Borehole Geophysical Characterization of Fractured-Rock Aquifers in West Virginia

When:
Track 1C: Insights into Groundwater
Thursday, October 12, 2017, 12:40 pm – 1:50 pm

Author(s):
Mark D. Kozar and Mitch McAdoo
U.S. Geological Survey
Charleston, West Virginia

Abstract:
Fractured-rock aquifers provide the majority of groundwater used for rural residential, commercial, and public supplies in West Virginia. These aquifers have not been well characterized in the past due to the cost and complexity associated with collecting the borehole geophysical data required for their assessment. To address this issue, the U.S. Geological Survey (USGS) Virginia-West Virginia Water Science Center and the West Virginia Department of Environmental Protection have implemented a 5-year research initiative to collect borehole geophysical logs from 20 wells annually. The intent is to establish a statewide dataset of geophysical logs from wells completed in fractured-rock aquifers to characterize fracture distribution, formation porosity, and groundwater-flow processes with respect to the broad range of lithologic, structural, topographic, and physiographic settings found in West Virginia. The project is currently in the third year of data collection, and is already producing substantive information which is helping to verify previously developed theories of fracture formation in the Appalachian region. To date, a total of 51 wells have been logged and, combined with existing logs and logs planned for ensuing years, will provide a dataset of about 120 wells for analysis. The types of borehole logging tools used for the project include acoustic and optical televiewers for documenting the strike, dip, depth and aperture of fractures in the aquifers, a full-wave sonic tool for documenting formation porosity, a 3-arm caliper tool for determining borehole diameter, a EM-induction and a multi-parameter logging tool for determining lithology, and an EM flow meter for determining flow of water within a borehole and to determine water-bearing fractures. A USGS aquifer test analysis method (FLASH) is being used to determine overall borehole transmissivity and the transmissivity of respective water-bearing fractures. Preliminary data from this statewide borehole geophysics project are being used 1) to expand the understanding of stress-relief fracturing in the Appalachian Plateaus Physiographic Province, 2) to understand the role of thin limestone units such as the Tonoloway and Helderberg limestone Formations as potential aquifers in the folded and faulted rocks of the Valley and Ridge Physiographic Province, 3) to better understand karst aquifer groundwater-flow processes for the Greenbrier Aquifer and for the Cambrian and Ordovician aquifers of the Shenandoah Valley, 4) to refine our understanding of subsidence fracturing in underground coal mine aquifers, 5) to investigate potential migration of saline waters to freshwater aquifers, and 6) to confirm that the majority of permeability within bedrock aquifers in West Virginia is derived from secondary fracture porosity, rather than from primary intergranular porosity. The information obtained as a result of this project will help to better characterize fractured-rock aquifers in West Virginia, which may aid in the delineation of aquifers, development of groundwater supplies, and protection of aquifers from contamination.