Mapping of hydrocarbon contaminants with Electrical Resistivity Tomography (ERT)

Summary

The Electrical Resistivity Tomography (ERT) method has proven to be an efficient way of differentiating areas of natural ground from those containing certain types of hydrocarbon pollutants. This method allows practitioners to understand the spatial extents of pollutants and plan effective remediation.

PROJECT

Method: Resistivity (in ERT mode)
Solution: ABEM Terrameter LS 2, 0.75 meters electrode separation.
Software for processing and interpretation: Resistivity inversion and visualisation made with Res2DInv from Aarhus GeoSoftware.

Challenge

The background information of the hydrocarbon contaminant stated it to be quite shallow (not more than 10 meters deep) but covering a larger area. This resulted in a need for a high-resolution mapping at a shallow depth.

Solution

Hydrocarbons most often (depending on the degeneration stage) increases the resistivity of the ground, and this effect can be mapped with resistivity investigations. To generate a high-resolution picture of the subsurface conditions and the possible hydrocarbon contamination the investigations were carried out as several parallel ERT (Electrical Resistivity Tomography) profiles.

Field work with the ABEM Terrameter LS 2.

Eight parallel ERT profiles, separated by 8 meters and each profile is approximately 55 meters long.

Results & Conclusions

The hydrocarbons in the investigation area gave a clear response in the resistivity data. In the figures the resistivity distribution is showed both in 2D, depth slice and in 3D as profiles and plotted as a fence diagram. In all figures, the low resistivities are illustrated in blue and the high resistivities in red. The hydrocarbon contaminated ground is delimited to the red, high resistivity areas.

ERT proved to be an efficient way to map hydrocarbons and the results correlated well with other investigations carried out (e.g. sampling). The results could also be used to estimate the volume of the contaminant and the max depth of the same.

Image showing excavation work as part of the remediation.

One example of an ERT profile. High resistivity values are shown in red.

A so-called fence plot of the eight investigated profiles. High resistivity values are shown in red

Acknowledgement

We would like to thank Geoaustral (www.geoaustral.com.ar) and Ignacio Borsani for sharing the information above.

Geoaustral is geophysical services company, based in Buenos Aires, Argentina. The company works with both geological data acquisition and processing to create geological models for a number of different applications. The work comprises of, for example: environmental, hydrological and mining projects.