Investigating Alpine Valley Fill With Reflection Seismics (2-D) And Georadar (2-D & 3-D)

The internal structures of Alpine valleys have been of particular interest to Swiss society since<br>it was first recognised that more than two-thirds of the country’s fresh water supplies are<br>contained in the unconsolidated sedimentary fill. Since most of the valleys are also densely<br>populated and industrialized, there is a high risk of ground-water contamination. In the Reuss<br>Delta near the shore of Lake Lucerne, several profiles have been surveyed with high-resolution<br>reflection seismic and/or georadar techniques. In one region, where the character of the georadar<br>images changed significantly, a 3-D georadar survey (40 x 80 m*) was also conducted. The<br>uniformly high quality of both the seismic and georadar data seems to have been influenced by<br>the presence of near-surface saturated gravels and sands. Dense geophone spacing (1.5 m) and<br>careful processing has enabled seismic reflections as shallow as - 15 m to be imaged. The deepest<br>seismic reflections image the dipping contact between unconsolidated sediments and the<br>underlying basement at 200 - 525 m depth. Distinctive reflection patterns in the georadar data<br>volume have enabled us to determine the local conditions of sedimentation in the upper -12 m of<br>the subsurface. For example, a distinctive ancient river channel has been identified overlying a<br>strongly reflective clay/silt layer deposited during a major flooding event.