Earth Imager 1 d Manual

Instruction Manual for

EarthImager 1D Version 2.0.5 1D Resistivity Inversion Software

Copyright © 2006-2014 by Advanced Geosciences, Inc. 2121 Geoscience Drive Austin, Texas 78726 Phone (512) 335-3338 Fax (512) 258-9958 Email: [email protected] Website: https://www.agiusa.com/

Instruction Manual of AGI EarthImager 1D, 2014-12-08

Copyright notice

This instruction manual is intended for use with the EarthImager 1D resistivity inversion software and related products, manufactured by Advanced Geosciences, Inc. This manual is protected by United States copyright law and may not be reproduced, distributed, transmitted, displayed or published without the prior written consent of Advanced Geosciences, Inc. Registered users of the EarthImager 1D resistivity inversion software are entitled to one copy of this instruction manual. No copies may be made without express written permission from Advanced Geosciences, Inc.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 TABLE OF CONTENTS 1. WELCOME ................................................................................................................................ 5 1.1 EarthImager at a Glance ................................................................................................................. 5 1.2 Where to Get Help............................................................................................................................ 5

2. INSTALLATION ....................................................................................................................... 7 2.1 System Requirements ....................................................................................................................... 7 2.2 Installation......................................................................................................................................... 7 2.2.1 Installation of EarthImager..........................................................................................................................7 2.2.2 Dongle driver installation............................................................................................................................7 2.2.3 Support for 64-bit Windows........................................................................................................................8

2.3 Software Protection with dongles or hardware keys..................................................................... 9 2.4 EarthImager Upgrade ...................................................................................................................... 9

3. GETTING STARTED WITH EarthImager............................................................................ 11 3.1 First Look at EarthImager ............................................................................................................ 11 3.2 Read a Data File.............................................................................................................................. 12 3.3 Array Types and Data Format ...................................................................................................... 14 3.4 Starting Model ................................................................................................................................ 16 3.5 Resistivity Inversion Settings......................................................................................................... 18 3.5.1 Inversion method.......................................................................................................................................18 3.5.2 Stop Criteria ..............................................................................................................................................18 3.5.3 Number of iterations .................................................................................................................................18 3.5.4 Maximum RMS error................................................................................................................................18 3.5.5 Error reduction ..........................................................................................................................................19 3.5.6 Damping factor .........................................................................................................................................19 3.5.7 Depth factor...............................................................................................................................................19 3.5.8 Data offset averaging ................................................................................................................................20

3.6 File Management and File Extension............................................................................................ 20 3.7 Run Inversion.................................................................................................................................. 21 3.8 Input Graphics Title....................................................................................................................... 22 3.9 View Measured Data ...................................................................................................................... 22 3.10 View Inverted Resistivity Model ................................................................................................. 22 3.11 View Inversion Result Image....................................................................................................... 23 3.12 Show Project Information Table................................................................................................. 23 3.13 Copy Image to Clipboard............................................................................................................. 24 3.14 Save Image..................................................................................................................................... 24 3.15 Save STG Data .............................................................................................................................. 24 3.16 Print Image.................................................................................................................................... 24 3.17 Inversion with Topography ......................................................................................................... 24

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 4. THEORY OF INVERSION..................................................................................................... 26 4.1 What Is Inversion? ......................................................................................................................... 26 4.2 Forward Modeling.......................................................................................................................... 27 4.3 Damped Least Squares Inversion.................................................................................................. 27 4.4 Smooth Model Inversion ................................................................................................................ 28

5. TOOLS...................................................................................................................................... 29 5.1 Forward Simulation ....................................................................................................................... 29 5.2 Combine 1D Raw Data Files.......................................................................................................... 29 5.3 Combine 1D Inverted Resistivity Models ..................................................................................... 30

6. MULTILINGUAL SUPPORT................................................................................................. 32 6.1 Introduction .................................................................................................................................... 32 6.2 Add your own language to EarthImager...................................................................................... 34

7. HELP........................................................................................................................................ 35 7.1 Instruction Manual in PDF............................................................................................................ 35 7.2 AGI Home Page .............................................................................................................................. 35 7.3 EarthImager User Group .............................................................................................................. 35 7.4 Upgrade ........................................................................................................................................... 35 7.5 About EarthImager 1D .................................................................................................................. 35 7.6 EarthImager 1D Release Notes...................................................................................................... 35

SUGGESTED READINGS ......................................................................................................... 36

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1. WELCOME We, all of us at Advanced Geosciences Inc. (AGI), thank you very much for purchasing our EarthImager 1D software. Resistivity data inversion with EarthImager 1D is easier, more fun and more productive than ever. We look forward to hearing your success stories through the use of our products. 1.1 EarthImager at a Glance AGI EarthImager 1D is a Windows 32-bit platform based computer program that interprets onedimensional (1D) electrical resistivity sounding data and produces a layered resistivity model that reveals subsurface geology. EarthImager 1D processes vertical electrical sounding (VES) data collected with Schlumberger, Wenner, dipole-dipole, pole dipole and pole-pole arrays. We will support any resistivity data format in addition to AGI’s STG files if a sample data file is available. 1.2 Where to Get Help This instruction manual is the ultimate resource for everything about EarthImager. This manual may be found in the installation folder as an Adobe Acrobat PDF file. AGI’s website will have the latest updates about EarthImager and free downloads with improvements and bug fixes. You are encouraged to go through the suggested readings at the end of this manual for in-depth understanding of resistivity data inversion. Your satisfaction is our top priority. If you find any feature is missing in our EarthImager, please let us know. We will add it for you as soon as possible if we think that your suggested feature would benefit other EarthImager users. This version of EarthImager will read AGI’s STG data files. If you have any resistivity data file format other than the STG format, please send us a sample data file. We will let EarthImager support your data format. EarthImager clients will have access to EarthImager User Group online. Download links to the latest version of EarthImager and document are available in the User Group. To enter the User Group, go the AGI website at https://www.agiusa.com/ and choose EarthImager User Group under the File Areas, or go to the page https://www.agiusa.com/files/eicust/earthimageruser.shtml directly. You will be asked to type in your user name and password that are given to you at the time of your purchase. Inside EarthImager, you may choose the menu item Help | EarthImager User Group to enter the User Group. Your user name and password should be shown. If your EarthImager is over six (6) months old, this is an indication that you have missed many new features. HTU

