Undersea cables are the latest seismic detection tools

These very words may have traveled through an undersea cable before they reached your eyeballs. Hundreds of thousands of miles of optical fiber cross the world’s oceans, transmitting emails, Netflix shows, and news articles as beams of light. And scientifically speaking, the boy has a story to tell – not so much about what happens on Earth, but what happens in the depths.

Last week the researchers wrote in Science how they used a 3,600-mile cable running between Halifax, Canada, and Southport, in the United Kingdom, to detect storms, tides and earthquakes. Because the cable lies on the sea floor, such disturbances create small but measurable disturbances in the optical fibres, changing how light speeds across the Atlantic. These changes give a reading of the location of an earthquake or other disturbance.

This technology – a form of interferometry sensing – is similar to another system increasingly popular among researchers: Distributed Acoustic Sensing, or DAS. Here scientists fire a laser through underground (but unused) fiber-optic cables and analyze what bounces back. If a car or person passes from above and causes interference to the cable, this vibration scatters some of the light back to its source. Measuring how long it takes the scattered light to travel gives an idea of ​​the size of an object passing overhead. The researchers also laid cables around Mount Etna, an active volcano in Italy, and used the DAS system to monitor its rumblings.

This new technology exploits a device in submarine cables called a repeater. (It looks like a slight bulge in the line). In the map below, those points are indicated by yellow dots. “Every 60 to 80 kilometres, you usually need an optical magnifier, which basically takes incoming light and amplifies it,” says Giuseppe Mara, a metrologist at the UK’s National Physical Laboratory and lead author of the new research paper. “So it propagates through the next range, and then there’s another speaker, and it continues that way to get to the other side.”

Courtesy of Giuseppe Mara

Each repeater boosts the signal to make sure it reaches its destination without damaging it. So Mara and his colleagues can send their own signals through the cable and analyze their shape as they reach each repeater. Unlike DAS, they do not attempt to analyze the perturbation that is reflected by a trace amount of light back to its source, but rather the frequency of the light that reaches the repeaters. “In the absence of disturbance, we get stable signals: the frequency we receive is the same as the one we sent,” Mara says. But if there is a disturbance, this frequency changes.

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