Mon Taj Geophysics Levelling

montaj Geophysics Levelling System Processing and Enhancing Geophysical Data Extension for Oasis montaj v7.1

TUTORIAL and USER GUIDE

www.geosoft.com

The software described in this manual is furnished under license and may only be used or copied in accordance with the terms of the license. Manual release date: 08/02/2010. Written by, Nancy Whitehead and Chris Musselman. Please send comments or questions to [email protected] © 2010 Geosoft Inc. All rights reserved. Geosoft is a registered trademark and Oasis montaj is a registered trademark of Geosoft Inc. Other brand and product names mentioned herein are properties of their respective trademark owners. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form, or by any means, electronic, mechanical, photocopying, reading, or otherwise, without prior consent from Geosoft Inc. The software described in this manual is furnished under license and may only be used or copied in accordance with the terms of the license. OMx.t.2010.02

Windows ®, and Windows NT are either registered trademarks or trademarks of Microsoft Corporation. Geosoft Incorporated Queen’s Quay Terminal 207 Queen’s Quay West Suite 810, PO Box 131 Toronto, Ontario M5J 1A7 Canada Tel: (416) 369-0111 Fax: (416) 369-9599 Web Site: www.geosoft.com E-mail: [email protected]

Contents Geosof Geosof t License Agreement

1

Finding more help information

4

Contacting Technical Support

Chapter 1: Before you begin

4

5

Creating Creating a proj ect

5

Loading the Levelling Levelling menus

6

Chapter 2: montaj Geophysics Levelling Overview

8

Ab ou t th is man ual

8

Who should use this manual

8

Understanding Understanding the problem

8

Recognizing Recognizing the solution

9

Levelling - the proc ess

10

Levelling other types of data

12

Chapter 3: Levelling Tutorials Tutori al 1: Evaluating data for syst ematic error and nois e

13 13

Preparing data for evaluation

14

Evaluate Data

17

Tutorial 2: Applying corrections

19

Correcting Systematic Errors in Geophysical Data

19

Applying Lag Corrections

19

Interpolate Data Following Lag Corrections

20

Evaluating the Effectiveness of Lag Corrections

Applying Heading Corrections

21

21

Tutori al 3: Statisti cal Levelling of tie lines

26

Tutorial 4: Full Levelling of survey lines

32

Tutorial 5: Evaluating your results

34

Chapter 4: Applying Corrections – Detailed Description

36

Recog nizing when corr ection s are required

36

Ap pl yi ng sy st emat ic co rr ect io ns

37

Ap pl yi ng mag net ic bas e stat io n c or rec ti on s

37

Importing Instrument Data Using Geosoft Table Files

38

Quality Control on Base Station Data

39

Applying Base Station (Diurnal) Corrections to a Database

40

Lag correction

41

Heading corr ection

41

Determining Heading Corrections

42

IGRF corr ections

42

Overview of Levelling problems and decision criteria

44

High Horizontal Gradients

44

High Vertical Gradients

45

Magnetic Storm Activity

45

Diurnal Variation

45

Block Level Shifts

46

Understanding statistical Levelling of tie lines Understanding Geosoft’s Implementation of Statistical Levelling

46 47

Understanding Geosoft’s implementation of full Levelling

48

Summary of simple Levelling

49

Chapter 5: Careful Levelling – Detailed Description

50

Understanding the careful Levelling process

50

Ap pl yi ng di ff eren t ty pes of tr end s

51

Understanding the Shift Function

51

Understanding the Tilt Function

51

Understanding the Spline Function

51

Understanding the Tensioned Function

52

Chapter 6: Empirical Levelling (Microlevelling) Geosoft microlevelling procedure To Microlevel Using Line Gridding and Oasis montaj

54 54 54

Appendix A: Levelling Menu

56

Glossary

58

Index

62

1

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4

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Chapter 1: Before you begin 5

Chapter 1: Before you begin This chapter describes how to begin working with the montaj Geophysics Levelling extension for Oasis montaj™. The topics discussed in this chapter include: • •

Creating a project Loading the Levelling menu

This tutorial uses sample data provided on the Oasis montaj CD and installed in your C:\Program Files\Geosoft\Oasis montaj\data\leveling directory. Before you begin the tutorial you need to create a working directory to store all your data. The system enables you to access files anywhere but it is a good strategy to carefully organize your data (project information and files) before carrying out any processing. To start this tutorial, please create a working directory called D:\Tutorial. A general rule to follow in working with Geosoft applications is to avoid working in the Geosoft directory. In these tutorials, we will follow this rule by keeping all the working data, found in C:\Program Files\Geosoft\Oasis montaj\data\leveling, in your working directory D:\Tutorial.

Creatin g a pro ject To work in Oasis montaj requires an open project. An Oasis montaj "Project" encompasses every item in your working directory; from the data files in your project (databases, maps, and grids), to the tools used (including auxiliary tools such as histograms, scatter plots etc.), to the project setup including the menus you have displayed and whether you are working on a map or profile and the state in which you left it the last time you used it. The project also controls your working directory. Projects are saved as (*.gpf) files. If you open an existing project from a directory, the system assumes that all your project files are located in the same directory. To streamline your work, as well as keep it organized, you may wish to make sure that your project file is in the same directory as the other files you want to use. We recommend that each project you work on have its own project (*.gpf) file. If you use a number of applications or addon tools in Oasis montaj that have different menus, you can use the project to display only the menus you require. The Project Explorer tool enables you to browse as well as open any project item. The Project Explorer has two tab windows, the Data window that includes all data files included in the project and the Tools window that organizes and maintains the project tools. To access the Tools window click the Tools bar on the bottom of the Project Explorer. To return to the Data window, click the Data bar on the top the Project Explorer.

6 Chapter 1: Before you begin T O C REATE A P ROJECT : 1.

Start Oasis montaj.

2.

On the File menu, select Project and then click New. The New Project dialog is displayed.

Note: 3.

4.

Oasis montaj assumes that your data is in the directory containing this project (i.e. D:\Tutorial).

Specify a name and directory for the project. For example, name the project Levelling and specify the working directory as D:\Tutorial. Click the [Save] button. The system saves the project and indicates it is open by adding menus to the menu bar, adding buttons to the Standard Short-cut bar and by displaying the Project Explorer window. These are visual clues indicating that you are ready to start working with the system.

Loading the Levellin g menus Before you can start working with the montaj Geophysics Levelling system, you have to load the Levelling System menu in your project. If you require more detailed information on modifying menus, refer to the Oasis montaj Online Help System ( Help|Help Topics). T O L OAD THE L EVELLING M ENUS 1.

On the GX menu, select Load Menu or click the Load Menu icon ( main toolbar. The Load Menu dialog is displayed.

) on the

2.

Select “Levelling_System.omn” from the list of files and click the [Open] button. The system adds the three Levelling menus on your menu bar ( LevUtil, LevCorr and LevTieLine).

Chapter 1: Before you begin 7

8 Chapter 2: montaj Geophysic s Levelling Overview

Chapter 2: mon taj Geophysics Levelling Overview This chapter is intended to provide an overview of the Levelling process for airborne and certain ground geophysical surveys. The chapter briefly describes the various sources of error for which Levelling attempts to compensate. Methods of correcting airborne data are also described briefly.

About this manual The montaj Geophysics Levelling extension is a software tool for processing and enhancing airborne magnetic and other geophysical data. The tool is accessed via a specialized menu in the Oasis montaj core software system that provides all functionality required to manage, manipulate, visualize and map spatially located Earth Science data. For more information about the core system, refer to the Oasis montaj online help system ( Help|Help Topics) or the Oasis montaj Quick Start Tutorials ( Help|Manuals and Tutorials). This manual provides an overview of the montaj Geophysics Levelling system and describes how to identify and correct errors in geophysical survey data. The manual is designed to help you learn and perform a variety of tasks efficiently. Specifically, it should help you: • •

•

•

•

Identify potential errors in your datasets Apply systematic corrections (including magnetic base station, lag, heading, and IGRF corrections) Find and edit intersections between any lines in a data set (lines can either be regular survey lines and tie lines) Perform conventional Levelling using simple (tie line and full Levelling), careful Levelling methods Perform empirical Levelling (Microlevelling)

A glossary is included to provide you with a quick reference of the geophysical Levelling terms used in this manual

Who sho uld us e this m anual This manual is intended for Geoscientists who are experienced using conventional Levelling of ground, marine or airborne geophysical datasets that have been collected on a grid system of lines and tie lines. The system may also be applied to other types of data that are affected by time-varying errors.

Understanding the problem Geophysical surveys collect data under constantly changing conditions that contribute to errors. The most obvious effect of such errors is an apparent level shift, or

Chapter 2: montaj Geophysics Levelling Overview 9

difference, in neighbouring parallel survey lines, which is a phenomena commonly referred to as “herring-bone” or simply Levelling error. Error in survey data can be classified into two types: • •

Systematic error Systematic noise

Systematic errors are measurable and can be corrected using simple mathematical formulae. Sources of systematic error include: diurnal variation of the earth’s magnetic field, aircraft heading, instrument variation, lag error between the aircraft and the sensor, and inconsistencies between flight lines and tie lines. Systematic errors are typically removed using known corrections (diurnal base station, lag, heading, and IGRF). Systematic noise is the residual error in the data that remains after systematic corrections are made. This noise is usually due to gradient or elevation differences in the data. Conventional Levelling and empirical Levelling methods are used to correct for systematic noise. This manual focuses on the conventional Levelling process, but it also provides a brief description of how to apply microlevelling procedures in Oasis montaj.

Note that the examples provided in this manual focus primarily on the Levelling of magnetic data. However, other time-varying errors can be corrected using the same techniques with only minor modifications to the Levelling parameters (as required to address the specific characteristics of the type of data with which you are working). For more information, please refer to the short description on Levelling Other Types of Data in this chapter.

Recogni zing the solut ion The first step in correcting airborne data errors is to identify and remove any systematic errors in the data. Time-varying magnetic errors are related to the diurnal variation of the earth’s magnetic field. A common technique used to correct magnetic diurnal variations is to measure the magnetic field at a fixed ground location (base station), which is near or within the survey area. The difference between the measured field at the base station and the known or assumed background value at the station represents the diurnal error that must be subtracted from the survey data. This technique assumes that the time variations in the magnetic field at the base station location will be the same at the location of the moving sensor. This is acceptable for small surveys, including most ground surveys, but this assumption can be poor for larger airborne surveys where the recording base station is often located far from the actual survey site. In such situations, it will be necessary to also perform tie line Levelling.


