Estimation of the accuracy and precision associated with a gravity survey is not an easy task. We must estimate not only the accuracy and precision associated with each observation of the Earth's gravity field, but also, because most modern surveys are acquired from moving platforms (or individual land stations are leveled as interconnected networks), we must estimate the spatial resolution of the survey. In one of the first attempts to understand this problem, Alan Herring of EDCON prepared the following graph, summarizing the estimated range of accuracies and spatial resolution of gravity acquisition systems. Individual estimates taken from specific surveys agree fairly well with this envelope, but vary depending on actual survey conditions. This graph plots survey accuracy as a function of shortest observable wavelength for various types of gravity acquisition systems.
Perhaps the best way to estimate survey accuracy and spatial resolution is to record “repeat lines,” that is, to make multiple observations of the gravity field over the same traverse. Fourier analysis of the repeat lines provides a good estimate of the spatial reproducibility of the survey and allows characterization of the noise statistics.
More commonly, gravity surveys are acquired as a series of intersecting profiles. Harmonization of the line intersections required to “level” the survey and the resultant “mis-tie” histograms often are quoted as indicative of the resolution of the survey, but they provide little information about the spatial resolution
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.