SEISMIC MONITORING OF KARST RECHARGE EVENTS – RESULTS FROM A PILOT EXPERIMENT

Seismic monitoring, long used to track earthquakes and study Earth structure, is now being employed to track surficial processes such as landslide and glacier movements. Recent seismic data collected near rivers demonstrate that changes in river flow and bedload transport generate observable seismic signals, providing the ability to remotely monitor these systems. Here we expand on seismic monitoring of flow processes to present data and analysis from a seismic monitoring pilot project for a karst aquifer system near Bear Spring, near Eyota, MN, USA. The pilot project consisted of seismically monitoring both controlled recharge experiments into a dry overflow spring, where injected water flowed underground until it discharged at nearby Bear Spring, as well as a fortuitous natural recharge event in the form of a large rainfall that supplied over 2 inches of rain within a few hours. These recharge events were recorded by 12 surface seismometers all placed within the ~4500 m² area containing the dry overflow spring and predicted underground flow path to Bear Spring. We clearly observe increased amplitudes of seismic energy during each controlled experiment and during the natural rain event, with different durations and dominant frequencies for the different events. Increased noise, or a tremor-like signal, dominates the early stages of the rain event on all seismic stations. Large amplitude signals identify a period when measured discharge at the spring increased above background levels. In addition, distinct large amplitude signals occur in the minutes prior to the largest increase in discharge and a change from subsurface to above-ground flow at the overflow spring. During the controlled experiments, we also see seismic evidence for pressure pulses, events observed in spring monitoring that occur as the water enters conduits with full pipe flow. We will furthermore highlight an upcoming experiment where seismometers, tiltmeters, and GPS, hydrologic, and meteorologic instruments will be deployed to characterize a well mapped Florida karst system geophysically.