| 08 Jul 2026
Fibre-optic strain sensing on an alpine glacier
Abstract. In-situ measurements of ice deformation are constrained by the spatial resolution of discrete sensors, leaving fundamental questions about small-scale ice rheology unresolved. Here we present the first application of fibre-optic Distributed Strain Sensing (DSS) in a glacier setting, deployed at Chessjengletscher, a small polythermal glacier in the Swiss Alps. We compare Brillouin and Rayleigh DSS techniques in a borehole drilled to 38.4 m depth, alongside Raman distributed temperature sensing. Both methods resolve highly variable strain rates at the sub-metre scale — detail unattainable with conventional tiltmeters or inclinometry. Fitting observed strain profiles to a plane-strain deformation model supports a near-linear ice rheology for temperatures within 1 °C of the pressure melting point, consistent with recent laboratory experiments on temperate ice. Corresponding rate factors align closely with accepted values – though likely tempered by incomplete borehole freeze-in, which decouples the cable from ice deformation over portions of the column and introduces uncertainty into our displacement estimates. Brillouin offers a simpler field workflow and kilometre-scale measurement range compatible with remote deployment while Rayleigh provides superior spatial resolution but requires a demanding calibration procedure ill-suited to autonomous field use, though it shows promise for laboratory-scale deformation experiments. DSS represents a step change in englacial deformation measurement with direct implications for the study of ice rheology, glacier dynamics, and hazard assessment at alpine and ice-sheet scales.
Competing interests: Matthias Buhler is employed by Marmota Engineering AG who were contracted by ETH Zurich to deploy and process strain data.
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General Comments
This is an interesting and well written study demonstrating a variety of fiber-based measurements techniques in temperate ice. The authors have done a very good job of both reviewing the techniques but also making the most out of a challenging measuring environment (a weakly freezing borehole!). The paper is a sound contribution.
As a general comment, I would suggest that the authors expand their comparison of resolution of these techniques to other vertically resolvable measurements, such as APReS, etc.
Specific Comments
Line 74. While Raman backscatter can be used independently, the backscatter return can be significantly altered by local strain on the fiber, requiring more sophisticated fiber geometries (double ended) to account for non -uniform attenuation. I would suggest noting this here that even Raman sensing is not independent of strain and strain distribution. A small point but helps demonstrate the linkage between strain and temperature in all of these techniques.
Line 163: an overstuff of 1% appears large by some industry standards. Can you please double check this?
Line 185: I do not see a reference for the borehole diameter; can you give the reader some sense of the expected thermal equilibration time following freeze in? Some further discussion here on the borehole thermal equilibration would be helpful.While this is discussed further in the manuscript, I believe pointing out that freeze in in a temperate setting such as this is quite challenging to give the reader a foreshadowing of some of the issues that will arise later on.
Line 316: This is the first appearance (I may have missed it earlier) of an assumption of fiber failure at 1% strain. This may be a bit confusing to some readers when compared with the 1% overstuff reported in the loose buffered cable for temperature. I would suggest devoting a greater discussion to the maximum strain that can be measured by typical tight buffered (or bare fiber that matter). This would be helpful for other future uses. For example, does your tight buffered fiber contain any strength elements such as Kevlar? If so, how would be impact the potential for viscoelastic slip along the fiber. It may be just as easy to use bare fiber in this case, to ensure that there is not strain resistance due to other components of the tight buffering.
And while on the subject, readers may ask if there are radial strain issues on the fiber during freeze in that could impact longitudinal strain measurements. Tyler et al ( https://doi.org/10.3390/photonics11070630) demonstrated very negligible impacts to single mode fiber under high stress conditions suggesting that freeze in is unlikely to impact at least Raman backscatter.