Observation for Seafloor Crustal Deformation Using Moored Buoy by GNSS-A Technique Considering the Heterogeneity of Sound Speed in Ocean

We started using the moored buoy off the coast of Ashizuri, Japan to observe seafloor crustal deformation continuously by GNSS-acoustic technique. The position of transponders set on the seafloor can be estimated by the acoustic ranging measurements. The main factor of positioning error is temporal and spatial variation of sound speed in ocean. Temporal variation is caused by the tide with a cycle of 12-24 hours and by the inertial gravity wave with a cycle of several 10 minutes. The spatial variation caused by the temperature, pressure, and salinity has large effect on the positioning in the area near the Kuroshio current because the temperature gradient is generated. Then, we introduced the sound speed structure model considering its temporal and spatial variation into the analysis. Proposed analytical method firstly determines the relative position of transponders using measurements of vessel and secondly estimates the displacement of transponders from initial position by using data observed by buoy with assumption that every single transponder moves in the same vector due to the crustal deformation. We applied this method to data observed by buoy for two weeks in April 2019 and discovered that the positioning accuracy has been greatly improved. The dispersion in the estimated position was within 2 cm, while it was 30 cm in the case of using sound speed model not considering its horizontal gradient. Thus, the proposed method is efficient in removing the effect of heterogeneity of sound speed in the analysis using short term observation data. We will evaluate it after accumulating long-term observation data.