I would like to make a distinction between the application of state-of-the-art aeromagnetics for geologic interpretation and micromagnetics as a hydrocarbon direct-detection method. Very few people promote micromagnetics as a directdetection method anymore. High-resolution aeromagnetics (HRAM) for structural interpretation, on the other hand, is enjoying something of a renaissance. Not only did magnetic methods survive last decade's industry slump, but they actually advanced through the period.
HRAM is descriptive of an exploration methodology targeted at very detailed high-resolution interpretation of the full geologic section. HRAM surveys are standard today because of an increased interest during the 1980s in sampling high-frequency, low-amplitude magnetic anomalies that many people thought were direct hydrocarbon indicators. The magnetic field was sampled at a 1/10-second increment, from an altitude of 80.150 m, along lines spaced 250. 500 m apart. The striking new data shook conventional wisdom, and caused a reevaluation of rules of thumb. Today, aeromagnetic contractors are busier than ever.
The conventional use of aeromagnetics, to map basement structure, has had to make room for new concepts. An explorationist now thinks of using highresolution aeromagnetics when:
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.