- Copyright © 2000 Society of Exploration Geophysicists
Application of 3-D prestack depth migration for subsalt exploration in the deepwater Gulf of Mexico requires efficient interaction between geophysical imaging and geologic modeling to obtain optimum imaging quality. When this process is applied to massive 3-D seismic volumes, time and cost concerns are critical. Salt-model building through conventional 2-D vertical section interpretation is too labor-intensive and error-prone to be cost-effective. The ability to visualize a full seismic volume greatly expedites interpretation while offering insight and perspective not possible with 3-D in-line and cross-line interpretations. 3-D visualization techniques, an essential component of our technical strategy, allowed us to build velocity models and complete PSDM processing over approximately 300 OCS blocks (7000 km2) in less than 12 months. In this paper, we discuss techniques applied across the entire project, but we can report our results on only the southern half.
In addition to the technical and logistical problems inherent in deepwater exploration, the Gulf presents an additional significant challenge: the presence of laterally extensive and substantially thick shallow allochthonous salt. Lateral salt is a particularly difficult seismic imaging problem. The velocity contrast between salt masses and underlying sediments is significant, leading to considerable mispositioning of time images and pro-nounced deviation from generalized CDP assumptions (Fagin, 1991). In addition, the shape (specifically the roughness or rugosity of the top or base surfaces of salt) further contributes to raypath dispersion and loss of coherent signal.
Interpretation in depth, rather than time, avoids a problem common in time interpretation—a “reasonable” interpretation in the time domain becoming unrealistic when converted to depth (Schultz, 1998). Particularly in the complex and poorly understood shallow allochthonous salt provinces of the Gulf of Mexico, deriving a velocity model based on time-domain interpretation can lead to untenable salt and subsalt geometries, as well as mislocation of reflectors representing sedimentary …