THE ESTIMATION RESULTS

The result of the estimation process is a velocity model with a velocity anomaly located around the midpoint location at 5.5 kilometers and centered at a depth of 1.6 kilometers. Figure  shows the resulting velocity model as the sum of two components: one background velocity profile, which is a function of depth but is constant in the lateral direction, and one anomalous model, which is a function of both depth and the lateral position. The anomalous model is shown with a contour plot superimposed to the result of migrating the stacked section using the background velocity. The contour interval is -40 m/s, and the maximum amplitude of the anomaly is 200 m/s. Migrating the stacked section using the background velocity as a velocity function did not correct the pull-down in the structure, although it approximately focused the data. Figure  shows that the estimated velocity anomaly is correctly positioned above the sag.

Figure  shows the result of migrating the stacked section using as a velocity function the estimated velocity model; that is, the background plus the anomalous velocity model. The inclusion of the anomaly has caused a considerable change in the positioning of the reflectors, as it can be noticed by comparing Figure  with Figure .

The two migration results are better compared by a look at the windows of the data that have been directly affected by the anomaly, as shown in Figure . The estimated velocity field corrects the mispositioning of the top of the anticline, both laterally and in depth. Furthermore, the migration with the background velocity did not perfectly focus the deeper reflectors; this misfocusing caused a decrease in the amplitudes of the migrated reflectors underneath the anomaly. This focusing effect is corrected in the migration that used the estimated velocity model as a velocity function.