The Rise and Fall of Early Oil-field Technology: The Torsion Balance Gradiometer
Robin E. Bell, R. O. Hansen, 1998. "The Rise and Fall of Early Oil-field Technology: The Torsion Balance Gradiometer", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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Today, elementary physics students take for granted such quantities as “big G,” the universal gravitational constant. In fact, in the late 1700s the value of this quantity was unknown, and the quest to determine it led to some of the earliest geophysical instrumentation. Just after the Revolutionary War in the United States, Henry Cavendish developed the first system to measure the universal gravitational constant, the familiar “big G.” Unfortunately, for geologists (at this time still mostly “gentlemen scientists”), this apparatus produced data which were difficult to interpret geologically, and it was far too large and cumbersome for field use. The geologic limitation was that the system measured only the horizontal derivative of a horizontal component of the gravity field, a quantity which by itself is difficult to interpret. Thus no applications of this elegant yet laboratory-bound instrument emerged.
Almost a full century later, the great Hungarian physicist Baron von Eötvös designed an instrument which would revolutionize the petroleum industry. As is often the case in revolutionizing technology, Eötvös used “new” fiber technology to significantly reduce the instrument's size and thereby increase portability. Eötvös also added a significant new feature. His master stroke was a design which suspended the weights on the torsion balance at different elevations. This modification made it possible to measure both the horizontal derivative of the horizontal field and the derivative of the vertical field (Figure 1). The vertical derivative was significantly easier to interpret geologically.
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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.