Seafloor geodesy and an ocean bottom cable system

Space geodetic technologies such as GPS, VLBI (very long baseline interferometry), SLR (satellite laser ranging), and SAR (synthetic aperture radar) have revolutionized geodesy on land. However, these technologies based on electric waves cannot be directly applied to seafloor geodesy because electric waves inclusive of light can hardly propagate in the seawater. We have to adapt sound waves and ocean bottom cables for underwater communication, positioning, power supply, and data recovery. Importance of crustal movement on the seafloor has been pointed out for long time, and at last the dream is coming true; there have been promising reports on GPS/acoustic seafloor positioning, repeated acoustic ranging on the seafloor, ocean bottom pressure monitoring, and tilt measurements in deep-sea boreholes. Long-term, continuous, and regional observation is crucial for geodetic monitoring of geodynamics. Now it is time for us to examine a new plan of scientific ocean bottom cable system. Although global coverage is difficult, trans-ocean monitoring systems or regional systems over seismogenic zones can be feasible. The latter can maintain the real time observations on the seafloor, which are crucial for the study how a big earthquake is prepared.