Lag error results when the survey positioning system location is significantly different from the physical sensor location (often the case in a 'towed bird' survey configuration). A lag correction simply adjusts the time base of the physical readings to match the positioning data. Heading errors are related to the magnetic field of the survey platform, which varies as a function of survey direction. A heading correction corrects data for systematic shifts in the data that change with the survey direction. Once all systematic errors that can be identified are removed, some error may remain in the data. Correcting for this error requires the use of tie lines. Tie lines are control lines that usually, but not always, run perpendicular to the regular survey lines. Some surveys run tie lines through low-gradient areas. If the data contain no Levelling errors, the tie lines and the survey lines will have the same values where they cross. If the values are different, the intersection is considered a mis-tie, and a Levelling correction is required. Levelling corrections can be applied both conventionally and empirically; the conventional approach requires evaluating tie line and regular survey lines together. First, all the tie lines are adjusted and then the regular survey lines are adjusted to match the leveled tie lines. Adjustments involve either applying a single type of trend to all tie and regular survey lines (simple Levelling) or applying one or more types of trends to single lines or groups of lines (careful Levelling). The Empirical Levelling approach is a grid-based approach in which the grid is effectively de-corrugated using mathematical filtering techniques. This approach can be used either on a standalone basis or in conjunction with conventional Levelling.

Levellin g - the proc ess Geosoft’s approach to correcting and Levelling airborne data is based on the types of error identified earlier: systematic error, systematic noise. Associated with each type of error is a set of procedures used to correct the data. The diagram below shows the Levelling process:

Chapter 2: montaj Geophysics Levelling Overview 11

Evaluate Data and Gri ds • Identify Systematic Errors • Identify Systematic Noise

Appl y System atic Corr ectio ns • Base Station Correction • Lag Correction • Heading Correction • IGRF Correcti on

Perform Conventional Leveling (Tie-Line) • Simple Leveling • Careful Leveling

Perform Empirical Leveling • Microleveling

The Leveling Process

Systematic corrections compensate for measurable errors in the data. Systematic errors follow regular patterns and are removed mathematically by the computer. After correcting for systematic errors, the user then applies tie line Levelling and/or empirical Levelling. These procedures correct discrepancies between the intersections of the survey lines and the tie lines. In the montaj Geophysics Levelling system, we refer to this process as ‘conventional Levelling’. The system provides two options for conventional Levelling: simple Levelling, and careful Levelling. Simple Levelling is a two-step process. First, tie lines are adjusted to match a statistical average or trend of the observed crossing survey lines. Then survey lines are adjusted to exactly match the tie lines. In careful Levelling, tie lines are also adjusted to an average of the survey lines, but a variety of trend surface options are available for adjusting survey lines to match the tie lines. Empirical Levelling, or microlevelling, is a gridding technique that uses mathematical filters to remove residual noise associated with flight lines. The user prepares data for empirical Levelling by first removing large anomalies or spikes in the data. Filters are then used to recognize small variations between lines. This technique is subjective as its success depends on your ability to interpret and understand the data. Identifying and isolating the remaining error from the true signal can be challenging - one must understand the data in question, as well as the methods available for separating the error from the signal.


Levellin g ot her types of data In addition to magnetic data, you can also level other types of data, such as airborne electromagnetic or ground survey data, using Geosoft’s Levelling System. Since each type of data has its own set of characteristics, it may be appropriate to consider a training course to optimize your results. A common problem when working with other types of data (especially ground geophysical data), is that the data set may lack tie lines. However, it is possible to use the gridding features in Oasis montaj to construct your own tie lines, and then apply Geosoft’s Levelling system. The basic method involves identifying areas of a grid that are free of geophysical noise. Then, using the grid, digitize the feature, and add new tie lines (Type T) to your database. You can then evaluate intersections and apply the tie line Levelling procedure provided in the system. This method assumes that the readings in a quiet part of the grid are representative of the background fields.

Chapter Chapter 3: Levelling Tutorials 13

Chapter 3: L eve evelli lling ng Tuto utorials rials These tutorials are designed to guide you through the basic Levelling process -evaluating data initially, applying corrections and performing normal tie line Levelling. Corrections described include lag and heading corrections. The normal tie line Levelling procedures show how to level tie lines (using Geosoft’s statistical trending algorithm) algorithm) and then how to adjust a djust survey lines to match the tie line intersections (using Geosoft’s full Levelling algorithm). a lgorithm). For information on advanced Levelling procedures, including applying specific corrections such as base station or IGRF corrections, applying careful Levelling or microlevelling, please refer to the related chapters which discuss these topics in detail. These tutorials are based on a 10-Mb dataset ( airmag.gdb). This magnetic dataset is in Geosoft database (*.gdb) format and contains X, Y and Mag channels. This database contains data with a number of problems. You may want to use this dataset on your own to experiment with the careful correction or microlevelling procedures described later in this manual. Note:

Base Station data is not included in the sample data set. However, we include a basic description of the base station correction procedure so those less experimental users have some guidelines for applying the base station correction effectively to survey data.

To help you learn how to use the Geosoft Levelling System we provide the following tutorials: Tutorial 1: Evaluating Data for Systematic Error and Noise (page 13) 13) Tutorial 2: Applying Corrections (page 19) 19) Tutorial 3: Statistical Levelling of Tie Lines (page 26) 26) Tutorial 4: Full Levelling of Survey Lines (page 32) 32) Tutorial 5: Evaluating Your Results (page 34) 34)

Tutorial 1: Evalua Evaluating ting data for sys tema tematic tic error and noise The two sources of Levelling error in geophysical data are systematic error and systematic noise. Systematic error is defined as error that can c an be corrected using simple mathematical corrections. Sources include diurnal variation, lag (instrument positioning), positioning), heading (changes in instrument magnetic fields due to direction changes) and IGRF (International Geomagnetic Reference Field).

14 Chapter 3: Levellin g Tutori als

Systematic noise is the error remaining following the application of corrections. This error is not coherent (i.e. predictable). This type of error can be removed using conventional Levelling or empirical techniques. Conventional techniques comprise simple Levelling (performing (performing tie and regular survey line Levelling Le velling on an entire data set) and/or careful Levelling of individual lines). Empirical techniques comprise microlevelling (grid-based de-corrugation de-corrugation on an entire data set). In this part of the tutorial, we describe one approach to identifying errors and noise in the sample data set. You may have different approaches but the following procedure provides a basic starting starting point. Preparing Preparing d ata for evaluation

Before we can begin to evaluate the data for systematic error and noise we need to open a database and then using the bi-directional gridding gridding method, grid the data. The grid will automatically be placed on a temporary map. We will then display a Colourshaded grid grid image on the temporary map. T O O PEN A D AT AB AS E : 1.

On the Data menu select Open database. The Open Database dialog is displayed.

2.

Select the airmag.gdb file and click the [Open] button. The database airmag.gdb will be opened and displayed in a spreadsheet window.

Chapter Chapter 3: Levelling Levelling Tutorials 15

T O G RID D AT A : 1.

On the Grid and Image menu, select Gridding| Bi-directional line gridding. The Bi-directional gridding of line data dialog is displayed.

2.

Using the Channel to grid dropdown dropdown list, select “Mag.” Specify the Output grid as “Air_Mag.” The Grid cell size can be left to the intelligent default.

3.

To access the Advanced gridding gridding options, click the [Advanced>] button. Click the [
4.

Click the [OK] button to grid the data and display the grid in a temporary map Air_Mag.


T O D ISPLAY A C OLOUR- SHADED G RID :

When working with the Levelling system it is sometimes useful to display the data as a Colour-shaded grid . 1.

On the Grid and Image menu, select Display| Colour-shaded grid . The Colourshaded grid image dialog is displayed.

2.

Using the Grid name dropdown list, select Air_Mag.grd. Click the [Current Map] button to display the Colour-shaded grid on the current Air_Mag map.

Chapter 3: Levelling Tutorials 17

Evaluate Data

You will notice that there is large-scale systematic noise that affects the entire data set (i.e. large scale vertical striping patterns). Later in the tutorial, we will look at how to use simple Levelling to reduce these effects. For now, we will concentrate on examining the data in detail to investigate less obvious effects. 1.

Select (highlight) the map to evaluate as Air_Mag.grd.

2.

On the Map Tools menu, select Profile. The Profile plot dialog is displayed.

3.

Using the Profile channel dropdown list, select “Mag” In the Profile scale (units/mm) box, specify “500” and in the Profile base box specify “55500.” Leave the remaining parameters to the intelligent default values and click the [OK] button. The No defined map scale dialog is displayed.


The No defined map scale dialog is displayed if you are drawing to a map for which a map scale has not been defined. This situation most commonly occurs when a map has been opened just to display a grid image. A map scale is required to determine appropriate scaling for drawing text, patterned lines, and other map graphic details. The displayed default scale has been chosen to fit the map to an area approximately 30x30 cm when plotted. Click the [OK] button to accept the default scale and the magnetic data profiles will be displayed on the current map.

Note:

4.

Tip:

5.

You may want to display the Line path via the Map Tools\ Line path menu option, along with the profiles. This will help you determine the amount of lag.

Using the Zoom box ( )button, zoom in on a strong, narrow linear anomaly that runs perpendicular to the regular survey line direction and crosses numerous lines (as shown in the figure below):

When examined at a detailed level (i.e. by plotting profiles and zooming into a portion of the grid), you will find that there are small-scale systematic-errors (i.e. chevron or zigzag patterns that indicate that lag corrections are required). Note that, you will only get a zigzag between lines if adjacent lines have been flown in opposite directions. Be careful to choose such a place in the data to get an accurate lag correction. A positive lag moves that data back in time, a negative lag moves the data forward in time. Most often we need to move the data forward in time to catch up to the aircraft position. Occasionally, the data collected in a sensor behind the aircraft still needs a positive lag due to the time stamping done by the data acquisition system. Only a lag test as described above will give the proper lag to be applied. Surveyors often


evaluate the lag of a system by flying in multiple directions back and forth over an anomaly that is located at a known position.

