An offshore observation buoy employing GPS data has been developed in Japan. The buoy system observes the offshore waves with the period range of swells to long period waves including tsunami components in the deep sea. The water depth at the station is about 1000m. For such deep-sea condition, several technical problems are under consideration. One of the important subjects is the relation of buoy motion and wave profiles. In this paper an experiment measuring the motion of buoy in the deepsea condition is described. The elastic spring is implemented in the experiment to reproduce the mass gravity of mooring chain under water. Experimental results reaveal the performance of buoy system installed in the deep-sea and the correlation between the buoy motion and wave profile.
An offshore observation buoy employing GPS data has been developed in Japan. Photograph 1 shows an example of offshore buoy station to measure the wave condition off the west island (Shikoku) in Japan. The heaving motion of buoy is obtained by GPS (Global Position System) and recorded in a main station located on the main island. The GPS buoy system becomes a tool suitable to measuring offshore waves in the ocean. Nagai et al. (2006) have proposed the offshore tsunami monitoring network employing several GPS buoys. The wave data obtained in deep water may be also useful to the integrated ocean observation system as the real time data is combined with other meteorological information like salinity, temperature, current etc. (NOAA, 2008). Meanwhile long period waves induced by the nonlinear infra-gravity waves bounded in offshore swells and tsunamis due to earthquakes on the continental plate boundary give great risk to the Japanese coastlines (Hiraishi, 2006, Okumura et al., 2007).