Environmental geophysics is a relatively new and rapidly expanding discipline. The role of geophysics in the site characterization and remediation process is evolving as site administrators become more familiar with geophysical techniques, and as geophysicists become aware of the complex nature of environmental problems. Nearly all of the geophysical techniques being applied to environmental problems were originally developed for mineral or petroleum exploration. For some routine problems they have proved to be adequate. In general however, the conditions under which these techniques are employed are far from routine, as there are often cultural noise sources (fences, power lines, automobile traffic, etc.) nearby which complicate the data collection and interpretation. Furthermore the nature of many environmental targets is very different from traditional exploration targets with pollutants having a small or negligible response to traditional methods. As a result there has been an increase in research activity to modify existing methods, as well as to develop new techniques for resolving subtle physical property variations in the near surface.
A survey of current applications demonstrates the wide range of problems being addressed by geophysical techniques: seismic methods to map paleochannels, voids, bedrock, and landfill boundaries; borehole methods to characterize aquifers, fractures, and contaminant plumes; tomographic methods to detect fractures and voids; electromagnetic methods to monitor contaminant plumes and saltwater intrusion, map paleochannels, and detect and characterize buried objects and landfills; and ground penetrating radar to locate buried objects, map the water table, and monitor contaminant migration. Some of these problems are simply variants of traditional ground-water exploration problems, while others are much more complicated and unheard of less than a decade ago. Similarly the geophysical tools being used span the gamut of well established to experimental. Shallow reflection seismology, borehole tomographic methods, high-frequency electromagnetics, ground penetrating radar, cone penetrometer surveys, and nuclear magnetic resonance are very active research topics where advances are being made in instrumentation, data collection and processing, and interpretation. These areas hold much promise for the future.
Acknowledgments. This work was supported in part by the Department of Energy under Inter-Agency Agreement (IAG) number DE-AI05-930R22104. The authors wish to thank Gary Olhoeft, Phil Nelson, Karl Ellefsen, Bob Horton, and Pete Haeni for their contributions.