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Strong lateral velocity variations occur in salt regions, which result into
non-hyperbolic moveout of reflection events.
Velocity-estimation methods that use stacking or other coherency
measures calculated over the whole cable length may therefore not resolve
local perturbations in the moveout of reflection
events. While having higher resolution, tomographic methods that use
picked traveltimes cannot easily be applied to salt data because
of the mentioned data quality problem.
Migrating the data partially solves the velocity problem: by incorporating
lateral variations in the migration velocity, the velocity-estimation
method only has to backproject residual moveout. However, as was shown
in Figure , the residual moveout pattern can still be
complex in salt data. Therefore, I use a local stacking operator
for the gradient calculations in the optimization, and I constrain
the velocity model by an interactive interpretation of the migrated
image. The next two sections discuss these procedures in detail.

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Stanford Exploration Project

1/13/1998