Copyright (1999-2001) M.H.Loke
RES3DMOD ver. 2.1
for Windows 95/98/Me/2000/NT
3-D resistivity & IP forward modeling
using the finite-difference and finite-element methods
(Wenner, dipole-dipole, pole-pole, pole-dipole,
Schlumberger, rectangular)
by
M.H.LOKE
March 2001
Freeware courtesy of M.H.Loke
Table of Contents
Topic Page No.
1 Introduction 1
2 Computer System Requirements 2
3 Theory 2
4 Program files 3
5 Data file options and model data format 4
6 Edit and display functions 8
7 Modifying the program settings 10
8 Calculating the potential values 11
9 Printing the results 12
References 12
1 2
Copyright (1999-2001) M.H.Loke
1 Introduction
This is a Windows based 3-D resistivity and IP modeling program This program
calculates the apparent resistivity values for a survey carried out with a rectangular
grid of electrodes (Figure 1) using the finite-difference (Dey and Morrison 1979) or
the finite-element (Silvester and Ferrari 1990) method.
Figure 1. The arrangement of the electrodes in a 3D resistivity imaging survey, and
two possible measurement techniques with the pole-pole array.
The maximum survey grid size the program can handle depends on the amount of
RAM available. It can handle a survey grid with up to 2500 electrodes (with the
finite-difference modelling method) if the computer has at least 128 MB RAM.
2 Computer System Requirements
This program requires the Windows 95/98/Me/2000/NT operating system. It must
have at least 32 MB RAM, and preferably at least 64 MB RAM. The hard-disk
should have at least 64 Megabytes of free space that the program might need for
temporary files. You should also use Windows in the SuperVGA 800x600 or
1024x768 (or even higher) graphics mode with 256 colors. The program will also run
with the 16-bit or 24-bit colors modes, but if you encounter problems it is
recommended that you switch to the 256 colors (8-bit) mode. To edit the 3-D
models on the screen, a sufficiently high resolution graphics mode must be used. If
you are still using the standard VGA 640x480 graphics mode, please switch to the
800x600 mode. It is expected that the computer used is a Pentium II or faster
system, although the program will also run on the older Pentium and 80486 system.
3 Theory
This program uses the finite-difference method. Figure 2 shows a schematic
diagram of the arrangement of the nodes in a 3-D mesh. In practice, the
RES3DMOD program uses many more nodes, with 2 nodes between adjacent
electrodes and 8 buffer nodes at the sides of the mesh.
Figure 2. Schematic diagram of 3-D mesh.
3 4
Copyright (1999-2001) M.H.Loke
To use this program, the user must supply the resistivity value of each cell in the 3D
mesh using an input text file. The relationship between the location of the
electrodes and the cells in the 3D mesh is shown in the following diagram.
Figure 3. An example 3D model that shows the location of the electrodes.
Note that there are two cells between adjacent electrodes, and the thickness of
each layer increases with depth. The resistivities of the cells at the sides and
bottom of the central region of model bounded by the grid of electrodes are
automatically extended to the sides and bottom of the finite-difference mesh used.
If the model has IP values, the IP value for each cell is arranged in a similar manner.
An option to use the finite-element method is also available. However, at present, it
is not recommended as it is much slower than the finite-difference method.
4 Program files
The RES3DMOD 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/2000/NT, click Start, and then Settings followed by Control
Panel, and then Add/Remove Programs. Then use Browse to navigate to the folder
where the SETUP.EXE program is located.
The RES3DMOD package should have the following files :BLOCK8.MOD
Example input model file with a small 8 by 8 survey grid.
BLOCK11.MOD Example input resistivity model file for a 11 by 11 survey grid.
BLOCK11X.MOD The above model with the calculated potential values. You can
read this file to display the apparent resistivity
pseudosections without the need to calculate them.
BLOCK17.MOD Example input resistivity model file for a 17 by 17 survey grid.
