Integration of Gravity and Magnetic Methods in the Risk-weighted Exploration Decision Process: Prospect Definition Stage
Robert Pawlowski, 1998. "Integration of Gravity and Magnetic Methods in the Risk-weighted Exploration Decision Process: Prospect Definition Stage", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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Exploration-risk parameters evaluated through the geologic integration of gravity and magnetic data during the prospect definition stage include:
At the prospect definition stage, a company has completed the fundamental basin reconnaissance work, has selected a prospect (or prospects), and is doing detailed evaluation of the prospect before deciding whether to commit drilling money. Gravity and magnetic methods can be applied again at this point to contribute to the evaluation of various risk parameters.
The most common application of gravity and magnetics at this stage of exploration is to better define geologic structure in areas of poor seismic data quality. For autochthonous sedimentary structures, magnetic data allow the geometry and depth of underlying basement structures to be determined. This is useful in structurally complex areas involving overthrusting and folding, such as that occurring in the Carpathian Mountains of Romania and the Dinaride Mountains of Yugoslavia. Potential-field methods often permit deep structure to be determined where the deep seismic image is poor because of the often high acoustic impedances and steep dips of the contorted structure within the overthrust units.
Similarly, where intrasedimentary igneous material occurs, as with the basalt flows of the Chaco-Parana Basin of Argentina, potential-field methods can be used to peer beneath the seismically reflective barrier presented by the basalt. Gravity and magnetic data also can be used to resolve seismic no-data zones caused by igneous intrusions or fossil volcanoes, such as those offshore the Netherlands in the North Sea (see also the St. George Basin study by Chapin et al. in this volume).
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.