CASE HISTORY 5: Ray-Trace Modeling for Salt Proximity Surveys
A Gulf Coast salt dome provides the setting for a case history using seismic ray trace modeling in the design and interpretation of a salt proximity survey. The salt proximity survey was undertaken in order to have an independent estimate of the position of the salt flank because an exploration well had penetrated porous sands but surface seismic of two different vintages gave conflicting interpretations of the amount of salt overhang. If sufficient salt overhang existed then a sidetrack well could be justified to test updip extension of the sand.
The salt proximity survey had two parts, one part to image the salt flank at shallow depths and the other to image deeper. Two computer models were constructed corresponding to the two surveys. Various shot locations were tested on the models for ascertaining the zone of salt imaging and associated time-depth curves. The graphic displays of ray patterns from the ray-tracing computer program can be used to develop quality control check routines useful in the field during the acquisition phase of the project. These routines show how some potential interpretative problems can be resolved by the example of inserting a caprock on one of the models to study the change of salt face imaging that occurs. An aplanatic analysis of many ray tracing trials were made in order to understand the effect of parameter changes on the subsequent analysis.
Finally, results of the field program are discussed. Results show that a salt proximity program can be successfully run and integrated with surface seismic information. A large part of the success of the survey was due to the presurvey planning and preparedness that came from the modeling process.
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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.