fieldmeasurements and depth of penetration are controlled by array geometry, and not by frequency. Thus near-field CSAMT E-field measurementsare equivalent to those of a dc resistivity survey, althoughwith considerably less logistical efficiency.

Far FieldResponse (r >>•, Ikrl

1).wFar fromthe

dipole source, equations (34) to (39) approach the following limits, with a quasi-staticassumption:

2,rrcrr3

(50)

I dl sin q>

Ez•

2,rr vr•r 2 ei•'/4

sin 4) Hr•I•rdl qlxcrcor 3e-i,rr/4

like it does in the

near-field zone; it is fully responsive to resistivity changesand to signalfrequency. As shown, the horizontal fields in the far-field zone

all decayas1/r3. Thusthewaveimpedance is independent of separation,r:

Z=

=

= •/Ixpco e+i,rr/4. (56)

As a slightdigression,note that the wave impedanceis very small for a conductive earth. For example, groundresistivitiesof 10 fl. m and a frequencyof 1 Hz result in an impedance of 0.003 fl. The impedance rises to highervalues for very resistivematerialsor at high frequencies.For example, with a groundresistivity of 1000fl. m and a signalfrequencyof 10 kHz, the wave impedanceis 8.9 fl. In free space,

Z-

e0C

-

= 377 fl

'

(57)

This low ground wave impedance at low frequencies provides the mechanismfor various waveform propagationmodesat the earth-airinterfaceandbetweenthe earth and the ionosphere. Equation (56) showsthat the far-field apparentresistivity can be determinedby measuringthe perpendicular E- and H-field strengths at a specified fre-

1

1 E, 2

ø- •toIzl2-•-• •rr n.m.

(58)

Note from equation(56) the phasedifferencebetween E and H is x/4 or 785 mrad. This is the homogeneous

'rro'F3

X/Ixco I dl cos4)

Hence, H does not "saturate"

quency:

I dl cos 4)

Er •

These equationspoint up several interestingfacts. For a homogeneousearth the E-field is independentof frequency and is directly proportional to resistivity. The near-fieldand far-field E-field values differ by only a factor of 2 in magnitude. The horizontal H-field componentsare frequency-dependentin the far field and, unlike their near-field counterparts, are also a function of the square root of ground resistivity.

(52) (53)

earth phase response. Identical resultsare obtainedwhen working in car-

tesiancoordinates andsolvingfor theEx/HyorEy/Hx ratios. For example:

1

Ex 2

P-2-rr•f • fl. m.

(59)

CSAMT


For field use it is often

convenient

to convert

from

723

"collinear" fashion. The areas in which the Cagniard

MKS units to units of mV/km for E and gammas(1•/=

values

10-2/4,rA/m = 1 nT) for H, in whichcasewe have:

far-field zone. For reasons of practicality, we define the far-field limit as the distanceat which the apparent resistivity is within _+5percent of the true resistivity value. Following this criterion, values are within _+5 percent of the true value at r > 4• for "broadside" measurements.Values are within -+5 percent at r > 5•

11. m.

(60)

This is commonly called the Cagniard resistivity, after the French geophysicist who was instrumental in the development of magnetotellurics in the 1950s (Cagniard, 1953). Cagniard resistivity is valid for far-field conditions

and is the usual relation used to calculate resistivities when far-field conditions

MT and CSAMT hold.

Resistivity can be defined in several ways, depending upon the parameters best suited for the application. Spies and Eggers (1986) summarize, contrast, and compare various definitions for resistivity and their uses. The Cagniard resistivity is commonly used for CSAMT measurementsto maintain the similarity between CSAMT and MT, and is so used here unless otherwise

noted.

Transition Zone Response (r • •).--Between the near-field and far-field zones, the behavior of E, H, and impedance are describedby the exact relations in equations (34) through (39). Data in this zone are smoothly transitional for the homogeneous-earthcase from near-field to far-field behavior. In nonhomogeneous environments, transition zone behavior becomes complex and dependent upon resistivity contrasts, as described in Section 5.

Field ResponseSummary.--Plotting the field responsesin the vicinity of a dipole sourceis instructive. Figures 5 and 6 show the results of one-dimensional (I-D) CSAMT models for a homogeneousearth of 100 tl.m resistivity at a frequency of 4096 Hz. The plots are presentedfor one quadrant only; the pattern can be mentally extended to show the full plan-view plot. Figure 5a shows the E-field vectors at each point consideredin the modeling. Each vector length represents magnitude, while direction indicates field orientation. Figure 5b shows the magnitude of the E-field vector as a function of field position. In a similar fashion, the vectors and magnitudes of the H-field vector are plotted in Figures 5c and 5d, respectively. Note that E and H are orthogonal, as anticipated. The total field resistivity is shown in Figure 5e. Note that the pattern is a complex function of distance and azimuthal angle from the source. The resistivity values converge to the true 100 tl.m homogeneous-earth value at smaller r separationswhen measured in the "broadside" configurationthan when measuredin the

are the same as the actual

for "collinear"

resistivities

is the

measurements.

Normal practice is to make CSAMT measurements in scalar setsoriented in the x and y directions. Figures 6a and 6c show the behavior of electric and magnetic fields for measurements oriented along the x and y axes. The E-field parallel to the source, Ex, showsa double-lobedappearance. The lobes are separatedby a "null" zone (shaded)in which Ex values go to zero. This change in values occurs because the E-field vector changesorientation in this region, as shown in Figure 5. Note that "collinear" Ex values along the x axis are half the "broadside" Ex values along the y axis. This difference in signal strength is one of the featuresthat makesEx measurementsin the broadside configurationpreferable.

The H-field component Hy, shownin Figure6c, showsa double-lobedappearancesimilarto that of Ex. The shadednull zone of low magnitudesis again due to the sign cross-over of the H-field vector. As seen with

Ex, signalstrength considerations makebroadside Ey measurementspreferable.

Figure6e showstheCagniard resistivityPxyresultingfromtheEx andHy components. The calculations are noisy in the null zone due to low signallevels. The data also are r-dependent close to the source. Both outside the far-field limit (r > 4• for broadside measurements, r > 5• for collinear measurements), Cagniard resistivity values are within about ___5 percent of the half-space resistivity, as observed before.

Figure6b showstheE-fieldvectorEy. The plothas a uniform appearanceexcept along the x and y axes, where the field strength drops off sharply (shaded zones). There is no double-lobed appearance to the

datasinceEy alwayshasthesamepositiveorientation. The plot for Hx, shown in Figure 6d, has the same

appearance. Predictably, theplotfor Pyx(Figure6f) is also much the same, with the data along the x and y

axesbeingundefined dueto thenullzonesfor Ey and Hx. Note that Cagniardresistivity is within -+5percent of the true half-space value for r >- 3•. This is a slight

improvement onthesituation for Pxymeasurements, in that far-field

data can be obtained

closer to the source

where signal strengthis greater. The cone of measure-

mentfor Pyxis approximately the sameas for Pxy; however, note that the signal strength is lower com-

pared to the broadsidePxy measurement. Hence broadside measurementsare usually chosen over di-

724

Zongeand Hughes •

i

ZO 40 '

r- -i-

o

-i-

zoo

--!

400

4OO


I

i

1

•

•

(b) E.,.(p.v/m)

"r -i

_

\

i

i

3O0

200

•00

-----

,

0

• 'T'x o

(c) H•Vector Plot -I

4OO

I I I III//

////•

I I I I /11///xx/

•

2o,ooo

•_q •_•__•m• (d) H•.(my)

o

200,000

.......

........

I j J / / / / / / / / / / / / / / ........ • i l l I1/I/////...

! IZ//I////..

i t 1,1,I, XX///•----

300

....

E 200

I00

" F?Txj•i "'ii', ' I I•1'1 ' r •I •

o

IOO

200

3oo

i•

, I

400

x(m)

(e) p-r (,g,' m)'""', /

---.

/

•

-'--.

/

/

"•

! I/•

'•

'%

•..

•.

•

',•_ '-"o

\

',

\

%\• ',\/ ',1/

••• Tx 0

• I00

,,, , i

200

,.

'x ,-

-•

',1/,, /•

\

'

',

',, ?• 500

, i

400

x(m)

Fig. 5. 1-D modelresultsfor a homogeneous earth, showingthe vectorE and H fieldsand impedances abouta shortgroundeddipolesource.Sourcelength= 10 m, current= 1000A, backgroundresistivity= 100f/.m, data density 20 x 20 m, frequency = 4096 Hz.

CSAMT

(;•)Ex(•V'/n•)'


,•oo-

•

00 -•

'

725

' l(b)E•(•,)/n•) '

'

'/

... :•.::.:...•• ,.:•.:,.........•..

:,, -....•.••••;:-r........•. ....... -...

200

I00

400 -

-

•00

200

I00

/

I

4oo}(e) Pxy (-f/" rn) 3OO

......'"•,'--..

I (

x(.Q'm)

_

;,,.(9o, •

'•' 200

x•

\\

",3",,

I00

• Tx 0

•

•

•

5&•

I00

200

300

400

x(rn)

- •' Tx 0

, I00

• 3• 200

_

'• 300

,400

x(m)

Fig. 6. 1-D model resultsfor a homogeneousearth, showingthe scalarE and H fields and impedancesabout a short groundeddipole source. The shadedarc indicatesthe location of the far-field zone. Shadingindicates zones of low signal strength in which field measurementsare difficult. Model parameters are the same as given in Figure 5.


726

Zonge and Hughes

agonalor collinear due to increasedsignalstrengthand logistical ease. Please note that the nulls seen in Figure 6 are an artifact of the measuring techniquesthat is, aligning the E-field and H-field detectors parallel and perpendicular to the transmittingdipole. If the E-field receiving dipole were aligned parallel to the source electric field, then the nulls would be eliminated (Zonge et al., 1980).

overall magnitudedrop of the E- and H-fields at higher angles in Figure 7c, is the result of the increased r-separation resulting from these angles. Vertical Magnetic Dipole Solution (Homogeneous Earth)

(broadside mode) as a function of frequency for varying values of r, p, and angle 4" = •r/2 - 4' from the transmitter bisector. These plots help summarize the

Occasionally it is necessary to use a horizontal ungroundedloop which provides a vertical magnetic dipole as a signal source instead of a grounded electric dipole. Wait (1951, 1962a), Bannister (1966), and Kaufman and Keller (1983) give the quasi-static field componentsat the surface of a conductive half-space

differences

as:

Figure7 showsthe behaviorof E•, Hy, and p•y

between

far-field

and near-field

behavior.

Figure 7a illustrates the r-dependence of Cagniard resistivity in the near field. The individual fields show the near-field saturation and the frequency shift of the transition zone as separation r decreases. The fields exhibit a sharpdecreasein field strengthas r increases. Note that the decrease in H with increase in r is greater on the far-field (right-hand) portion of the plot, while it is less on the near-field (left-hand) portion. This re-

E• =0

(61)

ilSn•xto • 2r2)e-ikr ] E4•= 32xk2r4 [1- (1+ ikr- 5k Ez =0

(62) (63)

flectsthechange from1/r3 to 1/r2 dependence when moving from the far field to the near field. The near-field effects are also a function of ground

Hr- 4,rr3 I• • K•

resistivity.Figure7b showstheE•/Hy broadside data for a fixed source-soundingseparation of r = 4 km. The incursion of the transition zone occurs along a 45 degree line from lower left to upper right in Figure 7b reflecting the fact that skin depth is proportional to resistivity and frequency to the same degree. An interestingpoint to note is the behavior of the individual fields

in the near

field.

E remains

sensitive

to

resistivity, while H becomesindependentof resistivity and frequency. This behavior is the result obtained from equations (42) to (47). Note that this effect produces the artificial 45 degree rise of the Cagniard resistivity curves in the near field.

(64)

_

9ISn

4 21. Hz- 2xk2r5 [1- (1+ ikr- 5k

-• l ik3r3)e-ikr].

Perturbations

like

those

shown

in the models

have

been observed in the field. Zonge et al. (1980) have shown that the variations with angle can be mitigated

by orientingthe field measurements in an E4•/Hr orientation for 4" > 15ø,insteadof usingthe Ex/Hy orientation. This procedure will extend the usable cone of measurement to at least -+30 degrees. The

(66)

The ISn term is the magnetic moment,

Figure7c showshow the E•/Hy databehaveas a function of angle 4", the angle of offset of the receiver from the perpendicular bisector of the source dipole. The far-field data are unaffected, but near-field data show significant changes at soundings far from the bisector. These changes are due chiefly to the strong curvature of the E- and H-field lines away from the bisector, an effect illustrated in Figure 5a and 5c. Perturbing effects are minimal at angles of 15 degrees or less, and increase dramatically at higher angles.

(65)

H, =0

M = ISn,

(67)

where I is the current in the loop, S is the area enclosedby the loop, and n is the number of turns. The

eitøtfrequency dependence is implicitfor M. Examining the near-field and far-field zones gives a better understanding of the fields.

NearFieldResponse (r <• •, Ikrl • 1).--The limit of equations(62), (64), and (66) for small induction numbers is derived in Kaufman and Keller (1983)' ilSnlxoo

E4, •

4,rr 2

1-2• (n-1)(n-3) (ikr) n-2 n!

n=4

(68)


CSAMT

(7

0

•

0

0

0

0

0

0

0 0

0 0

0 --

--

--

0

0

0

0

0

--

g

--

(•)

--

0

--

I

727

c• u c•

ß

¸

I

0

I

--

0

0

--

0

--

I

(XuJI ple!_-Io{leu6o• I

'•.

I

I

0 --

--. 0

728

Zonge and Hughes ISn

HF •

Z =

4•rr 3

-

4

(77)

'

(78)

.


Solving for resistivity:

x (ikr) 2+•(ikr)41n•

+'"

r E,

(69) ISn

4•rr 3

o• (n - 1)(n - 3)2

I +2 •

(ikr)n-2

n=4

(70)

Reducingthe equations furtherfor Ikrl

1 we find:

Notice that equation(78) is half the resultobtainedfor the grounded dipole case (equation 49). Thus, the apparentresistivitymeasuredcloseto a loop sourceis, like its dipole-sourcecounterpart, insensitive to frequency. Hence we are no longer "sounding" in the parametric sense; soundings are only achieved by varying the r-separation.

Far-FieldResponse (r >> •, Ikrl >> 1).•For large induction numbers, the field componentsbecome:

ilSnlxto

E, •- 4•.r2

P=

(71)

3ISn

E,= 2•.•rr 4

ilSntx•oo

Hr •

16•rr

(72)

ISn

Hz• 4m.r 3.

(73)

When comparingthese equationswith the equations for a grounded dipole, some major differences are

seen.For theloop,near-field E, varieswithfrequency and is not a function of earth resistivity, whereas for

the dipolecase(equation43), E, is independent of frequencybut is a function of resistivity. For the loop, near-field Hr is a function of both resistivity and frequency, whereas its counterpartfor the grounded dipoleis independentof both. H z remainsindependent of both frequency and conductivity for both cases. Upon taking the limits for the dc case (to = 0), we obtain:

E, = 0

(74)

Hr = 0

(75)

Hr=-2,n.•/p, crtor4 e-i•r/4 (80) 9ISn

e -i•/2 Hz= 2•r•rtor5 .

(81)

Comparingtheseresults[equations(79), (80), and(81)] with those for the dipole [equations(50) to (55)], reveals that the fields due to a loop source drop off faster with r than those due to a dipole. Of course, an exact comparisondependsupon the source size, current, and relative source-receiver position. Hence there are some instancesin which the loop signalmay be preferred over the signalfrom a dipole, as will be shown in a later section.

Solving for the far-field impedanceover a uniform half-spacewe find:

Eoo=qp, to/(tei•r/4 (82)

(76)

H z, the only non-zero component at dc, and is independent of earth conductivity. It is not surprisingthat the horizontal field componentsdisappearat dc, since thesefieldscan be thoughtof as the time derivativesof the horizontal fields for a groundeddipole. Assumingthe quasi-staticcase,where displacement

currentsare negligible (k =V'-ilx•rto),we can use equations(68) and (69) to determinethe impedancefor a plane wave propagatingin the z-direction (vertically downward). For the near-field case (r • 8) this becomes:

3ISn

and the equation for resistiviW is:

ISn

Hz= 4m.r 3.

(79)

1 E, 2 The Cartesian coordinateequivalent is thus:

p=

(n. m).

(84)

Equation(84) givesthe sameresultasthat obtainedfor the groundeddipole [equation(60)]. Hence, for large electricalseparations½ • g) the resistivi• measured using a magnetic loop source is the same as the resistivi• measuredwith a grounded dipole source.


CSAMT

729

TransitionZone Response(r • •).--As was observed for the grounded dipole, transition-zone behavior for a loop over a homogeneous earth changes smoothly

layered and varies gradually from high to low to high resistivities. Current was adjusted in the loop to match field intensities with the dipole at the higher frequen-

from the far-field

cies.

to the near-field

zones. The behavior

is more complex in nonhomogeneousenvironments, as illustrated

later.

Field ResponseSummary.--The behavior characteristics of the individual field components and the calculated Cagniard resistivity for a loop are similar to the case of a grounded dipole. However, there are differences in both the near-field

and transition

zones that

must be considered when comparing loop and dipole data sets.

To demonstrate the similarities and differences, a

field comparisonbetween a loop and dipole sourceis shown in Figure 8. These data were acquired over a test site near Tucson, Arizona, where the geology is

SOURCE loop

ipole I

SOUNDING

Notice that the loop and dipole responsesare identical in the far field, but that the E- and H-fields from the loop enter the transition zone and near field earlier than the fields due to the dipole. This behavior can be

explained by analyzing the valid range of the far-field approximationsfor the loop and dipole equations, and is predicted in Bannister (1966). Notice also that the fields from the loop tend toward zero at low frequencies, whereas they tend toward a constant value for the horizontal electric dipole, as predicted in the near-field equations. NonhomogeneousEarth Solutions

All geophysicalproblems of practical value concern layers and finite bodies with different media properties. The theory then becomes considerably more complex, and special techniques and modeling capabilities must be employed for the interpretation.

I I

,

-"I o=46m

r=2.16km

I

J $05m

iO-2

I

i

Electric

i

I

i

Field

E

i

i

i

i

i

i

and 2:

x

dipole

I

I

I

I

I

I

i

I

I

I

I

,I

I

I

Cagniard \

I

I

I,

Restivity p

I

I

I

I

IO-

i

4

i

8

I

• 3I2

(Tangential H continuous),

(86)

(Normal D discontinuous),

(87)

Etl = Et2

(Tangential E continuous),

(88)

J• = J•2

(Normal J continuous),

(89)

V 1 =V 2

(Electric potentials continuous), (90)

U 1 =U 2

(Magnetic potentials continuous). (91)

In these relations, qv is the charge density at the boundary. When the frequency is low, the fields and potentialsare related by E =-V.V and H = -V.U.

,

64

,

,

128 256

i 10,24 , , 2048

512

Frequency(Hz)

Fig. 8. Comparison between loop and dipole transmitter sources,from field tests in a uniformly layered test site near Tucson, Arizona.

Htl -- Ht2

xy

\loop • ,•

(85)

I

IOO

di

Bnl = Bn2 (Normal B continuous),

Dnl -- Dn2 = qv

f••tic FieldHy

IO0

Boundary Conditions.•The behavior of the propagating field can changewhen it encounterschangesin media properties. Boundary conditions describe the field as it traverses the boundary between medium 1

1-D Considerations.inHomogeneous-earth theory can be extended to the 1-D or layered-earth caserather easily. For example, in the two-layer case (Figure 9a), we can write for the quasi-static, far-field E and H fields in layer 1:

Ex1= El+e-iklzq-El-e iklz

(92)

730

Zonge and Hughes El+

l•

T]I

E 1-

e-ik•z

e ikiz.

(93)

resistive


For the field in layer 2 we can write:

Ex 2= E 2+e-ik2z,

(94)

E2+

Hy 2-

x12

e-#,2z,

(95)

where xli = Ei/H i. The impedancemeasuredat the earth's surfacewill dependupon the propertiesof both layers:

1+p•2 - p• -ik•

Zo =(ilxotop1)1/2ei(•/4) P•2 +• e --. 1- 0•2+OVr•e

(96)

xr[01np

(97)

Note that for a homogeneousearth, 0 In p/0 In to = 0, and the phaseis xr/4rad or 785 mrad. As layering and other inhomogeneities are encountered, the phase fluctuates about the homogeneousearth value. For example, if resistivitylayering is high over low (positive slope),the secondterm in the bracketsof equation (97) is positive,and the phaseis thereforegreaterthan xr/4. Conversely, if the layering is low over high (negativeslope), the secondterm is negativeand the phase is less than xr/4. Figure 10 illustratesthe resistivityand phasebehavior of a two-layered earth for various resistivity contrasts, assumingan infinite source-soundingsepara-

or conductive

basement.

The far-field

resis-

tivity cannotmore than doublebetween two consecutive, binary-intervalfrequencies,and the phase does not deviate from the homogeneousearth value by more than _+x r/4. Thus larger changes,often observed in far-field data, are not attributable to simplelayering, but rather to more complex 2-D or 3-D geology, current channeling,or anisotropy. However, with controlledsourcesystemsoperating over an ideal layered earth, it is possible to attain phase shifts greater than xr/2 or less than zero, and resistivity changeswith slopesgreater than _+45degreesin thevicinity of the transitionzone. An example of thesechangesis shownin Figure 21 andis discussed further

where D is the thicknessof the top layer. Phaseis related to the slopeof the resistivity (Bostick, 1977)by:

4=• - l+olnto

far-field resistivity changeswhich are greater than the corresponding frequencychange.This characteristicis seen in Figure 10 for the case of either an infinitely

in the Source Effects section.

An asymptoticevaluationof a layered earth developed by Bostick (1977) helps define the effective penetrationdepth of MT and far-field CSAMT surveys. Figure 11 showsa plane-wave,two-layer model with a 100 lI-m layer overlying an infinitely conductive basement.At high frequenciesthe apparentresistivity is equal to the resistivity of the top layer; at lower frequencies,apparentresistivity lies along the asymptotecontrolledby the basement:

Pa= p'D2toD,

(98)

where D is the depth to the basement.The frequency fo at which the basementis "seen" by the sounding may be taken to be the pointwhere Pa2 = Pal- At this point, the bulk apparentresistivityPa is related to D by

Apparent

Resistivity

Phase

Difference

tion.

A characteristic of resistivity calculated from a plane wave field in a horizontally layered, 1-D environment is that it is theoretically impossibleto obtain

(a) 2-Layered

Earth

(b) n- Layered Earth

•1• ø

••

i

z=O

z=0

Pl D

o

Di P2

D2 Dj-i Di DN_• PB

Fig. 9. Nomenclaturefor modelinga layered earth.

•oo

•o

•!o

Higher Frequency--•

o•

•oo

i •o

i0

HigherFrequency--•

Fig. 10. Apparent resistivity and phase differenceover a two-layeredearth. Modelsassumean infinitesource-sounding separation (all data in far field). After Keller and Frischknecht(1966).

CSAMT

tot> IXtra tot>

(99)


From equation (28) we then have

(1986)providesa goodcollectionof key MT papersof the past two decades. Butterworth (1988) presents someinterestingresultsof 3-D controlledsourcemodeling. In 2-D and 3-D areas, which are most common in

D=g/•/-• 356½ =

731

(00)

meters

which is the depth at which the basement is sensed,

exploration,the scalarimpedancedefinitionof equation (22) does not adequately define the resistivity structure of the ground. An extension is to use an impedancetensor:

andhenceby convention g/X/• is takento be the effective depth of investigationof MT and CSAMT

Ex

surveys.

Ey - Zyx ZyyZyz ß Hy .

In the more general case of an n-layered earth (Figure 8b), the solutionin any layerj is:

E• = EJ+e-i•'Jz + EJ-ei•'J •, ß

E j+

H• =

TIj

Z• Zxy Zx•

E•

Z• ZzyZzz

Hx

(104)

H•

(101)

Except for very highfrequencies,the Ez componentis very small and is never measured. The tensor then reducesto two equations:

(102)

Ex -- ZxxHx q-ZxyHy

(105)

Ey '- ZyxSx q-ZyyHy.

(106)

E j-

e-ik•z

eik•z.

and

The impedanceis:

Zj + i + xlj tanh (ikjhj)

Zj=xlj xlj+Zj+1tanh (ikjhj)' (103) In equation(103) Zj is the measuredor apparent impedance at thetopof layerj, andyljis theintrinsic impedanceof layerj. The multi-layeredcaseis solved by computingthe impedanceof the bottom layer and working upward in recursivefashiontoward the top. Wait (1953) and Hohmann(1983)providesomeexamples of this process.Several computer algorithmsare availablewhich solvethe n-layeredcase(forward and inverse)for either an infinite or a finite source. 2-D and 3-D Considerations.--2-D

and 3-D structure

requiresconsiderablydifferenttreatmentthan the 1-D case. MT interpretationfor 2-D and 3-D environments hasbeenthe subjectof a great deal of attention.Vozoff

These relations show that finite structure imposes a mutual dependenceof the E and H terms. For exam-

ple, theEx termis partlyinducedby Hy, resulting in theimpedance termZxy. In natural-source MT and AMT measurements, the

signalsourceis assumedto have an infinite number of polarizations.In sucha case,the EM field can be fully

defined by thetensordatasetof Ex, Ey, Hx, Hy, and Hz. On the other hand, controlled-source measurements are uniquely polarized due to the finite location and orientation

of the source.

In this case two source

polarizations, preferably at right angles to one another, are required to fully define the impedance tensor.

The net result

is that the MT

tensor

data set

must be measuredwith both sources, requiring ten componentmeasurements.Equations(105) and (106) then become:

/

/

Exl -- ZxxHxl q-ZxyHyl,

(107)

Ey1 -- ZyxHxl q-ZyyHyl,

(108)

Ex2 -- ZxxHx2q-ZxyHy2,

(109)

Ey2 -- ZyxHx2q-ZyyHy2,

(110)

Pa = PI

I

10,

I

in which the subscripts i and 2 refer to the two sources,and E and H are in units of V/m and gammas, respectively(or equivalent). For the sake of clarity, we define

Frequency (Hz)

Fig. 11. Resistivity responseover a two-layered earth, illustratingthe derivationof effectivedepth of penetration.

the coordinates

such that source

i is ori-

ented along the x direction, and source 2 along the y direction,as shownin Figure 12. From equations(107) to (110) we obtain the tensorimpedances:

732

Zonge and Hughes


Zxx =

Zyy =

=

(111)

Eyl Hx2 - Ey2Hxl , Hx2Hyl -- Hxl Hy2

(112)

Ex2Hxl - Exl Hx2

Ey2Hyl - Eyl Hy2

it is known.

CSAMT

work

it is convenient

to

surements

are made in this orientation

and the four

resistivities Pxy,Pyx,Pxx,and pyyare calculated. In (113)

order to obtain the resistivitiesparallel and perpendicular to strike, the xy coordinatesare rotated througha positive angle 0 to provide new rotated impedances:

.

(114)

Z•c x = • [(Zxxq-Zyy) q-(Zxx-Zyy)

1

ZYXHx2 Hyl--Hxl Hy2

Z can be solved for directly with controlled source measurements, since we use two known orthogonal sources, the desired apparent resistivities can be obtained from the Cagniard relation [equation (60)]:

x cos20 + (Zxyq-Zyx) sin20] t

1

g•cy: • [(Zxy- Zyx)q-(Zxyq-Zyx) x cos20 - (Zxx- Zyy) sin20]

1

Pij=• //jl2.

In

initially reference the x and y axes according to the source orientations, as shown in Figure 12. The mea-

,

ZxYHxl Hy2 - Hx2 Hyl =

althoughpreferably oriented with the geologicstrike if

Ex•Hy2 - Ex2Hyl , Hxl Hy2 -- Hx2Hyl

(115)

t

Normally resistivity calculations are made for the xx, yy, xy, and yx modes. In the homogeneousor horizon-

tally-layered earthcases,Pxx= Pyy= 0 andPxy= Pyx. In the2-D casePxx= Pyy= 0 butPxy• Pyx'In the3-D

x cos20 - (Zxx- Zyy) sin20] t

ponent resistivities. A primary objective of tensor work is to define structure, and this can be done by rotating the impedances to principal strike direction, as in MT data. The procedure is as follows. Orthogonal sources are laid out on the ground in a somewhat arbitrary orientation,

1

gyy= • [(Zxxq-Zyy) - (Zxx- gyy)

case, all components are in general nonzero and unequal. Ranganayaki (1984) demonstrates that the

determinant resistivity (@det)is relatively stable in a 2-D or 3-D environment, when compared to the com-

1

gyx=• [(-Zxy q-Zyx)q-(gxy-gyx)

x cos20 - (Zxyq-Zyx) sin20],

(116)

or, conversely,

Xxx= • [(Zxxq-gym)q-(Xxx- gym) !

x cos20 + (Z•cyq-Zyx) sin 20]

Zxy: • [(ZJcy - Zyx)q-(Zxyq-Zyx) !

x cos20 - (Z}x - gyy) sin 20]

gyx = • [(-gxy q-gyx) q-(gxy- gyx) !

x cos20 - (Z}x - gyy) sin20]

Zyy= • [(Zxxq-Zyy) - (Zxx-- Zyy) !

x cos20 - (Zxyq-Zyx) sin 20].

(117)

The rotation angle 0 is usually chosento maximize the relation

Z•:y 2 + Z}x 2, and to minimize

Fig. 12. Rotation of principal axis from the xy orientation (in which the measurementsare arbitrarily obtained) to the x'y' orientation along principal geologicstrike. The rotation occurs through angle 0 as described in the text.

(118)

the relation

Z•:x 2 + Z}y 2.

(119)

This can be done by differentiating these relations (Swift, !967). Vozoff (1972) obtains the solution'

CSAMT 1

means of determining geologic strike. It is calculated from equation (123). A related quantity is the skew, defined in Swift (1967) as:

-1

0 = • tan


x

(Zxx

)(Zx

+(Zxx

iZxx _Zyyl2_Zxy+Zyxl2

733

+Zx)

ß

(120)

S=

In a 1-D earth, no rotation is necessarysincethere is

Zxx q-Zyy . Zxy - Zyx

(126)

no strike.

Skew is invariant with rotation, and is another mea-

In a 2-D earth, the rotation is usually well-defined, and strike can be determined relatively easily. Following the notation of Figure 12, with strike along the y'

sure of three-dimensionality. For the 1-D and 2-D cases, S = 0. In the 3-D case, S varies from 0 (special case along the axis of symmetry of a body) to large values. Skew increasesat lower frequencies. A third quantity of interestis the ellipticity e, (Ting

axis,Pxyis the resistivityperpendicular to the 2-D structure, and is referred to as TM mode propagation

(ZTM= Zxy).Theresistivity parallelto strike,Pyx,is knownas TE modepropagation (ZTE = Zyx).Hoh-

and Hohmann, 1981);

mann (1983) and Wannamaker (1983) show examples

e=

of 2-D models of TM and TE mode data.

Zxx • Zyy . Zxy q-Zyx

(127)

In a 3-D earth, the meaningof strike becomesmore ambiguous,and additional information is needed to help with the transformation. A usefulparameter in defining2-D and 3-D structure is the "tipper." Simsand Bostick(1968)expressH z as a linear relationshipbetween the horizontal H compo-

Ellipticity is the ratio of the major to minor axis of the ellipsedescribedby the impedances.For the 1-D and 2-D cases,e = 0. As with skew, e = 0 along the axis of symmetryof a 3-D body but otherwiseassumesposi-

nents:

increasing frequency.

H z = KzxHx + KzyHy,

(121)

whereKzxandKzyareunknowns. Forthegeneral 3-D case,tipper magnitudeis definedas (Juppand Vozoff, 1976)

Irl - (Igzx12+ Igzy12) •/2.

(122)

With the definitions Kzx = a + ib andKzy = c + id, tipper direction can be definedas

(a2+c2)arctan(c/a)+(b2+d2)arctan(d/b) (123)

Both T and q>Tare invariantwith rotation. For the simple1-D case,H z = 0 and T = 0. For the 2-D case,T - 0 for the TM modesinceH z = 0; for TE mode, tipper and tipper direction are: HZ

•T=O

increase with

Examples of tensor interpretationare found in papers compiledby Vozoff (1986)and in privately published work by Hohmann (1983) and Wannamaker (1983). We commentfurther on tensorCSAMT in the Far-field Data Interpretation section. Vector CSAMT Apparent Resistivity Vector CSAMT

measurements are useful in areas

with complexgeology,and can track the field rotation causedby stronglateral resistivity contrasts.By measuringall horizontal field components,vector surveys record the actual orientation of the electric and mag-

r2

r •

tive values in the 3-D case. Values

(124)

(125)

Thus, tipper is an indicationof the tilt of the magnetic field out of the horizontal plane. Its value is typically less than unity, and it increasesat lower frequencies. Sinceit is dependentuponH-field measurements only, it is insensitive to static effects. The tipper direction

netic fields. Complete information about horizontal field components is obtainedregardlessof orientation of the measurement coordinate system.

Any method for calculatingvector CSAMT apparent resistivity should preserve this insensitivity to coordinate system orientation. Unfortunately, the techniquesfor obtaininga rotationallyinvariant impedancefrom an impedancetensorare not applicable. Vector CSAMT data do not contain enough information to determine a full impedancetensor, since measurementsare made with only one source-fieldpolarization.

The

best

alternative

is to use a scalar

indicates the direction to the inhomogeneity. Wanna-

impedancebasedon polarizationellipseorientations. Both E and H are complex vectors which define polarizationellipses.By choosingscalarfield componentsorientedalongthe major axis of the polarization ellipse, a rotationallyinvariant scalar impedanceis

maker (1983) finds that the tipper directionis a stable

obtained. The result, which is independent of the


734

Zonge and Hughes

measurement coordinate system, depends only upon the orientation of polarization ellipses. Calculating a vector apparent resistivity is a multiple-step procedure. The first step is to determinethe phaseof the polarization-ellipsemaximum. The phase of the electric field polarization-ellipsemaximum is:

q> =0 ß5. tan-' [Im •ee (-•x (E• 2 2++Ey2)] Ey2).] ' (128) The secondstep is to determine the orientation of the ellipsemaximum. The orientationis determinedusing:

Re[Ex'ei•] O=tan-1 {Re[Es ei*] }.

(129)

Next, the electric field componentparallel to the major axis of the ellipse, Em, is obtainedfrom:

Em = Ex ßcos 0 + Ey ß sin 0.

(130)

The magnetic field component, H m, parallel to the major axis of the magnetic field polarization-ellipseis obtained in a similar way. Since the controlled source fields are highly polarized, the electric and magnetic field componentsE m and H m are nearly orthogonal.A scalar impedance can be formed from the two scalar field components Em gm = •. Hm

(131)

Finally, an impedance phase and a vector Cagniard apparent resistivity can be obtained from the scalar impedance,Zm.

The measurementsite is typically located 5 to 10 km from the source, ideally in the far-field zone. Measurements are made of the electric and magnetic field components.Typical broadside scalar measurements involve the E-field parallel to the source and the H-field perpendicularto the source. The E-field is sensed by an electric dipole whose length is usually between 10 and 150 m. The dipole is terminated at both ends by nonpolarizing potential electrodes("pots"), which are buried in holes wetted by clean water. A third pot can be located at the dipole center in order to provide a ground reference for common-mode noise rejection. The E-field measurement consistsof the potential difference between the pots and the associatedphase angle reference to the source signal. For vector or tensor measurements, two orthogonalE-field dipoles are utilized. Different types of pots are available, but the most common type is a glazed ceramic cylinder with an unglazed, porous bottom. The cylinder is filled with a saturatedcopper sulfate solutionwhich diffusesslowly through the porous bottom, providing a nonpolarizing current path through the pot. The signalis picked up by a copper probe inserted into the copper sulfate solution. Another type of electrode in use is a solid lead/lead-chloride/plasticdevice containedin a plastic sleeve with the bottom open to expose the electrode material. The signal is picked up by a lead probe insertedinto the saturatedplaster base. The H-field is sensedas an induced potential in a magneticfield antenna. The measurementconsistsof a voltage and an associatedphase angle which is referenced to the sourcesignal.The signaldetectedby the antenna

FIELD

SURVEY

PLANNING

AND

LOGISTICS

Basic Logistics

The CSAMT field set-up involves a source (transmitter) and a sounding(receiving) point, as shown in the tensor CSAMT illustration of Figure 1. In the more common scalar CSAMT approach, a single source dipole is laid out on the ground.The sourceis typically 1 to 3 km in length--long enoughto provide the needed signalstrengthand short enoughto not causelogistical problems. The dipole is grounded at both ends by a network of buried aluminum foil pits or stakesdriven into the ground. The electrodesare wetted thoroughly with salt water to improve their electrical couplingto the ground. Insulated wires of 14 gauge (1.63 mm) or larger connect the electrodes to the transmitter, which for convenience is normally located near the center of the sourcedipole. An ungroundedloop may be usedas an alternative signal source when the contact resistance of the grounded dipole is too high to permit adequate current transmission.

is fed

back

to the

insulated wire for simultaneous

receiver

over

measurement

a short with the

E-field. For vector and tensor measurements, multichannel antennas are deployed. Antennas are usually partially buried in order to minimize the noise produced by wind vibration. After the necessary connections and preparations are made, the receiver operator instructs the transmitter operator to energize the source with current at a specified frequency. Stacking-and-averagingis performed on the E- and H-field signals until data of a specifiedprecisionare obtained. The next frequencyis selected,and the crew works methodicallythroughthe range of frequencies selected for the survey. With current technology this process normally takes about 30 minutes

to collect

data across the 1 to 8192 Hz

range. When data collection is completed, the crew moves the equipment and wires to the next sounding site. In the case of tensor measurements, a second current sourceis energizedas well to provide a second set of measurements.

Considerabletime is savedby carefully planningthe


CSAMT

field logistics prior to arrival in the field. A number of factors such as land holdings, site geology, access,etc. must be weighed in order to deploy field personnel most efficiently. An important concept in running an efficient survey is to minimize the time the receiver is not collecting data. This can be done by setting up receiver dipoles in advance of the receiver, increasing the efficiency of receiver operation, use of efficient access logistics, etc. Long work days help minimize the relative percentage of set-up time compared to data collection time in the course of the survey. Finally, proper maintenance and treatment of equipment saves many hours of "down time." Instrumentation

CSAMT measurements require modern digital instrumentation. The receiver should be capable of both magnitudeand phase data acquisitionin the 0.1 Hz to 10 kHz frequency range, and should have two or more measurement channels for maximum efficiency. Analog-to-digital conversion rates of 25 Ixs or less per conversion ensure adequate signal digitization at high frequencies. Proper antialias filtering is an essential part of receiver performance. Multinotch filters for powerline frequency rejection, and telluric-tracking filters are often required for acquisition of high quality data. High input impedance, low noise in active components, and low cross-talk between channelsare also necessary characteristics. The receiver should be capable of real-time apparent resistivity calculation and data display, some type of immediate viewing or hard-copy output of data and statistics,and display of field signalson an internal or external oscilloscope. These provisions enable the operator to maintain data quality in adverse noise conditions,to monitor proper equipmentfunction, and to optimize logistical procedures. At least daily plotting of the data (and preferably real-time plotting) is an essentialpart of maintainingcontrol over data quality. In addition, the equipment should permit convenient data transfer to a separatecomputer at the end of each field day. During the past decade, several digital receiver systems have become commercially available for CSAMT work. Digital CSAMT systems were manufactured and introduced in 1978 and 1987 by Zonge Engineering & Research Organization, Inc., in 1981by Bureau de Recherches Geologiques et Minieres or BRGM, and in 1984 by Phoenix Geophysics, Ltd. The transmitting unit should be current controlled and provide a stable waveform across a broad frequency range. Output of 10 to 100 A and 1000 V is desirable. In order to obtain quality phase data, the transmitter should be synchronizedto the receiver by

735

high-precision crystal clocks or a radio link. Since transmitters are subjected to high currents and voltages, ease of maintenance is an important consideration.

A high-gain, field-durable magnetic antenna is required for the H-field measurements.The usual type is a ferrite or iron core, dB/dt design with its resonant frequency above 1 kHz. A minimum antenna response of 0.1 mV/•t-Hz (0.1 mV/nT-Hz) is normally required for low frequencies, and a maximum response of 200 mV/•t for frequenciesabove 100 Hz. We

have

demonstrated

an occasional

additional

requirement for CSAMT instrumentation (Zonge et al., 1980; Zonge and Hughes, 1985). Tests carried out in Arizona showed that high-frequency electric field measurementsare dependent upon the contact resistance between earth and the potential electrodesat the ends of the electric dipole, and the length of the dipole. This electrode

contact resistance or ECR effect is due

to capacitive pickup between the earth and the dipole wire. The ECR effect is predictable by an equivalent circuit model and is readily reproduced in field experiments. When high contact resistance and long receiver dipoles produce high frequency signal distortion, the solution requires that operational amplifiers be placed at the pots. The amplifiers drive the dipole wires with a low impedance and reduce the RC time constant of the receiving antenna. This "active pot" system clears up the problem. As a general rule, for a given dipole length L (in km), frequency f (in kHz), and contact resistance Rc (in kf•), active pots are neededwhen L'f'Rc > 2. Electrical

Noise

Despite a generally strong signal source, CSAMT measurementsencounter significant amounts of electrical

noise. Noise

sources can be broken

into five

categories: (1) operator error, (2) instrumentation noise, (3) cultural noise, (4) atmospheric/telluricnoise, and (5) wind noise. Examples of operator error are: incorrect cable connection, undetected signal saturation, improper gain or groundpotential adjustment,use of insufficient bits of accuracy in the analog-to-digital conversion, wrong alignment of the E- and H-field detectors, incorrect reporting of transmitter currents, failure to use "active" pots when necessary,use of pots which are not freshly charged with a saturated electrolyte, etc. Many of these problems are avoided by maintaining an establishedset-up procedure, sufficientrecordkeeping,daily calibrationchecksof the receivers, and certain standardchecks at each receiver set-upprior to data acquisition. Semi-automatic instrumentation can aid significantlyin checking for operator error.


736

Zonge and Hughes

instrumentation error can result from noisy components, low input impedancein the receiver, inadequate stackingand filteringcapabilities,impropergrounding, etc. Many of these problems are avoided by proper operator training and by selecting adequate digital equipment. Problems with instrumentationor electrical component failure can be detected by periodic systemresponsechecksand by careful analysisof data as they are acquired. "Cultural"

noise

arises

from

man-made

metallic

structures and radio stations which carry electric signals. Pipelines constitute one of the worst sources of cultural noise, especially when they carry cathodic protection signalsto prevent corrosion. These signals often have a half- or full-wave rectified powerline waveform which varies considerablyin amplitude and frequency. Fortunately, arrangingto have the protection circuitry turned off for the duration of the field survey is often possible. Pipelines can also channel extraneous powerline and atmospheric noise, which can hamper data collection. Broadcast radio stations, air and sea navigation beacons, switching and control signals on railroads, powerlines, etc., constitutethe most commontypes of radio interference. Some signals,suchas powerline or railroad control signals, often occur within the top portion of the CSAMT data band. Operating in areas where there is strong interference may necessitate building special band-reject filters. High-voltage, cross-countrypowerlines and underground cables are a major source of cultural noise. In most of the world, 50 Hz and its odd harmonics (150, 250, 350, 450,... Hz) are the noise sources; in the United States, Canada, and parts of South America and Japan, 60 Hz and its first odd harmonics (180, 300, 420,540,... Hz) are the noise sources. In some cases, specific modes of transmission and modulation may produce unexpectedly high noise levels at higher harmonics, necessitatingcustom notch filtering. An additional problem is load shifting in the powerlines which can produce time and frequency varying ac signals. These signals make extended stacking-and-averaging very difficult. Large direct current (or self potential) shifts can be caused by irrigation pumps and underground mine workings which run on direct current or fully rectified attenuating current. These shifts often make data acquisition quite difficult in the immediate vicinity unless a "bad point" editor is engaged, a system of smart stacking is used, or the operator is fortunate enough to acquire data between noise bursts. Sometimes data collection directly over a mine can be delayed until mine operationsare minimal, suchas on holidays or at shift changes. Cultural noise is best dealt with by avoiding the

noise sourcesor by laying out the wires in a minimum coupling configuration (perpendicular to culture). However, this configuration is not always possible. Cultural noise is then countered by notch filtering, band-pass and low-pass filtering, and by extended stacking-and-averaging.Scouting field sites prior to starting a survey, and using a portable spectrum analyzer in highly cultured areas to evaluate the electrical noise intensity and sourcesis advisable. Atmospheric noise consistsof high-frequencyspiking due to thunderstorm activity and static effects in the air and clouds. The noise is readily observed by amplifying the natural-source signal picked up by an E-field dipole and observing it on a portable oscilloscope. Noise is best dealt with by appropriate lowpassfiltering, by maximizing the signal source, and by sufficient stacking-and-averaging.The stacking strategy dependsupon the noise frequency. If noise spikes are occasional, a few long stacks are sufficient; if they are frequent, numerous short stacks are often more effective.

Slow ground potential drift or "telluric" noise is caused by worldwide thunderstorm activity and by micropulsationsin the earth's magneticfield due to the impinging of the solar wind. Telluric noise typically falls in the dc to 1 Hz frequency range. While telluric noise is the primary signal source for natural-source MT work, it can be a nuisance to CSAMT surveys. Sometimes solar-related noise can be anticipated by keeping abreast of recent solar activity reports published in the United States by the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado. Digital telluric filters, which "track" a slowly-changingvoltage drift, help prevent equipment saturation and speed up data acquisition at low frequencies. Wind noise can strongly affect magnetic field measurements by physically rocking the antenna. This rocking can be avoided by burying or shielding the antenna. Making measurementsin a forest on a windy day can range from difficult to impossible due to ground motion induced by the roots of moving trees. Wind may also cause a suspendedE-field dipole wire to sway with the wind, producing a small but detectable

inductive

effect

as the

wire

oscillates

in the

earth's magnetic field. This effect is avoided by keeping the wire close to the ground. Related problems, such as static charge from drifting snow, are usually minor.

Other

sources of noise include

vibrations

from ve-

hicular traffic on nearby roads, as well as effectsfrom the moving metallic mass of the vehicles, if the magnetic sensoris set up too close to a road.

CSAMT


Cultural

Contamination

In additionto producingor radiatingelectricalnoise, grounded structure also causesspuriouscurrent channeling in the ground which can obscure the desired geologic effects. These cultural effects can render a data set completelyuninterpretableif care is not taken in field logistics. Three situations must be avoided at all costs: (1)

makinga direct connectionbetween the sourceand the soundingsite, (2) placing the E-field dipole parallel to a nearby grounded metal structure, and (3) placing an electrode

close to a cultural

feature.

The direct con-

737

The key to minimizing culture contamination in CSAMT data is to thoroughly scout the field site prior to finalizing the survey logistics. The scout should obtain maps of powerlines, undergroundcables, pipelines, sewer lines, and so forth from local utilities and private industry. With these features plotted on the survey map, the scout can then optimize logistics to meet the geologic constraints while minimizing cultural interference. A personal inspection of the field site to corroborate information obtained from maps is usually advisable. Once the survey is under way, difficult configurations may be avoided by offsetting or skipping some

nectionproblem is by far the worst possibleconfigura-

stations. If data must be obtained near a cased well or

tion to use in a controlled-source

a configuration channels current from the source directly to the sounding point, regardless of geologic trends. The result is repeatable but highly abnormal

other metal feature, the influence can be minimized by placing the soundingpoint on the source-sideof the feature, e.g., between the sourceand the feature. This placement avoids the "shadow effect" illustrated in

behavior

the Source Effects

in the E-field

or H-field

measurement.

measurements.

Such

Af-

fected data are almost impossible to interpret. The second-worst configuration is orienting the E-field dipole parallel to nearby culture. This orientation results in maximum signal coupling between the wire and the culture. A perpendicular orientation that provides minimum coupling is always preferred. Experience has shown there is great difficulty in predicting what the coupling effects of culture are goingto be prior to makingmeasurements.The effects can be minimal to devastating, dependingupon geometry and electrode grounding conditions. A general rule in parallel measurementsis to maintain a dipoleto-culture separationof at least 100 m or one E-field dipole spacing, whichever is greater. Finally, avoid groundingan electrode into a cultural feature. This restriction appliesto both the sourceand the soundingdipoles. In general, culture can be grouped into three categories, according to severity of contamination. The worst group (1) includes grounded metal pipelines, underground phone and power cables, and major cross-countrypowerlines. The intermediate group (2) includes medium-duty powerlines, train tracks, steel well casings,and culverts. The least disruptive group (3) includes metal fences and overhead telephone lines. While these features can vary radically in disruptive effects, those in the first two categoriesgenerally cause the most concern in establishingthe survey line locations. A particularly troublesome situation is an interconnected network of pipelines, powerlines, cased wells, and other features. Such networking is common, for example, in oil and gasfields. This "mass culture"

effect can make accurate

measurements

im-

possible in some areas. In some cases the pipes are buried and very little evidence of their existence is discernible

at the surface.

section.

Culture can produce problemsfor a CSAMT survey, but the reverse can be true as well. For example, current transmission near telephone lines (especially the old ground-return systems)can disrupt communication, prompting an unexpected visit from the local phone company. In general, when using currents exceeding a few amperes, the source should be kept at least 100 m from telephone lines. Tensor, Vector, and Scalar Measurements CSAMT

measurements

can include from two to as

many as ten individual-componentmeasurements,dependingupon the geologiccomplexity and economic constraints. Surveys may be categorized as scalar, vector, or tensor, depending upon the number of components measured and the number of sources used, as shown in Figure 13. Tensor CSAMT.mTensor

CSAMT

is defined as a

five-component (Ex, Ey, Hx, Hy, andH z) measurement using two sourcepolarizations. Two sourcesare required because, unlike natural source magnetotellutics, the CSAMT source is not omni-directional, but fixed in a specificpolarization. As a result, true sensor CSAMT must employ at least two distinct source polarizationsto fully define the impedance tensor. A total of ten componentmeasurements,five with each source, are required. Tensor measurements

are best used in areas where

the structure is very complex and when soundingsare far apart relative to the size of geologicfeaturesunder investigation.As soundingsare made closer together, tensor quantities such as tipper become less important because geologic features are mapped directly by virtue of the higher survey resolution. Hence, since

738

Zongeand Hughes


(a) Tensor CSAMT, Separated Sources

(b) Tensor CSAMT, Coincident Sources Source

Source

I

I

Source2

Hxl

Eyl

Hx2

Eyl

Hxl Hx2 Exl Ex2

Hyl

(c) Partial-Tensor CSAMT, Separated Sources Source I

(d) Vector

CSAMT

Source Source

2

Hx2

Hx

Hyl

(e) Scalar

Hy

(f) Scalar CSAET

CSAMT

Source

Source

Hy

Fig. 13. Definitionsof tensor,vector,andscalarCSAMT. (a, b) Full-tensorsurveysusetwo sources andmakefive measurements per source.(c) Partial-tensorsurveysmake two to three measurements per source.(d) Vector surveysmake five measurementsbut from only a singlesource.(e) ScalarCSAMT makestwo measurementsfrom a singlesource.(f) ScalarCSAET makesonlyE-fieldmeasurements in onedirectionfromonesource;occasional H-field measurements are obtainedto convertthe E-fielddatato approximate resistivities.

CSAMT

tensor CSAMT is relatively expensive, increasingthe soundinggrid density and making less expensive sca-


lar

or

vector

measurements

is

sometimes

better

(Uchida et al., 1987). We have found that even in complex areas, high-density scalar or vector data define structure nearly as well as lower density tensor.

The chief exception is in areas with strong regional anisotropy, for which a full tensor solution to the impedance may be preferred. An important consideration, discussedlater in this section, is that tensor CSAMT imposescertain limitations on where

the measurements

can be made with

respect to the sources. This imposition places greater logistic restrictions on tensor CSAMT than tensor MT. As shown in Figure 13a and 13b, tensor CSAMT can be run in either a separated-sourceor a coincidentsource configuration. The separated-sourceconfiguration provides two distinct, widely separatedpolarizations and approximates the "uniform source" of natural-source measurements. Unfortunately, this separation requires two different source set-ups, a logistic disadvantage. An additional complication is that a problem with differential "source overprint" can arise if the geology differs significantly between the two source sites (see Source Effects section). The coincident-source configuration is more efficient logistically, since the transmitter need not be moved to energize both source antennas. In addition, a differential source overprint is less likely if the sourcesare coincident. Although the distinctivenessof widely separated sourcesis compromisedwith coincident sources, a problem does not usually exist if the survey is designedproperly. Orient the tensor sourcesparallel and perpendicular to geologicstrike, if known. This orientation simplifies the interpretation by directly providing TE and TM mode information. An arbitrary orientation is adequate in caseswhere strike direction is unknown. Also the signal strengths provided by the various source orientations, as illustrated later in this section, should be considered.

When the general strike is known and deviations from strike are sought, "partial-tensor" CSAMT (Fig-

ure 13c)is sometimesused.The H z measurement may be skipped if soundingsare obtained in a dense grid network. In general we do not find partial-tensor satisfactory, since vector work provides nearly identical information Vector

at a lower cost.

CSAMT.mVector

CSAMT

is defined

as a

four-or five-component (Ex, Ey, H x, Hy, andoptional H z) measurementusinga singlesourcepolarization. As such, vector data provide definition of 2-D and 3-D structure, but less uniquely than in a tensor survey. Since vector CSAMT involves 50 percent less data

739

acquisitionand processing,vector is less expensiveto run than tensor. Vector CSAMT is useful for defining complex geology when regional anisotropy is not strong, and is often preferred to tensor CSAMT in 2-D areas due to economic

Scalar

considerations.

CSAMT.--Scalar

CSAMT

is defined

as a

two-component (Ex andHy or, alternatively, Ey and Hx) measurementusing a single source polarization. Scalar is adequate in 1-D layered environments or in 2-D

environments

where

the direction

of strike

is

known. In more complex environments, scalar may or may not be sufficient, depending upon data density. Single survey lines are especially risky in complex geology when using scalar CSAMT. For example, a linear, steeply dipping fault will be easily detected by scalar data if the dipole orientation happens to be perpendicular to the fault (TM mode); however, if the E-field dipole is in a parallel (TE mode) orientation, fault interpretation and location becomes difficult. As a result, scalar measurements over 2-D and 3-D areas

are usually made on a dense grid network. A densely gridded survey partly overcomes the lack of multicomponentdata inherent in vector or tensor. Again the main exceptionis where regionalanisotropyis strong. In such casestensor or vector data may be preferred. The

main

attraction

of scalar

CSAMT

is its rela-

tively low cost and high production speed. These attractions probably explain why the vast majority of CSAMT

data obtained

so far have been scalar.

CSAET.•Controlled-source audio-frequency electrotellurics (CSAET) is a simplifiedversion of CSAMT which dispenses with regular measurements of the

magnetic fieldandmeasures onlyEx orEy. Occasional measurements

of H are made to normalize

the E-field

measurementsto approximate Cagniard resistivities. CSAET can work well for reconnaissancemapping in areas where the H-field is fairly uniform, where the survey area is conducted within a +_15 degree cone about the perpendicular bisector of the transmitter dipole, and where geologyis layered and not complex. However, CSAET

can lead to trouble where these

assumptionsare not valid. For example, one common method of obtaining CSAET data uses a seven-channel machine

to measure

six consecutive

E-field

di-

poles, and to measure one H-field, typically in the middle of the array. Unfortunately, such a system often trades data interpretability for economy. Figure 14 shows

the

results

of two

1-D

models

over

the

upthrown and downthrown blocks of a typical graben fault. Figure 14a shows that the H-field response is higher at station 6, located over the downthrown block, than at station 4. This rather large difference, once squared for the Cagniard resistivity calculation,


740

Zonge and Hughes

can produce a large resistivity error. For example, Figure 14b shows the response obtained at station 6 from CSAMT and CSAET. The CSAMT response (dark line) accurately delineatesthe high/low/highgeology. But the same station, normalized by the lower H-field measured at the center of the CSAET array, produceserroneousresults. The layering appearsto be high/low/high/low/high, and the maximum error is nearly 100 percent. The CSAET data create a false high resistivity feature beneath the alluvial cover. Conversely, the wrong choice of where H is measured can also remove a real feature. Since this example applies to relatively simple geology, obviously more complex situations can yield even worse results. In general, we do not recommend the use of CSAET except for reconnaissancein the most simple geologic environments. CSAET can be used successfullyas an anomaly locator, to be followed with full CSAMT measurements. However, measuring two or three E-fields for each H-field provides an efficient compromise between speed and accuracy.

Optimum ComponentsFor Measurement

When scalar or partial-tensor surveys are conducted, a choice must be made as to the measurement configuration. Theoretically, for a given x-directed

source,Ey/Hx mightbe slightlypreferredin a homogeneous earth because the measurements can be obtained within three skin depths of the source and yet

remainin thefar field."Broadside"Ex/Hy measurements require a minimum separation of four skin

depths,and"collinear"Ex/Hymeasurements havea smaller measurement cone and signal strength, and require a five skin depth separation.

In actualfieldpractice,broadside Ex/Hy dataare usually obtained because signal strengths are maximized and this configuration is more efficient logistically. For example, the Ex dipole is oriented in the direction in which lines are run, making it unnecessary to reorient the wire at each succeeding station. The

Ey/Hx configuration doesnot have this advantage, unless lines are run along the y direction. With lines run along the y direction, the signal would grow weaker as the line moves away from the source or would

(a) Magnetic Field on Both Sides of Fault 1

I

I

I

I

I

I

I

-

I

I

2

I

3

4

I

5

I

S•x Eheld measurements•

;

I

6

One Hheld measurement •

I0_--

on the near field zone

as it moves

tage when the soundingsare to be obtained in a grid which is small with respect to the source-sounding separation, or when land holdings or other factors make data collection along the source bisector impos-

D:O

,. :•.•r•-,•r•,z•

encroach

towardthe source.Hence,broadside Ex/Hydataare mostoftenpreferred.Ey/Hx couldbe usedto advan-

.00m

• ---••.. ISøunding el

sible. CollinearEx/Hy data are almostneverused

:_

unlessrequired by peculiar field conditionsor unlessa crossed dipole tensor measurement is required.

,

ø"i

Plan-View Coverage Considerations

,ound.n

_

(b) Cagniard Resistivity: CSAET versus CSAMT I ooo

-

I

i

•

i

I

I

i

i

i

I

I (Erroneous CSAETResponse

I

I

I

I

-

Sounding 6 using H at station 3.5)J

Response J

(Sounding 6 using H at station 6)

_

I

I

I

I

I

i

2

4

8

16

I

I

I

I

I

I

:52

64

128

256

512

1024

I 2048

I 4096

Fre•luency (Hz)

Fig. 14. Errors in apparent resistivity resulting from CSAET.

(a) H field measurements at two stations on a

six-dipoleCSAET array. (b) Soundingsat station6, comparing the CSAMT data (E and H both measuredat station 6) to the CSAET data (E measured at 6 but H measured only at the center of the array, or station 3.5). Note the large interpretation error resulting from insufficientH field sampling.

The presence of a finite source in CSAMT measurements imposeslimitations on the zones where data can be reasonably collected in plan view. Three factors define the permissible exploration zone: (1) the minimum source-soundingseparation rmin, dictated by incursion of the near-field zone; (2) the maximum

separationrmax, dictated by the threshold of signal detectability; and (3) dependence of signal strength upon angle 4. The minimum separation,rmin,between the source and soundingis fixed by skin depth criteria. The ideal situation

is to remain

in the far field zone for all the

frequencies used in the survey. For an x-directed source, the following restrictions are imposed:

rmin > 4a for Ex/Hymeasurements in "broadside" configuration

rmin > 5• for Ex/Hy measurements in "collinear" configuration

rmin > 3gfor Ey/Hxmeasurements.

CSAMT

These restrictions

can be relaxed

if measurements

in

the transition zone are permitted'

741

From equation (51) for broadside measurements, and for r >> dl, we have:


rmin > 0.5g for all modes. In planning the survey, obtain the best estimate of the characteristicground resistivity in the survey area. This estimatedvalue is determinedby sampleresistivity measurementson a field scoutingtrip, by well log resistivities, or by generalpast experiencein the area. Knowing the desired penetration depth of the survey, the lowest required frequency can be calculated by rewriting equation (30):

rmax• ,rEmin'

(13 3)

in which Emin is the minimumdetectableelectricfield in a given noise condition. For example, consider the case of 10 fl.m ground, a source whose parameters

areI - 30 A andlengthL = 2 km, broadside Ex/Hy

where p is the average resistivity expectedin the area of interest. Then the rmin conditionappropriateto the survey can be selected, thus giving the minimum source-soundingseparation. Figure 15 is a convenient graph for making this determination. The maximum separationrmax is controlledby the

measurements(4• = 90ø), and a random ambient noise level of 10 IxV. Most digital CSAMT systems are capable of stacking-and-averaging successfully in purely random noise conditions at signal-to-noiseratios of up to 1:100. Hence, with Emin = 0.1 IxV (1/100 of 10 IxV), we obtain rmax = 12.4 km. This distance increases with higher resistivities and lower noise levels, but deteriorates in opposingconditions. We shouldnote that there is a practical limit to Emin imposed by the resolution of the measuring system

1/r3 drop-off in signalmoving awayfromthesource.

itself:

fr -

p

(132)

I I I I I I I I I I I I I I I )1 IOO

rmin For Strict Criterion (rmin >4•

(All data in far-field

, , Relaxed , , , Criterion , , , , (rmin , ,>(•.5•), ( 'b ) irmi,n For

zone)

(All data in far-field

,

or transition zones)

o.I

O.Ol

0.0010.125 • i,0•.5 • 2• • 8• • 32• • 128• • 512• i 2048• • 8192I Minimum'Survey Frequency (Hz)

0.125

0.5

2

8

32

128

512

2

8

8192

Minimum Survey Frequency (Hz)

Fig. 15. Plot for determiningthe minimum source-soundingseparation.(a) rmin for far-field data. (b) rmin for far-field and transition-zonedata. Note that these plots are valid only when there is no source overprint present.

742

Zonge and Hughes

ever,in the casewherebothE•,/HyandEy/Hx are


Emin > 2(tq+M_ •_ •),

(134)

in which V is the maximum input signalin volts for the analog-to-digital(A/D) converter, N is the A/D resolution in bits, M is the number of bits in the receiver gainfunction, and R is the required signalresolutionin bits. For example, for 5 volts input to a 16-bit A/D (N = 16), a maximum usable gain of 4096 (M = 12), and a minimum resolution of at least one bit (R = 1), the minimum detectable signal is 0.075 •xV. But the presence of cultural and other noise can limit the maximum gain M, so we frequently find that Emin > 0.5 •xV is a more practical limit. The third controlling factor for plan-view coverage is the minimum and maximum azimuthal angle 0' from the bisector of the source. This problem can be appreciated by studying the plots of Figures 5 and 6. In general, avoid making measurementsin the "null" zones where signal levels are low. This dictates specific "cones"

in which measurements

Depth of Exploration

As observed in equation (30), the depth of exploration is related to ground resistivity and signal frequency. Theoretically, given the characteristicground resistivities in an area, the frequency range necessary to achieve the desired penetration for a given geologic target can be determined. Equation (132) gives the minimum frequencyf• neededfor a given penetration D, and Figure 3b provides a useful diagram for this purpose. In general, to "sound down" a few frequencies lower than the target frequency is desirable and assures

can be made.

Ex/Hy

coverage.

P

f• = 4.0 _.•-- (for data in far-fieldzone) (135)

E•/Hy scalarmeasurements, andFigure16bshows the corresponding zonesfor Ey/H•,scalarmeasurements.

(ca) Scalar CSAMT,

proper

While equation (30) suggests that penetration of many tens of kilometers is possible, in practice this is not the case. As observed earlier, the finite distance of the source imposes additional limitations. In many cases, the lowest frequency used on the survey is limited by the need to stay out of the near-field zone. This "cut-off frequency" f• is related to the maximum source-soundingseparationrmax set by noise criteria:

Figure 16 sumsup the primary plan-view limitations for scalar CSAMT coverage. The patterns shown will change with different ground and logistical parameters. For example, we have noted that in most surveys there is sufficient 2-D and 3-D current scattering to eliminate any effective null zone. Figure 16a shows the zones of data acquisition for

These zones are broad and they allow considerable flexibility in where the measurementsare taken. How-

obtained or where more than one source is used, the

permissiblemeasurementzones for data overlap become rather narrow for the homogeneoushalf-space.

rmax

p

(for data in far-field

f• = 0.06/'max 2 ortransition zones) (136) ' (b) Scalar CSAMT,

Ey/H x

Fig. 16. Acceptablezonesof measurementfor scalarCSAMT configurations:(a) ScalarEx/H;Y (b) ScalarE/Hx Y ß Measurement zones will vary in each survey, dependingupon ground resistivity, current, instrumentationhmits, and layout geometry. Note that this figure is for a homogeneous earth; measurement zones in 2-D and 3-D environments are usually broader.

CSAMT

These relations include a number of assumptions (worst-case signal-to-noise ratio of 1:100, receiver gains not severely limited by cultural noise, etc.). But


we believe

that the relations

are at least useful

for

illustrating the point that equation (30) cannot be used blindly in CSAMT exploration. Figure 17 shows the expected limitations on penetration depth imposedby the cut-off frequency; again, this limitation is based upon a number of assumptions which vary with survey conditions and instrumentation. Note that penetration is most stronglylimited in noisier environments(requiring higher Emin), and in situationswhere the explorationist requires only farfield data.

As a very rough rule, we have found through practical experience that CSAMT is most useful for penetrating to about 2 to 3 km. Some environmentspermit deeper investigation, especially when transition zone data are allowed, but 2 to 3 km is a good conservative figure for CSAMT. Note that CSAMT has a shallowpenetrationlimit as well. Typically the top 10 m or so is too shallow for CSAMT, dependingon surfaceresistivities. Thus very shallow targets are the domain of higher frequency portable EM methods.

743 Resolution

The

vertical

i

Ioo

resolution

of subsurface

features

i

i i i I II I

-

•

-

I

I I I I 3

o

I

IO

I

I

(.9

I I I

I

I I I 1.0

0.1

1.0

Penetration Depth (krn) (a)

I0

de-

pends upon their lateral size, thickness, depth, and resistivity contrast with respect to background. The resolution of a conductive layer is easier than resolution of a resistive layer. A very rough rule-of-thumb is that a conductive layer will be resolved if its thicknessto-depth ratio exceeds0.2 times the squareroot of the ratio of layer resistivity to background resistivity. A resistive layer will be resolved if its thickness-to-depth ratio exceeds 0.2, given a resistivity contrast of 10:1 or better. Thick buried layers, such as basement, are better resolved than intermediate layers. 2-D and 3-D bodies are often harder to resolve than layers at an equivalent depth. Both TE and TM mode (E-field parallel and perpendicularto structure) data are sometimes needed to properly define 2-D and 3-D structures. Further information on resolvability of various types of features is provided in the Far-Field Data Interpretation section. The horizontal or lateral resolutionis related primarily to the size of the E-field dipole. As a general rule, the lateral resolution for TM mode is roughly equal to the dipole size. Smaller targets may be detectable,but resolutionof their positionis still dependentupon the

IOOO

o

Considerations

0.1

1.0

I0

Penetration Depth (krn) (b)

Fig. 17. Limits of CSAMT penetration,with the assumptions notedin the text. (a) Limits on Penetrationfor Strict Criterion (all data in far-field zone). (b) Limits on Penetrationfor Relaxed Criterion (all data in far-field or transition zone). These plots may vary considerablydependingupon the measurementsystemand noiseconditions.


744

Zonge and Hughes

E-field dipole size. Hence, effectivesurvey planning requiresoptimizationof the dipolesizeto the expected minimum target size and to the degree of resolution desired in defining the edges of interestingfeatures. Some examples of lateral resolution are shown in Figures18 and 19. The field data in Figure 18 demonstrate the increased resolution obtained with decreas-

ing E-field dipole size. Since the received signalis proportionalto the length of the E-field dipole, the practicallimitson lateralresolutionare determinedby backgroundnoiseandthe limitsof signaldetectability. Figure 19 compares60 m dipole-dipoleresistivitydata with 15 m CSAMT resistivity data along the sameline

whichpassedover a near-vertical,subcropping pyritic dike-like

feature.

Be aware that lateral resolution at depth is a function of signal wavelength and array size. Resolution decreasesat lower frequenciesdue to the expansionof the investigationzone of the longer wavelengths.

0 WEST I 2048

Hz

1024

Hz

2 I

4 I

782

824

,,,

,,, I

+

+•

The issue of data density is related to resolution both in a vertical and a horizontal sense. Adequate

density is needed to properly define the exploration target, but there is always a trade-off between full target definitionand economiclimitations. Vertical density is normally dictated by the frequency range (e.g., 1, 2, 4, 8,... Hz) typical of modern digital instrumentation.Although the lowerfrequency data are much more sparsely spacedthan the higher-frequencydata in terms of linear depth, remember that resolution is also degraded at lower frequencies.This result makes extremely high data densityat low frequenciessomewhatsuperfluous.It is possibleto improveresolutionby increasingthe number of frequencies or by using harmonic waveform decompositiontechniquessimilar to those used in complexresistivity measurements(Zonge and Wynn,

6 I

$22

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Data Density Considerations

8 I

506

+

+

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....... •:,•:::• •i::iiiiiiii::.--'!i•::::i::::..-'•iiiii•i• ................... .-..:•:•,•.-..-.:..-•:-• 5+98

512 Hz 256 Hz 128 Hz 64 Hz

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Hz

1024

Hz

512

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256

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128

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64

Hz

WEST 2:048

Hz

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512

Hz

256

Hz

128

Hz

64

Hz

883

3 I

4 I

5 I

42

341

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I

4 I

I

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844

96

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746

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.........

6 I

7 I

I

8 I

9 lEAST A = 30m

lEAST 522

•.-;,.<.,;• '-:•.•:•:•::.•:::•::•:•:•:..;::::,•:.•::.

A=15m

t'{*"•" '•$,li::*:t:-;40 '*" ,-'-.**?;•.....'.... "*•-, •' 389 2+933+232+95

Fig. 18. A field exampleshowingincreasingresolutionwith decreasingdipolesize.

12 lEAST A =60m

491 415 +


CSAMT

1975; Zonge and Hughes, 1981). However, the chief advantageof the latter would be in terms of acquisition speed, not data density. Horizontal or lateral density is especiallyaffected by the trade-off between geologicnecessityand economic constraints. At a minimum, the survey should be designed to obtain a sufficient quantity of data over both the features under investigation and over "background." For running a line over an area of interest, a general rule is to gather 50 percent of the data over the feature of interest, and 25 percent on either end of the line to establish the background response. The data base should be sufficient to fully define anomalous features. Generally this definition of features is accomplishedby selectinga dipole size smaller than

the smallest

feature

to be resolved.

The

data

density should allow true geologic effects to be distinguishedfrom spuriouseffects due to culture, topography, differences in TE/TM mode behavior, etc. Finally, regular station spacingsare desirable in order to avoid biasing effects in the interpretation.

0

I

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4113

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1309

4

I

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!

430

6

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1166

1159

I

818

745

Topographic Considerations

Any E-field measurementwill be affected by current density changes resulting from topography, and CSAMT is no exception. Higher current densities are found in valleys, resulting in artificially high resistivities, while lower current densities are found in hills, resulting in artificially low resistivities. While these effects are not easily avoided in the field, it is prudent to consider them when laying out survey line locations. If topographic problems are anticipated, the survey crew should obtain enough data to distinguish topographic and nontopographic anomalies. As pointed out in the Far-Field Data Interpretation section, topographic effects can be removed in data processing,and if not removed can strongly influence the interpretation process. It is important to identify all stationson a topographicmap to aid data interpretation.

GeologicConsiderations

Careful consideration of geologic trends prior to designingthe survey is important. This consideration is particularly critical in areas where a prevailing geologicstrike exists. The geologywill dictate whether scalar measurements will suffice, or whether more complex measurementsare needed. Geology also will determine

the orientation

needed to achieve TE or TM

mode measurements, whichever is desired. As with all

groundedelectrical techniques,care must be exercised to avoid coupling the source and soundingareas to the same conductive feature. As with culture, this cou-

pling can result in hard-to-interpret current-channeling effects. Highly conductive faults, steeply dipping beds with high clay or graphite content, and linear alteration zones are examples of geologic features requiring extra I

so,,,;;, ;o '

2

;.o

/ 4.3 4.0 4.44.3|•

3 I

I

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I

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I

I

NOSE

1.02.12.9 1941 3.03.9 2048Hz

Volues in K,Q

L52.7 4.1 2.9 6.•7• 4.7

%8 Ibl5'2 6,•.• 1.3

care.

4

' •Y•.8 3•Hz

Fig. 19. A field example comparinga dipole-dipoleresistivity pseudosection(top) with a CSAMT resistivity pseudosection (bottom) along the sameline. A-spacingfor the dipoledipole survey was 60 m; the E-field dipole size for the CSAMT survey was 15 m.

Loop-versus-DipoleSources

While the far-field resistivity and phase are the same for either a grounded dipole or a loop source (in a homogeneous earth), the signal strengths are not. Generally the signal from a loop is weaker than that from a similarly sized grounded dipole. On rare occasions, however, it is difficult to achieve a workable set

of electrodes for the dipole. This problem can occur, for example, on uncoveredbasaltsor in extremely dry, weathered alluvium, where high electrode contact resistanceseverely limits the amount of current which can be transmitted into the ground. In such cases it may be advantageousto use a large, ungroundedloop for a signal source, avoiding the contact resistance problem entirely. In order to determine which source gives the best

746

Zongeand Hughes

signal,the ratioof the fieldstrengths for a dipoleanda loopneedsto be calculated. Fromequations (51)and


(79) we have for the electric field

(E,) dipole 21oLr sin4) (Eq,)loop 3IL Sn '

In general,a dipolesourcewill coupleenergyinto the ground better than a loop source in most field (137)

and from equations(53) and (80)we have for the magnetic field

(Hq,)dipole21o Lr sin q> (H,)loop 3I• Sn

thesource, 4)= 90ø(4)'= 0ø)andE4,= Ex. Whenthe dimensionsof the dipole and loop are much smaller thanr, the E- andH-fieldsfor the dipoleandloopare equal under the followingconditions:

I•

ceeds500f•. Also, the dipoleallowsfieldpolarization control. Thus the loop is rarely used in CSAMT or long-offsetTEM exploration.

Normal transmitterwire (e.g., 14 gauge)has a self inductanceof approximately 2.6 mH/km. Usingthe reactance

Xz, = 2'rrfL

(140)

where L is the total inductanceof the wire, we can see that at highfrequenciesthe ac resistanceor inductive reactanceof wire usedfor loopsor dipolescouldhave an impedancevalue much higher than its dc resistance. For example, at 4 kHz the ac resistancewould be 65 f•/km due to self inductance,which would limit To calculate the effect wire resistance and reactance

(139)

Equation (139) is plotted in Figure 20 for various

distances r andcurrentratiosIo/Iœ, assuming a singleturn loop (n = 1) and a reasonablerangeof source-

will haveonlimitingcurrentin a loopor dipolesource, proceedasfollows.For a loop, the effectiveresistance is

gEL= V/[(Rw ' mw)2q-(XL' mw)2], (141)

geometryratiosS/L in meters.The practicaluseof this

plotin thefieldwouldbe asfollows.Suppose a crewis requested to runa scalarsurveyover100f•.m ground and the client wants data to 4 Hz.

formula

the currentthat couldbe usedat highfrequencies.

3Sn

2Lr'

situations unless the electrode contact resistance ex-

Wire Impedance

(38)

In theseequations, Io is the currentin thedipole,Iœis the currentin the loop, L is the lengthof the source dipole,S is the area of the sourceloop, andn is the number of turns in this loop. For the case of a measurementdirectly on the perpendicularbisectorof

Io

involvedin puttingin a loop, landpermittingrestrictions, amount of wire available, etc.

The minimum

source-soundingseparation of 48 is then 10.2 km.

Whileputtingin electrodes for a grounded dipole,the crew encounters high contact resistances of 250 f•

despiteeffortsto improvethe electrodequality.Their transmitteris limitedto a maximumoutputvoltageof 1000volts, allowinga maximumcurrentoutputto the dipoleof 1000/250 = 4 A usingOhm'sLaw. Knowing thatthe sametransmittercoulddrivea loopwith 40A, the crewchiefwishesto knowwhethera loopsource would provide more signal.

Fromthe amountof wire available(say,2000m), he can either use a 2000 m dipole or a 500 x 500 m single-turnloop. Therefore, his initial Sn/L ratio is set

to 125m. Goingto Figure20, he seesthat the point definedbyIo/I œ= 0.1 andr = 10.2kmliesto theright of the Sn/L = 125line, indicatingthat he is betteroff usinga dipole.A loopwouldbe moreeffectiveonlyif Io/I œ = 0.02, or Iœ = 200 A. Since attainingthis current level is impossible,he now has the choice of

gettingmorewire to make a biggerloop, or simply beinghappywith 4 A in the dipole.In makingthese choices, the crew chief must consider the minimum

signal needed for data acquisition,the extra time

where

Rw = wire resistanceper unit length Xœ = wire reactanceper unit length Lw = length of wire. For a dipole, the effective resistanceis

R•:o= V/[(Rw ' Lw + RG)2+ (XœßLw)2], (142) where R G is the total groundingresistanceof the electrodes

in ohms.

To demonstratethe severityof this problem,for a 2000 m dipole, with a total grounding(electrode) resistance of 25f•, andtotalwire resistance of 16f• (8 f• perkilometer)themaximumcurrentthatcanbeput into the groundat 1 Hz (no appreciablewire inductanceeffects)with a 1000V transmitteris 24 A (1000 V/41 ohms = 24 A). However, at 4 kHz the wire reactance is 130 f•, so the total impedancefrom equation(142) is 134 f• which will limit the current at 1000 V to 7.5 A.

A techniquewe usedto mitigatethis effectis to run two transmitterwiresin parallel,separatedby approximately1 m. The net effectis a doublingof the usable currentat the highfrequencies, in thiscaseincreasing the currentat 4 kHz to 15 A. Runningparalleltransmitter wires will becomecommonpracticeas high

CSAMT

higher frequencies.

volvesa frequencyrangeof 1 to 8192 Hz. In moderate resistivity ground, scalar soundingsover this frequencyrangerequire30 minutesto an hourto acquire.

Exploration Economics

Vector

The economicsof CSAMT explorationdependupon a numberof factors: type of receiver, frequencyrange, groundresistivity,ambientnoise,chargeratesfor the survey work, etc. Typical CSAMT exploration in-

hours per sounding,dependingupon specific survey logistics.For purposesof comparison,MT presently averagesone or two stationsper day (keepingin mind that MT achievesfar deeper penetration). In compar-

powered EM techniquesare pushedto higher and


747

and tensor

measurements

take

one to two

io

loop ,•00. • dipole

better•

better

dipole

loop

better\ i.o

better

Ooo

op

etter

•

better\•,00 %

loop

i•iePt•ele r

be O.I

loop better

0.0110 0

ipole better

I000

I0 000

I00 000

r(m)

Fig. 20. Plotsof equalelectricfield strengths for dipoleversusloop sources.The plot servesas an aid for determining whena loopsourceisbetterthana dipolesource; seetextfor anexplanation of theprocedure forusing this plot.

748

Zonge and Hughes


ison with reflection seismics, for scalar CSAMT coverage of five stations per kilometer, scalar CSAMT would average one-third to one-fifth the cost of seismics, using present day costs.

When

to Use CSAMT

addressed

with

Differences

MT.

Within the 10 rn to 3 km depth range, CSAMT must be compared with natural-source AMT, offset TEM, in-loop TEM, and galvanic resistivity soundingsbefore selecting the best technique for the field project. In our experience, the following observations are helpful in making this selection. First, we must considerwhether ground polarization data are required. If so, induced polarization becomes the best choice, as extraction of polarization information from EM soundings is still in its infancy. But if ground resistivity is the main parameter of interest, other criteria such as data quality, resolution, efficiency, etc. must be considered. CSAMT will normally acquire data faster and produce significantlybetter data quality than AMT or MT. Also, in environments typified by high cultural noise, the stronger artificial signal often gives CSAMT an advantage over the natural source methods. Frequency-domain filtering provides a better signal-to-noise ratio than is usually possible with time-domain EM methods; hence CSAMT is often preferred over TEM in heavily cultured or otherwise noisy areas. Resolution is a significant advantage of EM sounding methods, especially for deeper targets. The lateral resolution of CSAMT, AMT, and MT is a function of frequency and dipole size. To investigate near-surface features, the dipole size can be kept as small as a few meters. TEM has similar advantages. EM techniques that provide vertical sounding based on time or frequency minimize the areal extent that is averaged into a particular sounding. But geometric sounding techniques such as galvanic resistivity surveys using Schlumberger, Wenner, dipole-dipole arrays, etc., must increase their array size in order to obtain deeper penetration, causing them to average a larger volume of ground in the measurements. These techniques

in vertical

resolution

between

EM

and

dc resistivity techniques are generally minor, given a layered earth environment and sufficient sampling. Differences in ability to estimate depths under these conditions

Every geophysical technique has both advantages and disadvantages, and CSAMT is no exception. Hence, consideringwhen use of CSAMT is appropriate and when it is not is prudent. At present, CSAMT penetration is limited to the range of about 10 rn to roughly 3 km, dependingupon ground resistivity and frequencies used. Pushing CSAMT beyond these limits is usually a poor idea. Shallower problems are better addressed with highfrequency shallow EM systems. Deeper problems are better

provide less resolution compared to EM sounding techniques.

are also minor.

Changes in surface geology can compromise interpretability of galvanic resistivity techniques due to their large arrays. But the smaller EM soundingarrays overcome this problem. Static effects are a significant problem in systems which measure the electric field. Hence, TEM is a

good technique in areas known to have statics problems. AMT, CSAMT, and MT data must be corrected

for statics(See Far-Field Data Interpretation section). Source effects sometimes plague controlled source surveys when the geology beneath the source differs significantly from that at the sounding site. This difference requires extra care in designingthe survey. Logistic efficiency (and thus cost) is a crucial consideration. CSAMT is logistically very efficient for local grid-type surveys, but becomes inefficient for very long cross-country traverses due to the need for numerous transmitter locations. In the latter case, natural-source methods and TEM methods may be more useful. But most geophysical studiescan be done

with one or two CSAMT sources. In flat, open areas, in-loop TEM systems move significantly faster than CSAMT. But in more hilly and vegetated areas, CSAMT becomes more efficient. It is worth noting that all the EM soundingtechniques within the 500 to 3000 rn depth range are more efficient than galvanic resistivity. In terms of data acquisition, CSAMT instrumentation is considerably more portable and less complex than present MT instrumentation; it is roughly as portable as AMT and TEM instrumentation. CSAMT stackingtime is typically faster than AMT for the same spectral band and data quality. Compared to TEM systems, CSAMT stacking time is faster in noisy environments due to better noise rejection in frequency domain. Offsetting this is the fact that TEM gathers its data more efficiently, stacking long transients, while CSAMT gathers data one frequency at a time.

In summary, the EM sounding methods often have strong advantages over galvanic resistivity methods. But the choice of which EM method to use depends upon a number of site-specific factors. We have also found that personal bias usually enters into the equation as well.

With

the full arsenal

of electrical

tech-

niques at our disposal, we encourage flexibility in survey design in order to best address the project

CSAMT

objectives. That

design sometimes will

include


CSAMT, and sometimes it won't.

Also, some projects benefit from the use of more than one technique. For example, AMT/CSAMT and MT make a good combination when surface-to-depth information is required. TEM is a good tool to team with MT when static effects are a problem. CSAMT or TEM and galvanic resistivity work well together to help solve conductivity thicknessproduct ambiguities. FAR-FIELD

DATA

INTERPRETATION

Types of CSAMT Data

CSAMT directly measures four basic field quantities: (1) electric field magnitude, (2) electric field phase, (3) magnetic field magnitude, and (4) magnetic field phase. These quantities may be measured as scalar, vector, or tensor quantities. Apparent resistivity and phase difference are calculated directly from these measuredquantities. Figure 21 showsthe behavior of these parameters in homogeneousand layered earths.

Electric field magnitude (E, MKS units of volts per meter or CGS units of millivolts per kilometer) is obtained from the potential difference measured across a grounded dipole. For ease of interpretation, all data should be normalized

for current.

E-field

data

are analyzed as a separate parameter in order to determine sourcesof noise or peculiar responsesand to identify the incursion of near-field source effects. E-field data can be adversely affected by "static" distortionsdue to edge effectsfrom small 2-D and 3-D bodies. They also can be affected by sourceoverprint and the electrode contact resistance (ECR) effect.

749

data to lateral resistivity contrasts, and therefore H-field inversions are most responsive to large-scale layering or gross 2-D and 3-D structures. Vertical H is more sensitive to lateral structure. Like E, H is useful

for identifying the incursion of near-field effects. As shown in Figure 21, the magnetic field for a homogeneousearth slopes at an angle of 27.5 degrees for the far-field, and saturates in the near-field. The

far-field slope stems from the dependency of H upon the square root of frequency, from equation (53). Interpretation is facilitated by using a transparent overlay with parallel lines oriented at 27.5 degrees. Excursions above the lines as the frequency decreases indicate transition from a low to a high resistivity environment; excursions below the lines indicate a

transition from high to low resistivity. Magnetic field phase (q•/•, units of milliradians) is the phase lag between the transmitted signal and the measured magnetic field signal. In a homogeneous earth, the magnetic phase is -,r/4 rad in the far-field zone and zero in the near-field zone, with intermediate behavior in the transition zone (Figure 21). Behavior becomes more complex in a nonhomogeneousearth due to slope changes in H. Magnetic phase is not normally used separately in interpretation.

Apparentresistivity(Pa, units of ohm-meters)is the quantity most used in CSAMT work. The parameter is calculated directly from E and H by use of equation (22). Various methodscan be used to calculate apparent resistivity (Spies and Eggers, 1986), but most often the simplified Cagniard relation (equation 60) is used for the calculation, as in MT. However, note that the Cagniard relation holds only for the far-field, or planewave zone. In the transition zone and near-field zones,

Electric field phase(q•œ,units of milliradians)is the phase lag between the transmitted signal and the measured electric field signal. In a nonpolarizable homogeneousearth, the electric phase is zero except in the transition zone, where a change of slope in E occurs. In a nonhomogeneousearth, q•œ is often nonzero due to slope changes from resistivity contrasts. E-field phase is not normally used as a separate interpretation parameter. But since the E-field phase contains a small contribution from ground polarization, it may prove to be more diagnosticin the future. Magnetic field magnitude,H (MKS units of amphere per meter, or CGS units of milligammas, where 1•/=

the Cagniard relation is invalid and yields unrealistic resistivity values. The E- and H-fields change character in the transition zone and the H-field is no longer a function of frequency or resistivity in the near-field. The result is that Cagniard resistivities frequently generate a notch in the transition zone and slope at an angle of 45 degrees in the near-field (Figure 21). This anomalous behavior is strictly an artifact of using an inappropriate equation for calculating resistivity outsideof the far-field. But to calculatethe exact apparent resistivity in the near-field and transition zone, the exact geometry of the receiver-transmitter locations

10-2/4,rA/m = 1 nT) is obtained fromthe voltage

Phase difference (q•, units of milliradians) is the phase of the impedance, and is calculated from the differencebetweenq•œand q•/•. As shownin Figure 21, the phasedifferencein a homogeneousearth is ,r/4 rad (785 mrad) in the far-field, and zero in the near-field. Phasedifferenceis proportional to the slope of the log of the resistivity (equation 97); values higher than ,r/4

measured in a high-gain antenna and is normalized for current and antenna characteristics. H is affected by source effects, but not by static effects. Hence, H-field inversions are sometimesuseful for producing staticfree resistivity data. However, bear in mind that horizontal

H-field

data are less sensitive

than E-field

must be known.


