THE PROBLEM OF WATER-COLUMN MULTIPLES FOR PROCESSING CONVERTED S-WAVES IN MARINE VSP DATA

Seismic data from marine VSPs contain converted S waves which can be u sed to provide information on the internal structure of the subsurface rocks. To make the best analysis of these waves it is necessary to ch oose a suitable offset to maximize the converted energy. Numerical mod elling 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 wave s 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 bec omes more pronounced for a large Poisson's ratio in the sea-bed sedime nts. Standard processing on the three-component recordings cannot be a dequately 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 unde rstand the multicomponent response must be weighed up against the offs et desired for optimal conversion.