Passive seismic monitoring of a karst system: Methodological approach and prospects

While karst systems are difficult to understand because of their heterogeneity and complexity, passive seismic methods can offer a non-invasive alternative for detecting variations in flow and storage. This study explores the potential of this method at the Glane karst spring (Dordogne, France), a site with high hydrodynamic amplitude, a known terminal conduit, and a 75 km² recharge area. A network of four Raspberry Shake seismometers continuously recorded seismic noise for more than two years in order to analyse signal amplitude variations and relative fluctuations in ground wave propagation velocity during the hydrological cycle, as deduced from cross-correlations. The results show that the 7–12 Hz band can be used to identify 29 floods over three hydrological cycles, with energy increases of up to +3 dB during peak flows of up to 9 m³/s. Seismic noise correlations also reveal velocity variations of up to ±2% in the 0.1–0.5 Hz band, which are inversely related to fluctuations in the piezometric level of the Bathonian aquifer, suggesting a mechanical response of the environment to hydraulic load.