Rising 11,000 feet over a million people, Mount Etna is one of the most closely monitored volcanoes on earth. Hundreds of sensors are dotted along its flanks, and for good reason: it’s Europe’s most active volcano, regularly spewing out lava and huge clouds of debris that ground planes and make life difficult for those living in its shadow.
But now scientists have been spying on Mount Etna with an unlikely new surveillance device: fiber optic cables, like the ones that bring you the internet. Researchers wrote in the journal Nature Communications last week described how they used a technique known as Distributed Acoustic Sensing (DAS) to pick up seismic signals that conventional sensors missed. This could help improve the early warning system that people in surrounding parts of Italy rely on. Millions more around the world are also at the mercy of active volcanoes wreaking havoc, whether or not they are big or small.
THAT is shaking up (sorry) science in a big way. As the Internet grew in the 1990s, telecom companies laid more fiber optic cable than they needed because the material itself was cheap compared to the labor required to bury it. This extra cable remains unused, or “dark,” and scientists can borrow it to conduct DAS experiments. Engineers use it to monitor land deformation, geophysicists use it to do that study earthquakesand biologists even use underwater cables to record them Vibrations from whale calls.
Optical fibers work by carrying signals from point A to point B as pulses of light. But if the cable is disturbed by, say, an earthquake, a tiny amount of that light will bounce back toward the source. To measure this, scientists use an “interrogator” that shoots a laser through the fibers and analyzes what comes back. Because the researchers know the speed of light, they can determine perturbations at different lengths along the cable: Something passing 60 feet away reflects light that takes a little longer to get to the interrogator than something passing 50 feet away happens.
These measurements are sensitive. For example, in spring 2020, in the early days of the COVID-19 lockdown, scientists at Pennsylvania State University used their campus’s buried dark optical fibers to monitor pedestrian and vehicle movement subsided and resumed. They were even able to identify the source of the above-ground disturbance by the frequency of its vibration: a human footstep is between 1 and 5 Hertz, while car traffic is 40 to 50 Hertz.
This new research focuses on the same idea, only these scientists did it on an active volcano. Because the telecommunications companies never bothered to run fiber-optic cables on Mount Etna, the researchers dug a three-quarters-mile-long trench less than a foot deep and dug their own trench not far from the volcano’s rim.
In the image above you can see how the fiber optic cable was laid, its two branches are outlined in white and black. (The red and yellow lines are errors.) The points that run along the cable lines are where the scientists had traditional sensors, such as seismometers, which use pendulums to detect movement, and geophones, which convert ground movements into electrical signals. Because these sensors and the cable were in these locations—at C666, C667, etc.—the researchers were able to compare how the different techniques monitored activity.
The image above shows what a volcanic explosion (not a full eruption) in September 2018 looked like to the DAS network. The measuring stations are indicated in the graphic above. Red and blue represent the deflection, or “strain rate,” at which the cable is expanding or contracting at any given time for every six feet along the length of the cable. “So if the cable itself is, say, lengthened or compressed, then we see that in the signals,” says Charlotte Krawczyk, a geoscientist at the German Research Center for Geosciences and the Technical University of Berlin, co-author of the article describing the work. “We don’t do that with all other seismic devices. We measure the acceleration of the surface or something like that.”
Note the darker vertical red and blue bands at C671, representing an increase in signal amplitude. If you look back at the map, you can see that C671 sits right on top of a bug. “This is probably an area where the density and the speed of the ground are different,” says geoscientist Philippe Jousset from the German Research Center for Geosciences, first author of the paper. This changes how the energy flows through the earth and how the THAT reads the event.
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