Paper:

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

## Motoyuki Kido^{*1,†}, Misae Imano^{*2}, Yusaku Ohta^{*2}, Tatsuya Fukuda^{*3}, Narumi Takahashi^{*4}, Satoshi Tsubone^{*5,3}, Yasuhisa Ishihara^{*3}, Hiroshi Ochi^{*3}, Kentaro Imai^{*6}, Chie Honsho^{*2}, and Ryota Hino^{*2}

^{*1}International Research Institute of Disaster Science, Tohoku University

468-1 Aza-aoba, Aramaki, Aoba-ku, Sendai 980-8572, Japan

^{†}Corresponding author

^{*2}Graduate School of Science, Tohoku University, Sendai, Japan

^{*3}Marine Technology and Engineering Center, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan

^{*4}Earthquake and Tsunami Research Division, National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan

^{*5}Interlink Inc., Nagoya, Japan

^{*6}Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

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 ∼46 cm (2σ) even only with “a single ranging” when the buoy was inside the array, while it degraded 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.

*J. Disaster Res.*, Vol.13, No.3, pp. 472-488, 2018.

- [1] Y. Kaneda, K. Hirahara, and T. Furumura, “New Research Project for Evaluating Seismic Linkage Around the Nankai Trough –Integration of Observation, Simulation, and Disaster Mitigation–,” J. Disaster Res., Vol.4, No.2, pp. 61-66, doi:10.20965/jdr.2009.p0061, 2009.
- [2] T. Hori, “Earthquake and Tsunami Scenarios as Basic Information to Prepare Next Nankai Megathrust Earthquakes,” J. Disaster Res., Vol.12, No.4, pp. 775-781, doi:10.20965/jdr.2017.p0775, 2017.
- [3] Y. Kaneda, “Resilience Science for a Resilience Society in Seismogenic and Tsunamigenic Countries,” J. Disaster Res., Vol.12, No.4, pp. 712-721, doi:10.20965/jdr.2017.p0712, 2017.
- [4] T. Kato, Y. Terada, M. Kinoshita, H. Kakimoto, H. Isshiki, M. Matsuishi, A. Yokoyama, and T. Tanno, “Real-time observation of tsunami by RTK-GPS,” Earth Planets Space, Vol.52, pp. 841-845, doi:10.1186/BF03352292, 2000.
- [5] T. Nagai, “Development and Improvement of the Nationwide Coastal Wave Observation Network,” Proc. Techno-Ocean’2002, Paper-T1-1-2, 4pp., 2002.
- [6] Y. Terada, T. Kato, T. Nagai, S. Koshimura, N. Imada, H. Sakaue, and K. Tadokoro, “Recent Developments of GPS Tsunami Meter for a Far Offshore Observations,” GENAH, IAG Symp., Vol.145, pp. 145-153, doi:10.1007/1345_2015_151, 2015.
- [7] L. Mervart, Z. Lukes, C. Rocken, and T. Iwabuchi, “Precise point positioning with ambiguity resolution in real-time,” Proc. of the 21st Int. Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), pp. 397-405, 2008.
- [8] R. Hino, D. Inazu, Y. Ohta, Y. Ito, S. Suzuki, T. Iinuma, Y. Osada, M. Kido, H. Fujimoto, and Y. Kaneda, “Was the 2011 Tohoku-Oki earthquake preceded by aseismic preslip? Examination of seafloor vertical deformation data near the epicenter,” Mar. Geophys. Res., Vol.35, pp. 181-190, doi:10.1007/s11001-013-9208-2, 2014.
- [9] K. Kawaguchi, Y. Kaneda, and E. Araki, “The DONET: A real-time seafloor research infrastructure for the precise earthquake and tsunami monitoring,” Proc. of OCEANS’08 MTS/IEEE KOBE-TECHNO-OCEAN’08, doi:10.1109/OCEANSKOBE.2008.4530918, 2008.
- [10] Y. Kaneda, K. Kawaguchi, E. Araki, H. Matsumoto, T. Nakamura, S. Kamiya, K. Ariyoshi, T. Hori, T. Baba, and N. Takahashi, “Development and application of an advanced ocean floor network system for megathrust earthquakes and tsunamis, Seafloor Observatories,” P. Favali et al., Springer Praxis Books, doi:10.1007/978-3-642-11374-1_25, pp. 643-662, 2015.
