RES3DINV ver. 2.14
for Windows 98/Me/2000/NT/XP
Rapid 3D Resistivity & IP inversion
using the least-squares method
(For 3-D surveys using the pole-pole,
pole-dipole, dipole-dipole, rectangular, Wenner,
Wenner-Schlumberger and non-conventional arrays)
On land, underwater and borehole surveys
Geoelectrical Imaging 2D & 3D
GEOTOMO SOFTWARE
August 2004
This software is distributed by :
Geotomo Software,
5 Cangkat Minden Lorong 6,
Minden Heights, 11700 Gelugor,
Penang,
MALAYSIA
Tel : +60 4 6574525, Fax : +60 4 6588437
email : geotomo@tm.net.my
Internet : www.geoelectrical.com
Table of Contents
Topic Page No.
1 Introduction 1
2 Computer system requirements 2
3 Copy protection 4
4 Theory 5
5 Disk contents 7
6 Using the program 8
7 File operations 9
8 Change program settings 11
9 Inversion of data 17
10 Display results 19
11 Topography 24
12 Help 24
13 Some field examples 24
14 Trouble shooting 30
Acknowledgements 30
References 31
Appendix A : 3D surveys with the pole-pole array 33
Appendix B : 3D surveys with the pole-dipole
array 37
Appendix C : 3D surveys with the dipole-dipole
array 39
Appendix D : Fixing Resistivities 40
Appendix E : IP data inversion 42
Appendix F : Speed of graphics display 45
Appendix G : Non-uniform survey grids 46
Appendix H : The equatorial dipole-dipole
(rectangular) array 48
Appendix I : Export model to Rockworks and
Slicer/Dicer format 49
Appendix J : Disabling computer power
management and screen saver 52
Appendix K : Inversion of large data sets,
hard-disk file format and
recommended PC systems 53
Appendix L : Non-conventional or general arrays 55
Appendix M : Using more than 1 GB RAM 56
Appendix N : 3-D underwater surveys 59
Appendix O : Cross-borehole data inversion 62
Appendix P : Non-rectangular (trapezoidal)
survey grids 66
Appendix Q : Special topography information
for mineral exploration surveys 69
Disclaimer 70
Support 70
Notice of Copyright
RES3DINV is copyrighted by (C) Geotomo Software, 2000-2004. All rights
reserved. No part of this document may be reproduced without written
permission of Geotomo Software.
Copyright (2000-2004) Geotomo Software
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1 Introduction
RES3DINV is a computer program which will automatically determine a threedimensional
(3-D) resistivity model for the subsurface using the data obtained
from a 3-D electrical imaging survey (Li and Oldenburg 1992, White at al.
2001). The electrodes for such a survey are arranged in a rectangular grid
(Figure 1). It should first be emphasised that full 3D surveys are not merely a
series of 2D surveys, but a different approach has to be used. More details on
the practical use of the pole-pole, pole-dipole and dipole-dipole arrays for 3D
surveys are given in Appendices A, B and C, as well as in the free Tutorial
Notes on electrical imaging (Loke 2002). The most common arrays used for 3D
surveys are the pole-pole, pole-dipole and dipole-dipole arrays. Other arrays
do not provide sufficient data coverage for a full 3-D inversion. On a computer
with 1.5 GB RAM, this program can handle survey grids with up to 77 by 77
(or 5929) electrodes!
Figure 1. A schematic diagram for one possible layout for a 3-D survey.
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2 Computer system requirements
RES3DINV is a 32-bit program that will run under Windows 95/98/Me/NT/
2000/XP. You will need a Pentium (Pro, II, II, 4) or compatible based
microcomputer to run this program. The minimum amount of RAM required is
64 MB, but 128 MB or higher is recommended. You will also need about 200
MB of free hard-disk space which the program can use to store temporary swap
files. If you have more than one hard-disk drive, the program will
automatically select the drive with the largest amount of free space as the drive
to store the swap files. This is a 32-bit Windows program that can access up to
4 gigabytes of memory. The more RAM and free hard-disk space you have, the
larger is the data set that you can process with this program.
It is also recommended that you use the 800 by 600 (for 14 and 15 inch
monitors), or the 1024 by 768 (for 17 inch monitors), or the 1280 by 1024 (for
21 inch monitors) with 256 colours SVGA graphics mode. If you are using the
SuperVGA modes with 16-bit or 24-bit colours, you should switch the display
mode to 256 (i.e. 8-bit) colours using the Display option in the Control Panel
folder in Windows 95/98/Me/NT/2000/XP if you encounter problems.
Graphics operations with the 256 colours mode will be faster, and more
memory will be available to your programs. If the graphics appear to be rather
slow on your computer, please refer to Appendix F.
There should not be any background programs running (such as word
processors, CAD programs) when executing the RES3DINV program. Please
shut down the other programs to free the memory for this program. This will
reduce memory to hard disc swapping which slows down the program. In
Windows 95/98/Me/NT/2000/XP, the active programs are usually listed on the
Start bar at the bottom of the screen.
The RES3DINV package comes in a single compressed installation file
SETUP.EXE. It is a Windows based installation program that will install the
program files. In Windows 95/98/Me/NT/2000/XP, click Start, and then
Settings followed by Control Panel. Then click the icon to Add/Remove
programs. After that, navigate to the subdirectory where the SETUP.EXE file
is located.
In Windows NT/2000/XP, it is necessary to install a system driver for the
dongle.
a). For users with the Rainbow Technologies Sentinel SuperPro key
Run the RainbowSSD5.39.2.exe file in the RAINBOW_SYSTEM_DRIVER
subdirectory in the CDROM. This program will automatically install the
system driver for the dongle. If you are using a Toshiba Laptop with the Win
XP operating system, there are special instructions together with a patch file
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Sentinel.sys in the Toshiba_XP.zip file. After installing the driver, restart the
computer for the driver to take effect. If you are using Windows 95/98/Me, it is
normally not necessary to install the system driver. However, if the program is
unable to detect the dongle, installing the appropriate driver might help in
situations with a non-standard configuration for the printer port. In case of
problems, use the SuperProMedic.exe program to check the condition of the
dongle. More information about the system drivers can be obtained at the
following web site : www.rainbow.com.
b). For users with the Keylok key
Please run the INSTALL.EXE program in the found in the subdirectory for the
Keylok system drivers. This program will install the appropriate driver for your
operating system into your computer hard-disk. It is usually not necessary to
run the INSTALL.EXE program for Win 95/98/Me, but if the RES3DINV
consistently fails to detect the dongle, try running the INSTALL.EXE program
to install the driver for Win 95/98/Me. More information about the system
drivers can be obtained at the following web site : www.keylok.com
If you are using Windows 95/98/Me, it is normally not necessary to install the
system driver. However, if the program is unable to detect the dongle,
installing the appropriate driver might help in situations with a non-standard
configuration for the printer port.
In Windows 95/98/Me/NT/2000/XP, a Recycle Bin program is sometimes used
to save recently deleted files. This feature might prove to be a nuisance when
inverting very large data sets as the program uses a number of temporary files.
Although the program deletes the files, they might end up in the Recycle Bin,
and quickly fill up your hard disc! The RES3DINV program will, by default,
use the hard disc partition with the largest amount of free space as the buffer
drive. It is recommended that you disable the Recycle Bin for this drive. This
can be done by clicking the Recycle Bin icon with the right mouse button, and
then click the Properties option. You can then configure the settings for each
hard disc drive.
For both operating systems it is recommended that you remove the CDROM
disc, if present, from the CD-ROM drive when using this program. Due
to the multi-tasking nature of these systems, sometimes the program on the disc
might be launched while RES3DINV is busy inverting a large data set!
It is also recommended that you disable the computer power management
functions (in Windows as well as in the system CMOS setting) and the
Windows Screen Saver program (in the Display Settings function within the
Control Panel). Please refer to Appendix J for more details.
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For all versions of Windows, it is recommended that you run the SCANDISK
program occasionally to check the condition of the hard disc, and also the
DEFRAG program to reduce file fragmentation.
Since RES3DINV is a Windows based program, all Windows compatible
graphics cards or printers are automatically supported. It has been tested with
video screen modes of up to 1600 by 1200 pixels and 256 colours.
3 Copy protection
The program uses a hardware key (dongle) for copy protection. Without the
dongle, you can use the program to invert data sets with up to 50 electrodes.
There is a limit of 3 iterations in the inversion routine. If you had purchase the
2D resistivity/IP inversion program, RES2DINV, you can process data sets
with up to 100 electrodes (with a limit of 5 iterations) using the dongle
provided. In both cases, topographic modelling cannot be carried out. With the
RES3DINV dongle, both restrictions are removed and the maximum number of
electrodes the program allows is 1849. This corresponds to a 43 by 43
electrodes survey grid that is much larger than those used in most field surveys.
The program should be able to detect the dongle if the parallel printer port
follows the IBM PC specifications. If it is unable to find the dongle, the printer
port is not completely IBM PC compatible. In this case, after connecting the
dongle to the computer, connect a printer to the other end of the dongle. Turn
on the printer and print out something, for example your AUTOEXEC.BAT
file, to confirm that the printer port is working. Leave the printer on, and then
try to run the RES3DINV.EXE program again. If the program is able to detect
the dongle, you will need a special adapter from Rainbow Technologies to use
the dongle without a printer (particularly for older laptop computers). If the
computer is a desktop model, you could change the adapter card with the
printer port, or set up a second printer port with another card. If you are still not
able to solve the problem, please contact the Rainbow Technologies or the
Keylok companies by email.
If you are using Windows NT/2000/XP, it is necessary to install a system
driver for the dongle (please refer to the previous section). A more common
problem is that the printer port was assigned to another device, such as a
scanner port, and not as LPT1 that is the default printer port. In this case, you
will need to reassign the parallel port as a printer port. In Windows
95/98/Me/NT/2000/XP, you have to select the Settings option on the Start
menu bar to access the Control Panel or the Printers folder.
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4 Theory
The inversion routine used by the program is based on the smoothnessconstrained
least-squares method (deGroot-Hedlin and Constable 1990, Sasaki
1992). A new implementation of the least-squares method based on a quasiNewton
optimisation technique (Loke and Barker 1996) can also used. This
technique can be more than 10 times faster than the conventional least-squares
method for large data sets and requires less memory. The smoothnessconstrained
least-squares method is based on the following equation
(JTJ + uF)d = JTg (1)
where F = fxfx
T + fzfz
T
fx = horizontal flatness filter
fz = vertical flatness filter
J = matrix of partial derivatives
JT = transpose of J
u = damping factor
d = model perturbation vector
g = discrepancy vector
One advantage of this method is that the damping factor and flatness filters can
be adjusted to suit different types of data.
In this program, you can also use the conventional Gauss-Newton method that
recalculates the Jacobian matrix of partial derivatives after every iteration
(Loke and Dahlin 2002). It is much slower than the quasi-Newton method, but
in areas with large resistivity contrasts of greater than 10:1, it gives slightly
results. A third option in this program is to use the Gauss-Newton method for
the first 2 or 3 iterations, after which the quasi-Newton method is used. In
many cases, this provides the best compromise.
Besides the above smoothness-constrain method, other types of inversion
methods are also available. The user can choose a method that will apply the
smoothness-constrain directly on the model resistivity values. This method will
produce an optimally smooth model (see section 8). On the other extreme, a
robust model inversion method is also available which tends to produce models
with sharp boundaries. The choice of inversion method to use should be guided
by the nature of the known geology of the survey area.
The inversion program divides the subsurface into a number of small
rectangular prisms, and attempts to determine the resistivity values of the
prisms so as to minimise the difference between the calculated and observed
apparent resistivity values. One possible arrangement used by Loke and Barker
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(1996) is shown in Figure 2a. Here, each block in the top layer has an electrode
in each corner. Besides this basic arrangement, two other arrangements are
supported by the program. One alternative is to divide the blocks in the top few
layers by half in the horizontal directions only (Figure 2c). The third alternative
is to divide the layers by half in the vertical direction as well (Figure 2b). Since
the resolution of the resistivity method decreases rapidly with depth, it has
been found that subdividing the blocks is only beneficial for the top two layers
only. In many cases, subdividing the top layer only is enough. By subdividing
the blocks, the number of model parameters and thus the computer time
required to invert the data set can increase dramatically.
Figure 2. The models used in the inversion. (a) Standard model where the
widths of the rectangular blocks are equal to the unit electrode spacings in the
x- and y-directions. (b) A model where the top few layers are divided by half,
both vertically and horizontally, to provide better resolution. (c) A model
where the model blocks are divided in the horizontal directions but not in the
vertical direction.
