Underground fiber becomes a weapon for scientists to predict earthquakes

Bio ndo Biondi is a professor of geophysics at Stanford University who studies energy and environmental science. He envisions a billion-level sensor observatory through dense networks to continuously monitor and analyze earthquakes.

In the past year, Biondi's team has demonstrated that it is capable of predicting the magnitude and direction of earthquake occurrences from disturbing fiber optic filaments. Researchers have installed instruments in a fiber-optic loop three miles from the Stanford University campus.

Thousands of miles of underground fiber travel through California's San Francisco Bay area, providing high-speed Internet and HD video to businesses and homes. Seismographs used by researchers to monitor earthquakes, although telecommunications arrays are more sensitive, are sparsely covered and expensive, and have significant challenges in installation and maintenance, especially in urban areas.

In contrast, seismic stations like Biondi suggested are relatively inexpensive to run. Biondi said: "Every meter of fiber in our network is like a sensor, and it costs less than $1 to install. You can never create a network of such density with conventional seismographs."

Such a network will enable scientists to better analyze earthquakes, especially small earthquakes, and to identify their sources more quickly. Larger sensor coverage also better measures the ground's response to vibration.

Eileen Martin, a graduate student at Biondi Labs, said: "Civil engineers can react to small earthquakes in billions of sensor arrays from these buildings and bridges and use this information to design buildings that can withstand greater vibration."

From backscatter to signal

The fiber is a pure glass wire harness, which is about the thickness of a person's hair. They are usually bundled together to form a cable that transmits data signals over long distances by converting electronic signals into light.

Biondi is not the first to use fiber to study the environment. A technology called Distributed Acoustic Sensing (DAS) has been used to monitor the health of oil and gas pipelines.

Martin said that DAS works by moving light along the fiber as it hits the various impurities in the glass and bounces back. If the fiber is completely stationary, the "backscatter" signal will always look the same. However, if the fiber begins to stretch in certain areas, the signal changes due to vibration or strain.

Earlier implementation of this sonic sensor, but requires expensive fiber to be attached to a surface or wrapped in cement to increase contact with the ground to maximize data quality. In contrast, Biondi's project at Stanford University, the Fiber Optic Seismic Observatory, uses the same fiber as the telecommunications company, which is unsecured and free-floating in hollow plastic pipes.

Perhaps a lot of people don't believe it, but since fiber optic seismic observations began working in Stanford in September 2016, it recorded and classified more than 800 events, from small to man-made events and barely felt local earthquakes, like the recent earthquake hit more than 2000. A fatal disaster in Mexico miles away. In an experimental underground array recorded signals from two places with small earthquakes of 1.6 and 1.8. Biondi said: "As expected, both earthquakes have the same waveform or pattern because they originate from the same location, but larger earthquakes have larger amplitudes."

The array detection distinguishes between two different types of waves that propagate through the Earth, called P and S waves. "One of our goals is to contribute to early earthquake warning systems, which will require the ability to detect P waves, which are usually less destructive to S waves, but arrive much earlier," Martin said. . Stanford's fiber-optic seismic observatory is only the main step in developing a wide-area seismic network in the Bay Area. Biondi said that there are still many obstacles to overcome if it shows that the array can function within the city.