Optical fiber is used on Mount Etna to monitor volcanic activity

Optical fiber is used on Mount Etna to monitor volcanic activity

Etna, which stands at 3,357 meters above a million people, is one of the most monitored volcanoes on Earth. There is a good reason that hundreds of sensors are scattered on its slopes: it is the most active volcano in Europe, and periodically erupts lava and huge plumes of debris that force planes to land and, more generally, complicate life at people who live in its shadow.

Now, however, scientists have a new and unlikely tool to monitor the volcano: fiber optic cables, the same ones that guarantee internet connections. In a paper published recently in the scientific journal Nature Communications, researchers explained that they used a technique known as distributed acoustic sensing, or Das, to collect seismic signals that conventional sensors cannot detect. The Das could help improve the early warning system on which people residing in the Etna area rely. Around the world, millions of people live at the mercy of active volcanoes, which are capable of wreaking havoc regardless of their size.

Das represents a remarkable novelty in the world of science. At the time of the expansion of the internet in the 1990s, telecommunications companies installed more fiber optic cables than needed, since the material was cheaper than the labor required to place it underground. The surplus cables remained unused, allowing scientists to rent them to perform Das experiments. Cables are used by engineers to monitor soil deformation and by geophysicists to study earthquakes, while biologists use underwater cables to collect the vibrations generated by the calls of whales.

The trench to bury the cable.

P. Jousset Optical fibers carry signals from point A to point B in the form of pulses of light. However, if the cable is disturbed, for example, by an earthquake, a small amount of light is bounced back to the source. To measure this phenomenon, scientists use an "interrogator", which emits a laser through the fibers and analyzes what comes back. By knowing the speed of light, researchers are able to determine disturbances at various lengths along the cable: an event that occurs twenty meters away will bounce a light that takes longer to reach the interrogator than a phenomenon that happens. at fifteen meters.

These measurements are extremely sensitive. For example, in the spring of 2020, at the start of the Covid-19 lockdown, scientists at Pennsylvania State University used unused fiber optics buried on the university campus to observe how the movement of pedestrians and vehicles first decreased and then increased again. They also managed to indicate the source of the disturbance on the surface thanks to the frequency of the vibrations: human footsteps are between one and five hertz, while car traffic is between forty and fifty.

Optical fiber to monitor the 'Etna The new research is based on the same idea, with the difference that scientists have applied it to an active volcano. Since telecommunications companies have never installed optical fiber on Mount Etna, the researchers dug a ditch about 1,200 meters long and 15-25 centimeters deep to bury their cables near the edge of the volcano.

P. Jousset In the image above you can see the positioning of the fiber optic cable, whose two branches are represented in black and white (the red and yellow lines instead indicate the faults). The dots scattered along the cable lines are where scientists had installed conventional sensors, such as seismometers, which use pendulums to detect motion, and geophones, which convert motion in the ground into electrical signals instead. The researchers were thus able to compare how different techniques monitored volcanic activity.

P. Jousset The image above shows what a volcanic explosion (not a complete eruption) looked like at the Das network in September 2018 The survey stations are indicated at the top of the graph. The red and blue represent the strain rate at which the cable stretches or contracts at any given time, at intervals of approximately 1.8 meters. "The signals allow us to establish whether the cable is stretched or contracted - explains Charlotte Krawczyk, a geologist at the German Research Center for Geosciences and the Technical University of Berlin, as well as co-author of the article that reports the experiment -. all the other seismic instruments, we don't. We measure the acceleration of the surface and things like that ".

WiredLeaks, how to send us an anonymous report The darker red and blue vertical band at point C671 indicates a increase in signal amplitude. Looking back at the map, we see how C671 is right on a fault: "This is probably an area where the density and speed of the ground is different," says geoscientist Philippe Jousset of the German Research Center for Geosciences, lead author of the article. This changes the way the energy reverberates through the earth and how the Das reads the event.

The cables have also detected other volcanic events, which conventional sensors have not picked up or barely recognized : for example, degassing events, in which the volcano emits a plume of water vapor and other gases, such as carbon dioxide. The people who were on Etna at that time recorded the phenomenon in some videos. The Das also recorded "single shock pulses", distinguishable from outgassing due to the lower frequency of their signal. According to researchers these impulses could be due to the movement of gas or liquid in depth, which in turn drives the degassing events.

The benefits of Das "One of the main advantages of Das, which often tends to be overlooked, is that it can detect events at many different frequencies, ”explains geophysicist Ariel Lellouch, who uses the technology at Tel Aviv University but was not involved in the study. An infrasound sensor, on the other hand, only picks up low-frequency sounds. Furthermore, the Das requires less maintenance: "The fiber stands still, while traditional sensors need telemetry, and sometimes batteries that must be replaced", adds Lellouch.

According to Marco Aloisi, who studies Etna at the National Institute of Geophysics and Volcanology but did not participate in the research, the Das could integrate traditional methods for monitoring volcanoes. Given the number of people living around the volcano, Etna is closely monitored through about two hundred monitoring stations. However, this activity requires a lot of manpower, and the less time people spend on an active volcano, the better. "The real challenge is being able to have a lot of human resources and reliable technology to allow continuous operation of the entire system", says Aloisi.

The Das, on the other hand, is a more passive system: you lay the cables and expects the arrival of the data. "In a sense, a seismic observatory is built with the fiber - says Lellouch -. It could even return after years, unless the fiber was melted by a huge eruption".

\ Developments future The authors of the article also want to try using cables of several kilometers, in order to obtain even more data. In the future, scientists could even take a full circle around a volcano, providing 360-degree data that can further advance early warning systems.

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Arrow A week before the explosive eruption of Etna in July 2001, for example, the data collected by gps instruments showed that the monitoring were moving away, a sign that Etna was filling with magma from the depths of the subsoil. Since there were no real-time monitoring systems at the time, it took scientists a few days to process the data and raise the alarm (fortunately, in that case it was known from the start that the eruption would not pose a threat. for the people). Perhaps, explains Aloisi, the Das could pick up signals that conventional sensors do not register, further refining the alarm system: "This technology allows the detection of mild signals, provides detailed structural images and offers a more acute understanding of the dynamics behind the processes. magmatic ", he adds.

Faster alarms allow a more timely evacuation, and therefore to save a greater number of lives. "The goal is to extend the time available to warn people and help them to move away from the event - explains Krawczyk -. Being able to better understand which processes could represent warnings and indicate new parameters for alarms would be a fundamental novelty".

This article originally appeared on sportsgaming.win US.

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