Aeromagnetic Interpretation of Southwestern Continental Shelf of Korea
Czango Baag, Chang-Eob Baag, 1998. "Aeromagnetic Interpretation of Southwestern Continental Shelf of Korea", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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Analysis of the Project Magnet aeromagnetic data acquired by the U. S. Navy in 1969 permits us to predict a new sedimentary basin, Heuksan Basin, south of the known Gunsan Basin in Block II, offshore South Korea. The basin appears to consist of three subbasins trending north-northwest'south-southeast. The results of our analysis provide not only an independent assessment of the Gunsan Basin, but also new important information on the tectonic origin and mechanism for the two basins as well as for the entire region. The basin-forming tectonic style is interpreted as rhombochasm associated with double-overstepped left-lateral wrench faults. From magnetic evidence, a few northeast-southwest-trending major onshore faults are extended to the study area.
We also interpreted the faults to be left-lateral wrenches. This new gross structural style is consistent with the results of recent Yeongdong Basin analysis (Lee, 1990). The senses of fault movement also are supported by paleomagnetic evidence that the Philippine Sea has experienced an 80 clockwise rotation since the Eocene. Based on a 2.5-D model study, the probable maximum thickness of the sediments in the Gunsan Basin is approximately 7500 m. We believe that the new Heuksan Basin was left unidentified because a high-velocity layer may overlie the basin. Because the overall structural configuration of the Heuksan Basin appears to be favorable for hydrocarbon accumulation, a detailed airborne magnetic survey is recommended in the area, to verify the magnetic expressions of both this thick basin and the tectonic style. A detailed marine gravity survey acquired in conjunction with a highenergy source seismic survey also is recommended, to delineate the sedimentary section and to acquire data supplemental to the magnetics.
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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.