Abstract

A linearized elastic migration/inversion (M/I) technique is applied to multicomponent offset VSP data collected in the North Sea. High-resolution elastic images of the target zone (P- and S-wave velocities and density) are obtained from upgoing P-P and P-S wave fields. These M/I images are an approximate representation of rapid variations of subsurface parameters. Three image confidence criteria provide a measure of uncertainty in the interpretation. VSP-CDP transform maps assisted in the definition of the confidence region in M/I images. M/I was helpful in delineating the Brent reservoir, and detecting a two-fault system. Preliminary interpretation indicates an increase in Poisson's ratio at the top of the reservoir. Surface data were not able to image this target due to strong multiples at the Cretaceous base. This is the first time elastic depth images have been obtained from M/I of offset VSP field data.Generally, image artifacts depend on the following conditions: (1) good quality data with satisfactory coverage of the target zone; (2) accurate, amplitude-preserving data preprocessing techniques for filtering out 'unwanted' waves; and (3) an elastic background model which adequately represents the main subsurface features. Our M/I approach is flexible, since it can be used with single or multicomponent prestack data, P-P, P-S, S-P, and S-S scattered waves simultaneously, an arbitrary acquisition geometry, and a two-dimensional heterogeneous background model. Computational efficiency results from concentrating the inversion on a target zone and using the paraxial ray method to compute ray-approximate Green's functions.

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