750

Zonge and Hughes

HOMOGENEOUS

EARTH

LAYERED

CROSS-

d=0 SECTION I• IO,•.m

SECTIONd=o

I• Near

I

IO

Zone

,TZ,

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I

IOm !•1100'•'m

lOOm •1.

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CROSS-

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Zone

<•NFr:•

,TZ.

Field

•:•=:=3FFc:• I

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, , , , , ,

/,

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, , , , , , ,

,

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•-:-ßI00•000 F Cacjniard Resistivity __• 1 I•000 / 45øNF Sløpe N •

-

•m

/145• øNFSlope ••

•

/

•

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[ , oool-•

<3: •

/ I I I I I I I I I•

I I I

Layered Earth

•

Homogeneous Earth

_

s,... ,.cr..•\

-

Response

-

I /•

O- •+..•,'• ,,., o.,. ••

•,,..r,

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'•o 7/' o 01_ • •

mogeneous

ß-

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•IT

Earth Response 0 ••

-,,,j FSaturation

Loy• Respan.

/ I 2I I 8I I 32I I 128 I I 512 I I 2048 I I

I 2I I 8I I 32I I 128 I I 5Ii2 I 2048 I I

Frequency(Hz)

Frequency(Hz)

Fig. 21. Behavior of the measuredand calculatedCSAMT parameters,from 1-D models.Homogeneousearth and four-layer curves are contrasted.


CSAMT

indicate high-over-low resistivity layering, while values lower than ,r/4 indicate low-over-high layering. Phase difference is a very useful quantity. Because phasedifference is related to the derivative of resistivity, it is insensitive to static offsets, and hence can be used to judge when such effects are present and to assistin applying proper corrections. Phasedifference is also useful in judging differencesin the character of the soundingsfrom one zone to the next, helping to visually sort out the various types of responsesfound in a given survey area. From a singleresistivity/phase measurement at one frequency, the data interpreter can tell if he is looking at a high over low resistivity environment(phasedifferencegreater than ,r/4) or low over high (phase difference less than ,r/4). Often phase differences greater than ,r/2 or less than zero are observed in field data. This variation is usually due to one of two sources: (1) anisotropy (see Figure 30), or (2) current gathering due to 2-D or 3-D effects. Phaseis helpful in evaluating noise and is also useful for identifying the location of the transition zone, where phase contours in a pseudosectiontake on an artificial "layered" appearance.Like •œ, phasedifference may prove of even greater use in the near future if efforts to recover polarization information succeed. This topic is discussedlater. Tensor quantitiesare useful for interpreting2-D and 3-D structure when high density data are not obtained. Tipper, which is related to the ratio of horizontal H to vertical H, indicates the presence of and direction to local inhomogeneities.Skewness and ellipticity also aid in such determinations. The procedure is outlined later.

Data Processing

Modern data processing capabilities allow a great deal of flexibility in manipulating data. But they also open the door to confusion or incorrect interpretation if great care is not taken to understandthe processing procedure and its inherent limitations. Processingcan modify the appearance of data, hence care should be taken to understand and validate the wide range of processingchoices available to the interpreter. Present CSAMT field systems store data in solid state memory or on magnetic tape. At convenient points during the survey, usually on a nightly basis, the data are transferred to a computer for processing. Processingis done in two stages:preprocessingand interpretive processing.Preprocessinginvolves an examination

of the data set for errors

and noise.

The

work utilizes magnitude and phase for all E- and H-field componentsmeasured, the calculated apparent resistivities and phase differences, and data scatter (preferably both standard deviation, and coefficient of

751

variation). This suite of data enables the data processor to track down any existing data problems. He may elect to reject stacks clearly in error or abnormally noisy. Helpful information in this refining processis a set of original notes by the field operator, a listing of transmitter current settings, a listing of the gain settings utilized by the receiver, and so forth. Data listings and terminal screen graphics are invaluable tools in this process. Once the data are in acceptablecondition, interpretive processing begins. Interpretive processing involves optimizing plotting conventionsfor the particular data set, as well as enhancing certain effects by various processessuch as normalizing, static correction, filtering, and derivative calculation. Normalization removes layering effects by subtracting, dividing, or otherwise deconvolving equal-frequency or equal-depth average values from a data set. Deconvolution is very helpful in removing regional effects and enhancingthe appearance of subtle lateral effects in a complexly layered environment. Static corrections are very helpful in removing the erratic appearance of static-affected frequency data. All existing static-correction processing(except techniqueswhich use independentdata sets) are somewhat arbitrary, so the processormust use caution in making the corrections.

Effective filtering makes a noisy or complex data set easier to look at, and helps evaluate overall trends in the data set. Conversely, if very specific, subtle features are to be emphasized,secondderivative plots are quite useful. Generally these data show a double-lobed (positive and negative) behavior at each change in curve shape. Derivative data have the disadvantageof enhancingany noise present in the original data set. Recalculating parameters often adds insight into the interpretation process. For example, calculating apparent resistivity from phase information produces static free data and is a good check for static offset problems. Phase information can also be calculated from the resistivity data. Upon comparing the resistivity-derived phase data with the measured phase difference, the possibilitiesof anisotropic or induced polarization effects can be investigated. Comparingprocessedresults to the original resistivity and phase information is important in order to maintain a sense of reality in processing, particularly in multiple-step processing. If the basic information contained in the processedresults cannot be observed in the original data, possibly in a less enhanced form, something is wrong. Processing should emphasize selectedtrends in the original data set, not create new ones.

752

Zonge and Hughes


Data Display

Three types of data display are normally used for CSAMT data: (1) soundingplots, (2) pseudosection plots, and (3) plan-view plots. Soundingplots (Figure 22) of magnitudeor phase data are quite usefulfor interpretation.Magnitudedata are usuallyplotted with the log of the magnitudeon the vertical axis and the log of the frequencyplotted on the horizontal axis (Figure 22a). Phasedata are plotted in a similar manner but with linear phase on the vertical axis. The normal convention in CSAMT is to plot higherfrequencies(shorterperiods)to the right; notice that this is the reverse and TEM

of the convention

used in MT

work.

the

Pseudosectionplots (Figures22e to 22g) show data plotted with stationnumber on the horizontal axis and decreasingfrequency or increasingdepth on the vertical axis. Pseudosectionsare representationsof vertical-soundingplots. These are not "cross-sections" and should not be viewed as such, since the data represent a bulk averageof effects from the surfaceto some depth D. The data must be inverted to yield a true cross-section.

Several

choices are available

for the vertical

scale in

pseudosections.Plots with data positioned at evenly spaced log-frequencyintervals (Figure 22b) are the easiestto read. However, sincepenetrationis proportional to the squareroot of frequency, this type of plot artificially expands the plot-point spacings at high frequencies and compressesthe spacingsat low frequencies. This expansion and compression has the effect of artificially enhancingminor near-surfacefeatures, however, the problem is diminished in a plot which

rescales

to representthe chief electricalcontactsin the ground. The geologistsand the geophysicistcan then work toward relating these electrical features to actual geologic features. Plan-viewdata maps(Figure22f) can be constructed on surveyswith griddedcoverage.Generally each plot consistsof magnitude or phase data at a given frequency or at a specifieddepth. The maps may be 2-D contouredplots or computer-generated,3-D perspective plots. Several notes about data contouring are in order. Magnitude data, which have only positive values, are often contoured in logarithmic intervals in order to accommodatethe large range of resistivitiesfound in

the vertical

axis

to account

for

the

square root dependence of depth of penetration on frequency (Figure 22c). Such a plot is much more useful for interpretation. However, it only accounts for the frequency parameter in the depth relation of equation(30), ignoringthe influenceof resistivityupon penetration. A third type of plot, the pseudo-depth pseudosection(Figure 22d), accountsfor both resistivity and frequencyinfluenceson penetrationby plotting the calculated depth on the vertical axis. This calculation must be done after the data have been corrected

for static effects, which can distort depth estimates. Further, this type of plot is valid only for far-field data. Special care must be taken with these plots to avoid literal interpretation of depths to certain features; again, inversion or modeling is needed before doing this.

Geoelectriccross-sections (Figure 22e) are obtained by inverting the pseudosectiondata using computer modelingtechniques,preferably aided by somesort of independentgeologiccontrol. Cross-sectionsare used

earth.

We

often

use

a contour

interval

of

10

contoursper decade(10.0, 12.6, 15.8, 20.0, 25.1, 31.6, 39.8, 50.1, 63.1, 79.4, 100.0, etc.). Contour interval is varied dependingupon desiredcontour density. Data which have both positive and negativevalues, such as phase difference, are contoured at linear intervals. Shadingor coloringcan be usedto emphasizefeatures of interest.

Certain symbols for culture and other features are used in pseudosectionand plan-view data. Figure 23 lists these symbols. Noise Analysis

CSAMT data quality depends primarily on the source current, the source-soundingseparation, and the ambient

noise level.

We have found that CSAMT

often shows better data quality than natural-source techniquesdue to the stronger signal. For example, most exploration data which we have collected have standard deviations under ___10percent. However, noise is always a factor to be examined carefully. In a properly designed system, most noise in a measurementwill be nonsystematic, or random, at least when considered over a long period of time. Hence, atmosphericand telluric noise, can be averaged out by stacking-and-averaging whereas lightning spikes and cultural noise often cause problems. The random error present in any measurement of the electric and magnetic fields can be estimated by obtainingn stacks,each of which containsthe averageof m consecutivewaveforms, and examining the divergence of each individual stack value xi from the averagevalue n

Xi

(143)

5;x,-

(!44)

X = -i=1

Sx= --1 i=1


CSAMT

-

INDIVIDUAL

STATION

PLOT

i•_ ((•) Log Resistivity-Frequency Plot

•n30.El'''''''

i

i

I

I õ 12

.._

•

• C•

I

'

2

I

4

I

I

8

16

PSEUDOSECTION

Pseudosection 9

I

I

64 128 2õ6 Frequency ( Hz)

i

-I 1024

I

I 2048

40•

(c) Squere RootFrequency (d)Pseudo-Depth Pseudosection

I0 I! 12 13

8 9

I0 II 17_13

8

4096 4096 •_•• 'ø"'"1-•'•.-•.W.,•.,--2"/• I ""2561-

,o.. r•:••.•////_.•. •

:32 16

---

>,

3•

ß

16

•.

i (f) PLAN - VIEW

i

PLOTS

(b) LogFrequency 8

I

•2

753

Pseudosection

(e)Geoelectricol

9 I0 II 12 13

8

Interpretorion

9 I0 II 12 13

I ,,o

I•0 ,,o1-.... •

,,o

.,o i••• •

•. -")0

)0

•' • )0

• }0

•,o

,•o•:

ii )o

i )o

,,•-,,,

'•'•

PLOTS

Fig. 22. Plottingconventions for CSAMTdata.Seetextfor explanation. Plotsshownarefor apparent resistivity; otherparameters,suchas phasedifference,alsomay be plottedin a similarfashion.

754

Zonge and Hughes


where sx is the standarddeviation. The coefficientof variation (Vx) is the percent change of the standard deviationcomparedto the averagemeasurementitself

Sx

Vx=--.,y x 100%.

(145)

A weightingfunction basedupon the numberof waveforms measuredper stack can be employed if desired, although we do not believe that such practices are necessaryin actual field conditions. The error present in the Cagniard resistivity calculation is found by propagatingthe error in E and H:

so=20

+ •--

.

1-D Interpretation

1-D interpretation is generally sufficient in simple stratified geology, or where the areal extent of lateral resistivitychangesare largewith respectto the survey grid density. When such conditions are met, use of existing computer modeling routines can give relatively fast inversions. The CSAMT section presented the basic theory for these algorithms.In addition, 1-D interpretationis simplifiedenoughthat standardcomputer-generatedtype curves(e.g., Figure 10) can also be used for rough interpretation.This is not the case for 2-D

and 3-D environments.

A useful characteristic of a 1-D environment is that,

in the far-fieldzone, apparentresistivitynever changes by more than the correspondingchangein frequency. Hence, a log-resistivityversus log-frequencyplot al-

(146)

ways showsslopes<1---45ø1 ß Larger far-fieldslopes

(147)

note that even in a 1-D earth, larger apparentresistivity changes and negative phases can occur in the transition zone. This subject is expandedupon in the

indicate that the environment is 2-D or 3-D. However,

The error in phasedifferenceis given by:

Case Histories

in whichS•E istheerrorin theelectricphasemeasurementandS0,His the errorin magneticphase.Equations (146) and (147) assumethat the noise sourcesin E and H are uncorrelated.In the caseof a nearby noise source such as a thunderstorm,this is not always the case, and some noise spikes are highly correlated. In such a case the error is modified by a correlation coefficientC, which varies from 0 (totally uncorrelated) to ___ 1.0 (totally correlated).The correlationof noise is difficult to determine without adding noise analysisequipmentto the survey, but such complications are rarely necessaryor desirable.

CULTURE

pipeline

0 well location

buried cable

ßoil well

majorpowerline

•

naturalgaswell

•

dry hole

'1" powerline T

DRILLING

phone line

-•--•- dryholewithoilorgasshows

fence

•0 injection well

railroad

o water well

'57 Chevy

A drillrig

Fig. 23. Symbolsused in CSAMT data plots.

section.

Resolution.--Figures24 and 25 provide someinsight as to the resolvability of layers with CSAMT. The figuresshow the resultsof 1-D modelingof a horizontal layer imbedded in a 10 fl.m half-space. The conductivity, thickness, and depth of the layer were varied in order to show the dependenceof the curves on these parameters.The amplitudesof the anomalies decreasewhen the layer becomesthinner, deeper, or less conductive, as expected. Note that resistivelayers of a given contrastproduce a less intense anomaly than conductorsof an equivalent contrast. Resistive layers are resolved by thickness only, whereas conductive layers are resolved by their conductivitythicknessproduct (Wannamaker, 1983). These findingsmay be summarizedby lookingat the maximum anomalyfor the curves, normalizedto background, and by parameterizingthe related variables t and dœ into the thickness-to-depth ratio t/dœ. The resulting relative anomaly curves for layers of resistivity contrastsof 0.01, 0.1, 1.0, 10, and 100 are plotted in Figure 26. In this plot, the shadedzone surrounding each curve representswhat we might call the "resolution envelope" of the curve. Field experience has shown that, in simple geologic environments with good signal levels, resistivity changescan be distinguishedif the resultant anomaly is at least 10 percent larger or smallerthan the backgroundresistivityvalue. This 10 percentcriterionmay be much higheror lower in certain environments,but it is a useful approximation for many areasof explorationinterest. The shaded zones in Figure 26 represent the 10 percent criterion


CSAMT

755

Conductive

Layer

PL=O.I ,O,.m ioo

I

1

I

I

I

I

_

!

I

I

pL= 1.0,0,'m I

I

I

I

I

t=O. Im

__

__

1.0

dL=õOm •

- 500 I0•'•.••'r=lOkm l

I

varies ß t{

i•d:O dL

-

IJ••,J dL+ t

' Tx '

[=lamp I

0.1

I00

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

i

I

I

I

I

I

I

I

•

-

dL=I00 m '• 500

-

I00 5OO I000 I

0.1 I00

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

i

i

i

i

i

i

i

i

-

I• I00

dL=200m '•

500

i

0.1

I00

i

i

i

i

-

dL= 400 m .'2

500 iooo

Z •.o -

IOO _

500 IOOO

0.1

I

4

16

64

2.56

102.4

4096

Frecluency(Hz)

I

4

16

64

256

102.4

4096

Frecluency(Hz)

Fig. 24. Response of a conductive layerimbedded in a 10•. m half-space. Plotsareforlayerresistivities PL= 0.1 •'m and 1.0 •. m (columns),andfor layer depthsdL= 50, 100,200,and400 m (rows).Layer thicknesst = 0.1, 0.5, 1.0, 5.0, 10, 50, 100, 500, 1000 m varied for each model.


756

Zonge and Hughes

Resistive Layer pL=I00 •'m

ioo

I

I

I

I

I

I

I

I

PL=I000,•,'m I

I

I

dL=õOm• PLAN •-

VIEW

r=lOkm T

1.0

;

,,

Tx .•=1

, r-•oo; •'

, , , • ,

CROSS-SECTION

Lt varies ß t• I•-•ddL+

-

km

T =1 (:Imp 0.1 I00

-

1000500 IO• dL=lOOm • m

I0

0.1

I00

I000

200 m 'g m

I0

01 I00

L

:400m

-

•

.-

•

•0

0.1

II I 4I I 16I I 64I I 256I I 1024 I I 4096I I Frecluency(Hi)

•I • 4• • 16I • 64• • 256• • 1024 • • 4 d9 6 Fre•luency (Hi)

Fig. 25. Responseof a resistivelayer imbeddedin a 10 fl. m half-space.Plotsare for layer resistivitiesPL = 100 and 1000 fl.m.

Other variables same as in Figure 24.


CSAMT

resolution envelopesas a first approximation. If any curve falls within anothercurve's resolutionenvelope, the two curves will not be distinguishedaccordingto the 10 percent criterion. Several results are of interest in Figure 26. First, note that conductive layers are resolved from the homogeneousearth (@L/@B = 1.0) caseonly when the t/dœratio is large. As a very roughrule, a moderateto high conductive layer will be resolved from background when

0.5 >p• ) •>

0.005

Pa

(148)

and a moderate to high resistivelayer will be resolved when

> 5

(149)

under the 10 percent criterion. Hence, resolvability of a layer is scaled to the thickness-to-depthratio. This fact is true for both conductiveand resistivelayers. However, note that while the resolvability of conductive layers is highly dependent upon the resistivity

Resistors

PL

757

contrast of the layer with respect to background, resolvabilityof resistivelayers is relatively independent of resistivity contrast. In other words, EM soundings not only "see" conductivelayers quite readily, they also readily distinguishbetween various degrees of conductivity;on the other hand, soundings"see" resistive layers, albeit less easily, but they do not readily distinguisha strong resistor from a moderate resistor. Again, this result demonstratesthat electrical techniquesare better usedat exploringfor conductors than they are at exploringfor resistors,althoughthey are regularly used for both applications. Determining the thickness of a conductive layer is difficult. The problem stems from the fact that the electrical response of any layer depends upon the combined effects of conductivity, depth, and thickness.The effort is aided if the interpreter can fix one or more of these three parameters by known geologic constraints.For example, if geologicinformation can be used to fix the depth of a body, and some limits can be placedupon its conductivity,its approximatethickness can be determined. However, this kind of control information is rare in typical exploration, and more often than not, the explorationist is faced with the conductivity-thicknessproduct ambiguity. Raiche et al. (1985) and others show that this ambiguity can often be solved by using two independent sounding systems, such as TEM and Schlumberger, and then using the data in a joint inversion program. Depth determination is also complicated by the conductivity-thickness product ambiguity. Conventional MT wisdom sometimes quotes depth discrimination as about +_10 percent to +-30 percent of the depth of burial of a target. In actuality, depth discrimination is tied to equations(148) and (149), thoughvery generally. A very conservative, empirical rule is that depths to a layer can be determined to an accuracy of

(

0.5

Conductors

>

pL ) >

0.005

(150)

for a conductive layer, and

dr• +_40percent(-•)dr (151)

oo oooo•

ooo•

o o•

o •

•o

Ratio t__ dL

Fig. 26. Maximum normalized anomaly due to layers of various resistivities, from the results of Figures 24 and 25.

Vertical axis is anomalyamplitude(9^) normalizedto background resistivity (9}•); horizontal axis is the ratio of layer thickness(t) to its depth (dr). Shadedregionsshow the 10 percent resolution criterion described in the text.

for a resistive layer. Complexly layered areas will make depth determinations to a layer much more difficult than suggestedby these relations. It is worth noting that the above discussion is provided only for purposesof developing a "feeling" for CSAMT's capabilities in a layered environment. For any particular field project, a series of models

758

Zonge and Hughes

shouldbe run to establishthe resolution capabilitiesof CSAMT for that particularset of geologicconstraints.

Note that the 2-D model, which does not account for

the finite length of the slab, shows radically different


TE mode behavior.

Data Inversion.--There are two basic techniques in use today for inverting 1-D data. One is the traditional method of calculating forward solutions generated from bestguesses,and then matchingthem to the field data. Anderson (1974) has an excellent library of open file computer programs for CSAMT modeling and inversion, and many of the models used here were generatedfrom his programs. Another method attributed to Bostick (1977) is a

type of direct inversion, which he calls an "almost exact" method of analysis. The resultant corrected resistivity valuesare closeapproximationsof the real component. Depths are determined from equation (30). (See Figure 22d.) This technique was first described in Niblett and Sayn-Wittgenstein (1960). Jones (1983) demonstrated that the Bostick and Niblett methods

are the same.

2-D Interpretation

Data interpretationover 2-D structureis muchmore complex than in the 1-D case because"edge effects" must now be considered.Edge effectsare encountered when a lateral resistivity contrast is present between two bodies in the earth. This edge effect results in a discontinuous normal electric field (equation 36) acrossthe boundarybetweenthe two bodies;there are two importantramificationsof this. First, resistivity soundingswith the E-field dipole oriented parallel to the 2-D strike (TE mode) differ from those with the dipole perpendicularto strike (TM mode). Second, when soundingthrougha conductivebody, TM mode

This behavior

indicates that 2-D

modelingfor the TE mode may not be adequate even for bodies with very long strike lengths. Note that in TM mode both 2-D and 3-D models give the same results, as expected. In contrast to the above discussion, our field experiencehas shown that finite areal extent of 2-D and 3-D bodies does not generate serious interpretationproblems, and often 1-D modelingon a station-by-stationbasisprovidesadequateinformation for accurate interpretation. The

difference

between

TE

and TM

mode

data

acquiredover the edgeof a slab is better shownin the nonlayered3-D model of Figure 28, also from Wannamaker et al. (1984). The TE mode provides a smooth transitionalbehavior acrossthe edge of the slab, while TM gives an "undershoot" and overshoot" behavior. Edge effects of this type are common and are a functionof signalfrequency, resistivity contrast,body size, and the sharpnessof the boundarywith respectto dipole size. For example, Figure 29 showsa moderate TM mode undershootat the edge of a resistive body. We note that although TE mode produces a

(a) Model

x

3..6• m-'""0km

resistivities will not recover to their true values be-

neath the body, as they do in 1-D soundings. The difference between TE and TM mode behavior

is illustratedby modelinga conductiveslabas donein Wannamakeret al. (1984). Figure 27 showsthe model and the modelingresultsfor 2-D and 3-D algorithms. Focusingon the 3-D results,note that both TE andTM mode results "see" the slab but fail to see through the slab and to recover at depth their true half-space values. Both modes are affected by the discontinuous E-field, showing an effect which is related to the surfacechargedensityq• (equation87). The TM mode offset is strongersince the discontinuityis along the edge being traversed (y-direction), while for the TE modethe tangentialE-field is continuous(equation88) and any offset is due to the more distant x-striking edge. Ting and Hohmann (1981) suggestthat the slab must be very large in horizontal extent, basically approaching1-D layering, before the data recover to their correct half-space value.

(b) Model Results 2-D

MODEL

3-D

MODEL

IOO

• -• , i , • -! • ' ' ' ••.• (TE) •. 0,• •

•o

=-$o

Pyx g 1.0 •_ o.o,

I00 I•-/00I \ I •-

IO

•

$o

I

I

I

I .

--I

IOJ I

I

I

I

I

30

.•.......•.:'•. 0.001

Fi•. •?. •-D and 3-D modelso•cr a ] km thick conductive slab buried in a la•crcd haE-spacc. Adapted from Wannamaker ct al. (

CSAMT


smoother edge response,TM mode produces a more exaggerated and thus more diagnostic response. Hence, if the main objectiveof a surveyis to delineate lateral contacts, then the TM mode is preferred.

Resolution.--The lateral resolvabilityof a 2-D body is related to the dimensions,depth, and conductivity of the body, as in the 1-D case. But in the 2-D and 3-D cases, it is also a function of survey scale (E-field dipole length and orientation).When the body is large with respect to E-field dipole size, its resolvability shouldbe similarto that of an equivalentlayer. As the body becomes smaller than the dipole size, however, resolvability is compromisedbecausethe soundingis taking a bulk-average responseof the body and the background material. Note that, if the body is very conductive, it may be detectable even if it is small compared to dipole size, but its position may not be resolvable.The standardrule is that the E-field dipole should be made

smaller

than the size of the smallest

features needingto be resolved. Estimating the dips of contactsand major faults is sometimes difficult, but useful information can be provided as to whether the offset is at a shallow, moderate, or steep angle, and as to its dip direction.

759

The dips of smaller, finite bodies are even more difficult to ascertainto any degreeof certainty.

Anisotropy.--Anisotropy is defined as direction-dependentelectrical characteristicsin earth materials. In the 2-D case anisotropy is often due to foliated or bandedmetamorphicssuch as schistsand gneisses,in which conductivity along bedding planes is much higher than conductivity perpendicular to bedding planes. Sedimentary rocks such as shales, graphitebearing shales, and limestonesmay also produce anisotropic effects. In addition, fault/fracture planes, mineralized or altered veins, and other structure may lead to significant anisotropy. Electrical measurements are most strongly affected where anisotropic materials outcrop and where current-channelingbetween source and soundingoccurs. The degree of anisotropy in a rock mass may be characterizedby comparingthe resistivity measured alongbeddingplanes, (Pc, longitudinalresistivity, the equivalent of TE mode) to the resistivity measured acrossbeddingplanes (Pt, transverseresistivity, the equivalentof TM mode)'

(152) (a) Model

Sharp boundary

Resistor

Undershoot 4

5

6

I

I

2048

512 256 128

(b) Model Results

64

overshoot

)

•.ooo 4oo•.•J.......... 'g IOO

•

•o

I0

4

T8

edge of

slab

12 .

16

20

24

28

X[km)

Fig. 28. 2-D model over a conductive slab buried in a homogeneoushalf-space.Note the edge effects in the TM mode. Results are for f = 0.032 Hz. Adapted from Wannamaker et al. (1984).

8

Fig. 29. Cagniardresistivitypseudosection (TM mode)over a resistive body. Note the undershootedge-effecton the sharp right-handboundary of the conductor.


760

Zonge and Hughes

in which ha is the coefficientof anisotropy.Values of ha rangefrom unity for isotropicmaterialsto 2 or 3 for mudstones,coal, graphitic shales,and shale/limestone sequences. Interbedded anhydrite/shale sequences produce coefficients as high as 7.5 (Keller and Frischknecht, 1966). Given this range in anisotropy and the frequency of materials which produce it, the effects of anisotropy cannot be ignored in many exploration

environments.

Anisotropic effects may be seen in CSAMT data from a survey in South Australia (Figure 30). Geology along the line consistsof steeply dipping, interbedded quartzites and schistsat stationsto the west of station 16, and fine-grained quartzites to the east. A 50 m weathered layer overlies fresh bedrock. Figure 30b, the phase difference pseudosection,displays a type of anisotropiceffect to the west of station 16. Notice how the phase increaseswith increasing depth, exceeding 3000 mrad in some areas. Under normal conditions,

the phase would approach zero at depth. This anomalous phase effect appears to occur when making measurementswith the primary E-field perpendicular to the bedding plane, (transmitter dipole oriented perpendicular to strike of bedding). The effect disappears when the primary E-field is oriented parallel to the beddingplane. Once again, the phase information, although not well understood at this time, provides additional information for the explorationist. The conductornear station 15 is a narrow, vertically bedded massive sulfide, confirmed by drilling. From the character observed in both the resistivity and phase data, more of these vertically bedded sulfide bodies can be expected along this line. Anisotropic effects can yield useful information if they are properly measured. Vector or tensor measurements may be required, or at least spot checks with two different

transmitter

orientations.

Care must

be taken in selectingthe position and orientation of the sources in order to obtain definitive

results.

3-D Interpretation

Interpretation of 3-D structure is complex and until recently has been unsupportedby modeling. Wannamaker et al. (1984) provide an excellent tutorial on 3-D interpretation. But, at this time, reliable, cost-effective, routine 3-D modeling capabilitiesare still a thing of the future.

2-D modeling can sometimes be used to approximate 3-D features, as shown in Figure 27. However, 3-D modeling would be required to solve problems with finite strike lengths, when operating in the TE mode. In general, the 3-D problem requires that all measurements be regarded as TM mode, with their attendant

effects due to discontinuous

normal E-field.

The resolution of 3-D features is subject to the same criteria mentionedin the discussionof 2-D interpretation. In general, resolution is no worse than the 2-D case but modeling complexity is significantlygreater. A General Comment on Modeling

Keep in mind that just because a model has determined that a geophysicalsurvey will (or will not) see a target, does not mean that the results will be the same under

features

actual

field

delineated

conditions.

More

often

than

not

in field data can be discriminated

with much more intensity than would be anticipatedby modeling. The difference between field data and model data is probably due to a poor geologicmodel, localized alteration, current gathering, or a combinationof these and other effects.

Static Effects

A particularly troublesome consequence of nearsurface 2-D and 3-D structure

is the so-called

"static

effect." The previous discussionillustrated a similar effect under a buried conductive slab, but static shift can be caused by topographic features and shallow lateral changesin resistivity. Charge distributions at the surfaces of these inhomogeneities can shift E-field data up or down by a factor which is independent of frequency. The charge distributioneffect shiftsthe apparentresistivity curves but the phase curves remain unaffected. If the apparent resistivity curves are translated up or down in value, and remain parallel, but the phase curves remain superimposed,then static effectsmay be present as definedin this discussion.The intensity of the effect can approach two orders of magnitude, resulting in a substantialerror in estimated depth, and complication in interpreting structure. Static effects can influence all electric-field

measure-

ments to some degree, but the problem has not been treated thoroughly in the literature. Larsen (1975, 1977, 1981) and Berdichevskiy and Dmitriev (1976) made early referencesto the problem. Ting and Hohmann (1981) illustrated the effect in 3-D modeling. Warner et al. (1983) and Wannamaker (1983) suggested a simple correction based upon the average resistivity curve in a given area, and Sternberg et al. (1984) corrected for statics in a similar manner by applying a moving average filter to average station resistivities. Andrieux and Wightman (1984) provided an interpretation of the phenomenonand suggesteda correction based upon either modeling or an independent TEM data set. Sternberg et al. (1985) provided a field example of this approach. Bostick proposeda spatialfiltering technique utilizing short-dipole E-field mea-


CSAMT

Illl

0

I

i111 I

•

I

i

i

I

i i

--

I

I

(ZH)

i

I

i

761

fill I

I111 I

I

01::>

001 •

I

1 i

ooo i

i i i

762

Zonge and Hughes

surements across the survey area (Bastick, 1986; Word et al., 1986; Shoemaker et al., 1986). We noted earlier that all boundary fields and poten-


tials are continuous across the interface of an inhomo-

geneousbody except the normal electric flux: Dnl -- Dn2 = q•. (Normal D discontinuous),

(153)

where qs is the charge density at the surface of the body. Equation (153) can be rewritten with D = eE (assuminge is scalarand e = e0) Enl-

En2 = qs/eo.

(154)

Using equations (2) and (89), with J = ere and assum-

ing an eitøtfrequencyrelationship,we canwrite (O'1 -- ieoco)Enl= (0'2 - ieoco)En2.

(155)

Equations (154) and (155) can be combined such that

on curve D, the phase responsefor all four curves will be identical, since phase is proportional to the derivative of resistivity. Static offset can be viewed in part as a resolution problem. When the ratio of wavelength to body size is moderate, and the soundingis directly over the body, the body is resolved directly but an offset in the apparentresistivity curve occursat lower frequencies. When the wavelength-to-size ratio is very large, and the soundingis on or off the body, the body is not resolved

but it induces

an offset into the measure-

ments.

Static shift dependsupon the mode of propagation, as observedearlier. In 2-D geology, TM mode data are most affected; in 3-D geology, both TE and TM mode data are affected according to the geometry of the body and where the measurementsare made. Static shift also depends upon ground resistivity, since resistivity scalesthe wavelength. Higher resistivities mean longer wavelengths, and static effects

(cr2-crl)(156)

qs=En2eO o'1- ieocø '

Applyingthe quasi-staticassumption(or>>e0co),equation (156) reduces to: (a) Sounding Locations

qs = En2eO

.

(157)

o. 1

Although this surface charge density is quite small, its effect on E is not negligible, as is shown in the expression of Ward and Hahmann (1988, Volume I):

E=-VV=-V• 4•r•01rl qsds,

(158)

where ds is the surface element over which the change occurs. The effects of this surface charge will be felt when the skin depth is much larger than the dimensions of the feature. This means that a small 2-D or 3-D feature at or near the surface can affect the entire

E-field sounding. Of course, deeper bodies can cause static shifts as well, but surface features are the most troublesome.

Figure 31 is a simplified illustration of static effects over a conductive half-cylinder imbedded in a layered half-space. Notice in Figure 316, soundingD over the center of the half-cylinder responds to the surface

(b) Sounding Results

Static Sh•ft

I

Sfaf•c Sh•ft

Behavior

Layered-"'.. •,,••

Log Frequency (Hz) •

feature, but never recovers to the resistive values

beneath it. Sounding C does not detect the halfcylinder at the high frequency end, but has its entire curve shifted downscale. Higher frequencies on this soundingwould have shown a surface conductor similar to soundingD. The other soundingson the edge and away from the half-cylinder all provide resistivity curves that are shiftedhigher than the true responseof the layered environment, without any changein curve shape. With the exception of the high frequency end

Log Frequency (Hz)

ß

Fig. 31. Diagrammatic illustration of the static offset effect. Sounding A shows the homogeneoushalf-space response. SoundingsB and C show static effects near the edge of the conductor. Sounding D shows the static effect at low frequencies(where wavelength is very large compared to body size), and a direct-resolution effect at high frequencies (where wavelength is smaller and the conductor is partly resolved).


CSAMT

tend to become evident even at the higher survey frequencies. Static shift is also influenced by the ratio of the E-field dipole length to the size of the body. Due to a spatial-filtering effect, large-dipole measurements could be less affectedby near-surfaceinhomogeneities than short-dipolemeasurements.In addition, the shift will be a function

of where

the E-field

of static

shift

electrodes

are

placed. An

illustration

comes

from

vector

CSAMT data obtained in Queensland, Australia. The surface topography is flat, and geology consistsof a thin alluvial cover overlying resistive basement. The predominantgeologicstrike of the mineralized zone in this area is northeast-southwest.

The vector measure-

ments utilized a grounded dipole source oriented N63øE

and located

5.3 km southeast

of the receiver

line. Crossed E-field dipoles were oriented east-west (TE) and north-south (TM). Figure 32 shows the soundingcurves at two adjacent stationsin the vicinity of the mineralized zone. In TE mode (short dash and dash/dot patterns) the resistivity curves are identical except at the highest frequencies, where the differences are due to station 35 apparently being located more directly over the near-surface conductive zone. Hence, TE data are resolving the surface conductor and display a small negative static effect (Compare Figure 31a, curves C and D). However, TM mode data (long dash and dash/dotpatterns) show two effects:an apparent resolution of the conductor at high frequen-

I

i

i

i

i

TM{ '•.

X N

_

:

I

--------35 ----.• Station 40

"•.N N

TE Mode:

\ '•.\•,\.

-

i

TM Mode'

•*•N. TE(•.

iooo

i

..... s,o,io.• Station40

-- .... ß

L \\

x

%

\,., I

I

I

I

16

32

64

128

\ --

i

I

256

512

i 1024

I 2048

Ix 4096

Frequency (Hz)

Fig. 32. Variations in the static effect for TE and TM mode propagation over a two-dimensional conductor, from data collected in Queensland, Australia.

763

cies, and a static offset due to the presence of the conductor once the frequencies are too low to resolve it directly. Correcting Static Offset.•In general, static effects cannot be avoided, nor can they be predicted in any consistentmanner. Hence it is necessary to correct for them for all surveyswhich rely upon horizontal E-field measurements. Three basic approaches have been devised: (1) make a theoretical calculation of the effect; (2) use processingtechniques such as spatialfiltering or phase integration; (3) use independent, static-free

measurements.

Calculating the theoretical static shift is straightforward in theory but untenable in actual field situations because too much a priori information is required about the causative body. We have found that static effects often arise from bodies which have no unique geologiccharacteristics.Hence the geometry and conductivity of the bodies is unpredictable. Even when statics arise from generally recognizable bodies, too much remains unknown to make any reliable correction.

Processingcorrections (Warner et al., 1983; Sternberg et al., 1984; Zonge, 1985) show considerably more promise. These approachesinvolve various spatial filtering techniques.The key idea is that statics are a local effect due to local causes, but that over a broad area the effects average out. Hence, local curves are normalized to some regional type curve or curves. For example, Sternberg et al. (1984) used a Hanning filter for correctingMT data. The approachis more successful with higher data density and shorter dipoles, making it more applicable to CSAMT and AMT, on the average, than to MT, unless supplementary measurements are made.

Perhaps the most straightforward spatial filtering approach for an area known to have layered surface geology, is to simply shift all the curves so that they match at a certain frequency near surface. Look for soundingswith resistivity offsetsbut which have nonanomalous phase behavior, and then determine the frequencies at which the offsets are primarily staticlike and are less affected by direct-resolution effects. But there are several dangersin this approach. First, the correction is somewhat arbitrary unless a good deal of contiguous data are available to establish nominal phase behavior. Second, the correction invariably under- or over-corrects stations which are anomalous due to nonstatic effects. Third, the correction removes

anomalies

which

look like static effects

but which are nonstaticin origin. For example, effects due to vertical structures, which could look like a static shift, would be removed or reduced in a simple


764

Zongeand Hughes

static correction. Hence, great care must be exercised in using this approach. An example of the shift procedure is found in CSAMT data obtainedover Trap SpringField (Hughes and Carlson, 1987), a study which is described more fully in the Case Histories section. The data (Figure 33) show layering effects due to stratigraphy in this area, a general trend toward lower resistivities toward the east due to higher groundwater salinity in that direction, and several pronounced lateral features. The phasedifferencedata echo the layering and eastF I ELD

Topography (Scale - I:1)

In order to make a correction, the soundingcurves were analyzed and groupedinto two basic types: (1) conductive (stations 14, 19, 22, 23) and (2) background. These curves (Figure 34) show a relatively consistentoffset at lower frequencies, which becomes larger at high frequencies.The best explanationfor this offsetis that the conductivesoundingsare seeing STATIC

I0

12

14

16

18

20

(c) Normalization/Offset (D,.m) 22

24

I I I I I I I I I I I I I I I I

Ioo •z4••/ \\II/ )1i[\ • oooXq**..

8

01 I

I0

I0

•oo ••o••

400•

•oo• •

16

18

20

22

Ioo

'•'.•• a

•o.o

600

) Phase Difference-

700

14

x xx /

600 •

70 •

12

I I I I I I•______..__•__•1 I I I__t I I I I

500

8

CORRECTIONS

East

(a) Apparent Resistivity (,Cl,'m) 0

tion are static offsets.

DATA

West

8

ward trend to lower resistivities,but do not show any lateral effects. This lack strongly suggeststhat the lateral features in the apparentresistivity pseudosec-

12

14

-

785.4 16

18

700 /

(mrad) 20

(d) Phase-Derived

22

8

•

I0

12

14

Correction (•-m) I•

18

20

22

,oo

• 400

•oo• 700 •

Fig. 33. Example of static correctionfrom CSAMT data obtainedat Trap SpringField, Nevada. (a) Cagniard resistivitydata; note staticshiftsat stations14, 19, 22, and 23. (b) Phasedifferencedata; note smoothappearance acrossstatic-shiftedstations.(c) Static-correctedresistivitydata, basedon a normalization/offset procedure.(d) Static-correctedresistivitydata, basedon a phaseintegrationprocedure.

24

CSAMT

both

a near-surface

conductor

and the static

offset

conductorsproduced. The successof the correction is tested by the close resemblance of the static-corrected resistivity data to the phase difference data. In this case the match is quite good. Two observations are of interest. First, had we made the wrong choice of frequencies used for the offset, the interpreted depth would also be wrong. Second, the methodology we used for the static correction has reduced the regional trend to lower resistivities to the east. This trend, which was observed in the original resistivity data, is caused by increasing ground water salinity to the east. Since trends such as this have the same character as static effects, they are removed alongwith the static effects. Thus this type of static correction must not be interpreted as a "final pseudosection," but should be examined along with the original resistivity and phase data. Bastick (1986) proposed a variation on spatial filtering in which small-spacing E-field measurements are made in conjunction with MT. This approach provides the required data density for the statics correction. The chief disadvantage is that a large number of additional measurements are required, increasing the data acquisition cost.


which results from the conductor, much as seen in

Figure 31, curve D. The phase difference data show that the curves for the two groups are similar at low frequencies and diverge at high frequencies, as expected from the resistivity curves. A third plot is of phase scatter, which shows the relative similarity of dq>/dfdata at stations 8 to 23. The scatter is large at 4096 Hz but is reasonable for other frequencies. The correction was made at 64 and 128 Hz, where

curve behavior is nominal for the area. Averages of the resistivities at 64 Hz and at 128 Hz were obtained;

these two values were averaged together to yield the offset resistivity Pa. The full data set was normalized on a station-by-stationbasis, then offset by Po, producing the "static-corrected" data of Figure 33c. These data show a marked

reduction

in lateral

effects

at the conductive stations (in fact, a slight overcorrection may have been done). Since no change in curve shape is imposed by the processingprocedures, the original curve shapeshave been preserved. The resultant section, then, shows the near-surface conductors

directly at the top of the section, but has in the first approximation removed the static offset which the

4O

"'

I

I

I

3O

- Apparent Resistivity

:>0

-

I

i

765

I

I

256

512

I

I

I

•

I

.c

10

o

'

"Background"stations

....

Average of stations 14, 19, 2.2, 2.3

900

•.

8oo

•

700

•

600

=

500

.--

400

Phase

Difference

300

Phase Scatter 0.5

I•T 8

'• 16

32

64

128

•

T

I

I

1024

2048

4096

Frequency (Hz)

Fig. 34. Average resistivity and phasedifferencecurvesfor "background" and static-shiftedstations,and average phase scatter curve, Trap Spring Field, Nevada. Figure 33a showsthe apparentresistivity pseudosection.


766

Zonge and Hughes

Zonge introduced a spatial-filteringmethod in 1985 which integrates the static-free phase data to produce static-corrected resistivity data. The technique can be used in two different ways. The first method integrates the phase difference data to obtain phase-derived resistivity data for each station:

P, =P•v exp --

4>-

din

, (159)

in which P/v is the constantof integration(the static offset or normalizing value), f• is the beginningfrequency,f•; is the lowest surveyfrequency,and 4>is the E/H phase difference. The phase integration is begun several frequency steps above the highest measured frequency, under the assumption that phase tends to ,r/4 at the highest frequencies. For the first approximation to P/v, a surface resistivity value is obtained from an area that has minimal

static effect. This value

is then used to normalize all integrated resistivity values in a given area. In order to strip off near-surface effects (such as the high frequency portion of curve D, Figure 31) while adjusting for statics, a frequency is chosen which correspondsto the shallowest depth of interest. Then all of the data above this frequency will be strippedoff. Stripping introduces two processing parameters: (1) the reference phase (4•r) associatedwith a reference station and frequency, and (2) the specificphase (4•s), which is the phase response at all other stations and frequencies. In order to obtain the 4)r value, examine the data carefully and select a station for which the data are not strongly affected by static shift. The reference phase, chosen from an area in which the

phase is relatively stable laterally, is designated4•r. The resistivity for that station, at that frequency, is

used as the normalizing resistivity P/v. The static correction is made station-by-station using these values:

Dstat: Da PN e (4/,r)(rb r -(bs), Pref

(160)

where Pa is the measuredapparentresistivityand Pref is the apparent resistivity at the reference frequency for each station. This static correction or stripping method can be used with the phaseintegrationmethod

of equation(159)by replacing Pawith This method makes full use of the phase difference data in making the static correction. The correctness depends largely upon the ability of the interpreter to

selectthe most appropriatevalue for 4)r and An example of the phase integration technique is provided for the Trap Spring example introduced earlier (Figure 33). The processingparameters were:

4)r : 741 mrad, P/v = 20 l•.m, fref = 64 Hz, reference station = station 8. The results, shown in Figure 33d, show an excellent removal of static effects: resistivity varies smoothly across static-affected stations, and layering information is still present. Note that the regional eastward gradient to lower resistivities has been smoothedbut is still present since the gradient is represented in the phase difference data. A third approach to the static shift problem utilizes supplementarytechniqueswhich are unaffectedby the static problem. Transient electromagnetic (TEM) methods are one solution. Andrieux and Wightman (1984), Sternberg et al. (1985), and others have used TEM

for

static

corrections

in MT

data.

The

latter

group performed joint inversions of central-loop TEM soundingsand MT data, checking the reasonableness of the results with well log resistivitiesfrom a nearby well. This approach, which shows promise in correcting the problem, unfortunately imposes an economic penalty by requiring the use of two separate field techniques instead of one. For this reason use of TEM in conjunction with the much more expensive MT technique rather than with less expensive techniques such as CSAMT is justified. There is the possibility that CSAMT H-field measurements may eventually be used in making the correction, making an independent data set unnecessary. Our early attempts have shown mixed results. The exact separation and orientation of the source dipole and receiver antenna must be known, and this data is often difficult or costly to determine. However, note that there is a bright side to the static effect. We have been conditioned a contaminant

to see static shift as

which must be removed

from the data.

However, in some instances, static offset can be a valuable clue for investigationsof near-surface geology. For example, considerthe problem of detectinga brine leak from an abandoned well or a gold-bearing, silicified

dike at 10 rn in a 100 l•.m

environment.

At

typical CSAMT frequencies, this depth is well above the investigationrange of the technique. But while the vertical extent of the target might not be "resolved" with CSAMT, the lateral extent could be mapped due to the effects of static shift. We have used this approach with good successin searchingfor both high and low resistivity, near-surface features. Topographic Effects

All electric-field measurementsare distorted by the presence of topographicfeatures. As illustrated diagrammatically in Figure 35a, equi-potential and equicurrent lines are dispersedin the vicinity of a hill, and are focused in the vicinity of a valley. Topography produces different types of behavior for TM and TE


CSAMT

mode data in the 2-D case. TM mode (Figure 35b) shows an artificially low resistivity anomaly at depth beneath the hill and a high resistivity anomaly at depth beneath the valley. It also shows characteristic undershoot/overshoot features at the edges of the anomalies. (Note that these effects in the figures are highly exaggerated for demonstration purposes.) TE mode (Figure 35c) showsa shallow, artificially high-resistivity anomaly beneath a hill and a shallow, low-resistivity anomaly beneath a valley. Wannamaker et al. (1986) present a good overview of the topographic problem. In general, TM mode topographic anomalies are significantly larger and more complex than TE mode anomalies. Andrieux and Wightman (1984) have shown that topographicfeatures that are small compared to skin depth provide the equivalent of a static shift: low resistivities under hills, and high resistivities under valleys. Wannamaker et al. (1986) show TM anomalies spanningan order of magnitude in resistivity for valleys and hills of typical sizes; their TE anomaliesare typically smaller in peak amplitude by a factor of about four. Reddig and Jiracek (1984) use a Rayleigh scattering approach to the problem; their results show a typical anomaly of 20 percent in TM and 4 percent in TE for a hill 100 m high and 2300 m wide (5 degree slopes). Wannamaker et al. (1985)

equ•potent•al TE

(a) SchematicCurrent Flow Patterns ,

i

]

'l

i

I

I

•

[

I•nes

= •TM

'•fl•r•?nnet s

ß

I

•

i

[

I

I

'

I

I

i

i

I

,

I

,

I

,

i

I

I

(b) TM Mode Pseudosections

(c) TE Mode Pseudosections

767

compare various modeling techniquesfor a hill, showing qualitative though not always quantitative agreement with the results of Ngoc (1980) and Reddig and Jiracek (1984). But the consensus is that TM topographic anomalies can occur over moderate terrain changes,while TE anomaliesare probably found over only very severe terrain changes. In addition to disruptions in current distribution, topography coupled with geologic changes can cause static offset as well. This effect can be complex, dependingupon resistivity distributionsand geometry of the array. An example of topographic effects comes from a CSAMT survey conducted in the western United States over a long, narrow sandstone mesa. The E-field dipoles were oriented perpendicular to the length of the mesa, resulting in TM mode data. The mesa is caused by differential weathering of three units: (1) a lower, electrically resistive sandstone;(2) a middle, conductive sandstone;and (3) a capping unit of electrically resistive sandstonesand shales. Figure 36a showsthe topography/geologyand CSAMT apparent resistivity data for the project. A strongcorrelation is seen between

the electrical

character

geologic units and the data. Since strong, apparently static-type offsets are observed in the data, and since they are correlated with geologic contacts, a static correction was performed using simple shift procedures outlined earlier. The static-corrected data (Figure 36b) show a neatly layered appearance except over the mesa, where the soundingsshow major character changes. Two effects can be observed in these data over the mesa: (1) topographic and (2) geologic. The stronglow resistivity zone at depth is almost certainly related to topography, much as illustrated in Figure 35b. A slighttendency toward high resistivity flanks on both sides may also be topographic in origin. The near-surfacechangesare very likely geologicin origin, as they correlate very well with surface geology.

Correcting Topographic Effects.--Wannamaker et al. (1986) show that topographic effects can be minimized by placing the E- and H-field sensors in a horizontal position. In some cases this is not feasible due to the survey objectives, and a direct correction is needed. Fortunately, topographyis always known and hence its effects can be calculated

Fig. 35. Diagrammatic illustration of topographiceffects on E-field measurements.(a) Distortion of current flow patterns over various topographic features. (b) TM mode resistivity pseudosectionsresultant from the topographicfeatures. (c) TE mode resistivity sections. All pseudosectionsare qualitative only; H = high resistivity, L = low resistivity.

of the three

and removed

from

the data. This removal is best done by a layered-earth model with topography. A homogeneoushalf-space topographic correction is also useful in making a first-order correction. Several researchers (Ku et al., 1973; Wannamaker, 1983; Reddig and Jiracek, 1984; Wannamaker et al., 1986) have developed 2-D and 3-D models for calculating topographiceffects.

768

Zonge and Hughes


IP Effects in CSAMT

Data

Hohmann et al. (1970) suggestedthat recovery of IP

The induced polarization (IP) effect occurs when a charge accumulation occurs at the interface of a metallic mineral and an electrolytic fluid or when ion motion is constrained by clays or pore spaces. The charge constraint results in energy storage and discharge much like that of a capacitor. IP has been a parameter of great interest in nearfield techniques such as complex resistivity, since it can be used to determine the presenceand character of economic sulfide mineralization, certain alteration and

geologic regimes, clays, etc. However, attempts to recover IP information from EM soundingtechniques have been inconclusive.

The first theoretical

studies of

the subject in the 1960s were encouraging. Work by

Resistive

ss/sh

(a)Apparent Resistivity (f/,.rn)•,•'

Conductive

ss

] 0

2

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•,• 128

,

'

4

6

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8

I0

12

14

16

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information

was limited to induction

numbers between

0.1 and 10 and relatively polarizable materials. Additional work has been done by Debroux (1985). Frischknecht et al., this volume, (1991) provide a brief but useful review of research into this problem, concluding that much more work needs to be done. IP information is contained primarily in the electric field phase data for CSAMT measurements,and hence in phase difference data as well. But in most areas where one might expect an IP response, any polarization contrast in the ground would be a few tens of milliradians, or less than 5 percent of the homogeneous earth phase response. Hence the data would have to be virtually noise-free in order to recover any IP information. Further, recovery of IP information would require an effective decoupling routine to discern between structuralphase effects and polarizationrelated phaseeffects. Presentanalysisindicatesdecoupling will be a difficult problem. The authors have experimented with comparing measured phase data with theoretical phase data derived from the derivative of the resistivity in looking for IP effects. Some results have been moderately encouraging,but much work remains to be done. Interpretation of Cultural Effects

4

õ

(b) Static- Corrected Resistivity (f/,.m) •'•'•'••••.•

ß •>._.. •6oo[: 1500•.

In general, cultural effects are less severe in CSAMT surveysthan with near-field IP and resistivity techniques.There are several reasonsfor this difference. First, IP and resistivity surveys involve close spacing between the source and the measurement point which maximizes current channeling into cultural features, and then these features act as secondary sources.The result is a strong culture-induced effect. CSAMT measurements are made far from the source, where current densities are much lower and the elec-

-ioo

-200

-300

-400 - 500

-600 - 7OO

Fig. 36. Example of topographic effects in TM mode CSAMT data obtained over a mesa. (a) Cagniard resistivity data. (b) Static-corrected resistivity data.

tromagnetic fields are plane-wave. In this case coupling to the culture is minimized and preferential current channeling is less intense. Second, culture disrupts CSAMT data to a lesser degree than IP becauseCSAMT has considerablyless "side-looking" capability than does IP, since depth penetration is controlled by frequency with CSAMT, in contrast to the dependenceupon geometry with IP. Hence, offline contributions from culture, which strongly influence IP measurements, are less in CSAMT measurements. A third considerationis directly related to the

way in which the measurementsare obtained and plottedfor analysis.For example,dipole-dipoleIP plot point locations,beingcontrolledby geometry,lead to the pronounced"pants-leg" featurescharacteristicof the IP pseudosectionplots. These features sweep out large wedge-shapedzones beneath near-surfacestruc-


CSAMT

ture, making interpretation of other effects in these areas quite difficult. In contrast, CSAMT basically sees vertically beneath the measurement site. As a result, only one or two soundings are affected by near-surface features, not a large triangular-shaped zone. With the affected stations edited out of a survey line, the CSAMT line is continuously interpretable, while the IP line is not.

Nevertheless, culture can strongly influence CSAMT interpretation. Recognizing cultural contamination is crucial. It is essential to know the types of culture present, the precise location of each feature, and the orientations of the features with respect to the survey lines. We prefer to present this information (alongwith true-scaletopographicprofile) at the top of each pseudosectionas an aid to interpretation. Of all cultural features, metal pipelines are the most common cause of spurious effects in CSAMT data. The degree to which this effect occurs probably depends upon a number of factors, such as pipe size, effectivenessof the pipelines electrical contact with the ground, etc. The effect is also controlled to a large degreeby the survey logistics,specificallythe absolute distance between the pipe and the electrodes, the relative position of the E field dipole with respect to the pipeline, the angle between the pipe and the electric field dipole, and the signal frequency. The worst effectscan be expectedon lines where the source and the soundingdipoles are close to the same pipeline. This nearness establishes a direct current channeling effect and seriously disrupts the electric field response. An extreme sample of this linkage is shown in Figure 37, which shows the apparent resistivity pseudosection for a project conducted in the western United States. The crew inadvertently set up the transmitter dipole over a buried oil-collection pipeline which also traversed the receiver line. The results are highly anomalous data in the vicinity of the pipeline. The peculiar data alerted the crew chief to the problem, and the sourcedipole was moved, eliminating the problem. An example of spuriouseffectsfrom pipelines which are at large anglesto the electric field dipoles is shown in the apparent resistivity data of Figure 38. The data were obtained

over Ashland

Gas Field in the Arkoma

Basin of central Oklahoma (Carlson et al., 1985). A number of collection pipelines cross the survey line, all at a roughly perpendicular angle. The data show disruptive effectsbeneath all the pipelines, althoughto varying degrees. The bulk of the effect occurs at 128 Hz, and is the result of 120 Hz cathodicprotection on the pipelines (the protection circuitry was turned off by the field operator later in the survey, resulting in dramatically improved data). The pipe crossing the line at station 5.6 creates the largest distortions, pro-

769

ducing a strongbut artificial high resistivity feature in the lower frequencies. The pipe at station 3.5 produces the opposite effect at depth, with consistently low resistivities. Other pipelines inexplicably show little effect at depth. This example illustrates the extreme variability of type and magnitude of effects often associatedwith apparently identical cultural features. Although pipeline effects can be significant, it should not always be assumedthat a given resistivity change is due to culture. The authors have examined data which upon first glance appeared culturally contaminatedbut which, upon closer examination, proved to be the result of geology or other factors. Powerlines produce extensive amounts of cyclical noise and also generate spuriouscontaminatingeffects in the data. More often than not, the effect of a powerline is a high-resistivity feature in the pseudosection. A typical example of this effect is shown in Figure 38, in which a 15 percent increase in apparent resistivity occurs beneath the powerline near station 16. On rare occasions the effect can be more severe,

producingstronghigh or low resistivity effects. The disruptive influences of steel well casings in induced polarization data have been described by many authors including Wait (1952, 1977) and Holladay and West (1984). Fortunately, well-casing effects are generally more subduedin CSAMT data sincethey are minimally coupledto the plane-wave source. However, well casingscan influence CSAMT data if the receiver dipoles are close enough to the wells. Fence effects are rare in CSAMT work. They sometimes appear as mildly conductive features at high frequencies beneath the fence. SOURCE

EFFECTS

Unlike the natural-source MT and AMT techniques, the CSAMT

source lies at a finite distance

from the

sounding sites. While this fact results in CSAMT's high data quality and speed of acquisition, it can also introduce significant complications to the interpretation process.These complicationsare known as transmitter or source effects. Three

such effects concern us

here: (1) nonplanewave effects due to the closenessof the source; (2) transmitter or source overprint, due to the geology beneath the source and between the source and sounding site; (3) the shadow effect, in which the effects of a body are projected away like a shadow from the source. In this section we continue to use the word "source"

to representthe physical location of the source of the EM waves measured in CSAMT.

The CSAMT

source

comprisesthe EM transmitterand the groundeddipole or horizontal loop, and is often referred to as just the "transmitter." The "sounding" means the location at

770

Zongeand Hughes

whichthe magneticsensorsandelectricfielddipolesare locatedfor acquiringthe electricalinformationto make the sounding,and is often referred to as the "receiver."


We also limit our discussion to conditions derived

from a groundeddipolesource,unlessotherwisespecified.

source.As noted in the CSAMT Theory section,this is the result of changingdependenceof E and H on the source-soundingseparationr. Far from the source,in the far-field zone, both E and H are proportionalto

1/r• , andresistivity isthusindependent of r. Closeto thesource, thenear-field zone,E decays as1/r• andH

as1/r2, making resistivity a function ofgeometry. At

NonplanewaveEffects

intermediate distances, in the "transition" zone, E

The most common source effect is the distortion of

apparent resistivity and phase difference close to the

decays as 1/r• andH decaysat an intermediate rate between 1/r2 and 1/r3.

Major pipeline {direct link between transmitter E• receiver}

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Fig. 37. Stronglycontaminated datadueto the directlinkageof a pipelinebetweensourceandsounding. This representsthe one of the extreme types of cultural contamination.


CSAMT

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Zonge and Hughes


Some confusion

exists in the literature

about what

exactly constitutesfar field and near field. To avoid misleadingor confusingstatements,our definitionis as follows. The far-field zone is essentially free from nonplanewavesourceeffects, i.e., the resultslook like MT soundings. In practice we designate this as the zone

where

source

effects

constitute

less than

10

percent of the total responsein the Cagniardresistivity parameter (see Figure 6). We definethe near-fieldzone as the area where E and H are completely saturated

andH decays as1/r2. Theareabetween is thetransition zone.

Figure 21 summarizes the effects of a finite source on the chief CSAMT quantities for both a homogeneous and a layered earth. In the far field, all parameters behave according to real resistivity changesin the ground. In the transition zone, E almost always exhibits a shoulder/notch/recovery character; H shows an inflection in the transition zone; resistivity shows the combined influence of E and H; and phase difference shows significant changes related to the slope of resistivity. In the near-field zone, E and H saturate, becoming invariant with frequency, and H becomes independent of resistivity; the Cagniard resistivity increases uniformly with decreasing frequency, and phase difference tends toward zero. Previously we demonstratedthat the saturationof E and

H

in the

near

field

makes

near-field

CSAMT

soundingsa function of geometry. This result is true for any offset source, whether in time domain or frequency domain. Near-field data, as we define the term, cannot be corrected or adjusted to rescue "hidden" frequency soundinginformation. But the transition zone does contain recoverable information, since

E and H are still sensitiveto frequency and resistivity.

Interpreting Transition-zoneEffects.--The transition zone is almost always recognizedby the rather distinctive "notch," which is an artificial decrease in resistivity. As shown in Figure 21, the notch can vary from very shallow in a homogeneousearth to very sharp and steep in a layered or 2-D/3-D earth. The notch is often accompanied by an artificially high-resistivity shoulder on its high-frequency side and an artificially steep rise toward the near-field frequencies on its low-frequency side. In some casestrue changesin ground resistivity can resemble this transition-zone behavior. This potential problem can be avoided by making soundingsto very low frequencies, which permits direct identificationof the

near

field

via

E-

and

H-field

saturation.

The

expected frequency of the transition zone notch can also be calculated, but as will be seen, source overprint can shift the notch to higher or lower frequencies.

Geology has a strong impact on the shape of the transition zone notch. For example, Figure 21 shows how the gentle homogeneous-earth notch becomes steeper in the presence of a conductive layer. This tuning effect is illustrated by the 1-D model results of Figure 39. The model shows the interaction of effects due to the transition zone and the conductive layer. Far from the source, the transition-zone notch and the effect of the conductive layer are cleanly separated. However, at soundings near the source, the two effects coincide in frequency. At this point the layer enhancesthe steepnessof the transition zone notch. The tuning effect is again demonstrated in the soundingplot of Figure 40a, which showsthe resistivity response to a three-layered earth as r is varied. Figure 40b fixes the source-receiverseparationat 2 km and varies the depth to the conductive layer. The curves show the effects of having insutficientreceivertransmitter separationfor deeper soundings.The transition notch and the response from the conductive layer are superposedfor shallow layers, and the responsebecomes unresolvable for deeper layers. Figure 40c showsthe results for a larger separationof 20 km.

Here

the

transition

zone

is moved

to lower

frequencies and the layer and notch are separated quite nicely for shallow layers. However, as the layer is deepened,its effects occur near the frequency of the notch effects, and the two are superposedfor deeply buried layers. Finally, Figure 40d shows the effectsof varying the resistivity of the middle layer. Note that the layer and notch are well separated for a 10 11.m layer, but that a 1 11.m layer has the effect of converging the two. We have found that the notch is more pronouncedin areas with electrically resistive basement, especially when there are strong resistivity contrasts, or where a conductive layer lies just above basement. But generalizations are difficult because the tuning effect is a complex function of resistivities, depths, thicknesses, and source-soundingseparation. An interesting observation is that tuning effects can result in negativephase values. For example, someof the resistivity models of Figure 40 show steep slopes from

the notch

to the near field.

In some cases the

slope of the resistivity is less than -,r/4 rad, which by equation (97) produces a negative phase. We have seen many field examples of negative phase due to extreme slopes in the transition zone. Another consideration that could be important in interpreting transition-zone data is a possible change in soundingpath. In the far field, the soundingpath is essentially vertical, with lower frequenciesproviding deeper penetration. In the near field, only the sourcesounding separation, not the frequency, determines the sounding depth, and the effective volume being

CSAMT

tion-zone and near-field data (Figure 42c and d) have a skewed appearance. Figure 42c shows a strong influence of transmitter overprint in the transition zone. The sounding data are strongly influenced by the conductor when it is placed anywhere between the transmitter and receiver sites. Figure 42d shows a

investigated is usually estimated to be between the transmitter and receiver sites. Now, consider the fact


773

that the progression from far-field to near-field is a continuous one as lower and lower frequencies are used. This analogy suggeststhat the soundingbegins as a vertical phenomenon in the far field, but then migrates toward the source in the transition zone. Finally, in the near field, the soundingreaches a fixed point dictated by the transmitter-receiver separation. The curvature of the sounding path means that effects from geology beneath the sounding point are being contaminated with effects from geology in the direction of the source. The potential for misinterpretation is illustrated in a very simplifiedform in Figure

similar but weaker

effect. The far-field

data also show

some asymmetry, but this is attributable to a shadow effect, which is described later in this section.

Correcting NonplanewaveEffects.•We see the impossibility of correcting, in the true senseof the word, for near-field effects. Of course the Cagniard resistivity for source-soundinggeometry can be "corrected",

41.

but in the near field the result will still be insensitive

Figure 42 was generated to try to illustrate the soundingpath curvature effect. These plots show the sensitivity of the soundingsto a small conductorwhich is moved to various points in the ground (indicated by dots) for a fixed source-sounding separation. In all cases the strongest anomaly occurs when the conductor is located near the surface directly beneath the soundingpoint, as expected, but note that the transi-

resistivity changes at depth beneath the sounding unless the array geometry is varied. Hence, a correction of near-field data is not necessary, but is some-

Distance Source I 2 4096

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zone data which

are more useful.

Except for very shallow surveys, most CSAMT data sets will contain some transition

From Source 24 ?

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774

Zongeand Hughes

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Fig. 40. Modelsof the tuningeffect.(a) Resultsfor variousr-separations. (b) Resultsfor variousdepthsto the intermediate conductive layer,plottedfor r = 2.0 km. (c) Sameasin (b), butwithr = 20 km. (d) Resultsfor various resistivities of theintermediate layer,fromconductive to resistive. Notethatthecharacter of thecurvescanchange rapidlyaccordingto variouscombinations of theseparameters.

CSAMT


the need to minimize the transmitter-receiver separation for maximum signal. A rough correction can then be made in one of three ways.

The first correctiontechniqueis to simplynormalize the data on a frequency-by-frequency basis. This normalization can be done, for example, by obtaining the average Cagniard resistivity acrossa certain frequency, dividing each individual Cagniardresistivity at that samefrequencyby that average,then doingthe sameto the other frequencies(Zonge et al., 1986). The normalized, unitless results then can be scaledback to resistivityunits by multiplyingby somerepresentative resistivity value. This process strips out layers, whether real (earth layering) or apparent (transition zone/near field changes)but works well only when the transition zone notch lies at the samefrequency along the line.

An example of the normalization approachis shown in a CSAMT structure-mappingproject in Michigan (Figure 43). The Cagniardresistivity data (Figure 43a) show a resistive

fault near station 8 and two rather

different geologiczones to the west and the east of the fault. The eastern zone shows a transition zone around

128 to 256 Hz. Figure 43b shows normalized data, scaled back to resistivities by the average Cagniard resistivity at 4096 Hz (89.1 f•. m). The artificial resistivity increasewith depth has been removed from the section, as has most of the layering. The transition zone remains, however, since it is not horizontally continuous across the line in the original data, but shiftsto lower frequenciestoward the west sideof the line. Simple normalization will not work in such a case. We could normalize on a best-fit skewed line, but

this normalization could produce a biased removal of horizontal layering in the far field. Note that data west of the fault have been overcorrected because a single

775

normalization has been uniformly applied to all the data, regardlessof character changes. A secondcorrectiontechnique calculatesthe impedance directly based upon the known transmitterreceiver separation, instead of using the Cagniard calculation, which assumesan infinite separation. Yamashita et al. (1985) and Bartel and Jacobsen (1987) describe this second approach. Macinnes (1987) describes a technique which starts with a far-field Cagniard resistivity as a "first guess" resistivity for solving the general impedanceequations. The calculated impedance is then compared to the Cagniard impedance, and the first guess value is changed iteratively until the impedancesare equal. The final resistivity value needed to achieve the impedance match is the near-field corrected apparent resistivity value. Figure 44 illustratesthe impedancecorrection. Figure 44a shows the results from a homogeneous earth model. The Macinnes algorithm corrects the near-field effectsvery well. However, Figure 44b showsthat the simple impedancecorrection fails to correct the transition zone notch in a layered case, because the algorithm does not account for tuning effects in the transition zone. Of course, this technique can be extended by interactively modeling l-D, 2-D, or 3-D effects which

could lead to a better

solution

to the

problem. However, the effort may not always be practical consideringthe a priori geologicinformation needed, the extreme sensitivity of the notch to tuning effects, and the heavy computer usagerequired. The Macinnes algorithm was applied to the data set from Michigan. The results (Figure 43c) show good removal of the artificial near-field doublingof Cagniard resistivity, but, as expected, the transition-zone notch remains. But note that this technique is superior to normalization because it does not strip out layers in the far-field

and it does not overcorrect

the zone west

of the fault.

Fig. 41. A simplified, hypothetical result of the curved soundingpath problem. Although sounding1 is located over the orebody, the orebody is not seen; sounding2 sees the orebody, but is not located over it. Failure to recognize this problem would result in a barren hole at station 2.

A third correction technique requires an independent measurement which is unaffected by nonplanewave effects. One approach would be to acquire natural-sourceAMT data along with the CSAMT. But we could argue that it would be better to use CSAMT for the far-field frequencies,then use AMT or MT for the remainder of the sounding. Otherwise the exploration cost would be higher than necessary.We could also compare forward-modeled well-log data from nearby wells to the CSAMT data, but this would require that the soundingsbe near a well. Hence these approachesare nice in theory, but they are not broadly applicableto most exploration needs. A fourth approach,and one we recently found best, is not to try to correct the near-field and transition zone effects, but to work with a combination of the

776

Zongeand Hughes


raw field data and inverted

sections derived from the

des sectionfor someexamplesof thisapproach,which

layered earth CSAMT equations. Excellent correlation with geologyand structurehas been obtainedby accounting for both the near-field and transition zones by inverting the data for each CSAMT stationalong a profile, usinga layered earth model, and then contouring the results in section format. See the Case Histo-

6km

we call "smooth modeling". In summary, we have seen that near-field and transition-zonedata are difficult to interpret. Rather than

trying to remove these effects, an exact impedance calculationcoupledwith interactivemodelingseemsto offer the best solution.

Separation

(a) 128 Hz (Far Field) 6km

o

•

(b) 16 Hz {Far Field) 6km

o

T

2. km Separation (c) 12_8Hz (Transition

Zone) ..•1

o

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(d) 16 Hz (Near Field)

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Fig. 42. 2-D sensitivitymodelsshowingthe influenceof geologybetweensourceand soundingfor near-fielddata. The modelplaceda 0.5 x 0.5 km, 100ll.m 2-D conductorin a 1000II. m half-spaceand movedit to all the points indicatedby dots. The sourceand soundingpoint were fixed at 6 km [(a) and (b)] and at 2 km [(c) and (d)]. In the nearfield (d) the dataare especiallysensitiveto materialtowardthe source.This is dueto the curvedsoundingpath describedin the text. Similar though smaller trends in the far field are due to the shadow effect.

CSAMT

777


(a) Cagniard Resistivity

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778

Zongeand Hughes


SourceOverprint

Zonge et al. (1980) first pointed out that geology beneath the source can influence sounding data in CSAMT. For example, transition zones sometimes occur at unexpectedly high frequencies, and sometimesthe transition-zonenotchbehavesunexpectedly. We can classify these and related effects as transmitter or source overprint. Figure 45 shows the results of a 2-D model developedby Macinnes (1987). Model A, performedover a horizontaltwo-layeredearth, showstypical transition zone and near-field behavior. Model B, which utilizes a source located where the basement is shallower, shows a much different response.First, the notch is shifted to a higher frequency which makes intuitive sense,becausethe overall groundresistivity appears to be higher, moving the transition zone up in frequency. We would get a similar result if the basement were shallow at the soundingsite. Note also that the notch is much steeperfor model B. Again, we would get a similar responseif the basementwere shallow at the soundingsite. Hence, the overprint phenomenon, which results from specific geology beneath the source, tends to mimic the response if that same geologywere placed beneaththe sounding.

Figure 46 provides a field example of sourceoverprint in CSAMT data obtainedover Trap SpringField

in Nevada(HughesandCarlson,1987).The Ex/Hy sounding(solid line) enters the transition zone at 8 Hz.

But a layered-earthinversion(dashedline) basedupon geologyfrom a nearby well log shows that the transition zone is higher than expected. This difference is apparently due to the resistive basementbeing shal-

lower beneaththe source(locatedon an alluvialslope) than it is beneath the sounding(located in an alluvial valley). A secondexample of sourceoverprint is shown in Figure47. The curvesrepresentsoundingsat a single site using two different sources. Source 1 is located over relatively recent andesiteswhich are coveredby

PLAN

VIEW

MODEL

A (Sol•d

-2km

200m

['•:•iii:i::iii::i::ii[iii!i .................. IO0,0.. m::!::i::i::i::111::i::i::i::i::ii

Source

•:100m

I :lamp

MODEL

%

•

•

CROSS-SECTION

•,•

IOO

r=2 km• i0•d=O L_•el Response]

. M•

A

fi

Curve)

Source ß

Sounding w +2.5km

.....-.•.,.•lO •.m• ido--OL 200m

(SheHow Basement I

I

• •

B• •em eht )

-•= I km

B (Dashed

I •'•

(SimplyLeyeredI

Source

o.

V +25kin d=O

...........................................

r=2k

(a) Homogenous Earth

_

Sounding

V

-2km

ßg

Curve)

Source

lOOm •1 ß•--

I0,000

•

'•//F X

•

/

X

IO

/

x

/

I Near-Field Correchon I I

I

I

I

I

I

I

I0

I

I

I

I

1

I

I

I

I

I

I

I

2

4

8

16

32

64

128

256

512

1024

(b) Two- Layered Earth

I

2048

I

4896

Frequency (Hz)

IOOO

i i i••.i i i i i i I •

•---.-.. ••odel Response J r=2•mT

•oo

-

_

_

ß •,.m d=O

//---'"''"- x

INeorField Correct,on '--• S'•rce o

"'

IO

Fig. 45. 2-D model of source overprint. Model B, performed over a basementhigh, showsan exaggeratedtransition-zonenotchwhichhasbeenmovedupwardin frequency. Model parameterswere selectedto generatea "worst case"

_

.5 II 2I 4I • 0.125 01

example. •:•

• • I

I

I

I

I

I

I

I

i

40

'• 30

i

i

i

i

i

i

i

i

i

i

i

i

No. I Trap Spring Best-Fit Model

I

16 32 64 1282565121024 2048 4096 Fre(luency (Hz)

•

•0 2048

Fig. 44. Two modelsshowingnear-fieldcorrections.(a) A goodcorrectionis madefor the homogeneous earth case.(b) The two-layeredcaseis well correctedin the near field, but the algorithm fails to correct the transition zone notch, which is enhanceddue to the layering.

409G

Frequency (Hz)

Fig. 46. Data and best-fit one-dimensional model near the

No. 1 Trap Springoil well. Mismatchat the low frequencies is interpretedto be the result of sourceoverprint.


CSAMT

779

a thin alluvial layer; source 2 is located over older andesiteswhich are covered by a thicker alluvial layer. The soundingsfor these two sources are nearly identical in the far field. However, below 16 Hz the soundingsshow divergent character. The notch for the source with shallow cover is at a higher frequency and is sharper than the notch for the source with deeper cover, which is consistentwith the modeling in Figure

notch. Sources 2 and 3, located over deeper alluvial cover, produce a gentler notch. Note that the responses from sources 2 and 3 are quite similar in shape, although they diverge at lower frequencies. Although we feel that the major portion of the differences discussedhere are due to source overprint, we must also keep in mind the fact that these different

45.

dipole parallel to the transmitter dipole, so it is possible that a portion of the differencesin these responses

measurements

were

made

So far we have considered source overprint to be a phenomenonof the nonplanewave zone, but Figure 48

is due to differences

shows that the far-field

sponses.

data can be affected

as well.

The data were obtained near Stoney Point Oilfield in Michigan (Carlson and Vugteveen, 1985). Source 2 was located 4.8 km from the soundings,and source 3 was 2.9 km away. Both sources were located over similar geology and on flat ground. In the far field, the soundingsat both stationsfrom source 2 are 20 percent lower in resistivity than those from source 3. This static shift is apparently due to a difference in structure between the sounding and the two sources, possibly enhanced by current channelling. We should note, however, that although we have observed this effect on several

occasions

it is not a common

occurrence

with

the receiver

in the TE

and TM

E-field

mode

re-

In some cases, overprint can cause the notch to occur at the same frequency independent of the separation between the transmitter and receiver. Normally increasing the transmitter-receiver separation lowers the frequency of the transition zone notch. But a fixed notch frequency tends to indicate an overprint problem.

Correcting Overprint.--Source overprint rarely affects the overall far-field interpretation of a project. However, it is sometimes useful to correct for the

effect. Unfortunately, correction techniquesare still in

and it is usually relatively minor. A final example of overprint comes from a vector data set over the Roosevelt Hot Springs geothermal resource in Utah (see the Case Histories section for a more complete discussionof this data set). Figure 49 shows two soundings, each obtained using three separate sources. The soundingswere located at approximately the same distance from all three sources, with the E-field oriented parallel to the source. Sources 2 and 3 are north-south and are located in valley sedi-

i

i

i

I

I

i

I

I

_ Station 68Source No. 2 -

i

I

i

• •Y

Source No.:5

--

/•.•'•

-_

-

:

--

.

_

-

\\ : _

ments.

Source

4 is east-west

and is located

near

a

graben fault at the boundary between sediments and mountains.

Although stations55 and 56 are in different geologies and show different far-field results, the effects of sourceoverprint are very consistent. Source 4, located on the graben fault, produces a strong transition zone

i

i

i

i'

i

i

i

i

Station

70

•

SourceNo. 2

/•

.•

Source No. 3••/•••••,/-

i

lOO Source

No. I

Source

No. 2

I

I

I

I

i

:•

4

8

I

I

I

I

16

$:•

64

1:•8

I

4 512

1024

2048

6

8

16

32

64

128

256

512

1024

2048

4096

Frecluency (Hz)

Fre(luency (Hz)

Fig. 47. Common-site CSAMT soundings using two sources.Mismatch at the low frequenciesis interpretedto be the result of source overprint.

Fig. 48. Common-site CSAMT soundings using two sourcesat Stoney Point Field, Michigan; data at two sites(68 and 70) are shown. Note the consistency of this source overprint effect between the two stations.

780

Zongeand Hughes


their infancy, and much more work is needed to provide an adequatesolutionto the problem. MacInnes (1987) has made some valuable early contribu-

the results; (2) normalize the measurements based

upondata from multiplesourcesitesor from recipro-

tions in this effort.

cal measurements;(3) make a direct correction based upon independent natural-source measurements.

There are three approachesto the overprintproblem: (1) minimize overprint in the field and live with

source where the depth to bedrock or resistive base-

I

I

I

I

I

'

Overprint effectscan be minimizedby locatingthe

I

I

I

Station

IOO

I

I

I

55

( Hydrothermally altered valley sediments) '

ß--- •

Source

No. 2_.

(invalley)

---- Source

No. 3

(invalley)

......

Source No.4

(on fault)

IO

Station

56

(Unaltered

bedrock)

I00

I0

I

I

I

I

I

I

I

I

I

4

8

16

:52

64

128

256

512

1024

I 2048

I 4096

Frequency (Hz) Fig. 49. Common-siteCSAMT soundingsusingthree sources;data at stations55 and 56 are shown. Note the radicalchangein characterof thenear-fieldandtransitionzonesbetweensoundings obtainedwith source4 dataand those obtained with sources 2 and 3.

CSAMT

ment is greater than at the soundingsite, or in simple geology similar to that at the sounding site.


When consistent

differences

occur in measurements

from several sources,the sourceeffect can be approximately removed from the data by deconvolution. By repeating several soundingsbetween sourcesand selecting a reference source, we can then correct the resistivity data to the reference source. The correction is best made by averaging the soundings for each source at the overlap point and deconvolving the difference

from the data for one of the sources. This

781

becomingmore diffusefarther from the body. MT and AMT

data show shadow effects but CSAMT

data are

perhaps more strongly affected because of the proximity and distinctive polarization of the source. Note that, even well away from the body, the shadow still persists. Hence, measurements could detect the influence from the body, even though it is not beneath the soundingsite. For example, a sounding placed at station 5 in Figure 51 would sense a conductiveanomaly startingat about 1 km in depth. A drillhole at this location would clearly be unsuccessful.

procedure requires at least two stations be measured

It should be noted that in actual field measurements

with both sources.

have never encounteredshadow effects of this magnitude. Part of the strong shadow effect of Figure 51 is

A variation on the normalizationapproachinvolves reciprocal source-receiver resistivity soundings. Zonge et al. (1980) show an example from CSAMT

RIOE

measurements made at a test site near Tucson, Ari-

zona. The geology consists of conductive alluvium overlying resistive volcanic bedrock, with bedrock becoming deeper toward the west. Three source/ sounding sites were used (Figure 50a). Site A was located over a thick section of alluvium; site C was located near a volcanic outcrop where the alluvial cover is considerably thinner. When measurements are made at intermediate site B, a source overprint occurs between sources A and C (solid curves). We might expect the solid curve of Figure 50b to be more representative of the geology beneath site B than that of Figure 50c. Since the B-C reciprocal measurementsare so similar in character, we could assume that the measurement at receiver site B is basically controlled by the underlyinggeologyat site C. With this assumption,we should be able to compare the two responsesand arrive at a residual which would more closely approximate the actual resistivity sounding at site B. The dotted curve in Figure 50c was obtainedby deconvolving the two curves and then normalizing the result to match the apparent resistivity at 2048 Hz of the solid curve of Figure 50b. Note that the deconvolved result strongly resembles the AB/BA curves.

•o J ,i' •

we

RIlE

urden •

I •J

-XX, •

, •e•e/ s• /outcro,•

(b) A-B

ires Reciprocity

IO0

Source ot BI •••'"•'••••

I Sounding at A_J

• •

••,'"•

Sounding atB

IO

(c) B-C Sites Reciprocity

•ß I0,000 \ \

._

\ ._

\ \

\

a)

1,000

ISSounding ource at atCI IO0

The Shadow

Effect

A variation on the source overprint problem is the "shadow effect" (Kuznetzov, 1982). The effect arises from local geologic features between the source and the soundingsite, as illustrated by Figure 51. The 2-D model

is of a 0.5

x 0.5 km conductor

buried

in a

homogeneoushalf-space. The data, which have been corrected for near-field effects by the Macinnes algorithm, show influences due to soundingthrough the conductor (the vertical shaded zone) and due to a shadow cast by the source (the shallowly-dipping shadedzone). The shadowdips away from the source,

I Deconvolution of BCFromCBI 4

8

16

3?_.

64

17_.8

?--56

517_.

107_.4

?_.048

Frequency (Hz)

Fig. 50. Field tests of source-soundingreciprocity at a test site in southeast Arizona. Note that sites A and B, which overlie similar geology, show reasonablereciprocity; sites B and C, which overlie very different geology, show a very strongstatic-like shift in the reciprocity measurements.The dotted line shows the result of deconvolving the dashed curve from the solid curve in (c), as described in the text.

782

Zonge and Hughes

probably an artifact of modelinga typical 3-D problem

4096 Hz data show a regional north-northwest lowresistivity trend but very little apparent casingeffect. However, the 8 Hz data appear to show both a regional north-northwest trend and a low-resistivity casingeffect. The casingeffect may produce the subtle shadow which points away from the source. Further work involving a second source location would be


in 2-D.

A field example of a possible weak shadow effect comesfrom vector CSAMT study over Tricentrol 2-14 Nancy Unit, a steel-cased dry hole at Little Nancy

Canyonoilfieldin Utah'sParadoxBasin.Ex/Hy and Ey/Hx measurements weremadewith a 76 m E-field

needed to confirm

dipole at points separated by 76 m; the source was located 7.2 km to the northwest. Figure 52 shows vector apparent resistivities at 4096 Hz and 8 Hz. The

Source

this.

The best solutionto the shadow effect problem is to obtain good plan-view coverage of the survey site. A

Distance

From

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Source 4.0

(km) 4.5

5.0

5.5

6.0

6.5

7.0

V

V

V

V

V

V

V

V

V

V

V

V

V

V

•

ß

I

Iß

ß

ß

ß

ß

ß

ß

Iß

I

'ß

I 8.9 ./ ;-

1.0

Source Shadow

2.0

3.0 x

4.0

Fig. 51. Shadow effectsfor a 0.5 x 0.5 km, 10 •.m body in a 100 •. m half-space,usinga 2-D model. Body locationand dimensionsare indicatedby the squarebeneathstation3.0. E-field dipolesare 0.5 km long and are oriented in the y-direction (TE mode). Data are corrected for near-field effects. "Shadows" due to the shadow effect are shaded.

Po- 4096 Hz

Plan Layout 18

(not to scale)

?-3

19

16

?_9

Source

t

' i

N

Grid

Fig. 52. Apparent shadow effect due to a cased dry hole in Little Nancy Canyon Field in Utah. Apparent resistivities

are calculated from vector data.

CSAMT

wedge-shapedanomaly dipping down and away from the

source

would

be an obvious

indicator

of the

shadow effect. Although usually inconvenient, we could illuminate the site from a different sourceto help


isolate

the effect.

We

could

also model

the effect.

783

resistivity behavior at depth is completely different, giving a picture of a body centered near the 30 Hz level. These changesat depth are mainly an artifact of enteringthe near-field at the lower frequencies.Notice that there is even a slight shadow effect to the right of

However, modelingpresupposesa knowledgeof earth featureswhich are being exploredfor in the first place. For this reason, modeling is of limited use.

/O •2.m

l'tzrgefB

( •.m }

•.•

Comparison with MT

I000

o

2000

3000

Pyx 4000

5000

6000m

IOOO

Far-field

measurements

in CSAMT

are often mod-

eled usingthe standardnatural sourceMT plane-wave equations. It can be shown that even in the far-field, the effects of the CSAMT finite source are weakly present. Figures from Butterworth (1988) demonstrate this phenomenonvery nicely. Figure 53 is a schematicof a 3-D model•a 10 f•.m block in a 100 f•.m halfspace. Figure 54 shows the

apparentresistivitypseudosectionincludingmeasurements over the conductive

300

I00

-

30

-

I0

-

block derived from a nor-

mally incidentplane-wave, simulatinga natural source MT measurement. Figure 55 displays the impedance phase for this model. Notice the uniform, symmetric responseto the block for both resistivity and phase. Figure 56 is the resistivity pseudosectionfor the same model using a finite, horizontal, electric bipole located on the left margin of the figure. Notice that the high-frequencyresponsebeneath station3000 roughly compares with the plane-wave field model, but the

3-

Fig. 54. Apparent resistivity pseudosectionover a conductive block in a uniform backgroundusing a normally incident plane-wave source. From Butterworth (1988).

Mop View /O•.m

Ttzrgef B

Source

• I000 IOOO

2000

3000

•yx(deg') 4000

5000

6000m ,,

•x

Receivers I00,0,-m

y I

300

' 500 m

I00

3O

Cross Section Source

• IO0,O,-m

•"'•.Receivers

/•'1o •-m

z

Fig. 53. Schematic of 3-D model for comparing finitesource and plane-wave sourcesfor CSAMT and MT. From Butterworth (1988).

Fig. 55. Impedancephasepseudosectionover a conductive block in a uniform background using a normally incident plane-wave source. From Butterworth (1988).


784

Zonge and Hughes

the main anomaly. The impedance phase as seen in Figure 57 respondsin a similar manner with abrupt changesat depth due to near-field effects. This type of model might not give a totally accurate representationof the differencesbetween finite-curved

and plane-wave sources, but does serve to provide insight into the differencesbetween the two types of measurements, and these differences are easily observed in the field.