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For further questions, you may contact our tech support team by phone or email listed below. Tech Support

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Email: [email protected] Phone: +1 512 335-3338 ext. 1026 AGI directory is listed here https://www.agiusa.com/directory.shtml. Snail mail: Advanced Geosciences, Inc. 2121 Geoscience Drive Austin, Texas 78726 U. S. A. Website: https://www.agiusa.com/ HTU

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2. INSTALLATION 2.1 System Requirements EarthImager will run on IBM PC and compatible models on any 32-bit Windows operating system that include Windows 2000/XP/Vista/7. It also runs on 64-bit versions of Windows, but then in 32-bit mode. EarthImager 1D does NOT require a lot of memory. Here are our recommended minimum system requirements. • • • •

Windows 2000/XP/Vista/7 Intel Pentium IV or up 10 MB or more disk space available on the hard drive 20 MB or more available (free) RAM

2.2 Installation 2.2.1 Installation of EarthImager We created a Windows standard setup program for ease of installation. Insert CD with EarthImager setup program into the CD ROM drive and the installation wizard should pop up automatically. One may also start the installation by double-clicking the AGIEarthImager1Dx.x.x.xxx.exe on the CD ROM (x indicates a variable version number). Before the installation of EarthImager, make sure that the dongle is unplugged and NOT connected to your computer. Follow the onscreen instructions until the installation finishes successfully. Read the EarthImager license agreement carefully during the installation. It is suggested that you install all AGI computer programs under C:\Programs\AGI\. If the installation starts with errors and is unable to finish successfully, you may need to install the “dongle driver” before your installation of EarthImager. The dongle driver may be found on the EarthImager CD or downloaded from the EarthImager User Group on the AGI website at https://www.agiusa.com/files/eicust/earthimageruser.shtml

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2.2.2 Dongle driver installation The dongle driver, i.e., Sentinel System Driver, is automatically installed onto your computer while installing EarthImager. If you observe any of the following error messages when you launch (start) EarthImager, this indicates that the dongle driver is NOT installed correctly. To fix the dongle driver problem, follow these steps:

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1) Go to the dongle vender website at http://www.safenet-inc.com/support/tech/sentinel.asp to download the latest version of the dongle driver: Sentinel System Driver 5.xx.x – MSI Installer version. 2) Install the downloaded driver. If you observe the following warning message, it should be safe to click “Continue Anyway” because the dongle driver is directly from the dongle manufacturer.

3) Start EarthImager again and see if the problem is fixed. 4) If this installation does not solve the problem, you must uninstall all Sentinel-dongle related programs, if any, which include EarthImager 1D, EarthImager 2D, EarthImager 3D, AGI Marine Log Manager, and other third party programs. At the end, uninstall Sentinel System Driver. These can be done by going to Control Panel | Add or Remove Programs. 5) Download Sentinel System Driver Cleanup Utility version 1.x.x from the above website at http://www.safenet-inc.com/support/tech/sentinel.asp. Install and run this driver cleanup program. 6) Reinstall EarthImager. Stop here if EarthImager runs OK. 7) If the problem persists, install Sentinel System Driver 5.xx.x – MSI Installer version. 2.2.3 Support for 64-bit Windows According to our tests, EarthImager will run on Windows XP 64-bit, Windows Vista 64-bit, Windows 7 64-bit and Windows Server 2003 x64. For any 64-bit operating system (OS), one must install the Sentinel SuperPro dongle driver manually. The driver, Sentinel Protection Installer 7.x.x.exe, can be found on the installation CD or from AGI EarthImager User Group. It may be downloaded from the SafeNet website http://www.safenet-inc.com/support/tech/sentinel.asp. AGI EarthImager is a 32-bit Windows application which cannot take advantage of expanded 64bit memory address space. EarthImager will still run in the 32-bit mode with a maximum memory space of 2GB.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 AGI offers a command line true 64-bit EarthImager 3D CL without any GUI. EarthImager 3D CL would run on any 64-bit Windows, access more than 2GB memory space, and carry out scalable parallel processing on a multi-core and/or multiprocessor PC. 2.3 Software Protection with dongles or hardware keys We use Sentinel hardware keys, i.e., dongles, to protect EarthImager. These are normally the USB type. Dongles are also available on special order for the parallel printer port but this is not recommended. Please specify the printer port type when ordering. Warning when using the printer port dongle : To connect both the hardware key (dongle) and the printer to the same parallel port, you must turn off both the computer and the printer first. Then connect the dongle and the printer to the parallel port. Make sure both dongle and printer are firmly connected. Finally, you may turn on the computer and the printer. Here is what the dongles looks like.

