Dec 12 2019
Telecommunications lines built for carrying internet and phone service can sense the rumble of thunder underground, potentially offering researchers with a new method of detecting environmental risks and imaging deep within the Earth.
Image Credit: AGU
The new study being presented currently at AGU’s Fall Meeting and reported in AGU’s Journal of Geophysical Research: Atmospheres marks the first time thunder has been audible underground in a telecommunications fiber optic array, according to the authors of the study.
The new research employed The Pennsylvania State University’s current fiber network for internet and phone service as a distributed sensor array to detect the advance of thunderstorms as they passed the campus.
Conventional seismometers have recorded ground motions induced by thunder, known as thunderquakes, vibrating in the infrasound frequency range, lower than 20 Hertz, which cannot be heard by the human ear.
The fiber array, buried 1 m (3 ft) underground, sensed a broader frequency range heard in a crash of thunder. The detected bandwidth, from 20 to 130 Hz, is consistent with microphone recordings of thunder and offers more information about the event, the researchers learned.
In April 2019, Penn State geophysicist Tieyuan Zhu and meteorologist David Stensrud got access to the university’s telecommunication fiber optic cable. They were listening for mild vibrations from a range of environmental effects, such as flooding and sinkhole formation.
Once we set up, we found a lot of very strong events in our fiber optic data, so I was very curious, what’s the cause of these signals?
Tieyuan Zhu, Geophysicist, Penn State University
The scientists identified a match upon synchronizing their results with data from the U.S. National Lightning Detection Network. “We thought, yeah, this is exactly the thunderstorm data, actually recorded by our fiber array.”
When lightning passes, the air is heated so rapidly that it forms a shockwave that sounds like thunder. Vibrations from loud events such as meteor explosions, lightning, and aircraft sonic booms pass from the air to Earth’s surface, vibrating the ground.
Fiber optic cables transport telecommunications information in bursts of laser light conducted by transparent glass strands measuring about as thick as a strand of human hair. Vibrations in the Earth — for example, those caused by earthquakes, thunderstorms, or hurricanes — compress or stretch the glass fibers, leading to a mild change in light intensity and the time taken by the laser pulse to move to its endpoint.
The scientists monitored these aberrations to track ground motion, changing the laser pulses back to acoustic signals.
The laser is very sensitive. If there is a subtle underground perturbation, the laser can detect that change.
Tieyuan Zhu, Geophysicist, Penn State University
Several kilometers of nonstop fiber underlay Penn State’s campus, which implies that the array can serve as a network of over 2,000 seismometers positioned every 2 m along the cable path. With this high density of sensors, the scientists can compute the location of origin of the thunder, potentially differentiating between cloud-to-cloud and cloud-to-ground lightning.
Compared to the seismometers, the fiber optic array can provide fabulous spatial, and also temporal, resolution. We can track the thunderstorm source movement.
Tieyuan Zhu, Geophysicist, Penn State University
According to the scientists, the new study shows fiber optic networks under urban areas are an unexploited resource for monitoring environmental risks. They can also be potentially used for exploring the crust and deep structures of the Earth, which cannot be measured openly.
Researchers learn about the inner side of the planet by noticing the way seismic waves from earthquakes change as they pass through it. Ground motions triggered by thunderstorms, which are a lot more recurrent than earthquakes on the east coast of North America, could help expose the concealed shapes of Earth’s interior, stated Zhu.
Sound of a thunderquake
Video Credit: AGU