Centimeter-level positioning on the seafloor

This paper reports the activity in the development of the observation system of seafloor crustal deformation for understanding mechanism of interplate earthquakes at subduction plate boundaries around Japan. Currently, the method for the seafloor geodesy is under development in order to overcome several difficulties. The position of the sea-bottom transponders at the vicinity of the Nankai trough and the Japan Trench is observed using a GPS/Acoustic measurement system. Early results from processed data show a scatter of 4cm standard deviation for repeated horizontal positioning from drifting survey vessel. This paper describes two 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.