[S-CG67_2AM1] Ocean Floor Geoscience

[SCG67-13] A summary of the achievement in the project for advanced GPS/acoustic survey

*Motoyuki KIDO¹, Hiromi FUJIMOTO¹, Ryota HINO¹, Yusaku OHTA², Yukihito OSADA¹, Takeshi IINUMA¹, Ryosuke AZUMA¹, Ikuko WADA¹, Syuichi SUZUKI², Fumiaki TOMITA³, Misae IMANO³ (1.IRIDeS, Tohoku Univ., 2.RCPEV, Tohoku Univ., 3.Sci., Tohoku Univ.)

Keywords:GPS/acoustic, moored buoy, autonomous surface vehicle, Japan trench

GPS/acoustic survey is known as a most probable way to measure the crustal deformation of seafloor far from the coasts, where dense GPS network is not available. We, Tohoku University, together with Nagoya University and Japan Coast Guard dedicated in GPS/acoustic survey for more than decade. MEXT has been strongly promotes our activities though financially support as governmental project. We summarize individual topics in the project.For the moored buoy, collaborating with JAMSTEC and JAXA, we have started long-term continuous and realtime seafloor geodetic survey at Kumano-nada. At the early stage of the project, we employed a small buoy, which can be also used as towing survey, at off-Miyagi site. Because the size of battery is limited, sea-trials was lasting only for two days. However, using this platform, we developed an automatic ranging system and simple on-demand operation technique via UHF communication. In 2012, we have started developing a automatic ranging system in a realistic working condition using a time-proven platform, m-TRITON buoy, operated by JAMSTEC. Together with JAMSTEC and JAXA, satellite communication part and GPS positioning part have been shared for multi-purpose. Tohoku University group concentrate acoustic ranging part and onsite data processing to compute precise traveltimes. Using a limited onsite resource in the buoy, we have eliminated unnecessary and redundant procedure and data as possible. The first sea-trial took place in 2013 for four months and the ongoing second trial has started in 2014 for six months. In the second trial, acoustic ranging data has been successfully transmitted to onshore station every week and we can monitor it from our laboratory.For the Autonomous Surface Vehicle (ASV) system, we aimed to develop an automatic survey system, which can also be used simultaneous measurements from other platform, such as a research vessel, for improve the ranging accuracy with multi-acoustic-paths. In our system, vehicle is like an unmanned boat (2.4m long and 400kg in weight), whose propulsion system is driven by electric power from onboard diesel generator lasting for a week. As the ASV system demonstrates sufficient performance for our use in GPS/acoustic survey, it can be a candidate of multi surface platform for simultaneous ranging to achieve high accuracy GPS/acoustic measurement taking the spatial sound speed variation into account.After the Tohoku-Oki earthquake in 2011, the project has an extra mission that significantly enhance the survey framework, especially in deep seafloor (>5000m) near the trench axis. In this extra mission, we have developed a new type of seafloor transponder that works at over 5000m depth and its acoustic communication range is greater than 15km. We made 86 transponders in total and constructed 20 new GPS/acoustic station along the Japan Trench in 2012. In addition, we chartered a research ship for about 50 days per year to construct and observe the new stations. At present we have carried out four times of campaign surveys during 2012-2013. At these new stations, we conducted both moving and stationary surveys, the former generally took several hours and the latter 12 hours for each station. We found a problem in acoustic property in the new transponders, which can be corrected with post-processing shown in Azuma et al. (2014, JpGU). Campaign surveys ranges only about one year, but we have observed post-seismic movement at selected stations. These results are reported in Tomita et al. (2014, JpGU). The new transponders are hybrid type so that Japan Coast Guard has started to make measurements with their own system at several stations above.This work has been supported by MEXT project for advanced GPS/acoustic survey. Stuffs in RCPEV and IRIDeS, Tohoku University gave dedicated support and collaborative operation in the onboard and GPS surveys. The construction and surveys in the new stations were collaboration with Nagoya University.