Copyright (1996-2002) M.H.Loke
RES2DMOD ver. 3.01
Rapid 2D resistivity forward modelling using the finitedifference
and finite-element methods
Wenner (alpha, beta, gamma), inline & equatorial dipole-dipole,
pole-pole, pole-dipole and Wenner-Schlumberger
by
M.H.LOKE
mhloke@pc.jaring.my
July 2002
Table of Contents
Topic Page No.
1 Introduction 1
2 Computer System Requirements 2
3 Freeware 2
4 Theory 3
5 File operations 5
6 Editing and displaying models 9
7 Options 13
8 Model Computation 13
9 Print 14
10 Help 14
11 Printing the pseudosections 17
12 Pitfalls 15
Disclaimer 15
References 16
Appendix A : Underwater surveys 17
Appendix B : I.P. models 19
Appendix C : Calculation of 2D sensitivity values
(limited availability) 20
Preview – What is this? 27
What's New 28
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Copyright (1996-2002) M.H.Loke
1 Introduction
RES2DMOD is a free 2D forward modelling program that calculates the apparent
resistivity pseudosection for a user defined 2D subsurface model. It is largely
intended for teaching undergraduates and postgraduate students about the use of
the 2D electrical imaging method. The program might also assist the user in
choosing the appropriate array for different geological situations or surveys.
The arrays supported by this program are the Wenner (Alpha, Beta and Gamma
configurations), pole-pole, gradient, inline dipole-dipole, pole-dipole and
equatorial dipole-dipole (Edwards 1977). Each type of array has its advantages
and disadvantages. This program will hopefully help you in choosing the "best"
array for a particular survey area after carefully balancing such factors as the cost,
depth of investigation (or equivalent depth), resolution and practicality.
The following figure shows an overhead view of the arrangement of the electrodes
for the different arrays.
Figure 1 Arrangement of the electrodes for different arrays.
You can use a model with a maximum of 151 electrodes (with 2 nodes per unit
electrode spacing) or 75 electrodes (with 4 nodes per unit electrode spacing). This
limit is set by the resolution of the computer graphics display.
2 Computer System Requirements
RES2DMOD is a 32-bit Windows based program that will run under Windows
95/98/Me/2000/NT. You will need a Pentium, Pentium Pro, Pentium II, Pentium
III, or Pentium 4 (or compatible) based microcomputer to run this program. The
minimum amount of RAM is 32 megabytes, but 64 megabytes is recommended!
You will also need about 128 megabytes of free hard-disk space that 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.
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/NT if you encounter problems. Graphics operations with the 256
colours mode will be faster, and more memory will be available to your programs.
The RES2DMOD 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/ NT, 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.
3 Freeware
This program is released as freeware. However, I am not liable for any problems
or possible damages arising from the use of this program. If you find this program
useful, please send an email to me at mhloke@pc.jaring.my. I will try to fix bugs,
and provide more features in future versions, but that will depend on my free time.
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Copyright (1996-2002) M.H.Loke
4 Theory
The 2-D model used by the finite-difference or finite-element method divides the
subsurface into a number of blocks using a rectangular mesh (Figure 2). Some
improvements were made to the Dey and Morrison (1979) finite-difference
formulation to improve the accuracy of the calculated apparent resistivity values
(Loke 1994). The finite-difference method basically determines the potential at the
nodes of the rectangular mesh that consists of L nodes in the horizontal direction
and M nodes in the horizontal direction.
Figure 2 Schematic diagram of the finite-difference or finite-element mesh used
by the program.
Note that the grid model has L-1 columns and M-1 rows of rectangular blocks.
The blocks can have different resistivity values. By using a sufficiently fine mesh,
complex geological structures can be modelled. The program uses a mesh with two
or four horizontal nodes per unit electrode spacing for a multi-electrode array
(Figure 3).
The program requires you to supply the resistivity values of the rectangular blocks
in between the mesh lines (and other information) using an input data file. The
format of the input data file is described in section 5. In general, you should try to
use "reasonable" model resistivity values that are not too small or too large.
The first electrode is placed at the 12th horizontal node from the left edge of the
mesh. Similarly there are 12 nodes between the last electrode and the right edge of
the mesh. Table 1 shows the vertical depths and horizontal offsets of the mesh
lines for an electrode spacing of 1 meter for the default model used by this
program. Note this example has 4 vertical mesh lines between adjacent electrodes.
