Velocity departures from the background can be seen in the left panel of Figure 5 as residual curvatures in angle-domain common image gathers (ADCIGs). This figure contains only the angles between and . Because of the small depth of the thermohaline reflections and of the missing near-offset information, information from incidence angles smaller than was obtained only from the offset-continuation fill. We discarded it, since it was characterized completely by our estimate of constant velocity. The angles larger than were unusable because of a loss of bandwidth during the transformation to ADCIGs. The small curvatures place the velocity anomalies well within the limits of the Born approximation. This means that WEMVA would be a suitable tool for resolving them.
WEMVA is an iterative inversion scheme that attempts to optimize the focusing of the migrated image Biondi and Sava (1999). Specifically, the result of wavefield-continuation migration is transformed to ADCIGs, the gathers are flattened, the difference from the unflattened image is taken to obtain an image perturbation, which is finally inverted into a velocity update. We plan to perform this procedure in the future, using moveout shifts computed by dip field integration Guitton (2003a) to flatten the gathers for the image perturbation.
Figure 5 ADCIGs between and after migration with a constant velocity of 1520 m/s.