Surface Scanning to Determine Extent of Soil Contamination at Oil Spill Site

Akin Jeffrey Adebiyi - Engineering Geophysics Coordinator, Fundamental Integrated SIte Appraisal Services (FISAS) Limited, Abuja.

Oil spill on land leads to hydrocarbon contamination of soil –  a form of environmental pollution.  Human activities that cause this form of pollution include loss of containment of crude or refined petroleum products at production site/processing plant or in storage or in transport via rail, road or pipeline.

Petroleum hydrocarbons are known to be very poor conductors of electricity, which means hydrocarbons highly resist electricity. Hence the resistivity values of hydrocarbon contaminated soil is anomalously high and that makes the Electrical Resistivity Tomography (ERT) a perfect geophysical tool for determining the extent of oil contamination in the soil. ERT simply means a 2D pictorial representation of soil electrical resistivity.

ERT is based on the principle of Ohm’s law which states, ‘that voltage across a given body is proportional to the current through it’. The constant of proportionality in ohm’s law is known as resistance. Three major parameters which include the current (I), the potential difference (V), and the resistance (R) are all that is needed to determine the extent of oil contamination of soil in an area.

The ERT method uses an advance resistivity meter and sets of 20cm stainless steel pins to inject a known current (I) into the soil and another set of stainless-steel pins to measure the potential difference, hence resistance is calculated. The resistance values are processed further to determine soil resistivity of the tested area.

ERT profile showing extent of hydrocarbon contamination at a spill site.

The obtained soil electrical resistivity values are compared with electrical resistivity values of the geological layers then areas with abnormally high resistivity values (possible oil contaminated areas) are deduced and depicted pictorially.

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