A variety of borehole geophysical logging techniques are being used to characterize aquifers and monitor contaminant plumes in the United States. Of particular importance are new procedures for characterizing fracture flow across borehole arrays, such as that developed by Paillet [1993]. In this four-step approach, 1) conventional electrical resistivity, gamma, and caliper logs are used to characterize lithology, areas of fractures, and areas of alteration within each borehole; 2) borehole wall image logs are used to identify and characterize the fractures intersecting the boreholes; 3) high-resolution flowmeter logs are used during pumping to identify the subsets of fractures associated with ground water inflow and outflow and the connections between boreholes; and 4) transient flow experiments are used to refine the hydrologic model. Paillet [1993] uses this approach to characterize fracture flow within an array of 13 boreholes in crystalline bedrock at the Mirror Lake, New Hampshire, test site.
A similar approach is used by Vernon et al. [1993] to characterize fractures and flow at the site of a planned water supply well in a crystalline bedrock aquifer near Dover, New Hampshire. In this case borehole fluid conductivity logs, taken after the fluid in the borehole was replaced by deionized water, were used to confirm the locations of water-producing zones in the borehole, and provide water-quality parameters for correlating flow between the 5 boreholes and estimating inflow from the overlying saturated surficial deposits.
A third example is provided by Paillet et al. [1993], who characterize a fractured dolomite aquifer at a contamination site in Illinois. To extend the scale of this study away from the 6 logged boreholes, many observation wells were used both to define flow domains within the aquifer by mapping the water-level elevation and to define preferred flow directions through aquifer drawdown tests.
A summary of recent trends and developments in geological applications of borehole logs [ Doveton and Prensky, 1992] suggests that newly developed techniques will have greater application to environmental problems in the future. Some new techniques such as electrical imaging methods have been quickly adopted by environmental scientists. Other techniques such as spectral gamma-ray logging, which has the potential of identifying clay minerals and oxidizing or reducing conditions, and geochemical logging, which can identify concentrations of elements, have not yet found environmental applications.