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From the menu item Help | About EarthImager 1D, you can find your dongle serial number, a unique number to identify your dongle. The Option Number tells what EarthImager 1D module you have purchased. Option 0 indicates a base version with most of EarthImager1D features. 2.4 EarthImager Upgrade We have different definitions for EarthImager update and upgrade. An update means changes from an older EarthImager version to a newer EarthImager version. AGI gives away free updates of EarthImager year-round. Please go to AGI EarthImager User Group https://www.agiusa.com/files/eicust/earthimageruser.shtml to download the latest version. An EarthImager upgrade means to add one or more special-purpose software modules to EarthImager. For example, EarthImager 2D has three modules: Time Lapse Inversion, CRP, and IP Survey Planner. In addition, one may add EarthImager 2D and 3D licenses to existing EarthImager 1D dongle. The software download is free from EarthImager User Group. After purchase of EarthImager 2D and/or 3D, we will email you an upgrade code to activate your dongle for both EarthImager 2D and/ or 3D.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 AGI charges an upgrade fee for adding a new module. It is easy and convenient to upgrade your EarthImager. You do not need to send the dongle (hardware key) back to AGI. Instead, AGI will send you a dongle upgrade code upon the receipt of your payment. To activate the new option(s) you purchased, please follow these steps 1) Connect the dongle to your computer 2) Start EarthImager 1D 3) Go to the menu item Help | Upgrade 4) Copy and paste (or type) the update code to the license code field.

5) Click the OK button to complete the dongle upgrade. One of the message boxes below will pop up to show the success or failure of the upgrade.

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3. GETTING STARTED WITH EarthImager 3.1 First Look at EarthImager This is the Main Window of EarthImager 1D that is based on Windows 32-bit platforms. Its user-friendly graphical user interface (GUI) consists of seven main menus and one toolbar with 14 tool buttons.

The function of a tool button is displayed when the mouse cursor hovers over a button. Here are the functions of all tool buttons. Read a raw data file. The AGI STG file is a default. Open inversion Settings Window so a user can set desired settings. Start inversion. Stop inversion. Save any on-screen image as a bitmap/JPEG file. Print out any on-screen image. Open the current trial folder in Microsoft Windows Explorer. This helps locate the current trial folder and browse/check the files in it. , Used to change length unit, feet or meter, for AB/2 distance and depth. Apparent resistivity is always displayed as Ωmeter. To set Window’s true type font for captions and labels of an on-screen image.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 To show or hide the chart border and a project information table at the lower right corner. To combine neighboring layers with like resistivity values or delete thin layers. The order of main menus is basically that of inversion steps. Resistivity data inversion with EarthImager generally consists of these steps: 1) 2) 3) 4)

Read a data file. Change inversion settings or read a settings file (*.ini). Start inversion. Improve the starting model based on the inversion results by Combine-Layers or manually modify the model. 5) Run inversion again. Steps 4 and 5 may be repeated a few times. 6) View inversion results. 7) Save and print the raw data and inversion results. There are three sets of data on the left chart. The black dots are field measurements (raw data). The red dots and line are predicted (modeled, calculated) data. The thick blue curve is the inverted resistivity model which may be dragged to alter layer resistivity and thickness. 3.2 Read a Data File To load a data file, you may either choose the Read Data menu item from the File menu or click the first tool button . Tool buttons are used for quick and easy access to some frequently-used menu items. You will be prompted to choose a data file to process. EarthImager 1D reads AGI Sting, MiniSting and SuperSting data files (*.stg). There are two synthetic data sets in the demo folders: Schlum1D.stg and Wenner1D.stg. Please consult AGI instrument manuals for more information about AGI Sting data format. EarthImager 1D also reads an AGI-defined DAT file format which supports Schlumberger, Wenner, dipole-dipole, pole dipole and pole pole arrays at this time. Refer to the next section for the description of this format. We will upgrade our program to support any ASCII data format from our customers. If you are interested in our program to read your non-STG data, please send us a sample data file. To find out if EarthImager 1D supports your data format, select Read Data, and check the Files of Type box at the bottom of Open File dialog. We created a Universal Resistivity (data) File (URF) format to facilitate processing of resistivity data collected with non-AGI instruments. Here is a sample URF file (Schlum1D.urf). ---------------------------------------A 1D sample Universal Resistivity data File (URF) Top two lines are comments and the third line defines the length unit. Unit: meters

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 ;Any line starting with a semicolon (;) is a comment line ;A line starting with a colon (:) defines a section header. ;A section header is required and must be spelled correctly. ;A blank line will be ignored ;Data entries are comma-separated. ;Electrode locations are defined with XYZ coordinates. ;X coordinates define electrode locations. ;Both Y and Z are optional and are always set to 0 :Geometry ;ID, X, Y, Z 1, -500, 0 2, -300, 0 …. 13, -5, 0 14, -3, 0 15, -2, 0 16, -1, 0 17, -0.5, 0 18, 0.5, 0 …. 31, 120, 0 32, 200, 0 33, 300, 0 34, 500, 0 :Measurements ;A, B, M, N, V/I(ohm), I(mA), Error(%) 15,20,17,18,1.70787E+01 14,21,17,18,7.38018E+00 13,22,16,19,5.30200E+00 12,23,16,19,2.03070E+00 ….. ---------------------------------------------------To read a URF data file, go to File | Read Data menu or click the Read-Data tool button . Then choose “Universal Resistivity Files” from the “Files of Type” box (see below). You may find other resistivity data file formats supported in EarthImager 1D. The raw data is shown as a profile on a log-log coordinate system. The horizontal axis is the logarithmic array span and the vertical axis is the logarithmic apparent resistivity. The array span is defined as AB/2 for Schlumberger array, as a-spacing for Wenner array, and as AN/2 for dipole-dipole array. Also shown to the right is a recommended starting model for automatic inversion. The white line in the layered model shows relative logarithmic resistivity values of all layers.

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There are two ways to edit the raw data. The first approach is to drag a data point up or down with the left mouse button after reading a raw data file. However, one can’t add or delete a data point in this way. In the second approach, a user can manually modify data values in a spreadsheet. After opening a data file, go to the menu Edit | Edit Data to launch a spread sheet. After all necessary changes, click the button “Update Data” to update the raw data plot. 3.3 Array Types and Data Format EarthImager 1D supports five common array types: Schlumberger, Wenner, dipole-dipole, poledipole, and pole-pole arrays. AGI defined a simple raw data format (*.dat) for EarthImager 1D users to input raw data manually. Go to the menu File | Input Raw Data and bring up this interface.

