In traditional seismology, researchers studying how the earth moves in the moments before, during, and after an earthquake rely on sensors that cost tens of thousands of dollars to implement. Because of that expense, only a few seismic sensors have been installed throughout remote areas of California, making it hard to understand the impacts of future earthquakes as well as small earthquakes occurring on unmapped faults.

Now researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have figured out a way to overcome these hurdles by turning parts of ESnet’s 13,000-mile-long testbed of unused fiber-optic cable, into a highly sensitive seismic activity sensor. 

Shaking up seismology with dark fiber

According to Jonathan Ajo-Franklin, a staff scientist in Berkeley Lab’s Earth and Environmental Sciences Area who led the study, there are approximately 10 million kilometers of fiber-optic cable around the world, and about 10 percent of that consists of unused, or ‘dark’ fiber.

These relics of the dot-com boom were installed when telecom companies rushed to supply vast networks of underground cable to meet the demands of a growing new industry. As the technology for transmitting data improved, however, fewer cables were needed, leaving behind a legacy of unlit dark fiber, waiting to be used.

In a 2017 study, the Ajo-Franklin group installed a fiber-optic cable in a shallow trench in Richmond, CA. This demonstrated that a new sensing technology called distributed acoustic sensing (DAS) could be used for imaging of the shallow subsurface. DAS measures seismic wavefields by shooting short laser pulses across the length of the fiber. A follow-up study demonstrated for the first time that fiber-optic cables could be used to detect earthquakes.

The current study uses the same DAS technique, but instead of deploying their own cable, the researchers ran their experiments on a 20-mile segment of the 13,000-mile-long Energy Sciences Network (ESnet) Dark Fiber Testbed that extends from West Sacramento to Woodland, California. 

“When Jonathan approached me about using our Dark Fiber Testbed, I didn’t even know it was possible [to use a network as a sensor]," said Inder Monga, Executive Director of ESnet and director of the Scientific Networking Division at Berkeley Lab. “No one had done this work before. But the possibilities were tremendous, so I said, ‘Sure, let’s do this!’”

Because the ESnet Testbed has regional coverage, the researchers were able to monitor seismic activity and environmental noise with finer detail than previous studies.

“Conventional seismic networks often employ only a few dozen sensors spaced apart by several kilometers to cover an area this large, but with the ESnet Testbed and DAS, we have 10,000 sensors in a line with a two-meter spacing,” said co-author Verónica Rodríguez Tribaldos, a postdoctoral researcher in Ajo-Franklin’s lab. “This means that with just one fiber-optic cable you can gather very detailed information about soil structure over several months.”

Digging deep for data underground

After seven months of using DAS to record data through the ESnet Dark Fiber Testbed, the researchers proved that the benefits of using commercial fiber are manifold.

“Just by listening for 40 minutes, this technology has the potential to do about 10 different things at once. We were able to pick up very low frequency waves from distant earthquakes as well as the higher frequencies generated by nearby vehicles,” said Ajo-Franklin.

The technology can also be used to characterize soil quality, provide information on aquifers, and be integrated into geotechnical studies, he added.

With such a detailed picture of the subsurface, the technology has potential for use in time-lapse studies of soil properties, said Rodríguez Tribaldos. For example, this tool could be used to detect long-term groundwater changes, the melting of permafrost, or the hydrological changes involved in landslide hazards.

All this means that researchers may no longer have to choose between data quality and affordability. “Cell phone sensors are inexpensive and tell us when a large earthquake happens nearby, but they will not be able to record the fine vibrations of the planet,” said co-author Nate Lindsey, a UC Berkeley graduate student who led the field work and earthquake analysis for the 2017 study. “In this study, we showed that inexpensive fiber-optics pick up those small ground motions with surprising quality.”

With 300 terabytes of raw data collected for the study, the researchers have been challenged to find ways to effectively manage and process the “fire hose” of seismic information.

Ajo-Franklin expressed hope to one day build a seismology data portal that couples ESnet as a sensor and data transfer mechanism, with analysis and long-term data storage managed by Berkeley Lab’s supercomputing facility, NERSC (National Energy Research Scientific Computing Center).

“Although it was completely unexpected that ESnet – a transatlantic network dedicated for research – could be used as a seismic sensor, it fits perfectly within our mission,” Monga said. “At ESnet, we want to enable scientific discovery unconstrained by geography.”

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