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The optimisation method tries to reduce the difference between the calculated
and measured apparent resistivity values by adjusting the resistivity of the
model blocks. A measure of this difference is given by the root-mean-squared
(RMS) error. However the model with the lowest possible RMS error can
sometimes show large and unrealistic variations in the model resistivity values
and might not always be the "best" model from a geological perspective. In
general the most prudent approach is to choose the model at the iteration after
which the RMS error does not change significantly. This usually occurs
between the 4th and 5th iterations.
5 Disk contents
The package containing the program should have the following files :
RES3DINV.EXE Main inversion program
ROOTS7.DAT A small field data set with a 7 by 7 survey grid.
SEPTIC.DAT A field data set from a survey over a septic tank
using an 8 by 7 grid.
PIPE3D.DAT Data from a field survey over an underground
pipe with an 8 by 9 grid.
GRID7X7.DAT A synthetic data set with a 7 by 7 grid with all the
possible measurements. (Pole-pole array).
BLK15PPL.DAT A synthetic data set with a 15 by 15 grid.
(Pole-pole array).
BLK15PDP.DAT A synthetic data set with a 15 by 15 grid.
(Pole-dipole array).
BLK15DDP.DAT A synthetic data set with a 15 by 15 grid.
(Dipole-dipole array).
MODEL3IP.DAT An example data file with resistivity and IP data.
GRID8X8.DAT A small synthetic data set with an 8 by 8 grid.
(Pole-pole array).
MOD3DFIX.DAT An example data file with where the resistivity of
a section of the model is fixed by the user.
BLOCK26W.DAT Example data set for the Wenner array
BLOCK26S.DAT Example data set for the Wenner-Schlumberger
array
BLOCK11T.DAT Example input data file with topography.
RES3DINV.HLP Windows help file for the program.
RES3DINV.CNT Support file for help file.
Depending on the type of field equipment you are using, the manufacturer of
the equipment will probably also supply you with supplementary programs and
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example files to convert the raw data files obtained with the instrument into the
format required by this program.
6 Using the program
After installing the program, just click the RES3DINV icon to start the
program. The program will first check the computer system to ensure that it has
the necessary resources that this program requires. It will check for the
available memory and hard-disk space. If the program displays a warning, you
should quit from the program and make the necessary changes. After checking
the computer configuration, the program will then display the following Main
Menu bar near the top of the screen.
You can select an option by clicking it with the mouse cursor. If this is the first
time you are using the program, try reading in one of the smaller files such as
ROOTS7.DAT provided with the program. Click the File choice, followed by
the “Read in data file” submenu choice. After that, if necessary, navigate to the
subdirectory where the RES3DINV program is installed. After reading in the
file, go to the “Inversion” option, and then choose the “Carry out inversion”
option. The inversion subroutine will start. Wait for a few minutes for the data
to be inverted. If you need to stop the inversion routine at any time, just press
the “Q” key and wait for a short while. By default, the program will carry out 6
iterations that can be increased if necessary.
After the inversion process has been completed, click the “Display” and then
the “Display inversion model” suboption. You will then be asked to select the
iteration number, type of model slice (horizontal or vertical) and type of
contour intervals. After you have made the appropriate choices, the program
will then display the model slices.
The following sections will describe the different menu options in more detail.
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7 File operations
When you select the “File” option, the following menu choices are displayed.
Read data file - When you select this option, the list of files in the current
directory which has an extension of DAT will be displayed. It is assumes that
the files follow the format required by this program. If not, you have to convert
the raw data files using the conversion program for the particular resistivity
meter system. You can use the mouse or keyboard to select the appropriate
file, or to change the subdirectory.
The apparent resistivity values must first be typed into a text file. You can use
any general purpose text editor, such as the Windows NOTEPAD program.
The data are arranged in an ASCII delimited manner where a comma or blank
space or LF/CR is used to separate different numerical data items. The program
requires the data to be arranged in a certain format. If there is a problem in
running this program, one possible cause is that the input data were arranged in
a wrong format. The format of the input data file is as follows using the
ROOTS7.DAT file as an example :ROOTS7.DAT
file Comments
garden-square-0.5m | Title
7 | x grid size
7 | y grid size
0.5 | x unit electrode spacing
0.5 | y unit electrode spacing
2 | array type, enter 2 for pole-pole
467 | Number of datum points
0.00 0.00 0.50 0.00 350.46 | For each datum point, enter
0.00 0.00 1.00 0.00 398.05 | x- and y- location of current elec., x- and y-
0.00 0.00 1.50 0.00 424.08 | location of potential electrode, apparent
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0.00 0.00 2.00 0.00 413.83 | resistivity value
0.00 0.00 2.50 0.00 373.76 |
. | Repeat for all datum points
. |
0 | Followed by a few 0's
0
0
0
0
In the local coordinate system used, the top-left electrode is located at (0.0,0.0).
For the pole-dipole array, the x- and y- locations of the C1 electrode, followed
by the x- and y- locations of the P1 electrode and then similar data for the P2
electrode are given. The file BLK15PDP.DAT is an example data file with the
pole-dipole array. Note that the array type number for this array is 6.
For the dipole-dipole array, the x- and y- locations of the C1 electrode,
followed by the x- and y- locations of the C2 electrode, followed by the x- and
y- locations of the P1 electrode and then similar information for the P2
electrode are given. The file BLK15DDP.DAT is an example data file with the
dipole-dipole array. Note that the array type number for this array is 3.
If the survey data has topography, the elevation of each electrode in the grid is
entered in the data file after the data for the apparent resistivity measurements.
The data file BLOCK11T.DAT is an example with topography which has an
11 by 11 grid of electrodes. The bottom part of the file with comments are
given below.
BLOCK11T.PPL file Comments
8.000 10.000 10.000 10.000 113.3131 | Last two resistivity
9.000 10.000 10.000 10.000 102.7262 | datum points
Topography | Word indicates topography present
2 | Type of x- and y- coordinate
0.00 0.00 0.00 -0.50 -1.00 -1.50 -1.00 -0.50 0.00 0.00 0.00 | Height of electrodes in
0.00 0.00 0.00 -0.50 -1.00 -1.50 -1.00 -0.50 0.00 0.00 0.00 | arranged row by row.
0.00 0.00 0.00 -0.50 -1.00 -1.50 -1.00 -0.50 0.00 0.00 0.00 |
0.00 0.00 0.00 -0.50 -1.00 -1.50 -1.00 -0.50 0.00 0.00 0.00 |
0.00 0.00 0.00 -0.50 -1.00 -1.50 -1.00 -0.50 0.00 0.00 0.00 |
The word “Topography” must be entered in the file immediately after the last
apparent resistivity datum point. Next, the value “2” in the above example
indicates that the x- and y- coordinates given earlier are distances along the
ground surface and not the true horizontal distances. This is probably the
normal case where a cable with a constant separation between the takeouts is
used. If a value of “1” is given, it indicates that the x- and y- coordinates given
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earlier are the true horizontal distances. Next the height of each electrode in the
gird is given, row by row in the x-direction. For example, the values in the first
row are the heights of the electrodes in the line at x equals to 0 metres. In this
example, since there are 11 electrodes in the x-direction, there are 11 values in
each row. This is followed by the heights for the second row of electrodes, and
similarly for all the rows.
If the data set contains topography, the inversion subroutine will automatically
incorporate the topography into the inversion model by using the finite-element
method (Silvester and Ferrari 1990).
Edit data file - This option will call up a text editor which you can use to edit
the data file.
Import data in .....format : This program also allows you to run other programs
to convert data arranged in a propriety format into the format required by this
program. This program is usually supplied by the manufacturer of the
resistivity meter system you are using.
8 Change program settings
The program has a set of predefined settings for the damping factors and other
variables that generally gives satisfactory results for most data sets. However,
in some situations, you might get better results by modifying the parameters
which control the inversion process. When you select this option, the following
menu is displayed.
When you select the "Inversion parameters" suboption, the following dialog
box is displayed. To change a particular setting, move the mouse cursor to the
appropriate box and then just click it. Where appropriate, type in the required
information into the box.
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Initial Damping Factor and Minimum Damping Factor- In this section, you can
set the initial value for the damping factor in equation (1), as well as the
minimum damping factor. If the data set is very noisy, you should use a
relatively larger damping factor (for example 0.3). If the data set is less noisy,
use a smaller initial damping factor (for example 0.1). The inversion subroutine
will generally reduce the damping factor in equation (1) after each iteration.
However, a minimum limit for the damping factor must be set to stabilise the
inversion process. The minimum value should usually set to about one-fifth to
one-fifteenth of the value of the initial damping factor.
Directly smooth model resistivity - The least-squares formulation used in
equation (1) applies a smoothness constraint on the model perturbation vector
d only, and not directly on the model resistivity values. In most cases, it will
produce a model with a reasonably smooth variation in the resistivity values. In
some cases, particularly for very noisy data sets, better results might be
obtained by applying a smoothness constraint on the model resistivity values as
well. The resulting least-squares equation is given by
(JTJ + uF)d = JTg - uFr (2)
where r is a vector containing the logarithm of the model resistivity values.
While for the same damping factors this will usually produce a model with a
larger apparent resistivity RMS error, this modification will ensure that the
resulting model shows a smooth variation in the resistivity values.
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Line search on every iteration - The inversion routine determines the change in
the model parameters by solving equation (1). Normally the parameter change
vector d will result in a model with a lower RMS error. In the event that the
RMS error increases, you have two options. This option enables you to perform
a line search using quartic interpolation (Fletcher 1987) to find the optimum
step size for the change in the resistivity of the model blocks. This will usually
give the optimum step size, but will require at least one forward modelling
computation per iteration. In some cases, the extra forward computation could
be worthwhile if it reduces the number of iterations needed to bring the RMS
error down to an acceptable level.
This setting will only affect the inversion process for the third iteration
onwards. For the first two iterations, where the largest change in the RMS error
usually occurs, the program will always carry out a line search in an attempt to
find the optimum step size to further reduce the RMS error.
Minimum % RMS change for line search - The line search method used can
estimate the expected change in the apparent resistivity RMS error. If the
expected change in the RMS error is too small, it might not be worthwhile to
proceed with the line search to determine the optimum step size for the model
parameter change vector. Normally a value between 0.1 and 1.0 % is used.
Thickness of first layer - This give the ratio of the thickness of the first model
layer to the smallest unit electrode spacing. For the pole-pole array, this is set
at 0.70 times the unit electrode spacing. For other arrays, the thickness of the
first layer will be adjusted according to the depth of investigation of the array.
Factor to increase layer thickness - Since the resolution of the resistivity
method decreases with depth, normally the thickness of the model layers are
also increased with depth. By default, a factor of 1.15 is used to increase the
thickness of the model layers. In this case, the thickness of each subsequent
layer increases by 15%. However, you can modify this factor within a limited
range. Normally, a value of between 1.05 to 1.25 is used.
Half Size Layer Option - This allows you to use a model where the widths and
thickness of the top few layers are divided by half (see Figure 2 in the ‘Theory’
section). Here you can choose the number of layers which are to be subdivided,
and also whether to subdivide them in the vertical direction as well (Figure 2b).
You can also adjust the flatness filter damping factor for these layers. It has
been found that the smaller the model blocks, the greater is the potential
oscillation in the model values after inversion, especially for noisy data sets. To
reduce the oscillations, you can use a slightly larger damping factor, for
example 1.10, for these layers.
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Number of iterations - This allows the user to set the maximum number of
iterations for the inversion routine. By default the maximum number of
iterations is set to 6. For most data sets, this is probably sufficient. When the
inversion routine reaches this maximum limit, it will ask the user for the
number of additional iterations if you wish to continue with the inversion
process. It is usually not necessary to use more than 10 iterations.
Number of iterations to recalculate Jacobian - In the Gauss-Newton method,
the Jacobian matrix is recalculated after each iteration, whereas in the quasiNewton
method the Jacobian matrix is never recalculated but estimated after
each iteration (see ‘Theory’ section). The fastest method is to use the quasiNewton
method to estimate the Jacobian matrix (Loke and Barker 1996). In
this case, put a value of 0 in the dialog box. This might be a good alternative to
use in the field where a slower laptop computer with more limited memory is
frequently used. The most accurate and slowest method is to recalculate the
Jacobian matrix after every iteration. This requires a fast computer with
preferably at least 32 megabytes of RAM, and about 256 megabytes free hard
disc space for very large data sets. To choose this method, just put a very large
value (for example 20) into the box. A third alternative is to recalculate the
Jacobian matrix for the first few iterations only, and use the quasi-Newton
updating method for subsequent iterations. The largest changes in the Jacobian
matrix usually occurs in the first few iterations. So in many cases, a limited
recalculation of the Jacobian matrix gives the best compromise between speed
and accuracy. By default, the program will recalculate Jacobian matrix for the
first 3 iterations only. However, the default settings can be changed by the user
in this menu option. The quasi-Newton option is useful to get a quick look at
the results, particularly in the field during the course of a survey using a laptop
computer. Another situation where this should be used is when you want to
remove the bad data points using the RMS Errors Statistics method (see the
‘Display’ section). For the final model, it is best to use the limited recalculation
option on a faster desktop computer with more RAM and free hard disc space.
