5-5　Developments of the measuring technology addressed for marine geodesy

 

To measure the crustal deformation on the seafloor is very important to know the distributions of the strain and stress in the vicinity of the plate boundary where large earthquakes occur. We developed GPS/acoustic seafloor positioning system (Fig.1). The precision acoustic transponder (PXP) system is the key component of the positioning system. We developed the PXP system for precise ranging over 10 km that is necessary for precise positioning of a point on the Pacific plate at a water depth over 6 km (Fig.2). The other key component is the system for the kinematic GPS positioning at the sea surface at a baseline length of several hundreds of kilometers. During the measurements, we tow a buoy system that install 3 GPS receivers, acoustic ranging instruments and a radio link to lower the acoustical noises generated by a research vessel (Fig.3).

We examined the system near Hawaii Island and seaward of the Japan Trench in 2001. We succeeded in receiving acoustic signals at slant ranges more than 15 km. The fluctuations of the positions are estimated to be less than 10 cm

 

Fig.1. Schematic diagram of GPS/acoustic seafloor positioning system. The distances between the transducer that is fixed on the bottom of the buoy and the precision acoustic transponder (PXP) systems on the seafloor are measured acoustically. The position of the transducer is calculated from the buoy-GPS data.

 

Fig.2. The precision acoustic transponder (PXP) system.

Fig.3. The buoy system that installs 3 GPS receivers and an acoustic transducer.