DESCRIPTION OF THE DATA

The data that I present in this report are part of a marine survey donated to SEP by AGIP. The data were recorded with steam-gun sources and a 2.35-km cable. The shot spacing is 25 m and the group interval is 50 m; consequently the midpoint coverage is 48 fold and the midpoint spacing is 25 m. I sorted the data in common-midpoint gathers and kept the 341 gathers that were full fold. The data were sampled every 2 ms and were wide-band in frequency, reflected energy was as high as 100 Hz. SEP received the data already gained and deconvolved; I then muted the data to eliminate the first arrivals and dip-filtered the constant-offset sections to remove some steeply dipping noise.

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.