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Monitoring Etna with a glass fibre cable

Installation of the DAS cable in a shallow trench. Summit of Etna in the background (photo: M. Weber, GFZ).
Column of ash from an explosion at the north-east crater of Etna, photographed from the deployment area of the DAS (photo: M. Weber, GFZ).

A team of volcanologists from Istituto Nazionale di Geofisica e Vulcanologia (INGV) Catania and the GFZ has installed a 1.5 km long glass fibre cable at the Italian volcano Etna. The deployment area is at 2.800 m elevation, ca. 550 m under the summit of the largest European volcano.

"The objective of our experiment is to test fibre optic technologies as a complementary and alternative tool for monitoring Etna volcanic activity" says Philippe Jousset from GFZ German Research Centre for Geosciences. The fibre cable is interrogated by a DAS Silixa Ltd instrument (Distributed Acoustic Sensors), installed at the Pizzi Deneri Observatory close to the summit of Etna. The experiment started 1st of July 2019 and will last until Mid-September 2019, thus enabling a continuous recording of Etna’s volcanic activity. „This is to our knowledge the first project using cutting edge fibre optic technology on an active volcano with an unprecedented spatial resolution“ says Jousset.

The technique allows to detect, locate and quantify strain generated by geophysical processes along the cable, allowing the discovery of unknown subsurface structural features and their dynamic processes and to monitor seismic and volcanic activity at high spatial (2 m) and temporal resolution (1000 Hz). To validate DAS measurements, 30 broadband seismometers and 3 infrasound arrays from the Geophysical Instrument Pool Potsdam (GIPP) were deployed.

Original study: Jousset, P., Reinsch, T., Ryberg, T., Blanck, H., Clarke, A., Aghayev, R., Hersir, G. P., Henninges, J., Weber, M., Krawczyk, C. (2018): Dynamic strain determination using fibre-optic cables allows imaging of seismological and structural features. Nature Communications, 9, 2509. DOI: doi.org/10.1038/s41467-018-04860-y.

Authors: P. Jousset (GFZ, Potsdam, Germany), G. Currenti (INGV, Catania, Italy), V. Parra (U. Jean Monnet, St. Etienne, France), M. Weber, C. Krawczyk T. Reinsch, Ch. Cunow (GFZ, Potsdam, Germany).

Scientific contact:
Dr. Philippe Jousset
Helmholtz-Zentrum Potsdam
Deutsches GeoForschungsZentrum GFZ
Telegrafenberg
14473 Potsdam
Tel.: +49 331/288-1299
E-Mail: philippe.jousset@gfz-potsdam.de

 

Funding of the project:
Project INFRADAS (expedition money GFZ) and project FAME, awarded through the EC EUROVOLC project (Funded under: H2020-EU.1.4.1.2, Project ID: 731070).

More information:
When a laser pulse is launched into an optical fibre, a fraction of the light is elastically scattered (Rayleigh scattering) due to random inhomogeneity distribution in the glass fibre material. The position of the scattering inhomogeneity within the fibre can be calculated based on the speed of light within the fibre (optical time domain reflectometry, OTDR). If a coherent laser pulse is launched into the fibre, with appropriate optical processing, not only the amplitude but also the phase of the backscattered photons can be analysed (phase-OTDR). For any section of the fibre, the phase-difference of photons scattered at both ends of that section is linearly related to the length of this section. State of the art DAS systems are capable of quantifying the frequency, amplitude, phase and location of dynamic perturbations anywhere along the sensing fibre. In other words: Tiny changes in length or curvature of the cable allow scientists to detect movements of the ground or shaking.