Rising 11,000 feet above a million people, Mount Etna is one of the most thoroughly monitored volcanoes on Earth. Hundreds of sensors are located on its flanks, and for good reason: it’s Europe’s most active volcano, periodically spewing lava and huge plumes of debris that keep planes grounded and generally make the lives of those who live in the shadows miserable. .
But now scientists have spied on Etna with an unlikely new surveillance device: fiber optic cables, like the ones that bring you the Internet. Last week, researchers wrote in the journal Nature Communications how they used a technique known as distributed acoustic sensing, or DAS, to pick up seismic signals that conventional sensors lacked. This could help improve the early warning system that people in the surrounding parts of Italy rely on. Millions of others around the world are also at the mercy of active volcanoes, creating chaos, whether large or small.
DAS (sorry) shakes up science in a big way. As the Internet grew in the 1990s, telecommunications ended up laying more fiber optic cable than needed because the material itself was cheap compared to the labor required to bury it. That extra cable remains unused, or “dark,” and scientists can rent it out to conduct DAS experiments. Engineers use it to track land deformation, geophysicists use it to study earthquakes, and biologists even use underwater cables to pick up the vibrations of whale calls.
Fiber optic works by carrying signals from point A to point B as pulses of light. But if the cable is disturbed by, say, an earthquake, a small amount of that light is reflected back to the source. To measure this, scientists use an “interrogator,” which shoots a laser through the fibers and analyzes what comes back. Because researchers know the speed of light, they can determine perturbations at different lengths along the cable: Something that happens 60 feet away will bounce back light that takes a little longer to get to the interrogator than something that happens 20 feet.
These measurements are: sensitive† For example, in the spring of 2020, during the early days of COVID-19 lockdowns, scientists at Pennsylvania State University used the buried dark fiberglass on their campus to observe how pedestrian and vehicular movement slowed and picked up again. They were even able to identify the source of the overhead disturbance by the frequency of the 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 telecom never bothered to lay fiber optics on Etna, the researchers dug a three-quarter-mile trench less than a foot deep and buried their own not far from the volcano’s rim.
In the image above you can see how the fiber optic cable lay, the two branches are outlined in white and black. (The red and yellow lines are errors.) The dots along the cable lines are places where the scientists had conventional sensors, such as seismometers, which use pendulums to detect motion, and geophones, which convert ground motion into electrical signals. Because these sensors and the cable were placed in those spots — at C666, C667, and so on — the researchers were able to compare how the different techniques monitored activity.
The image above shows what a volcanic explosion (not a full-blown eruption) in September 2018 looked like for the DAS network. The detection stations are listed at the top of the image. The red and blue represent the deformation, or “strain rate” at which the cable elongates or contracts, at any given time for every six feet along the length of the cable. “So if the cable itself is, say, extended or compressed, then we see that in the signals,” said Charlotte Krawczyk, a geoscientist at the German Research Center for Geosciences and the Technical University of Berlin, who co-authored the paper in which the work is described . “We don’t do that with all other seismic equipment. We measure the acceleration of the surface or things like that.”
Note the darker vertical red and blue band at C671, which is an increase in the signal’s amplitude. If you look back at the map, you’ll see that the C671 is right on the mark. “This is probably an area where the density and speed of the ground are different,” said geoscientist Philippe Jousset of the German Research Center for Geosciences, lead author of the paper. That changes how the energy ripples through the Earth and then how the DAS reads the event.
