April 12, 2010 10:53
Sound waves from the blast that ripped apart the Navy corvette Cheonan on March 26 were detected as far away as Cheolwon, Gangwon Province, making it equivalent to 260 kg of TNT. That is 44 percent more powerful than the initial intensity (170 kg to 180 kg of TNT) of the blast gauged by sensors on Baeknyeong Island.
According to the Korea Institute of Geoscience and Mineral Resources on Sunday, a 5.418 Hz sound wave was detected at an observation station in Gimpo 177 km away at 9:30.41 p.m. on March 26, and a 2.532 Hz sound wave at another station in Cheolwon, 220 km away, at 9:32.53 p.m. Earlier, at 9:21.59 p.m., just one second after the seismic wave, an observatory on Baeknyeong Island also captured 6.575 Hz sound waves. In fact, two separate sound waves were detected, 1.1 seconds apart, supporting the accounts of survivors that there were two explosions over a short period.
Five hours after the blast, the KIGMR sent a report to the military and a government agency with its analysis that the intensity of the blast based on seismic readings (1.5 on the Richter magnitude scale) was equivalent to 180 kg of TNT, while the intensity of the explosion based on sound wave readings, on the premise that a torpedo or mine exploded 10 m below the Cheonan, was equivalent to 260 kg of TNT. The deeper it gets, the weaker the power of the blast that is reverberated outwards, and a torpedo or mine that explodes at a depth of 20 m produces the equivalent of 710 kg of TNT in terms of intensity the center said.
"The sound of the blast was probably not heard, since it was beyond the audible frequency of humans," an official at the center said. "Seismic waves are transferred through several vehicles, but sound waves aren't, so estimates of the intensity of the blast based on sound waves are probably more accurate."
If a torpedo had caused the Cheonan to sink, military experts believe it would have been a mid-sized torpedo, which has a larger warhead than a light one. A mid-sized torpedo can split a 1,200-ton vessel like the Cheonan in half and even sever a 7,000-ton warship. Light torpedoes mounted on North Korean semi-submersibles have only 50 kg warheads. In contrast, a small North Korean submarine of the 300-ton class carries 533 mm mid-sized torpedoes, which reportedly have 300 kg warheads. One type of North Korean torpedo that travels in a straight line toward its targets carries 150 kg to 300 kg warheads.
South Korean military officials speculate that any North Korean attack would have involved Shark class mini-submarines that are capable of operating 30 m underwater. The Yugo class mini-submarines (85-ton class) are smaller but also capable of being equipped with mid-sized torpedoes in certain cases, but South Korean intelligence services believe all of them were retired three years ago.
Some experts speculate North Korea imported its mid-sized torpedoes from Iran, which maintains close ties with the communist country in the development of nuclear weapons and missiles, as well as submarines and other types of underwater weapons. In 2006, Iran developed a new high-speed torpedo called Hoot, which uses a rocket propulsion system that makes it three to four times faster than a conventional torpedo, while jointly developing with North Korea an upgraded version of the Yugo class sub.
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