That Was Then, This Is Now: Fiber Optic Cables Find New Use as Seismic Sensors

ROBIN RILEY, WEB EDITOR, robin.riley@photonics.com

The decade that gave us the Sony PlayStation also gave us dark fiber — an excess of optical fiber cables installed underground, mostly in the 1990s, before advances in data transmission reduced the need for all those cables. Now, research teams on the earthquake-prone West Coast of the U.S. are putting dark fiber optic cables to use as sensor arrays for seismic monitoring.

Map shows location of a three-mile, figure-8 loop of optical fibers installed beneath the Stanford campus as part of the fiber optic seismic observatory. Courtesy of Stamen Design and the Victoria and Albert Museum.

Scientists at the Lawrence Berkeley National Laboratory (Berkeley Lab) and Stanford University have shown that dark fiber networks can be used for sensing earthquakes, the presence of groundwater, changes in permafrost and a variety of other subsurface activity.

In a project using laser interrogators provided by OptaSense, Stanford researchers have recorded more than 800 seismic events by monitoring a three-mile loop of optical fiber installed in existing telecommunications conduits beneath the Stanford University campus. A parallel study, conducted by Berkeley Lab scientists, utilized two networks installed in Richmond, Calif. and Fairbanks, Alaska to acquire similar datasets using a laser interrogator provided by Silixa Ltd. The two teams worked together to compare results, as discussed in a recent article in Geophysical Research Letters. The teams told Photonics Media that ongoing experiments are being conducted in California’s Central Valley and the Mojave desert in collaboration with several other institutions.

The two teams have reported seismic monitoring results comparable to those achieved with conventional seismometers using distributed acoustic sensing (DAS), a technology that measures seismic wavefields by shooting short laser pulses across the length of the fiber. Tiny impurities in the fiber cause the laser light to scatter. If the fiber is stationary, the backscatter signal stays the same. But if the fiber starts to stretch in some areas, due to vibrations or strain, the signal changes.

Shan Dou (from left), Jonathan Ajo-Franklin and Nate Lindsey were on a Berkeley Lab team that used fiber optic cables for detecting earthquakes and other subsurface activity. Courtesy of Berkeley Lab.

“When the fiber is deformed, we will see distortions in the backscattered light, and from these distortions, we can measure how the fiber itself is being squeezed or pulled,” Jonathan Ajo-Franklin, a researcher at Berkeley Lab, said.

“We got interested in DAS due to research at LBNL in a related technology that uses fiber to measure temperature — distributed temperature sensing,” Ajo-Franklin told Photonics Media. “We had used DTS to monitor deep wells which are difficult to instrument. Since I’m a seismologist, when DAS started developing we immediately jumped on the technology to help instrument wells for seismic measurements . . .

“Oil and gas wells . . . are tough on traditional sensors (geophones). Fibers, when correctly packaged, can be very rugged and handle these environments, hence the DAS technology is a great match for borehole geophysics,” Ajo-Franklin said.

The Berkeley Lab and Stanford teams conducted two studies related to the use of DAS for seismic sensing.

Using DAS for Near-Surface Seismic Monitoring

In the first study, documented in a recent article in Scientific Reports, the teams demonstrated the efficacy of near-surface seismic monitoring using DAS-recorded ambient noise. Results of the study showed that as a low-cost, dense array, DAS could be useful in establishing smarter systems for monitoring the Earth’s near surface.

“The idea is that by using fiber that can be buried underground for a long time, we can transform traffic noise or other ambient vibrations into usable seismic signals that can help us monitor near-surface changes such as permafrost thaw and groundwater-level fluctuations,” said researcher Shan Dou, also from Berkley Lab.

DAS and Dark Fiber for Earthquake Detection

In a follow-up study, Berkeley Lab and Stanford researchers demonstrated the viability of using fiber-optic cables for earthquake detection.

Using DAS, the two teams took independent measurements on fiber-optic arrays at two locations in California — including the Stanford University campus — and one in Alaska. In all three cases, DAS proved to be about as sensitive to earthquakes as conventional seismometers, despite its higher noise levels. Using the DAS arrays, the Berkeley Lab and Stanford teams assembled a catalog of local, regional and distant earthquakes and showed how novel processing techniques could be used to take advantage of DAS’ many channels, in order to better understand where earthquakes originate.

According to researchers, a seismic recording approach using DAS, in contrast to traditional seismometers, would be relatively inexpensive to implement and operate.

“Every meter of optical fiber in our network acts like a sensor and costs less than a dollar to install,” said Biondo Biondi, a professor of geophysics at Stanford. “You would never be able to create a network using conventional seismometers with that kind of coverage, density and price.”

The Fiber Optic Seismic Observatory successfully detected the 8.2 magnitude earthquake that struck Central Mexico on Sept. 8, 2017. Courtesy of Siyuan Yuan.

Since the fiber optic seismic observatory at Stanford began operation in September 2016, it has recorded and cataloged more than 800 events, ranging from manmade events and small local temblors to events like the 2017 earthquakes in Mexico, which were more than 2,000 miles away from the observatory. In one particularly revealing experiment, the underground array picked up signals from two small local earthquakes with magnitudes of 1.6 and 1.8.

“As expected, both earthquakes had the same waveform, or pattern, because they originated from the same place, but the amplitude of the bigger quake was larger,” Biondi said. “This demonstrates that the fiber optic seismic observatory can correctly distinguish between different magnitude quakes.”

The array also distinguished between two different types of waves that travel through the Earth, called P and S waves.

“One of our goals is to contribute to an early earthquake warning system. That will require the ability to detect P waves, which are generally less damaging than S waves but arrive much earlier,” Stanford graduate student Eileen Martin said.

Dark Fiber Benefits

Berkley's Ajo-Franklin said that dark fiber allows for dense spatial sampling, because data points are only meters apart, whereas traditional seismometers are typically separated by many kilometers.

Traditional seismometers are expensive to install and maintain, but dark fiber is installed everywhere, including in subsea locations.

One end of the fiber needs to be physically accessible, researchers told Photonics Media, so that there is a place where the laser pulse can be initiated. But the rest of the fiber can be anywhere — down a deep well, on the bottom of the ocean or inside a concrete block, for example.

“Fiber has a lot of implications for earthquake detection, location and early warning,” said Nate Lindsey, who is a UC Berkeley graduate student. “Fiber goes out in the ocean, and it’s all over the land, so this technology increases the likelihood that a sensor is near the rupture when an earthquake happens, which translates into finding small events, improved earthquake locations, and extra time for early warning.”

The researchers told Photonics Media that the primary challenge to monitoring dark fiber undersea is the limits to the distance over which DAS can be utilized. Each interrogator can “sense” between 10 and 50 kilometers depending on the specifics of the technology, and undersea cables can be thousands of kilometers long.

The work done by the Berkeley Lab and Stanford teams is the first step toward developing a city-wide seismic network in the San Francisco Bay area.

The research was published in Scientific Reports (doi: 10.1038/s41598-017-11986-4). Geophysical Research Letters (doi: 10.1002/2017GL075722), and The Leading Edge (doi: 10.1190/tle36121025.1).

The research is also being presented at the American Geophysical Union Fall Meeting, December 11-15, 2017. There are two presentations: Dark Fiber and Distributed Acoustic Sensing: Applications to Monitoring Seismicity and Near-Surface Properties, and Earthquake Recording at the Stanford DAS Array With Fibers in Existing Telecomm Conduits.