SEISMIC RESPONSES DURING RECHARGE EVENTS IN KARST AQUIFERS: POTENTIAL RECORDS OF PRESSURE PULSES?

Seismic signals have been recorded during recharge events in karst aquifers, including responses to both anthropogenic injection experiments and natural recharge events. Here, we highlight records that were collected near Bear Spring in southeastern Minnesota, USA. Temperature and electrical conductivity monitoring during a natural recharge event suggest that the spring is sourced by at least a couple of conduit flow paths. During this natural recharge event, surface displacements of up to 2 µm were recorded. The largest displacement corresponded to a time when spring discharge underwent a significant increase. It is likely that this displacement was caused by a pressure pulse or surge signal within one of the conduits in response to the recharge event, either from the conduit transitioning from open channel to full pipe flow or by a flood wave arriving at an already, fully submerged conduit. While both velocity and discharge generally increase as water levels rise in a conduit with open channel flow, both decrease as water levels approach and reach full pipe flow conditions. If this transition happens quickly, pressure increases, potentially producing a pressure surge or a water hammer effect. Regardless of whether the measured displacement resulted from the transition of open channel to full pipe flow or from a flood wave reaching a conduit full of water, pressure perturbations would travel at ~1500 m/s in conduits full of water. This particular displacement was first recorded at seismometers near the spring and successively later at seismometers further from the spring and at higher elevations, potentially suggesting much faster signal propagation through the water in the conduit than through the thin overlying rock and soil. Another series of displacements were recorded during one of the artificial recharge events, but the order of arrival times at the seismometers was reversed, likely due to lower water levels and only portions of the flow path full of water that led to slower pressure transmission through the conduit. Environmental seismology enables monitoring of dynamic flow processes in karst aquifers, and ongoing research is assessing how this information may be used to improve karst aquifer characterization.