Abstract

There are two principal indicators for the presence of hydrocarbons in groundwater: an increase in the electrical resistivity of the groundwater saturated layer and an induced polarization effect (IP). The electro-osmosis type IP superimposes on the time domain electromagnetic (TDEM) soundings an effect that distorts the signals. The parameter affecting the electro-osmosis IP is the polarizability η which is the ratio of surplus electrical conductance and electrical conductance of the pore fluid. The value of the decay constant τ is controlled mainly by the radii of pores. Theoretically, the process of electro-osmosis type IP can occur in different types of sediments. In reality, the IPeffect can only be observed in fluid-saturated sediments with an imperfect relationship between the components if the pore size is in the range 1 ⋅10 -5 – 5 ⋅10 -4 m. The interpretation of TDEM soundings was carried out using the Sτ version as an indicator of the IP effect, and mathematical modeling provided the forward calculation of the electromagnetic field in multilayered polarizable media. The TDEM sounding results could successfully be used to delineate hydrocarbon contamination of groundwater in a few areas of South Africa and Russia. Follow-up drilling results confirmed that the contaminant plume could be mapped by identifying the IP effects.

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