Prestack split-step Fourier depth migration algorithms and parallel implementations on Cray T3E

Seismic depth migration within complex geological structures is the fundamental process used to obtain images of the subsurface geological formations. For accurate seismic depth imaging, the seismic wave field recorded at the surface of the earth needs to be backward propagated using wave-theoretical approaches. This requires the use of a 3-D prestack depth migration algorithm that accurately accounts for the waves propagating within the medium. However, the use of such an algorithm is still expensive and only with the availability of massively parallel processor (MPP) systems has the application of 3-D prestack depth migration become feasible. To address this important aspect of exploration geophysics, I have developed three prestack split-step Fourier depth migration algorithms based on different imaging principles for 2-D and 3-D seismic data and implemented them as parallel algorithms.;In the first approach, the source and receiver wave fields are downward continued into the subsurface independently by the split-step Fourier operator. In the second approach, travel times obtained by a solution of the Eikonal equation from the source location to each subsurface position are used as an alternative to downward continuation of the source. For the third approach, unlike the shot record methods the prestack split-step Fourier depth migration algorithm is developed based on double downward continuation of the source and receiver wave fields simultaneously for a laterally varying media. Depending on the data, available hardware platform and complexity of geology, any of these imaging algorithms can be utilized for prestack depth migration.;For parallel implementation, a variety of optimizations were applied to reduce the computation time and memory requirements of the migration algorithms. In the implementation, the main parallelization is staged over frequency so that each processing element can carry out the wave field extrapolations independently for its specific range of frequencies.;I have applied these three migration methods to synthetic 2-D and 3-D seismic data. I illustrate that the migrated outputs obtained by these prestack split-step Fourier migration algorithms reveal the structures present in the models accurately. This indicates that given a macro model, these methods can successfully map the recorded seismic data to the depth domain. The efficiency of the parallel implementation makes the full wave field algorithms feasible for 3-D prestack depth migration.