Geophysical Logging of Fresh- and Saline-Water-Producing Zones in Fractured Bedrock of the Northern Appalachian Basin

Geophysical logging was used to characterize fresh- and saline-water-producing zones in fractured bedrock at selected upland and valley drilling sites in the northern Appalachian Basin. The logged sites included several stratigraphic test holes drilled by the Pennsylvania Geological Survey on upland ridges, a top hole for an oil and gas well on an upland slope, a domestic water-supply well in a tributary valley, and a geothermal wellfield in a major river valley. The geophysical methods included nuclear, electric, induction, and acoustic logging; video and televiewer imaging; and specific conductance, temperature, and flow logging under ambient, pumped, and recovery conditions. Integrated analysis of the geophysical logs with drilling data provided qualitative and quantitative information on bedrock lithology, fracture orientation and transmissivity, and hydraulic head and salinity of water-producing zones. Such information is critical for monitoring and protection of the groundwater resources during energy development.

The geophysical logging delineated water-producing zones in the penetrated interbedded sandstone, siltstone, and mudstone at fractures subparallel to bedding and at steeply dipping fractures related to jointing. In the upland boreholes, the most transmissive fractured zones were penetrated between 50 to 300 feet below land surface. These zones produced freshwater and generally had hydraulic heads significantly higher than zones at intermediate depths. Minimal fracture transmissivity was penetrated in these boreholes below a depth of about 700 feet, and the few deep fractures produced small inflows of saline water. In the valley boreholes, fractured zones that produced saline water were penetrated at 175 and 250 feet below land surface. The saline-water-producing zone at the river valley site was very transmissive and extended for at least 300 feet across the wellfield.