Electrical resistivity tomography

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Electrical resistivity tomography (ERT) is a geophysical method used to image the subsurface using differences in measured electrical resistivity at the surface. These differences in resistivity can be tied to the porosity, fluid content, and degree of water content in the subsurface[1]. The data collected in these surveys is then inverted to give an image of the subsurface electrical characteristics.

Figure 1: An example cross section of an ERT profile comparing freshwater to saltwater in the subsurface, the saltwater has lower resistivity values (colder colors), the areas with no groundwater have higher resistivity (warm colors). Modified from Rao et al 2011.

Background

Data is collected by using various Electric resistivity surveys, most of the methods used are applied from electrodes at the surface. Electrical methods were first used in the 1920's as a one dimensional measure of the resistivity, they were later changed to 2D and 3D methods[2]. The data is then inverted, which means a model is formed based on the data if there is good knowledge of the subsurface, a forward model can be applied before inversion, this will help the inversion be more accurate unless the forward model is wrong. These inversions create the tomographic profile, or a cross section of the subsurface directly below where the survey is collected. After the profile is created, the subsurface is interpreted based on the electric resistivity values. These surveys can be used for geologic features that have a difference in surrounding resistivity like groundwater detection, porosity comparison, and archeological finds. ERT is often coupled with other Near surface geophysical methods to offer complimentary data.

Applications

Figure 2: An ERT profile along an exposed outcrop, with varying lithology of chert and claystone. The water content and porosity differences lead to the differences seen in the ERT profile. Modified from Hisham et al 2017.

Groundwater Detection

The presence of Groundwater in the subsurface would be indicated by an area of lower resistivity compared to somewhere that has no groundwater. Changes in the composition of groundwater also relate to the resistivity measured, such as the presence of salt or other contaminates. Many coastal areas use this method, along with some well data to determine the presence of salt water in the water table[3]. ERT has also been successful in karst regions at identifying water and clay filled caverns[4]. More advanced ERT application involves 3D to 4D groundwater evaluation. Figure 1 shows an example of a groundwater survey.

Porosity Comparison

Porosity is tied to how much water or less resistive fluid can be present in in a rock, and this can be used in a variety of near surface applications. For example ERT can be used to see if different lithologies are present if they have different porosity values. An example of this would be the different in porosity causing a difference between chert and claystone, the chert has a higher resistivity due to its composition and lack of pore space compared to the claystone, which has a low resistivity[5]. Figure 2 shows the an ERT profile that was conducted on the top of an outcrop, so a lithology comparison could be made. In this profile the chert corresponds to the higher resistivity seen.

Figure 3: Comparison of the archeological model of the site (top), the ERT profile (middle) and the GPR profile (bottom). The ERT does a good job of locating the fill areas identified in the model, seen as areas of high resistivity. Modified from Deiana et al 2018.

Archeological

One of the advantages of Electric resistivity surveys is they are non-invasive to the ground beneath them. This helps with archeological surveys to get an idea of what is down there before digging starts without disrupting the near surface. Some archeological areas of interest that would show up nicely in ERT are tombs, or cavities in the ground[6]. Areas with high metal content would also show up nicely in ERT. Figure 3 depicts an archeological survey that found fill zones in the subsurface without digging, ground penetrating radar was also used to prove the ERT profile.

See also

External links

References

  1. ↑ Loke, M.H. (2004). Tutorial: 2-D and 3-D electrical imaging surveys (PDF). Retrieved 2019-10-14.
  2. ↑ Loke, M.H. (2004). Tutorial: 2-D and 3-D electrical imaging surveys (PDF). Retrieved 2019-10-14.
  3. ↑ Gurunadha Rao, V.V.S., Rao, G.T., Surinaidu, L. et al. Water Air Soil Pollut (2011) 217: 503. https://doi.org/10.1007/s11270-010-0604-9
  4. ↑ Tassy, A., Maxwell, M., Borgomano, J. et al. Environ Earth Sci (2014) 71: 601. https://doi-org.ezproxy.lib.ou.edu/10.1007/s12665-013-2802-4
  5. ↑ Hisham, H., Nordiana, M., & Ying Jia, T. (2017). Evaluation of Semanggol Formation (Permian Facies) Using Electrical Resistivity Tomography and Seismic Refraction Tomography Parameter. IOP Conference Series: Earth and Environmental Science, 62(1), 6.
  6. ↑ Deiana, R., Bonetto, J. & Mazzariol, A. Surv Geophys (2018) 39: 1081. https://doi-org.ezproxy.lib.ou.edu/10.1007/s10712-018-9495-x
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