With a view toward understanding the mechanism of interplate earthquakes at subduction plate boundaries around Japan, remarkable progress was made in the long-term observation system of seafloor crustal deformation. The method for the seafloor geodesy has been exploited in order to overcome several difficulties. The position of the sea-bottom transponders on the Kumano Trough was repeatedly observed using a GPS/Acoustic measurement system in May and August 2000. Early results from processed data show about 2 cm differences between May and August with a scatter of 4 to 10 cm in horizontal positioning. This paper describes techniques developed to realize precise acoustic ranging, automatically detecting and correcting the Doppler shift of acoustic propagation signal due to ship's drifting and attitude motion, and theoretically forming a pulse of one wavelength in cross correlation between transmission and reception of long-coded signals. Furthermore, by using GPS surface positioning, acoustic ranging and sound speed profiling data, the temporal changes of sound speed profile were estimated with sufficient accuracy for acoustic positioning.
