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August 2007    Download the Entire Issue (PDF) Available to the Public Vol. 33, No. 8   RSS Feed for Undercurrent Issues
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Eliminating Underwater Terrorists

excruciating sound will drive them from the water

from the August, 2007 issue of Undercurrent   Subscribe Now

September 11 boosted terror alerts everywhere, including underwater. Now, the FBI is asking the nation’s scuba instructors to watch for potential terrorist threats. Its Joint Terrorism Task Force recently alerted dive shops around the country to look out for divers seeking advanced training, including diving in murky water and in sewer pipes. The FBI said the advisory is routine and was not prompted by any threat, but it did ask instructors to be aware of “odd inquiries” inconsistent with recreational diving, such as advanced navigation techniques, deep diving and using underwater vehicles.

You may picture the image of hooded divers with spearguns chasing each other around underwater on self-propelled vehicles, like James Bond in Thunderball.But just as Agent 007 was fictional, so is that scenario. Capturing and killing divers by hand only happens in the movies.

The most promising non-lethal diver
weapon is a low-frequency sound.

With hundreds of thousands of certified divers out in the water, how can the military differentiate between those intent on carrying out terrorist attacks and those who just innocently stumble into restricted areas? And do they refrain from injuring, even killing, unsuspecting divers unintentionally? The good news is armed forces are developing remarkable non-lethal weapons that separate the good from the bad.

Passive sonar is used to listen for sounds like propellers, motors and marine mammals, but divers are harder to track. Those using open-circuit scuba gear produce periodic noise that can be detected and classified, but divers using rebreathers don’t produce the same amounts of noise and so are extremely difficult to locate on a sonar screen.

According to a 2002 report commissioned by the Navy’s Space and Naval Warfare Systems Center in San Diego, the most promising long-term solution for a non-lethal diver threat weapon is a low-frequency sound in the 20 to 100 megahertz range. That would be just enough to cause a lot of discomfort to divers’ ears and lungs but no physical damage. The report’s recommendation was “spark gap sound sources” that store electric charge in a large, high-voltage bank of conductors, then release all the stored energy in an arc across electrodes in the water. This spark discharge creates a highpressure plasma and vapor bubble in the water that expands and then collapses, making a loud sound similar to those from air guns and underwater explosions.

According to a New Scientist article, the U.S. Navy’s Anti- Terrorism Afloat program is developing and testing a sparkgap system to deploy from patrol boats or control remotely from the sea floor. Ideally, the device will emit an audible, low-powered warning when an intruder is sighted to make him surface, but also create more severe effects if an intruder persists. But it won’t be lethal. This is partly because of the risk of accidentally targeting innocent divers, but also “because you can learn much more from people if they are alive,” Tom LaPuzza, a spokesperson for the Space and Naval Warfare Systems Center, told New Scientist.

“There’s a tremendous emphasis now on underwater terrorism protection,” says Rob Williamson, marketing director for Marine Sonic in White Marsh, Virginia. Marine Sonic’s top product is Sea Scan PC, a navigational computer that uses sonar to transmit sound into the water and GPS to let users search, record and locate any objects of interest. Sea Scan PC can also use transducers to let law enforcement scan for moving divers swimming through sonar beams. “They can see divers but the divers can’t see them,” Williamson says. Sea Scan only uses low-frequency sonar tuned so low even divers won’t hear it. “We certify to the military that the system will not hurt a diver. We’d never use high-frequency because that would turn divers’ internal organs into jelly.”

British security company Westminster International has created Enforcer, a sound generator combining high-resolution sonar with powerful loudspeakers that can emit intense bursts of noise to detect divers. If the intruder fails to surface, an Enforcer user switches on a high-power signal that can create panic, sickness and confusion. Westminster says Enforcer was recently used to defend the coastline of a city in the Middle East during an international summit and is under evaluation by the U.S. government as protection for pipelines.

In January, researchers at the Georgia Institute of Technology released a sensor that detects the direction from which a sound is coming underwater. Francois Guillot, a research engineer who helped devise the sensor, says it could allow the Navy to develop compact scanners that detect quiet underwater divers. “Our sensor detects small sounds over the noise of the ocean and also provides directional information, an important improvement over current technology.”

The sensor uses fiber optics, a technique inspired by how fish hear underwater. A fish’s ear has thousands of tiny hairs that move when a sound wave passes through the fish. The hairs communicate with the nerves to allow fish to detect sound and avoid getting eaten.

Currently, the Navy uses expensive hydrophones, long lines towed behind boats using sonar to listen to underwater sound but lacking knowledge about its direction. “The hydrophones are thousands of feet long, making it difficult to maneuver the ship,” says Guillot. “Since we can cut that length by a factor of five, it will cost less money to operate and be easier to handle.” The project was funded by the Office of Naval Research.

But the spread of acoustic weapons underwater concerns marine biologists. There is evidence that sonar can kill or injure whales because it forces them to surface quickly, giving them the equivalent of the bends. Sound can also travel farther underwater than in air, disrupting communications between animals even hundreds of miles away, which could create huge exclusion zones in the ocean for fish and marinebased animals. Last spring, a group including the National Resources Defense Council and the California Coastal Commission filed lawsuits against the Navy for its intention to use sonar near the Channel Islands National Marine Sanctuary in Southern California. It’s the first time such a lawsuit has been brought by a government agency. LaPuzza says new systems will be thoroughly evaluated for their effects on the marine environment. Each possible site will be evaluated, deployment will be on a case-by-case basis and they will not be used where there is a risk to sea creatures – or recreational divers.

As technology improves, you can breathe easier that the military is more able to detect and catch that rebreatherwearing diver 30 feet underneath a cruise ship, and less likely to set its sights on you.

- - Vanessa Richardson

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