The joint nonhyperbolic moveout inversion of PP and PS data in VTI media

Abstract

Nonhyperbolic moveout of P‐waves in horizontally layered transversely isotropic media with a vertical symmetry axis (VTI) can be used to estimate the anellipticity coefficient η in addition to the NMO velocity Vnmo,P. Those two parameters are sufficient for time processing of P‐wave data (despite a certain instability in the inversion for η), but they do not constrain the vertical velocity VP0and the depth scale of the model. It has been suggested in the literature that this ambiguity in the depth‐domain velocity analysis for layer‐cake VTI media can be resolved by combining long‐spread reflection traveltimes of P‐waves and mode‐converted PSV‐waves.Here, we show that reflection traveltimes of horizontal PSV events help to determine the ratio of the P‐ and S‐wave vertical velocities and the NMO velocity of SV‐waves, and they give a more accurate estimate of η. However, nonhyperbolic moveout of PSV‐waves turns out to be mostly controlled by wide‐angle P‐wave traveltimes and does not provide independent information for the inversion. As a result, even for a single‐layer model and uncommonly large offsets, traveltimes of P‐ and PSV‐waves cannot be inverted for the vertical velocity and anisotropic parameters ε and δ. To reconstruct the horizontally layered VTI model from surface data, it is necessary to combine long‐spread traveltimes of pure P and SV reflections.