Abstract

The conventional method used to estimate velocities for converted-wave (C-wave) prestack time migration is awkward because the P-wave velocity vp comes from P-wave processing, the velocity ratio gamma (vp/vs) is estimated from C-wave data, and the S-wave velocity vs is then derived from vp and gamma. Instead, by using the C-wave velocity vc, effective gamma γeff, and anisotropy parameter χeff, velocity updating becomes straightforward and more reliable. To update vc for converted-wave time migration, one can carry out hyperbolic moveout analysis on the hyperbolic-moveout-migrated-common-midpoint (HMO-MCMP) gathers. However, the errors in initial γeff and anisotropy parameter χeff can only be corrected by trial and error. In this article, we propose to remove the effects of initial γeff and χeff in the HMO-MCMP gathers by inverting the moveout related to the initial γeff and χeff. This enables a full nonhyperbolic velocity analysis to update not only vc but also γeff and χeff. To obtain reliable χeff, we also develop a simultaneous PP/PS anisotropic-parameter estimation method so the ηeff estimated from P-wave data is compared immediately with the ηeff derived from χeff by using C-wave data. This provides a better constraint for estimating anisotropy parameters. The method has been tested and shows consistent improvement in converted-wave prestack time-migration velocity estimations.

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