By Raymond L. Straub Jr., PG
The resolution of the array or the ability of the array to measure a specific interval is proportional to one-half the receiver dipole spacing of M and N. So, if the RX electrodes are spaced 2 meters apart, the resolution would be 1 meter. The Wiener array has a MN = 1⁄3 AB configuration, so the resolution on a 1000-meter single array would be 166 meters— not a very fine resolution.
In order to overcome some of the shortcomings of the Wiener array, numerous other arrays have been designed to achieve better range and resolution. The Schlumberger array is similar to the Wiener array, but the RX electrode spacing is one-tenth the spacing of the TX electrodes, creating a little better resolution.
Advanced Geosciences Inc. (AGI) has extended the Schlumberger array theory even further by developing the Strong Gradient 8-channel array. Through the use of specially designed equipment and proprietary software, the electrical signal is migrated across an equally spaced array moving the A-B and M-N electrode location based on a specific pro- grammed path.
The strong gradient uses the same concept of MN = 1/10 AB, but can migrate the spacing at will across the array. AGI offers several sizes of arrays ranging from a 28-electrode array up to a 112-electrode array. The proprietary software can keep track of each electrode and its spacing by designating a specific digital location. Once the software has processed a specific location, the array can be moved perpendicular or laterally to extend the width or length of the area of investigation.
The resistivity characteristics of a specific interval is pro- portional to the ability of the material to pass an electrical current. Each material or soil type has a specific resistance characteristic based on the composition of the material and its liquid content. Therefore, areas of uniformity show up as uniform resistance and areas of heterogeneity show up as non-uniform resistance. The electric field must go around areas of high resistance, creating distortion. This distortion is known as an anomaly. Air-filled caverns, sinkholes, and cavities can show up as anomalies.
When asked by a fellow student in the class, “How do you resolve anomalies?” Langmanson responded, “You drill anomalies. Anomalies are what we look for.”
Using numerous electrode arrays across hundreds and thousands of square meters of area and multiplying that with depth, the shear amount of data points can be staggering. This method can produce hundreds of thousands of measurements that can only be managed through computer software and data processing. AGI has developed proprietary software to collect, process, evaluate, and display this vast sum of data into a meaningful and interpretable format.
What the Pros Say
Not being familiar with all the uses and complexities of ERI, I asked several professionals who use this method regu- larly for some pointers and advice.
My first questions were what are the benefits to using these tools and secondly what are the limitations of the technology.
I had always understood it to be useful for groundwater and contamination investigations, but many use it for other applications: dam and levee surveying; fault and fracture detection; karst, cave, and sinkhole investigations; and even aboveground pit liner leak detection.
James W. Ward, Ph.D., PG, an associate professor of geol- ogy at Angelo State University, says this is how he uses ERI.
I have primarily used ERI to find brine water spills and buried historic brine water pits. It has proven useful in delineating brine plumes and in providing target areas for remediation. I have also used ERI successfully in locating subsurface karst conduits.
I personally prefer ERI to most all shallow geophysical methods. It is relatively easy to conduct an ERI survey, but it can be a little manual labor-intensive at times. I have found it works really well in areas of high clay content where other methods such as GPR fall short. Also, it is a great tool to run prior to drilling to allow targeting of specific locations, thus saving money and time conducting a remediation program on sites.
The process of conducing ERI surveys is relatively straight- forward. I find the most challenging part is determining where to conduct the survey along with how to design out your probe spacing to image depths properly. Further, it is hard to conduct ERI surveys sometimes in West Texas due to the amount of metal infrastructure around sites which will affect survey results; thus one must always be cautious when running surveys. (Ward 2016)
The class was well attended by people from across the United States. I was surprised to see so many people I recog- nized as well. One of the people there was Doug Laymon, PG, manager of geophysical services at Collier Consulting Inc. in Stephenville, Texas. Doug and I go way back and he provided his thoughts on ERI to me.
I have used electrical resistivity imaging for a multitude of ap- plications and find it to be a useful tool. Some of the applica- tions where I have found the method to be useful include karst mapping, brine and other contamination delineation, aquifer mapping, levee assessment, fracture delineation, and mapping of lithology.
Electrical resistivity imaging is a tool and like all tools requires the right application of that tool. Sometimes resistivity is used in concert with other tools, such as another geophysical method like seismic imaging. The two methods often complement each other. Additionally, ground-truthing through the use of test borings is an important component to using a geophysical tool like resistivity.
One of the most technically challenging issues in interpreting resistivity data is when the target has a non-unique electrical signature. For example, a water-filled cave can have similar electrical signature to a shale or clay zone. This is an example of where the use of test borings can provide ground- truth that is invaluable to correctly interpreting a resistivity data set. Additionally, understanding site hydrogeology is also essential to successfully implementing a tool such as electrical resistivity for any site characterization. (Laymon 2016)
It is always nice to talk to the right people about a complex subject. Lagmanson is considered one of the best in the field. His class was informative. In trying to further understand this method in hopes of eventually using its benefits, I asked him if he had any final thoughts on the subject.
It is another tool in the toolbox for geoscientists to better understand a complex geologic/hydrogeologic situation in the subsurface that isn’t readily apparent. It has its advantages and disadvantages.
One interesting application I saw years ago is from the guys at KIGAM that spurred a lot of thought into difference inver- sion and its applications to hydrogeology. What they did was scan between two boreholes, then inject a weak solution of NaCl, then scan the same two boreholes. With the two data- sets, we could look at only the changes between them in time. It was like subtracting out the geology and only looking at the connected water passages that connected the two boreholes.
ERI works well in many conditions. The more challenging conditions would be when the ground conditions are very resistive (electrically) like in dry sand dunes and frozen per- mafrost or when it is physically difficult to install electrodes like in cement or hard rock. A little bentonite goes a long way to decrease the contact resistance on hard rock. Water is often used on sand dunes. There isn’t much you can do about per- mafrost (although we do have many successful case histories). (Langmanson 2016)
The results of the survey of the test site were interesting, to say the least. Having used drilling logs, cutting sample logs, and aerial data for years to design maps and models, I had hopes of seeing the full data set from the survey, but I did not get to. The Angelo State Geosciences Department had not finished conducting the survey before our portion of the class was complete.
However, we did get to see a couple of processed lines from the survey. It took me a little while to fully realize the image I was looking at was a subsurface image of electrical resistance and not necessarily a layer cake image of various formations.
As with all tools, the user must work to become skilled in their use and interpretation. That’s kind of like a drilling rig— just because you may be able to drive it to the site doesn’t mean you can get the derrick up and make it drill!
Driscoll, Fletcher, Ph.D. 2003. Groundwater & Wells, Second Edition. St. Paul, Minnesota: Johnson Screens.
Langmanson, Markus, Ph.D., PG. 2016. Personal interview by author on December 19, 2016.
Laymon, Doug, PG. 2016. Personal interview by author on December 21, 2016.
Ward, James. Ph.D., PG. 2016. Personal interview by author on December 19, 2016.
Raymond L. Straub Jr., PG, is the president of Straub Corp. in Stanton, Texas, a Texas-registered geoscience firm and specialized ground-water services firm. He is a Texas-licensed professional geoscientist and holds master driller licenses in Texas and New Mexico and a master pump installer license in Texas. He can be reached at firstname.lastname@example.org.