Image gathers of SV‐waves in homogenous and factorized VTI media

Abstract

One of the main problems in the velocity analysis of P‐wave data for VTI (transversely isotropic with a vertical symmetry axis) media is the need for a priori information in building a model for depth imaging. Including SV‐wave moveout in the parameter‐estimation procedure, either alone or in combination with P‐waves, can help in positioning the reflectors at the correct depth using only reflection traveltimes. Here, in order to develop a foundation for shear‐wave migration velocity analysis (MVA) in VTI media, we study SV‐wave image gathers obtained after prestack depth migration. For purpose of the moveout inversion of SV‐waves, it is convenient to parameterized the model in terms of the normal‐moveout (NMO) velocity Vnmo of horizontal SV events, the anisotropic parameter σ, which largely controls SV‐wave anisotropy, and Thomsen parameters ε and δ. The moveout of horizontal events on image gathers is close to hyperbolic and depends just on Vnmo out to relatively large offset‐to‐depth ratios of about 1.7. The influence of the parameter σ on the migrated depth rapidly increases as the offset‐to depth ratio approaches two. However, estimation of σ (and, therefore, the vertical velocity) for SV‐wave moveout is hampered by the tradeoff between σ and ε on long‐spread gathers. For factorized v(z) VTI media with a constant SV‐wave vertical‐velocity gradient kzs, flattering of both horizontal and dipping events requires kzs, and the parameters σ and ε. on the whole, the ambiguity in the estimation of σ and reflector depth from SV‐wave moveout highlights the need to combine P‐ and SV‐wave data in migration velocity analysis for VTI media.