You can also set your own depths for the mesh lines. If you choose to set the
depths of the grid lines, use a smaller vertical spacing (∆zj in Figure 1) between
adjacent horizontal mesh lines near the surface and larger spacings for the deeper
mesh lines to ensure that the results will be sufficiently accurate. In particular, the
vertical spacings between the top 3 horizontal mesh lines should not be greater
than the horizontal spacings between adjacent vertical mesh lines. The files
FAULT.MOD and MODEL41.MOD are example model files with user defined
depths for the mesh lines.
If this is the first time you are using the program, try reading in the example model
data file TIXALL.MOD provided with the program using the "File" option in the
Main Menu. Then select the "Model Computation" option to calculate the
apparent resistivity values for this model. After that, use the "Edit/Display" option
to take a look at the 2-D model and the apparent resistivity pseudosection. In the
following sections, a more detailed description of each item in the above menu is
given.
Figure 3 Part of the finite-difference or finite-element mesh showing the location
of the electrodes.
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Copyright (1996-2002) M.H.Loke
Table 1 : Locations and spacings between the nodes in the 2-D finite-difference
grid model using a predefined set of depths for the horizontal mesh lines. The first
electrode on the surface is located at the 12th horizontal node. Subsequent
electrodes are placed at 4 horizontal nodes apart. The electrode spacing is 1 metre.
The horizontal spacings between the last 11 nodes after the last electrode are the
same as that for the first 12 nodes (except in reverse order).
Horizontal Node Vertical Node
Index Location Spacing Index Depth Spacing
i xi dxi j zj dzj
1 0.00 64.0 1 0.25 0.25
2 64.00 32.0 2 0.50 0.25
3 96.00 16.0 3 0.75 0.25
4 112.00 8.0 4 1.00 0.25
5 120.00 4.0 5 1.55 0.55
6 124.00 2.0 6 2.16 0.61
7 126.00 1.0 7 2.82 0.67
8 127.00 0.50 8 3.55 0.73
9 127.50 0.50 9 4.36 0.81
10 127.75 0.25 10 5.24 0.89
11 128.00 0.25 11 7.02 1.77
12 128.25 0.25 12 10.56 3.54
13 128.50 0.25 13 17.64 7.09
14 128.75 0.25 14 31.82 14.17
15 129.00 0.25 15 60.16 28.35
16 129.25 0.25 16 116.85 56.69
17 129.50 0.25 17 230.23 113.38
5 File operations
On clicking the ‘File’ option on the top Main Menu bar, the following submenu
will be displayed.
Read data file with forward model - In this option, you read in a model data file.
When you select this option, the list of files in the current directory (or the last
subdirectory from which you read in a model file) which has an extension of
.MOD will be displayed.
The program requires the resistivity model values to be typed in separately in a
text file. For an example of the input data format, you should make a printout the
MODEL41.MOD data file. The parameters for the finite-difference model are
arranged in the following format.
Line 1 - Name of model
Line 2 - Number of electrodes (Maximum of 101)
Line 3 - Number of pseudosection data levels (Absolute maximum of 32 but also
depends on the type of array and the number of electrodes).
Line 4 - Flag for underwater survey. Enter 0 for now.
Line 5 - Unit (or smallest) electrode spacing
Line 6 - Flag for type of grid model. Enter 0 or 1 for a grid model with the default
depths for the mesh lines. Enter 2 for user defined depths for the mesh lines (see
the file MODEL41.MOD). The file MODEL25.MOD gives an example of a
model file which uses the default (option 0) depth values given in Table 1. If
option 1 is used, the depths are scaled according to the depth of investigation of
the array used.
Line 7 - Offset of first block in user-defined model section from the first
electrode. Normally enter 0 to avoid problems.
Line 8 - Number of blocks in user-defined model. To avoid problems, use a value
of (Number of electrodes - 1) * Number of nodes/unit electrode spacing which
will cover all the blocks between the first and last electrode.
Line 9 - Number of resistivity values in the model (maximum of 16)
Line 10 - Number of nodes per unit electrode spacing (2 or 4). You can use a
value of 4 for up to a maximum of 75 electrodes.