- [11] N. Takahashi, K. Imai, M. Ishihara, K. Sueki, R. Obayashi, T. Tanabe, F. Tamazawa, T. Baba, and Y. Kaneda, “Real-Time Tsunami Prediction System Using DONET,” J. Disaster Res., Vol.12, No.4, pp. 766-774, doi:10.20965/jdr.2017.p0766, 2017.
- [12] K. Uehira, T. Kanazawa, S. Noguchi, S. Aoi, T. Kunugi, T. Matsumoto, Y. Okada, S. Sekiguchi, K. Shiomi, M. Shinohara, and T. Yamada, “Ocean bottom seismic and tsunami network along the Japan Trench,” 2012 AGU Fall Meeting abstract OS41C-1736, 2012.
- [13] F. I. González, H. M. Milburn, E. N. Bernard and J. C. Newman, “Deep-ocean Assessment and Reporting of Tsunamis (DART): Brief Overview and Status Report.” Proc. of the Int. Workshop on Tsunami Disaster Mitigation, 1998.
- [14] H. Tsushima, R. Hino, H. Fujimoto, Y. Tanioka, and F. Imamura, “Near-field tsunami forecasting from cabled ocean bottom pressure data,” J. Geophys. Res., Vol.114, B06309, doi:10.1029/2008JB005988, 2009.
- [15] Y. Ohta, T. Kobayashi, H. Tsushima, S. Miura, R. Hino, T. Takasu, H. Fujimoto, T. Iinuma, K. Tachibana, T. Demachi, T. Sato, M. Ohzono, and N. Umino, “Quasi real-time fault model estimation for near-field tsunami forecasting based on RTK-GPS analysis: Application to the 2011 Tohoku-Oki earthquake (Mw 9.0),” J. Geophys. Res., Vol.117, doi:10.1029/2011JB008750, 2012.
- [16] H. Tsushima, R. Hino, Y. Ohta, T. Iinuma, and S. Miura, “tFISH/RAPiD: Rapid improvement of near-field tsunami forecasting based on offshore tsunami data by incorporating onshore GNSS data,” Geophys. Res. Lett., Vol.41, pp. 3390-3397, doi:10.1002/2014GL059863, 2014.
- [17] H. Tsushima and Y. Ohta, “Review on near-field tsunami forecasting from offshore tsunami data and onshore GNSS data for tsunami early warning,” J. Disaster Res., Vol.9, No.3, pp. 339-357, doi:10.20965/jdr.2014.p0339, 2014.
- [18] F. N. Spiess, “Suboceanic geodetic measurements,” IEEE Trans. on Geoscience and Remote Senssing, GR-23, pp. 502-510, doi:10.1109/TGRS.1985.289441, 1985.
- [19] F. N. Spiess, C. D. Chadwell, J. A. Hildebrand, L. E. Young, G. H. Purcell Jr., and H. Dragert, “Precise GPS/acoustic positioning of seafloor reference points for tectonic studies,” Phys. Earth Planet. Inter., Vol.108, No.2, pp. 101-112, doi:10.1016/S0031-9201(98)00089-2, 1998.
- [20] K. Tadokoro, M. Ando, R. Ikuta, T. Okuda, G. Besana, S. Sugimoto, and M. Kuno, “Observation of coseismic seafloor crustal deformation due to M7 class offshore earthquakes,” Geophys. Res. Lett., Vol.33, L23306, doi:10.1029/2006GL026742, 2006.
- [21] M. Kido, H. Fujimoto, S. Miura, Y. Osada, K. Tsuka, and T. Tabei, “Seafloor displacement at Kumano-nada caused by the 2004 off Kii Peninsula earthquakes, detected through repeated GPS/Acoustic surveys,” Earth Planets Space, Vol.58, pp.911-915, doi:10.1186/BF03351996, 2006.
- [22] M. Fujita, T. Ishikawa, M. Mochizuki, M. Sato, S. Toyama, M. Katayama, K. Kawai, Y. Matsumoto, T. Yabuki, A. Asada, and O. L. Colombo, “GPS/Acoustic seafloor geodetic observation: method of data analysis and its application,” Earth Planets Space, Vol.58, pp.265-275, doi:10.1186/BF03351923, 2006.
- [23] Y. Matsumoto, M. Fujita, T. Ishikawa, M. Mochizuki, T. Yabuki, and A. Asada, “Undersea co-seismic crustal movements associated with the 2005 Off Miyagi Prefecture Earthquake detected by GPS/acoustic seafloor geodetic observation,” Earth Planets Space, Vol.58, pp. 1573-1576, doi:10.1186/BF03352663, 2006.
