High-resolution Aeromagnetic Interpretation over Sierra and Yoyo Reefs, Northeastern British Columbia
John Peirce, Erwin Ebner, Nathalie Marchand, 1998. "High-resolution Aeromagnetic Interpretation over Sierra and Yoyo Reefs, Northeastern British Columbia", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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There has been a sudden resurgence of aeromagnetic data acquisition in western Canada since 1994 because the new high-resolution aeromagnetic (HRAM) surveys are now able to map intrasedimentary faults and fractures on a regional scale, whereas previously, aeromagnetic data were used only to delineate basement features. This increase in resolution has been achieved primarily because of more precise navigation and positioning of data using the Global Positioning System (GPS), and also because of flying in controlled drape mode close to the ground to enhance shallow sources. Better instrumentation in the aircraft and improved software for analysis and visualization also have helped.
In 1994, the first HRAM survey in western Canada was flown for Focus Seismic Corp. by Questor Surveys, Ltd., over the Sierra and Yoyo Reefs in northeastern British Columbia. Although these Devonian reefs are well known and have been producing gas for years, the area is also one of active exploration for smaller pinnacles and carbonate bank plays.
This paper discusses many of the concepts applicable to designing, flying, processing, and interpreting an HRAM survey, using the Sierra survey to demonstrate the points. Particular emphasis is placed on editing the data culturally, choosing a line spacing appropriate to the complexity of the expected results, and the value of depth analysis on the profiles to resolve subtle magnetic anomalies associated with faults and fractures.
Such linear features can be mapped on adjacent profiles and traced for many kilometers. The consistency of the fault interpretations from profile to profile is an essential factor in sorting out geologically significant features from mathematical artifacts.
We hypothesize that the cause of these fault-related intrasedimentary anomalies is magnetization in the plane of the faults or fractures. Recent work (Pierce et al., 1998) has demonstrated the importance of the sulfur geochemistry in catalysing reactions involving iron-bearing minerals in fractures in shales. Specifically, authigenic macroscopic pyrrhotite has been observed in shales at shallow depth. Pyrrhotite is important magnetically because it has a similar susceptibility to that of magnetite. Pyrite, which often is considered nonmagnetic but actually has a small susceptibility, also may contribute significantly to fracture magnetization when the surrounding sedimentary section is nonmagnetic.
This approach to magnetic interpretation allows one to map regional faulting patterns over large areas at relatively low cost. The final product, a magnetic structural-grain map, allows one to define exploration fairways and to position seismic programs more efficiently, and it usually reveals several new exploration leads.
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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.