Fiber-Optic Cables Help Study Offshore Earthquakes and Hidden Geologic Structures

Researchers can potentially investigate offshore earthquakes as well as geologic structures buried deep under the surface of the ocean, thanks to fiber-optic cables constituting a global undersea telecommunications network.

A research team from Lawrence Berkeley National Laboratory (Berkeley Lab), the University of California, Berkeley (UC Berkeley), Rice University, and Monterey Bay Aquarium Research Institute (MBARI) has demonstrated an experiment in which a 20-km undersea fiber-optic cable was converted into the equivalent of 10,000 seismic stations along the seabed. The study has been recently published in the Science journal.

During the experiment, which was performed for four days in Monterey Bay, the scientists recorded a 3.5 magnitude quake as well as seismic scattering from fault zones located under the water.

The researchers’ method, which they had earlier tested using fiber-optic cables on land, can offer the much-needed information on earthquakes that take place under the ocean. However, only a few seismic stations are present under the sea, which leaves 70% of the surface of the Earth without earthquake detectors.

There is a huge need for seafloor seismology. Any instrumentation you get out into the ocean, even if it is only for the first 50 kilometers from shore, will be very useful.

Nate Lindsey, Study Lead Author and Graduate Student, University of California, Berkeley

The experiment was headed by Lindsey and Jonathan Ajo-Franklin, a faculty scientist at Berkeley Lab and a geophysics professor at Rice University in Houston, along with Craig Dawe of MBARI. The fiber-optic cable is owned by MBARI.

The fiber-optic cable stretches 52 km offshore to the first seismic station ever deployed to the Pacific Ocean floor. Both MBARI and Barbara Romanowicz, a Professor of the Graduate School in the Department of Earth and Planetary Science at UC Berkeley, had put the seismic station in the Pacific Ocean floor 17 years ago.

In 2009, a permanent cable to the Monterey Accelerated Research System (MARS) node was also laid. In March 2018, 20 km of this cable was utilized in this test while keeping it off-line for annual maintenance.

This is really a study on the frontier of seismology, the first time anyone has used offshore fiber-optic cables for looking at these types of oceanographic signals or for imaging fault structures. One of the blank spots in the seismographic network worldwide is in the oceans.

Jonathan Ajo-Franklin, Geophysics Professor, Rice University

Ajo-Franklin informed that the ultimate objective of the team’s efforts is to utilize the dense fiber-optic networks globally—perhaps over 10 million kilometers together, on both under the sea and land—as sensitive measures of the movement of Earth. This helped in monitoring earthquakes in areas that lack costly ground stations like those present in most of the Pacific Coast and California prone to earthquakes.

“The existing seismic network tends to have high-precision instruments, but is relatively sparse, whereas this gives you access to a much denser array,” added Ajo-Franklin.

Photonic Seismology

The scientists used Distributed Acoustic Sensing, a method in which a photonic device is used. This device transmits short pulses of laser light down the fiber-optic cable and locates the backscattering produced by strain in the cable that is induced as a result of stretching. Using interferometry, the scientists can quantify the backscatter every 2 m (6 feet), thus successfully converting a 20-km fiber-optic cable into 10,000 separate motion sensors.

“These systems are sensitive to changes of nanometers to hundreds of picometers for every meter of length,” stated Ajo-Franklin. “That is a one-part-in-a-billion change.”

The researchers reported the outcomes of a six-month trial earlier this year. The test was performed on land using a cable of 22 km close to Sacramento. The Department of Energy emplaced the cable as part of its 13,000-mile ESnet Dark Fiber Testbed.

Dark fibers are essentially optical cables installed beneath the ground, but remain unused or leased out for an interim use when compared to the “lit” internet that is actively used. The scientists successfully tracked environmental noise and seismic activity and achieved subsurface images at a larger scale and a higher resolution than would have been possible with a conventional sensor network.

“The beauty of fiber-optic seismology is that you can use existing telecommunications cables without having to put out 10,000 seismometers,” added Lindsey. “You just walk out to the site and connect the instrument to the end of the fiber.”

While performing the underwater test, the researchers quantified a wide range of frequencies of seismic waves from a 3.4 magnitude earthquake that took place 45 km inland close to Gilroy, California. They subsequently mapped many known as well as earlier unmapped submarine fault zones, which were part of the San Gregorio Fault system.

In addition, the researchers detected steady-state ocean waves—what is known as ocean microseisms—and also storm waves, which collectively matched land and buoy seismic measurements.

We have huge knowledge gaps about processes on the ocean floor and the structure of the oceanic crust because it is challenging to put instruments like seismometers at the bottom of the sea.

Michael Manga, Professor, Earth and Planetary Science, University of California, Berkeley

Manga continued, “This research shows the promise of using existing fiber-optic cables as arrays of sensors to image in new ways. Here, they’ve identified previously hypothesized waves that had not been detected before.”

Lindsey informed that seismologists are keen to record the ambient noise field of Earth. This field is caused by interactions between the continental land and the ocean: in essence, waves that splash near the coastlines.

“By using these coastal fiber optic cables, we can basically watch the waves we are used to seeing from shore mapped onto the seafloor, and the way these ocean waves couple into the Earth to create seismic waves,” added Lindsey.

To utilize the lit fiber-optic cables in the world, both Lindsey and Ajo-Franklin have to demonstrate that they can ping laser pulses through a single channel without disrupting other channels in the fiber carrying independent data packets.

At present, the researchers are performing experiments using lit fibers. They have also planned the fiber-optic monitoring of seismic events in a geothermal region located in the Salton Sea, south of Southern California in the Brawley seismic zone.

The study was funded by the U.S. Department of Energy via Berkeley Lab’s Laboratory Directed Research and Development program, the National Science Foundation (DGE 1106400), and the David and Lucille Packard Foundation.

The final analysis was supported by the National Energy Technology Laboratory of the Department of Energy as part of the GoMCarb project (DE-AC02-05CH11231).

Source: https://www.berkeley.edu/