Line 11 - The model resistivity values.
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Line 12 - Number of rows of rectangular blocks in the model (maximum of 29).
Note that the number of vertical mesh lines is equals to the number of rows of
blocks plus one.
Line 13 - The depth to the horizontal grid mesh line.
Lines 14 to 27 - The resistivity of the model blocks are given row by row.
A number "0" means the block has the first resistivity value, "1" for the second
resistivity value and so on. For the 11th to the 16th resistivity values, use the
letters "A" to "F".
Lines 28 - Type of array. 1 for Wenner, 2 for two-pole, 3 for dipole-dipole,
4 for Wenner Beta, 5 for Wenner Gamma, 6 for inline pole-dipole, 7 for WennerSchlumberger
and 8 for equatorial dipole-dipole array. For the equatorial dipoledipole
array, you will also need to enter the separation between the current
electrode pair. It is assumed that the potential electrode pair will have the same
separation. Please refer to the file MODELEQ.MOD for an example of this array
format.
Lines 29 and 30 - Enter 0 for both. These are flags for calculated apparent
resistivity and potential values which are used for the output data file produced by
this program.
Lines 31, 32 and 33 - Enter 0 for now. They are reserved for possible new
features in future versions of this program.
Note that you only have to set the resistivities for a limited section of finitedifference
grid model. The program assumes that the resistivity of the blocks to the
left side of the first electrode is the same as that of the first model block for which
you have set the resistivity value. Similarly the resistivity of the blocks on the right
side are set to be the same as that of the last block of the rightmost column in the
user-defined model section. Normally it would probably be most convenient to set
the resistivities of all the blocks between the first and last electrode of the multielectrode
array. In this case, the offset of the first block in your model from the
first electrode is 0 (data for line 7 above). If the total number of electrodes is E and
the number of nodes per unit electrode spacing is n, then the number of blocks
between the first and last electrode is (E-1)*n. For an example, see the data file
MODEL25.MOD.
As an example of model file where the depths to the mesh levels are specified, the
upper part of the FAULT.MOD file with comments are given as follows.
Data in FAULT.MOD file Comments
Fault and block model | Title
51 | 51 electrodes in line
15 | 15 data levels
0 | Normal land survey, i.e. not underwater
1.0 | Unit electrode spacing
2 | 2 for user defined mesh lines depths
0 | Offset of first block form first electrode
200 | Total number of blocks in user model
3 | Indicates three model resistivity values
4 | Four nodes between adjacent electrodes
10.0,50.0,100.0 | The model resistivity values
14 | Fourteen rows of model blocks
0.2500,0.5000,0.8125,1.1875,| The depth to the base of each row starting from the
1.6875,2.3125,3.1875,4.4375,| top. Note the smaller distance between the top
6.4375,10.4375,18.4375,34.4375,| two mesh lines, and the large spacings near the
66.4375,130.4375 | bottom
00000000000000000000000000000| The usual model codes start here
The file LANDFILL.MOD gives an example of an input model file which also
contains measured apparent resistivity values from a field survey. The required
information is placed in after the value for the type of array. Below is part of this
model file with comment about the format of the data (the format is similar to that
used by the RES2DINV inversion program).
Data in LANDFILL.MOD Comments
999999999999999999999999 | Last part of
999999999999999999999999 | model codes.
1 | Wenner array flag
0 | Put 0's in these 2 lines. These are flags used by
0 | RES2DMOD for the output file it produces.
2 | Put 2 here for only measured app. resis. values.
334 | Number of datum points
1 | 0 for location of first electrode, 1 for midpoint
0 | Put zero here, will be used later for I.P. flag
4.50000, 3.00000, 84.90 | The following lines give the field data, arranged
7.50000, 3.00000, 62.80 | as x-location of datum point, electrode spacing,
10.50000, 3.00000, 49.20 | apparent resistivity value. See manual for inversion
13.50000, 3.00000, 41.30 | program for details. At the end of the file, put four 0's.
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One common problem encountered in using this program is mistakes in the input
data file. If the program stops with an error message it could be due to a mistake in
the input data file. First check that the data in the file is arranged according to the
format described above.
It is important that you do not edit the output files produced by the program. The
program saves the apparent resistivity and potential values calculated in the 'Model
Computation' option in its own format.