Schlumberger Array

Wenner Array

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To input your raw data on this window manually, choose the array type, the distance unit, and the number of data points. Apparent resistivity values are not needed and not saved in the data file and they are calculated/updated by clicking the Update button or double-clicking the left mouse button in the spreadsheet. Only one of apparent resistivity and V/I columns are required. The other column can be “updated”. Here are column headers of five arrays: -

Schlumberger array: Wenner array: Dipole dipole array: Pole dipole array:

Data#, AB/2, MN/2, V/I Data#, a-spacing, V/I Data#, a-spacing, n, V/I Data#, a-spacing, n, V/I

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 -

Pole pole array:

Data#, AM, V/I.

Here are two sample DAT files for the Schlumberger array and Wenner array respectively. Schlumberger Array Unit=meters ;# AB/2 MN/2 1, 2, 1, 2, 3, 1, 3, 5, 2, 4, 8, 2, 5, 10, 2, 6, 12, 4, 7, 20, 4, 8, 25, 4, 9, 30, 6, 10, 50, 6, 11, 80, 6, 12, 120, 10, 13, 200, 10, 14, 300, 10, 15, 500, 16,

V/I 17.0787 7.38018 5.302 2.0307 1.27373 1.81631 0.668214 0.441833 0.485544 0.207408 0.099555 0.086385 0.027763 0.008765 0.003118

Dipole-dipole or pole-dipole array

Wenner Array Unit=meters ;# A V/I 1, 0.3, 52.7267 2, 0.46, 35.953 3, 0.76, 21.2928 4, 1, 16.0521 5, 1.5, 10.4878 6, 2.26, 7.05041 7, 3, 5.3731 8, 4.5, 3.60188 9, 9, 1.90944 10, 15, 1.31389 11, 21, 1.17109 12, 30, 1.10146 13, 45, 1.08453 14, 60, 1.03488 15, 75, 0.944382 16, 90, 0.876246 17, 125, 0.732803

Pole-pole array

3.4 Starting Model A 1D inversion constructs a layered resistivity model from the field measurements. However, the number of layers is generally unknown. The non-uniqueness of inversion further complicates the 1D inversion. The inversion algorithm in EarthImager 1D resolves layer resistivity and thickness values based on a known number of layer, which stay unchanged during the inversion process.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 A spreadsheet of the starting model consists of five columns. The first column is a sequential number indicating the layer number. The second column shows the bottom depth of each layer. The depth of the bottom layer, which should be infinite, shows the maximum depth of investigation for the current data set or electrode layout. The first two columns are read-only. The third column is the layer thickness whose unit is determined by the Meter/Feet tool button on the main window. The fourth column “Resis (Ohm-m) is the resistivity value of each layer. The resistivity unit depends on the “Resistivity Unit” setting to the right and it can be Ohm-m or Ohm-feet or Ohm-cm. The last column “Fix” constrains a constant layer resistivity during inversion if a lock icon is shown. The Reset Layers button restores a previous starting model and discards any changes in the spreadsheet. Clicking the OK button will alter the starting model and this process is not reversible.

After reading the data, EarthImager 1D recommends a layered model with many layers, e.g. 12, for automatic inversion. At the end of automatic inversion, this starting model will be replaced with the inverted model. At this point, a user has following options to update the starting model. 1) Combine Layers. This will combine neighboring layers based on a user-defined logarithmic resistivity contrast threshold. The default threshold is 10%. “Combine Layers” also deletes thin layers if a neighboring layer is 20 times thicker. “Combine Layers” may be executed/clicked multiple times. There are two access points of Combine Layers: a tool button is on the tool bar and another “Combine” button on the Settings Window. 2) Manual update on the spreadsheet of Starting Model. A user may construct a new starting model manually based on the inverted resistivity model from automatic inversion and any a-priori information at the survey site. One may change the number of layers, layer thickness and resistivity values, delete any specific layer. 3) Manual update of the starting model by dragging the blue inverted model curve on the main window. This dragging update utility won’t change number of layers, but it is useful to alter layer resistivity and thickness. Then combine layers for better results. An instant forward modeling is carried out after dragging and the modeled apparent resistivity curve is updated right away. Page 17 of 36


Double-clicking a cell in the column of “Fix” would turn an unlock icon The resistivity value of a locked layer stays unchanged during inversion.

into a lock icon

.

3.5 Resistivity Inversion Settings 3.5.1 Inversion method Forward modeling or forward simulation generates a synthetic data set from a synthetic model and a user-defined electrode configuration. The damped least squares inversion method constructed a subsurface layered resistivity model from measured resistivity data.

3.5.2 Stop Criteria Resistivity data inversion is a nonlinear iterative process. This group of parameters tells when to stop the iteration. Three factors within this group have a logical OR relation, that is, if any of the criteria is met, the inversion will stop. To activate or enable a rule, check the box in front of parameter. 3.5.3 Number of iterations This sets a maximum number of iterations for an iterative nonlinear inversion stop. If the inversion diverges, it will stop after the prescribed maximum number of iterations. Thus infinite loops can be avoided. Inversion usually converges in 3 ~ 5 iterations. 3.5.4 Maximum RMS error The primary objective of inversion is to reduce data misfit between field measurements and calculated data of a reconstructed model. Measurement errors usually range from 1% to 5%. But do not forget about numerical errors and rounding errors. This number should be equal to or larger than percentage of noise estimate. 3% is a default setting.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 One of the objectives of the resistivity inversion is to find a resistivity model whose response (predicted data) best fits the measured data. The goodness of the fit may be characterized by the root mean squared (RMS) error in percent (%) defined below.

 d iPr ed − d iMeas  ∑ d iMeas i =1  N N

RMS =

2

   × 100 %,

where N is the total number of measurements, d Pred is the predicted data and d Meas is the measured data. It is clear that the RMS error depends on the number of bad data points and how bad each bad data point is. P

P

P

P

Note that the RMS error is an average data misfit over all data points. A single erroneous data point could lead to a large RMS error. A large RMS error is the result of 1) 2) 3) 4)

Data noise, Numerical modeling error, Inability to model 2D and 3D objects with a 1D modeling program, and Poor inversion settings.

