Tech

As the underground fiber optics spies on the men moving above it

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When last spring lockdown calmed the Penn State campus and the surrounding city of State College, an instrument ruled by the jury was “to be heard.” A team of university researchers had exploited an underground telecommunications fiber-optic cable, which works two and a half miles across campus, and transformed it into a sort of scientific surveillance device.

By shining a laser through the optical fiber, scientists have been able to detect vibrations from above the ground thanks to the way the cable is still so slightly deformed. When a car turns through the underground cable or a person passing by, the earth transmits its unique seismic signature. So without visually monitoring the surface, scientists could paint a detailed picture of how a once-animated community would stop, and slowly come back to life as the block became easier.

They could say, for example, that foot traffic on campus almost disappeared in April after the blockade began, and went on until June. But after it initially declined, vehicle traffic began to pick up. “You can see people walking is still very minimal compared to normal days, but vehicle traffic has actually returned to almost normal,” says Tieyuan Zhu, a Penn State seismologist, lead author in a new letter describing the work in the journal The Seismic Register. “This fiber optic cable can distinguish such a subtle signal.”

More specifically, it is the frequency in the signal. A human step generates vibrations with frequencies between 1 and 5 hertz, while vehicle traffic is more similar to 40 or 50 hertz. Vibrations of construction machinery jump from more than 100 Hz.

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Fiber optic cables work by perfectly trapping light pulses and transporting vast distances as signals. But when a car or a person passes by, the vibrations introduce a disturbance, or imperfection: a small amount of that light. spread back to the source. Because the speed of light is a known amount, Penn State researchers could flash a laser through a single fiber optic chain and measure vibrations at different cable lengths by calculating the time it took for scattered light to travel. . The technique is known in geoscience as distributed acoustic sensing, or DAS.

A traditional seismograph, which records tremors with the physical movement of its internal parts, measures only activity in one place on Earth. But using this technique, the scientists were able to sample more than 2,000 points along the 2.5 kilometers of cable – one every six and a half meters – giving them a superficial resolution of activity above ground. They did so between March 2020, when the blockchain was installed, and June 2020, when State College businesses began to reopen.

Only from those vibrational signals could DAS show that on the west side of campus, where a new garage was under development, there was no industrial activity in April when construction stopped. In June, the researchers not only detected the vibrations of the restarted machine, but were able to actually select the construction vehicles, which rang at a lower frequency. However, they noted, by this time pedestrian activity on campus had barely recovered, although some pandemic restrictions had eased.

DAS could be a powerful tool for tracking people’s movement: Instead of sifting through mobile phone location data, researchers could instead exploit fiber-optic cables to track the passage of pedestrians and cars. But technology can’t exactly identify a car or a person. “You can tell if it’s a car, or if it’s a truck, or if it’s a bicycle.” But you can’t say, “Oh, this is a Nissan Sentra, 2019,” says Ariel Lellouch, a geophysicist at Stanford University who uses DAS but hasn’t been involved in this study but made it a peer review. “DAS anonymity is one of the biggest advantages, really.”


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