Scientific analysis shows signs of a powerful underwater explosion
By Oh Cheol-woo, science correspondent
An article has been published in an international academic journal arguing that the explosion that sank the South Korean Cheonan warship in March 2010 may not have been from a North Korean torpedo, but from a mine discarded by the South Korean navy.
This is the second scientific study on the Cheonan sinking published in an academic journal, the first being a seismic analysis published last year by Yonsei University Department of Earth System Sciences professor Hong Tae-kyung. That study supported the findings of the government’s joint investigation team.
In the study published in the international academic journal “Pure and Applied Geophysics,” Korea Seismological Institute director Kim So-gu and the Geophysical Institute of Israel’s Yefim Gitterman wrote that analysis of the seismic waves, acoustic waves and bubble frequency made it clear an underwater explosion took place.
They said the seismic magnitude of the explosion was 2.04, that of 136kg of TNT and equivalent to the individual yield of the large number of land control mines abandoned by the Korean navy after they were first installed in the 1970s.
The findings are noteworthy in that they differ greatly from those of the Civilian-Military Joint Investigation Group (MCNJIG), which found the cause of the sinking to be a North Korean CHT-02D torpedo with a yield of 250kg of TNT exploding at a depth of six to nine meters, producing a seismic yield of 1.5.
In the thesis, the research team analyzed the cause of the underwater explosion through equations, models and simulations examining the frequency of gas bubbles that expand rapidly after an explosion and the amount of explosive yield needed to produce them.
The repeated expansion and contraction of bubbles, which expand quickly with an explosion but then contract due to water pressure, causes damage to a ship.
The time it takes for one expansion and contraction is called the bubble pulse period. In their observed data, Kim and Gitterman calculated the bubble pulse period – a value needed to determine explosive yield and explosion depth – to be 0.990 seconds.
Kim and Gitterman then made calculations based on various explosive yields and depths and found that an explosion of 136km of TNT at 8m in depth would produce the bubble pulse period in the observed data.
Kim and Gitterman said confirmation attempts using several methods showed that an explosion of 250kg of TNT produced results too discordant with the observed bubble pulse period.
MCMJIG also considered the possibility that the explosion was caused by a land control mine.
According to the MCMJIG findings report published in 2010, the Korean navy – following a 1985 decision that they were no longer necessary – abandoned its land control mines on the ocean floor after a process of deactivation that involved the cutting of their long fuse lines. The mines were placed around Korea’s West Sea islands along the Northern Limit Line in 1977.
MCMJIG excluded the mines as a possible cause of the explosion, saying that a land control mine with a yield of 136kg of TNT would have been unable to cut a ship’s hull in two at 47m, the water depth at which the incident took place.
Kim said, “The results of the MCMJIG study did not sufficiently reflect the basics of underwater explosions and bubble dynamics. As other possibilities are being raised, there should be a reinvestigation to scientifically study the cause of the explosion.”