Research On Direct Pre-stack Depth Migration Under The Undulating Topography Circumstance

In recent years, as the investment for national infrastructure increases, the demand of engineering investigation grows rapidly. Shallow reflection seismic exploration (SRSE) is going to play a more and more important role in engineering investigation thanks to its prominent vertical and horizontal resolution and wide depth rang. However, it is inevitable to carry out the SRSE in undulating terrain conditions, such as mountains, hills, deserts. These areas always mean serious uneven surfaces and strong lateral velocity variations, both of which pose challenges to the conventional seismic data processing.As to overcome the uneven surface problem, the usual solution is static correction process(SCP). Unfortunately, the SCP bases on the assumption of surface consistent and vertical wave path near the surface which are almost impossible to be satisfied in those uneven areas, as a result, a large number of residual statics will be left. For the second problem, the pre-stack migration can be introduced to overcome the difficulty, but it still will be affected by the uneven surface. So it is necessary to develop a new method that the SCP can be thoroughly avoided. The direct pre-stack depth migration (DPDM) bases on actual shot and receiver elevations, by which the SCP is completely avoided before migration, hence the imaging precision will be remarkably improved.Here we model the propagation of acoustic wave in the undulating topography medium with high-order staggered-grid finite-difference method, and a series of artificial shot gathers are obtained as well. Since the large amount of regular interferences contained in the shot gathers, the SCP must be performed to support the interference elimination by apparent velocity filtering method. Then every trace of these worked shot gathers should be removed back into their original positions by reverse static correction. Then all the shot gathers will be migrated respectively with pre-stack migration method followed by imaging condition. Obviously, in this case the static correction only affects the regular interference decrease rather than pre-static migration. Finally a group of model tests are presented, which show that the DPDM can ideally turn away the mistakes caused by the SCP and increase imaging precision of shallow and middle layers.