CASE HISTORY 1: Seismic Modeling Beneath a Salt Flow
A high relief structure was noted on seismic data beneath a salt overhang. The prospective nature of this subsalt feature hinged upon the ability to discriminate between a structural anticline in depth and the illusion of an anticline in time. Imaging distortion brought about by: (1) velocity pull up due to the high velocity of salt; and (2) raypath bending associated with salt boundaries, complicates structural interpretation. A depth model was created from an interpreted time-migrated line to confirm the presence of a true anticline in depth. The resulting two-dimensional (2-D) synthetic seismic section compared well with the original seismic line. Finally, depth migration was performed on the original seismic data, with a result similar to that of the modeling. This result demonstrated the use of a simple modeling technique to validate a prospective exploration target.
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Seismic interpretation apparently is becoming primarily a geologic rather than a geophysical skill. This observation has been true from the moment seismic reflection data were displayed as a continuous record with the intention of creating an image of subsurface structure. The imaging advances that have occurred in the past two decades only reinforce the tendency. More effective migration algorithms making use of faster and less expensive computers, as well as high-fold and, in particular, 3-D data all serve to make the seismic picture better. As the image increasingly reveals more geology, the geologic skills become more crucial to the task of extracting the information made available. As seismic artifacts such as multiples, sideswipe, and raypath distortion effects are successively eliminated from the image, the geophysical sophistication of the interpreter becomes increasingly less important. At first glance it would seem that these tendencies can only intensify as these technological trends continue.
And yet the depiction of complex structures remains elusive. Migration programs have been developed that can manage the severe raypath bending attendant with complex structures. Moreover, the ever decreasing costs of computation make the application of these programs increasingly more feasible. Unfortunately, to benefit from these imaging approaches requires, a priori, an increasingly more precise definition of the velocity field which often is, in itself, an expression of geologic structure. Therefore, before we can create the image, we require an understanding of what the image is supposed to show. This circumstance implies that the preparation of the seismic image has become, and will likely remain, inextricably bound up with its interpretation.