3.5.5 Error reduction

It is generally expected that data misfit, e.g. RMS error, be reduced from iteration to iteration. The first several iterations have a larger RMS error reduction than later iterations. When RMS error reduction is very small or RMS error increases, the inversion should be terminated. This parameter sets a threshold for this purpose. It is recommended that the inversion stop when error reduction is smaller than 5%. Sometimes it is normal that the convergence of inversion is not monotonic. This error reduction factor may cause termination of the inversion prematurely. This is why the box in front of this setting is unchecked by default. 3.5.6 Damping factor

This factor suppresses the effect of small eigenvalues of the inversion kernel matrix at earlier iterations so that it stabilizes the inversion process. The default value is 100. 3.5.7 Depth factor

EarthImager 1D defines the depth of penetration based on the median depth values defined by Edwards (1977). The median depth ranges from 15% to 25% of the array span depending on the array type for any four-electrode array. However, these median depth values seem underestimating the depth of investigation for 1D VES. We also considered the depth suggested by Zohdy (1989) to set the layer depth values in EarthImager 1D.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 We added this depth factor so that a user can adjust the penetration depth to reflect the geoelectrical characteristics of the survey area. The penetrating depth is defined as the product of the median depth and the depth factor. 3.5.8 Data offset averaging

For a vertical electrical sounding with Schlumberger array, the potential electrode spacing is increased as the signal strength becomes very small. As a standard practice, two measurements are made at the same AB but different MN during the transition. These two apparent resistivity values differ slightly due to the different MN distance, creating an offset on the 1D apparent resistivity vs. AB/2 curve. “Data offset averaging” will reset both measurements to the average of two different apparent resistivity values.

3.6 File Management and File Extension

A resistivity survey project has a project name such as DEMO. The project name may be used as the top-level folder name for data processing. All data files for this project should be stored in this project folder. Each data file would correspond to a sounding point. Under the project folder, a subfolder will be created for each data file automatically when you open a data file. This subfolder will be named after the data file name without extension. Schlum1D, for example, will be the subfolder name for data file Schlum1D.stg. For every inversion trial, a trial subfolder is created automatically under the sounding folder with a folder name of “trial” plus a trial sequential number. The folder hierarchy will look like the figure below. In a trial folder, these files are saved with the default names. The INI file is the inversion settings file and may be read back into the EarthImager for inversion of other data sets. The OUT file is an inversion log file containing inversion settings, raw data, mesh definition, inverted results of all iterations. The STG file or DAT file contains the raw data used in the inversion and it may also be saved optionally by the user from the menu File | Save STG Data. The JPG file is an image file. The MOD file is an ASCII text file of inverted resistivity model.

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3.7 Run Inversion

One may start inversion by either clicking the green tool button or choosing the menu Inversion | Start Inversion. Two files will be created and saved automatically in the current trial folder. The INI file contains the settings for current inversion. OUT files are inversion log files with inversion settings and results for all iterations. At the end of inversion, you may use the mouse wheel to navigate through inverted results at all iterations. 1D inversion is an interactive process in which a user improves the layered resistivity model iteratively from one inversion to the next. Obtaining a small RMS error is generally not a difficult task in a 1D inversion because of nonuniqueness of 1D inversion. However, searching for a geologically-sensible resistivity model can be very challenging. At the end of one inversion, a user often changes the number of layers and layer resistivity value based on the previous inversion results, and starts a new inversion with an improved model. This process can go on and on. The Iterative Processing in EarthImager 1D automates this iterative process by 1) Starting the inversion with many layers, e.g., 12 layers. 2) Combining neighboring layers with a small resistivity contrast based on a user-specified threshold.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 3) Repeating the last two steps until no layers can be combined. 3.8 Input Graphics Title

Choose View | Graphics Settings | General Settings. Type the desired titles in the box below.

3.9 View Measured Data

This will show measured apparent resistivity data in a spreadsheet. The data shown in the spreadsheet may be saved as an ASCII text file and printed as a hard copy. Note that the column header may vary depending on the measurement array configuration. The distance unit can be changed from the control in the toolbar. The resistivity unit may be changed from a setting at the bottom of this spreadsheet.

3.10 View Inverted Resistivity Model

This option lists the inverted resistivity model in a spreadsheet. The layered model shown in the spreadsheet may be saved as an ASCII text file and printed as a hard copy.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 3.11 View Inversion Result Image Use the menu command View|View Inversion Result Image to open a dialogue where you can inspect the inversion result image. This dialog shows the PNG formatted result file from the selected inversion trial. You can navigate to any trial by clicking the ellipsis button. Here is an example screen:

3.12 Show Project Information Table

Click this tool button to show or hide a project information table placed at the lower right corner of the main window. To change the project information, go to the menu View | Project Information. You must check “Show Project Info to view the project information at the lower right corner of the screen. Please note that the text field has a fixed length, so you should keep your input short. To fit more characters in the table, you will have to reduce the font size.

