How underground fiber optics spy on humans moving above

When last spring The shutdown calmed the Penn State campus and the city around State College, where a jury-rigged machine was “listening”. A team of researchers from the university has taken advantage of a fiber-optic cable for underground communications, which runs two and a half miles across the campus, and turned it into a kind of scientific monitoring device.

By shining a laser through optical fibers, scientists can detect vibrations from above ground thanks to the way the cable is slightly deformed. When a car rolls through the subterranean cable or a person walks near it, the Earth will transmit its unique seismic signature. So, without visually observing the surface, scientists can paint a detailed picture of how a once crowded community has come to a halt, slowly returning to life as the lockdown eases.

They could say, for example, that foot traffic on campus almost disappeared in April after the lockdown began, and kept going until June. But after initially dropping, car traffic started to pick up,” says Penn State seismologist Tiyuan Zhou, lead author on new research. paper Description of the work in the magazine seismic record. “This fiber-optic cable can actually distinguish such a fine signal.”

More specifically, it’s a file Frequency in reference. Human footsteps generate vibrations at frequencies between 1 and 5 Hz, while car traffic is similar to 40 or 50 Hz. Vibrations from construction machinery jump above 100 Hz.

Fiber-optic cables work by trapping light pulses and transmitting them over vast distances as signals. But when a car or a person passes through the sky, the vibrations cause a disturbance or a defect: a tiny amount of that light Spreads back to the source. Because the speed of light is a known quantity, researchers at Penn State can shine a laser through a single optical fiber strand and measure vibrations at different lengths of cable by calculating the time it takes the scattered light to travel. This technology is known in Earth sciences as Distributed Acoustic Sensing, or DAS.

A conventional seismograph, which records vibration by the physical movement of its internal parts, only measures activity in one location on the Earth. But using this technology, scientists can sample more than 2,000 spots along 2.5 miles of cable – one every six and a half feet – giving them super-resolution of aboveground activity. They did so between March 2020, when the shutdown began, and June 2020, when businesses at State College began reopening.

Just from those vibrating cues, DAS can show that on the west side of the campus, where a new parking garage was under development, there was no industrial activity in April as construction halted. In June, researchers not only detected vibrations from the restarted machines, but were actually able to identify the construction vehicles, which were rushing along the lower frequency. However, they noted that by this time pedestrian activity on the campus had hardly recovered, although some epidemiological restrictions had eased.

DAS can be a powerful tool for tracking people’s movement: Instead of sifting through cell phone location data, researchers can instead make use of fiber-optic cables to track pedestrian and car traffic. But technology can’t exactly do that Specify car or person. “You can say if it’s a car, or a truck, or a bike. But you can’t say, ‘Oh, this is a Nissan Sentra, 2019’,” says Ariel Lelloch, a geophysicist at Stanford University, who uses DAS but was not involved this study but did a review. “Anonymization of the DAS is actually one of the biggest benefits.”

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