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Despite the significant advantages of combining PP and PS reflection data in anisotropic parameter estimation, application of this approach has been hindered by the inherent complexity of PS‐wave moveout. To overcome this problem, V. Grechka and I. Tsvankin suggested a model‐independent procedure to construct the traveltimes of pure SS‐wave reflections from PP and PS data. Here, we apply their method and anisotropic multicomponent stacking‐velocity tomography to a 2‐D line acquired over the lower Tertiary Siri reservoir in the North Sea. The computed traveltimes of SS reflections from several horizons are used to estimate the effective SS‐wave normal‐moveout (NMO) velocities, which are combined with the NMO velocities of PP‐waves in the tomographic velocity analysis. Comparison of the vertical and NMO velocities of the PP‐ and SS‐waves provides clear evidence of anisotropy in the section above the reservoir. The interval parameter estimation is performed under the assumption that the section is composed of transversely isotropic layers with a vertical symmetry axis (VTI media). Since the subsurface structure is close to horizontally layered, the reflection data cannot be uniquely inverted for the VTI parameters without additional information (e.g., the vertical velocities found from borehole data). The parameter‐estimation algorithm produces a family of equivalent VTI models that fit the PP and PS (or SS) traveltimes equally well. Although the range of variations in ε and δ for the equivalent models is rather wide, it does not include isotropic media (ε = δ = 0), which implies that accurate matching of both PP and PS data is impossible without accounting for anisotropy. To overcome the nonuniqueness in the inversion of reflection data and build a VTI depth model, we use P‐wave check shots acquired in the only borehole drilled on the processed line. Throughout the section ε > δ, with the largest values of both anisotropic coefficients observed in the depth range 0.7–1.5 km where ε reaches almost 0.25. Time sections of the original PS data computed using the VTI model have a much higher quality than the conventional isotropic sections.

The technology of ocean-bottom surveys has put mode-converted waves at the forefront of seismic exploration and reservoir characterization. For example, PS-waves proved effective in imaging offshore reservoirs screened by gas clouds that cause high attenuation/scattering of compressional energy (e.g., Thomsen, 1999). Also, information about shear-wave velocities contained in converted modes can be used to separate the effects of saturation and pressure and reduce uncertainty in predicting lithology and fluid saturation. The high sensitivity of PS-wave reflection coefficients to shear-wave velocity and density makes converted-wave AVO analysis potentially powerful for detecting hydrocarbon-saturated rocks. Conventional (isotropic) processing of mode conversions, however, often is inadequate because the influence of anisotropy on PS-wave moveout and amplitude is much more substantial than that on P-wave signatures. In particular, mis-ties between PP and PS sections (such as different depths of reflectors) are difficult to remove without taking anisotropy into account. Assuming a purely isotropic overburden often causes smearing of the conversion point, which leads to errors in building common-conversion-point (CCP) gathers and poor focusing of PS images. Also, shear-wave splitting in anisotropic media makes it necessary to rotate PS-wave displacement components prior to imaging or AVO analysis. The difficulties in applying isotropic processing techniques to mode conversions underscore the importance of anisotropic velocity analysis of PS data. In the presence of anisotropy, it is especially beneficial to combine PP-and PSwaves in model-building algorithms because a certain subset of the medium parameters influences both P-and S-wave propagation. For transverse isotropy with a vertical symmetry axis (VTI media), signatures of P-and SV-waves depend on the P-wave vertical velocity V P0 and Thomsen parameters ε and δ; additionally, SV-wave kinematics is a function of the shear-wave vertical velocity V S0. It should be emphasized that reflection traveltimes of PP-waves alone typically are insufficient for resolving individual values of V P0 , ε and δ.

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