Imaging groundwater ‘steps’ in push moraines by georadar

Georadar has proved to be a valuable new geophysical technique for non-destructive groundwater exploration in the Netherlands, yielding detailed continuous images to depths of 40 m under favourable conditions. During the Saalian, glacial action in the Netherlands deformed the fluvial deposits and created ice-pushed ridges or push moraines. Most of these displaced fluvial sediments are coarse-grained sands and gravels, but thin clay layers which acted as sliding planes during the glacial deformation are also present. The ice-pushed ridges are infiltration areas and the thick coarse sediments are important aquifers of the Netherlands. To be able to adequately manage these groundwater resources and forecast their depletion and contamination, the groundwater flow must be modelled in detail. Steeply dipping clay layers impede groundwater flow and create ‘steps’ in groundwater levels. Georadar surveying enables water tables to be imaged precisely and detects the exact location of such groundwater steps and vertical barriers.

During a georadar survey near the village of Epe on one of the major ice-pushed ridges of the Netherlands, a sudden and spectacular offset in the water-table reflector was observed. The jump in the reflector could be traced along a number of parallel lines. A series of nine boreholes drilled to verify the georadar results confirmed the presence of important steps in the water table and proved the georadar predictions on depths and offsets to be remarkably close to reality.