For deep-water Gulf of Mexico, accurate salt geometry iscritical to subsalt imaging. This requires the definition of bothexternal and internal salt geometries. In recent years, externalsalt geometry (i.e., boundaries between allochthonous saltand background sediment) has improved a great deal dueto advances in acquisition, velocity model building, andmigration algorithms. But when it comes to defining internalsalt geometry (i.e., intrasalt inclusions or dirty salt), no efficientmethod has yet been developed. In common industrypractices, intrasalt inclusions (and thus their velocityanomalies) are generally ignored during the model buildingstages. However, as external salt geometries reach higherlevels of accuracy, it becomes more important to considerthe once-ignored effects of dirty salt. We have developed areflectivity-based approach for dirty salt velocity inversion.This method takes true-amplitude reverse time migrationstack volumes as input, then estimates the dirty salt velocitybased on reflectivity under a 1D assumption. Results from a2D synthetic data set and a real 3D Wide Azimuthdata set demonstrated that the reflectivity inversion schemesignificantly improves the subsalt image for certain areas.In general, we believe that this method produces a better saltmodel than the traditional clean salt velocity approach.
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