MODELIP.MOD Example input model with both resistivity and IP. An 11 by 11
survey grid is used.
BLOCK10F.MOD A model with remote electrodes.
RES3DMOD.EXE The forward modeling program.
COLOUR.COL Color configuration file
RES3DMOD.CFG Configuration file for RES3DMOD
RES3DMOD.CF2 Alternative configuration file for RES3DMOD
5 Data file options and model data format
After starting the program, you can test it by choosing the File option. On
selecting the file option, the following menu will be displayed.
Read model data – This option enables you to read in a data file containing the
model parameters. As an initial test, select the example file BLOCKS8.MOD. The
program uses a finite-difference mesh with an x- or y-spacing that is half the unit
electrode spacing. You basically need to specify the resistivity of the sub-blocks
within the area covered by the grid of electrodes. As an example of the data format,
the contents of the BLOCKS8.MOD file is listed below with a description of the
input parameters.
5 6
Copyright (1999-2001) M.H.Loke
BLOCKS8.MOD file | Comments
-------------------------------------------------------------------------------------------------
2 BLOCKS | Title
8,8 | Number of electrodes in x and y directions, i.e. survey grid size
1.0,1.0 | Electrode spacing in x,y directions
3 | Number of model resistivity values
1.0,10.0,100 | The model resistivity values
7 | Number of layers in model
LEVEL 1 | Following is data for layer 1
11111111111111 | Resistivity of blocks in layer 1, note 2 blocks per
11111111111111 | electrode spacing
11111111111111 | 0 stands for first resistivity value, i.e. 1.0
11111111111111 | 1 stands for second resistivity value, i.e. 10.0
11222222221111 | 2 stands for resistivity first resistivity value, i.e. 100
11222222221111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
LEVEL 2 | Model data for layer 2
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11222222221111 |
11222222221111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
. | Repeat data for all layers
. |
. |
LEVEL 7 Last layer
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
11111111111111 |
1 | A "1" here to indicate user defined depths for the model layers
0.30 | Followed by the depth to the base of each model layer.
0.40 | Note there are 7 values given here for the 7 model layers
0.90 | in this example.
1.60 |
2.40 |
3.50 |
5.00 |
0 | Followed by a few 0's
0 |
0 |
0 |
This survey grid in this example has 8 electrodes in the x-direction, thus there are
(8-1)*2=14 model blocks between the first and last electrode. Similarly for the ydirection.
Try reading this file using the ‘File’ option as described above, and then
click the Calculate option to calculate the apparent resistivity values. On a 200 Mhz
Pentium Pro microcomputer, the calculations take about 22 seconds. Since this
model has a rather small electrode survey grid, choose the Pole-pole array if you
wish to display the apparent resistivity values. To save the calculated apparent
resistivity values, click File and then the "Save results in RES3DMOD format"
suboption.
The file MODELIP.MOD is an example model file with IP values as well. The IP
models values are listed after the resistivity model values. Part of the
MODELIP.MOD file is shown below with comments.
7 8
Copyright (1999-2001) M.H.Loke
MODELIP.MOD file | Comments
-------------------------------------------------------------------------------------------------
11111111111111111111 | Last 2 lines of the model resistivity
11111111111111111111 | values.
IP Model Values | This line to show IP values are present
3 | Number of IP values
0.10,0.00,0.20 | The IP values
LEVEL 1 | Following is IP data for layer 1
11111111111111111111 | IP of blocks in layer 1
11111111111111111111 | The format is the same as that
11111111111111111111 | used for the model resistivity values.
11111111111111111111 | The number of layers must be the same
11112222221111111111 | as that used for the resistivity model.
This model shows interesting “anomaly inversion” effects in both the apparent
resistivity and apparent IP values.
Note that the model IP values are given as V/V values, and not mV/V values. If you
use chargeability values of greater than or equals to 1, the program will assume
that they are given in the mV/V unit, and it will divide the values by 1000 to convert
them into V/V values.
