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Abstract

With recent advances in the acquisition of multicomponent data, including the technology of ocean bottom surveys, converted waves find an increasing number of applications in seismic exploration. For example, PS-waves help in imaging hydrocarbon reservoirs beneath gas clouds, where conventional P-wave methods suffer due to the high attenuation of compressional energy (Granli et al., 1999; Thomsen, 1999). Also, converted waves provide information about shear-wave velocities (including the ratio of the P- and S-wave vertical velocities; see Gaiser, 1996) and other medium parameters that cannot be constrained using P-wave data alone. This advantage of mode conversions becomes especially important in anisotropic media due to the large number of unknown parameters and the ambiguity in estimating reflector depth from surface P-wave data. It should be emphasized that the influence of anisotropy on PS-wave moveout and amplitude is usually more significant than that on P-wave signatures, and isotropic processing methods often fail to produce accurate convertedwave images.

A key difference between converted and pure reflections in common-midpoint (CMP) geometry is that mode conversion can make the moveout curve asymmetric with respect to zero offset (i.e., traveltime is no longer an even function of offset). Only in the special case of horizontal reflectors and a medium with a horizontal symmetry plane, does the converted-wave (e.g., PS-wave) reflection traveltime remain the same if we interchange the source and receiver. The asymmetry of the converted-wave moveout can be further enhanced by angular velocity variations in anisotropic media. Therefore, in general the

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