Introduction
This tutorial is designed to lead you through the steps to build
a simple forward model. Forward modeling is useful to determine system response
and target detectability. Knowledge of these parameters is valuable for survey
design that involves two key decisions, where to place the electrodes and what
arrays to use. The location of electrodes is primarily based on signal
strength vs cost and a knowledge of instrument and site parameters such as
output current, noise and target type and size are necessary. The choice of
arrays depends on such parameters as distance of target from nearest
electrodes, time available to collect readings and availability of
sophisiticated interpretation tools such as ERTLab inversion algorithms.
For complex systems and sites forward modeling is not only important, it is
necessary. This tutorial addresses a relatively simple situation and is
designed to provide a solid foundation in the practice.
Getting Ready
The first step is to gather knowledge of the host and target and
determine what types of electrode installation will be permitted. Parameters to
consider for the host are bulk resistivity, geological layering, extraordinary
features such as extremely resistive or conductive layers and site conditions
that may require special (i.e. costly) electrode installation methods. For the
target, consider the nature of target, size, form and resistivity. For the
electrode installation consider the goals of the survey (characterization vs
monitoring), the type of system that will be used (multi-channel,
multi-electrode), the number of electrodes that may be hooked to the system at
once, and the environmental conditions at surface and depth (activity at the
site, power-lines, surface paved or grass, underground utilities).
Target Configuration
In this simple case we will assume a clean host environment, a
homogenous (non-layered) halfspace host, a simple rectalinear three
dimensional target, willingness to install boreholes for electrode
installation and multi-electrode ERT system. The target will be 2 X 4 X 5 m
relative conductor in a typical vadose zone host with a 4:1 constrast. We will
use 200 ohm-m resistivity for the host and 50 ohm-m for the target with an
intrinsic IP of 100 mV/V. We will arrange 4 boreholes with 15 electrodes
(requiring a 60 electrode multiplexed system) in a square around the target,
with the target located closer to one of the boreholes (offset from the center
of the boreholes. The electrode strings will be arranged ideally, maintaining a
2:1 aspect ratio and extending 50% below the maximum target depth.
Overview of steps
The following steps will be followed:
-
Configure the borehole arrangement using the ERTLab Sequencer.
-
Schedule a sequence in the ERTLab Sequencer.
-
Configure the host and target in the ERTLab Solver.
-
View the configuration with the ERTLab Viewer.
-
Prepare and run the forward model without the target.
-
Prepare and run the forward model with the target.
-
Compare the models in Excel or other Spreadsheet.
-
View the model with target using the viewer.
-
Invert the forward model with the target.
-
View the inversion with the ERTLab Viewer.
Step 1: Configure the borehole arrangement using the ERTLab Sequencer
-
Start the ERTLab solver environment by selecting it from the start menu.
-
Start the ERTLab Sequencer by selecting
- Tools>ERTLab Sequencer
- or clicking the
button.
- or by starting the ERTLab Sequencer directly from the Start menu.
-
The ERTLab Sequencer starts at the cable entry screen.
-
Change the electrode count to 15. Notice a yellow prompt appears requesting
enter to update. Press enter.
- notice the X cable end changes to 14 m.
-
Locate the borehole checkbox and click it so that it is checked (notice the
cable end is now adjusted in Z rather than X and is negative).
-
Since we will locate the first borehole at 0,0 X and Y coordinates leave these
and push the Insert button. A grid and dot representing the bottom electrode
will appear in the 3D View area.
-
To orient the 3D View
-
Press the ortho button in the move area.
-
Click in the upper, mid-right of the 3DView and drag the mouse to the lower,
mid-left.
-
Hold the Ctrl button and click in the upper, mid-left area, pan the view to the
center, mid-left and release.
-
To insert another cable:
-
Enter 7 in the Y cable start and push enter
-
Enter 7 in the Y cable end and push enter.
-
Push the insert button.
-
Another cable appears on the screen.
-
Another way to insert a cable is to move it after insert.
-
Push insert again. Notice that while nothing appears to have changed in the
image, the cable count is now 3. This is because the new cable was inserted
directly on top of the last cable.
-
Move the new cable 7 meters in X by pushing the
right X button in the Move Cable frame.
-
Yet another way to insert a cable is to adjust it after insert.
-
Push insert. A new cable is inserted on top of the last one.
-
Change the Y start to 0 and push enter
-
Change the Y end to 0 and push enter
-
Push Update Cbl.
-
The new cable moves to the coordinates indicated.
-
Any cable may be adjusted by selecting it.
-
to select any cable double click on any of the cables electrodes.
-
Adjust the cable using any of the methods above.
-
To recenter the view, click the Grd. Auto check box at the bottom center of the
screen so it is checked.

