Electrical Earth Resistivity Surveys in Geophysics Measures Soil and Rock Resistance

Earth resistivity measurements in geophysics

are often used to map changes in electrical resistivity

below the ground’s surface. Engineering geophysics incorporates electrical earth resistivity methods in various applications, which range from characterizing geologic conditions at a depth of 100’s of feet to measuring the electrical resistivity of a sample placed in a small handheld container. As an example, earth resistivity measurements can be used to map out clays, sands, gravels, frac sand deposits, types of rock, levee areas, groundwater, fracture joint systems, faults, karst features, variations in porosity, changes in cementation, voids, graves, archaeological sites, groundwater contamination, and saltwater intrusion, to name a few.

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Earth resistivity equipment

uses little electrical power to measure the resistivity of near surface soils. Determining the electrical resistivity of the subsurface for a ground plane around a large power plant or for geologic conditions 100’s of feet below grade usually demands a larger more stable power supply than a unit powered by a few AA batteries.

Larger earth resistivity meters that utilize automobile sized batteries or even gas powered generators generally fall in to the category of geophysical instruments and can be rented from K.D. Jones Instruments. This type of geophysical equipment is designed not only for greater depths of penetration, but for adverse weather, rugged field conditions, and poor site conditions. In my opinion, good electrical resistivity equipment is built by a hand full of manufactures. Two U.S. companies that have worked hard at meeting the geophysicists’ needs are AGI and Geometrics.

Multielectrode Earth Resistivity Array

Multielectrode Earth Resistivity Array

Geometrics OhmMapper Capacitive Coupled Resistivity Array

Geometrics OhmMapper Capacitive Coupled Resistivity Array

One of AGI’s most popular units is the SuperSting R8. In addition to being capable of acquiring profile data or resistivity soundings, it is a programmable multiple electrode resistivity MER system capable of acquiring 1000’s of readings with a single array setup.  The arrays come in unlimited lengths and configurations. An 84 electrode system with takeouts spaced 5.25 meters apart can reach 100’s of feet deep. A 2-D resistivity line can be “rolled” end to end for as far as you have space. The same system can be configured to detail the top 10 feet of soil or create a 3-D electrical resistivity representation of the subsurface. The unit is programmable for multiple array configurations (e.g. Wenner, Schlumberger, Dipole-Dipole, Pole-Dipole, Pole-Pole, or user defined). While the SuperSting provides a variety of possibilities, it is mostly used along a single straight line and can not be towed. In contrast, Geometerics OhmMapper is a towed array that is cable of covering spatially large areas, such as, fields, parking lots, landfills, and large commercial sites. The unit is generally towed by a person (sometimes a vehicle) and is pulled across a site from one end to the other. Using the OhmMapper to acquire closely spaced lines, allows the geophysicist to map lateral changes across a site. Multiple lines along a single transect can yield cross-sections of electrical resistivity, much like the SuperSting.


Electrical Earth Resistivity Surveys in Geophysics Measures Soil and Rock Resistance — 37 Comments

  1. Hi, i am using GPR method(sir-3000) to get image underground to depth 30m but i found that this radar cant go down more than 6-7m.
    now can you advise me if earth resistivity can give a good image with good resolution and can shows the caves at this depth if the earth is sand, silt, gravels, and clay over bedrock.

    • Electrical resistivity is often a great alternative to ground penetrating radar GPR. The GSSI SIR-3000 works well. I am curious to know what frequency antenna are you using? What are the dimensions of the caves or void spaces you are looking to locate?

  2. I would like to map groundwater contamination by Nitrogenous fertilizer. What equipment would best suit my work.?
    Thanks in advance.

    • Geophysically, I have documented changes in electrical resistivity or electrical conductivity. Electrical resistivity methods provide some of the greatest detail in cross-section, using multielectrode electrical resistivity imagining methods. I have produced great results using AGI’s SuperSting R8 system with 84 electrodes. Electrical terrain conductivity measurements using a Geonics EM-34, for deep penetration, or Geonics EM31 for shallow surveys works well to survey spatially large areas or the lateral extent of contaminiation. In either case, you need a electrically detectable contrast that correlates well with the contamination from the Nitrogenous fertilizer. This is generally dependent on the soil, rock, and/or groundwater conditions, as well as access.

    • Thanks for the support. Good geophysical equipment, like AGI’s SuperSting R8 and the multielectrode resistivity arrays for imaging the subsurface, makes a difference. Good luck to you at AGI.