Save results in RES2DMOD format - In this suboption, you can save the apparent
resistivity values into a file with the format used by the RES2DMOD program.
Besides the model information, the apparent resistivity and potential values will be
saved. By reading this file again, you can display the apparent resistivity values
without having to recalculate the potentials.
Use RES2DINV format - This will save the apparent resistivity values into the
format used by the RES2DINV inversion program. Note that you cannot read this
output file with the RES2DMOD program since it is intended for the RES2DINV
program.
6 Editing and displaying models
On selecting the ‘Edit’ option on the main menu bar, the following menu will be
displayed.
Edit model - Under this option, you can change the resistivity of the blocks in the
model interactively using the mouse or keyboard. The individual rectangular
blocks and the finite-difference grid together with the apparent resistivity
pseudosection are displayed.
To change the resistivity of a single block, move the "+" shaped cursor with the
mouse to the centre of the block. Then click the right mouse button. The colour of
the block should change to white. After that move the cursor to one of the
rectangular blocks in the legend just above the top left of model grid section. Press
the left mouse button and the colour of the small rectangular block within the
model should change to the colour you have chosen. You can change the
resistivity of several blocks to the same resistivity value by first clicking them with
the left mouse button, and then click the appropriate block on the legend. The
following keys are also used :Left
mouse button or "4" key - Change the resistivity of 4 blocks to the right of the
cursor.
"D" key - Change the resistivity of a whole column of blocks downwards to the
bottom edge of model.
"]" key - Change the resistivity of a whole row of blocks rightwards to the right
edge of model.
"[" key - Change the resistivity of a whole row of blocks leftwards to the left edge
of model.
"}" key - Change the resistivity of all the blocks to the right and below the cursor.
"{" key - Change the resistivity of all the blocks to the left and below the cursor.
After editing the model, use the "Model Computation" option in the Main Menu to
calculate the apparent resistivity pseudosection. After the calculations are
completed, you can use the "Edit/Display" option again to take a look at the
apparent resistivity pseudosection as well as the model section. Figure 4 shows an
example of a model with its apparent resistivity pseudosection.
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Figure 4. Model with apparent resistivity pseudosection.
Display model - Under this option, the model without the grid lines and the
apparent resistivity pseudosection will be displayed.
Change array type – This option enables you to change the array type, as well as
the number of ‘a’ spacings, and the ‘n’ values (for the pole-dipole, dipole-dipole
and Wenner-Schlumberger arrays). On selecting this sub-option, the following
dialog box will be shown.
To change the array type, just click the button next to the appropriate array type.
This is useful to investigate the effect of the type of array used on the shape of the
anomalies in the apparent resistivity pseudosection. This will hopefully assist you
in selecting the "best" array for a particular problem.To change the number of ‘a’
spacings or ‘n’ values, just type in the number needed inside the relevant box.
Horizontal plotting scale - This allows you to change the horizontal scale, in terms
of number of pixels per unit electrode spacing. This option is useful when you
want to plot the results from different survey lines with different numbers of
electrodes, with the same scale.
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. If you enter a value of 0.0,
the program will use a default scaling factor so that the display can fit into the
display screen.
Restore default colours - This will reset the colour scheme used for colouring the
sections to a default system used by the program.
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7 Options
In this option, you can change certain parameters which affects the calculation of
the apparent resistivity values.
Apparent Resistivity Normalisation - If this option is enabled, the program will
first calculate the potentials for a homogeneous earth model. These values will
then be used as correction factors for the potentials calculated for the input model.
In most cases, there will be a slight increase in the accuracy of the calculated
apparent resistivity values.
Finite-Element or Finite-Difference Method - In this suboption, you can select the
finite-element (Silvester and Ferrari, 1990) or finite-difference method which will
be used to calculate the apparent resistivity values. The file MODEL2.MOD
contains a comparison of the results obtained with the finite-difference and finiteelement
methods for a two-layer model. For most models, there is not much
difference in the apparent resistivity values calculated by the two methods.
However, some of the older finite-element implementations contain an interesting
error whose effect shows up more prominently for certain arrays (see the lower
section of the MODEL2.MOD file).
