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Amoco recently acquired four surface-seismic shear-wave lines, four multicomponent VSP's and three crossed-dipole sonic logs in south-central Wyoming. A feature of this region is that areas of increased gas production correlate with areas of increased fracturing. After seismic processing of the shear-wave lines, it was observed that reductions in the relative amplitude of the slow shear-wave (S2) seismic section with respect to the fast shear-wave (S1) section correlate with areas of greater gas production. These amplitude anomalies are attributed to anisotropy due to the preferential alignment of vertical fractures where S1 is polarized parallel to the fracture azimuth and S2 is polarized perpendicular to the fractures. Furthermore, preferred directions of fast shear-wave polarization, and hence fracture strike, derived from crossed-dipole sonic logs and multicomponent VSPs are in agreement with the directions determined from the surface seismic. Modeling of wave propagation effects due to the anisotropy illustrates how enhanced open fracturing may cause shear-wave amplitude anomalies. Seismic waveforms are sensitive to the fracture density and orientation and the nature of the material within the fractures. The modelling also shows that the range of offsets used in stacking shear-wave data must be carefully selected. Locations of polarity reversals and the onset of critical reflections can vary dramatically with fracturing.

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Preserving amplitudes in converted wave prestack migration is important for subsequent AVO processing and interpretation. However, it has not received as much attention as amplitude preservation for P wave migration. In this paper, we derive common offset true amplitude weight functions in a v(z) medium for two and one half, and three dimensional converted wave Kirchhoff migration. The weight formulas are then simplified into the terms invovling travel time, ray path and velocity functions for efficient computation. The simplified weight functions are also examined with numeric modeling data for depth migration, which demonstrated well-controlled amplitude behavior in offset and depth.