SHALLOW SEISMIC-REFLECTION STUDY OF A GLACIATED VALLEY

Shallow seismic reflection data were recorded along two long (>1.6 km) intersecting profiles in the glaciated Suhre Valley of northern Switz erland. Appropriate choice of source and receiver parameters resulted in a high-fold (36-48) data set with common midpoints every 1.25 m. As for many shallow seismic reflection data sets, upper portions of the shot gathers were contaminated with high-amplitude, source-generated n oise (e.g., direct, refracted, guided, surface, and airwaves). Spectra l balancing was effective in significantly increasing the strength of the reflected signals relative to the source-generated noise, and appl ication of carefully selected top mutes ensured guided phases were not misprocessed and misinterpreted as reflections. Resultant processed s ections were characterized by distributions of distinct seismic reflec tion patterns or facies that were bounded by quasi-continuous reflecti on zones. The uppermost reflection zone at 20 to 50 ms (similar to 15 to similar to 40 m depth) originated from a boundary between glaciolac ustrine clays/silts and underlying glacial sands/gravels (till) deposi ts. Of particular importance was the discovery that the deepest part o f the valley floor appeared on the seismic section at traveltimes >180 ms (similar to 200 m), approximately twice as deep as expected. Const rained by information from boreholes adjacent to the profiles, the var ious seismic units were interpreted in terms of unconsolidated glacial , glaciofluvial, and glaciolacustrine sediments deposited during two p rincipal phases of glaciation (Riss at >100000 and Wurm at similar to 18000 years before present).