Migration velocity analysis by perturbation for converted waves

Maria Donati and Robert R. Stewart

ABSTRACT

Prestack depth migration can handle dipping reflectors and lateral velocity variations, but its success depends on a more accurate velocity model than may be obtained from simple velocity analysis methods, i.e. normal moveout. For converted waves, imaging is more complex due to prestack depth migration requiring different propagating velocities, P-wave (downgoing) and S-wave (upgoing) velocity fields.

This paper shows an approach to P-S prestack depth migration for converted waves, based on an extension of the method proposed by Liu (1993). This method works on common-image gathers (CIG) and migration velocity is obtained through residual moveout analysis. The migration velocities are updated by a perturbation approach. When the correct migration velocity is obtained, reflectors in CIG gathers, after depth migration, will be nearly horizontal.

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