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Indonesia, a country of thousands of islands, is vulnerable to earthquakes and tsunamis. WHOI research engineer Lee Freitag and his colleagues developed a new sound-based underwater communications system to warn of a possible tsunami. The system detects tsunami waves offshore and sends an acoustic signal through a natural undersea sound channel  to people on shore. (Cabell Davis, Woods Hole Oceanographic Institution) [ Hide caption ]

A New Tsunami-warning System

Scientists devise way to relay sound signals under water

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After successfully testing a long-range underwater communications system that worked under Arctic Ocean ice, an engineering team at Woods Hole Oceanographic Institution (WHOI) adapted it for a very different environment—the tropics—and for a different purpose—to provide warnings of impending tsunamis.

While the Arctic sound-signaling system lets researchers communicate with robotic vehicles operating beneath sea ice, the tropical system, tested in 2016 off Indonesia, is designed to relay signals “from an undersea sensor network to shore, where they can be used to estimate the level of the potential tsunami,” said Lee Freitag, the WHOI engineer who led the project.

“There’s a threat of near-shore tsunamis in this area,” Freitag said of the Indonesian archipelago, which has large population centers right at sea level. “If there’s an earthquake here, there’s only thirty minutes before a tsunami would reach shore.”

In the aftermath of a 2004 undersea Indian Ocean earthquake that generated a tsunami and caused extensive damage and deaths in Indonesia, scientists from multiple countries expanded the ocean-based warning system called DART (Deep-ocean Assessment and Reporting of Tsunamis) to include more of the Indonesian archipelago. DART consists of pressure sensors on the seafloor that detect quakes and tsunamis. The sensors relay sound signals to buoys at the ocean surface that transmit data via satellite to shore so officials can trigger an alert.

But instruments floating atop the ocean are vulnerable. In the Arctic, ice and polar bears can crush them. Fish can bite instruments, and wind, waves, and currents pummel them. If the equipment is visible and accessible, people may take instruments or even remove entire buoys. Several Indonesian DART buoys have met that fate, hobbling the network’s ability to warn Indonesia’s emergency disaster response agencies.

Freitag and his team are working with Indonesian and United States researchers, led by disaster researcher Louise Comfort at the University of Pittsburgh, to send tsunami alerts without the surface buoys. In early 2016, they successfully tested a new system off Indonesia that uses sound signals traveling under water, out of sight.

The scientists are taking advantage of naturally occurring sound feataures in the ocean that efficiently channel sound waves, allowing them to travel farther without losing energy. These features consist of layers of water of different densities. The sound waves don’t easily penetrate the boundaries; instead, they refract away from them and continue on with minimal loss.

In Indonesia, surface water is always warmer and less dense than water near the bottom, which is colder and under higher pressure and therefore denser.

Freitag’s team put a pressure sensor on the seafloor about a mile deep and about 60 miles from land. They sent sound signals from it at a frequency of 3 kilohertz. That’s a higher frequency than they used in the Arctic, and so it doesn’t travel as far under water, but it can carry more information.

The sound traveled upward, bounced off the warm layer boundary near the surface, and continued back down to the sea bottom some 15 to 18 miles away. There, another modem on the seafloor received the sound and sent it out again. In this way, the signal hopped, or jumped, from modem to modem toward shore.

If implemented in the future, the system would send signals only in the event of a quake that generates a tsunami, and weekly for 'health checks,' so only very rarely would there be any signals.

“We think we’ve proven the acoustic side will work,” Freitag said. “Now we just need a seafloor cable to link to shore and make the final connection to the rest of the Indonesian early-warning system. That’s the next step.”

This project was funded by the National Science Foundation, Interdisciplinary Research in Hazards and Disasters (Hazards SEES) Program.

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