hankyoreh

director of the Korean Seismological Institute

By Oh Cheol-woo, science correspondent

Q: It must have not been easy publishing a study rebutting the findings of the Multinational Civilian-Military Joint Investigation Group.

A: This is a very important matter. I have a lot experience in this field. I majored in seismology and I wrote my dissertation on methods of distinguishing explosions from natural seismic tremors or earthquakes. I conducted a six-month ocean survey with a group of American scholars in the south Pacific. I also conducted an in-depth profile of the ocean floor in the West Sea. I established the Seismology Institute at Hanyang University. When I heard about the ROKS Cheonan, I felt immediately that there were some problems there. As a scholar in the field and an expert, I felt responsible to straighten out the records. And that is why I decided to take on the study myself.“

Q: Can you tell us about your findings?

A: The first conclusion is that the sinking of the Cheonan was caused by an underwater explosion. This conclusion is based on three findings: in an explosion the pattern of seismic wave is different and the characteristic that we saw in the initial motion was a compression wave pattern. Secondly, two things happen during an underwater explosion. One is bubbles caused by the explosion gas and the other is that reverberation (a kind of underwater sound wave, the wave that is exchanged from the floor to the surface) occurs. Both were apparent this time. And thirdly, midair sound wave and infra-sonic frequency, which are not found in natural seismic tremors, were also detected.

Q: You determined through your study that the local magnitude of the explosion was 2.04 at a depth of 8m with seismic yield of about 136kg. This is different from the findings of the investigation group.

A: In 2000, when the Russian nuclear powered submarine Kursk exploded in the Barents Sea, they were able to find the scale and depth of the explosion from the bubble pulse period of the underwater sound waves (which is the time it takes for the bubble to expand and shrink). If we can find this measurement, we can then find the scale and depth of the explosion. Our data showed that the bubble pulse period was 0.990 seconds. From a comparative study using various methods of simulation, underwater explosion formula and boundary element method (BEM), we found that this bubble pulse period is found in an explosion with TNT yield of 136 kg at 8m depth. The bubble pulse period becomes too big at TNT yield of 250 kg, the yield indicated by the investigation group. 　　

Q: Your study has the explosion scale reduced to magnitude of 2.04 and the TNT yield reduced to 136 kg.

A: Our study points out that the findings of the MCMJIG are wrong. An underwater or submarine earthquake also happens in the bowels of the earth and it is observed through seismic waves. But this explosion happened not underneath the floor but inside water. You therefore cannot use an ordinary formula to measure a submarine earthquake. In addition, because of the characteristic of water, the explosive energy does not scatter easily, like land explosion does. For this reason, the scale of the explosion of the same amount of explosive will become larger.

Q: The bubble pulse period does seem very important as a means of measurement. But why was the measurement different? The investigation group said that it was 1.1 seconds.

A: There is a given formula and the result cannot be different. Using the data that we observed, we used analysis of high resolution frequency to calculate the bubble pulse period.

Q: What is the biggest difference between your study and the findings of the investigation group?

A: We have put together data of the seismic waves, mid-air sound waves and sonar and dealt with it as an underwater explosion and accordingly used hydromechanics and kinetics. We also confirmed our findings by using three methods including Cole’s method, boundary element method (BEM) and 3 dimensional simulations.

Q: You specified 136 kg of TNT yield, why?

A: The calculation came out to about that. There are records of sea mines with 136 kg TNT yield that have been installed and they are still operational. For this reason, using BEM and simulation, we tested it, and it all fit.

Q: What is the next study you are undertaking?

A: I am still working on it. I am looking at the hull, the body of the ship and the bubbles, what kind of interaction there was. I am analyzing the simulations we worked on. The direction that the propeller bent will also provide a good clue for the study.

■Underwater Explosion (UWE) Analysis of the ROKS Cheonan Incident (So Gu Kim and Yefim Gitterman)

http://www.springerlink.com/content/c861379488372070/

Pure and Applied Geophysics

2012, DOI: 10.1007/s00024-012-0554-9

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