The Application of Ground Conductivity and Offset Wenner Resistivity Soundings to Optimise the Investigation of a 300 ha Site in the West Midlands

Abstract

Abstract In order to evaluate quickly and cost effectively the detailed shallow sub-surface geology of a 300 ha area in the West Midlands, geophysical techniques were used in conjunction with shell and auger drilling. Since the area contains one of the largest landfill sites of its type in Europe and is crossed by a river which is abstracted for drinking water within a short distance downstream, it was important to determine the geology and hydrogeology in some detail. Previous geological surveys of the area, which included the drilling of over 80 boreholes for sand and gravel extraction, indicated the presence of up to 5 m of highly variable sandy overburden overlying Keuper Marl. It was apparent from an initial desk study that despite the large number of boreholes the geology had been completely misinterpreted. A ground conductivity map was produced using a Geonics EM-31 which rapidly gave a picture of the general structure and identified targets for drilling and Offset Wenner resistivity soundings. The geophysics revealed the presence of a major buried channel cutting deeply into the marl, which was later confirmed in several places by drilling. Surface outcrops of clay or marl could be identified using the ground conductivity values alone. The use of the geophysical techniques has considerably reduced the cost and time of the investigation and has allowed the major effort to be concentrated in problem areas.