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The Seikan Tunnel is the longest submarine tunnel in the world located under the Tsugaru Straits, connecting the mainland of Japan with the island of Hokkaido. It lies 100 m under the seabed about 150 m below sealevel and is as long as 23 km even only in submarine length. The geological formation consisting of soft-sandy mudstone made the constructing job of the tunnel extremely difficult, but the pilot tunnel was successfully driven through in January 1982. The Japan Railway Construction Republic corporation, by whose responsibility the century project is being carried out, estimates that the main tunnel will be completed in 1985. Geological survey of the west end of the strait where the tunnel was planned to go through started in 1946. Geolgical exploration by a submersible core drill, which played an important role for the preparation of the marine geological map, was conducted in 1958, 1968 and 1978, The submersible drilling equipment I am going to mention herein is lowered to the seabottom by a winch mounted on a working vessel, sitting on it, and practising core sampling by means of power-supply and remote control cables connecting the drill to the vessel. The first exploration work by a submersible drill for the construction of the Seikan Tunnel was conducted in 1958 by an "Ogawa" Type drill at 25 points along both the coasts of the strait. The sample was very valuable for the geological analysis of she site in tellng us that the zone 1-15 cm below the surface of the seabed generally withered and faded. The second survey was practised under collaboration between The Oceanographic Reserch Institute of The University of Tokyo and The Japan Railway Construction Public Corporation. The drilling equipment used then was a Marine Drill Model "MD-150S" manufactured by KOKEN BORING MACHINE CO., LTD. We could successfully get many pieces of core samples, the longest one of which was 4.53 m. The third one was met with another Marine Drill Model "MD-500S" (manufactured by KOKEN BORING MACHINE CO., LTD.). This drill is powered by a special battery, and driven either by sonar remote control system from the vessel or by a timer incorporated in the drill. Nothing but the lowering/lifting wire rope, therefore, is connected with the drill sitting on the seabed, which ensures the safest and most efficient work to lower and lift the drill unit. Core boring was practised to 50 points at the central area of the strait with "MD-500S" mounted on a 498-ton working vessel with an A-frame to the good result of getting cores of 2.01 m length in average and 5.5 m length in maximum, with the core recovery rate being 72%. The main purpose of the third exploration was to make an intense survey of how Kuromatsu formation is distributed at the 8 km-long central section of the target route of the tunnel. We could get valuable data which prove the correctness of the other survey results including those from geophysical prospecting previously practised. In conclusion, we may say that the third exploration was fairly successful with good result that 50 points were drilled in one and half months in the Tsugaru Straits under adverse working conditions where the maximum speed of tidal current is 4.8 knot and the maximum seadepth is about 150 m. 20 *日 本鉄道建設公団青函建設局 **鉱 研試錐工業株式会社 昭和 59 年 6 月 津軽海峡西 口附近 に於け るマ リン ドリルの施工 につい て 67

Core Sampling by Marine Drill at the West End of the Tsugaru Straits