An alternative residual‐curvature velocity updating method for prestack depth migration

Abstract

Residual-curvature analysis of prestack depth migrated common image gathers (CIGs) is widely used for updating the whole velocity model in areas of complex geology. The velocity is estimated by maximizing the flatness of these events in either the time domain or the depth domain. In order to perform conventional reflection tomography, the reflector positions are needed and are either guessed or estimated. Therefore, the backprojection operator is often incorrectly calculated because of the incorrectly estimated reflector positions. This process may slow down the convergence of inversion. Van Trier (1990) points out that, instead of using reflectors in the depth domain as the reference events, we may use the reference events in the time domain. Based on the principle that ray tracing (modeling) undoes migration, these reference events in the time domain are the true events and don’t change with velocity variations. Hence if we can convert the depth deviations into adequate time deviations, there is no need to use reflector positions in the inversion. With this in mind, we use specular ray tracing to obtain all the necessary information. The true reflector positions need not be guessed or estimated: they follow naturally from the migration results. Instead, the backprojection operator incorporates reflector movement and ray-bending effects. A simple synthetic example shows that the algorithm discussed holds promise.