Step 2: Schedule a sequence in the ERTLab Sequencer.
Before a file can be saved in ERTLab format at least one sequence must be
built. Follow these steps to build a simple reciprocating dipole-dipole
sequece.
-
Select Sequence > Make-Multi-borehole sequence from the menu.

-
Push the Dip-Dip button on the Multi-borehole array menu.
-
Notice both common-cable and cross-cable choices are selected. Common cable
confines the transmitter and receiver to a single cable. Cross cable forces the
transmitter and receiver to be on opposite cables.
-
Notice the V mode is chosen for both Tx and Rx. V means the dipole is oriented
vertically. H means it is oriented horizontally.
-
Notice the a List dipole length. Dipole length is specified in terms of
electrode spacing on the cables. Multiple 'a' spacings may be generated at
once, and the default is 1,2 and 3 electrode spacings. To keep things simple
for now, change this to read '1' only.
-
Notice in Tx, Rx distance there is a list of 'n' spacings that will be
generated and this is filled to n=12. An alternative syntax is available.
This is startingN:step:endingN, so for the list shown the syntax would be
1:1:12. In order to ensure that we get all possible 'n' spacings change the
list shown to read 1:1:50.
-
Push the
make sequence button. A message will pop up indicating the number of
quadrapoles generated.

-
To view the quadrapoles select Sequence>View Quadrapole List from the
sequence menu, or push the F11 keyboard button.
-
Click any quadrapole in the list and the transmitter electrodes will be shown
in red, the receiver electrodes in green.

-
Multiple sequeces may be generated, or the sequences generated may be erased by
returning to the Make Multi-borehole sequences menu.
-
The tools menu provides tools such as removal of duplicates, addition and
removal of reciprocals and optimization for multi-channel ERT instruments.
-
To save the sequence choose File>Save>ERTLab Format from the file menu.
-
Use a file name like ForwardSequence1.schd (the extension is added
automatically).
-
Close out of the ERTLab Sequencer using File>Quit on the File menu or by
pushing the X in the upper right corner of the window.
Setp 3: Configure the host and target in the ERTLab Solver.
You are now ready to configure the host and target.
Step 3:Configure the host and target in the ERTLab Solver.
-
Return to, or start the ERTLab Solver environment.
-
From the choose File>New Project > Survey Design from the file menu or
push the
survey design button.
-
Under Data input click on input Schedule. Find the file generated above
(ForwardSequence1.schd) and open it.
-
The screen and configuration settings will be updated as shown.

-
Scroll through the configuration settings using the scroll bar on the right of
the list. Note that you can jump to grouped items using the Select item menu to
the left of the list.
-
Click on Mesh settings under the Select Item, or scroll to Mesh settings.
-
Notice that Interior Grid Element X size (m) is .5 m and Interior grid element
sizes for Y and Z are also .5 m. These are set automatically and are based on
the electrode spacing in the data (i.e.) 0.5 m is half the smallest electrode
separation, which is 1 m in the Z direction.
-
Since we have a relatively wide well-well separation of 7 m, we know we can
probably use a corase mesh in X and Y. However we are constrained by the
element size in Z. Stability is compromised if we use a ratio > 2:1 for any
cell dimension. For optimal run speeds we can enter 1 m in both the X and Y
directions.
-
Double click on the 0.5 value for Interior Grid Element X size (m). A separate
menu is presented that permits changing of the value to 1. Click OK to accept
the change.

-
Double click on the 0.5 value for Interior Grid Element Y size (m). A separate
menu is presented that permits changing of the value to 1. Click OK to accept
the change.
-
Scroll a little farther down the list to find Adapt Grid to Electrodes and
change the value from 2 to 1 by double-clicking.
-
No other parameters in the configuration settings need to be changed for this
model.
-
On the right side of the ERTLab Solver display notice the Survey Design
Explorer menu. If this is not visible use View > Explorer Window from the
menu bar to select it.
-
In the explorer menu click Make Resistivity IP Model.
-
The resistivity IP model workspace will be presented.