There are two flavors of the project information table. The new “Custom Project Information Table” gives the user more options to define his or her own project information table. Most fields Page 23 of 36

Instruction Manual of AGI EarthImager 1D, 2014-12-08 except data file name and processing software name are editable. This would be valuable for users whose language is not English. 3.13 Copy Image to Clipboard

You may copy an on-screen image to the Windows clipboard by choosing Copy Image to Clipboard menu item from the Edit menu or from this button on the toolbar. Then the image on the clipboard may be pasted to any Windows program such as MS Word, Powerpoint, Paint, and Photo Editor. 3.14 Save Image

The onscreen image may be saved as a bitmap, JPEG or Windows metafile file at any of three or resolution levels. To save an onscreen image, you may either click Save Image tool button choose Save Image menu item from File menu. In the Save as type box at the bottom, choose your desired image file type and resolution. You have three graphics file types to choose from. A suggested image file name may look like this: Schlum1D_trial3_DataFitInvModel5.jpg. Schlum1D is the prefix of the STG file name (Schlum1D.stg). The current trial is trial 5. 3.15 Save STG Data

Choose Save STG Data from the File menu. A saved STG data file will be in the 3D (x, y, z) STG format. When one reads in a non-STG data file, this feature may be used to convert a nonSTG file into an STG file. 3.16 Print Image

or choose Print To print an onscreen image, you may either click Print Image tool button Image menu item from File men. For custom printing, choose Page Setup from File menu. You are given options for page orientation, margins, and output resolution. High resolution is the highest DPI (dots per inch) of the printer. Medium resolution is one half of printer’s DPI. Low resolution is one quarter of printer’s DPI. The printed image will be stretched to fit the settings in Page Setup, but the orientation, aspect ratio, and size may differ from the onscreen image. 3.17 Inversion with Topography

EarthImager 1D (version 1.x.x) can invert a data set collected on an uneven ground surface. You have two ways to tell EarthImager your electrode geometry. First, you may use horizontal projected distance that is equivalent to the horizontal coordinate. Second, you may use tape measure or slope distance. In this case, the distance measure in the raw data file and the command is also defined by the tape measure or slope distance. The second option is much easier to implement and used more commonly. Here is a sample terrain file (Wenner1D.trn). ; TRN File unit= Meters 2

// Mandatory header - TRN or trn is required in this line // Unit of distance measure // 1 = horizontal distance, 2 = tape measure or slope distance

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 -500, 101 -200, 100 -190, 108 -165, 110 -148, 113 -120, 105 -87, 103 -46, 98 -14, 95 0, 97 22, 100 56, 99 72, 101 133, 106 150. 102 169, 99 194, 101 234, 102 1000, 101

// X, Elevation (positive upward)

The terrain file (*.trn) is a two-column comma separated text file that may be created in any text editor such as the NotePad. To process a data with topography, follow these steps: 1) 2) 3) 4) 5)

Read Data. Read a terrain file from the menu File | Read Terrain File. Set a starting model. Start inversion or Start Iterative Processing. Repeat Steps 3 and 4 for a geologically-sensible model.

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4. THEORY OF INVERSION 4.1 What Is Inversion?

Resistivity surveys measure injected current (I) through transmitting electrodes and potential difference (voltage V) between two receiving electrodes. Measured current and voltage together with electrode geometry (K) may be converted into apparent resistivity (ρ a ). Normalized voltage by current (V/I) and apparent resistivity ρ a are data in the inversion. V/I and ρ a data are equivalent quantities that can be transformed back and forth with the help of a geometric factor K. B

B

B

B

B

B

The goal of resistivity survey is to image a subsurface resistivity distribution which is closely correlated with subsurface geology. The subsurface resistivity distribution (or its reciprocal electrical conductivity) is the model parameter in the inversion. The model is the partial differential equation that governs the relationship between data and model parameters. Forward modeling is defined as the process of predicting the data on the basis of the known distribution of model parameter, electrode configuration and model. It is a mapping from the model space to the data space. Forward modeling creates synthetic data sets. Forward modeling is also known as forward simulation, forward problem, and forward solution. Inversion is defined as the process of determining the estimates of the model parameter on the basis of the data and the model. Inversion is a mapping from data space to model space, and it reconstructs the subsurface resistivity distribution from measured voltage and current data. Inversion is also known as inverse modeling, inverse simulation, and inverse problem. The resistivity data inversion proceeds as follows. 1) A starting resistivity model is constructed based on either the average apparent resistivity, or apparent resistivity distribution, or user assumption, or a-priori knowledge of subsurface resistivity distribution. 2) A virtual survey (forward modeling) is carried out for a predicted data set over the starting model. The initial root mean squared (RMS) error at the zero-th iteration may be calculated at this step. 3) Solve a linearized inverse problem based on the current model and data misfit for a model update (∆m). 4) Update the resistivity model using a formula like this: m i+1 = m i + ∆m. The model parameter m consists of electrical conductivity of all model blocks in the finite difference or finite element mesh. The symbol i is the iteration number. 5) Run a forward modeling (virtual survey) based on the updated model for an updated predicted data set. 6) Calculate a new RMS error between the predicted data and the measured data. B

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B

B

B

Instruction Manual of AGI EarthImager 1D, 2014-12-08 7) If any of inversion stop criteria is satisfied, stop the inversion. Otherwise, repeat the Steps (3) to (7). For a thorough understanding of inversion theory, the user is encouraged to go through the suggested readings at the end of this manual. 4.2 Forward Modeling

EarthImager 1D (version 1.x.x) is a simplified version of EarthImager 2D for 1D layered earth models. We are dealing with a 2.5D or 2D/3D problem here, that is, a 2D earth model but a 3D electrical field due to a point source. It has become a standard practice to solve 2.5D resistivity forward modeling problem using numerical methods by discretization of the domain of investigation. The governing 3D partial differential equation is Fourier-transformed into a 2D equation to reduce computing time. The forward solution can be obtained by solving the 2D partial differential equation in the Fourier transform domain:

