Document Type

Article

Publication Date

1998

Abstract

Characterization of fractured-bedrock aquifcrs is onc of the most difficult problems in hydrogcology. Successful characterization of aquifers at the site scale requires the effcctive integration of the three basic tools at our disposal: 1) surface geophysical soundings provide full non-destructive coverage of the aquifer volumc but are generally ambiguous in interpretation and fail to identify individual fracture conduits; 2) geophysical measurements in borcholes can churacterizc fractures in detail but only adjacent to iridividual borcholes; and 3) hydraulic measurements in boreholes can be used to generate dircct relationships between geophysical and hydraulic properties for the rock immediately adjacent to the borcholc. 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. Iné contrast, several different geophysical propertics can be measured using a conventional suitc of gcophysical wcll logs. Thesc data can be used to gcnerate sitc-specific interpretation models that rclatc a spccific geophysical sounding response to several independent rock properties. quantitative rclation between the gcophysical log paramcters (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 wherc such samples can be recovered. Relatively new, high-resolution flowmeter and borchole-dilution logging techniques offer more readily available and morc flexible methods for relating borchole hydraulies to geophysical properties. This report provides specific cxamples of studics fractured-bedrock aquifer sites where gcophysical and hydraulic measurements are effectively integrated using this combination of techniques.

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