Gravity and Magnetic Exploration: Principles, Practices, and Applications

Book Preface

Objectives of this book

Investigations of terrestrial gravity and magnetic fields are among the oldest methods for determining the nature and processes of the Earth. Despite the development of an increasing number of additional complementary investigative methods, some of which have better subsurface resolution, the gravity and magnetic methods continue to have an important, often decisive role, in a wide variety of terrestrial investigations. In contrast to the several available texts on specific topics in gravity and magnetics, this book provides an overall, modern resource on the principles, practices, and applications of both the gravity and magnetic methods to exploring the Earth. Although these aspects of the gravity and magnetic methods are well grounded in widely described and accepted principles, they are continually undergoing practical improvements and expanded understanding. The continued improvements result from enhanced technology for acquiring, processing, and interpreting data made possible largely by increasing computational power and new techniques. Special emphasis in gravity and magnetic exploration is being placed on high sensitivity mapping of anomalies at the extremities of their spectra, both shorter and longer wavelengths, increasing the vertical and horizontal resolution of individual anomaly sources, and investigating the temporal variations in the gravity and magnetic fields of the Earth which permit new insights into Earth processes. It is hoped that this book which sets a benchmark in our current knowledge of gravity and magnetic exploration will encourage further developments in these methods and applications.

Both the theory and practice of gravity and magnetic methods applied to subsurface investigations are described in this book. The book considers the methods from the planning and organization of surveys through the analysis and interpretation of observations by digital computations incorporating both physical and geological principles. Case histories in supplemental chapters, which are available on an accompanying website, illustrate the advantages and limitations of the methods in a variety of applications including near-surface engineering and archaeological studies, mineral and energy resource investigations, and planetary crustal and subcrustal studies. A final chapter of this book summarizes these applications.

This presentation of gravity and magnetic methods differs substantially from existing books. For example, it takes into account the rapidly accelerating availability of subsurface, terrestrial, marine, airborne, and satellite measurements that the current information age is mapping in prodigious volumes. Further, it develops strategies on the combined use of these anomaly fields for solving subsurface problems. It also describes the newest standards and methods for reducing gravity and magnetic data to a usable form, as well as modern instrumentation for observing both absolute and relative total field and vectorial components. The book emphasizes practical procedures and applications, and the physical properties and geological principles that constrain subsurface analyses of gravity and magnetic fields. It describes all major modern topics of gravity and magnetic methods that are likely to be of general interest. More general descriptive material and procedures are highlighted, and reference is made to alternate procedures or approaches. However, necessarily numerous computational and interpretational procedures described in the geophysical literature which may have a more limited application are not included because of space limitations. Wherever possible, examples of these methods are briefly described and pertinent literature is cited to guide the interested reader to additional information. Derivations of equations are restricted to those of general interest and details of many procedures are not included in the descriptions. Therefore the reader of this text will need to make abundant use of the many references cited for more detailed and comprehensive treatment of specialized topics.

The book can serve university courses in exploration and general Earth geophysics, gravity exploration, and magnetic exploration at the advanced undergraduate and beginning graduate levels. It uses mathematics up to and including basic differential equations, but develops the methods in simple digital array operations with minimal use of arcane and complex notations from integral and differential calculus. Thus, this book is much more computationally oriented than most previous works because the reader can readily implement and explore analytical results with electronic computing. Consistent abbreviations and terminology are used throughout the book.

The book is not only a textbook, but also serves as a reference for practioners, that is for professional geophysicists, geologists, engineers, and other scientists who have limited experience in the application of the gravity and magnetic methods to subsurface problems. Especially useful in this regard are the supplemental case histories, which have extensive reference lists. The explanation of the gravity and magnetic methods along with the case histories will also facilitate the efforts of project managers to develop optimal strategies for implementing geophysical methods in solving subsurface problems. Thus, the book serves a wide audience ranging from advanced undergraduate and beginning graduate students in the Earth and planetary sciences and engineering to professional geoscientists and engineers in academia, government, and industry.