SHALLOW 3-D SEISMIC-REFLECTION SURVEYING - DATA-ACQUISITION AND PRELIMINARY PROCESSING STRATEGIES

A comprehensive strategy of 3-D seismic reflection data acquisition an d processing has been used in a study of glacial sediments deposited w ithin a Swiss mountain valley. Seismic data generated by a downhole sh otgun source were recorded with single 30-Hz geophones distributed at 3 m x 3 m intervals across a 357 m x 432 m area. For most common-midpo int (CMP) bins, traces covering a full range of azimuths and source-re ceiver distances of similar to 2 to similar to 12.5 m were recorded. A common processing scheme was applied to the entire data set and to va rious subsets designed to simulate data volumes collected with lower d ensity source and receiver patterns. Comparisons of seismic sections e xtracted from the processed 3-D subsets demonstrated that high-fold (> 40) and densely spaced (CMP bin sizes less than or equal to 3 m x 3 m) data with relatively large numbers (>6) of traces recorded at short ( <20 m) source-receiver offsets were essential for obtaining clear imag es of the shallowest (<100 ms) reflecting horizons. Reflections rich i n frequencies >100 Hz at traveltimes of similar to 20 to similar to 17 0 ms provided a vertical resolution of 3 to 6 m over a depth range of similar to 15 to similar to 150 m. The shallowest prominent reflection at 20 to 35 ms (similar to 15 to 27 m depth) originated from the boun dary between a near-surface sequence of clays/silts and an underlying unit of heterogeneous sands/gravels.