An Introduction to ERI, IP, and SP

You may have already noticed the common factor between the three methods we're discussing. They all rely on an electric current. Of course, there are other geophysical methods, but in our experience, electrical methods have provided either the best results—or have served as a complement to other methods. 

Electrical Resistivity Imaging

ERI, which you may also know as Electrical Resistivity Tomography (ERT), is the measurement of ground variations gathered by applying a small and highly-controlled electric current across an array of electrodes. In the simplest of terms, you're testing what is resistive to an electric current. This geophysical technique is used to create an image of a specific portion of the Earth’s subsurface. This image's size, depth, and resolution can be controlled by the number of electrodes used, and the spacing of the dipoles. 

Induced Polarization

IP is the Earth’s capacity to hold an electric charge over time. IP measures the voltage decay curve after the injected current is shut off. The higher the IP, the longer the charge is held over time. IP always decays and disappears over time, typically a few seconds—but sometimes up to a couple of minutes. IP is a derivative of resistivity—so in order to measure IP, first, you must measure resistivity.

Streaming Potential

You may have seen various iterations of "SP". SP can stand for Self Potential, Spontaneous Potential, and Streaming Potential. Since Self Potential and Spontaneous Potential are mostly used in mineral exploration, for the purpose of discussing bioremediation, we're only referencing Streaming Potential. 

SP is an electrical surveying method used when fluids or electrolytes—typically water—move through porous areas in the ground. This creates a change in voltage that can be read using a voltmeter and two electrodes.

What are these methods generally used for?

Electric-based geophysical methods are used for various applications. Again, the versatility of these methods is one of the main reasons why Advanced Geosciences only manufactures equipment that does this kind of work. Below are just a few of the many applications these methods can be used:

• Construction Projects
• Groundwater Exploration
• Geotechnical Applications
• Dam, Levee, and Pond Monitoring
• Saltwater intrusion mapping
• Pollution and Waste Mapping
• Cathodic Protection
• Cave Exploration
• Subsidence, Karst, and Landslide Mapping

You may be able to already see how some of the applications listed above could translate to agricultural use cases and bioremediation projects.

For instance, the basic principles of a groundwater survey and a saltwater intrusion mapping project can translate directly into a subsurface wastewater exploration project. The same methodology one would take for dam monitoring would be taken for heap leach pond monitoring or water well monitoring. You can see what we mean by the versatility of these methods.  

Why use these methods for bioremediation?

There are a few reasons why you may want to use these methods—either alone or in conjunction with other subsurface imaging methods.

Precise Mapping & Monitoring

Electric geophysical methods can give you a clear interpretation of what is below the surface. This is especially useful when looking for a specific target like an underground plume or an aquifer. After all, half of the battle of bioremediation is finding the target.

Low Environmental Impact

Devices, such as the SuperSting™, that let you employ these electrical methods run on electricity themselves. As such, there is little to no carbon footprint associated with these methods. It is also categorized as an in-situ solution—in which the equipment is set up on-site and does not require heavy machinery or invasive techniques.

Reduces the cost of Bioremediation

All three of the methods we're discussing can be deployed by using one device—a SuperSting™ System. With the precision of these methods, the risk of starting treatment in the wrong place—or even worse, missing a location that needs treatment—is greatly reduced. Reducing the chance of mistakes will save a lot of money, effort, and environmental impacts. And as we mentioned above, this process doesn’t require massive drilling machines or other invasive equipment.

Low Barrier to Entry

Devices like the SuperSting™ are all-in-one machines when it comes to using ERI, IP, and SP. As such, there isn't much equipment that one would need to learn how to use to operate a system. The system itself can fit in a couple of pelican cases, so there's no heavy machinery to worry about taking up a lot of space. And as we'll get into later, anyone can be trained to use these methods for the purpose of bioremediation. 

How can these methods be applied to bioremediation?

ERI, IP, and SP are great for two aspects of bioremediation: mapping and monitoring. It's an unfortunate truth that most projects require mapping (i.e, after a problem occurs), but we're strong advocates for monitoring projects that can catch problems before they arise. In fact, an aspect of these geophysical methods that could use more awareness is that they can be used in time-lapse monitoring—where one can keep track of changes over time.

Mapping:

Wastewater, Saltwater, & Similar Contaminants 

In environments like farms, wastewater is a common issue. Similarly, saltwater intrusion into a freshwater aquifer can have devastating effects on soil. Manure contains about 5 to 10 percent salt by dry weight, which comes from salt supplements in cattle feed. (Source: Roger A. Eigenberg, Bryan L. Woodbury (2011), "With Feedlot Manure, It Pays To Be Precise" May/June 2011 issue of Agricultural Research magazine) Because salts are an excellent conductor of electricity, ERI surveys can be used to map out underground wastewater. 

Monitoring:

Pond/Leach Field Monitoring

Structures like feedlot runoff ponds and leaching fields have a high tendency to pollute the surrounding soils. As such, it's important to keep a constant eye on these structures in the event that a leak or seepage occurs. In addition to monitoring wells, ERI and SP have proven to be effective ways to monitor any movement of liquids below the surface. Whereas monitoring wells are dependent on groundwater, ERI can detect seepage in the soils near and around ponds. (Source: Woodbury, B.L., Eigenberg, R.A., Koelsch, R. 2015. Early warning wastewater pond monitoring system. Nebraska Agricultural Experiment Station Bulletin. Date first used: September 2015.) 

Hydrocarbons

Hydrocarbons such as chlorinated solvents and oils need to be monitored at certain facilities. For example, there are many countries that produce olive oil. As such, olive oil mills can be a serious source of contamination—especially mills that dispose of waste into open ponds. In Greece, there have been experiments to use a saline tracer, coupled with time-lapse ERI monitoring, to track and monitor oils in the subsurface. (Source: S. Kirkou, et all, “3D time-lapse ERT monitoring of an experimental simulation of olive-oil mills' waste movement“ Berichte Geo/. 8.-A., 112, ISSN 1017-8880, 3rd Internal)

Aquifer, Water Table, & Water Well Monitoring 

 As we mentioned above, ERI can be used in time-lapse monitoring. This means that with ERI and SP, you can track underground water levels over time. Not only that, but you can track when and where contaminants have polluted fresh groundwater. 

Who is qualified to use these methods?

It may seem like you would need to employ the help of a geophysicist to complete a survey that uses these methods. While that would definitely help, it's not totally necessary. 

For instance, when Advanced Geosciences sells a SuperSting™ System to our customers, we always include a training seminar with the purchase. We have trained hundreds of users to use the system within the span of 8-24 hours. The backgrounds of our customers can vary from geotechnical engineers to ranchers.

With that said, seeking out help from local experts in the geoscience field can help projects go smoothly. Making connections with professors, geotechnical engineers, and geophysicists are recommended. Even a cursory knowledge of your surrounding geology goes a long way.