Seismic Processing in Areas of Complex Near-Surface Velocity Fields: a Case Study

The ability of a number of processing techniques to accommodate a complex near-surface velocity field resulting from a highly irregular water bottom has been evaluated on seismic data from the Timor Sea. Conventional processing is not appropriate where such large lateral velocity variations exist. One approach is to effectively remove the influence of the irregular water bottom by replacing the water layer with higher velocity material and recomputing seismic travel times. Conventional processing can then proceed more successfully. The water replacement statics technique is a simple but efficient method for computing corrective time shifts. A more accurate algorithm to compensate for propagation effects of the near-surface velocity field is utilised in the wave-equation layer replacement technique. Both water-layer replacement techniques produce sections with increased horizon continuity and less structural distortion when compared to those generated using only conventional processing techniques. Although results from the two methods are similar in the present case, the wave-equation method is generally preferred because of its more realistic computation of travel-time corrections.

As an alternative to the water-layer replacement methods, prestack depth migration (PSDM) can be used to image seismic data in areas characterised by irregular water-bottom topography. However, PSDM is sensitive to the interval velocity model used for the migration. To achieve results which justify the computational expense of PSDM, considerable effort must be spent deriving an interval velocity model. In this study, an iterative migration/linear inversion scheme is used to construct an accurate model. PSDM based on this model produces a seismic section superior to those generated from the replacement techniques. Incorporation of high-resolution reflection tomography into the model-building process is suggested as a means of further improving PSDM results in areas of complex near-surface velocity fields.