How the underground optical fiber monitors humans moving above

Last spring The blockade calmed the Pennsylvania State University campus and the surrounding town of State College, and the instruments operated by the jury were “listening.” A team of researchers at the university developed an underground telecommunications fiber optic cable that spans two and a half miles across the campus and turned it into a scientific surveillance device.

By shining a laser into an optical fiber, scientists can detect vibrations from the ground, thanks to the way the cable is slightly deformed. When cars drive over underground cables or someone walks by, the ground will transmit their unique seismic signals. Therefore, without visually monitoring the surface, scientists can describe in detail how a once bustling community stopped and then slowly returned to life as the blockade eased.

For example, they can see that in April after the lockdown began, the flow of people on campus almost disappeared, and it continued until June. But after the initial decline, vehicle traffic began to pick up. Zhu Tieyuan, a seismologist at Penn State University, said: “You can see that compared with usual, people still walk very few times, but vehicle traffic has actually almost returned to normal.” paper Describe the work in the journal Earthquake record“This optical cable can actually distinguish such subtle signals.”

More specifically, it is frequency In the signal. The frequency of vibration produced by human footsteps is between 1 and 5 Hz, while car traffic is more like 40 or 50 Hz. Vibration from construction machinery exceeds 100 Hz.

The working principle of fiber optic cables is to perfectly capture light pulses and transmit them as signals to long distances.But when a car or a person passes overhead, the vibration can introduce interference or defects: a small amount of light Back to the sourceSince the speed of light is a known quantity, researchers at Pennsylvania State University can irradiate laser light through an optical fiber bundle and measure the vibration of different lengths of cables by calculating the time required for the scattered light to propagate. This technology is called Distributed Acoustic Sensing or DAS in earth sciences.

Traditional seismographs record vibrations through the physical movement of its internal components, and only measure the activity of one location on the earth. But using this technique, scientists can sample more than 2,000 points — one every six and a half feet — along a 2.5-mile cable to provide them with ultra-fine resolution of ground activity. They did this between March 2020 (the lockdown began) and June 2020 (the state college businesses began to reopen).

From these vibration signals alone, DAS can indicate that a new parking lot under development on the west side of the campus has no industrial activity in April due to the suspension of construction. In June of this year, researchers not only detected the vibrations of restarted machines, but they could actually pick out construction vehicles that buzzed at a lower frequency. Nevertheless, they pointed out that although some epidemic restrictions have been relaxed, pedestrian activities on campus have hardly resumed at this time.

DAS can be a powerful tool for tracking people’s movements: Researchers can use fiber optic cables to track the passage of pedestrians and cars, instead of filtering mobile phone location data.But the technology can’t be complete confirm A car or a person. “You can say it’s a car, a truck, or a bicycle. But you can’t say,’Oh, this is a 2019 Nissan Sentra,'” said Stanford University geophysicist Ariel Lellouch. He Used DAS, but did not participate in this study, but it was peer reviewed. “In fact, the anonymity of DAS is one of the greatest benefits.”