Application of High Resolution Electromagnetic Frequency Soundings in Mapping of Oil Contamination Flow Routes

Small coil separation (hand-held) electromagnetic (EM) mapping systems are frequently used in environmental and hydrological surveys. Multi-frequency capability allows shallow earth resistivity soundings to be conducted. Apparent depth of the response is approximately square root of the skin-depth value. In frequency domain true measurement accuracy and system stability factors are important because earth responses can be small in magnitude. Measurement height needs to be taken into account in absolute values and in data inversion. Case study concerns with mapping of shallow overburden variations for light (diesel) oil spill flow routes in eastern Finland. Oil contamination occurred years ago and current interest was to map potential flow routes along bedrock surface or in bedrock depressions where flow along the groundwater layer surface can happen. Survey was taken with electromagnetic frequency domain instrument. Data was interpreted with converted apparent resistivity maps, 2-layer 1-D model inversion, calculating electrical conductance maps and producing bedrock surface elevation map. Results indicate that EM surveying is useful method in detailed hydrological studies. Results were used with success to place new investigation holes and install groundwater standpipes into potential flow routes.