According to Biondo Biondi, a professor of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences, such dense networks could be turned into an inexpensive “billion sensors” observatory for continuously monitoring and studying earthquakes. Instead of relying on dedicated and expensive seismic sensors placed far apart, the researchers install a laser interrogator, an instrument provided by company OptaSense, at one end of an optical fiber.
The instrument sends pulses of laser light into the fiber and monitors the backscattered light. Any changes in the timing of that backscatter can be correlated to displacements of the fiber as it stretches or contracts when the ground moves during an earthquake. And a single interrogator can cover some 40 kilometers of fiber, according to Biondi, akin to monitoring a virtual sensor every couple of meters.
“We can continuously listen to the Earth using pre-existing optical fibers that have been deployed for telecom purposes,” Biondi explains.
As a proof-of-concept, the researchers have been recording seismic tremors in a 3-mile loop of optical fibre installed beneath the Stanford University campus. Since the fiber optic seismic observatory began operation in September 2016, it has recorded and catalogued more than 800 events, ranging from manmade events and small, barely felt local temblors to powerful, deadly catastrophes like the recent earthquakes that struck more than 2,000 miles away in Mexico. In one experiment, the underground array picked up signals from two small local earthquakes with magnitudes of 1.6 and 1.8.