Fibre-optic exploration of the cryosphere

SUMMARY The icy parts of the Earth, known as the cryosphere, are an integral part of the climate system. Comprehensively understanding the cryosphere requires dense observations, not only of its surface, but also of its internal structure and dynamics. Seismic methods play a central role in this endeavour. Fibre-optic sensing is emerging as a valuable complement and alternative to well-established inertial seismometers. Offering metre-scale channel spacing, interrogation distances of up to $\sim$100 km and a bandwidth from mHz to kHz, it has enabled new seismological applications, for instance, under water, in cities and on volcanoes. Cryosphere research particularly benefits from fibre-optic sensing because long cables can be deployed with relative ease in icy environments where dense arrays of seismometers are difficult to install, including glaciers, ice sheets and deep boreholes. Intended to facilitate future fibre-optic seismology research in the cryosphere, this Expository Review combines a classical publication review with theoretical background, a practical field guide, a cryospheric signal gallery and open-access data examples for hands-on training. Following a summary of recent findings about firn and ice structure, glacial seismicity, hydrology and avalanche dynamics, we derive the ideal instrument response of a distributed fibre-optic deformation sensor. To approach this ideal in field experiments, we propose numerous practical dos and don’ts concerning the choice and handling of fibre-optic cables, required equipment, splicing in the field at low temperatures, cable layout and trenching, and the deployment and coupling of cables in boreholes. A cryospheric signal gallery provides examples of data from a wide range of sources, such as explosions, land and air traffic, electricity generators, basal stick-slip icequakes, surface crevassing, englacial icequake cascades, floating ice shelf resonance, surface water flow and snow avalanches. Many of these data are enclosed as an open-access training resource, together with code for reading, visualization and simple analyses. This review concludes with a discussion of grand open challenges in our understanding of cryosphere structure and dynamics, and how further advances in fibre-optic sensing may help to overcome them.