Tutorial 2: Applying corrections Correcting Systematic Errors in Geophysical Data

This section of the tutorial describes the basic sequence of steps required to apply corrections to geophysical data and remove systematic errors as defined previously. To keep this tutorial general in nature, we have focused on lag and heading corrections. If you are working with magnetic data, you will find additional information on base station and IGRF corrections in later sections of the manual. Applying Lag Correcti ons

If you are working with magnetic data, you typically apply base station corrections and then lag corrections. If you are working with electromagnetic data, the lag correction may be your first correction. Lag corrections are required to compensate for positioning errors that typically occur because the actual measuring position is offset from the actual data recording position. Lag effects are most often visible as “chevron” (i.e. zigzag) patterns between lines (when looking for lag effects, remember that level shift is caused by heading, not lag. It is easy for the novice interpreter to mistake lag error for heading error on a map). As described previously, you must first examine the data in detail to identify these patterns. The next step is to determine the lag offset in fiducials by comparing equivalent points on anomalies on different lines and determining how many fiducials the anomaly is offset. The correction is this value divided by two and with a negative sign applied (i.e. the instrument is located behind the survey position – the typical case). You then apply the corresponding correction value in fiducials (-10 / 2 = -5). You will normally know the lag corrections from the survey platform design. Note:

You may either correct the data channels to match the x, y position channels, or you may correct the x, y position to match the data. Correct the data if you have multiple data channels with different lag corrections. Correct the x and y channels if all data channels share the same lag.

T O A PPLY L AG C ORRECTIONS TO Y OUR D AT AB AS E : 1.

On the LevCorr menu, select Lag. The Lag Correction dialog is displayed.


2.

Using the Channel to lag dropdown list, select “Mag” and then in the Output channel specify “Mag_lag.” In the Lag shift (fiducials) box, specify “–5.” The Lag shift represents the lag to be added to the start fiducial, normally a negative number.

3.

Click the [OK] button to calculate the lag correction and place the corrections in the database.

4.

To display the new channel, select (highlight) the first empty header cell, right click and from the popup menu, select List . From the list, select Mag_lag and click the [OK] button. The () channel is displayed in the spreadsheet.

Interpolate Data Follow ing Lag Corrections

When you examine the data in your database after you have applied the Lag correction, you will notice that dummy values have been added ( *) to the first row of all the lines in the Mag_lag channel. Depending on your processing preferences, you may want to interpolate your data to the ends of the line. T O I NTERPOLATE D AT A F OLLOWING L AG C ORRECTIONS 1.

Select the database to interpolate airmag.gdb.

2.

On the LevUtil menu, select Interpolate. The Interpolate Dummies dialog is displayed.

3.

Specify the parameters as shown above making sure that you choose “Yes” for the Interpolate dummy edges? box.

Chapter 3: Levelling Tutorials 21 4.

Click the [OK] button. The system places edge interpolated values in the Mag_lag channel.

E VALUATING THE E FFECTIVENESS OF L AG C ORRECTIONS

The final step that we recommend is to verify that the corrections were applied successfully by regridding the data in the Mag_lag channel and re-displaying the magnetic profiles as before. The following comparison shows the results of using a value of –5 on the data.

Peaks on adjacent lines now occur at the same fiducial following lag correction and the gridded anomaly assumes a linear appearance as opposed to the original chevron or zigzag appearance. Applying Heading Correcti ons

A heading correction corrects data for systematic shift in the data that is a function of the direction of travel on a survey line. This is most often required in magnetic surveys because the survey aircraft produces a magnetic field that changes based on survey direction.


In modern surveys, heading corrections are normally included as part of the aircraft compensation corrections that are performed automatically by the survey data acquisition system. Older surveys, or improperly compensated surveys, may still require a heading correction as part of data processing. For airborne magnetic surveys without automatic compensation, the heading corrections must be determined from a heading test flight. This involves flying over the same ground location at a relatively high altitude in the four possible flight directions. The location and height should be chosen so that the magnetic field has no horizontal gradient. The test flight should also be conducted under quiet diurnal conditions. A heading test flight is then flown in a clover leaf pattern over the test location and the magnetic field is rated for the four flight directions. If there is doubt about diurnal variation, the test should be repeated in reverse order and the two values can be averaged. This data is then used to construct a heading table file that will be used to apply the heading correction. There are three steps required to construct a heading table and apply the correction: •

•

•

Determine mean magnetic values in each grid direction and subtract these from the heading test value. This gives a correction value in each direction. Create a heading correction table and populate the table with the correction values for each direction. Apply the correction.

The following procedures describe each of these processes in detail. T O D ETERMINE M EAN M AG NET IC V AL UES IN E AC H D IRECTION:

The test data set you are supplied with has N and S (normal) lines as well as E (tie) lines. There are no tie lines flown in the W direction. We assume an average heading test value of 56920.53. Before you can create a heading correction table, you must first calculate the mean magnetic values in each of the grid directions. The process is as follows: 1.

Move the cursor to the Mag channel and click the left mouse button three times to highlight the entire channel (required for statistics calculation in step 3).

2.

On the LevCorr menu, select Select direction. The Select lines on direction dialog is displayed.


3.

As our test data includes N and S lines we will specify “0” as the direction, in degrees, that corresponds to one of the grid directions. We will leave the remaining parameters to the defaults and click the [OK] button. The Levelling system selects all lines oriented in this direction.

4.

On the LevUtil menu, select Statistics. Although the entire Mag channel is highlighted, the system only performs statistics on the selected lines. The results should be similar to the following.

5.

Record the mean values and subtract them from the average obtained from the heading test (in this case, we are using 56920.53).

6.

Repeat steps 2, 3 and 4 for the remaining line directions (90, 180 and 270 – optional since no West oriented lines in database). T O C ONSTRUCT A H EADING T AB L E F IL E :

1.

Access your text editor and create a file as follows:


2.

Save the file as to your working directory ( D:\Tutorial) and name it Mag_heading.tbl.

The processing steps to this point have obviously revealed the presence of heading errors in this data set. Note that the heading errors shown here are atypically large for a magnetic survey; they were exaggerated on purpose for this tutorial. The next step is to attempt to remove the heading error. T O R ESELECT THE L INES IN A D AT AB AS E

Before we can apply the heading corrections we first need to reselect all of the lines in the database. 1.

Select (highlight) the database to process airmag.gdb.

2.

Select and right-click the Line channel header.

3.

On the pop-up menu, select the Selection Tool. The Line Selection Tool dialog is displayed.

4.

Select all lines and click [OK].

Chapter 3: Levelling Tutorials 25 T O A PPLY H EADING C ORRECTIONS TO A D AT AB AS E 1.

On the LevCorr menu, select Heading. The Heading correction dialog is displayed.

2.

Using the button, select the Heading table name as Mag_heading.tbl. Then using the dropdown lists, select the X and Y reference channels as X, Y and select the Channel to correct as Mag_lag. Then specify the Output corrected channel as “Mag_lag_head.”

3.

Click the [OK] button and the system enters the corrections in the output channel.

4.

The final step is to evaluate the effect of the corrections. To do this simply, grid the Mag_lag_head channel and display the grid as follows:


You will notice that the first two steps have removed a significant portion of the systematic error in the data. You are now ready to advance to removing the systematic noise that remains.

Tutorial 3: Statistical Levelling o f ti e lines The statistical Levelling method corrects for intersection errors (mis-ties) that follow a specific pattern or trend. Statistical Levelling corrects these errors by modelling a trend surface to the intersection errors and removing the trend from the tie line data. To level the tie lines, we assume that the difference between the tie line and all crossing survey lines can be used as an estimate of the remaining diurnal error along the tie line. Provided there are many crossing survey lines, it is not unreasonable to assume that the tie line should, on average, match the crossing lines. Any single line may be out of level, but the average of all lines should follow the correct tie line. The Statistical Levelling procedure fits a zero order (level shift), first order trend or second order (polynomial) trend to the crossing differences and applies that surface as a correction to the tie lines. The following procedures describe how to perform Statistical Levelling using the sample data provided with the system. Before you can complete this exercise please make sure that you have applied the systematic corrections as described previously. T O C REATE A T IE L INE I NTERSECTION T AB L E : 1.

On the Database Tools| Channel Tools menu, select Copy channel. The Copy a channel dialog is displayed.

2.

Using the Copy FROM dropdown list, select the channel Mag_lag_head. Then in the TO box, specify “Mag_tie_corr.”

3.

Click the [OK] button. The system copies the data from the Mag_lag_head channel to the new channel Mag_tie_corr. This step is to protect your previous work.

Chapter 3: Levelling Tutorials 27 4.

5.

On the LevTieLine menu, select Intersections. The system displays the Intersection Information dialog box.

Specify the parameters as shown above. Make sure that for the Tie lines or All lines “Tie Lines” are specified and specify an Output Intersect Table as “Air_tie.tbl.”

6.

Click the [OK] button. The system creates an ASCII table file. . For more information about the table contents, please refer to the Summary of Simple Levelling page 49. T O I MPORT THE T AB L E I NTO THE D AT AB AS E :

The next step is to extract intersections along each line and place the intersection information in new channels of the database. Note:

Prior to proceeding, you may want to review the intersection table to determine the magnitude of gradients and the maximum value to use as a cut-off. You can enter this information in the Maximum gradient field or simply accept the default value (0), which loads all intersections.

When ready: 1.

On the LevTieLine menu, select Load corrections. The Make a level channel dialog is displayed.


2.

Specify the parameter for the dialog as shown above and click the [OK] button. The system reads the intersection table and extracts intersection information for tie lines.

You now have three new channels in your database ( CROSS_LEVEL contains survey line values at the intersection points, CROSS_DIFF contains differences between tie line and survey line values and CROSS_GRAD contains gradient values at the intersection points). T O C REATE A “ L EVELLING V IEW ” :

After loading the Levelling channels, you can display the level channels in Profile Windows to look for and remove suspicious ties. We recommend the following procedure for displaying the Levelling information: 1.

Display a “Tie line” (ex. T90150) in the database.

2.

Display 3 profile windows at the bottom of the spreadsheet window. You can do this by positioning your cursor at the bottom of the profile window; you should see a two-headed arrow, drag the cursor upward to display a new profile window. Repeat this process on the new window until you see three profile windows.

3.

Select (highlight) the first profile window and then move the cursor to the Mag_tie_corr channel, right-click and from the popup menu, select Show Profile. The profile is displayed.

4.

While keeping the focus in the same profile window, move the cursor to the CROSS_LEVEL channel and select Show Symbols from the popup menu. The survey line values at the intersections are displayed as solid coloured circles.

5.

While still in the first profile window select the Profile menu and then select Profile Options. The Profile dialog is displayed.


6.

Click on the available profile options tab and change the Data Info, Line Styles, Colours, Symbols and Grid as required. Click the [OK] button to close the Profile dialog.

7.

