New UW Photonic Sensing Facility will use fib

The fiber-optic cables that travel underground, along the seafloor and into our homes have potential besides transmitting videos, emails and tweets. These signals can also record ground vibrations as small as a nanometer anywhere the cable touches the ground. This unintended use for fiber-optic cables was discovered decades ago and has had limited use in military and commercial applications.

A University of Washington pilot project is exploring the use of fiber-optic sensing for seismology, glaciology, and even urban monitoring. Funded in part with a $473,000 grant from the MJ Murdock Charitable Trust, a nonprofit based in Vancouver, Washington, the new UW Photonic Sensing Facility has three decoder machines, or “interrogators,” that use photons traveling through a fiber-optic cable to detect ground motions as small as 1 nanometer.

“Fiber-optic sensing is the biggest advance in ground-based geophysics since the field went digital in the 1970s,” said principal investigator Brad Lipovsky, a UW assistant professor of Earth and space sciences. “The UW Photonic Sensing Facility and its partners will explore this technology’s potential across scientific fields — including seismology, glaciology, oceanography and monitoring hydrology and infrastructure.”

The new center — the largest in the United States and the first of its kind in the Pacific Northwest — is among a handful of research hubs around the world that are beginning to explore fiber optics for sensing ground motion. This approach to monitoring could expand the amount of seismic data by thousands of times.

For now, one of the three UW interrogator machines is hooked up to a “dark fiber,” or unused cable, that runs between the UW campuses in Seattle and Bothell. The researchers will soon also connect to a similar underwater cable across Alaska’s Cook Inlet to sense volcanic, oceanic, glacial and tectonic systems there. The other equipment will be used for temporary deployments.

When the ground vibrates — due to a heavy truck, construction work, or an earthquake — the seismic waves travel out from the source like ripples on a pond. When a seismic wave reaches the fiber-optic cable, the cable stretches very slightly, and that disrupts photons that are naturally reflected back to the source. The researchers can detect this disruption in the returning light waves and determine where the cable was disturbed.

The technique is known as “distributed acoustic sensing,” or DAS, because the system is spread out and can be used to monitor both sound waves and ground motion.

The same technology can also record more gradual motions. Lipovsky, who studies glaciers, and UW graduate student John-Morgan Manos carried equipment up to Easton Glacier on Mount Baker to monitor the rate of surface melt. The team installed a cable and used an interrogator to see how much snow was melting on the glacier.

In other pilot projects, UW researchers with the Pacific Northwest Seismic Network are exploring uses for seismology, including earthquakes, volcanoes and landslides. UW oceanographers will use fiber-optic cables connecting to a seafloor observatory to monitor ocean faults and even eavesdrop on whales. UW civil engineers will study whether this technology could monitor traffic collisions or building and bridge infrastructure.

The facility will include semi-permanent observatories in Seattle and other unused “dark” fibers, including a cable that runs to Whidbey Island. The team also plans to lay cables for temporary field deployments at Mt. Rainier and is exploring projects at a fjord in Greenland and at McMurdo Station in Antarctica.

“We’re getting to the ‘smart Earth’ concept, where we can listen to the Earth,” said Marine Denolle, a UW assistant professor of Earth and space sciences. “This technology allows seismic sensing to go to places you could not go before — where it was too hard, or too expensive, to deploy sensors. The other aspect that’s new is a density of sensors beyond what we had before.”

Today’s seismometers record ground motion at a single point, whereas fiber-optic cables take measurements at many points along the cable — the test cable has 15,000 data channels. Denolle will use computing and machine learning to make sense of this new mountain of seismic data.

“In seismology, our data used to be just wiggles,” Denolle said. “This is the first time we can get 2D images, and even videos, of data streaming in.”

The grant was awarded in late 2021. Researchers have used the funds to hook up and test the equipment last spring, and a data-visualization room on campus is coming soon.

“Thank to the MJ Murdock Charitable Trust’s support, the UW is the first university to acquire so much equipment for this technique,” ​​Lipovsky said. “This is in the pilot experiment stage, and we are excited to see where it goes.”

Other funders are the UW and the UW-based Pacific Northwest Seismic Network.

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