In areas with large resistivity contrasts, where the largest apparent resistivity
value is more than 10 times the smallest value, recalculating the Jacobian
matrix produces models with boundaries which are sharper than those
produced by the quasi-Newton method.
Convergence limit - This sets the lower limit for the relative change in the
RMS error between 2 iterations. By default, a value of 5% is used. In this
program the relative change in the RMS error, rather than an absolute RMS
value, is used to accommodate different data sets with different degrees of
noise present.
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Select robust inversion - The conventional least-squares method will attempt to
minimise the square of difference between the measured and calculated
apparent resistivity values. The 'Robust data constrain' option will attempt to
minimise the absolute difference (or the first power) between the measured and
calculated apparent resistivity values is minimised can be used (Claerbout and
Muir 1973). This method is less sensitive to very noisy data points. In a similar
way, the 'Robust model constrain' option will attempt to produce where
absolute spatial changes in the model resistivity values is minimised, compared
to the normal smoothness-constrained method where the square of the changes
is minimised. This robust model option tends to produce a model with sharper
boundaries. The two options are shown in the following dialog box.
Limit range of model resistivity - When you select this option, the following
dialog box will be shown.
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This option allows you to limit the range of resistivity values that the inversion
subroutine will give. In the above example, the upper limit for is 20 times the
average model resistivity value for the previous iteration while the lower limit
is 0.05 times (i.e. 1/20 times). The program uses “soft” limits that allows the
actual resistivity model values to exceed the limits to a certain degree.
However, this option will avoid extremely small or large model resistivity
values that are physically unrealistic. The user can also choose to use the
inversion model obtained at the first iteration as the reference model.
Change model depth range - This option allows you to increase or decrease the
depth of the layers in the inversion model used by the program. The depths are
multiplied by a scaling factor given by the user.
Choose logarithm of apparent resistivity - By default, the program will use the
logarithm of the apparent resistivity values as the data parameter when carrying
out the inversion. For most cases, this gives the best results. In some cases, for
example with negative or zero apparent resistivity, this is not possible. This
option enables the apparent resistivity value by itself to be used for such
situations.
Change number of nodes - By default, the program uses a finite-difference or
finite-element grid with 2 nodes between adjacent electrodes. This option
allows you to use 3 or 4 nodes between adjacent electrodes for greater
accuracy. Using more nodes will increase the accuracy of the forward
modelling subroutine, but the computer time and memory required will also
increase.
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9 Inversion of data
This option enables you to carry out the inversion of the data set that you had
read in using the "File" option. A default set of inversion parameters that
control the inversion process is normally used. You can change the settings
using the "Change Settings" menu described earlier. On selecting this option
the following menu will be displayed.
Carry out inversion - This option will start the least-squares inversion routine.
You will be asked for the name of the output data file in which to store the
results. After the inversion process has started, you can stop it by pressing the
Q key, and then wait for a short while.
Type of optimisation method – This option allows you to choose two different
methods to solve the least-squares equation (1). On selecting this menu option,
the following dialog box will be shown.
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By default the program uses the ‘Standard Gauss-Newton’ least-squares
method, particularly if the number of data points and/or model cells is small
(less than a few thousand), where an exact solution of the least-squares
equation is calculated. If the number of data points and/or model cells is large
(more than 3000), the time taken to solve the least-squares equation could be
the most time-consuming part of the inversion process. To reduce the inversion
time, an alternative method that calculates an approximate solution of the leastsquares
equation using the ‘Incomplete Gauss-Newton’ method can be used.
The user can set the accuracy of the solution. For most data sets, an accuracy of
about 1% to 2% (i.e. a convergence limit of 0.01 to 0.02 in the above dialog
box) seems to provide a solution that is almost the same as that obtained by the
‘Standard Gauss-Newton’ method. Setting a higher accuracy, for example
0.5%, will in theory give a result that is even closer to the ‘Standard GaussNewton’
method but this is at the expense of a longer inversion time. For very
large data sets and models (over 15000 data points/cells), a “data compression”
option is provided with the ‘Incomplete Gauss-Newton’ method. This can
significantly reduce the inversion time. As an example, the inversion of a
model with about 30000 cells and 60000 data points was reduced from more
than a week to less than 2 days on a 2 GHz P4 computer with 1 GB RAM.
Use finite-element method - This program allows you to use either the finitedifference
or finite-element method to calculate the apparent resistivity values.
By default, the program will use the finite-difference method, which is faster, if
the data set does not contain topography. If the data set contains topography,
the default choice is the finite-element method. The finite-element method is
significantly slower than the finite-difference method, so it is recommended
that you use the finite-difference method if the data set does not have
topography.
Optimise damping factor - If you select this option, the program will attempt to
find the optimum damping factor u in equation (1) which gives the lowest
RMS error in each iteration. By optimising the damping factor, the number of
iterations the program requires to converge can be significantly reduced.
However, the time taken per iteration will be increased. For small to medium
size data sets, this can significantly reduce the overall computer time needed to
invert the data set. For very large data sets with more than 2000 data points,
the time taken in each iteration could be significantly increased as it is
necessary to solve the least-squares equation more than once per iteration. Use
this option with care as the model with the lowest RMS error is not always the
“best” model, particularly for very noisy data sets (see the “Theory” section).
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Type of IP inversion method - To invert a combined resistivity and IP dataset,
you can choose to invert both data sets concurrently in each iteration or to
invert the IP data sequentially after the inversion of the resistivity data.
IP damping factor – The damping factor used in the inversion of IP is usually
smaller than that used in the inversion of the corresponding resistivity data set.
If a value of 1.0 is used, then the same damping factors will be used for both
the resistivity and IP inversion routines. Normally a much smaller value of
about 0.05 to 0.25 is used. Alternatively, you can choose the option to let the
program automatically calculate the damping factor.
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10 Display results
In this option, you can read in a data file or the output file produced by the
inversion subroutine and display the measured apparent resistivity
pseudosections and the model sections. Within this option, you can change the
contour interval used for drawing the pseudo and model sections and the
vertical scale of the pseudosections. You can also change the colour scheme
used by the program. Two methods to remove bad data points from a 3D data
set is also provided.
If you had earlier read in an input data file or used the inversion option in the
Main Menu, the program will first read the last file used. After selecting this
option on the Main Menu bar, the program will switch to the Window for
displaying the pseudosections and model section. To return to the Main menu,
you will need to Exit from this window. This window has the following top
menu bar.
The “File” option will bring up the following submenu.
Read data file with inversion results - You can read in the output data file
produced by the inversion subroutine or an input data file.
Save model in XYZ format - You can save the inversion results into a disc file
with a format used by many contouring programs, such as Geosoft. This is a
separate program sold by GEOSOFT Inc. which enables you to use more
sophisticated contouring techniques than that used by the RES3DINV program.
Clicking the “Display” option on the top menu bar will bring up the following
submenu.
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Display inversion model - Select this option to plot the model. You can plot the
model in the form of horizontal or vertical slices through the earth.
Display model with topography - If the data file contains topographical
information, this option will display the model in the form of vertical slices
with the topography.
Display apparent resistivity - This option allows you to display the apparent
resistivity (or IP) pseudosections for measurements made in the x- or y-
directions.
The “Change display settings” option enables you to change the parameters
which control the way the apparent resistivity pseudosections and the model
sections are displayed. The following submenu will be displayed when you
click this option.
Reverse colour scheme - Normally the program uses a colour scheme where
blue is used for the low resistivity and red for the high resistivity values. This
option allows use to reverse the colour scheme so that blue is used for the high
resistivity values and red for the low resistivity values.
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Customise colour scheme - This enables the user to change the colours used for
the sections.
Read customise colour scheme - You can change the colour scheme to a set of
customise colours that was earlier saved in a disk file. In some cases when
running this program in the background, the colour scheme might change to the
palette used by the foreground program. You can use this option to restore the
colour scheme used by this program.
Display resistivity or IP - You can choose to display the resistivity or IP in the
model sections or pseudosections if the data set contains IP measurements as
well.
Colour/Black-White display - By default, the program will display the
pseudosections and model sections in colour. This option allows you to display
the sections in black and white, possibly for making printouts on normal laser
printers. The program provides two choices for the black and white display.
The first choice is to use pure black and white colours together with a dithering
method to provide the different tones. Such screen images can be printed
on almost any printer. The second choice is to use a grey scale where different
shades of grey, ranging from pure white to pure black, is used. You will
probably need a laser printer to make reasonably good printouts of the grey
scale images.
Vertical display scaling factor - This option allows you to specify the ratio of
the vertical scale to the horizontal scale, i.e. the vertical exaggeration factor, in
the display. Convenient values to use are 2.0, 1.5 and 1.0.
Type of pole-dipole display - This option is only relevant for pole-dipole data
sets with measurements with the “forward” and “reverse” pole-dipole arrays
(see Appendix B). Here you can choose to display the apparent resistivity
values measured with the array in the “forward” or “reverse” configuration.
The “Edit” option will bring up the following submenu.
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Large 3D surveys can involve many thousands of datum points. In the most
general case, the array for a particular measurement can be orientated in any
direction. To obtain a good inversion model, the “bad” data points must be
removed from the data set. One possible way of removing the “bad” data points
from the data set is by statistical techniques. In the program, two methods are
used. On clicking the “Edit data” suboption in the top menu bar, the following
suboptions are provided.
Apparent resistivity statistics - This suboption just displays the distribution of
the apparent resistivity values as a bar chart. It is meant to be first used to
detect data points that obviously have wrong values. This could be caused by
hardware problems, such as very poor ground contact at the electrodes or a
break in one of the wires. This data points will probably have apparent
resistivity values that are several orders of magnitude larger or smaller than the
other data points. These “bad” data points can be removed by moving the left
blue or right green cursor lines.
RMS error statistics - This suboption just displays the distribution of the
percentage difference between the logarithms of the observed and calculated
apparent resistivity values. It can only be used after carrying out a trial
inversion of the data set. In this case, you should set the “Number of iterations
to recalculate Jacobian matrix” in the “Change Inversion Parameters” dialogue
box (described earlier in section 8) to 0. In this case, the program will use the
quasi-Newton inversion method. On selecting this suboption, the distribution
of the RMS errors will be displayed in the form of a bar chart. It is expected the
“bad” data points will have relatively large “errors”, for eg. above 100 percent.
To remove the points with large errors, just move the green cursor line to set
the cut-off error limit to remove the “bad” datum points.
The “Print” option will bring up the following submenu.
This option enables you to save the screen image as a BMP or PCX graphics
file, or make a direct printout with a printer by using the Windows Paintbrush
program. After the program has completed drawing the model or pseudosection
(selected from the “Display” option described earlier), click the Print option.
This will display options to save the screen as a BMP file, a PCX file or to call
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up the PaintBrush program to print the file. If you need to print the screen
image together with some text, it might be more convenient to save the screen
image as a BMP or PCX file. You can later insert the picture directly into the
document if the word processor, such as Microsoft Word for Windows or
Lotus Amipro or WordPerfect, allows you to import graphics files.
11 Topography
If the data set contains topography, the effect of the topography is
automatically incorporated into the inversion model by using a distorted 3D
finite-element grid. This is probably the most accurate method if the near
surface regions contain significant resistivity variations, although it is probably
the slowest method. Techniques based on the “correction factors” of a
homogeneous earth model with surface topography, in theory, will only be
exact if the actual subsurface is also homogeneous. The “correction factors”
method will correct for changes in the geometric factor caused by the
topography, but will not correct for changes in the distances of the electrodes
from the subsurface bodies. If there are large resistivity variations near the
surface, the “correction factors” method can cause significant distortions in the
results (Tong and Yang, 1990; Loke, 2002). In any case, the “correction
factors” method cannot be used for underwater surveys with topography.
12 Help
This option allows you to call the Windows help file for the program.
13 Some field examples
Some examples of field data sets that were inverted with this program are
given below :Birmingham
field test survey - UK
This field test was carried out using a multielectrode system with 50 electrodes
commonly used for 2-D resistivity surveys. The electrodes are arranged in a 7
by 7 square grid with a unit electrode spacing of 0.5 metre (Figure 3). The two
far electrodes were placed at more than 25 metres from the grid to reduce their
effects on the measured apparent resistivity values. To reduce the survey time,
the cross-diagonal survey technique was used (Appendix A). The subsurface is
known to be highly inhomogeneous consisting of sands and gravels. Figure 3
shows the horizontal and vertical sections of the model obtained at the 6th
iteration. The two high resistivity zones in the upper left quadrant and the
lower right corner of Layer 2 are probably gravel beds (Loke and Barker 1996).
