Prestack depth imaging and velocity analysis for P-P and P-S data with nonstationary integral extrapolators

Practical parallel implementation of the nonstationary integral wavefield extrapolators is presented. Two approaches, the windowed split-step algorithm and full integral algorithm, were implemented. The integral extrapolators are suitable for large-step wavefield extrapolation, even when lateral velocity variations are strong, provided that the static phase-shift and the focusing phase-shift velocities are replaced by time-average and depth-average velocities, respectively. Linear vertical wavefield interpolation between reference wavefields produced by large-step extrapolation algorithms can be used to compute the intermediate depth image. This dual algorithm significantly reduces the run time of the integral wavefield extrapolators.; Nonstationary extrapolators are found to be very capable of imaging shallow events at large dipping angles as well as at deep events, even when severe topography variations and high near-surface velocity is present. The Marmousi synthetic data set and the Alberta Foothills synthetic data set were migrated and superior depth images were achieved.; P-P prestack depth imaging techniques can be conveniently applied to converted-wave data based on the primary-only P-S wave propagation model. Prestack P-S shot gather migration with the dual algorithm produced a very good depth image for the 1997 Blackfoot 3C-2D survey.; The concept of downward-continuation migration velocity analysis (DMVA) is proposed. It can be used to partially eliminate the assumption of laterally invariant velocity in the established migration velocity analysis techniques and hence provide a better velocity estimation.