    • Electrical earth resistivity measurements are one of the best geophysical methods to use when looking for sinkholes. However, the sinkholes need to be fairly large. Small sinkholes at depth are nearly impossible to locate. However, the dissolving processes that creates the sinkholes may impact a very large area. Back in March, Florida news stations announced that a man was swallowed up by a sinkhole. I do not remember which station it was, but they showed a crew of geophysicists using a multi-electrode earth resistivity array to investigate the surrounding area.

  3. Hallo
    I a Geophysicist and I like to know what the price of the system and to work vertical electrical sounding ground to depths of 300 meters

    • Where are you located? Do you have any electrical earth resistivity results from the past to help determine what equipment you may need? Are you interested in acquiring 2-D resistivity images, cross-sections, or just soundings? What type of electrical resistivity survey are you interested in (e.g. dipole-dipole, wenner, schlumberger,….)?

        • While electrical resistivity measurements can be acquired to great depths, it is often difficult and time consuming. It may be easier and more cost effective for you to use Time Domain EM TDEM. Geonics makes a Protem system that may be a little more friendly to use in this geophysical application. If you are serious about imaging or acquiring soundings for these depths using earth resistivity measurements, you will likely need to consider using a larger power supply, one that uses a gasoline motor.

  4. It looks like you know AGI’s equipment, what can I do with a dataset that is from rolling 6 times.

    • Large earth resistivity datasets can be hard to manage. AGI points out that “A marine resistivity survey with a boat-towed array produces thousands of measurements with thousnds of dynamic electrodes in an hour.” While this is extreme the solution for inverting or processing the data is the same. AGI suggests using an optional module or add-on called Continuous Resistivity Profiler (CRP). With this module “EarthImager divides the long section into many subsections and inverts subsections one after another.”

  5. I am looking to map bedrock. My client wants to see to about 150-200 feet below the surface. The overburden is mostly sands and gravels. I was thinking GPR, but would seismic be the way to go.

    • Ground penetrating radar will not likely penetrate 150 to 200 feet. Seismic refraction can but there is often low velocity layers or thinly bedded layers that can complicate the interpretation. If you are thinking of doing a seismic survey, you may want to consider a seismic reflection survey. Electrical resistivity or earth resistivity measurements are capable of those depths, as well as, electromagnetic methods (either time domain EM like Geonics Protem or frequency domain EM with a Geonics EM-34). If the sands and gravels have little clay or are not saturated, EM methods may not work very well. I am the least confident that GPR will yield desirable results.

    • The answer leans towards yes and no. Earth resistivity measurements can get a little complicated for this application. A soil resistivity meter that is known to acquire good data can measure the unit resistivity of a formation, if you have direct access. If you don’t, then you will likely need to model or invert the data to interpret what the unit resistivities may be. After that one needs to correlate the measurements to various rock types and site conditions. After that is determined, charts often provide a range of values that correlate to geologic ages. Depending on your point of view, the groupings can cover a wide range of ages, for example, Precambrian, early Paleozoic, Carboniferous, Mesozoic, Eocene, Miocene, Pliocene, and Quaternary. This may or may not help you out.

  6. I read that there are geophysical instruments that can help me with my crops. I have 1000 or so acres and want to do a better job. Should I try a soil resistivity meter?

    • Earth resistivity measurements can assist with delineating areas that have more clay minerals, moisture, conductive chemicals, and other from areas with less. However, typical 4 probe measurements or even multielectrode resistivity systems take time to cover a spatially large area. A Geometrics OhmMappaer is a capacitance coupled resistivity instrument that is dragged across the ground. There is picture above of the capacitance coupled array. The spacing between the transmitter and the receiver and the lengths of the transmitter and receiver arrays are varied to obtain different depths of penetration. This equipment can be pulled behind a vehicle and integrated with DGPS to make maps. You may also want to consider using a Geonics EM-38, the EM38 is an electromagnetic terrain conductivity meter that measures apparent conductivity. Apparent conductivity is the inverse of apparent resistivity. Geonics data acquisition software is also able to record DGPS.

        • The time of year can make a difference, especially if there are noticeable variations in the moisture of the soil. Heavy rains and spring thaws may saturate the soil. This is not necessarily a bad thing. Generally, the deeper you penetrate the less effect the near surface material has on your apparent resistivity measurements.

