Seismic Observations of Possible Overpressurized Air Pocket Release in a Karst Aquifer

Environmental seismology is a growing field that enables novel observations of many near and Earth surface processes, including those associated with mass movements, sediment transport, river and glacier flow, and ocean waves. Additional research in aquifers has demonstrated the capability to monitor for water level or groundwater storage changes using ambient seismic noise. Here, we report on seismic responses during a recharge event in a karst aquifer. We deployed twelve seismometers and hydrologic sensors near and in Bear Spring in southeastern Minnesota to monitor for flow processes and hydrologic changes. During a 5 cm rainfall event, spring discharge underwent a few changes and ultimately increased from 100 L/s to more than 300 L/s, where the total flow included the perennial spring as well as a nearby overflow spring that became active during the recharge event. There were also a few changes in water temperature and electrical conductivity, and the various hydrologic responses suggest that the spring is fed by a few conduit flow paths. All seismometers recorded signals in response to the rain event, and the largest signals generated displacements on the order of 1 μm at all seismometers over a 5 sec period after the overflow spring began flowing during the significant increase in discharge. We locate these signals, finding that they were generated from multiple locations, with timing suggestive that the signals were unique events, rather than originating from an individual location and then propagating along the flow path to initiate subsequent signals. Based on these observations and accompanying calculations, we suggest these largest signals resulted from the release of overpressurized air pockets in karst conduits as water levels were rising, where energy was released as the pressurized air encountered pathways to escape from the conduits. Thus, environmental seismology enables monitoring of fundamental flow processes and dynamics beyond what is possible with traditional monitoring strategies, and identifying the source of these signals enables karst conduit delineation. This subsurface characterization has implications ranging from water resources (monitoring of flow and associated processes) to hazard assessment (delineation of subsurface cavities, highlighting areas susceptible to collapse).