Stanford scientists turn fibre optic network into a massive earthquake sensor

Scientists from Stanford University have devised a way to make use of fibre optic networks as earthquake sensors. While fibre optical cables have been used to transmit high speed data reliably for years now, this is the first time that a network has been put to use as a sensor in this manner.

Professor Biondo Biondi, a geophysicist, has been developing this system for over a year now, reported Spectrum. A 4.8km testing loop was setup at the university to record disturbances and vibrations from the Earth. The trick was to separate the vibrations coming from other sources, like traffic and ambient disturbances.

Since the time it was setup in September 2016, the report pointed out that the team was able to record 800 events. This included the massive quake that hit Mexico on 19 September this year, killing over 220, as well as rumbles caused by explosions in quarries in California.

The detection system can also differentiate between the two types of earthquake waves – the P and S waves. P waves travel faster, but S waves cause more damage. So knowing the difference is an integral part of any early warning system, noted the report.

Eileen Martin, a graduate student working on the project, said, "People didn't believe this would work. They always assumed that an uncoupled optical fibre would generate too much signal noise to be useful."

The report mentioned that while using fibre optic cables to detect seismic movements is not new technology, this is the first time such cables have been used without encasing them in cement, or stabilising the cables and balancing them using pre-installed pipelines, for example. Oil rigs and gas companies have used similar methods.

Biondi's project simply uses loose cables encased in plastic pipes like regular communications cables.

The technology works because there are inherent impurities, or flaws, in optical fibres and each of them can be used as a sensor, noted the report. This is done by passing beams of laser through one end using an "interrogator" and studying the "backscatter" that arrives at the other end of the cable. There are changes in the backscatter that occur when there are changes in the wire, like stretching or contracting, and this tends to happen when the Earth shakes.

One interrogator can cover up to 40km and can monitor the sensors at fixed intervals. Biondi said that the technology is getting better with higher resolutions and sharper results.

"A network of millions or billions of sensors already exists in the telecom lines," he added, saying that it just needs to be tapped into.

Biondi further noted that while the images put out by dedicated seismic instruments are going to be a lot more accurate and crisper than the virtual ones, they are very expensive as well.

According to him, this is why earthquake networks cannot afford to have enough of them constantly at work. "I'm talking about having a sensor every few metres. So, an array of these sensors can be more sensitive than an individual seismic sensor. It might let us see small earthquakes that can't be identified with normal sensors."

His team is preparing to test out this system over a broader network starting next year.