Passive seismic signatures of a fast-flowing Alpine glacier

A multi-disciplinary shallow geophysics experiment was carried out near a prominent break in surface slope of the Grubengletscher, Valais, Switzerland, during the summer melt season of 2007. High-resolution reflection and refraction seismics, differential GPS, passive seismology, seismoelectric sounding and GPR profiling were undertaken. Here we present the initial results of the passive seismic experiment. An array of 6 high-frequency SAQS data loggers with 4.5Hz 3-component geophones was deployed on the surface of the glacier, around the break-of-slope, near the terminus of the glacier. An inter-station distance of around 50m was used, comparable to the ice thickness, with one instrument deployed at the centre of a ring of 5. Data were recorded continuously at 3000sps with GPS timing. Due to the rapid ablation rate the array required daily maintenance involving the re-seating of recording equipment, batteries and solar panels, as well as redeployment of the geophones to maintain correct orientation. Geophones were buried in small pits with a flat base chipped out of the ice with an ice-axe. Holes were then drilled for the three spikes of the sensor casing to achieve maximum coupling. The sensors were then covered with ice and overlain by rocks to inhibit surface melt. With no snow cover at the surface, deployment of the geophones became the most critical aspect of the deployment. Spurious resonances were observed in the data as a result of significant tilt of the geophones. With surface-melt in the cm/day range this kind of deployment would not be possible without daily attention. Dozens of events per hour are observed on all stations at all times of day and night. These signals encompass surface events such as rockfalls, and also crevassing and basal events. To allow the characterisation and identification of signal waveforms, the timing of known events such as rockfalls, footfalls, sledgehammer blows and "felt" glacial events were recorded. Initial results indicate a range of event types from simple impulsive crevasse-like events to highly complex large-amplitude events associated with rock-falls. When combined with the other geophysical data, the extraction of basal events from the plethora of data will allow the subglacial mechanism sustaining the flow instability of the glacier to be isolated. The findings are significant because the flow velocity of the Grubengletscher can reach some 45 m / year, which is fast for an Alpine glacier; thus, the glacier is anticipated to serve as a small-scale analogue for fast-flowing outlet glaciers from larger ice caps or ice sheets.