Shallow 3-D seismic reflection surveying: Data acquisition and preliminary processing strategies

A comprehensive strategy of 3-D seismic reflection data acquisition and processing has been used in a study of glacial sediments deposited within a Swiss mountain valley. Seismic data generated by a downhole shotgun 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-midpoint (CMP) bins, traces covering a full range of azimuths and source-receiver 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 various subsets designed to simulate data volumes collected with lower density source and receiver patterns. Comparisons of seismic sections extracted 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 images of the shallowest (<100 ms) reflecting horizons. Reflections rich in frequencies >100 Hz at traveltimes of similar to 20 to similar to 170 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 boundary between a near-surface sequence of clays/silts and an underlying unit of heterogeneous sands/gravels.