Seismic reflection imaging of a Warm Core Ring south of Hokkaido
Mikiya Yamashita 1 4 Kanako Yokota 1 2 3 Yoshio Fukao 1 Shuichi Kodaira 1 Seiichi Miura 1 Katsuro Katsumata 21 Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan.
2 Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan.
3 Technology Center, Taisei Corporation, 344-1 Nase-cho, Totsuku-ku, Yokohama, Kanagawa 245-0051, Japan.
4 Corresponding author. Email: mikiya@jamstec.go.jp
Exploration Geophysics 42(1) 18-24 https://doi.org/10.1071/EG11004
Submitted: 8 July 2010 Accepted: 22 December 2010 Published: 25 February 2011
Abstract
A multi-channel seismic reflection (MCS) survey was conducted in 2009 to explore the deep crustal structure of the Pacific Plate south of Hokkaido. The survey line happened to traverse a 250-km-wide Warm Core Ring (WCR), a current eddy that had been generated by the Kuroshio Extension. We attempted to use these MCS data to delineate the WCR fine structure. The survey line consists of two profiles: one with a shot interval of 200 m and the other with a shot interval of 50 m. Records from the denser shot point line show much higher background noise than the records from the sparser shot point line. We identified the origin of this noise as acoustic reverberations between the sea surface, seafloor and subsurface discontinuities, from previous shots. Results showed that a prestack migration technique could enhance the signal buried in this background noise efficiently, if the sound speed information acquired from concurrent temperature measurements is available. The WCR is acoustically an assemblage of concave reflectors dipping inward, with steeper slopes (~2°) on the ocean side and gentler slopes (~1°) on the coastal side. Within the WCR, we recognised a 30-km-wide lens-shaped structure with reflectors on the perimeter.
Key words: multi-channel seismic reflection survey, prestack migration, seismic oceanography.
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