Joint inversion ofPPandPSreflection data for VTI media: A North Sea case study

Abstract

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 PPand 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 2 and δ for the equivalent models is rather wide, it does not include isotropic media (2= 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 2 > δ, with the largest values of both anisotropic coefficients observed in the depth range 0.7–1.5 km where 2 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.