ERTLab Forward Modeling Tutorial
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.
- 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.
I'M HERE. WANT TO ADD SOMETHING ABOUT ADDING RANDOM NOISE AND WHICH
COLUMN IT GETS SAVE TO.
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 8: 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 9: 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