CASE

HISTORIES

IO•'m

Target B

rx

I000

2000

3000

,Oy x(,O..rn) 4000

5000 I

6000rn '

I000 '•II I 3OO

ioo

30

•o 3

Petroleum Exploration (Structure Mapping)

The reflection seismicmethod offers superiorresolution to all electrical techniques,including CSAMT. But some situations, imposed either by geology or economics, make electrical a good complement or replacementfor seismic. An example is illustrated in data obtained over Trap Spring Field (Hughes and Carlson, 1987). The field is located in Railroad Valley of east-centralNevada. Oil is producedfrom fractured Oligocene ignimbrites.The trap results from truncation of the volcanics in an updip direction by a high-angle, basin-marginfault. The top seal is provided by a heavily argillized, unweldedzone at the top of the volcanics. The volcanics are overlain by 300 to over 3000 rn of unconsolidatedvalley fill material, and are underlain by Paleozoic shales, dolomites, and limestones. Over six million barrels of oil have been recovered from the field to date.

Fig. 56. Apparent resistivity pseudosectionover a conductive block in a uniform background using a horizontal electric bipole. From Butterworth (1988). IO•'m

Target B

•

Tx

I000

I000

i

2000

3000

I

•yx(deg.) 4000

i

5000

6000rn

I

300

ioo

30 I0

3

I

Fig. 57. Impedance phase pseudosectionover a conductive block in a uniform background using a horizontal electric bipole. From Butterworth (1988).

Exploration in Nevada is difficult. There are numerous graben-typesub-basins,each of which has its own unique lithology, structure, reservoir rocks, source rocks, thermal history, and so forth. Drilling throughout the area is sparse. Seismic exploration faces numerousdifficulties,suchas multiplesfrom volcanic lenses,inability to resolvecomplex structures,unpredictability of the form of favorable structuraltargets, and poor signal coupling in some areas. Due to the high cost of seismicsin this area, a less expensive structure-mappingtool would be useful in the early stagesof exploration. A single line of scalar CSAMT was run across the southern portion of the field (Figure 58). Cagniard resistivity and phasedata are shown in Figure 59a and 59b. Most data below 16 Hz are in the transition zone, as indicated by the characteristic notch at the lower frequencies. The notch occurs at a higher frequency than expectedpossiblydue to a sourceoverprint (the source is over a shallower

basement

than the sound-

ings). The notch is also steepenedconsiderablyby a tuning effect, which results from an extreme low-overhigh resistivity contrast at the top of the volcanics, where a conductive argillized zone overlies resistive ignimbrites. This electrical "marker" makes the CSAMT data especially sensitiveto changesin layering and lateral displacementsdue to faulting in this

area, and providesan effectivemethodof mappingthe resistive basement.


CSAMT

785

induction logs available near the survey line. We compared the data sets directly by using the induction log data as input for a series of forward 1-D models, which were then compared to the actual CSAMT measurements.Since the logs were run at depths of 100 m or greater, the shallow CSAMT data were inverted to complete the upper-layeringpicture for the forward models. Figure 60 shows a comparisonof the log-constrained models and the actual CSAMT resistivity data. In all four wells, the CSAMT data and the electric log data compare quite favorably in the far field. More field data are required to adequately define

A distinct lateral discontinuity exists in the transition-zone data beneath station -2. The discontinuityis correlated with the main basin-margin fault which bounds the western edge of the field. As illustrated in the interpreted electrical section of Figure 59c, the data indicate: (1) the fault dips at a moderately steep angle to the east; (2) the east block is downdropped approximately 200 m with respect to the west block; (3) the east-block basement deepens steadily to the east, the result of successivestep-block faulting; and (4) the main fault probably extends to the surface. These findingsare in agreementwith information from drilling, seismic studies, and aerial independent photographs. The shallower data show complex layering in the Trap Spring area. The layers represent changesin the depositionalenvironment as valley fill sedimentswere laid down. These data agree quite well with deep-

the transition

zone.

Given the four to one cost advantage of CSAMT over seismic in this area, the study suggests that CSAMT would be an effective tool for mapping structure on a reconnaissancebasis prior to detailing with seismics.

R56E

LOCATION MAP

R57E

Source

TION T9N

CSAMT Survey, Trop Spring Field Nye County,

Nevodo 3

2

I

6

Structure: Top of Unconformity"A" SCALE

• 0 0

I

•

I

• I

I

2I mi

I 2

3 km

I

I

.•).* •P7'$/

10

12

7

• /

+/20•

-.

....

.

.

,. ,,,

19

..

., T9N

Fig. 58. Location map for the CSAMT survey at Trap Spring Field, an oil producer from Railroad Valley of east-centralNevada. Structure showsthe top of the volcanics(unconformity "A"), from Duey (1979).

786

Zongeand Hughes CSAMT

Data

(a) Cagniard Resistivity (•l.m) East


IP STATIONS O I

I

I

CSAMT

I

5 I

I

•0

I

I

15

I

20

I

I

I

I

I

I

i

I

STATIONS -12 I

-I0

I

I

-8 I

I

-6 I

I

-4 I

I

0 I

I

I

I

i

i

I

2

4

Illill

6

I

I

8

I

I

I0

I

I

12

I

I

14

I

I

I

16 I

I

18 I

I

20 I

I

22 I

I

4096 51; ) I;)8 64

N

(b) Phase Difference i 4096 512 128 64 32 N

-12 i

i

-I0

i

i

-8

I

i

(mrad)

-6

i

-4

i

i

i

-2

0 i

-

_•oo--••.,

:

;,,,•....•-.:--•-.-----• ---•.,

......... _•.•_':_-_.., ..... •

2

lit

4

i i i i

-• -,

i

6 I

8

I

i

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I0 i

12

i

i

I,

14 i

i,

16 i

i

.............................

•:.....•

18 i

20 i

•oo-_

22

i

i

1

•

...____•.::•-------.. --•

-

....

-

i

......

•

_

(c) CSAMT•Drillhole Interpretation -12 ß

I

I

-I0 I

-8

I

I

I

-6 I

I

-4 I

I

-2 I

I

I

0

2

I

I I I

4 I

I I

6

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8 I

10

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I

12

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14 I

16

18

20

22

I

o•:::•.•.•..m...:•.-.[•.;--•`-•C-•-.•.•.•.E•-•-•:;•"•::•:•.•.;;• ............. -::,-: .•:,9•_m___'::-.': ....Z'_U_'.--1

,oo[ ,,•m

p-a• •',,

ALLUVIUM ,,,

-.

,;,.--:'.:_' .......... . ...........................

m .......... ",-•,- --,, 15,0, ........... -'-'-:'-'---_

200 r (Qal •Thc) I 50,•,rn• \ \

,oo• ........ •

',,-k-', .............

........... \

•o.,• \

.oo 500

'•

600

"

700

•'

"

PALEOZ01CS •

900 Qal-Qu-arternaryalluvium I000

•

__

I100

'''-.

,•(.3,....

• ...... --• ? % ',,,•

'"'••,••-..... '•,, I

,O-•O•m

i

(alluvium)

--1

- ...... --.-.-

15,0, m

.........

-... O,0. m

o .oo-

12oo

'::::::::::.-_::--•.o ......

er \

30,0.

"-'\

• ......... .........................

\?

....

Tov-sc= Stone Cabin Formation ] • slim"[ (01igocene volcanics)

Tov-ps = Pritchards

1300 14 O0 0

Station Formation

(01igocene volcanics)

I000

2000

3000

4000

5000

6000

7000

8000

9000

Fig. 59. Apparent resistivity data, phase differencedata, and interpretedgeological/electrical cross-sectionfor Trap SpringField. Somedetailsof the interpretationare fixed by independentgeologicaland geophysicaldata; otherdetails,suchas someof the subsurface faulting,are new informationprovidedby the CSAMT survey.

CSAMT


Geothermal Mapping and Monitoring

In recent years, CSAMT has becomea prominent tool for mapping reservoir brines and structureassociatedwith geothermalresources.Sometypical examples are found in Sandbergand Hohmann (1982), Bartel and Jacobsen (1987), and Hughes and Maas (1987).The techniqueis particularlyeffectivefor mapping the top 2 km in conductivegeothermalareas. Hughesand Maas (1987)presentdata from a reservoir monitoring project over the Roosevelt Hot

Springsgeothermalarea, located 19 km northeastof Milford, in southwestern Utah. The resource is a high-temperature, water-dominated, hydrothermal system. Conductivefluids are found in an extensive fault and fracture system associatedwith the sediments, metasediments, and intrusives of the Mineral Mountain Range. Figure 61 showsthe geologiccrosssection,which is typified by alluvial sandsand clays overlyingbedrock. Figure 62 showsthe surfacegeology in the KGRA area. The primary structuresare the north-southOpal Mound Fault and the intersecting,

l.•

I

I

I

I

I

I

I

I

I

I

5o I No. 2Trap Spring |

MODEL •

' I--I

east-westNegro Mag Fault. The Opal Mound controls the geothermalfluids,whichrangein depthfrom 300to at least 2200 m. There is extensive hydrothermal alteration in the area, •including clays. The reservoir is extensive,judging by heat flow data

(Figure 63).The100mW/m 2 heatflowcontour (Ward et al., 1978)suggestsa resourcearea of approximately

175km:. Wrightet al. (1985)alsonotethatheatflux data suggestover 60 MW of continuouslyrenewable power supply, a very significantnumber by normal standards.

Two partial-tensorCSAMT surveyswere run over the field in October 1984 and November 1985 (Hughes and Maas, 1987). Both surveys utilized identical sources. Sources 1 and 4 were oriented in an east-west

direction near the north-southtrending structural axis of the resource. Sources 2 and 3 were oriented in a

north-south direction in the deeper valley fill sediments west of the survey area. The electric dipoles were always oriented parallel to the source dipole beingenergized.The electricdipole spacingwas 152m and the frequency range was 4 to 4096 Hz. Data from both surveys are of high precision. Av-

eragedatavariancesare - 1.3percentfor survey1 and _ 1.5 percentfor survey 2. Most individualvariance values were less than -1 percent. Somewhat noisier data were often recorded at 64 Hz and 128 Hz due to

/

strong60 Hz powerlinenoisefrom the nearbypower plant, and at 4096 Hz due to high-frequencyatmosphericnoise. Absoluterepeatability,determinedby repeatingstationsperiodically,was found to average -_-5percent. The causeof this increaseis not known, thoughwe suspectthat changingsurfaceconditionsin the semifreezingweather and perhapschanginglevels

5o No. 3Trap Spring )DEL M(: -,•

'• 20

787

/

of cultural noise contributed to the overall noise level. I

We regard any resistivity changesof less than +-5 percentas statisticallyunreliable.

'; I No. ITrap Spring •' 5o

As mentioned in the discussion of source effects, the

• LMOD.•E,• ,.•

data show evidence of source overprint between the east-west and north-south sources. This overprint is explainedin somedetail in Hughesand Maas (1987). The datapresentedin this discussionthusincludeonly

20

io

I

I

50 1NO. 13 Trap Spring L.

information obtained with sources 1 and 4. In addition,

DATA

'

io

Fi ';'

i

I

I

I

4

8

16

32

I

I

I

I

I

64

128

256

51';'

1024

I 2048

I I 4096

Fre(•uency (Hz)

Fig. 60. Comparisonof well-log derived resistivitymodels with CSAMT data for Trap SpringField. Dashedlines show the 1-D model results, which are obtained from a joint inversionof the log resistivitiesand CSAMT surfaceresistivities. Solid lines show the soundingclosest to the well; shadedzones show the range of data at stationswithin the immediate vicinity of the well.

the data also show significant anisotropy, but the anisotropyis limited to stationsobtainedover faulted bedrock areas. The analysis by Hughes and Maas suggests that scalarCSAMT is sufficientin thisparticular casefor both mappingthe field and for monitoring changesin the field. Figure 64 showsthe apparentresistivity resultsat 256 Hz for the second survey. Conductive values are associatedwith the geothermalfield and the controlling Opal Mound Fault. Moderate resistivities are found over alluvium toward Milford Valley, and high resistivities in the mountains east of the field. Along

788

Zonge and Hughes

tive values are associatedwith all present and former producing wells, and resistive values are associated with dry wells. Structural features, especially the Opal Mound Fault, as well as alteration products, probably also contribute to the response. The moderately shallow southernpocket appearsto


the conductive trend are three conductive pockets. Data at 4 Hz (Figure 65) indicate that only the middle pocket has considerabledepth extent, while the southern pocket and particularly the northern pocket are shallow

features.

A good overview of the three anomalouspockets is provided by the pseudosectiondata of Figure 66. The pseudosectionspresent only data for 64 Hz and above for purposesof this discussion.The northernpocket is clearly the shallowestin the resistivity data, while the middle pocket is the deepest. Phase data (Figure 66b) show this very well. Note how the southern and northern pockets are characterized by decreasing phases, indicating that the conductive zone has been penetrated and more resistive or at least homogeneous material is being seenat depth. In contrast, the middle pocket is characterizedby high values well above xr/4 radians, indicating steadily decreasingresistivities at depth. These findings agree with our current geologic understanding of the resource. The shallow northern pocket is located near the Roosevelt Hot Springs steam vent, the source of considerableshallow hydrothermal alteration, principally clays. The complex fault and fracture system, which comes to the surface, probably also contributes to the response. The deep, middle conductive pocket is nearly centered on the zone of production, where hot reservoir brines are found at depths of about 800 to 1600m. The correlation with productive wells is excellent; conduc-

be the result of several influences.

Reservoir

fluids are

shallow in this region (one well encountered fluids at 243øCat a depth of only 382 m). The fluids are known to be part of the reservoir system due to strong hydraulic conductivity noted between wells. Fluids probably account for much of the response of the southern pocket. The most conductive values are found near wells which were initially producers, but now serve as observation wells; resistive values are

associated with dry wells. The resistivity contours tend to close off right at the southern edge of the reservoir, and they strongly resemble the heat flow contours (Figure 63). But some conductive values are traced to the south-southwestpast the reservoir along the Opal Mound Fault, suggestingthat there are also strong structural influences. The high data quality and the care taken to repeat measurementprocedure exactly permits a direct comparison of results from the two surveys. Figure 67 shows the percent change in resistivity from survey 1 to survey 2 at each frequency. Positive numbers indicate a change to higher resistivities over the oneyear interval between surveys; negative numbersindicate a change to lower resistivities. Many of the East

West

Mineral Mountains

Roosevelt

Hot

Springs Qrf

Milford

Qcal Q,I•

Valley

+2

o

o /\

x

ßQolz . ß . ß

ß

.

••t

Se0 Level

Igneous Heat

Sour-ce

-2

minor sands '.Q.• Silica cemented alluvium 'Q• Sand, gravel, boulders'.Q'• Clays, •

Rhyolite flows

rocks Granite togranodiorite _m•Metamorphic

Fig. 61. Generalized east-westgeologiccross-sectionacrossthe Roosevelt Hot Springsreservoir, adapted from Ward et al. (1978). Scale: 1' 1. The geothermal fluids are found in faults and fractures associatedwith the Opal Mound Fault, at depths generally below 300 m.


CSAMT

SU R F/•CE

789

GEOLOGY

•'" ,:,.-.,.:',•/

oJ•• Alluvium, siliceous sinter •.•

Rhyolite domes

,_•,•

1,• Rhyolite flows •• Gronite, cluortz monzonite, syenite P•

Bonded gneiss

ß. --

i

............ ' %'"-/

,•.',•..,.::. f

ß. /"

.

'-•,::;,_'

,.-,,

- -, z. -' ,'-': l•f_•:,x•.', ;_•; ;',,' ,,/, i •

xl

i

•

T•?S

--

/

/ /

I

!

',

::.'.

•,, ,

/__,•

] Xl

I

,• 0

0

I

•

/

•

I

i

0.5 1.0 1.5 km•

•

0.5

- -" -;•;..

•_,;,.,,

' , ,:.,.• ...... . .....-.• / !•.--. _,-..'•!.• •_

. ß .

ß .

I.?mi

Fig. 62. Surface geology at Roosevelt Hot Springs, adapted from Sibbett and Nielson (1980). Numerous geophysicalstudieshave shown additional faulting beneath the alluvial cover.

790

Zongeand Hughes

changesare smaller than the -+5 percent criterion we have adopted for statistical significance. The highfrequency data show changes which are probably


associated

with variations

in alluvial

water

saturation

over the year between the surveys. Brine injection at

HEAT

injection well 82-33 may have produced the more conductivevalues noted in that region at 8 to 32 Hz. Of particular interest are changesto higher resistivities observedover the reservoir in the deeper data, (Text continued on page 797)

FLOW

Valuesin mW/m 2

,

/

,Xxx'•ø •

L

•

"::iii

•/

SCALE 0 t o

0.5 I

1.0 I 0.5

• 1.5 • l i.o•i ,

Fig. 63. Heatflowdataat Roosevelt Hot Springs, fromWardet al. (1978).Valuesin mW/m2.


CSAMT

791

,

APPARENT RESISTIVITY

(Near-

field

Corrected)

Survey

•:

256 Hz

RI0W

,

R9W

13

8.3 /

•

•/

24

XI,•

T26•

SCALE 0

0.5

I

I

1.0 I

0

0.5

t

I

1.5•m I

I.O•i I ,

Fig. 64. Apparentresistivitydataat 256Hz for survey2, usingsources1 and4. Note the threeconductivepockets along the axis of the Opal Mound Fault. Producingwells are darkened circles; former or minimal producersare slashedcircles; injection wells have arrows; dry wells are enclosedby a cross.

792

Zongeand Hughes


,,

APPARENT RESISTIVITY

(Neor-

field

Corrected)

Survey

2

4Hz

R 10W R9W

304

ß

420

• /3•7934, \ / •

'

I• ,/

/•,o.•

•/ ••'75

,

Tz•S

,50

• '••, •i ,•//-' /

/ ,,,•-••0-,,• • ß

SCALE 0

0.5

I

I

0 I

1.0 I

0.5 '

I

1.5 km I

I.Om I

Fig.65.Apparent resistivity dataat4 Hzforsurvey 2,using sources 1and4.Onlythemiddle conductive pocket remains in these deeper data.


CSAMT

o

o

{ m) qidaQ opnesd

0

•

793

0


794

Zongeand Hughes

Geology

+3

2__56Hz

-3

ß

128Hz

ß

64Hz -2

-$

ß

+1

-4

-4

o +7 +. +• :.•

ß.• *.• *• ß.•

+: +.• -I

-j

+3

,? ß

ß• '

ß•

+3 0 ß

-; +• +• ß I'

_.•

o

ß

+5+6+5-I -iß

+1

ß

+1 ß

+2

ß

ß

1-12

-. E-•3

-I

+,•

o ß +1

o ß

ß

E+553

•-2

+:•

-4

o

ß

ß

+S +1 ß

+.4 +.2

i ß

0

6+_7 +8 +:o.+4 '-.6•)• '

+4

+4 ß

16Hz

+;

0

8Hz

ß

-:5 ß

+$

ß

-2

ß

+••

.0

.6

-.•

'

+.

:52_ Hz

'

0

ß

-6

4Hz

ß

-4

_

ß

+1

-5

_ ,,i..e?

+.• +.4 +.e _,, .

-.• .4ß•f.._

*.• ..•'

o,+• +f+.•_.:/_

ß

. •,-""--q7

+.• +'6

•.

+1 +1 +4 -a 'l'•'!J'• ß

ß

ß

ß '

+j +.4'{• -.•

•,_•.•

,•/ *••Z•, o

o

+••

'+?

ß

+,• +4

+1 ß

+,•

o '%,' +<.ADJ +1

ß

ß

-I

•7•%• ' +1•

ß -.4•

Fig. 67. Percentchange in resistivity fromsurvey1to 2 asa functionof frequency. Positivevaluesindicatesurvey 2 resistivities werehigherthanon survey1. Oneinterpretation is thata steamcaphasincreased the apparent resistivitiesat depth, but the changeis too smallto confirmthis.


CSAMT

primarily at data below 32 Hz. The pattern changes from plot to plot but the overall trend is toward positive values. The changeto more resistive values is sharply cut off to the north at the Negro Mag Fault. The 4 Hz data in particular suggest increases in resistivity along the trend of this fault. Toward the south, the change over the reservoir merges with positive changesover the bedrock areas. At an average resistivity of 20 f/.m, the top of the zone which has become more resistive is estimated to be about 250

m. Overall, the changeover the geothermalproduction zone appears to be between +5 percent and q-10 percent.

Conceivably the change to higher resistivities over the field may indicate the developmentof a steam cap in the reservoir. Steam had indeed been developed between the two surveys, accordingto reservoir engineers. The location and depth of the resistivity change identified by CSAMT is consistentwith what is known about the reservoir, but at present we are unable to draw any firm conclusions. Further phases of this monitoringstudy are plannedto investigatethis matter more thoroughly. Massive Sulfide Exploration

The Crandon massive sulfide orebody is located in the Rhinelander-Ladysmith greenstonebelt of northern Wisconsin.

795

source-sounding separation(this was in the early days of CSAMT, when source effects were not as well understood). Near-field saturation occurs at about 64

Hz. Superimposedupon the transition-zone and nearfield data is a strongly conductive feature near station 0. The conductor is well correlated with the location of

the sulfide body. The data show a sharp southern boundary, indicating a strong resistivity contrast between the sulfidebody and the footwall breccia to the south. The northern boundary is much more gradual, due in part to the northerly dip of the orebody. Figure 69c showsthe normalized apparentresistivity data, which resultsin a more cosmeticallyrealistic pseudosectionand which also strips out the shallow layering due to more conductive surface till. Gold Exploration, Mapping Epithermal Alteration

The Metal Mining Agency of Japan has been active in CSAMT exploration for gold since the early 1980s. Yokokawa (1984) and Kawasaki et al. (1986) report on work done over the world-classHishikari gold deposit on Kyushu island in southern Japan. Hishikari is an epithermal vein type deposit in which hot fluids still predominatedue to recent vulcanism(Abe et al., 1986; Ikeda, 1987). Fluids rich in silica and precious metals were depositedin fissuresand tension cracks in andes-

As described in a review in Schmidt et

al. (1978), the body is estimated to have reserves of some 62 million metric tons of ore, consisting of 5 percent zinc, 1 percent copper, and low grades of silver, gold, and lead. The sulfide content of the ore zone averages about 65 percent. Sulfides consist of primarily pyrite, with moderate amountsof sphalerite and chalcopyrite, and small amountsof galena, quartz, chlorite, and sericite. The ore zone is an elliptical, lens-like, nearly vertical tabular structure averaging about 30 m in width and having a length of about 1500m. It is covered by 50 m of glacialoverburdenand extends to a depth of about 500 m. Figure 68 shows an idealized geoelectric cross-section. The authors obtained both complex resistivity (CR) and CSAMT data over this orebody. The CR apparent resistivity data (Figure 69a) show a strongly conductive feature over the orebody. The relatively large 90 m a-spacingbroadens and weakens the strengthof the anomaly, but the anomaly is still plainly discernable. A moderate polarization high is also associated with the high sulfide content of the body.

0 5OO

2 '•: Glacial

4

6

8

I0

12

14

Overburden '

400

300

200 Crondon Sulfide Bad

I00

Footwall Breccia

Brecciated Tuff

ScalarEx/HyCSAMTdatawereobtained alongthe CR line using an E field spacingof 30 m and a 765 m source placed 2.8 km from the line. The Cagniard resistivity data (Figure 69b) show artificially high near-field resistivities, which are the result of the close

-I00

Fig. 68. Geologic cross-sectionof the Crandon ore body, from Schmidt et al. (1978).

796

Zongeand Hughes

itic hostrocks,forminga seriesof quartzveinscarry-

exploration. A line of scalar CSAMT was then run to detail the anomaly.

ing high-gradegold and silver. Ore is found in four

major veins and numeroussmallveinlets.The major

Figure 70 showsthe subsequently developedgeo-


veins vary in thicknessfrom a few centimetersto 10 rn

logic cross-section and the scalar CSAMT data over the ore zone. High resistivities are associated with

andhavea total strikelengthof over700m. The grade is extremely high. Peak gold is nearly 1500g/t (54 oz/ton);averagegradesare 80 g/t (2.9 oz/ton)for gold

surfacerocks, particularlythe Kurozonsanrhyolite and the HokusatsuShinki volcanics.At a depth of about 100 m, hot epithermal fluids cause an overall drop in resistivity.The strong, vertical resistivity anomalybetweenstations16 and 18 is the anomaly whichwasinitiallydrilled.As indicatedin thegeologic discussion in Abe et al. (1986), this anomalyis associated with extensivevertical fracturesin the pa-

and about 50 g/t (1.8 oz/ton) for silver.

Abe et al. (1986)describethe explorationhistoryof the deposit.From 1975to 1981,initialregionalexplorationby the Metal Mining Agencyof Japan(MMAJ) includedgravity, Schlumbergersoundings,and airborne EM and magnetics.These data showeda lowresistivity, high-gravity zone which needed further CR

leoandesites which are filled with hot water and have

DATA

(a) Apparent Resistivity (•-m) -12

-9

I

-6 I

-3

o

3

452:

9

12

650

679

E94

2-

484

3609

4-

293

?87

5-

ß

276

6-

ß

CSAMT

DATA

(b) Cognic•rd Resistivity -4 I

I

I

(c) Norrnolized Apporent Resistivity (•.m) 2_.

-2 I

I

I

4 I

I

6

-4

I

I

2048

2048

512

512

256

256

128

ß-!-

6

i

-2 I

I

I

,""%

o

2

I

I

4

6

•

.•, 12:8

64

(..3

c- 32

•

16

•

•6

8

Fig. 69. Electricaldataobtained overthe Crandonorebody.(a) CR apparent resistivity froma dipole-dipole survey;a = 91 m. (b) CSAMT Cagniardresistivityfrom a scalarsurvey;a = 30 m, r = 3.3 km. (c) Normalized CSAMT Cagniardresistivity.


CSAMT

extensive clay alteration. These same fractures contain the ore in the Shirnanto Group. Hence, the CSAMT has directly identified the structures which host the gold. Subsequent work provided a plan-map of CSAMT apparent resistivities in which the conductive zone corresponds well to the ore veins and to geologic structures. Kawasaki et al. (1986) describe 1-D inversions which produced electrical cross-sectionsin excellent agreementwith the presently understoodgeology presented in Abe et al. (1986). Following precise location of the target with CSAMT, a drilling program was commenced by the Sumitomo Mining Company. Initial core drilling tests were favorable. In 1982 several high-gradeveins were encountered, and preparations for underground mining began. Sumitomo did considerablefollow-up geophysical exploration, including three CSAMT studies, gravity, rnagnetotellurics, and induced polarization. Mining began in 1983, reaching full capacity by 1986. Hishikari is a truly world-class deposit. Recent estimates of ore reserves are 5 Mt, with an average recovery of about 200 t/day. This gives the mine a 100-year life with a total reserve of 400 000 kg (14 million oz) of gold alone. Operating costsare presently about 20 percent of the gross revenue (Stocks and Hodinott, 1987). At presentprices ($430/oz), the mine would yield about $5 billion net revenue over its lifetime. The successof this project spurred extensive use of CSAMT in gold exploration in Japan by domestic and Australian companies(Yokokawa, 1984;Ikeda, 1987; Austpac, 1987).

797

Gold Exploration: Mapping Silicified Reefs

Cannon mine is a major gold producer in central Washington, USA. Gold mineralization occurs in hydrothermally altered, silicified zones of the Swauk Formation, which is composed of arkosic sandstones and conglomerates.The silicified zones occur in long, semicontinuous

linear trends which

have a southeast-

erly strike. The structure is steeply dipping and usually is concealed by surface cover. Figure 71 shows a CSAMT apparent resistivity pseudosectionacross a known silicified outcrop several kilometers southeast of the mine. A strong electrically resistive feature is observed directly over the silicified reef. The resistor is flanked by conductive zones, which are thought to be conductive clay alteration surroundingthe silicification. Hence the CSAMT data show a distinct electrical "signature" over the silicified

zones.

Additional

CSAMT

lines were

run in areas with

surfacecover. Resistive trends were mapped alongthe predominant strike well to the southeastof the known siliciousoutcrops. This information directly identified specific drill targets, reducing the cost of the exploratory drilling program. Gold Exploration: Mapping Structure Under Alluvial Cover

An excellent example of mapping gold-bearing structure

beneath

alluvial

cover

comes

from

work

done by Phoenix Geoscience(pers. comm., 1988). The survey was done in north-central Nevada. The geology, shown in Figure 72a, consists of alluvium over silicified

Paleozoic

sediments.

The bedrock

interface

(a) Geology (2.'1 vertical scale exaggeration) $

24

Cloy-

Altered Stn.

Silicified

(conductive) / siljcic'"--7/-'---

Zone --It •-'(conduct

400-6--•-•• 8

i

{b) CSAMT Apparent Resistivity Data {Q-m) f •, ; .,o .,, .,. .,,, ... ;o ,,2

Clay-

Zone Altered (resistive) Zone

9

/ 9u.t. crøp //

.•.•11 //

apple

orchard

,.,, o 200 ..

IOO

Fig. 70. CSAMT results from Hishikari Mine survey (Yokokawa, 1984). (a) Geology (Abe et al., 1986;Yokokawa, 1984). (b) CSAMT apparent resistivity.

Fig. 71. CSAMT apparent resistivity data over an outcropping silicified reef southeastof Cannon mine, Washington. Silicification is resistive, while clay-altered flanks are conductive. Further mapping was done over buried reefs, as described

in the text.


798

Zonge and Hughes

varies in depth due to graben faulting typical of the Great Basin (see the Trap Springcasehistory earlier in this section). The gold deposit is found in the fracture zone, which shows argillic alteration. Mineralization is typical of hydrothermal Carlin-type depositsin central Nevada. The deposit has been proven by drilling but

ern fault showsan apparent eastward dip and a significant offset. Basement

on the east side of the fault is

less resistive, as expected since it is unaltered (not silicified).

has not been mined as of 1988.

Uranium Exploration

Figure 72b shows the CSAMT apparent resistivity data, obtained with 60 rn dipoles. The data map the alluvial basementcontact quite well. One-dimensional inversions of the resistivity data agree closely with basement depths obtained from drilling. Two major graben-type faults are observed. The western fault appears to be steeply dipping. Conductive material is indicated on its downthrown side, correspondingto the altered fracture zone. The east-

The uranium depositsat McLean Lake in Saskatchewan represent a good CSAMT target. The geology consists of approximately 170 rn of high-resistivity pre-CambrianAthabaskasandstonesoverlyingporous regolith. Uranium and sulfide ore bodies are found in highly fractured and altered regolith. Phoenix Geoscience performed extensive CSAET work over two orebodiesusing 30 and 60 rn dipoles (Yamashita and Hallof, 1985). The data show a good

(a} Geology -120

-60

I

0

i

0

60

120

i

i

i

180

240

I

I

i

i

3 O0 I i

360 I i

420 , ,

4 80

Iß

I i

540

,

Alluvium

-

..

Argillized

•• .•

FractureZone

..- ....

I00 ,,=.

..

Silicified Bedrock

"

Unaltered

Resistive ['' Bedrock

2OO

i

Bedrock

(b) CSAET Cagniard Resistivity 2048

512

256 128

•

64

-

8

--

Fig. 72. CSAET results over buried basement structure associatedwith a hydrothermal gold deposit in northcentral Nevada. (a) Geology obtainedfrom drilling, with someinterpolationfrom the CSAET results. (b) CSAET Cagniardresistivity data. Geology and data courtesyof Phoenix Geophysics(pers. comm., 1988).

--


CSAMT

correlation to geology, and both ore bodies traversed on the work were well defined in the apparent resistivity data. Figure 73 showsthe 60 m dipole data over one of the ore bodies. These data are near-field corrected apparent resistivity data, as described in Yamashita and Hallof (1985). The geologic contact at 170 m is readily seen at the highest frequencies. The ore body lies between stations 700S and 800S; note the excellent correlation with the conductor. The conductor appears to extend upward toward the top of the regolith, but its strongestresponse probably starts at about 300 m in depth. Two dimensional finite-element modeling done by Yamashita and Hallof producean anomaly which is consistent

799

between the E- and H-fields, or the impedance phase. The deep resistive zone is manifestedby the low phase numbers in that area, but notice that there is very little evidence of the resistive feature beneath station 11.5,

leading us to believe that this is an artifact of a near-surface resistive block, or static offset effect.

Figure 76 is a static corrected resistivity pseudosection. Notice

that the dike-like

station

with the field results.

Comparisonof Dipole-Dipole Resistivityand CSAMT

ObservedCagniardResistivitõ ß;

The following brief case history is included to demonstrate some of the data processingprocedures and to give a comparison of dipole-dipole resistivity and CSAMT over the same line using the same E-field dipole spacings(300 ft). Figure 74 shows the observed Cagniard resistivity along the line with several prominent features. A deep zone can be seen centered

beneath

station 6

and a strongly resistive dike-like zone beneath station 11.5. A deep conductive area appears to flank the dike-like

beneath

11.5 has disappeared, substantiating our contention that this was a near-surface feature, and diminishing its interest as a gold target. Figure 77 shows the results of inverting the static corrected pseudosection. Notice the deep resistive target is now centered beneath station 5.5 and there is

Measurements

resistive

feature

feature.

Either

1.5 Z.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 lZ.5 13.$ 14.5 15.5 16.5 17.5

2848 -

1024 -

S12

....

...

32

16

.... .•i:'L...•.....-" /• Fig. 74. Observed Cagniard resistivity.

of the resistive features could

represent silicified structure that would be of interest from a gold exploration standpoint.

ObservedImpedancePhase 5.5 6.5 7.5 8.5 9.5 10.5 11.5 ILS 13.5 14:5 5.5 16.5 17.5

Figure 75 is apseudosection of the phase difference '• .-, 1024

McLean South

East

l,

,•oos I

I

,ooo I

I

eoo I

Uranium

I

600 I

Pod

I

•oo I

I

•oo I

I

o I

• s12

128•

•

,•• • •{•.)• "--/

• -

Fig. 75. Pasediffee ceb - - ß SCenic-corrected ResistiviC•

• g

.•

i• 8

Fig. ?3. CSAET near-field corrected data over McLean Lake uranium deposit (redrawn from Yamashita and Ha]]of, 1985).

2.53 . s.s6.57.. 8.59•5 11.5 15.5 16:5 12.5 13.•14.5

1 ,•

•2•

. ......

•

Fig. ?6. Static corrected resistivity pseudosection.


800

Zonge and Hughes

a broad, very low resistivity (alteration?) zone stretching from station 10.5 to the end of the line. In comparison, Figure 78 shows the dipole-dipole resistivity results over the same line. Notice that the data are mostly affected by near-surface geology. The

resistive

surface

block

beneath

station

11.5

(0 + 11.5N) appears as a pant-leg which masks the conductive zone on that end of the line. The deep resistive

zone beneath

station 5.5 is not evident

at all.

The IP data along this line was relatively uninteresting, but had there been diagnosticIP data, this is one area where the dipole-dipole survey would have provided information that could not be obtained by

bullion. Drilling at the new site predicts a total estimated yield of over 600,000 oz. Figure 79 shows the location of the Golden Cross mine in the Coromandel Kaimai volcanic zone.

Figure 80 showsthe generalgeologyof the area, and Figure 81 is a geologiccross-sectionalong line 4850 N. There are four main groupingsof rock types which determine the geophysical signature of the Empire deposit (Hay, 1989): Silicified

ore zone

Surrounding alteration zone Unaltered

host material

Overlying andesite.

CSAMT.

Mapping Alteration and Silicification Golden Cross Mine, New Zealand

Epithermal gold deposits are characterized by strong variations in electrical resistivity. This is typically due to alteration of silicate minerals to clays and results in increased conductivities; whereas remobilized silica depositionresultsin increasedresistivities. Contrasts of four orders of magnitudecan be observed within hydrothermal alteration zones. The new Golden Cross orebody is the only major epithermal gold discovery in New Zealand this century. Earlier mining activities southof the new deposit produced a documented 386,000 oz of gold-silver

The silicified ore zone is characterized by lowporosity high-resistivity (1000 f•-m) rocks containing 2 to 3 percent sulfides, and no magnetite. Surroundingargillic altered rock is low resistivity (• 1 f•-m) due to clays in the heavily altered zones. The formation containsup to 5 percent sulfidesin the stockwork above the ore zone, and up to 3 percent sulfidesin the clay-altered zones. No magneticminerals are found in this area.

The propylitic altered zones are characterized by low resistivities(• 10 f•-m), up to 1 percent sulfides, and a lack of magnetic materials. Unaltered host rocks and overlying andesiteshave a resistivity of about 100 f•-m and are variably magnetic.

Smooth-modelInversion o½ Static-corrected 1.S

ZS

3.S

Fig. 77. Results pseudosection.

4.S

S.S 6.S

of

7.S

Resistivit!j

B.S 9.S 10.S 11.S 1ZS 13.S 14.S 1S.S 1•.5 17.S

inverting

the

o

2

static

corrected

This new orebody was discovered as a result of ground follow up over an area delineated as lowmagnetic relief by an airborne magnetometer survey flown in 1978. Ground follow up consistedof geological mapping,rock chip and soil sampling,and vertical diamonddrilling designedto follow the gently dipping stockwork down dip from its outcrop position. After the initial discovery hole was completed, a number of different geophysicaltechniqueswere run over this area in an effort to find some method

n:4

n=õ n:6

.,,

Fig. 78. Dipole-dipole resistivity results over the same line.

that


CSAMT

would detect the orebody. Air and ground magnetics tended to outline the outcropping altered zones. Resistivity-IP (time-domain dipole-dipole and gradient) mapped alteration but did not delineate the highresistivityore zones. The IP data were very noisy and

801

inconclusive, and subtle IP highs were drilled without findinggold. Perhapsthe highly altered rock overlying the stockwork shielded the IP response. None of the surface techniques used at Golden Cross located the Empire vein zone.

NORTHLAND

North

Island

GOLDEN CROSS

TAS

M

AN

SEA EAST

COAST

TARANAKI

AWKE 'S BAY

NEW

ZEALAND

MARLBOROUGH

South Is/and

P

A

0

C

C

I

F

I

E

A

N

C

Fig. 79. Locationof the GoldenCrossminein the Coromandel Kaimaivolcaniczoneon northislandof New Zealand.

802

Zonge and Hughes


In 1986 we were

asked to run a test CSAMT

line

I

I

I

•,000E

I

I

•,200E

•,400E

LINE

4850N

over the orebody to see if this technique would provide useful information. Figures 82 and 83 show the observed Cagniard resistivity and phase difference measured along line 4850N. Notice the strong contact to the east of station

3000E

and the resistive

zone

beneath station 3200E which is flanked by low-resistivity material on either side. The phasepseudosection showssimilar activity: a contact east of station 3000E, a resistive zone beneath station 3200E with conductive

zones coming near to the surface on either side, and a conductive zone extending deeper to the east. Figure 84 gives the results of smooth-model inversion of the Cagniard resistivity. This section shows a break in the conductive layer beneath station 3200E with a depth extent of several hundred meters. This feature coincides exactly with the Golden Cross orebody. This test survey provided the only surfacegeophysical information that could be used for delineating the Golden Cross orebody.

Fig. 81. Geologic cross-sectionalong line 4850N (after Hay, 1990).

Observed Cagniard Resistivit g

I•L I- /- .t

Golden

::*;: ..'/:.:. ...

Cross

Line

4850N

4096

q

7t'_t

&'-7/-,4 ß

r,

I"r' <'/t"/ß i • L> V

...

2048 -

ß

) 024

< >, LEGEND

^ - •YoungerAndemite 4 <• OlderAnde=ite ^ ^ t. • ...--Un½omformity ...

.:;:.. ..

.::..

128

^z.t,• ![-•Propyliti½ Alteration

•4

ßvr v.; !•Argilli½ Alteration

32

cu•-7 Gold c• , v •
...

':::.

•.'• •"

512

1G 8

........17 .*

/"

'71/'7/,._ T>.• t,.-7L,4• < A Empire V•n 7'one

Fig. 82. Observed Cagniard resistivity for line 4850N.

.

•' 7L6 t-PI•I"''lv- P

t•.7t•>l-v<•'t-•'•<

Observed ImpedancePhase

/..7 A I,,.n 2800 E

3000 E

Golden

Cross

Line

4850N

2)200E

3400 E

3600 E

3800 E

4000 E

4200 E

4096 i

I' "'/% x/ t_ ß<•'•

/- < - ß

v

1024

512 :::::::.-:}. r. ^ ,• v v .• / ................. •t. ,. •,•n-Sj:iii:::::::::::::::' • -•'" -' < ,i i ....... ............. ::...: ........ < <• /

......................

........................

_ .

t

• ^ •

,

:::::::::::::::::::::::::^ ^ >^a•

/

.. < t. '1 ß t, v_,•,, < -, • ßß •

i-•l

j

•

•28

•00m

•4

I

32

8

Fig. 80. General geology'of the Golden Cross/EmpireVein Zone (after Hay, 1989).

Fig. 83. Observed phase difference for line 4850N.

CSAMT

803

Smooth-modelInversion of' Cagniard Resistivity Golden

Cross

Line

4850N

We thank Cyprus Gold New Zealand Ltd. for permission to use this data set.


500

400_ 300_

Detecting SubsurfaceWater and Structure

200_

-•oo_

An engineering firm was contracted to perform a hydrologic investigation of a potential commercial developmentof a desert area north of Phoenix, Arizona. The purpose of the project was to determine

-200

whether

•oo_

o_

Fig. 84. Modelling resultsfor line 4850N.

or not sufficient

water

was available

on the

property to support development. The presenceof a local water supply on the approximate square-mile property would save enormousexpenseand eliminate

EXPLANATION

QTb, basalticflows,agglomerate, tuff, & cinders

pCgr, granite, quartz monzonite, granodiorite, • quartz diorite pCsc,schistose units Drywater well (500'depth) Producing water well(1500' depth) Fig. 85. Location map of the Arizona groundwatersurvey.

804

Zongeand Hughes

the potential legal problemsof piping in off-property


water.

The subsurfacegeology was relatively unknown at the start of the project, with the exception of some regional gravity data and surface geology. Figure 85 shows the surface geology. Granitic outcrops trend approximately N50øW. The outcrop is discontinuous alongSkunk Creek, suggestingsomestructuralcontrol in this area.

Based on an examination of the surface geology, two drillholeswere placed in the southernpart of the property. The holes, placed about 1500 feet apart, were drilled to a depth of 500 feet. Water was encountered near 300 feet at hole B and near 140 feet near

hole A, but pressureheadswere minimal and saturated layers were thin. At this point three surveylines of CSAMT data were run in order to provide information about subsurface water and structure. The apparent resistivity data (Figure 86) indicatea probablefault at station8, which lies in Skunk Creek. All three lines show the fault as a

north-northeasttrendingfeature. This previouslyunmapped fault probably accounts for the break in the

steepnesswas interpreted to be the result of decreased resistivity at depth near the fault. Computer modeling indicated that a conductive layer near 1000 ft could change the curve behavior in this manner. This modelingsuggested that an unsuspectedaquiferlay deeper than the testing done to date. The decision was made to deepen hole B to investigate the conductor near 1000 feet.

Drilling encountered a significant water-saturated zone at 1040feet (Figure 88). The saturatedlayer was 200feet thick and showeda pressureheadof 360psi on a 6 inch borehole. It is estimated that with a 17 inch

borehole,the well will producefrom 600 to 1000gpm. The CSAMT surveywas significantlylessexpensive than the drilling program in this area, and provided information

which was crucial to the success of the

project. Drilling to 1000ft would be very risky in this area without somegeophysicalbasis. Based upon the geophysics,however, continueddrilling located water which will support commercial development of the property. The survey helped avoid premature abandonmentof the property, or pursuingan expensiveand complicatedpipeline construction.

outcrop.

The data exhibit two types of behavior, as shownin Figure 87. The stationswell away from the fault show type A behavior, with the shallow notch near 64 Hz.

This notchis a geometricartifactof the way the survey is laid out on the ground. However, the notch is also affected by geology. The stationsnear the fault show type B behavior, with a steepernotch. This increased

Mapping Brine Leaks From Injection Wells

Thousandsof abandoned,improperly plugged oil and gas wells exist throughout the United States. In situationswhere the producinghorizonis overpressurized due to secondaryrecoveryoperations,thesewells can allow injected oilfield brines to migrate up the boreholeinto shallowerdrinking-wateraquifers. This infiltration can also occur at abandoned well sites over

FAULT

West•

2 I

•

4 •

•

6 •

I

I0 I

12 I I

I

14 I

I

16 I EQst

4096 1024

'

/

256

a period of time. CSAMT was usedto map brine contaminationin the vicinity of the Prue Sand Unit, located on the Sac and Fox tribal lands of Lincoln County, Oklahoma. Oil has been produced from the Prue sands since the 1930s, and brine injection has been used for enhanced recovery since the 1950s.

128 N

64

-

150

-B

Resistivity

behavior

DEEP

32

SHALLOW NOTCH

•2

6'4

Resistivity away

l•'8

'

256

FREQUENCY

Fig. 86. Apparent resistivity data from one of the CSAMT lines. Vertical axis representssignalfrequency;lower frequenciescorrespondto greaterdepths.

behavior

from

the

'

512

foult

I

1024

20

i48

I

4096

(Hz)

Fig. 87. Comparisonof resistivityresponsesnear and away from the fault.

CSAMT

A CSAMT survey grid was completed in selected portionsof theproblemarea. Figure89 showsa typical apparentresistivitypseudosection near an abandoned well. The data delineate a deeper plume of conductive material coming close to the surface. Other plumes were located on additional survey lines. Bromide/

The Vamoosa Formation is a major supplyof drink-


ing water in the area. The base of the fresh water rangesfrom 40 to 135 m in depth. Total dissolved solids in the Vamoosa are less than $00 mg/1. Test wells drilled in 1979 indicated anomalouslyhigh brine content in the aquifer over the oilfield.

GENERALIZED

AT

805

SECTION

LITHOLOGI½ HOLE

B

AS

OF

dAN.

1987.

LAND SURFACE

I00'

\) BASALT I•ANDESITE FRAGS. / W/QUARTZ I• PINKGRANITE /

200'

/

Drilled before Geophysics. Stopped

Oct. 86. Water

first

w/minimal

encountered

•

3

PREDOMINATELY QUARTZ

W/PINK GRANITE

pressure head 400'

CLEAR QUARTZ W/FEW VOLCANIC FRAGS.

500'

6oo'

Water

at ~360

HIGHLY

psi

OF

15' thick

FRACTURED

GRANITIC

ZONE

GRAVELS

700'

800'

Drilled Restatted

after

Geophysics.

Jan.

900'

87.

I000'

Water

at ,,, 360 psi-••

(6"casing ) Drillers

600-

estimate

•loo' of

I000 gpm

with 17" casing.

) PERMEABLE CONGLOMERATE

,•oo'

/ /

Lithologic logs I• pump

testsin progress-Feb. 87 •3oo'

ß

.

] ? .

Drillin(] Stopped

Fig. 88. Summaryof drillingbeforeand after the CSAMT survey.

806

Zonge and Hughes chloride

ratios from

two test wells drilled

after the

survey indicated that the source of Vamoosa contam-


inants is indeed the Prue Sand brines.

Tracing Injection Fluids

-5 Hz

I

4096 2048

1024 512

Resistivity

Key

256

128

r•> 8.0ohmmeters I"• 4.0- 8.0ohm-meters • 3.2- 4.0ohm-meters

64

32

Fig. 89. Apparent resistivity data near an abandonedinjection well.

CSAMT has been successfully used to trace the migration of conductive fluids for in-situ mining projectsand injectiontests. In this example, an in-situ leachingproject was recoveringonly 10 percentof the injectedfluid and it was necessaryto determinewhere the rest of the fluid was going. Figure 90 shows the resultsof modelingCSAMT data which were acquired over the area and processed to give resistivities at fixed levels. The heavy shading indicates conductive zones, with lighter shades being more resistive. The conductive fluid is easily identified as being centered near the 1600 level, which was above the collection area. The fluid was moving slower than the hydrologistsexpectedand two surveyswere run over this area one month apart to see if the fluid motion could be traced. Results of the survey demonstrated that in some areas the apparent resistivity at depth was changingin excess of 1 percent per day. CONCLUSIONS

....

IOOO LEVEL

....

1300

LEVEL

....

1600

LEVEL

....

1900 LEVEL

....

2200

....

2500 LEVEL

LEVEL

(Collection Area)

Fig. 90. CSAMT depth-level modeling results for insitu leaching survey.

CSAMT has been used in exploration for over a decade, and has demonstratedoutstandingcapabilities in exploring for massive sulfides, base and precious metals, geothermal resources, petroleum, and ground water contamination, as well as for geotechnicalinvestigations. Its chief advantagesare high data quality, data repeatability, simultaneoushigh lateral resolution and good depth penetration, and logistical simplicity. Its chief disadvantagesare transmitter overprint and static effects. CSAMT is best used for exploring to depthsof 2 to 3 km. Deeper targetsare better explored with magnetotellurics, and targets shallower than about 10 rn are better explored with shallow EM systems. As with any other geophysicaltechnique, CSAMT is one tool that should be used when appropriate. We hope that this discussionpresentssufficient information to help the explorationist intelligently select (or not select) the CSAMT method. Several challenging interpretational difficulties remain to be solved, particularly the removal of source overprint, removal of static offset, and extraction of polarization information from the measurements.The solutions to these problems will require a concerted effort. We believe that suchefforts will expand present CSAMT capabilities to a wider range of exploration and engineeringuses over land and sea, and in downhole and underground studies. We hope this discussion will contribute the essential background summary needed to trigger some new developments.

CSAMT ACKNOWLEDGMENTS

807

ern Michigan: Zonge Engineering& Research Organiza-


tion, Inc.

We acknowledgeand thankthe unnamedminingand oil companieswho provided the data for use in this work. We also thank Scott Macinnes for his help in conceptualizingand modeling many of the effects presentedherein.We are indebtedto JamesWait, Phil Wannamaker, Norm Carlson, Keeva Vozoff, and Charles Swift for their careful reviews and helpful

comments.Special thanks are owed to Anne Urizar and Helen Baribeau for their outstanding drafting. Finally, our thanksto Misac Nabighian, who waited so patiently for a final draft to materialize. REFERENCES

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Austpac Gold N. L., 1987, Quarterly report to 31st December 1987: Auspac Gold N. L., GPO Box 5297, Sydney, NSW

2001.

Anderson,W. L., 1974, Electromagneticfieldsabout a finite electric wire source, PB-238-199: U.S. Dept. of Comm., NTIS.

Bannister,P. R., 1966, Quasi-staticfields of dipole antennas at the earth's surface: Radio Science, 1, 1321-1330.

Bartel, L. C., 1981,Resultsfrom usingthe CSAMT geophysical technique to map oil recovery processes:56th Ann. Fall Technical Conf. and Expo. of the Society of Petroleum Engineers of AIME. •1982, Potential use of the CSAMT geophysicaltechnique to map UCG processes:Proceedingsof the 8th Underground Coal Conversion Symposium,59-70. Bartel, L. C., and Dobecki, T. L., 1982, Geophysicalapplications in coal exploration and mine planning: Electromagnetics:SME-AIME Ann. Mtg., Dallas, Texas, SME Reprint 82-97, 1-9. Bartel, L. C., and Jacobson, R. D., 1987, Results of a CSAMT survey at the Puhimau Thermal area, Kilauea Volcano, Hawaii: Geophysics, 52, 655-677. Berdichevskiy, M. N., and Dmitriev, V. I., 1976, Basic principlesof interpretationof magnetotelluriccurves, in Adam, A., Ed., Geoelectric and geothermalstudies:Akademini Kiado, 165-221.

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Carlson, N. R., Hughes, L. J., and Zonge, K. L., 1985, Delineation of oil and gas fields using a high-resolution electrical technique: NSF/SBIR Grant #EAR-8460562. Carlson, N. R., and Vugteveen, R. Wm., 1985, Final report on CSAMT survey at Albion-Scipio and Stoney Point fields, Hillsdale and Jackson counties, Michigan--the evaluationof CSAMT for petroleumexplorationin south-

Carlson, N. R., Zonge, K. L., and Hughes, L. J., 1986, Applicationof CSAMT to oil and gas exploration:a case history: Technical programme and abstracts of papers, 48th Mtg. European Assn. Expl. Geophys., 83. Debroux, P.S., 1985, The influenceof induced polarization in inductive methods of electrical prospecting:M.S. thesis, University of Arizona. Duey, H. D., 1979, Trap Spring Oilfield, Nye County, Nevada: Rocky Mountain Associationof Geologists/Utah Geol. Ass. Basin and Range Symposium, 469-476. Duff, B. M., and Lepper, C. M., 1980, A high-resolution controlled-sourceaudio magnetotelluricsystemfor mining

applications:50th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 3449-3467. Eadie, E. T., Silic, J., and Hungerford, N., 1987, Marionoak--a very deep conductive target (abs.): Exploration Geophysics, 18, 39-41. Fryberger,J. S., and Tinlin, R. M., 1984,Pollutionpotential from injectionwells via abandonedwells: Proceedingsof the 1st National

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Spring Field, Nye County, Nevada: An evaluation of CSAMT for Great Basin petroleum exploration: Zonge Engineeringand ResearchOrganization, Inc. Hughes,L. J., and Carlson,N. R., 1987, Structuremapping at Trap SpringField with controlledsourcemagnetotellurics: First Break, 5, 403-418.

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808

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Sandberg, S. K., and Hohmann, G. W., 1982, Controlledsource audiomagnetotelluricsin geothermal exploration: Geophysics, 47, 100-116. Schmidt, P. G., Dolence, J. D., Lluria, M. R., and Parsons, G. III, 1978, Geology of Crandon massivesulfidedeposit in Wisconsin: Skillings' Mining Review, 8-11. Shoemaker, C. L., Shoham, Y., and Hockey, R. L., 1986, Interpretation of natural source electromagnetic array data: 56th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 63-65. Sibbett, B. S., and Nielson, D. L., 1980, Geology of the central Mineral Mountains, Beaver Co., Utah: Univ. of Utah Res. Inst., Earth Sci. Lab., Rep. 33. Sims, W. E., and Bostick, F. X., 1969, Methods of magnetotelluric analysis: Univ. of Texas at Austin Tech. Rep. 58.

Spies, B. R., and Eggers, D. E., 1986, The use and misuseof apparentresistivity in electromagneticmethods:Geophysics, 51, 1462-1471.

Spies, B. R., and Frischknecht, F. C., 1991 (this volume), Electromagnetic sounding, in Nabighian, M. N., Ed., Electromagnetic methods--Theory and practice, vol. 2: Soc. Expl. Geophys. Sternberg, B. K., Buller, P. L., Kisabeth, J. L., and Mehreteab, E., 1984, Electrical methods for hydrocarbon exploration II, magnetotelluric (MT) method, in Davidson, M. J., and Gottlieb, B. M., Eds., Unconventional

methodsin exploration for petroleum and natural gas III: Southern Methodist Univ. Press, 202-230. Sternberg, B. K., Washburne, J. C., and Anderson, R. G., 1985, Investigation of MT static shift correction methods: 55th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 264-267. Stocks, J., and Hodinott, P. J., 1987, Underground mining in Mining Annual Revue 1987: The Mining Journal Ltd. Strangway, D. W., Swift, C. M., and Holmer, R. C., 1973, The application of audio-frequency magnetotellurics (AMT) to mineral exploration: Geophysics,38, 1159-1175. Strangway, D. W., Ilkisik, O. M., and Redman, J. D., 1983, Surface electromagneticmapping in selectedportions of northern Ontario, 1982-1983: Ontario Geol. Surv. misc. paper 113. Syed, T., Zonge, K. L., Figgins, S. J., and Anzzolin, A. R., 1985, Application of the controlled sourceaudio magnetotellurics (CSAMT) survey to delineate zones of groundwater contamination--a case history: Presented at the Surface and Borehole Geophysical Methods in Ground Water Investigations, 2nd Nat. Conf. and Expo., Fort Worth.

Swift, C. M., 1967, A magnetotelluric investigation of an electrical conductivity anomaly in the southwestern United States: Ph.D. dissertation, Mass. Inst. Technology.

Tikhonov, A. N.,

1950, Determination of the electrical

characteristicsof the deep strata of the earth's crust: Dokl. Akad. Nauk SSSR, 73, 295-297. Ting, S.C., and Hohmann, G. W., 1981, Integral equation modelingof three-dimensionalmagnetotelluricresponse: Geophysics,46, 182-197.


CSAMT

Tinlin, R. M., Hughes, L. J., and Anzzolin, A. R., 1988 (in press), The use of controlled source audio magnetotellurics (CSAMT) to delineate zones of ground-water contamination--a case history: in Collins, Ag., and Johnson, A. J., Eds., Field methods for ground-water contamination studies and their standardization, Am. Soc. for Testing and Materials. Uchida, T., Yokokawa, K., and Nishikawa, N., 1987, Test survey of tensor CSAMT at the Akenobe Mine, central Japan: 57th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 217-219. Valla, P., 1982, A test field survey using the Melos electromagnetic method: 52nd Ann. Internat. Mtg., Soc. Expl. Geophys., 395-397. Valla, P., and Gasnier, S., 1984, Broadband frequency electromagneticsoundingswith absolutephase reference: 54th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 121-123. Van Blaricom, R., and O'Connor, L. J., 1987, The geophysical responseof the Red Dog deposit: Presentedat the 3rd Internat. Conf. on Geophys. and Geochem. Expl. for Minerals

and Groundwater.

Vozoff, K., 1972, The magnetotelluricmethod in the exploration of sedimentary basins: Geophysics, 37, 98-141. •(Ed.), 1986, Magnetotelluric methods: Soc. Expl. Geophys. Wait, J. R., 1951, The magneticdipole over the horizontally stratified earth: Can. J. of Physics, 29, 557-592. •1952, The cylindrical orebody in the presence of a cable carrying an oscillating current: Geophysics, 17, 378-386.

•1953, Propagation of radio waves above a stratified ground: Geophysics, 18, 416-422. •1961, The electromagneticfields of a horizontal dipole in the presence of a conducting half-space: Can. J. of Physics, 39, 1017-1028. •1962a, The propagation of electromagnetic waves alongthe earth's surface, in Langer, R., Ed., Electromagnetic waves: Univ. of Wisconsin Press, 243-290. •1962b, Electromagnetic waves in stratified media: Pergamon Press, Inc. •1977, Multiple scatteringbetween a buried line conductor and the earth's surface: Geophysics, 42, 14701472.

•1982, Geo-electromagnetism:Academic Press, Inc. Wannamaker, P. E., 1983, Interpretation of magnetotelluric data: Geophysical workshop notes, Electrical Methods in Oil and Gas Exploration, Univ. of Utah Res. Inst., 1. Wannamaker, P. E., Hohmann, G. W., and Ward, S. H., 1984,Magnetotelluricresponsesof three-dimensionalbodies in layered earths: Geophysics, 49, 1517-1533. Wannamaker, P. E., Stodt, J. A., and Rijo, L., 1985, PW2D finite element program for solution of magnetotelluric responsesof two-dimensional earth resistivity structure, user documentation: Earth Science Laboratory, Univ. of Utah Res. Inst., DOE contract #DE-AC03-84SF12196.

•1986,

Two-dimensional topographic responses in

.magnetotellurics modeledusingfiniteelements: Geophys-

ics, 51, 2131-2144.

Ward, S. H., and Hohmann, G. W., 1988, Electromagnetic theory for geophysicalapplications, in Nabighian, M. N., Ed., Electromagnetic methods--Theory and practice, Vol. 1, 131-311: Soc. Expl. Geophys.

809

Ward, S. H., Parry, W. T., Nash, W. P., Sill, W. R., Cook, K. L., Smith, R. B., Chapman, D. S., Brown, F. H., Whelan, J. A., and Bowman, J. R., 1978, A summary of the geology, geochemistry, and geophysics of the Roosevelt Hot Springs thermal area, Utah: Geophysics, 43, 1515-1542. Warner, B. N., Bloomquist, M. G., and Griffith, P. G., 1983, Magnetotelluric interpretations based upon new processing and display techniques: and biographies, 53rd Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 151-154.

Wayland, J. R., Jr., Lee, D. O., and Cabe, T. J., 1984, Mapping of a streamfloodin a Utah tar sand by controlled source audio magnetotelluric survey: Soc. Petr. Eng./ Dept. of Energy, 4th Symp. on Enhanced Oil Recovery. Wayland, J. R., Jr., and Leighton, A. J., 1985, Mapping technology:a key to EOR control: Oil and Gas J., Dec. 2, 109-115.

West, R. C., and Ward, S. H., 1988, The borehole controlled-source audiomagnetotelluric response of a three-

dimensionalfracture zone: Geophysics, 53, 215-230. Word, D. R., Goss, R., and Chambers, D. M.,

1986, An

EMAP case study: 56th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 61-63. Wright, P.M., Ward, S. H., Ross, H. P., and West, R. C., 1985, State-of-the-art geophysicalexploration for geothermal resources: Geophysics, 50, 2666-2699. Yamashita, M., and Hallof, P. G., 1985, CSAMT case historieswith a multi-channel CSAMT system and discus-

sion of near-field data correction: Phoenix Geophys., Ltd. Yamashita, M., Hallof, P. G., and Pelton, W. H., 1985, CSAMT case histories with a multi-channel CSAMT system and near-field

data correction:

55th Ann.

Internat.

Mtg., Soc. Expl. Geophys., Expanded Abstracts, 276278.

Yokokawa, K., 1984,The exploration systemand equipment of the CSAMT method (in Japanese): Geophys. Expl. (Butsuri Tanko), 37, 279-286.

Zonge, K. L., 1985, Internal company report on removal of static shift effects.

Zonge, K. L., and Hughes, L. J., 1981, The complex resistivity method, in Advances in induced polarization and complex resistivity: Univ. of Arizona Press, 163-208. •1985, The effect of electrode contact resistance on electric field measurements: 55th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 231-234. Zonge, K. L., Figgins, S. J., and Hughes, L. J., 1985, Use of electrical geophysics to detect sources of groundwater contamination: 55th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 147-149. Zonge, K. L., Hughes, L. J., and Emer, D. F., 1986, The use of IP, CSAMT, and TEM in mineral exploration: 2nd Symp. on Expl. Geophys. Abstracts, Xian. Zonge, K. L., Ostrander, A. G., and Emer, D. F., 1980, Controlled source audio-frequency magnetotelluric measurements: 50th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 5, 2491-2521. Reprinted in: Vozoff, K. (Ed.) (1987), Magnetotelluric methods: Soc. of Expl. Geophys., 749-763. Zonge, K. L., and Wynn, J. C., 1975, Recent advances and applications in complex resistivity measurements: Geophysics, 40, 851-864.




CHAPTER

AIRBORNE

10

ELECTROMAGNETIC

METHODS

G. J. Palacky*andG. F. West*

INTRODUCTION

nique could not be adapted to airborne use becauseof its inherent need for electrode ground contact, the secondalternative appeared feasible in concept. The first attempt known to the authors to mount EM equipment in an aircraft was made in 1946 by Hans Lundberg. This early EM system, which consisted of two coils mounted inside the helicopter cabin, was flown in northern Quebec and Ontario, but conductors could be detected only if overflown at an altitude of 5 m, a rather impractical proposition (Collett, 1986). Nonetheless, the idea flourished. A more successfulairborne EM (AEM) development was initiated in 1949 by Stanley Davidson, a geologistwith Stanmac Ltd., and McPhar Geophysics Ltd., a newly formed geophysical company in Toronto, which has developed a portable ground EM system. Experiments carried out on a frozen lake proved that the conceptwas feasible and International Nickel Co. of Canada (INCO) contracted with McPhar to designand build the world's first operational AEM system. The aircraft used was a wooden-skinned An-

The Story of Airborne Electromagnetics After the end of World War II, the reconstruction of

war-ravaged economies fueled a great demand for natural resources. The emerging Cold War caused explorationists to seek secure supplies in countries geographically and politically close to the United States. With vast areas that were then little explored, Canada was one obvious choice. These circumstances

provided a great incentive to develop geophysical methodswhereby a sparselypopulatedcountry, where the climate is often harsh and frigid for part of the year, could be scanned quickly and effectively for depositsof strategicbase metals, suchas copper, lead, zinc, and nickel. Airborne magnetometer systemsthat were developed from early war-time prototypes used in submarine detection became widely used in mineral exploration in Canada. However, it soon became obvious that the magnetic information was of more value indirectly in aiding geologicreconnaissancethan it was directly in ore exploration. The abundance of magnetic bodies in deformed metamorphic terrains with base metal potential made it difficult to select specific targets for more detailed exploration on the ground. An alternative or additional technique was, therefore, required to carry out prospectingfrom the

son with an onboard transmitter

and receiver

towed in

a "bird". Airborne testing was carried out over the Whistle Mine near Sudbury, Ontario, which was subsequently used by many AEM system developers for similar purpose. The field surveys carried out with the INCO-Anson system in 1953 in the Bathurst-Newcas-

air.

tle area in New Brunswick, Canada, became an instant

Pre-war geophysicalsurveyshad shownthat several types of mineral deposits were highly conductive, particularly the metamorphosed massive sulfides of the Precambrian greenstone (metavolcanic) belts which are an important source of lead, zinc, copper, silver, gold, and nickel. Ground resistivity and electromagnetic (EM) surveys were beginningto be used regularly for their detection. While the former tech-

success.

One of the AEM

anomalies

drilled

in 1954

was the Heath Steele lead-zinc deposit 65 km southwest of Bathurst. It was the first in a long series of triumphs for the new AEM method. In the early 1950s, many inventors in Canada and elsewhere attempted to meet the potential of the booming market and at least 10 types of AEM systemsbecame operational before the decade was over. A reader interested in the history

*Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8.

*Universityof Toronto,Department of Physics, Toronto,Ontario,CanadaM5S 1A7. 811


812

Palacky and West

of AEM techniques will find many fascinating details in Pemberton (1962), Collett (1986) and Becker et al. (1990). An extensive bibliography of papers published on the subject can be found in Palacky (1986c). In 1953, a two-frequency quadrature system designed by Puranen, Kahma, and Ronka was tested by the Geological Survey of Finland and a year later systematic surveying using the system was begun. In 1972, Finland became the first country to have a complete AEM coverage (Peltoniemi, 1986). In 1955, a modified version of the system was introduced to Canada and offered there commercially by Aeromagnetic Surveys Ltd. (which later became Hunting Surveys Ltd.). The surveying technique was successful and six orebodies

were

found

as a result

of AEM

surveys.

Among the novel systems tested in the 1950s was the

airborne

version

of AFMAG

which

measured

changes in the orientation of natural EM fields and thus dispensed with the need to carry a transmitter (Ward, 1959). While this development did not become commercially successful, AFMAG's ground and airborne form ultimately led to the design of VLF (very low frequency) EM systems, which used radio communication signalstransmitted by powerful stationsin the 10-20 kHz range as the primary EM field. Airborne VLF systemsbecame immensely popular in the 1970s as standard add-on equipment in airborne magnetic and radiometric surveys (Herz, 1986). The beginning of the most successfulAEM system, the time-domain, towed-bird, INPUT © (INduced PUlse Transient) system can also be traced to the late 1950s. The novel design, in which the primary EM field was transmitted in short pulses rather than continuously, was pioneered and patented by Dr. A. R. Barringer. The first field surveys were carried out in 1959, and the technique became widely used by explorationists

from

the

mid

1960s

when

it was

made

available in the contract surveying market (Barringer, 1962). In the 1970s, the INPUT system accounted for 70 percent of all AEM surveys carried out in the free-market and developing countries but its share declined considerably in the 1980s when demand for base metals and uranium slumped. From the introduction of AEM methodology in the early 1950s until the mid 1970s, the principal market force was the demand for base metals. AEM surveying profoundly changed the way base metal exploration was conducted in Canada and Scandinavia. Perhaps the most spectacular AEM discovery of all time was made in 1959 near Timmins, Ontario, where the world's richest massive sulfide orebody was found at Kidd Creek using a helicopter AEM systemdeveloped ©RegisteredTrademark of Barringer Research Ltd.

by Texas Gulf Sulfur that same year (Becker et al., 1990). The reserves of this orebody at the time of the discovery were 157.8 Mt of ore grading 6% Zn, 2.5% Cu, 0.2% Pb, and 72 g Ag/t. In the early days of AEM prospecting for massive sulfides, one of the main problems was the large number of graphitic conductors encountered in some of the areas being prospected. Many schemes were devised to try to discriminate between the two types on the basis of their geometry, distribution, and resistivity characteristics. Discrimination between them still remainsa problem, but it took surprisinglylong, in Canada at least, to recognize that there was much valuable stratigraphicand structural geologicinformation to be had by tracing the graphitic conductors. Especially in the Nordic countries and the USSR, AEM surveys frequently have been carried out in combination with magnetic surveys as much for geologic mapping as for direct base metal prospecting purposes.

In Canada by 1980, most of the more accessible geologicregions containing structuresfavorable to the occurence of massive sulfide deposits (the so-called greenstone belts) were surveyed not only once, but several times; sometimes just for different clients, other times with new instruments having deeper penetration or better data interpretability. Eventually, the skimming of easy-to-find, large tonnage orebodies by this saturation surveying has taken its toll and fewer orebodies have been found by AEM during this last decade. Even at an early stage, exploration stategies like the one pioneered by the Mattagami Syndicate in 1956 (Paterson, 1970), which leaned more heavily on airborne and ground geophysicsthan on geology, fell out of favor, and exploration managers started to emphasizethe need for detailed geologicstudiesas the mining camps matured and the ore forming processes that produced the depositsbecame better understood. The early used exploration sequence, consisting of AEM coverage, conductor classification, ground geophysical follow-up, and immediate drilling of targets which met certain interpretation criteria, was perceived as much too simplistic in the light of new geologic theories. Geophysicists often continued to classify anomalies simply as bedrock conductors or overburden at a time when geologistswere interested in obtaining more detailed information on bedrock lithology and stratigraphy. Perhaps because of the inadequateinterpretationof data, the usefulnessof the AEM technique was downgraded in the minds of explorationistsand its potential was underestimated. Initially, the Canadian and Fennoscandian Shields were the principal sites of AEM surveying. However,


Airborne

the 1970s saw many attempts to apply the methodology in other parts of the world. Extensive AEM surveys were carried out in Australia, Brazil, C6te d'Ivoire, India, Kenya, and Saudi Arabia. By and large, the efforts in tropical areas were much less successfulthan those in the formerly glaciated shield terrains. In the former, near-surface materials in the weathered layer are highly and irregularly conductive and

massive

sulfide

ores

are

often

weathered

into

nonconductiveforms to depths exceeding 100 m (e.g., Australia). Furthermore, experience with base metal exploration in rocks of lower metamorphic grade than is typical of the Precambrian shields has shown that there is a lower likelihood in such terrain of the target ores exhibiting a very large conductivity contrast with their host rocks.

sulfide base metal

ores has attenu-

ated, the effort of system designershas shifted toward making their systemsmore versatile. The trend started in the early 1970s, when multicoil helicopter AEM surveys were pioneered by Dighem (Fraser, 1972). The survey results were often compiled as resistivity contour maps rather than schematic maps using symbols (Fraser, 1978). This more quantitative approach which allowed superior geologic correlation has ultimately led to new applications in mineral exploration (particularly gold), engineeringstudies, and groundwater prospecting. Among the early pioneers of new applications and compilation of resistivity maps were geophysicists at the 6eological Survey of Canada (Collett, 1970, Dyck et al., 1974). In the late 1970s, instrumentation development began to follow two distinct lines, high-resolution helicopter surveying and deep-penetration, fixed-wing measurements. Demand for deep-penetration surveys came in Canada from new users uranium exploration companies. In the Athabasca Basin of Saskatchewan, where a relatively undisturbed sandstoneoverlies a deformed and metamorphosed Proterozoic basement, uranium orebodies are found near the basement unconformity, associated with clay alteration in fracture zones and the paleoregolith and with conductive graphitic metapelites subcroppingat the basement surface. Both of these geologicmaterials are conductive in comparison with the sandstone and the rest of the basement rocks,

and they can be detected by EM methods (Palacky, 1990). The most spectacular discovery was made in 1981near Cigar Lake, when an extremely rich orebody (12.2 percent U3Os with reserves of 110 000 t) was found under 420 m of Athabasca sediments(Fouques et al., 1986). In 1988, conductorsover 600 m deep were

813

identifiedafter surveyswith the GEOTEM © system(J. Hawkins, pers. comm.). The need to detect deeply buried conductors stimulated renewed activity in AEM design in the late 1970s. The successful INPUT was redesigned using digital technology (Lazenby and Becker, 1984) and is now being operated by Questor Surveys under the name QUESTEM. Geoterrex gave its new version the name, GEOTEM. New-generation AEM systems using powerful transmitters and the latest in computer technology were tested in the early and mid 1980s: COTRAN © (Collett et al., 1983), PROSPECT © (Annan, 1986), and Sweepem (Best and Bremner, 1986). SPECTREM

is a South African

version of the PROS-

PECT system. However, the slump in demand for uranium

As the early enthusiasm for AEM as a direct search tool for massive

EM

and base metals in the mid 1980s has caused

the interest in deep-penetration systemsto wane, and only GEOTEM, QUESTEM, and SPECTREM were in service

in 1990.

In the 1980s, the attention of many mining companies was drawn to gold exploration. In the Canadian Shield, the spectacular discoveries of the Hemlo and Casa Berardi deposits were indications of the untapped potential for finding precious metals. Both camps and many geologically similar areas have been covered in detail (line spacing of 100 m) using multifrequency/multicoil helicopter AEM systems (Hogg and Boustead, 1990). Detailed high-sensitivity magnetic and conductivity (or resistivity) maps were found invaluable in unravelling the often complex geology of the two

areas.

One

of the

orebodies

in the

Casa

Berardi area (Estrades) is associated with massive

sulfides and, hence, is directly indicated by AEM techniques. One of the ironies of geophysical exploration is that this orebody was only found by routine drilling for assessment work purposes in 1981, although a clear, isolated AEM anomaly can be identified on INPUT maps publishedby the Quebec government in the 1970s. The role of the combined

airborne

EM and magnetometer surveys (which often included other sensors, as well) in geologic mapping of deformed crystaline terrains like the shields has only been recognized rather lately in Canada and the USA. Nontraditional applications of AEM systems, such as in groundwater exploration, were pioneered by some researchers already in the 1960s (e.g., Collett, 1966, Baudoin et al., 1970). In the 1970s, AEM surveys began to be used in prospecting for kimberlites in South Africa (Macnae, 1979), in geologic mapping in Brazil (Palacky, 1981), and in geothermal exploration in the USA (Hoover and Pierce, 1986). In the 1980s, new applications included mapping of Tertiary and

©GEOTEM, COTRAN, and PROSPECT are RegisteredTrademarks of Geoterrex Ltd., Barringer ResearchLtd., and A-Cubed Inc., respectively


814

Palacky and West

Quaternary sediments in France (Deletie and Lakshmanan, 1986), detection of buried paleochannels and salinity mapping in Australia (O'Connell and Nader, 1986), and shallow-water bathymetry and sea-ice thickness determinations in the USA (Won and Smits, 1986; Zollinger et al., 1987; Kovacs and Vaileau, 1990). Significant research on interpretation of AEM data for groundwater exploration has been carried out in Germany (Sengpiel, 1986). New applications for AEM methods were the subject of two symposia (Palacky, 1986a, Fitterman, 1990). AEM surveying in North America and Australia was to a large extent generatedby mining companieswhich kept their data proprietary. Because of this dependence on mineral exploration AEM activities always have been highly volatile. Figure 1 illustrates this development. In the peak years (1967-73 and 1979-81) over 500 000 line km of AEM surveys were flown annually. Because of low prices for base metals and uranium, the extent of AEM surveying slumped in 1982 to about 200 000 line km. In the late 1980s, when prices for base metals reached new highs, the demand for AEM surveys temporarily strengthened, but declined again when prices softened. The cyclical nature of exploration expendituresis one characteristicof the mining industry. Another approach has been taken in Finland where the government initiated a systemic mapping program using airborne geophysicalmethods. No detailed geologic mapping or ground geophysicalwork is done by the Geological Survey of Finland before completion of airborne geophysical coverage of any given area. Between 1954 and 1972 the whole country was covered by combined magnetic, radiometric and AEM surveys with 400 m line spacingand a flight height of 150 m. When more advanced multiparameter geophysical equipment became available in 1972, a new coverage was initiated using a 200 m line spacingwith the aircraft flying at a lower altitude (30-40 m). Until the end of 1985, some 1 476 000 line km of AEM surveys (Peltoniemi, 1986) had been completed. Two Scandinavian countries (Norway and Sweden) started similar programs more recently. In the U SSR, selected regions (e.g., Ural Mountains) had been systematically covered using AEM systems (Kamenetsky and Yakubovsky, 1985). At present, Canada, Finland, China, and the U SSR are the only countries manufacturing AEM instruments. Only the former two offer such equipment for export. Geophysical contractors currently offering AEM

services

are based in Australia

government AEM surveys in such countries are currently included in annual SEG statistics on which Figure 1 was based. Classificationof AEM Systems

Despite their tremendous diversity, all AEM systems share a rather simple principle of operation. Active systems use an artificial source to create a primary time-varying magnetic field, which causes eddy currents to flow in conductors. A secondary EM field is generated by those currents and the emf due to suchfield can be measuredusingcoils. Figure 2 shows the principle for a towed-bird system. Active AEM techniques can be classifiedaccording to the type of transmitter they use and how the emf due to the secondarymagnetic field is measured (Figure 3).

•000 km 600

--

500

400• 300 f 200 f

aøøfl IIIIIIIIIIIIIIIII 0960

1965

1970

1975

1980

1985

Fig. 1. Contract airborne electromagnetic surveys in the world. Annual totals between 1961 and 1987 (based on

Palacky, 1986, and updated).

TRANSMIT

EIVER

GROUND

and Canada.

Governments (usually geological surveys) of the following countries own and operate AEM systems: Austria, China, Finland, Germany, India, Norway, Sweden, and the USSR (Palacky, 1986b). Few of the

CONDUCTOR '""• • J

Fig. 2. Principle of airborne electromagnetic surveying. The illustrated system is of the towed-bird type.


Airborne EM 815


816

Palacky and West

Most active AEM systems have been designed to employ a current-carrying loop or coil transported or towed by the aircraft as the source. Only early in the AEM development some systems used sources located on the ground. The Soviet BDK-70 developedin 1955 used a grounded cable and survey lines were flown perpendicularto this linear source(sketch of the survey configuration in Figure 4). The difficulty with the system was the presence of both galvanic and inductive effects, which led to complicationsin interpretation. Identification of bedrock conductors was difficult in most, except highly resistive, areas and reliable estimates of conductivity were impossibleto make. Some improvements occurred when a nongroundedloop was used to generatea primary field (an airborne equivalent of Turam), but difficulties persisted in separating large-scale from local induction.

FIXED

TRANSMITTER Flight

lines

Transmitting loop

MOVING

able. One solution

I

h=100m

surements.

h•

120rn

l RIGID BOOM ] TIP

HELICOPTER

T

h•-60m

The best results so far have been achieved

systems.

I, •-105m•

T

WING

in one

by the designers of the PROSPECT and Sweepem

i TOWED BIRD I [•-

is to install the transmitter

aircraft and the receiver in another. Systems using sucha concept were used in the 1950sand 1960sin the USSR and Sweden. The arrangement had several drawbacks, e.g., the expenseof operating two aircraft to carry out a survey and difficulty in maintaining a constant separation and a stable flying height of the two planes (Ward, 1967). The early efforts in Canada and Finland in the 1950s were directed to developing the towed-bird configuration. The transmitter was on the survey aircraft and the receiver placed in a small aerodynamic container ("bird") towed by a cable. In the early systems, the primary EM field was generated by transmitting a continuous-wave signal at two or more frequencies in the audio range (100 to 5000 Hz). Most of these systemstransmittedtwo (and later up to 5) frequencies and measured only the quadrature component of the secondary magnetic field. As the in-phase component is more affected by a change in the relative birdaircraft position than by conductorsin the ground, its measurement is not practical when using this configuration. Despite numerous efforts, nobody has succeeded in measuring the relative bird-aircraft motion with sufficient accuracy to correct the in-phase mea-

-•=30m

TRANSMITTER

I ,wo

Scintrex introduced a Canadian version of this system (Turair) in 1968, but it never gained popularity in the North American mining community (Becker, 1979). All presently used active AEM systemsuse moving transmitters. Basically, there are four ways to mount transmittersand receivers on airborne platforms (Figure 4). To achieve deep penetration, a substantial separationbetween transmitter and receiver is desire-

•m

1•-6lorn I I== 30m

Fig. 4. Transmitter-receivergeometry of five basic types of active airborne electromagneticsystems.

Other possibilitiesof isolating the secondary magnetic field from the primary field were investigated. Secondary-field measurements can be carried out in the absence of the primary field if a time-domain approach is taken to EM measurements. Then the primary EM field is generated not by a continuous wave at a given frequency, but by a relatively short pulse (duration of the order 1 ms) followed by a transmitter shut-offperiod during which the secondary magnetic field is measured. This approach was exploited by the INPUT systemwhose superiorpenetration was consideredimportant in prospectingfor deep deposits of massive sulfidesand uranium. GEOTEM and QUESTEM are based on the same principle, and unlike the analogINPUT (Marks 5 and 6) use the latest technologyin recordingand processingof the data. The towed-bird designhas remained the favorite of system designers when they need transmitters with large dipole moments. The original Puranen, Kahma,


Airborne

and Ronka patent on a two-frequency towed-bird AEM system contemplatedusing the lower frequency as a reference channel for monitoring changes in geometriccouplingbetween the receiver and the transmitter. This approachdid not prove practical originally but advances in creating wide-band systemshave now made it possible. The approach which has been explored by at least two research groups (Annan, 1986; Best and Bremner, 1986) can be used effectively either in the frequency or in the time domain. Several digitally based, time-domain/frequency-domain hybrid systems were under development in the 1980s. Because of the simultaneousslump in uranium and base metal mineral exploration activities, which were the main markets for deep-penetration surveys, the new systems have not entered commercial service despite encouraging preliminary testing. However, the projects can probably be revived if the demand strengthens enough to warrant large-scale exploration. All towed-bird systems have one characteristic in common. The coil system is asymmetric which causes different responsesdependingon the flight direction. Although the coil may contribute to easy determination of the dip of thin steeply-dipping conductors, which is an advantagein some applications, asymmetry is a definite drawback in conductivity (resistivity) mapping. The development of wing-tip, rigid-boom systems measuring in-phase and quadrature responses was pursued at about the same time as that of the towedbird quadrature systems. By the mid 1950s, several companies offered in-phase/quadrature, rigid-boom surveys. While the penetration of such systems is smaller than that achieved with towed-bird configurations, the increased resolution and the relative econ-

omy of surveying compared with helicopter surveys have contributed to their short-lived popularity which peaked in the 1960s (Pemberton, 1962). Multiplefrequency operationswere pioneeredby Scintrex Ltd. and their Tridem system has been used by two governments (China and India) for geologic mapping (Johnsonand Seigel, 1986). The Geological Survey of Finland has been using a single-frequency wing-tip system since 1972 (Peltoniemi, 1986). Both components, in-phase and quadrature, can be reliably measured and because of the symmetry of the coil configuration, identical conductivity values can be obtained irrespective of the flight direction. At present, such systemsare not operated in North America. Presently, the most popular mode of AEM surveying is one that relies on helicopters. Not only the maneuverability is greatly increased, but the distance between the sensorsand the ground can be reduced. Using electronic navigation, one can achieve an accuracy of better than 5 m in determining the location of

EM

817

any survey point. This increased accuracy eliminates the need for ground EM follow-up in many applications, which is another reason for helicopter AEM popularity. Unlike the previously described systems, both the transmitter and the receiver are placed in a kevlar bird towed on a 30 m long cable (Figure 4). The transmitter-receiver separation is typically between 6 and 10 m. In older publications, the vertical coaxial coil configurationwas describedas one normally used in helicopter surveys. Since the late 1970s, most helicopter AEM systemsuse two coil configurations, vertical coaxial and horizontal coplanar, and multiple (3 to 7) frequencies. Such systems, which have been pioneeredby Dighem, are used in most nontraditional applications, usually because of the possibility to compile conductivity (or resistivity) maps and sections (Fraser, 1972, 1978, 1979, 1986). In the early 1980s the INPUT system was briefly

operatedwith a helicopteras well, but becauseof the high cost this variation was never as popular as fixed-wing operations (Konings and de Carle, 1985). Unlike the helicopter systems previously described, INPUT employed a geometry reminiscent of the towed-bird systems.The transmitting loop was wound on the helicopter and the receiver was towed in a bird. An interesting concept of using a superconducting coil (Unicoil) as both transmitter and receiver was pioneered by researchersat the University of California, Berkeley. While the penetration at a low frequency (40 Hz) is theoretically superior to more conventional systems, the interpretability of the data consisting of only quadrature response is limited in most environments. The cost of field operationswould have been extremely high because of the need to frequently replenish the liquid helium supply required to keep the coil at very low temperaturesand the need for a large and expensive helicopter (Morrison et al., 1976).

Since the early days of AEM development, the idea of using natural electromagnetic fields as a source for the primary field appeared very attractive. The first functioning systemof this kind, AFMAG, dating from 1955, used natural fields generated by thunderstorms in tropical regions. Despite some claimed advantages, such as deep penetration, problems due to poor interpretability and operations unpredictability prevented the system from becoming widely used (Ward, 1959). In the 1960s, a global network of powerful VLF (very low frequency) transmitters was established by the U.S. Navy to facilitate communication with submarines. Shortly thereafter, these stations became widely used in geophysicalexploration. VLF transmitters operate almost continuously(except for scheduled maintenance shutdowns) and provide a reliable primary-field source. Unlike the dipole sources whose


818

Palacky and West

EM field was difficult to model mathematically, particularly before the advent of powerful computers, the responseof bodies illuminated by a plane-wave excitation is easy to calculate. While the early VLF-AEM systemsused one frequency, the more recent ones(e.g., Herz Totem 2A) use two (Herz, 1986).A variety of VLF systems has been designed by Canadian instrument manufacturers(BarringerRadiophase,GeonicsEM-18, McPhar KEM, Scintrex Deltair and SE-90, Sander VLF-EM II, and Herz Totem 1A and 2A). The quantities measuredvary accordingto the system:total field and quadrature (Totem), dip angle (KEM), in-phase and quadrature components (SE-90). One system, E-PHASE ©, measuredthe electric rather than magnetic component of the secondary EM field (Palacky and Jagodits,1975). Currently, there are about as many line kilometerssurveyedusingVLF instruments,which are an easy add-on to other geophysicalsystems(notably magnetic and radiometric), as with all other AEM systems combined. However, they do not appear in statistics on geophysicalactivities,becausethe listingis done under the main sensor. VLF techniques,both ground and airborne, are coveredin anotherchapterof this book (McNeill and Labson, 1991, this volume) and will not be discussed here. DESIGN

CONSIDERATIONS

To design any kind of high-performanceEM exploration system is a challengingtask, and to create a system that will function well from an airborne platform is especially daunting. The task of the early systemdesignerswas mainly to find a way of achieving a reasonably high system signal/noise ratio so that conductors in the ground could be detected with certainty. More is wanted of modern AEM systems. They must also provide data of sufficientdiversity and quality to allow the interpreter to decipher the form and characteristicsof relatively complicatedconductivity structures in the ground. In order that the interpreter can deal with as many different geologic scenarios as possible, he has an insatiable list of performance demands for the designer of a new system' increased sensitivity to small and deep conductors, broader and more complete spectralcoverage so the system will respondto and provide discriminatory information for a wider range of target conductivities, a wider variety of coil configurations to sort out the geometric characteristicsof conductors, greater data accuracy to withstand the rigors of quantitative numerical data processing,and, finally, compatibility of the AEM system with a variety of other geophysical and navigational sensors in simultaneous use; all this ©RegisteredTrademark of Barringer Research Ltd.

to be achievedwithout appreciablenegativeimpact on the flight performanceof the chosenaircraft and within the inevitable practical constraints of a reasonable operational and capital cost budget. With the aid of modern electronic technology, many of the characteristicshoped for in an optimum EM system are at least conceptually achievable from an aircraft platform. However, they are not all compatible with one another. Inevitably, many compromises have to be made in comingto a final design.Depending on which geophysical characteristics are most highly valued in the system'sexpected applications,the best design could vary widely. There is no globally optimum AEM system which is best suited to all tasks and circumstances.Nonetheless, the designer of each new system has the chance to stand on the shouldersof his predecessors by learning from the capabilities and problems of earlier systems,and by availing himself of the latest developments in instrumentation technology.