    • Contact resistance plays an important part in measuring soil resistivity or rock resistivity. Standards often state something like not to place the polished electrode greater than 10% of the electrode spacing. Others state that the contact resistance should be less than 2000, 3000, or 10,000 ohms. These are all good suggestions, unless the soil or rock has a unit resistivity greater than these resistivity values. In any case, if the instrument is working properly I think it is best to keep the contact resistance as low as possible. Geophysicists often use water, saltwater, and other electrically conductive liquids to reduce the resistance between the electrode and the ground. One manufacture states that laying temporary electrodes like metal ground mats or coiled chains with water poured over them may yield desirable results. The idea is that there is more surface area in contact with the ground, than with a spike. This may not be a bad idea for electrodes spaced far apart. One may need to do some preliminary testing to determine how far the electrodes need to be spaced to avoid acquiring erroneous values.

    • Megger makes many different products and earth resistivity testers. The specs for the DET5/4D & DET5/4R are Test voltage: Maximum 50 V peak Test Current (constant current within a range): 20 ½ Range: 10 mA a.c. r.m.s.,200 ½ Range: 1 mA a.c. r.m.s. 2 k½, 20 k½ Range: 100 μA a.c. r.m.s.

      In comparison, a geophysical instrument like AGI’s SuperSting has an Output current intensity of 1mA to 2000 mA continuous and output voltage of 800 Vp-p. They point out that the actual electrode voltage depends on transmitted current and ground resistivity. The total output power is 200W.

      While the two markets overlap, the SuperSting earth resistivity system can provide a substantially greater amount of power, when needed.

  7. Hi, we have a job in Canada and we would like to rent a 2D resistivity system complete with 10m cables and electrodes and a copy of RES2DINV. We are aiming for 50m penetration in sand/silt and clay over bedrock. Could you recommend your most suitable electrical resistivity system. We need the gear to be shipped to Labrador, Canada to arrive …..

    • In the geophysical market, many of the multi-electrode earth resistivity systems utilize 56 electrodes. I believe that to get approximately 50 m of penetration over approximately half the length of a 56 electrode dipole-dipole resistivity array one would likely need to use a 10 m electrode spacing. In contrast, I believe that an 84 electrode AGI SuperSting 8 channel earth resistivity system with a dipole-dipole array and 5.25 m electrode spacings provides about the same coverage at a depth of 50 m. Both systems provide less coverage at depth when a Wenner array is used. However, the 84 electrode system with 5.25 m electrode spacings offers greater detail above 50 m. This detail can be used to better map lateral and vertical variations that are beneficial to the inversion process. In my opinion, the more detail you have from above your target depth the more likely the earth resistivity model will reflect known conditions at depth. As with any geophysical survey certain factors determine whether or not you will get desirable results. In this case, earth resistivity methods (or soil resistivity methods) do not yield unique results and the depth of penetration is dependent on the true unit resistivities and thickness. K.D. Jones Instruments rents the SuperSting 8 channel system ships with Earth Imager for no extra charge. Earth Image is easy to use and is useful for correcting the geometry, splicing data, editing the data, and creating models. Res2DInv is an excellent software package. The SuperSting administration software, which is used to program the SuperSting, would assist you with designing your multi-electrode resistivity survey and confirming my above comments.

    • Geophysical electrical resistivity methods work well for finding sand and gravel. Please tell me more about the geology and where is the water table?

        • Electrical resistivity methods often work well for mapping sand and gravel. You need a large enough deposit to detect and it must have an unit resistivity that is different from the adjacent clay. If the sand and gravel is saturated it will be less resistive than if the sand and gravel was unsaturated. A Geonics EM-34 often works well for mapping apparent conductivities (inverse of apparent resistivity) over large areas (100’s and 1000’s of acres). If the water table is below the top of rock, the overburden generally a constant thickness, and the rock is more or less homogeneous then decreases in apparent conductivity may correlate well to areas with less clay minerals. Areas with lower apparent conductivity would likely be of interest. Multi-electrode electrical resistivity survey lines over areas of interest would provide greater detail. A SuperSting 84 electrode system could assist with determining variability within the sand and gravel deposit, the thickness, and the lateral extent.

          • Yes, if it has enough power. You may not be happy if your resistivity meter runs off of a few flashlight batteries. You will likely need more power (e.g., ministing or Sting R1). It will also be difficult to acquire enough geophysical data to create a detailed cross-section. You would likely need a multi-electrode system for detail along a single line. Your earth resistivity meter may be the most useful for acquiring apparent resistivity data with one or two array geometries. You could acquire data on parallel lines over the entire site and create a contour map of the results. The idea would be to find areas with high apparent resistivity. These areas may have the greatest chance of being the sand and gravel you are interested in.

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