[Narrator]
Today is environmental site managers have many tools to choose from when conducting
site characterization and remediation.
Geophysical tools can provide noninvasive ways to see inside the earth, much like how
medical imaging lets us see inside the human body.
The USGS Scenario Evaluator for Electrical Resistivity (or "SEER") is a quick and simple
tool practitioners can use to assess the likely outcome of using two-dimensional electrical
resistivity imaging for site characterization and remediation monitoring.
Electrical resistivity imaging is a widely used geophysical method for environmental
site management studies.
The method is sensitive to fluid conductivity, interconnected porosity, saturation, clay
content, and metallic materials.
But is 2D electrical resistivity imaging the right tool for your project?
This depends on the site properties, the goals of the survey, and the survey design.
The Scenario Evaluator for Electrical Resistivity, or SEER, is a spreadsheet-based tool that
allows the user to simulate conceptual site models for electrical resistivity imaging.
Usually when you go into the field, you have an idea of what you are looking for, and you
know something about the site geology and hydrogeology.
With this information, you can use SEER as a type of quick geophysical feasibility study
to see if electrical resistivity imaging is the right tool for the job.
First, you use your site conceptual model to create a model of the anticipated electrical
resistivity conditions at the site.
Then, you use this to create a hypothetical dataset, like what you would expect to collect
in the field, which is called a forward model.
Next, you analyze the hypothetical dataset as if it were actual data collected in the
field, giving an inverse model that provides a realistic idea of the best obtainable result
from a geophysical survey in the field.
Although you might assume the inverse model would always match the true resistivity model
we created based on our conceptual model, it inevitably will not.
Unlike in popular movies, real-word geophysical surveys donít provide magical 20/20 vision
of the Earthís subsurface, but instead provide images that can be blurry and imperfect yet
still useful.
Letís see how desktop feasibility studies with SEER can help you decide whether electrical
resistivity imaging surveys are a go/no-go for a particular environmental investigation.
The SEER spreadsheet allows the user to select a survey design to model, Using dropdown menus,
the user can: Select from common study scenarios, such as
delineating dense nonaqueous-phase liquid (DNAPL) or light nonaqueous-phase liquid (LNAPL)
releases in the subsurface or imaging an underground storage tank;
Select electrode spacing and survey geometry; Estimate measurement errors, to give us something
close to real-world results; and Decide whether to add borehole electrodes.
Then you click on "simulate" to compare the true resistivity model and the predicted inversion
results.
This will give you a sense of whether your target is likely to be resolved using a given
survey design.
In another example, you can experiment with a survey design to detect a conductive DNAPL
plume.
First, model a survey using only surface electrodes.
Next, model the same target and survey, but add borehole electrodes.
This example shows that, in this scenario, you might want to add borehole electrodes
in order to develop a more accurate model of the subsurface conditions to inform remediation.
However, our project sites are more complex and electrically heterogeneous than the simple
example models shown so far.
That is why SEER allows you to freely manipulate resistivity values in cells of the worksheet.
You can change the target shape, extent, and depth, or the electrical resistivity of the
surrounding earth materials, and immediately view the effect on the predicted results.
Detailed help files provide instructions on how to use SEER, as well as background information
on electrical resistivity surveys and modeling.
SEER can provide environmental site managers with a quick and easy mini feasibility study
to assess realistic outcomes from electrical resistivity imaging for site investigations
and remediation monitoring.
SEER and additional documentation are available from the USGS web site.
https://doi.org/10.5066/F7028PQ1
[CREDITS]