FRACTURED-ROCK AQUIFER CHARACTERIZATION WITHIN A REGIONAL GEOLOGIC CONTEXT: RESULTS FROM THE BUCKNELL UNIVERSITY HYDROGEOPHYSICS TEST SITE

An integrated and multiple-scale approach to fractured-rock aquifer characterization has been undertaken at the Bucknell University Hydrogeophysics Test Site. The research site is on the north limb of a west-plunging, second-order anticline in the Valley and Ridge province of central Pennsylvania, and consists of five 175-ft deep, open-hole wells in the carbonate rock and shale of the Silurian Wills Creek Formation. Data collected at the well field include water-level responses during drilling operations, acoustic- and optical-televiewer logs, caliper logs, slug tests, borehole flowmeter surveys, straddle-packer injection tests, and multiple-well pumping tests. To place the site-scale characterization within the context of the regional geology, two nearby Wills Creek outcrops in similar structural position to the well field were studied.

Three major fracture sets were identified at the Wills Creek outcrops: (1) bedding-sub-parallel fractures, (2) cleavage, and (3) joints. The same three sets were identified in borehole acoustic- and optical-televiewer logs from the well field. In addition, carbonate-solution features were apparent in outcrop and borehole-televiewer images. Although the angle between average bedding planes varies up to 20 degrees between sites, orientation analyses of outcrop and borehole data indicate that the angles between the fracture sets and bedding are similar.

Hydraulic, geophysical, and structural observations were combined to identify transmissive fracture zones in the well field. Several transmissive zones were penetrated by each of the wells, and these zones provide hydraulic connections between the wells at multiple depths. Observed fractures within the transmissive zones include the three fracture sets, although bedding-sub-parallel fractures are most prominent and show the most oxidation. The strong correlation between borehole and outcrop observations should facilitate the extension of the site-scale hydrogeophysical results to other similar geologic settings in the Valley and Ridge.