Results from a High-resolution, 3-D Marine Gravity Gradiometry Survey over a Buried Salt Structure, Mississippi Canyon Area, Gulf of Mexico
Lincoln F. Pratson, Robin E. Bell, Roger N. Anderson, Dan Dosch, John White, Clive Affleck, Andrew Grierson, Bryant E. Korn, Ronald L. Phair, E. K. Biegert, Peter E. Gale, 1998. "Results from a High-resolution, 3-D Marine Gravity Gradiometry Survey over a Buried Salt Structure, Mississippi Canyon Area, Gulf of Mexico", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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The first test of the Bell Aerospace gravity Gradiometry Survey System (GSS) for geologic applications was conducted in April 1994 in collaboration with the U.S. Navy. The GSS is a recently declassified gravity sensing system that contains the world's only moving-base gravity gradiometer. The system measures both gravitational acceleration and gravity gradients, yielding six measurements that define the local gravity field and its gradients in three dimensions a technologic advance in measuring gravity analogous to the advance from 2-D to 3-D seismic profiling through the towing of multiple rather than single hydrophone arrays. The gravity gradiometry test survey was conducted over a buried salt structure southsoutheast of New Orleans in water depths of ˜1500 m. The quality of the survey data is excellent. In declassified grids of the data at 2-km wavelengths, gravity gradients are resolved to 0.5 and gravity to 0.07 mGal. Simple models are used to illustrate the power of this data in subsurface structure definition. The potential utility of gravity gradiometry in oil and gas exploration then is demonstrated through application of the survey data in improving a geologic model of a part of the survey area derived from 3-D seismic data.
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The idea for this book came from a perceived lack of recent, instructive examples of exploration-oriented interpretations of gravity and magnetic data. The Society of Exploration Geophysicists two volumes, Geophysical Case Histories, are probably closest in philosophy to this book. Published in 1948 and 1956, many of the examples in the Case Histories are relatively dated and specific to particular areas. We hope this new book provides an update that includes lessons about gravity and magnetic exploration that can be applied to many parts of the world. The Utility of Regional Gravity and Magnetic Anomaly Maps (SEG, 1985, W.J. Hinze, editor) contains some excellent papers dealing with tectonics that have clear bearing on hydrocarbon exploration, but no paper shows the relationships among hydrocarbon accumulations, exploration, gravity, and magnetics. Geophysical texts focusing on gravity and magnetics, including L.L. Nettleton's classics, include only a few (albeit often excellent) case histories, and many are dated.
Thus, this book's target audience is geologists and geophysicists in operations offices, actively involved in exploration at any level from basin analysis to prospect generation. Although most of the papers deal with hydrocarbon exploration, several papers relate to gravity and magnetic data in mining and environmental applications. A final section is included on new developments, the state of the art.
The book is not intended for gravity and magnetics specialists (although we hope they will find it interesting), or for geophysicists interested in theory, acquisition, and processing, unless those aspects are important to the geologic exploration problem and to the decisionmaking process.
We believe that the philosophical approach to interpretation is almost as important as some aspects of a technical interpretation itself. This book reveals the diversity of philosophies that gravity and magnetic interpreters embrace, as well as the common threads to which all interpreters aspire.
This book is not a textbook, although we have tried hard to highlight the exploration lessons inherent in each technical paper. Additional instructional aspects of the book are the glossary of gravity and magnetic terms, provided by Integrated Geophysics Corporation (with assistance from Richard Hansen of Pearson, DeRidder & Johnson) and an annotated bibliography, which has pointers to the rich literature of gravity and magnetics. Other short "lessons" can be found in stand-alone illustrations or short features throughout the book.
We thank Ray Thomasson for continual encouragement, suggestions, and prodding. Reviewers, whose efforts are appreciated greatly, include Dale Bird, Bill Pearson, Mark Odegard, and several anonymous reviewers. We appreciate the help of the AAPG, especially Ken Wolgemuth, in this, the first effort at serious book publication by the coeditors.