Gradient field of undersea sound speed structure extracted from the GNSS-A oceanography
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After the twenty-first century, the Global Navigation Satellite System-Acoustic ranging (GNSS-A) technique detected geodetic events such as co- and postseismic effects following the 2011 Tohoku-oki earthquake and slip-deficit rate distributions along the Nankai Trough subduction zone. Although these are extremely important discoveries in geodesy and seismology, more accurate observation that can capture temporal and spatial changes are required for future earthquake disaster prevention. In order to upgrade the accuracy of the GNSS-A technique, it is necessary to understand disturbances in undersea sound speed structures, which are major error sources. In particular, detailed temporal and spatial variations are difficult to observe accurately, and their effect was not sufficiently extracted in previous studies. In the present paper, we reconstruct an inversion scheme for extracting the effect from GNSS-A data and experimentally apply this scheme to the seafloor sites around the Kuroshio. The extracted gradient effects are believed to represent not only a broad sound speed structure but also a more detailed structure generated in the unsteady disturbance. The accuracy of the seafloor positioning was also improved by this new method. The obtained results demonstrate the feasibility of using the GNSS-A technique to detect a seafloor crustal deformation for oceanography research.
KeywordsGNSS-A Seafloor geodesy Sound speed structure Ocean disturbance GNSS-A oceanography
The authors would like to thank O. L. Colombo of the NASA Goddard Space Flight Center for providing us with the Interferometric Translocation (IT) kinematic GNSS software and the Geospatial Information Authority of Japan (GSI) for providing us with the high-rate GNSS data for the kinematic GNSS analysis and the daily coordinates of the sites on the GSI’s website. JCOPE2 data was provided by JAMSTEC’s JCOPE website. We thank R. Zhang for speaking about and providing advice regarding the JCOPE2 data. We also thank the editor and two anonymous reviewers for their comments, which improved this article. Many among the staff of the JHOD, including the crews of the S/V Takuyo, S/V Shoyo, S/V Meiyo and S/V Kaiyo, have supported our observations and data processing.
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