Interpretation of Magnetic Anomalies at Low Latitudes: Potential Pitfalls
D. E. Bird, S. A. Hall, J, F. Casey, P. S. Millegan, 1998. "Interpretation of Magnetic Anomalies at Low Latitudes: Potential Pitfalls", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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The vector nature of the Earth's magnetic field dictates that interpreters must take care to understand pitfalls related to the orientation of the field (i.e., magnetic inclination and declination), and the relationship of the magnetic field to a region's geology. The case history presented here demonstrates one such pitfall. Present models for the formation of the Grenada Basin vary from north-south extension to northeast-southwest extension to east-west extension. Gridded magnetic anomalies over the basin provide a picture of the Earth's field that contributes to this spectrum of possible extensional origins.
The Grenada Basin is a back-arc basin located near the eastern edge of the Caribbean Plate. The basin is bounded on the east and west by the roughly north-south-trending active Lesser Antilles and remnant Aves Ridge Island Arcs, respectively. Although this physiography, as well as gravity data, supports formation by near east-west extension, magnetic anomalies over the basin exhibit predominantly east-west trends. The crust of the Grenada Basin and of other back-arc basins forms similarly to the crusts of ocean basins. If the observed magnetic anomalies over the basin are produced by sea-floor spreading, then the orientation of extension may be complex. Extension in most back-arc basins is roughly normal to their trenches and subduction zones, but some basins appear to exhibit oblique extension. A careful interpretation of magnetic profiles reveals low-amplitude magnetic anomaly trends, oriented subparallel to the island arc, over the southern part of the Grenada Basin, which supports a model for basin development by near east-west extension.
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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.