Researchers Use Underground Optical Fiber Networks To Detect Earthquakes

Optical fibers are used across the globe to transmit data. They enable superfast internet speeds and form the basis of the World Wide Web. They bring you everything from the latest leak of a Game of Thrones episode to live updates on terror attacks. Now, a team of researchers from Stanford University has used it to detect earthquakes and possibly develop a warning system.

Biondo Biondi, a professor of geophysics at Stanford University, used a 3-mile loop of optical fibers on the Stanford campus to record seismic vibrations caused by tectonic activity. His team successfully isolated vibrations from nearby sources to focus only on seismic activity and feel that this breakthrough could give us the most efficient earthquake warning system yet.

According to a report published in the Institute of Electrical and Electronics Engineers Spectrum website, "his team has recorded 800 seismic events using this fiber optic seismic observatory since September 2016, including signals from the recent Mexico earthquake and vibrations from blasting at quarries in the area."

The team also added that the fibers can distinguish between the two types of earthquake waves, the P-wave and the S-wave. This is crucial for earthquake warning system because P-waves travel faster but S-waves cause more damage. Detecting the P-waves early is pivotal in predicting earthquakes.

Though this aspect of optic fiber technology is already used by oil and gas companies, this study is the first comprehensive look at how to implement it on a wider scale. The companies use the fibers by attaching them to a surface, like a pipeline, or encasing them in cement. The team, however, used loose fiber optic cables lying inside plastic pipes, mimicking a standard optical communications installation.

The team faced a lot of skepticism when it came to this project because of the far-fetched idea it posed. Using well-established, previously used technology for a purpose that is so widely different from why it was created caused a few raised-eyebrows, said Eileen Martin, a graduate student on the project. "They always assumed that an uncoupled optical fiber would generate too much signal noise to be useful," she added in the report.

The team used a device called an "interrogator" to conduct their study. When attached at one end of the optic fiber, it emits a pulse of light that travels through the fiber and bounces back. The changes in the timing of backscattering were studied and the differences were calculated. The team found that when the fiber stretches and contracts — something that happens when the ground moves during an earthquake — there were changes in this value.

The quality of the reading while using optical fiber-based virtual sensors will be always worse quality than the standard sensors a seismologist has out there, according to Biondi. But, the use of optic fiber is very inexpensive compared to traditional seismographs. Logically, there can only be one sensor every 20 km or so. "With fiber optics, I’m talking about having a sensor every few meters. So, an array of these sensors can be more sensitive than an individual seismic sensor; it might let us see small earthquakes that can’t be identified with normal sensors," he said in the report.

The team wants to have another test array over a broader area starting in 2018. This, he hopes, will allow his group to identify earthquakes that conventional networks in the area miss. "That would be a big step forward to getting the community and the government to pay attention." The team hopes that the wider data set will set bells ringing all over on how effective and efficient this can be.

A comprehensive paper describing the project will be published in the December issue of the geophysics journal, The Leading Edge, and details on the signal processing methods used will appear in the January issue of the IEEE Signal Processing Magazine.