8 Model Computation
After making the necessary changes to your model, select this option to calculate
the apparent resistivity values. The model section will be displayed when the
calculations are being carried out. After calculating the apparent resistivity values,
you can save the results in a disk file in a format that can be read by this program.
If you read this file at a later time, it will not be necessary to recalculate the
apparent resistivity values. With present day Pentium class and faster computers,
the calculations probably takes less than a minute for most models.
9 Print
Besides displaying the pseudosections on the screen, you might make a printout of
the model sections. On selecting the ‘Print’ option on the main menu bar, the
following list of suboptions will be displayed.
Saving the screen image into the form of a BMP or PCX graphics file allows you
to edit the picture with a bit-mapped graphics program, such as the Windows
PaintBrush program, before printing it. The graphics file can also be embedded
into most word processor documents such as Microsoft Word for Windows.
10 Help
The following list will be displayed when this option is selected.
Help – This brings up the online Windows Help file.
Program Info – This displays some information about the program.
Editing keys – This displays the keyboard keys that can be used in the ‘Edit’
model mode.
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12 Pitfalls
While this program will show you the apparent resistivity pseudosection for a
given model and array, there are many other factors that should be considered
before selecting an array for actual field measurements. One important factor is the
potential that will be measured by the potential electrodes. All field instruments
have a finite accuracy in measuring the potential caused by the input current used.
This is further degraded by the presence of random noise. This is particularly an
important factor for the inline dipole-dipole, equatorial dipole-dipole, pole-dipole
and Schlumberger arrays. You can determine the potential by dividing the
apparent resistivity value by the geometric factor (see Figure 1) for the array and
multiply this by the current used. An interesting and common mistake, particularly
in North America, is the use of the inline dipole-dipole array in many situation
where it is not suitable (especially with "n" values exceeding 6). Please refer to
Appendix A in the RES2DINV manual, or the free lecture notes "Electrical
imaging surveys for environmental and engineering surveys" or the more technical
tutorial notes on electrical imaging). You can download these documents from the
www.geoelectrical.com web site.
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
distributor will assume responsibility for any damage or loss caused by the use of
this program.
Since this is a free program, no warranties are given. I will be grateful for bug
reports and every effort will be made to correct the bugs. If you wish to contact
me, please send an email to mhloke@pc.jaring.my or geotomo@tm.net.my.
References
Dey, A. and Morrison, H.F., 1979, Resistivity modelling for arbitrary shaped twodimensional
structures. Geophysical Prospecting, 27, 1020-1036.
Edwards, L.S., 1977, A modified pseudosection for resistivity and inducedpolarization.
Geophysics, 42, 1020-1036.
Loke, M.H., 1994, The inversion of two-dimensional apparent resistivity data.
unpubl. Ph.D. thesis, Un. of Birmingham (U.K.).
Loke, M.H. and Barker, R.D., 1995. Least-squares deconvolution of apparent
resistivity pseudosections. Geophysics, 60, 1682-1690.
McGillivray, P.R. and Oldenburg, D.W., 1990. Methods for calculating Frechet
derivatives and sensitivities for the non-linear inverse problem : A
comparative study. Geophysical Prospecting, 38, 499-524.
Silvester P.P. and Ferrari R.L., 1990. Finite elements for electrical engineers (2nd.
ed.) . Cambridge University Press.
Zhou, B. and Greenhalgh, S.A., 2000. Cross-hole resistivity tomography using
different electrode configurations. Geophysical Prospecting, 48, 887-912.
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Appendix A : Underwater surveys
The following diagram shows the various possible situations for underwater
resistivity surveys.
The present program only supports the situation shown in Case 1 with a water
layer of constant thickness over a flat sediment surface (although the inversion
program RES2DINV supports all 3 cases). The file WATER.MOD is an example
model data file with a water layer. The initial part of the file with format used is as
follows :WATER.MOD
file Comments
Model with water layer | Title
51,16 | 51 electrodes, 16 datum levels
1 | 1 to indicate water layer, normally 0
50.00,-100.00,200.00,-4.0 | Water resistivity, left limit of water layer,
| right limit, water thickness
1.00 | Unit electrode spacing
2 | Number of model resistivity values
| The rest of the file follows the standard format
The left and right horizontal limits of the water layer is not used by the present
version of the program which only supports Case 1 in Figure 5. It is included for
future use with Case 3 where the water layer has a finite horizontal extent. Note
that the water layer thickness is given as a negative value. The depth of the
electrodes are set at 0 m., i.e. the depth datum level, and the depth values are
positive downwards. Since the top of the water layer is above the 0 datum level,
following this convention, the water thickness has a negative value.
