Application of Multichannel Analysis of Surface Waves (MASW) to improve the characterization of an ice-rich rock glacier

The Flüela rock glacier is located in the Eastern Swiss Alps, at the top of the Flüelapass (Grisons). Previous geophysical studies indicated the presence of an ice-rich frozen layer (Boaga et al. 2024, Bast et al. 2024) towards the central area of the rock glacier at ~ 5 m depth and absent close to the front. In August 2023, we collected electrical resistivity tomography (ERT) and seismic data along a longitudinal line (48 electrodes/geophones; spacing 3 m) in the central part of the lower rock glacier. The ERT results confirm the presence of the ice-rich frozen layer, but the P-wave velocities (Vp) obtained from the seismic refraction tomography (SRT) are surprisingly lower than the typical velocities of an ice-bearing sediment. The SRT results indicate, in fact, the typical Vp values of liquid water (~1500 m/s). Consequently, we hypothesised the presence of a shallow water-saturated sediment layer (supra-permafrost flow) that prevents P-wave penetration. Since the seismic survey was carried out with low-frequency geophones (4.5 Hz), we additionally ran a multichannel analysis of surface waves (MASW; Park et al., 1999) to retrieve the S-wave velocities (Vs), which are insensitive to the liquid phase in the medium. Another advantage of the MASW analysis, compared to the common SRT applied in permafrost environments, is that it allows detecting velocity inversions in the subsurface (i.e., a lower velocity layer between two higher velocity layers). The obtained Vs profiles agree with the ERT results and confirm the presence of a shallow high-velocity layer (Vs = 2000 m/s) in the upper part of the rock glacier, between 5-10 m depth and absent towards the front.

To confirm our results, we conducted full-wave seismic modelling, using a subsurface structure akin to that proposed for the Flüela rock glacier. This model consists of no permafrost in the first half and features a 5 m thick ice-rich layer and supra-permafrost water in the second half. The synthetic shot gathers were compared to the real ones, both in terms of surface wave dispersion and Vp first-arrival times. In both cases, we found a high correlation between synthetic and real data, confirming the reliability of the proposed rock glacier structure. Therefore, we encourage data acquisition using low-frequency geophones (e.g., 4.5 Hz) for future seismic surveys within mountain permafrost environments. This ensures that both the traditional SRT analysis and the MASW approach can be applied.

 

References

Bast, A., Pavoni, M., Lichtenegger, M., Buckel, J., & Boaga, J.: The Use of Textile Electrodes for Electrical Resistivity Tomography in Periglacial, Coarse Blocky Terrain: A Comparison With Conventional Steel Electrodes. Permafrost and Periglacial Processes, https://doi.org/10.1002/ppp.2257, 2024.

Boaga, J., Pavoni, M., Bast, A., and Weber, S.: Brief communication: On the potential of seismic polarity reversal to identify a thin low-velocity layer above a high-velocity layer in ice-rich rock glaciers, The Cryosphere, 18, 3231–3236, https://doi.org/10.5194/tc-18-3231-2024, 2024.

Park, C. B., Miller, R. D., & Xia, J.: Multichannel analysis of surface waves. Geophysics, 64(3), 800808, https://doi.org/10.1190/1.1444590, 1999.