The effects of mine tailings on local groundwater have been recognized for decades. The oxidation of newly exposed mine tailings, particularly sulphide minerals such as pyrite, releases acids and heavy metals into the environment. The rate of acid production is far higher than that caused by natural leaching and erosion of solid rock, and is beyond the environment capacity to absorb. As a result, acid mine drainage has been identified as the largest single environmental problem facing the Canadian mining industry (Filion et al., 1990).
Recent mining practices have reduced the amount of waste rock in tailings ponds through activities such as backfill and reprocessing, and improved site engineering has reduced the flow of contaminants beyond the confinement area. Federal restrictions on groundwater acidity and heavy-metal concentration levels regulate conditions in the immediate vicinity of the mine, and Ontario mines have a respectable track record for compliance (Environment Canada, 1988). Abandoned mines, however, generally date to an era before environmental regulation, and thus may pose a considerable risk.
The high acid and heavy-metal concentration in the leachate make the groundwater extremely conductive and an excellent electromagnetic geophysical target. The example shown has been extracted from a larger mineral exploration survey conducted for the Ontario Geological Survey. The survey was flown with helicopter electromagnetics (HEM)/magnetometer/ VLF-EM sensors at a nominal terrain clearance of 30 m (HEM) and 45 m (magnetometer and VLF-EM) and a 200-m line spacing. Observations were positioned by radar ranging techniques, and results were presented originally as EM profiles at 1:20,000. The actual location and orientation of the data have been disguised.
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.