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 fibre. The instrument sends pulses of laser light into the fibre and monitors the backscattered light. Any changes in the timing of that backscatter can be correlated to displacements of the fibre as it stretches or contract when the ground moves during an earthquake. And a single interrogator can cover some 40 kilometres of fibre, according to Biondi, akin to monitoring a virtual sensor every couple of meters.
“We can continuously listen to the Earth using pre-existing optical fibres 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 fibre 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.
The array was proven to distinguish between two different types of waves that travel through the Earth, P waves which are generally less damaging but arrive much earlier, and S waves. This capability could be used as an early earthquake warning system.
Although seismometers used today are more sensitive than the proposed telecom array, their coverage is sparse and they can be challenging and expensive to install and maintain, especially in urban areas. In comparison, a seismic observatory like the one Biondi proposes would be relatively inexpensive to operate, with every meter of optical fibre in the network acting like a sensor, something that could not be achieved economically with seismic sensors.
The researchers envision that such a network would allow scientists to study even small earthquakes in greater detail, pinpointing their sources more quickly than is currently possible. The data could also be used by Civil engineers to visualize how buildings and bridges respond to small earthquakes.
Stanford University – www.stanford.edu