Through the new Distributed Acoustic Sensing technology, a fiber optic cable has been transformed into a long series of sensors to capture the signals generated by the volcanic activity of Etna.
A fiber optic cable was buried in the summit area of Etna to measure the variations in deformation associated with seismic and volcanic activity. The cable, interrogated by a sophisticated DAS (distributed acoustic sensing) device, was able to detect signs of Etna's volcanic activity on a fiber for the first time.
These are the results of a study conducted by researchers from the National Institute of Geophysics and Volcanology (INGV) and the Deutsches GeoForschungsZentrum - GFZ - in Potsdam (Germany), just published in the journal 'Nature Communications'.
Using DAS technology, fiber optic cables are able to record earthquakes. This technique, particularly useful in geoscience applications where the goal is to measure ground vibrations, is based on a measurement principle that involves sending a pulse of light into a fiber and detecting the back-scattered signal from its imperfections. It is precisely the analysis of the signal that provides information on the deformations that the cable undergoes at each of its points.
"In the work just published”, explains Gilda Currenti, researcher at the INGV Etna Observatory, “we have demonstrated the high sensitivity and accuracy of DAS devices in measuring seismic-volcanic signals and the contribution that this technology can provide in advancing the understanding of volcanic processes. The cable, buried in a layer of slag", continues the researcher, "was able to record deformations associated with Etna's activity, such as explosions, degassing of the summit craters, local earthquakes, as well as atmospheric phenomena, including hail and lightning."
“DAS measurements have been validated by comparison with traditional sensors such as geophones, seismometers and infrasonic sensors”, adds Philippe Jousset, researcher at the GFZ.
The detailed and abundant observations, impossible with other measurement techniques, allow to detect and characterize the volcanic explosions, the propagation of the generated acoustic waves and their non-linear interaction with the ground. From the analysis and modeling of the signals it was possible to identify hidden volcanic-tectonic structures, as well as identify and characterize volcanic events with high accuracy.
“The spatial resolution afforded by DAS measurements made it possible to extract and amplify very faint but important signals that would otherwise be out of reach for quantitative analysis”, adds Benjamin Schwarz, research co-author and GFZ researcher.
“This study demonstrates that DAS can be used to monitor volcanic activity and we believe this technique could soon become a standard for monitoring" continues Gilda Currenti.
Thanks to the ability to interrogate cables even at long distances, up to tens of km, DAS devices can be installed in safe places far from active craters, transforming the fiber into a series of distributed sensors that are easier to manage than traditional sensors that require power and data transmission systems at the installation site.
“The installation and use of fiber optic cables crossing the sides of the volcanoes from the summit areas up to the inhabited villages would provide a unique opportunity to deepen the understanding of the response of the volcanic edifice to magmatic processes in order to understand the origins”, observes Philippe Jousset.
“The application of DAS on submarine networks of fiber optic cables, nowadays widely used for data transmission, could also provide thousands of sensors to study submarine magmatic systems, otherwise inaccessible”, concludes Lotte Krawczyk, Director of Program-Oriented Funding (POF) at the Helmholtz Association.
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Photo 1 - Connection of fiber optic cables on the Etna volcano (Piano delle Concazze; 2800 m above sea level) (Photo MA Gutscher, 2018).
Photo 2 - Installation of the fiber optic cable in the volcanic slag layer at Etna (Photo P. Jousset, 2018).
Photo 3 - Excavation of the track for the burial of the fiber optic cable at Piano delle Concazze (Etna, 2800 m asl), with the Pizzi Deneri Observatory in the background (Photo P. Jousset, 2018).