-
Change the background resistivity to 200 ohm-m and click Apply Background.
-
From the Survey Design Explorer select Make Mesh. The Mesh Workspace will be
presented.
-
Since we already changed the X and Y element sizes in the foreground region,
and we have no other changes to make click Apply Settings.
-
The Make Mesh area is presented. There is a list of node locations and the
ability to add nodes at selected places. Notice the list is separated into -
and + background nodes with foreground nodes listed in the center. Any node
location may be changed by double-clicking on it. Use the X, Y and Z buttons to
switch between directions. Note that it is not usually necessary to make
changes to node locations, or to add nodes.
-
Click Make Mesh. The program works for a few seconds to a few minutes depending
on processor speed and mesh size, then the View Mesh bottun becomes available.
Click View Mesh to look at your model.
Step 4: View the configuration with the ERTLab Viewer
-
If the View Mesh button did not automatically run the viewer then select the
viewer button to run the viewer or select the ERTLab Viewer from the Start
menu.
-
A file called ViewMesh.Dat was created in the ERTLab application directory.
-
Open this file using File>Open in the ERTLab Viewer.
-
The initial view of the model is looking down from the top.
-
Left click and hold on the screen and move the mouse to modify the Point of
View (rotate the model).
-
Left click and hold while holding the shift key to pan the view.
-
Left click and hold while holding both the shift and ctrl key to zoom the view.
-
Click the
button from the toolbar to display the processing tool menu.
-
Select options like axes by clicking in the select box to modify the items
shown.

