Advantages of applying the Multichannel Analysis of Surface Waves (MASW) in ice-rich rock glacier environments: A case study

Rock glaciers are typical landforms of mountain permafrost, composed of a surface of boulders and debris insulating an ice-bearing sediment layer, overlying a glacial till deposit and/or bedrock. It is well-known and documented that the degradation of mountain permafrost is influencing the triggering of slope mass movements (e.g. rock falls, debris flows, and floods), and the stability of infrastructures (e.g. ski resorts). Consequently, a reliable characterization of rock glacier’s structure is a key aspect for evaluating the risk related to their presence.

Since boreholes are challenging and expensive to realize in high mountain environments, geophysical methods are widely used to characterize the internal structure of rock glaciers. Electrical Resistivity Tomography (ERT) and Seismic Refraction Tomography (SRT) are among the most applied techniques to retrieve the electrical properties and the compressive wave velocities (Vp) in the subsurface.

In this work, we propose the application of the Multichannel Analysis of Surface Waves (MASW) to complement the information brought by ERT and SRT, and to overcome some limitations of the SRT method. For this purpose, the seismic data should be collected with low-frequency geophones (i.e., with 4.5 Hz natural frequency). The main advantage of the MASW approach is the possibility of obtaining shear wave velocity (Vs) profiles and to reveal velocity inversions in the subsurface, i.e., a lower velocity layer between two higher velocity layers (e.g., the unfrozen till deposit between the ice-bearing layer and bedrock). Furthermore, Vs are insensitive to the liquid phase in the medium, therefore MASW approach could be used to detect the ice-rich layer when it is surmounted by a water-saturated sediment layer (supra-permafrost flow), that could prevent P-waves from penetrating deeper.

In this work, we successfully tested the MASW method at the Flüela rock glacier (Engadine, Switzerland). ERT results clearly suggest the presence of an ice-rich layer, but the SRT analysis surprisingly does not show P-wave velocities consistent with this interpretation. The Vp model reveals in fact the typical values of liquid water. On the other hand, the Vs profiles retrieved from the MASW approach are in very good agreement with the ERT outcomes. Therefore, we hypothesise the presence of a thin water-saturated sediment layer on the top of the ice-rich layer, that would prevent P-waves penetration. In order to support our hypothesis, we performed a seismic full-wave forward modelling: the synthetic shot gathers are consistent with the real ones, both in terms of surface wave dispersion and P-wave first-arrival times.