Select (highlight) the second profile window and then move the cursor to the CROSS_DIFF (error) channel, right click and from the popup menu, select Show Symbols. The system displays the errors (differences between tie and survey line values) in the profile window. If required, change colours / symbols (as in Step 6).

8.

Select (highlight) the third profile window and then move the cursor to the CROSS_GRAD (horizontal gradient) channel and display the profile symbols for this channel. The system displays the horizontal gradient values in the profile window. If required, change colours / symbols (as in Step 6). You should now have a view that looks something like the following:


T O E VALUATE AND R EMOVE “ S USPICIOUS” T IE LINE V AL UES :

The Levelling View (above picture) is ideal for inspecting the level errors along each survey line. If there are block shifts in the differences, that is, corresponding to where the tie line goes through different flights, the shifts must be corrected first. This should have been done during the diurnal base station correction. At this moment in the process, we are about to level the tie lines. In the Levelling view ( above), we are looking at a tie line and the differences between the tie line and all of the survey lines that it goes through. If there are any particularly large differences, we may wish to exclude them from the process so that they do not skew the chosen trend. 1. 2.

Select the suspicious point in the cross-difference ( CROSS_DIFF) channel. Press the space bar. The difference value will be replaced by a dummy value and the coloured circle will disappear from the middle profile window in the Levelling View. The system does not process intersections that use dummies as crossdifferences.


T O A UTOMATE “ S USPICIOUS” T IE LINE R EMOVAL :

For large datasets, it may be time-consuming to evaluate intersections manually. In this case, you may want to consider an automated procedure. The database provides access to a built-in math formula editor that enables you to query the database and remove values that lie above a certain threshold. For example, if you want to dummy values that have gradients larger than 20, you could specify: CROSS_ DI FF = ( CROSS_GRAD>20) : DUMMY: CROSS_DI FF

This formula places dummy values in the difference channel depending on the corresponding gradient. You can also design more advanced queries that also compare the corresponding CROSS_DIFF value to determine whether it is greater than an acceptable tolerance. As with any techniques such as this, you should try it on a sample database first. T O C REATE AN I NTERSECTION – GRADIENT V IEW

Another view that you may find useful is a map that shows flight lines and symbols corresponding to intersection points. You can then add proportional symbols to the map to show the highest amplitude gradients present and use the map to navigate in the database (via dynamic linking). This procedure is not detailed in full; however, the basic method is to copy the intersection table and edit the new table so that only the x, y and cross gradient values are shown. Then import these into a new database. Create a map and display flight lines. Select the new cross gradient database and plot symbols to show the intersections. Use the proportional symbol GX to plot symbols showing the cross gradients. You can then activate the dynamic links in the map and use the links to quickly locate high-gradients in the database, and remove suspicious cross difference values from statistical trending. T O P ERFORM S TATISTICAL L EVELLING :

After removing suspicious intersections, you are ready to level the tie lines: 1.

On the LevTieLine menu, select Simple Levelling| Statistical Levelling. The Statistically level a channel dialog is displayed.


2.

Specify the parameters for the dialog box as shown above. Ensure that “1” is specified for the Trend order (0 to 9) box. The system calculates a first order trend surface that best fits the observed differences and then adjusts the tie line to the trend. This approach is valid for Levelling the tie line if the crossing survey lines can be assumed to represent a statistical sampling to the correct field during the tie line flight.

3.

Click the [OK] button. The system levels the tie lines. Tie lines can now be assumed to be correctly levelled and will form the base on which to tie the regular survey lines. Note:

The system places corrected tie line values in the Mag_tie_corr channel. If you compare tie line values between the Mag_lag_head and Mag_tie_corr values, you will notice that they are different. Survey line values remain the same – only changing after the full Levelling process is complete.

Tutorial 4: Full Levelli ng of s urvey lin es In the previous tutorial, we removed a first order trend (linear) from each tie line. The result of the Statistical Levelling process is a set of trended tie lines that are either shifted, linearly trended, splined or b-splined. The objective of the next process (Full Levelling) is to adjust the Survey lines so that all lines match the trended tie lines exactly at each intersection that has been included in the process. The Full Levelling procedure assumes that the tie line values are correct where they cross the survey lines. Therefore, you must start by creating a new intersection table. T O P ERFORM F ULL L EVELLING : 1.

On the LevTieLine menu, select Intersections. The Intersection Information dialog is displayed.


2.

Complete the fields as shown above and click the [OK] button. The system creates a new intersection table based on the data in the levelled channel. The table contains the crossing differences between the levelled tie lines and the regular survey lines to be levelled.

3.


4.

Complete the fields as shown above making sure to specify “Line” for the Process line types.

5.

Click the [OK] button. The system extracts the Levelling difference information from the intersection table and creates a second set of cross-level, cross-difference and cross-gradient channels.

6.

Construct a second Levelling view using the data in the second cross-difference channel (CROSS_DIFF2). Although the tie lines have been levelled, you may still want to question differences which are unusually large, or which differ significantly from other differences on the same line. The cross-gradient value


may also be helpful for evaluating the quality of a specific tie point. The larger the horizontal gradient, the greater the uncertainty in the tie point. 7.

Adjust cross-difference (error) values manually. To remove a tie po int from Levelling, mark the cross-difference channel at that point and press the space bar to replace the difference by a dummy value. You can also manually insert difference values wherever you would like to interactively adjust the level of a line.

8.

If you would like a smoother correction along each survey line, you might consider first interpolating the difference channel ( CROSS_DIFF2) using a n Akima or minimum curvature spline. You can do this from the Utility menu by clicking the Interpolate menu option and selecting an appropriate spline, and creating a new interpolated channel. You would then perform the next step usi ng the interpolated channel to correct the survey line data.

9.

After completing any required manual adjustments selec t the LevTieLine menu and then select Simple Levelling| Full level. The Level to cross lines dialog is displayed.

10.

Specify the parameters as shown above. Make sure to select “Line” as the Apply to line types.

11.

Click the [OK] button. The system adjusts the data by adding the difference at each point that a difference value is defined. Linear interpolation is used to correct data values between difference points.

Tutorial 5: Evaluating your results Generally, you should find that the initial three steps (la g, heading and simple Levelling) significantly improve the quality of your dataset. Levelling is rarely perfect from the first attempt. We recommend creating a grid from the levelled data and plotting it together with the line path and evaluate the quality of the Levelling. Make a note of where the data has been over-corrected or undercorrected. You can then open the database and adjust the level differences manu ally by creating a new Levelling channel with desired values and then selecting the Full


level menu option. This process should help obtain a final levelled product that is

acceptable. Subtracting a levelled grid from an unlevelled grid can give a good representation of the noise that you have removed. However, be careful. If you subtract the levelled data from the data before lag correction, then a lot of seemingly geological signals will appear in the difference grid. This high frequency signal has not been removed from the data, just re-located by the lag correction. It is best to look at the differences after lag correction. The grid statistics will provide you with information about the magnitude of the applied corrections. If you feel that the magnitude of the corrections is too great, window the grid on the Z value to set a clipped maximum correction. This ‘clipping’ can also be achieved within the database, using channel operators prior to gridding. B-splining the data, with some tensions, would be one way of achieving clipping smoothly. If you have only a few lines that are problematic, you may want to use the careful Levelling functionality provided in the system. For more details about these capabilities, please refer to the Chapter 5: Careful Levelling – Detailed Description section of this manual. If your lag- and heading-corrected, normal-leveled product appears to have only minor problems that apply to the entire data set, you may want to apply the Empirical Levelling (Microlevelling) technique. For more details about the Empirical Levelling technique, please refer to the Chapter 6: Empirical Levelling (Microlevelling) section of this manual.

36 Chapter 4: Applyin g Corrections – Detailed Descriptio n

Chapter 4: Applyi ng Correction s – Detailed Description Many types of geophysical data contain a time-varying error that is fundamental to the type of data being measured. A variety of factors can cause errors: diurnal variation of the earth’s magnetic field, instrument heading, instrument variation, lag error between the aircraft and the sensor, inconsistencies between flight lines and tie lines. Spectrometer surveys commonly have time varying components that are a function of environmental conditions during the survey. Before you apply corrections or process your data, we recommend importing the downloaded base station data into an Oasis montaj database and evaluating it on a line-by-line basis. The objective is to examine the data in spreadsheet and profile views, and correct any noise or extreme values prior to applying the actual base station corrections to survey data. Note: This exercise provides instructions on how to apply corrections to your data.

Note that the procedures in this chapter are not part of the previous tutorials and no sample data is provided for them.

Recogni zing when co rrections are required The earth's magnetic field intensity will vary naturally as the earth rotates in the ionosphere of the sun. This variation is called “Diurnal”, because the variation has a natural period of one day. To correct this error, base station values representing diurnal variation are subtracted from the survey instrument data. Base station corrections are recommended for any magnetic survey where base stations are used. As the figure below demonstrates, the effect of lag on the data shifts the data points behind where they should be. The time it takes the sensor to reach the position of the aircraft determines the required log correction. This causes straight-line features (those with common value points) on a map to appear as “zigzag” or “chevron” patterns when adjacent flight lines are flown in opposite directions.

Flight Path C or r ec t ed

U nc o rr ec t ed

Chapter 4: Applyin g Corrections – Detailed Descriptio n 37

Heading error causes regular and noticeable shifts in the data that are a function of the direction of the survey lines. The figure shows the effect of heading error on a profile with equal data values for all lines. At each line, the values increase or decrease as the aircraft changes direction.

Line 1

Line 2

Line 3

IGRF Corrections are used to remove the background magnetic field of the earth, which typically appears as a regional gradient in the data. IGRF corrections will vary with the location and date of the survey.

Applying systematic corrections Since systematic errors follow regular and predictable patterns, the system applies mathematical corrections to the entire data set at once. Systematic corrections can be made in any order. The rest of this chapter explains how to make specific types of systematic corrections.

Heading Correction

Lag Correction

Systematic Corrections

Base Station Correction

IGRF Correction

When making systematic corrections to your data channels, Oasis montaj protects your original data by copying the corrected data into a new channel. As long as the channel is not protected, you can use the same name as the input channel. Protected channels are indicated by a black triangle in the channel header cell. To change this status, click the right mouse button on the header cell and make sure there is no check mark beside the Protected option.

Applying magn eti c base station corrections Most diurnal error, if not all diurnal error, can often be corrected by establishing a magnetic base station at a fixed ground location. The base station site should be located away from any natural or man-made magnetic sources, and should be tested to insure that the background magnetic field is relatively constant over a distance of 10 meters or more.


