I sliced the beam stacks at the traveltimes corresponding to the reflector at about 4.5 km in the migrated section of Figure . Figure shows semblance, as a function of the transformed offset and midpoint location, and for a fixed ph equal to 0.076 s/km and py equal to zero. The thinner line superimposed to the semblance plot corresponds to the transformed offsets predicted by the background velocity, and the thicker line corresponds to the transformed offsets predicted by the estimated velocity. The peaks of the beam stacks are well predicted by the final results of the estimation.
One of the main advantages of estimating velocity with local stacking operators, such as beam stacks, is the possibility of inverting data that depend locally on the moveouts of the reflections. This advantage would be only theoretical if the resolution of beam stacks were not sufficient to obtain independent measures for different offset ray parameters. Figure shows that it is actually possible to measure the different effects of the velocity anomaly on the reflections having different offset ray parameters. The three panels show slices of the beam-stacked data taken at three midpoint locations, and for the same reflector as in Figure , but as a function of ph. In this figure the thinner line superimposed to the semblance plot corresponds to the transformed offsets predicted by the background velocity, and the thicker line corresponds to the transformed offsets predicted by the estimated velocity. The first and the third panel correspond to midpoints located on the side of the anomaly, and consequently the reflections with higher ph were more strongly affected by the anomaly than were the ones with lower ph. On the contrary the panel on the center corresponds to a midpoint located at the center of the anomaly, and consequently the reflections with lower ph were more strongly affected by the anomaly than were the ones with higher ph. It is useful to notice that the beam stacks are zero for ph higher than 0.206 s/km because the corresponding reflections would have been recorded off the end of the cable.