A long-term seafloor experiment using an acoustic ranging system: Precise horizontal distance measurements for detection of seafloor crustal deformation

We have developed a seafloor acoustic ranging system to monitor local seafloor crustal deformation. In August 2007 we carried out an experiment to estimate the long-term stability of acoustic measurements collected by the system. We deployed four precision acoustic transponders with about 920 m and 590 m spacing at the depth of 2035 m in Kumano-nada, southeast coast Japan, and we collected acoustic ranges for 4 months. The observed round-trip travel times showed a variation with peak-to-peak amplitude of about 0.75 m in the range. It was confirmed that most of the variation could be explained by changes in sound speed. The standard deviation in the acoustic measurements was about 1.0 cm after the correction for sound speed, and the average value of the ranges remained constant within 1 cm.

► A seafloor acoustic ranging system was developed for the study of seafloor crustal movements.
► This system is direct path using acoustic measurement between a pair of PXPs.
► This was a long term experiment needed to estimate the stability of the system.
► The standard deviation in the acoustic measurements was about 1.0 cm after corrections.