A digital recording base station magnetometer is set up at the site and the magnetic field is measured continuously during the course of the survey, normally at a rate of 10 to 60 times per minute, depending on the specifications of the survey. The base station should be within the survey area away from any large magnetic sources. Care must be taken if the base station is a distance from the survey (often the case with marine magnetic data). Importin g Instru ment Data Using Geosoft Table Files

You must import your airborne or ground survey data into an Oasis montaj database before you can make systematic or Levelling corrections. The instructions in this manual assume that you have already done this prior to beginning the Levelling process. More information on importing survey data into an Oasis montaj database is available in Chapter 2: Working with Data of the Oasis montaj Quick Start Tutorials or the online help system ( Help|Help Topics). The base station magnetometer will output an ASCII or binary file containing timing and magnetometer values. This information must be formatted as follows: An example of base station data is an ASCII file containing 2 columns: GPSTIME and MAG. The GPSTIME is ideal, because it is read from the satellite and has a precise match in the database. No longer do you need to try to synchronize your systems - they all get their time from the same place. To import recorded base station readings into Oasis montaj, you must format the base station information formatted in a specific way. An example of the standard Base Station Table File (*.bas) used in Oasis montaj is provided below: / Geosof t Base St at i on Dat a / / MAGBASE. BASE=64545 / / = Dat e: dat e: I / = Ti me: t i me: I / = Cor r ecti on: r eal / = Comment s: st r i ng20 / / Dat e Ti me Cor r ect i on Comment s 93/ 12/ 23 23: 56: 57 25. 5 you 93/ 12/ 23 23: 56: 59 24. 6 can 93/ 12/ 23 23: 57: 01 24. 3 wr i t e 93/ 12/ 23 23: 57: 03 23. 9 char act er s 93/ 12/ 23 23: 57: 05 23. 5 i n t hi s 93/ 12/ 23 23: 57: 07 23. 1 col umn

A base station table file (*.bas) consists of a comment header, which is indicated by '/' characters at the start of each line, followed by columns of data. The comment header of a table file is scanned to find lines that begin with '/=', and these lines are interpreted to determine the contents of the data in the file. Each column must have a '/=' header line that describes the data using the syntax:


/= name:type[:i] Each column has a name and a type, which can be 'real', 'double', 'int', 'date', 'time', 'line' (line name) or 'string'. The optional ':i' indicates that a column should be used as an index for looking up data in the table. In this case, corrections are looked up based on a date and time index. For the column type 'string', (indicating that the column contains characters, not numbers) the header line will contain a number indicating the width of the column in characters. For example, to create a 'string' column with a width of 80 characters, the header line would be: /= name:string80 The base station table will use linear interpolation between times to determine a correction for any required time. A table file header may also contain Oasis parameter block settings that are entered into the current Oasis parameter block when the table is read. Parameter settings are indicated by header lines that begin with a '//'. In the example above, the parameter MAGBASE.BASE is defined for information only. It indicates that the established magnetic base station value is 64,545 nT. A base station table is normally created by a separate program that converts the native dump format of a recording base station magnetometer to the Geosoft table format. Quality Control on Base Station Data

As mentioned in the section above, you must initially download base station data from the instrument into an ASCII file. Before applying corrections, you require a Geosoft table file (*.tbl) containing header information, and date, time and base station magnetic values. As an interim step (i.e. before creating the base station table file), we recommend first importing the downloaded base station data into an Oasis montaj database and evaluating it on a line-by-line basis. The objective is to examine the data in spreadsheet and profile views, and correct any noise prior to applying the actual base station corrections to survey data. If you detect extreme values (i.e. spikes) in line data, for instance, you may want to remove these using a non-linear filter. Alternately, you may find high-frequency instrument chatter that is most effectively removed using a low-pass filter. When you are satisfied that the base station data is of sufficient quality to continue, use the Export tool to export data to a new ASCII file. You can then add the header information as described previously and start correcting survey data for diurnal variations.


Applying Base Statio n (Diur nal) Correcti ons to a Datab ase

Diurnal corrections are applied to survey data contained in an Oasis montaj database. The system uses the date and the time of each line in the database to find the appropriate diurnal correction for the corresponding line in the base station table. By default, the system protects your original data by copying the corrected base station data into a new channel. Note:

An alternative method for importing total field information from a diurnal table (*.tbl) file into a database is to use a table lookup file. For example, create a table lookup with GPSTIME as the index, and then create a diurnal correction channel in the database (*.gdb). This method gives you full control over editing and interpolating the diurnal corrections if required.

T O A PPLY D IURNAL C ORRECTIONS TO A D AT AB AS E : 1.

Edit the base station readings from the magnetometer so that the data are in the base station table file format (*.bas). Refer to the section above for instructions on creating a base station file.

2.

On the LevCorr menu, select Base station. The Base Station Correction dialog is displayed.

3.

Using the

4.

Using the dropdown list, select the Time channel. This is the channel in the database that contains the time each data measurement was taken.

5.

Select the Input data channel for correction from the dropdown list. This is the channel containing the data you wish to correct for diurnal variation.

6.

Specify a name for the Output corrected channel. The system will write the corrected data to this channel.

7.

Click the [OK] button. The system enters the corrections in the output channel.

button, locate the Base station table file (*.bas).

Magnetic survey specifications often place limits on the variability of the diurnal field. These limits should be based on the time it takes to fly the distance between tie lines since we assume that the magnetic field varies smoothly during that time.


To view the amount of diurnal correction for each line in Oasis montaj, create a new channel and simply subtract the corrected magnetic channel from the original magnetic channel using the Oasis montaj math capabilities. This profile may then be viewed on a time base in the profile window. You may also wish to export the diurnal correction data to an XYZ file or plot it to a map.

Lag correction A lag correction corrects data for the distance between the measurement sensor and the positioning sensor. For example, in a towed-bird helicopter configuration, the positioning system is usually located in the helicopter and the geophysical sensors are towed on a cable that trails the aircraft. The time it takes for the sensors to move to the position of the aircraft, which is a function of the aircraft speed and the distance between the aircraft and the sensors, is known as the lag. A lag correction subtracts the lag (in fiducials) from the start fiducial of the data channel. For example, consider a helicopter platform towing a magnetometer sensor on a cable 50 metres behind the helicopter, at a speed of 100 km/hr, and using a fiducial increment of 1 second. The helicopter moves at 27.8 m/sec, so the lag correction would be (-50m/27.8m/s) or -1.8 seconds. Although the lag correction can be any real value, we recommend that the lag be a multiple of the fiducial sample interval in order to improve the efficiency of processing the data. If the lag is not an even multiple of the sample interval, Oasis is forced to re-sample channels as they are used. Lag can also be introduced by the data recording system. Some instruments need time to gather a signal. By the time a signal is recorded, the result might belong halfway back to the start of the signal collection. This would be positive lag. Sometimes the instrument designer builds this factor into the data acquisition system (DAS), sometimes not. Note:

For specific procedural information, please refer to the Tutorial. For dialog parameter information, please click the

button on the dialog of interest.

Heading corr ection A heading correction corrects data for systematic shift in the data that is a function of the direction of travel on a survey line. This is most often required in magnetic surveys because the survey aircraft produces a magnetic field that changes based on survey direction. In modern surveys, heading corrections are normally included as part of the aircraft compensation corrections that are performed automatically by the survey data acquisition system. Older surveys, or improperly compensated surveys, may still require a heading correction as part of data processing.


Determinin g Heading Corrections

Heading corrections can be determined by flying over the same point at a high altitude in the four cardinal compass directions (North, South, East and West), or in directions parallel and perpendicular to the planed survey lines. The average of the field in the four directions is the correct field value, and the difference between the average and the measured values in each direction is the heading correction. The heading corrections (in degrees true) must be compiled into a small Geosoft heading table as follows: / Geosof t Headi ng Corr ect i on Tabl e / / = Di r e ct i on: r eal : i / = Cor r ect i on: r eal / / Di r ecti on Cor r ecti on 0 - 3. 25 90 - 0. 42 180 +3. 19 270 +0. 48 360 - 3. 25

The directions are with respect to the Y axis direction of the survey co-ordinate system, and a heading correction in the 0 and 360 degree directions must be specified. If the heading test is flown in the directions of the planned survey lines, you must calculate a 0 and 360 degree correction for the table by linearly interpolating between the observed corrections. Following is an example for a heading test flown for survey lines 30 degrees East of the Y-axis: / Geosof t Headi ng Corr ect i on Tabl e / / = Di r e ct i on: r eal : i / = Cor r ect i on: doubl e / / Di r ecti on Cor r ecti on 0 - 5. 05 30 - 10. 88 120 - 4. 87 210 +9. 13 300 +6. 63 360 - 5. 05

The directions listed in the table must be in the true heading. The correction in directions between those specified is linearly interpolated. Note:

For specific procedural information, please refer to the Tutorial. For dialog parameter information, please click the


IGRF corrections The International Geomagnetic Reference Field (IGRF) is a mathematical model of the normal magnetic field background of the earth. This model is a function of date, location and elevation, and the model is updated every five years based on magnetic observations from base stations located throughout the world. Once updated, the


model is termed the Definitive Geomagnetic Reference Field (DGRF). A magnetic survey can be corrected for the IGRF by subtracting the IGRF model values at each point in the survey. The montaj Geophysics Levelling extension enables you to calculate an IGRF channel, which you can then subtract from your magnetic channel, or you can calculate a value for a single point. Note:

When dealing with a variety of data with different ages, it is important that the data are corrected using the same IGRF coefficients, otherwise large Levelling errors may result.

T O C AL CUL AT E A N IGRF C HANNEL : 1.

Determine a range of longitude, latitude that covers your survey area. Skip this step if you are using channels containing the Longitude and Latitude information.

2.

On the LevCorr menu, select IGRF channel. The Compute GRF Channels dialog is displayed.

3.

Complete the parameters as required, and click the [OK] button. The system calculates the IGRF model at the long, lat points specified in the lat and long channels. The field is calculated for May 17, 2004 at an elevation of 500 metres. The IGRF strength (nT) is placed in the Total Field channel and Inclination / Declination results are placed in the specified channels.

Subtract the IGRF field from the final levelled and corrected magnetic channel. Note: For specific procedural information, please refer to the Tutorial. For dialog 4.

parameter information, please click the


After all known systematic error corrections have been made, conventional Levelling procedures are used to perform final Levelling of the data. This process, also known as tie line Levelling, corrects discrepancies between intersecting regular survey line and tie line data points. These discrepancies are called mis-ties. The simple Levelling


process attempts to reveal the cause of mis-ties, so that corrections can be applied. Simple Levelling basically involves two steps: 1.

