As part of a project for tracking down overflowed oil, Enviroprobe conducted a geophysical survey to delineate the bedrock topography around a tank field. The seismic refraction was the primary investigation method and an EM31 survey was also conducted as a quick complementary method.
The 2D seismic refraction survey was conducted along parallel lines and the results were interpolated for the bedrock topography. One of the important discoveries was that the elevations of the bedrock top around perimeters of the tank field were low, although the tank field itself appeared to be at the top of a hill.
The EM31 data appeared to agree with the seismic survey results although it was severely affected by underground utilities and other ground features. This is based on the assumptions that the overburden soils are electrically more conductive than the underlying bedrock, and the overburden and the bedrock are homogeneous.