∂  ∂V  ∂  ∂V  2 σ  + σ  − k σV = − I ⋅ δ ( x) ⋅ δ ( z ) , ∂x  ∂x  ∂z  ∂z  where V is the scalar electrical potential in the Fourier transform domain, and I is the electric current source. k is the wave-number in the transform domain. σ is electrical conductivity as a function of (x, z). We implemented both finite difference and finite element methods. The newer version 2.x.x of EarthImager 1D conducts its forward modeling through an analytical solution by digital filtering. 4.3 Damped Least Squares Inversion

For a nonlinear inverse problem, the data vector, d, is a nonlinear function of the model parameter vector, m, that is, d = g(m). The objective of least squares inversion is to minimize a weighted data misfit,

S(m) = (d obs − g (m)) T Wd (d obs − g (m)) d obs is the observed data, g(m) is the calculated data. W d is a data weighting matrix. The nonlinear inverse problem is solved iteratively. At each iteration, a model update vector ∆m is obtained by solving the following linearized system of equations. B

(J

B

T

B

)

Wd J + λI ∆m = J T Wd ⋅ (d obs − g(m) ) ,

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B

Instruction Manual of AGI EarthImager 1D, 2014-12-08 ∂g(m) is Jacobian (sensitivity) matrix. λ is a damping factor whose effect is to ensure ∂m that the inversion resolves primary features at the early iterations. Our damped least squares inversion algorithm was adapted from Rijo et al. (1977).

where J =

4.4 Smooth Model Inversion The resistivity inverse problem is inherently nonunique. Additional constraints, or regularization, must be imposed on the model to single out one optimal solution. The smooth model inversion, also known as Occam’s inversion, finds the smoothest possible model whose response fits the data to an a-priori Chi-squared statistic. The true model must be at least as, but never less complex than, the smooth model obtained through smooth model inversion. Smooth model inversion is based on the assumption of Gaussian distribution of data errors. The objective function of smooth model inversion is given by S(m) = (d obs − g (m)) T Wd (d obs − g (m)) + α ⋅ m T Rm α is a Lagrange multiplier and a stabilizing factor. It determines the amount of model roughness imposed on the model during the inversion. R is a roughness operator. The smooth model inversion algorithm is not available anymore in EarthImager 1D.

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5. TOOLS 5.1 Forward Simulation This is a forward modeling tool which generates a synthetic data set from a layered resistivity model built by the user. Follow these steps to run Forward Simulation. 1) Read Command File from the menu File. 2) Build a layered resistivity model from the menu Settings | Starting Model 3) Start the Forward Simulation from the menu Tools. The synthetic model (*.mod) and data (*.stg) are saved in the current trial folder.

5.2 Combine 1D Raw Data Files This utility helps combine 1D raw data files to form a 2D data set for 2D inversion in EarthImager 2D. The inversion result of a combined 2D data set in this way will be probably disappointing due to limited data coverage. A 2D resistivity imaging survey is recommended for a high resolution 2D cross-section image. Raw data files may be in different formats such as STG, URF and DAT. The combined 2D data set will be in the AGI STG format only. Here are the procedures to combine multiple 1D sounding data files to form a 2D data set. 1) Start EarthImager 1D and choose the menu Tools | Combine 1D Raw Data Files. 2) Click the button Add Files to add raw data files. One may add one or more files at a time and add files multiple times. One may also add files of different formats (STG, DAT) by Page 29 of 36


3) 4) 5) 6)

selecting different file extension. On the screenshot below, two STG files (Test1.stg and Schlum1D.stg) were added first. Wenner1D.dat was added later by choosing a new file extension DAT. All raw data files may have the same sounding location (Mid-X) in the files. To combine them together, we must transform them to the same global coordinate system by reassigning a new Mid-X to each sounding. Choose a distance unit for Mid-X which is the mid point of 1D sounding. However, the combined 2D data file will always use the meter unit. Input Mid-X coordinates for each 1D sounding. Combine all raw data files together to form a 2D data set by clicking the Combine button.

5.3 Combine 1D Inverted Resistivity Models This utility combines 1D inverted resistivity model files together to form a data set which may be used to generate a 2D cross section image in a graphics program such as Surfer by Golden Software. The combined file will have three columns in X, depth and resistivity value format. At the end of 1D inversion with EarthImager 1D, an inverted resistivity model file (*.mod) is saved in the current trial folder. A model file consists of three columns. The first column is a layer number, the second column is the bottom depth of each layer, and the last column is the resistivity value of a layered earth. A mid depth of each layer is calculated and saved in a combined file. The coordinate of the mid point of each sounding is typed in by the user. Here are the procedures to combine 1D inverted resistivity models. 1) Start EarthImager 1D and choose the menu Tools | Combine 1D Inverted Resistivity Models. 2) Click the button Add Files to add model files (*.mod). One may add one or more model files at a time and add files multiple times from different folders.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 3) All model files have no information about the sounding location, i.e., mid point of sounding (Mid-X). To combine model files together, one must input these Mid-X coordinates manually at the Step (5) below. 4) Choose a distance unit for Mid-X which is the mid point of 1D sounding. The Mid-X coordinate and mid depth of each layer in a combined 2D file will bear the distance unit specified here. 5) Input Mid-X coordinates for each 1D sounding. 6) Combine all model files to form a single data set by clicking the Combine button. The combined file will have three columns. The first column is the x coordinate of a sounding location, the second column is the mid depth of each layer and the last column is the resistivity value of each layer.

