U010-03
Monitoring Glaciers with Fiber Optic Cables

Thursday, 10 December 2020: 05:41
Patrick Paitz1, Fabian Walter2, Dominik Gräff3, Fabian Lindner3, Manuela Köpfli3, Małgorzata Joanna Chmiel4 and Andreas Fichtner1, (1)ETH Zurich, Department of Earth Sciences, Institute of Geophysics, Zurich, Switzerland, (2)ETH Swiss Federal Institute of Technology Zurich, Laboratory of Hydraulics, Hydrology and Glaciology, Zurich, Switzerland, (3)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (4)ETH Zurich, Laboratory of Hydraulics, Hydrology and Glaciology, Zurich, Switzerland
Abstract:
Similar to medical ultrasound tomography of the human body, seismologists study the internal structure of the Earth through the analysis of elastic waves propagating through it. This class of methods, broadly known as seismic tomography, illuminates processes ranging from shallow changes in soil properties to long-term geologic processes near the Earth’s core-mantle boundary.

During the past decade, a new paradigm for the recording of seismic waves has emerged: Distributed Acoustic Sensing (DAS). In contrast to conventional seismometers, DAS is based on interferometry of laser pulses transmitted through a fibre-optic cable, which may be several kilometres long. The effective channel spacing can be as small as 1 m, or less. This provides thousands of deformation sensor along the cable, an ideal condition for tomography studies. DAS is particularly attractive in environments that are difficult to instrument using traditional seismometers, including urban areas, the ocean bottom and glaciers.

To investigate the usefulness of DAS on glaciers, we conducted a pilot experiment on Rhônegletscher, a temperate glacier in the Swiss Alps. Surprisingly, a 1-km long cable, simply covered by few centimetres of snow, could record a wide range of signals, which included noise sources generating high-frequency (>1 Hz) surface waves, ideal for imaging the ice column with tomography techniques. Other recorded microseismic signals include rock falls and stick-slip motion along the interface between ice and bedrock. The high spatial sensor coverage of DAS allowed us to locate these sources with significantly higher accuracy compared to conventional seismometer networks and provided valuable insights into ice dynamics and basal structure.

The data quality and ease of cable installation of the initial Rhônegletscher study suggested that seismic monitoring of an entire Alpine glacier with DAS is possible. We therefore returned to the glacier to install another cable of 9 km length, from the glacier’s accumulation area down to the tongue. The results of this summer-long follow-up study are still preliminary. Yet with Terabytes of data coming in every week, this unique dataset has the potential to illuminate a wide range of glacial processes from crevasse opening to glacier sliding and ice-quake source mechanisms.

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