PERIODIC HYDRAULIC TESTS IN A BEDROCK FRACTURE NETWORK

Better understanding of groundwater flow through bedrock fracture networks is critical for the emerging field of enhanced geothermal systems, as well as traditional hydrogeologic characterization. Periodic hydraulic testing has shown promise for its sensitivity to local heterogeneity and, therefore, may provide useful information about flow channelization and short circuiting. Unlike conventional steady-rate pumping or injection tests, periodic tests create a disturbance such that heads in the pumping and observation wells are always in the transient state. The volume of hydraulic influence of the oscillating flow increases with period of oscillation. Thus, different portions of the formation may be interrogated even with a single well pair. We recently performed periodic pumping tests at an experimental fractured rock hydrology field site in the Northeastern United States. Head in one well was oscillated while heads in five monitoring wells 30 to 60 m away were monitored. Periodicity of the induced signal was varied in order to investigate different volumes of the formation. Drawdown data from the monitoring wells were processed with the fast Fourier Transform, which enabled use of responses that were too small or noisy for curve fitting methods. As expected, the volume of hydraulic influence increased with period, but well response was not strictly a function of distance from the source well. This anomalous response is attributed to variation in network hydraulic connectivity.