What Is Electrical Resistivity Imaging (ERI)?
If you need to understand what is happening beneath the ground, digging or drilling is not
always the best place to start.
Electrical Resistivity Imaging (ERI) is a geophysical surveying method used to map changes in
subsurface conditions without having to excavate the entire area being investigated.
By measuring how strongly different underground materials resist the flow of electrical current,
geologists can develop a cross-sectional image of variations beneath the ground. These
variations can provide valuable information about geology, groundwater flow, shallow rock,
fractures, voids, soil conditions, and other subsurface features.
At HydroGeo, electrical resistivity is one of several geophysical tools that may be used to help
property owners, engineers, environmental professionals, developers, and other clients better
understand what lies below the surface.
How Does Electrical Resistivity Imaging Work?
Different subsurface materials conduct electricity differently.
Soil, clay, groundwater, weathered rock, fractured bedrock, and competent bedrock can have
different electrical properties depending on factors such as moisture content, porosity, mineral
composition, clay content, and the chemistry of water within the soil or rock.
During an electrical resistivity survey, a series of electrodes is placed in the ground along a
survey line.
A small electrical current is introduced into the subsurface through certain electrodes while
other electrodes measure the resulting electrical potential.
By taking many measurements using different electrode combinations and spacings, geologists
collect information about how electrical resistivity changes both horizontally and vertically
beneath the survey line.
The data are then processed using specialized software to create a subsurface resistivity model
or image.
What Does an ERI Image Show?
An electrical resistivity profile typically appears as a colored cross-section of the ground.
The colors represent differences in calculated electrical resistivity.
However, the image should not be interpreted as a literal photograph of underground materials.
Instead, geologists evaluate the patterns and anomalies in the data and compare them with
what is already known about the site.
A resistivity change might be associated with differences in:
• Moisture or groundwater conditions
• Depth-to-rock
• Clay content
• Soil type
• Weathered versus competent bedrock
• Fractures or fracture zones
• Porosity
• Salinity or groundwater contamination
• Certain voids or karst features
Because several different materials can produce similar resistivity signatures, professional
interpretation is important.
What Can Electrical Resistivity Imaging Be Used For?
ERI can be useful in a wide variety of environmental, geological, groundwater, and engineering
investigations.
Depending on the site and project objective, electrical resistivity may help investigate:
Groundwater and Well Siting
In fractured-bedrock environments, ERI can help identify changes in the subsurface that may be
consistent with weathering, fractures, or other geological conditions that warrant further
investigation as potential groundwater targets.
Electrical resistivity has long been used in groundwater prospecting and aquifer
characterization.
Bedrock Depth and Weathering
ERI can help distinguish areas of shallower competent bedrock from zones where rock may be
more deeply weathered.
Knowing where competent rock sits, and where it doesn't, informs well siting, foundation
design, excavation planning, and environmental assessments.
Fractures and Geologic Structure
Changes in resistivity can help identify faults, fractures, or zones of fractured rock.
These features can be important because bedrock fractures influence groundwater flow and
subsurface contaminant migration.
Karst Features and Possible Voids
In certain geologic environments (ex: Virginia’s Shenandoah Valley), electrical resistivity can be
used to map karst features, sinkhole-related conditions, and subsurface voids.
Because these features produce electrical contrasts with the surrounding material, ERI can help
identify areas that may warrant additional investigation.
Environmental Investigations
Electrical resistivity is also useful when investigating changes associated with contamination,
salinity, groundwater conditions, or remediation.
It can provide a broader view of subsurface conditions that can help determine where
additional sampling, drilling, or investigation may be most useful.
Environmental geophysics is often used to guide more invasive investigation methods rather
than replacing them entirely.
Does Electrical Resistivity Actually “See” Underground?
Not in the way a camera takes a photograph.
This is an important distinction.
ERI measures electrical properties and uses those measurements to create a model of the
subsurface.
A low-resistivity area does not automatically mean “groundwater,” just as a high-resistivity area
does not automatically mean “bedrock.”
Geologists interpret the data within the context of:
• Local geology
• Site history
• Surface observations
• Existing boring or well information
• Other geophysical data
• The specific purpose of the investigation
That is why the interpretation of the survey is just as important as collecting the measurements.
How Deep Can Electrical Resistivity Imaging See?
There is no single depth that applies to every ERI survey.
The depth of investigation depends on several factors, including:
• Electrode spacing
• Length of the survey line
• Electrode configuration
• Subsurface electrical conditions
• Site conditions
• The target being investigated
Generally, increasing electrode spacing can increase the depth of investigation, while closer
electrode spacing can improve resolution closer to the surface.
There is a tradeoff between depth and resolution, which is why the survey needs to be
designed around the project's specific objective.
Is ERI the Same as Ground-Penetrating Radar?
No.
Although both are geophysical methods used to investigate below the ground, they operate
differently.
Ground-penetrating radar (GPR) transmits electromagnetic waves into the subsurface and
records reflections produced where electrical properties change.
Electrical resistivity imaging introduces electrical current into the ground and measures
changes in electrical resistance.
Because the two technologies respond to different physical properties, one method may
perform better than another depending on the target and site conditions.
In some investigations, using more than one geophysical technique can provide a more
complete understanding of subsurface conditions.
Why Would a Geologist Use ERI Instead of Drilling?
Drilling provides direct information at the location of the borehole—but only at that location.
An ERI survey can provide information continuously along a larger survey line.
That broader picture can help identify areas that deserve more focused investigation.
For example, geophysical data may help a project team determine where to place:
• Monitoring wells
• Soil borings
• Test pits
• Exploratory drilling
• Water wells
• Additional survey lines
This can make subsequent investigation more targeted.
Can ERI Identify Exactly What Is Underground?
Not always.
Electrical resistivity should not be viewed as a stand-alone identification tool.
Different materials can produce similar resistivity values, and the same material can produce
different values depending on moisture, mineral content, porosity, and other conditions.
That means geologists look for patterns, contrasts, and anomalies rather than simply assigning
one color on a resistivity image to one specific material.
A strong investigation combines geophysical data with knowledge of the geology, site
conditions, and other available information.
Ask a Geologist: What Is Electrical Resistivity Imaging?
Electrical Resistivity Imaging is a geophysical method that measures variations in the electrical
properties of the subsurface to help geologists understand how underground conditions
change across a site.
It can provide valuable information about groundwater conditions, bedrock, weathered zones,
fractures, geologic structure, possible voids, and environmental conditions.
But ERI does not simply identify underground objects or materials automatically.
Its value comes from combining the survey data with professional geological interpretation.
Better Subsurface Decisions Start With Better Data
What lies beneath the ground can significantly influence groundwater development,
environmental investigations, construction planning, and site assessment.
Electrical Resistivity Imaging can help provide a clearer picture of those subsurface conditions
before more invasive investigation begins.
HydroGeo Environmental uses state-of-the-art geophysical equipment and professional
geological interpretation to help clients gather better subsurface information and make more
informed decisions.
Trusted Data. Confident Decisions.
Talk to a Geologist About Your Site
Need to better understand the conditions beneath your property or project site?
Contact HydroGeo Environmental to discuss whether electrical resistivity or another
geophysical method may be appropriate for your investigation.
434-255-1617
HydroGeoEnvironmental.com
Technical Resources: HydroGeo Environmental field experience and project methodology; U.S.
Environmental Protection Agency environmental geophysics resources
