Scientists spy on Mount Etna with fiber-optic cables

One of the most thoroughly monitored volcanoes on Earth is Mount Etna, which is 11,000 feet above a million humans. Hundreds of sensors dot its flanks, and for good reason: it's Europe's most active volcano, and it frequently emits lava and debris that makes life miserable for those living in its shadow.

Scientists have been using fiber-optic cables like the ones that bring you the internet to spy on Etna. Researchers described how they used a technique known as distributed acoustic sensor, or DAS, to pick up signals that conventional sensors missed. The early warning system that people in the surrounding parts of Italy rely on could be improved by this. Millions of people around the world are at the mercy of active volcanoes, which can cause chaos even if they are small.

DAS is shaking up (sorry) science in a big way. When the Internet was growing in the 1990s, telecoms ended up laying down more fiber-optic cable than they needed, since 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 run DAS experiments. Engineers use it to monitor land deformation, geophysicists use it to study earthquakes, and biologists are even using underwater cables to pick up the vibrations of whale calls. Advertisement

The signals from point A to point B are sent as light waves. A small amount of the light gets bounced back to the source if the cable is damaged by an earthquake. Scientists use an interrogator, which fires a laser through the fibers, to measure this. Researchers know the speed of light, so they can determine if there is a problem along the cable by looking at how long it takes to get to the interrogator.

These are sensitive. Scientists at Pennsylvania State University used buried dark fiber optics to observe as pedestrian and vehicle movement waned and picked up again during the early days of COVID-19. A human footstep is between 1 and 5 hertz, whereas car traffic is 40 to 50 hertz.

The scientists did it on an active volcano. The researchers dug a ditch less than a foot deep and buried their own, not far from the volcano's rim, because telecoms never bothered to lay fiber optics there.

The fiber-optic cable's two branches are outlined in white and black in the image above. The red and yellow lines are not straight. There are dots running along the cable lines that have conventional sensors, like seismometers, which use pendulums to detect movement andgeophones, which convert ground movement into electrical signals. The researchers could compare how different techniques were monitoring activity because the sensors and cable were colocated at those spots.

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The image above shows a volcanic explosion in September of last year. At the top of the graphic is a note about the sensing stations. The red and blue are the strain rates at which the cable contracts at a given time. We measure the surface's speed.

The red and blue band is an increase in the signal's amplitude. If you look at the map, you will see that C671 is on a fault. The energy ripples through the earth and the DAS reads it.