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Appendix B : I.P. models
The file MODELIP.MOD gives an example of a model file where the I.P. values
(note in terms of chargeability with units of V/V which is 1000 times smaller than
the more usual unit of mV/V). Essentially an extra section is included following
the section with the model resistivity values. The lower part of the
MODELIP.MOD file is shown below with comments.
WATER.MOD file Comments
1111111111111111111111 | Last line of model resistivity codes
IP present | keyword 'IP present' to indicate IP model
3 | Three model IP values
0.00,0.10,0.20 | The chargeability values
0000000000000000000000 | The IP codes
Appendix C : Calculation of 2D sensitivity values (limited availability)
This option is currently undergoing tests, and its present use is limited to
registered users of the RES2DINV software. You will need to attach the
RES2DINV dongle to the computer in order to use this feature.
This option calculates the 2-D resistivity sensitivity values for a homogeneous
half-space for electrode arrays where the electrodes can be on the surface or
underground. The sensitivity pattern shows the areas of the subsurface which have
the greatest influence on the apparent resistivity measurement. This essentially
shows the regions mapped by an array. The sensitivity function basically tells us
the degree to which a change in the resistivity of a section of the subsurface will
influence the potential measured by the array. The higher the value of the
sensitivity function, the greater is the influence of the subsurface region on the
measurement. Mathematically, the sensitivity function is given by the Frechet
derivative (McGillivray and Oldenburg 1990). For a homogeneous half-space, the
3-D and 2-D forms of the sensitivity function can be calculated analytically (Loke
and Barker 1999).
There are two main situations this type of calculation is useful. The first situation
is in understanding the behavior of borehole arrays (Zhou and Greenhalgh, 2000).
It will show the area ‘scanned’ by a borehole array, and hopefully lead better
designs for cross-borehole surveys. The second situation is in designing streamers
for mobile surveying systems, such as in water covered areas. In this problem, the
number of nodes in the cable is very limited (usually less than 10) and the current
electrodes are fixed at two of the nodes. The challenge is to design a measurement
sequence that gives the most information about the subsurface within limitations
imposed by the available equipment.
To use this option, the appropriate dongle must be attached to the computer before
starting up the RES2DMOD program. After stating up the RES2DMOD program,
click the ‘Model computation’ option on the top menu bar, and the following menu
items should be displayed.
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Clicking the ‘Calculate 2D sensitivity section’ option will bring up the following
dialog box.You can choose to type in the parameters for a surface or borehole
array, or read in the parameters from a text file. The text file option is particularly
useful if you want to calculate the sensitivity values for several array
configurations at the same time.
(a). User input surface array.
This option is for the calculation of the sensitivity values for a single standard
surface array. When this option is selected, the following dialog box will be
shown.
The standard arrays supported are the Wenner Alpha, Wenner Beta, Wenner
Gamma and Pole-Pole; as well as the dipole-dipole, Wenner-Schlumberger and
pole-dipole. For the dipole-dipole, Wenner-Schlumberger and pole-dipole arrays,
the value of the ‘n’ dipole separation factor is also needed. The general array
format where the locations of the C1, C2, P1 and P2 electrodes are specified by
the user is also supported. By default, the program sets the locations of the
electrodes such that the array length is 1 meter with the first electrode at the 0
meter mark and the last electrode at the 1.0 meter mark. Next the range of the x
(horizontal) and z (vertical) values of the subsurface to calculate the sensitivity
values is needed, as well as the spacing between the calculation points. To avoid
the singularity at the electrodes, the minimum z value must be greater than 0.0
meter.
For plotting purposes, the program multiplies the sensitivity values by a numerical
scaling factor, usually between 100 to 100000, so that the final maximum
sensitivity value is between 100 and 1000. By default, the program will calculate
the scaling factor automatically, but the user can also fix the scaling factor.
(b). User input borehole array.
This option is for the calculation of a single borehole array. On selecting this
option, the following dialog box is shown.
In this case, both the x and z coordinates of the four electrodes must be specified
by the user.
