Moveout Analysis and Velocity Determination for Angle-domain Common-image Gathers

For the case that a prestack depth migration has been performed with inaccurate velocities, it is shown that second order approximations can be used to predict the residual moveout of angle domain common image gathers. A residual depth correction is proposed; the relevant corrections can be determined from the migrated data. The residual moveout determined can be used both to improve the signal to noise quality of the migrated data and the velocity model for the migration. Although developed for small reflection angles, several realistic examples demonstrate that satisfactory approximations to the residual moveout curve can be obtained over a relatively wide range of reflection angles. A zero offset inversion is applied to the estimated residual radii of curvature. The inversion is applied layerwise; for each layer the velocity is assumed to vary according to v(x,z)=f(z)+w(x) with f(z) known and w(x) to be determined. Iterative applications of the suggested approach have rendered satisfactory results for several representative velocity models.