Desirable GeophysicalCharacteristics

At least from a theoretical point of view, EM surveying from an airborne platform has one special advantageover ground surveying. The objective of an AEM survey is an overview of the earth's threedimensionalconductivity structureto a spatialresolution of some tens of meters and to a depth of many tens or even several hundreds of meters. Systems can certainly be constructed with adequate sensitivity to detect reasonably sized target conductors at the required distances, but because geophysical systems employ diffusive or potential fields, they necessarily respond much more strongly to nearby objects than to distant ones. The strong shallow responsesmay easily mask a weaker but otherwisedetectableresponsefrom a deeper target. As a consequence, it can be very advantageous for the coils of an EM system to be locatedat a distancefrom the nearestpossibleconductor (i.e., the overburden), at a height which is a significantfraction of the desired depth of exploration. Although the additional distance will reduce the sensitivity of the systemto all targets somewhat and will also reduce the spatial resolution to some extent, it will tend strongly to equalize the responses of the system to shallow and deeper conductors and give a picture of ground conductivity structure that has more similar spatialresolutionin shallow and deeperparts. In comparison with ground EM systems, there are also practical advantagesto the airborne platform. A considerable amount of electrical power is usually available and the weight limit for apparatus greatly

Airborne EM


exceedsthe limit for a pack-portablegroundEM unit. If the aircraft can be permanentlycommittedto AEM surveying, the power and weight limitations are usually lessstringentthan if the systemmustbe installable in a standard charter aircraft.

The desirablecharacteristicsof an AEM systemdo not differ fundamentally from those wanted in a ground EM system. They are:

819

of the system)is a function of theseparametersalso. The rangeof conductivity(in a giventargetbody) over which the noise level is exceeded is known as the

channel conductivity aperture. An example of sensitivity graphsfor some idealized systemsis shown in Figure 5. They are a usefulway of comparingperformancebetweenconceptualand actual AEM systems. Broad Spectral Coverage.--Since EM response is

High Sensitivity.---Thesensitivityof a specificEM system is usually documentedas the average noise level in each data channelunder typical flight conditions, given as a fraction of primary signalstrength. Suchinformationis usefulonly in comparingsystems of a similar type. Sensitivityis better measuredas the ratio of the peak responseobtained over some idealized target conductorto the peak or rms noise level obtained in ordinary survey conditions over nonresponsiveground.For a givenform of targetconductor, sensitivity also can be expressedas the maximum depth or minimum size which will yield a response equalto the systemnoiselevel. Of course,the target responsewill be dependenton frequency(or its equivalent, delay time, in time-domain systems)and on target conductivity. Thus, the sensitivityof a system (or more precisely the sensitivity of one data channel

I00

,,,,

frequencydependent,and thereis a closerelationship betweenfrequencyand the conductivityof conductors to which a given data channel will respond, and becausethe configurationof geologicfeatures which have quite different conductivities and dimensions

often shouldbe determined,the EM systemdesirably operatesover a broad range of frequencies(or delay times)sothat eachchannelhasa differentconductivity aperture. If that is not possible,the frequency of the systemmust be chosenvery carefully in regard to the specificexploration problemsand conditionsthat the system must address.

Optimal Coil Configurations.--The receiver-trans-

mitter coil configurationof an EM systemhas strong influence on the relative strength and form of the responseprofile obtained over conductors of different

......

HELICOPTER E•M

INPUT

.

I ........ "''''•' '"1I00,000 L;lOm

H=30m

VERTICAL

SHEET

-

MODEL

•

-

•

_

•• I• VERTIO*L SHEET MODEL S=240m,W=240, D= 30m •100

IN-

I00

•o

•oo :.

o.•

•.o

•o

•t(s)

•oo

t•

•o

•oo

tooo

•t(s)

Fig. 5. Sensitivityaperturefor somedata channelsof idealizedAEM systems(for a verticalthin plate target conductor)as a function of target conductance.


820

Palacky and West

shape and attitude. The coil configuration is also strongly related to what types of measurementcan be made with sufficient sensitivity to be geophysically useful. If flat-lying conductorsare an important target,

compromisesmay be required between AEM capability and survey cost factors. The choice of aircraft and its flight speed and range may have a very significant effect on how much and where surveying can be done,

the receiver

and these factors will have to be taken into account in

and transmitter

coils should have their

axes vertical, because coils in this orientation are

maximally coupledwith horizontally flowinginduction currents and because, if the coils are not greatly separatedlaterally from each other, the responsewill be independent of the direction of flight. On the other hand, if the target conductors are expected to be steeply dipping, a variety of other geometriesmay be preferable. In many cases, data from more than one coil configuration are helpful because then the form and attitude of the conductor often can be interpreted by comparing the data from different coil configurations.

Data Fidelity and Completeness.--Inmost modern AEM systems, the sensitivity is sufficiently high that many typical responsesfrom the ground are tens or hundreds of times the system noise level. In such cases, and especially if several types of AEM data have been recorded simultaneously, a good deal of interpretative information may be extracted by quantitative analysis of the recordings. For this to succeed, the data must conform closely to a clearly defined specification or model of exactly what the AEM system measures. Systematic data errors such as baselevel drifts, errors in amplitude calibration, incorrect phase references, or ill-defined smoothingof the profile data can easily defeat the best of processingand interpretation algorithms. Such algorithms often will require ancillary data, such as the sensorheight above ground and the attitude in space, and it is vital that these data be provided with sufficient accuracy. To design an AEM system, the foregoing characteristics which are to be achieved must be specified quantitatively. Naturally, the target specificationswill be chosen on the basis of what has already been achieved by existing systems and on what new technologies or innovations the designer can introduce. A good deal will also depend on what exploration tasks are to be undertaken with the planned system, or, put in other terms, on what conductivity structures the system likely will encounter. Quantitative data about the conductivity/resistivity characteristics of typical structures

in different

areas can be hard to obtain and

may best come from experience with earlier AEM surveys (Palacky, 1988). In addition to the EM characteristics, there are a

number of other important considerations:

Operational Characteristics.--Airborne surveyingis expensive, and it is important to realize that important

the system design. An important considerationis the type of terrain to be flown, which determines what kind of aircraft can be contemplated, and what flight altitude will be possible. A related question is whether the aircraft can be fully committed to geophysical surveying or whether the system needs to be installed and removed frequently. Also to be consideredis the range of survey line spacingsthat is contemplatedand whether it is important that the AEM system has a significant sensitivity to off-line earth features.

Navigational Capability.--High-quality AEM measurements would be of little value without achieving an adequate capability to navigate the aircraft along a safe path as close as possibleto the specifiedsurvey line and without having a reliable method of recovering the actual flight path after the survey. In-flight navigation control has traditionally been done by a visual navigator beside the pilot who assistsby following the flight on large-scale photomosaic maps of the area with the desired flight profiles drawn on them. The information provided by visual navigator is supplemented by a variety of electronic navigation aids such as gyrocompassheading, doppler radar measurements of precise ground velocity (which can be timeintegrated to give relative position along track), and radar altimetry for altitude control. During flight, all quantitative navigation data are recorded, along with continuousphotographyof the ground, and these data are combined during data processingto give the best possibleestimateof the aircraft's actual track. Details on presently used means of navigation are given in the Field Operations and Data Processingsection. Compatibility with Other Systems.--Few AEM systems are flown without at least a magnetometer being operated simultaneously, and in many cases other sensors such as gamma spectrometersand/or VLFEM receivers also are included. All modern systems rely at least to some extent on electronic navigation aids in addition to some form of ground photography. The requirements of these other systems must be taken into account in the designof the AEM systemto prevent interference. An example is a proton magnetometer, which uses a transient polarizing current in its sensor coil. This interference easily can be picked up by the EM receiver. Even more serious, the transmitted field of the EM system can resemble the very weak audio frequency magnetic fields which the magnetometer sensor is looking for.

Airborne

EM

821


Constraintsin DesigningAEM Systems Sensitivity is the essential feature of any AEM system. Obviously achieving other desirable characteristics is useless unless a sensitivity sufficient to detect reasonably sized target conductors at useful depths can also be obtained. There are two main factors to be considered:receiver noise and geometry noise. The diversity in AEM systems is largely an outgrowth of different schemesto minimize the deleterious effects of geometry noise. Beyond sensitivity limitations, further difficulties related to calibration may arise in maintainingthe systemfidelity. Receiver

Noise.--The

maximum

level of noise in an

AEM data channel, given as the magnetic field strength which would generate a similar signal level, determines what magnetic moment will be necessary in the transmitter in order to achievethe desiredtarget sensitivity. Receiver noise, originating from several sources, can usually be made insignificantby careful design. Receiver noises are:

the audio-frequencyrange is largely a superposition

of

numerous

transients

called

sferics

which are due to lightningdischarges.The lightning may be local (within 200 km) or occur at great distanceswhere propagationis efficient in the earth-ionospherewaveguide. The local discharges have a very sporadic and irregular nature, and the frequency of their occurrence depends strongly on local climatic and weather conditions. The distant discharges produce weaker sferics, but they are more frequent and constitute a more or less steady noise background that has a strong frequency dependence due to characteristicsof the E-I waveguide (Fig. 6). The amplitude level has a seasonalvariation to its intensity due to world climate patterns (Watt, 1967). A single sferic lasts less than one millisecond, but because of built-in integrators its duration may appear much longer on recorded AEM data. A typical sferic event and its frequency spectrum are shown in Figure 7. (6) The magnetic fields of power lines and other man-made installations, such as radio stations

(1) The basic thermal agitationnoise of the receiver

coil, which is largely a matter of its size and weight. (2) Electronic noise generated in the initial preamplifier, which, like the thermal noise, can be made negligible in a relative sense by using a large enough receiver coil. (3) Motion noise, which is generatedby irregularities in the motion of the receiver coil throughthe earth's magnetic field. The average aircraft motion is not all that is important. Noise due to any kind of local vibrations or oscillations and by other microphonicpropertiesof the receiver coil may be equally or even more troublesome. The noise can be minimized by good mechanical design and by acoustical and vibrational isolation. However, motion noise becomes unavoidable at very low frequencies, below about 100 Hz, because of the aircraft response to turbulence

and electric railways, generally will be a problem only when such sources are directly overflown by the AEM system. Rejection of the various steady harmonic components of these signals normally will be a part of the basic system design. In predicting noise levels, it is improtant to realize that, becauseof the flight speed of the aircraft, AEM measurementsmust be taken at a rapid rate such as 10 samplesper second. Thus the effective bandwidth of any measuringchannel cannot be reduced to less than a few hertz. Sferics, which are the most potent kind of natural noise may be removed either in the receiver

I

in the air.

G•_•GN ETIC

SPHERIC BAND

(4) Spurious magnetic field noise that is generated

by systemson the aircraft, such as generators, electric motors, solenoid actuators, relays, and possiblyeven by vibration of large metal aircraft parts in the earth's magneticfield. Also included is interference from other geophysical instruments, suchas the polarizing current of a proton magnetometer. Generally, these noise sources are best minimized by suppressiontechniques applied to the noise source and by careful location of the receiver

coil.

(5) The natural magnetic noise field of the earth in

POWER

10-2_

LINE

uJ z

10-4_

I

10 -2

i

I FREQUENCY

i

10 2

-.•.•LF .V

10 4

(Hz)

Fig. 6. Spectral density of the EM noise fields that affects surveying.


822

Palacky and West

apparatus by interrupting measurement or in the data processing if a high data sampling rate is employed (see Field Operations and Data Processing section). This irreducible componentsof receiver noise are the distant sferic field, plus some motional noise at low frequencies and some transient man-made noise when flying over built-up regions. Even with the removal of local sferics, the sferic activity can sometimes be a very serious problem for good quality surveying in some localities and in some seasons, particularly in equatorial regions during summer. In some cases, flying must be restricted to a few favorable hours a day. Examination of Figure 6 indicates that a minimum transmitter moment is neededfor AEM systemsoperating in the 1-2 kHz frequency range. A somewhat higher moment seemsto be needed at higher frequencies, but a higher background noise level is offset to someextent by the fact that EM responsesare likely to be larger at high frequencies than at low ones, therefore a lower sensitivity may be tolerable. However, to carry out surveys at low frequencies, much larger moments will be needed, not only to overcome both the sferic noise and possible motion noise, but also because the sensitivity required of the system in a low-frequency channel likely will be high, since most conductorresponseswill be weak. Most of the current helicopter AEM systems, which have been designed for exploration depths of less than 100 m, operate at frequencies above 500 Hz using transmitters with

momentsof about 100 A.m•. The time-domain INPUT system, which had been designedfor a substantially greater depth of exploration, used a base frequency of about 144 Hz (i.e., harmonics 144,432,720 etc.) and employed an rms moment about a thousand times greater than helicopter AEM systems.

Geometry Noise.--In all active AEM systems, the primary (free space) field strength at the receiver due to the relatively nearby transmitter is enormous in comparisonto any expected secondaryfield strengths. A key element in system designis selectingthe means by which secondaryand primary signalswill be distinguished. There are three basic approaches to the problem. The first is to control very accurately the relative position of receiver and transmitter coils so that the primary signal is constant and therefore removable with some sort of cancelation system. The second is to make do with those kinds of secondary field measurementsthat do not depend on the receivertransmitter coupling. The third approach is to try to find a' means of measuring the transmitter-receiver coupling from some combination of the AEM measurements or from other sensors, and use this measurement

to correct

(1) Rigid coil coupling: This approach is taken in most helicopter AEM systems, and also in a number of fixed-wing systems.In the rigid-boom systems built so far, measurement has always been carried out in the frequency domain, although there is no fundamental reason why time-domain

0

-258

!

i

i

I

i

i

!

6 B

TIME

d

i

!

8

i

i

10

i

12

i

measurement

could not be used. In

all such systems, a cancelation system is employed to null out the primary signal in the receiver. The system has to be very carefully designed to be independent of the transmitter current and as free as possibleof any drift. The key questionis how well the primary cancelation can be maintained under flight conditions. The main causesof change in coil coupling are: the inevitable mechanical flexure of the system under the variable accelerations of flight, thermal changes in dimensions, and motion of the system with respect to the aircraft which generally is a conductor. Helicopter AEM systems solve this latter problem by putting all the coils in a

E

o

the rest of the data.

i

14

(me)

nonconductive

bird that is carried

far from the

aircraft.

-120

IO0

I ooo

Io ooo

FREQUENCY

(Hz)

I O0 000

Fig. 7. Typical sferical event and its frequency spectrum (courtesy of A. P. Annan).

In designing a rigid-boom system, an interesting design question is whether to use the maximum practical coil separation.A large coil separationrelaxes the required precision of primary field compensationfor a given target sensitivity. In general, for close coupled systemswhere the coil separation is significantlyless


Airborne

than the usual coil height, the required compensation precision falls inversely as the cube of the coil separation. Mispositioning due to thermal expansion will, on the other hand, rise only as the first power of separation.Elastic flexibility problemsmay, however, increase much more rapidly with separation, usually favoring designs with short separations. If a large separationis employed, such as is obtained with coils located on wing tips, the important modes of flexure will need to be identified

and some scheme devised to

minimize their effect on coupling. In wing-tip systems with coils coaxial in the line of flight, this adjustmentis done by mounting the coils on supports which carry them below the wing surface. When the wing flex and reduce the separation between wing tips, the coil mounts are bent outward by the flexing and make a compensatingextension of the separation. In helicopter AEM systems using coils in a horizontal tube suspended below the aircraft, the selection of the suspensionpoints on the tube can be quite important. In most present-day rigid-boom systems, the uncompensatedflexural noiseis significant,but is not the limiting factor for system performance, although its time behavior along a profile may be similar to the ground response. Thermal and other similar drifts are usually of much greater amplitude. Being drifts, they can be largely determined by calibration and data processingprocedures and then subtracted in a reasonablysatisfactorymanner (see Field Operationsand Data Processing section), but there is usually an irregular component that cannot be so removed. This irregular component of drift usually limits the system performance. More frequent calibration, done for instance by taking the AEM system at the end of each line to sufficient altitude where the ground response practically disappears, can sometimes be extremely beneficial, but in other circumstances does not help much because the change in height may subject the systemto more severe and irregular temperature variations than would otherwise

EM

823

four-frequency systems were later used with success.

The time-domain approach to solving the variable coupling problem is epitomized in the INPUT system and is somewhat akin to the multifrequency quadrature method. The transmitter current (and therefore the primary field) is turned on only during a short time interval of not more than a few milliseconds, and measurement

is confined

to a similar interval

immedi-

ately following the cessation of transmitter current (Figure 8). During the measuringinterval, the receiver records a voltage proportional to the time rate of change of the secondary magnetic field. The designer of the INPUT system chose a half-sine wave repeated periodically with oppositepolarity for the transmitter current waveform, presumably because a high-amplitude current of this form was comparatively easy to generate in an inductive transmitter coil. Other forms are certainly feasible. Repetition of the transmitter current with alternating polarity removes its zero frequency components from the signal and permits rejection of the noise component near zero frequency. This is desirable with any primary waveform. Quadrature and off-time measuring time-domain methodsalways have a diminishingsensitivity to both very high and very low conductivity targets, whereas an in-phase measuring system reponds strongly to all targets that have a conductivity greater than some threshold value. To obtain a response characteristic somethinglike a frequency-domainin-phase response

I'HPL• TRANSMITTER CURRENT T•.6.67 - II.Ims;I

EP •

be encountered.

(2) Flexible coupling: In a frequency-domain sys-

tem, the classical solution to problems of variation in primary coupling is to measure only the quadrature component of the received signal since geometric couplingchangescan only alter the in-phase component. (An equivalent measurement is the phase difference between the total field and a primary field reference because anomalies are always a small fraction of the primary field except in a null-coupled coil configuration.) This procedure was employed in many early AEM systems. The first systems operated at only one or two frequencies but

TIME Es•

•,

RECEIVED VOLTAGE

I \

I'• (SECONDARY' MAGNIFIED)

I \

II'l-h•

•

l'•ø•7•øø•o ,•o ,.•o

•s

2

Fig. 8. Transmitter current, primary signalwaveforms, and receiver samplingtimes for the INPUT AEM system.


824

Palacky and West

with a time-domain system, the system must measure during the time of the primary current transient in addition to the off-time or must integrate the received signalinto the form it would take if the receiver were a magnetometerrather than an induction coil. In such a case, the data become sensitive to the transmitter-

receiver coupling in a manner very similar to a multifrequency, in-phase measuringsystem. (3) Coupling compensation:Even if the transmitter-

receiver coupling cannot be held sufficiently constant or, equivalently, cannot be measured independently so that coupling corrections can be applied, some important in-phase data will remain

if measurements

are taken over a broad

frequency range using the same coil pair. Expressed in the frequency domain, the coil coupling is real (in-phase) and frequency independent. Thus, differences between the in-phase secondaryresponsesat different frequenciescan always be measured, even if the absolute levels cannot.

Where

the minimum

measurement

fre-

quency is low enough that no substantive inphase secondary response is likely to be recorded, the lowest frequency data provide a reference level for the others (Figure 9). In some early AEM systems, orthogonal pairs of transmitter-receiver coils were mounted together in an attempt to solve the variable coupling problem. The idea was that sought-aftertargetswould respondmuch better to one coil pair than the other, because of the different coupling geometries, whereas differencesin the coil separation would affect both pairs equally. This is a workable concept if just the simple discovery of conductors is of interest, but it is inadequate when any sort of seriousdata interpretation is contemplated because a priori assumptions are required about the

'E)O

FOt? 0 AND FOt?

i

2

IP OFA NON-MAGNETIC FREQUENCY (kHz)

3

4

BODY

Fig. 9. Use of a low-frequency channel as a reference for other data to remove geometric coupling noise.

geometry of all the conductors which will be detected, assumptionsthat can rarely be assured in practice. The method is no longer used for correcting coupling changes. But differences in the ground responseseen by different coil pairs can be of great value in interpreting the geometry of the earth structure, if the responsescan be measured independently. A typical aluminum airframe is an excellent conductor which will have a near saturation (inductive limit) response to the AEM system. The secondary signal produced by the airframe will not degrade system sensitivity if the signal remains constant, but any unpredictablechangesconstitutea noise. The problem is indistinguishablefrom ordinary coupling noise in an aircraft-mounted rigid-coil system. The noise is eliminated in helicopter AEM systemsby the large separation between the bird in which the coil system resides and the aircraft. A few systems have been built on wooden aircraft to reduce the problem. In towed-bird systems, especially those that use a horizontal transmitter coil around wingtips, nose, and tail, the aircraft responsemay be so strong that the primary field seen by the ground is appreciably reduced or geometrically distorted. To the receiver, the noise appearsmuch like primary coupling noise because of its strong in-phase nature, and the noise generally does not upset wideband primary signal estimation schemes very seriously.

Calibration and System Fidelity.mAs mentioned, because the interpretation of data from an AEM systemis based on a comparisonto model responses, the system characteristics must be essentially constant throughout the survey and be known quantitatively. Many early generations of AEM systems were very poor in this regard. Most modern systems have a variety of internal calibration devices to check the zeroing and sensitivity factors of the channel outputs. Usually these provide only static checks of sensitivity and factors suchas orthogonality of phase references, but dynamic tests that check the time responseof the measuring system can be implemented in the same way. The calibrators are usually internal to the receiver electronics and only check certain aspects of the overall system. The fundamental problem is in devising absolute calibration schemesthat can determine if there is any systematic error beyond what an internal device can detect. Examples are error in the phase reference of a frequency-domain system and sensitivity changes caused by the airframe EM responses.In towed-bird systemsto know exactly where and in what attitude the bird is flying relative to the aircraft is difficult. Even relatively minor changesfrom the assumed configuration may have a deleterious effect on interpretation. This problem can be serious

Airborne


when doing conductivity mapping because sensitivity changesdue to short term fluctuationsin coil geometry can modulate the ground responseand be interpreted as lateral variations in subsurface structure (Annan, 1983). EXAMPLE

AEM

SYSTEMS

The three specific operational AEM systems describedare examplesfrom current practice (late 1980s) and illustrate some of the generalities discussedin the Design Considerations section. There are many systems, so the choice was rather arbitrary. The first two examples are of frequency-domain, rigid-boom AEM systems--one a wing-tip, single-frequency, EM system which is one componentof a multisensorairborne geophysicalsystem used by the Geological Survey of Finland for detailed systematicsurveys--and the other a four coil-pair helicopter AEM system flown for contract surveys by Aerodat Ltd. The third is a time-domain, towed-bird system known as GEOTEM and flown for contract surveys by Geoterrex Ltd. Most AEM systems are custom made either by the organizationoperating the systemor by a group working in close associationwith the operator. The systems are usually in a state of continuous evolution and improvement. The airborne geophysical systemsused by the Geological Survey of Finland (GSF) have been described in Peltoniemi (1986) and Vironm/•ki et al. (1982). The current system originated in 1980 and is installed in a DHC-6 Twin Otter STOL aircraft. The system is mainly used for detailed systematic mapping surveys where the flight lines are separated 125-200 m and the mean terrain

clearance

is 30-40

m. The multisensor

geophysicalmapping is undertaken partly to assist in the production of detailed geologic maps of both the Precambrianbedrock and the Quaternary overburden, and partly for mineral, hydrological,and fuel exploration purposes.The systemitself is shownin Figure 10a and its main features are tabulated in Figure 10b. The EM system operates at a nominal frequency of 3.2 kHz, and has a coplanarcoil configurationwith coil axes along the line of flight. The observedEM data are the in-phase (IP) and quadrature (Q) componentsof secondaryfield in units of the primary field strength. The systemis optimizedfor geologicmappingon the Fennnoscandian Precambrian Shield, and the frequency has been chosenin order to obtain quadrature responsefrom the thicker surficialdepositsof peat and clay, and an in-phase responsefrom bedrock conductors which are mainly graphitic-sulfidicmetasedimentary formations (so-called black schists). A lower frequency was employed in earlier AEM systems which conducted less detailed surveys.

EM

825

High spatial resolution magnetometry is the key capability for the GSF aerosurvey system, and to maximize the flight line spacingwithout causingexcessive spatial aliasing, proton magnetometersare operated in both wing tips and on the tail of the aircraft at a sampling rate of four measurementsper second. Because of the low flight altitude and the relatively magnetic nature of the Finnish bedrock, a large dynamic range is required of the magnetometersbut high sensitivity is not necessary. Compensation errors of a few nanoteslascan be tolerated. An interesting engineering feature of the system is that the EM coils are mounted on the same wingtip pods that contain the magnetometers, within about a meter of each other. Interaction has been reducedto an acceptablelevel by careful orientation of the EM and magnetometer coils to give a null coupling. The pods are fastened below the wing surface by an amount such that wing flexure does not change the separation of the EM system coils. The AEM system, as originally designed, includeda sfericsmonitor (a detector of transientsignals in the receiver coil at frequencies unrelated to the primary signalor expectedinterferences)and monitors of temperature, acceleration, and the configuration of the aircraft control surfaces. As the system has evolved, most of the predictable interferenceswith the EM measurement have been reduced to the point where, in practice, these monitor signalsare not much needed in routine data analysis. The GSF system also includes a large volume scintillometer for gamma spectrometry mounted in the cabin of the aircraft and, more recently, a VLF system which is mounted in the tail boom along with a third magnetometer.

Multicoil frequency-domain, helicopter AEM systems are operatedby severalcompanies.The Aerodat Ltd. system which is described here and shown in Figure 1l a is an outgrowth of systemsbuilt for Aerodat by Geotech Ltd. The coils are carried in a Kevlar bird suspendedfrom the helicopter'scargohook. The configurationof the coils in the bird is shownin Figure 1lb. Separations are about 7 m. The Aerodat AEM system is designed for both conductivity mapping and conductor search applications, especiallywhere good spatial resolution is necessary. The system operates at three frequencies, 0.93, •4.5, and 33 kHz, using a coaxial configuration at the lowest frequency for locating deep steeply dipping conductorsin the bedrock, a coplanar system at the highestfrequency for near-surfaceconductivity mapping and both configurations at the middle frequency for resolution of conductor geometry. An alternative system with the 0.93 kHz coaxial system replacedwith 0.5 kHz coplanar systemis available for surveyswhere conductivity mapping is a more impor-


826

Palacky and West

(a) AIRBORNE GEOPHYSICAL SYSTEM OPERATED BY THE GEOLOGICAL SURVEY OF FINLAND AIRCRAFT

DATA RECORDING

DeHaviland DHC-6 Twin Otter, with demountablewing-tip pods and tail boom added; instrumentsand operator console in demountableracks in cabin;typicalflightheight35 m; speed 180 km/h (50 m/s) GEOPHYSICAL

Via 16 bit micro computerto 800 bpi 1/2 inch magnetictape EM data--12 bit digitalsampling,4 per second Data monitoring--analogstrip chart All data--178 variables,totaling512 bytes recordedeach second

SYSTEMS

NAVIGATION

Magnetometers (3sensors, for total field and horizontal gEr•dients); singlefrequency, in-phase and quadrature measuring

Cockpitonly--visual navigationfrom air photo mozaics Cockpitand recorded--radioand barometricaltimeter;gyro sensingof heading, pitch and roll; Doppler radar navigation Recorded--continuousstrip groundphotography;navigator visuallyfixes locations

system;gamma spectrometer;VLF system EM SYSTEM

Configuration--vertical coplanarcoilsmountedon wing-tipswith axes in line of flight Ultimatedata--IP and Q componentsof the secondaryfield, primaryfield normalized(ppm) Frequency--3.2 kHz oil Separation--21.44 m

MAGNETOMETERS

Three independentprotonprecessionmagnetometersin wing-tip pods and tail boom, aircraft compensationapplied in data processing.

Transmitter moment--105 Am•; Power--50 W

Nominalnoise level--14 ppm (IP); 7 ppm (Q); Dynamic range-+_12,000 ppm Primaryfield cancellation--analogelectronicsubtractionof a signalderivedfrom a referenceresistorin serieswith the transmitter

GAMMA

SPECTROMETER

6 x 256 cu. in. sodium iodide scintillator/photomultiplier units (total volumeabout 25 I) into a 256 channelanalyzer, 120 channels recorded once per second

coil.

Phase reference--as for primaryfield cancellation Signaldetection--analogelectronicsynchronousdetectorwith

VLF SYSTEM

fractional second time constant

Multicomponentmagnetic(H) and electric (E) field sensing

Sfericsremoval--in data processing(deconvolutionof detector outputfilter, followedby median filtering,followedby smoothing filter) Base level adjustment--manualin aircraft,retrospectivein data processing

GENERAL

Total weight of geophysicalsystem 600 kg, power consumption 1100 W

(b) Fig. 10. (a) Geophysical survey aircraft of the Geological Survey of Finland. The AEM system coils can be seen mounted on the wing-tip pods which contain the magnetometers.The tail boom contains a magnetometerand VLF system. A gamma spectrometer is carried in the cabin. The system is demountable so the aircraft need not be committed to geophysicalsurveying all year round. (b) Description of the AEM system in (a).

Airborne EM

827

COIL SYSTEMS //•USPENSION CABLE

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950

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4600

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(b) EM SYSTEM

Configuration--4 coilpairs,operating independently on separatefrequencies, coilsverticalcoaxialor horizontal coplanarin the lineof flight Ultimatedata--IP and Q components of the secondary field,primaryfield

normalized(ppm) Typicalcoilset for mineralexplorationin Canadianconditions--0.93and 4.6 kHz, verticalcoaxial;4.3 and 33 kHz, horizontal coplanar;separations 6.45 or 7m

Transmitter moments--in therange15-200Am2, depending onfrequency

Nominalnoiselevels--<1 ppm in all channelsin low sferic,low-moderatewind conditions. Operationalnoiselevelin the lowerfrequencies dependson sferic level, at higherfrequencieson bird motionrelativeto tow cable

Primaryfieldcancellation--differential sensingof the magneticfield

Phase reference•derived

from transmitter coil via a one turn reference coil

Signaldetection--analog synchronous detectorsfor eachcoilpair,outputsare anti-aliasfilteredat 2.5 Hz for digitalsamplingat 10 samplesper second Sfericsremovalin data processing Baselevelsetting--underoperatorcontrol,and retrospectively in data processing

Instrument set-upand calibration--phase referenceadjustments, inter-channel crosstalk compensator for channelsat closelyspacedfrequencies DATA RECORDING

Digitalonto1/2 inchdigitalcassettetape,via a generalpurposeaircraftdata acquisition systemusing12 bitA/D conversion withautomatic gainrangingto

give 16 bits Visual monitorrecordsprovidedin aircraft NAVIGATION

Cockpitonly--visualnavigation fromairphoto mozaicsif elecronic navigation not in service, standard aircraft instruments.

Cockpit andrecorded--radar andoptional laseraltimeters, electronic positioning (radarrangingto 3 speciallyestablished transponders - MotorolaMiniranger II, Falcon,or Sercel Syledis)

Recordedonly--videophotography (VCR)of flightpathalongthe ground

(a)

MAGNETOMETER

AERODAT LIMITED---HELICOPTER

AEM SYSTEM

on tow cable

AIRCRAFT

Aerospatiale350B, 350D, or Bell 206L, with EM bird slungfrom the cargo hook,demountable

instrument consolein helicoptercabin;typical aircraftflightheight60 m; speed 100 km/h GEOPHYSICAL

Continuously recording caesiumvaportotal-field magnetometer, in separatepod

SYSTEMS

VLF SYSTEM

Magnetic(H) fieldsensing(Herz Totemor similar),in separatepodon tow

cable

GENERAL

Total systemweight<500 kg, powerconsumption approx.600 W

4 EM coil pairs(frequencydomain,IP and Q measuring),optionalmagnetometer, VLF bird(see diagram)

Suspended on 30 m towcable,magnetometer podat 15 m, VLF podat 7 m; totalweight= 250 kg

(c)

Fig. 11.(a) The Aerodathelicopter AEM system.Similarkindsof EM birdareusedby othersurveycompanies, witha varietyof coilconfigurations. Magnetometer andVLF-EM arenotattachedin thispicture.The apparatus is relativelyeasyto mountand demountfrom the aircraft.(b) Configuration of the EM coilsin the bird of the AerodatAEM. (c) Descriptionof the AerodatAEM system.


828

Palacky and West

tant function than bedrock exploration. A key part of the bird designis the spreaderbar, to which the tow cable and aerodynamic drag elements are rigidly fastened. The bird is elastically suspended from the spreaderbar to minimize vibration and flexure. Details of the system are listed in Figure 1l c. Electronic navigationusingthree locally established transponders, for instance with the Motorola Miniranger or the Sercel Syledis radar ranging systems, is the preferred mode of operationespeciallyfor detailed surveys. A magnetometercan be carried with the EM system, afixed to the tow cable midway between the EM bird and the aircraft

in order to minimize

aircraft

effect on the magnetometer and interference between the magnetometer and the EM unit. A VLF system can be carried in another pod attached to the tow cable

organizations, both having their established routines. Rather than explaining in detail such procedures we focus on the needs of geoscientists who design and monitor AEM surveys or use geophysical maps. We explain technicalitems, which shouldbe specifiedin a contract and monitored during the survey execution, and describepost-flightdata processingand compilation of final products (maps, composite profiles). Examples are given of products currently available for three AEM systems discussed in the previous section (GSF system, helicopter AEM system, and GEOTEM). Digital tapes or disks constitute an integral part of the processed material supplied by the contractor or made available by governments. However, the scope of this book is not to describe the details of handling data in a digital format.

nearer the aircraft.

The GEOTEM system of Geoterrex Ltd. stands in marked contrast to the Aerodat AEM system. The GEOTEM is designedfor maximum depth of penetra-

tion and is an offspring of the Barringer INPUT system. Shown in Figure 12a and described in Figure 12b, the GEOTEM

has an enormous transmitter coil

energized with the INPUT current waveform and uses a towed-bird

receiver

that

is connected

to a time-

domain sampling-averaging receiver. Unlike most other AEM systems,the receiver is fully digital; all the basic signal processing is carried out in flight by a digital computer in the aircraft. The ability to make substantivechangesin the system's output and to the aircraft's data recording system, simply by changing software, adds flexibility. For instance, the pulse repetition can be changed to accommodate different powerline frequencies in different survey areas or different survey requirements;the data displaycan be chosento suit the geologicconditions.The operator's display of data can be selected to better reveal certain quality control factors rather than beingjust a limited, in-flight version of the data delivered to clients. The great power of the transmitter is not without costs. Because the transmitter cannot be powered from the normal aircraft electricgenerators,a separate auxiliary power unit is required. Also to reduce the interference of the broad band EM transmitter signal with the magnetometeris not trivial without sacrificing the loop size or the ability to compensatethe magnetometer well enough for surveys of high sensitivity. Obviously, to demount and reinstall the systemin the aircraft to make the aircraft fully available for other purposes is not easy. FIELD

OPERATIONS

AND DATA

PROCESSING

Virtually all airborne geophysicalsurveys are carried out by specialized contractors or government

Survey Design and Specifications Among the factors to be considered in the selection of AEM equipment are the purpose of the survey and the geologic and topographic conditions. When the aim is to prospectfor deep, highly conductivetargets, a system with a large penetration (e.g., time-domain, towed-bird variety) is required. When the emphasisis on high resolution (e.g., definition of narrow conductive zones), a helicopter AEM system is more appropriate. In regions with a conductive surficial cover (clay-rich glaciolacustrine sediments, or thick weathered layer), a systemoffering a deep penetration may be advantageous, but if a definition of near-surface layers is desired, a multifrequency, multicoil helicopter system may be more suitable. In rugged areas, low-altitute fixed-wing surveys cannot be safely carried out and use of the helicopter remains the only option. An early paper (Paterson, 1971)was published comparing AEM systems, in which several generally valid evaluation criteria were postulated. After selectingthe equipmentfor a given problem, the geophysicistmust specify various survey parameters. Most of the specificationswill be part of the contract between a client and a company carrying out the survey. Particular attention should be paid to the following:

(1) (2) (3) (4) (5) (6) (7)

Survey layout; Means of navigation; Navigation tolerance; Data acquisition system; Acceptable instrument noise levels; Lag, calibration and nulling tests; and Ground monitoring stations.

Survey Layout.--Usually, the results of AEM surveys are compiled in the form of maps and composite (stacked) profiles. To obtain a systematic coverage,

Airborne

EM

829

GEOTEM system (Geoterrex Ltd) Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

mounted

on a

Casa C-212

aircraft

12 (a) GEOTERREX

LIMITED•GEOTEM

SYSTEM

AIRCRAFT

Casa C 212 twinturbopropfixed-wingSTOL aircraftwithspecialwing-tip,foreand aft boomsto carrythe EM transmitter coil;EM receiverin a towedbird'typicalaircraftflightheight120 m, speed 200 km/h

GEOPHYSICAL

SYSTEMS

Time-domaintowed-birdEM, high-sensitivity magnetometer, gammaspectrometer EM SYSTEM

Multichannel time-domain systemusingalternating,equispaced, half-sinusoidal, currentpulsesand secondarysignalrecordingin the current-offperiods;current-oninterval1.0 ms, off interval>2.0 ms, base frequencies150 or 125 Hz (60 or 50 Hz countries)

Transmitter--vertical axis,3 doubleturns,peakcurrent600 A, peakmoment450,000Am2

Receiver--singlecoil,axis in line of flight(verticalon specialtests),mountedin birdflownabout 105 m behindand 80 m belowthe transmittercoil on a 135 m (max.)cable

Dataacquisition--fully digital,16 bitsamplingat 256 samplesperfullperiod;secondary signal

binning andaveraging doneinrealtimebyfastdigital computer, alongwithspurious si.•nal

filtering(sfericsand powerlinenoise),compensation of aircraft-effect, base-levelcorrections

Sampling--software selectable,typicallya set of windowsapproximately logarithmically spacedafter the cessationof transmitter current,givingthe averagevalueof the secondaryvoltagein each time window,averagedover integralfull transmittercyclesfor •0.1 s Ultimatedata--alternativeformatsavailable;typically12 continuous secondarysignalchannels normalizedto the peak primaryvoltageinducedin the receiver(ppmor ppk) In-flightmonitors--aselectionof the data channelsplusseveralqualitycontrolvariables

DATA RECORDING

Recordingon 1/2 inch,9 trackmagnetictape;35 EM variablesincludingmonitorsignals,transmitter waveform,and receivedsignal;data fromothersensorsand navigationdevices NAVIGATION

Cockpitonly--airphotomozaicsand standardaircraftinstruments Cockpitand recorded--radarand barometric altimeter,Dopplerradarand gyrocompasselectronic navigator,optionalLoranC, GPS navigation

Recordedonly--videoVCR and photographic recording of the flightpathalongthe ground MAGNETOMETER

Caesiumvapormagnetometerin tail stinger;within-flightaircraftcompensator GAMMA SPECTROMETER

Optionalgammaspectrometerwith largevolumescintillator(•50 I) in aircraftcabin

12 (b) Fig. 12(a). The GeoterrexGEOTEM time-domain,towed-birdAEM system.Note the hugetransmitterloop strung aroundthe wingtips,noseand tail. Not shownis the receiverbird and tow cablewhich fliesfrom the lower rear of the aircraft. The extendedtail stingercarriesa high-sensitivitymagnetometer;a gammaspectrometercan be carried in the aircraft cabin. (b) Description of the GEOTEM AEM systemshown in Figure 12(a). ,


830

Palacky and West

flights are carried out along parallel lines which are perpendicularto structuresexpectedto causepredominant geophysicalresponse.The area of interest may be divided into segmentswith two or three different flight directions. In mappingprograms,traverse lines spaced 200 m are standard (e.g., government-sponsoredsurveysin Canadaand Finland). in areaswhere highresolutionis required(e.g., goldminingcamps),a 100 m spacingis more appropriate. A decade ago, a line spacingof 400 or 500 m was more common, but experiencehas shown that such low density of flight lines resultedin missingexplorationtargetsof interest. Unlike other geophysicaltechniques,the lateral coverage of AEM systemsis very limited (Best, !985). High-quality contour maps are difficult to produce in areasof a rapid conductivitychangeusinga flight line spacinglarger than 200 m. Control lines (tie lines) are usually flown every 4-5 km in a direction perpendicular to traverse lines and at the extremes of the survey area.

AEM surveyscan be usedin applicationsother than systematic mapping. In this chapter, an example is given of surveysconductedalongroads (transects)in northern Ontario, Canada. The minimum requirements for any nonsystematicwork is at least two lines flown in oppositedirectionwith a spacingof 100 m or less.

Means of Navigation.--At present, three procedures can be used in aircraft positioning and subsequent flight-path recovery. Normally, visual navigationwill complementone of the other two except in areaswith no topographicfeatures (e.g., dense tropical forest, sea). (a) Visual navigationrequiresthe navigatorto select the flight path usingphotomosaics(the usual scalefor flying is 1:10 000), which are prepared by joining existing aerial photographs to provide a complete coverage of the survey area. This commonly used approach presents several dificulties. Often the aerial photographsare obsoleteand do not reflectthe ground truth (e.g., deforestation, erosion, new roads, new subdivisions,or cultivated areas). Becauseof distortions near edges, photographsoften cannot be accurately joined to producea photomosaic.The navigator has to be experienced to guide the pilot onto the correct path. During the flight, an on-board camera photographsthe flight path. Particularly in rugged areas, the camera is not always pointing downward, and the resultingphotographdoesnot reflectthe exact position of the aircraft at a given moment. The best guessof the true path of the aircraft is recoveredafter the return to the base by matchingidentifiablepoints on the film and photomosaics.This tediousprocedure becomesunreliablein featurelessterrains(e.g., forest,

desert).Insteadof a trackingcamera,a VCR camerais often used. Matching of the planned and actual flight path is then doneon a VCR screen.This approachhas some advantages,particularly in documentingmovement of the bird which causes many false AEM anomalies(bird-swingnoise). (b) Electronic navigation (e.g., Motorola Miniranger, Serce (Syledis) eliminatesmost of the drawbacks of the visual approach. Before commencing flying, transpondersare set up in the survey area. As the low-flyingaircraft mustremainin the direct sightof the transponders,the procedurebecomescomplicated (multiple setupsare required) or even impossiblein ruggedareas.The automaticpilot guidesthe aircraftto position along the selectedtrack (survey line). Any variation from the preselectedpath is recorded and used in the recovery of the flight path. A precisionof 5 m can be achieved in recovering any survey point alongthe flight line. In offshoreareas, Loran or Decca networkscanbe used,but their accuracyis poor (e.g., more than 100 m). In the 1990s, accuratenavigation and flightpath recoverywill be achievedby usingGPS (Global PositioningSystem), a system based on a network of satellitesbeaming signals. (c) Doppler effect is the third techniquecommonly used for navigation and flight path recovery. Unlike the Miniranger and some other types of electronic navigationwhich require settingup of transponderson the ground, the Doppler navigation system is fully installed aboard the plane. The Doppler shift is measured using four transmitters which send signals slightly deviating from the vertical in the forward, aft, and side directions. Starting the flight from a given positionand with an initial bearing, the positionof the plane can be reconstructedfor any givenmomentfrom the continuousrecord of the Doppler shift. While the precisionof the flight path recovery is lessin the case of Miniranger or Syledis (50 m or more), Doppler navigationis advantageousin ruggedor remote areas. A radar altimeteris normallyusedto keep track of the aircraft clearanceabove the groundin AEM surveying. However, its accuracyis not sufficientfor specialtypes of AEM surveys,suchas sea-icethicknessdetermination, for which a laser altimeter must be used. The AEM

responseis highlysensitiveto changesin distanceseparating the receiver and conductive horizontal layers. Most altimetersbecome inaccuratein denselyforested areasbecausethe beam often locks on the tree canopy. NavigationTolerance.•An AEM contract will typically specify the allowed deviation from the planned flight path and the normal terrain clearance. Both specificationsdepend on the type of the airborne platformandthe AEM systemused.The normalflying height is 120 m for a fixed-wing aircraft with a towed-


Airborne

bird (e.g., GEOTEM, QUESTEM, SPECTREM). The permitted deviation would be typically ---30 m, but such conditions often cannot be met in rugged areas. Some clients specify only that the maximum flight height should not exceed 150 m. The usual spacingof traverse lines is 200 m; the permitted deviation using visual navigation will be 25 to 50 percent of the line spacing. Reflights will be requested if the deviation exceeds

the limits over a distance of 1 km or if two

parallel lines are more than one and one-half line spacingsapart. The navigation requirements are more stringentfor helicopter AEM surveys. The mean terrain clearance of 30 m for the bird shouldbe kept within 10 m, except for very rugged areas (deep valleys, steep mountains). In mineral exploration surveys, often only the upper limit may be specified(e.g., 50 m). The contractor will usually be able to meet the requirement of maximum deviation of 25 percent of the line separationfrom the planned flight path. As in fixed-wing surveys, the maximum permitted distancefor a deviation is usually fixed at one and one-half times the line spacing. Data AcquisitionSystems.--In the past, the output of all on-board geophysical instruments was registered using an analog recorder. While such devices are still used for in-flight monitoring of the performance of somesystems,they are not usedfor acquisitionof data suitable for post-flight processing.Present-day monitors often do not display simultaneouslyall channelsof recorded geophysicaldata and the operator selectsthe parameters of interest. When manual calibration and nulling are required (e.g., for helicopter AEM systems), use is made in the procedureof on-board analog recorders.

The digital data acquisition system should be capable of handling all measured parameters plus navigation information.

Because

of the short time constants

presently used in all AEM systems, the data acquisition device shouldbe capableof handlingmultichannel information (20 or more) 10 to 100 times per second. At present, most airborne data acquisition sytems use cartridges or a combination of hard disk and cartridges. Acceptable Instrument Noise Levels.--The acceptable receiver noise level depends on the signal over selected targets (e.g., vertical dike). A time-domain, towed-bird system which has a larger separation between

the receiver

and the transmitter

has a much

stronger signal and also a higher receiver noise level [expressedin parts per million (ppm)] than a helicopter AEM system with a closely spaced transmitter and receiver. The instrument noise whose origin was described in the Design Considerations section should

EM

831

not be confused with sferics which have much higher amplitudes. For a helicopter AEM system, the receiver noise should not exceed 1 ppm; for a timedomain, towed-bird system the threshold level is about 40 ppm or less. All AEM systemsexperience base-level (zero-level) drifts, but some (e.g., GEOTEM, QUESTEM, SPECTREM) compensatefor them automatically.At present, drift correctionshave to be done in helicopter AEM surveys. The drift can be determined as a difference between two backgroundlevel checks performed periodically at a high altitude (e.g., 400 rn above terrain). Usually, it is assumed that the drifts are linear and correctionsare made at the data processingstage. In reality, the drifts are often nonlinear and seriouserrors may appear in conductivity (resistivity) maps without additional corrections. If the drift on any channel exceeds an acceptablethreshold (e.g., 50 ppm/hour for helicopterssystems),surveyingshouldbe suspended. The effect of sferics, which are by far the most significantsource of AEM noise, can be largely eliminated in real time by a suitable receiver design (e.g., GEOTEM, QUESTEM, SPECTREM) or by postflight data processing (helicopter AEM systems). Sferic removal by post-flight processingis only possible if the receiver time constant and data acquisition sampling rates are sufficient (10 times per second). However, once sferics become very frequent (more than 3 or 4 times per second), post-flight removal becomes inaccurate and surveying should be suspended. Similar conditions may cause suspensionof time-domain towed-bird surveys as well. A significant source of noise is the relative movement of transmitter

and receiver.

This source of noise

is usually negligible in rigid-boom AEM systems, unless the target is a deep conductor in a resistive terrain, where it becomes a limiting factor. Noise resultingfrom the coil movement is very important in all towed-bird configurations.It was argued in the past that time-domain AEM systemsrecording in the transmitter-off

time

were

insensitive

to bird

motion

but

detailed studies by Annan (1984) showed otherwise. Becker et al. (1989) made a comparisonof INPUT data obtained with analog and digital receivers. Older analog INPUT surveys contain many false anomaliesdue to changesin the relative position of the aircraft and bird. The bird swing dependson many factors, suchas the bird design(high-dragbirds are more stable), wind conditions, and the pilot's ability to keep the bird steady. In areas of low ground conductivity, anomalies due to bird swing are very small as the overall responseis close to zero. Therefore, little attention has been paid to this serious survey problem in highly resistive areas, such as the Canadian Precambrian Shield. Anomalies related to bird swingoften resemble


832

Palacky and West

a dampened sinusoid and can be removed by postflight processingusing iterative notch filters (Annan and Lobach, 1985). However, the procedure is nontrivial as the bird-swingfrequencyvaries dependingon factors such as wind speed and direction and time elapsed after turn. Fortunately, the bird-motion effect can be minimized when systemswith digital receivers are used. In analyzing towed-bird AEM data, the interpreter may find it difficult to recognizebird-swing noise in areas with a large number of EM anomalies. At present, no post-flightremoval of bird-swingnoise is routinely done by contractors. Lag, Calibration, and Nu!!ing Tests.--A contract should specify calibration proceduresfor all geophysical equipmentused in the survey. Lag tests previous to survey are carried out in two directions that are not parallel over a distinct anomaly. Their purpose is to determine the time difference (lag) between the magnetometer, EM readings, and the operation of the flight-path camera. The lag test will be carried out at the standard flight height (e.g., 30 m for helicopter AEM systems; 120 m for fixed-wing, towed-bird operations). Radar altimeter will be calibrated by flying at predetermined altitudes above the airbase strip. The usual values for helicopter operations are 20, 30, 50, 70, and 100 m; for fixed-wing aircraft 100, 120, 140, and 160 m. The results of the test will be submitted

for

approval to the technical inspectorprior to commencing routine surveying. In the case of helicopter AEM surveys, a preflight calibration is performed before each flight using an external calibration coil. As a result of this procedure the ratio of the secondary-to-primarymagneticfield (in parts per million) and voltage will be established.The correct responsein parts per million will be obtained by multiplying the survey data by the calibration factor. The external calibration coil is placed in positions defined for each transmitter-receiverpair. The calibration checks are run for 10 s for each position. Calibration also may be performed during the flight using internal calibration coils, but such calibration is less reliable and should never be carried out instead of

the preflight checking routine. In-flight calibration is used only as an additional means of checking the calibration when doubts exist about the proper functioning of instruments. Nulling is a procedureto assurethat the helicopter AEM system response is zero when no conductor is present. Such condition occurs when the system is flown at a sufficientaltitude as not be be affected by any ground conductors (in practice the flight height is 400 m except over seawater, where 500 m is required). The test should be carried out at least once an hour, but sometimesmore frequent tests may be required in

highly conductive areas, or when the drift is significant. Unfortunately, the benefits of frequent nulling at high altitude are not unambiguous.As the temperature at 400 m and at the standardflight height for helicopter surveys (30 m) may differ substantially, the zero level may changefast after the descent to low altitudes. The drift (change of zero level with time) must be removed at the data processingstage. Data processingproblems occur when the drift is fast (i.e., exceeding 50 ppm/ hour for helicopter AEM systems) and/or nonlinear. Significantdrift also may be symptomatic of the system's malfunctioning. A fully digital AEM receiver (e.g., GEOTEM, QUESTEM, SPECTREM) is self-calibratingand selfnulling, hence no pre-flight calibration and high-altitude nulling are required. Sferics and any other noise whose characteristicscan be specifiedin advance are removed in real time. Annan and Lobach (1985) described such procedures.

Ground Monitoring Stations.--While an active AEM system does not require any ground monitoring, additional geophysicalsensorscarried aboard the aircraft almost always do. During AEM surveys the total magnetic field is recorded and sometimesVLF as well. In the present surveying practice, a magnetic ground monitoring station is mandatory and a VLF ground station is optional. A ground magnetometer recording the total field with sensitivity of 0.25 nT must be placed in a location free of magnetic noise. A ground VLF station (usually an identical receiver to that used in the survey) is set up to monitor the changesin the total VLF

field. Both stations will be connected

to an

analog and digital data acquisition systemrecordingall signalsat least once a second. The analog output will be used for visual detection of magnetic storms and other disturbances;the digital record will be used for post-flightdata processing. Data Processing

Until the early 1980s,the compilation of AEM data was largely based on handling the output of chart recorders. At present, most contractors are in the transitional phase; all data are recorded digitally, but not all AEM receivers are of digital design. Some tasks that can in principlebe achievedin real time duringthe flight (e.g., removal of sferics) have to be done in the first phase of post-flightdata processing.Therefore, a short description of such techniques is in order. In principle, AEM surveysmust be repeatable. Any type of response that may differ between surveys conducted

at different

times should be removed

at the

data processing stage. The single most important sourceof noise are sferics (atmosphericdisturbances


Airborne

describedin the Design Considerationssection). They are randomly spaced events of short duration whose amplitude varies over the whole sensitivity range of AEM receivers. If maintained in proper working conditions, all modern AEM instruments have only a negligiblereceiver noise. However, nondigital receivers which record the emf due to the secondary magnetic field in the presenceof the primary field, experience base-level (zero level) drifts which must be removed at the data processing stage (helicopter AEM). Various authors (e.g., Ward, 1967) used different definitionsof noise and often the responseof noneconomic conductors (glacial sediments, weathered layers) was called geologic noise. As explained in the Design Considerations section, the term noise should be reserved for nonrepeatableevents which are due to changes in atmospheric conditions and equipment performance. The nature of airborne surveying does not allow precise retracking of a previous line to determine the precision of measurement. Repeating a flight path exactly in terms of altitude and x-y position is impossible. The accuracy as specifiedin Navigation Tolerance means that the measured AEM response may vary from one acquired during another flight. Data processing leads to minimizing this inevitable problem. Maps and other products obtained from different surveys, even with identical equipment, will never be exactly the same, but the main conductivity features shouldbe preserved. Post-flightdata processingconsistsof several steps:

EM

833

several specific programs. Figure 14 illustrates the removal of sfericsusing a sequenceof sferic rejection and smoothingfilters for helicopter AEM data. The resultsare comparedwith conventionalfiltering of raw data (lower two traces). Superimposedare the results of another survey flown on a day when the sferic activity was low. The combinationof a sferic rejection filter followed by a smoothingfilter produced results superior to other types of filtering. The final output strongly resembles survey data acquired on a quiet day. First-pass leveling of data is achieved by using values obtained during high-altitude nulling procedures. As the zero-level drift is often not linear,

additional leveling may be required. This leveling is achievedby visual identificationof areaswith very low conductivitywhich shouldhave a zero AEM response. If the AEM readings differ from zero, the measured responseis used for a manual correction of the backgroundlevel. This proceduremay becomeimpractical in areas of high surficial conductivity (e.g., regions with a thick clay cover). In such a case, more frequent

REMOVAL OF SFERICS (STOCHASTIC

DRIFT

FILTER)

REMOVAL

REDUCTION OFPOWER-LINE &OTHER INTERFERENCE

(1) Removal of sferics and drift, (2) Recognition of man-made disturbances,

CORRECTED FLIGHT

elimination

of sferics.

The

most

effective

removal

routine consists of event identification and replacement by the average of two readingsat the neighboring undisturbed points. A suitable routine for anomaly identification, which was described originally in Palacky and West (1974), was used in the design of

DATA

ALTIMETER,

(3) Merging of flight path and geophysicaldata, (4) Presentationof AEM data as profiles, and (5) Presentationof AEM data as maps. Removal of Sferics and Drift.--The first step in processingAEM data, which were not acquired with a fully-digital receiver, is the removal of sferics (Figure 13). In the past, when receiver time constants were long and data were recorded only once a second, the recognition of sferics was sometimes ambiguous and their removal very difficult. With high recording density of data, sferics can be recognized as events of short duration. Conventional filtering would lead to a distortion of genuine anomalies and only incomplete

CORRECTED

MAGNETIC

PATH

ETC.

PRELIMINARY LINE PLOTS FOR

INSPECTION

.

CORRECTED INTERMEDIATE

(TAPE INTERPRET DISCRETE

I

CONDUCTORS •r

_

AND APPROVED DATA BASE

OR DISC) INTERPRET

I

•

,

LAYERING

I , CALCULATE ! I IDENTIFY BEDROCK ,I, I CALCULATE or, CONDUCTIVITY

CONDUCTORS

DEPTH,

IF NECESSARY

DIP

"•[ CORRECTE SET OF

MAGNETIC

AUXILIARY

ALL PARAMETERS

AND

DATA

(PLOT AND TAPE) IJ Fig. 13. Steps in processingof helicopter AEM data. Each operator may use a slightly different outine. Anomalies due to discrete (bedrock) conductors will be treated separately from AEM responsesdue to lithologic units and overburden.


834 PalackyandWest


Airborne

in-flight nulling should be performed and surveys suspendedif the drift is excessiveor highly nonlinear. Figure 15 illustrates the removal of drift (leveling) by manual intervention. The data are acquired with a multicoil, multifrequency, helicopter AEM system. At the top, the coaxial AEM trace at the frequency of 4600 Hz is shown.

In the middle

is drift which

was

determined by interpolation between two high-altitude tests. At the bottom, the drift has been removed•the data have been leveled.

Recognitionof Man-made Disturbances.--Man-made objects, such as power-lines, telephone cables, drainage pipes, groundedfences, and railways, cause significant EM response. Whether they should be called noise, a term we prefer to reserve for nonrepeatable events (sferics, receiver and bird-swing noise) is a question of semantics. In our opinion, the term manmade disturbance (or feature, or object, or simply "culture") is preferable. Recognition of some manmade disturbancesis easy--most AEM systemscarry on-boardpower-line monitors, and electrical transmission lines are also easily visible on aerial photographs. Railways and groundedfencescan be identifiedreadily on tracking camera or video film (even though the process may be quite time-consuming).Buried telephone cablesare not visible and groundchecksmay be necessaryto explain some suspicious(i.e., elongated) anomalies, particularly if they follow roads or are in densilypopulatedareas. Also drainagepipescannotbe identified on aerial photographs.Moreover, they can form irregular patterns and a detailed ground examination is often required. The best telling feature is their AEM response indicating a narrow, very shallow

EM

835

conductor, but in many areas bedrock conductorsmay cause similar AEM responses. Figure 16 shows the response of a major transmission line near Fraserdale, Ontario. The power line affects the four recorded EM channels differently. On the high-frequency coaxial channel, the AEM data are noisy over a distance of 2 km, but the most affected segmentis 0.5 km wide. For the compilation of conductivity maps, the worst affected area must be removed, but the flanks can be retained after low-pass filtering. On the low-frequency coaxial channel, the distortion does not consistof high-frequencyrepetitive oscillations, but of randomly shaped response. If the high-frequency coplanar channel stood alone, without other EM channelsor power-line monitor, it would be difficult to identify the power-line response without additional information. The slight decrease in amplitude on both the in-phase and quadrature data has the same appearance as an anomaly due to geologic formation. Obviously, calculated conductivities (shown for the two coplanar channels) are meaninglessover a wide zone surroundingthe transmissionline and the data should be removed before the compilation of the conductivity map. All man-made disturbances should be depicted on interpretation maps using proper symbols. Their disturbance response should be retained in all products displaying the measured data (e.g., composite profiles), but must be removed from the conductivity (resistivity) maps which are supposedto reflect only the underlying geology. Merging of Flight Path and GeophysicalData.--As indicated in Figure 13, the first stepsof post-flightdata

ppm

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Fig. 15. Removal of zero-level drift for a helicopterAEM system.Three traces are shown:uncorrected,drift, and corrected (courtesy of Aerodat Ltd.).

836

Palacky and West

processingconsist of removal of sferics, leveling, and


reduction

of

transmission-line

disturbances.

The

"clean" AEM data shouldbe mergedat this stagewith the navigation information. The flight lines are usually plotted on a topographicbase or a photomosaicusing information from the navigation systems (e.g., Doppler, Miniranger) and/or visual picks on strip films. A topographic base has become more common with the increasinguse of electronic navigation systems. Frequently, the identification of points is not 100 percent accurateand a discontinuityof magneticcontoursmay indicate errors in the flight path recovery, which then should be rechecked

and corrected

if shown

to be

wrong.

Line plot maps, usually at a scale twice the final presentation (e.g., 1'10 000 when the final scale is 1:20 000), are used to check the effectiveness of lev-

eling and sferic removal. Typically, two map setswill be produced, one with the raw data and the other with the corrected data. The superpositionof the two maps reveals errors and deficienciesthat may have escaped

the inspectorduring the previous processingstages.If no corrections are required, an intermediate data base is created which is stored on a disk and used for the

subsequentdata manipulation. If a detailed monitoring of airborne surveys is done by inspectorson behalf of the client, this point is a usual inspectionstep. Depending on the purpose of the survey, further data processing may assume a variety of forms. In mineral prospecting surveys (massive sulfide detection) bedrock conductorsare identified, and quantitative interpretation is carried out for selected anomalies•determination of conductortype and selectionof the interpretation model (see Data Interpretation section). In parallel with this we have the calculation of conductivity (or resistivity) for all points. Conductivity or resistivity maps are becoming standard for all applications, including mineral prospecting. As the model used in conductivity and resistivity calculations is not suitablefor the totality of anomalies, the calculated values are in many instances referred to as "apparent" values. If the leveling was not done cor-

150-

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Fig. 16. Effect of a high-voltageelectrical transmissionline on helicopterAEM data. The survey was flown by Aerodat Ltd. for the Geological Survey of Canada.

Airborne


rectly, calculated values may show trends unrelated to geologic changes. Additional leveling corrections will be required in such situations. Presentation

of AEM

Data

as Profiles.--The

most

common way of presenting all measured and calculated parameters along survey lines is in composite (stacked) profiles. Figure 17 shows a sample profile from a helicopter AEM survey in the Monts Stoke area, Quebec. From the top, the following parameters are displayed (horizontal scale is the same as for the contour maps, usually 1:20 000 when the line spacing is 200 m or less):

EM

837

field)•solid line, scale in nanoteslasper meter or nanoteslas.

Composite (stacked) profiles for a towed-bird, timedomain AEM survey are shown in Figure 18. The data were acquired with the GEOTEM system in the Lynn Lake area, Manitoba. From the top, the following parameters are displayed (original horizontal scale 1'20 000, vertical scale proportionately reduced)' (a) Radar altimeter (vertical scale 25 m/cm). (b) Calculated apparent conductivity (logarithmic scale, 2 cm/decade, maximum range 1-100 mS/ m). The homogeneous half-space model was used in the calculation.

(a) VLF

traces (total field, quadrature) for two transmitters. Not all AEM surveys record VLF responses.The traces have not been subject to any filtering. (b) EM noise trace obtained by subtractingraw and filtered data (in-phaseand quadratureon vertical coaxial configuration, frequency 4600 Hz). This trace replaces the previously used sferics monitor whose performance was found less satisfactory because its spectral characteristics differ from the actual AEM

channels.

Another

trace is

the power-line monitor (or subtracted powerline noise•the third step in the data processing chart, Figure 13). (c) Filtered EM data (in-phase, quadrature)for horizontal coplanar configuration (frequency 4175 Hz), full vertical scale 300 ppm. (d) Filtered EM data (in-phase, quadrature)for vertical coaxial configuration (frequency 4600 Hz), full vertical scale 75 ppm (notice the difference in scale between coplanar and coaxial configurations). (e) Filtered EM data (in-phase, quadrature) for ver-

tical coaxial configuration (frequency 935 Hz), full vertical scale 75 ppm. (0 Apparent conductivity calculated from the coplanar data using a homogeneous half-space model (logarithmic scale, maximum range 0.11000 mS/m). If coplanar data were available at another frequency, apparent conductivity for both frequencies would be displayed simultaneously. The same strip can also be used to display the conductancesestimated for bedrock conductors (as narrow bars centered at the conductor location). In this case, the scale would be in seimens, but the range (0.1-1000 S) remains the same.

(g) Manual fiducials recovered in the map and time (in seconds). (h) Radar altimeter (dashed trace, scale in meters) and vertical magneticgradient (or total magnetic

(c) Total-field magnetic measurements;the data are displayed at two scales, 20 nT/cm and 200 nT/ cm.

(d) Twelve channels of GEOTEM measurements, starting with Channel 1 at the top. The respective time delays after the transmitter switch-off for the centers of the contiguous 12 channels are: 360, 470, 570, 670, 790, 920, 1050, 1210,

1350, 1520, 1700, and 1880 Ixs.The vertical scale is the same for all channels-400 ppm/cm. Each channelis offset from the previous one by 5 mm. (e) Peak response symbols (circles) over bedrock conductors. Anomalies on each line are designated by a letter. The shadingof circles indicates the number

of channels

with

visible

AEM

re-

sponse,ranging from dark (11 or 12 channels)to empty (3 or 4 channels). A star depicts a 1-2 channel response.Identical symbolsare used in the GEOTEM maps. (f) Fiducial numbers of identified picks which appear on all geophysical maps. (g) EM noise monitor (scale 30 mV/cm).

All composite (stacked) profiles are inspected for errors. The checks include the accuracy in anomaly location, the correlation between the analog records and the composite profiles, and the suitability and correctness of vertical scales. There should be no inverse correlation between the altimeter trace and the

calculatedconductivity which is theoretically independent of altitude

variation.

The correctness

of conduc-

tivity calculations also can be ascertained by spot checks: (a) Estimates of conductivity for selected points using responseaperture diagrams described in the Data Interpretation section. (b) Calculated conductivity must be higher for points which have a higher in-phase responsewith the quadrature being the same (for helicopter AEM data) or for points where the decay rate (change of amplitude from channel to channel) is slower (for GEOTEM and QUESTEM data).


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Presentationof AEM Data as Maps.--AEM data can be presented as maps in several forms. The simplest one which was in vogue, when helicopter AEM systems were operated at only one frequency and with one coil configuration, is a map of profiles showingthe in-phase and quadrature components and selected anomalies as symbols. Figure 19 illustrates such a presentation for coaxial data at 935 Hz from the Casa Berardi gold camp in Quebec. The map shows clearly the 30 km long Casa Berardi fault zone with which the Golden Pond orebody is associated. This east-west fault zone is within a unit of graphitic sulfide facies and, therefore, appears as a well-defined conductor. The negative in-phase anomalies indicate the position of a strongly magnetic iron formation near the southern edge of the map. Using colors, the in-phase and quadrature components for three frequencies and two coil configurations (coaxial at 935 and 4600 Hz, coplanar at 4175 Hz) and the total-field magneticdata can be presentedas profile maps. While the scale along the flight lines can be maintained, the distance between the lines had to be

increasedto provide enough spacefor the presentation of seven parameters. In Figure 20, the nine easternmost lines of the Casa Berardi map are depicted. In addition to the bedrock conductors, which are indicated by circles with letters, the different patterns of in-phase and quadrature anomalies reflect lithologic changes. While a strong positive EM response indicates sediments, the negative in-phase anomalies are associated with an iron formation (near the southern edge of the map). The same formation causes strong magnetic anomalies depicted by the dark colors in the bar presentation. Cold colors (blue, green) correspond to low values of the magnetic total field. The advantage of this presentation is the possibility to show simultaneouslyall measuredparameters. Its drawback is that the map scale cannot be maintained in the direction perpendicular to the flight lines. A combination

of color and black contour

lines can

be used to present two contoured parameters in the same map. Figure 21 depicts a combination of apparent resistivity (contour lines), depth to bedrock (color), and selected bedrock conductors(circles) plotted along the flight lines. A thick horizontal layer model was used to calculate apparent resistivity and depth to bedrock. A prominent anomaly in the upper central part of the map suggestsa thickening (to over 70 m) of moderately conductive Quaternary sediments(mostly silt having an apparent resistivity of 100 12.m, or conductivityof 10 mS/m). For many applications,such as mineral exploration, this may be the most useful way of data presentation. AEM data presentation as profile maps of one measured component routinely has been done on

systematicsurveys in Finland. The systemused by the Geological Survey of Finland has been summarized in Figure 10. Figures 22 (a) and (b) show respectively, in-phase and quadrature-response maps originally published at the 1:20 000 scale (sheet 3333 02 Tihvonjfirvi). The data were acquired using a fixed-wing aircraft with EM coils mounted on wing tips (separation 21.44 m, frequency 3222 Hz). The plane was flown at an averagealtitude of 39 m, with a cruisingspeedof

49 m/s. The parameter scaleof 1000ppm per 1 cm (at the original scale) results in overlap in areas of strong EM response, but the shading allows for a clear identification as to which line the response belongs. The measured values, such as the in-phase and quadrature components, can also be presented as contour maps, and some anomaly trends become more clearly visible in such presentation. Compare the contourand profile mapsof the quadraturecomponent (Figures 22b and 23). Color conductivity maps have become the standard output of AEM surveys sponsoredby the Geological Survey of Canada. Figure 24 is a color conductivity map compiled from helicopter AEM data. The survey was flown with the EM bird at an average height of 30 m. Other birds containing two magnetic sensors (vertical gradiometer) and VLF receivers were towed simultaneouslyat a higher altitude. This survey, carried out in 1986 by Aerodat Ltd., was the first of its kind---combinedhelicopter AEM and magneticgradiometer measurements.Joint interpretation of conductivity and magnetic maps presented at the same scale (originally 1:50 000) is useful for geologic mapping. Figures 24, 25 and 26 depict geophysicalmaps of the Monts Stokes area, 25 km northeast of Sherbrooke, Quebec. All three maps (apparent conductivity, total magnetic field, and magnetic gradiometer) reflect changesin lithology. The Monts Stokes massif, which is composed of rhyolite lavas and tuffs (Ascott-

Weedon Formation of Ordovician age), has a higher magnetic susceptibility than the sedimentary formations (shales, siltstones). Conversely, while the conductivity of the massif is low (generally less than 0.5 mS/m), higher values (over 5 mS/m) are typical of the sedimentary formations, particularly shales. As Schwarz et al. (1988) pointed out, numerousanomalies within the known formations, which at present cannot be correlated to known features, contain information that can be used to improve the geologicmap (examples of geologic interpretation of AEM data are discussedlater in a section entitled Survey Examples). For mineral exploration, a simultaneouspresentation of apparent conductivity contours and bedrock anomalies shown as symbols may be more useful. The (Text continued on page 850)


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848

Palacky and West

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850


western

Palacky and West corner

of the Monts

Stokes area is shown in

such presentation in Figure 27. A compositeprofile of one of the lines (2120) was illustrated in Figure 17. Data acquired in the course of time-domain towedbird surveys also can be presented as color conductivity maps and maps of bedrock conductorsand apparent conductivity contours. Figure 28 showsthe results of a GEOTEM survey in the Lynn Lake area in Manitoba, which was carried out in 1988by Geoterrex Ltd. for the Geological Survey of Canada. A major structure can be recognized on the conductivity contours and also from the pattern of bedrock anomalies. Because of low-pass filtering to remove the effect of the transmitter-receiver asymmetry on the calculated conductivity, the conductivity contours based on GEOTEM

measurements

are

smoother

than

those

based on a helicopter AEM survey. The symbols depicting bedrock conductorsare shadedas a function of channel response. They have been mentioned earlier in conjunction with line processingof GEOTEM data and a composite(stacked) profile for line 91 was previously shown in Figure 18. In addition to a letter indicating the position of the selected anomaly along the survey line, three interpreted parametersare given (clockwise):apparent conductance(in Siemens),depth (in meters) and dip (in degrees), all based on interpretation using a response aperture diagram for the dipping half-plane model. Details about how quantitative interpretation is commonly carried out is given in the following section. DATA

INTERPRETATION

Principles

The basic concept that underlies the extraction of geologicinformation from AEM data is a quantitative or qualitative comparison of the AEM observations with the results of modeling which show how the systemwould respondto different earth modelshaving certain conductivity and magnetic permeability. If a resemblance

can be found between observational

and

model data, the actual earth structure is likely to resemble

the

model

structure

in some

discernible

ways.

In the early days of AEM surveys, the model data consistedonly of educated guessesabout the form and intensity of the response; such guesseswere obtained by qualitative physical reasoning guided by scale model studiesof limited extent and theoretically calculatedresponses,and by field data from flightsover a few known geologictargets. Numerical EM modeling has improved markedly over the last two decades(see review by Hohmann, 1988), but even now it is impossible to calculate in a simple way the AEM response over a conductivity structure of arbitrary form. EM

interpretation is far behind gravity, magnetic, and seismicmethodswhere excellent computermethodsof modelingare now widely available and can be used on personal computers or work stations. However, practical quantitative EM modeling methods do now exist for a number of elementary shapes, such as horizontally layered structures,sphericalconductors,and thin plates of various forms. These, plus a few investigations of what happens to system response in more complicatedsituations•for instance, where the target is magnetically permeable or where overburden and host rock are conductive•have provided a sufficient base on which to build the processingand interpretation schemesthat can analyze the data from currently used AEM systems. They may, however, be inadequate for fully interpreting the more diverse and accurate data that forthcoming AEM systems may provide. At the fundamental level, interpretation of AEM data doesnot differ significantlyfrom interpretationof ground EM surveys. However, there are numerous differences in emphasis. As airborne surveys, which are often performed in areas where detailed geologic knowledge is scarce, typically generate large quantities of data, the interpretation work is directed to qualitative identification and delineation of as many geologicfeatures as possible. Until recently, detailed groundfollow-up EM surveysalways were carried out prior to drilling, but accurate positioning, increased resolution, and improvementsin interpretation of helicopter AEM surveys may eliminate this intermediate step in the near future. The principle need in AEM interpretation is for economical methods of analysis which are robust (i.e., are reasonablystablein the face of inevitable data error and mistaken assumptions about earth structure), and can be applied routinely to the measureddata. After completionof data processing, as described in the previous section, interpretation of AEM data consistsof quantitative parameter estimation

for

selected

features.

This

estimation

is

usually done by some form of automated but perhaps interactively controlled model fitting, and generally must take account of correlationsbetween responses on adjacent flight lines. Some features may be reinterpreted at a later stage by detailed interactive model fitting for various geologic hypotheses. AEM Modeling

Model studiesprovide necessaryinformation for the following tasks: (a) designfor new AEM systems,(b) basiscomparisonof the geophysicalcharacteristicsof existing systems, (c) setting of data processingand inversion schemesused in data displays mentioned in Field Operations and Data Processingsection 4, and


Airborne

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851

ANOMALY Conductance

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Fig. 27. Southwest portion of the Monts Stoke survey. Map depicting topograph, (grey), flight lines (blue), contours of apparent conductivity (red) and symbols indicating bedrock conductors (black). Original map scale 1:20 000. (GSC Open File Report 1591).


852

Palackyand West

Bedrock

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contours

Fig. 28. A portionof the GEOTEM AEM surveyat Lynn Lake, Manitoba.Topography is in grey,flightlinesin blue,apparent conductivity contours in red,andbedrockconductor symbols in black.Originalmapscale1'20000. Flyinganddatacompilation wereperformedby GeoterrexLtd. for the Geological Surveyof Canada(NTS map sheet 20365G).


Airborne

(d) basis for detailed quantitative interpretation of selected features. Analytical, numerical, and analog scale modeling techniques still are all employed, although the modern trend is toward computer simulation wherever possible. Except for modeling airborne VLF or AFMAG surveys, where a uniform source field can be assumed, all AEM model responseshave to be calculated for a local EM source which usually approximates a magnetic dipole. This is a significant complicatingfactor as it makes direct numerical simulation by gridding methods much harder. So far, computer simulation has had to rely mainly on evaluating analytical solutions or on numerical integral equation techniquesdesignedfor rather specificcases. Few of the current EM modeling algorithms are simple enough to use interactively as is the case in magnetic and gravity interpretation. Instead, it has been necessary to study a range of simple models a priori, and then summarize the model data in tabulations or graphsof anomaly "characteristics" (such as peak amplitude) versus model parameters (such as conductor depth or size). Comparisonof the "characteristics" of the survey data with corresponding "characteristics" of the model responsespermits estimation of the model parameter values which will, in some sense, make the model data fit the observations. Although this may be sound and a practical approach for the more routine and simpler interpretation tasks, interactive modeling remains a very desirable future objective for interpretation of special cases. Available Models.raThe most important model structures for which computer simulation is possible are: a uniform or layered conductive half-space, a conductive half-plane (thin infinite dike) in an insulating host medium, a conductive and/or permeable sphere in an insulating host, and a conductive thin rectangular plate in an insulating space. Other cases have been treated in research studies but (at the time of writing) the modeling methodologiesemployed are neither easy to implement, nor widely available to interpreters. Generally, to model time-domain methods is more laborious than modeling frequency-domain EM methods because a full spectral frequency response IF(to)] or the equivalent exponential spectral responseIF(s)] must first be calculated

and then transformed

to the

time domain. Because all time-domain AEM systems use a repetitive transmitter current waveform of somewhat complicated shape and short period, to model AEM observations as a pure discontinuity response (step or impulse) is not sufficient.Rather, to convolve the theoretical time-domain response (i.e., the pure discontinuity response) with the transmitter current waveform or its derivative is necessary. Alternatively,

EM

853

the periodic response waveform may be obtained by inverting the product of the transmitter Fourier line spectrum with the frequency spectral responseof the earth. If the receiver system includes any linear filtering before sampling (which is the case of several AEM systems),the convolution or multiplication must be between the transmitter signal(primary signal), as it would be seen by the receiver sampling device if the receiver were left running continuously, and the impulse response of the AEM system to the model (source magnetic moment to secondary magnetic field at the receiver). This is illustrated in Figure 29. If the receiver outputs (channels) are time averages over appreciable intervals, integrating the convolved signals over sampling windows also will be necessary. The effect of the periodic transmitter waveform and the sampling windows is interesting. As described in Macnae et al. (1984) and Becker and Cheng (1988), each channel of receiver output samplesthe theoretical continuousfrequency responseof the EM system through a different comb filter which has acceptance lines at odd harmonics

of the transmitter

waveform.

Weights for the early-time channels concentrate more on the high-frequencyparts of the spectrum whereas weights for the late-time channels concentrate on lower frequencies. Depending on the nature of the primary signal (more pulse-like or more step-like), the channelsweightsmay emphasizethe quadratureor the in-phase spectral response. The calculation of frequency spectral response requires complex arithmetic. If a time-domain discontinuity responseis to be calculated, the complex arithmetic can be avoided by using the Gaver-Stehfest (Stehfest, 1970 a, b) method of numerical Laplace transformation, where only the values of F(s) for real s are calculated, or by expansion of an analytical solution in partial fractions followed by an evaluation of residues at poles to obtain the pure transient discontinuity response directly as an algebraic series. Convolution for an arbitrary source current waveform will then have to be done in the time domain.

Stratified Medium.mComputer programs for calculating the responseof a dipole sourcedipole receiver (small loop) system over a horizontally layered structure are widely available. They are based on well known analytical solutions described in Ward and Hohmann (1988) and take the form

F(h, ,, h) exp [-h(z•

+ z2)]J (•. r)dh,

(1)

where F(X, •, h) contains all the information about the earth model in its parameter vectors and J(Xr) is a Bessel function

of order zero or one.


854

Palacky and West

Integrations such as this which cover an infinite range and in which the integrand contains an oscillating factor such as J (kr) can be difficult. Those arising from diffusive EM problems, where the function F is well represented when logarithmically samped in •, are usually attacked by the convolutionalfilter method (e.g., Koefoed et al., 1972; Anderson, 1979; Johansen and Sorensen, 1979). Fortunately, AEM problems are usually a simple case. Becausethe horizontal distance r between the coils is generally smaller than the combined heights of the transmitter and receiver coils above the earth (z• + z2), the exponential damping term generally dominatesthe integrand at larger values of X, and evaluation of the integrals by standard quadrature methods is satisfactory. Because of the limited ability of most current AEM systemsto resolve even moderately complicatedlayering, simple models, such as a uniform half-space, a conductive layer over an insulating or uniformly conductive medium, or an isolated thin conductive layer, are of greatest utility.

ConductiveSphere.--This classicalproblem in EM theory has been the subject of many papers and also has been reviewed in Ward and Hohmann (1988). Theoretical solutionscan be derived in the frequency or time domain for cases where the sphere is in a nonconductive host medium, to take account of a dipole source, magnetic susceptibility, radial variations in conductivity, and polarizability (frequency dependent conductivity). Although less studied, an Tx

Conductive Half-plane.•AEM surveys over resistive, metamorphic terrains, like the Fennoscandian and Canadian Shields, detect many sinuous conductive bands of metasediments which may be rich in graphite and/or massive sulfides. The earliest used model for such cases was the perfectly conductive half-plane. As for stratified earth, the original analytical solution is the work

CUTTING

MOMENT

I(co)

M(co)

i(,)

re(t)

has extended

The solution

the solution

to include

finite

surface

conductance by using an analytical numerical technique, but his algorithm has not yet been widely used for AEM modeling. FIELDS

SIGNAL

RECEIVER

HP(co) hP(t) FREE

of Sommerfeld.

was put into geophysicalcontext first by Wesley (1958 a, b) and later by Grant and West (1965). Although the solution provides only the response at the inductive limit, this model made possiblethe prediction of many geometricaspectsof response,suchas altitude, strike, and dip dependence. More recently, Weidelt (1983)

MAGNETIC

Tx

CURRENT

infinite horizontal cylinder can be analyzed in an analogous manner (Mallick, 1973). Usable solutions for a sphere in an infinite, uniformly conductive host medium also have been worked out (Singh, 1973), as have solution methodsfor a half-space or layered host medium (Lee, 1975; Thio and Gleeson, 1979). However, only the uniformly conductive and permeable sphere in an insulating host have been used so far in practical AEM modeling(Lohda and West, 1976;Best and Shammas, 1979; Dyck et al., 1980).

TO

MEASURING SYSTEM

H(co) h(t)

DATA

OUTPUT

r(t)

SPACE

(Pri.amplitude

Yl(t)

attenuation) P

Yz(t) ',.-..CN

Hs(CO) hS(t)

EARTH RESPONSE I E (co) e(t)

FREQUENCY

'

DOMAIN

R (co) = RP(co)+ RS(co)

TIME

r(t)

= I(co)Yl(co)Y;,(co)[P+E(co)] R(.•)

I+ I

Rp(co) -• E(co)

:

rP(t)

DOMAIN

+ rS(t)

= i (t)*yl(t),y•,(t),[P+e(t) ]

rS(t) = i(t)*y(t)*e(t)

f_ [•-d FOR A COIL-

i(t )], e(t) COIL

EM

SYSTEM

Fig. 29. Linear systems representation of an EM system showing the convolution effect of the transmitter waveform, system response and sampling scheme on the channel responsesobserved by a time-domain EM system.


Airborne

Laterally inhomogeneous conductive overburden, typically the weathered layer, can interfere seriously with deeper exploration. A horizontal half-plane of finite conductance is a good model for studying the edge response of conductive overburden. Studies of a less abrupt edge can be based on Price's formal solution

for a thin sheet of variable

surface conduc-

tance (Smith and West, 1987), but all algorithms offeredto date are computationallyintensiveand have not yet been widely used. Conductive

Plates.•Several

numerical

schemes

have been developed that allow rapid calculation of the EM induction responseof a conductiverectangular thin plate in an insulatinghost medium to an arbitrary sourcefield (e.g., magneticdipole). They are basedon an integral equation formulation of the problem. The best known of them is Annan's (1974) method, in which the induced current, represented by a stream potential expandedin a polynomial basis, is expressed as a truncated seriesof eigenpotentials.This algorithm has been incorporated into the computer modeling program PLATE (Dyck et al., 1980), which has been widely used in AEM model studies. Several algorithmsfor calculatingthe responsedue to a thin conductive plate situated in a layered conductive host medium have been constructed using different variations of the integral equation technique (Lajoie and West, 1976; Hanneson and West, 1984). They require innumerable evaluationsof Green' s functions for the host medium, each of which involves a nontrivial numerical integration, and therefore are rather laborious to run. However, a few AEM modeling studies have been carried out with them.

EM

Interpretation of Helicopter AEM Surveys

Modeling and interpretation schemesmust take account of the actual configurations and dimensions of the specific AEM system with which data will be acquired. We can, therefore, only give examples of what might be done in practice for data from different systems. In this section we assume that the AEM system is of the rigid-boom, frequency-domain, in-

phase, and quadraturemeasuringtype, with two coil configurations:a coplanar coil pair with vertical axes, and a coaxial pair with axes in the line of flight. These coil pairs operate at close frequencieswhich from an interpretation point of view can be considered the same. If additional coils are used, as is accepted practice today, data can be recorded at severalwidely separatedfrequencies. Because the separation of all the coils is small compared to the flight height, any modestchangein coil separationwould affect only the primary field observedby the receiver. Thus, only the parts per million (ppm) amplitude of the observed anomalies would be altered, not any of their other characteristics.Generally speaking, this applies if the coil separation is less than about a third of the flight height. Systemswith sucha small separationare often referred to as "coincident dipole" or "double dipole" configurations and have the property that the phase componentsof an anomalousresponsedue to simple eddy current induction must always be of the same sign, irrespective of conductor geometry. In the examples given in the following pages, the normal flight height above the ground surface is 30 m, and the separationof all coil pairs is 10 m. Isolated Local Conductors.--To

Other Models.•The foregoing list is intended to describe those cases for which modeling algorithms are fairly readily available and can therefore be used to build up a basic interpretationcapability for any controlled source AEM system, or to compare the response characteristics of different systems. Although not treated here, scale model studies have been a

standby in the past and are still the easiest way to study some problems. The subject has been reviewed by Frischknecht (1988). Extensive sets of scale-model data were compiled by Ghosh and West (1971) for frequency-domain AEM systems and Palacky (1974) for time-domain AEM systems. The research literature abounds with many theoretical and some numerical studies that at least in principle bear on AEM interpretation. However, to adapt the results to the actual geometries and frequencies or sampling time windows of current AEM systemsso they can be used in quantitative analysis is difficult.

855

illustrate

the basic

geometricaspectsof the anomalousresponseof a local conductor in a resistive (unresponsive) host medium, anomaly profiles of the system responseto a rectangular plate oriented vertically or horizontally and to a spherical conductor are shown in Figure 30. In the plate model, the induced current vortex can flow only in one plane, whereas in a sphere it can have any orientation. All the anomalies shown have even symmetry along the line of flight, but the spherical conductor producesanomalieswith a central peak in both the coaxial and coplanarcoil systems,whereas a plate produces anomalies which generally have a central peak in one coil systemand a central minimum point in the other; a vertical plate gives a central peak in the coaxial coils, whereas the horizontal plate gives a central peak in the coplanar system.Dipping plate-like conductorsproduce asymmetric anomaly profiles with a different symmetry recorded by the coplanar and coaxial systems (Figure 31). There are also major

856

Palacky and West

differences in the relative amplitude of the anomalies recorded by the two coil systems. The geometric form of the anomaly profile is not only dependent on the form and attitude of the con-


ductor, but also, in the case of a horizontal conductor,

on its dimensions along the flight axis. This is illustrated in Figure 32 where responseprofiles over horizontal plates of various widths are shown. The coplanar coil system always gives a simple, single peak anomaly whose width and amplitude reflect the width of the conductor, but the coaxial system gives a more complicated profile. The current induced in the sheet, when coaxial coils are symmetrically located over it, must have a double vortex (figure of eight) configura-

tion, whereas only a simple vortex need be present when the coils are at the edge of the plate. A single current vortex is more easily establishedin a narrow, poorly conductivebody than is a double one, therefore the responseis reduced in amplitude and phase over the center. Over wide conductors, "shoulders" are producedat the edges.As the sheet is made wider, the central responsefrom either coil system convergesto that of an infinite

sheet.

The above examples show that changes in conductivity or frequency affect mostly the phase ratio and the overall amplitude of response, rather than the profile geometry. Changesin the conductor depth and dimensionsand in the strike angle of the flight profile may have a strong effect on the response amplitude, but only a lesser effect on phase ratio and geometry. In

600[

•oo[1 • •øø oII • bJ

C+

•

•P

IOO

O

Lu 200 I 0 0 ' •'

>_ IO

•

•

- 200

o 03

STRIKEEXTENT 25Om

SPHERE

STRIKE EXTENT 2:50 m

•Z

T 125m

o •

e S•IKE EXTENT250m

,oor .....

• IOta • 50[

øL ' •

:

Fig. 30. Helicopter AEM profilesover isolatedconductors of differentshape:a horizontalplate, a sphere,and a vertical plate. For each model, in-phase (IP) and quadrature (Q) anomaly profiles are shown for three frequencies:20 kHz (top), 2 kHz, and 0.4 kHz. Note the differencesin the form the coaxial and coplanar coil profiles in the three cases. All

Fig. 31. HelicopterAEM profilesover plateconductorswith differingdips. For each model, in-phase(IP) and quadrature (Q) anomalyprofilesare shownfor threefrequencies:20 kHz

traverses are over the center of the conductor and normal to

(top), 2 kHz, and 0.4 kHz. All plateshave a conductanceof

strike. The plate conductorshave a conductanceof 10 S and the sphere has a conductivity of 2.5 S/m. (Courtesy of

10 S. Note the relatively weak responseof the vertical plate relative to a horizontal one; also the strongerresponseof the horizontalcoplanarsystem.(Courtesyof RichardS. Smith).

Richard S. Smith).

50


Airborne

such cases, the amplitude and phase ratio effects are most easily described by plotting peak amplitudes of anomalies in phasor form (on linear or logarithmic scales)as a function of conductivity (or frequency, or responseparameter) and the other variable under test. Figure 33 shows the well-known coaxial-coil, peak response for a vertical half-plane as a function of conductanceand depth-to-top, plotted logarithmically. Variants of this responseaperture diagram drawn up for different kinds of AEM apparatus have been keystonesof anomaly interpretation since the 1960s.The diagrams allow the determination of conductor depth and conductance. Figure 34 shows responseaperture diagrams for smaller plates with different geometric factors. The following symbols have been used to denote dimensionsof the plate and the system' H flying height of the transmitter-receiver,D - depth of the top of the plate, W- depth extent of the plate, S - strike extent of the plate, t - thickness of the plate. In the diagrams, all dimensionsare normalized to the coil separation L. In Figure 34 and subsequent illustrations, interpretations are made for dimensionless responseparameter a rather than plate conductance. a is a product of conductivity •r, magnetic

1600[-•lorn

•

EM

857

IN- PHASE (ppm)

Fig. 33. Peak anomaly amplitudes are plotted for a vertical half-plane conductoras a function of plate conductanceand depth to top. Response aperture diagram for a helicopter vertical coxial system on a central profile normal to strike (after Gosh, 1972).

?t

•L'---• I

103 1..(a)

(d) -C'"' -C-'" H

S•-2(H+D)

Y

•

z

o

4øø1-

oI

::::)•oo

o

(fi+œ) /L -- 12

161 id I

IO0

IOI

IO2

IO3

IN-PHASE (ppm) --15Oral'.-

--.I lOOmI.--

--t

200m

I--

iO3 STRIKE EXTENT

z

o

( c)

200m

400[ -••Om •-

O:

s-.-

i

( b)

:

s.=•(H+D)

ß

ß

....:: ....

.....

........

,:••g.A:,% "'

• IOI

r•

•

ioø (H+D)/L

il2

'

(H + D)/L = 12 '

:

ß

.

ß

ß

16•

i(•I

i0ø

i01

102

103

IN-PHASE (ppm)

Fig. 32. Helicopter AEM profiles over flat lying plates of differingwidth. In-phase (IP) and quadrature(Q) responses are shown for frequenciesof 20 kHz (top) and 2 kHz. The plates have a conductance of 2 S. The vertical coaxial system gives a much smaller response than the horizontal systemand shows strongeredge effects. Narrow conductors give not only narrower but much weaker anomalies than wide ones. Also, their anomalieshave a lower IP/Q phase ratio. (Courtesy of Richard S. Smith).

.

........ i ........ i ........ i .......

i0ø

i01

102

103

IN-PHASE (ppm)

STRIKE 4 = 90 ø , DIP O= 90 ø, W=4(H+D)

Fig. 34. Response aperture diagrms for a vertical coaxial system on a central profile normal to strike (a-c). Peak anomaly amplitudesare plotted for vertical plate conductors of different size as a function of plate conductanceand depth to top (after Ferneybough, 1985). Geometry of the system and the conductor, and the responseparameter a are given in (d).


858

Palacky and West

permeability Ix0, angularfrequencyco,and two dimensions (thickness t for the plate models and sum of the flight height H and depth D). If a model has more than three important parameters, the number of aperture response diagrams needed to characterize the peak in-phase and quadrature anomaly amplitudes (not to mention any other anomaly characteristics) as a function of all the model parameters can become unwieldy. However, some of the geometric parameters usually act more or less independently of the others, especially of the model conductivity, and sometimesfinding simplemathematical formulas that representtheir effects, at least over reasonable ranges of the other parameters, is not too difficult. Examples are shown in Figure 35 for the effect of strike angle on the response of a coaxial system to a dipping plate model, and in Figure 36 for the relative strengthsof the two peaks of the coplanar system anomaly of a dipping plate as a function of dip and depth. Coupled with a few tabulationscorresponding to the graphical responsediagrams, such formulas can make it practical to describe the main response characteristicsof an AEM system to a given model as an algorithmic function of model parameters. The algorithms then serve as the basis of simple inversion schemes where the model responses may be fitted automatically to the survey data. The method of "characteristics" (Grant and West, 1965, West and Bailey, 1989) can in principle provide direct inversion of field data to model parameters as long as the chosen model has the same or a fewer

EFFECT OF DIP AND STRIKE ON PEAK AMPLITUDE

2.o

number of parameters than there are reasonably independent and measureable characteristics of the data. In general, n well chosen characteristics of the data can be inverted to n model parameters, but there may be some regions of the data (characteristic) space, where there is no corresponding model, and some, where a point in the data space correspondsto more than one model or to a range of models. In general, the inversionmust proceedfirst by narrowingthe rangeof some of the less critical parameters by using information not found in the principal response profiles. Examples are strike angle and strike extent for a plate model or offset from the principle profile for a sphere, parameters that can be estimated or bounded by examiningthe correlation between profiles. Then, the rest of the model parameters can be estimated from the characteristicsof the principle responseprofiles or, if necessary,by hypothesis.Of course, the first task will be to decide which, if any, models are appropriatefor fitting to the data. It will often be necessaryto separate the responsesattributable to a local conductor from a backgroundresponsefrom conductive overburden.

MagneticBodies.•If a geologicformation has a high magnetic susceptibility, but a relatively low conductivity, no eddy current will be induced in it, and an AEM system will register an appreciable in-phase response.The systemacts like an in-situ susceptibility meter. In a coincident loop AEM system, magnetic response has the opposite polarity to a conductive responseand is therefore easily recognized (field examples in Figures 19 and 20). Bodies having sufficient susceptibilityto produce a visible magnetic response will also produce a strong anomaly in the total magnetic field, but the AEM and magnetic responseswill usually reflect different aspectsof the causative body.

EFFECT OF DIP ON ANOMALY

I02,•,

,

,

t3 •F• '"•,,.• ',."©.. 0 '.. --• •

I0

¾ z 0

o.5

96=50ø a =1.59

0 30 ø

• 45 ø

60 ø

75 ø

90 ø

DIP (8) Fig. 35. Relative anomaly peak amplitude of the vertical coaxial response to a plate conductor as a function of conductor dip and strike. Profiles cross above the center of the top edge of the plate (after Ferneyhough, 1985).

L

•"

•

'..

o

•: i0o:50 ø

'-..

•'..

•.• I

45 ø

, •

'•"• '...

SHAPE

I

60 ø

L=lorn

H+D=40m

-- H+D= 80m

.-.-H,D--/•O,• s=.•oom :_

w=/5o r•--

•'-:'e.

"•]•'• 75 ø

90 ø

DIP (0)

Fig. 36. Relative amplitude of the two peaks of the horizontal coplanar coil response over a dipping conductor at different depths. (Central profiles normal to strike.) An exponential relationship represents the data adequately in the range 30-90 degrees (after Ferneyhough, 1985).


Airborne

The magneticanomalymay be causedpartly by remanence. Its induced componentis excited by the relatively uniform geomagneticfield, in contrast to the highly localized primary field of the AEM system. Thus the magneticanomalyprofile is likely to reflect a deeper portion of the causative structure than the AEM response.Determinationsof susceptibilityfrom AEM data may be of value in appraisingiron ore deposits(Fraser, 1973, 1981;Seguin,1975). Stratified Medium.--The responseof coincident dipole AEM systemsto horizontal layering is particularly easy to evaluate because the Bessel function in the equation(1) has a vanishingargumentand it drops out of the integrand or can be representedby a few terms of a seriesexpansion.An interestingfeature of the coincident dipole response of any horizontally layered half-space is that horizontal and vertical axis coil systemsyield identicalresponseexceptfor a scale factor (Fraser, 1979; Mundry, 1984). The anomaly amplitude(in ppm) from the vertical coplanarloopsis half the amplitudefrom the horizontalcoplanarloops; that of vertical coaxial loops is one quarter. This is perhapssurprising,in view of the markedly different responsesthe two coil systemselicit from many kinds of local conductors.This relationshiphasbeen usedto distinguish the responses of local conductors from layeredresponses.A trace hasbeen incorporatedinto multicoil helicopter AEM systems to display, after appropriatescaling,the differencesin the responseof horizontaland vertical coil pairs (Fraser, 1979). Although modeling of layered earth responsesis very straightforward, there is no single optimum methodof inversionof AEM responsedata to layered earth parameters. The one selected necessarily dependson the explorationtask andthe amountandtype of informationthe chosenAEM systemcan provide. In mineralexplorationand geologicmappingsurveys, all datawill be invertedwith the understanding that the obtainedparameters(conductivity,thickness)are in most situations only apparent. The parameters obtained by inversionmay be subjectto further qualitative interpretation.In groundwatersurveys,the horizontal layering assumptionis often stratified and the interpreter desires to obtain as accurate information on layering as possible. A transformational approach to inversion can conveniently be based on a half-space(or thin layer) modelwhich, havingonly two parametersand a very simpleform, can be fitted to any in-phaseand quadrature responseof a coincidentdipole system.Figure 37 showsthe modeldata. The transformationwill give an apparentconductivity(or conductance)and an apparent depth, irrespectiveof the actualcauseof response. It may be applied independentlyat each frequencyto

EM

859

the observed(or slightlysmoothed)data profileson a point-by-pointbasis. Usually, it is applied only to horizontalcoplanarcoil data. It canbe appliedequally well to data from other coil orientations, but the

resultsare lesslikely to be directlymeaningfulwhere there is much lateral variation in conductivity structure. When the in-phaseresponseis in the noise level, either the apparentconductivityor the apparentdepth mustbe restrictedto a (lower or upper)boundin order to havea stableresult.Wherewarranted,comparisons of results at different frequencies can lead to an interpretationof layering,for instanceby followingthe proceduresof Mundry (1984)or Sengpiel(1988). A direct model-fittingapproachto inversionby way of the least-squareserror criterion or any similar method based on the generalized linear inverse of a matrix may be appropriate where there is a priori knowledgeof the possibleearth layering. An advantageis that the effectof data error and model ambiguity on the outcomeis easierto track. It allowshandling of problemsof over- and under-determination by the standardmethodsof inversetheory (West and Bailey, 1989). In current practice, few attempts have been madeto fit modelsmore complicatedthan one conductive layer over a half-space,and even for this case, one of the parameters(suchas the depth to the top of the layer or the lower half-spaceconductivity)is often set from non-EM considerations.A four-parameterearth

O 102 30m

/ -

O

10ø

ß

//////•/////// HALF SPACE

.i, HOR. SHEET ........

iO-2

iO-I

iO0

I0 [

IO2

RESPONSE PARAMETER (ou/.z.o-r 2or cu/.z.o-tr)

Fig. 37. The responsesof a helicopterAEM system to an infinite horizontal sheet and a half-spaceconductor. Note that the half-spaceresponseundergoesa slower transition from resistive

to inductive

limits than the sheet. The curves

are scaled to a horizontal coplanar system with 10 m separation but fundamentally are dependent only on the dimensionlessresponseparameter.The axes can be appropriately shiftedfor other cases(after Mundry, 1984).


860

Palacky and West

model should, in principle, be appropriate for data from a two-frequency AEM system but, in reality, the data provided by one specific system can constrain only all four parameters over very limited parameter ranges. For wider ranges, more frequenciesor more a priori constraintson someof the model parametersare required. Paterson and Reford (1986) used an iterative leastsquares inverse modeling method stabilized by the SVD (singular value decomposition)matrix inversion procedure. In their example, they used three-frequency helicopter AEM data assuming a layer and half-space models (4 parameters). Six values, which were used in the inversion for each location, reduced

the sensitivity of the interpreted model to leveling errors in the data. Another advantageof this approach is that error ranges are determined for the model parameters. The method is, however, substantially more computationally intensive than the simple table look-up procedure that is usedin an evenly determined (n data, n parameter) inversion. An evenly determined formulation of an inverse problem is just an algorithmic implementation of the graphical method of "characteristics". The sought after set of model parameters is the solution of a nonlinear set of equations rn(p) - d = 0, where rn, p, and d are, respectively, each n dimensionalvectors of model data, model parameters, and observed data. The vector of functions rn(p) is computed either by interpolating tables of precomputed models or by direct computation of the layered earth responseintegrals. Standard numerical methods of root finding may then be applied to obtain p from d. This approach has been followed in inverting data from hydrographicand ice-thickness surveys, where a layered model is likely to be an accurate representation of the real conditions over most of the survey area (e.g., Won and Smits, 1986; Holladay et al., 1986). Inversion of multiparameter EM data to the parameters of a multidimensionalearth model requires accurate data. The results can easily be upset by modest systematic errors that are not obvious when one examines the data qualitatively. Examples in addition to the well-recognized problem of leveling are poor calibration of amplitudes, slight mis-phasing of the in-phase and quadrature components, and tilt of the EM coil system. If radar altimetry is employed to set one parameter of the model, its error (possibly arising from poor calibration, tilt, early reflection from trees and hillock tops, or overpenetrationin dry soilsor ice) may have a detrimental effect on the overall result, particularly since the AEM response has a cubedependency on altitude.

Discrete

Conductors

under

Conductive

Overbur-

den.mWhen local anomalies are superimposed on a more widespread regional response, an interpreter's likely reaction will be to strip the "local" response from the "regional background" and apply interpretation techniques to the residual anomaly, such as previously described for isolated discrete conductors. However, the effect of the overburden on the residual

local response may be significant. If the regional response is due to a conductive overburden and the local feature lies in a resistive host medium beneath it,

the overburdenwill act like a low-passfrequency filter and will shift the phase and reduce the amplitude of the local residual response from that which would be observed in free space (Lowtie and West, 1965). Ferneyhough (1985) has shown that the filtering effect can be accurately estimated for AEM systems,at least by calculating the frequency-dependent phase shift and attenuation of the magnetic fields produced by dipole sourcesat the transmitter and receiver locations at a representative point in the middle of the local conductor. An example for a vertical plate is shown in Figure 38. Thus, if the conductanceof the overburden can be estimated from the regional background responselevel, the value of the filter transfer function due to the overburden

can be estimated.

Then

the

80-

60

•

2o

::3 O'

o

•

-;;:)0

,.o_, H+D=40m L=9m

'

ø / -i +=FILTER (/o-'/.Z.oCO O'otH

(/p=/.t•p t(H+ D) e= SCALE MODEL

-

I Tx

I Rx

ß

80

..... 6'0 8b

IO0

' I,0 ' IO'

IN-PHASE (ppm) Fig. 38. The effect of a conductive overburden on the local helicopter AEM responseof a vertical plate conductor. Note that it is mainly a phase rotation for modest overburden conductance(after Ferneyhough, 1985). The symbolspreviously have been described in the text (Figure 34). The

response parameters of overburden and plate,% and Otp, respectively, are defined in the figure.


Airborne

observed local residual response amplitudes can be correctedbefore free-spaceinterpretation methodsare applied. Without the correction,significantsystematic errors in estimatingthe conductivity or conductanceof the local feature may be made as well as errors in its depth or size. An uncorrectedresidualanomalywill be uninterpretable if the polarity of the quadrature responsehas been inverted. Separationof a responseinto "local" and "regional" componentsis far more complicatedthan in magnetic or gravity surveys. In general, one wants the regional responseto be that which would have been generated by the overburden and bedrock of the region, if the conductivity that produced the local feature were not present. Thus defined, the regional response most likely will not be smooth along the profile and the stripping problem becomes one of separatingtwo different anomalousresponses.When the objective of a multifrequency survey is to locate bedrock conductors, which may be confused with local variations in the near-surface structure, an appropriate strategy may be to use the high-frequency data to define the surficial regional responseby local fitting of a simple layered model and then to extrapolate that response to lower frequencies where the responseof bedrock conductorswill be more evident. The extrapolatedresponsethen definesthe "regional background" for strippingpurposes.Examples of the recognitionof discrete conductorsunder difficult conditions were given by Freneyhough (1990).

EM

861

conductor thanon a poorone(e.g.,ap = 60).Thus,a small poor conductor might be misinterpreted as a large good conductor, but a good conductor is less likely to remain unrecognized. However, when such stronghost conductivity effects are present in the local anomaly, the direct responsefrom the host medium also will be strongand easy to recognize, therefore the interpreter shouldbe aware of the possibleproblems. With helicopter AEM systems the total response should remain in the first quadrant even if the residual response of the local feature shifts to the fourth or even the third quadrant. Nevertheless, unless the regionalbackgroundresponseis a smallfraction of the local responseamplitude, the interpreter shouldanticipate that appreciable host medium and overburden effectsmay be presentand try to take accountof them in the interpretation of local features. Since overburden and host medium responseusually attenuate very rapidly with decreasingfrequency, the most practical approach often will be to restrict application of those interpretation methods that are based on discrete, isolated conductor models only to the lowest frequency data, or to check for consistency between interpretationsat different frequenciesbefore accepting parameter estimates.

Interpretation of Time-domain, Towed-bird AEM Surveys

resistivehost(e.g.,ap = 2) andthefilteringeffectcan

Time-domain EM systems intrinsically provide wide-band spectral coverage for each transmitter-receiver coil geometry. In the standard towed-bird configuration, only one transmitter loop is possible. The receiver, which may be multicoil, is located at a substantialdistance from the transmitter compared to the flight height.As a result of the lower altitude of the receiver, the response profile for a body which is symmetric along the flight direction is asymmetric. Unlike coincident-loopsystems,there is no limitation on the polarity of the inductiveresponse.Both factors make quantitativeinterpretationmore complicatedfor time-domain, towed-bird surveys. By necessity, the interpretationmethodologyis more closely tied to the characteristicsof a particular AEM system. Thus we can only give examples in this paper, and we shall confine ourselves to studying the INPUT, GEOTEM, and QUESTEM systems,which use half-sine primary pulse and record output in the form of channels(6 or 12). Input waveform is depicted in Figure 8. All three systemsroutinely use one receiver-coil configuration (horizontal axis dipole), which is sometimes called

lead to a larger rotation in phase; but proportionally there will be a smaller effect on the responseof a good

been used only sporadically.

Discrete Conductors in Conductive Host Medium.--If

the regional response is due to conductive material, which is in good contact with the local feature (e.g., conductor embedded in conductive host), all or a part of the local responsemay be producedby channeling of regionally induced currents rather than by direct induction in it the local conductor. Also, there may be filtering effects on deeper aspectsof the responselike those caused by an overburden. Modeling and interpretation of channeling effects is relatively difficult becausethe EM responseis sensitive to the detailed nature

of the electrical

connection

between

the host

medium and the local feature, and this can be quite variable in nature. However, some idea of the type and

possiblemagnitudeof the effects can be gained from consideringa vertical plate conductor in a conductive half-space. In the example in Figure 39, the channelingeffects of the host half-spacecan lead to more than a doubling of the responseamplitude from a poor conductorin a

"vertical"

receiver.

The

vertical-axis

receiver

has


862 Palacky and West

o


Airborne

Isolated Local Conductors.--To a first approximation, the responsefrom a simple conductive feature will have the same profile form on each channel, with the relative profile amplitudesdecayingprogressively from channel to channel in accord with their respec-

EM

863

tive samplingdelays. Figure 40 shows the configuration of a towed-bird AEM system and geometric characteristicsof a dipping half-plane. The symbols explained here will be used in subsequentfigures. Palacky and West (1973) summarizedthe relationship between channel amplitudes and model conductance for the INPUT waveform in response aperture diagrams.Figure 41 showsone suchdiagramfor a vertical half-plane and the horizontal dipole receiver (Hx). In manual interpretation, peak amplitudes for all six channels are plotted on a transparent paper and matchedwith the diagramcurves.Today, this cumbersomeprocedureis replacedby computercurve matching (DeMoully and Becker, 1984). While the response decay rate decreasesmonotonically with increasing model conductance,the responseamplitudereachesa maximum over a limited range. To study the geometryof responseprofileswe need examine only one profile for each model. Response profilesfor plate conductorsof differentdip are shown in Figure 42. The profile may have one or two major peaks, and the conductor is not necessarily located directly under a peak. The largest response is observed for horizontal plates (0 = 180ø).As alternate

TRACE - Y ••TxHz-• FLIGHT PATH Ti CONDUCTOR TRACK

Fig. 40. Configurationof a towed-bird AEM system.H x is horizontal-axisreceiver(standard),H 2 vertical-axisreceiver (seldomused). Geometriccharacteristicsof a dippinghalfplane of conductancecrt.

VERTICAL COMPONENT

-200 -ioo

o

ioo 200 300 FLIGHT TRACK (m) _

o

•

-5

NSE

•

I

/ 2

I

I

I

I 5

IIIII

I 10

/ 20

/

/

! il//, 50

\ \

-.

•..-18o o -15

•

lO 3

/

100

/

/

/ /

"/

/

'

/

/,'

2

•oo •oo ,oo •oo

øz 0

PLATE MODELS -I0

$= 300m ,W = 150m

/8o o

102

8= = VARIABLE ,• 20 = 90S ø D 30 m,crt =

5

lO

I

-•oo•,•::•ooo

o

5

/

HORIZONTAL COMPONENT

h

10 4

10000.

/ /

c.•E• •O_•ES •=lOOm..=I•On,,V=•On,

10

•

1000

/ /

5

10

o t

20

50

100

(s)

Fig. 41. Responseaperturediagramfor the INPUT system and a vertical half-planeconductor(after Palacky and West, 1973). The peak anomaly amplitudesare plotted on a transparent paper usingthe scaleon the right. The points are then translatedto fit the respectivecurvesand the positionof the

105markwill indicate conductance anddepth.

Fig. 42. Comparisonof time-domain,towed-bird response profilesof plate modelsof differentdip. (Symbolsspecified in Figure 40, flight direction from right to left.) Only the horizontalcomponentis measuredroutinely by the systems in operation.Vertical scalein ppk (partsper thousand).Note the doubleor triple lobedform of someof the profilesand the lower sensitivityto a steeplydippingplate than to a shallow dippingone; also that the peak responseis not always over the conductorupper edge (data plotting point is under the receiver coil). (Courtesyof Richard S. Smith).


864

Palacky and West

may arise between different versions of the same apparatus. Ferneyhough (1985) has suggesteda way of representing families of model decays where one model responseof the family is selected as a reference case. Instead of fitting a power law or an exponential directly to the channel response amplitudes versus mean channel delay time for each model case, he substitutesthe reference model amplitude responsefor the delay-time axis. Thus normalized, the channel amplitudesR of all model responsesobey a power-law relationship to one another to a good approximation, and the parameters of the powerlaw are less dependent on details of the AEM system. For convenience in plotting families of normalized decays on one set of (logarithmic) axes and for tabulating the model parameters, ordinate and abscissa quantities also are both divided by the channel 3 reference response. An example of the approach applied to vertical plate models is shown in Figure 44. This approach can greatly reduce the task of tabulating model data for routine computerizedinterpretation schemes.Ferneyhough has shown that characteristic response diagrams showingthe relative variation of responsewith

flight lines are flown in different directions to aid dip determination, adjacent recorded profiles may differ substantially even if the conductive feature is uniform along strike. Responseprofiles for sphericaland plate conductors are compared in Figure 43. Even when data from horizontal and vertical coils are available, some cases

are not easily distinguishable. An important factor to consider is the conductor depth. A typical time-domain, towed-bird AEM system can detect large bodies buried several hundred meters, and the response profiles for such deep features differ in more than just amplitude from those of shallow bodies.

DecayCurve Analysis.--Several attemptshave been made to fit empirical functions to response decay curves, either those observed in the field or computed for models (e.g., Palacky and West, 1973). Simple two parameter representations, such as exponentials or power laws, are tempting, but they do not adapt automatically to account for variations in the form of the time decay causedby differencesin the transmitted waveform or the sampling scheme, differences that

INPUT RESPONSE PROFILES (L= lOOm,H = 120m,V=70m) (6 CHANNELS)

HOR.

-lOOm I

I

0 •

•

lOOm •

I

-lOOm I

0 •

•

COM

lOOm I

I

0 •

•

•

I00 •

•

200m I

•

D=Sm (• 1.75 D=30m •0 S 20••SD=30m S/m a=45m

: 300m

W= 150m

S:300m • W=150m

Fig. 43. Comparisonbetween time-domain, towed-bird responseprofiles of plate and spheremodels. (Symbols for the systemgeometryand half-planemodelspecifiedin Figure40. D--depth of sphere,a--its radius.)Even with data from orthogonalreceiver coils it might not be easy to distinguisha sphericalconductorfrom a shallow dippingplate. (Courtesy of Richard S. Smith).

Airborne EM


changesin depthandconductance for differentsizesof plate are all very similar to one another, the differences associated with plate size being mainly expressedin the referencemodel responses.

I

i

I

I I I II I

I

I

I

I I III1

I

I

I

i

l lll. _

_

H • D = 120m

........

H • D = 220

ß crl

_

m

= •$

ß crl = lOS• ßcrl =50S

_

6}"

S=900m

•.•'" of'

fact that calculation

of

stratifiedearth responsesfor a time-domain, towedbird system is harder than for a helicopter AEM system,seemssofar to have inhibitedroutinequantitative interpretation of layering from INPUT, GEOTEM, and QUESTEM data. Althoughthere is no fundamental difference between the systems in how

PEAK CHANNEL RESPONSE Ai

ioI

Stratified Medium.--The

865

o

•l[" '?'

Z

....•:'

ß

_

ß

the problemmightbe approached,there are practical differences.Helicopter AEM data sufferfrom a baseline drift, which is particularly significanton the inphasecomponent.Time-domain,towed-birdsystems do not have this problem, but the data are often affectedby bird swing(see Field Operationsand Data Processingsection). Therefore, the relative channel amplitudesare more reliable data for inversionthan the absoluteamplitudes.Routineconversionof survey data to a half-spaceapparentconductivityand apparent depth is possiblein principle by determininga decayrate andamplitudefor any two or morechannels of data. Examples of layered earth interpretations usingcomputerdiagrammatchingwere given in DeMoully and Becker (1983), O'Connell and Nader

ß

(1986), and in Zollinger et al. (1987), the last for ocean I

I0 -2

I

I I•lll

I

•

I'111•11

-•

I0

IO

I

I

I

I I •

bathymetry surveys.

0o

Ri/R 3 (a) PEAK

ANOMALY

RESPONSE

I.O

• = 90o,•=90 o

-

37= 900 m, •= 450m

0.5

..2=•t {s;

• o •l•1-

• •- •5 C•SE -

o

•

_

R/=Z958ppm •'

20 G

-0.5_R•=//7/

-•

_-

&=545 •20 270 220/70 /20=H+P{m)R5=247 _

- I 0 ,

-4.0

• •

=/o• ,•1,•,,I,, -3.0

_ t•l,,,, -2.0

-I .0

I,,,, 0

.0

LOG o

(b)

Fig. 44. Relative responseamplitude obeys a power-law relationship for several vertical plate models. (Symbols specifiedin Figure40). The fit to the decaycurveis shownin (a); the relationship of the power-law parameters to the modelparametersin (b). The exponentn is a directindicator of the relative conductance of the plates and is nearly independentof other parameters.This displayis an alternative for representingmodel responseto the responseaperture diagram of Fig. 41.

Inversion techniquesare valuable when a continuousestimateof parameters,suchas layer conductivity and thickness,is required. Huang and Palacky (1991) proposeda dampedleast-squaresinversionmethod with singularvalue decomposition(SVD) to map overburden. Assuminga two-layer model, they estimated overburden thicknessand resistivity from survey data acquiredwith the Chinese-madeM-1 systemwhich resemblesthe INPUT. Barongo (1989) applied a similar techniqueto INPUT surveydata from Kenya and demonstratedthe usefulnessof such approach to geologic mapping in tropical regions.

Effect of Magnetic Bodies, ConductiveOverburden, a time-domain, towed-bird system recordschannelamplitudesonly in the transmitter off-time (as INPUT, GEOTEM, and QUESTEM do), it is insensitive to nonconductive, magnetic bodies becausethe induced magnetic field due to the susceptibility effect is presentonly at the sametime as the and Host Medium.--If

primary field. Therefore, there is no analog of the reverse-signin-phaseanomaliesthat are seenin helicopter AEM systems. As Palacky (1975) demonstrated,polarity reversalsin local residualanomalies are generatedby current channelingin conductive overburden. They affect particularly the responsesat

early delaytimes. Similarresponsesalsocanbe dueto current channeling in the host rock (Ferneyhough, 1985). The effects of conductive host medium on time-domain surveys are demonstratedin Figure 45.

866

Palacky and West


Because

of the enormous

transmitter

moment

and

large transmitter-receiver separation, time-domain, towed-bird systems are capable of achieving a great depth of penetration. However, the potential for detailed quantitative interpretation data has not yet been as fully exploited as in helicopter AEM systems.The advent of new digital systemswith multicoil receivers and different transmitter waveforms and sampling schemes coupled with a resurgent interest in base metal exploration should bring new development in the technique, which in the 1970swas the most widely used mode of AEM surveying.

SURVEY

EXAMPLES

that set them apart from the surrounding geologic formations. In geologicmapping, all AEM signalshave to be explained in terms of lithology and structure. When mappingQuaternary sediments,the emphasisis on determining their composition and depth to bedrock and the underlying resistive geologic formations may not be considered in interpretation. For the last application describedhere, shallow-oceanbathymetry and sea-ice thickness determinations, EM results may be presented as vertical sections or contour depth maps. Present AEM capabilities permit a variety of other applications, which, for the time being at least, are less common. Many such applications were described in Palacky (1988).

Prospectingfor Volcanic-associatedMassive Sulfides

Processingand interpretation of AEM data varies accordingto the survey objectives. From this point of view, applications can be divided into three broad categories: mineral prospecting, geologic mapping, and bathymetric charting. In mineral prospecting,particularly for volcanic-associatedmassive sulfides, targets are usually discrete bodies with electric properties

AEM systemswere developed primarily as a prospecting tool for massive sulfide deposits in resistive environments, like the Canadian and Fennoscandian

PrecambrianShields. Perhapsthe best example of the exploration strategiesbased on AEM surveys in the 1950sand 1960swas one pioneered by the Mattagami Syndicate (Paterson, 1970). The prospecting sequence

10 4

INPUT RESPONSE OF AVERTICAL

PLATE IN A UNIFORM MEDIUM

'"I'"hOST

•

103

HOST

D- 50rn

S=600rn •

ß ,,..,}

102 _

W =450 rn

la.I

Crh=5m$/m I

tl

I

I

I

I

I

i0 4 crh: 20mS/m

Oh: I0 mS/m

/ •

'-.•

103

%t: 2s

•

: lOS

$

= 50S

o z

io 2

IO

o

I

I

I

0.5

I

I.$

| 2

MEAN CHANNEL DELAY TIME

I

I

I

0.5

I

I.$

• 2

(ms)

Fig. 45. Amplitude of time-domain, towed-bird responsefrom a vertical conductive plate in a conductive

half-space, fordifferent plateconductance %t andhostconductivity (rh. (Symbols specified in Figure40).Thehost responseis indicatedby dottedline, the strippedresponseof the local conductorby solidline. Strippingis explained near the top of the figure. Note the reversal of the early time local responseand the addition of a channeling component to the local anomaly which decays like the host-medium response.

Airborne


consisted of AEM coverage, selection of targets for

follow-up, ground geophysicalsurveys, and drilling. Each step in the sequence was a major decisionmaking point resulting in selection of targets. The reduction ratio was approximately 10:1 at each stage. Perhapsthe most critical elimination processwas the first one, the selection of AEM anomalies for follow-

up. Generally accepted selectioncriteria were based on survey experiencerather than on thoroughquantitative geophysicalanalysisof AEM signatures.As the equipmenthas improved and AEM surveyshave been applied to a greater variety of terrains, new approaches have evolved. Two previously published case histories

are discussed

in this section.

In both

instances,AEM surveysled to a discovery of massive sulfides,even thoughthe responseof the target did not meet the classical"Mattagami" criteria. The first case history, Detour deposit in Quebec, is typical of AEM applicationin the CanadianPrecambrianShield. The secondexample, Itapicuru greenstonebelt, describes lithologic interpretation of AEM data using statistical methods. This latter approach based on concepts developedin geochemistrywas foundeffectivein parts of the world where conductive saprolite was not removed by glacial erosion and scouring.

Detour Polymetallic Deposit, Quebec.--Many of Canada's polymetallic mines are located in the greenstone belts of Manitoba, Ontario, and Quebec. Volcanic-associated massive sulfide orebodies are usually much more

conductive

than unweathered

host rock

thoughsomeexceptionsexist (Palacky, 1988). Most of the more accessible greenstone belts have been surveyed more than once using state-of-the-art AEM instrumentation. Reed (1981) described the discovery of the Detour zinc-copper-silver deposit, which is located approximately 100 km west of Mattagami, Quebec. The area was surveyed using a dual-frequencyquadraturesystemin 1958.The depositcaused visible AEM anomalies, but they were not selectedfor ground follow-up at that time because they did not meet the empirical "Mattagami" criteria. Another AEM survey was commissioned by Selco Mining Corp. in 1971, this time using a time-domain, towedbird system (INPUT). Figure 46 shows a geologic and AEM map of a portion of the survey including the deposit (zones A and B). Felsic and mafic volcanic rocks underlie most of the survey area but outcrops are scarce and occur only along the Wawagosic River. A granitic massif crops out in the northeastern corner, but the exact location

of its contact

with

volcanic

rocks

was un-

known. The INPUT survey was flown in the northsouth direction with a line spacing of 600 m. The results were compiled in the form of a schematic

EM

867

conductormap. Each anomaly circle correspondsto a peak on the AEM channelsand each is shadedaccording to the number of channels on which a departure from

the

zero

level

can

be measured.

Full

circles

correspondto 6-channelresponses,3/4 full to 5 channels, half to 4 channels, open circle to 2 channels. No correction has been made for the system's asymmetric response;dipping conductorsare typically depicted as two circles on updip flights and one circle on downdip flights. At the time of the survey, routine conductance and depth of estimate were not made. Two broad conductive anomaly trends striking NWW-SEE indicate the position of intervolcanic sediments rich in graphite. The sediments do not outcrop but were intersected in drill holes (the only places where geology is indicated apart from the river valley). Quaternary sediments(gravel, sand, clay, silt) have variable conductivity and thickness (up to 60 m), but their

response hasbeenomittedfromthe conductor map. The underlying Precambrianformations were exposed during glaciation and fresh rocks occur immediately below the glacial sediments. Therefore, there is no AEM responsedue to conductive saprolite. Zones

A and B are clusters

of AEM

anomalies

isolated from the main conductive trend. Experience has shown that such setting may indicate massive sulfides.Figure 47 showsAEM responsesover the two

zones.Six INPUT channels andtotal-fieldmagnetometer data are displayed. There is no visible magnetic responseassociatedwith AEM anomalies. Response aperture diagrams developed by Palacky and West

Fig. 46. Discovery of the Detour deposit, Quebec. Schematic map of INPUT AEM conductorsand geologybasedon outcrops(1--mafic volcanicrocks, 2--felsic volcanicrocks, 3•granite). The shadingof the conductorsdarkenswith an increasein apparent conductance(after Reed, 1981).


868

Palacky and West

(1973) were used to determine conductance for selected conductors (Figure 41). Assuming a vertical half-plane model, zone A had a conductanceof 6 S and zone B of 1 S. In the 1970s, the "Mattagami" criteria were already discredited and the conductors were selected for ground follow-up even though they were nonmagnetic and poorly conductive. Figure 48 shows the results of horizontal-loop EM measurements carried out at Zone A with a coil separation of 100 rn and frequency of 2400 Hz. An irregularly shaped conductor dipping north and having a conductanceof 4 S was drilled at two locations (B-1 and B-2). Massive sulfides were intersected

in both holes.

After

further

detailed

exploration, a feasibility study indicated reserves of 35.4 Mt of ore grading 2.3% zinc, 0.4% copper, and 34.5 g/t silver. The subsequentlydrilled Zone B has reserves of 3.4 Mt grading 0.8% zinc, 4.5% copper, and 36.9 g/t silver. Before drilling, which was carried out in 1974, the INPUT survey was made more detailed by flying fill-in lines, thus reducing the line spacing to 300 m. The successof this program can be attributed largely to increased geologic input, which resulted in disregardingsome of the geophysicalrules of thumb (now considered obsolete), and also to improved instrumentation (better data resolution and interpretability).

6

N

FLIGHT •.-

DIRECTION

CH.•

lOOO

3 2

ppm

INPUT

AEM

1

Itapicuru GreenstoneBelt, Bahia, Brazil.--Palacky and Sena (1979) described the use of airborne and ground EM surveys in prospectingfor massive sulfides in Brazil. The Itapicuru greenstone belt in Bahia, 180 km NNW of the state capital Salvador, was identified by the geologists of DOCEGEO (a stateowned mining company) in the mid 1970s. The belt consists of isoclinal, westerly dipping folds whose axes strike predominantly north-south. Kishida and Riccio (1980) defined three major geologic units: (1) Metavolcanic mafic unit (metabasalts, graphitic schists, iron formations, and intrusive gabbro dikes and sills), (2) Metavolcanic felsic unit (felsic and intermediate volcanic rocks, felsic lavas, volcanic breccia and tuffs), and (3) Metasedimentary unit (conglomerates, arkoses, pelites, greywackes with frequent graphitic layers). The three units form a sequence that has intrusions of pillow lavas at its base and fine pelitic sediments at the top. The greenstone belt is almost completely surrounded by gneissesand granites of the Cara•a Group, which has been dated as being 2.5 Ga. A complete airborne geophysical coverage of the

greenstone belt (2400km2) wascarriedout in early 1976 using the INPUT time-domain AEM system and proton magnetometer. The flight line spacing was 500 m. Magnetic data were presented as contour maps. Due to insufficient susceptibility contrast of most lithologic units, the magnetic maps could not be used to upgrade geologic maps, which were not very detailed at the time of the survey. The AEM data were compiled by the contractor as a schematic map of conductors, whose widths were indicated by bar lengths. Anomalies were classified

nT

59

750

TOTAL MAGNETIC

8W

4W

BZONE •15965ø

0

4E

;

FIELD

',

6 5

S

•

N

•-

' '•:? ....'". i

B-1

0 I

lOOm I

•100m(• f = 2400

RATIO

A ZONE

Hz

SECONDARY/PRIMARY

IN-PHASE

o I

QUADRATURE

lkm I

Fig. 47. INPUT AEM and total-field magnetic responses recorded over Zones A and B of the Detour deposit, Quebec (their location is depicted in Figure 50), after Reed (1981).

Fig. 48. Ground electromagnetic data (in-phase and quadrature) obtained with a horizontal-loop EM system over Zone A of the Detour deposit, Quebec. Also depicted is the location

of two drillholes

zone (after Reed, 1981).

and the outline

of the mineralized


Airborne

according to the number of channels on which the AEM response was visible on analog records (similar to Figure 46). Recommendations were made for ground follow-up based on criteria which were commonly used in poorly-explored areas (the "Mattagami" approach). The first field campaign has shown that such criteria were inadequate and the company geoscientistsdeveloped a new approach more appropriate to local geologic and weathering conditions. The INPUT data were recompiled by DOCEGEO staff in the form of conductivity maps. Response aperture diagrams for half-space and vertical halfplane (Figure 41) were used for conductivity and conductancedeterminations. It soon became apparent that conductor patterns could be related to geology. Narrow anomaliestypical of bedrock conductorswere encountered mostly in the sequence of pelitic sediments. Large-amplitude anomalies were found to correlate with the metavolcanic

mafic units. A statistical

evaluation of apparent conductivities determined for the half-space model indicated that the values are fairly consistentfor individual units. Figure 49 showsa geologicmap of the northwestern part of the Itapicuru

EM

869

greenstonebelt. The pelitic metasedimentaryunit appears to have an average conductivity of 130 mS/m with

a standard

deviation

of 42 mS/m.

Anomalies

within this unit can be ascribedto frequently occurring graphitic schists. The metavolcanic mafic unit is slightlylessconductive,with a mean apparentconductivity of 95 mS/m and standard deviation of 23 mS/m. Scattered AEM anomalies were detected over portions of the metavolcanicfelsic unit, but in other parts, the felsic unit appeared resistive. The standard deviation calculated over this unit was relatively high (20 mS/m) compared with the mean (42 mS/m). Distinct AEM anomalies were recorded over amphibolite lenses, but the data were insufficient to permit a meaningful statistical analysis. Granites and clastic sediments appeared resistive to the INPUT system, which has a rather limited bandwidth and is insensitive to bodies with conductivities less than 15 mS/m.

The most difficult practical problem at that time was how to separate anomalies caused by saprolite from those caused by bedrock conductors. Because of its coil configuration (horizontal-axis receiver), the INPUT system is not sensitive to the geometry of conductors (see Figure 43). Palacky (1975) demonstrated that vertical

dikes and horizontal

ribbons cause

essentially identical anomalies when only this coil configuration is used. A scheme had to be designed that would permit conductor identification using existing AEM data, without carrying out ground EM surveys over all anomalies (in view of their number, over 3500, that would have been economically prohibitive). Some types of volcanic-associated massive sulfides are found in the sequenceof felsic volcanic rocks close to the contact with mafic volcanic rocks (Palacky, 1988). Sulfides are known to be more conductive than saprolite layers formed over felsic rocks. It was reasoned that anomalies due to sulfides and saprolite could be separated by statistical analysis of conductivities

• I1• •

GRANITE - GNEISS - RESISTIVE AMPHIBOLITESMALL ANOMALIES IN GRANITE FELSlC VOLCANIC ROCKSRESISTIVE

•

FELSIC VOLCANIC ROCKSSCATTERED ANOMALIES

•

BASIC VOLCANIC ROCKS-STRONG, WIDE ANOMALIES

j•

CLASTIC SEDIMENTS - RESISTIVE

I•

PELITIC SED.WITHGRAPHITE--NARROW ANOMALIES O

r•

STATISTICALLY MEANINGFUL ANOMALIES

FOLLOWUP TARGET D

Fig. 49. Geologic map of the northwestern part of the Itapicuru greenstonebelt in Bahia, Brazil. Averageapparent conductivity (in mS/m) and standarddeviation are given for several geologic formations (R denotes resistive units). After Palacky and Sena, 1979.

determined

over the metavolcanic

felsic unit.

Several such targets were identified, none of them selected previously for follow-up in the course of the traditional anomaly rating. Figure 50 shows the INPUT response and geologic profile over one such target which was highlightedby a square in Figure 49. The flight direction was east-west. Four AEM anomalies can be seen on the profile, but none appear to have the classic desired shape (narrow with a large amplitude). Three anomaliescoincide with the mapped unit of metavolcanic mafic rocks and were interpreted as due to saprolite. The fourth anomaly, in the center of the line, is located in a sequence of metavolcanic felsic rocks; ground geophysical measurementswere carried out in order to determine its origin. Its conductance (21 S) was determined usinga vertical half-plane response aperture diagram (Figure 41). Figure 51


870

Palacky and West

shows selected results of a horizontal-loop EM survey (one coil separation and one frequency are shown). The ground EM anomalies have a different character on the four lines depicted. On line B, there is a "classic" horizontal-loop EM response over a dikelike, westerly dipping conductor. Two conductors have been detected on lines C and D, and they appear deeper on the latter line. Nonzero backgroundvalues visible on all lines are due to conductive saprolite. In the Itapicurti greenstone belt, irregularly shaped anomalies

have

been

found

to be associated

with

sulfides. By contrast, EM anomalies due to graphitic layers were found to extend for several kilometers without a significantchange in conductor parameters. A decision

was made

to test the conductor

illus-

trated in Figure 51 by diamond drilling. Massive sulfides (20-30 percent), mostly pyrite and pyrrhotite with minor chalcopyrite, were intersected in several layers, the thickest coincidingwith the EM conductor. The dip of the body was to the west, as interpreted from ground EM data. Drill cores were analyzed for base metals, but the grades were found to be uneconomic. The thickness of the weathered layer was locally 25 m. This case history demonstrates that an innovative approach to interpretation (geologic correlation, sta-

tistical analysis) can lead to identification of massive sulfide targets. The AEM survey led to the discovery of the Araci gold deposit in 1976. The mineralization is associatedwith arsenopyrite disseminated in the sequence of chlorite schists and graphitic pellites. Mining operations started in 1984 and the present production is approximately 450 kg of gold per month. Reserves

at the Fazenda

Brasileiro

mine

have

been

estimated at 19 Mt of ore grading 7.3 g Au/t, and the geologic potential of the Faixa Weber formation is even greater (CVRD, 1985). The success of AEM surveys in the Itapicuru greenstone has resulted in an increased use of the technique in Brazil. Excellent correlation between certain anomaly types and lithologic units stimulated further research on electrical properties of the weathered layer and have led to a more widespread application of AEM methods in geologic mapping. GeologicMapping

Many lithologic formations have distinct AEM signatures. Some rock types are inherently conductive, e.g., shales and graphitic sediments, while others can

400

W

i

300 i

200

100

i

0

i

i

lO0

200

i

E

i

FIDUCIAL

i i i • illl

illl

98

i i i i i i i i i i i i i i i i i i i i i 1911 97

96

•

5

.....%2•..o., •-•. •..,'•-iLL

•'--•-o

.._16 •• 7

5

I

7

I

7 _

,,

-40

I C)-f :21S

<2• AEM

-20•

•t=5

5S

-40

INPUT

INTERPRETATION

-2

,.o[,.[ --o--=o--

W A A A• TYPES

OF

•

AEM

A•E

RESPONSE

GEOLOGY

Bedrock 'conductor

•Saprolite

conductor

•

layer

Resistive

•]

Felsic volcanic rocks

rx-x-I Mafic volcanic rocks

0

05

HOR

IN-PHASE HORIZ.

SCALE

--<>---o-QUADRATURE o 5o lOO15•Orn

150 rn (• f=444

HZ

GEOLOGY

•

Rhyolitic lava

.•

Pyrite and pyrrhotite (20-30%)

.';l•'• Andesitic lava

•

Interpreted conductor

lkm

SCALE

Fig. 50. INPUT AEM and total-field magnetic responses over a zone indicated in Figure 49, Itapicuru greenstonebelt, Bahia, Brazil (after Palacky and Sena, 1979).

Fig. 51. Ground electromagneticdata (in-phaseand quadrature) obtained with a multifrequency horizontal-loop system over a mineralized zone in the Itapicuru greenstone belt, Bahia, Brazil. Also depicted is a geologic section obtained by drilling on profile B (after Palacky and Sena, 1979).


Airborne

be identified due to characteristic conductivity of the weathered layer. The weathering mechanism and formation of saprolite, whose conductivity varies according to the type of parent rock, were explained in Palacky (1988). The previously described mineral exploration case hislary highlighted the importance of interpretingAEM data in lithologicterms, but an AEM survey can be carried out solely for geologicmapping. In areas of poor outcrop and/or difficult access, such surveys are a cost-effective alternative to more traditional mapping approaches. Moreover, detailed information on conductivity distribution, which relates to lithology and tectonics, cannot be obtained by other means.

Labrador Trough, Quebec.•A case history describeslithologic interpretation of INPUT AEM data from the Labrador Trough in Canada. The trough which is located in northern Quebec extends for over 1200 km from the border

with the Grenville

Province

(latitude 53ø30'N) to the Ungava Peninsula (latitude 62øN). It bordersthe Superior Province in the west and Churchill and Nair Provinces in the east. Early Proterozoic igneous and sedimentary rocks of the trough overlay an Archean basement. The southern part of the trough is of economic importancesiron ore was mined near Schefferville from the 1940s until 1983 and mining operations continue near Wabush. Since the 1930s, detailed geologic studies have been carried out in the area by the Geological Survey of Canada, Quebec Ministry of Energy and Resources,and several mining companies, particularly Iron Ore Companyof Canada. In 1983,the Quebec Ministry of Energy and Resources commissioned extensive airborne geophysical surveys to explore other mineral resourcesof the region. A total of 14 836 line km was flown with the time-domain

INPUT

system and 36 075 line km with a multifrequency, multicoil helicopter AEM system (Lefebvre et al., 1986). Some of the INPUT data were reprocessedby A-Cubed Inc. in the form of stacked magnetic and EM profiles. Palacky (1989) describedinterpretation of data from the ThompsonLake area. Figures52, 53, and 54 show, respectivelygeology, total magneticfield and pseudochannel 6 derived from INPUT data (Vaughan, 1990). The advantageof this presentationof geophysicaldata is that the original anomaly shape, which is important for line-to-line correlation, can be seen. Responses due to poor conductors(presumablyQuaternary sediments) are suppressedby displayingChannel 6 (delay time 2.28 ms). Rocks belonging to three groups of Proterozoic age were identified in the depicted area. The oldest is the Doublet Group which is represented here by three formations (Murdock, Thompson, and

EM

871

Willbob). The geologic descriptions given below are based on Dimroth (1978). The Murdock Formation, which crops out in the southwest corner, coincides with an area of strong magnetic activity. The formation is made of predominantly pyroclastic rocks with subordinate basalt flows and sedimentary rocks. A strong AEM trend consisting of five anomalies indicates the true extent of the mapped gabbro (Montagnais Group). Most likely, the AEM anomalies are due to saprolite developed over this rock type. Numerous

AEM

anomalies

are

associated

with

pelitic schistsof the Thompson Lake Formation. The schist sequence consists of graphitic or pyritic shales, and locally, massive pyrite beds were observed. All mentioned rock types may be highly conductive. As they are poorly exposed and sometimes weathered (particularly graphitic shales), EM results can be used to improve the accuracy of geologic maps. The Willbob Formation, which covers almost half of

the illustrated area, consists of pillowed and massive basalt flows, with interbedded thin layers of graphitic or pyritic shales. Geologic reports mention that basalt is frequently weathered (light grey-green color), suggesting the possibility of saprolite presence. Both shales and saprolite usually are conductive. No magnetic anomalies, but numerous strong AEM trends can be observed in areas underlain by this formation. The Doublet Group is separated by a thrust fault from the Laport Group which crops out in the northeastern corner of the described area. Strong EM anomalieswere detectedjust northeast of the fault, but the existing geologicmap did not offer any explanation for their existence. The local lithology has been described as a sequence of metaquartzite, metamorphosed dolomitic sandstone, aluminous pelite, amphibolite, and ultramafic rocks. As neither pyritic nor graphitic schistsare known to occur in this formation, the detected

AEM

anomalies

are assumed to be due to

saprolite developed over amphibolite or ultramafic igneousrocks. Existence of weathered layers has been repeatedly observed in the Labrador Trough. The Montagnais Group includes all poorly defined gabbroicand ultramafic igneousrocks in the Labrador Trough. Originally, the rocks were thought to be intrusive, but recent studies have shown that many are of extrusive origin and thus can be included with their host formations. Strong magnetic anomalies are associated with peridotite and magnetic surveys have been used extensively to map their extent. Gabbro does not appear to cause magnetic anomaliesbut EM response has been recorded.

Chloritization

has been described

in the area and chlorite-rich layers can be conductive. A careful analysis of AEM data can contribute to better discriminationof units in this poorly understood group.


GEOLOGY

LEGEND

0 I

Pleistocene

Doublet

Group

:./. "...•Sand,gravel, till

,•,• Willbob Formation

Montagnais Group •/.•_,•• Pe ridot ite Gabbro Laporte Group

I •

Thompson Formation (pelitic schist) Murdock Formation (agglomerate)

//• Paragneiss

•

Thrustfault

5 km i

•

,

•

I

(basalt)

4:• Lake Fig. 52. Geologic map of the Thompson Lake area, Labrador Trough, Quebec (based on Dimroth, 1978). TOTAL

MAGNETIC

FIELD

nT

ß -

20

000

10

000

0

0

5 km

I

I

I

,

,

I

Fig. 53. Magnetic map (total field) of the ThompsonLake area, Labrador Trough, Quebec. The data were obtained

in the courseof an airborneINPUT EM and surveyfundedby the Governmentof Quebec.Reprocessedmap courtesy of A-Cubed Inc. OUTPUT

PSEUDO-CHANNEL

6

lOOO

500

0 I

I

I

I

"

5 km I

Fig. 54. OUTPUT pseudo-channel 6 map of the ThompsonLake area, LabradorTrough, Quebec.The original INPUT AEM data acquiredby QuestorSurveysLtd. were reprocessedby A-Cubed Inc.

Airborne EM


Kapuskasing, Ontario.--The second case history describesthe applicationof helicopter AEM surveys to mappingof Quaternary sediments.Information was required along selected transects in northern Ontario and two lines, each 50 m from the center of selected

roads, were flown. The resultsof airborneand ground EM surveys near Kapuskasing, Ontario, were originally presented in Palacky (1989). Helicopter AEM data were processed in the form of composite (stacked) profiles (such as Figure 17) and color bar maps of apparent conductivity and depth to bedrock.

CONDUCTIVITY

BAR

f:

This novelway of data presentationwas developedto cope with the unusualsurvey characteristics(selected transects rather than a compact area).

Figure 55 depictsapparentconductivityalong one segment 8 km long, which is located 35 km south of Kapuskasing. The color coding varies from blue for resistive materials to green, orange, and red for conductors. Conductivity values were calculated from the horizontal coplanar data (frequencies 4175 and 32 000 Hz) using a thick horizontal layer model. The

samedata set was usedto calculatedepth to bedrock

to Kapuskasing

DEPTH

! INSIDE

$73

4175

BAR

f:32

BEDROCK

to

Kapuskasing

!

Hz

INSIDE OUTSIDE

TO

BAR

f-46OO

Hz

kHz

OUTSIDE

BAR

f-32

kHz

BROAD

CLAY-FILLED

BROAD

DEPRESSION

CLAY-FILLED DEPRESSION

mS/m

1259.O metres 316.O

TWO

135

BEDROCK

CONDUCTORS, C

80.0

2OOm

TWO

APART

2OOm

C

105

20.0

5.0

o,:o 75

1.3

BEDROCK CONDUCTOR

SHEAR

ZONES

0.45

Casselman

IN

6O

BEDROCK

Tpo

APART

B

BEDROCK CONDUCTOR

A

BEDROCK

CONDUCTORS,

120

B

SHEAR

.-

ZONES BEDROCK

45

0.11

Casselman

IN

Tp.

,

0.04

30

Slack

Tp.

Fenton Tp. / c

0.01

1000m I

Staples

Tp.

Fenton

Tp.

15

Slack

= ,=

Tp.

,=o o

0

ß

Fig. 55. Conductivity bar map of segmentKAP-3, south of Kapuskasing, Ontario. The values of apparent conductivity were calculatedfrom coplanarAEM data at two frequencies. The helicopterAEM survey was performedby Aerodat Ltd. for the Geological Survey of Canada (Mineral Development Agreement with the Province of Ontario).

1000m

I

Fig. 56. Depth-to-bedrock map derived from helicopter AEM data, KAP-4 segment, south of Kapuskasing, Ontario. The depth values were calculated from AEM response assumingconductivity of 30 mS/m.


874

Palacky and West

fixing the conductivity at 30 mS/m. The data are displayed also as color bars, with warm color for areas of deep bedrock (Figure 56). AEM data in the form of composite profiles are shown in Figure 57. From the top, one can see in-phase and quadrature components for vertical coaxial data (low and high frequency), then in-phase and quadrature response for the horizontal coplanar configuration (low and high frequency), and finally apparent conductivity traces derived from the horizontal coplanar data. The original compositeprofile presentation included also VLF data (total field and quadrature) for two transmitters, sferic and power-line monitors, altimeter, and total magnetic field trace.

Interpretation of AEM data resulted in identification of two types of anomalies: (a) bedrock conductors A, B, C (B and C have been followed up on the ground), (b) anomalies caused by changesin lithology and thickness of Quaternary sediments (all other features). Ground follow-up was carried out alongprofiles of 1 km length (their location is depicted in Figures 55-57). A multifrequency horizontal-loop EM (HLEM) system (APEX Max Min I) was used in the survey with a coil separationof 100 m. Readingswere recorded at frequencies 110,220,440, 880, 1760, 3520, 7040, and 14 080 Hz. On the first profile (Figure 58), four shear zones were identified

which are character-

ized by anomalies wider on the quadrature component. The AEM system could not discriminate between clays in subvertical shear zones and Quaternary

cover and recorded one wide anomaly. The thickness estimate based on AEM data (horizontal coplanar responseat 4600 Hz) is fairly accurate, about 9 m. A vertical

reverse-circulation

drill

hole

at

-550

m

reached bedrock at 8.5 m. Quaternary sedimentsconsist of sand and clay. The secondHLEM profile (Figure 59) confirmed the existence

of two

bedrock

conductors

which

were

interpreted from AEM data at -515 and -275 m. Using a helicopter AEM response aperture diagram for a vertical half-plane (such as Figure 33), their conductanceswere estimated at 10 S (for coaxial coils and f = 935 Hz). On the ground data, the two anomalies have a trough shape, which is typically observed over vertical dikes, and a characteristic reversal on the high-frequencyquadrature responseindicating a conductive surficial layer. A vertical drill hole at -500 m was loggedas having 6 m of poorly sorted sediments, 14 m of sand, and 15 m of till and silt before reaching mylonitized bedrock. The thickness derived from horizontal-coplanar AEM data at 4600 Hz was an underestimate (24-27 m), because of the unrealistic conductivity assumption(30 mS/m is too high for sand and till). The third HLEM profile (Figure 60) outlined a broad depression filled with clay (conductivity estimated from AEM data 14-20 mS/m). Accordingto the ground HLEM data, the greatest thicknessis between stations -500

and -700

m. A drill hole at -650

m intersected

12 m of massive clays, 6 m of varved clays, and 17 m

S

N 60

60•I • (• --I "• "•

SHEAR ZONES

......... ....... . ppm • .................. 4600 Hz--" .•-. .... '5"---: 0

•

240• {

{

• • •

,• •

,-, CLAY-FILLED DEPRESSION //•'...? ',,

BEDROCK

CONDUCTORS

{ • ,

{

'S1S2 S3S4' A

{ {

• ...............

..../ ..•-,'-'-"---•

-•--._.---_ ........ •

•

•

I

0

- 240

-

•

ppm o

•- 60

.... •-'•

0

QUADRATURE • ....................

T- o

mS/m

100-I

CONDUCTIVITY •

10-1

32

kHz

--100

•/ _.•/•4175Hz

--.........'-

0.1

0

.............,--10

---' '------'-<•••• 0.1

1000m

Fig. 57. Compositeprofile of segmentKAP-4, which are depictedin Figures 56 and 57. The bars indicate areas where ground follow-up was carried out.


Airborne

of sand. Low-frequency, horizontal-coplanar AEM data indicated a thickness of 30 m. Considering that only a two-layer model was used (conductiveclay and resistive bedrock), the results are reasonable. What can be realistically interpreted from AEM data without an a priori knowledge of ground EM results and drilling?Three bedrock conductors(marked A, B, C) were unambiguouslyrecognized. An increase in overburdenconductivity marked a thicker sequenceof clays in a depression (third profile). Even the most modern AEM systems have a limited resolution and multiple shear zones are the most difficult target to identify on AEM data. It should be noted here that neither of the simultaneous VLF surveys detected the shear zones because of a thin horizontal clay layer. An increase in conductivity clearly indicated the presence of clays: highestvalues (10-14 mS/m on low-frequency and 14-20 mS/m on high-frequencyhorizontal coplanar data) were recorded over a depressionwhere 18 rn of clays were intersected. In the middle profile, tills and silts were encountered with a conductivity of 5-7 mS/m. The area of shearing had an even lower conductivity (3-5 mS/m). The approachused in calculating depth to bedrock was rather crude as only one

EM

875

layer of fixed conductivity (30 mS/m) was considered. Joint inversion of all AEM parameters can be used to improve the accuracy of conductivity and thickness estimates (Palacky et al., 1990). Bathymetric Charting and Sea-iceMeasurements

The recent developmentsof AEM instrumentation have made possible applications in shallow-ocean bathymetry and determinations of sea ice thickness. Experimental surveys carried out for bathymetric chartingwere describedin Won and Smits (1986) and Zollinger et al. (1987) and summarized in Palacky (1988). In this section, we focus on the more complex surveysin the Arctic which were analyzed in Kovacs and Valleau (1990). Rather than dealing with one conductivelayer (seawater) as in bathymetric studies, in the case of snow and ice cover we have to consider

three layers. The first layer, snowand ice, is not really seenby the AEM system.The thicknessis determined as a difference between a laser altimeter reading and the calculated

distance between

the EM

sensors and

the conductive layer (seawater). The accuracy of the laser altimeter is 10 cm. Because of the relatively low

50

5o t

lOOm 40

f(Hz) 30

14080

20¾• ','%•,_'-..._•-•-1408 0

[/3

.

<•

10

7O4O

'-I-

0

13.

z_ 3520

_

,-

"3520

'

-10-

.

-20.

-;30-

-2O

o%' '--•o6' tgo6' '•o6'

'--gob' tgo6' '•o6'

--•1oob''_•o6'L•o6' -•o6' L•Jofi' L,•o6'L4•o6' L•,ofi'L:•o6''-1'o6''fi

'--:•o6' '--•o6' '1'o6 ' ' 6

DISTANCE(m)

DISTANCE(m)

20

10•

/7040

;

o

10

.-3520 -1760 -880

•

ß

•-2o•

-

.<:

O_30 •q

-40•

•

••

Drill j hole ]

hole

Shear zones•• /

....... ......

-•ooo -9oo

-•oo

-700

-soo

-soo' L•o•

14080

.

0-50-

-7o: LJo•' L•o•'L

'•

DISTANCE(m)

Fig. 58. Groundelectromagneticdata (in-phaseand quadrature) obtained with a multifrequency horizontal-loop EM system(APEX MaxMin I). Four anomalies(Si, S2, S3, S4) are due to shear zones. Their location is indicated in Figures 55-57 (after Palacky, 1989).

conductors 5o•' L•o6' L•o•' L-)o•' L•o•' L•L;edrock 'LA•' Lj'CLeon' L•'o•' '•) DISTANCE(m)

Fig. 59. Groundelectromagneticdata (in-phaseand quadrature) obtained with a multifrequency horizontal-loop EM

system(APEX MaxMin I). The existenceof two bedrock conductorsidentified on AEM data (B and C) was confirmed by ground surveys(after Palacky, 1989).


876

Palacky and West

frequency used, the AEM systemdoesnot see sharply small features which are averaged over an area equal to the footprint (about 40 m, or 1.25 times the flight height). That means that the AEM system detects an ice ridge 10 rn wide only imperfectly. On the other can be determinedwith an error of only 1 percent. The

AEM surveys in the Beaufort Sea. Figure 61 shows profilesof ice thicknessand water depth derived from AEM data. The survey was carried out over a snowfree, relatively uniform refrozen lead, whose thickness was measured by drilling (75 cm). Interpretation of AEM data indicated an average ice thickness of 65 cm. The seawater conductivity was estimated at 3 S/m,

second value that can be determined

while direct measurements

hand, the thickness of extensive snow and ice cover from AEM

data is

the thicknessof seawater (bathymetric charting). The error in thickness determination varies according to the water depth: it is -+30 cm for 10 m, 1 rn for 15 m, and 2 rn for 20 m. The third parameter that can be estimated from AEM results is the conductivity of seawater. Comparisonsbetween interpreted and measured values suggestan accuracy of better than 20

indicated

2.5 S/m. The sea

bottom was found rather flat, with an average depth of 18 m. Determinations

of seawater

thickness

based on

AEM data were not confirmed by direct measurements.

Figure 62 illustrates a more complex situation, a profile across a large, grounded second-year rubble formation. Bathymetric charts indicate that this for-

percent.

Determinations of the mentioned parameters were carried out by computer matching the measureddata to response aperture diagrams. Another alternative would be AEM data inversion.A descriptionand error analysisof this techniquecan be found in Holladay et al. (1986). In this study, we showtwo examplesof data presentationand interpretation. Kovacs and Valleau (1990) describedtest helicopter

-

10

WATER

20

0

2890

O'w --3.0

2910

"3520

"-"

• 0•;_.•..........

.....

2930

•

2970

2990

3OI0

NUMBER

,-1760

:::::::::::::::::::::: '880

"r_20• z .

2950

FIDUClAL

Sire

500

1000m

14080 Fig. 61. Thickness of ice and seawater determined from helicopter AEM data, Beaufort Sea (after Kovacs and Vai-

-50-

-40•

leau, 1990).

.

-50.

-s-ø•oob' L•o•' L•lo•' L•o6' L•lo•' L.4o6 ' L,;o6' L•o3' D•S•Uc[

(•)

20

•-20

D••

7040

,• o

• -5o

.I-

o -80

I-

WATE• o* w -2.6 $1m _

4

Q. UJ

-70

-80

_

C• 8 UJ I-

- .. 1

-•ooo -• -•oo

_

L• D•ST•UC[ (•)

Fig. 60. Ground electromagneticdata (in-phaseand quadrature) obtained with a multifrequency horizontal-loop EM system (APEX MaxMin I). Massive clays which caused a strong airborne and ground EM responsewere intersected by a drill hole located at -650 (after Palacky, 1989).

'•

12

_

, "•x•r'•'•'• ='x'•'""SEA BED 5340

5360

5380

_

5400

FIDUCIAL

5420

5440

5460

--

5480

NUMBER

250

500m

Fig. 62. Thicknessof first-year sea ice and a large grounded ice-rubble formation as determined from helicopter AEM data, Beaufort Sea (after Kovacs and Valleau, 1990).

Airborne

mation


AEM

rests on a shoal 8 rn below the sea surface. indicated 2-3 rn of seawater

measurements

beneath the rubble formation. The discrepancy could be explained either by a presenceof unfrozen seawater within the submergedice bed, or by the AEM system responding to seawater to the side of the ice keel (a shortcoming caused by the already mentioned large footprint). The maximum ice thickness (over 20 m) appears reasonable from the ice-surface elevation measurements and the shoal depth (bathymetric charts). Drill hole measurements at 5390 indicated a water depth of 11.5 rn versus the AEM estimate of 10 m, and a thickness of sea-ice and snow of 1.75 m, the

latter determination being virtually identical to that obtained by AEM interpretation. Also the conductivity of seawater determined from the AEM data agreed with the measured values (2.6 S/m). The described test surveys indicated that AEM technology has a tremendous potential for measuring thicknessof sea-iceand seawater, and conductivity of seawater. The accuracy of measurements can be improved by upgrading the hardware and refining the data processingroutine. The performance of the helicopter AEM systemin field tests was rated higher than that of ground radar systems (Kovacs and Valleau, 1990). ACKNOWLEDGMENTS

Geological Survey of Finland, A-Cubed Inc. and Aerodat Ltd. of Mississauga, Ontario, and Geoterrex Ltd. of Ottawa, Ontario, provided specifications of AEM systems and examples of processedAEM data. Aerodat staff, particularly D. Pitcher and M. Steiner, have been helpful in explaining details of routine helicopter AEM data processing.R. S. Smith of the University of Toronto calculated several response profiles specifically for this publication. Reviewers of the manuscript, Prof. A. Becker of the University of California, Berkeley and Dr. D.C. Fraser of Dighem Surveys and Data Processing Inc. made useful comments which when implemented improved the final text. Many of the previously unpublished surveys are from Open Files of the Geological Survey of Canada. Illustrations were drafted by S. Davis at the Geological Survey of Canada and K. Khan at the Department of Physics, University of Toronto. Geological Survey of Canada Contribution 41288. REFERENCES

Anderson, W. L., 1979, Numerical integration of related Hankel transforms of orders 0 and 1 by adaptive digital filtering: Geophysics, 44, 1287-1328. Annan, A. P., 1974, The equivalent source method for electromagnetic scattering analysis and its geophysical

EM

877

applications, Ph.D. thesis, Memorial Univ. of Newfoundland.

•1983, Effect of differential transmitter-receiver motion on airborne transient EM interpretation' 53rd Ann.

Mtg., Soc. Expl. Geophys., Expanded Abstract, 622-623. •1984, Compensation of towed-bird AEM system data for

differential

transmitter-receiver

motion:

54th

Ann.

Mtg., Soc. Expl. Geophys., Expanded Abstract, 80-81. •1986, Development of the PROSPECT I airborne electromagnetic system, in Palacky, G. J., Ed., Airborne resistivity mapping:Geol. Surv. Can. Paper 86-22, 63-70. Annan, A. P., and Lobach, J., 1985, Experiments with a new general purpose digital EM receiver: Exp. Abstr. 55th Ann. Mtg., Soc. Exp. Geophys., Expanded Abstract, 234-236.

Barongo, J. O., 1989, Application of transient airborne electromagnetic and ground resistivity methods to geological mapping in tropical terrains' Ph.D. thesis, McGill Univ., Montreal. Barringer, A. R., 1962, The INPUT electrical pulse prospecting system, Min. Cong. J., 48, 49-52. Baudoin, P., Durozoy, G., and Utard, M., 1970, Study of a freshwater-saltwater contact in the Rh6ne Delta using airborne electromagneticmethods, in Morley, L. W., Ed., Mining and groundwater geophysics/1967' Geol. Surv. Can. Econ. Geol. Rep. 26, 626-637 (in French). Becker, A., 1979, Airborne electromagnetic methods: in Hood, P. J., Ed., Geophysics and geochemistry in the search for metallic

ores'

Geol.

Surv.

Can.

Econ.

Geol.

Rep. 31, 33-43. Becker, A., Barringer, A. R., and Annan, P., 1990, Airborne electromagnetics 1978-1988, in Fitterman, D. V., Ed., Developments and applications of modern airborne electromagnetic surveys' U.S. Geol. Surv. Bull. 1925, 9-20. Becker, A., and Cheng, G., 1988, Detection of repetitive electromagnetic signals, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics, Vol. 1, Theory' 442-466. Best, M. E., 1985, A systematic approach for evaluating airborne electromagnetic systems' Geophys. Prosp., 33, 577-599.

Best, M. E., and Bremner, T. G. T., 1986, The Sweepem airborne electromagnetic system, in Palacky, G. J., Ed., Airborne resistivity mapping' Geol. Surv. Can. Paper 86-22, 71-77.

Best, M. E., and Shammas, B. R., 1979, A general solution for a spherical conductor in a magnetic dipole field: Geophysics, 44, 781-800 (errata, 44, 1622). Collett, L. S., 1966, Airborne electromagnetic survey over the Winkler aquifer, Manitoba: Geol. Surv. Can. Paper 66-2, 6-9.

•1970, Resistivity mapping by electromagnetic methods; in Morley, L. W., Ed., Mining and Groundwater Geophysics/1967' Geol. Surv. Can. Econ. Geol. Rep. 26, 615-625.

•1986, Development of the airborne electromagnetic technique, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 9-18. Collett, L. S., Barringer, A. R. and Loveless, A. J., 1983, COTRAN electromagnetic survey of the NEA/IAEA Athabasca test area in Cameron, E. M., Ed., Uranium Exploration in Athabasca Basin, Saskatchewan, Canada: Geol. Surv. Can. Paper 82-11, 179-190. CVRD, 1985, The gold mine at Fazenda Brasileiro: Revista da Companhia do Vale do Rio Doce (CVRD), 6(19) (in Portuguese). Deletie, P., and Lakshmanan, J., 1986, Airborne resistivity surveyingapplied to nuclear power plant site investigation in France, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 145-152. DeMoully, G. T., and Becker, A., 1983, Automated interpretation of airborne electromagnetic data: Geophysics, 49, 1301-1312.

878

Palacky and West

Dimroth, E., 1978, Region of the Labrador Trough between


latitudes 54 30' and 56 30" Min. des richesses naturelies,

Qu6bec, Rapport G6ologique 193, 1-396 (in French). Dyck, A. V., Becker, A., and Collett, L. S., 1974, Surficial conductivity mapping with the airborne INPUT system' Can. Inst. Min. Metallurg. Bull. 67(744), 104-109. Dyck, A. V., Bloor, M., and Val16e, M. A., 1980, User manual for programs PLATE and SPHERE: Rsch. Appl. Geophysics, 14, Univ. Toronto. Ferneyhough, A. B., 1985, The quantitative interpretation of airborne electromagnetic data: Rsch. Appl. Geophysics, 34, Univ. Toronto. Fitterman, D. V., 1990, Ed., Developments and applications of modern airborne electromagnetic surveys' U.S. Geol. Surv. Bull. 1925, 216. Fouques, J.P., Fowler, M., Knipping, H. P., and Schimann, K., 1986, Cigar Lake uranium deposits' Discovery and general characteristics' Can. Inst. Min. Metallurg. Bull., 79(886), 70-82 (in French). Fraser, D.C., 1972, A new multicoil aerial electromagnetic prospecting system; Geophysics, 37, 518-537. •1973, Magnetite ore tonnage estimates from an aerial electromagnetic survey: Geoexploration, 11, 97-105. •1978, Resistivity mapping with an airborne multicoil electromagnetic system: Geophysics, 43, 144-172. •1979, The multicoil II airborne electromagnetic system: Geophysics, 44, 1367-1394. •1981, Magnetite mapping with a multicoil airborne electromagnetic system: Geophysics, 46, 1579-1593. •1986, Dighem resistivity techniques in airborne electromagnetic mapping, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can., Paper 86-22, 4954.

•1990, Electromagnetic anomaly recognition within geological and cultural noise, in Fitterman, D. V., Ed., Developments and applications of modern airborne electromagnetic surveys' U.S. Geol. Surv. Bull., 1925, 53-64. Frischknecht, F. C., 1988, Electromagnetic physical scale modeling, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics, v. 1, Theory' 36•. •41. Ghosh, M. K., 1972, Interpretation of airborne EM measurements based on thin sheet models: Rsch. Appl. Geophysics, 4, Univ. Toronto. Ghosh, M. K., and West, C. F., 1971, AEM analoguemodel studies' N. Paterson & Assoc., Toronto. Grant, F. S., and West, G. F., 1965, Interpretation theory in applied geophysics; McGraw Hill Book Co. Hanneson, J. E., and West, G. F., 1984, The horizontal loop electromagnetic response of a thin plate in a conductive earth: Part 1--Computational method: Geophysics, 49, 411-420.

Herz, A., 1986, Airborne EM instrumentsoperating at VLF and higher frequencies, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 55-61. Hogg, R. L. S., and Boustead, G. A., 1990, Estimation of overburden thickness using helicopter electromagnetic data, in Fitterman, D. V., Ed., Developments and applicationsof modern airborne electro magneticsurveys:U.S. Geol. Surv. Bull. 1925, 103-115. Hohmann, G. W., 1988, Numerical modelingfor electromagnetic methods in geophysics, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics, V. 1, Theory: 312-363. Holladay, J. S., Valleau, N., and Morrison, E., 1986, Application of multifrequency helicopter EM surveys to mapping of sea-ice thickness and shallow-water bathymetry, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 91-98. Hoover, D. B., and Pierce, H. A., 1986, Airborne EM mapping of geothermal systems in the Basin and Range and Cascade provinces, in Palacky, G. J., Ed., Airborne resistivity mapping' Geol. Surv. Can. Paper 86-22, 139144.

Huang, H., and Palacky, G. J., 1991, Damped least-squares inversion of time-domain airborne electromagnetic data using singular value decomposition: Geophys. Prosp., 39 (in press). Johansen, H. K., and Sorensen, K., 1979, Fast Hankel transforms: Geophys. Prosp., 27, 876-901. Johnson, I. M., and Seigel, H. O., 1986, Tridem resistivity mapping for natural resources development, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 125-129. Kamenetsky, F. H., and Yakubovsky, J. V., 1985, Electrical prospecting using induction methods in the mid 1980s: Geologia i razvedka, 28(7), 87-97 (in Russian). Kishida, A., and Riccio, I., 1980, Chemostratigraphyof lava sequences from Rio Itapicuru greenstone belt, Bahia: Precambrian Rsch., 11, 161-178. Koefoed, O., Ghosh, D. P., and Polman, G. J., 1972, Computation of type curves for electromagnetic depth soundingwith a horizontal transmitting coil by means of a digital linear filter: Geophys. Prosp., 20, 406-420. Konings, M. H., and DeCarle, R. J., 1985, Helicopter input in British Columbia: Presented at Ann. Mtg. Can. Inst. Min. Metallurg., Vancouver. Kovacs, A., and Valleau, N. C., 1990, Airborne electromagnetic

measurement

of

sea-ice

thickness

and

sub-ice

bathymetry, in Fitterman, D. V., Ed., Developments and applicationsof modern airborne electromagnetic surveys: U.S. Geol. Surv. Bull. 1925, 165-169. Lajoie, J. J., and West, G. F., 1976, Electromagnetic response of a conducting inhomogeneity in a layered earth: Geophysics, 41, 1133-1156. Lazenby, P. G., and Becker, A., 1984, Redefinition of the INPUT system: Part B--Signal processing and field results (Grant 003), in Milne, V. G., and Barlow, R. B., Eds., Exploration technologydevelopment program of the Board of Industrial Leadership and Development, Ontario Geol. Surv., Misc. Paper 120, 5-16. Lee, T., 1975, Transient electromagnetic response of a sphere in a layered medium: Geophys. Prosp., 23, 492512.

Lefebvre, D. L., Dion, D. J., and Keating, P., 1986, Airborne electromagnetic coverage of the Province of Quebec, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 169-173 (in French). Lodha, G. S., and West, G. F., 1976, Practical airborne EM interpretation using a sphere model: Geophysics, 41, 1157-1169.

Lowrie, W., and West, G. F., 1965, The effect of a conducting overburden on electromagnetic prospecting measurements: Geophysics, 30, 918-924. Macnae, J. C., 1979, Kimberlites and exploration geophysics: Geophysics, 44, 1395-1416. Macnae, J. C., Lamontagne, Y., and West, G. F., 1984, Noise processingtechniquesfor time domain EM system: Geophysics, 49, 924-948. Mallick, K., 1973, Conducting infinite horizontal cylinder in electromagnetic INPUT field: Geophys. Prosp., 21, 102108.

McNeill, J. D., and Labson, V., 1991, Geological mapping using VLF radio fields, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics,Applications, Vol. II, Morrison, H. F., Dolan, W., and Dey, A., 1976, Earth conductivity determinationsemploying a single superconducting coil: Geophysics, 41, 1184-1206. Mundry, E., 1984, On the interpretation of airborne electromagnetic data for the two layer cases: Geophys. Prosp., 32, 336-346. O'Connell, M.D., and Nader, G. L., 1986, Conductive layer mapping by computer processingof airborne electromagnetic measurements, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 111124.


Airborne

Palacky, G. J., 1974, The atlas of INPUT model profiles: Barringer Research Ltd., Rexdale. •1975, Interpretation of Input AEM measurementsin areas of conductive overburden: Geophysics, 40, 490-502. •1981, The airborne electromagneticmethod as a tool of geological mapping: Geophys. Prosp., 29, 60-88. •.Ed., 1986a, Airborne resistivity mapping:Geol. Surv. Can. Paper 86-22, 195. •1986b, Airborne electromagneticsat crossroads, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 5-8. •1986c, A bibliography of airborne electromagnetic methods: Instrumentation, interpretation, and case histories, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 175-180. •1988, Resistivity characteristicsof geologictargets, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics,V. 1, Theory, 52-129. •1989, Advances in geological mapping with airborne electromagneticsystems,in Garland, G. D., Ed., Proceedings of Exploration '87: Third Decennial International Conference on Geophysicaland GeochemicalExploration For Minerals and Groundwater: Ontario Geol. Surv., Special Vol. 3, 137-152. •1990, Airborne electromagnetics,geologicalmapping, and prospecting for nontraditional targets, in Fitterman, D. V., Ed., Developments and applications of modern airborne electromagneticsurveys: U.S. Geol. Surv. Bull. 1925, 89-101.

Palacky, G. J., and Jagodits, F. L., 1975, Computer data processingand quantitative interpretation of airborne resistivity surveys: Geophysics, 40, 818-830. Palacky, G. J., and Sena, F. O., 1979, Conductor identification in tropical terrainswCase historiesfrom the Itapicuru greenstone belt, Bahia, Brazil: Geophysics, 44, 19411962.

Palacky, G. J., and West, G. F., 1973, Quantitative interpretation of Input AEM measurements: Geophysics, 38, 1145-1158.

•1974, Computer processing of airborne electromagnetic data; Geophys. Prosp., 22, 490-509. Palacky, G. J., Holladay, J. S., and Walker, P. W., 1990, Use of inversion techniquesin interpretation of helicopter electromagnetic data for mapping of Quaternary sediments near Kapuskasing, Ontario, Canada: 60th Ann. Mtg., Soc. Expl. Geophys., Expanded Abstract, 689-692. Paterson, N. R., 1970, Exploration for massive sulphidesin the Canadian Shield, in Morley, L. W., Ed., Mining and groundwater geophysics/1967:Geol. Surv. Can. Econ. Geol. Rept. 26, 275-289. •1971, Airborne electromagneticmethodsas appliedto the search for sulphide deposits: Can. Inst. Min. Metall. Bull. 64(705), 29-38. Paterson, N. R., and Reford, S. W., 1986, Inversion of electromagneticdata for overburdenmappingand groundwater exploration, in Palacky, G. J., Ed., Airborne resistivity mapping, Geol. Surv. Can. Paper 86-22, 39-48. Peltoniemi, M., 1986, Systematic airborne electromagnetic surveys in Finland: an overview; in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 159-167.

Pemberton, R. H., 1962, Airborne electromagneticsin review: Geophysics, 27, 691-713. Reed, L. E., 1981, The airborne electromagneticdiscovery of the Detour zinc-copper-silver deposit, northwestern Quebec: Geophysics, 46, 1278-1290. Schwarz, E. J., Palacky, G. J., Stephens, L. E., Lefebvre, D. L., Church, H., and Gravel, C., 1988, Helicopter magnetic and electromagnetic survey of Monts Stoke,

EM

879

Quebec, and preliminary geological interpretation: Geol. Surv. Can. Paper 88-1B, 23-27 (in French). Seguin, M., 1975, Quantitative interpretation of a combined EM and magnetic helicopter survey for the search of economic magnetic taconite: Geophys. Prosp., 13, 471491.

Sengpiel,K. P., 1986, Groundwaterprospectingby multifrequency airborne electromagnetictechniques, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 131-138. •1988, Approximate inversion of airborne EM data from a multilayered ground: Geophys. Prosp., 36, 446459.

Singh, S. K., 1973, Electromagnetic transient responseof a conducting sphere embedded in a conducting medium: Geophysics, 38, 864-893. Smith, R. S., and West, G. F., 1987, Electromagnetic induction in an inhomogeneous conductive thin sheet: Geophysics, 52, 1677-1688. Stehfest, H., 1970a, Algorithm 368, Numerical inversion of Laplace transforms: CommunicationsAssoc. for Computing Machines, 13, 47-49. •1970b, Remarks on algorithm 368, Numerical inversion of Laplace transforms: Communications Assoc. for Computing Machines, 13, 624. Thio, Y. C., and Gleeson, L. J., 1979, Electromagneticfields in a half-spacescatteredby a buried sphereby the method of transformation of local elements: Geophys. J. Roy. Astr. Soc., 59, 671-681. Vaughan, C., 1990, A novel approach to airborne electromagnetic data compilation, in Fitterman, D. V., Ed., Developments and applications of modern airborne electromagnetic surveys: U.S. Geol. Surv. Bull. 1925, 81-88. Vironmfiki, J., Multala, J., and Peltoniemi, M., 1982, The multisensor aerogeophysicalequipment of the Geological Survey of Finland: 44th Mtg., Europ. Assoc. Expl. Geophys., Cannes, Abstract 60-61. Ward, S. H., 1959, AFMAGwairborne and ground; Geophysics, 24, 761-789. •1966, The search for massive sulphides•Introduction, in Mining GeophysicsI: Soc. Expl. Geophys., 117129.

•1967, The electromagnetic method, in Mining Geophysics II: Soc. Expl. Geophys., 224-372. Ward, S. H., and Hohmann, G. W., 1988, Electromagnetic theory for geophysicalapplications, in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics, V. 1, Theory, 130-311. Watt, A.D., 1967, VLF radio engineering:Pergamon Press. Weidelt, P., 1983, The harmonic and transient electromagnetic response of a thin dipping dike: Geophysics, 48, 934-952.

Wesley, J.P., 1958a, Response of a dike to an oscillating dipole: Geophysics, 23, 128-133. •1958b, Responseof a thin dike to an oscillatingdipole: Geophysics, 23, 134-143. West, G. F., and Bailey, R. C., 1989, Inverse methods in geophysicalexploration, in Garland, G. D., Ed., Proceedings of Exploration '87: Third Decennial International Conference on Geophysical and Geochemical Exploration for Minerals and Groundwater: Ontario Geol. Surv., Special Vol. 3, 191-212. Won, I. J., and Smits, K., 1986, Application of the airborne electromagneticmethod for bathymetric charting in shallow oceans, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. Can. Paper 86-22, 99-104. Zollinger, R., Morrison, H. F., Lazenby, P. G., and Becker, A., 1987, Airborne electromagneticbathymetry, Geophysics, 52, 1127-1137.




