Fiber foods

Scientists spy on Mount Etna with fiber optic cables

Rising to 11,000 feet more than a million humans, Etna is one of the most watched volcanoes on the planet. Hundreds of sensors dot its flanks, and for good reason: it’s Europe’s most active volcano, periodically spewing out lava and huge plumes of debris that hover on the ground and generally make life miserable for those living. in his shadow.

But now scientists are spying on Mount Etna with an unlikely new monitoring device: fiber optic cables, like the ones that bring you the internet. Writing last week in the newspaper Nature Communicationresearchers describe how they used a technique known as distributed acoustic sensing, or DAS, to pick up seismic signals that conventional sensors missed. This could help improve the early warning system that people in surrounding regions of Italy rely on. Millions more around the world are also at the mercy of active volcanoes, which create chaos, big or small.

DAS shakes up (sorry) the science in a big way. When the internet was growing in the 1990s, telecoms ended up laying more fiber optic cables than they needed because the material itself was cheap compared to labor needed to bury him. This extra cable sits unused, or “dark”, and scientists can rent it to conduct DAS experiments. Engineers use it to monitor ground deformation, geophysicists use it to study earthquakesand biologists even use undersea cables to pick up the vibrations of whale calls.

Dig a trench to bury the DAC cable

Photographer: P. Jousset

Optical fiber works by carrying signals from point A to point B in the form of light pulses. But if the cable is disrupted by, say, an earthquake, a tiny amount of that light is reflected back to the source. To measure this, scientists use an ‘interrogator’, which fires a laser through the fibers and analyzes what comes back. Because researchers know the speed of light, they can determine disturbances at different lengths along the cable: something happening 60 feet away will reflect light that takes a little longer to get to the interrogator. that something happens 50 feet away.

These measures are sensitive. For example, in the spring of 2020, at the start of the Covid-19 shutdowns, scientists at Pennsylvania State University used their campus’ buried dark fiber optic to observe the decline and resumption of pedestrian and traffic movement. vehicles. They could even tell the source of the air disturbance by the frequency of its vibration: a human footstep is between 1 and 5 hertz, while automobile traffic is 40 to 50 hertz.

This new research is centered on the same idea, only these scientists have done it on an active volcano. Because telecoms never bothered to lay fiber optics on Mount Etna, researchers dug a three-quarter-mile ditch less than a foot deep and buried their own, not far from the edge. of the volcano.

Illustration: P. Jousset

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