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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 67 (1983)

Issue: 8. (August)

First Page: 1339

Last Page: 1339

Title: Imaging Beneath Complex Structure: A Case History: ABSTRACT

Author(s): Bruce Gibson, Ken Larner, Ron Chambers

Article Type: Meeting abstract

Abstract:

Migration is recognized as the essential step in converting seismic Previous HitdataNext Hit into a representation of the earth's subsurface structure. Ironically, conventional migration often fails where migration is needed most--when the Previous HitdataNext Hit are recorded over complex structures. Processing Previous HitfieldNext Hit Previous HitdataNext Hit shot in Central America and synthetic Previous HitdataNext Hit derived for that section, demonstrates that time migration actually degrades the image of the deep structure that lies below a complicated overburden.

In the Central American Previous HitexampleNext Hit, velocities increase nearly two-fold across an arched and thrust-faulted interface. Wave-front distortion introduced by this feature gives rise to distorted reflections from depth. Even with interval velocity known perfectly, no velocity is proper for time migrating the Previous HitdataNext Hit here; time migration is the wrong process because it does not honor Snell's law. Depth migration of the stacked Previous HitdataNext Hit, on the other hand, produces a reasonable image of the deeper section. The depth migration, however, leaves artifacts that could be attributed to problems that are common in structurally complicated areas: (1) departures of the stacked section from the ideal, a zero-offset section, (2) incorrect specification of velocities, and (3) loss of energy transmitted through t e complex zone.

For such an inhomogeneous velocity structure, shortcomings in CDP stacking are related directly to highly nonhyperbolic moveout. As with migration velocity, no proper stacking velocity can be developed for these Previous HitdataNext Hit, even from the known interval-velocity model. Proper treatment of nonzero-offset reflection Previous HitdataNext Hit could be accomplished by depth migration before stacking. Simple ray-theoretical correction of the complex moveouts, however, can produce a stack that is similar to the desired zero-offset section.

Overall, the choice of velocity model most strongly influences the results of depth migration. Processing the Previous HitdataTop with a range of plausible velocity models, however, leads to an important conclusion: although the velocities can never be known exactly, depth migration is essential for clarifying structure beneath complex overburden.

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Copyright 1997 American Association of Petroleum Geologists