Figure shows a typical common midpoint gather (CMP) of the data set, together with its beam-stacks.
Figure shows the stack of the data. The velocity anomaly causes the large pull-down visible in the stacked section near the midpoint location of 5.5 km. The data set does not need to be migrated before stack, because the reflections underneath the anomaly stacked coherently, even if their moveouts were not hyperbolic. On the other hand, a poststack depth migration and a good velocity model are needed to position correctly the reflectors. Depth migration is particularly needed for correctly positioning the top of the anticline, which has been flattened by the velocity anomaly. The conventional methods for estimating interval velocity from stacking velocities cannot be used for this data set because the anomaly width, which is about 2 kilometers, is smaller than length of the cable. Therefore this data set is a good test case for a tomographic velocity estimation method such as the one using beam stacks.