The two low resistivity linear features at the lower edge of Layer 1 are due to
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roots from a large sycamore tree just outside the survey area. The vertical
extent of the gravel beds is more clearly shown in the vertical cross-sections
across the model. The inverse model shows that the subsurface resistivity
distribution in this area is highly inhomogeneous and can change rapidly within
a short distance. In such a situation a simpler 2-D resistivity model (and
certainly a 1-D model from conventional sounding surveys) would probably
not be sufficiently accurate.
Sludge deposit - Sweden
Most commercial 3D surveys will probably involve grids of at least 16 by 16 in
order to cover a reasonably large area. A 16 by 16 grid will require 256
electrodes which is larger than many multi-electrode resistivity meter systems.
One method to survey such large grids with a limited number of electrodes is to
extend the roll-along technique used in 2D surveys to 3D surveys. In the 3-D
roll-along method used by Dahlin et al. (2002), a number of parallel multielectrode
cables were used. A survey was carried out at Lernacken in Southern
Sweden over a covered sludge deposit. Seven parallel multi-electrode cables
were used to cover a 21 by 17 grid with a 5 metres spacing between adjacent
electrodes. Initially the cables are laid out in the x-direction, and measurements
were made in the x-direction only. After each set of measurements, the cables
were shifted step by step in the y-direction until the end of the grid. Next, the
measurements were made with the cables laid out in the y-direction. In surveys
with large grids, such as in this example, it is common to limit the maximum
spacing for the measurements. The maximum spacing is chosen so that the
survey will map structures to the maximum depth of interest. In this case, the
maximum spacing was 40 metres compared to the total length of 100 metres
along a line in the x-direction. The model obtained from the inversion of this
data set is shown in Figure 4. The former sludge ponds containing highly
contaminated ground water show up as low resistivity zones in the top two
layers (Dahlin et al. 2002). This was confirmed by chemical analysis of
samples. The low resistivity areas in the bottom two layers are due to saline
water from a nearby sea.
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Figure 3. Horizontal and vertical cross-sections of the model obtained from the
inversion of the Birmingham field survey data set. The location of observed
tree roots on the ground surface are also shown.
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Figure 4. The 3D model obtained from the inversion of the Lernacken Sludge
deposit survey data set. The model is shown in the form of horizontal slices
through the earth.
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Copper Hill - Australia
This is an interesting example of a 3-D resistivity and IP survey provided by
Arctan Services Pty. and Golden Cross Resources, Australia. Copper Hill is the
oldest copper mine in NSW, Australia. An earlier survey was conducted in
1966 using mapping, rock chip sampling, an IP survey and 7 drill-holes (White
at al. 2001). Copper porphyry with minor gold and palladium mineralization
were found to occur in structurally controlled fractures and quartz veins.
However, due to the very complex geology, large differences in ore grades
were found in drill-holes that were less than 200 meters apart.
To map the ore deposit more accurately, a new 3-D resistivity and IP survey
using the pole-dipole array was used. The survey covered a large (1.6 x 1.1km)
area using a series of 1.6 km lines with a spacing of 25m between adjacent
electrodes. Figure 5a shows the arrangement of the transmitter and receiver
lines. Currents of up to 7 Amps were used. The entire survey took 10 days
giving a total of over 7000 measurements. Further details about the survey
layout and procedures used to improve the data quality as well as to reduce the
survey time are described in the paper by White at al. (2001).
The data was inverted with the RES3DINV program that produced a 3-D
resistivity as well as a 3-D IP model for the area. The 3-D IP model that shows
the location of the mineralized zones more clearly is shown in Figure 5b. The
inversion model output from the RES3DINV program was rearranged into a
VRML format that could be read by a 3-D visualization program (please
contact Arctan Services at www.arctan.com.au for the details) that enables the
user to display the model from any direction. The 3D IP model in Figure 5b
shows two en-echelon north-south trends and two approximately east-west
trends forming an annular zone of high chargeability. The results from existing
drill-holes that had targeted the shallower part of the western zone agree well
with the resistivity and IP model. A drill-hole, CHRC58, intersected a 217m
zone with 1.7 g/t gold and 0.72% copper coincided well an IP zone of greater
than 35mV/V. The lower boundary of the western zone with high chargeability
coincides well with low assay results from existing drill-holes. The eastern
zone with high chargeability and resistivity values do not outcrop on the
surface and very little drilling has penetrated it. Further surveys, including
drilling, is presently being carried out.
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(b)
Figure 5. (a) Electrodes layout used for the 3-D survey of the Copper Hill area.
(b) The IP model obtained from the inversion of the Copper Hill survey data
set. Yellow areas have chargeability values of greater than 35 mV/V, while red
areas have chargeability values of greater than 45 mV/V (White et al. 2001).
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14 Trouble shooting
A brief compilation of possible problems and their possible causes and
solutions is given in this section.
Insufficient memory : If the program cannot find sufficient memory, one
possible reason is that it has been reserved for other programs. In this case, you
will need to remove the other active programs. In Windows 95/98/NT, the
active programs are usually shown on the Start menu bar at the bottom of the
screen. If you have less than 64 megabytes RAM, it is recommended that you
buy some RAM.
Program crashes when it tries to invert the data set : This could be due to
data arranged in the wrong format or other errors in the data set. When the
program reads the data file, it will check for some common errors such as zero
or negative apparent resistivity values. However, there might be some new
unexpected errors. If you still cannot find the error, please send a copy of the
data set to the company who sold you this program (or the author of this
program) by email or by post using a 3.5" disc.
Program cannot find dongle : If the program cannot find the hardware key, it
will run as a demonstration program. If you had securely attached the dongle
provided to the printer port and the program still cannot detect it, please refer
to section 2 for a method to check for the source of the problem. The two most
likely causes are a faulty printer port, or one which is not completely IBM PC
compatible (this problem is more common in older laptop computers). Another
possibility is that the parallel port has been reconfigured by a program as
another port instead of as a printer port. If you are using Windows NT, it is
necessary to install a system driver for the dongle (please refer to sections 2and
3). The system driver file provided (SYSDRVR.EXE) also contains the drivers
for Windows 3.1 and Windows 95. Install this driver if the program is unable
to detect the dongle with Win 95/98.
Acknowledgements
Dr. Torleif Dahlin of Lund University in Sweden kindly provided the
Lernacken field example. A special acknowledgement to Ron Barker of the
School of Earth Sciences, University of Birmingham for the 3D Birmingham
field test data set. Rockworks and Slicer/Dicer are registered trademarks of
Rockware Earth Science Software and Visualogic Inc. Last, but not least,
permission from Arctan Services Pty. and Golden Cross Resources to use the
Copper Hill 3-D example is gratefully acknowledged.
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References
Claerbout, J.F. and Muir, F., 1973. Robust modeling with erratic data.
Geophysics, 38, 826-844.
deGroot-Hedlin C. and Constable S.C. 1990. Occam's inversion to generate
smooth, two-dimensional models from magnetotelluric data.
Geophysics, 55, 1613-1624.
Dahlin, T., Bernstone, C. and Loke, M.H., 2002. A 3-D resistivity investigation
of a contaminated site at Lernacken, Sweden. Geophysics, 67, 1692-
1700.
Dey, A. and Morrison, H.F., 1979. Resistivity modeling for arbitrarily shaped
three-dimensional shaped structures. Geophysics, 44, 753-780.
Fletcher R. 1987. Practical methods of optimization. John Wiley & Sons.
Fox, R.C., Hohmann, G.W., Killpack, T.J. and Rijo, L., 1980. Topographic
effects in resistivity and induced polarization surveys. Geophysics, 45,
75-93.
Li, Y. and Oldenburg D.W., 1992. Approximate inverse mappings in DC
resistivity problems. Geophysical Journal International, 109, 343-362.
Loke, M.H., 2000. Topographic modelling in resistivity imaging inversion.
62nd EAGE Conference & Technical Exhibition Extended Abstracts,
D-2.
Loke, M.H., 2002. Tutorial : 2-D and 3-D electrical imaging surveys.
(available for download from www.geoelctrical.com).
Loke M.H. and Barker R.D., 1996. Practical techniques for 3D resistivity
surveys and data inversion. Geophysical Prospecting, 44, 499-523.
Loke, M.H. and Dahlin, T., 2002. A comparison of the Gauss-Newton and
quasi-Newton methods in resistivity imaging inversion. Journal of
Applied Geophysics, 49, 149-162.
Nelson, G.D. and Van Voorhis, 1973. Letter to the editor regarding the paper
“Complex resistivity spectra of porphyry copper mineralization”.
Geophysics, 38, 984.
Silvester P.P. and Ferrari R.L., 1990. Finite elements for electrical engineers
(2nd. ed.) . Cambridge University Press.
Summer, J.S., 1976. Principles of induced polarization for geophysical
exploration. Elsevier Scientific Publishing Company.
Tong, L. and Yang, C., 1990. Incorporation of topography into twodimensional
resistivity inversion. Geophysics, 55, 354-361.
Van Voorhis, G.D. Nelson, P.H. and Drake, T.L., 1973. Complex resistivity
spectra of porphyry copper mineralization. Geophysics, 38, 49-60.
White, R.M.S., Collins, S., Denne, R., Hee, R. and Brown, P., 2001. A new
survey design for 3D IP modelling at Copper hill. Exploration
Geophysics, 32, 152-155.
Witherly, K.E. and Vyselaar, J, 1990. A geophysical case history of the Poplar
Lake Copper-Molybdenum deposit, Houston Area, British Columbia.
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in Fink, J.B., McAlister, E.O., Sternberg, B.K., Wieduwilt, W.G. and
Ward, S.H. (Eds), 1990, Induced polarization : Applications and case
histories : Investigations in Geophysics No. 4, Soc. Expl. Geophys.
Xu, B. and Noel, M., 1993. On the completeness of data sets with
multielectrode systems for electrical resistivity survey. Geophysical
Prospecting 41, 791-801.
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Appendix A : 3D surveys with the pole-pole array
The pole-pole electrode configuration is commonly used for 3D surveys, such
as the E-SCAN method (Li and Oldenburg 1992). The apparent resistivity
value for the pole-pole array is given by
ρ = 2 pi a R
where R is the measured resistance, a is the spacing between the C1 and P1
electrodes.
For a given number of electrodes, the pole-pole array gives the maximum
number of independent measurements, nmax, which is given by
nmax = ne (ne -1) / 2.
Where ne is the number of electrodes. In the measurement sequence shown in
Figure 6a, each electrode in turn is used as a current electrode and the
potentials at all the other electrodes are measured. Note that because of
reciprocity, it is only necessary to measure the potentials at the electrodes with
a higher index number than the current electrode in Figure 6a. For a 5 by 5
electrode grid, a complete data set (Xu and Noel 1993) will have 300 datum
points. For 7 by 7 and 10 by 10 electrode grids, the numbers of measurements
are 1176 and 4500 respectively. It can be very time-consuming to make such a
large number of measurements with typical single-channel resistivity meters
commonly used for 2D surveys. For example, it could take several hours to
make the 1176 measurements for a 7 by 7 survey grid with a standard low
frequency earth resistance meter. To reduce the number of measurements
required without seriously degrading the quality of the model obtained, an
alternative measurement sequence has been tested (Figure 6b). In this proposed
"cross-diagonal survey" technique, the potential measurements are only made
at the electrodes along the horizontal, vertical and the 45 degrees diagonal lines
passing through the current electrode. The number of datum points with this
arrangement for a 7 by 7 grid is reduced to 476. For large survey grids, such as
the sludge deposit field example described earlier, it is also common to limit
the maximum spacing used in the measurements to about 8 to 10 times the
minimum electrode spacing.
The file GRID7X7.DAT is an synthetic data file for a pole-pole survey over a 7
by 7 grid with all the possible measurements. The file ROOTS7.DAT is from a
field survey with a 7 by 7 grid where the cross-diagonal measurement sequence
was used.
To map large areas with a limited number of electrodes in a multi-electrode
resistivity meter system, the roll-along technique can be used (Dahlin et al.
2002). Figure 7 shows an example of survey using a multi-electrode resistivitymeter
system with 50 electrodes to survey a 10 by 10 grid.
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Figure 6. The location of potential electrodes corresponding to a single current
electrode in the arrangement used by (a) a survey to measure the complete data
set and (b) a cross-diagonal survey.
Initially the electrodes are arranged in a 10 by 5 grid with the longer lines
orientated in the x-direction (Figure 7a). Measurements are made primary in
the x-direction, with some possible measurements in the diagonal directions.
Next the entire grid is moved in the y-direction so that the 10 by 5 grid now
covers the second half of the 10 by 10 grid area. The 10 by 5 grid of electrodes
is next orientated in the y-direction and the measurements are made between
the electrodes in the y-direction (Figure 7b).
