The problem of water-column multiples for processing converted S waves in marine VSP data

Abstract

SUMMARY Seismic data from marine VSPs contain converted S, waves which can be used to provide information on the internal structure of the subsurface rocks. to make the best analysis of these waves it is necessary to choose a suitable offset to maximize the converted energy. Numerical modelling with a hard sea-bottom shows that even for the moderate offsets which provide the best conversions at the sea-bed in a shallow-water environment, it is not possible to jointly compensate the P and S waves for their effective seismic signatures using a single common scalar function. This is due to a difference in the periodicity of the water-column multiples associated with the two wavetypes. the difference becomes more pronounced for a large Poisson's ratio in the sea-bed sediments. Standard processing on the three-component recordings cannot be adequately performed except for subvertical incidence. Consequently, P and S waves must be separated before signature deconvolution, or else a more sophisticated multicomponent operator must be deployed. In the process of acquisition design, the detailed modelling required to understand the multicomponent response must be weighed up against the offset desired for optimal conversion.