When spring passed the lock was silenced by the Penn State campus and the surrounding city of State College, an instrument set up by the jury to “listen”. A team of researchers from the university eavesdropped on an underground telecom optical cable that runs two and a half miles across campus and turned it into a kind of scientific surveillance device.
By shining the laser through an optical network, the scientists were able to detect vibrations above the ground thanks to the way the cable is so slightly deformed. As the car rolled over an underground cable or a person passed by, the ground would give off their unique seismic signature. Thus, without visual surveillance of the surface, scientists could portray in detail how the once vibrant community came to a halt and slowly revived as the locking loosened.
We could say, for example, that pedestrian traffic on campus almost disappeared in April after the lock began and was left without June. But after initially declining, vehicle traffic began to accelerate. “You can see that people are still walking very minimally compared to normal days, but vehicle traffic has actually returned to almost normal,” says Penn State seismologist Tieyuan Zhu, lead author of the new paper description of work in a journal Seismic record. “This optical cable can actually distinguish such a subtle signal.”
More precisely, it is frequency in the signal. The human step generates vibrations with frequencies between 1 and 5 hertz, while car traffic is more like 40 or 50 hertz. The vibrations of construction machines jump over 100 hertz.
Optical cables work by perfectly capturing light pulses and transmitting them over long distances as signals. But when a car or a person passes over them, the vibrations introduce a disturbance or imperfection: a small amount of that light it disperses back to the source. Since the speed of light is a known quantity, researchers in Penn State could illuminate a laser through a single optical beam and measure vibrations at different cable lengths by calculating the time it takes for scattered light to travel. The technique is known in geoscience as distributed acoustic sensor or DAS.
A traditional seismograph, which registers vibration during the physical movement of its internal parts, measures activity in only one place on Earth. But using this technique, scientists could sample over 2,000 points along a 2.5-mile cable – one every 6 and a half feet – giving them super-fine resolution activity above ground. They did so between March 2020, when locking began, and June 2020, when businesses at State College began reopening.
Only from these vibrational signals could the DAS show that on the west side of the campus, where a new parking garage was under construction, there was no industrial activity in April because construction stopped. In June, researchers not only discovered the vibrations of restarted machines, but were actually able to single out construction vehicles, which were humming at a lower frequency. Still, they noted, by then, walking activities on campus had barely recovered, although some of the pandemic’s constraints had eased.
The DAS could be a powerful tool for tracking people’s movements: Instead of searching for cell phone location data, researchers could instead touch optical cables to track the passage of pedestrians and cars. But technology can’t do just that identify car or 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 Stanford University geophysicist Ariel Lellouch, who uses DAS but was not included in the study but reviewed it. “The anonymity of the DAS is actually one of the biggest benefits.”