Contributions of Surface Geophysics to Environmental Site Investigation of Former Oil Distribution Terminals

A former industrial site, last used some twelve years ago to house oil distribution terminals, is to be redeveloped following any necessary remediation. Before the start of invasive site investigations, a trial geophysical survey was conducted involving magnetics and electromagnetics. This trial demonstrated that the electromagnetic technique, involving a Geonics EM31 soil conductivity meter, was capable of detecting subsurface metal pipes and contaminant concentrations resulting from the oil terminal works. The EM31 is directionally dependent and yields different results when its boom is either parallel or perpendicular to subsurface linear anomalies. This directionality can lead to misleading interpretations. By taking the second derivative of the EM31 quadrature response, a new directionally independent map is presented with no such interpretation difficulties. A full, detailed electromagnetic survey revealed a labyrinth of subsurface pipes about 1 m deep together with a number of anomalously low conductivity regions. Trial pits and trenches validated the geophysical interpretation, even to the extent of finding tarry contaminants emanating from the ends of broken pipes. This has led to a two-stage remediation programme in which the pipes will first have to be carefully extracted.