Abstract
A GPS/Acoustic seafloor positioning system has been newly developed for observation at 6, 000 m water depth and 300 km offshore from the coast. The system copes with three critical problems: precise underwater acoustic ranging on the deep seafloor of the Pacific plate, long baseline KGPS positioning over the trench axis at about 200 km offshore, and compatibility of precise acoustic ranging and controlling the ship's position. The observation system was evaluated during the sea trials off Hawaii Island and near the Japan Trench. The newly devel oped system demonstrated acoustic ranging capability with a sub-centimeter precision at slant ranges over 15 km. Precise KGPS positioning was available in the offshore seaward of the Japan Trench. A towed buoy with an absorber above the acoustic transducer is a useful surface platform to reduce the acoustic noises from a survey vessel. The developed observation system can be used for observation of seafloor crustal movements at almost any site in the subduction zones around Japan even at deep seafloor near the trench axis.