CHAPTER

DRILL-HOLE

11

ELECTROMAGNETIC

METHODS

Alfred V. Dyck*

INTRODUCTION

the geometric restrictions of the drill hole (for example, tilt of the field cannot be measured by geometric nulling). Relatively large-scale prospecting systems which

Application of drill-hole electromagnetic(EM) techniques used in exploring for bodies of conductive mineralization, can detect and define the conductive

could be considered

target itself or other geologic features (for example, fault zones) which may lead indirectly to discovery of a mineralized deposit. Drill-hole applications can be broadly defined to include any configuration where either the source or the receiver (or both) is submersed in the lower half-space. While this conceptual definition includes underground work in mines with conventional surface equipment, often man-made installations (metallic pipes, tracks, etc.) preclude suchwork. Accordingly, a drill hole is required to allow entry of all or part of the prospecting system into the ground. Such drill holes may be collared either on surface or in underground workings. Drill-hole applications could be consideredthe basis for the generalized problem of EM prospectingin that no geometric specificationscan be predefined. Surface applications are more restricted because, as is known

tinction

of the hostile environment

is more

than

a matter

of scale:

the

latter

devices, surrounded by the medium to which they are sensitive, yield a record of physical properties (in this case electrical) of the rocks in the immediate vicinity of the hole, i.e., an in situ rock-property measurement. EM prospecting systems, on the other hand, are sensitive to conductive bodies lying at some distance from the drill hole such as target A shown in the geoelectric section of Figure 1. An EM logger would be sensitive only to the conductive mineralization virtually intersected by the drill hole. Of the variety of methods shown in Figure 2 some have been tried in the past, and some are presently in use or exist only in the research and development stage. The method which uses a large, fixed but relocatable transmitter loop and a mobile, singlecomponent magnetic-field sensorcoaxial with the drill hole (large-loop method) shown in Figure 2c is the only

confinedto the surface (or slightly above it in airborne applications)and therefore can be assumedto lie in the same, roughly horizontal, plane. In principle, similar restrictions in geometry cannot be assumedfor borehole applications. Therefore instrumentation, operational procedures, and interpretation facilities must be designedto accommodatemany different geometries. However, any particular system is, at least in part, confinedeven more tightly than on surface.Therefore, from an operational point of view, borehole methods could be considered specialized. Specialized equipbecause

of surface methods are

emphasized here. By comparison, inductive well-logging techniquesemployed in hydrocarbon exploration and evaluation have seenrelatively little use in mineral prospecting and are mentioned only briefly. The dis-

beforehand, both the transmitter and receiver are

ment is needed

extensions

method which could be considered in routine use, a

fact reflected in the emphasis here on interpretation aids and field examples. Other methods ranging from the large-scale version of the induction logger (a) to hole-to-hole wave propagation (f) are also shown in Figure 2. An overview of each of these methods is given, including the configuration, the instrumentation, field procedures, and the type of response expected. Interpretation methods for large-loop EM

of

the drill hole, the relatively low occurrence of application compared to surface methods, and because of

*Queen's University, Department of Geological Sciences, Kingston, Ontario K7L 3N6. 881

882

Dyck

(LLEM)

are discussed separately in a subsequent


section.

The series of field examples presented include the Trillabelle and Gertrude deposits in the Sudbury, Canada region, which have served as test sites for many different methods, and several examples which have recently appeared in the literature. These examples illustrate the range of approachesthat are taken to interpretation. HISTORICAL

DEVELOPMENT

The course of development of EM methods applied in drill holes has been treated in some detail in Dyck (1975b) and only highlights are given here. EM methods were first applied in the well-logging industry (induction loggers). Doll (1949) describedthe basisfor interpretation of induction logs as the "geometrical factor" concept in which noninteracting(i.e. resistive limit) eddy currents are induced in the medium. The eddy currents are considered to be concentric with the drill hole for the simplest configurationin which both transmitter and receiver are close together (of the order of 1 m) and lie coaxial with the hole (configura-

EM

RECEIVER

TRANSMITTER

:.i.i:i:!:OVERBURDE.. N AND/OR WEATHERED

tion a, Figure 2). The use of this rather large-diameter device has not spread to mining exploration. An exception is a recent development in France of a similar short-spacingdevice designedspecificallyfor slim-hole detection of highly conducting metallic sulfides (Frignet, 1986). One of the first large-scale prospecting systems designedto detect conducting sulfide bodies at some distance

from

the

hole

was

introduced

in Noakes

(1951). The prototype system was similar to the surface Turam method (Parasnis, 1991, this volume) in that the transmitter was a large, fixed loop and the receiver

a dual-coil

device

that measured

the differ-

ence in magneticfield at two points about 15 rn apart. This equipment, by demonstrating that the method worked, became the forerunner of modern frequencydomain (FEM) drill-hole systems using large-loop transmitters. A similar concept was describedin Vekseer and Plyusnin (1957). Concurrently, Ward and Harvey (1954) developed a tilt-angle method of downhole EM surveying in which the configuration was minimum coupled (of the type depicted in Figure 2b). While this method is not presently in use, it has characteristicswhich suggestthat it shouldbe revived. A third type of large-scale system employs the fixedseparation, coaxial-coil configuration akin to the induction logger (Figure 2a). Elliot (1961, 1966) de-

(a)

(b)

SHALLOW TARGET

•Tx

XRx MINOR

SULFIDE STRINGERS

(c)

Tx

(d)

(MODERATE-HIGH)

%.x

SENSOR DEEP TARGET

CONDUCTIVITY

• (1-10 HIGH 3 S/m) '"i• ?. (10-3-1 MODERATE S/m)

•

(e)

MINERALIZED

(f) E, H

HORIZON

LOW

(10 -5 - 10-3S/m)

Fig. 1. Geoelectric section illustrating use of large-loop drill-hole EM method for massive-sulfideexploration in highly resistive environments such as Precambrian rocks. The system consists of a transmitter (Tx), a receiver (Rx), and a downhole sensor. Clearance (C) is the critical distance in a drill-hole exploration problem involving a highly conducting target buried at depth (D). There may also be other bodies which are conductive.

A host formation

other than

tight, crystalline (Precambrian) rocks could have significant (i.e. moderate) conductivity.

Electric

Dip

Fig. 2. Drill-hole EM systems: (a) dipole-dipole EM; (b) rotatable transmitter EM (with transmitter coil shown in

edge view); (c) large-loop EM; (d) hole-hole dipole EM [variation of (a)]; (e) remote transmitter, e.g., VLF radio transmitter--downhole measurementof electric and/or magnetic field; (f) hole-hole wave propagation.


Drill-hole

EM

883

scribed such a system which operated at 1230 Hz and measured in-phase and quadrature components of secondaryfield with a sensitivityof 100ppm. Versions which could operate at coil separationsof up to 150 rn (Smith and Hallof, 1971) were subsequently available but to the best of my knowledge the only presently operative systemis the somewhatsmallerscaleequipment (coil spacing up to 20 m) described in Frignet

package is also important as is the elimination of groundloops introducedby multiple groundingpoints (see Figure 3 for generalconsiderations).More precise information regarding the techniques for reduction of spuriouseffectsis consideredproprietary by the manufacturers; commercially available systems, by and large, have these effects under control.

(1986).

Dipole-dipole EM

At present, large-loopsystemswith a single-component magnetic-field receiver in either time-domain (TEM) form (one example described in Woods and Crone, 1980) or in FEM version are the downhole EM methods enjoying the greatest popularity in routine application. Nabighian (1984) reports that a drill-hole version of the Newmont EMP system was first tested in South Africa in 1970. Three-component sensors (Worthington et al, 1981) are a very recent innovation which will, hopefully, be incorporated in the next generation of commercial equipment. Other techniques described briefly in the next section, have been of little traditional interest in mining exploration and are still in the researchand development stage. They are included for possible future interest.

DESCRIPTION

instrumentation

are discussed in

Becker and Cheng (1988). Drill-hole EM sensorsand transmitters(coils) are no different in principle but are constructed

to suit the constraints

of the hole.

device is not necessary,nor is it possibleto determine direction to the conductor (i.e. its azimuthal location

with respectto the drill hole) without some additional information, such as dip of the conductor, to give a frame

of reference.

The most recently operative (to circa 1977) dipoledipole EM in North America was the McPhar SP-700 unit. Specificationsfor the systemare given in Table 1. The separation between transmitter and receiver is kept fixed by a number of short-length fiberglass spacerrodsjoined togetherin a manner similarto drill

cross-section.

Coils coaxial

with

the hole are, therefore, the most common and furthermore obviate the need for azimuthal

orientation

TRANSMITTER

LOOP

field E in additionto

magnetic field H)///

///

AV2ommon-mod !

The

main consequenceis, that in order to achieve appropriate sensitivity, the coils must have small crosssectional area, many turns, and thus relatively long axes. The need for water-tight housing, for use at considerabledepths in water-filled holes, further reduces the available

about the drill-hole axis, azimuthal orientation of the

OF METHODS

Various methods shown schematically in Figure 2 are described, including, where appropriate, the instrumentation, operational procedures, data reduction, and type response for simple situations. More extensive sets of model responses(for the large-loop method) are left to the interpretation section. The basics of EM

Principles of the dipole-dipole EM system with coaxial coil configuration are identical to those of a surface-modehorizontal-loopelectromagnetic(HLEM) system.Interpretation proceduresare likewise comparable. Because the system has cylindrical symmetry

of the

sensor.The intimate physical proximity of the sensor and the surroundingmedium which results in capacitive coupling makes the requirement for electrostatic shielding even more stringent for drill-hole sensors than for surface sensors.Proper shieldingessentially reduces the electric-field sensitivity of an elongated coil to the point where the coil behaves strictly as a magnetic field sensor. Reduction of common-mode voltageof the sensorrelative to the surfaceelectronics

Electric field at • / / / / ///•

sensor •'/••• / "'/

/

- •UN D L%OP

Fig. 3. Schematic illustration of parasitic voltages of importance in drill-hole EM (magnetic field) measurements: (a) electric-fieldpickup, which can be reduced by electrostatic shielding;(b) common-modevoltage, which can be reduced by shielding, differentially wound sensor and differential input to receiver; (c) ground loops, caused by multiple groundingpoints in system (manifestedas parasitic voltage on cable causedby magnetic-fieldinduction in loop which is

completedby groundreturn). The effect can be reducedby use of a preamplifier in the downhole probe.


884

Dyck

rods. The transmitter precedes the receiver down the hole. Assembly (shown in Figure 4) is carried out section by section as the probe is inserted into the hole, while the portion already assembledis held fixed in the top portion of the hole by a clamp attached to the drill-hole casing. The operating frequency can be selected remotely at the surface console so that, normally, one frequency is used on the trip down the hole and the other on the way up. Since the primary field

at the

receiver

is bucked

out to achieve

the

specified measurement accuracy, the field strength must be calibrated at the beginningof each run. Setup is a fairly laborious procedure because of the time required to assemblethe probe. However, once operations are underway, readings can be taken very quickly stopping the probe at each station or, with a suitable recording system, can be done on a continuous or finely digitized basis. The probe also can be pushed into almost horizontal or upwardly inclined drill holes with nonconductivepush-rods.If performed manually, however, this procedure can severely limit the length of hole surveyed. Measurements are normally taken at stations spaced either 3.05 or 6.10 rn (10 or 20 feet). Station locations are not necessarily repeated exactly if one frequency is used per trip. Readings are plotted midway between transmitter and receiver, usually as in-phase and quadrature of the secondary magnetic field as a percentage of the primary field. Total received signalalso can be plotted (in arbitrary units with 40 IxA being the nonanomalous value). Figure 5 shows characteristic responses(obtained from scale-model experiments, Drinkrow, 1975) for two types: where the probe passes through the conductor (intersection anomaly) and where it passes nearby (near-miss anomaly). Distinguishing between these two situationsis probably the most useful aspect of the device, parametric interpretation of small conductors (roughly equivalent in size to the coil spacing) being the other. A complementary suite of responses

for a conductive target in a conductive host also is available (Drinkrow and Duffin, 1976). A summary of results was published in Drinkrow and Duffin (1978). The relatively small scale of the system means that the system has relatively large sensitivity to small, economically insignificant, conductors. This sensitivity to geologicnoise, togetherwith the geometricnoise (relative motion of transmitter and receiver causedby flexing of the probe, estimated to be about 1 percent), constitute

the main limitations

to the method because

they exceed the specified instrumental sensitivity of 0.1 percent. For an overall sensitivity of 1 percent the search radius is limited to 0.5 of the coil separation (see, for example, co-axial Airborne EM diagram published in Brant et al., 1966, reproduced here in Figure 6). For the coil separation of 30 rn (Table 1), which

is the maximum

that

can be handled

conve-

niently, the search radius is limited to about 15 rn under good conditions. More recently short- and long-spacing induction loggingdevices, ROMULUS and ERIC, respectively, have been developed in France (Clerc et al., 1983; Frignet, 1986). The ROMULUS probe is a short spacing device of 0.85 m operating at 4 kHz and calibrated for magnetic susceptibility (in-phase) and conductivity (quadrature); the ERIC probe has a choice of either 5, 10, or 20 m spacingand an operating frequency of 1 kHz. Figure 7 shows the spectral response of a dipole-dipole system to a conductive, nonmagnetic whole-space with the response of the ROMULUS and ERIC probes indicated for a conductivity of 1 S/m. Rotatable-transmitter

EM

Another type of system also employs a dipole transmitter and dipole receiver but unlike the dipole-dipole system described previously, the transmitter remains in a fixed location at the collar of the hole. The label,

rotatable transmitter, describesthe procedure of mea-

Table 1. SpecificationsMcPhar model SP-700 drill-hole EM unit. ,

Operating frequencies: Coil separation (nominal): Measurement accuracy: Quantities measured:

0.1% of received primary field. In-phase and quadrature components of the secondary field.

Meter display:

Amplitude of in-phase and quadrature componentsare read directly

335 and 1340 Hz, switch-selectable on front panel of console. 30.5 m (100 ft).

from two meters on the receiver

console at switch-selectable

sensitivities of either 0.3, 1.0, 3.0, 10, or 30% full scale.

Down-hole probe size:

Maximum 28.6 mm (1 1/8 in.) diameter for use in EX (38 mm) or larger hole.

Depth capability:

910 m (in vertical water-filled holes).

Down-hole cable length:

760 m.


Drill-hole

surementwhich dependson rotation of the transmitter coil about an axis parallel to the plane of the coil. The axis, as shown in Figure 8, is horizontal and perpendicular to the page. The method is the drill-hole analog of the tilt-angle null method in that the transmitter is oriented

to achieve

a null in the downhole

EM

885

The most recently operative system also employed only one surface coil and is the system described here. Such a system (Table 2) was custom built by McPhar Geophysics for Anaconda Copper under the auspices

sensor. At

the null orientation a component of the primary field balances the secondary field detected by the coil at

Case 2•

that location.

The original published description of the method (Ward and Harvey, 1954) shows that the system used a second surface coil to generate the nulling field and that the transmitter

coil was fixed in orientation

1.0 -0

.Case I•__N•_10. 0

for the

length of each profile. The advantageof this method of operation is that the couplingbetween transmitter and any conductors nearby remains constant, thus rendering the secondary fields easier to map. The disadvantage is that two high-power coils are needed. A system describedin Salt (1966) required only one surfacecoil.

Case 0

1.0

Case 3-•.__100 Case4 •

1.0 -0

GEOMETRIES

ASSOCIATED CASE

WITH

EACH

NUMBER

Fig. 5. Type responses for dipole-dipole EM. The case numbers are entry points into an extensive suite of scalemodel experiments for tabular, rectangular bodies and thin and thick, circular disk models cataloged in the original publication. (Reproduced from Drinkrow, 1975.) -10 4

-10 =

-101 -101

-10 •

_ 0=

-10 4

-10 •

IN PHASE (PPM)

Fig. 4. In-field assemblyof McPhar SP-700probe. The unit is assembledsection by section similarly to drill rods. A clamp at the casing is used to prevent the accidental loss of the tool during assembly.

Fig. 6. Generalized interpretation diagram for coaxial dipoledipole EM system. In-phase versus quadrature peak responseis plotted for an infinite-sheet conductor of thickness t, lying perpendicular to the hole at a distance h from the EM system of coil separation L. g is the skin depth in the conductor. Reproduced from Brant, et al., 1966, diagram for the Newmont-Aero Coaxial Coil airborne system.


886

Dyck

of J. Betz (pers. comm.) whose personal experience with the system is the basis for the following discussion. Figure 9 shows the system in operation. The equipment can be used in two different modes, minimum coupled and maximum coupled. In minimum-coupled mode the transmitter is rotated for a minimum induced voltage in the receiver. In this mode the tilt angle of the transmitter (i.e. the angle between

in the vicinity (i.e., crosstalk). To minimize this condition, the frequency carried up the line from the sensoris the beat frequency producedby the operating frequency, 735 Hz, and a signalof 2400 Hz produced

0 - TX ROTATION

the plane of the transmitter coil and the plane of the Ix••_••• voltage (atminimum coupling)is recorded asamen// •• X X Hp drill-hole)is recordedas a measureof the in-phase secondary fieldat the receivercoil. Alsothe residual

'•"•

•

sure ofthesecondary field inquadrature with the

/[

primary field. In maximum coupled mode the receiver voltage is recorded as a measure of the total field (primary plus secondary). Spurious effects are possible becausethe electronic measurement package and the transmitter are both located close to the collar of the hole, a situation

'' \

conducive todirect pickup ofthe strong magnetic field

• so

(• QUADRATURE 106x Hs/ Hp

of

ductor

(• IN PHASE • IN PHASE/QUADRATURE

Fig. 8. Magnetic fields producedby rotatable transmitterand

,02• /:

/

'

J ß

-

target conductor. The transmitter coil isrotated about an

•0' •....... //•/ ß. ß-: j --///,0ø•_ 4 - -t • ....

position shown. Therefore a counterclockwise(+0) rotation

,0-' •L-/ •// -]-

of transmitter isrequired toachieve anull by offsetting H sthe with acomponent ofHe.This assumes little change in

•0.2•/ •x

.

• •

•,l• •

•I•..•.-• ••

• ß

', -. •

! •

_•_ •J_•_•3•! ...... - i

....

•

01

102

103

10•.

105

downward) component ofsecondary field Hsatthe receiver

transmitter-conductor coupling asthetransmitteris tilted--a

,o-3• , _•,• ................ 1 , ,'...... J .................... 100

axisperpendiculartøth Thecønd

tor to the right of the drill hole produces a negative (i.e.

06

107

condition which may be violated when the conductor is near

minimum-coupled with thetransmitter inthenontilted posi-

tion.

(.00 L2/4

Table 2. Specificationsrotatable-transmitter EM

ROMULUS

system.

ERIC

- 5 m

ERIC

- 10 m

Transmitter power:

3 kva maximum

ERIC

- 20 M

Dipole moment:

0.02 to 6055 A-m squared, continuously variable

Operating frequency:

735 Hz

o' = 1 MHO/M

Fig. 7. Responseof coaxial dipole-dipoleEM system to a conductive,nonmagnetic,whole space.The ratio of secondary to primary field measuredat the receiver is plotted as a function of induction number (product of conductivity, angular frequency, and square of the coil separation). The ROMULUS

and ERIC

values are indicated

for a conductiv-

ity of 1 S/m. (Reproduced from Frignet, 1986)

Bandwidth:

10 Hz (3 dB point)

Dynamic range: Overall sensitivity:

20 to 10,000 (E-06) gamma

1% of primary field strength at depth 600 m in plane of transmitter.


Drill-hole

EM

887

by the probe preamplifier. The filter circuit on the voltmeter is tuned to the beat frequency of 1665 Hz. The uniquenessof the systemlies in its operation in minimum-coupledmode. Measurementsare made on a station-by-station basis at intervals of approximately 10 m or less. Normally the transmitter is setup in two mutually orthogonal directions in order to explore in

is assumed, the transmitter tilt is such that the bottom

all directions

of the transmitter

from the hole. Where

the hole is vertical

coupling, it is possiblethat the coupling could, in fact, reverse to produce erratic or ambiguous results. Salt (1966), in fact, cites an example in which the results were tenuous becauseof sudden and sharp changesin the tilt-angle curves which reflected the proximity of the conductor. Notice that whatever sign convention must be moved

in the direction

of

the two setups are made parallel and perpendicular to the dip of anticipated targets should such information be available. Where the hole is dipping, one setup is made so that the plane containing the drill hole and

the target to achieve the null when the sensor is at the peak of the anomaly. When the conductor has been intersected by the hole, the signof the anomaly is related to the dip of the

transmitter

conductor.

is vertical.

For a hole in a nonanomalous

area, the extended plane of the transmitter coil contains the hole and there is no residual voltage. For a vertical hole, the plane of the transmitter will be vertical. For a bending hole, the plane of the transmitter (tilt) will follow the position of the probe. It sometimesbecomesnecessaryto correct for this effect usingindependentinclination and azimuth data for the drill hole in order

to authenticate

an anomalous

re-

sponse.

Figure 8 illustrates the responseof the system to a tabular

conductor

near the hole.

The

For weak secondaryfields (i.e. small tilts), the angle of tilt 0 is related to the in-phase response of the conductor by: In-phase anomaly = 3 0 x 100,

as a percentageof primary field. The amplitude of the residual voltage in the receiver at minimum-coupled orientation is a measure of the quadrature response of the conductor as:

field lines are

drawn on the assumptionthat coupling between transmitter and target changes little as the transmitter is tilted, an assumption which is not strictly true. The assumptionis best when the secondaryfields are weak but not valid when the secondaryfields are strong. For instance, if a conductor is situated near minimum

Quadrature anomaly = 2

Vresidual x 100,

V•max

as a percentageof primary field, where V•max is the primary voltage in maximum-coupled orientation at that depth. Precision of readings is between 0.1 and 1.0 percent

Fig. 9. Rotatable-transmitter EM in operation (Photograph courtesy of J. Betz).


888

Dyck

of the primary field strength at the downhole sensor from surface to depths of 750 rn. The main purposeof the maximum-coupledmode is to ensure that conductors which are poorly oriented for the minimum-coupledmode do not go undetected. Results are plotted as a percentage of the calculated primary field at a given depth. Effects of hole deviation are relatively small. Precision of the readings is 1 percent of the primary field at depth. The advantage of the system is its transverse transmitter moment which permits extraction of directional information. Its small physical size, furthermore, makes it convenient for undergroundoperations. The compromise lies in the difficulty in achieving a sufficiently strong field at great depth, especially for a low-frequency system. A rather laborious rate of production is a further consequenceof the null method of measurement. Certainly continuous profiling is not possible and wideband (i.e. multiple frequency) operation would

material which is a consequenceof the very limited use of the method. Furthermore, production of type curves in the laboratory has constraints similar to those encounteredin field operations, namely tedious, painstakingproduction becausethe procedure cannot be automated.

This

lack of automation

is a serious

drawback in covering the vast range of cases necessary for interpreting borehole data. Furthermore, computer modeling is awkward because of the feedback nature of the response,i.e. at the transmitter is rotated to achievethe null, the inducingfield impingingon the conductor also changes. Large-loop Electromagnetic Systems(LLEM)

Figure 10a is a block diagram of the large-loop drill-hole EM system showing the geometric configuration common to all three types: impulse-type, stepfunction type (which are both TEM) and multifrequency systems(FEM). The classificationrefers to the received primary waveform or "system function". Only a brief summary of systemspresently available

be tedious.

Only the most rudimentary considerations have been made here becauseof a dearth of interpretational

TIMING

(a)

TRANSMITTER

/

,

<

/

.__

.

ITXI

/..1-.,

..IDYl

< /

///

,, •

'4/ ? .• C •

STEP

IMPULSE

FREQUENCY

(b) TRANSMITTER

CURRENT

PRIMARY FIELD

(SYSTEM

FUNCTION) IN-PHASE

LOCATION SECONDARY Io•1 (.0 •

FIELD

log

CO

•..

LOCATION

c

•,"'" t

t o •

Fig. 10. Block diagram of LLEM systemsshowing: (a) Primary fields in relation to secondaryfields produced by the target conductor. Location A would be encounteredin surveyingan intersectingdrill hole. Location C would be encounteredin surveyinga drill hole which bypassedthe target; (b) Secondaryfield responsesat locationsA and C. The signof the secondaryfield is considered,by convention,relative to that of the primary field. The timing link may be hard-wire, radio, or clock synchronization.

Drill-hole

and those details specific to borehole work are dis-


cussed.

The transmitter loop is laid out on the surface or underground, if a suitable path can be found through mine workings. The loop is of a size comparable to the depth of the hole and commonly ranges from 100 to 1000 rn per side. Using a priori knowledge of the geology, the transmitter loop is usually placed in several positions chosento maximize expected differences in transmitter-target coupling. The size is, thus, a compromise between the need for a large area to provide a strongfield over an extensive zone at depth, and the desire for logistic convenience and some flexibility in coupling with suspectedtargets. Commercially available systems employ a singlecomponentinduction-coil sensor,coaxial with the drill hole, which is linked to the surface electronics by way of a cable. All systems are essentially adaptations of surface equipment so that most of the system components (transmitter and receiver) can fulfill a dual role. A comparisonbetween surface and borehole versions is shown in Table 3. The solepurposeof this tabulation is to demonstrate that similar system capabilities are available

for borehole

use as are available

EM

889

Figure 10b also demonstrates the relationship between transmitter current, primary field, and secondary field for a particular system. In each case the system function (i.e. primary field) is the time derivative of the transmitter current. Eddy currents are induced so as to produce a secondary field which opposesthe change in primary field at the face of the conductor (i.e. at location A). The secondary fields at location C are just opposite in sign to those at location A.

The sign conventions for all types of large-loop systems can be derived by following the above relationships. Other aspects of the sign conventions, including the direction of transmitter current and coordinate system, are arbitrary, provided one system is used consistently. The layout demonstrated in Figure 10a will be referred to as the "borehole standard geometry". If the responseof one systemin the standardgeometryis known, that systemresponsecan be used to derive the response for other systems and situations without returning to first principles each time. A detail description of the principles for each system type follows.

for their

Waveforms for each system type are tabulated in Figure 10b. Locations A and C refer to the situations where the probe either passesthrough A or nearby C, a target conductor shown in Figure 10a. The starting point for the waveform conventions depicted in the diagram is current flowing counterclockwise and abruptly terminating at time t = 0. The upwarddirected component of the primary field is shown, for each system type, in the region beneath the transmitter. The signs of the field are related as follows: the impulse function is the time derivative of the step function and the frequency spectrum is the Fourier transform of the impulse function. Likewise the secondary fields are consistent with each other on the

Impulse-typeEM.mThe following descriptionof one commercially available pulse EM equipment (Crone PEM) is representative, in principle, of impulse systems with drill-hole capability although significant operational differencesmay exist between the various implementations (including more recent versions of the Crone system). In addition to Crone PEM, Geonics EM-37, and SIROTEM systems listed in Table 3, two pulse systemsstill not commercially available are the Newmont EMP system (Boyd and Wiles, 1984) and the Soviet MPP-4 (Buselli, 1980). The transmitted current is a repetitive step and the systemfunction is, therefore, a seriesof pulses. Timing is supplied by either radio, wire link, or synchronized crystal oscillators. Secondary fields caused by eddy currents in nearby conductors are detected by means of sampling windows between pulses (see Figure 11 for waveform and samplingspecifications).The systemfunction is zero duringthe samplingtime so the

same basis.

channel values will be zero in the absence of second-

respective surface equivalent. Other important factors such as signal-to-noise ratio and transmitter-sensor design constraints (West et al., 1984) would necessarily be included in any fair comparison between systems.

Table 3. Comparisonbetweenboreholeand surfaceversionsfor somecommerciallyavailable TEM systems.

' System Crone PEM

4,000

GEONICS EM-37 SIROTEM

UTEM *attenuation

Effective area (m2) Surface

= 30.

100

Bandwidth (kHz)

Drill hole 4,000

3,000/30*

Surface

Drill hole

8

8

43

26

10,000

10,000

22

45

63,000

63,000

45

45


890 Dyck

N

w

z

w¾

wO


Drill-hole

ary fields. Thus the influence of the primary field is removed by time separation of the primary and secondary fields. This feature contributes to the sensitivity of the system and is considered by many to be particularly advantageousfor borehole systemswhere there is greater uncertainty in the geometry (of the receiver coil with respect to the transmitter). The advantage gained is offset to some extent by the requirement for larger dynamic range in the instrumentation (cf. step function and FEM systems)and may be negated entirely when the survey zone has a large backgroundresponse. Figure 12 shows various approachesto layout of the transmitter. If a group of surface holes or a single undergroundhole is to be surveyed, a single optimally

SHAFT

I I CONDUCTOR

(a)

EM

891

placed transmitter loop is usually expedient. A set of five transmitter loops placed centrally and in each of four quadrants is necessary to maximize information from survey of an isolated drill hole. Reconnaisance profiling down the hole is usually carried out at stations 10 to 20 m apart, especiallyunder the pressureof routine production. Station spacing may need to be reduced to as little as 1 m for detailing zones of particular interest. The systemcan be operated by adjustingthe gain at each station to give constant primary-pulse height which automatically normalizes the channel data to the measured pulse amplitude. This procedure produces a relatively constant system noise level of approximately 1-2 ppk (parts per thousand) of the primary voltage to a depth comparableto the transmitter loop dimensions.Another mode of operation is by constant gain, which is advantageousfor direct plotting of undistortedprofilesfor interpretation purposes. The former method is recommended in order to optimize the sensitivity over the whole profile and yet the data can be plotted later at constant gain (i.e. point normalized), which is a straightforward process if a computer is available for playback of the data. Another recommendation is that the polarity switch on the PEM receiver (used in conjunctionwith the polarity connectionof the loop wire with the transmitter) be set so that the primary field (pulse) would be positive for the standard geometry of Figure 10a. The use of this convention for other geometries as well, helps to circumvent potential uncertainties in anomaly sign which, as we shall see, is of great importance for interpretation. With reference to Figure 10b, position A gives a secondary field transient which is of the same sign as the primary field at that location, hence a positive anomaly. Position C, off to one side of the conductor, gives a transient of opposite sign and hence a negative anomaly. The predominant effect this negative anomaly has on field observations is shown schematically in Figure 13a. The negative peak between two positive flanks which occurs when the probe bypasses the conductor

allows

this

situation

to be differentiated

readily from that when the drill hole intersects the conductor.

CONDUCTOR

(b) Fig. 12. Transmitter layoutsfor surveying(a) a group of drill holes or a hole collared underground; (b) a single, isolated drill hole (from Crone, 1986).

An intermediate

case occurs when the drill

hole intersects a conductor near its edge (position B). Then a transition from an intersection-type anomaly to a near-miss anomaly is observed as the induced eddy currents migrate through the conductor with the passageof time after the pulse. Field examples of Type A, B, and C are shown in Figure 13b. The migratory nature of eddy-current behavior is discussedfurther in the interpretation section. Figure 13b shows the data plotted on a linearlogarithmic amplitude scale. Logarithmic compression


892 Dyck

$ I

o• o•

,

iii

OCr

'r

m•


Drill-hole

is one means, commonly usedfor plotting Crone Pulse EM data, of accommodatingthe large dynamic range of impulse systems (the variable, sample-dependent gain factor built into the Crone receiver provides further dynamic compression).The linear portion is necessary to accommodate change in sign. The transition from linear to logarithmic scale is done at a scale factor of

10 linear units = logarithmic cycle/(ln 10) = (0.4343) x logarithmic cycle which can be shown to be necessaryto ensure that the derivative

is continuous

at the transition.

The use of

logarithmic compressionof EM data requires that the following be kept clearly in mind: -anomaly shapesare distorted, -apparent anomaly size dependson the strengthof backgroundresponseon which the anomaly rides, making comparison of anomaly amplitudes more difficult.

Another common means of plotting impulse-type data is to provide separate axes for each channel or group of channels with sensitivity increasing as a function of delay time of the sample (eg. Geonics EM-37 or Newmont EMP data, Boyd and Wiles, •9s4•.

Step-functionEM.--The UTEM system,as the only implementationof the step response,has been studied extensively for surface surveys (West et al., 1984). The transmitter generates a repetitive triangular current waveform so that the system function, with an induction coil sensor, is a series of repetitive steps of alternating sign. Synchronization between receiver and transmitter is provided by low drift, crystal oscillators. The decaying secondary field is measured as a function of time with binary-spaced windows of binary-variable width as shown in Figure 14. In this case the decay is sampled in the presence of the primary field which must be removed by calculation usingprecise knowledgeof the geometryor by direct measurement of a late-time channel which is relatively unaffected by the transient. We again refer to Figure 10 for the application of signconventionsin a manner consistentwith the other systems. Position A gives a negative anomaly (the transient is of opposite sign to the change of state of primary field), while position B produces a positive anomaly. This result is equivalent to the convention illustrated in Figure 14, that a ratio of total field to primary field greater than one is a positive UTEM anomaly. Note that anomaly signs for the step re-

EM

893

sponse are thus reversed compared to the impulsesystem response.

Implementation of the system is comparable to other TEM systemswith the exception of two technological innovations: (1) The data link between the downhole sensor and the surface receiver is fiber-optic cable, a feature which improves data quality by ensuring electric isolation of the sensor (Lamontagne and Macnae, 1986).

(2) Data acquisition and winch operation can be performed automatically under microprocessorcontrol by programming survey specificationsinto the system, including automatic fill-in of intermediate stations where required. Frequency-domainEM (FEM).--Borehole FEM in LLEM configurationis most often an adaptationof the common Turam approach (see Parasnis, this volume) which uses some form of comparator receiver. In modern versions, such as the Scintrex DHEM-5/SE-77

system, the comparator is a phase-lock amplifier requiringa strong,clean signalto drive the signaldetection circuitry and provide amplitude and phase reference. The configuration thus chosen for borehole operation, shown in Figure 15a, is surface placement of a reference coil (which can be the one used for

normal Turam operation)and a single,mobile receiver

LTX

I.

.•ALF CYCLE.I

'-•.

d•TIME,18-40 as']

' A-..,"•-.

•

-

••

'-'

t

•

SAMPLING WINDOWS

-BINARY SPACING-

t

SECONDARY

FIELD

2 %Hs:H-H P

Fig. 14. Waveform specifications for UTEM system. A positive UTEM anomalyoccurswhen the ratio of total field to primary field is greater than one. (From West et al., 1984)

894

Dyck


REFERENCE

r

I

COIL AUTOMATIC

INTERFACE

BRIDGE

WINCH

UNIT

RECEIVER

'

SE-77

1

TRANSMITTER LOOP / ITRANSMITTER

TSQ-2M/500W

I i i

I i

', I Block diagram SE-77

..j

,i Z<

of

I

TSQ-2M/500W

and

DHEM-5

Im Hs

(:3

c

I

Drill Hole EM Logging System

• DHEM-5•J

Hs OBSERVED AT DEPTH

HP

FIELD

Re Hs REFERENCE AT

In-phase

(a)

i

component

FIELD

SURFACE

- g

Phase - A(;b + arctan

Re Hs+HP 1 glmH 1

(b)

HR

= &• + g lm Hs

for small phase anomalies,

HR

where g and A•

g- HR / HP

are gain and phase shift. g varies in steps by factors

of two.

Fig. 15. (a) Borehole FEM block diagram (Scintrex). (b) Phasor diagram of fields measured with SE-77 and DHEM-5 logging system.

Drill-hole


to travel

down

the hole.

No

additional

reference

or

synchronizationwith the transmitter is necessary.The receiver thus suppliesamplitude of the in-phasecomponent of the total field (primary plus secondary) normalized to the reference field strength, and the phase difference between total field and reference field. As Turam receivers are normally restricted in range (ratio: 0.5 to 2.0; phase +_20degrees) an interface is required to provide several decades range of amplication of the downhole signal and a full 360 degreesphase rotation. In the absence of conductive material, only the normal primary field change with depth (geometric attenuation) will be observed, with no phase change. The zero levels are completely arbitrary unless calibration is carried out (not usual). Secondaryeffects are seen as perturbations on the normal primary field variation. For small anomaliestheseperturbationsare, approximately, in-phase and quadrature components of the secondaryfield relative to the primary field at the depth measured (see Figure 15b). If the primary field is not first removed the display of profiles is convenient on logarithmic amplitude, linear phase axes in order to "normalize" the data to primary field strength. Measurement at position A in Figure 10 results in negative anomalousamplitude values and at position C produces a positive anomaly. The senseof these anomalies is the same as for the step-response system.

Figure 16 demonstrates the plotting of field data from all three types of system at the Gertrude site (Sudbury, Canada) where there is an off-hole conductor at a depth of 305 m. Anomaly A is the responseto that conductor. The sourcesof the other anomalies (B and C) are not known. Figure 48 (under "field examples") illustratesdetails of the geologicsectionand of the interpretationof the Pulse EM anomaly. Three-componentSystems.--Three-component (magnetic field) sensors are a significantimprovement to the large-loop method with the single-component probe and represent the next generation of borehole equipment. This improvement is still in the development stage and because published results are scarce the discussionhere is limited to a few perfunctory remarks.

A computer-model study reported in Dyck and West (1982) clearly demonstrates the additional capability afforded by measurementof three orthogonal components of the secondary field. As with the magnetic method (usingthe static field of the earth as shown, for example, in Levanto, 1959; Hattula, 1986) obtaining unequivocal location of the source body is possible. Figure 17 shows the results of an EM computer simulationfor a deep orebody in the Noranda, Quebec

EM

895

mining area. The signsof the transverse components, X and Y, depend uniquely on which quadrant the target occupies with respect to the drill hole. In this hypothetical case simple vector summationof X and Y peak responsescan be used to determine azimuth of the conductor's center to within about 5 degrees. As shown in the Interpretation section, use of a single axial componentin this regard is not nearly so straightforward.

One successfulprototype 3-component system has been described in Pantze, et al. (1986). It is an FEM system which delivers 4 A at 1 kW into a standard transmitter loop up to 1 x 1 km, at two frequencies200 and 2 000 Hz.

The three sensors are mounted

in a 32

mm diameter probe as shown in Figure 18. Gravity provides the orientation reference so that Y is always horizontal, X is along the axis of the probe and Z is orthogonal to the other two. This principle of orientation, which is the same orientation used in the ABEM 3-component borehole magnetometer, unfortunately does not function in a vertical hole. The coils are tuned

to a bandwidth of 150 Hz about the center frequency, and the signals are preamplified and transmitted up a coaxial cable to the detector

which has a bandwidth

of

1 Hz. Amplitude and phase are determined by the instrument to accuracies of +_2 percent and +_2 degrees, respectively, for signalswithin the range 0.01 to 16 nT. The information is stored digitally and transferred to a mainframe computerfor further processing. The residual fields (after removal of the primary field and background response caused by the host) are routinely plotted in two different ways for interpretation. Figure 19 showsthe in-phaseresidualsplotted in profile form for three components measured in borehole 103 at the Kankberg site in Sweden. Figure 20 shows data from boreholes 101 to 104, represented by vectors. Notice that the vector plot permits visualization of the secondary field lines around the conductor, and that the pattern of vectors observed in borehole 103 is clearly indicative of the conductor to the west of the hole. Most

of this information

is contained

in the

Y-componenttrace (Figure 19) which shows a characteristic antisymmetric anomaly whose sense is dependent on the location of the target. The relatively small excursionin the Z-component indicates that the plane of the conductor (i.e. its dip) is parallel to the Z-component sensor. Borehole

EM

with Remote

Source

This category includes borehole very low frequency (VLF) using remote military transmitters as sources, and borehole audio frequency magnetics (AFMAG) (Text continued on page 900)


896 Dyck

j

Ill

o -•

I i I I

0 o 0 o 0 o 0 o

I I I


Drill-hole EM

897

--

FC266

FC266

I

I

I

{

-tO

0

tO

IOO

I

-tOO Pulse

M

600

M

--700

M

--

500

M

--

600

M

--

700

M

--

500

M

--

600

M

--

700

M

-D246

--D219

I"

I

I

I

-IOO

-IO

o

IO

EM Amplitude (PPK)

219o I00

500

m

_ 0D246

Y

•;}D240 PLATE

(o-s = IOOS)

;X

D219(• 0D246

• IOOrn •

D240

SPHERE

(o- = I00 S/m)

Fig. 17.Computersimulation of three-component magnetic fieldmeasurements for an orelensin the Millenbach Mine, Noranda,Quebec.Directionto the conductorcouldbe obtainedfrom the ratio of peaksof the two components transverse to the drill hole (X and Y). The modelresponses were generated usingPLATE and SPHERE (Dyck et al., 1980;Dyck, 1981)computermodelsdiscussed in the modelingsection.

898

Dyck i


8005

i

I

I

-

_

1000s

ß . '• '" "-.. lJ" '

-

'.[:: •,'•"

ß

1•)4 102101 103 12005

i

i

5600w

i

I

5/-,00W

5200W

Fig. 20. In-phase residual data at 200 Hz for Kankberg boreholes 101to 104 plotted in vector form. Interpretation of these data determined the location, to the west of borehole

Y

103, of the conductive zones shown here in plan at a depth of 350 m, and their dip roughly perpendicular to the boreholes (from Pantze, et al., 1986).

z

x

Fig. 18. Three-component borehole EM sensor of Boliden Mineral AB, Boliden, Sweden, showing the arrangement of the coils and the definition of the X, Y, and Z directions. The transverse sensors are free to rotate as a pair about the longitudinal axis of the probe so that Y is always horizontal. This method of orientation functions only in nonvertical boreholes (from Pantze, et al., 1986). / P ,ase and I

100

COIL

':-_-..-• .,,'..-."'/ -'-,

'-"¾.....,

x

,eeleelelee •

7 Conductor. I

I

-so

'

%

//

-'

!

•.--y

Neop.... shielded

\

jacket.

\

10mseparation

/

H

H

.•

PRIMARY FIELD

PROBE

-loo

200

300

400

500

600

Borehole depth (rn)

Fig. 19. In-phase residual data at 200 Hz for a 3-component survey at the Kankberg site, Sweden (borehole 103), plotted in profile form for each component (from Pantze, et al.,

Fig. 21. Prototype borehole VLF system built for the Geological Survey of Canada by Mineral Exploration Research

1986).

Institute, Montreal.


Drill-hole

z

E E

E E

i.u

z

EM 899

J

o

E

0 ß

ß

ß

N

E

i.u

oc

-r

I.U

I-

IN

i

i

Xo

ß ,0

E

ß N


900

Dyck

using natural sources such as tropical lightning activity. The source must be sufficiently far removed that the field in the vicinity of the target can be considered uniform. Experiments have been carried out with both types of source (Roy, 1984; Hayles and Dyck, 1989; St. Amant, 1984) but only the VLF method is discussed here. In addition to the uniform source, the

VLF method offers one other novelty: the measurement of electric field as well as the usual axial magnetic component. Figure 21 illustrates a Geological Survey of Canada prototype system (Roy, 1979) which measures both magnetic-fieldand electric-field axial componentsproduced by remote VLF sources. The receiver at the surface continuouslycomparesthe downhole signalto a surface reference signal, both in amplitude and phase. The in-phaseand quadrature-phaseoutputsare recorded digitally and displayed as a function of distance down the hole, via a microcomputer system. The normal (or undisturbed)componentsof electric and magnetic fields are horizontal and orthogonal to each other. There is evidence that the dominant

mode

of VLF excitation of large, conductive fracture zones

BOREHOLE NAA

0

CR-I

Ez/Hy

Ez/Hy -25

VLF,

NSS

+25

-25

0

+25

• 50M--

•-

•- >-

FF •

O

-J

•

o

o

•

•

•

•

•

o

O:7FF OT o

T

T

•

ß

ß

ß

F

T

•

ßßßFT F

is current channeling(Scott, 1987). Regional currents, driven by the horizontal electric field, are distorted by relatively local conductivity inhomogeneitiessuch as fracture zones. Figure 22 showsthe expected currentchannelingresponseproducedby an oblate spheroidal inhomogeneitynearby a drill hole in which the electric potential difference is measured. The S-shapedanomaly is theoretically diagnostic of the size and relative location of the conductor. An example of field data (Figure 23) showsa seriesof S-shapedanomalies each coincident with fault zones intersected by the borehole.

Wave Propagation Methods

Whereas all other methodspreviously discussedare implementations of EM induction, methods such as radio detection and ranging (RADAR) exploit the phenomenonof EM wave propagation.Strictly speaking, RADAR is concerned with obtaining reflections from off-hole targets. More generally, EM wave propagation in boreholes is concerned with both reflection and transmissionof EM energy. Two sets of equipment have been recently developed, both sets can operate in either mode. One set (Nilsson, 1986; Olsson and Nilsson, 1986) has separate units for transmitter and receiver antennae, and can thus be used for both single-holeand crossholemeasurements. The other set (Wright et al., 1986) employs receiver and transmitter in one hole and, if crosshole operation is desired, a transponder (active reflector) in an adjacent hole. The principle of geotomographic imaging (see for example, McMechan, 1983), illustrated in Figure 24, is

T

lOOM -COMPUTER PROCESSING

J • T 150M

--

200M

I

250M

--

INVERSION

F

• IN

SIGNAL CONDITIONING ACQUISITION AND STORAGE

ßßßFT

•

ßßßFT

•

••

PHASE

•

ß

AND

GEOTOMOGRAPHIC

{

•

j COLUMNS

I WS

F

Ez, Borehole oxiol component H•

•a•netic

re•eren•e

ol

s•rJo•

RESPONSE

Fig. 23. VLF electric-field data from Chalk River, Ontario, Canada. The major anomalies, which appear with either of the two orthogonal field directions, are indicative of intersected

fracture

zones

and coincide

with

other

borehole

anomalous measurements such as resistivity and neutronporosity logs, tube-wave events (T), and in-hole television observations(F) (from Hayles and Dyck, 1987).

Fig. 24. Schematicof a geotomographicimagingsystem. The inversion processconsidersthe effect of velocity or attenuation that each cell has on the rays passingthrough it.

Drill-hole


•

60

50 x

'• o%

H MEASURED

= SCREENING COEFFICIENT

•,•\•••///' •......•-30 20

OBSERVED

•2

•'•

host and other

conductors.

These

modes

can be combined in many different ways, some of which are illustrated in Figure 26. The total response can be described, albeit nonrigorously, in terms of four

H NORMAL

-14o NORMAL

D•TANCE

conductive

contributions:

3 1% Copper

H

901

diagnosis. This section provides a basis for understanding of important effects but does not present suites of model responses. For purposes of further discussionthe mechanisms of conductor excitation are separated into component modes which include the complications introduced by

LEVEL 1

COcP•PEE R• OBSpEo•V•ION • TRANSMITTER

*fn

EM

,

2o

(m)

Fig. 25. Radio-absorption experiment applied in the Mailaram copper mine, India (Rao and Rao, 1983). The system was operated at 1 MHz, delivering three watts into a 50 ohm load, to transmit at distances up to 20 m in 26 ohm.m host rocks. The screening coefficient (1/attenuation) is correlatable with the amount of copper (as a measure of sulfide content).

(1) Free-space inductionsInduction in an isolated, confined target located in an insulator is describable in terms of an induced vortex or eddy current which flows entirely within the target (Kaufman, 1978; Nabighian and Macnae, this volume). The vortex gives rise to a secondary field

"moment",H s whosecontribution to theobserved fieldat the measurement pointis H2. Free-space induction in nearby conductors, for example, powerlines or other artificial structures, produces a

background secondary fieldIt B. (2) Half-space inductionsInduction in the medium which hosts the target is describable in terms

of a vortex,drivenby electricfieldE]/2 which to cover a block of ground as completely as possible with a multitude of ray paths. Interpretation of the data, using either velocity or attenuation as the parameter, is accomplished through inversion on a cell-bycell basis. Daily (1984) and Somerstein et al. (1984) have applied the technique to monitoring of oil-shale retorts. Contour-map presentation of results (especially when in color as shown in Daily, 1984) is particularly striking. Rao and Rao (1983) have shown that a similar approach (but without tomographic inversion) can be usefully applied to mining exploration underground (see Figure 25). The result of their experiment was a correlation between sulfide content of a deposit and the screeningcoefficient of transmitted radio waves. This experiment also is known as the radio shadow-zone method (Buselli, 1980). TOOLS

OF INTERPRETATION

Models

An intuitive understanding of borehole EM responsescan be achieved with models of simple shape. While such simple models may sometimes seem geologically unrealistic, they can often supply physically satisfactory explanations of the important aspects of the induction processin geologicconductors. Furthermore, the gross characteristics of field survey anomalies are often reproducible with sufficient fidelity to satisfy the explorationist's requirements for target

recedes from the surface and expands indefinitely, hence the descriptive term "smoke rings" (Nabighian, 1979; Nabighian and Macnae, this volume).

The contribution, H 1/2 to thefieldat the observation point depends on the relative location of the smoke ring. (3) Resistive interactionsDistortion of a current flow by an inhomogeneity which is either more

HP ] •

E'/• H p / \ •HP\\\ '\ \:\JE), i// / \ ',..,

HEH • /•Hr / H• H

H• /

•••;:•td

• HT H•

H

Channeled

C.....t

Fig. 26. Modes of conductor excitation (induction and current channeling) illustrated for the large-loop system. Symbols are definedin the accompanyingtext. Note the variance in polarity with which a particular mode contributes to the responseat different locations.

902

Dyck


conductive

or more resistive

than the host medium

(referred to variously as current channeling or gathering) can be described in terms of anomalous electric field arising from induced charges on the body (see Jones, 1983, for detailed discussion). In

Figure26, channeling of currentdrivenby E •/2 produces a resistive contribution, H E. (4) Inductive interaction--Mutual coupling between current vortices induced in the target, the host medium, and any adjacent confined conductors. The coupling can be thought of as a contribu-

tion,H3, arising fromthetertiaryfieldmoment, HT, which is induced by the secondary fields of neighboring conductors. A rigorous treatment of the complete problem is found in West and Edwards (1985). The following section focuses on induction

in confined conductors

in

Confined Conductors.--Three useful examples of confined conductorsare a simple loop, a plate (thin, rectangulartabular body), and a sphere. Together they constitute a complete range of three-dimensional(3-D) conductorsin a physical sense(Dyck and West, 1984), as follows: the loop confines the induced current to a filament; the plate confinesthe eddy current to a plane; and the sphere constrains the volume in which the currents flow but has no influence

realism

Some

of the models

are also

discussed in Nabighian & Macnae in this volume. Table 4 lists sources of modeling facilities and results which are specific to borehole EM interpretation.

in either

one.

S(t)=c

dressed, but some discussion of the other modes of is included.

of

A loop has the simplest responseof all conductors; its responseto a step change in magnetic field at time t = 0 is just a single exponential decay:

free space, as this is the situation most readily adexcitation

on the orientation

those currents. All three are important models for understandingborehole EM responses.In particular, plate and sphere models can be used in complementary fashion to overcome the obvious lack of geologic

exp(-t/,),

t>O

where -r = L/R (inductance/resistancefor the loop), and c, the coupling coefficient, is a measure of the couplingof the circuit with the primary magnetic field, i.e.

Table 4. Summary of sourcesof modeling facilities and results for borehole EM interpretation. C--Computer model

P--Physical (scale) Models

Free-space Simple loop

Intersected plate

model

References

Taylor (1985), Barnett (1984) Dyck and West (1984), Boyd and Wiles (1984), Fullagar (1987), Duncan (1987). Woods (1975), Woods and Crone (1980) **Drinkrow (1975) *Drinkrow and Duffin (1976, 1978)

Plate

Woods (1975)

Annan (1974; algorithm and core program), Dyck (1981), Dyck and West (1984), Dyck et al. (1980) Sphere

Nabighian (1971, theoretical solution; also others), Dyck and West (1984), Dyck (1981), Dyck et al. (1980)

Oblate Spheroid

McNeill (1980), Fromaigeat (1985)

Blocks and wedges

Dyck (1981)

Multiple plates Conductive

host

Plate

*Drinkrow and Duffin (1976, 1978)

Plate (2-D)

Eaton and Hohmann (1984)

Plate (3-D)

West (1986), West and Ward (1988)

Half-space and buried conductor

Levy and McNeill (1986)

*Pertaining to dipole-dipole EM method.

Drill-hole


C •

•s Bßda

and S is the area enclosed by the loop. Kaufman (1978) has shown that the response of a 3-D conductor can, in general, be reconstructed from an infinite number of equivalent current-carrying loops (eigencurrents) each having a distinct time constant,

,i-n and couplingcoefficient,c,. The step responseof such a conductor

can thus be written

as the sum of

magnetic fields created by these eigencurrents:

S(t)= •cn exp(-t/xn).

EM

903

tion) because there are no boundaries at which charge can accumulate (Eaton and Hohmann, 1984). The full 3-D solution, which includes inductive and galvanic interaction, shows the relative importance of those two contributions (San Filipo and Hohmann, 1985). Furthermore San Filipo and Hohmann admit that 3-D EM methods are in their infancy, so perhaps these results shouldbe treated especially warily for borehole responses.Approximate results published in Levy and McNeill (1986) deliberately omit the inductive interaction term from their 3-D models but are, nevertheless, useful in deriving some appreciation for the complexity of the interpretation problem when the host is conductive.

As pointed out in Macnae and Lamontagne (1986)

Appendix A and Appendix B briefly outline solutions for two simple models, plate and sphere, which illustrate the foregoing. Properties of eigencurrents are discussed in Kaufman (1978) and Dyck and West (1984); the latter discussion is reproduced here in Appendix C. In the case of an infinite

thin sheet there is an exact

"equivalent" current which is representative of the external magnetic fields (Grant and West, 1965); and in the case of conductive host media (half-space) there is an approximate equivalent current (Nabighian, 1979); but in the case of confined (3-D) conductors, we need a whole set of equivalent currents (an infinite set referred to above as eigencurrents). In practice, the EM response of simple conductive models can be computed to sufficient accuracy by using a limited set of approximate eigencurrents. In addition to computational convenience the eigencurrent approach facilitates analysis of EM responses. To appreciate borehole EM responsesin detail, particularly for the large-loop systems, study Appendix C.

Targets in ConductiveHost.--Modeling of such targets may be accomplishedthrough scale-modelexperiments or numerical methods. The latter approach provides most of the few results available at present

some surface

EM

results

found

in the literature

are

adaptable to borehole situations. One useful result is shown in Figure 27, as adapted from McNeill et al. (1984) by 90 degrees rotation of the diagram and transpositionof the components so that Z corresponds to the usual axial component. While not strictly applicable because the air-earth boundary in the model (unrealistically) parallels the borehole, the diagram nevertheless

illustrates

the relative

contributions

of

the inductive (free-space) and galvanic excitations. For a borehole situated on the opposite side of the conductor the sign of the galvanic contribution would be of opposite sign. The influence of the galvanic componentwould also alter the crossoverposition and hence the apparent dynamic characteristics of the anomaly. The half-spaceresponsedecaysaccordingto a power law (the exact exponent depending on component and systemfunction, Nabighian, 1979; Nabighian and Macnae, this volume); the field of the target conductor decays, of course, exponentially. Large Loop EM

tailed shapes of bodies at depth are unimportant for numerical modeling. Of course this is not strictly true for borehole surveys, where details at depth are not only of interest but indeed critical to the response

The fixed transmitter of the LLEM method gives us the opportunity to study magnetic fields which can be mapped, or observed in profile, without the added complication of a mobile transmitter. A sound appreciation of eddy current behavior is best acquired through use of eigencurrentsand visualization of the dynamic effects produced by current migration (or diffusion). To optimize use of dynamic effects higher order eigencurrents (or multipoles) must be excited. This procedure is in contrast to the approach of Nabighian and Macnae (this volume) who advocate use of large-loop transmitters to minimize high order excitation and thus improve estimation of bulk param-

when near the drill hole.

eters. The discussion

Two-dimensional (2-D) models include inductive interaction with the host but, by their nature, exclude current gathering (the contribution of galvanic interac-

evant to borehole applicationsis illustrated with model responsesfor a variety of conductor shapes using parameters of the Crone PEM system. Unless other-

and these are limited to models of heuristic nature,

because of the prodigious computational effort involved (see Hohmann, Vol. 1, for full treatment). Hohmann (1983) stated that high resolution of structures at depth cannot be achieved because of the diffusive nature of EM induction, and therefore, de-

of induction

characteristics

rel-


904

Dyck

wise indicated, plate and sphere model responsesare computed using programs PLATE and SPHERE, respectively (Dyck, 1981; Dyck et al., 1980). Anomaly characteristics are analyzed through observation of the behavior of peaks and crossovers.

hole introduces asymmetry into the anomaly which can changethe shapefrom the standardnegativepeak with positive flanks to an S-shapedresponsewhen the current plane is parallel to the hole. The thin plate and the simple loop are the only models for which the

Crossovers

orientation

are sensitive indicators

because the sensor

of the conductor

is the sole control

located at a crossoveris null coupled to the principalorder eigencurrent. In this position the probe's location is indicative of the grossgeometric characteristics of the principal-order eigencurrent but is able to respond to the higher order eigencurrents. The great sensitivity to all influences means, however, that crossover position may be strongly altered by background effects (Macnae and Lamontagne, 1986). In many cases it is necessary to resort to a less sensitive indicator, such as the behavior of anomaly peaks.

sphere model discussedin the next section. The main effects of increasing clearance (defined as distance of closest approach of the drill hole to the conductor)are reduction in amplitude and a widening of the anomaly. At great distance from the plate, the secondary field will, of course, resemble that of a dipole, but close to the plate, variations in anomaly amplitude and width are less sensitive to clearance than for the equivalent dipole field. A compilation of crossover separationas a measure of anomaly width is displayed in Figure 28b as a function of clearance and plate dimensions. Guidelines for determining the search radius can be obtained by combining the dependence of amplitude on clearance with the effects of geological, ambient, and instrumentalnoisefor a particular environment. In ideal conditions (i.e. a limit of anomaly detectability

Effect of Target-Drill Hole Relationship.--The dominant determiningfactor in the grossanomaly shapeis position of the profile with respect to the eddy current vortex. Figure 28a shows the importance of the effect for a plate conductor of various orientations with the influence of transmitter position minimized. As expected, inclination other than perpendicularto the drill

() LOOP

0

/

i

/ SPACE FREE

o

/

/f PRIMARY• FIELD,•

/ ! i

/\1

150m [ ', I• ',

\

0.2

0.4

0.6

i\

i

i

', \

•

0.8

/

1

X

'-, \ Component I '-X //

•.Component

PLATE 200 X 600 rn

I

s

I

0'= 0.01 S/m

I I

•

over

the orientation of the induced vortex, in contrast to the

0'=0

..... GALVANIC VORTEX

EARTH-AIR

INTERFACE

Fig. 27. Free-space-inductionand galvaniccomponents(for a GeonicsEM-37 at 3 ms, adaptedfrom McNeill et al., 1984). Note that the transmitter loop and air-earth boundary are not realistically placed for a borehole model (see text). What represented,in the original diagram,the horizontal componenton a surfaceprofile is now labeledZ as the axial componentin a vertical boreholeprofile.


Drill-hole EM 905

o•

o o o

ii ii ,1

NOI.I.VaVd3S a3AOSSOa3

X

906

Dyck

equal to 1 ppk, the instrumental limit) the search radius is approximately equal to the dimensionsof the target (see, for example, Dyck and West, 1984, Figure


16).

Effect of Transmitter-Target Coupling.--Another important, relatively model dependent factor in the grossanomaly shapeis the distributionof primary field over the conductor. In a plate model only the component of primary field, which is perpendicular to the plane of the plate, induces current. The range of primary field variations in Figure 29 (six different transmitter positions) illustrates that variations in response are primarily a function of the degree of excitation of the principal eigencurrent. Transmitters 2, 3, and 4 excite the plate relatively uniformly and in the same direction so that the principal eigencurrentis the dominant contributor and very little variation in response is visible. Transmitter 6 excites the plate uniformly in the opposite direction with a resulting 4

5

C

•

(D

©

©

I

(D

6

•)

(D

•)

Tx

3

2

G

A

(D

Lx

polarity change in the observed anomaly but little change in gross shape. The secondary field could thus be said to have a "moment" which is always perpendicular to the plate, irrespective of transmitter position. The primary fields from each of transmitters 1 and 3 change signover the plate with the result that the second-order eigencurrent, which has a figure-eight shape (see Figure A-2, Appendix), is the dominant contributor to the response. In both casesthe width of the anomaly is noticeably reduced. Also, the early time responsecaused by transmitter 1 is of opposite sign becausethe field nearest the drill hole is reversed, although at later times the responseis of normal sign indicating that the principal eigencurrent once again dominates.

The effect of variation in transmitter-targetcoupling is startlingly different for a sphere model as shown in Figure 30. Whereas the eddy currents are confined to the plane of a plate, they are free to assume any orientation in a sphere, and thus flow in planes perpendicular to the main direction of the primary field. The secondarymoment producedby the eddy currents is thus parallel to the primary field. When the moment is parallel to the drill hole, the standard, symmetric anomaly shape is produced (eg. transmitter 2 and 5). When the moment is perpendicular to the drill hole an antisymmetric anomaly is created (eg. transmitters 1, 3, 4, and 6). See also Figure 31 for another illustration of this important effect. Variation in anomaly shape with transmitter location is thus one of the principal diagnosticfeatures of the method. Multiple transmitter layout as a routine procedure is necessaryto ensure good coupling with a planar conductor of unknown attitude. Furthermore, the characteristic behavior of anomaly shape as a function of transmitter position can help to distinguish between planar and sphere-like conductors, as well as to provide information for conductor location to be discussed in the next section.

4

5

%h • 411 I/I

%' F

i

6

', '11//// i

i

t

t

i

Pulse EM Amplifude (PPK)

Fig. 29. Study of transmitter-targetcouplingfor a flat-lying free-spaceplate model equal in size to the transmitterloop. The plate is shown in section for six different locations relative to the transmitterloop. The plottedresponseis for a plate conductanceof 100 S when the plate dimensionis 100 m. Amplitudesare point normalizedto the total primary field at the midpoint of the plate edge nearest the drill hole.

Principles of Determining Conductor Location.-Location includesnot only distancefrom the hole but, perhaps more importantly, azimuthal direction. If the location is to be determined using the standardaxialcomponentsystemfrom one drill-hole profile (i.e. one drill hole, one transmitter position) then the orientation of the secondary"moment" with respectto the drill hole will determine

the outcome as illustrated

in

Figure 31. If the moment is aligned with the drill hole the anomaly will be symmetric and its sign independent of the location of the conductor.

If the moment is

directed perpendicularlyat the drill hole the anomaly will be antisymmetricand of opposite sign to that produced by a moment directed away from the drill hole. An axial-componentsensorcan provide direc-


Drill-hole

EM

through judicious choice of transmitter-loop position. However, the secondary moment induced in a platelike conductor is controlled only by the orientation of the target and not by transmitter position. Therefore the opportunity to determine azimuthal location is largely dependent on fortuitous nonperpendicular relation between drill hole and target. If borehole EM is intended as part of the exploration procedure it is, perhaps, less desirable, and certainly less critical, that a drill hole be oriented perpendicular to a planar target. However, even when the target has the "wrong" orientation, it may be possibleto estimate its

tional information only if a significant asymmetric contribution exists in the anomaly. It should be noted that measurement of transverse components, for which a prototype system was discussedearlier, provides information complementary to that provided by the axial component. Transverse components are included on some of the model diagrams presented here to illustrate this point. Clearly, whether the proper conditions can be arranged for determining azimuthal location or not is model dependent. The orientation of the moment induced in a sphere-like conductor can be controlled

i

¸

907

2

• ©

11 -A©

Lx

B

-A

-B

-C

I

-I000

I

I

I

I

I

I

-I00

-I0

0

I0

I00

I000

Puls• EM Amplitude (PPK)

Fig. 30. Study of transmitter-targetcouplingfor a free-spacespheremodel with diameter equal to the length of one side of the square transmitter loop. The sphere is shown in section for six different locations relative to the transmitter loop. The responsesare for a sphere of conductivity 1 S/m when the diameter is set at 100 m. Amplitudes are point normalized to the total primary field at the center of the sphere.

908

Dyck


location by coupling considerations, an approach which is aided by using multiple transmitter positions. Dynamic Response of Plate and Sphere Models.-Dynamic responseis defined as the effectsof migration of eddy currents (induced current patterns changing with time) which result from the diffusive nature of the induction phenomenon. Diffusion effects can be observed directly in the time domain as will be shown here with TEM examples. Comparable FEM effects are eddy-current patterns that vary with frequency. Figure 32 illustratesdynamic effectsfor the two simple models.

Eddy-current migration in an inductively thin plate (Figure 32a) manifests itself as outward crossover migration. At early time (near the inductive limit) when the concentration of current is near the edge of the conductor, a relatively narrow anomaly is observed in a nearby drill hole. As the response progresses toward the resistive limit (late time), the current vortex collapsestoward the center of the plate and its near axis away from the drill hole. The current ring also becomes more spread out in the process

(Annan, 1974). The late-time anomaly is therefore spread relative to early-time. Since crossover separation is dependenton size of the plate and the clearance between it and the drill hole (see Figure 28c), the crossover migration distance also is dependent on those parameters. Under favorable geometric conditions, crossover migration should be detectable if the anomalousresponsecan be observed over a range of about one decade of delay time (or frequency). Where the plate is not perpendicular to the drill hole, inward collapse of the vortex is observed in a nearby drill hole as a lateral displacement of the anomaly along the profile (Figure 32b). Peak migration occurs in the direction of dip, and migration of the dip-side crossover is enhanced while the other is suppressed.The computed models of Figure 28 show that a dip of -+30 degreesinfluencescrossovermigration only slightlyand producesminimal peak migration which might be too small to be observed in field data. A dip of +_60degreesshould produce a readily visible effect.

PRIMARY FIELD

/

PRIMARY FIELD

/

••AXIS OF

(a)

EDDY

CURRENT

(maximum) (a)

(b)

Z(•)

I ,

x(

-)

I

I

UNIFORM PRIMARY FIELD

PRIMARY FIELD

(b)

/

(c)

Fig. 31. Schematic illustration of inductive responsefrom point of view of eddy current "moment". (a) The induced moment is parallel to the profile; (b) the induced moment is perpendicular to the profile. Orientation of this moment with respect to the drill hole is the main control on whether or not conductor location can be determined from a single profile. Note that the transversecomponent(X) suppliesinformation which is complementary to that of the axial (Z) component.

(d)

Fig. 32. Schematic illustration of dynamic responseof plate and spheremodels showingsectional view of migratingeddy currents and resultant migrating secondary fields; (a) thin plate; (b) dipping thin plate; (c) sphere in uniform field (no externally visible dynamic response); (d) sphere in nonuniform field (asymmetric distribution of induced current represents contribution of higher order multipoles to secondary field).


Drill-hole

A sphere model produces different dynamic effects depending on whether or not the inducing field is uniform, a distinction made in Figure 32c and d. When the primary field is uniform the eddy current vortex is symmetricallydistributedabout the plane of the equator, as defined by the direction of the field. The inward collapse of the vortex exhibits the same symmetry with the result that the external magnetic field is symmetric and dipolar at all times. This is a consequence of the fact that a uniform field excites only the principal-order (dipolar) componentof the secondaryfield moment, which then decaysin amplitudebut does not vary geometrically. Production of externally visible dynamic effects depends on excitation of higher order multipoles which can only be accomplished when the primary field is nonuniform over the volume of the sphere. Contribution of the nondipolar terms to the external secondary field allows us to observe the processas diffusion of the field (or the eddy currents) through the sphere in a direction away from the transmitter (i.e. the direction of decreasing primaryfield strength). There can be noticeable migration of both crossovers (unlike the plate, dippingaway from the hole, for which only the lower crossovermigrates)and the peak

EM

909

of the anomaly. Computed models in Figure 29 show the degree of migration in relation to the size of the sphereand the degreeof nonuniformity of the primary field crossing it (i.e. proximity to the transmitter). Diffusion effects appear to be readily visible over a range of one decade of delay time. Blocks and Wedges.--Consider the response of a block conductorwhich is both inductively and geometrically thick, i.e. skin effect is significant, at least at early delay time or high frequency, and in which migration of eddy currents is substantial. Figure 33 shows the responseof a block 50 rn thick and located 30 rn (clearance) from the drill hole. Two different primary field directions are considered. The differencein anomaly shapecomparedto that of a thin sheet is produced by the added effect of induction in the edge face (edge-faceresponse).The edgeface contribution is of opposite polarity for the two primary field directions shown, while the contribution of the broad face is of the same polarity in both instances. When the primary field originates from above, the result is reduction and enhancement of upper and lower crossover migrations, respectively, comparedto the thin plate response.Some downward

•-750m •

50m /

PRIMARY FIELD

30111

750

x

(7' = I

300

x

50

200m

m

300m

S/m

CH

1

-CH3 CH

5

CH

7

-1000

-100

-10

0

10

100

-100

-10

0

10

Pulse EM Amplitude (PPK) (a)

(b)

Fig. 33. Pulse EM responseof slab conductor(geometricallythick). The scale-modelresponseis shownfor primary fields impingingfrom two different directions, (a) from a transmitter above, and (b) from a transmitter offset to the right. The effect of primary field variation is to changedynamic responseas explained by "rotating moment".


910

Dyck

shift of peak response also occurs. When the primary field impingesfrom the right side, there are similar but

coupled with the transmitter. Again, this phenomenon should be useful in determining direction of the con-

reversed

ductor.

effects which

are even more substantial

than

those previously noted. The mechanism may be thought of as an induced moment which rotates with time. First the edge face and then the broad face are the dominant contributors to the response (rotatingmoment effect). The rotating moment effect could provide an important clue in locating the conductor (i.e. determining its azimuth with respect to the drill hole) because of the dependence of the effect on primary field direction. Figure 34 shows the effect of a wedge-shapedconductor (tapered block) when the thin edge is nearest the drill hole. The anomaly is very wide, particularly when the broad face is well-coupled to the transmitter as when the field impinges from above. This configuration also produces a rapid migration of eddy currents away from the drill hole which is manifested in enhanced crossover migration of the same senseas for a thin plate. Both breadth of the anomaly and crossover migration are reduced when the primary field impinges from the right, in a manner consistent with the fact that the best part of the conductor is less well

Note that should the thick edge of the wedge be nearest the hole (not shown) the case is very similar in character to the rectangular block (Figure 33) previously discussed. It is expected that the two cases would be virtually indistinguishablebecause the bulk of the conductor lies nearest the hole, thus dominating the response.

Multiple Conductors.--Significantinductive interaction can occur between multiple conductorsif they are sufficiently well coupled to each other, i.e. their mutual inductance is large. Figure 35 shows this interaction for a pair of plates, of either 10 S or 40 S conductanceeach, which are separated by a distance which is small compared to the size of the plates, thus making their mutual inductance large, even when the conductance is relatively poor. As the plotted transients show, interaction slows the decay compared to the single plate values. Also the spatial resolution is better at higher induction number (early time or larger conductance) because the currents are concentrated

75m I

30m

PRIMARY

200m

FIELD

•

300m 400m

I

-100

-10

0

10

100

-100

I

-10

ii

0

Ii iII II

10

i

!

100

Pulse EM Amplitude (PPK)

(a)

(b)

Fig. 34. Pulse EM responseof wedge-shapedconductor (or= 1 S/m, thin edge nearest drill hole). The scale-model responseis shown for primary fields impingingfrom two different directions, (a) from a transmitter above, and (b) from a transmitter offset to the right. The large crossover migration is indicative of conductivity-thickness increasing away from drill hole.


Drill-hole

EM

Figure 36 demonstrates the variation in response attributable to the relative position of the drill hole and the eddy currents for an intersected conductor. The left to right progression on the diagram is for a drill hole near the plate's edge rangingtoward the center of the plate. The drill hole nearest the edge is inside the eddy current vortex only at the earliest time as indicated by the entirely positive channel-one response. Further from the edge, an increasing number of channels are positive. This result is the most dramatic evidence of the migration of the axis of current con-

nearer the edge of the plates. The movement of the dominant peak from the upper to the lower conductor gives the appearance of diffusion through the pair of conductors. The response differs from that of a thick body, however, because the laminated nature of the conductor means the absenceof an edge-faceresponse and, therefore, of a rotating-moment effect. Intersected Conductors.--Diagnosis of anomalies which arise when the drill hole passes through the conductor is often made very difficult because irregular distribution

of conductive

material

911

centration

exists close to

toward

the center

of the conductor.

the hole. This material causes a response of high spatial frequency (noise) to be superimposedon the broad response of the bulk of the conductor. However, there are three potentially useful features discussed here: (1) the relative signs of the various channel responses;(2) the broadnessof the flanks of the anomaly; and (3) the existence of systematic sharp discontinuitieson either side of a plate-like conductor.

reversal

is visible.

The extent and slope of the flanks of an intersection anomaly are dependenton the size of the conductoras shown in Figure 37. Even when a drill hole has been terminated short of intersection, the upper flank may furnish a rough estimate of target size.

--200m

PRIMARY FIELD

Os -40S

Os :1 OS

--300m 10S

40S

--400m

i

i

-lOOO

i

-lOO

-lO

i

I

o

lO

i

lOO

Pulse EM Amplitude (PPK) -1000

-1000

Os -10S -lOO

-IO0

PPK

PPK

'•L&U-U L&U-L

-lO

--

-lO

LorU

I

0

I

I

I

I

I

2

3

4

5

6

I

I

7

8

0

I

I

•

I

I

I

I

I

I

2

3

4

5

6

7

8

CHANNEL

(LOG

NUMBER

DELAY)

Fig. 35. Pulse EM response(scalemodel)of double-plateconductors,each either 10 S or 40 S in conductance. Transientsare plottedfor peakresponses at U (Upper)andL (Lower) plate.L and U signifythe combinedresponse includinginteraction;L and U--U meansthat the U plate responsehas been subtracted.Absenceof "rotating moment" effectis evidencethat two platesbehavetogetherlike a laminatedthick body, i.e. one in which currents are allowed to flow only parallel to the laminations.

The

drill hole nearest the center must be entirely within even the late-time vortex because only a hint of sign


912

Dyck

Grant and West (1965) showed that there is a discontinuity in the component of the secondary magnetic field which is tangential to a thin plate. A component of that tangential field is sensed by an axially oriented receiver coil when the angle of intersection is not 90 degrees. Figure 38 shows the tangend=?.6m

15.2m

22.9m

tial discontinuity component superimposed on the smoothly varying part of the response. The discontinuity becomes larger as the angle of intersection becomesmore acute. The plus/minus senseof the sign reversal is a consequenceof most of the plate being updip from the drill hole. The sign change would be reversed for a plate located downdip.

30.5m

Large-loop Responsesin Conductive Media

200m

Figure 39 is a compilation of the results of Levy and McNeill (1986) for the EM-37 responseto a conductive host with and without a buried plate conductor. Examining the response of the half-space alone reveals that the decay pattern of the secondaryfield is dependent on the location of the drill hole with respect to the transmitter loop. For example, the drill hole is entirely within the smoke ring produced by transmitter C at all times so the secondary field is always negative. [The signof theseanomaliesis consistentwith that specified in Figure 10 for the absolute secondary field (in nV/

300m

o

-10

10

100

1000

PULSE EM AMPLITUDE

ß

(PPK)

460m

m2)for a sensorwhichis on axisof the eddy-current

// as - 52 S/m

Fig. 36. Plate responseof pulse EM systemalong intersecting profiles: transition of anomaly shape is dependent on position of drill hole with respectto eddy-currentconcentration. The change in anomaly sign and shape occurs as the eddy-current ring migrates past the drill hole in diffusing from the edge of the conductor towards its center. (Scalemodel experiments from Woods, 1975).

loop, (location A) i.e., normalization with respect to the primary pulse amplitude would reverse the plotted polarities.] Transmitter A createsa smokering which, at early time, lies external to the drill hole thereby producing a positive responsefor gates 2 to 8. However, at later time (gates 11 to 17) the responsechanges polarity becausenow the smoke ring is intersectedby the drill hole. The detail in the gate-2 profile for transmitter B is caused by the proximity of the smoke ring to the drill hole at early time. The transmitter E) = 60 ø

SxW = 460x305

300

m --

400

m --

500

m --

274x183

m

-•o

m

183x122

•oo•o•o

PULSE EM AMPLITUDE

O= 30 ø

m

(PPK)

300rn

--

400m

--

500m

--

-100

-10

I PULSE

I

0

10

I

I

100

1000

I

I

EM AMPLITUDE

(PPK)

15.2mN••-• // Os = 52 S/m

'1 / 15.2 rn ,/

Fig. 37. Influence of size of a plate conductoron the extent and slope of the flanks of an intersectionanomaly. (Scalemodel experiments from Woods, 1975).

460m

Fig. 38. Detection of the surfacetangential component,by an axial-componentreceiver, of the secondaryfield producedby a plate conductor.The conditionunder which this occurs is that the surface be nonperpendicularto the drill hole. (Scale-modelexperimentsfrom Woods, 1975).


Drill-hole

position is certainly more important at early time and less so at late time when the responses become smaller, deeper, and broader. The dashed lines are the late-stage (resistive limit) responseof the plate (Figure 39a, plate only; Figure 39cplate plushalf-space,no interactionincluded).The highamplitudeof the half-spaceresponseat early time virtually obliterates the plate contribution so no attempt was madeto showthe latter. The plate response is strongestfor transmitter A, a result of its strong couplingwith the transmitteras well as the relatively diminished coupling between the half-space (smoke ring) and the receiver at late time. Figure 40 is a computed2-D resultfrom Eaton and

EM

913

•x

300m

LINE SOURCES

(a)

•

-

/ 280m

HALF-SPACE

(Y =.01

S/m

2m __,

x=160m (a)

X (Y=10 S/m ,

200m--•

x=220m

PLATE IN FREE SPACE EM-37

RESPONSE

nV/m 2 -20

o

-10

0

10

I I '•'

-

TXA

200 m

(b)

/

• •.•-•

HALF-SPACE

400m 11 x,• I

I

(b)

II

O=

0.02

S/m

/

HALF-SPACE (EARLY TIME) EM-37 RESPONSE nV/m 2

-10

0

10

20

-30

,,x;oo,, /

-20

-10

0

-8

/,,x;oo,,/

-6

-4

0

-2

/ 'x,aoo' / 8

100m

(a),o,

100m

,•, ,•'

(b)

,

,0

100

I//

t3øøml-

",•\13ooml-

400m

400m

II I I I / '[ • • , • (C)

'•

HALF-SPACE (LATE TIME) -3- PLATE half-space

EM-37 RESPONSE nV/m2

_• _•

-4

-2

0

-20

-15

-10

-5

0

x10

TXA[\ :, t''' /'''/TXB I I[lOOm I-

I

.-"

,

N

-20

-15

-10

-5

,

x10

./--' l-' ,H•øøS•[ .'

I

,

/

lOOm t,,1t•øø•t -

•

only

plateadded

:4•

0

I

(c)

I((',/

(d)

':oo I

TXC ,,

,......

4,, 4oo. t..-.....4oo. tl

//

[

I

I

X•lt

II

L

Fig. 39. EM-37 response(gates 2, 5, 8, 11, 14, and 17 signifyingincreasingdelay time) for a conductivehalf-space with and without a buried plate conductornearby. Note that

amplitude is plottedasnV/m2 for a transmitter currentof 1 A (i.e. not normalizedwith respectto the primary pulse). At late time the resistive-limit response of the plate is merely added to that of the half-space,all interactionsincludingthe galvaniccomponentbeingignored.The plateis positionedso that energizationby Tx A is oppositeto that of Tx B and C. (Adapted from Levy and McNeill, 1986).

Fig. 40. Computedresponse(finite-differencealgorithm)for targetin a half-space:(a) the 2-D model; (b) the 2-D impulse responsewith the delay for each profile shown in ms; (c) profilesfor the correspondingfree-spacetarget (3-D plate with conductance200 S, strike-length 1600 m and transmitter strike-extent of 1500 m. (From Eaton and Hohmann, 1984).


914

Dyck APPROACHES

Hohmann (1984) which shows the complicationsarising when the full solution is considered. A free-space responsefor the corresponding3-D plate is included for comparison. When the full interaction is included the target conductor "experiences" the field at later time as a gradual increase and then decrease of primary field (i.e. the pulse is smeared out in time as the field diffuses into the half space) thus producing a target anomaly which changesin polarity. Diffusion of the electric field into the half-space may be more readily appreciatedby viewing the processin vertical sectionas provided in Eaton and Hohmann (1984) and, in color in Oristaglio and Hohmann (1984).

METRES

METRES

0

AND

FIELD

EXAMPLES

Approaches to Interpretation

The reasonablywide range of interpretational material available meansthat there is some flexibility in the approachtaken, dependingon the interpreter's experience or inclination.

Circumstances

also dictate what

type of information is required and how much effort is to be expended. For example, the goal of the survey may be limited to detection of a conductive target (or alternatively, to evidence which proves its nonexistence) in which case minimal and strictly qualitative

METRES

0

TO INTERPRETATION

METRES

0--

o--

PLOT

DEPTH

/

P2 ß

ß ---,PLOT

,,• ½1 ß

IOO

IOO

I00

--

ioo /

/'

/ !! / ! i

•.zoo

200

Bonds of intersected

sulphides

$oo

:5oo-

:500 :500 400

400

400

-

400

-

•

Torõet depth (est. from

500 1 I

0

•0

I

I

40

I

60

80

CHARGEABILITY

RESISTIVITY

mv/v

KO-m

....

500

500

--

I0,000 gommos MAGNETIC

( VERTICAL

FIELD

COMPONENT)

-40%

-20

•

I

0

20

IN - PHASE

I I00

40% LOG

( o , 12.2m)

%

AMPLITUDE

QUADRATURE LARGE

POLE-DIPOLE

Contocl

500--

I

I00

Norire

25-40m drill hole)

DIPOLE-DIPOLE

EM

f = 2855

LOOP

EM

Hz.

SEPARATION = 33m. f - 1340 Hz.

(o)

(b)

(c).

(d)

Fig. 41. A comparisonof four boreholegeophysical methodsusedat the Trillabelletestsite(DDH 34619),Sudbury Basin, Ontario, Canada; (a) resistivity/induced polarizationwith a downholepole-dipolearray; (b) fluxgate magnetometer(vertical component);(c) dipole-dipoleEM operatingat 1340 Hz; (d) large-loopFEM system operatingat 2835 Hz. The latter is clearly the least susceptibleto the geologicnoiseof the scatteredsulfidesin this

areaandthe only oneof the four methodswhichwasableto detectthe previouslyknownmassive-sulfide deposit at 400 m depth.

Drill-hole


interpretation may be satisfactory.On the other hand, quantitative results could be desirable in instances where they are used to guide detailed exploratory drilling of either a new prospect or around an existing mine structure.

Interpretation is sometimescarried out on a strictly empirical basis; sometimes an automated inversion process may be feasible, for example, in the implementation of a simple-loopmodel (Boyd and Wiles, 1984; Taylor, 1985). Most often a blend of intuitive reasoning and model-aided analysis is used. This approach is no different than in the processof interpreting surface data but different influences may be at work because drill-hole methods are applied more selectively and at a later stage in the exploration sequence. Furthermore, no standardapproach has yet come into practice as we are still on the steepportion of the learning curve and because no two cases seem to be alike.

A number of field examples were chosento illustrate various aspectsof the foregoingdiscussion.Some are new examples. Some previously published materials are included to provide the opportunity for pursuinga greater understanding of those cases. They are presented briefly and only in sufficientdetail to illustrate what the interpreter's specific approach was able to achieve. As such, many of the comments represent the interpreter's opinion at the time the original interpretation was made but do not precludedrawing your own inferences as to the meaningof the data. Many of these examplesare from test sitesand not actual exploration cases. Commentary has thus been added to show how the borehole results and interpretation might have been used in an exploration sequence.

EM

915

prospectof successin detectingblind targets in this environment, whereas the other anomalies respond primarily to the geologic noise such as bands of intersected sulfides. The resistivity/induced polarization responseis, for example, relatively independent of the array size (an array of double the spacingshown producedremarkably similar response).This does not indicate, however, that the induced-polarization method

is not useful elsewhere

200

FREQUENCY IN

M--

DOMAIN

PHASE

••300

•ø••

M-

S MALL

Tri!!abelle•A

PLATE 15 x 15 M

Comparison of Methods

3

Trillabelle, an INCO property on the west side of the Sudbury basin, is the site of a massive-sulfide deposit made available as a test case for a study of borehole geophysicalmethodscarried out by the Geological Survey of Canada (Dyck, 1975a). The host rocks are highly resistivebut scatteredsulfidesabound in the vicinity of the deposit which lies on or near the norite contact, the favorable horizon in the basin. In this environment, when the problem is considered from the point of view of geophysical grassroots exploration, detection of targetsis of primary concern. Reduced costs for systematic drilling of the contact could result from confidence in a geophysicalmethod for exploring between holes. Profiles

of

four

different

borehole

methods

are

shown in Figure 41. The clean, 80 percent anomaly in response to the known target at 400 m depth shows that the LLEM method could provide a reasonable

in the basin or more

readily applied at great depths (Krause, B., pers. comm., 1977). Knowledge of the conductor through the large-loop FEM survey would probably have been sufficientinterpretation to warrant a higher density of drill holes in the immediate vicinity. Figure 42 compares the responses of two EM systems to a large and small plate conductor. These model calculations show, with reference to Figure 4 l c and d, why a LLEM system is more useful than a dipoledipole system of fixed spacing (33 m) in detecting large, distant conductors (massive-sulfidetargets) in the presenceof small ones (scattered sulfides)close to the drill hole. The large-loop system discriminateson the basis of both anomaly amplitude and wavelength; the dipole-dipole system does not, as its responseis

LARGE

PLATE 150 x 150 M

•- 200M--

PULSE EM (LARGE LOOP TRANSMITTER)

I •

t)

•oo••

I

-100

4

I

I

I

400

M --

II

-IO 0 +lO +100-100-I•) •)+l•) NORMALIZED

SECONDARY

FIELD

I

+ I00

(PPK)

Fig. 42. Model responses(calculated with program PLATE) for a dipole-dipole FEM system (profiles 1 and 2) and a large-loop TEM system (profiles 3 and 4). Comparison between a small plate (profiles 1 and 3, representinga small, inconsequentialoccurence)and a large plate (profiles 2 and 4, representingan interestingtarget) demonstratestheoretically the advantage of the large-loop layout in geologically noisy environments,an example of which is shown in Figure 41.

916

Dyck

controlledby that portionof the plate nearestthe hole, whether the conductoris large or small.


Example of Multiple-loopSurvey

massivebody known from other drilling was not evidentin the core from the hole surveyed.The test was to determineif the massivebody could be detected in the presenceof numerous scatteredbandsof

A test survey was performed (Crone, 1986) at a lead-zincprospectin Virginiausingthe multipletrans-

nor copper values) intersectedby the hole from 75 rn

mitter-looplayoutshownin Figure 12b.An adjacent

downward.

mineralization(pyrite, sphalerite,and galenawith mi-

½TX

Pulse

EM Amplitude

-100

-50 i

lOOm

-

200

-

-20 i

(PPK) -10

i

r 122m

•oo-

SampleI • •i 4 5 Ii 1 II (a)

Pulse EM Amplitude O

,. IO

+ 20

I

I

,50

•

(PPK)

,100

t

I•, INTERSECTED SULFIDES

-

lOOm

-

2OO

-

3OO

I 22m

(b) Fig. 43. Detectionandinterpretation of a massive-sulfide body,at a Virginialead-zinc prospect, usingtheCrone PulseEM systemin multiple-transmitter mode.Response fromtwoof fiveloopsareshown:(a) westloop;(b)east loop; (interpretationby Crone, 1986).

Drill-hole

EM

917


Profiles from two of the five loops are shown in Figure 43 as representativeof the complete set. The

/, ,]'

INTERPRETED

/I

/!

/I

•,•?•,•0.• others are similar inshape, differing only inamplitud BODY

,-.