2.

First, looking at the differences at the crossings of all the level tie lines in the survey lines. This assumes that there are a sufficient number of crossing lines to properly model the tie line error, which is further assumed to be a simple base level shift or some higher order polynomial surface. After tie lines have been levelled, all survey lines are adjusted to match the tie lines values at their intersection.

For surveys that collect data on evenly spaced parallel lines with more widely spaced perpendicular tie lines, it is conventional to use the tie lines only as an aid to Levelling the regular survey lines. In these cases, the tie lines are normally not included when gridding the data. However, on uniform grid surveys and many marine surveys where line locations appear arbitrary, all line crossing errors must be corrected in order to be able to use all data in gridding.

Overview of L evellin g pro blems and decisi on cr iteria Simple Levelling process often requires a certain amount of manual intervention. Types of intervention may include rejecting specific mis-ties that are a problem, or applying artificial tie corrections where real ties are missing or inadequate. Also, where miss-ties cannot be used, perhaps because of excessive gradients, tie lines and survey lines will not match and should not be forced to match. Tie line and survey line errors can occur for a variety of reasons: High horizontal gradient, high vertical gradient, magnetic storm activity, and normal diurnal variation are all sources of Levelling error. The cause of the Levelling error will determine which Levelling method you can use to correct the data. This section explains how to identify some common Levelling problems and correct them. High Horizontal Gradients

High, unconstrained differences between tie lines and regular survey lines are a source of error. This type of error is the result of high horizontal gradient and positioning error. If the tie lines or survey lines are measured in a region of high horizontal gradient, a positioning error will produce a mis-tie. For example, if the horizontal gradient is 1 nT/m, and the positioning error is ±5 m, the potential mis-tie from the maximum error on both lines is ±10 nT. For this reason, mis-ties in high gradient areas should not be used to level data.


±5m

Maximum error intersection point

±5m Tie Line

Ideal intersection point Survey Line

High Vertical Gradients

If the tie line and regular survey line elevations are different, the mis-tie will be a function of the vertical magnetic gradient. Such mis-ties can often be rejected by rejecting ties based on horizontal gradients, which are often related to the intensity of the vertical gradient. Tie Line

Elevation d ifference

Incorrect line position

Survey Line

Correct line position

Magnetic Storm Activity

Short-duration magnetic variations caused by magnetic storm activity can produce variations in the magnetic field that do not vary smoothly in the time it takes to cross two tie lines during the survey. Since Levelling assumes a smooth variation, such errors can lead to invalid corrections. Normally, survey data should be rejected if they are not collected during periods of relatively quiet magnetic activity. Diurnal Variation

Normal diurnal variation in the magnetic field can cause tie lines and regular survey lines to have different measurements at the same point. If tie lines have been corrected for diurnal base station recordings it may not be necessary to level the tie lines. This decision depends on how close the survey area is to the base station site, and on the smoothness of the diurnal variation during the tie line flights. The smoother the diurnal, the better the base corrections will be.


If the data has not been corrected for a base station the tie lines may contain diurnal error themselves and should be levelled themselves before applying Levelling corrections to the normal survey lines. Block Level Shifts

When working with your own data (especially with Electromagnetic data which may be affected by temperature drift), you may want to check that there are no block level shifts to apply since these should be corrected prior to tie line Levelling. If there are block level shifts, use a batch method to correct these values automatically.

Understanding statistic al Levelling of t ie lines The statistical Levelling method corrects for intersection errors (mis-ties) that follow a specific pattern or trend. Statistical Levelling corrects these errors by modelling a trend surface to the intersection errors and removing the trend from the tie line data. To use statistical Levelling effectively, it is important to understand when to use it. The following diagrams illustrate a typical Levelling situation where statistical Levelling would be useful.

Tie Line (T) L1

L2

L3

L4

L5

L6

L7

The diagram above shows a vertical profile of a single tie line (solid line) showing where it intersects with the regular survey lines (indicated by circles). The profile shows several regular lines that intersect the tie line in correctly. The intersections at lines 1, 2 and 4 require minimal Levelling since they cross the tie line correctly. Line 3, is problematic since its position is much different from the rest of the lines. Since Line 3 appears to be a single outlier rather than part of a trend, requires either manual adjustment or removal (make it a ‘dummy’ value). Intersections at lines 5, 6, 7 show an increasing trend that can be removed through statistical Levelling.

Statistical Trend Tie Line (T) L1

L2

L3

L4

L5

L6

L7


In the second diagram (above), a statistical trend is modelled for the tie line as it intersects lines 4, 5, 6 and 7. Note the intersection point for Line 3 has been ‘dummied’ to remove it from the data. Dummying an intersection point means that the corresponding errors will not be used for statistical Levelling.

Modified Tie Line (T¹)

L1

L2

L3

L4

L5

L6

L7

In the statistical Levelling process, the system models a trend surface to the intersection errors and removes the trend from the tie line data. The final result is statistically leveled tie line data. Understanding Geosoft ’s Implementation of Statistical Levelling

In Oasis montaj, there are five procedural steps required to implement Statistical Levelling of Tie lines: 1.

Create a tie line intersection table. This table is constructed using tie lines only, and includes fiducial, x, y, level (CROSS_LEVEL0), errors (CROSS_DIFF) and horizontal gradient (CROSS_GRAD) columns. The latter three columns play an important role in removing anomalies tie values that may affect the quality of the final product.

The output intersection table file tabulates every intersection between tie lines and regular survey lines. It includes the exact ground location of the intersection point, the tie line and survey line numbers, the recorded value on each line, and the horizontal gradient of the data at that location. Following is an example of an intersection table: / Intersection Table / / X

Y

Tie# TFid

TZ

TDZ Line# LFid

LZ

LDZ

988094.09 815336.51 T90 1306.9765 64180.236 0.068 L470 574.1339 64197.888 0.463 988106.99 821893.43 T90 1409.5507 64207.285 0.445 L471 2276.9579 64266.167 0.211 988049.27 797721.71 T90 1030.2008 64069.941 0.556 L510 599.39139 64065.831 0.155 988082.4 806688.47 T90 1170.7415 64116.57 0.421 L490 2304.9

64130.017 0.526

987983.82 780078.85 T90 777.00052 64078.934 0.558 L550 649.57509 64069.698 1.376 988058.46 787854.91 T90 876.39104 64050.944 0.132 L530 2282.7486 64075.181 0.009 987997.32 762392.99 T90 472.45185 63904.933 0.759 L590 881.50135 63852.423 1.427

The columns of the table are: • • •

X Y Tie#

X coordinate of the intersection Y coordinate of the intersection Tie line number

48 Chapter 4: Applyin g Corrections – Detailed Descriptio n • • • • • • •

TFid TZ TDZ Line# LFid LZ LDZ

Tie line fiducial Tie line reading Line gradient (per fiducial) Line number Line fiducial Line reading Line gradient (per fiducial)

This is the basic information required to determine correction for tie line Levelling. 2.

Load the tie line intersection table data into the database. This process creates CROSS_LEVEL (magnetic value on the intersecting line), CROSS_DIFF (intersection errors) and CROSS_GRAD (horizontal gradient) channels.

3.

Create a “Levelling View”. This view recreates the following figure graphically within the Oasis montaj system.

Tie Line (T) L1

4.

L2

L3

L4

L5

L6

L7

Examine and manually edit the data to remove “suspicious” intersections from the Levelling process. In practice, this is done by dummying (i.e. removing) values in the CROSS_DIFF channel. The system uses the CROSS_DIFF channel in the next step to determine whether use a certain intersection in calculating a trend.

Apply the Statistical Levelling menu option using tie lines only. You have four trend options for applying Statistical Levelling including 0 (shift), 1 (linear), 2 (spline) or 3 (b-spline) orders. The system uses this value to calculate a new trended set of tie line data which you will use to level survey lines. Note: Levelling (i.e. STATLEV and FULLLEV) uses the DIFF channel to rewrite magnetic data. To avoid overwriting "raw" magnetic channel data, we suggest that you copy the raw data to another channel before starting to level. 5.

Understanding Geosoft ’s im plementation of fu ll Levelling Levelling normal survey lines involves correcting each survey line to match the level at each tie line. This procedure assumes that the tie line values are correct where they cross survey lines. In Oasis montaj, the INTERSCT, XLEVEL, and FULLLEV GX processes are used to carry out the Full Levelling process. The process is as follows:

Chapter 4: Applyin g Corrections – Detailed Descriptio n 49 1.

Since the tie lines have been changed, it is necessary to calculate a new intersection table. This table will have new error (CROSS_DIFF) values that you may wish to edit manually (although this may not be necessary).

2.

Load the new intersection table into the database.

3.

Create a “Levelling View” as described previously.

4.

Examine, and if necessary, manually remove suspicious intersections.

5.

Apply the Full Levelling menu option.

Summary of s impl e Levellin g Levelling is rarely perfect from the first attempt. You should create a grid from the levelled data and plot it together with the line path and evaluate the quality of the Levelling. Make a note of where the data has been over-corrected or under-corrected. You can then open the database and adjust the level differences manually by creating a new Levelling channel with desired values and then using the XLEVEL GX. This process should help obtain a final levelled product that is acceptable.

50 Chapter 5: Careful Levelling – Detailed Description

Chapter 5: Careful Levelling – Detailed Description In simple Levelling, the objective is to obtain a coarsely levelled set of lines that remove a significant portion of the Levelling error. As mentioned in the tutorial, however, simple Levelling may or may not be successful initially. If after examination of the normally levelled data, there remains a significant error component, there are several alternatives: •

•

•

Re-examine and further correct tie line intersections and re-run the Full Levelling menu item Select individual tie lines, lines or groups of lines and then use the Careful Levelling menu item to create a correction channel and apply the corrections in the correction channel to the magnetic data Apply microlevelling techniques

This chapter provides a detailed description of Careful Levelling. Careful Levelling can be used to correct seemingly ‘rogue’ lines in areas where data coverage is sparse and you need to use all available lines. Careful Levelling techniques can be applied in many types of situations, including processing magnetic data for mineral exploration or petroleum applications, or for processing electromagnetic data in Unexploded Ordnance (UXO) applications where block shifts or temperature effects are visible in the data. Careful Levelling techniques are also very useful when combining data from different ages, and with varying (often-dubious) quality.

