[China Instrument Network Instrument Development] Although the current seismograph can accurately pick up slight tremors, their distance is relatively short. However, research at Stanford University now shows that a wide range of seismic detection networks can already exist at our feet: fiber optic cables carrying high-speed Internet.
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Since the optical fiber works by bounce the optical signal on a cable that is transparent like a glass plate, it can measure the small interference of the signal. This technology, called distributed acoustic sensing (DAS), has already worked in the oil and gas industry.
Eileen Martin, co-author of the study, said: "The working principle of DAS is that as light travels along the fiber, it encounters various impurities in the glass and rebounds. If the fiber is completely still, then the opposite The scattered signal always looks the same, but if the fiber starts to stretch in some areas - the signal will change due to vibration or strain.
To test whether these cables can be used to monitor and measure earthquakes, Stanford installed three miles (4.8 kilometers) of fiber in the shape of a number eight, equipped with a laser interrogator to record any motion.
It is said that the optical fiber seismic observation station set up by Stanford University can record more than 800 events in its first year of operation. This included explosions from nearby quarries, small earthquakes, and even a massive 8.2-magnitude incident on September 8 this year, which destroyed central Mexico and was set approximately 2,000 miles (3,220 kilometers) from Stanford University. On one occasion, the sensor picked up two earthquakes from the same source and its magnitude was 1.6 and 1.8.
Lead researcher Biondo Biondi said: “As expected, the two earthquakes were all the same waveforms or patterns because they originated in the same place, but the magnitude of the earthquake was even greater. This shows that the optical fiber seismic stations can correctly distinguish different magnitudes. ."
Similarly, seismic stations can also distinguish the difference between S-waves and P-waves, and shock waves with ripples at different speeds. P-waves are usually much earlier than S-waves, but are much weaker, so detecting them is the key to an effective early warning system.
Although their fiber optic seismic stations have shown some preliminary results, the researchers pointed out that traditional seismographs are still more sensitive to monitoring earthquakes, but fiber-optic seismic observation systems have other advantages, such as economy, wide-range, and so on.
Biondi said: “We can use the pre-existing fiber already deployed for telecommunication purposes to constantly listen to and monitor the earth. Every metre of fiber in our network is like a sensor, costs less than a dollar, and traditional seismographs create The network cannot have such coverage density and cost advantages."
(Original title: Stanford University: Fiber optic cable can be used to monitor earthquakes)
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