Fiber optic cables A University of Michigan researcher says ocean floors could provide a less expensive, more comprehensive alternative to existing buoys that act as early warning systems for tsunamis.

A system called DART, or Deep Ocean Assessment and Reporting of Tsunami, consists of special buoys that monitor tsunamis. Overseen by the National Oceanic and Atmospheric Administration, the buoys cost about $500,000 to install, with another $300,000 a year to maintain. Thirty-two detection buoys dot the Pacific Rim, resulting in millions of dollars in annual maintenance—expensive, but important maintenance.

Now, a UM seismologist Zach Spica And colleagues at the California Institute of Technology have used a cheap and ubiquitous technique called distributed acoustic sensing, or DAS, to monitor natural disasters: nearly 1 million miles of fiber-optic cables. Cross cross sea ​​floor.

“Telecommunications companies have been laying these fiber-optic cables for the past 30 years, and have spent hundreds of billions of dollars doing it,” said Spica, a UM assistant professor of earth and environmental sciences. “Now, thanks to advanced photonics and tremendous computing power, we can turn fiber-optic cables into very dense, high-fidelity sensors.”

A tsunami is a series of large waves generated by a sudden displacement of seawater, usually caused by a sudden land movement of the ocean floor. Tsunamis can be minor, or they can be catastrophic, such as the 2004 Indian Ocean tsunami, which killed approximately 228,000 people.

In a study published in Geophysical Research Letters, Spica and colleagues report that fiber-optic cables can be used as an early warning system for tsunamis.

“Unlike earthquakes that occur suddenly and can hardly be avoided, even though there are some early warning systems, tsunamis usually take longer to form and reach the coast,” Spica said. “This means that early warning systems for tsunamis are more effective. However, it is difficult to predict the magnitude of a tsunami before it reaches the coast. Hence, offshore equipment is needed, which is expensive and difficult to maintain. Is.

Over the past five years, Spica and his fellow researchers have installed DAS interrogator units at fiber-optic telecommunications companies in Alaska, Japan, Spain and Lake Ontario that tap into underwater fiber-optic cables. Using one of the instruments stationed in Florence, Oregon, the team was able to detect a tsunami that originated from an island chain about 1,300 miles east of the tip of South America.

“It was a big earthquake in the Sandwich Islands that produced a big tsunami. It wasn’t even in the same ocean as the cable and device we detected it on,” Spica said. “By the time the tsunami reached Oregon and Alaska, it was only a few centimeters, causing no damage.”

The DAS technique works by monitoring photons—particles of light—that travel through fiber-optic cables. As light travels in waves through cables, some photons are refracted back to the beginning of the cable. These photons are refracted backwards and at a given time, the amount of light that returns to the interrogator is proportional to the error along the cable.

Researchers initially used these cables to detect earthquakes. Earthquakes release large amounts of energy in a short period of time. Spica said the big question was whether the cables could detect the more subtle movements of tsunamis. The time between wave peaks in a tsunami can be incredibly long – tens of minutes and several miles between waves.

“Earthquakes typically have a lot of energy and move very quickly, while tsunami waves are very wide,” Spica said. “So the question was, can we use these techniques to monitor long-term waves?”

Researchers aren’t sure what feature of the tsunami caused the shift in the fiber-optic cables. Deformation caused by excess water pressure on the cables can cause the fibers inside them to stretch, changing how photons are refracted. Temperature can cause a similar change, but Spica says more research is needed to determine how the fibers are affected.

DAS systems could offer telecommunications companies an alternative to using fiber-optic cables in the future, as satellites replace cables as the primary means of delivering the Internet. The cables could be used for military surveillance, ship tracking, internal wave measurements, ocean temperature monitoring and climate change research, Spica says.

“These telecommunications companies have heard about this sensing, 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 infrastructure.”

The study builds on Spica’s previous research to determine whether fiber optics could detect ground motions from earthquakes. Next, Spica says, software needs to be developed to simulate tsunami detection information from fiber-optic cables in real time.

Source: University of Michigan