Onboard Realtime Processing of GPS-Acoustic Data for Moored Buoy-Based Observation

Realtime observations of vertical/horizontal seafloor movements and sea surface height associated with a huge earthquake are crucial for immediate recognition of its causal fault rupture, so that tsunami early warning can be issued and also the risk of subsequent ruptures can be evaluated. For this purpose, we developed an offshore monitoring system using a moored buoy platform to measure, in realtime, the three observables mentioned above and operated it on a trial basis for a year. While operating the system, GPS-acoustic observation of horizontal movement on the buoy was especially a new challenge. To achieve realtime GPS-acoustic observation under conditions of the limited power supply and narrow bandwidth in satellite communication, we developed special hardware suitable for use on a buoy and software to minimize onboard computational procedures and data transmission. The system functioned properly through the year; 53 regular weekly measurements and 55 on-demand measurements at arbitrary timings. Each measurement consisted of 11 successive acoustic rangings. The buoy tended to drift far from the preferred position for GPS-acoustic measurement, i.e., the center of the seafloor transponder array, due to strong current. The accuracy of the GPS-acoustic positioning achieved similar to 46 cm ( 2 sigma) even only with a single ranging when the buoy was inside the array, while it degraded to similar to 1.0 m when the buoy was outside the array. Although the 1.0 m accuracy is a detectable level of possible displacement due to a M8-class earthquake in the source region, further improvement to keep the drifting range smaller despite the current will enhance the utilization of the system.