Feb 21, 2019

Fiber-Optic Cables Provide New Tool as Early Earthquake Sensors

Fiber-optic cables, like those in CENIC’s CalREN network and used for internet services everywhere, might eventually serve dual purposes — used not only to transmit data, but also to provide earthquake measurements, and maybe even safety alerts.

Using 20 miles of unlit fiber stretching from West Sacramento to Woodland, California, scientists at the Department of Energy’s Lawrence Berkeley National Laboratory have shown that fiber-optic cables can detect seismic activity with more spatial detail than traditional seismometer networks.

The study, published in February in Nature’s Scientific Reports, was the first to use a regional fiber-optic network for seismic sensing purposes. The study was conducted over seven months on Energy Sciences Network’s Dark Fiber Testbed, a 13,000-mile network of short and long haul telecommunication fiber designed for testing novel network communication equipment and protocols. “The unique part of the work here is that it had not been done over long distance telecommunication fiber before,” said ESnet Executive Director Inder Monga. “Telecommunications fiber gets installed in conduits, and there is a certain way to get it done. Previous research involved scientists laying their own fiber-optic cable.”

How It Works

Distributed acoustic sensing (DAS) is a novel technology that measures seismic wavefields by shooting short laser pulses across the length of fiber-optic cables. Distortions in the laser light can be used to measure small changes in the length of the cables as the ground is pushed and pulled by seismic waves. “You can use the approach to measure things like temperature, or strain, or in this case, acoustic waves,” said Jonathan Ajo-Franklin, LBNL Geophysics department head, and staff scientist. “You can basically use a very long fiber cable as a series of distributed seismometers.”

Recording was conducted on slightly more than 20 miles of fiber with a measurement every two meters, equivalent to 10,000 point sensors — all on ESnet’s dark fiber network. Ajo-Franklin and his team were able to monitor everything from the immediate soil conditions to nearby car and train activity to distant earthquakes in Peru. The wide-ranging coverage of ESnet’s testbed provided a more detailed and larger-scale picture of underground activity than seismometers. Ajo-Franklin and his team collected 300 terabytes of raw data.

This graph shows Berkeley researchers’ recordings of earthquakes on ESnet’s dark fiber testbed. Seismic events are sorted by increasing epicentral distance from Sacramento. (For details, see Figure 5 in the journal article.)

Faster, Cheaper Than Seismometers

Fiber-optic sensing could provide a faster method for earthquake detection than traditional seismometers. Using fiber decreases processing delays because earthquakes can be detected right where they occur and warnings can be immediately transmitted. Fiber can be used to detect P waves, which are weaker shock waves that arrive earlier than the more destructive surface waves of an earthquake. “You could potentially get notice of the earthquake a little bit earlier, and shave five or six seconds off the alert process, which in some cases is useful,” Ajo-Franklin said.

Fiber optic sensing is also more cost-efficient than seismometers. Seismometers can be difficult to install and expensive to maintain, whereas fiber-optic cables are already widespread in the ground. A glut of “dark” or unused fiber-optic cables in the United States resulted from telecommunication companies’ rush to install the infrastructure during the dot-com boom in the 1990s. Plus, technology has improved since that time, so fewer cables are needed. Once refined, fiber-optic sensing technology could potentially be applied to the West Coast’s early earthquake warning system.

Measuring Other Subsurface Activity

The hardest-to-reach areas, such as seismic activity in the ocean responsible for tsunamis, would especially benefit from fiber-optic sensing. Installing seismometers off-shore in the ocean or urban areas can be a challenge. “The offshore environment is one with almost no seismic sensors because it’s very expensive and cost prohibitive to install geophones or seismometers off the coast,” Ajo-Franklin noted. “But there are quite a few submarine fiber-optic cables, which could potentially be used for sensing. So, that’s a really exciting opportunity.”

Fiber can also be used to measure other environmentally and agriculturally important subsurface activity. The technology can be used to monitor groundwater levels, and soil and permafrost conditions by measuring near-surface seismic wave velocities. “This technique can be used to measure many other things that will be important to California and California farmers,” noted Monga.

Dual-Purpose Sensing and Data Transfer

Now that researchers know that a large fiber-optic network provides such detailed seismic and subsurface sensing, they plan to test the use of fiber for both sensing and data transmission. “The next step is using a narrow slice of wavelengths to do the measurement so that you can run it at the same time you’re moving data back and forth on the same fiber,” Ajo-Franklin said. “It hasn’t been done yet, but we’re hoping to do a proof of principle test in the next year if we can find the funds for it.”

Whether fiber-optic earthquake detection technology moves forward may depend on companies’ willingness to offer use of their cables at minimal or no cost. ESnet, which provides high-performance unclassified network infrastructure to DOE research sites across the country, including facilities for high-performance computing (HPC), data-intensive infrastructure, and nanoscience, offered free use of its fiber to Berkeley researchers for the study. ESnet continues to actively seek fiber-optic suppliers who might incorporate this capability into their equipment such that it does not interfere with data transmission.

“If the next step, which is having the sensors co-exist with data channels, comes to pass, then this would be very relevant for CENIC,” said Monga.

Related Content:

• Caltech: The Moving Earth, Micro to Mega
• Caltech: Q&A — Creating a Virtual Seismologist
• Caltech: Fast, Simple New Assessment of Earthquake Hazard

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