USGS Groundwater Information: Branch of Geophysics
John H. Williams, U.S. Geological Survey, 425 Jordan Road, Troy, NY 12080-8349
Frederick L. Paillet, U.S. Geological Survey, Denver Federal Center MS 403, Lakewood, CO 80225-0046
Aquifer tests, which entail stressing a centrally located borehole and measuring drawdown in a series of observation boreholes, are commonly used to define hydraulic properties and connections in the subsurface. However, the heterogeneity and discrete nature of permeability in fractured-rock aquifers make it difficult to design effective aquifer tests without some preliminary understanding of the fracture-flow geometry. The crosshole-flowmeter method provides an efficient means to characterize hydraulic connections among discrete fractures penetrated by boreholes.
In the crosshole-flowmeter method, the transient flow in observation boreholes is measured in response to short-term (typically 10 to 20 minutes) pumping or injection in a stressed borehole. To reduce ambiguity in the interpretation of the flow response, a borehole that either penetrates a single, permeable fracture zone, or a borehole in which the individual zones are isolated with straddle packers, is stressed. Initial delineation of which fracture zones to monitor in the observation boreholes is not necessary because the flowmeter measurements that are made in the open boreholes reveal the identities of the individual fracture connections as part of the analysis.
The measured flow response in each observation borehole is compared to characteristic type curves developed by model simulation for simple classes of fracture connections. Once the appropriate type curve is identified, the fracture transmissivity (initially estimated from previous single-borehole flowmeter tests) and the fracture storage coefficient are adjusted until the simulated flow matches the measured flow. The analysis can be used to characterize fracture zones that are connected in series or parallel and to discover hidden cross-connections among fracture zones that are not otherwise identified. Even in those situations where more extensive aquifer tests are planned, an initial series of crosshole-flowmeter experiments can greatly increase the quality of such tests by optimizing their design and by improving our ability to interpret their results.
The crosshole-flowmeter method is part of ongoing investigations at fractured-rock research wellfields in the Potsdam sandstone in upstate New York; and in Valley and Ridge carbonate and Mesozoic-basin clastic rocks in Pennsylvania. The design and analysis of a series of crosshole-flowmeter tests that was recently completed in fractured shale at a VOC-contaminated site in eastern New York will be presented.
Final copy as submitted to Fractured Rock 2002 for publication as: Williams, J. H. and Paillet, F. L., 2002, Crosshole-flowmeter method for the characterization of hydraulic connections in fractured rock, in Fractured Rock 2002, Denver,Colo., March 13-15, 2002, Proceedings [abs.]: Westerville, Ohio, National Ground Water Association.