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The file PIPE3D was from a survey using such a roll-along technique. It was
carried out with a resistivity-meter system with only 25 electrodes, with the
electrodes arranged in an 8 by 3 grid. The long axis of this grid was orientated
perpendicularly to two known subsurface pipes. The measurements were made
using three such 8 by 3 grids so that the entire survey covers an 8 by 9 grid. For
each 8 by 3 sub-grid, all the possible measurements (including a limited
number in the y-direction) for the pole-pole array were made. In this survey,
the second set of measurements in the y-direction (as in Figure 7b) was not
carried out to reduce the survey time, and also because the pipes have an
almost two-dimensional structure.
For practical reasons, the number of field measurements in some surveys might
be even less than the cross-diagonal technique. Another common approach is to
just make the measurements in the x- and y- directions only, without the
diagonal measurements. This is particularly common if the survey is made with
a system with a limited number of independent electrodes, but a relatively large
grid is needed.
For relatively small grids of less than 12 by 12 electrodes, the pole-pole array
has a substantially larger of possible measurements compared to other arrays.
The loss of data points near the sides of the grid is kept to a minimum, and
provides better horizontal data coverage compared to any other array. This is
an attractive array for small survey grids with relatively small spacings (less
than 10 metres) between the electrodes. However, it has the disadvantage of
requiring two “far” electrodes that must be placed at a sufficiently large
distance (at least 10 times the maximum electrode spacing used) from the
survey grid. Due to the large distance between the two potential electrodes, this
array is more sensitive to telluric noise.
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Figure 7. Using the roll-along method to survey a 10 by 10 grid with a
resistivity-meter system with 50 electrodes. (a) Surveys using a 10 by 5 grid
with the lines orientated in the x-direction. (b) Surveys with the lines orientated
in the y-direction.
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Appendix B : 3D surveys with the pole-dipole array
This array is an attractive alternative to the pole-pole array for surveys with
medium and large survey grids (12 by 12 and above). It has a better resolving
power than the pole-pole array, and is less susceptible to telluric noise since
both potential electrodes are kept within the survey grid. Compared to the
dipole-dipole array, it has a significantly stronger signal strength. The
arrangement of the electrodes is shown below.
Figure 8. The arrangement of the electrodes for the pole-dipole array for
measurements in the (a) “forward” direction, (b) with a larger spacing between
the potential electrodes and (c) in the “reverse” direction.
Unlike other common arrays, it is an asymmetrical array. Over symmetrical
structures the apparent resistivity anomalies are also asymmetrical. In some
situations, the asymmetry in the measured apparent resistivity values could
influence the model obtained after inversion. One method to eliminate the
effect of this asymmetry is to repeat the measurements with the electrodes
arranged in the reverse manner (Figure 7c). By combining the measurements
with the “forward” and “reverse” pole-dipole arrays (Figures 7a and 7c), any
bias in the model due to the asymmetrical nature of this array would be
removed.
The apparent resistivity value for the pole-dipole array is given by
ρ = 2 pi n (n+1) a R
where R is the measured resistance, a is the spacing between the P1 and P2
electrodes and n is the ratio of the distances between the C1 and P1 electrodes
to the P1-P2 spacing. The voltage measure between the potential electrodes
decreases with the square of the n factor. For large values of the n factor, the
signal to noise ratio might become too small for a reliable measurement to be
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made. To increase the signal strength, the distance between the P1 and P2
electrodes is increased (Figure 7b). One possible field technique is to make all
the possible measurements with the P1-P2 spacing equals to 1a (i.e. the unit
electrode spacing), followed by measurements with P1-P2 spacing equals to 2a.
This will increase the data density. In some cases, particularly in noisy areas, it
has been found by having such redundant measurements improves the
resolution of the resulting inversion model.
Concerning the data format, the upper part of the example data file
BLK10PDP.DAT is shown below with some comments. Note that the array
number is 6 for the pole-dipole, and the coordinates of the C1, P1 and P2 (i.e.
three electrodes compared to only two electrodes for the pole-pole array) are
given.
BLK10PDP.DAT file Comments
2 BLOCKS | Title
10 | x grid size
10 | y grid size
1.00 | x unit electrode spacing
1.00 | y unit electrode spacing
6 | array type, enter 6 for pole-dipole
1466 | Number of data points
0.00 0.00 1.00 0.00 2.00 0.00 29.9669 | For each data point, enter
0.00 0.00 2.00 0.00 3.00 0.00 30.0220 | x- and y- location of C1 current elec.,
0.00 0.00 3.00 0.00 4.00 0.00 30.2216 | x- and y-location of C2 current elec.,
0.00 0.00 4.00 0.00 5.00 0.00 30.3957 | x- and y-location of P1 potential elec.,
0.00 0.00 5.00 0.00 6.00 0.00 30.4765 | apparent resistivity value
0.00 0.00 6.00 0.00 7.00 0.00 30.4459 |
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Appendix C : 3D surveys with the dipole-dipole array
This array can is recommended only for grids that are larger than 12 by 12 due
to the poorer horizontal data coverage at the sides. The arrangement of the
electrodes are shown below.
Figure 9. (a) The arrangement of the electrodes for the dipole-dipole array. (b)
The use of a larger spacing between the C1-C2 (and P1-P2) electrodes to
increase the signal strength.
The apparent resistivity value for the dipole-dipole array is given by
ρ = pi n (n+1) (n+2) a R
where R is the measured resistance, a is the spacing between the P1 and P2
electrodes and n is the ratio of the distances between the C1 and P1 electrodes
to the P1-P2 spacing. This array has the advantage of very good horizontal
resolution, but its main disadvantage is its relatively low signal strength. The
voltage measured by the P1-P2 pair is inversely proportional to the cube of the
n factor. Normally, the maximum value for the n factor is 6. To get a deeper
depth of penetration, the spacing between the C1-C2 (and P1-P2) pair is
increased. One method to reduce the effect of noise is to make redundant
measurements using different combinations of the n factor and the a spacing.
As an example, consider the case where the unit electrode spacing is 1 meter.
First make all the possible measurements with a equals to 1 meter, and with n
equals to 1, 2, 3, 4, 5 and 6. Next make the measurements with a equals to 2
meters and also with n equals to 1, 2, 3, 4, 5 and 6. If the grid is sufficiently
large, another series of measurements with a equals to 3 meters can be carried
out.
The file BLK10DDP.DAT is an example dipole-dipole array data file. Note
that the array number is 3, and that the x- and y-coordinates of the C1, C2, P1
and P2 electrodes are given in that order.
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Appendix D : Fixing resistivities
In some cases, the true resistivity of a section of the subsurface might be
known, for example from borehole resistivity measurements. This program
allows you to fix the resistivity of up to 10 sections of the subsurface. The
shape of the section to be fixed must be rectangular.
Figure 10 : Fixing the resistivity of a rectangular region of the inversion model.
The data to fix the model resistivity values are entered into the input data file
after section with the topography information (if topography is present). As an
example, part of the example data file MOD3DFIX.DAT is listed below.
MOD3DFIX.DAT COMMENTS
7.000 9.000 8.000 9.000 9.8828 | Last three data lines
7.000 9.000 9.000 9.000 9.8237 | Note no topography information
8.000 9.000 9.000 9.000 9.9337 | in this example
FIXED REGIONS | Header to indicate model has fixed regions
1 | Number of regions to be fixed
R | Shape of fixed region, R for rectangular
2.0,4.0,0.7 | X, Y, Z coordinates of top-left-back corner
6.0,7.0,1.5 | X, Y, Z coordinates of bottom-right-front corner
1.0 | Resistivity of fixed region
2.0 | Damping factor weight for fixed region
0
0
The first item is the header “FIXED REGIONS” to indicate that the model has
at least one region where the resistivity is to be fixed. This is followed by the
number of fixed regions, 1 regions in the above example. Next, the shape of the
region is given, which is R for rectangular. For the rectangular region, the X, Y
and Z coordinates of the top-left-back and bottom-right-front corners of the
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rectangular prism are given, as shown in the above diagram. After the
coordinates of the region to be fixed are given, the next data item is the
resistivity of the region. After that, the damping factor weight for the resistivity
of the region is needed. This parameter allows you control the degree in which
the inversion subroutine can change the resistivity of the region. There is
usually some degree of uncertainty in resistivity of the region. Borehole
measurements can only give the resistivity of a very limited zone near the
borehole. Thus, it is advisable that the program should be allowed (within
limits) to change the resistivity of the region. If a damping factor weight of 1.0
is used, the resistivity of the region is allowed to change to the same extent as
other regions of the subsurface model. The larger the damping factor weight is
used, the smaller is the change that is allowed in the resistivity of the "fixed"
region. Normally, a value of about 1.0 to 2.0 is used. If a relatively large value
is used, for example 10.0, the change in the resistivity of the region would be
very small during the inversion process. Such a large value should only be used
if the resistivity and shape of the region is accurately known.
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Appendix E : IP data inversion
The data collected from IP (Induced Polarization) surveys consists of two sets,
the normal apparent resistivity measurements and an apparent IP data set. So,
the number of datum points is twice the number for a normal resistivity survey.
One possible method to invert such a joint data set is to treat the two data sets
as completely separate data sets. The inversion of the resistivity data is
completed before starting on the inversion of the IP data set. In this case, the
resistivity model obtained at the last iteration of the inversion of the resistivity
data set (which usually has the lowest RMS error) is used in the inversion of
the IP data set. This is probably the most efficient method if the resistivity
model is a good representation of the true subsurface resistivity. However, it
has been found that even in the inversion of computer generated data sets, the
model obtained can contain distortions particularly if a large number (more
than 5) of iterations is used in the inversion of the resistivity data set. This is
because the inversion is based on a finite number of datum points. The
distortion becomes worse when the data contains noise. Many users tend to use
a large number of iterations together with relatively low damping factors in an
effort to reduce the RMS error to the lowest possible value. If a resistivity
model with distortions is used in the inversion of the IP data, then the
distortions will affect the IP models for all iterations of the IP inversion.
The inversion method used in this program is a step-wise refinement of IP
inversion model. The inversion of the resistivity and IP data sets are still
carried out separately. However, immediately after an iteration of the inversion
of the resistivity data, an iteration of the IP inversion is carried out. The
resistivity model obtained at the particular iteration of the resistivity data is
used for the inversion of the IP data at the same iteration. Since the resistivity
models at the earlier iterations are less distorted by noise in the data, the
corresponding IP models are also less influenced by the distortions in the
resistivity models.
The files MODEL3IP.DAT is an example data files with both resistivity and IP
data. This program supports three different types of IP data; (i) time domain
chargeability measurements, (ii) frequency domain percent frequency effect
measurements and (iii) metal factor IP values. The file MODEL3IP.DAT has
the IP data as chargeability values. The first part of the file with comments is as
follows :MODEL3IP.DAT
file Comments
RESISTIVITY & IP MODEL | Title
8, 8 | x and y grid size
1.00, 1.00 | x and y unit electrode spacing
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2 | Code for pole-pole array
712 | Number of datum points
IP present | Word IP to indicate IP is present
Chargeability | Type of IP data
mV/V | IP unit
0.1,1.0 | Delay, integration time
0.000 0.000 1.000 0.000 10.0544 0.3702 | For each data line, the x- and y|
location of the C1 electrode
0.000 0.000 2.000 0.000 10.1311 0.8952 | followed by the x- and y-location of
| the P1 electrode, then
0.000 0.000 3.000 0.000 10.1860 1.3004 | the apparent resistivity value and the
| apparent IP value
0.000 0.000 4.000 0.000 10.1808 1.3179 |
The word IP, placed after the data line with the number of datum points, is
used to indicate the data set contains IP measurements. This is followed by the
type of IP data. The word Chargeability shows that the IP data is given in
terms of chargeability. Many IP instruments measured the chargeability be
integrating the area under the IP decay curve. In this case, the unit is in msec
(millisecond). The chargeability value obtained by this method is normally
calibrated to the Newmont M(331) standard (Summer 1976) so that the
chargeability value in msec has the same numerical value as the chargeability
given in mV/V. The chargeability in mV/V is defined as the ratio of the
secondary voltage immediately after the current is cut off to the primary DC
voltage. The next line gives the type of IP unit, in this case in mV/V. If the IP
measurement was made in msec, then the word msec should be used. Finally
two parameters relevant to the IP measurement are given. For time-domain
chargeability measurements, these are the delay and the integration time.
IP values that are given in other units are converted into chargeability values
internally by the program during the inversion process. The relevant
conversion equations may be found in the papers by Van Voorhis et al. (1973)
and Nelson and Van Voorhis (1973).