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6. MULTILINGUAL SUPPORT 6.1 Introduction We added multilingual support to our data processing software such as AGI SuperSting Administrator, EarthImager 1D, EarthImager 2D and EarthImager 3D. To change the language of labels and captions on EarthImager GUI, choose your language option from the Language menu if available. However, if your language is not supported, you can translate EarthImager GUI into your own language by yourself. To add support for a new language, all you need to do is to translate the language file ENGLISH.LNG into your language.

A “Language” menu was added on the main menu of EarthImager. Supported languages are listed under the language menu. In the screenshots above, both English and Chinese are

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 supported as well as 7 other languages. More languages will be adde. The language files are installed, by default, in the folder: C:\Programs\AGI\EarthImager1D\lang. All supported languages would have a language file listed in this folder. To display EarthImager user interface in a different language using non-Latin characters, one needs either a local-language version of Windows or appropriate setups for the local language. These settings are located in Control Panels | Regional and Language Options. Two screen shots below show the English-version Windows setups for display of the EarthImager user interface in Chinese on a Windows XP computer. On other Windows versions the dialogs may look differently.

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Instruction Manual of AGI EarthImager 1D, 2014-12-08 6.2 Add your own language to EarthImager To turn EarthImager GUI in your language, simply make a copy of ENGLISH.LNG as MYLANGUAGE.LNG, for example, FRENCH.LNG, and translate the key words in the new language file into your language. Your language will be automatically added to EarthImager without any change inside EarthImager. Please do not delete or modify the English language file ENGLISH.LNG. Here are first few lines of ENGLISH.LNG: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)

[Info] Language=English [Font] Name=MS Sans Serif Charset=1 [TfrmMain] Caption="AGI EarthImager 1D" tbSettings.Caption="Settings Window" tbSettings.Hint="Settings Window" tbPrintImage.Caption="Print On-screen Images"

The line numbers from 1 to 10 are not part of the language file and they are added for illustration purpose only. On the second line Language=English, change the word “English” into your language name. This name will be the menu item name under the Language menu. On Line 4, choose a font name of your native language. Finally, translate all quoted text on the right hand side of “=” sign from Line 7. That is, you should translate "AGI EarthImager 1D", "Settings Window", etc. into your own language. The double quotation signs should be kept and your translation should be kept inside the double quotations. The ending quote MUST be preserved! Here is a sample translated Chinese language file (CHINESE.LNG): [Info] Language=Chinese [Font] Name=MS SONG Charset=1 [TfrmMain] Caption="AGI EarthImager 1D" tbSettings.Caption="反演参数" tbSettings.Hint="反演参数" tbPrintImage.Caption="打印图像"

Once you complete the translation of English language file into your language, please email us a copy of your new language file ([email protected]). If you find any inappropriate translation in any language file, please inform us or email us the corrected language file.

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7. HELP 7.1 Instruction Manual in PDF This menu item brings up a PDF help file in Acrobat Reader. 7.2 AGI Home Page This will lead to the website of Advanced Geosciences, Inc. at https://www.agiusa.com/. 7.3 EarthImager User Group This is an AGI EarthImager support page with software updates for users to download. The website address is https://www.agiusa.com/files/eicust/earthimageruser.shtml . Refer to the Section 1.2 for more detail. HTU

UTH

7.4 Upgrade The menu item Help | Upgrade brings up a dialog box for user to enter an upgrade license code. An EarthImager upgrade means to add one or more special-purpose software modules to EarthImager and AGI charges an upgrade fee to add a new module. Refer to the Section 2.4 for more detail. The license code below is invalid and it is for illustration only.

7.5 About EarthImager 1D This will display a message box showing program name and copyright information. The AGI logo is also a link to the AGI website. 7.6 EarthImager 1D Release Notes This menu item under the Help menu brings up the EarthImager 1D release notes to the default internet browser.

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SUGGESTED READINGS Constable, S., Parker, R.L., and Constable, C.G., 1987, Occam’s inversion: A practical algorithm for generating smooth models from electromagnetic sounding data, Geophysics, 52, 289300. DeGroot-Hedlin, D. and Constable, S., 1990, Occam’s inversion to generate smooth, twodimensional models from magnetotelluric data, Geophysics, 55, 1613-1624. Dey, A., Morrison, H.F., 1979, Resistivity modeling for arbitrarily shaped two-dimensional structures, Geophysical Prospecting, 27, 106-136. Edwards, L.S., 1977, A modified pseudosection for resistivity and induced polarization: Geophysics, 42, 1020-1036. Golub G.H., and Van Loan, C.F., 1996, Matrix computation, Johns Hopkins. Huebner, K.H., and Thornton, E.A., 1995, The finite element method for engineers, John Wiley and Sons. Lines, L.R., and Treitel, S., 1984, A review of least squares inversion and its applications to geophysical problems, Geophysical Prospecting, 32, 159-186. Golub G.H., and Van Loan C.F., 1996, Matrix computations, third edition, John Hopkins University Press. Menke, W., 1989, Geophysical data analysis: Discrete inverse theory, Academic Press. Rijo, L., Pelton, W.H., Feitosa, E.C., and Ward, S.H., 1977, Interpretation of apparent resistivity data from Apodi Valley, Rio Grande Do Norte, Brazil, Geophysics, 42, 11, 811-822. Tarantola, A., 1987, Inverse problem theory: Methods for data fitting and model parameter estimation, Elsevier. Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990, Applied geophysics, 2 nd Edition, Cambridge University Press. Tikhonov, A.N., and Arsenin, V.Y., 1977, Solutions of ill-posed problems, John Wiley & Sons, New York. Ward, S.H. (Editor), 1990, Geotechnical and environmental geophysics, Volumes I, II, and III, Investigations In Geophysics, No. 5, Society of Exploration Geophysicists. Zohdy, A.A.R., 1998, A new method for the automatic interpretation of Schlumberger and Wenner sounding curves, Geophysics, 245-253. P

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