Researchers have discovered that telecommunication lines, which carry internet and phone service, can pick up the rumble of thunder underground, an advance that can offer scientists a new way to detect environmental hazards, and image deep inside the Earth. The researchers, including those from The Pennsylvania State University (Penn State) in the US, said this is the first time thunder has been heard underground using a telecommunications fiber-optic array.
Explaining how the sound was produced in nature, the passage of lightning heats up the air so fast that it creates a shockwave we hear as thunder. Vibrations from loud events like lightning, meteor explosions, and aircraft sonic booms pass from the air to the Earth's surface, shaking the ground. The study, published in the Journal of Geophysical Research-Atmospheres, used the university's existing fiber network for internet and phone service as an array of distributed sensors to observe the progress of thunderstorms as they crossed the campus.
The researchers said conventional seismometers have recorded ground motions evoked by thunder, called thunder quakes. These thunder quakes vibrated in the infrasound frequency range below 20 Hertz (Hz) which is inaudible to the human ear. The study reported that the fiber array buried 1 meter underground picked up a wider range of the frequencies heard in a peal of thunder.
The sounds detected ranged from 20 to 130 Hz and were consistent with microphone recordings of the thunder, the measurements made this way provide more information about the event.
Tieyuan Zhu, study co-author from Penn State said, 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?.
The researchers said they found a match when they synchronized their results with data from the US National Lightning Detection Network.
Zhu said, we thought, yeah, this is exactly the thunderstorm data, actually recorded by our fiber array.
The study noted that fiber optic cables carry telecommunications information in bursts of laser light conducted by strands of transparent glass about as thick as human hair. Vibrations such as those created by thunderstorms, earthquakes, or hurricanes stretch or compress the glass fibers, causing a slight change in light intensity, and the time the laser pulse takes to travel to its destination. Using these aberrations, the researchers monitored ground motion, converting the laser pulses back to sound signals.
Zhu said, the laser is very sensitive. If there is a subtle underground perturbation, the laser can detect that change.
The researchers said Penn State's campus was underlaid with several kilometers of continuous fiber, which act like a network of more than 2,000 seismometers emplaced every two meters along the cable path.
Using this high density of sensors, the scientists said they can calculate the location where the thunder originated, helping to distinguish between cloud-to-ground and cloud-to-cloud lightning.
Zhu said, compared to the seismometers, the fiber optic array can provide fabulous spatial, and also temporal, resolution. We can track the thunderstorm source movement.
According to the researchers, the fiber optic networks under urban areas are an untapped resource for monitoring environmental hazards, also helpful in studying the crust and deep structures of the Earth, which cannot be measured directly.
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