For frequency domain measurements, the IP value is usually given in Percent
Frequency Effect (PFE). This is given as
PFE = 100 (ρDC - ρAC) / ρAC
where ρDC and ρAC are the apparent resistivity values measured at low and high
frequencies. For such measurements, the format would be as follows :IP
present | Word IP to indicate IP is present
Chargeability | Type of IP data
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PFE | IP unit
0.3,5.0 | Lower and upper measurement frequencies
IP values are also given as phase angles. In this case the unit normally used is
milli-radians (mrad). In this case, the IP data section would be given as follows
:IP
present | Word IP to indicate IP is present
Phase angle | Type of IP data
mrad | IP unit
0.0,5.0 | Lower and upper measurement frequencies
IP data is sometimes given as a metal factor (MF) value. The MF value can be
calculated from either time domain or frequency domain measurements. In the
time domain, the metal factor is value is given by
MF = 1000 M / ρDC
where the chargeability M is given in terms of millisecond (Witherly and
Vyselaar, 1990). In the frequency domain, it is given as
MF = 100000 (ρDC - ρAC) / ρAC
2
An example format for metal factor measurements is as follows :IP
present | Word IP to indicate IP is present
Metal factor | Type of IP data
0.001 ms/ohm.m | IP unit
0.1,1.0 | Delay and integration time for time-domain measurements
The inversion program tries to minimise the difference between the observed
and calculated equivalent apparent chargeability values. IP values in percent
frequency effect and phase angles are directly proportional to the equivalent
chargeability values. However, the metal factor value also involve a resistivity
value. The model which gives a minimum RMS error for the chargeability
values sometimes does not give a minimum RMS error for the metal factor
values. However, the actual effect on the inversion model is usually very small.
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Appendix F : Speed of graphics display
As RES3DINV is a Windows based program, graphics operations by the
program are handled via the Windows display driver for the computer graphics
card. Thus the program will work with any graphics card as long as it has a
Windows driver. This is different from DOS based programs where
compatibility problems with the high resolution Super VGA graphics modes
are common. One big advantage of this program is that high resolution
graphics modes, such as the 1024 by 768 pixels with 256 colours graphics
mode, can be easily used by this program. This is important for data collected
from surveys with more than several hundred to several thousand electrodes (a
survey with about 5000 electrodes, 65000 data points and 32000 model cells
now holds the record) where the standard VGA 640 by 480 pixels graphics
mode is clearly inadequate.
Since graphics operations are carried out via the display driver, the speed of the
graphics display is dependent on the efficiency of the display driver. For some
graphics cards, the earlier versions of the drivers are relatively inefficient
which results in painfully slow graphics operations such as in drawing the
pseudosections. If this happens on your computer, the cause is almost certainly
due to an early version graphics driver for the display card. The best way to
overcome this problem is to replace the Windows graphics card driver with a
newer version that should be more efficient. The drivers can be downloaded
via the Internet from many shareware sites or from the manufacturer’s web
site. Using the latest version of the display driver can result in significant
improvements in the speed of the graphics operations. This will affect not only
the speed of the graphics display in the RES3DINV program, but will also
improve the response of practically all graphics intensive Windows programs
(such as CAD and animation programs).
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Appendix G : Non-uniform survey grids
In some areas, it might not be possible to place the survey lines at exactly the
same distance apart. One possible situation is shown in Figure 11 where the
inner survey lines are 0.5 metres apart while the lines at the edge are 1.0 metres
apart. To handle such a situation, the x- and y-location of each survey line must
be specified. The file NUGRID.DAT is an example data file with a nonuniform
survey grid. The first part of the file with comments is as follows :NUGRID.DAT
file Comments
2 BLOCKS | Title
9,9 | x and y grid size
Nonuniform grid | Header to indicate non-uniform grid
x-location of grid-lines | Header for x-location of grid lines
0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 | x-location of grid lines
y-location of grid-lines | Header for y-location of grid lines
0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 | y-location of grid lines
2 | Array type
992 | Number of datum points
0.000 0.000 1.000 0.000 29.934 | First datum point
0.000 0.000 1.500 0.000 29.918 | Second datum point
. | Format of the rest of the data file is
. | the same as normal data file
. |
. |
4.000 5.000 5.000 5.000 29.820 | Last data point
0 |
0 |
0 |
0 | End of file
The program will check whether the x- and y-locations of each data point must
fall on one of the grid lines when it reads in the file. If the difference is
significant (more than 0.1%), the program will reject the data point.
This feature can also be used to modify the arrangement of the model blocks so
as to add extra belts of blocks at the edges of the survey grid. This might be
useful if there are strong anomalies near the sides of the survey grid. This is
done by adding dummy electrodes outside the actual survey grid.
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Figure 11. Arrangement of the survey grid lines in the data file NUGRID.DAT.
Note the wider spacing at the sides.
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Appendix H : The equatorial dipole-dipole (rectangular) array
This array differs from other arrays in that the electrodes are not arranged in a
straight line but in a rectangular manner (Figure 12). This rectangular
arrangement appears to fit in with the rectangular grid electrode arrangement
used for 3D surveys. For a 3D grid of electrodes, it appears to have a better
coverage at the sides compared to the inline dipole-dipole array, so it might be
useful for small survey grids, particularly if high resolution at shallow depths is
needed. The file EQUADIP3.DAT is an example data with this array for a 10
by 10 electrodes survey grid.
Figure 12. The arrangement of the electrodes and geometric factor “k” for the
equatorial dipole-dipole or rectangular array.
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Appendix I : Export model to Rockworks and Slicer/Dicer formats
In the "File" option within the "Display" window, there is a suboption to save
the model resistivity or I.P. values into the format used by two 3D graphics
display and modelling programs, the Rockworks 99 program by Rockware
Earth Science Software (www.rockware.com) and the Slicer/Dicer program by
Visualogic Inc. (www.visualogic.com).
For the Rockworks program, the model values are saved in the XYZG format
that is very similar to the usual XYZ format (section 10). In this case, the file is
read in as a text data file by the Rockworks program. In the XYZG file, the X,
Y, and Z coordinates of the centre of each model block together with the
resistivity is given. In place of the resistivity, you can also save the model
conductivity or I.P. values.
For the Slicer/Dicer program, the data is saved as a generic metadata text file
with a "gm" extension. The following dialog box is displayed when this option
is selected.
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The Slicer/Dicer program assumes that the unit spacing in all 3 directions are
the same. This is not a problem in the X and Y directions if the survey is
carried out with electrodes arranged in a rectangular grid with a constant unit
spacing in the X and Y directions (i.e. a uniform gird).
If the unit spacing in the Y-direction is different from that in the X-direction,
you can set different plotting scales within the Slicer/Dicer program so that the
correct relative dimensions are obtained for the resulting display. The main
problem lies in the Z-direction since the RES3DINV program normally uses a
model where the thickness of the layers increases with depth. To overcome this
problem, the RES3DINV program has an option to subdivide the model into
layers with a constant spacing in the Z-direction. The program subdivides the
model into layers that are thinner than the actual layers in the model. In the
comment section of the generic metadata file, the appropriate relative scale
factors to use in the X-, Y- and Z-directions for the display within the
Slicer/Dicer program is also given.
There are two options for the type of output file in the Slicer-Dicer format. In
the first option, all the information is stored in a single text file. The main
advantage of this option is that the file can be copied to other drives or
subdirectories, and still be directly read by the Slicer-Dicer program. The main
disadvantage is that the size of the file becomes very large if there are a large
number of model values, or if you choose option to use a uniform sampling
spacing in all directions. In the second option, the model values are stored in a
binary file (with a “bin” extension) while a small text metadata file with a
“gm” extension stores information about the model values. The second line of
this metadata file has the location (drive and subdirectory) of the binary file. If
the files are moved to a different drive or subdirectory, you will need to edit the
metadata text file so that the second line shows the correct drive and
subdirectory where the corresponding binary file is located.
The file ROOTS7.GM is an example output Slicer/Dicer generic metadata
format file for the Birmingham field test (section 13) inversion model. Note
that in this file, the base 10 logarithms of the model resistivity values are
stored. Figure 13 shows a 3D plot of the Sludge deposit (section 13) inversion
model obtained with the Slicer/Dicer program.
Slicer-Dicer normally displays units for the labels on the x, y and z axes in units
of pixels. To change it into the true units used, for example meters, click the
‘Names and Units’ option under the ‘Options’ menu. In the top half of the
dialog box under ‘Coordinates’, change the starting and ending values for the x,
y and z coordinates to the actual values. Lastly, click the ‘Show values’ option
in the ‘Coordinates’ section, and select ‘in dimensional units’.
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Figure 13. A 3-D view of the model obtained from the inversion of the
Lernacken Sludge deposit survey data set displayed with the Slicer/Dicer
program. A vertical exaggeration factor of 2 is used in the display to highlight
the sludge ponds. Note that the colour contour intervals are arranged in a
logarithmic manner with respect to the resistivity.
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Appendix J : Disabling computer power management and screen saver
Practically all computers have an in-built power management system which
slows down and eventually shuts down the computer system if the
keyboard/mouse or hard disc is not accessed after a certain time limit. This
might be a bit inconvenient if you are inverting a large data set since this
program spends most of the time calculating using the CPU and coprocessor
only. You will need to disable the power management feature in your computer
to stop the computer from automatically slowing or shutting down. If the
computer shuts down the system automatically, it will normally start again
from the last program used, the next time it is switched on again. If this
happens, you should exit from the program and restart it before using any of
the program options. In some computer systems, the contents of all the memory
might not be fully restored which could cause the program to crash. A similar,
but fortunately less severe problem, exists with screen savers which kick in
after a period of time if the keyboard or mouse is not used. Normally, a setting
of several minutes is used for the screen saver. Since the calculations for a
large model can take longer, it is recommended that you disable, the screen
saver. To disable the power management and screen saver, you need to take
the following steps.
1). Shortly after the computer is switched on, it usually runs a memory check.
During this time, you need to go to the CMOS Setup program in the computer
BIOS. For most computers, this is done by pressing the "Del" key. The CMOS
setup screen will next be shown. Go to the section which concerns power
management, and disable the power management options for the computer.
This is to ensure that the computer does not shut down or slow down
automatically if the keyboard or mouse is not used after a fixed time.
2). After Windows starts up, go to the Control Panel. Click the Power
Management icon. Here it will probably show the number of minutes after
which Windows will automatically shut down the computer if the mouse or
keyboard is not used. Disable this feature.
3). Next, still within the Control Panel, click the Display icon. In the Display
program, click the Screen Saver option and disable the screen saver. This will
ensure that Windows does not automatically run a screen saver.
After disabling the power management and screen saver options, you can
physically switch off the monitor if the computer is left running overnight.
Please also shut down other active programs (such as word processors, CAD
programs, spreadsheets) before running the RES3DINV program. This will
ensure that the maximum amount of memory is available to the RES3DINV
program, and also it will not be interrupted by another program during the
course of inverting a large data set.
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Appendix K : Inversion of large data sets, hard-disk file format and
recommended PC systems
Recent advances in microcomputer technology and numerical algorithms have
made it possible to carry out the inversion of very large data sets and grid
models (over 30,000 data points and model cells). Even as recently as 5 years
ago, 3D inversion of such large data sets would have required a fast mainframe
or workstation computer system. Now it can be carried out on commonly
available microcomputers with RES3DINV.
While carrying out the inversion of such large data sets, the RES3DINV
program stores some of the information into temporary files on the computer
hard-disk. In some cases, the size of such a file could exceed 4 GB
(gigabytes). The Windows 98/Me operating systems use a file format system
called FAT32 that has a maximum limit of 4 GB for files. So, the inversion of
large data sets that require temporary files of greater size cannot be done
efficiently under the Windows 98/Me operating systems. Windows
NT4/2000/XP supports a newer file format system called NTFS that allows the
use of much larger files. The maximum possible size of a file under the NTFS
system presently exceeds that largest hard-disk commercially available for
microcomputers. Computers using Windows NT4/2000 can use either the older
FAT32 or the newer NTFS file format system. If the FAT32 system is used,
the 4 GB file size limit still remains. According to Microsoft, it is possible to
convert the file format system from FAT32 to NTFS without losing the data in
the hard-disk by using a utility program supplied with Windows NT4/2000.
The details can be found at the following web site.
http://www.microsoft.com/windowsxp/pro/using/itpro/managing/convertfat.asp
Windows XP is the latest Windows operating system, so the hard-disk that
comes with this operating system is usually formatted with the NTFS file
format system.
The recommended microcomputer system for the inversion of very large data
sets is as follows.
Intel P4 CPU with speed of at least 2 GHz
40GB hard-disk with at least 16GB free hard-disk space (80 GB
recommended)
At least 1 GB RAM (2 GB recommended)
Windows NT4/2000/XP operating system with NTFS file format system
There have been some reports of problems with systems using the AMD
Athleon processors when running inversions that take more than a few hours. It
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is recommended that you use a PC with an Intel P4 CPU together with an Intel
motherboard.
