Geophysical characterization of fractured rock aquifers: Accounting for scale effects and putting hydrology in the geophysics

Characterization of fractured-bedrock aquifers is one of the most difficult problems in hydrogeology. Successful characterization of aquifers at the site scale requires the effective integration of the three basic tools at our disposal: 1) surface geophysical soundings provide full non-destructive coverage of the aquifer volume but are generally ambiguous in interpretation and fail to identify individual fracture conduits; 2) geophysical measurements in boreholes can characterize fractures in detail but only adjacent to individual boreholes; and 3) hydraulic measurements in boreholes can be used to generate direct relationships between geophysical and hydraulic properties for the rock immediately adjacent to the borehole. The most common drawback of surface geophysical soundings is the ambiguity that results when regions of anomalous response might be related to fracture permeability, but may also be attributed to alteration, rock texture, or lithology. In contrast, several different geophysical properties can be measured using a conventional suite of geophysical well logs. These data can be used to generate site-specific interpretation models that relate a specific geophysical sounding response to several independent rock properties. A quantitative relation between the geophysical log parameters (gamma activity, resistivity, acoustic velocity, etc.) and the hydrologic properties of the formation (permeability, storage, and water quality) can be defined by regressing hydraulic measurements in the borehole versus the geophysical properties of the appropriate intervals. In the past, these regressions were accomplished using cumbersome and time-consuming straddle-packer isolation methods, or by conducting laboratory experiments using small core samples from intervals where such samples can be recovered. Relatively new, high-resolution flowmeter and borehole-dilution logging techniques offer more readily available and more flexible methods for relating borehole hydraulics to geophysical properties. This report provides specific examples of studies at fractured-bedrock aquifer sites where geophysical and hydraulic measurements are effectively integrated using this combination of techniques.