Understanding the careful Levellin g pr ocess The careful Levelling process is a two-step process as follows: •

•

Create corrections (i.e. correction channel) using a selected trend type (shift, tilt, spline or b-spline) Examine, modify if necessary and apply

The basic procedure in Oasis montaj is to: 1.

Copy the target data to a working channel.

2.

On the LevTieLine menu, select Intersections. The Intersection Information dialog is displayed.

3.

Specify a name for the Output Intersect Table to create a table of intersections and cross differences. Click the [OK] button.

4.


5.

Click the database.

button and select the Intersection table to load the channels into the

Chapter 5: Careful Levelling – Detailed Description 51 6.

Examine the intersections and cross-sections via a Levelling View (described in the Tutorial).

7.

Select individual tie lines, survey lines or groups of lines for processing.

8.

On the LevTieLine menu, select Careful Levelling and then select Shift correction. The Create a level shift correction channel dialog is displayed.

9.

Specify the parameters as required to create a correction channel based on a certain trend type.

10.

Examine the correction channel to evaluate the corresponding trend to be applied.

11.

On the LevTieLine menu, select Careful Levelling and then select Apply correction to apply the trend in the correction channel.

12.

Re-examine the data and repeat the corrections process until data fall within the desired limits of error reduction.

Applying different types of trends There are four main types of orders of trends you can use in the system: 1.

Shift Correction: Applies a specific positive or negative constant value to

selected lines. 2.

Tilt Correction: Applies a linear trend to selected lines.

3.

Spline Correction: Applies a spline function to selected lines.

4.

Tensioned Correction: Applies a b-spline function to selected lines.

Understanding t he Shift Functi on

The Shift Correction function is simply a linear, positive or negative constant value applied to selected lines. This function is used to account for uncorrected heading errors and the longest wavelength features. The correction is calculated as the average of the level errors (differences) at each crossing. Understanding t he Tilt Functi on

The Tilt Correction function is simply a straight, inclined line that fits the data using least squares fitting. Use the tilt function to create a tilt level correction channel from an error difference channel to account for other long wavelength errors such as diurnal or navigation errors. The correction is calculated as the first order trend through the level errors (differences) at each crossing. Understanding the Spline Functi on

This Spline Correction function is used to create a smooth splined level correction channel from an error difference channel. The correction is calculated by passing a

52 Chapter 5: Careful Levelling – Detailed Description

spline function through the error points. The spline function is used to account for shorter wavelength errors, such as diurnal or navigational errors. Understanding the Tensioned Functi on

The B-Spline or Tensioned spline option is often used to apply least-squares smooth fitting to scattered, noisy data. The B-spine function is used to correct the same errors as splines, but it can moderate the amount of correction. The Tensioned spline works like a metal band secured by horizontal and vertical springs. •

•

You can adjust horizontal tension using a roughness control. The Roughness (S) controls how well the spline will match the observed data within the standard deviation. A small roughness (0) is smoother than a large roughness (1). A very large roughness will cause the spline to match the data. Very small values are necessary when trying to refine the magnitude of the spline fit while trying to keep a relatively smooth curve. You can adjust vertical tension using a Tension (Tau) control. . No tension (T=0) results in a minimum of curvature. A tension of 1 results in piece-wise linear interpolation between points.

In the following example, we show the results of various combinations of horizontal and vertical tension.

The top profile shows the error in the CROSS_DIFF2 channel with S=0 and T=0, and a default error value. The computed function matches the data almost exactly (within rounding error). In the bottom profile, we see b-splines with S=1, T= 0 (bold line), S=0.5, T=0.5 (dotted line) and S=1, T=1.

Chapter 5: Careful Levelling – Detailed Description 53

For best results, we recommend experimenting with various settings until you are satisfied that the correction ideally models the errors in the data.

54 Chapter 6: Empirical Levelling (Microlevelling)

Chapter 6: Empiric al Levelling (Microlevelling) When working with complex datasets, such as airborne magnetic or radiometric data, a key data processing objective is to eliminate Levelling problems which were not removed during regular data processing (such as lag corrections, tie line Levelling, base level corrections, etc.). Geosoft's Microlevelling procedure is designed to help you remove effects rapidly and effectively. To use this procedure, you require Geosoft's Gridding Toolkit (141W), Advanced Grid Utilities (4015W) and Oasis montaj Data Processing Environment (4000W).

Geosoft micr olevellin g proc edure Before you start you should be aware that Geosoft’s Bi-directional Line Gridding program changes the storage sense of your data. For example, if your lines are oriented in the X direction and you run Line Gridding, the output grid is oriented in the Y direction. The kx parameter controls the storage direction of your grid and can be examined in Grid Stats, if required. The reason that it is important to know about the grid storage sense is that this controls the direction in which you are applying filters for Levelling. For instance, if you use Line Gridding to apply a filter while gridding from profile data, the program applies the filter down the lines. If you regrid and apply a second filter, the program applies the filter across the lines. This note currently applies only to lines oriented east-west or north-south. To Microlevel Using Line Gridding and Oasis mont aj 1.

Run Line Gridding on your raw Z channel to create a starting grid. This creates a Grid (G1) oriented perpendicular to the lines (e.g. kx=1 or row-oriented, or in the X direction, for north-south lines).

2.

Run Line Gridding on your raw Z channel and apply a low-pass filter (i.e. filter down the lines). This creates an initial Regional Grid (G2) oriented perpendicular to the lines. This filter length should be long enough to separate high-frequency geological information from the long-wavelength Levelling error and the longwavelength geological signal. Use a filter length 4-10 times the line separation (10 is recommended).

3.

Run Re-grid a grid on the Regional Grid (G2) and apply the same non-linear and low-pass filters (i.e. filter across lines). This creates a filtered Regional Grid (G3) oriented parallel to the lines (e.g., kx=-1 or column sorted in the case of northsouth survey lines). This removes the Levelling error from the regional grid.

Chapter 6: Empirical Levelling (Microlevelling) 55 4.

Smooth the filtered Regional Grid (G3) using a Hanning filter with 3 to 5 passes. This creates a Hanning filtered Regional Grid (G4) oriented parallel to the lines.

5.

Restore the geologic and regional signal by using the Add Grids tool. This creates a decorrugated grid (G5 = G1 - G2 + G4). Examine this grid to determine if the result is acceptable. If not, repeat steps 2 to 4 until an acceptable result is achieved.

6.

Determine the Levelling errors using the Add Grids tool. This creates a Levelling Error grid (G6 = G1 - G5) or (G6 = G2 - G4).

7.

Create a Levelling Error channel from the Levelling Error Grid (G6) by running the GRIDSAMP GX (Sample a grid…) in montaj. This GX uses the X and Y values in the database to sample a specified grid (G6) and create a new database channel.

8.

Create a Filtered Levelling Error channel by running the LOWPASS GX (Low pass filter…) in montaj to remove any remaining geologic signal.

9.

Create a Corrected channel by applying a math formula in montaj (i.e. Corrected channel = Raw channel - Filtered Levelling Error channel).

10.

Regrid the Corrected data using Line Gridding. This creates the final Microleveled Grid (G7) ready for further processing or interpretation.

56 Ap pen di x A : L evel li ng Menu

Appendix A: Levelling Menu The montaj Geophysics Levelling System includes the menu file Levelling_System.omn , which can be loaded to the main menu bar by selecting the Load menu icon () from the shortcut bar or by selecting the Load menu option on the GX menu and then select the Levelling_System.omn file. The Levelling menus provide rapid access to a number of GX processes that are useful for Levelling. The following table summarizes the menu structure. LevUtil Statistics...

stat.gx

Calculates range and standard deviation of marked data. This can be useful for determining the range of crossing errors from the cross differences.

Interpolate...

interp.gx

This can be used to interpolate correction errors between crossings before applying corrections using the Full level... menu option. Normally, the error is assumed to vary linearly between crossing points. If a smoother error is required, you can interpolate the error channel using a cubic or linear spline.

Trend...

trend.gx

As with interpolation, the trend GX can be used to establish a statistical trend surface through a number to intersection differences. The Statistical level ...menu option calculates a trend automatically to determine a smooth error surface.

Sample a grid...

gridsamp.gx

This option can be used to extract Levelling errors from a Levelling error grid. The Apply levels... menu option can be used to apply the corrections to the raw data.

Apply levels...

level.gx

Levels a channel to the errors defined in another channel.

Table lookup...

lookup1.gx

This can be used to lookup errors from a Levelling table that you have created yourself. Refer to the LOOKUP1 GX documentation for more information.

Base station...

magbase.gx

Apply time based Levelling corrections from a base station table. See Applying magnetic base station corrections p.37

Lag...

lag.gx

Apply a time lag correction to a channel. See Applying Lag Corrections p.19

Select direction

Seldir.gx

Selects lines based on line direction.

Heading...

heading.gx

Apply a heading correction to a channel. See Applying Heading Corrections p.21

IGRF Channel

igrf.gx

Computes the IGRF or DGRF field strength, inclination and declination from given longitude, latitude and optional elevation channels. The line date is used as the

LevCorr

Ap pen di x A : L evel li ng Menu 57

model date. igrfpt.gx

Calculates IGRF and DGRF model values for a single point at a specified date.

Intersections...

intersct.gx

Finds intersections between tie lines and regular lines, and creates an ASCII file intersection table.

Load corrections...

xlevel.gx

Loads corrections from an intersection table into correction channels of the database.

Statistical level...

statlev.gx

Statistically levels a channel by fitting a trend surface to the cross errors and adjusting the channel data to the trend.

Full level...

fulllev.gx

Corrects a channel by forcing a match at the locations in a cross level channel.

Shift correction…

lvshift.gx

Creates a level shift correction channel from an error difference channel. The correction is calculated as the average of the level errors (differences) at each crossing.

Tilt correction…

lvtilt.gx

Creates a tilt level correction channel from an error difference channel. The correction is calculated as the first order trend through the level errors (differences) at each crossing.

Spline correction…

lvspln.gx

Creates a smooth splined level correction channel from an error difference channel. The correction is calculated by passing a spline function through the error points.

Tensioned spline…

lvtspln.gx

Creates a smooth (B-splined) level correction channel.

Apply correction…

lvcorr.gx

Adds a level correction channel to a raw data channel to produce a new leveled channel. The level correction channels are created using LVSHIFT, LVTILT, LVSPLN or LVTSPLN.

