Geophysical methods were used to map buried paleochannels that could serve as conduits for contaminant migration at two hazardous waste sites in Colorado. Irons et al. [1991] used shallow seismic reflection profiles to map paleochannel deposits at the Department of Energy's Rocky Flats Plant. Nicholl and Cain [1992] characterized the containment system of a contaminant plume at the Rocky Mountain Arsenal by using seismic refraction lines to locate the base of the unconsolidated alluvial deposits and an EM survey to map channels in the paleosurface of the Denver formation.
Hole-to-hole radar tomography has been used to map fractures in crystalline rock at the U.S. Geological Survey (USGS) Mirror Lake, New Hampshire test site [ Wright et al., 1993; in press a,b]. The use of radar for fracture detection is an important area of research as fractures, which are often the principal fluid flow paths in crystalline rocks, may not intersect well bores. The presence of such undetected permeability can play havoc with attempts at hydrologic modeling. Haeni et al. [in press] compare the use of various surface geophysical techniques used for mapping fracture orientation at the Mirror Lake site.
A great deal of research has been conducted on the use of geophysical techniques for the detection of tunnels [ Miller and Dennis, 1993]. While most of the tunnel detection work has been focused on the use of hole-to-hole radar systems, there has been some work done on using seismic tomography and seismic wavefield analysis [see Miller and Dennis, 1993].
At a site near Hutchinson, Kansas, Miller et al. [1993] used areas of decreased coherency in shallow seismic reflection profiles to identify the subsurface extent of roof failure associated with dissolution mining of salt. The area of failure, confirmed by drilling, defines the potential future enlargement of an existing sinkhole. In a similar study, Miller and Steeples [1991] used seismic reflection to detect voids in an underground coal mine near a growing sinkhole. It was determined that the sinkhole was unrelated to the voids, but was the result of surface water draining into a vertical shaft. Miller and Steeples [1994] review the previous two studies, and present a third in which seismic reflection is used to identify faults and dissolution structures in the vicinity of a leaking chemical storage pond.
Microgravity surveys were used by Ghatge [1993] to identify covered shafts of an abandoned copper mine that posed a subsidence hazard in a residential area of New Jersey. Of 19 shafts located on the basis of gravity lows ranging from 0.02 to 0.05 milligal, 15 were confirmed by drilling. Cohen and Donahue [1994] used a shallow seismic reflection survey at an U.S. Environmental Protection Agency Superfund waste disposal site in Pennsylvania to select sites for monitoring wells. The wells were designed to monitor organic contaminants migrating into underground tunnels of an abandoned coal mine. Disrupted zones in the seismic reflection profiles were used to identify and locate the mine tunnels.