Turning the Optical Fiber Network into a Giant Earthquake Sensor
The best earthquake monitoring system is already installed under our feet, say Stanford researchers
Optical fibers can do more than transmit data—they can actually sense what’s going on around them, including the earliest rumbles of an earthquake.
For the past year, Biondo Biondi, a professor of geophysics at Stanford University, has used a 4.8-kilometer (or 3-mile) test loop of optical fibers installed on the Stanford campus to record vibrations caused by earthquakes, and distinguish those from vibrations caused by other sources, such as passing cars.
His team has recorded 800 seismic events using this fiber optic seismic observatory since September 2016, including signals from the recent Mexico earthquake and vibrations from blasting at quarries in the area. The fibers can distinguish between two types of earthquake waves, the P wave and the S wave. That’s important for earthquake warning systems, because P waves travel faster but S waves cause more damage.
Using optical fibers to monitor seismic events is not a new technology—it’s standard operating procedure for oil and gas companies. However, this involves first stabilizing the fibers by attaching them to a surface, like a pipeline, or encasing them in cement. Biondi’s project used loose fiber optic cables laying inside plastic pipes, mimicking a standard optical communications installation.
“People didn’t believe this would work,” said Eileen Martin, a graduate student on the project, in a prepared statement. “They always assumed that an uncoupled optical fiber would generate too much signal noise to be useful.”
Both stabilized and loose optical fiber seismic monitoring systems work by using innate impurities in optical fibers as virtual sensors. An “interrogator” installed at one end of the line sends pulses of laser light into the fiber and monitors the light that bounces back—the backscatter. Changes in the timing of backscattering occur when the fiber stretches and contracts—something that happens when the ground moves during an earthquake.
A single interrogator can cover some 40 kilometers of fiber, Biondi says, and monitor a virtual sensor every couple of meters. The resolution of these systems is steadily improving. His project has demonstrated, he says, that “a network of millions or billions of sensors already exists in the telecom lines,” he says, we just have to tap into it.
Biondi, whose main research interest has involved turning seismic data collected by oil and gas exploration companies into images of geological structures, started thinking about optical fibers about five years ago, when his home was wired as part of an early trial of the Google Fiber project. “It seemed so simple to install,” he told me. Then Martin, during a summer research stint at a national laboratory, told him about researchers she’d met who were planning to use fiber optic cables to monitor the thawing of the permafrost in Alaska. [READ MORE]