- [24] M. Sato, H. Saito, T. Ishikawa, Y. Matsumoto, M. Fujita, M. Mochizuki, and A. Asada, “Restoration of interplate locking after the 2005 Off-Miyagi Prefecture earthquake, detected by GPS/acoustic seafloor geodetic observation,” Geophys. Res. Lett., Vol.38, L01312, doi:10.1029/2010GL045689, 2011.
- [25] M. Sato, M. Fujita, Y. Matsumoto, T. Ishikawa, H. Saito, M. Mochizuki, and A. Asada, “Interplate coupling off northeastern Japan before the 2011 Tohoku-oki earthquake, inferred from seafloor geodetic data,” J. Geophys. Res. Solid Earth, Vol.118, No.7, pp. 3860-3869, doi:10.1002/jgrb.50275, 2013.
- [26] M. Sato, T. Ishikawa, N. Ujihara, S. Yoshida, M. Fujita, M. Mochizuki, and A. Asada, “Displacement above the hypocenter of the 2011 Tohoku-oki earthquake,” Science, Vol.332, pp. 1395, doi:10.1126/science.1207401, 2011.
- [27] M. Kido, Y. Osada, H. Fujimoto, R. Hino, and Y. Ito, “Trench-normal variation in observed seafloor displacements associated with the 2011 Tohoku-Oki earthquake,” Geophys. Res. Lett., Vol.38, L24303, doi:10.1029/2011GL050057, 2011.
- [28] S. Watanabe, M. Sato, M. Fujita, T. Ishikawa, Y. Yokota, N. Ujihara, and A. Asada, “Evidence of viscoelastic deformation following the 2011 Tohoku-Oki earthquake revealed from seafloor geodetic observation,” Geophys. Res. Lett., Vol.41, pp. 5789-5796, doi:10.1002/2014GL061134, 2014.
- [29] T. Sun, W. Kelin, T. Iinuma, R. Hino, J. He, H. Fujimoto, M. Kido, and Y. Osada, “Prevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake,” Nature, Vol.514, pp. 84-87, doi:10.1038/nature13778, 2014.
- [30] F. Tomita, M. Kido, Y. Ohta, T. Iinuma, and R. Hino, “Along-trench variation in seafloor displacements after the 2011 Tohoku earthquake,” Science Advances, Vol.3, No.7, e1700113, doi:10.1126/sciadv.1700113, 2017.
- [31] K. Tadokoro, R. Ikuta, T. Watanabe, M. Ando, T. Okuda, S. Nagai, K. Yasuda, and T. Sakata, “Interseismic seafloor crustal deformation immediately above the source region of anticipated megathrust earthquake along the Nankai Trough, Japan,” Geophys. Res. Lett., Vol.39, L10306, doi:10.1029/2012GL051696, 2012.
- [32] Y. Yokota, T. Ishikawa, S. Watanabe, T. Tashiro, and A. Asada, “Seafloor geodetic constraints on interplate coupling of the Nankai Trough megathrust zone,” Nature, Vol.534, pp. 374-377, doi:10.1038/nature17632, 2016.
- [33] K. Yasuda, K. Tadokoro, S. Taniguchi, H. Kimura, and K. Matsuhiro, “Interplate locking condition derived from seafloor geodetic observation in the shallowest subduction segment at the Central Nankai Trough, Japan,” Geophys. Res. Lett., Vo.44, pp. 3572-3579, doi:10.1002/2017GL072918, 2017.
- [34] M. Kido, H. Fujimoto, R. Hino, Y. Ohta, Y. Osada, T. Iinuma, R. Azuma, I. Wada, S. Miura, S. Suzuki, F. Tomita, and M. Imano, “Progress in the Project for Development of GPS/Acoustic Technique Over the Last 4 Years,” GENAH, IAG Symp., Vol.145, pp. 3-10, doi:10.1007/1345_2015_127, 2015.
- [35] C. D. Chadwell, “GPS-Acoustic Seafloor Geodesy using a Wave Glider,” AGU Fall Meeting, abstract G14A-01, 2013.
- [36] M. Kido, Y. Osada, and H. Fujimoto, “Temporal variation of sound speed in ocean: a comparison between GPS/acoustic and in situ measurements,” Earth Planet Space, Vol.60, pp. 229-234, doi:10.1186/BF03352785, 2008.
