One of the difficulties in seeing beneath salt is that the migration velocity in the salt and above it is not well known. This can lead to defocusing of migration images beneath the salt. In this paper, we show that reduced-time migration (RTM) and interferometric migration (IM) can partly mitigate this problem. RTM time-shifts the traces with the time difference between the calculated arrival time τsgref and the natural arrival time τ~sgref of a reference reflection, where s and g denote the source and receiver locations on the surface, respectively. We use the terms natural and calculated to represent, respectively, the arrival times that are velocity-independent (traveltimes directly extracted from the data without knowledge of the velocity model) and velocity-dependent (traveltimes calculated by ray tracing through a given velocity model). The benefit of RTM is a significant reduction of defocusing errors caused by errors in the migration velocity. IM, on the other hand, requires extrapolation of the surface data below salt using the natural arrival times τ~sgref of the subsalt reference reflector, and migration of the extrapolated data below the salt. The benefit with IM is that no salt velocity model is needed, so the model-based defocusing errors are, in theory, eliminated. To reduce computational time, we implement IM with a seminatural Green's function (combination of model-based calculated and picked natural traveltimes). Because no explicit data extrapolation is needed, IM with seminatural Green's functions is more cost-efficient than the standard IM. In this paper, we tested both RTM and IM with seminatural Green's functions on a synthetic and a field common-depth-point (CDP) data set, the latter from the Gulf of Mexico (GOM). Results show that both RTM and IM can remove the significant kinematic distortions caused by the overburden without knowledge of the overburden velocity.

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