Save results in RES3DINV format – After calculating the potential values, you
can save the apparent resistivity values into the format used by the RES3DINV
inversion program.
Save results in RES3DMOD format – After calculating the potential values, you
can save the model parameters, as well as the calculated potential values into the
format used by this program. This saves you some time in calculating the potential
values in future for the same model. This is particularly useful if the model has a
very large grid.
6 Edit and display functions
These are a number of functions that enable you to edit the model and to display
the apparent resistivity values. On selecting this option on the main menu bar, the
following list of options will be displayed.
Edit resistivity model – This option will display the input model in grid form
(Figure 3). By using the mouse, you can change the resistivity values of the cells.
Click ‘Help’ in the Edit Model Mode to see the mouse and keys to use.
Figure 3. An example 3D model that shows the location of the electrodes.
9 10
Copyright (1999-2001) M.H.Loke
Display apparent resistivity – This option enables you to display the apparent
resistivity values in the form of vertical or horizontal pseudosections. The
following dialog box will be displayed when you select this option.
Figure 4. Dialog box showing the different options for displaying the apparent
resistivity values.
In this option, you can select the array type, the measurement direction of the
apparent resistivity values to be displayed, the choice of horizontal or vertical
pseudosections, and the apparent resistivity or IP values.
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.
Save contour values – This option saves the values used to draw the contours in
the pseudo and model sections into a file. This is useful when you want to use the
same contour values in different displays.
Default colour scheme - This will reset the colour scheme used for colouring the
sections to a default system used by the program.
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.
Use Text Editor - When you select this option, the text editor (by default
NOTEPAD) will start up. To return to the RES2DINV program, you must first exit
from the text editor program.
Run another program - This option enables you to run another program. It is
mainly intended for cases where the program had not been installed and thus not
in the list of programs recorded in the Windows Start and Program option.
7 Modifying the program settings
You can change the setting used by the program by selecting the ‘Modify
Parameters’ option on the main menu bar. When you select this option, the
following menu list will be displayed.
Normalisation –The potential values calculated for your model is normalised by
using the potential values for a homogenous earth model. You can disable this
normalization procedure in this option. By default, the normalisation procedure is
used.
11 12
Copyright (1999-2001) M.H.Loke
Geometric factor – This option is only relevant for the pole-pole and pole-dipole
arrays where the location(s) of the remote electrode(s) are given in the model file.
In this case, you can choose to use the geometric factor for the ideal pole-pole or
pole-dipole array, or the true geometric factor by taking into consideration the
locations of the remote electrodes, when calculating the apparent resistivity
values.
Finite element – When you select this option, the following dialog box is
displayed.
Firstly, you can choose to use either the finite-difference or finite-element method
to calculate the apparent resistivity values. By default, the program uses the finitedifference
method that is much faster. For the finite-element method, you can
choose to use standard tetrahedral elements (Silvester and Ferrari 1990), or a
modified symmetrical formulation with tetrahedral elements.
8 Calculating the potential values
When you are ready to calculate the potential values for your model, just click the
‘Calculate’ option on the main menu bar. The calculation will take a few seconds to
tens of minutes depending on the speed of your computer processor and the grid
size of your model. For each electrode position, the program calculates the
potential at all other electrodes due to a current electrode at that position. Thus
once the potential values have been calculated, the apparent resistivity due to any
array can be determined without the need to recalculate the potential values. You
only need to recalculate the potential values if you change the resistivity of the
cells in the model.
9 Printing the results
When you select this option, the following menu will be displayed.
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, click the
Print option. This will display options to save the screen as a BMP file, a PCX file
or to call 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.
References
Dey, A. and Morrison, H.F., 1979. Resistivity modelling for arbitrary shaped twodimensional
structures. Geophysical Prospecting, 27, 1020-1036.
Silvester P.P. and Ferrari R.L., 1990. Finite elements for electrical engineers (2nd.
ed.) . Cambridge University Press.