"'

(-,

•

to the east-loop response shown in Figure 43b. The west-loop response(Figure 43a) is again similar but opposite in sign. The following interpretation taken from Crone (1986) is basedprimarily on considerations garneredfrom heuristic model studies. The early-time induced-current paths are completely differentfrom thoseat late time, as evidenced

",,<,"

/;

/,.-

•

•

SOUTH

BAY

by the different anomaly patterns, implying that there

are two separateconductingbodies. The bodieswere all interpreted to be tabular in shape which would

-700

LEVEL

a' Mine: Vline: •a horizontal hori; nta projection projec:ion of c three Fig. 44. South Bay :li holes •annel 4 4 Pulse Pul EM upward-inclineddrill holes and and 1 the• C Channel e] d cond• •ctor •was Ls1( :ated o .• the response.The inferred conductor located on the 1basisof :i' anom; .lies observed , ,se• ,ed in in SB533 SB5! the reverse-polarity anomalies and SB528, and the greater ;[ ter amplitude am )litud, and an( detail of the thl latter (after Reed, 1986).

(a) NORTH

LOOP

SOUTH

LOOP

FE •1100%

"%N•

NORTH

•6 •

SOUTH

LOOP

LOOP

Om

100

..............................

(a) NORTH

LOOP

SOUTH

. :..•. _.•.._-_?'TL - -=--.:_--;-=. _--. .•_-:_•_

LOOP

•'•_

_

_. •..-

'-:::'~ L•.-

½ •,•7 •W: •'- -• :•-"

2OO

.......... :•.•:7

./, ß , ..:-:-•::.__• _' .s..

.

= "-

--.

..•------:•••-• . _ _. ß.•

.•--

-----'----- :'_L-..••

..........:-

,

(b)

300

(b) i'i_. •400 •,

_•=•.•------•

.

Fig. 45. Ruttan Mine, Manitoba:(a) layoutof the PulseEM surveyand plate-conductormodels(generatedwith program PLATE) used to simulate the expected dip of the sulfide bodieslyingnearandintersectedby the surveyeddrill holes. The circular zone around 400 m depth is shown in greater detail to the left; (b) plate responsesshowing the sign

-_--"

.

- ........... ::•:{=•: •.-..............

•'5."--"':-•;::;:-•-':':-"• z: .........

•

f" -looo

......

•-•

I -loo

PULSE

............

•t•

I

-lo

o lo

loo

EM

AMPLITUDE

looo

dependenceon both transmitterlocationand on conductor location.A platewhosedip liesin the zonemarked+ + in (a) would have the samepolarity of excitationby both transmit-

Fig. 46. Ruttan Mine (West Anomaly area) (a) projected sectionwith an iron-histogramrepresentationof the conduc-

ters.

tive sulfides;(b) Pulse EM responseof drill-hole 1361

918

Dyck

allow little variation in flow path of the induced


currents as the field direction is altered, hence the

similarity in anomaly pattern for all transmitter loops. The upper anomaly (at 155 m) was interpreted as an edge-type response of a conductor of relatively poor quality (conductance estimated 10 S) whose bulk lies updip from the hole. This was the basisfor the upward extrapolation of the intersected bands as shown in Figure 43. The lower anomaly (at 225 m) is caused by a body of greater conductance (estimated 60 S) which, by considerationof couplingwith the various transmitters, was interpreted to lie below the drill hole as shown. The reversed anomaly obtained in responseto the west loop can be explained by reversed energization as shown in the magnetic field diagrams. The lower conductor could be a separate body or a downfaulted portion of the upper conductor. South Bay Mine, Ontario An extensive Pulse EM survey was undertaken in the South Bay Mine (an airborne and ground EM discovery) operated by Selco Mining Corporation (Reed, 1986). The single-transmitter method of Figure 12a was employed. A total of 85 holes, all collared underground, were surveyed in an effort to extend the life of the mine by finding new reserves. Useful data were acquired 1100 m below surface in the deepest of these holes. Regrettably no new mineable ore was found but several previously unknown sulfide lenses were discovered. The account in Reed (1986) de-

SIMPLIFIE•"• • •

scribesmany difficulties in operating undergroundand offers as well a plethora of field examples of various types. The interpretation was carried out by curve matching of field data with Woods' (1975) model results. While responseswere often complicated and difficult to interpret, many of the off-hole sources(of secondary field) were seen from more than one hole. One such example is presented here. Figure 44 showsthe horizontal projection and Channel 4 PEM responseof three upward-angled drill holes which happened to be drilled parallel to a nearby sulfide lens. This is a very unusual case as most holes are designedto cross geologic strike. The antisymmetric anomalies recorded in each of the holes give clear indication of the relative position of the target. Notice that the response in SB533 is opposite to that in the other two holes. The extent of the body was estimated to be approximately the distance between the peaks of the anomaly in SB528. An additional hole was drilled to investigate the inferred conductor and intersected stringersand narrow veins of sulfides, mostly pyrite. Ruttan Mine, Manitoba

Pulse EM surveys were carried out at Ruttan Mine by the Geological Survey of Canada to help determine the problems of application in mining environments in general, and feasibility for Ruttan in particular (Dyck, 1984). Interpretation problems turned out severe for two reasons: (1) the distribution of conductors is exceedingly complex; (2) the conductivity of the mine

500

1 GEOLOGY A/B 'øø lO

PLATE

100

and

EXTENSION

o

HORIZON

vw•v'vv •

-1ø 1 INFERRED FAULT

- J''•,•.-.. • EXTENSI0 N

10 0

-10

- 1 •oo

lO-] _. o

5001

1oo ///• PLA TE ONLY

10

SCALE -lOO

MODEL

0

-10

lOOO

,o o

lOO

PLATE •

,o o I I

-lO -lOO

-lOO

D^T^

•'---

FIELD

I

I

PLATE-'•I EXTENSION

Fig. 47. An example in the use of a specificscale-modelexperiment to provide an answerto an exploration problem. The extension to the sulfide body was inferred from the absence, in the field data, of a tangential-component discontinuity (from Macnae and Lamontagne, 1986).


Drill-hole EM

919

structureitself produceda hugebackgroundresponse which distorts all anomalies. Interpretation under these conditionsproved difficult becausethe simple sign relations discussedearlier are not necessarily

conductor and also on location of the conductor. A

relevant.

Anomaly area and PEM profiles taken in drill-hole 1361for each of two transmitterloops. Apparently the characteristicbehavior predictedby the modelsis not generally evident in the field data. Why the sign dependenceon transmitterposition is absent is not clear. One explanationis that the dip of the targets lies outsidethe range shown in Figure 45 (i.e. in the hashedarea) althoughthe explanationis not consistent with the geologicallyinferred dip of the lenses. Another speculationis that the fieldsare so distortedby

sphericalconductorat the samelocationsproduced comparable effects. Figure 46 shows a projected section of the West

Quantitativeinterpretationbasedon simplemodels was examined using computed responsesshown in Figure45. Thesemodelsare specificto the surveyand to the orientation and shape of well known (from numerousmine-developmentdrill holes)sulfidebodies in an area of the mine called West Anomaly. A small,

thin, squareplate 10 m across,and lying both above and below the drill hole, shows the expected lack of variation in anomaly shape. However, the signof the anomalyis dependenton whichtransmitterexcitesthe

the whole network of conductors that the usual con-

(a)

(b)

SOUTH LOOP

NORTH LOOP

SOUTH LOOP

NORTH LOOP

:500M

400M

I -IOO

I -IO

I o

I IO

I -IOO

i -IO

I o

I IO

t IOO

(d) (c) PLATE 150x150 o's

= 50

S

M

SOUTH

500

M•

LOOP

400M

?

(a)SPHEREGERT:-9266

GERT -9266

:N

NORTH, LOOP

Fig.48. Gertrude sitemcomparison of fieldprofiles andmodels of thePulseEM response. Theplateandsphere modelswereadjusted in size,position, andconductivity to providea bestfit to thefielddatain termsof crossover andpeakpositions andanomalyamplitude. (a) spheremodel(e = I S/m);(b) fielddata;(c) platemodel (conductance = 50S);(d)planviewof looplayout;(e)sectional viewof Gertrude conductor withsphere andplate models. (From Dyck and West, 1984).


920

Dyck

siderationsare just not applicable.Becausethe quantitative approach did not appear to be feasible, a strictly empirical approach was adopted. In general, there is a proliferation of anomalies which reflects the distributionof intersectedsulfides(asportrayedby the Fe histogramaffixed to the PEM plot). While many individual anomalies are correlatable directly with intersections,othersare not. An exampleof the latter is the D series of anomalies which occurs across the

FREQUENCY i

i

28:55

945

(Hz)

i

i

I

:315

105

:55

IOO%

top of all holes.A progressionof increasingamplitude and degree of detail from west to east (as well as greater apparentresolution of D4 comparedto D3) led to the inference of a conductor lying above the holes and plunging slightly to the east. Two holes drilled upward on that basis intersected subeconomic sulfides.

Scale-modelExample

Figure 47 showsan example presentedin Macnae and Lamontagne (1986) in which a problem-specific scale-modelexperiment was used to test a possible interpretation. It had been intended that drill hole B should intersect the nose of a fold which was known to

•

,,,,.g•,

••"""---.•,•e

<.

IN PHASE

FREQUENCY

•-•.

SPECTRUM20•

be the site of significant thickening of sulfides in the horizon. When B did not produce the anticipated intersection, the question arose as to whether the fold thickening lay between A and B or whether it had been displaced laterally by the shallowly dipping fault marked in the d•agram. The drill holes were surveyed with a Crone PEM systemand the data interpreted by modelingof a plate with and without a folded extension. The data from drill hole A show characteristic differences between

the two models which can be compareddirectly with the field data. If only the uniform plate is presentthe inducedcurrents migrate from the edgeof the conductor past the drill hole and produce a large tangential discontinuity as shown (discussedearlier under intersected conductors). When the extension is in place, the currents are kept further from A and no tangential discontinuity can be seen. The absence of such a discontinuity in the field data implies that more sulfides do lie in the zone between 9

8

7

UTEM

6

5

CHANNEL

4

3

2

NUMBER

"'"•;.• 2

PULSE

"

I

I

I

I

I

i

3

4

5

6

7

8

EM

SAMPLE

NUMBER

Fig. 49. Frequency spectra, step and impulsetransientsfor three models,and field data for the Gertrudeanomaly(peak responseat 310 m, north loop). Only the layered sphere(see Appendix C for definition of parameters) produces the plateau which is observed in the field data.

Gertrude (Sudbury), Canada

The Gertrude massive-sulfide deposit, another INCO property, is situated on the south side of the Sudbury basin and has been the site of many borehole geophysical tests. While the deposit appears, from drilling, to be a simple, tabular conductingbody with some thickeningin the middle, embeddedin a highly

HOMOGENEous ,•,••,

,••

the two holes.

resistive Precambrian host, the EM responsesare anything but simple. Comparison of pulse EM field and model data in Figure 48 reveals that, for example, neither the crossoverbehavior nor the peak migration characteristicsare readily explainable in terms of a simple model. Results from the Gertrude test site are included here as an example of what can be accomplishedby quantitative analysisand computer modeling specific to an exploration problem. While the profiles have been published before (Dyck and West, 1984),the presentdiscussionaddsa comparativestudy of the decay behavior to the picture.


Drill-hole

Figure 49 is a compilation of the transient or spectral behavior for all three system-functiontypes (frequency, step, and impulse) from Dyck (1981). The frequency axis and the UTEM and PEM time axes have been lined up for approximate equivalence. There is clearly a two-stage decay (or two-part frequency dependence) evident in the field data which is comparable for all systems used. The two-stage decay is probably another manifestation of the complicated migration patterns evident in the profiles. Certainly it cannot be emulated by simple, homogeneousmodels. Only a layered sphere with the core 100 times more conductive than the shell exhibits a similar behavior,

suggesting that the Gertrude deposit has a highly conducting inner region. The best-fit plate model coincides well with the portion of the body nearest the hole but is not indicative of the body's thickness or complexity. While a sphere of the size required seemsunrealistic geologically, the model is indicative of the nontabular character of the target. Such information would certainly be beneficialin exploring a highly mineralized contact. The value of fitting both plate and sphere models is thus demonstrated.

The resistive nature of the host precludes ascribing the complexity of the observationsto anything but the body itself. The effect of an apparent halo of lesser sulfides in stringer and disseminated form, lying mostly in the hangingwall, certainly bears evaluation. In cases where more detail of interpretation is re-

EM

921

quired, homogeneousmodels of simple geometry are clearly not adequate. Survey in a Conductive Environment (Tasmania SIROTEM Example)

An example from the West Coast of Tasmania is shown in Figure 50 (G. Buselli, pers. comm.). The very high conductivity of the ground (>1 S/m, as inferred from the EM data) results in considerable

responseon all 32 channels of the system. As a result of this survey, a second drill hole was recommended to test the interpretation that a significantconductor was indicated by the cross-over of the earlier channels at 120 m. The drill hole intersected a major pyritic conductor in the anticipated position. The similarity with the geometry and responseshown in Figure 39 for transmitter A would tend to bear out this interpretation. STATE

OF THE

ART

In the last ten to twenty years, great strides have been made in borehole EM methods. Experimentation and development have been carried out with a variety of techniques. As far as traditional EM induction techniques are concerned, the large-loop technique has become the most popular, as reflected by the emphasis placed on large-loop interpretation and examples in this chapter. Certainly the equipment available for exploration purposeshas become much more EAST

LOOP

(ZOOxZOOm)

DEPTH

(m) AST

LOOP

IOO -

O'HOST
-

-

SI LTSTONES • SULPHIDES 200

-

-I0000

TASMANIA SITE (a)

•

-I000

-I0

I0

I000

I0000

SlROTEM RESPONSE (•V/A) (b)

Fig. 50. SIROTEM example (Tasmania). The conductivity of the host was inferred from the drill hole EM data (Compiled by H. Rutter, Geophysical Exploration ConsultantsPty Ltd., on behalf of CSIRO Division of Mineral Physics, G. Buselli, Pers. comm.)

922


reliable

Dyck and warrants

the confidence

which

is often

placed on the field results. The commercial availability of systems is direct evidence that this is true. Salt (1966) expressed frustration in the general lack of interpretation aids available. Fortunately, much improved resources are now available. Surveys in conductive environmentspresentinterpretationalenigmas as formidable as those presented by surveys in resistive environments twenty years ago. Details of conductors and complexity of the induction phenomenon in geologic conductors, which are otherwise not observed, become evident in drill-hole EM surveys. In my opinion, we must take advantage of current knowledge of and experience with these details to fine-tune interpretationswhen solvingexploration problems. Future advanceswill depend on how well this can be accomplished.

EM probe' Commonwealth Sci. Ind. Res. Org. Investigation Rept., 108. Drinkrow, R. L., and Duffin, R. H., 1976, Scale model type curves for down-hole electromagneticloggingin a conductive environment: Commonwealth Sci. Ind. Res. Org. Investigation Rept. 120. •1978, Scale model results for inductive logging in the region of singleand multiple conductive bodies: Geophysics, 43, 804-818. Duncan, A. C., 1987, Interpretation of down-hole transient EN data using current filaments: Austral. Soc. Expl. Geophys. Conf., Expanded Abstracts, Exploration Geophysics, 18, 36-39. Dyck, A. V., 1975a, Borehole logging (electrical), in Report of Activities' Geol. Surv. Can. Paper 75-1A, 81. •1975b, Electrical borehole methodsapplied to mineral prospecting, in Dyck, A. V., Ed., Borehole geophysics applied to metallic mineral prospecting' A review: Geol. Surv. Can. Paper 75-31, 13-29. •1981, A method for quantitative interpretation of wideband, drill-hole EM surveys in mineral exploration: Ph.D. thesis, Univ. of Toronto, available as Research in Applied Geophysics 23, Geophysics Lab., Dept. of Physics, Univ. of Toronto.

ACKNOWLEDGMENTS

I thank Carol

Luce

and Ela Rusak Mazur

•1984, Report of preliminary results, drill-hole pulse EM surveys, Ruttan mine: Geol. Surv. Can. in-house for their

fine efforts in drafting the diagrams. Constructive criticisms of the manuscript provided by Ben Sternberg, Michael Asten, and Jules Lajoie also are gratefully acknowledged. REFERENCES

Annan, A. P., 1974, The equivalent source method for electromagnetic scattering analysis and its geophysical application: Ph.D. thesis, Memorial Univ. of Newfoundland.

Barnett, C. T., 1984, Simple inversion of time-domain electromagnetic data: Geophysics, 49, 925-933. Becker, A., and Cheng, G., 1988, Detection of repetitive electromagnetic signals in Nabighian, M. N., Ed., Electromagnetic methods in applied geophysics, Vol. 1, Theory: Soc. Expl. Geophys., 443-468. Boyd, G. W., and Wiles, C. J., 1984, The Newmont drillhole electromagneticpulse system--Examples from eastern Australia: Geophysics, 49, 949-956. Brant, A. A., Dolan, W. M., and Elliot, C. L., 1966, Coplanar and coaxial EM tests in Bathurst area, New Brunswick, Canada, 1956: in Mining Geophysics,Volume I, Case Histories, Soc. Expl. Geophys., 130-141. Buselli, G., 1980, Electrical geophysicsin the U.S.S.R.' Geophysics, 45, 1551-1562. Clerc, G., Frignet, B., and Tabbagh, A., 1983, Transmitterreceiver induction techniquesand probes developed for surface and borehole measurements' Application to archaeology and mineral exploration: J. Geomag. Geoelectr., 35, 443-454. Crone, J. D., 1980, Field results using borehole pulse EM methods' Case History Note, Crone GeophysicsLtd. •1986, Field examples of borehole pulse EM surveys used to detect and outline conductive ore deposits, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnicalapplications: Geological Survey of Canada, Paper 85-27, 59-70. Daily, W., 1984, Underground oil-shale retort monitoring using geotomography:Geophysics,49, 1701-1707. Doll, H. G., 1949, Introduction to induction logging and application to loggingof wells drilled with oil base mud: J. Pet. Tech., 1, 148-162. Drinkrow, R. L., 1975, Scale model studies at a down-hole

report.

Dyck, A. V., and West, G. F., 1982, The interpretative value of three-componentEM surveys in mineral exploration' A model study' Presented at the 52nd Ann. Internat. Mtg., Soc. Expl. Geophys. •1984, The role of simple computer models in interpretation of wide-band drill-hole electromagnetic surveys in mineral exploration' Geophysics, 49, 957-980. Dyck, A. V., Bloore, M., and Vallee, M. A., 1980, User manual for programs PLATE and SPHERE' Research in Applied Geophysics 14, GeophysicsLab., Dept. of Physics, Univ. of Toronto. Eaton, P. A., and Hohmann, G. W., 1984, The influence of a conductive

host on two-dimensional

borehole

transient

electromagnetic responses: Geophysics, 49, 861-869. Elliot, C. L., 1961, An electromagnetic drill hole instrument for the detection

of conductive

sulfide bodies: Presented at

the 31st Ann. Internat. Mtg., Soc. Expl. Geophys. •1966, Electromagnetic method and apparatusof geophysical exploration: Canadian Patent 743,665. Frignet, B., 1986, Induction logs applied to mineral exploration and development, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnicalapplications:Geological Survey of Canada, Paper 85-27, 89-100. Fromaigeat, L., 1985, Methode electromagnetique en forage' application de la mesure des 3 composantesmagnetiques en exploration miniere' D.Eng. thesis, Institut National Polytechnique de Lorraine, Nancy, France. Fullagar, P. K., 1987, Inversion of down-hole TEM data using circular current filaments: Exploration Geophysics, 18, 341-344.

Grant, F. S., and West, G. F., 1965, Interpretation theory in applied geophysics' McGraw-Hill Book Co. Hattula, A., 1986, Magnetic three-componentborehole measurements in Finland, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnicalapplications:Geological Survey of Canada, Paper 85-27, 237-250. Hayles, J. G., and Dyck, A. V., 1987, Drill-hole electromagnetic surveysas an aid to structuralmapping, Chalk River, Ontario, -A feasibility study, in Thomas, M.D., and Dixon, D. F., Eds., Geophysicaland related geoscientific research at Chalk River, Ontario: Atomic Energy of Canada Ltd. Report AECL-9085, 309-327. Hohmann, G. W., 1983, Three-dimensional EM modeling' Geophysical Surveys, 6, 27-53. •1988, Numerical modeling for electromagnetic methods of geophysicsin Nabighian, M. N., Ed., Electromag-

Drill-hole


netic methods in applied geophysics,Vol. 1, Soc. Expl. Geophys., 313-364. Jones, A. G., 1983, The problem of current channelling:A critical review: Geophysical Surveys, 6, 79-122. Kaufman, A. A., 1978, Frequency and transientresponsesof

electromagneticfields created by currents in confined conductors: Geophysics, 43, 1002-1010. Lamontagne, Y., and Macnae, J. C., 1986, Fiber optic data links for borehole EM application, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnicalapplications: Geological Survey of Canada, Paper 85-27, 101-106. Levanto, A. E., 1959, A three-componentmagnetometerfor small drill-holes and its use in ore prospecting: Geophysical Prospecting, 7, 183-195. Levy, G. M., and McNeill, J. D., 1986, Transient electromagnetic borehole logging, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnicalapplications: Geological Survey of Canada, Paper 85-27, 71-78. Macnae, J. C., and Lamontagne, Y., 1986, Interpretation of conductor complexity with borehole applications, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnicalapplications:Geological Survey of Canada, Paper 85-27, 323-336. McMechan, G., 1983, Seismic tomography in boreholes: Geophys. J. Roy. Astr. Soc., 74, 601-612. McNeill, J. D., 1980, Geonics Technical Note TN-7, Geonics Ltd., Mississauga, Ontario. McNeill, J. D., Edwards, R. N., and Levy, G. M., 1984,

Approximatecalculationsof the transientelectromagnetic responsefrom buried conductorsin a conductive halfspace: Geophysics, 49, 918-924. Nabighian, M. N., 1971, Quasistatictransient responseof a conductingpermeabletwo-layer spherein a dipolar field: Geophysics, 36, 25-37. •1979, Quasistatictransient responseof a conducting half-space--An approximaterepresentation:Geophysics, 44, 1700-1705.

•1984, Forward and Introduction, Special Issue-Time-domain electromagnetic methods of exploration: Geophysics, 49, 849-853. Nabighian, M. N., and Macnae, J. C., 1991, Time domain electromagnetic prospecting methods, in Nabighian, M. N., Ed., Electromagneticmethodsin applied geophysics: Applications, Volume II, Soc. Expl. Geophys. Nilsson, B., 1986, A new borehole radar system, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnical applications: Geological Survey of Canada, Paper 85-27, 189-195.

Noakes, J. E., 195l, An electromagneticmethod of geophysical prospectingfor applicationto drill holes:Ph.D. thesis, Univ.

of Toronto.

Olsson, O., and Nilsson, B., 1986, Some examples from borehole radar measurements, in Killeen, P. G., Ed., Boreholegeophysicsfor miningand geotechnicalapplications: Geological Survey of Canada, Paper 85-27, 197-206. Oristaglio, M. L., and Hohmann, G. W., 1984, Diffusion of electromagnetic fields into a two-dimensional earth: A finite-differenceapproach: Geophysics, 49, 870-894. Pantze, R., Malmqvist, and Kristensson, G., 1986, Directional EM measurements in boreholes, in Killeen, P. G., Ed., Borehole geophysicsfor mining and geotechnical applications: Geological Survey of Canada, Paper 85-27, 7%88.

Parasnis, D. S., 1991, Large-layout harmonic field systems, in Nabighian, M., Ed., Electromagnetic methods in applied geophysics: Applications, Volume II, Soc. Expl. Geophys. Rao, V. M., and Rao, I. B. R., 1983, The radio wave absorption technique in Mailaram copper mines, India: Geophysics, 48, 391-395. Reed, L. E., 1986, A borehole electromagneticsurvey of the South Bay mine, Ontario, in Killeen, P. G., Ed., Borehole

EM

923

geophysicsfor mining and geotechnicalapplications:Geological Survey of Canada, Paper 85-27, 307-322. Roy, J., 1979, Borehole VLF prototype system user's manual: Geol. Surv. Can. in-house report on DSS Contract 1SQ76-00016. •1984, Electrical methods in mineral well logging: Ph.D. thesis, McGill Univ. Salt, D. J., 1966, Tests of drill hole methods of geophysical prospectingon the property of Lake Dufault Mines Limited, Dufresnoy Township, Quebec, in Mining Geophysics, Volume I, Case Histories, Soc. Expl. Geophys., 206-226.

San Filipo, W. A., and Hohmann, G. W., 1985, Integral equation solution for the transient electromagnetic responseof a three-dimensionalbody in a conductivehalfspace: Geophysics, 50, 798-809. Schelkunov, S. A., 1948, Electromagnetic waves, Van Nostrand.

Scott, W. J., 1987, VLF-EM surveys at Chalk River, Ontario, in Thomas, M.D., and Dixon, D. F., Eds., Geophysicaland related geoscientificresearchat Chalk River, Ontario: Atomic Energy of Canada Ltd. Report AECL9085.

Smith, R. J., and Hallof, P. G., 1971, A new deep drill-hole electromagneticsystem: Presentedat the 41st Ann. Internat. Mtg., Soc. Expl. Geophys. Somerstein, S. F., Berg, M., Chang, D., Chung, H., Johnson, H., Richardson, B., Pissicara, J., and Salisbury, W. W., 1984, Radio-frequency tomography for remotely

probingthe interiors of operatingmini- and commercialsized oil-shale retorts: Geophysics, 49, 1288-1300. St. Amant, M., 1984, Drill-hole AFMAG method: Geol. Surv. Can. in-housereport on DSS Contract 1SQ81-00080. Taylor, S. I., 1985, A simple inversion method to aid in the interpretation of borehole TDEM data: M.Sc. thesis, Queen's Univ. Vekseer, V. I., and Plyusnin, M. I., 1957, Low-frequency electromagneticinvestigationof the surroundingsof boreholes: Translated by Syers, K. from Izv. Akad. Nauk SSSR, ser. geofiz. No. 7,934-939 (Reprint order no. GEO 115).

Ward, S. H., and Harvey, H. A., 1954, Electromagnetic surveyingof diamonddrill holes:CanadaMining Manual, National Business Publications, 19-30.

West, G. F., and Edwards, R. N., 1985. A simpleparametric model for the electromagnetic response of an anomalous body in a host medium: Geophysics, 50, 2542-2557. West, G. F., Macnae, J. C., and Lamontagne, Y., 1984, A time-domain electromagnetic system measuring the step responseof the ground: Geophysics,49, 1010-1026. West, R. C., 1986, Borehole transient EM response of a three-dimensional

fracture

zone

in a conductive

half-

space:M.Sc. thesis, Univ. of Utah. West, R. C., and Ward, S. H., 1988, The borehole transient EM responseof a three-dimensionalfracture zone in a conductivehalf-space:Geophysics,53, 1469-1478. Woods, D. V., 1975, A model study of the Crone borehole pulse electromagnetic (PEM) system: M.Sc. thesis, Queen's Univ. Woods, D. V., and Crone, D., 1980, Scale model study of a borehole pulse electromagneticsystem: Can. Inst. Min. Metallurg. Bull., 73, No. 817, 96-104. Worthington,M. J., Kuckes, A., and Oristaglio,M., 1981,A borehole induction procedure for investigatingelectrical conductivity structure in the broad vicinity of a hole: Geophysics, 46, 65-67. Wright, D. L., Watts, R. D., and Bramsoe, E., 1986, Single-holeshort-pulseboreholeradar experimentsand a crossholetransponder,in Killeen, P. G., Ed., Borehole geophysicsfor miningand geotechnicalapplications:Geological Survey of Canada, Paper 85-27,207-216.


924

Dyck

APPENDIX

INTEGRAL

EQUATION IN

A

SOLUTION A THIN

A recapitulation of Annan's (1974) numerical technique follows. The basic integral equation for the currents induced in the conductingplate is:

Ks/o's-i.,•offI•(r')a(r, r')d2r' =Eo . (A-l)

As the sheet is inductively thin (current density does not vary acrossthe thickness of the plate), the current

hasbeenrepresentedas a surfacecurrentdensity,Ks; crs is the conductance of the sheet (conductivity-

FOR EM INDUCTION

PLATE

where N is the maximumpolynomialdegreeof the test functions, Tn is a Chebychevpolynomial of the first kind and order n, ( = xi/ai, and xl = x2/a2. In matrix notation U = ((1))(C) where ((1)) is a row vector with entries•I)nmand (C) is a columnvector, of the sameordering, of unknownsto be determined. The Galerkin formulation of equation (A- 1) is:

[(1/Crs)(F_) - icolxo(a2/al)a 1(_L)](C)= -ico0 al (H) (A-3) where

thickness, ors), d -•l

i.e. E = J/or - Js/crs = Ks/es. In equation (A-I), to - frequency of alternating field (time dependenceexp (-icot), tx0 - magneticpermeability of plate = permeability of free space, g(r, r') is the Green's function for the free-space induction problem = 1/(4,fir - r'l) Eo = primary electric field in plane of sheet. The geometric parameters are given in Figure A-1. The conditionsV.K s = 0 and Ks'e3 = 0 ensurethat K s is solenoidal(no galvanic sourcesin the plate) and does not cross the boundaries of the plate (conductorinsulator interface), respectively. Since Ks lies in the plane of the sheetand V.K s = 0, K s may be expressed in terms of a scalar potential, U, as

I

OX1

a2

OX1

Odl)(XlX2)' Odl)(XlX2) dxldx2. Ox2 Ox2 (A-4)

g(xl , X'l, x2, x•) 1•alf_'2 f•/i f_2

[_L] =2--•1

ßJ -al

a2

--

I

a2

X{O(•)(xlx2) r[oa) (X'lX) dxl dx2 dx'l dx} OXl

OXl

0(I)(x 1X2) OX2

OX2

(A-5)

Ks = curl (U e3) = -e 3 x grad U.

U is zero on the edge of the plate since Ks.e 3 --- 0. The eigenfunctionresponsefor the solenoidal-current potential is formulated following the Galerkin approach. The potential, U, is expanded in terms of a set of trial

functions (I)nm = (1 -- (2)(1-x12)Tn(()rm(x i) as

U- E CnmCI)nm, n + m_
(A-2)

[HI=2-•11 a-a,- 2

(A-6)

X [(I)(XlX2)]r H03 (XlX2)dXl dx2.

In equations (A-4) through(A-6) the symbol[]T implies the transpose. As an operator equation, (A-3) becomes

[_Z][C]= [V]

(A-7)

Drill-hole


where

EM

925

The solution of equation (A-3) is

[Z_]= [_R]= [_X]= [V] =

[C] = [S_]_D[-iot/(1 - icXl3na2/al)][s_]T[H]

[_R]+ i[X_] 1/crs[_F] -cotxo (a2/al)al [_L] -ico•0a 1[H].

(^-• •)

for a frequency-domain source, and

Diagonalization of [_Z] is a two-stage process. Treated as a weighted eigenvalue problem'

[Z][C] = k[8][C ].

(A-S)

[_F]is diagonalizedfirst so that

D_ -l/2(f i)[v•r[F_][_v]D_ -l/2(f i) = 1 where D_is a diagonalmatrix of eigenvalues(fi) of [_F] and [_v]is a unitary matrix whose columns are the correspondingeigenvectors.Applying the transformation (A-9) to equation (A-8) we get

[C] = [_S]_D{-exp [--t/(xna2/al)]/[13nXn(a2/al)2]} x [S_] r[H]

(A-12)

for a time-domain impulse source, where a tOlXo•sal,and -rn = iXo•sal •n' The individual eigenpotentials may be computed from [U] = [•][_S] where the nth element of the row vector [U] corresponds to the eigenpotential whose

eigenvalueis [3n. A complete set of eigenpotentials need be computed only once for each width/length value of the plate; the number of eigenpotentials dependson the value of N. Eigenpotentialsfor a2/al = 0.5 and N - 4 (a total of 15) are mapped in Figure A-2. The eigencurrentstreamlinesare parallel to the potential contours in this diagram. The final step in the analysis is the computation of secondarymagneticfields associatedwith the induced

{liars q- [_L']}[C']- h(l/crs) [C'] where

[_L']= D_-l/2(fi)[_v]r[_L][v _]D•-1/2(ft') and

currentsaccordingto Ha(r) = [HS][C] where

[C']: D_ 1/2(fi)[v_lT[c].

3

We now have a standard eigenvalue problem

[H]s= {-[q•]•(x3)e3 - E [H}]e/} i=1

-itolxo(a2/a1)a1[_L'][C'] = (h - 1)l/•rs[C'].

, [H'•]= [H/•(r), H'Io(r)H•i (r),' ......... HoN(r)] i

(A-10)

The cigcnvalucs,[3n,of [_L']arc definedby -itoDo(a:/al)al•.

= 1/•r•(•.-

1)

4.rl r - •1 O 2 fal dPnm(X•X•) ' H}m(r) =OXi OX3 - I•'_2 a2 -' - dx•dx•

or

}kn = 1 - icolXoO'sa 1(a2/al)•n.

X3 UNIT VECTORS

The transformationis defined by [S_],the matrix of eigenvectors, as

[s_]T[z_][_S] = _D(•.n) and D_is the diagonal matrix of eigenvaluesof the weighted eigenvalue problem (A-8) and

x

\

X•2al

[_S]= [_v]D_-l/2(fi)[S_'l where [S_']is matrix of eigenvectorsof [L_'].

Fig. A-1. Thin sheet geometry.


926 Dyck


Drill-hole

APPENDIX

EM INDUCTION

EM

927

B

IN A CONDUCTING, TWO-LAYER

We presenta review of Nabighian's(1971)solution by separationof variables for the EM responseof a conducting, permeable, two-layer sphere (core and shell)in a dipolarmagneticfield. The main stepsof the solution are reviewed here but details of the expressions may be referred to in the original publication. The spherecenter is placed at the origin of a spherical coordinatesystem(r, 0, •) and a pole of strength(m) at (r = 1, 0, 0) as shownin FigureB-l; the diagramalso definesthe sphereparameters.At time t = 0 the pole is abruptly removed (i.e. primary field is reduced to zero by step turn off). The responseto a dipolewill be obtained by differentiation as the last step of the solution. The polar symmetry results in an electric field with only a • component.Using the scalarstream potentialdefinedby Schelkunoff(1948, p. 403)

PERMEABLE,

SPHERE

{•n(kr)

ql= •n (kr)Pn(cOs 0)exp(itot) (B-2) where]n (kr)= X/•rkr/2 In+i/2(kr),and•n (kr)= X/2kr/•r Kn+l/2(kr),arethemodified Bessel functions as definedby Schelkunoff(1948, p. 52) andPn (cos0) are the Legendre polynomialsand

k2 = i•rtxto. The stream function for each of the three regions can be expressed(for t > 0) as

• An•n(klF)/•n(klb) n=0

X Pn (cos 0 ) exp (itot) (r > b)

E• = (11r)O•/00. The stream potential is related to the magneticpotential, U, (H = -grad U) through

• [Bn]n (k2F)•n (k2b) n=0

OU/Ot = -(1/!x)O•/Or.

q-Cn• n(k2F)/•n(k2b)]

The other field componentsare thus (Nabighian,1970)

X Pn (cos 0 ) exp (itot) (a < r < b)

OHr/Ot= 1/(•xr• sin0)0/00(sin0 0•/00) OHo/Ot= 1/(!xr)O2qdOOOr Er = Eo =H,

= O.

The potential, •, satisfiesthe equation

r202t• 1 O( 0•) /]. =r2 •!x• +elxO-•T

zTm (

0-•-+ sin0 00 sin0 •-•

(r,O,•)

(B-l)

In the quasistatic(low frequency)approximationthe second term on the right side may be neglected. By separation of variables, the elementary solutions of equation(B-l) are determinedas follows:

•1

Y cr=O

x

Fig. B-1. Geometryfor spheresolution.

928

Dyck

Ul=mZ

Z Dn[n(k3r)/[n(k3a)

n=l

n=O


n n+l

b2n+ 1

n+• 1n+•' P,•(cos 0)Fn(t)

where

x P,(cos 0) exp (itot) (r < a) where

k•2= iitYi lxito.

(B-3)

s=l

In region 1 where tr - 0 we have

lim •n (kr)/•n (kb) = (b/r)n k--> O

and

ZAn (b/r)nPn(cos O)exp(itot). n=O

Continuity of tangential E and H at r - a and r = b give, for the stream potential, q/i : q/i+l

1 Oq•i Ixi Or

=

1 i•i+l

OqJi+ 1 Or

-X•/ty31•3 s a2,s-1,2,.

Negative, real valuesfor icos lead to a responsewith a simple decay without oscillation. The variables Vn, E n, and X3s are functions of the sphere parameters b/a, cr2/tY3, [[2 and •3, only. The orderof the multipole expansion is (n); each multiple has an infinite set of eigenvaluescounted by (s). Response to a dipolar field may be obtained by differentiation according to derivative expressions given by Grant and West (1965). The form of these expressions,representedhere by the derivative operator, •i, dependson the dipole orientationwhere i = 1, 2, 3 signifiesthree orthogonal components of the source dipole. The correspondingexternal magnetic field componentsof the sphereresponseare, thus,

i=1,2

Hi =-•i

which are applied to equation (B-3) to find the coefficientsA n, Bn, Cn, Dn. The systemof 4 homogeneous equationsin 4 unknowns has a nontrivial solution only when the system determinant is zero, i.e. in order to satisfy the boundary conditions, we must choose values of ki (or to) that satisfythe characteristicequation (Nabighian, equation 10). The roots of this equation form a discrete set of eigenvaluesof the problem:

itos =

Fn(t)=-2(2n+ 1)•3/•o Z V•2/Enexp(itost).

....

After considerablealgebraic effort the solutionfor the magneticpotential, U, in the exterior region is written

grad U•

from which the response to an arbitrarily oriented dipole may be obtained. Correspondencewith variables occuring in Equation (4-4) is made through dnm(x, y, z) = H i (r, 0, 4)) (after coordinatetransformation)

rt b2n+l

= --•)i grad m n+lr

n+•1n+• ' P • (cos0 )

qnm= -2(2n + 1)tx3/bo V•2/E• =X3•

A=a

B=b


Drill-hole

APPENDIX

PROPERTIES

OF

EM

929

C

EIGENCURRENTS

(After Dyck and West, 1984)

Eigencurrent Expansion

Some properties of induction in isolated, confined conductors (i.e., 3-D conductors in free space) were summarizedby Kaufman (1978) and will be reviewed briefly here to provide insight into drill-hole model responses.Secondaryfields generatedby circulating eddy currents (induced vortex) can be separatedinto time and spatial variations. The time variation of a freely decaying current system J(r, t) set up by the sudden termination of a magnetic field can be expressedas an infinite sum of componentcurrent distributions in the form (Annan, 1974; Kaufman, 1978):

J(r, t)= • Jn(r)Cn • exp(--t/Xn) , (C-l)

s n •, exp (-t/x), n H[(t)= • CnC

(C-2)

n=l

where c• is the magneticfield componentin the i direction and is known as the secondary field coefficient for eigencurrent n. Kaufman (1978) pointed out that 3-D conductors

have, in general, an infinite number of eigencurrents each having a distinct time constant. The principal mode is the eigencurrent with the maximum time constant(= ,• when the seriesis enumeratedin order of decreasingtime constant). The utility of the eigencurrent concept stemsfrom the fact that the coupling •Cn • form a convergent, but not coefficientsCn -- Cn uniformly convergent, series, i.e.,

n=l

Hi(O)

or as the sum of simplefractionsin the corresponding frequencydomain. The componentcurrentshave fixed geometryJn(r) and an exponentiallydecayingtime variation. Each represents a noninteracting current which has the electrical properties of a simple loop filamentwith time constant'rn (=loop inductance/loop resistance).The componentsare called eigencurrents becausethey independentlysatisfy the homogeneous (source-free) differential equations for the fields in a given conductivity distribution.The eigencurrentsand their associated time constantsare thus functions only of the conductivity distributionand are independentof whatever source field creates the eddy current system. The excitation level (at instant t - 0) of each eddy current is expressedby the amplitudecoefficientCn • (known as the primary field excitation coefficient). The strength of the appropriate secondary field componentat a given receiver point can be calculated usingthe Biot-Savart law separatelyfor each eigencurrent and expressed as

is finite. For large t the whole responseis controlled only by the lowest order mode, i.e., largest time constant, that has nonzero coupling. For a given time delay, conductivity distribution, and source and receiver configuration,any desired degree of accuracy may be achievedby includinga sufficientnumber of terms in the calculation. In practice, the EM response of simple conductive models can be computed to sufficientaccuracy by using a limited set of approximate eigencurrents. Properties of Eigencurrents

It is instructive to examine expressionsfor three ideal systemfunctions--step, impulse, and frequency domain•as

shown in Table 1 in order to understand

the nature of wide-band EM response, i.e., progression from inductive to resistive limit. The expressions are consistentwith the convention of Nabighian (1971) for the responsegenerated by the sudden turn-off


930

Dyck

(step) of a primary magnetic source, the impulse response (negative impulse) obtained by differentiation, and the frequency spectrum by Fourier transform. The amplitudefactor cn is referredto as the nth coupling coefficient for the eigencurrent with time constant 'rn.

If the eigencurrents are orderedso that -rn > -rn+1, then in generalthe magnitudesof cn decreasesystematically, but not necessarily uniformly, with n. For example, the antisymmetric plate eigencurrents are not excited by a uniform field. Each plate eigencurrent has a different geometryas shown in Figure A-2; the more complex the pattern, the more rapid is the time decay and usually the geometricattenuation.A similar

situationexists for the spherewith the exceptionthat all the eigencurrentsfor one multipole have the same external magnetic field geometry. Table C-1 shows that the system function dictates the form of the weightingfunction for each term in the series. We see also that in the inductive limit (early time or high frequency), the principal mode (n = 1) is important but the total responseis modifiedby contributions from higher order modes. The step response

and frequency-domain in-phase response have the same inductive limit. Likewise the impulseresponse and frequency-domainquadrature responseshave the same form in the inductive

limit. In the resistive limit

(late time or low frequency) the principal mode (n = 1) is dominant.The time-domainexpressionsreduce, by definition,to one term and differ only by the amplitude coefficient. In the frequency domain there is always somecontributionfrom the higherorder terms, even at very low frequency. Eddy-current migration thus exists for all types of excitation, but the amount will depend to some degree on the system function. For example, the impulse response exhibits the greatest dynamic effect. Implications for accuracy of computationmay also be gleanedfrom the above: the worst convergencecan be expected to occur for the inductive limit and when receiver and transmitter

are both close to the conduc-

tor. Limitationsto precisionand applicationguidelines were discussed by Dyck et al. (1980). Clearly the dynamic properties of eddy current migration will be underestimated

if the series is truncated

too short to

give proper representationof the true currentpattern.

Table C-1. Expressionsfor fundamentalresponses of confinedconductors. SYSTEM

FUNCTION

INDUCTIVE

LIMIT

RESISTIVE

LIMIT

STEP t_>O Step (t) - HE Cn(x,y,z)exp(-t/rn), n

= 0,

E Cn

c• exp(-t/z'•) where r• > z'n

t
n Im(t)--H E cnI•(t)-(1/l'n)exp(-t/l'n) 1, t->O E Cn/l' n

n

= O,

where r I > 'rn

t
cs/rs exp(-t/z'1 )

SPECTRUM

rn2 +i•oECnrn n•cn+i/(o E Cn/'r n •o2ECn• n

n

n


CHAPTER

ELECTRICAL

EXPLORATION

12

METHODS

FOR

THE

SEAFLOOR

AlanD. Chave,*StevenC. Constable,* andR. NigelEdwards õ INTRODUCTION

mid-ocean ridge ore genesis as an analog to terrestrial occurrences; see Teleki et al. (1985), Brimhall (1987), and Rona (1987) for comprehensive reviews of these topics. The sulfide depositswere located visually with submersibles or using near-bottom survey tools such as ANGUS (Phillips et al., 1979). While these methods

Recent developments in instrumentation and submarine geology have spawned increasing interest in the use of electromagnetic (EM) methods for seafloor exploration. Previously, little attention had been given to their use in the marine environment, due both to the

are capable of examining surficial geology, they are not able to adequately assessthe actual extent of the depositsand the nature of the geological structures in which they are found. Seafloor conductivity mapping is one of the few geophysical tools suitable for this purpose,just as the EM methods are one of the major geophysicaltechniquesused in mineral exploration on

successof the seismic techniques in delineating subsurface structure and to a pervasive belief that the high electrical conductivity of seawater precluded the application of EM principles. Marine EM exploration of the solid earth has progressed substantially in academic circles over the past two decades; the adaptation of this technologyfor commercial purposesis only beginning. Over three-fifths of the earth's surface is covered by oceans. Even though petroleum is producedfrom huge deposits on the relatively shallow continental shelf, the immense area of the ocean represents a largely unexplored and unexploited resource base. Until recently, little economic interest was shown in the ocean floor environment; with the possible exception of manganese nodule fields (Sorem and Fewkes, 1979) and the heavy metal-rich brines found in intracratonic basins like the Red Sea (Degens and Ross, 1969), the seafloor was assumed to be essentially barren. However, the recent discovery of intense hydrothermal activity and polymetallic sulfide deposits of unprecedented concentration

land.

Over the past few decades, the searchfor petroleum reserves

has been extended

from

the continents

off-

shore into progressively deeper water, making the continental shelves a focus for geophysical exploration. The principal geophysical tool for this is the seismic method, and the success of the seismic approach is attested to by the level of offshore drilling activity and the subsequent production of oil. However, there are marine geological terranes in which the interpretation of seismic data is difficult, such as regions dominated by scattering or the high reflectivity that is characteristic of carbonate reefs, volcanic cover, and submarine permafrost. Alternative, com-

and scale on the crest of the East

plementary geophysical techniques are required to study these regions. In recent years, significant advances have been recorded in theory, methodology, and instrumentation for the marine EM methods. Many of the seafloor techniquesare adaptationsof standard terrestrial EM approaches, while others represent new directions.

Pacific Rise (Hekinian et al., 1983; Spiess et al., 1980; Ballard et al., 1981), the Galapagos Ridge (Corliss et al., 1979), the Juan de Fuca Ridge (Normark et al., 1983; Koski et al., 1985), and the Mid-Atlantic Ridge (Rona et al., 1986) has aroused interest in the possibility of deep-sea mining and spurred research into

*AT&T Bell Laboratories, Murray Hill, NJ 07974, and Scripps Institution of Oceanography, La Jolla, CA 92093.

*Scripps Institution of Oceanography, La Jolla,CA 92093. õDepartment of Physics, Universityof Toronto,Toronto,OntarioM5S 1A7CANADA. 931


932

Chave, et al.

This paper emphasizesthe differences between seafloor and terrestrial EM applications, especially with regard to noise, resolving ability, and apparatus. The ocean environment and its impact on EM measurements and equipmentis discussedin somedetail, and then the specificpractice of magnetotelluric,dc resistivity, magnetometric resistivity, self potential, and frequency- and time-domain controlled source EM sounding is described. A comprehensive review of pertinent literature is included. Most of the existing work on seafloor EM has been motivated by solid earth problems, as opposedto exploration ones. The real data discussed reflect this difference, which is

principally one of scale. THEORY

There are numerousapproachesto the theory of EM induction in conducting media by finite or distant sources; see Ward and Hohmann, Vol. I (1988) for a review. From the point of view of both computational ease and preservation of essentialphysics, the modal form of the induction equations for one-dimensional (I-D) media is especially appealing and has been adopted here. The EM fields for a 1-D conductivity structure may be separatedinto independenttoroidal and poloidal magnetic (TM and PM) modes about the vertical

axis.

The

TM

modes

are

associated

with

electric currents flowing in loops containingthe vertical, and possessno vertical magneticfield component, while PM modes are driven by electric current systems which are always horizontal, and have no vertical electric field component. Because of this distinction, the sensitivity of the two modes to electrical structure is quite different. This difference can be demonstrated most vividly by imagining a horizontal insulatinglayer buried in a half-space, and deducing the behavior of vertical and horizontal currents in its presence. Due to the existence

of vertical

electric

currents

and conse-

quent galvanic interactions, TM modes are strongly affected by relatively low conductivity zones, being unable to penetrate them very effectively, while the PM mode is quite insensitive to such regionsdue to its entirely inductive nature. Both modes are influenced by relatively high conductivity material. A summaryof mode theory appearsin the appendix, includingGreen functions

which

account

for seafloor and sea-surface

boundary effects explicitly, and will be referred to as needed.

is a very different environment. The seafloor is covered by up to 11 km of seawater that has conductivity ranging in value from as much as 5 S/m near the surface to about 3.2 S/m below the main thermocline;

the latter typically occurs at depths of a few hundred meters in the deep ocean. The conductivity of seawater is controlled almost entirely by temperature and salinity, with a negligible pressure effect (Horne and Frysinger, 1963). To a reasonableapproximation, the conductivity of seawater is given by cr(T) = 3 + T/10

where tr is the conductivity in S/m and T is the in situ temperature in degrees centigrade. This conductivity value is higherthan that of most of the materialswhich lie at or below the seafloor. The uppermost sediments under the ocean are usually water-saturated, and have conductivities

of 0.1-1.0

The

OCEANIC

continents

ENVIRONMENT

are in direct

contact

S/m.

On

the

continental

shelves, this value decreases as lithification and di-

agenesisreducethe porosity of the sediments,until the deep structure of the shelves is much like that of the rest of the continents. By contrast, sediments in the deep ocean are rarely thicker than a few hundred meters, and are underlain by a basaltic crust and peridotitic mantle whose conductivity decreasesfrom about 0.1 S/m near the top to values that may be more than 1000 times smaller at depths of 10 km or less. The highly conductive ocean acts as a low passfilter for fluctuating EM fields generated above it in the ionosphere and magnetosphere. Little power is presentat the seafloorat frequenciesabove a fraction of a hertz in water greater than a few hundred meters deep from these types of sources. Contaminationby man-made or cultural sources is substantially reduced by the seawater layer, and need not be consideredin most deep-oceanexperiments. However, this may be an important noise source on the continental shelf close to cities, where signalscould propagateoffshore through low conductivity seafloor layers. The ocean filtering effect may be quantifiedby the use of a simple model. Consider a seawater layer of thickness H and conductivitytr0 overlyinga 1-D earth and underlying an insulatingatmosphere. A vertical magnetic dipolelike source of intrinsic frequency to a large distance above the water producesnormally incident magnetic fields at the sea surface; this range is assumed sufficient that the source produces only plane waves. Chave and Filloux (1984) give expressionsfrom which the ratio of seafloor-to-sea

THE

(1)

surface horizontal

electric

(Esf/Ess) and magnetic(Bsf/Bss) fieldsmay be comwith

a near

insulator, the atmosphere, which allows the nearly instantaneouspropagationof EM signalswith limited attenuation. To the electrical prospector, the seafloor

puted

Esf

PM

(1 + RL )e

-•o •

Ess 1 + RLPMe -2•øn '

(2)

Electrical

Methods

and

933

ocean to the continental shelf, and for earth half-

spacesof conductivity 0.005 and 0.05 S/m. The hori-

)e-•O" Bsf (1 - R LVM

zontal

Bss - 1- R•me -2•ø" Downloaded 09/30/13 to 134.7.248.132. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

for the Seafloor

(3)

whereRL PMis thePM modeseafloor reflection coefficient (see A-16 in the appendix), and

•/0= •/itolx0cr0

(4)

is the induction parameter or the reciprocal of the complex skin depth. Due to the large conductivity contrast

between

seawater

and crustal

rock

and the

assumption of zero source field wavenumber, the magnitudeof the reflectioncoefficientis closeto unity. Equations (2) and (3) reduce to simple exponential field attenuationat a depth H in an infinitely deep

ocean whenRL PM= 0. Figure1 shows thetwoexpressions(2) and (3) evaluatedfor ocean depthsof 5000, 500, and 100 m, covering the range from the deep

field ratio is insensitive

to the conduc-

tivity beneath the observation site and displays marked attenuation only when the water layer becomes electrically thick (•/0H >> 1). By contrast, the horizontal magnetic field ratio is quite sensitive to earth conductivity and is attenuated more than the electric field even at very long periods. The vertical magnetic field behaves like the horizontal electric field, and is attenuatedonly at relatively shortperiods. The natural, ionosphericand magnetospheric,electromagnetic spectrum is highly nonstationary, varying over severaldecadesfrom day-to-dayin all parts of the spectrum, and displays a strong dependenceon location, increasing at high latitudes and around certain geologicanomalies.For this reason, there is no "typical" EM source spectrum from which a seafloor model may be deducedby applyingthe filters given as equations (2)-(3). However, in the deep ocean, the attenuation of magnetic fields at periods smaller than

100 s is very large,exceeding 106 in power,and

10 0

..10-1 ,., 10-2

10-3

10 5 10 4 10 3 10 2 101 10 0 10 -1

Period (s) 10 o

•10 -1 rn 10_ 2

electric

:_

__

_:

ooo

\

,, \

virtually no EM energyreachesthe seafloorbelow this period. In shallow water, such severe magnetic field reductionoccursat periodsof around1 s. By contrast, the attenuationof the electric field is only a factor of 100 at around 10 s period in the deep ocean, and is not observableat periodslonger than 1 s in shallowwater. The ocean is in continual motion on all spatial and temporal scales, generating EM fields by dynamo interactionwith the earth's magneticfield in a manner identical to the principle of an electric generator. EM field generationby water motion is a complex subject. Since the Maxwell equations constitute a linear set, the fields occur on space and time scales similar to those of the variability of the ocean, covering periods from years to seconds and wavelengths from thousandsof kilometersto centimeters.However, the large spatial scalesgenerally correlatewith the long period disturbances,and problemswith oceanicEM fields as a noise sourceare completelydifferent for long period magnetotellurics(MT) and high frequency controlled source

10-3 10 5 10 4 10 3 10 2 101 10 0 10 -1

Period (s) Fig. 1. The ratios of the seafloor-to-seasurface horizontal

EM.

Long period (> 1 h) EM fieldsin the deep oceanare discussedin Cox et al. (1971), Cox (1980, 1981), Chave and Filloux (1984), Chave (1984a), Chave and Filloux (1985), Lilley et al. (1986), and Chave et al. (1989), while a major physical oceanographic experiment

electric(top) and horizontalmagnetic(bottom)fields as a

based on their measurement

functionof period in the zero sourcefield wavenumberlimit. The earth is modeledas an insulatingatmosphereoverlying an ocean layer of conductivity 3.2 S/m and thickness5000, 500, and 100 m and underlainby a halfspaceof conductivity 0.05 S/m (solidlines)or 0.005 S/m (dashedlines). Attenuation of the vertical magnetic field is similar to that of the

al. (1987, 1990). At periods longer than three to five days, mesoscale eddy activity and a background barotropic(depth-independent) water velocity component produce intense electric fields, reducing the coherence between seafloor electric and magnetic measurementssubstantially. This is particularly pro-

horizontal

electric

field.

is described

in Luther

et


934

Chave, et al.

nounced in the westward portion of ocean basins because mesoscale eddies are continually being shed by the intense currents (e.g., the Gulf Stream) that dominate the dynamics of those regions. In addition, the ocean tides produce sizable electric and magnetic fields (Larsen, 1968). Becauseof the strongmotional contribution to the EM field, it is not possibleto utilize external source sounding methods at periods longer than a few days. It is also probable that the ubiquitous oceanic internal waves produce a magnetic signature that affects seafloormeasurementsat periods of 1-4 h, dependingon the level of geomagneticactivity and the

consequence,sharply peaked EM spectra having an amplitudeone to three decadesabove the background and centered on 0.1-0.3 Hz are observed; multiple peaks can occur due to the presence of swell from distant storms combined with local wind forcing and reflections from nearby coastlines. Webb and Cox (1986) presented measurements of the seafloor electric field due to microseisms, and the evolution of its spectrum during a storm. Webb and Cox (1982, 1986) have also investigated the EM effects of seafloor acousticand seismicdisturbances.They describethe

latitude.

results in acoustic energy being trapped near the seafloorand multiple peaks in the EM spectrum.The backgroundnoise between these narrow-band features in the electric field spectra is probably due to smallscale(0.1 m) turbulenteddies(Cox et al., 1978).Figure 2 illustratesall of these high frequencyphenomena.It should be emphasized that the background oceanic noise at high frequencies is complex and not completely understood,with a strong dependenceon sea state and local geology.

Additional complications appear on the continental shelves:the water velocity field is substantiallymore energetic and complex than at the deep ocean floor, and possible topographic effects from rugged shelf topography may appear at high frequencies. The dynamics of the circulation on the continental shelves is not well understood, but new classes of waves that

are trapped to the topography are possible on many spatial and temporal scales, and interactions of warm currents with topography can generate new, smallscalewater currentswhose EM effectswill be important at the seafloor. The generally higher velocities will combinewith rough shelf topographyto producelocal turbulence. The EM fields generated by oceanic turbulence are discussedin Cox et al. (1971); the electric fields associatedwith turbulence may be locally intense, while the magnetic signatureis negligible. At higher frequencies, surface gravity waves, microseisms, swell, and wind waves produce velocity and pressure fluctuations at the seafloor that in turn induce local EM fields. At periods longer than 40 s, direct forcing by surface gravity waves produces a rapidly rising noise spectrum toward longer periods even in the deep ocean. Measurements indicate that the fluctuatingionosphericcomponentin seafloorelectric field measurementsoften dominates the gravity wave part, but the latter servesas a relatively stationary base below which the spectrumcannot decrease, determining the ultimate noise level (Webb and Cox, 1986). At periods of 10-40 s, the spectrumof electric field variations on the seafloor decreases as the wave-

effect of resonances

The effect of these two main noise sources•exter-

nal and oceanic•is differentfor the passiveand active EM methods. Seafloor MT suffers from a band-limited

source, being caught between attenuated external sourcefieldsat high frequenciesand contaminationby

oceanic noise at long periods. The dc, self potential (SP), and controlled source methods are not influenced

by long period oceanic phenomena and (except in shallow water) by ionospheric noise, but may be affected by the higher frequency sourcesdiscussed.

10-16

10 -18 10 -2o 10-22 10-24

length of the gravity modes becomes smaller, but a background noise component in the electric field of indeterminate origin remains (Webb and Cox, 1986).

Cox et al. (1978)conducteda thoroughexperimental and theoretical study of the EM fields produced by microseisms,which are causedby the nonlinearinterference of opposingsurface gravity wave trains. This produces a pressure disturbance at twice the intrinsic frequency of the waves which propagatesto the seafloor even in deep water, resultingin horizontalwater motions which generate ambient EM fields. As a

of seismic surface waves that

10-2

10-1

10 0

Frequency (Hz) Fig. 2. Electric field power spectrumcomputedfrom measurementstaken 200 km southwestof San Diego in 3700 m of

water.The •f-4 slopeat low frequencies is causedby ionospheric activity and forcing by long surface gravity waves. The peak near 0.1 Hz is due to microseismactivity at the seafloor, while the higher frequency peaks are due to Rayleigh wave-induced motion of the seafloor antenna. See text for details.

Electrical

Methods

For all but one oceanic EM method (SP), the electric

or magneticfieldswhich constitutethe signalare much


reduced over the values which would be encountered

on land. The conductive water layer which is responsible for this decrease in amplitude produces some compensatory benefits; the measurement of these smaller signalsis possibleif correct advantageis taken of the marine environment.

Potential

electrode

noise is

reduced throughoutthe spectrumbecauselow impedance contact with seawater is easily established, and the ocean provides thermal and saline stability at a level never observed

on land. Transmitter

electrodes

are also less of a problem than on land, where considerable labor is often required to obtain resistancesto groundof 10 1•. At sea, a groundingresistanceof 0.1 1• can be obtained merely by lowering a long, uninsulated cable or pipe into the water. Only magnetic sourcesand receivers can be towed over land, but these must be well off the ground for safety reasonsand are usually attached to an aircraft. By contrast, in the ocean both transmitter and receiver electrodes may be towed through the water and close to the seabed. Near-bottom towing introduces new sources of noise caused by antenna cables moving in the earth's magnetic field and streamingpotentials or turbulent flows that affect potential electrodes, but these problems are outweighed by the greatly increased areal coverage that can be achieved. This makes new types of survey tools possiblein the ocean that have no counterpart on land. Another advantageoffered by the conductive ocean is its smoothing effect on electric fields. Except for regions of extreme bottom relief (e.g., mid-ocean ridges and the continental slope), the uniformity and great electrical thickness of the water column, combined with the large conductivity of seawater, will dominate the effect of small-scale irregularities in the near-bottom rocks, yielding electric field measurements that are homogeneousover large areas. This is quite apparent in Atlantic array data at periods of several days, where very good coherence between electric field stations separated by over 100 km was observed (Cox et al., 1980; Cox, 1980), and should be contrasted with the usual situation on land, where

near-surfaceheterogeneitydegradesthe electric field coherence substantially, even when the distance between measurements is quite small. Operating at sea does create specialtechnicaldifficulties associated with maintaining equipment in a hostileenvironment. Most of the packagingtechniques used are common to all of the methods described later.

Seafloor equipment must be housed in watertight vesselscapable of withstanding 100-1000 atmospheres of hydrostatic pressure. Made out of aluminum or glass,these vesselsare usually sphericalor cylindrical

for the Seafloor

935

for mechanical strength and volumetric efficiency. Although it is possible to connect floating surface packages to seafloor equipment, such moorings are very costly and time consumingto deploy and recover. Instead, most receiving instruments are self-contained, operatingoff battery power and collectingdata under the control of a small computer. Measurements are usually stored on magnetic tape, although solid state memory is beginning to supplant tape. A means to orient the measurementswith respect to the earth after instrument emplacement is sometimesnecessary; photographic recording of a compass is commonly used. The packagesare buoyant to facilitate recovery, and attachment of a heavy anchor allows them to be lowered or dropped in free fall to the ocean bottom. When necessary, instruments may be located at the seafloor using standard acoustic ranging techniques. After data collection a timed or acoustically triggered release causes the instrument to part from the anchor and float to the surface, where it is recovered with the aid of radio and strobe light beacons. Seafloor electric field receivers, as with terrestrial types, consist of a pair of electrodes connected to a recordingvoltmeter. The standardoceanicelectrodeis of silver-silver chloride (Ag-AgCI) type, and their construction is discussed in Filloux (1973, 1974, 1987) and in Webb et al. (1985). A great deal of progress in electrode

noise reduction

has been made

in recent

years, but this factor looms large in limiting the sensitivity of oceanic electric field measurements.The principalnoisesourcesincludeelectrochemicaleffects associated either with contamination during construction or with natural processesin the ocean, temperature and salinity fluctuations associated with smallscale turbulence, and streaming potentials or other electrokinetic phenomena. Most of these effects are not well understood, and a largely empirical approach to their reduction has been employed. The ultimate limitation

is due to the Johnson noise associated with

the electrode, antenna, and input impedance of the

amplifiers; this noiseis about10-19V2/Hzfor the Webb et al. (1985) high frequency electrodes. MAGNETOTELLURICS

The magnetotelluric(MT) method is a standardway of determiningthe electrical conductivity distribution beneath and around a measuringpoint using the natural electric and magnetic fields induced in the earth by ionosphericand magnetosphericelectric current systems. The theory and principles behind MT are covered both in Vozoff (1972, and this volume) and in Kaufman and Keller (1981), and are not changed appreciably for marine applications. However, oceanic MT measurements

must be made on the seafloor


936

Chave, et al.

because magnetometermotion has to be eliminated; this arrangementprecludesoperation from either subsurfacemooringsor surface ships. The two major physical phenomenawhich differentiate marine from terrestrial MT techniques are the high-frequency cutoff of the source spectrum at the seafloor caused by induction in the conductingocean and the generationof EM fieldsby water motion. This differentiation results in a seafloor EM spectrum for solid earth MT purposes that is band-limited in the deep ocean, being cut off at both high and low frequencies by different phenomena. While the amplitude reduction of the seafloor fields is an important constraint in instrumental and experimental design, the seafloorMT responsefunction or impedancedoes not depend on the thickness of the water layer and is a measure only of the suboceanic conductivity structure. However, the applicationof MT methods in the deep ocean is generally limited to periods longer than a few minutes, and the audiomagnetotelluric(AMT) method

cannot

be used even on the continental

shelf.

The extent and importance of contamination of existing, low-frequency MT data by motional fields is not well-understood, and its potential as a problem at the higher frequenciesof interest in exploration geophysics is not well appreciated. It should be noted that motional sources produce EM fields that are both mixed mode and short spatialwavelength in nature, so that they do not serve as a good geophysicalsource,

=

(5)

where •0 is the integratedoceanconductivityandPe is the integrated resistivity of the basement to some depth where it drops abruptly (typically, 30-50 km). Values for L range from 200-2000 km for basement

conductivities of 10-2 to 10-6 S/m,andtheinfluence of this boundary layer is independent of frequency. However, the evidence from MT data requires a boundarylayer only a few hundredkilometers in width at most. When consideredtogetherwith the controlled sourceresults of Cox et al. (1986), which indicate a very resistive uppermost mantle for moderate age lithosphere away from the continental margin, this boundary layer suggestsmarked lateral heterogeneity of basementconductivity. It is probable that conductive pathways from the ocean to the deep earth exist throughout the ocean basins, most likely where the oceanic lithosphere is quite young and hot or in fluid-saturatedzones at the continentalmargins.While the polarization effect is not a seriousproblem in the deep ocean, it will profoundlyinfluenceMT measurements near coastlines

and on the continental

shelves.

This influence means that interpretation of seafloor MT data near the continents requires at least a twodimensional(2-D) modelregardlessof the natureof the geological structure of interest. The instrumentation

used for seafloor MT

studies is

subsequentinterpretation could be profound. Turbulent flow associatedwith continental shelf topography could be a seriousnoiseproblem for shallowwater MT work, primarily through contaminationof the electric field. While many of these complicationscan be minimized by avoidingregionsof extreme bottom relief, a better understandingof these phenomena awaits the

discussedin Cox et al. (1971)and Filloux (1973, 1974, 1980a). A comprehensivereview of all aspects of oceanic EM apparatusis given in Filloux (1987). In general, seafloorelectric and magneticfield recording is done with separate instruments. Seafloor magnetometers are an adaptationof their terrestrial counterparts to the deep ocean environment, while oceanic electric field sensorspresent someunique challenges. Seafloor MT apparatusis usually self-contained,with

actual

no direct connection

and their

influence

collection

on MT

measurements

of measurements

and their

on the continental

shelves.

Another problem in oceanic MT is associatedwith the large electrical contrast between the oceans and continents and the probable existence of highly resistive material in the upper oceaniclithosphere.A model for the polarization of the flow of electric current induced by external sources at the ocean-continent boundary, which is the oceanic counterpart of the well-known coasteffect, was presentedin Cox (1980), Ranganayakiand Madden (1980), and Chave and Cox (1983). This model may introduceanisotropyinto the MT response function. The component of electric current in the ocean flowing normal to the coastline must

either

enter

the

resistive

continent

or be de-

flecteddownwardinto the oceaniccrust and mantleby boundary electric charges.The width of the zone over which the latter effect will be important is given by

to the sea surface.

Because

of the

very good temperaturestability at the seafloor,where variations of only a few millidegrees are expected, good thermal sensitivitycharacteristicsare usually not a critical factor during design.However, magnetometer motion is a real problem, as the benthic boundary layer is a regionof intenseand variable activity (Armi and D'Asaro, 1980), suggestingthe use of compact packageswhich present only a limited surface area to the ambientcurrents.By contrast,inductioneffectson electric field instrumentationcausedby motion of the receiving antenna are negligible for the frequencies used in seafloorMT sounding. Two types of magneticsensorsare in current use for seafloorMT experiments--the suspensionunits developed by Filloux and reported in Filloux (1967, 1980a, 1987) and the fluxgate units with usage reported in White (1979), Law and Greenhouse(1981), and Seg-

Electrical

Methods


awa et al. (1982, 1983). Filloux's instrumentation is

based on magnets suspended from torsion fibers of special design and placed in a feedback coil. Optical detectors sensethe angular position of the magnet, and the current in the coil is adjusted to null the magnet position, serving as a measure of the field variations. This design has the dual advantages of having no natural noise contributions except for plastic flow or creep of the torsion fiber, which can be controlled during manufacture, and making minimal demands on the stability of the associated electronics and power supplies. The main disadvantageis the custom nature of the sensors, which are not commercially available. The suspension magnetometers are housed ia selfbuoyant cylindrical pressure cases, sit 1 m off of the seafloor on tripod anchors, and are capable of digital recording of the magnetic variations with a least count of 0.1-0.2 nT at a rate of 32 times per hour for six months.

Fluxgate sensorshave the advantage of commercial availability and the disadvantages of requiring relatively large amounts of power and placing fairly stringent stability constraintson the associatedelectronics. Both Law (pets. comm., 1985) and White (1979) have adapted commercial fluxgate sensors to the seafloor, while Segawa et al. (1983) used the newer ringcores which are now appearing on the market. These units have a least-count sensitivity of 0.5-1.0 nT and display long term drift, but fluxgate technology is changing quite rapidly and these limitations are expected to diminish

with time.

Two types of electric field instrumentation have been used for seafloor MT work--(1) long wire units and (2) a short arm, salt bridge apparatus utilizing chopping techniques to reduce noise. The former is simply a long (typically, 500-1000 m) insulated wire with Ag-AgCI electrodes attached at the ends and connected to a recording package. The extended antenna is used to raise the signal level well above the electrode noise level, and has the additional advantage of averaging out the electric fields generated by turbulence and temperature-salinity variations, which usually occur with a spatial scale of a few meters. The main disadvantage is the difficulty of deployment, which requires a ship equipped with winches and takes a great deal of time, involving the dynamic straightening of the wire as it slowly sinks to the seafloor. Examples of long wire electric field measurementsare given in Cox et al. (1971), Filloux (1973), and Webb et al. (1985).

The short span instruments have an electrode spacing of a few meters, utilizing salt bridges(hollow tubes attached

to the electrode

at one end and the sea on the

other) to connect the Ag-AgCI electrodes, which are placed together on the instrument package, to the

for the Seafloor

937

water. A mechanical device, called a water chopper in Filloux (1974), is used to physically reverse the seawater

connection

to the electrodes

between

measure-

ments. This procedure works in a manner similar to electronic chopping amplifiers by moving the operating frequency to a quieter part of the noise spectrum. Chopping also eliminates baseline drift so that the dc electric field is physically interpretable. A major advantage of the short arm instrument is the ease of deployment; it need merely be hoisted over the side of a ship and released. The major disadvantage is the difficulty of fabricating a water chopper that maintains very high resistance isolation of the electrodes, yet operates reliably under the high ambient pressures at the seafloor. Figure 3 shows a short span instrument being deployed from an oceanographic research vessel. This type of instrument is capable of measuring both horizontal electric field components32 times per hour for 300 days with a least-count value of 0.02 i•V/m. Water chopper instrumentshave recently been developed to measure the vertical electric field for oceanographic studies. The use of a chopper is essential if short span electric field measurements are to be interpretable at periods of an hour or more, as electrode drift can easily dwarf the natural field variations without

one.

All of the oceanic MT work performed to date has been done with the purpose of probing the deep lithospheric and asthenosphericstructure to depths of hundreds of kilometers (e.g., Filloux, 1981, 1982; Law and Greenhouse, 1981). The data are interpretable in terms of a I-D conductivity structure in the deep ocean away from continents, and the results indicate low conductivity in the upper 50-100 km followed by an abrupt rise to 0.05-0.2 S/m below this. The resolution of the data is generally quite low (e.g., Oldenburg, 1983), reflecting the strongly band-limited nature of the seafloor EM spectrum. In particular, the data contain little or no information on the conductivity of the oceanic lithosphere above depths of •30 km. A possible correlation of the depth to good conductor with lithospheric age has been noted (Filloux, 1980b; Oldenburg et al., 1984), and has been interpreted in terms of partial melting in the presence of water-rich fluids (Tarits, 1986). The ability of MT to resolve lateral changes of structure is better than its sensitivity to vertical variations in conductivity, and array experiments are beginning to exploit this property. The first such effort is the EMSLAB experiment covering the Juan de Fuca plate, and is summarized by EMSLAB Group (1988). The substantial improvement in resolving power from combining both land and seafloor MT data is evidenced by modeling the EMSLAB data, where the top of the subductedJuan de Fuca plate is clearly imaged (Wannamaker et al., 1989). It is likely


938

Chave, et al.

that seafloor MT or geomagneticdepth sounding will be especially useful around mid-ocean ridges, where lateral conductivity variations associated with the thermal evolution of the lithosphere are substantial. In spite of the limitations of the MT method in the oceans, it is presently the only way of obtaining conductivity information from deeper than 30 km or so. The low electrode noise and drift, the homogeneity of the electric fields, and the applicability of the I-D approximation all help to temper the problem of reduced signal levels. It is unlikely that MT sounding will ever be useful for marine geophysicalexploration purposes, except possibly for delineating regional (as opposed to small-scale) geological structure on the

continental shelves. However, it must be appreciated that the pioneering electrical work in the ocean was done to obtain

information

about both the solid earth

and about water currents, and that the technology so developed has since been adapted to other more exploration-related purposes. DIRECT

CURRENT

RESISTIVITY

The direct current (dc) resistivity method is one of the simplestEM methods available to the explorationist. A direct current is passed into the earth through a pair of source electrodes and the resulting potential difference is measured between another pair of elec-

Fig. 3. A short span electric field instrument being deployed from the R/V Alexander Agassiz in 1976. The aluminum pressure case houses the recording electronics, while the four horizontal arms are salt bridges that connect Ag-AgCI electrodes, located on the water chopper near the base of the pressure case, to the ocean. The tripod anchor is releasedunder timer control, and the remainder of the instrumentreturns to the surface under slight positive buoyancy.

Electrical

Methods

trodes. Under the static approximation, electric current has no sources or sinks, and the governing equa-


tion is that of conservation

of electric

current.

This is

easily solved for simple electrode geometries and electrical structures, and more complicated ones may be treated using numerical approaches. At a first impression, it appears absurd to attempt the use of dc methods in a highly conducting medium like the ocean because the proportion of the signal which is due to the seabed is given to a first approxi-

mation by the ratio trl/tr0, where tr• and tr0 are the conductivities

of rock and seawater.

less in unconsolidated duced

in hard

rock.

sediments The

This ratio is 0.1 or and is further

attenuation

becomes

reeven

greater if the electrode array is not located on the seafloor (Lagabrielle, 1983). To detect a 10 percent change in the seafloor conductivity requires seafloorbased measurementswith a precision of 0.3 percent or better. The requirement for high precision is counteracted in part by the advantagesof working in the ocean with potential and transmitter electrodes (low noise and high source current). In spite of these advantages, particular care must be taken in assessingthe effects of topography due to the small size of the dc resistivity anomaly in the ocean. The dc method was developed in France by the Schlumberger brothers between 1912 and 1926, and their brilliance is attested to by the fact that they conducted the first resistivity surveys over water only a few years later (Schlumberger et al., 1934). The object of this work was the determination of seabed structure for a harbor engineering project, and would be accomplished more accurately and inexpensively by acoustic means today. More recent oceanic dc resistivity work has focussed on delineation of sulfide mineral bodies, measurements of extent of submarine

permafrost in the arctic, and examination of the porosity structure of the oceanic crust. Neither sulfide bodies nor small porosity changespossessa significant seismic signature, but both have a pronounced effect on electrical conductivity. Early attempts to use dc techniques to detect the offshore extension of sulfide deposits on the Cornish coast are reviewed in Francis (1985a). More recently, Francis (1977) developed a Wenner array about onehalf kilometer in length which was towed on the sea surface for shallow water resistivity sounding in the same area. A source current of 2000 A was provided by the generators of a Royal Navy minesweeper and over 2000 km of survey lines were covered. Bottom profiling by acoustic means allowed correction for the influence of seafloor bathymetry. Several sizable resistivity anomalies were detected which could only be accounted for by sulfide mineralization, but confirmation of the finds by dredging or drilling is lacking.

for the Seafloor

939

Francis (1985b) also described a modified Wenner array that can be deployed from a research submersible. A 50 rn long cable is towed behind a research submersible

and

contains

both

source

and

receiver

electrodes. The unit is weighted to hang vertically below the submarine, and is stretched along the seafloor during the measurement process. The apparatus was designed for the assessment of the massive, polymetallic sulfide ore bodies that have been discovered on mid-ocean ridges. A trial experiment was conducted on the East Pacific Rise in early 1984, and substantial conductivity anomalies were observed around known hydrothermal fields, with local rock conductivity values as large as that of seawater. This should be contrasted

with the much lower

conductiv-

ity (<0.1 S/m) of seawater saturated basalt. Another related application of a dc method for mineral exploration is proposed in Wynn (1988), who designed a system for the study of induced polarization in titanium-bearing sands. The apparatus consists of a towed 4 Hz dipole source with Ag-AgC1 receiver electrodes 30 rn in length for use in shallow (=20 m) water. Preliminary results were inconclusive, primarily due to the lack of a suitable deposit in the test area. Because of the large electrical conductivity contrast between resistive permafrost and unfrozen sediment, galvanic electrical methods are suitable for the determination of the depth to and thickness of permafrost below the seafloor. Corwin (1983) described the results of a seriesof trial experiments conducted in an area of known structure off Prudhoe Bay, Alaska, using a 100 rn long Schlumberger array towed on the surface behind a small boat. Under good conditions, the method was capable of detecting the depth to permafrost with an accuracy comparable to that of seismic refraction. Resistivity techniques can detect thin permafrost layers quite precisely in shallow water, and can be operated in a rapid, towed, survey mode. Owing to the relative homogeneity of the upper oceanic crust, large-scale borehole resistivity techniques offer useful measurements of bulk rock conductivity. This technique differs from conventional well-logging because large-scale averages of the conductivity for hundreds of meters around the borehole are obtained, rather than the more typical small-scale values measured by logging tools. The method has been applied in Deep Sea Drilling Project (DSDP) holes which penetrate the basaltic oceanic crust. Neglecting mineralized and hydrothermal zones, the conductivity of surface rocks depends primarily on the degree of water saturation, and hence porosity, and all of the experiments performed to date have been aimed at determining this important parameter. The experimental set-up for the large-scaleresistivity experiment is relatively simple. One current elec-


940

Chave, et al.

trode is placed near the surface ship (or maybe the surface ship's hull), while the secondis placed near the bottom of the borehole at a depth of 500-1000 rn below the seafloor. The potential electrodes are spaced 10150 rn apart, a much larger separation than is used in conventional logging tools, in order to measure conductivity averages over large volumes of rock. This practice practically eliminates the effect of the zone of anomalous resistivity and porosity around the borehole that is produced by the drilling process, and also reduces the influence of small-scale and laterally inhomogeneous structures in the oceanic crust. Francis (1982) developed the theory and original apparatus for the large-scale downhole resistivity experiment, which was tested in DSDP Hole 459B in the Mariana Basin. While the first trial was only partially successful, it did demonstrate the feasibility of the concept. Two additional experiments have been conducted

in DSDP

Hole

504B near the Costa Rica Rift

south of Panama. Von Herzen et al. (1983) measured

the large-scale resistivity to a depth of 836 rn below the seafloor during DSDP Leg 70, and several years later, after the hole was extended to nearly 1.5 km depth, the experiment was repeated (Becker et al., 1982; Becker, 1985). Figure 4 shows some results from the latter attempt. The resistivitiesobtained from the large-scale apparatusagree in form with the results obtained from commercial tools, but are generally lower and smoother because of the averaging properties of the technique and the elimination of drilling disturbance effects on the outcome. The experiment clearly demonstrates an ability to detect the boundary between the uppermost pillow lavas and the deeper sheeted dikes at 850-1050

10

100

.....

10 I

........

the seafloor.

The former

have a

bulk conductivity as high as 0.1 S/m, while the deeper layer is much more resistive, averaging about 0.005 S/m in conductivity. The resistivity indicates a substantial reduction in porosity from surface values of 7-10 percent to less than 3 percent at depth, and has implications for the depth of hydrothermal circulation

APPARENT RESISTIVITY 2OO

rn below

10 I

(ohm-m)

.......

10 '1

........

10 I

10 I

........

'

SEDIMENT I

3OO

400

'•'

5OO

PILLOW LAVAS and

MINOR FLOWS-

6OO

700

70 O

8oo

T'

900

-Y

TRANSITION

12 1000

]

1100

5-m AVERAGES

SHEETED DIKES

SCHLUMBERGER 1200

-

k

SPHERICALLY-FOCUSED

LATEROLOG

1300

I

I I I •titI

I

I I I Illll

I

ELECTRODE

PAIR 1-2 ...........

•

2-3

and

MASSIVE

3-' 4

LEG 83

I I I 10

100

1000

1000

1000

Fig. 4. Large scale resistivity experiment resultstaken on DSDP Leg 83 and comparedto an earlier experiment on Leg 70 and standard logging results. Shown from left to right are 5 m averages of Schlumberger Laterolog resistivities, three independentlarge scale measurementsfrom Leg 83, and two experimentsfrom Leg 70. All of the traces are plotted on equivalent semi-logarithmic scalesand are offset by one decade. The apparent resistivities are about 10 f•.m in the uppermost pillow lavas and increase to about 1000 f•.m at the base of the hole (taken from Becker, 1985).

UNITS


Electrical

Methods

in the oceanic crust. More recently, Becker (1988) described a large scale resistivity experiment from Ocean Drilling Project (ODP) Leg 109in which a 500 rn deep hole in the Atlantic was reoccupied. The results give a different resistivity structure than for Hole 504B, but the inferred porosity structures are similar: this result is attributed to the markedly disparate

temperature profiles between the two holes. MAGNETOMETRIC

RESISTIVITY

The principles of the magnetometric resistivity (MMR) method are described by Edwards and Nabighian (this volume). The method is based on the measurement of the low-level static magnetic fields of a grounded electric source. Edwards et al. (1981, 1984) introduced a modification of the method, a natural extension of cross-hole MMR known as MOSES, an

acronym which stands for Magnetometric Off-Shore Electrical Sounding. The transmitter is a vertical longwire bipole extending from the sea surface to the seafloor. A cornmutated current is fed to two large electrodes: one near the sea surface, the other on the bottom. The return current is through seawater and

the subjacent crust. The receiver consists of two orthogonal horizontal magneticfield detectors located on the seafloor. The measured data are two components of the magnetic field as a function of frequency and transmitter-receiver separation. Accurate estimates of seafloorconductivity are possible using this configuration because the horizontal magnetic field is proportional to the current which enters the crustal material. For dc resistivity sounding, only the value of the TM mode reflection coefficient (equation A-16 in the appendix) at the seafloor,which has a magnitude very close to unity for most crustal conductivities, is obtained. By contrast, the MOSES method

measures

the

transmission

coefficient

for the Seafloor

941 TM

2'rrf•dkJl(k9) 13 21 B,-- •x01 k2 +2 1 - 2e -•H + e -2•3H(6)

1 + RL TMe-213H

where [3 is given by equation (A-17) in the appendix, I is the sourcecurrent, • is the source-receiverrange, and H is the water depth. The seafloorcan be modeled as a half-space of conductivity tr• and approximate solutionscan be obtained for the case trl small compared with the seawater conductivity tr0 can be derived. A simple useful formula can be obtained provided two additional approximationsare made. First, the range • must be large comparedto the depth of the ocean H. Second, the integrated conductivity of the sea layer tr0H must be large compared with the parameterproduct tr• •, so that the bipole current is channeled out to relatively large distances by the sea layer. At the static limit, a simple expressionfor the azimuthal magnetic field results [xolo' l H

13 6=4,rrcr0p 2(1- itolx0crlp2/2). (7) Computationsand graphspresentedin the theoretical papers already referenced indicate that the field strength is small. However, the field need only be measured to an accuracy of 10 percent to obtain the seafloor conductivity to the same degree of approxi-

1 +

RL TMwhich,in thestaticlimit,is proportional to the crustal conductivity. The principle is illustrated in Figure 5. Ampere's circuital law is applied to a horizontal path on the seafloor,centered at the base of the current bipole. The total current flow, for a uniform sea layer above a uniform layered earth, has axial symmetry about a vertical line defined by the bipole. The azimuthal magnetic field is constant in magnitude around the Ampere circuit and is due only to the current which crossesits plane (i.e., the current which enters the seafloor). This current may be shown to be proportional to the ratio of the sedimentconductivity to that of seawater. Consequently, the associated magnetic field is a direct measure of the basement conductivity.

The azimuthalmagneticfieldB0 in the quasistatic limit is given by

i

I

Ikm

MOSES

Fig. 5. Schematic illustrating the principle of the MOSES method. The current flow is axisymmetric about the bipole source. The relatively small amount of current entering the resistive crust is proportional to the ratio of the crustal to seawater conductivities. By Ampere's circuital theorem, only this current contributesto the azimuthal magneticfield measured at a point on the seafloor.

942

Chave, et al.


mation. Estimates of seafloor conductivity are obtained at all transmitter-receiver separations,with the larger spacings yielding averages encompassing greater depths.

In the static limit, apparentresistivity curves similar to standardSchlumbergersoundingcurves without the sea layer may be constructed to aid in layered earth interpretations. Traditionally, an apparent resistivity

fields detected by the magnetometers are filtered, digitized, stacked, and recorded in solid state memory under the control of a microprocessor. The bucking of the fluxgate instruments and the timing of all experimental operations are also under software control. The first successful sea test of a MOSES system is described in Edwards et al. (1985). The test area, Bute Inlet, is located approximately 200 km NW of Vancou-

formula

ver, British Columbia, Canada. The inlet is more than

is a function

of the actual field measurements

multiplied by a numerical factor derived for the special case where the ground volume studied is homogeneous and isotropic. In the MOSES case, no simple formula that is valid over a wide range of model

50 km long and averages 3 km in width. It is a vee-shaped valley containing seawater about 650 rn deep overlying sediment estimated to be 600 rn thick by extrapolating the shape of the adjacent topography

parameters exists.Expression (7)for thefieldB•, with

downward

frequency to set equal to zero, may be inverted to yield

The field data in Figure 9 were used to compile the apparent resistivities and associated errors shown in Figure 10 taken along an axial profile down the inlet. The range of separationof the transmitter and receiver is 150-2000 m. The averaging time at each station was I h. The operating frequency was 0.125 Hz, which is just low enough to allow the effects of induction to be

a formula

used to transform

field data into a domain

where they may be compared with model curves for the whole range of parameters encountered. The apparent resistivity formula is txolH

ß

(8)

P"- 4'rrcr0p2B,

The expression reduces exactly to the resistivity of a half-space beneath the sea only when the conditions for the validity of expression (7) hold. A suite of apparent resistivity curves for a layer over a half-space is shown in Figure 6. Experimental MOSES systems have been developed for deep crustal sounding (Edwards et al., 1985; Nobes et al., 1986), for mapping sulfide depositsin the vicinity of mid-ocean ridges (Wolfgram et al., 1986), and for determining the physical properties of permafrost beneath the Beaufort Sea (Edwards et al., 1988). The apparatus for a MOSES experiment depends on the application, but in general divides into two broad categories: (1) that required to perform the actual sounding, and (2) that required to determine position

beneath

the sea.

104

,.•3000

>-103 •1000

>

• 300

LI..Ii02 n-

• -. _ _ • _ ....

I00• • • '"'" •

Z

at sea.

Of particular interest are the designsof the seafloor magnetic receivers which are described briefly. For deep crustal sounding experiments, the seafloor receiver is the remote microprocessorcontrolled device shown in Figures 7 and 8. The receiver is dropped in free fall from a surface vessel. On deployment, the shape of the anchor ensures a landing within one or two degreesof the vertical in well-sedimentedregions. At some preprogrammed later time, it releases its anchor

and returns

to the sea surface.

