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Last spring the blockade calmed the Penn State campus and the surrounding State College town, which was “listening” to an instrument placed by the jury. A team of university researchers introduced an underground fiber-optic telecommunications cable, two and a half miles across campus, and turned it into a sort of scientific surveillance device.
By shining a laser through an optical fiber, scientists can detect vibrations on the ground thanks to the way the cable is slightly so deformed. Like a car crossing a subterranean cable or a person walking by, the earth would transmit its unique seismic signature. So without visually examining the surface, scientists were able to make an accurate portrait of how a noisy community at one time stopped and slowly came back to life when the blockade was eased.
They could say, for example, that pedestrian traffic on campus disappeared almost in April after the closure began, and stopped until June. But after the initial downturn, vehicular traffic began. “People are still seen to be walking very low compared to normal days, but vehicle traffic has almost returned to normal,” says Penn State seismologist Tieyuan Zhu. paper describing the work in the journal Seismic record. “This fiber optic cable can distinguish a subtle signal.”
More specifically, it is frequency at the signal. A human step produces vibrations with frequencies between 1 and 5 Hz, while car traffic is 40 or 50 Hz. The vibrations of the construction machinery rise above 100 hertz.
Fiber optic cables work by capturing light pulses perfectly and transporting them over long distances as a signal. But when a car or a person goes through the head, the vibrations cause a disturbance or error: a small amount of that light. it returns to the source. Because the speed of light is known, Penn State researchers can shine a laser through a single fiber-optic wire and measure vibrations at different cable lengths by calculating the time it took for the scattered light to travel. The technique is known as a distributed acoustic sensor or DAS.
The traditional seismograph, which records vibrations with the physical movement of internal parts, measures activity only at one location on Earth. But using this technique, scientists were able to taste more than 2,000 points along the 2.5 kilometers of the cable (one every six and a half meters), giving a fine resolution of activity on the ground. They did so in March 2020, when the lockout was imposed, and in June 2020, when the State College businesses began to reopen.
From these divisive signs alone, DAS was able to show that a new parking garage was being developed to the west of the campus when there was no industrial activity in April when construction stopped. In June, researchers were able to not only detect vibrations of restarted machinery, but also construction vehicles with a lower frequency to pick up rumors. However, they noted that pedestrian activity on the campus had barely recovered by then, although some pandemic reductions had eased.
DAS could be a powerful tool for tracking people’s movement: instead of analyzing cell phone location data, researchers can turn to fiber-optic cables to track pedestrians and cars. But technology can’t exactly identify car or person. “You can tell if it’s a car, a truck or a bicycle. But you can’t say ‘Oh, this is the Nissan Sentra, 2019’, ”says Stanford University geophysicist Ariel Lellouch, who uses DAS but has not been involved in this research but has reviewed it.“ Really the anonymity of DAS is one of the biggest advantages. ”
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