P. S. Millegan, 1998. "General Philosophy 6: A Different Approach (Mining and Environmental)", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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Although gravity and magnetic surveys for hydrocarbon exploration typically are designed with relatively wide data spacings, to define relatively deep-seated features, environmental and mining projects are concerned with information from much nearer the surface. One of the biggest differences in these two kinds of surveys is, therefore, line or data spacing, which is much closer for shallow-target work. Ground magnetometry commonly is used instead of airborne magnetics, so that the sensor can be closer to the small target anomalies.
Magnetic data still find their greatest application in mapping buried metallic objects such as waste drums, but excellent magnetic surveys have been used archaeologically to define tepee rings, hearths, and other structures. Similar approaches may contribute to understanding soil and channel distribution in aquifers and other environmentally interesting situations. An important use has been finding abandoned steel-cased well bores, which have serious impact on subsurface fluid flow. Gravity data can help locate buried void spaces and can assist in the delineation of subsurface features such as channels. A high-tech superconducting gravity meter can discern between a “filled” aquifer and a mostly empty one.
Mining studies are concerned with mineral distributions, and their distinctive gravity and magnetic signatures can assist with such interpretation. Traditionally, mining applications (especially using magnetics) have ranged from frontier exploration to ore-body delineation. Tectonic studies also bear on miningexploration problems, and for diamond exploration, kimberlite pipes are often almost directly detectable by their characteristic
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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.