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

Two types of electrical conductivity sensors were evaluated to prospect circular ditches surrounding former Bronze Age burial mounds, complementing aerial photography. The first sensor was based on the electrical resistivity (ER) method, while the second sensor was based on frequency-domain electromagnetic induction (FDEM). Both sensors were designed with multiple receivers, which measure several depth sensitivities simultaneously. First, the sensors were tested on an experimental site where a rectangular structure with limited dimensions was dug in a sandy soil. The structure appeared as a higher conductivity anomaly in the low-conductivity sand. Then, both methods were applied on two Bronze Age sites with different soil properties, which were discovered by aerial photography. The first site, in a sandy soil, gave only very weak anomalies. Soil augering revealed that the ditch filling consisted of the same sandy material as the surrounding, therefore this filling was not able to cause a high-conductivity contrast. Due to its lower sensitivity to noise in the low-conductive range, the ER-sensor produced a more pronounced anomaly than the FDEM-sensor. The second site was located on top of a ridge with a shallow substrate of Tertiary, coastal sediments. The ditch was very clearly visible on the sensor maps as a conductive low. At this location, the soil augering revealed that the ditch was dug through an alternating clay-sand layer and subsequently filled up with silty material from the topsoil. Overall, the shallow receiver separation produced anomalies that were both stronger and that corresponded better to the geometry of the ditches. The other receiver separations provided more information on the natural soil layering, and in the case of the ER-array they could be used to obtain a cross-section of the actual electrical conductivity with 2-D inversion modelling. The results of this study proofed that conductivity sensors can detect Bronze Age ditches, with varying contrast depending on the soil geomorphology. Moreover, the sensor maps combined with soil observations by coring provided insight in the environmental conditions that influence the contrast of the anomalies seen on the aerial photographs and the sensor maps.