GEOPHYSICAL CHARACTERIZATION OF KARST AQUIFERS USING DYNAMIC RECHARGE EVENTS

Water flowing through karst aquifers produces geophysical signals and deformation as the water interacts with conduit networks, matrix porosity, and fracture systems. Preliminary work in a karst aquifer in Minnesota, USA demonstrated that seismic signals were generated during artificial recharge experiments and a natural recharge event. Seismic signals result from pressure pulses, which commonly occur as recharge enters conduits with full pipe flow. Pore pressure changes in fractures and the rock matrix as well as the interaction of turbulent flow with the conduit wall rocks likely serve as additional sources of geophysical signals that facilitate karst aquifer characterization. We have run forward models to identify the magnitude and spatial extent of deformation induced by water recharging karst aquifers. The models suggest the water pressure increase in conduits associated with recharge events will produce measureable deformation at the surface. For example, a pressure increase of 0.1 bar (i.e., equivalent to increasing hydraulic head by 1 m) in a 20 m diameter circular conduit 30 m below the surface yields displacements up to 3 μm at the surface. The same pressure increase in a rectangular conduit 30 m wide × 0.1 m high buried at 30 m depth produces surface displacements up to 9 μm. Models also predict measureable tilt due to mass loading-induced subsidence and recovery with the rising and falling of water levels. For example, adding 1 m of water over an area with dimensions 400 m × 40 m will cause vertical displacements up to 25 μm and tilts up to ±100 nrad. To further evaluate these results, we will instrument the Santa Fe River Sink-Rise system in Florida, USA with seismometers, tiltmeters, GPS instruments, hydrologic sensors, and a weather station to record recharge-induced responses for two years, starting summer of 2018. This experiment should characterize sources of geophysical signals that arise during recharge events in karst aquifers and evaluate their utility for aquifer characterization and regionally integrated flow monitoring. We hypothesize that geophysical remote sensing of all of these processes in karst systems during recharge events can be used to facilitate conduit delineation, to document flow and transport processes, and to constrain the structure of the aquifer system.