Glaciologists usually judge a glacier's health from the surface, yet much of the action happens within the ice. Researchers led by Thomas Hudson of ETH Zurich's Environmental and Exploration Geophysics Group, together with the Swiss Federal Institute for Forest, Snow and Landscape Research and the University of Oxford, laid a single fibre-optic cable across the Gorner Glacier in Valais and turned it into a dense seismic array. The study appears in Science Advances.
An interrogator sends light pulses along the fibre and records the scattered light that returns; seismic waves from microearthquakes, such as crevasses cracking open, deform the cable minutely and betray their location and depth. Processing these signals produced an ultrasound-like image of the ice down to about 25 metres. Around 8 per cent of the ice volume in the studied crevasse field turned out to consist of water- or air-filled fractures, more damage than the team expected.
Internal crevasses matter because meltwater can deepen them and lubricate a glacier's bed, accelerating its movement, and hidden damage influences when ice breaks away. One cable provides hundreds of measurement points at once and is quicker and safer to deploy than conventional seismometers on crevassed terrain. The researchers now plan to apply the technique to other Alpine glaciers and to the polar ice sheets, where such monitoring bears directly on sea-level forecasts.