Evaluating the Multiple Coil Configurations of the EM38DD and DUALEM-21S Sensors to Detect Archaeological Anomalies

The multiple coil configurations of two electromagnetic induction sensors were tested on afield with strong electrical and magnetic contrasts. The first sensor, EM38DD, measures either the apparent electrical conductivity (ECa or sigma(a)) or the apparent magnetic susceptibility (MSa or chi(a)) of the soil at two coil orientations. The second sensor, DUALEM-21S, measures both ECa and MSa at two coil orientations and two coil separations. The goal was to test if measuring with the multiple coils resulted in a better detection of near-surface artefacts and the natural soil variability. The ECa of all coil configurations was closely related to the depth of a clay substrate beneath the topsoil sandy loam, which was verified by soil augering. Configurations with a shallower theoretical depth of exploration were less influenced by the clay substrate. Combining two coil configurations revealed important ECa anomalies, not visible on individual measurements, associated with a brick wall foundation and a former ditch. The MSa maps showed totally different anomaly patterns, related to anthropogenic disturbances in the soil, such as the filling-in with brick rubble of a former pond. Depending on the depth and thickness of the disturbance and the relative response of the sensor configurations, the MSa anomalies were entirely positive for one configuration but other configurations also had negative anomalies. It was concluded that multiple coil configurations provide a better insight into the buildup of the soil profile and are better able to detect anomalies than single measurements. Copyright (C) 2009 John Wiley & Sons, Ltd.