Please remember that you need to install the driver for the dongle so that the
RES2DINV/RES3DINV program can detect the dongle under Windows
NT/2000/XP.
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Appendix L : Non-conventional or general arrays
This feature is to cater for electrode arrangements that do not fall under the
usual array types or electrode arrangements (such as the gradient array) or a
mixture of data from different arrays. There are two main differences in the
data format. Firstly the array type number is 11, and it also has a sub-array type
to accommodate the case where the data is from one of the conventional arrays.
The second difference is that in each data line, the number of electrodes used in
the measurement is given. The file 3d_mixed_arrays.dat gives an example of
the data format. This file contains a mixture of data from the dipole-dipole and
pole-dipole arrays.
3d_mixed_arrays.dat file | Comments
Mixed dipole-dipole and pole-dipole data | Title
10 | X grid size
10 | Y grid size
1.00 | X unit electrode spacing
1.00 | Y unit electrode spacing
11 | Array type 11
0 | Sub-array type
Type of data (0=apparent resistivity,1=resistance) | Header
0 | Data in apparent resistivity
2480 | Number of data points
4 1.0 0.0 0.0 0.0 2.0 0.0 3.0 0.0 29.968 | Each data line has Number of electrodes,
4 1.0 0.0 0.0 0.0 3.0 0.0 4.0 0.0 30.096 | X and Y loc. of C1,C2,P1,P2 electrodes
4 1.0 0.0 0.0 0.0 4.0 0.0 5.0 0.0 30.438 | followed by the apparent resistivity value.
4 1.0 0.0 0.0 0.0 5.0 0.0 6.0 0.0 30.810 |
. |
. |
. |
3 0.0 0.0 1.0 0.0 2.0 0.0 29.964 | Note number of electrodes is 3 for
3 0.0 0.0 2.0 0.0 3.0 0.0 30.022 | pole-dipole array
. |
. |
3 9.0 9.0 9.0 7.0 9.0 6.0 29.947 | Last 2 data points
3 9.0 9.0 9.0 8.0 9.0 7.0 30.013 |
0,0,0,0,0 | Ends with a few zero’s
Note that the sub-array number is 0 since there is no single fixed array type in
this data set.
To accommodate the gradient array, the non-uniform grid option (Appendix G)
can be used to add extra survey grid lines so as to include the remote C1 and
C2 electrodes into the survey grid. In this case, two of the outer grid lines are
likely to have a wider spacing from the interior grid lines.
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Appendix M : Using more than 1 GB RAM
For the inversion of some data sets, even 1 GB RAM might not be sufficient!
This situation occurs with large surveys grids (with more than one thousand
electrodes) in an area with topography together with use of the 3 nodes or 4
nodes between adjacent electrodes option. The 3 or 4 nodes option is used for
greater accuracy. It is necessary to use the finite-element method when the data
set has topography. For the inversion of such large data sets, the Windows
NT/2000/XP operating system must be used so that temporary files larger than
4 GB can be used by the program (see Appendix K).
There are two main obstacles in using more than 1 GB RAM on standard PCs.
The first obstacle is the number of memory slots on the motherboard. Most PCs
have two or three memory slots which can accept 512 MB or 1 GB RAM
modules. At the present time (March 2003), 512MB RAM modules are fairly
common and inexpensive, while 1 GB RAM modules are comparatively rare
and much more expensive. If there are three memory slots, then a maximum of
1.5 GB can be installed using the 512 MB RAM modules. A problem arises if
there are only two memory slots. Then there is a choice of limiting the
maximum RAM to 1 GB using 512 MB RAM modules, or using the more
expensive 1 GB RAM modules.
In order for a program to access more than 1 GB memory, the virtual memory
setting in Windows NT/2000/XP must be modified. Windows probably has a 1
GB virtual memory default setting. To use memory above 1 GB, the
SDVIRTUALMAXMEMORY environment variable in Windows must be
changed. The following section describes the steps needed in Windows XP to
change this setting. Similar steps are used in Windows NT and 2000.
First click the ‘Start’ button and then ‘Control Panel’ icon. In the ‘Control
panel, click the ‘System’ icon, as shown below.
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In the ‘System Properties’ window, click the ‘Advanced’ tab, and the following
settings will be shown.
In this window, click the ‘Environment Variables’ option, after which the
following window is displayed.
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In the ‘System variables’ section, click the ‘New’ button. Fill in the ‘New
System Variable’ dialog box as shown below. The variable name should be set
to ‘SDVIRTUALMAXMEMORY’, while the variable value is the size in
megabytes for the virtual memory setting. The example below uses a value of
1536 for a system with 1.5 GB RAM.
After filling in the box, click the OK button for this box, follow by the OK
buttons for the ‘Environment Variables’ and ‘System Properties’ windows.
After that, restart the computer so that the new setting will be used by
Windows.
While 32-bit processors such as the Pentium series can physically address up to
4 GB RAM, only the bottom 2 GB can be used by application programs in
Windows. The upper 2GB memory address space is reserved for Windows.
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Appendix N : Underwater 3-D surveys
This section describes the data format for a survey where some or all of the
electrodes are underwater. In such a survey, it is important that the effect of the
water layer is accurately accounted for by the inversion program. The elevation
of all the electrodes, as well as the water surface, must be measured. In
addition, the resistivity of the water layer must be accurately measured. This is
usually done with a hand-held conductivity meter that frequently also measures
other physical variables such as the temperature and water pH.
The file WATER3D.DAT gives an example of the data format used. Sections
of the file with comments are given below.
WATER3D.DAT file | Comments
Underwater survey example | Title
12 | 12 electrodes in x-direction
12 | 12 electrodes in y-direction
2.00 | Unit electrode spacing in x-direction
2.00 | Unit electrode spacing in y-direction
3 | 3 for dipole-dipole array
1440 | Number of data points
2.0 0.0 0.0 0.0 4.0 0.0 6.0 0.0 106.811 | x- and y-locations of C1, C2, P1, P2
2.0 0.0 0.0 0.0 6.0 0.0 8.0 0.0 84.895 | electrodes, followed by the apparent
2.0 0.0 0.0 0.0 8.0 0.0 10.0 0.0 97.331 | resistivity value.
. | Note same format as land survey.
. | Other data points
22.0 8.0 22.0 2.0 22.0 14.0 22.0 20.0 101.143 | Last 2 data points
22.0 10.0 22.0 4.0 22.0 16.0 22.0 22.0 97.808 |
Topography | Header for topography
2 | Type of x- and y- coordinate
0.5 0.0 0.1 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 0.0 0.0 0.0 | Elevation of electrodes
0.3 0.2 0.0 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 0.0 0.0 0.0 | Given x-row by x-row
0.2 0.1 0.0 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 0.0 0.0 0.0 |
0.1 0.0 0.0 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 0.0 0.0 0.0 |
0.0 0.0 -0.1 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 0.0 0.0 0.0 |
0.0 0.0 -0.2 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 0.0 0.0 0.0 |
0.0 -0.1 -0.2 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 0.0 0.0 0.0 |
-0.1 -0.2 -0.3 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 -0.2 -0.1 0.0 |
-0.2 -0.3 -0.4 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 -0.4 -0.2 -0.1 |
-0.3 -0.4 -0.5 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 -0.4 -0.3 -0.2 |
-0.4 -0.5 -0.5 -1.0 -1.5 -1.5 -1.5 -1.0 -0.5 -0.4 -0.3 -0.3 |
-0.5 -0.5 -0.5 -0.5 -1.0 -1.5 -1.5 -1.0 -0.5 -0.5 -0.4 -0.4 |
Underwater survey parameters | Header to indicate underwater survey
Resistivity of water layer | Header
50.0 | Resistivity of water layer
Rectangular limits of water layer | Header
0.000,0.000 | Minimum x- and y-coordinates of water covered area
22.000,22.000 | The maximum x- and y-coordinates
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Elevation of water surface | Header
-0.2 | The elevation of the water surface
Type of geometric factor (1=surface,0=exact) | Header
1 | 1 to indicate surface geometric factor
0,0,0,0 | End with a few zeros
Firstly note that the format for the initial section of the data file, covering the
apparent resistivity measurements and the topography sections, is the same as
that used for normal land surface surveys. The only additional section is
towards the end of the file where the parameters of the water layer are given.
The most important parameters are the water resistivity and elevation of the
water surface. In many surveys, all the electrodes are underwater. However,
this program will also handle a situation where some of the electrodes are
above the water level. The program determines this by comparing the elevation
of the electrodes as given in the topography section with the elevation of the
water surface. The data format also requires the minimum and maximum xand
y-coordinates of the water covered section. However, this information is at
present not used by the inversion program, but is included for possible new
options in future.
If the data file has a ‘Fixed’ regions section (Appendix D), the information for
the fixed regions should be given after the topography information section but
before the underwater survey information. The present version of RES3DINV
requires the topography information section for an underwater survey. This is
probably not a significant problem since the river/lake/sea bottom usually has
some height variations.
The file WATER3D.INV gives the inversion model for the WATER3D.DAT
data set. A plot of the vertical sections across the model with the topography is
shown in the following figure.
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Figure 14. Plot of the model sections with topography from the
WATER3D.INV file. The water layer is indicate by the blue zone at the top
part of the sections. Note that some of the electrodes are above the water level,
i.e. this data set has electrodes below as well as above the water layer.
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Appendix O : Cross-borehole data inversion
The resolution of electrical surveys carried out with electrodes on the ground
surface decreases exponentially with depth. One method to obtain reasonably
good resolution at depth is by making measurements with the electrodes in
boreholes. Cross-borehole surveys are much less frequently carried out
compared to surface based surveys, so unlike surface surveys, there are at
present no common standards with regards to the arrangement of the
electrodes. The cross-borehole option in this program is intended for shallow
surveys with the arrangement of the electrodes placed in a certain general
pattern. The surface electrodes must be arranged in a rectangular grid. The x
and y location of a borehole must coincide with the location of one of the
surface electrodes in the grid.
The program divides the subsurface into a number of quadrilateral blocks. The
x and y location of the boundaries of the blocks are set using the surface grid of
electrodes (Figure 2a). The vertical position of the boundaries are set at the
depths where the borehole electrodes are located. The program has the
restriction that the number of electrodes in all the boreholes must be the same,
and the borehole electrodes are located at the same depths from the surface.
The file BOR9ALL.DAT is an example data file with cross-borehole
measurements using the pole-pole array. A description of the format used is
given below in the comments section. Note that in this example, the
measurements are made with the pole-pole array where only the C1 and P1
electrodes are used. This example has a 9 by 9 surface grid and 4 boreholes
with 8 electrodes in each borehole. Thus the total number of electrodes is 123.
BOR9ALL.DAT file Comments
1 BLOCK | Title
9 | x surface grid size
9 | y surface grid size
1.00 | x unit electrode spacing
1.00 | y unit electrode spacing
12 | array number 12 for borehole survey
Number of boreholes | Header
4 | The number of boreholes
Borehole 1 | Header
8 | Number in electrodes in borehole 1
2.0 2.0 1.0 | x, y and z location of 1st electrode in borehole
2.0 2.0 2.0 | x, y and z location of 2nd electrode in borehole
2.0 2.0 3.0 | x, y and z location of 3rd electrode in borehole
2.0 2.0 4.0 |
2.0 2.0 5.0 |
2.0 2.0 6.0 |
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2.0 2.0 7.0 |
2.0 2.0 8.0 | x, y and z location of last electrode in borehole
Borehole 2 | Header for 2nd
borehole
8 | Number of electrode in 2nd
borehole
6.0 2.0 1.0 | The following section gives the x, y and z location
6.0 2.0 2.0 | of the borehole electrodes starting from the first
6.0 2.0 3.0 | borehole electrode.
6.0 2.0 4.0 |
6.0 2.0 5.0 |
6.0 2.0 6.0 |
6.0 2.0 7.0 |
6.0 2.0 8.0 |
Borehole 3 | Header for 3rd
borehole
8 |
6.0 6.0 1.0 |
6.0 6.0 2.0 |
6.0 6.0 3.0 |
6.0 6.0 4.0 |
6.0 6.0 5.0 |
6.0 6.0 6.0 |
6.0 6.0 7.0 |
6.0 6.0 8.0 |
Borehole 4 | Header for 4th
borehole
8 |
2.0 6.0 1.0 |
2.0 6.0 2.0 |
2.0 6.0 3.0 |
2.0 6.0 4.0 |
2.0 6.0 5.0 |
2.0 6.0 6.0 |
2.0 6.0 7.0 |
2.0 6.0 8.0 |
6328 | Number of data points
2 0.0 0.0 0.0 1.0 0.0 0.0 99.39 | For each datum point, the first parameter is
2 0.0 0.0 0.0 2.0 0.0 0.0 98.50 | the number of electrodes used (2 for pole-pole).
