Development of instruments for seafloor geodesy

We have developed systems for measuring differential displacements across a fault zone, and examined their resolutions through seafloor experiments at relatively short baselines. A system for a seafloor extensometer makes use of precise acoustic ranging with a linear pulse compression technique. The system has a resolution better than 1 cm in acoustic ranging over a baseli ne of at least 1 km. The most critical problem is correction for temperatur e variations, and we estimate that the effect can be corrected with cm-orde r accuracy in the case of a deep-sea experiment. We have also examined a le veling system on the seafloor using an array of ocean bottom pressure gauge s and an ocean bottom gravimeter to detect differential vertical motion, Th e system is estimated to have a resolution of several centimeters in vertic al displacement. These system will be useful for triangulation and leveling on the seafloor, but we need further studies over a longer baseline and to achieve better long-term stability.