Evaluation of GPS/Acoustic Seafloor Positioning at Suruga Bay, Central Japan

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• Ikuta, R.
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• Ando, M.
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• Tadokoro, K.
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• Okuda, T.
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• Besana, G. M.
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• Nagao, T.
;
• Sayanagi, K.

Abstract

Observations of seafloor crustal deformation is very important to understand the dynamics of plate boundary that include the strain accumulation processes, great interplate earthquakes mechanisms, and submarine volcanic activities. We have been developing an observation system with GPS/Acoustic combination technique to monitor seafloor crustal deformation at Suruga Bay from 2002 [Tadokoro et al., 2003]. In this system, repeated measurements of seafloor transponders could reveal directly the seafloor crustal deformation in focal area of the subduction zone. The Suruga Bay is an appropriate site to evaluate the seafloor positioning because we can set the reference GPS station with short baseline (< 20 km). Such distance could produce an error on kinematic GPS positioning of about 1-2 cm. In addition, the convergence rate at Suruga Trough is 2-4 cm/year. For the above reasons, this study would evaluate the results of seafloor positioning using our observation system FY 2005 undertaken at Suruga Bay area. In our observation area, three acoustic transponders are installed as an array on the seafloor at depths of about 800 m. These transponders are located within radius of about 300 m. Using the low-engine-noise vessel from 2005, we can control and keep the distribution of acoustic measurements for each transponder. In parallel with the acoustic measurements, the_@sound speed profile was also measured in seawater using a CTD profiler three times per observation that lasted for about 5 hours. The data analysis consists of three main components; (1) kinematic GPS analysis, (2) cross-correlation of acoustic waves to calculate round-trip travel time between the transducer attached on the side of the vessel and the seafloor transponder, and (3) a combination of results from (1) and (2) to estimate positions of seafloor transponders. Assuming the sound speed is constant for a given length of time and undergoes changes during some certain time span throughout the observation period, we estimate the correction coefficient of sound speed and the position of three seafloor benchmarks simultaneously for each time span [Ikuta et al., 2004]. First observation for this evaluation is undertaken on 22 and 23 June 2005. The repeatability of weight center positioning in short time was 3.3 cm and 4.6 cm in horizontal and vertical positions, respectively. We will report the results of positioning of the seafloor transponders and evaluate the error of positioning_@and validity of estimation of temporal variation in sound speed.