-
We will return to the viewer when the model is more exciting. For now simply
exit.
Step 5: Prepare and run the forward model without the target.
-
Return to the FM Workspace by selecting FM Workspace from the Survey Design
Explorer.
-
Under Forward Modeling
-
check the Model IP data checkbox.
-
Click run forward solver
-
The Progress Info box first indicates preliminary tasks, then forward modeling
starts and the number of iterations and final residual values are presented for
each electrode.
-
Finally the forward model results are listed for each quadrapole.
-
the message ....Forward Modeling completed indicates the end of the task.
-
To save the data we must indicate the data columns in the Configuration
settings.
-
Click Data I/O Config. from the Select Item: menu
-
Scroll down the master list until Item Description reads Calculated Res. Column
-
change from the default -1 to 10 by double clicking the value and entering 10
in the menu that pops up.
-
Scroll down the master list until Item Descripition reads Calculated IP Column.
-
change from the default -1 to 11 by double clicking the value and entering 11
in the menu that pops up.
-
The configuration settings may be saved with the data, or separately. To save
the configuration settings separately click Save Configuration in the
Configuration settings area. To save the configuration with the data click the
Include config. checkbox in the Save Results area. For this tutorial please do
both using the default for the configuration file and
ForwardModelHalfspace.schd for the data file name.
-
Notice the default extension of the configuration file is .cfg
-
Notice the default extension of the data file is .schd. Choose a file name like
ForwardModelHalfspace.schd for the data file.
Step 6: Prepare and run the forward model with the target.
-
Return to the ERTLab solver.
-
Close the current project using File > Close Project from the file menu, or
by clicking the X on any open workspace menu.
-
Start a new survey design project
-
From the choose File>New Project > Survey Design from the file menu or
push the
survey design button.
-
Under Data input click on input Schedule. Find the file generated above
(ForwardModelHalfspace.schd) and open it.
-
If the configuration settings were included with the file, they should be
automaticaly loaded with the data. If not, then load the configuration
file (ForwardModelHalfspace.cfg).
-
Add the target
-
If the survey design explorer is not visible use View > Explorer Window from
the menu bar to select it.
-
In the explorer menu click Make Resistivity IP Model.
-
The resistivity IP model workspace will be presented.
-
Make sure the background resistivity is 200 ohm-m
-
Add a resistivity and IP anomaly
-
In X min (m) type -100
-
In X max (m) type 100
-
In Y min (m) type -100
-
In Y max (m) type 100
-
In Z min (m) type -9
-
In Z max (m) type -7
-
In the Resistivity box type 50
-
In the IP box type .01
-
Push insert anomaly.
-
Notice the anomaly may be deleted but not changed. Also, the IP and resistivity
anomalies are recorded independently, so they must be deleted independently.
-
Select the IP anomaly and push delete. The IP anomaly is deleted. Push the
resistivity button to confirm that the resistivity anomaly.
-
Add a second resistivity and IP anomaly
-
In X min (m) type 2
-
In X max (m) type 4
-
In Y min (m) type 2
-
In Y max (m) type 4
-
In Z min (m) type -9
-
In Z max (m) type -7
-
In the Resistivity box type 200
-
In the IP box type 20
-
Push insert anomaly.
-
The combination of these anomalies produces a resistivity layer with a hole.
The hole is filled with a chargeable body.
-
Return to the FM Workspace by selecting FM Workspace from the Survey Design
Explorer.
-
Save the configuration (ForwardModelWithTarget.cfg)
-
Under Forward Modeling
-
make sure the Model IP data checkbox is checked.
-
Click run forward solver
-
The Progress Info box first indicates preliminary tasks, then forward modeling
starts and the number of iterations and final residual values are presented for
each electrode.
-
Finally the forward model results are listed for each quadrapole.
-
the message ....Forward Modeling completed indicates the end of the task.
-
To save the data we must indicate the data columns in the Configuration
settings.
-
Click Data I/O Config. from the Select Item: menu
-
Scroll down the master list until Item Description reads Calculated Res. Column
-
change from the default -1 to 10 by double clicking the value and entering 10
in the menu that pops up.
-
Scroll down the master list until Item Descripition reads Calculated IP Column.
-
change from the default -1 to 11 by double clicking the value and entering 11
in the menu that pops up.
-
The configuration settings may be saved with the data, or separately. To save
the configuration settings separately click Save Configuration in the
Configuration settings area. To save the configuration with the data click the
Include config. checkbox in the Save Results area. For this tutorial please do
both using the default for the configuration file and
ForwardModelHalfspace.schd for the data file name.
-
Notice the default extension of the configuration file is .cfg
-
Notice the default extension of the data file is .schd. Choose a file name like
ForwardModelWithTarget.schd for the data file.
Step 7: Compare the models in Excel or other Spreadsheet.
-
Open the two files ForwardModelHalfspace.schd and ForwardModelWithTarget.schd
using Excel or similar spreasheet.
-
Notice that because the data portion of the files is tab delimited the data are
automtically separed into columns.
-
Copy the two files to separate worksheets in the same spreadsheet.
-
Create a third worksheet to perform the compares.
-
Create difference and percent difference columns.
-
Bin the data and create histograms or analyse by simply sorting on the
difference or percent difference columns
-
Using knowledge of instrument sensitivity it is possible to make judgements
about the detectability of the target.
Step 8: View the model with target using the viewer.
-
In the FM Workspace click IP, then click View Model.
-
If the View Mesh button did not automatically run the viewer then select the
viewer button to run the viewer or select the ERTLab Viewer from the Start
menu.
-
A file called ViewMesh.Dat was created in the ERTLab application directory
(save for reference in the tutorial subdirecty as ViewMeshIP.dat).
-
Open this file using File>Open in the ERTLab Viewer.
-
The initial view of the model is looking down from the top.
-
Left click and hold on the screen and move the mouse to modify the Point of
View (rotate the model).
-
Left click and hold while holding the shift key to pan the view.
-
Left click and hold while holding both the shift and ctrl key to zoom the view.
-
Click the
button from the toolbar to display the processing tool menu.
-
Select options like axes by clicking in the select box to modify the items
shown.
-
Click the
section button to add a section.
-
In the submenu that appears click YZ section, remove the show lines check under
contours, turn clipping off and change the mode to cell scalars as shown
-
Insert another section, this time selecting XZ with all the other choices
indicated above.
-
Insert another section, this time selecting XY with all the other choices
indicated above.
-
The display should be similar to the following
-
Sections may be moved by selecting the appropriate section in the Post-process
tool bar then pressing the
double arrow buttons, or by direction entering a position.
-
In the FM Workspace click Rho, then click View Model.
-
If the View Mesh button did not automatically run the viewer then select the
viewer button to run the viewer or select the ERTLab Viewer from the Start
menu.
-
A file called ViewMesh.Dat was created in the ERTLab application directory
(save for reference in the tutorial subdirecty as ViewMeshRho.dat).
-
Open this file using File>Open in the ERTLab Viewer.
-
The initial view of the model is looking down from the top.
-
Left click and hold on the screen and move the mouse to modify the Point of
View (rotate the model).
-
Left click and hold while holding the shift key to pan the view.
-
Left click and hold while holding both the shift and ctrl key to zoom the view.
-
Click the
button from the toolbar to display the processing tool menu.
-
Select options like axes by clicking in the select box to modify the items
shown.
-
Click the
volume button to add a volume.
-
In the submenu that appears insert 50 in the min area and 60 in the max area.

