A high-resolution 3D seismic reflection survey and prestack depth imaging in the Nankai Trough off southeast Kii Peninsula

  • Park Jin-Oh
    Ocean Research Institute, University of Tokyo
  • Tsuru Tetsuro
    Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology (Currently) Cosmo Energy Exploration & Development Ltd.
  • No Tetsuo
    Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology
  • Takizawa Kaoru
    Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology
  • Sato So
    JGI, Inc.
  • Kaneda Yoshiyuki
    Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology

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Other Title
  • 紀伊半島南東沖南海トラフでの高分解能3次元反射法地震探査と重合前深度マイグレーション処理
  • キイ ハントウ ナントウオキ ナンカイ トラフ デノ コウブンカイノウ 3ジゲン ハンシャホウ ジシン タンサ ト ジュウゴウマエ シンド マイグレーション ショリ
  • High-resolution 3D seismic reflection survey and prestack depth imaging in the Nankai Trough off southeast Kii Peninsula

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Abstract

 In late 2007, the IODP NanTroSEIZE project will commence drilling into the seismogenic portion of the megathrust along which the 1944 Tonankai earthquake (M = 8.1) has occurred off the Kii Peninsula, southwest Japan. In order to figure out high-resolution structure and physical property of the subducting Philippine Sea Plate (PSP) consisting of oceanic crust and incoming sedimentary strata, we have performed a high-resolution, three-dimensional (3D) multi-channel seismic (MCS) reflection survey (KR06-02 cruise) in the Nankai Trough off Kii Peninsula using R/V Kairei of the Japan Agency for Marine-Earth Science and Technology in March, 2006. This Kairei 3D survey area covers three drilling sites of the NanTroSEIZE. For the high-resolution 3D MCS survey, we used a ∼5-km, 204-channel streamer and ∼100 m separated dual source, each of which is composed of two G-guns plus a Gl-gun. Flip-flop shooting with 30 m interval yields ∼50 m separated two CMP lines, resulting in 3.5 x 52 km 3D seismic volume. Recording length with 1 msec sampling interval is 10 sec. Depths of source and streamer cable are 5 m and 8 m, respectively. We used REFLEX system for onboard real-time 3D QC binning as well as SPECTRA navigation system. For the 3D data, we applied bandpass filter, amplitude recovery, deconvolution, multiple suppression, 3D geometry, flexible binning, and NMO velocity analysis. Then, we have constructed and updated interval velocity volume model for 3D prestack depth migration (PSDM) following 3D prestack time migration using those CMP bin gathers. A velocity uncertainty test demonstrates that the final 3D PSDM velocity has a ∼5% maximum velocity uncertainty at ∼6 km depth. Final 3D PSDM result shows high-resolution seismic images of three Miocene to Pliocene Shikoku Basin sedimentary units (Units A, B, and C) including three remarkable turbidites. The oceanic crust of the subducting Philippine Sea Plate underthrusts the overlying the accretionary prism, and is traceable over the entire inlines. In particular, Unit C beneath the trough turbidite fill shows unusual increase of thickness and anticline structure, which have never been reported in the Nankai Trough. We observe a noticeable, strong reflector (Reflector-R) immediately beneath the anticline structure, which is located at almost midst level of the Unit C. We interpret that both the anticline structure and thickness increase might be resulted from duplication of stratigraphic section following oblique slip up the sidewall ramp.<br>

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