USING BOREHOLE GEOPHYSICAL METHODS TO ASSESS THE DEPTH DEPENDENCE OF PERMEABILITY IN SHALLOW FRACTURED-CRYSTALLINE ROCK AQUIFERS OF THE PIEDMONT AND BLUE RIDGE PROVINCES OF NORTH CAROLINA

The depth dependence of permeability in shallow (<150 m) fractured rock aquifers in the Piedmont and Blue Ridge provinces is analyzed using optical televiewer, caliper and heat pulse flow meter logs. We analyze characteristics (i.e., attributes, distributions, orientations) of fractures that intersect boreholes in conjunction with flow profiles to investigate the depth evolution of groundwater systems in crystalline rocks. A total of 570 fractures that intersect 26 bedrock wells previously drilled in various crystalline rocks are analyzed. The results reveal that the dominant fracture types are foliation parallel fractures (FPFs) (42%), whereas other fractures (32%) are intermediate, and the least dominant fractures are sheet joints (26%). The dominant fractures in the upper 35 m of the shallow subsurface are sheet joints, whereas FPFs and other fractures are dominant below 35 m. Fractures with the largest apertures are mostly restricted to shallow depths, where sheet joints dominate. Flow intervals identified from heat pulse flow meter logs reveal that sheet joints are the major fracture type through which the majority of flow is observed and are therefore likely to control most of the flow in the shallow subsurface. In contrast, FPFs and other fractures are the dominant conduits for channeling flow at greater depths (>35 m). Relative interval transmissivity profiles derived from heat-pulse flow-meter logs generally show that the most of the flow (~80%) in fractured crystalline rock aquifers in the two provinces occurs at depths shallower than 75 m. This result suggests the existence of a bounding depth in the shallow subsurface below which permeabilities of fractured crystalline rocks are significantly reduced.