Shape of Gulf Coast Salt and the need for 3‐D imaging

Abstract

In the Gulf Coast, there is a high contrast in velocity between the Tertiary sediments and the Jurassic salt. As a result, seismic energy is strongly refracted at the salt boundaries. For imaging beneath salt domes, lenses, and ledges, it is therefore necessary to have accurate knowledge of the salt/sediment interface. Extensive studies of salt bodies show that simple models of salt shape do not adequately represent the salt/sediment interface. Salt bodies rarely can be represented by either “rounded” topologies, or two dimensional or radial symmetries. Salt bodies are best represented by a combination of curvilinear and angular segments in 3 dimensions. Structural deformation of salt and sediments in combination, cause highly angular faulted “corners? and sometimes injection of salt into the fault zones. Detailed time migrations that handle sharp horizontal amplitude changes and steep to overturned dip have been crucial in the development of the accurate understanding of salt shapes. Seismic imaging below salt requires an accurate representation of the 3D topology of the salt/sediment interface. Although there are additional complications of seismology below salt (i.e., multiple energy), we believe that the predominant factor in imaging is the correct representation of the 3D kinematics (travel times). Observations of reflectors below salt show that as the 3-dimensional angularity of the salt increases, the need for detailed salt representation also increases. Comparisons of 2D and 3D subsalt imaging show the need for 3D representation. Good subsalt images, therefore, cannot be produced without knowledge of where the energy is going in 3 dimensions. Smooth and simple models, or sparse grid representations of salt are not sufficient to produce reliable subsalt imaging.