On the trail of volcanic signatures: A successful test on Mount Etna shows that fiber optic technology could significantly expand the options for monitoring volcanoes. A sensor cable laid there was able to record the finest activities of the volcano and reveal hidden structures by means of fiber optic measurements. With its unprecedented sensitivity, the method could make an important contribution to researching and monitoring the earth’s dangerous mountains of fire, say the scientists.
Many volcanoes still remain enigmatic and difficult to calculate: Despite the great advances in volcanology and seismology, many physical processes before and during eruptions are still unclear. It is also still difficult to recognize the signs of volcanic phenomena in good time using conventional observation methods such as seismometers. Above all, the sensitivity and accuracy leaves a lot to be desired. Because the detection of weak activity can be crucial when it comes to predicting and assessing the risk of volcanic eruptions.
A long feeler in Etna’s flank
An international research team led by Philippe Jousset from the German Research Center for Geosciences Potsdam (GFZ) has now explored the extent to which fiber optic cables are suitable for measuring subtle volcanic activity on the Sicilian volcano Etna. This was based on earlier successes of the so-called fiber optic acoustic measurement (Distributed Acoustic Sensing, DAS) method in earthquake monitoring. The conventional fiber optic telephone network is used to obtain information about ground movements based on interference in the light signals in the lines. In order to record volcanic processes, however, Jousset and his colleagues laid a fiber optic cable: the 1.3-kilometer-long line was installed in 2018 about 2 kilometers from the summit craters of Mount Etna, about 20 centimeters deep in a layer of slag.
The scientists are now reporting on the results that the sensor cable has delivered since then. As they explain, the process involves sending successive pulses of light down the fiber optic cable. The light partially backscattered by the natural features of the fibers is then captured and analyzed. The first step is the control signature of the cable. The sensor function is then created by the fact that subtle ground movements, acoustic waves or temperature changes slightly deform the fibers and thus change the propagation time of the light in the cable. According to the scientists, the method makes it possible to detect such events at every meter along the line and thus provide extremely detailed data.
Promising test results
“The cable laid in the cinder layer was able to measure and localize strain changes associated with volcanic activity on Etna, such as volcanic explosions, small volcanic vents, local volcanic-tectonic earthquakes, and even atmospheric phenomena such as hail and Thunderstorms,” reports Jousset. The DAS data were confirmed by measurements using conventional sensors – geophones, broadband seismometers, infrasound sensors. However, the spatially very dense measurement data was only possible with the DAS method. “The unprecedented spatial resolution makes it possible for the first time in this form to specifically separate and evaluate weak signals that have hitherto been of little practical use,” says co-author Benjamin Schwarz from the GFZ.
As the researchers continue to report, resonance phenomena were also detectable underground. They are triggered when, as a result of volcanic explosions, acoustic waves propagate and interact with the near-surface scoria deposits. This data could be incorporated into methods for detecting hidden structural features in the subsoil. The DAS method can thus identify volcanic activity and also reveal near-surface structures. “Our study thus shows that DAS, with its high sensitivity and accuracy, can be used to efficiently monitor volcanic activity,” summarizes Jousset.
The scientists therefore also see great potential in the process for improving risk assessment. Their importance is particularly evident in the case of Mount Etna, which is home to more than a million people. Again and again the volcano causes fear and terror in the region with its explosive eruptions and lava flows. “The results of our study are a new contribution that advances the understanding of volcanic processes. And we are convinced that the technique will become a standard for volcano monitoring in the years to come,” says co-author Gilda Currenti from the Istituto Nazionale di Geofisica e Vulcanologia in Rome.
Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences, specialist article: Nature Communications. doi: 10.1038/s41467-022-29184-w