Application of seismically derived tilt signals to characterize groundwater flow regimes: An example from a constant-rate pumping test in Taiwan

Aquifer volumetric changes induce ground deformation. In hydrogeology, a constant-rate pumping test is used to characterize an aquifer system and its flow regimes. However, data only from one or two wells in a pumping site may lack detailed spatial information for a heterogeneous site. We propose that ground tilts detected by seismometers may provide additional spatial coverage. Here, we installed nine closely spaced broadband seismometers around a pumping well within 3–38 m to measure hydraulic-induced ground tilts during a 24-h constant rate pumping conducted in northeastern Taiwan. The tilts are overall consistent with an inverted cone shape analytical solution of ground vertical displacement due to water pressure perturbation. We found that ground subsidence, water table drawdown, vertical displacement, and ground tilt are linear with each other. However, inconsistent tilt directions show that local heterogeneities at each station affect the deformation pattern. The tilts at the stations near the pumping well were approximately oriented in a northwest-southeast direction, parallel to the dip directions of the fractures and the bedding plane. Tilt-estimated flow dimensions at seismic stations are spatially consistent with the drawdown-estimated flow dimensions at two wells. Preliminary hydrogeological surveys and seismic analyses show that this groundwater system combines sub-radial and spherical flow and has impermeable boundaries. The network is restricted by impermeable strata at greater depth, and the steep dipping fractures and the bedding plane. We demonstrate that the time series of tilts can be used to estimate flow dimensions at the tested site. Multiple closely spaced seismometers help to characterize details of the fractured groundwater network for constraining flow regimes and hydrogeological conditions.