Evaluating the multiple coil configurations of the EM38DD and DUALEM‐21S sensors to detect archaeological anomalies

Abstract

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.

The 3-D reconstruction of medieval wetland reclamation through electromagnetic induction survey

“…The multi-receiver EMI sensor we used, allows simultaneously recording the apparent electrical conductivity (σ a ) and apparent magnetic susceptibility ( κ a ) of four different soil volumes12. Whereas in non-saline environments σ a can be directly related to soil texture (clay, silt, sand), and is influenced by soil organic matter and water content1314, κ a anomalies are often the result from ferrimagnetic soil enrichment1516, the anthropogenic disturbance17 of top soils (e.g.…”

Prospection of two circular Bronze Age ditches with multi-receiver electrical conductivity sensors (North Belgium)

“…Both the coil orientation and the coil separation determine the volume (and thus depth) of exploration of the instrument, so that the DUALEM-21S measures four different soil volumes (the depth sensitivity curves similar to Fig. 2 were given in Simpson et al, 2009). The height of the sensor and thus of the coils above the soil surface is also very important and often neglected in the interpretation.…”

“…1b), while the surveys in Belgium were carried out using an all-terrain-vehicle pulling a sled with the sensors (at 0.16 m above the soil surface) as in Simpson et al (2009). Both platforms were georeferenced by a dGPS with a 0.1 m pass-to-pass accuracy.…”

Non-standard electromagnetic induction sensor configurations: Evaluating sensitivities and applicability

“…These systems, which are also called ground conductivity meters (GCM), are often used in archeological prospection (Tabbagh, 1986;Lück and Eisenreich, 1999;Simpson et al, 2009), soil science (Corwin and Rhoades, 1982;Abdu et al, 2007), precision farming applications (Doolittle et al, 1994;Yoder et al, 2002;Gebbers et al, 2009), as well as for soil salinity mapping (Triantafilis et al, 2000;Nogués et al, 2006) and detecting buried objects such as utility pipes (Nelson and McDonald, 2001). Most of the surveys are acquired by using one fixed loop configuration and one selected direction of data acquisition.…”