Comparison between electromagnetic induction and fluxgate gradiometer measurements on the buried remains of a 17th century castle

This study aimed to evaluate the different configurations of an electromagnetic induction (EMI) sensor, the EM38DD (Geonics Limited, Canada) with fluxgate gradiometer measurements on an archaeological site. The EM38DD allows measuring both the apparent magnetic susceptibility (MSa or χa) and the apparent electrical conductivity (ECa or σa) in two different coil orientations. A gradiometer measures the lateral variations of the vertical magnetic field gradient, caused by the induced and remanent magnetisations. An archaeological site where historical documents indicated the presence of a 17th century brick castle was selected as a test area. The results of the first survey with the EM38DD showed very strong magnetic anomalies in the central field, which were caused by the brick remains of the castle. Therefore, a smaller area was chosen within this field to compare the different configurations of the EM38DD with the gradiometer at the same measurement resolution. The most useful results with the EM38DD were obtained from the MSa measured in a vertical coplanar orientation. Its anomalies corresponded well with the gradiometer anomalies. The gradiometer anomalies were sharper defined than the EM38DD anomalies, but were complicated by the bipolar response pattern. The MSa map in horizontal coplanar orientation was very difficult to interpret, due to the less optimal spatial sensitivity. The wall remains were not visible in the ECa map in horizontal coplanar orientation, although other interesting anomalies were detected.