Project title: Monitoring water flow in karst using passive seismic ‘listening’
Researcher: Dr Haleh Karbala Ali
Detecting and quantifying water flow structures in heterogeneous karst through geophysical imagery of karst structures, is open to large uncertainties. Here we determine that subterranean flow-induced seismic signal originating in karst subsurface water pathways such as fractures, conduits, and caves can be detected as persistent frequencies on spectrograms. Conducting passive seismic surveys on two karst sites with shallow and deep conduits shows that these frequencies are site dependent, and differ from what might be expected from surface water flow in rivers. We apply an amplitude-based location method to locate this flow-induced micro-tremor. Numerical simulation of the elasto-acoustic wave propagation in a simplified karst model shows that resonance may play an important role, excited by specific frequencies in the flow-related seismic spectrum. This study introduces a novel approach for detecting the flowing structures in karst by directly imaging the source itself rather than inferring flow pathways through geophysical imaging of karst structures.
The main objective of this project is to locate the flowing conduits in karst by considering the subterranean hydraulic processes such as turbulence of water-flow, bed-load transport, and other mechanisms as the sources of strong vibrations. After some pilot experiments in 2020, we undertook the main fieldwork in karst between 15-20 Dec. 2020 on Pollnagran Cave, Frenchpark, Co. Roscommon. Following successful findings of this experiment and to test if the method also works in deeper conduit settings, we did another experiment between 29-30 March 2020 on Polltoophill-Polldeelin Cave, Gort, Co. Galway. Once located, we can indirectly monitor the groundwater flow dynamics by looking at seismic ground vibrations over time. We showed that seismology has the potential to indirectly monitor the flow dynamics by correlating the seismic monitoring station near Avoca River, Co. Wicklow with the permanent OPW flow gauge data. In the latest experiment, we deployed two water-level loggers at the entrance and resurgence of Polltoophill-
Polldeelin Cave, Gort, Co. Galway, in Nov. 2020. We also left three broad-band and one short-period seismic monitoring stations in that site over the winter 2020 and spring 2021. We collected the seismic stations at the end of March 2021 and retrieved the water level To support the observations in the field, we also carried out numerical simulation considering a geological model similar to Pollnagran Cave, Co. Roscommon.