CASE HISTORY 4: Seismic Modeling of Fault-Related Folds
Peter F. Morse, Guy W. Purnell, Donald A. Medwedeff, 1991. "Seismic Modeling of Fault-Related Folds", Seismic Modeling of Geologic Structures: Applications to Exploration Problems, Stuart W. Fagin
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We present examples from a class of geologic models for compressional tectonic regimes along with their corresponding seismic expressions on unmigrated and migrated seismic profiles. The geologic models are based upon fault-related fold theory (Suppe 1983, 1985). For most of our seismic examples, we use modeling and migration programs based on the acoustic wave equation. Such wave-equation techniques, while not as computationally fast, often handle complex models more realistically than geometric raytracing and provide output better suited for subsequent data processing.
Synthetic 2-D zero-offset sections, computed using the wave-equation exploding-reflector approach, lead to (1) recognition of patterns associated with different fault-related folds, and (2) prediction of some of the difficulties in working with unmigrated and migrated seismic sections. Synthetic multioffset shot records, generated using the 2-D acoustic wave equation, demonstrate difficulties in imaging beneath fault-propagation folds using the conventional CMP method. A synthetic 3-D zero-offset survey across a 3-D fault-bend fold demonstrates that conventional 2-D seismic lines acquired in the fault-slip direction are insufficient for correct structural imaging and interpretation; hence, 3-D data acquisition and processing are necessary.
Finally, we examine a case history from Lost Hills field, San Joaquin Valley, California. This study demonstrates that geologically plausible interpretations consistent with both well and seismic data can be generated by iterating between geologic interpretations and synthetic zero-offset sections.
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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.