The multiple coil configurations of two electromagnetic induction sensors were tested on a field with strong electrical and magnetic contrasts. The first sensor, EM38DD, measures either the apparent electrical conductivity (ECa or s a ) or the apparent magnetic susceptibility (MSa or x a ) of the soil at two coilorientations.The secondsensor,DUALEM-21S, measuresboth ECa and MSa at two coilorientations 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 formerditch.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.

No abstract

Multiple apparent electrical conductivity (EC a ) measurements with an electromagnetic induction (EMI) sensor frequently reveal analogue patterns caused by conductive features in the soil. A procedure was proposed to highlight different archaeological anomalies based on combinations of the simultaneous EC a measurements with the DUALEM-21S instrument. After selection of a 3.5 ha study site, 0.79 ha has been recorded by archaeological excavation. Since the majority of the archaeological features were found between the plough layer and 1.0 m below the soil surface, a set of four equations were developed to model the EC within that predefined depth interval. This set of four equations employed the four depth response curves specific to the four DUALEM-21S coil configurations. The modelled conductivity between 0.5 and 1.0 m (EC Ã 2 ) showed a larger variability across the archaeological features than the raw EC data. To quantify the added value of this modelled conductivity, EC Ã 2 and measured EC a were compared with the rasterized map of the archaeological traces. Finally, the EC Ã 2 map proved to be better able to distinguish between the archaeological features and the 'empty' background. This technique allowed the highlighting of vague anomalies in the simultaneous DUALEM-21S EC a measurements. m, mean in mS m À1 after conversion to a reference temperature of 25 C; CV, coefficient of variation in %; RD, relative difference in %.

No abstract

No abstract