-
The display should be similar to the following
-
Notice that because cell scalars are not available for volumes, the viewer has
'smoothed' the edges of the hole so that it appears larger than it really is,
and rounded. Similarly the layer is thinner than it actually is. This may be
confirmed by increasing the max value for the volume from 60 10 100, in which
case graded colours will appear showing the rest of the volume. This effect is
purely an artifact of the 3D engine chosen. The actual model used in the Solver
is the one that was input.
Step 9: Invert the forward model with the target.
The ultimate way to check the resolving ability of the array and electrode
layout used is to perform an inversion of the forward results.
-
Close the survey design project
-
Use File>New Project > Data inversion or the
data inversion button to start a new inversion project.
-
Scroll down the configuration settings list to find the IP data column item and
change from the default -1 to 11.
-
Click Input schedule and select the ForwardModelWithTarget.schd.
-
Select mesh settings from configuration settings Select Item List.
-
Adjust Interior Grid Element X Size (m) to 1.
-
Adjust Interior Grid Element Y Size (m) to 1.
-
Leave Interior Grid Element Z Size (m) at 0.5.
-
Set Adapt grid to electrodes to 1 (Yes)
-
Scroll to Inversion Parameters
-
Set Simple Inversion to 2 (No)
-
Set Maximum Internal PCG Iterations -Rho to 12
-
Set Maximum Internal PCG Iterations - IP to 20
-
Scroll to Noise
-
Set Rho Data Percent Errors to 3 %
-
Set IP Data Percent Errors to 10 %
-
Set Use Robust Inversion (data errors reweight) to 2 (No)
-
Scroll to Saving and Plotting parameters and set Save inverted model for each
iteratino to 2 (No).
-
In the inversion area select the IP inversion checkbox.
-
Save the configuration as Inversion.cfg
-
Click Run Data Inversion
-
Change the progress file name to Inversion.prg
-
The inversion will proceed and results will be presented in text form in the
progress window box and in graphical form in the Inversion progress graph.
Notice that for each iteration there will be several trials (up to 4) performed
in selection of the optimal roughness factor. If simple inversion was chosen
then the selection of optimal roughness factor only occurs for the first
iteration, then the roughness factor is simply deccreased by a factor equal to
the parameter 'Multiplier for changing roughness factor (in rho and IP). This
parameter is usually set to 10.
-
Notice that the Resistivity inversion proceeds first and is completed before
the IP inversion starts.
-
If all parameters and anomalies were set as described, then this inversion will
complete in 3 iterations for both resistivity and IP. Elapsed time will be on
the order of 15-30 minutes, depending on processor speed.
-
The ultimate screen at completion of the IP inversion is shown below.

Step 10: View the inversion with the ERTLab Viewer.
-
Close the Inversion progress window by clicking the X in the upper left or by
clicking the activation button
.
-
To save the mesh for viewing:
-
In the SAVE RESULTS area of the DI workspace click Rho, then click save model.
Assign an appropriate name such as InversionRho.dat. If the Save XYZ file
checkbox is selected then an XYZ file is written having resistivity values
posted at each node of the inversion mesh.
-
In the SAVE RESULTS area of the DI workspace click ip, then click save model.
Assign an appropriate name such as InversionIP.dat. If the Save XYZ file
checkbox is selected then an XYZ file is written having IP values posted at
each node of the inversion mesh.
-
To save the modeled results for each quadrapole:
-
In the CONFIGURATION SETTINGS area scroll to Data input / output format, find
calculated Res. column and change this from -1 to 12.
-
Find the calculated IP column parameter and change this from -1 to 13.
-
In the SAVE RESULTS area click Save Schedule and assign an appropriate name
such as Inversion.schd
-
To view the IP model, under MESH Parameters select IP then click View Model, or
run the viewer using the
viewer button, then open the InversionIP.dat file saved above.
-
Rotate and position the image as before.
-
Click the
button from the toolbar to display the processing tool menu.
-
Select options like axes by clicking in the select box to modify the items
shown.
-
Click the
section button to add a section.
-
In the submenu that appears click YZ section, leave show lines checked under
contours, turn clipping off and leave the mode as contours as shown
-
Insert another section, this time selecting XZ with all the other choices
indicated above.
-
Insert another section, this time selecting XY with all the other choices
indicated above.
-
The image should look like one shown below:

-
To view the Rho model, under MESH Parameters select Rho then click View Model,
or run the viewer using the
viewer button, then open the InversionRho.dat file saved above.
-
Rotate and position the image as before.
-
Click the
button from the toolbar to display the processing tool menu.
-
Select options like axes by clicking in the select box to modify the items
shown.
-
Click the
volume button to add a volume.
-
In the submenu that appears leave the min Rho value alone and insert 60 in the
max area.

-
The display should be similar to the following

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