Locating subglacial cavities and investigating basal conditions on glaciers with ambient seismic noise: toward acquisition optimization.

Subglacial cavities may trap a considerable quantity of liquid water, causing devastating outburst floods in densely populated mountain areas. Both active and passive geophysical methods are employed for the glacier-bedrock interface and intra-glacial characterization, including Ground Penetrating Radar (GPR), refraction seismic, borehole measurements, and surface nuclear magnetic resonance (SNMR). Ambient seismic noise can be collected by light and dense arrays at a relatively moderate cost, and allows to access some mechanical properties of the glacier, including the detection and localization of ice cavities and, possibly, basal detachment, taking advantage of spectral anomalies in the horizontal-to-vertical-spectral ratio (HVSR) and in the Vertical-to-Horizontal spectral ratio (VHSR). Specifically, a peak in the VHSR indicates a low impedance volume beneath the surface 1,2. As a simple picture, we can refer to the bridge vibrating mode, where the vertical displacement in the middle of the bridge largely dominates other components of the movement. Antunes et al. 2 furthermore noticed that the VHSR gives information about seismic energy anomalies generated by fluids in reservoirs since the wavefield is polarized mainly in the vertical direction.In this work, we apply the HVSR and VHSR techniques to locate asubglacial water-filled cavity in the Tte Rousse glacier (Mont Blanc area, French Alps), using 15 days of data collected in may, 2022 3. The results also confirm the general basal conditions of the glacier suggested by other methods, locating temperate areas of the glacier where basal detachments are possible. We evaluate the optimal seismic noise record duration to obtain a reliable and stable mapping of the VHSR over the glacier to properly locate the main cavity (or secondary cavities). In our case, results suggest that 6 days of record are enough to detect and locate a cavity 1 Saenger, E-H. et al:A passive seismic survey over a gas field: Analysis of low-frequency anomalies, Geophysics,74(2), O29O40 (2009). 2 Antunes V. et al:Insights into the dynamics of the Nirano Mud Volcano through seismic characterization of drumbeat signals and V/H analysis.Journal of Volcanology and Geothermal Research,431(2022). 3A. Guillemot, N. Bontemps, E. Larose, D. Teodor, S. Faller, L. Baillet, S. Garambois, E. Thibert, O. Gagliardini, C. Vincent:Investigating Subglacial Water-filled Cavities by Spectral Analysis of Ambient Seismic Noise: Results on the Polythermal Tte-Rousse Glacier (Mont Blanc, France), Geophys.Res. Lett.accepted (2024). DOI:10.1029/2023GL105038