Bedrock detection and talus thickness assessment in the European Alps using geophysical methods

Three different geophysical techniques (ground-penetrating radar (GPR), 2D-resistivity, seismic refraction) were applied to measure sediment thickness of alpine talus slopes. The eight study areas covered a range of different rock types, aspects and elevations and are mainly situated in the north-eastern part of the European Alps. A total of more than 30 scree slopes were investigated. The emphasis was on the application of GPR. The other two methods were used additionally in five areas to verify the GPR results and to check the concurrence of the results.With all the techniques applied, certain difficulties of bedrock detection arose. GPR surveys using several frequencies seem to be the most powerful tool for detecting the bedrock surface and internal structures in these high-alpine environments. However, there frequently is no radar wave reflection at the debris–bedrock interface due to the rather similar dielectrical properties. This problem was overcome by using the characteristic reflection patterns of the subsurface units for delimitating debris from glacigenic sediments and bedrock. Typical radar facies are presented. The other two methods applied were impeded by the broad overlap between resistivity and seismic velocity values, respectively, of the subsurface layers. The widespread presence of compacted and/or glacigenic debris layers between surficial scree and bedrock rendered the interpretation difficult. The three methods coincide well in straightforward settings (e.g. debris superimposing bedrock). Additional sediment units like till, wet or compacted debris lead to considerable ambiguities and deviations between the results. Thus, the combined application of two or three techniques is strongly advisable to verify and improve the interpretation.The calculated Holocene rockwall retreat rates lay between 200 and 850 mm/ka which is well within the framework of previous measurements. The weathering rates at north faces seem to be roughly two times higher than at south faces in the areas of investigation.