Shallow 3-D seismic refraction surveying; data acquisition and preliminary processing strategies

Abstract

Abstract 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 mX3 m intervals across a 357 mX432 m area. For most common-midpoint (CMP) bins, traces covering a full range of azimuths and source-receiver distances of approximately 2 to approximately 125 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 < or =3 mX3 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 approximately 20 to approximately 170 ms provided a vertical resolution of 3 to 6 m over a depth range of approximately 15 to approximately 150 m. The shallowest prominent reflection at 20 to 35 ms ( approximately 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.