Fiber optic cables are an effective way to detect tsunamis, researchers say

Fiber optic cables lining the seabed could provide a cheaper and more comprehensive alternative to current buoys that serve as early warning systems for tsunamis, says a University of Michigan researcher.

A system called DART, or Deep-ocean Assessment and Reporting of Tsunamis, is made up of specialized buoys that monitor tsunamis. Overseen by the National Oceanic and Atmospheric Administration, the buoys cost about $500,000 to install, with an additional $300,000 a year for maintenance. Thirty-two sensor buoys dot the perimeter of the Pacific Ocean, representing millions of dollars a year in maintenance – costly but vital maintenance.

Now, UM seismologist Zack Spica and his colleagues at the California Institute of Technology have used a technique called distributed acoustic sensing, or DAS, to exploit a cheaper, more ubiquitous way to monitor natural disasters: the approximately 1 million kilometers of optical fiber. cables that crisscross the seabed.

“Telecommunications companies have been laying these fiber optic cables for 30 years and have spent hundreds of billions of dollars doing it,” said Spica, an assistant professor of earth and environmental sciences at UM. “Now, thanks to advanced photonics and big computing power, we can transform fiber optic cables into super-dense, high-fidelity sensor networks.”

Tsunamis are a series of massive waves triggered by a sudden displacement of ocean water, most often caused by the sudden movement of the seafloor. Tsunamis can be minor or devastating, such as the 2004 Indian Ocean tsunami, which killed nearly 228,000 people.

In a study published in Geophysical research lettersSpica and colleagues show that fiber optic cables can be used as a tsunami early warning system.

“Unlike earthquakes that occur suddenly and are difficult to prevent, even if there are early warning systems, tsunamis generally take longer to develop and reach the coast,” Spica said. “This means that early warning systems are more effective for tsunamis. However, what is difficult is to assess the magnitude of a tsunami before it reaches the coast. Therefore, offshore instruments are required, which are expensive and difficult to maintain.”

Over the previous five years, Spica and his fellow researchers installed DAS interrogators at fiber-optic telecommunications companies in Alaska, Japan, Spain and Lake Ontario that operate undersea fiber-optic cables. Using one of the devices placed in Florence, Oregon, the team was able to detect a tsunami originating from a chain of islands located nearly 1,300 miles east of the tip of America from South.

“This was a major earthquake in the Sandwich Islands that generated a large tsunami. It wasn’t even in the same ocean as the cable and device we detected it on,” he said. Spica said. “By the time the tsunami hit Oregon and Alaska, there was only a few inches of runoff, which caused no damage.”

The DAS technique works by monitoring photons (particles of light) that travel through fiber optic cables. As light travels as a wave through the cables, some photons are refracted toward the beginning of the cable. These photons are refracted backwards and at any given time, the amount of light returning to the interrogator is proportional to the deformation along the cable.

Researchers initially used these cables to detect earthquakes. Earthquakes release a huge amount of energy in a very short time. The big question, Spica said, was whether the cables could detect the much more subtle movements of tsunamis. The period between wave crests during a tsunami can be incredibly long: up to tens of minutes and several kilometers between wave crests.

“Earthquakes generally have much higher energy and shake very quickly, while tsunamis have very large waves,” Spica said. “So the question was: can we use these techniques to monitor very long period waves?”

Researchers aren’t sure exactly what feature of the tsunami causes fiber-optic cables to change. Pressure-induced deformation from additional water above the cables could cause the fibers within them to stretch, changing the way photons are refracted. Temperature could cause a similar change, but Spica says more research is needed to determine exactly how the fibers are affected.

The DAS system could offer telecommunications companies another way to use fiber optic cables in the future, as satellites replace cables as the primary means of delivering the Internet. Spica says the cables could be used for military surveillance, boat tracking, measuring internal waves, tracking ocean temperatures and for climate change research.

“These telecom companies have heard about this detection, but it’s still very early,” Spica said. “But if we think big, if we think big over the next 15 years, they should probably try to reinvest in their own infrastructure.”

This study builds on previous research conducted by Spica to determine whether fiber optics could detect ground movements caused by earthquakes. Next, Spica says software must be developed to transcribe the information needed to detect tsunamis from fiber optic cables in real time.