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 of those cables. Now, researchers on the earthquake-prone West Coast of the U.S. are putting dark fiber optic cables to use as sensor arrays for seismic monitoring. Scientists at the Lawrence Berkeley National Laboratory and Stanford University have demonstrated 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, Stanford researchers have recorded more than 800 seismic events by monitoring a 3-mile loop of optical fiber installed in existing telecommunication conduits beneath the school’s campus. A parallel study conducted by Berkeley Lab scientists utilized two networks installed in Richmond, Calif., and Fairbanks, Alaska, to acquire similar data sets using a laser interrogator. The two teams compared results, which were included in an article published in Geophysical Research Letters. According to the researchers, ongoing experiments are being conducted in California’s Central Valley and the Mojave Desert, in collaboration with several other institutions. This map shows the location of a 3-mile, figure-eight loop of optical fibers installed beneath the Stanford University campus as part of the fiber optic seismic observatory. Courtesy of Stamen Design and the Victoria and Albert Museum. The seismic monitoring results are comparable to those achieved with conventional seismometers that employ distributed acoustic sensing (DAS). This is 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. However, if the fiber starts to stretch in some areas due to vibrations or strain, the signal changes. “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,” said Jonathan Ajo-Franklin, a researcher at Berkeley Lab. He noted that his team began studying DAS following research of “a related technology that uses fiber to measure temperature — distributed temperature sensing” (DTS), which they had used to monitor deep wells that are difficult to instrument. “When DAS started developing, we immediately jumped on the technology to help instrument wells for seismic measurements,” Ajo-Franklin said. “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.” Near-surface seismic monitoring The researchers demonstrated the efficacy of near-surface seismic monitoring using DAS-recorded ambient noise. Their results showed that as a low-cost, dense array, DAS could be useful in establishing smarter systems for monitoring 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 Berkeley Lab researcher Shan Dou. In a follow-up study, the 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 on the Stanford 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, a catalog of local, regional and distant earthquakes was assembled. This demonstrated how novel processing techniques could be used to take advantage of DAS’ many channels to better understand where earthquakes originate. According to the researchers, a seismic recording approach using DAS is relatively inexpensive to implement and operate compared to traditional seismometers. “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.” Since the fiber optic seismic observatory at Stanford began operation in September 2016, it has recorded and cataloged more than 800 events, ranging from those that were man-made and small local temblors to events such as the 2017 earthquakes in Mexico (more than 2000 miles 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. The fiber optic seismic observatory at Stanford University successfully detected the 8.2-magnitude earthquake that struck central Mexico on Sept. 8, 2017. Courtesy of Siyuan Yuan. “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,” said Eileen Martin, a graduate student at Stanford. “That will require the ability to detect P waves, which are generally less damaging than S waves but arrive much earlier.” Berkeley’s Ajo-Franklin said that dark fiber allows for dense spatial sampling because data points are only meters apart. Traditional seismometers are typically separated by many kilometers; they also are expensive to install and maintain. But dark fiber is installed everywhere, he said, including in subsea locations. One end of the fiber needs to be physically accessible 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, a graduate student at Berkeley. “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 note that distance limitations with DAS are the primary challenge to monitoring undersea dark fiber. Each interrogator can sense between 10 and 50 km depending on the specifics of the technology, and undersea cables can be thousands of kilometers long. The work done by Berkeley Lab and Stanford is the first step toward developing a citywide 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).