HR: 0830h
AN: OS51C-0882    [PDF]
TI: High Resolution, Deep-Tow Seismic Survey to Investigate Methane Hydrate-Bearing Sediments, Nankai Trough, Offshore Japan.
AU: * Ward, P
EM: pward@jgi.co.jp
AF: JGI Inc., Meikei Building, 1-5-21 Otsuka, Bunkyo-Ku, Tokyo, 112-0012 Japan
AU: Asakawa, E
EM: asakawa@jgi.co.jp
AF: JGI Inc., Meikei Building, 1-5-21 Otsuka, Bunkyo-Ku, Tokyo, 112-0012 Japan
AU: Shimizu, S
EM: shimiz-s@jnoc.go.jp
AF: Japan National Oil Corporation, Technology Research Center, 1-2-2 Hamada, Mihama-Ku, Chiba, 261-0025 Japan
AB: A 2D deep-towed seismic survey was carried out over part of the Nankai Trough, Offshore Japan, in 1996. The area is known to have a major accumulation of methane hydrate; conventional seismic surveys have mapped thousands of sq. km. of BSR events, and the presence of methane hydrate was confirmed by a MITI research well in 2000. The objective of the deep-towed survey was to obtain high resolution stack and velocity profiles of the methane hydrate-bearing sediments down to the BSR, which occurs at around 300 msec below the sea bottom in this area. The data was acquired using the "Deep-Towed Acoustics/Geophysics System (DTAGS), developed by the Naval Research Laboratory, USA. Both the source and the hydrophone cable are towed close to the seabed, allowing a "close look" at the shallow sediments. The source is a vibrator-type Helmholtz transducer with a 0.25 sec sweep from 250-650 Hz. The hydrophone cable has 48 channels and a maximum offset of 620m. This paper focuses on the data processing, carried out in 1998 and 2002. The main challenge in the processing was to handle the floating acquisition datum with source and receiver depths being maintained at typically 200-300 m above a varying seabed. The measured depths had to be corrected by residual statics derived from picks of the sea bottom reflection event. A total of around 200 km of data was processed, with water depths of 700-1400 m. The results showed that it is possible to obtain high resolution profiles and velocity data, and that it is also possible to fail to achieve such data. Success largely depends upon control of the depths during acquisition; they should be kept smoothly varying and measured as accurately as possible. The deep-tow system has the advantage over conventional data in water depths of around 1000 m or deeper, for zones close (within 100's m) to the seabed. The advantage of the system becomes greater with increasing water depth.
DE: 3025 Marine seismics (0935)
DE: 4219 Continental shelf processes
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting