Wide‐angle seismic reflections from two‐dimensional random target zones

Abstract

We have analyzed the shot gather seismic response of a buried zone of isotropic random velocity fluctuations. Scattering analysis predicts that events at offsets greater than the depth of the target zone will appear more continuous and coherent than events at zero and small offsets due to an effective dip filter inherent to the source‐target‐receiver geometry. The scattering theory is confirmed with finite difference synthetic seismograms calculated for an idealized crustal model. The closely spaced synthetic seismograms show bright events having the appearance of refraction arrivals which suggest a horizontally layered structure in the target zone. These effects appear to be independent of the magnitude of the velocity fluctuations, provided that the magnitude of the fluctuations is small (σ< 3.0% of the mean velocity). We have made a second finite difference calculation of a crustal scale model to reinterpret previously published wide‐angle data from the Black Forest, Germany [Sandmeier and Wenzel, 1986]. The response of the lower crust seen in the conventionally recorded field data can be explained by a lower crustal model of isotropic, small‐amplitude, random velocity fluctuations. We present this in contrast to a laterally homogeneous model of a finely layered lower crust of alternating high and low velocities proposed by Sandmeier and Wenzel [1986]. The synthetic studies and the scattering analysis demonstrate that the geometry of the wide‐angle experiment produces signals which appear to be from layered target zones, even where no layering exists. They also underscore the importance of conducting seismic surveys which make use of closely spaced receivers and which integrate vertical incidence and wide‐angle recording.