Seismic array response in the presence of a dipping shallow layer

Abstract

Seismic arrays are commonly used to enhance vertically traveling signals and attenuate horizontally traveling noise. However, variations within the array length can degrade the array response. This paper studies the effect of a dipping shallow layer across the array length on the array response, by modeling plane Ricker wavefronts incident on 12-element equally and triangularly weighted arrays. To analyze the dip effect, shallow layer dip angles of 0°, 15°, 30°, and 45° are used, while wavefront incidence angles of 90°, 70°, 45°, 20°, and 5° are used to analyze the effect of dip on various types of signals and noises incident on the array. In the presence of a dipping shallow layer, an array designed to attenuate a wave with a certain dominant frequency will actually attenuate a wave with a different frequency, so degrading the array performance. The effects of incidence and dip angles on the array response are opposite. The degradation in the array performance depends largely on the combination of incidence angle of the targeted signal and shallow layer dip angle. However, the degradation in the array response became more severe as the incidence angle decreased. Thus, vertically incident signals are more severely affected than horizontally incident noise (e.g., ground roll). A spatial filter to compensate for these effects is proposed, which requires knowledge of the incidence and dip angles. Since a dipping bottom boundary of the shallow layer cannot be inferred from the surface topography, it can affect the array response without being detected. Therefore, the dip of the shallow layer bottom should be estimated prior to array layout. Once the dip angle is known, a spatial filter can be designed to account for the effects of incidence and dip angles.