While the range of environmental problems is very broad, they typically involve the need to determine the location and nature of fixed or mobile environmental hazards. Fixed hazards include buried waste containers and discarded objects. Spilled contaminants or fluids leaking from tanks or barrels constitute mobile hazards. In the case of mobile hazards knowledge of the geologic and hydrologic conditions that effect the transport of contaminants is often required. The integrated process of data collection, interpretation, and modeling needed to address environmental problems is called site characterization. Site characterization should not stop once remediation starts, as there is a continuing need to monitor and understand how the remediation effort is progressing. Although the nature of the environmental ``targets'' is typically different from traditional resource exploration targets, geophysical techniques often can provide a noninvasive means of obtaining the information required to characterize a site, be it a gas station, a landfill, or other hazardous or toxic waste site.
Some of the more common characterization problems relate to transport of contaminants by means of ground-water flow. They include: 1) determination of geologic zones and boundaries that control fluid flow such as sand lenses and clay layers; 2) detection of fractures that control flow in crystalline rock; 3) detection of inorganic and organic contaminant plumes that may originate from industrial operations, landfills, or abandoned mines; 4) detection of saltwater intrusion in aquifers; and 5) detection of abandoned wells which can serve as conduits for contaminant transport.
There are other environmental targets that do not move, but none the less pose a current or potential environmental threat such as: 6) abandoned mines and natural dissolution structures (sink holes) which can collapse and thereby endanger man-made structures built over them, 7) abandoned underground storage tanks and drums containing fluids that will eventually leak, and 8) unexploded ordnance (UXO) that needs to be removed from military reservations before the land can be returned to civilian functions. Targets of this sort must be located before remedial action can be planned.
The list of targets enumerated above is by no means complete, however, it is sufficient to give an idea of the scope and nature of the problems being addressed.