How Basement Lithology Changes Affect Magnetic Interpretation
Patrick S. Millegan, D. E. Bird, 1998. "How Basement Lithology Changes Affect Magnetic Interpretation", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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Many people are intimidated by the magnetic method, but they are quite willing to attack gravity interpretation. This is usually a result of the inclination/declination issues with magnetics, and the fact that most people view a gravity map as a structural map. Magnetic maps are not structural maps; they are a contouredrepresentation of magnetization changes in the geology. This is harder for people to “visualize” in terms of real rocks and structure. Gravity interpretation seems easier because a gravity high usually correlates with a structural high (except in the case of salt), but simple criteria such as that can cause problems if the interpreter is not careful
On the other hand, some aspects of the magnetic method are quite straightforward. One is the direct mathematical relationship between anomaly wavelength and source depth. Another important relationship is that basement lithology changes typically have amplitudes of hundreds of nanotesla (nT), but structural changes usually cause anomalies of only tens of nT (see Gibson's magnetic susceptibility versus structure illustration later in this volume).
It is also important to note that, depending on magnetic inclination, a fault and a lithology change can have distinctly different magnetic signatures. One might cause a symmetrical anomaly while the other causes an asymmetrical anomaly. This underscores the importance of preliminary models prior to undertaking any gravity or magnetic interpretation.
Figures & Tables
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.