Conventional gravimeters are extremely sensitive weighing devices routinely capable of distinguishing anomalies less than 1 part in 107 of the Earth's ambient field, and gradiometers, which measure the first derivative of the gravitational field, to about 1 part in 3000. Observations of the Earth's gravity field are made with a variety of instruments from different types of platforms, each with its own characteristic measurement accuracy, spatial resolution, and cost of acquisition. Applications of gravity within the oil industry are just as varied, impacting all phases of the business from global tectonic studies to frontier exploration to prospect selection to seismic processing to well logging to reservoir monitoring to site hazard and engineering studies to environmental analysis. Advances inthree technologies, satellite altimetry, Global Positioning System (GPS), and gravity gradiometry, have dramatically improved our ability to acquire higher resolution gravity data. This, in turn, has increased the impact that analysis of gravity data has on exploration applications.
In southeast Asia, inexpensive satellite gravity predicts previously undiscovered minibasins for the frontier exploration group, allowing it to relocate its seismic lines to the more prospective areas.
In the deep-water Gulf of Mexico, drilling operations are suspended while Eddy Whopper, a large vortex of swirling water shed from the Loop Current, passes the drill ship. The drillers, warned by predictions from the ocean circulation model constrained by satellite gravity determinations of the local geoid, are
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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.