Analysis of multimode seismic conversions from high-velocity layers

Jitendra S. Gulati, Robert R. Stewart

The seismic imaging of targets lying beneath or within a high-velocity sequence can be unsuccessful when using conventional techniques. We study a model based on log data from the northeastern area of Ireland which is covered by high-velocity basalt over a large area. The Zoeppritz response at the top and bottom of the high-velocity layer of the given model predicts strong multimode conversions for an incident P wave.Raytracing demonstrates the complicating effect that the high-velocity surface has on the recording of vertical and horizontal component data. Synthetic seismograms of the various multimode arrivals from the high-velocity layer of the model give an idea of the complexity of the problem. When offsets are large, the effect of wave conversions is even more important, and the possibility of confusing converted waves with multiples of primary P-wave reflection increases.