MULTIPLE-SCALE CHARACTERIZATION OF FRACTURED BEDROCK FLOW PATHS USING A BOREHOLE FLOWMETER

Fractured bedrock aquifers are an important resource in the northeastern USA, but effective characterization of such aquifers requires piezometers completed into individual water-producing fractures. Isolation of individual fractures with packers or elaborate well completions and multiple-borehole aquifer testing is expensive and time consuming. Packer installation may not be practical when the geometry of large-scale flow paths is unknown, or when borehole conditions make packer installation impossible. We propose using borehole flow measurements in open bedrock boreholes to: 1) identify water-producing fractures intersecting individual boreholes, and 2) characterize aquifer-scale flow paths in the regions between boreholes. The transient evolution of flow in observation boreholes when an adjacent borehole is pumped is analyzed to characterize the rate of propagation of the cone of water level drawdown throughout the large-scale fracture network. The flow distribution in each observation borehole at each time step is inverted to give the relative water level difference between each fracture zone. The absolute depth to water in each zone is then computed using the open-borehole water level as a reference under the assumption that the water level is the transmissivity-weighted average of the water level in each zone. We applied this technique to a typical bedrock aquifer by using existing water supply wells to stress the aquifer, and measuring flow in a series of fully screened observation wells. The method gave acceptable estimates of the hydraulic head in four different fracture zones as a function of time during the aquifer tests, and a fully transient forward model demonstrates that negligible error was introduced by using a quasi-steady borehole flow inversion for data that were changing over time.