Characterizing inter- and subglacial properties of a 3700 m plateau on the Grenzgletscher with active seismics

Located in the Monte Rosa massif in the Swiss Alps, the Grenzgletscher is one of the largest glaciers in the Alps, extending over approximately 2000 meters in height, with an accumulation zone that reaches up to 4500 meters and an ablation zone that descends to around 2500 meters. While its basal temperature reaches values of -13 °C at high elevations (Colle Gnifetti, 4450 m), it is temperate in the ablation zone, hence exhibiting at least one transition from frozen to temperate bed. As part of the ERC-funded project PHAST, a surface geophysics field campaign aimed at identifying the location of the frozen-to-temperate basal transition was conducted between 2024 and 2025. In this contribution we focus on the analysis of an active seismic survey conducted in 2024 to aid the characterization of basal conditions on a roughly 500 m x 500 m plateau at approximately 3700 m. The ELVIS-7 surface vibrator source was used to produce single-shot P-wave sweep signals along two lines of 48 geophones each, covering a total of 235 m, both parallel and perpendicular to the glacial flow. A velocity analysis was performed on the measured refracted waves, providing information on the upper part of the ice column and the depth of the firn layer. Deeper layers, the ice thickness, as well as the basal conditions, were studied via CMP/NMO processing and a phase-polarity analysis of the reflected waves. Finally, a post-stack migration was performed to obtain an accurate image of the glacier's subsurface along the receiver lines by accounting for possible steep-dipping interfaces or other structural complexities. In addition to revealing new information about the inter- and subglacial properties of the Grenzgletscher at high altitudes, the findings will be useful for identifying suitable drilling locations to study the physics of sliding onset in a natural laboratory, one of the main goals of PHAST.