Sea bottom topography of an active margin – where a pushing oceanic plate converges with a continental plate – is characterised by trenches, island arcs and marginal seas. Delineation of the outer limit of the continental shelf beyond 200 nautical miles at an active margin is impossible without information on the crustal structure because of the complexity of sea bottom topography. The Government of Japan decided to conduct a detailed investigation of crustal structure around Japan as part of a continental shelf survey.
Our company has conducted seismic reflection and refraction investigations, as part of a continental shelf survey, under contract of the government since 2004. Surveys at sea using one geophysical ship and three work boats were successfully completed in October this year and now we are tackling a huge amount of seismic data. The total track line length of navigation by these four ships is about 280,000km.
Before we started these investigations, we made maximum efforts to prepare the best software and hardware, including tuned air-gun array for deep seismic penetration in the crust. We also developed seismic refraction processing software. The ship and boats were very carefully checked and every effort was made to keep them in their best condition.
However, our main concern was always the safety of the operation and physical condition of crew and surveyors because the survey area was often more than a thousand nautical miles away. It would take more than a few days to rescue them in an emergency. Of course, human life is a matter of highest priority but if the operation were delayed, the cost would be huge. For practical reasons, it was not possible to have a doctor on board the ship during the survey.
We checked the physical health records of each crew member and the surveyors before they boarded the vessel. A safety supervisor was appointed for each ship for each cruise. Meetings on board were opened and closed with the old slogan ‘Safety First’. Seminars on safety and healthcare were held on board for all surveyors. The seminars included a three-hour training session with an Automated External Defibrillator (AED).
Fortunately, our four-year survey work at sea has finished successfully, without major problems during the survey operation. Nobody opened the red AED box on the ship, although recently one of the bosses of our co-operative company had a sudden heart attack while he was walking in front of his office. Luckily, a friend found him soon after he had fallen to the ground and lost consciousness, and he was given first aid with an AED. The boss has since recovered and is back at work. It seems as though the land can be more dangerous than the sea.
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