When salt tectonism is at an advanced stage, it can cause overhang structures. If velocities within the surrounding sedimentary strata increase rapidly with depth, it is possible to record reflections from the underside of the salt overhang. These reflections are associated with turning waves that first travel downward after they bounce off the underside of the salt overhang before returning to the surface. Turning-wave reflections give rise to a dipping event on the stacked section that conflicts with the reflections associated with the surrounding gently dipping strata. Nevertheless, dip-moveout correction alone does not strictly preserve the dipping event associated with the turning waves, since these waves propagate at angles greater than 90 degrees. They can be preserved only by making use of its abnormal moveout behavior during stacking and imaged properly by making use of the evanescent energy associated with dips greater than 90 degrees during migration (principles of dip-moveout correction).
Figure 5.0-13 shows a field data example of turning-wave migration. The salt boundary from the image based on phase-shift migration of the conventional stack can be inferred only from the termination of the reflectors associated with the surrounding sedimentary sequence. Whereas, the image based on the turning-wave migration of the stack that preserves the turning-wave energy shows the overhang structure on either side of the salt intrusion, distinctively.