2 0.0 0.0 0.0 3.0 0.0 0.0 97.63 | This is followed by the x, y and z location of the
2 0.0 0.0 0.0 4.0 0.0 0.0 97.14 | current electrode C1, and then the x, y and z
2 0.0 0.0 0.0 5.0 0.0 0.0 97.23 | location of the potential electrode P1.
2 0.0 0.0 0.0 6.0 0.0 0.0 97.73 |
2 0.0 0.0 0.0 7.0 0.0 0.0 98.35 |
2 0.0 0.0 0.0 8.0 0.0 0.0 98.92 |
2 0.0 0.0 0.0 0.0 1.0 0.0 99.39 |
2 0.0 0.0 0.0 1.0 1.0 0.0 98.76 |
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.
.
.
2 2.0 6.0 7.0 2.0 6.0 8.0 99.38 | Last datum point, followed
0 | by a few 0’s. If there is topography,
0 | the topographical information should be entered
0 | after the apparent resistivity data using the same
0 | format as for standard surface surveys.
The file BORHOLE3D.DAT gives an example where the pole-dipole array is
used. The format for the initial section is the same as for the above example,
the only difference is the x, y and z location of the P2 electrode is also entered
in the data section. Below part of the data section is shown with comments.
BOREHOLE3D.DAT file Comments
2642 | Number of data points
3 0.0 0.0 0.0 1.0 0.0 0.0 2.0 0.0 0.0 100.273 | The format for each datum point
3 0.0 0.0 0.0 2.0 0.0 0.0 3.0 0.0 0.0 100.255 | is :-
3 0.0 0.0 0.0 3.0 0.0 0.0 4.0 0.0 0.0 99.094 | Number of electrodes used in
3 0.0 0.0 0.0 4.0 0.0 0.0 5.0 0.0 0.0 96.780 | measurement; x-, y- and z-
3 0.0 0.0 0.0 5.0 0.0 0.0 6.0 0.0 0.0 94.732 | location of C1 electrode;
3 0.0 0.0 0.0 6.0 0.0 0.0 7.0 0.0 0.0 93.977 | x-, y- and z- location of P1
3 0.0 0.0 0.0 7.0 0.0 0.0 8.0 0.0 0.0 94.352 | electrode; x-, y- and z- location
3 0.0 0.0 0.0 0.0 1.0 0.0 0.0 2.0 0.0 100.273 | of P2 electrode; apparent
3 0.0 0.0 0.0 0.0 2.0 0.0 0.0 3.0 0.0 100.254 | resistivity value.
3 0.0 0.0 0.0 0.0 3.0 0.0 0.0 4.0 0.0 99.094 |
If 4 electrodes are used in a measurement, the x-, y- and z- location of the C1,
C2, P1 and P2 electrodes are given in that order.
The array number is 12 for cross-borehole survey data with the measurements
given as apparent resistivity values. You can also enter the data as resistance
measurements, in which case the array number is 13. This might be more
convenient as most resistivity meters give the readings as a resistance value in
ohm, and furthermore the geometric factor for arrays with subsurface
electrodes is different from the geometric factor for conventional surface
arrays.
If IP data is present, the format is similar to that used for normal surface
surveys. The type of IP information is entered after the number of data points
and before the apparent resistivity and IP values.
For a computer system with 512MB RAM, the maximum number of boreholes
allowed is 12. The maximum total number of electrodes in the boreholes is
500. For example, if there are 10 boreholes, then each borehole can have a
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maximum of 50 electrodes. These limits are higher for systems with more
RAM.
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Appendix P : Non-rectangular (trapezoidal) survey grids
In areas with rugged and forested terrain, it is frequently not possible to place
the survey lines in a perfect rectangular gird. This option allows moderate
departures from a perfectly rectangular arrangement of the electrodes. Figure
15 gives a sketch of a possible electrodes arrangement that deviate from a
rectangular grid and effectively forms a trapezoidal grid. Along each survey
line, the number of electrode still must be the same. The example has a 7 by 5
grid, so there still must be 7 electrodes in each row. The survey lines need not
be orthogonal, but deviations of greater than 30 degrees will probably not be
advisable.
In the data file, the x-, y- and z-coordinates of each electrode in the grid must
be given. The x and y coordinates must be horizontal coordinates, and not
distances along the ground surface, while the z coordinate is the elevation. The
file TRAPEZOIDAL.DAT gives an example of the data format used.
TRAPEZOIDAL.DAT file Comments
2 BLOCKS with Shifted Y lines | Title
14 | x grid size
14 | y grid size
Trapezoidal Grid used | Header to indicate trapezoidal survey grid
Location of electrodes given line by line | Header
Line 1 | Header for Line 1
0.0,0.0,0.0 | (x,y,z) coordinate of first electrode on Line 1
3.0,0.0,0.0 | (x,y,z) coordinate of second electrode on Line 1
6.0,0.0,0.0 | Same for other electrodes on Line 1
9.0,0.0,0.0 |
12.0,1.0,0.0 | Note shift of 1 m. for
15.0,1.0,0.0 | these 3 electrodes
18.0,1.0,0.0 | in y-direction
21.0,0.0,0.0 |
24.0,0.0,0.0 |
27.0,0.0,0.0 |
30.0,0.0,0.0 |
33.0,0.0,0.0 |
36.0,0.0,0.0 |
39.0,0.0,0.0 | (x,y,z) coordinate of last electrode on Line 1
Line 2 | Header for line 2
0.0,3.0,0.0 | (x,y,z) coordinate of first electrode on Line 2
3.0,3.0,0.0 | (x,y,z) coordinate of second electrode on Line 2
. | Similar information is given for all
. | electrode and survey lines until last line
. |
36.0,39.0,0.0 | (x,y,z) coordinate of second last electrode on Line 14
39.0,39.0,0.0 | (x,y,z) coordinate of last electrode on last Line
Type of geometric factor (0=horizontal, 1=linear, 2=user defined) | Header
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0 | 0 to indicate horizontal distance used to calculate geometric factor
2 | Array type. Pole-pole in this case. Rest of data follows the
| standard format.
1274 | Number of data points
0.00, 0.00 0.00, 3.00 101.4090 | Location of electrodes and apparent resistivity
0.00, 0.00 0.00, 6.00 102.4530 | Same for the rest
0.00, 0.00 0.00, 9.00 103.3240 |
.
.
One possible ambiguity when the ground surface is not flat is in the geometric
factor used, i.e. whether it is calculated using the horizontal coordinates alone
or linear distances where the z-coordinate is also used in the calculation of the
distance between the electrodes. The type of geometric factor is probably not
important, but the program must use the same geometric factor as that used by
the user in calculating the apparent resistivity value from the potential and
current values. In the data format, the type of geometric factor used must be
specified to avoid any ambiguity. Most survey will probably calculate the
distance between the electrodes using the (x,y) coordinates only, which can
usually be derived from GPS readings. In the above example, a value of 0 is
used to indicate that the geometric factor is calculated using horizontal
distances. Another possibility is to use the linear distance between the
electrodes by using the (x,y,z) coordinate (value of 1 in the above data format)
of each electrode.
In some surveys, the geometric factor might be calculated using the distance
along the ground surface, or some other criteria. To accommodate for this
possibility, a user-defined option is available. In this case the value of the
geometric factor must be entered into the data file after the apparent resistivity
value (or after IP value if present) for each data point. The file
BLOCKS_TRAP.DAT is an example with a user defined geometric factor
(actually calculated using the surface distance).
In the above TRAPEZOIDAL.DAT file, the elevation of the electrodes is set at
0.0 in which case there is actually no topography. If topography is present, the
elevation is given as the z coordinate of the electrode, as in the example
BLOCKS_TRAP.DAT file.
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Figure 15. Schematic diagram of a survey where the electrodes are displaced
from a rectangular grid pattern.
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Appendix Q : Special topography information for mineral exploration
surveys
The RES3DINV software was originally written for surveys by engineering
and environmental geophysicists. Thus the default settings in the program
follow the practices in this field. One important difference is that in
engineering/environmental surveys, the electrodes are laid out such that the
distance along the ground surface is constant by using a multicore cable with a
constant spacing between the takeouts. This saves time and the expense of
carrying out a detailed topographical survey before the resistivity survey. The
RES2DINV program assumes that the x-locations of the electrodes are given in
terms of distances along the ground surface.
However for mineral exploration surveys (with bigger budgets) it is common to
carry out a detailed topographical survey before the resistivity and IP survey,
and the electrodes are placed such that the true horizontal distance (rather than
the surface distance) between the electrodes is constant. In areas with steep
topography, it is possible for the vertical distance between two adjacent
electrodes to exceed the horizontal distance (but still less than the surface
distance). For surveys where the true horizontal distance is used for the x- and
y- locations of the electrodes, the indicator in the topography data section for
the type of x- and y- coordinates must be given as 1 instead of 2 (which
indicates surface distances are used). A description is given in pages 10 and 11
of this manual.
In very rugged areas, it is possible that the survey grid is not exactly
rectangular in shape. For such a situation, the non-rectangular or trapezoidal
survey grid data format must be used (see Appendix P). In this data format, the
true horizontal x-, y- and z- coordinates of all the electrodes must be given.
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DISCLAIMER
This software is provided "as is" without any express or implied warranties
including its suitability for a particular purpose. Neither the author nor the
subagent will assume responsibility for any damage or loss caused by the use
of this program. In the event of program bugs, every effort will be made to
correct the bugs and a replacement program will be provided for a nominal
charge to cover postage and handling. Faulty dongles or program disks will be
replaced free of charge within the first 6 months from the date of purchase to
registered users. Registered users are entitled to free updates and support
within the first year.
SUPPORT
Please contact your distributor if you have any questions concerning this
program. A frequent source of queries is when the results obtained are not what
you expect, or the program refuses to accept your data file. In such a case,
support will only be provided if a copy of the data file is sent together as an
attachment with your email, preferably compressed into a ZIP file. The most
common causes of problems are (a) the data in the DAT file is not given in the
correct RES3DINV format (b) excessive noise in the data set (particularly for
dipole-dipole surveys and/or I.P. data sets) (c) instrument malfunction or field
survey errors. It is only possible to trace the source of the problem if a copy of
the data file is provided.
Please note that support is only available for registered users of the full version
of the program. It is not possible to provide technical support for users of the
demo version of this software. For technical support, you can contact the
software provider by email at the following addresses. Please attach a copy of
your data file to your email.
geotomo@tm.net.my
support@geoelectrical.com
The following information will be required before technical support is
provided.
1). Your dongle ID number. This ID number is displayed near the bottom of
the initial information box when the RES3DINV program starts up if the
dongle is attached to the computer.
2). The name of your company or institution.
3). Approximate date (month/year) your company or institution purchased the
software.
Copyright (2000-2004) Geotomo Software
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What’s New
Version 2.00 - Upgrade to Windows 3.1 and 95.
Version 2.01 - Code optimisations for Windows 95/NT systems. Maximum
grid size increased to 43 by 43 (1849) electrodes for systems with 128 MB
RAM.
Version 2.03 - Support for non-uniform survey grids
Version 2.04 - Support for robust inversion.
Version 2.05 – Support for equatorial dipole-dipole (rectangular) array.
Options to save the model resistivity and I.P. values in the Rockworks and
Slicer/Dicer programs added.
Version 2.06 - Option to use 4 nodes between adjacent electrodes added. This
gives greater accuracy, but at a huge increase in the computer time, for cases
with large resistivity contrasts near the surface. The user can now stop the
inversion process at the end of the current iteration by pressing the 'N' key.
After displaying the model sections, the user can view the results from the later
or earlier iterations by pressing the 'PgUp' or 'PgDn' key.
Version 2.07 – Options to carry out the inversion of IP data sequentially and to
determine the IP damping factor automatically were added.
Version 2.10 – Incomplete Gauss-Newton inversion option added.
Version 2.11 – Data compression option for the incomplete Gauss-Newton
method added. Maximum number of electrodes increased to 5041, and
maximum number of data points to 67500, for systems with 1 GB RAM. The
computer must have Win NT/2000/XP and a hard-disk using the NTFS file
system format.
Version 2.12 – Option to use 3 nodes between adjacent electrodes added.
Option to save model values in a binary format for the Slicer-Dicer program
also added.
Version 2.13 – Support for 3-D underwater surveys added. Optimisation made
to inversion code for data from surveys with multi-channel instruments. For
such surveys, the inversion subroutine is about 20-50% faster than version
2.11.
Version 2.14 – Support for borehole surveys and trapezoidal survey grids
added.