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How underground fiber optics spy on moving humans

last spring Lockdowns have quieted down the Penn State campus and surrounding State College towns. The jury’s instrument was “listening.” A team of university researchers used underground telecommunication fiber-optic cables. which runs two and a half miles across campus. and turn it into a kind of scientific surveillance device.

by projecting laser light through optical fiber Scientists can detect tremors from the ground Because the cable is slightly deformed as cars pass by underground cables or passers-by The ground transmits their unique seismic waves, so without visual observation of the surface, Scientists can paint a detailed portrait of this once bustling community that has come to a halt. and slowly resurrected As the lockdown eases

For example, they could tell that campus traffic almost disappeared in April after the lockdown began and remained until June. but after the first drop Car traffic is starting to improve. “You can see very few people walking compared to a normal day. But vehicle traffic is almost back to normal,” said Penn State seismologist Tieyuan Zhu, lead author on a new paper describing the journal work. earthquake record. “This fiber optic cable really can distinguish subtle signals.”

In particular, it is frequency In the signal, human footsteps create vibrations with frequencies between 1

and 5 hertz, while car traffic is over 40 or 50 hertz, construction machinery tremors exceed 100 hertz.

Fiber optic cables work by perfectly trapping the pulses of light and transporting them over vast distances as a signal. But when a car or a person passes overhead The oscillations cause interference or imperfections: a small amount of light is scattered back to the source. Because the speed of light is a known quantity, Penn State researchers were able to project a laser light through a single optical fiber and measure the vibrations at different lengths of the cable, calculating the time it takes for the scattered light to travel. The technique is known in geosciences as diffuse acoustic sensing, or DAS.

Traditional seismograph, which records vibrations with the physical movement of internal parts. It only measures activity at a single location on Earth. but with this technique Scientists were able to sample more than 2,000 points along a 2.5-mile cable every six and a half feet, giving them excellent resolution of above-ground activity. They did this between March 2020 when lockdown and June 2020 when businesses at State College began to reopen.

Based on those vibration signals, the DAS was able to show that on the west side of the campus, where a new parking garage is under development. There was no industrial activity in April as construction was halted. in june The researchers not only detected vibrations from restarted machines. but also be able to choose vehicles that are actually used in construction which hums at a lower frequency. at this time Pedestrian activity on campus has barely recovered. Although some epidemic restrictions are eased

DAS could be a powerful tool for tracking people’s movements: instead of filtering cell phone location data Instead, researchers could use fiber-optic cables to track pedestrian and car corridors. But technology certainly can’t. digest Cars or people: “You can say this is a car, a truck, or a bike, but you can’t say, ‘Oh, this is the 2019 Nissan Sentra,'” says Stanford geophysicist Ariel Lellouch, who used the DAS. but was not involved in this study. “The anonymity of DAS is one of its greatest benefits.”

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