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(c). Read array parameters from file.
This option allows the user to calculate the sensitivity values of a number of array
configurations. The input file must have a TXT extension. When this option is
selected, the following input dialog box is shown to prompt the user for an array
data file.
Basically, the x and z coordinates of each array configuration is typed into a text
file. The ARRAYS.TXT file is an example input file. Below is a description of the
file format .
ARRAYS.TXT file | Format Description
-----------------------------------------------------------------------------------------------------------------
Example configurations | Title
Number of arrays | Header for number of arrays data
3 | The number of configurations in this file
Unit electrode spacing | Header
1 | The unit electrode spacing value
Coordinates of electrodes (xc1,zc1,xc2,zc2,xp1,zp1,xp2,zp2) | Header
array 1 = surface dipole-dipole | Header for first array
1,0 | x and z location of C1 electrode
0,0 | x and z location of C2 electrode
3,0 | x and z location of P1 electrode
4,0 | x and z location of P2 electrode
array 2 = surface pole-dipole | Header for second array (pole-dipole)
0,0 | x and z location of C1 electrode
-1000,0 | C2 electrode, note very large x distance
2,0 | x and z location of P1 electrode
3,0 | x and z location of P2 electrode
array 3 = borehole to surface bipole-bipole | Header for third array (borehole array)
0,1 | x and z location of C1 electrode (in borehole)
0,2 | x and z location of C2 electrode (in borehole)
2,0 | x and z location of P1 electrode (on surface)
3,0 | x and z location of P2 electrode (on surface)
Starting and ending x-location to calculate sensitivity values, spacing | Header
-2.0,6.0,0.02 | The x calculation range and spacing
Starting and ending z-location to calculate sensitivity values, spacing | Header
0.02,5.0,0.02 | The z calculation range and spacing
Material resistivity (ohm.m) | Header
1 | The subsurface resistivity
Maximum current (Amp) | Header
1.0 | The maximum current value
Minimum potential (mV) | Header
0.1 | The minimum potential value
Note that in theory, the C2 electrode for the pole-dipole array is at infinity. For
calculation purposes, it can be placed at a large distance from the other electrodes.
After the program has completed the calculations, it will prompt the user for the
name of the output files to store the sensitivity values. The purpose of the files is
described in the following section. After saving the sensitivity values, the program
will then ask the user to select an array configuration to plot the sensitivity section.
For example, after calculating the sensitivity values for the ARRAYS.TXT file,
the following dialog box will be shown.
Note that although the file has only three array configurations, there are four
sections. The fourth section has the integrated sensitivity values of the first three
individual arrays. After selecting the appropriate array number, the program will
then plot a contour section for the sensitivity values, such as in Figure 2 for the
first array configuration in the ARRAYS.TXT file. The program will
automatically select a scaling factor so that the sensitivity values when multiplied
by this factor will have a reasonable range. However, you can change the scaling
factor if the resulting contoured section is not suitable.
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If you want to display the section for another configuration, you will need to read
the output file containing the sensitivity values (for this example, the output file
will be ARRAYS_OUT.TXT) by using the “Read array parameters from file”
option described earlier. The program will detect that the output file already has
the sensitivity values and will not carry out the sensitivity calculations again.
Figure 2. The sensitivity section for the first array configuration in the
ARRAYS.TXT file. Note that it is a dipole-dipole array with ‘n’ equals to 2.
Example array files
Besides the ARRAYS.TXT file, the following example files might be of interest
for various applications.
DIPOLES.TXT : The dipole-dipole array with large ‘n’ values ranging from 7 to
11. Shows that the dipole-dipole (and pole-dipole) becomes less sensitive to
deeper parts of the subsurface as the ‘n’ values becomes too large.
BOREHOLES.TXT : Various cross-borehole array configurations.
STREAMERS_WSEX.TXT : A streamer with 11 nodes, with all electrodes on the
ground or water surface. This streamer is based on the Schlumberger type of
configuration. The integerated sensitivity section that shows the section of the
subsurface mapped by the streamer system is of interest.
STREAMERS_WSEX_UW.TXT : The same streamer but underwater. For those
planning an underwater survey with the cable dragged along the river/lake/sea bed.
