Military searches for Cheonan evidence

The military is striving to secure critical evidence to substantiate its tentative conclusion that a “non-contact underwater blast” ripped apart and sank the Navy corvette Cheonan in the West Sea on March 26.

The multinational investigation team said Sunday that a torpedo or a sea mine appears to have exploded under the bottom of the vessel’s portside without directly hitting the hull, which further fanned speculation over North Korea’s involvement in the tragedy.

Defense Minister Kim Tae-young said that he thinks a “bubble jet impact” -- caused by a mid-sized torpedo with a warhead weighing more than 200 kilograms -- is most likely to have caused the 1,200-ton ship to be torn into two.

Experts have raised the possibility that “Yu-3G” -- a passive acoustic homing torpedo with a warhead weighing roughly 205 kilograms -- may have been used in the disaster that has left 40 dead and six still unaccounted for.

The torpedo, which was developed by China in the 1980s and has been imported by the North, is guided to its target by the sound of a ship’s engine or screw propellers.

The investigation team consisting of some 130 people, including experts from the United States, Britain, Sweden and Australia, has been analyzing hundreds of fragments collected from the scene of the incident and the salvaged wreckages of the ship.

The military said that it has yet to secure any fragments that appear to have come from a torpedo or a sea mine.

Officials said that the team would take three-dimensional pictures of the fracture lines in the salvaged stern and bow of the splintered ship from different angles and put them together to find out how the fracture lines in the wrecks were created.

Based on the pictures, the team plans to conduct simulation tests to determine what type of weapons was used, and the location, direction and intensity of the explosion.

With the “non-destructive testing” -- scans using ultrasonic waves and x-rays, the team will analyze fragments and gunpowder residue that may remain on the surface of the breached areas. The team will also use liquid chromatography, gas chromatography and mass spectrometry to examine the wreckages, officials said.

With electron microscopes, the team plans to find out how many shocks the hull sustained following the explosion. The analysis with microscopes will focus on the portside to help find out from where the fractures started, officials said.

To collect fragments and debris, the military plans to mobilize fishing vessels equipped with nets having dozens of hooks to scrape the sea floor. It also plans to use fishing boats with dragnets.

It has also deployed minesweepers and unmanned underwater probes to search for any floating matter within a 500-meter radius of where the explosion occurred. However, the search operations, which the military expects to continue for at least one month, have been hampered by strong tidal currents and rough seas.

The investigation team has ruled out the possibility of an internal explosion, a fatigue fracture and a collision with a reef because there are no traces of burns or melting inside the wreckage, and there has been no major puncture found.

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