Radar facies: recognition of facies patterns and heterogeneities within Pleistocene Rhine gravels, NE Switzerland

Pleistocene braided-river deposits in river valleys constitute a large fraction of natural groundwater reservoirs in Switzerland. A key for estimating the residence times of water and for determining the extent of macrodispersion, which describes large-scale mixing processes in the aquifers, is a knowledge of the distribution of hydraulic conductivities. In many contamination problems, sedimentological information is sparse and drill-core descriptions and pumping-tests only give a limited picture of the geometry of inhomogeneities. Ground-probing radar (GPR) offers the potential to resolve sedimentary structures and lithofacies in gravel deposits. For example, the geometry of characteristic sedimentary structures and the textures of late Pleistocene Rhine gravel are portrayed on GPR reflection images. Because of the ability of the GPR method to detect changes in water content, the reflection image can be related to small changes in the degree of sediment saturation, which may also reflect a change in sediment composition. This allows a distinction between several characteristic lithofacies that are typical of late Pleistocene braided-river depositional systems. The main limitation of the GPR method is the rapid attenuation of electromagnetic waves in the ground, especially in clay-covered regions.