USGS Groundwater Information: Branch of Geophysics
Carole D. Johnson and John H. Williams
USGS, Office of Ground Water, Branch of Geophysics
Geophysical surveying techniques provide important information for ground-water investigations (Zohdy and others, 1974; Keys, 1997; Haeni and others, 2001). Subsurface-geophysical methods are used to delineate and characterize hydraulically active zones; the extent of contamination, and contaminant sources; identify geologic features; optimize monitoring well placement; and guide remediation efforts. Borehole-geophysical methods provide information about the physical, chemical, and hydraulic properties of rock, sediments, and fluids in the subsurface and provide important information on subsurface bedrock structures including lithology, rock fabric, location, orientation, and hydraulic properties of fractures (Keys, 1990).
Effective use of geophysical data requires that the data be interpreted in the context of known local and regional geology and hydrogeology. In addition, because of the complexity and heterogeneity of crystalline-rock aquifers, a suite of borehole-geophysical methods is used to determine the location, extent, and nature of fractures and other structural features in the bedrock aquifer. The geophysical data from each borehole and method are analyzed together to provide an integrated interpretation, thereby reducing the ambiguity that can occur by interpreting each geophysical log individually (Shapiro and others, 1999).
Previous work using borehole geophysics to characterize ground-water availability in crystalline-rock aquifers includes Chapman and Lane (1996), Mack and others (1998), and Johnson and others (1999). Other investigations that focused on contamination in fractured-rock aquifers used geophysical methods to relate highly transmissive features to structural features in the bedrock (Hansen and Lane, 1995; and Lane and others, 2002).
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Final copy as submitted to the Georgia Geologic Survey for publication as: Johnson, C.D., and Williams, J.H., 2003, Hydraulic logging methods - a summary and field demonstration in Conyers, Rockdale County, Georgia, in ed. Williams, L.J., Methods used to assess the occurrence and availability of ground water in fractured-crystalline bedrock - an excursion into areas of Lithonia Gneiss in eastern metropolitan Atlanta, Georgia: [Atlanta, Georgia], Georgia Geologic Survey, Guidebook 23, p. 40-47.