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Abstract:
Underground aquifer pore fluid volume changes can induce ground deformation, detectable by seismometers as ultra-long period tilt signals. To study evolution of the hydraulic-induced ground deformation, we deployed a dense seismic array during a water pumping experiment, which withdrew groundwater from a nearby borehole well in Shuangxi, mountainous region in northern Taiwan. Nine shallow borehole broadband seismometers were installed around the pumping well within 3-40 m away. We conducted two stages of pumping tests: a step-drawdown pumping for 6 hours and later on a 24-h continuous drawdown with a constant pumping rate. In the first stage, the tilt signals from four seismometers near the pumping well show ground bulge with radially directional tilts up to 10 μrad for a drawdown of 10 meters. We interpret the bulge as the crustal response to the unloading of discharged groundwater in this relatively homogenous crust, as imaged by our reflection seismic survey. In the second stage, the tilt signals show that the spatially complex deformation patterns are related to local fracture attitudes. However, the arrayed observation can still detect the deformation enhanced by supplementary water from outside of the array after 6 hours of the continuous pumping. Our study demonstrates that dense broadband seismic arrays may offer a relatively less expensive and non-invasive tool to study temporal and spatial evolution of ground motions and groundwater flow patterns during pumping tests.
