Transient ground bulge derived from a dense broadband seismic array during an aquifer step-drawdown pumping test

We deployed nine broadband seismometers within 32 m of a well where a four-hour step-drawdown pumping test was conducted at a site near Shuangxi, Taiwan in August 2018. The whole seismic array simultaneously recorded hour-long seismic signals that we interpret as ground tilt evolution during and after the pumping. Here we document data from the six seismic stations within 10 m of the boreholes to show the ground deformation closest to the pump site. Instead of ground subsidence due to groundwater over-draw as documented in many long-term and even short-term GPS and tiltmeter studies, four out of the six seismometers show ground bulge at the pumping site with radial directional tilts up to 1.0 x 10-5 rad for a drawdown of 10 m. We interpret the bulge as the dominant transient crustal response to the unloading of discharged groundwater, outpacing the minor subsidence due to reduced effective stress from reduced pore fluid head. After the pumping stopped, the ground tilt signals started to level back, though not simultaneously: some signals recorded delays up to an hour, possibly related to replenishment of groundwater from far-field or different aquifers and shallow depth hydrological heterogeneity including minor faults and fractures. We conducted seismic reflection surveys and found mostly layered structures without large fault offsets. Using an analytical solution that calculates the ground tilt due to surface load changes, we calculated the effects of unloading on ground deformation. The predicted tilt signals are in the range of 10-5 rad, the same order as the seismically derived tilts from this study. For 10 m of drawdown we estimate 10-5 to10-4 m of vertical ground uplift for this particular site. 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 thus the ground water flow patterns, during pumping tests.