Abstract

Shallow seismic reflection surveys can assist in determination of velocity and/or thickness variations in near-surface layers. Static corrections to seismic reflection data compensate for velocity and thickness variations within the 'weathered zone.' An uncompensated weathered-layer thickness variation on the order of 1 m across the length of a geophone array can distort the spectrum of the signal and result in aberrations on final stacked data. P-wave velocities in areas where the weathered zone is composed of unconsolidated materials can be substantially less than the velocity of sound in air. Weathered-layer thickness variation of 1 m in these low-velocity materials could result in a static anomaly in excess of 3 ms. Shallow-reflection data from the Texas panhandle illustrate a real geologic situation with sufficient variability in the near surface to significantly affect seismic signal reflected from depths commonly targeted by conventional reflection surveys. Synthetic data approximating a conventional reflection survey combined with a weathered-layer model generated from shallow-reflection data show the possible dramatic static effects of alluvium. Shallow high-resolution reflection surveys can be used both to determine the severity of intra-array statics and to assist in the design of a filter to remove much of the distortion such statics cause on deeper reflection data. The static effects of unconsolidated materials can be even more dramatic on S-wave reflection surveys than on comparable P-wave surveys.

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