Thousands of miles of buried optical fibers crisscross California’s San Francisco Bay Area, delivering high-speed internet and HD video to homes and businesses. Biondo Biondi, a professor of geophysics at Stanford University’s School of Earth, Energy & Environmental Sciences, dreams of turning that dense network into an inexpensive “billion-sensor” observatory for continuously monitoring and studying earthquakes.
Over the past year, Biondi and his team has shown it’s possible to convert the jiggles of perturbed optical fiber strands into information about the direction and magnitude of seismic events.
The researchers have been recording those seismic jiggles in a three-mile loop of optical fiber installed beneath the Stanford campus with instruments called laser interrogators (provided by OptaSense, a partner in this research effort).
“We can continuously listen to the Earth using preexisting optical fibers that have been deployed for telecom purposes,” Biondi says.
At present, geologists monitor earthquakes mainly with seismometers, which are more sensitive than the proposed telecom array. However, their coverage is sparse, and they can be challenging and expensive to install and maintain—especially in cities.
By contrast, a seismic observatory such as the one Biondi proposes would be relatively inexpensive to operate. “Every meter of optical fiber in our network acts like a sensor and costs less than a dollar to install,” he says. “You will never be able to create a network using conventional seismometers with that kind of coverage, density, and price.”
The network will let scientists study earthquakes—especially smaller ones—in greater detail, and pinpoint their sources more quickly than is currently possible. Greater sensor coverage would also provide higher-resolution measurements of ground responses to shaking.
Civil engineers could take what they learn about how buildings and bridges respond to small earthquakes from the billion-sensor array and use that information to design buildings that can withstand greater shaking.
From Backscatter to Signal
Optical fibers are thin strands of pure glass about the thickness of a human hair. They are typically bundled together to create cables that transmit data signals over long distances by converting electronic signals into light. Biondi is not the first to envision using them to monitor the environment. A technology known as distributed acoustic sensing (DAS) already monitors the health of pipelines and wells in the oil and gas industry.
With DAS, light travels along the fiber and any impurities in the glass cause the light to be reflected back (backscatter). If the fiber were to remain stationary, that “backscatter” signal would always look the same. But if the fiber starts to stretch in some areas due to vibrations or strain, the reflected signal changes.
Previous attempts at kind of acoustic sensing required that the optical fibers be expensively mounted to a surface or encased in cement to maximize contact with the ground and ensure the highest data quality. In contrast, Biondi’s project beneath Stanford, dubbed the fiber optic seismic observatory, uses the same optical fibers as telecom companies, which lie unsecured and free-floating inside hollow plastic piping.
Some engineers did not believe this approach would work. They assumed an uncoupled optical fiber would generate too much signal noise to be useful.
But the fiber optic seismic observatory has been in operation since September of last year. Over that time, it has recorded and cataloged more than 800 events, ranging from manmade events and small, barely felt local tremors to powerful, deadly catastrophes such as the recent earthquakes that struck more than 2,000 miles away in Mexico. 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 says. “This demonstrates that fiber optic seismic observatory can correctly distinguish between different magnitude quakes.”
Crucially, the array also detected and distinguished between two different types of waves that travel through the Earth, called P and S waves. One goal of the project is to improve early earthquake warning. This will require the ability to detect P waves, which are generally less damaging that S waves but arrive much earlier.
The fiber optic seismic observatory at Stanford is just the first step toward developing a Bay Area-wide seismic network, Biondi says. There are still many hurdles to overcome, such as demonstrating that the array can operate on a city-wide scale.
