To better understand and predict volcanic events, a better understanding of the diverse subsurface processes involved is required. A new way to detect such processes, even if they are very subtle, is to use fiber optic cables as sensors. The analysis of light that is scattered back in them when the cables are deformed, for example by shaking, has now made it possible for the first time to determine the volcanic signature of the Sicilian volcano Etna very precisely. An international team led by Philippe Jousset from the German Research Center for Geosciences in Potsdam (GFZ) and Gilda Currenti from the Italian Istituto Nazionale di Geofisica e Vulcanologia (INGV) reported on this today in the journal Nature Communications. The method, known as Distributed Acoustic Sensing (DAS), has been able to remotely identify seismo-acoustic volcanic activity and map hidden near-surface volcanic structural features. With its high sensitivity and accuracy, it proves to be the basis for improved volcano monitoring and risk assessment.
The challenge of predicting volcanic eruptions
The understanding of the physical processes before and during a volcanic eruption has greatly improved in recent years. However, it is still difficult to detect very subtle triggering mechanisms of volcanic phenomena using conventional observation methods such as seismometers. On the one hand, the accuracy of the measuring methods is not sufficient to detect and identify all processes within the volcano, on the other hand, unknown subterranean structures distort the measured signals observed. However, knowledge of weak activities can also be crucial when it comes to predicting and assessing the risk of volcanic eruptions, for example on the Sicilian volcano Etna, the largest, most active and most visited volcano in Europe, more than a million people live on its flanks and in the immediate vicinity. Etna's volcanic activity is characterized by frequent explosive eruptions with lava flow.
Testing a new measuring method on the Etna volcano in Sicily
Philippe Jousset from the German Research Center for Geosciences Potsdam GFZ and Gilda Currenti from the Italian Istituto Nazionale di Geofisica e Vulcanologia INGV and their colleagues have investigated the extent to which fiber optic cables are suitable for measuring very weak seismic and volcanic activity. To do this, they laid a 1.3-kilometer-long fiber optic cable in a joint experiment on the Etna volcano about 2 kilometers from the summit craters, a good 20 centimeters deep in a layer of slag, and measured the changes in the cable's strain caused by the various activities.
The measuring principle with glass fibers
The so-called DAS principle was used for the measurement. DAS stands for "Distributed Acoustic Sensing". Using a laser, successive light pulses are sent into an optical fiber via an interrogation device. The light, partially backscattered at the natural "imperfections" of the fiber, is then analyzed. The transit time of the light changes when the fiber deforms or stretches - for example due to minute ground movements, acoustic waves or temperature changes. Such events can be detected every meter along a fiber and measured very precisely.
It was already known that fiber optic telephone lines can be used to record earthquakes: since this was demonstrated by GFZ researchers in Iceland in 2018, the technology has been used in various places around the world, including the USA, Switzerland and Japan , for monitoring seismic activity, but also in other geoscientific applications where ground movements or vibrations are to be measured.
The new glass fiber method is suitable for the analysis of volcanoes
The researchers have now demonstrated that the method is also suitable for the precise analysis and monitoring of volcanoes. "The cable, laid in a cinder layer, was able to measure and localize strain changes associated with volcanic activity on Mount Etna, such as volcanic explosions, small volcanic outgassing, local volcanic-tectonic earthquakes, and atmospheric phenomena such as hail and thunderstorms ", explains Philippe Jousset, first author of the study and scientist in the section "Geophysical mapping of the subsoil" at the GFZ.
The DAS strain rate measurements were validated with measurements from conventional sensors - geophones, broadband seismometers, infrasonic sensors. The spatially very dense measurements, which cannot be carried out with other methods, make it possible to recognize and characterize both volcanic explosions and resonance phenomena in the subsoil. The latter are triggered when acoustic waves propagate as a result of the explosions and interact non-linearly with the near-surface slag deposits.
Standard analyzes of volcanic seismology, modeling tools for wave and extensional propagation, and techniques such as wave field separation and reconstruction were used to quantify subsurface structural features hidden from the measured data and to detect and locate volcanic events with accuracy.
Benjamin Schwarz, GFZ scientist and also involved in the study, says: "The unprecedented spatial resolution of the recordings, which is made possible by fiber-optic strain measurements, makes it possible in this form for the first time to target weak and previously hardly usable signals separate and evaluate."
"Our study shows that DAS, with its high sensitivity and accuracy, can be used to efficiently monitor volcanic activity," summarizes Jousset. "This is a new contribution advancing the understanding of volcanic processes. And we are convinced that the technology will become a standard for volcano monitoring in the years to come,” expects Gilda Currenti.
A special advantage of the fiber optics: the most precise measurements are also possible from large distances
Thanks to their ability to measure long distances—currently at least tens of kilometers—an interrogator can be deployed in a remote location, making observations with fiber optics easier and more secure than traditional sensor arrays that require telemetry, on-site power supplies, and regular maintenance. "Fiber optic cables running from the volcano's summit to remote locations would provide unique opportunities to deepen the understanding of ground response, including estimation of travel path effects, and to better understand the origin of volcanic phenomena," says Lotte Krawczyk, spokesperson for the Helmholtz research program PoF IV and co-author of the study.