The

d

02=VCIF

.I

consistof two orthogonal, horizontal componentflux-

4pT/X/-•zz at 1 Hz. However, whenoperated closeto the other electronic equipment forming the receiver, the noise level increases almost tenfold. Magnetic

I

IO

IOO

r/'d

sensors

gate magnetometers based on the Scintrex MFM3 design. The inherent noise level of each sensor is

P•=100

Fig. 6. MOSES apparent resistivity curves for the model of a layer over a half-space beneath the sea as a function of the source-receiver offset normalized by the water depth. The layer is assumedto have a thicknesscomparableto the depth of the ocean and a resistivity contrast of 100 with the ocean. The resistivity contrast with the lower half-space is variable as marked.


Electrical

Methods

neglected. Errors in apparent resistivity at short spacings are principally due to uncertainty in position, whereas those at large separation are due to errors in magnetic component estimation. The local geology beneath the sea can be modeled by one layer over a half-space. The layer represents the conductive sediment and the half-space the relatively resistive basement rock. The best fitting model is inset into Figure 10; the correspondingtype curve is shown as a very heavy line. An independent estimate of the coarse error bound (Jupp and Vozoff, 1975) in

..

for the Seafloor

943

both the layer conductivity and thickness is calculated as 9.2 percent. The error in the half-spaceresistivity is illustrated by plotting the family of type curves shown in Figure 10. By inspection, the ratio of resistivities Oh/Pcbetweenthe lower half-spaceand the layer must be at least 10 and could be larger. The estimates of the sediment thickness and resistivity determined by the technique are reasonable. The sediment resistivity of 1.9 ll-m correspondsto a porosity of about 42 percent, which is in the range of that measured on core samples. The thickness of the sediments, estimated at 560

:.

Fig. 7. The MOSES deep water receiver mounted in its concrete anchor. The instrument is housed in a spherical aluminum pressurecase that floats to the surface when the concrete base is released. A 27 MHz radio beacon and a flasher unit are attached to the left and right sides of the pressure case, respectively, as an aid in instrument recovery The small black circular ring in front of the strobe is an acoustic transponder.


944

Chave, et al.

the sedimentoverlying a relatively resistive half-space approximating the fractured basalt. When this model is fit by least squares to site 1 data, the sediment thickness and resistivity are obtained as 0.82 _+ 0.06 fi.m and 1800 _+300 m, respectively. The sediment porosity which corresponds to the quoted resistivity is of the order of 30 percent. The resistivity of the fractured

m, is less than the upper estimate of 600 m obtained by extrapolating the shape of the adjacent topography downward beneath the sea. The value is also in good agreement with that obtained by reflection seismology. The interpreted range of basement resistivity does include that of typical crystalline rock. The MOSES method has also been used in deep water. Nobes et al. (1986) determined the resistivity and porosity of the sediment and fractured basalt layers in the Middle Valley of the northern Juan de Fuca Ridge system. The valley is generally flat and featureless. Two sites within the valley were studied; the first site is in the central region, which is filled with a thick sequence of clay turbidites, while the second site is in the southern region, where the sedimentsare much thinner. A geologically reasonable model for both sites is a relatively conductive layer representing

basalt

is indeterminate.

When

the

same

model

least 1000 m thick.

The field technique used in the Arctic experiments was very different from that previously described, and

.-.:

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............... .::.... •--. ..... •

.....

}.'.";.•..,: ';....,., ;;: :... :;½5;';21::?.::-.:. "?:;:;,'-•:•:•.;' ; .'..'..":: :....: . .- "--. .':

' ...:

,- ...

.... .......

.... ;?'::" .-->'•

..•:.

.;:.

....

..:..:;:;.-:u;•;;•,•g, ......... j;•::

. ............ ...••••••.•,:'•½,,:.....

Z',. :'"•..."

.........

:*. ...... ,:.: ............ ..... . "':,..,.,;;.-. .:..;,;::57.;-**.,•. .........-.,•,-.-:-,;:-

? .... ..... -.::: *'•" .

:

::....

ß....

...

ß .......:....,.: ..............

is

adjusted to fit site 2 data, the thickness of the sediment is obtained as 200 _+ 50 m on average provided the resistivity of the layer is specifiedas that determinedat site 1. The resistivity of the basalt is 30 percent higher at a value of 8.5 _+ 3.4 fi.m, corresponding to a porosity of about 8 percent. The fractured zone is at

:•r•`•:::.:•:::•:•:::.*?•::•*•::•.•...*...•*.:•$•*•:•:•:::•:•:•:•.?• :....:.;..• ..•.*,;:,,;'•t•a:,,..,.::.,•.........?.;•. •g,•.:......-½ .....

-:

--.•½?:?'

Fig. 8. The interior of the MOSES deep water receiver showingthe two orthogonalfluxgate magnetometersand the microcomputer boards.


Electrical

Methods

the design of the sensor reflects this. The instrumentation had to be lowered and subsequentlyrecovered through a 10 inch hole in the sea ice which covers the surface until the late spring of every year. The ice sometimes forms a very stable floating platform for geophysicalexperiments. The sensor, shown in Figure 11, is folded up and lowered vertically into the hole. It is rigged to unfold on passingthrough the hole to form the horizontal square framed arrangement shown in the figure. The unit is then gently lowered to the seafloor.

The field sensors are coils wound

iron laminated

core and are contained

on a soft

in stainless steel

jackets. The coil and its current amplifier have a flat frequency response from 0.5 to 200 Hz. The Arctic experiment (Edwards et al., 1988)repre-

I

I

I

I '1 I I1 I

I

I

I

I

I I I I

5

• 10_2 :LLI

for the Seafloor

945

sents the first application of MOSES to a problem of importance to the petroleum industry. The target is the permafrostlayer under the Beaufort Sea, a seismically rigid layer from 100-600 m thick underlying shallow water typically 10-100 m deep. Detailed knowledge of the location and physical properties of the permafrost layer is essentialfor accurate interpretation of seismic reflection data. The permafrost can contain pockets of gas hydrate. The gas hydrate is both a possible resourceand a hazard to drilling operations.A local map of the permafrost zone is essential geotechnical information required prior to the construction of an offshore structure or pipeline. The test soundingwas conducted northwest of Tuktoyaktuk at the location marked with a cross on Figure 12. The depth of water beneath the ice at the site was only 16 m, a little too shallow to demonstrate the full potential of the technique. The data are displayed in the form of magnetic field amplitude and phase shift curves in Figure 13. The interpretation was accomplished using an algorithm developed in Constable et al. (1987). The method seeks to minimize the roughness of the conductivity-depth profile, defined as the integrated square of either the first or second derivative of the profile. The two curves that result are shown in Figure 14. The resistivity clearly increases rapidly with depth. The profiles are consistent with a two-layer model of the seafloor composed of soft recent marine sediments with a porosity in excess of

-

CD z

_

o

-

CD

-

ioo

_

LLI z

-

i(:f• _ _

_

_

_

_

_

_

I

IOO

I

I

i

IOOO

I

I

I

11

I0000

SEPARATION (m)

Fig. 9. The azimuthal magnetic field amplitude at a frequency of 0.125 Hz as a function of transmitter-receiver

separationfor the Bute Inlet MMR experiment describedby Edwards et al. (1985). The magnetic field is normalized by the transmitter current and plotted as a function of transmitter-receiver separation.The numberscorrespondto the sites where measurements were made; see Edwards et al. (1985) for details.

SEPARATION

(m)

Fig. 10. The apparent resistivity as a function of transmitter-receiver separation for the Bute Inlet MMR data of Figure 9. The curves are for the model of a layer over a more resistive halfspace beneath the sea shown in the inset. The numberscorrespondto the measurementsites of Figure 9.

946

Chave, et al.

35 percent overlying deltaic sands of lower porosity which are probably partially frozen.


SELF

POTENTIAL

The measurement of electric self potential (sometimes called spontaneouspolarization or SP) is an establishedmethod of geophysicalprospectingon land and is used primarily in the search for sulfide mineral deposits. The origin of SP fields is not certain. The most acceptable mechanism is that of Sato and Mooney (1960) who suggestthat SP fields result from electric currents that are produced when a conducting body connects regions of different electrochemical potential. Seawater has a reduction-oxidation (redox) potential (Eh) of +200 to +400 mV, while marine sediments

have Eh values of - 100 to -200

mV a small

distance below the water-sediment interface, and the seafloor serves as a strong redox boundary. A con-

ducting object, such as a mineral deposit, that pierces the contact will produce an electric current; the return current occurs in a diffuse zone surroundingthe body, causing an SP anomaly.

Corwin (1975) has examined the principles of SP prospecting in the ocean, and notes the advantages already discussed of using electrical methods in the ocean. It is convenient to tow a pair of electrodes through the ocean, making rapid SP surveys of large areas quite feasible. SP apparatus is very simple, consisting of a pair of nonpolarizing potential electrodes and a recording voltmeter. However, as for the dc method, the SP effect is reduced by a factor of

crl/cr0, where cr• and cr0 are the conductivities of sediment and seawater, but this attenuation is not

large where the near-bottom marine sediments are water saturated and have an electrical conductivity approaching that of seawater. Corwin (1975) reports that wave- and tow-induced potentials are the most significant types of contamination

in oceanic

SP.

Surface

wind

waves

and

swell

produce EM noise both by direct induction and by producing periodic motion (strumming) of cables that are moved through the earth's magnetic field. Both of these are high frequency, narrow-band processes,and can be reduced substantially by filtering. Tow noise is

Fig. 11. The ICE-MOSES folding sensor.


Electrical

Methods

caused by changesin the nearly dc potential produced by moving a conducting cable through the geomagnetic field when the heading is altered. This noise is large only during turns, and can be corrected by careful navigation. Turbulence is another likely source of backgroundnoise. However, noise levels below the millivolt level have been obtained, so that SP anoma-

lies of only a few millivolts can be differentiated from spurious effects. Corwin et al. (1970) and Corwin (1975) describe an apparatus designed to be towed at the sea surface for shallow water SP surveys. The first apparatususes salt bridges to connect shipboard Ag-AgCl electrodes to the ocean and measures the vertical potential difference. The second design uses Ag-AgCl electrodes at the ends of conducting cables and separated horizontally by 10-100 m. Both units record the SP potential on shipboard, and are towed at speedsof $-10 knots. Brewitt-Taylor (1975) has developed an SP apparatus that is towed

a short distance

above

the seafloor

for

use in the deep ocean. A pair of Ag-AgCl electrodes were placed on a weighted streamer 50 rn apart and connected to a near-bottom recording package. More recently, Schultz (pers. comm., 1988) has described three-dimensional (3-D) model studies of sulfide ore bodies on the mid-ocean ridge, and suggested that simultaneous

measurement

of the horizontal

for the Seafloor

947

The number of oceanic SP surveys performed to date is quite limited. Corwin (1975) describes the results of shallow water (<20 m) surveying in Penobscot Bay, Maine, in which the offshore extension of known land sulfide mineral deposits were successfully detected, with SP anomalies as large as 300 mV. Brewitt-Taylor's (1975) deep ocean trials were inconclusive.

With

the interest

that seafloor

sulfide

miner-

alization is arousing, it is probable that SP will be applied more frequently in coming years. CONTROLLED

SOURCE

EM

METHODS

Controlled source EM methods utilize time-varying electric and magnetic dipole sources of known geometry to induce electric currents inside the conducting earth. The electric or magnetic signature of the currents can be detected and can yield a measure of the electrical conductivity of the underlying rock. The four fundamental source/receiver types for controlled source

work

are the vertical

and horizontal

electric

dipoles (VED and HED) and the vertical and horizontal magnetic dipoles (VMD and HMD), and there are many practical combinations of them. The earliest development work on seafloor con-

and ver-

10o

tical gradient fields offers real advantages.

ORIGINAL

DATA

AT 5.6

•

ond39Hz

.'..'..'.'.,'::'?." 7ZzZzZZzZzZZzZZZZzZzZz.ii.?Z.': .• 0 'c,.',•

rr i(f3 _

BEAUFORT SEA

[

l•

IO-4 __

_

i

i

i

i i i iiI

i

i

i

I i

O5.6

,, -

u')

Hz

39 Hz

-40

-80

!

I01

!

i

i i 1111

i

io2

i

i

i i i i i

io3

SEPARATION (m)

Fig. 12. The location of the Beaufort sea sounding(marked by the cross at upper left) superimposedon a regional map. The shaded areas indicate where permafrost has been mapped acoustically. The water depth contours in feet are also shown.

Fig. 13. The magnetic field amplitude and phase at two frequencies as a function of transmitter-receiver separation for the Beaufort sea experiment. The magneticfield has been normalized by the source current.


948

Chave, et al.

trolled source systems appears to be due to Drysdale (1924), who describes an extensive program to measure the magnetic field and electric current around a submarine cable for use in a ship guidance system of World War I vintage, and shows that many of the difficulties of working in the ocean have not changed over time. In an accompanying paper, Butterworth (1924) computed the fields about the cable and over an insulating seafloor for comparison to the measurements. More recently, Bannister (1968a, 1968b) calculated the seafloor fields produced by an extended HED placed on the sea surface and a seafloor HMD, both with the purpose of determining the seabedconductivity. Coggon and Morrison (1970) modeled a seafloor VMD source with both electric and magnetic receivers, emphasizing exploration of the uppermost few hundred

meters

of the seabed.

Kaufman

and Keller

(1983) computed seafloor sounding curves for VMD and HED sources with a vertical magnetic field receiver.

Oceanic controlled sourceproblems differ from their terrestrial counterpartsin important ways: both source and receiver are always immersed in a conductive medium, and the electrical structure both below and

above the medium influences the induction problem. Furthermore, some system geometries require the explicit inclusion of the ocean-atmosphere boundary in the theory. The importance of interactions with the

20[[ [ [[]][[[[ .....

•Q::: 51• /

F, rsf der,vof,ve

--- -- Second der•vahve

sea surface when using seafloor-based controlled source systems in electrically shallow water is discussedin Coggon and Morrison (1970) and Chave and Cox (1982), and will be especially important on the continental shelves. The finite depth of the ocean may be neglected at frequencies where the water is many skin depths deep if the source-receiver separation is small enough. TM

and PM modes can be associated

with the four

basic types of sources: (1) the vertical electric dipole (VED) generates only TM modes, (2) the vertical magnetic dipole (VMD) induces only PM modes, and (3) the horizontal electric dipole (HED) and (4) horizontal magnetic dipole (HMD) are more general, and can produce both modes. An EM exploration system is made up of some source-receiver combination. It is of no use to generate a desirable mode with the source if the receiver is incapable of detecting it. For this reason, symmetric systems in which the source and receiver are of the same type are commonly used. In our notation, when HED, VMD, and HMD refer to systems, they are the collinear horizontal electric

dipole-dipole, coplanar vertical magnetic dipole-dipole, and coaxial horizontal magnetic dipole-dipole combinations.

The theory associated with controlled source methods for terrestrial exploration is well-understood(e.g., Spies and Frischknecht, this volume) and the theoretical properties of some common systems on land can be grouped as follows. The VMD and HMD types detect only horizontal current flow (PM modes) in a I-D earth, hence are relatively insensitive to thin resistive zones. The HED system combines TM and PM modes and is preferred when resistive zones have to be mapped. The grouping of these three systemson the seafloor is fundamentally different. They are now buried inside a conductive medium rather than lying on a conductive half-space, and preconceptionsbased on their terrestrial use can be quite misleading. The VMD system still is based only on a PM mode, but the HED and HMD systemsgenerate and receive both PM and TM modes. Furthermore, the secondary EM fields due to induction

in the crustal

material

are measured

near the interface of a good conductor (seawater), so a system like the VMD, in which a component of a field which vanishes at the surface of a good conductor is measured, is unlikely to display sensitivity to a resisOCCAM'S INVERSION

tive seafloor.

This is not true for the HED

and HMD

systems, which are both quite capable of accurately measuring the conductivity of the seafloor in the

2 -

common

instance

than rock. DEPTH ( m )

Fig. 14. The inversion of the Beaufort sea data to continuous resistivity/depthmodelsof minimum slopeand minimum second derivative (curvature), respectively.

where

seawater

The less common

is more conductive

circumstance

of a rela-

tively conductive seafloor is analogous to the terrestrial case, and systems like the VMD type are then sensitive to seafloor conductivity. The choice of operating an EM system in either the


Electrical

Methods

frequency domain, transmitting a set of discrete frequencies one or a few at a time, or the time domain, transmitting a square or triangular step and measuring the transient response of the seafloor-ocean system, also exists. The physics of the two methods are identical, the response in one domain being the Fourier transform of the response in the other domain. Because of the finite and inexact nature of practical measurements, this transformation cannot usually be made

outside

the

realm

of theoretical

studies.

The

choice of one system over another must be made on the basis of practical and logistical considerations. Frequency Domain EM

Chave and Cox (1982) developed the theory for the frequency domain HED method using the modal formulation given in the appendix, and some details will be summarized to illustrate the behavior of frequency domain controlled source EM (CSEM). Only the electric field will be considered here. Using the Green functions from equations (A-14) and (A-15), taking the limit of an infinitely deep ocean (H--> =:), and utilizing the cylindrical symmetry to convert from a Fourier to a Hankel transform, the radial, azimuthal, and vertical

electric fields may be written

P

cos0 Ep= 4zrtro

dk Jo(kp)k[3R• PM- -J1 (kp)

for the Seafloor

949

P

cos4) Ez = 4'fro'

dk J1(kP)k2

x [RLTMe-13(z +z')T-e-131zz'l]

(11)

where p is the source dipole moment in A-m, • is the azimuthal angle measured with respect to the source, p is the horizontal range, z and z' are the receiver and

source heights, RJM andR•TMaretheseafloor modal reflection coefficients given by equation (A-16), 13is given by equation (A-17), and Y0 is the self-induction parameter given by equation (4). The upper sign in equation (11) holds for z > z' and vice versa. The first terms in equations (9)-(11) represent propagation in the underlying rock and along the sea-rock interface, while the second terms, which can be evaluated in

closed form (Chave and Cox, 1982, Appendix B), represent propagation in the ocean. The electrical conductivity structure beneath the seafloor enters the

problem onlythrough thereflection coefficients RL TM andR•TM, and equations (9)-(11)are wavenumber expansions of the fields. It is helpful to model the seafloor as a half-space of conductivity o'i and obtain approximateanalytic solutions to equations(9)-(11) for o-1 <• o-0. The reflection coefficients, equation (A-16), are expanded in powers of o-i/o-0, and only the lowest order terms are retained. Letting the source and receiver both occupy the interface (z - z' - 0) and evaluating the Sommerfeldtype integrals yields the horizontal components

-13(: + z')

[3R• TM +•- R•_ Me

J0

P

4zrtro cos 0 1

P

dk

k2

p

p

COS4) Ep• 2'fro'0

e-131z- z'l

Jl (kp)

+ (9)

+

s0(/p)0R

p

e-YlP

(12)

P

3

e-Y•P

(13)

where Yl is equation(4) with trl substitutedfor it0. To get an expression for the vertical electric field, an additional approximation discussedin Cheesman et al. (1987) and valid at ranges comparable to or larger than a skin depth in the lower medium must be invoked, yielding

1

TM

L

3

e -'vop

p3+1) e-y0p Psin • [(y0p 2(ylp +1) ]

dk

k

3

E, = 2'fro-0

sin 4)f:

E0- 4'rrO-o

P

p+1) [(7op

J 1 (kp) [3RL TM

P

P sin 0f:dk

O'1 Y0(YlP + 1)

cos 4) E:-• 2'fro'0 o'0 p2

4zrtro

e --•tlPo (14)

Note that equations (12) and (13) are lowest order in

k 1 k2

J0(kp)•0 2•'ør-p J1

it l/it 0, but equation (14) is a first order term, so that any observed vertical electric field indicates a nonzero

e -131: (10)

seafloor conductivity, as is required by conservation

950

Chave, et al.

of electric current. The first terms in equations (12) and (13) correspond to a disturbance propagating in the ocean and along the seafloor, and vanish rapidly


for p >>I01-', Fora frequency of 1 Hz, theseawater skin depth is 270 m, and the oceanic component is negligible beyond about 1 km. By contrast, the second terms correspond to a similar disturbance below and along the seafloor, and attenuate much more slowly

withrange,dominating thesolution whenp >>•/01-1

Forranges p• •/l-l, thefield decays slowly (asp-l'

ing to the conductivities are 270 m in seawater and 2.3 or 7.1 km in rock. At ranges below a skin depth in the lower medium, the source looks like a quasi-static dipole, and the horizontal electric field attenuation is largely controlled by the conductivity of the ocean. At a range of about one skin depth, the effect of the lower half-space becomes noticeable, but the conductivity dependence of the field is weak and the attenuation is not sharp. At larger ranges, the attenuation becomes exponential as in equations (12) and (13), so that the

to p-3), whileat largerrangesthe exponential term

differences

controls

offset grows. By contrast, the vertical electric field is sensitive to the lower medium conductivity in the quasi-static limit, as seen in equation (13), and a smaller rock conductivity results in weaker fields. At even larger ranges, propagation effects yield more rapid attenuation as the seafloor conductivity increases. The vertical electric field is always substantially smaller than the horizontal componentswhen the ocean conductivity exceeds that of the underlying rock. By the reciprocity theorem, the vertical electric field produced by an HED is equivalent to the azimuthal electric field generated by a VED, so that the HED always yields larger fields for a given range, frequency, and source dipole moment. However, it is clear from Figure 15 that the vertical electric field is sensitive to the conductivity of the half-space at smaller ranges, and that the relative changes in ampli-

the attenuation.

This can be contrasted

to the

terrestrial case, where the disturbance propagating in the low conductivity air is much larger than the geophysically useful one in the underlying rock at all ranges.

Figure 15 shows the radial and vertical electric fields, obtained by integrating equations (9) and (11) numerically, as a function of source-receiver separation at a frequency of 1 Hz and for an ocean half-space of conductivity 3.2 S/m overlying rock half-spaces of conductivity 0.05 and 0.005 S/m. The behavior of the azimuthal

electric

field is similar

to that of the radial

part except for the different angular dependenceseen in equations (9) and (10). The skin depths correspond-

tude of the vertical

10-5 10 -7

x ",,

,005

10-9

10-•

•

5

•

10

I

2.10

15

Range (km)

km

10-5

•

10-7

_• 10-9

• 10_•• 1 0 -2

1 0 -1

10 ø

1 01

10 2

Frequency (Hz)

Fig. 15. The upper panel shows the radial and vertical electric fields per unit of sourcedipole moment as a function of range at a frequency of 1 Hz and for lower half-spacesof conductivity 0.05 and 0.005 S/m. The lower panel showsthe radial electric field as a function of frequency at rangesof 2, 5, and 10 km for a lower half-spaceof conductivity 0.05 S/m. The ocean conductivity is taken as 3.2 S/m, and the radial electric

between

field is measured

off of the end of the source.

the two

models

and horizontal

increase

electric

as the

fields are

comparable for a given change in the medium conductivity. Figure 15 also shows the radial electric field as a function of frequency for several ranges and a lower half-space of conductivity 0.05 S/m. At frequencies corresponding to skin depths larger than the sourcereceiver spacing,the behavior is that of a static dipole, with only slight attenuation as the frequency rises. The attenuation and attenuation rate increase at frequencies corresponding to skin depths smaller than the range. The phase of the electric field (not shown) behaves similarly, with attenuation at a radian per skin depth at long ranges and high frequencies and little variation at the other extreme. Similar relationships exist for the magnetic field components; see Chave and Cox (1982) for details. It

is instructive

to examine

the

behavior

of the

horizontal electric field for geometric (range-dependent) and parametric (frequency-dependent) soundings in the presenceof the simplest structural complication, a buried layer. In each case, a specific model consisting of a half-space of conductivity 0.05 S/m containing 1 km thick layers either ten times more or less conductive and centered at depths of 1.5 and 5.5 km is considered; these values are intended only to be illustrative. Figure 16 shows the geometric sounding


Electrical

Methods

curves. The low conductivity zone behaves as a 1ossy waveguide which traps and guidesthe signal, resulting in slower attenuationwith rangewhen comparedto the half-space case. The deep buried layer produces a smaller effect, as expected from the diffusion nature of EM induction, and requires a larger range for the trapping to become apparent. If the buried layer has a higher conductivity than the surrounding material, greater attenuation will ultimately result at long range, but the low conductivity waveguide created between the seafloor and the layer results in an increase in signal strength at intermediate distances. The HED method is preferentially sensitive to relatively low conductivity zones due to the presence of the TM mode.

The

existence

of a minimum

usable

source-

receiver spacing of 1-3 times the burial skin depth, depending on the senseof the conductivity contrast, is also apparent. Longer ranges are required to detect low conductivity material. Figure 17 showsparametric soundingcurves for the same model at rangesof 5 and 10 km. The relationships discussed for Figure 16 are observed, with the greatest sensitivity to high conductivity zones occurring at frequencies correspondingto the skin depth that equals the range, while the low conductivity zone is sensed at the largest ranges and highest frequencies.

for the Seafloor Time

Domain

10 -5

<

10-6

rock. A second, later transient is a measure of the

conductivity of seawater, and the ultimate measured field is just the steady-state, dc value. The senseof the separation of the parts of the transient is opposite to that in air, where the direct part propagating in the atmospherearrives almost instantaneously.The quantities plotted in Figure 18 are the dimensionlessmagnetic fields obtained by normalizing the transient by the late-time dc response, and the dimensionlesstime, obtained by normalizing the actual time by the EM diffusion time constant in seawater given by

'r0 -- IXoO'op 2

iT I

hi

10-6

>• 10-7 '-'

10-8

10-9 10-1o

(15)

10-5

:::t 10 -7 -o

EM

The increasing interest in time-domain EM (TEM) systems for terrestrial mineral exploration led Edwards and Chave (1986), Cheesman et al. (1987), and Edwards and Cheesman (1987) to investigate the response of a variety of seafloor transient systems. Figure 18 illustrates the TEM response for a coaxial HMD system in the simple case of a uniformly conducting seafloor half-space. The HED response behaves in a qualitatively similar way. For these two systems, and when the seafloor is more resistive than seawater, the position in time of the initial rise of the step-on transient is indicative of the conductivity of

10-4 E

951

\•\\\

10_8

L• 10-9 5I

1I0

I 15

2•0

'" 10_10 0 -2

Range (krn)

1 0 -1

10 ø

101

10 2

Frequency (Hz)

10-4 10-6 E

10 -5 E

< v

10-6

I

m

•

:::t 1 0 -7

-o

10-8

iT

10-9

I

10-•o

10-7 10-8

v

10-9 m

I

I

5

10

,

I

15

iT 10 -1ø I

x_

20

Range (kin)

'.' 10_11 10 -2

10 -1

10 ø

1 01

10 2

Frequency (Hz)

Fig. 16. The radial electric field as a function of range at a frequency of 1 Hz for an ocean half-space of conductivity 3.2 S/m and a lower half-spaceof conductivity 0.05 S/m containing 1 km thick layers at 1 and 5 km depth. In the upper panel the layers have a low relative conductivity of 0.005 S/m, while in the lower panel the conductivity of the layer is high (0.5 S/m).

Fig. 17. The radial electric field as a function of frequency at a range of 5 km (bottom panel) and 10 km (top panel) for the shallow models of Figure 6. The notations lo and hi refer to the relative conductivity of the 1 km thick layer buried at 1 km, while hs is the responsefor a half-space of conductivity 0.05 S/m.


952

Chave, et al.

where p is the source-receiver separation. This results in quantities whose amplitudes and shapes are independent of range, simplifying the conceptual interpretation. Figure 18 shows this dimensionless response for various values of the conductivity ratio between

seawater. Clearly, EM energy arrives at a point in the seajust above the seafloorat two distinct times by two different mechanisms.As time progresses,both vortices expand and finally, at the late time limit, the contours match those of a 2-D static dipole, and no

seawater and rock, and also indicates the true time

current

scale for a system operating on the seafloor with a source-receiver separation of 100 m. The system responses are distinctly different for varying values of the conductivity ratio. Cheesman et al. (1987) also

A simplified treatment of both layered earth models and those containing some type of higher dimensional complexity may be obtained for a transient system by recognizing that when the seafloor is more resistive than seawater, the form of the responseat early time is dictated principally by the electrical conductivity of the seafloor, and so the ocean may be regarded as a perfect conductor. At later times, the response is determined by the conductivity of seawater, and then the seafloormay be modeled as an insulator. A given response may be approximated by the simple superposition of the early and late time effects. However, as the late time response is nearly independent of the conductivity of the seafloor, the behavior for different crustal models may be compared by examination only of the early time transient. Cheesman et al. (1987) investigated the coaxial HMD response for models containing a crustal layer over very resistive and highly conductive halfspaces and one containing a thin resistive layer. In each case, the upper ocean half-space is assumed to be perfectly conductive as explained above. The presence of a resistive basement, illustrated in Figure 20, was found to shift the half-spaceresponseto earlier time, with the amount of the shift beinga strongfunction of the depth to basement and larger changes corresponding to smaller basement depths. This shift occurs because the resistive basement provides a faster path for the transient signal, yielding earlier arrivals. The shift is minimal for depths to basement exceeding one-half of the transmitter-receiver separation, establishing a rough bound on the depth of investigation. The conductive basement model displays an increased transient amplitude as the thickness of the overlying layer is reduced, with an asymptotic approach to a change by a factor of two. This change occurs because the images of the transmitter current in two very good conductors (the ocean and basement) reinforce the horizontal magnetic field; this effect becomes more intense as the two conductors are closer together, or as basement depth decreases. The arrival time of the transient is not appreciably changed as the depth to basement is altered. The resistive layer displays a much stronger effect on the TEM response than the simpler resistive basement model. In the latter instance, the transient shape is not appreciably changed as basement depth is altered, while the resistive layer model shows marked amplitude and shape variation. For instance, a layer buried at a depth of one tenth of

discuss the less common case of a conductive seafloor,

and show that the VMD and HMD types are sensitive to its conductivity, while the HED responseis markedly attenuated. The VMD system appears to be the most suitable choice if conductive material is being investigated. A visual impressionof outward diffusionof an initial transient in the EM field into a double half-space model as a function of time is shown in Figure 19. The sourceis a two-dimensional (2-D) electric dipole. Each contour map of the electric current stream function represents a snapshot at the times indicated following a unit increase in the source dipole moment. At the instant the source is activated, two current vortices form, one above and one below the source. The lower

layer vortex circulates in the crust and in a narrow band of seawater

near the seafloor. The streamlines

at

large range in the vicinity of the interface strikingly resemble

the wavefronts

of a seismic head wave.

The

upper vortex is more compact and confined to the 2.0

HRHR .'--'-'

/ w 1.5

-

'rx

/

/

/ /

//

/

/

/

,' , ,, / i...-

o-) 0.5

/ / / ,,,//,'//

,',,',, /

//

/,/, •.,/, ••1 o 10 -•, 10 -3

/

%• \

or.

F•x

h

/-%\...............................

//,'// //

10 -z

•½,'

/

/

P--

.....

...... numerical

,,,I ........ I ........ 10 -• 10 ø 10

DIMENSIONLESS TIME(t/T o) I,,,,,I

, , ,,,,,,1

10 -5

, ,,,,,,,I

, , ,,,,,,I

........

I

, , I

10 -•' 10 -3 10 -z 10- • TRUE TIME (p=lOOm) (s)

Fig. 18. The normalized step-on response for the coaxial horizontal magneticdipole-dipole(HMD) systemcomputed analytically (dashed) and numerically (solid) for a range of the ratio of seawater to seafloorconductivity. The magnetic response is normalized by the late-time or dc value, while the time axis is normalized by the diffusiontime in seawater (15). A dimensional time axis for a range of 100 m is also shown.

crosses the interface.

Electrical

Methods

for the Seafloor

953

t =0.3 ms

•oo =0.33•' m


IO0 m

t : O. ims

/o• --IO.OD,.m

t=ls

ß

Late

time

Fig. 19. The outward diffusionof electric current into a doublehalf-spacemodel is shownin the time domain. Each contour map of the current stream function representsa snapshotat the times indicated following a unit increase in the moment of a 2-D electric dipole source.The numberson the contourshave units of mA/km. The amplitude of the current density at a point is the local gradient of the map. The direction of the current density is parallel to

the contour. Within the shadedarea, the contourlinesare too denseto showclearly. The conductivities•0 and o'1 have values of 3 S/m and 0.1 S/m, respectively. Notice that the density of contours in the vicinity of a 2-D target shown in black is much greater at early time than at the late time limit. One can therefore deduce that the size of any anomaly over the target is largest at early times (From Edwards, 1988).


954

Chave, et al.

the source-receiver separation shows a transient amplitude of four times the free space value. Cheesman et al. (1987) also investigated the response of the HMD system to a finite conductive vertical dike. If the dike is positioned between the transmitter and receiver, the responseis found to be insensitive to their actual locations, and the dike

serves to delay the arrival of the received signal. The incremental delay causedby the dike is linearly related to its conductance.This relation suggeststhat the time delay can be inverted directly to give somemeasureof the anomalousintegrated conductivity of the seafloor which lies between

the source and receiver. 1 is a collection of the most useful formulas

Table

for the frequency and transient (step-on) responseof two half-spacesin contact for a variety of geometries. In all instances, the variable s may be set to icoto get the frequency domain response; the remaining variables are self-explanatory. Experimental CSEM

A submarine HED system has been proposed and developed at Scripps Institution of Oceanographyfor deep soundingof the oceanic lithosphere (Cox, 1980, 1981;Chave and Cox, 1982; Young and Cox, 1981;and Cox et al., 1986). This system utilizes a long, insulated seafloor transmitting antenna with bared ends that is energized at frequencies near 1 Hz and a series of horizontal electric field receivers emplaced on the seafloor at ranges of 1-200 km from the source. The HED approach offers several unique advantagesfor deep electrical conductivity studies. The system is sensitiveto both low and high conductivity material in different ways due to the presenceof both TM and PM

HMD

1.5

d/•o = I •

0.5

0

, , ,,,,,,I

10 -3

....I

10 -2

10 -•

........ I

10 ø

........

10 •

DIMENSIONLESS TIME(t/r I) Fig. 20. The step-on response of the double half-space model modified by including a resistive basementat depth d for a range of values of the normalized layer thickness. The ocean has been approximated as a perfect conductor; see text for details.

The

seafloor-based

transmitter

is connected

to a

surface power source by an insulated cable. The receivers are separate, self-contained, seafloor-based horizontal electric field recorders. This apparatus has been used successfully in the deep ocean on three occasions, with the East Pacific Rise results reported in Spiesset al. (1980) and Young and Cox (1981), and more

recent

data from

the North

Pacific

collected

during 1983 and 1984 covered in Cox et al. (1986). The transmitter is shown schematicallyin Figure 21. A surfacegeneratoris located either in a moored buoy or on board a research ship and supplies power through an insulated, single conductor cable to an underwater unit. The cable must be strong enough to support the weight of the bottom package and wire during deployment and to withstand the complex, dynamic forces on the buoy and ship during operation; high voltage (•2000 V) alternating current (•64 Hz) is used to minimize

both ohmic losses and the amount of

copper required. A seawater return completes the circuit. Under the direction of a CPU, surface elec-

tronics sendcontrol signalssuperimposedon a 20 kHz carrier throughthe same cable to control the operation of the underwater

unit.

The seafloorinstrumentpackagetransformsthe high voltage ac to a lower voltage (• 100 V) and detects the control signalfor use by a local processor. The electronics

switch

a set of silicon

controlled

rectifiers

connected as a full wave bridge to synthesize waveforms in the range 1/16-16 Hz with nearly equal first and third harmonics and minimal power at other frequencies. The transmitting dipole antenna is a 5001000 rn insulated

co

• •

10-4

modes, and a horizontal electric source in a high conductivity region (the ocean) couples to a low conductivity region (the seafloor)better than a vertical electric or magnetic source by a factor of the ratio of the conductivities of seawater to rock, which is typically 100 or more.

cable terminated

in stainless steel

electrodes 15 rn long, has an intrinsic resistance of about 1 1/, and has a small resistance with respect to seawater. This antenna yields a source current of about 100 A from a generator of 15-30 kW capacity. The electric field receivers utilize Ag-AgC1 electrodes to couple to seawater and act as recording voltmeters, similar to the operation of seafloor MT instrumentation.There are two basic types of electric field recorders in use: the first is a free-fall type which uses a pair of rigid, orthogonal antennae of 9 rn span, while the second measures the potential between the endsof a 200-3000 rn long, insulated,copperwire. The first type is called an ELF (Electric Field recorder) and was the only receiver used in the 1979 RISE experiment (Young and Cox, 1981). The fixed pair of receiving dipoles forms part of the heavy anchor assembly.

Electrical

Methods

for the Seafloor

955

Table 1. The frequencyand transient step-onEM responsesof two half-spacesin contact. The Coaxial Magnetic Dipole-Dipole


IAA

Hø(s) =2xrp3(n'o - n' 1)s

x{-X/•oS(•ls)3Kl(•s)[X•os('rl -'ro)S +(2'r• -5'ro)s -12•0s -12] exp (-X•oS)+ [X•lS(2'ro -'r•)s +(2•o -5•,)s -12V•lS -12] exp (-X•lS)}, H•(t) =2xrp3(,r 0_'r•)-'r! exp- W•/2,•/2 +(•o- • +12t) exp-

-

exp-•

+(2'to +'r•-12t)erfc

- (•o+2• - 12t) erfc

.

The Vertical Coplanar Magnetic Dipole-Dipole IAA

+ (•oS)3]exp(-•oS)- [9+ 9X•lS + 4'r•s + (•y/•s) 3]exp{- x•s)). Hz(s)=2xrp 3(• o- • )s{[9+ 9•oS+ 4'ros

•-• 9IAA t [(•o)

HzS(t) - 2xrp 3q'0 --q'l x 1+

1 -

erfc

+

1+

exp

(-•t) -(1-•t)erfc (••)-•

exp -• .

The Vertical Magnetic Field of an Electric Dipole /Al

1

(-•1s)-(3+ 3•oS+ 'roS)exp (-V•oS)]. H•(s) - :•rp ns•o- • [(3+ 3•s + 'rls)exp 31xolA/

t

V•exp (-•t)-(1-•) erfc ('r•t) -V•exp (-•t)]'

HzS(t) - 2'fro2 (,to _'r•)

The Vertical Magnetic Field at the Center of a Finite Loop m•(s) = where

the time constant

a('ro

[(•s + 3•1s+ 3)exp (- •s)-(•oS+ 3•/•oS + 3)exp (-•oS)]

is defined as

,ri = !Xoo. ia2.

(-•t) -(1 -•t)erfc (V•t) -• exp (-•t)l' 31t [('r•tt) ('r•t ) exp

H•(t)=--•

a (%-'r•)

1-

erfc

The Horizontal Coaxial Electric Dipole-Dipole

+

956

Chave, et al. Table

1. Continued.


The Horizontal Magnetic Field of a Vertical Magnetic Dipole

(bv•) , Hp(s) =4--•p 3(%- 'rl)S I2(aV/•)K2(Dv/•)•c•- 1[1(aV•)K1 where

a

•

• and

b •

•

IA.•4 (,r0 - ,rl)

H•(t) - 4n.rp3 2t exp

A

SURFACE

•

12 8t

(x

8t

UNDERWATER

UNIT

UNIT

MOORING

NEAR

OR STEP-UP

STEP

TOWING

TRANS

TRANSFORMER

450v • 60

-FORMER DOWN I

SCR S

ELECTRODE

CABLE

2000v

Hz

FAR

•

ELECTRODE

60Hz

'•0 kHz

I CLOCK

I

I

T

I SIGNAL

I CONTROL CIRCUIT

CONTROLLER

B-2

B-1 RIPPLE

DUE

TO

FULL-WAVE

//RECTIFIED 60Hz •_

i•

81,11ø

••U

i

360 ø

.-

•

(x.I

tt

FUNDAMENTAL PERIOD

-I

t3

t5

t7

19

21

23

25

Harmonic

Fig. 21. Schematicof the HED transmitter designused at ScrippsInstitution of Oceanography(A). The lower panel (B-I) showsthe transmittedwaveform, obtainedby switchingthe sourcehalf cycles. The phaseof the switchingis chosento maximize the power at the fundamentaland third harmonic(B-2).

27


Electrical

Methods

The Ag-AgCI electrodes are fixed to the ends of the antenna arms and weighted to hold them as close as possible to the seabed, minimizing noise from water motion. Special low-noise amplifiers are used to increase the signal level for internal processing and recording. Much of the measurement noise is generated by the electrodes, and greater sensitivity may be achieved by substantially increasing the antenna length. The second type of instrument, called a LEM for Long antenna EM recorder, has an antenna consisting of 200-3000 rn of 6AWG insulated copper wire terminated by large (0.5 m) Ag-AgCI electrodes, and is described in Webb et al. (1985). The unit is deployed dynamically by streaming the antenna behind the ship and then lowering the instrument on a winch with enough way on the ship to maintain a straight antenna (typically, 1-3 knots); the system is then released within a few meters of the seafloor.

These instruments

were used to accomplish the longer source-receiver spacingsused in the 1983 and 1984 experiments (Cox

for the Seafloor

957

et al., 1986). Figure 22 shows a LEM instrument in its launching cradle. Common

features

of the ELF

and LEM

units in-

clude the use of special, low noise, low impedance Ag-AgCI electrodes which differ from the usual MT design by being much larger (Webb et al., 1985), and the application of acoustictechniquesto allow location and (for the LEM's)

orientation on the seafloor to be

determined from the surface ship. Data are collected on digital magnetic tape under the control of a CPU; this CPU allows the use of synchronous stacking and block averaging techniques to reduce the necessary tape capacity. Figure 23 shows the layout for an ideal experiment. The ELF instruments are placed 5-20 km from the transmitter, while the LEM units are deployed up to 100 km or more away. The transmitter is then lowered to the seafloor and either connected to a surface buoy if a moored configuration is used or towed slowly (0.5-2 knots) behind a surface ship. A potential problem associated with towing the transmitter is that the

Fig. 22. Photographof a LEM instrumentin its launchingcradle. The aluminum pressurecase is contained in a protectiveplastictube containingglassball flotation, radio beacons,and light strobesto aid in locatingthe package after release. The instrument sits on a steel sled which is dropped under internal timer control at recovery time. The receiving antenna is attached at the right end of the unit.


958

Chave, et al.

phase coherence necessary for stacking techniques to operate correctly may be lost at high frequencies where the wavelength (i.e., the skin depth)is small, although there was no indication of this at the greatest frequency (24 Hz) used in the 1983 and 1984 experiments. A preprogrammed set of transmitting frequencies, synchronized with the receiver stacking algo-

4300 rn of water. There is only limited (<30 m) sediment cover in this area, so that attenuation of the fields

by the surface layer is negligible. The transmitted frequencies in this example were 8 and 24 Hz. Data were collected during this experiment over ranges of

rithm in the seafloor instruments, is then run for a

A

period of several days. This run is followed by recovery of all instruments. The RISE experiment (Spiess et al., 1980) yielded the

first

results

from

the

HED

method.

An

2

ELF

receiver was placed 19 km from a ship-towed transmitter and data were collected in the frequency range 0.25-2.25 Hz. A signal-to-noise (S/N) ratio of better than 5 was achieved at all frequencies by stacking the data to an equivalent bandwidth of 1 cycle per hour. There is essentially no high conductivity sediment cover on mid-ocean ridges, and the uppermost basalt layer of the oceanic crust is predicted to have a conductivity of =0.05 S/m to a depth of at least 1 km from the results of the downhole resistivity experiment (e.g., Becker et al., 1982). The results of the RISE experiment require that this layer be no more than 1-1.5 km thick and that it be underlain by a layer of conductivity <0.004 S/m with a thickness of several kilometers (Young and Cox, 198!). Figure 24 shows a typical measurement collected using LEM instrumentsduring the 1984experiment in the North Pacific near 29 degrees north, 122 degrees west over 20 million year old oceanic lithosphere in

-3

•

•

•

0.5

0.0

•5

1.0

•

1.

2.0

seconds B

1.2Io

_

o 0.8 x

E

Ilhl,,,i.,,h ...... I..I,..,, ......... I....... ti,II ....... I....I,,....,....... i,,i..... I,Iii....I......... •,.

0.0 I

0

I

I

I

10

I

20

I

I

.30

Hz

Fig. 24. Typical data collected by a LEM instrument in the North

Pacific near 29øN, 122øW in 4300 m of water in 1984.

The transmitted frequencies were 8 and 24 Hz, and the source-receiver separation was about 60 km.

Fig. 23. Typical layout for an HED deep soundingexperiment. A surface ship or buoy suppliespower to the seafloortransmitter (A) througha singleconductorcable with a seawaterreturn. The transmitter switchesthe input waveform to synthesize the signal shown in Figure 21 and drives an insulated antenna (with bared ends) of about 600 m length. Either ELF (B) or LEM (C) receivers are placed at rangesof 5 to 70 km or more from the transmitter. Acoustic transponders(T) are used to locate all of the seafloorcomponentsfrom a surface ship.

Electrical

Methods

for the Seafloor

959


20-70 km and frequencies of 0.06-24 Hz, and an interpretation is reported in Cox et al. (1986). Figure 25 shows some simple models which fit these data. The results indicate that an extremely resistive upper man-

tle,witha conductivity of about10-5 S/m,underlies a 5 km thick crustof conductivity •10 -3 S/m. The thickness of the resistive mantle region is not welldetermined, but laboratory data for olivine conductivity (Duba et al., 1974) may be used to limit it to a maximum depth of 30 km. Cox et al. (1986) inferred an upper limit of about 0.1 percent by volume for free water in the uppermost mantle based on these data. The controlled source group at Scripps has also developed and tested a system for EM surveying on the continental shelf. Figure 26 shows the layout of this towed system. The layout is based on the same frequency domain dipole-dipole system used for deep sounding, so the theory developed by Chave and Cox (1982) and Chave (1984b) is directly applicable. In particular, it may be shown that resistive features such as permafrost layers and basalt flows can be mapped using frequencies and source-receiver ranges attainable by the experimental system (Constable et al.,

10-=

10-•

::::1 10-4 o

10 -5

1986). 0

20

40

Depth (km) Fig. 25. Models which fit the 1984 North Pacific data acceptably well. Structure below the second layer has been constrained to fit high and low extremes of mantle conductivity suggested by laboratory data (see Cox et al., 1986). Thus, although the conductivity of the second, very resistive layer, cannot be determined independently of the deep 6 5 structure, it can be bounded between 10- and 10- S/m.

•

• ..... •

Transceive :ii.:.': i::i[;:::ii ii:.!i! ::ii :!i![..-:: i:i!' i::i !.'i::ii!i:;::: i::•:.

The receiver for the survey array is towed in tandem with the transmitter at the end of a floating rope which may be winched in or out to vary the source-receiver spacing. The receiver records the data internally on magnetic tape, but a sample of the data is also transmitted to the surface ship over a radio link so that the experiment may be monitored in real time. It is important that the power from the transmitter not be too large to avoid overloading the receiver at short ranges, low frequencies, or over resistive seafloor, and careful oversight of this parameter is necessary. The receiver

Float & Radio

•

Transmitter

::!::i:: :! i::!:::. :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::!i::!::!:::i!i ............................................................................ !i!i'•!!iii!ii. ;i'::: i:ii •:i. ::: ::: ::: ::!:.i: :::!ii i:i!i:iii! ::: i::i i•::iii::::-i:i :!.::::: i:i:i:. !iili::i !i!ii'"•:--•-..-. -:•u..-'i:-/ '•"" ":" '-' -""

/ -'"-'--• :•:-•:.:..: :::.\ -i'i-: :.!iii!'i:--. i......... ::!': '•:i! i:i: ::•!i! iii::iiiii!!i ii:[.-' ii•i•!• i::i!•:.![i ii•i:: i::•ii! ::::: i::ii i!il ::.: i'i:::i: i:: i:: !::!::!::: ::! !ii ?:i::!::i ;!ii ::!::!i!::ii: !i:.:::i ::!::• i:.[::.if!.: ii::i :iii': :i:::ii i!i::!i:!. :-:i.:-iiiii: ::•:: i:i: ii•i: i:!:: ii•!ii!! !!i-: :'ri/ariS ponder

::::i::i::::::i::i:.i::::ii:!•::i::![::ii::i:i-i'A-C•':iJstic :• :.:::':.ii!:i!i':!i':i:::!;ii!!!i:i!i:::[::i !::::::i! ! :::::. :: [ !;;::::::::::::.i!i ::i!!!:::i:::[::::ii!i::11;::!i::::[ii::i[:.!::i::::::ii:[: ':.::::ii:::!::::.:::.::::::::•[i::::.i.::ii:i::i::::.:::i)::ii::':' .....•.•.•:•:•:•:•:•:•;•:•[•.•:•;•.•.•:•}•}•:•.•}•.•;•.•.•;•}•.•:•[•[[•:•}•}•.•:;•.•:•.•[}•::•}•[•

i::::?:::[i;::!11ii!::!ili:::.ii'::iiiii. ii::[!i!ii:i ......s...l•... .............. ::.::i:: .......' Unili:i!i!:::i•;.iiiii::i:?::i!: ............ '.::::-:::•:.:-•::-A--.':'"i:-"......... ii::::::11i:.::ii!i[i::::::i:::::::.iii::':..:i:.?.::-::::i:::ii:: •)•i•i•i•i•i•i•!iii•i•i•i!i•ii•ii•i•i•!•ii•!i•!•iiii• Receiver Antenna [i!iiiii•i!ii!!i•ii•!•i.•!i•i!!!•:!•iii!•i•i•!iiii!.ii!i.[.i.i![.•i!!.•ii!!iii!ii.!•[i:i[iiii[iii•i•...•.. :'-:'.'ii::!::::i!!:i::i[!:!.ii!i."'...... ::.... ::: ::... ' .-:-:[""•-:::'!-::!:::[:!ii.ii:i:::!:i:i'i.. --'""['.-i."" -...... ' :i":'":"!!iiii':':':"i!!iiii!ii:!i:':'iiiii!•!ii•!/':i!iii:'/':iii!"....... ..... ....... . ."':"" "'": •.•....!i•...........•.........•.....•.....•...•.•.iii!!•i.i•i•iii.•iii!!::i•i•ii[i ..'i':::..

........................... •••:••

:'•••'-<. •'•.......................................................... ' •'•'"'"

":•:"•:••'-:•••• .-:4 •!

Fig. 26. Sketch showing the towed frequency-domain profiling system developed at Scripps Institution of Oceanography. The source antenna is towed immediately behind a research ship and powered by the ship's generators. The receiving antenna is towed farther aft from a radio-equipped float and consists of an array of Ag-AgCI electrodes with acoustic transponders for location.


960

Chave, et al.

antenna must be dragged in contact with the seabed and so must withstand a high degree of abuse, but by maintaining a smooth and streamlined antenna surface problems seem to be obviated. The operation of the transmitter is much like that of the deep sea unit except that the shorter towing cable and higher frequency of operation allow the transmitted waveform to be generated on board ship rather Record

Jl12'1

3.50E+06

3.00E+06

2.50E+06

•

2.00E+06

WWWVVVVWWW

o

1.50E+06

than synthesizedin the submergedelectronic package. Thus, high voltage and low current at the appropriate frequency is sent via the towing cable to the underwater unit where it is simply transformed to lower voltage, making about 100 A available for the 50 m transmitter

antenna.

The

transmitter

electrodes

are

7 m lengthsof copper tube about 7 cm in diameter. The entire operation is monitored and navigated by means of acoustic transponders attached to the transmitter, recording unit, and receiver antenna. An initial test off of San Diego demonstrates the feasibility of the technique. Figure 27 shows the received signal at 7 consecutive locations spaced about 100 m apart. The transmitted frequency is 64 Hz and the source-receiver spacing is 500 m. The excellent S/N ratio is clear from inspection of the records. The signalsare up to 6 V peak-to-peak after amplification and stacking, correspondingto responsesof 3 to 6 x 10-11 V/A-m'- or a I to 0.8 fl.m half-spaceresistivity. As the seafloorresistivity decreases,the amplitude of the signal decays and the phase advances. Further work with this towed survey system is planned. Experimental CSEMiTime

Domain

1.00E+06

5.00E+05

0.00E+00

0

100 Data

200 Number

Fig. 27. The received signal in instrument counts for the towed frequency-domain system at 7 points spaced about 100 m apart. The transmitted frequency is 64 Hz. See text for details.

The prototype of a time-domain HMD system, shown in Figure 28, has been constructedand tested at the University of Toronto (Cheesmanet al., 1988). The magnetic dipole transmitter consists of a fiberglass cylinder, 2 m long and 1 m in diameter, in which 100 turns of wire are embedded and evenly spaced along its length. The cylinder is open ended and has three internal radial fins running its entire length for strength and hydrodynamic streamlining. The front end is tapered slightlyto prevent scoopingof bottom sediments from occurring as it is dragged forward. Current to the

•[!iiillli[llill RELIEF

TRANSMITTER 5Om

CABLE

COIL

• STRAIN

ill RECEIVER

COIL

Fig. 28. Sketch of the HMD transmitter designused at the University of Toronto. A transmitter coil containing 100 turns of wire is connected to a surface ship by an electric cable. The receiver, a coil wound on an iron core, is streamed behind the transmitter and encased in a protective plastic sleeve.

Electrical

Methods


transmitter coil is supplied by a transmitter and transformer on the ship which is powered by a pair of automotive batteries. The polarity of the current is reversed every 5 ms to provide the EM transient. A modified

iron-core

coil serves as the receiver.

The

coil is enclosed in a protective polycarbonate tube, and is joined to the transmitter by a 50 m cable. The entire array is towed along the seafloor by a heavy wire spooled on a winch so that its length may be varied with the water depth. Unshielded conductors within the wire carry both the transmitted current and the received voltage signal. An analog delay line installed in the receiver retards the returned signal by 2 ms, a point well after the main effect of the transmitter voltage transient. The cost-effective solution eliminates the need for a specially constructedcable containing either shieldedwires or fiber optics. A test survey was performed in the Trincomali Channel near the southeast end of Vancouver

could be ascertained.

961

10.24 ms signal record consistedof 1024 samples, and was triggered directly from the transmitter. An average signal waveform was produced by stacking 512 signals. The acquisition of an averaged signal took about 90 s. Signal amplitudes ranged from 30 to 160 mV, with a background noise of about I mV in the stacked signals. The averaged signal waveforms were stored digitally on floppy disks. It is necessaryto keep the receiver stationary while measurements are being made

to avoid

motional

emfs

induced

in the

coil.

However, it was possible for the ship to maintain a constant way by paying out the tow cable whenever a measurement was made and then reeling it in between sites.

Interpretation of the data began with the production of a set of theoretical type curves. The model chosen is a double half-space, representing seawater over a homogeneous conductor. Figure 30 shows the tran-

Island at

about 48ø 55'N, 123ø 27'W (see Figure 29) where the water depth ranges from 30 to 100 m. The seafloor consistsof bedrock overlain by varying thicknessesof mud, ranging from zero to tens of meters, so that the response of the system to varying thicknesses of sediment

for the Seafloor

0.15

::>

0.10

A total of 38 measure-

ments were made at 19 sites along 3 lines. The signal was digitally sampled at 100 kHz. Each

0.05

lO

,

1.0

2.0

.0

time (ms) Fig. 30. Theoretical signals for selected values of the seafloor-to-seawater resistivity contrast c•, synthesized by convolving the impulse response of the model with the actual transmitted

current

and the receiver

coil transfer

function.

Note that both the shape and the amplitude of the transient are diagnostic of the seafloor resistivity.

0.15

-

9

>

0.10

0.05

10

, , , , I , , , ,1.0I , , , , I , , I •1.0 .... i , • , :3.0 i time (ms) Fig. 29. Location map showing the site of a test of the HMD time-domain apparatus. Measurements were made along three lines, at positions indicated by the open circles. The water depth contours are in meters.

Fig. 31. Actual measured signals from selected sites with interpreted values of the resistivity contrast. These should be compared to the Figure 30 theoretical responses.


962

Chave, et al.

sient response of the system for models in which the resistivity contrast is 1, 3, 10 and 20. The output of a coil is proportional to the time rate of change of the magnetic field. A relatively resistive seafloor produces a shorter and stronger signal than a more conductive seafloor

4oo •oo •oo

•oo •ooo

Position (m) Fig. 32. Interpreted values of the resistivity contrast on Line A. This line covered a relatively featureless area of thick mud which

is reflected

in the constant

seafloor

resis-

tivity.

10 2

because

of the

differences

in the

•10 •

1

0

0

200 400 600 800 101 Position (m)

Fig. 33. Interpreted values of the resistivity contrast on

of

diffusion. Thus, both the signal shape and amplitude are indicative of the seafloor resistivity. By matching the delay time and amplitude of an actual signal to the theoretical curves, a value of the resistivity contrast at that site may be determined. Figure 31 shows three of the recorded signalsalong with the interpreted values for the seafloor resistivity. The results were also compared with other data. A 3.5 kHz acoustic recorder measuredthe bottom topography and indicated the presence of thin sediment cover. Line A (Figure 29) traversed a relatively flat, acoustically featureless area of thick mud. The resistivity contrast showed little variation; values of the ratio of seafloor-to-seawater resistivity o• ranged between 2.5 and 3.0 (Figure 32). Line B was chosen because it crossed an area where the acoustic

o

rates

recorder

indicated that the underlying rock was covered by only 10-25 m of mud. Interpreted resistivities directly correlate with the thickness of the relatively less resistive mud, with values of o• varying from 3 where the mud was 25 m or more thick, up to 12 where the mud was about 10 m thick (Figure 33). Line C showed the highest contrasts, with values of o• as high as 20 in a region where the acoustic measurements show no covering layer of sediment (Figure 34). All of the measurements

are consistent

with a model

in which

a

varying thickness of 10 D.m mud overlies rock with a resistivity of about 60 D.m.

Line B. This line crossed an area with minimal seafloor mud,

and the maximum in seafloor resistivity corresponds to the thinnest

ACKNOWLEDGMENTS

sediments.

The authors

thank K. Becker

for information

on the

downhole resistivity experimentsand C. S. Cox, J. H. Filloux,

10 2

and S.C.

Webb for valuable discussions on

many aspectsof seafloor EM. EM research at Scripps Institution of Oceanographyis supportedby the Office of Naval

Research

and the National

Science

Founda-

tion, while work at the University of Toronto is funded by the Natural Sciences and Engineering Research Council

of Canada. REFERENCES

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APPENDIX:

B-0

Lett.,

8, 1043-1046.

INDUCTION

EQUATIONS

be shown, using the properties of analytic functions of a complex variable, that the modal scalars satisfy the differential equations

(A-I)

V x E + OrB--0

V x B-

965

M. N., 1986, Polymetallic sulfide exploration on the deep sea floor: The feasibility of the MINI-MOSES technique: Geophysics, 51, 1808-1818. Wynn, J. C., 1988, Titanium geophysics:the application of induced polarization to seafloor mineral exploration: Geophysics, 53, 386-401. Young, P. D. and Cox, C. S., 1981, Electromagnetic active source soundingnear the East Pacific Rise: Geophys. Res.

ELECTROMAGNETIC

The approach taken here closely follows the treatment of Chave and Luther (1990). The Maxwell equations in the quasi-static limit with all electric current terms except the conduction current neglected are V.

for the Seafloor

= Ix0E-

(A-2)

p•0o'E= P•0J

(A-3)

where E and B are the electric field and magnetic

induction, P•0 is the magnetic permeability of free space, cr is the electrical conductivity, and J is the impressedsourceelectric current density. Using a Mie representation (Backus, 1986), the magnetic induction may be written 9

B = • x V/,II + VhO:• - V7,•z

(A-4)

where Vh denotesthe horizontal gradient operator and II and ß are scalar functions representing toroidal and poloidal magnetic (TM and PM) modes, as discussed in the text. The source current in equation (A-3) may be decomposedin a similar way to equation (A-4)

J = • x V/, Y + V/,T + E•

(A-5)

where E is the vertical part of the sourcecurrent and Y and T are scalar functions which satisfy the Poisson equations

V•,Y= (V x J). • 2

(A-8)

V2•-

(A-9)

p•ocrOt•= P•oY

and the electric field is given by

E = • x V/,Ot• - Vh[(O:II/P•o + T)/cr]

+

-

(A-I O)

The differences between equation (A-8) find equation (A-9) are caused by the nature of the electric currents associated with the two modes. Equation (A-8) reduces to that for dc resistivity sounding in the zero frequency limit, while equation (A-9) is the usual diffusion equation of EM induction. The modal equations can be solved conveniently by constructing Green functions which incorporate the necessary EM boundary conditions at the seafloor and sea surface, which are assumed to be flat interfaces.

Assuming ei•t timedependence, expressing the horizontal spatial components as the Fourier transform pair defined by

(A-6)

VhT= V h ß J.

pooO'O-(T/cr)

?(xl, • ) =

dx dy f( x, y)ei('qx +

(A-7) and

If the electric

field

is also written

in terms

of three

scalars, the conductivity profile is assumed to vary only vertically, and the Cartesian componentsof equations (A-2) and (A-3) are written out explicitly, it can

f(x, y)-

(2,rr) 2 dxl d••(xl, •)e-i('qx +•')(A-1 1)

966

Chave, et al.


and assumingan ocean depth H and conductivityrr0, then placing the coordinate origin (z - O) at the seaflooryields

fl=

RL TM= ([3K/cro -1)/([3K/cro + 1) nn=

dz' g.z, (A-12)

qr=txo f•dz' g,(z, z');f(z, z')

(A-13)

functions

are

g•(z, z') = -[e -•lz-z'l + R•TUe-•(z +z')

- e-213% 13(z +z')_ R•TUe-213He 131zz'l]/ X [213(1 + R•TMe-213H)]

(A-14)

g,(z, z') = -[e-131z-z'l + R•.u e-13(z +z') + R.•u e -213% 13(z +z') ß-•PM• PMe -213H e 131z - z'l]/ +lx.• •xL

x [2[3(1- R• • RLPMe-213H)]. (A-15) The

reflection

coefficients

(A-16)

where K and A are TM and PM mode response

+ If dz' z')Y(z') the Green

+

R• M= ([3A- 1)/([3A+ 1) functions

where

-

at the

seafloor

surfacein equations(A-14) and (A-15) are

and

sea

which

contain

all of the

information

on

conductivitybelow the seafloornecessaryto solvethe problem and the inductionparameteris

[3= x/k 2+ ico•0cr 0 (A-17) withthecomposite wavenumber givenby k = (•12+ t52) 1/2.Expressions forthemodalresponse functions for both layered and continuousconductivityprofiles are given in Chave and Cox (1982), and their different functionalforms are causedby the disparatesensitivity of the two modesto electrical structure. Since the reflection coefficientsare in generalcomplex, the EM inductionphenemenathey representare complicated, involving leaky surfaceor evanescentwaves. The EM fields producedby any controlled source type follow by integratingequations(A-12) and (A-13) over the vertical dimensionsof the source,invertingto the spatialdomainusingequation(A-11), andcompleting the source integration. The EM fields are then computedusingequations(A-4) and (A-10). The Fourier transformmay be convertedto a Hankel transform for sourcesof simplesymmetryusingthe usualprocedure of converting equation (^-11) to polar coordinates. Becauseof the complexityof equations(A-14) and (A-15), evaluationof these closed-formexpressions must be done numerically.


INDEX

B and dB/dt measurements compared 289

adjustment distance 662 AEM modeling 850-853 AEM surveys Australia

Backus-Gilbert ments

Arctic

Cote d'Ivoire

Beaufort

813

Saudi Arabia

USA

813

characteristics

search with 825

813,817

airborne 811

slate 202

anisotropy 349, 759 apparent conductivity 358, 837 apparent permeability 112 apparent resistivity calculations

417

417

conversion

archeological investigations 234

417 417

Cole-Cole

Saudi Arabia Bidah

Mineralization

201 district

201

asymmetric configurations 108 Athabasca Basin, Saskatchewan 813 atmospherics/spherics646 automatic interpretation 467 location

907

289

relaxation

induction

buried

number

resistance

layer detection 327 objects location 230 valley model 161 Burnt Nubble pyrrhotite 196, 197 Bute Inlet, Canada 93

Cagniard resistivity 723

347

17

resistivity 342-343 compositeprofiles apparent conductivity 837 helicopter AEM 837 VLF

traces

837

thin sheet 435

of airborne equipment 832 of EM sounding equipment 354 system fidelity 824 Brunswick

model 343

conducting half space 431 sphere 437

calibration

search for natural

of Economic

in drill hole EM

defined 904

common-mode voltage 883 complex conductivity 342

technique 108 tilt angle measurements258 traversing 112

New

clearance

coil configurations819

Canada

studies 113

481

catalog look-up 365, 366 Cavendish Test Range, Ontario 264 central loop curve shape 314 charge density q 7 Cigar Lake discovery 813

main effects of increasing 904

to 358

290

616

739

clock circuits

459

formula for calculating 293 limiting cases 551,552 transformation of measured quantities

azimuthal

transmitter

SYSCAL-EM

alpha-center 77-78 Ampere's law 8, 50, 56 anisotropic earth soundingsover 286

Wadi

law 50, 52, 56, 73 borehole standard geometry 889 BRGM (Bureau de Recherches Geologiqueset Minieres) MELIS

Fault

Seismos 615

castle waveform

broadside

456

Association

171

equation 9

MELOS

Prakla CASET

method

Fluxtra

Faso 616-620

Gloucester

Biot-Savart

design 816 principle of operation 819 towed-bird system 819 Aerodat AEM systems conductivity mapping with 825

EM

875

biomagnetism50

819

620-621

Burkina

814

Bieler-Watson

AEM systems

permafrost 376 using VLF method 615-626 case histories, VLF method Agricola Lake 623 Australia

Sea 875

bathymetry and sea-icedetermination,

Kenya 813

TEM

875

laser altimeter

India 813

conductor

366-368

bathymetric charting and sea-ice measure-

813

Brazil 813,868-869

AFMAG

method

811

Timmins, Ontario 812

capacitive coupling 115 Casa Beradi deposit 813 case histories

coal and petroleum 376 deep crustal sounding374-375 geothermal 375 groundwater 376

967

half plane 854 host 36, 156, 181 host medium 54, 278,861 host rock 218

811 resources

conductive

mapping with LIN 113 media 442

mineral deposits 105, 112 ore bodies

164

overburden 31, 181, 187, 214, 446, 592, 855,859, 865 sphere 854 conductivity anisotropy 161


968

Index

conductivity (continued) aperture 819 imaging 374 maps 836, 840 mapping 825 mapping with LIN 113 soil 234

thickness product 54 confined conductors 437, 442 continuous controlled

normalization source EM

458

methods

horizontal electrical dipoles 947 horizontal magnetic dipoles 947 vertical electrical dipoles 947 vertical magnetic dipoles 947 Copper Creek, Arizona 90

manipulating 751 processing118,751,832 decay curve analysis 864 decay plots 459 deep-sea mining 931 depolarization 11 depth detectability 191 exploration 742 imaging 373 penetration 282 range 291 Detour Lake Gold deposit 265 dielectric

constant

dielectric

limit 719

349

Cork Tree Well, Australia 90

diffusion depth 187,292 Dighem 817

correlation and parameter statistics

dikes

366-368

COTRAN

Crone

PEM

description of system 185 digital receiver 269 fixed transmitter loop 269 loop profiling 269 slingram 269 crosshole MMR 49, 90 crossover

indicator, defined 904 migration 908

separation as measure of anomaly width 904

cross-ring system battery powered transmitter 260 orthogonal aircore loop receiver 260 used to measure

CSAET

wavetilt

260

cultural

contamination

cultural

effects 350

737

cumulative responses 308 current channeling 166, 865,900 concept 48 number 12, 75, 81, 166 current dipole 81 current

vertical

599

in a conductive

medium

19

direction finding methods broadside tilt angle 175 dip angle 174 strike angle 175 tilt angle method 176 tilt angle receiver 177 vertical loop method 177 VLEM,

174

displaced loop, 239, 342 displacement currents 239, 542 343

and propagation effects 346-349 downhole resistivity 940 drifts 832-833 drill hole 455

electromagnetictechniques 881 MMR techniques 49 dual frequency method 335 dual loop configuration 454

E-phase 818 early-time, far-zone, wave stage 292, 357 eddy currents complex resistance 18

currents

fine structure

galvanic 154 vortex eddy 154 curve matching techniques 363,364

ideal circuit

induction

18 13

number

14

inductive limit 14, 16

inductive responseparameter 14

acquisition systems 831 fidelity 820 filtering techniques 611 interpretation, AEM 850 leveling 833

954

wave tilt 541

electrical conductivity 240 electrical

noise 735

electrical permittivity and EM measurements 225

electromagnetic noise electric power 458 geomagnetic noise 350-351 microphonics 458 resonances

351

time constant vortex

electromagnetic soundingmethods controlled source EM sounding287 Eltran

method

287

history 287 electronic navigation 817 Doppler navigation 828 laser altimeter

828

Motorola Miniranger 828 828

Sercel Syledis 828 transponders 828 using transponders 826 electronic

noise 821

electrostatic shielding 883 elevation

corrections

277

elliptical cylinder 441 ellipticity, 168, 191 EM 34-3,260 EM 38, 260

EM 60 (LBL system), 398-402 EM migration, 374 EM profiling methods, 105-270 emf, 430 embedded prism, 574 energy resource exploration geothermal, 221 fossil fuel resources, 221 use of LIN instrument, 221 temperature measurements, 221 permafrost terrain, 221 petroleum reservoir leakage, 221 environmental investigations abandoned mine workings 227 acid water

228

crude oil 228

real conductors 17 resistive limit 14 data

receivers

radar altimeter

sources 7, 8

distortion

channeling 662 gathering 662 current filament approximation 185 current gathering 23, 47, 350, 443 current migration 903

9

capacitive coupling 115

spherics 351

vertical, limited depth 596 dip determinations 470 dipping conductive half-plane 81, 109, 128 layers 336 dipole

defined

739

CSAMT 307,713-809

field

behavior

wind noise 458

resistive

813

electric

Schumann

multiple conductive 599

coupling compensation 824 Crone JEM and CEM systems 264-265

eigencurrents 439, 903 electric dipoles 947

16

154

effect of

conductive host medium 278, 281

half-space conductivity on measured Turam ratios 280, 281

disposal lagoons 227 groundwater supplies 227 hazardous waste site investigations228 injection wells 227 landfills

227

leachate plume 228 migration of pollutants 228 minetailings and spoils 227 equatorial and in-line electric dipole-dipole 307

equi•,alent currents 903 equivalent current filament 433

Index

GEM5, GEM8 systems 168, 171 generalized inversion 366

IGS/EM-4 system 255 image resistivities/pseudo-depths424 impedance 651 impulse and step 484 impulse response 431 India (AEM systems) 814-817

GENIE

255

induction

GENIE

TM-2

equivalence, problems with 366-368 erroneous dip 471

galvanic currents 109, 112, 113, 154, 156,

errors

gamma spectrometry 825

associatedwith using a half-plane model 118


969

cultural

effects 458

electromagnetic noise 458 geometric 457 improper adjustment and calibration of instruments

113

in coil orientation 191, 192

in sounding333 in TEM

measurements

457-458

interpretation 166 exploration in conductive host rocks 218 galvanic currents 218 California massive sulfide deposit 218 exploration through conductive overburden Elura deposit 216 glaciated regions 214 glaciolacustrinedeposits 214 influence

of alluvium

overburden

214

of weathered

salts accumulation

rock 214

216

far-field response722, 726 far zone 24, 292 asympotic forms 300 long-offset soundings315 Faraday's law 8

field intensity, electric/magnetic6 field TEM techniques457 field measurementconversion procedure interpretation 5, 6 modeling 5, 6 finite thin conduction plate 441 finitely conducting half-plane in free space 128

Finland

first AEM coverage 812 Geological Survey of geophysicalsys-

VMD

347

Fourier transform 59, 68, 83

free-space induction 901 frequency differencing methods 105 conductive targets 178 EM-4 system 178 errors

EM

303 253

transmitter

255

logging devices 4, 884

geologic mapping 112, 237, 817 geologic noise 109, 113, 161, 191,335 Geologic Survey of Canada 840 geomagneticnoise 351 geometric attenuation 895 geometric sounding290, 309 geometry noise 822 GEONICS EM-31,259 GEONICS EM 37, 266, 484 Geonics PROTEM 47 and PROTEM

57,

266

GEOTEM, 427, 816, 831 case history 509-513

survey, Lynn Lake Area, Manitobe 850 system 813,828 geothermal exploration in USA 813 geotomographicimaging 900 Gertrude (Sudbury) Canada 900-921 Geysers area case history 408-410 Gironville-MELIS system survey 420 Gofobic Range 208 graphitic conductors 202, 812 greenstonebelts 811 ground loops 883 ground monitoring stations 832 grounded horizontal electric dipole 719 grounded source induction effects 27 groundwater exploration and development 223-225, 813-814

GENIE system 178 geologic noise 178, 179, 181 massive sulfide glacial overburden 179 178

scale model measurements

frequency domain systems 171, 191,287, 300, 823,949

frozen ground mapping 225

inductive

coupling 187 effects 49

interaction

902

zone 292

infinite thin sheet response31 in-line technique 108 in-loop central

induction

338

central loop 454 INPUT 514-520, 813,816-817, 828,867-868 instrument

822-823,

noise

bird swing 83! drift corrections

831

drifts 831 INPUT

831

sferics 831

instrumental error, sources 334

Integrated GeoSciences, Inc., LOTEM 412 interfering fields 113 intermediate zone, active region 24 interpretation of AEM

data 850

of TEM

measurements

459-472

intersection anomaly, near-miss anomaly 884

H-type section 319

calibration

Half Mile

central-loop configuration370

Lake

sulfide zone 219

374

half-space induction 901

distortions

Hankel transform 53, 80

overshoots/undershoots

hazardous waste site investigations

static shift 370

LIN

TEM

instruments

228

LIN slingramprofiles 230 total dissolved

solids 230

urban areas in or near 228

Heath Steele (lead-zinc deposit) 811 helicoper AEM surveys 831,855 systems 817, 822, 825 Hemlo deposit 813 homogeneous earth 300

half-space293

107

plate 142 profiling 285 host medium

370

measurements

372 459

TEM tipper 371 TDEM

caveats

470

interpreteddips with TDEM system471 intrinsic impedance 541 inverse power law 31 inversion Backus-Gilbert

method

368

catalog look-up 365 computer 365 curve-matching techniques 363 generalized 366 iterative

methods

367

joint 370

coplanar measurements338 layered earth 436 magnetic dipole primary field components

179

number 97, 113,292, 300, 721 parameter 129

interpretation, 1-D 307-371

horizontal

178

misorientation

GEFINEX

landfills 228

tems 825

systemicmapping program 814 fixed source technique 108 fixed transmitter, roving receiver 455 flat-lying body 141 flexible coupling 823 flux density, electrical/magnetic 6 forerunners

187, 211

865

host rocks 109, 113

Master curves, 365 Monte

Carlo method

366

induced polarization (IP) 342-343 effects 450, 768 iron ores and magnetic bodies exploration 207

Itapicuru Greenstone Belt, Bahia, Brazil 861

970

Index

iterative

methods

of inversion

366-368

noise, cause 821 permeability 112, 121,452


K-type section 319 Kallkyla, Finland Cu-Zn bearing pyrrhotite-pyrite deposit 197-198 Kapuskasing, Ontario 873 Kidd

Creek

812

kimberlite pipes 212 Kirpatrick #1 well, Oregon 412

Labrador Trough, Quebec 871 lag tests 832 large fixed source layouts 271 large TEM anomalous responses471 large-layout harmonic field methods Sundberg or compensator 271-274 measurements

271

large-loop EM method 881 large-loop systems impulse-type 888 step-function type 888 multifrequency systems 888 system function 888 large-scale resistivity experiment 939 Lasail copper deposit, ultanate of Oman 269

lateric

soil 342

late-time (near-zone) 292, 358 Leitrim, Ontario (MMR survey) 84 leveling of AEM data 833

641-711

in Finland

840

receivers

840

as conductive target 105 bodies 105, 109, 166, 221,269, 812 deposits 193, 219 matched filter processing374

Mattagami Syndicate exploration strategy 866

Maxiprobe system 421 MaxMin systems258 GEM5

and GEM8

battery powered transmitter 261 depth soundings264 ellipticity 261 fixed source 263

moving source 263 receiver

shootback

metal detecting instruments 112

local transmitters

microseisms

and EM

measurements

286

in sedimentary rocks 349 response of thin layers 319 loop configurations 106 loop-loop configurations290-291 loop plane of orientation 106-107 loop size (TEM) 456 loop spacing 112 loop versus dipole sources 745 LOTEM 412, 424 low induction number (LIN) 111, 114, 308 Lynn Lake, Manitoba 850

loss mechanisms

261

data 263

tilt angle 261 use to obtain polarization ellipse parameters 261

934

mineral resource exploration 192 halo 31

coupled modes 886 detectable

electric

field 741

separation 740 MIP, magnetic induced polarization 87-90 misorientation of loops 116 MMR

alpha center 75 anomalies 48, 57 conducting plate 79 crosshole

sections 471

mutual

coupling 357 impedance 289 inductance

910

ribbon-like

conductors

142

navigation electronic navigation 820 flight path recovery 830 photomosaics830 tracking camera 830 VCR

camera

830

visual navigation 830 navigation systems Doppler 836 Miniranger 836 navigation tolerance flight path 830 GEOTEM

831

helicopter AEM surveys 831 QUESTEM 831 SPECTREM

831

clearance

830

near-field response 721,726 near-surface inhomogeneities335 near zone 24, 292, 300 Newmont

EMP

427-473

Night Hawk Geophysical Test Range, Ontario 257, 266 noise

analysis 752 cancellation

352

735

measurement

113

rejection 483 nonmassive sulfide and vein depositsexploration 206 normal magnetic field 51, 58 Norway and Sweden AEM surveys 814 nulling 832 number of layers, determination of 366

84

Occam's

inversion

423

current dipole 72-74 dipping contact 63

oceanic controlled

filled sinks and channels 69

offshore MMR surveys, Bute Inlet, Canada

48

surface

current

thick vertical

source 948

93

of anisotropicearth 59 outcroppinghemisphericaldepression69 small target 74 235

307

electrical

minimum

methods

magnetic bodies 858,865 dipoles 9, 947 field 8, 430

measurements

mineralized

MT

terrain

data interpretation 118 instruments 114, 123,224, 259, 260 in high conductivity areas 115 slingram profiles 123 line spacingfor TEM surveys 456, 830 626

MOSES 49, 84, 96, 942 motion noise, cause 352, 821 Motorola Miniranger 828

narrow

magnetic gradiometer 840

LIN

logarithmic compression891-893 long period EM fields 933 longitudinalconductivity

search inversion

365

multiple conductive dikes 599 multiple conductors 910

theory 292 manganesenodule 931 man-made disturbances, AEM 835 map of profiles Casa Beradi, Quebec 840 conductivity maps by GSC 840

McPhar

Carlo or random method

multiconductor

magnetometry 825 magnetotelluric

VLF

62

contact

multicoil helicopter AEM surveys 813

47-99

massive sulfides

Turam 271,274 two-frame

susceptibility 235,260, 338 viscosity 234, 338,340 magnetite content 208 magnetometer 820, 828 magnetometric resistivity method (MMR)

method

dike 66

vertical

Monte

sferics 821

joint inversion 370-371

thin conductive

74

dike 64

off-time

TEM

481

on-time

TEM

482

one-dimensionalinterpretation 754 orthogonality adjust 113

Index check

radar altimeter

113

overburden 109, 113


overburden

bedrock

structures

599

PEM 184, 428,490

perfectly conductinghalf-plane 126 periodicity 482 permafrost 225,939 permittivity 225, 240 perpendicular loops 307 phase rotation

166

reference

289

phase-lock amplifier 893 physical scale model 110 PLATE

model 904

PLATE program 442, 484 plotting point 108 plotting soundingdata 362-363 point normalization 458 polarization ellipse methods 172, 254, 355-356 parameters 355, 636 vectors

7

poloidal current 51,443 polymetallic sulfide deposits 931 powerline noise 253 preamplifier 289 Precambrian

shields

109

832

ratio measurement interpretation dielectric properties 174 groundwater toxic waste studies 174

SPHERE program 177 tilt angle 177 sign convention 889, 893 signal-to-noiseratio 456

ratio methods

silver-silver

ratiometer

Panther Canyon, Grass Valley, Nevada 398 parametric interpretation 884 parapotential 76 partial EM fields primary 9 secondary 9

971

167

114

receiver console 255, 258 receivers, senso type induction

coils 289

grounded wires 289 loop sensors289 magnetometers 289 recursion process downward

53-54

instruments

398

interaction

901

resistive vertical dike 596, 599 resistivity limit 308 resistivity structure 648 resolution, considerations 743 resolutionand equivalence315, 317, 366-368 response

aperture diagram 850, 856 PEM sphere 185 SPHERE program 143 two-dimensional targets 599 reversed

energization 918 polarity anomalies 606 rigid-boom systems817, 822 rigid-coil coupling 822 rotatable

transmitter

rotating-momenteffect 910 Ruttan Mine, Manitoba 918-920

196

programs, computer PLATE

SPHERE

903

propagation velocity 718 PROSPECT 428,813,816 pseudo-randombinary sequence388-398 autocorrelation

909

pure anomaly MMR method 51, 84

Insco orebodies

164

164

ratiometer receiver

111 111

reference signal 111 111

slingram method electromagnetic/HLEM 110 HCP configuration 110 horizontal loop 110 loop-frame 110 110

VCP

110

secant chaining 116 secondary field moment 906-907 sediment hosted uranium deposits 212

slingramresponseand interpretation homogeneousearth model 120 homogeneoushalf-space 119 layered earth 119 magnetic permeability 120 phasor diagram 121 variation in apparent conductivity with height 121

self potential/spontaneous polarization/SP

small induction

Schlumbergercurves 55 Schumann

resonances

351

SE-88 system 255 search radius 884, 906 932

946

sensitivity, high AEM 819 Sercel Syledis 828 separatedTEM loops 454 sferics 646, 825,833 shadow effect 781 shallow

quadrature susceptibility 235 system (two-frequency) 812 quasi-static 293, 718 Quaternary sediments, France 814 QUESTEM 813,816, 818,831

164

quantitative interpretation of profiles 161 skin depth 164 target not electrically thin 164 slingram loops mutual coupling 111

110

389

University of Toronto 392 pulse EM response of blocks and wedges

ISO/New overburden

VCA

seawater

903

conductance

depth extent 162 finite strike length 162 galvanic currents 164

PERP

114

116

258

technique 105 slingram data interpretation depth estimate frequency dependent

scalar CSAMT

739

error

measurements

salinity mapping, Australia 814 scale factor

287

measurement

transmitter

884

Precambrian volcanogenic massive sulfides

predictabilities, MT 680 primary field components 107 processingand interpretation of results current channeling 184 late-time decay 184 profiles, TEM 459 profiling results imaging 240

935

skin depth 12, 24, 292, 545,710 Slingram configuration 265

repeat measurements,variations in 236 resistive

electrode

Skeels-Watson transformations 58, 59, 60, 61

upward 53-54 remote-referencegeomagneticnoise cancellation

chloride

single-componentmagneticfield receiver 883 single loop TEM 453 singular value decomposition 859, 865 SIROTEM 428, 506-509

investigations 292 soundings353 shootback

measurements

shootback

method

258

176

Crone JEM and CEM systems 264 responseof layered earth 177 responseto a sphere 177

number

292

small loop frequencies 113 small loop profiling technique 105 smoke ring 434 soil

conductivity 234 salinity 232 sounding techniques frequency 285 geometric 285 parametric 285 time domain

285

source effects 668,769 source overprint 778, 780 source receiver geometries 290, 292

972

Index


sources, EM

electric dipoles 287 grounded wires 287 in frequency domain systems287 in-time domain systems 287 loops 287 magnetic dipoles 287 moment

287

on a half-space 21 South Bay Mine, Ontario 918 Spargoville Deposit, Western Australia 87 spectral coverage 819 spectral response 853 SPECTREM 813,831 SPHERE program 143,442, 904 square plate, MMR 80 static effects 666, 760, 763 static shift 335, 373

interpretation 76 plane wave responsefrom targets 606 tilt angle 152, 168, 174-177, 219, 264, 289, 355, 886 time constant (L/R) 16, 467, 470 time-domain

depth imaging processing403 University of Toronto 402-403

EM 181,951

frequency domain hybrid system 817 HMD system 960 INPUT system 812, 823 methods

181-184

results 291,309

sampling, averaging receiver 828 system waveform 287 812

stratified

torodial

853

induction

444

towed bird

configuration 816, 861 828

systems816, 817 transition sounding terms 292 transition zone, response 723,729 transmitter

design 354

configuration 427, 453-455 design 353,456-457 layout 830 Sveriges Geologiska Slingram 259 Sweepem 813,816 symmetric configurations 108 system function 889

-target coupling 889

ore 208

Tasmania, west coast of 921 TDEM 408-411,470-472

822 time correction

conductivity 349 and longitudinal resistivity 286 315

Tridem system 817 915

Turam method 219, 271,274, 275,277 conductors

447

configurations453-456

two-dimensional interpretation 758-769

methods

two-dimensional

181

tensor CSAMT terrain

737

conductive

and far-field

models 41

two-frequencytowed bird AEM system817 two layer models 300

systems 479-483 meters

111

dikes 576

thin vertical dikes 577, 587 three component sensors 833,895 3-D

body response 153

Unicoil concept 817 University of Utah 14-frequency EM system 171,377, 386-388 uranium exploration Athabasca

shield 867

water chopper 937 wave

impedances541 length 718 propagation 900

ellipse 253 measurement

method

error

basin 208

sources

172

167-168

Widgiemooltha Area, Australia, MIP 87 windows

479-480

wire impedance 746 wing-tip system 817, 823,825 Woodlawn, New South Wales 221

thermal agitation noise 821 thick vertical

massive sulfides

Precambrian

zone 292, 300 wavetilt 107, 355

Tulks East deposit 205 Turair system 816 two-dimensional

TEM

transmitters 523,817, 818 volcanic-associated

487-489

transverse

Trillabelle

taconite

traces 837

vortex current 81, 112, 164, 444

moment

resistance

EM systems 813 magnetic field response611 receivers 522, 840 signal and noise levels 524 system 825

Detour deposit in Quebec 867 Matagami Syndicate 866

481

survey

turn-off

contact-H/E polarization 560-564 coplanar measurements309, 338 dike 587, 592,596 ellipse parameters 289, 355 magnetic dipole 107,726 thin prism 156 viscosity 338

Canadian

battery powered 258 characteristics

vertical

VLF

magneticfield 51 and poloidal magnetic965

receiver

vector

CSAMT apparent resistivity 733 plots 459

profiling 184

topographicand geometric corrections, misalignment361 topographiceffects, corrections766-767

medium

427

case histories 497-506

AEM systems 850, 861 approach 816, 823

Stefanescu, S.S. 47, 48, 58, 59, 60, 76 step response 431

bird swing 865 coincident dipole 858 inversion of AEM response data 858 singular value decomposition 859, 865 subsurface fields and currents 543,555 Sundbergor compensatormethod 271,274 explanation of responsesin 274 general phase diagram 273 Superior Area, Arizona MMR survey 86, 87 supermagnetism340 superparamagneticeffects 452 surface gravity waves 934 surface impedance 542, 549

UTEM

apparent depth 403

towed-bird

station spacing 112, 456

deposits, sediment hosted 212 Key Lake 208 USGS frequency domain system 406-407 USSR Ural Mountains AEM survey 814

zero levels 115

Zonge Engineering CSAMT

414-417

Electromagnetic Methods2 in Applied Geophysics

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