DIPOLES_OM.TXT : A mobile surface survey system, i.e. another type of
streamer. In this case there is a limitation that the dipole length is fixed at 1.0
meter. To get vertical resolution, the ‘n’ factor is varied from 0.25 to 2.0.
Output files
After calculating the sensitivity values, the program will save the data into a
number of files. One file stores all the data into the internal format used by the
RES2DMOD PLUS program, while the rest stores the sensitivity values for each
array configuration in the format used by the SURFER 2D plotting program. As an
example, for the ARRAYS.TXT input file, the program will store the sensitivity
values for all the arrays into an ARRAYS_OUT.TXT file. This file can be read by
the RES2DMOD PLUS program with the ‘Read array parameters from file’ option
described earlier. In this case, the program will by-pass the step of calculating the
sensitivity values. There will also be three other output files
(ARRAYS_OUT_1.DAT, ARRAYS_OUT_2.DAT and ARRAYS_OUT_3.DAT
for the ARRAYS.TXT input file) which has the sensitivity values in the SURFER
format. A fourth file, ARRAY_OUT_TOTAL.DAT, has the integrated sensitivity
values in the SURFER format.
Use of Surfer format data file
For each array configuration, the program will store two files that can be used by
the SURFER 2D contouring program. As an example, the files
ARRAYS_OUT_1.DAT and ARRAYS_OUT_1_POST.DAT are generated for
the first array configuration in the ARRAYS.TXT file. To create a contour plot,
follow the following steps.
1). Create contour file. Click 'Grid' on the top menu bar, and then 'Data' to read in
the ARRAYS_OUT_1.DAT file. After reading in the data file, SURFER will show
the 'Scattered Data Interpolation' dialog box. In the 'Grid Line Geometry' section,
the number of lines that SURFER uses to interpolate the data values into a
rectangular grid is shown. For this data set, the default values are probably 50 lines
in the X direction and 31 lines in the Y direction. You will need to change the X
and Y spacing values to the values used in calculating the sensitivity values (as
given in the ARRAYS.TXT files). In this example, both values are 0.02. After
changing the spacings, the following dialog box should be shown.
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The SURFER program will create a grid file ARRAYS_OUT_1.GRD. Next click
'Map' on the top menu bar and then 'Contour'. In the 'Contour' option, read in the
default file ARRAYS_OUT_1.GRD. Next the 'Contour Map' dialog box will be
shown. In this dialog box, click the 'Fill Contours' option. Next click the 'Load'
button, and then read in the FRECHET1.LVL file to set the contour values and
colours. If you wish to display the colour scale and smooth the contours, click the
'Color Scale' and 'Smooth Contours' options. Next click OK, and SURFER should
start drawing the contour section.
If you want to plot the positions of the electrodes as well, click the 'Post' command
in the 'Map' menu, and then read in the ARRAYS_OUT_1_POST.DAT file (which
contains the location of the electrodes) in the 'Open Data' dialog box. After
reading in this file, the 'Post Map' dialog box will be shown. In this dialog box,
select the symbol shape and size and other options you want to use, and then click
OK. After that, press F2, and then click the 'Overlay Maps' command in the 'Map'
menu. Next click the figure and then the 'Edit Overlays' command in the 'Map'
menu. Click 'Post' and the 'Move to Front', and the electrodes will be drawn on top
of the model section.
After drawing the section in the SURFER window, you can use other options
within SURFER to improve the model section, such as changing the font, labels,
titles etc., before printing the results or exporting it to another format. SURFER
supports a large variety of formats, including bit-mapped graphics formats such as
PCX, BMP, GIF etc.; and well as vector-based formats such as the AUTOCAD
DXF format.
Preview – What is this?
Have you carried out this type of survey?
What's New
Version 1.x - Old version using expanded memory.
Version 2.0 - New version using a DOS extender and more friendly user interface.
The rectangular (equatorial dipole-dipole) array has been added.
Version 2.10 - An option to read the inversion results files produced by the
RES2DINV.EXE program was added to the RES2DMOD program. An option to
use the Finite-Element method (with first order triangular elements) was also
included.
Version 2.20 - Support for underwater surveys with a water layer of constant
thickness was added.
Version 3.00 - Windows version.
Version 3.01 – Calculation of 2D sensitivity sections added. Only available to
registred RES2DINV users.