IGRF at a point…

Tie line

58 Glossary

Glossary channels

Array of elements which are consecutively related based on a fiducial. In the Oasis montaj spreadsheet, a channel is essentially a column that contains a specific type of data.

contour

Line connecting points of equal value, or representing the locus of a constant value on a map or diagram

diurnal variation

The daily variation in earth’s magnetic field intensity as it rotates in the ionosphere of the sun.

database

See Oasis montaj database

desktop

Background area in the Oasis montaj project. You can open and display a virtually unlimited number of Spreadsheet, Profile and Map windows in this area.

empirical Levelling

A gridding technique that uses mathematical filters to remove large anomalies or spikes in the data.

fiducials

Points accepted as fixed bases of reference or marks which indicate points of simultaneity.

conventional Levelling

A process that corrects discrepancies between the intersections of regular survey lines and tie lines.

Geosoft Database (GDB)

A proprietary binary database architecture that stores spatial data in a format that enables rapid access, processing and analysis of high volume data (tested up to 10 gigabytes).

Graphical User Interface

Interactive software environment where functions are performed by selecting graphic objects.

grid

Collection of points along rows and columns that define a two-dimensional rectangular area on some plan, usually a ground plan.

Glossary 59

Grids or Grid file

Files containing location (X and Y) and data (Z) values. Values are typically interpolated to create a regular and smoothly sampled representation of the locations and data.

groups

A set of graphics elements that make up a graphic component of the map. For example, a line path plot, a contour plot or a profile plot would all be separate graphics groups within the Data View.

GX or Geosoft eXecutable

Programmed process (identified by the *.GX file extension) used to process data in Oasis montaj.

heading error

Systematic shifts in the data that change with the survey direction.

IGRF

The International Geomagnetic Reference Field (IGRF) is a mathematical model of the earth’s normal magnetic field background.

Images or Image file

Files containing location (X and Y) and colour values. The values are not interpolated. Standard PC file types created using imaging or electronic photo-editing techniques.

imaging

Process of creating and manipulating digital pictures derived from gridded earth science data. Used for visualisation and analysis of information.

line

Linear array of observation points

lag error

Positioning error caused when the actual measuring position is offset from the actual data recording position.

Main window

Primary tool used to create and maintain databases, display data and process data. Oasis montaj is a Graphical User Interface (GUI) system that provides all functionality required to process and display virtually any type of Earth Science data.

Map (*.MAP)

Geosoft-developed file that integrates all graphics elements (lines, polygons and text) and

60 Glossary

layers (basemaps, data, grids, plots and images) constructed in Oasis montaj menu (*.OMN)

Graphical list of commands or functions that a user may choose from

microlevelling

See empirical Levelling

mis-tie

A point where a tie line and a regular survey line cross but do not intersect due to Levelling error.

Oasis montaj

Oasis montaj is the core software platform that provides all functionality required to manage, manipulate, visualize and map spatially located Earth Science data.

profile line

Line along which measurements are made

project (*.GPF)

Binary file that defines a desktop environment. It is essentially an “electronic briefcase” that helps organize data and the activities related to a data processing project.

random XYZ data

Located XYZ data that cannot be grouped naturally into separate lines. For example, regional gravity surveys or geochemical surveys are commonly considered random data because the locations appear somewhat random in nature.

statistical Levelling

A Levelling method that corrects for intersection errors (mis-ties) by modelling a trend surface to the intersection errors and removing the trend from the tie line data.

survey lines

Parallel lines created by connecting a series of data points where measurements were taken.

systematic error

Measurable errors in survey data that are removed mathematically by a computer. Sources of error include diurnal variation, lag, heading and IGRF.

systematic noise

The error remaining after systematic corrections have been applied. This error is not predictable

Glossary 61

and must be removed using conventional Levelling or empirical techniques. table file

An ASCII file that used to correct survey data. Table files contain the date, time and value of each survey measurement.

tie line Levelling

A Levelling process that corrects discrepancies between intersecting regular survey line and tie line data points.

trend-reinforced grid

Grid in which the line direction of a grid is not square to the map co-ordinate system. Oasis montaj reinforces strong line-to-line correlation by joining all features in a preferred strike direction.

template

A file used to manage data in a particular format.

62 Index

Index Cross-gradient value, 32 Add Grids tool, 54 Corrections, 21 Akima spline, 33 Applying base station corrections, 36 heading corrections, 24 lag corrections, 18 systematic corrections, 36 ASCII files creating, 26 exporting, 38 importing, 37, 38 Base station applying corrections, 36 data, 12 magnetometer, 37 Base station table files (*.bas) creating, 38 description, 37 Bi-directional line gridding, 53 Block level shifts, 45 Block shifts, 49 B-spline function, 51 Calculating first-order trends, 31 heading corrections, 41 IGRF channels, 42 intersection tables, 48 Careful Levelling, 34 Careful Levelling, 49 Channels correction, 49 IGRF, 42 protecting, 36 Chevron patterns. See Lag error Column name, 38 Column type, 38 Comment header, 37 Correction channel, 39, 49, 50, 56 Corrections base station, 36 heading, 9 IGRF, 36, 41 lag, 9, 17, 20, 40, 53, 55 Levelling, 9 mathematical, 36 Corrections menu, 55 creating project, 4

Data base station, 12, 36 editing, 47 electromagnetic, 18 evaluating, 12 evaluating geophysical, 7, 18, 35 exporting, 38 importing instrument, 37 interpolating, 19 over-correcting, 33 raw magnetic, 47 under-correcting, 33 Database creating channels, 27 importing tables, 26 Oasis, 39 Decorrugated grid, 54 De-corrugation, 13 Definitive Geomagnetic Reference Field, 42 DGRF. See Definitive Geomagnetic Reference Field Displaying Magnetic profiles, 17 Diurnal error, 36 Diurnal field variability limits, 39 Diurnal variation, 44 Dummy value, 33 Dummy values adding, 19 interpolating, 19 Electromagnetic data, 18 Empirical error, 9 Empirical Levelling, 12, 13, 53 definition, 10 overview, 9 using line gridding, 53 Empirical Levelling, 34 Error cross-gradient, 32 diurnal, 36 empirical, 9 heading, 36 lag, 35 Levelling, 8, 9, 12, 43, 49, 53, 54, 55 small-scalesystematic, 17 systematic, 8 time-varying, 8 Errors

Index 63

intersection, 25, 45 positioning, 18 systematic, 12, 13 Evaluating systematic error, 12 systematic noise, 12, 13 Evaulating lag corrections, 20 Export tool, 38 Exporting data, 38 Extreme values, 38 Fiducials, 18, 40 Field intensity magnetic, 35 Filter length determining, 53 Filters Hanning, 54 low-pass, 38, 53 non-linear, 38, 53 FormalLevelling procedure, 42 Full Levelling performing, 31 process, 47 FULLLEV GX, 47, 56 Geological signal, 53 Geophysical data, 12 Geosoft project file, 4 Geosoft table file (*.tbl), 38 Grid microleveled, 54 Gridding bi-directional line, 53 line, 53 Grids decorregated, 54 GRIDSAMP GX, 55 GXs descriptions, 55 FULLLEV, 47, 56 GRIDSAMP, 55 HEADING, 24, 55 IGRF, 55 IGRFPT, 56 INTERP, 19, 55 INTERSCT, 26 INTERSCT, 47 INTERSCT, 56 LAG, 55 LEVEL, 55

LOOKUP1, 55 LOWPASS, 54 LVCORR, 56 LVSHIFT, 56 LVSPLN, 56 LVTILT, 56 LVTSPLN, 56 MAGBASE, 55 STATLEV, 30, 55, 56 TREND, 55 XLEVEL, 26, 47, 48, 56 Header comment, 37 Heading correction table, 41 Heading corrections, 9, 12, 18, 55 applying, 20, 24 definition, 20 determining, 41 Heading error, 36 HEADING GX, 24, 55 Herring-bone, 8 Horizontal gradients, 43 Horizontal tension, 51 Horizontalgradient, 33, 46 IGRF. See International Geomagnetic Reference Field corrections, 36, 41 IGRF channel calculating, 42 IGRF GX, 55 IGRFPT GX, 56 Importing survey data, 37 tables, 26 Instrument data importing, 37 International Geomagnetic Reference Field, 12, 41 INTERP GX, 19, 55 Interpolatedchannels creating, 33 Interpolation linear, 38 INTERSCT GX, 26, 47, 56 Intersection error, 25 Intersection errors, 45 Intersection point, 27, 46 dummy, 46 Intersection table, 46, 48, 56 Intersection tables, 26 creating, 26, 31 Lag error, 9, 35

64 Index

offset, 18 Lag correction, 9, 17, 20, 40, 53, 55 Lag corrections evaluating, 20 LAG GX, 55 LEVEL GX, 55 Level shift correction channel, 56 LEVEL32.OMN, 55 Levelling careful, 34 corrections, 9 empirical, 9 error, 8, 9, 12, 43, 49, 53, 54, 55 filters, 53 full, 31 statistical, 25 Levelling menu, 5, 55 Levelling view creating, 47 using, 29 Levelling View creating, 27 Line gridding, 53 Linear interpolation, 38 Linear trends, 50 Linevalues removing, 29 Load correction channel, 56 loading menus, 5 LOOKUP1 GX, 55 Low-pass filter, 38 LOWPASS GX, 54 LVCORR GX, 56 LVSHIFT GX, 56 LVSPLN GX, 56 LVTILT GX, 56 LVTSPLN GX, 56 MAGBASE GX, 55 Magnetic data, 47 Magnetic profiles Displaying, 17 Magnetic Storm activity, 44 Magnetometer base station, 37 Math formulas applying, 54 Mathematical corrections, 36 Maximum gradient field, 26 menu Levelling, 5 Minimumcurvaturespline, 33 Mis-tie, 9, 25, 43, 44 Mis-ties.

Noise evaluating systematic, 13 systematic, 8 Non-linear filter, 38 Normal Levelling, 49 procedure, 43 summary, 48 Oasis database, 39 Parameter block settings, 38 Patterns vertical striping, 16 Profile windows, 27 project creating, 4 Project Creating, 5 Protecting channels, 36 Removing linevalues, 29 tiepoints, 33 Selecting tie lines, 49 Shift systematic, 9 Shift function, 50 Spikes, 38 Spline function, 50 Splined correction channel, 56 Statistical Levelling implementing, 46 performing, 25 tie lines, 45 STATLEV GX, 30, 55, 56 Survey airborne data, 21 direction, 9, 20, 40 Survey data importing, 37 Survey lines regular, 45 Surveylines, 47 crossings, 43 Systematic error, 8 noise, 8 shift, 9 Systematic corrections, 10 applying, 36

Mon Taj Geophysics Levelling

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