CASE HISTORY 8: Seismic Modeling of a Pinnacle Reef: An Example from the Williston Basin
K. W. Rudolph, S. M. Greenlee, 1991. "Seismic Modeling of a Pinnacle Reef: An Example from the Williston Basin", Seismic Modeling of Geologic Structures: Applications to Exploration Problems, Stuart W. Fagin
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By using two- and three-dimensional ray-trace seismic modeling, the subtle seismic response of a small Winnepegosis (Devonian) pinnacle reef from the Williston Basin was successfully modeled. Because of the small size and steep stratigraphic dips, the crest of the reef acts as a scattering point and generates a low amplitude diffraction on the unmigrated seismic data. Other seismic manifestations include disruption of deeper reflections, paired diffractions sourced from the toe of the slope, velocity pullup of deeper reflections, and nucleation on a structural terrace. Modeled seismic criteria are only interpretable on the actual data when the data is redisplayed using a squeezed gray-scale display. Synthetic seismic sections shot at the toe of slope and off-buildup positions show that recognition of the buildup crest is difficult without a line shot perpendicular to the axis of the hyperbolaes that represent the apparent reef crest. Three-dimensional ray-trace modeling also shows that pinnacles are detected by seismic sections shot off of the buildup. In these cases, two-dimensional migration may then result in the false imaging of a buildup along a line where none occurs. These modeling criteria are particularly useful in recognizing the occurrence and in mapping the crest of small carbonate buildups.
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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.