CHARACTERIZATION OF FRACTURES AND FRESH- AND SALINE-GROUNDWATER FLOW ZONES IN SELECTED BOREHOLES, NORTHERN APPALACHIAN PLATEAU

Fractures and fresh- and saline-water flow zones penetrated by selected boreholes in three northern Appalachian Plateau settings were characterized through the collection and analysis of drilling and geophysical logs. The boreholes, which penetrated lower Pennsylvanian, Mississippian, and upper Devonian clastic-rock strata, included stratigraphic test holes drilled by the Pennsylvania Geological Survey, holes for a closed-loop geothermal wellfield, and a tophole for an oil-and-gas well. The drilling logs included measurement of blown-discharge rate and specific conductance and inspection of drill core or cuttings. The geophysical logs included petrophysical and fluid-property measurements and optical and acoustic images. 

At high-relief upland-ridge sites in north-central Pennsylvania, multiple fresh-groundwater flow zones primarily associated with bedding fractures were penetrated at 50 to 425 feet below land surface. The deepest fresh-groundwater flow zones were penetrated at 550 to 700 feet below land surface. Petrophysical log analysis indicated increasing formation water salinity below this depth. Saline-groundwater flow zones associated with a few poorly permeable bedding fractures were penetrated at 900 to 1,000 feet below land surface. At a river-valley bottom site in south-central New York, the deepest fresh-groundwater zone was penetrated at the fractured top of bedrock at 140 feet below land surface. Highly transmissive saline-groundwater zones, each associated with an interconnected series of bedding fractures, were penetrated at 255 and 310 feet below land surface.

At an upland-slope site in northwestern Pennsylvania, multiple fresh-groundwater flow zones associated with bedding and subvertical fractures were penetrated at 30 to 150 feet below land surface. The deepest fresh-groundwater flow zone was penetrated at 250 feet below land surface. Petrophysical log analysis indicated increasing formation water salinity below this depth. Transient-electromagnetic soundings along a topographic profile centered on the logged tophole suggested that the deepest fresh groundwater was shallower below the broad upland ridge and the tributary-valley bottom than that below the slope, which is consistent with the stress-relief model of fracturing in the Appalachian Plateau.