When it comes to optical fibers, the underground optical cables that transmit tons of information at a time are more familiar to us. But few would ever associate optical fibers with earthquake detection.
Recently, researchers from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) developed a Distributed Acoustic/Vibration Sensing (DAS/DVS) system with independent intellectual property rights. Using the existing communication optical cables, this system can be applied to earthquake detection, geological hazard prediction, subsurface structure imaging and so on.
When an earthquake occurs, the optical cable will be stretched or compressed due to the strain from seismic waves. As a result of elasto-optical effect of coherent laser in fiber optic medium, the amplitude as well as the phase of Rayleigh Scattering will be changed. Thus, by receiving and demodulating Rayleigh backscattering, information about seismic waves can be obtained.
The DAS technology takes advantage of the mentioned elasto-optical effect to detect and transmit external oscillation and sound wave signals. With a series of merits like excellent anti-interference performance, high integration level and long-distance transmission, DAS technology has created a new path for earthquake detection and subsurface structure imaging.
The key technology of this system includes DAS laser sources, detection of coherent light, demodulation of beat frequency signal and algorithm. And the equipment now has the capacity of detecting frequency between 10 mHz to 20 kHz, with positional accuracy of 3.5 m and detection length of up to 40 km.
Since deployed at Mount Zipeng in Hefei, the DAS equipment has detected a magnitude-2.3 earthquake in Dingyuan County, a 2.7-magnitude earthquake in Xuancheng, a magnitude-6.6 earthquake in Philippines and many other regional and global earthquakes.
Earthquakes detected by the DAS system. (Image by CHEN Yang et al)
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