- [37] M. Kido, “Detecting horizontal gradient of sound speed in ocean,” Earth Planet Space, Vol.59, e33-e36, doi:10.1186/BF03352027, 2007.
- [38] N. Takahashi, Y. Ishihara, H. Ochi, T. Fukuda, J. Tahara, Y. Maeda, M. Kido, Y. Ohta, K. Mutoh, G. Hashimoto, S. Kogure, and Y. Kaneda, “New buoy observation system for tsunami and crustal deformation,” Mar. Geophys. Res., Vol.35, pp. 243-253, doi:10.1007/s11001-014-9235-7, 2014.
- [39] N. Takahashi, Y. Ishihara, T. Fukuda, H. Ochi, J. Harada, T. Mori, M. Deguchi, M. Kido, Y. Ohta, R. Hino, K. Mutoh, G. Hashimoto, O. Motohashi, and Y. Kaneda, “Buoy Platform Development for Observation of Tsunami and Crustal Deformation,” GENAH, IAG Symp., Vol.145, pp. 97-103, doi:10.1007/1345_2015_114, 2015.
- [40] N. Takahashi, K. Imai, Y. Ishihara, T. Fukuda, H. Ochi, K. Suzuki, M. Kido, Y. Ohta, M. Imano, and R. Hino, “Real-time and on-demand buoy observation system for tsunami and crustal displacement,” AGU Fall Meeting, abstract NH23A-0220, 2017.
- [41] M. Kido, H. Fujimoto, and Y. Osada, “Utilizing motion sensor data in past seafloor geodetic measurements,” J. Geodetic Soc. Japan, Vol.54, pp. 163-179, doi:10.11366/sokuchi1954.54.163, 2008 (in Japanese with English abstract).
- [42] M. Imano, M. Kido, Y. Ohta, T. Fukuda, H. Ochi, N. Takahashi, and R. Hino, “Improvement in the Accuracy of Real-time GPS/Acoustic Measurements Using a Multi-purpose Moored Buoy System by Removal of Acoustic Multipath,” GENAH, IAG Symp., Vol.145, pp. 105-114, doi:10.1007/1345_2015_192, 2015.
- [43] International Association of Oil and Gas Producers (IOGP), “Coordinate Conversions and Transformations including Formulas,” IOGP Publication 373-7-2, Geomatics Guidance Note number 7, part 2, p. 142, 2004.
- [44] T. Vincenty, “Direct and inverse solutions of geodesics on the ellipsoid with application of nested equations,” Survey Review, XXII, Issue 176, pp. 88-93, doi:10.1179/sre.1975.23.176.88, 1975.
- [45] T. Fukuda, Y. Ishihara, H. Ochi, M. Deguchi, N. Takahashi, K. Imai, M. Kido, Y. Ohta, M. Imano, S. Miyoshi, and H. Yamada, “Development of an on-demand monitoring buoy system for crustal deformation,” BlueEarth Sympo., BE17-P69, CST Nihon University, Tokyo, 2017.
- [46] R. Azuma, F. Tomita, T. Iinuma, M. Kido, and R. Hino, “Development and examination of new algorithm of traveltime detection in GPS/acoustic geodetic data for precise and automated analysis,” Earth Planet Space, Vol.68, No.143, doi:10.1186/s40623-016-0521-2, 2016.
- [47] C. Honsho and M. Kido, “Comprehensive analysis of traveltime data collected through GPS-acoustic observation of seafloor crustal movements,” J. Geophys. Res., Vol.122, pp. 8583-8599, doi:10.1002/2017JB014733, 2017.
- [48] M. Imano, M. Kido, Y. Ohta, N. Takahashi, T. Fukuda, H. Ochi, C. Honsho, and R. Hino, “Accuracy of real-time GPS/Acoustic measurement using a slackly moored buoy,” JpGU-AGU joint meeting, SOG72-15, 2017.
- [49] H. Ochi, Y. Ishihara, T. Fukuda, M. Deguchi, N. Takahashi, K. Imai, M. Kido, Y. Ohta, M. Imano, S. Miyoshi, and H. Yamada, “Development of an on-demand monitoring buoy system –acoustic data transfer for seafloor pressure gauge–,” BlueEarth Symp., BE17-P70, 2017.
- [50] H. Ochi, Y. Ishihara, and T. Sugiyama, “Concept of a tsunami warning buoy system and an acoustic telemetry system for high current area,” 2012 Oceans-Yeosu, pp. 1-3, doi:10.1109/OCEANS-Yeosu.2012.6263646, 2012.

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