At the most basic level, electrical resistivity (ER) (as used in geophysics) is the measurement of ground variations gathered by applying a small and highly controlled electric current across an array of electrodes.
Electrical resistivity imaging (ERI)—also known as electrical resistivity tomography (ERT)—is a geophysical technique used to create an image of a specific portion of the Earth’s subsurface. It is created through the use of automated geophysical instruments that gather thousands of resistivity measurements via an electrode cable and multiple electrodes.
In this article, we’ll explore what electrical resistivity surveys are used for and seven important things you should know about electrical resistivity.
Why Electrical Resistivity Surveys?
There are a number of situations in which you can conduct an electrical resistivity survey, including:
- Groundwater exploration
- Imaging leaks and weak zones in dams and levees
- Cave and sinkhole detection
- Marine dredge planning
- Cross borehole tomography
- Landfill volume and leakage monitoring
- Monitoring of heap leach mining operations
- Gold and mineral exploration
- Borehole-to-borehole tomography
- Marine towed and stationary measurements
- Tree trunk imaging
- Wall, tunnel, or pyramid imaging
- Grave locating
- CO2 sequestration time-lapse monitoring
An electrical resistivity survey can assess a number of geotechnical targets that engineers or builders may be interested in, and it is used to answer the following questions:
- “When we build, how much earth will we have to dig through before we get to the bedrock to place the footers for a foundation?”
- “Are there sinkholes or hidden voids at the construction site that would impact the property value?”
- “Where’s the best place on this property to drill a well?”
- “Is there an unknown object—like an old landfill, tunnel, or mine shaft—at the site that we don’t know about?”
7 Things To Know About Electrical Resistivity
- To create an electrical resistivity image, you need multiple electrodes. Electrodes are stainless steel rods that can transmit currents and measure voltage. Any company you purchase electrodes from has a different base number they offer. For example, AGI’s base number is 28—you may use as few as 28 electrodes and as many as several hundred. These electrodes are hammered into the earth in precisely-measured intervals and attached to an electrode cable. When these electrodes are installed, they’re used to build up a picture of the subsurface.
- You can create a two- or three-dimensional resistivity image, depending on your survey goals. If you deploy electrodes in a line, you’ll gather the data for a 2D slice of the subsurface. You can think of this like an X-ray of the earth. If you deploy the electrodes in a rectangular grid, you’ll gather the data with a 3D cube of the earth’s subsurface. This is similar to an MRI of the body, because it shows three dimensions. You can view your 2D or 3D resistivity image using AGI’s EarthImager software.
- Electrical resistivity imaging can create a time-lapse of biological or hydrological events. There are many different situations where it may be critical that you leave your grid in place and have it measure resistivity multiple times over days, weeks, or months to capture percentage change (+/-) of dynamic conditions. Consider these two scenarios:
- If you need to examine how water discharges underground at low tide and comes back in at high tide, you could submerge or bury an electrode cable along a shoreline and see how the water moves in phases. This may be important if you are studying groundwater budgets or drilling for freshwater in an area surrounded by saltwater.
- If a toxic or dangerous chemical spills, you can monitor how it’s moving over time and where it’s going. This allows you to look at a base condition and assess the positive or negative percent difference on the subsurface over time, so you can target the right areas for cleanup and decontamination.
- Electrical resistivity data can be gathered in a very short amount of time. If you’re on a very short time table and need to gather electrical resistivity data quickly, you can do so. For example, during a survey for a fracking operation, the SuperSting gathered 967 data points in 30 minutes—and a total of 3,354 unique measurements in 1 hour and 44 minutes.
- Properly deployed resistivity surveys are safe. The cables are monitored by an operator in real-time during an electrical resistivity survey in order to ensure the safety of everyone on the job and in the surrounding area. If a cable is left for a time-lapse survey, it is typically buried or attached to a non-conductive borehole.
- You can account for varying topography when doing a resistivity survey. Anytime electrodes move over a depression or topographical variation—like a hill or a valley—the data becomes distorted. If a topographical map of the area isn’t available, survey-grade real-time kinematic (RTK) GPS with sub-centimeter accuracy can be used instead. The topographical data may be included in the post-processing so the EarthImager can present an image which is free from topographic distortion.
- You can improve your final plot by adding various data during post-processing. You can add a great deal to your survey during post-processing to improve the model in EarthImager software, including resistivity topographic correction, time-lapse resistivity imaging, or underwater resistivity imaging(both bathymetry and topography). This helps you get a more exact look at the plot of that you’re measuring. Consider these two scenarios:
- Consider if you’re in the field and collect five sets of 2D line data over a two-day period—and all along those lines you collected the elevations with the GPS. Once you have this information, you could go back to the office and merge your five 2D lines of data into a 3D grid (so long as they were close together and in a rectangular grid). The EarthImager 3D can then process the data into a terrain corrected 3D volume, which you can look into, turnaround, slice, and filter in your computer to see every detail of the subsurface.
- Your survey area could include many different topographical variations and mediums. Your electrode cables could run over sand dunes, down a shore face and under water. During post-processing, you could add the time-lapse imaging of tidal effects, topography over the sand dunes, and the shape of the the shoreline with the water layer atop it.
Do you need to conduct an electrical resistivity survey?
Whether you’re determining where the best place to drill a well is or whether there’s a sinkhole beneath your future construction site, conducting a resistivity survey is an effective way to determine the best course of action.
Advanced Geosciences, Inc. (AGI) is the leading developer and manufacturer of geophysical imaging systems. Since 1989, we’ve been producing state-of-the-art electrical resistivity meters and products for resistivity and induced polarization (IP) imaging. If you conduct your electrical resistivity survey using an AGI imaging system, our staff will be there to help you from start to finish with free technical support.
Ready to discuss your project?