Reconstructing palaeotopography at the beginning of the Weichselian glacial stage using an electromagnetic induction sensor

During the last glacial period (Weichselian), wind-blown loess was deposited over the undulating landscape of central Belgium, which had been formed in surfacing Tertiary marine sediments. Since valleys were covered with a thicker loess layer than were elevated areas, the present topography is much flatter. Reconstructing the palaeolandscape at a detailed scale is almost impossible by conventional procedures based on soil augering. Therefore, the use of the EM38DD electromagnetic induction sensor was evaluated as an alternative for mapping the depth to the Tertiary clay substrate. On our 2-ha study site, strong non-linear relationships (R-2 = 0.85) were found between the apparent electrical conductivities (ECa) measured by the vertical orientation of the EM38DD and the depth to the Tertiary clay (z(clay)) on the one hand and between the combination of the EC(a)s measured by the vertical and horizontal orientations of the EM38DD and z(clay) on the other hand. These predictions were validated by independent observations of the depth to the Tertiary clay, and r values of 0.84 (using only the measurements in the vertical orientation) and 0.85 (using both measurements in the vertical and horizontal orientations), with an average error of 0.26 m, were found. Our dense ECa measurements (2 m x 2 m resolution) allowed us to build a three-dimensional surface of the depth to the Tertiary substrate, reconstructing the palaeotopography beneath the loess cover and revealing distinct erosion patterns. The continuity of these was confirmed by an analysis of surface flow patterns on the reconstructed palaeotopography. The non-invasive, quick, and cost-effective electromagnetic induction sensor offers new possibilities in reconstructing and analysing the Quaternary palaeotopography beneath the loess cover