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doi:10.2204/iodp.proc.314315316.102.2009 3-D seismic dataAcquisitionWe contracted with Petroleum Geo-Services (PGS) to acquire the 3-D survey with the M/V Nordic Explorer from April 2006 to May 2006. As the sound source, we deployed two arrays, each with 28 Sodera G-guns totaling 51 L (3090 in3), fired alternately at 37.5 m shot intervals. Modeling of the far-field signature of the G-gun arrays shows that the source was minimum phase with primary energy of 68.2 bar-m, peak-peak energy of 141.4 bar-m, and primary to bubble amplitude ratio of 86.7. The survey used four receiver cables spaced 150 m apart, each 4500 m long with 360 receiver groups at 12.5 m spacing (Fig. F3). This 4-streamer/2-source array geometry yielded 8 source-receiver common midpoint (CMP) lines per sail line at a spacing of 37.5 m and nominal 30-fold data. The resulting 3-D data set covers an ~12 km × 56 km area that extends from the Kumano Basin seaward to the frontal thrust in the dip direction and extends along strike ~4 km northeast and ~8 km southwest of the NanTroSEIZE drilling transect (Fig. F4). The long axis of the survey is oriented at 330.1°, and the total area of the survey was ~585 km2. The inline direction is parallel to the long axis of the survey and the cross-line direction is parallel to the short axis (60.1°). Navigation included a differential Global Positioning System (GPS; Fugro StarFix) for the survey vessel, with each G-gun array and tail buoy also having GPS receivers. A Sonardyne SIPS2 acoustic transponder array was deployed on the G-guns and streamers such that the location of each receiver was sensed and recorded for each shot. Onboard navigation processing was carried out using SPRINT software. Relative accuracy of the shot and receiver locations is estimated to be ±5 m. Acquisition was made difficult by the Kuroshio Current, which had maximum velocities during the survey of 4–4.5 km from the west-southwest with the main axis approximately perpendicular to our survey direction. Neither the current’s direction nor its magnitude were constant, so the feather angle of the streamers varied, thus making it impossible to perfectly match the offsets on adjacent passes, and therefore requiring significant additional passes to fill in gaps in data coverage. Turns at the southeast end of the survey were also complicated by the strong current, thus preventing most of the CMP lines from reaching the trench axis. Processing3-D seismic processing consisted of three stages. In the first stage, PGS provided a 3-D stack and poststack migration, which gave a “quick look” at the 3-D data volume that was useful for understanding the regional seismic reflection characteristics for choosing parameters for more detailed processing. During the second stage, the data set was processed through 3-D prestack time migration (PSTM) by Compagnie Générale de Géophysique (CGG) in Kuala Lumpur, Malaysia (see the “Appendix”). Because of the variable streamer feathering, several processing steps were necessary to fill and regularize all of the bins. Strong seafloor multiples also required several multiple attenuation steps in the premigration stage. The third stage consisted of 3-D prestack depth migration (PSDM) performed at Japan Agency for Marine Earth Science and Technology (JAMSTEC)-Institute for Research on Earth Evolution (IFREE). The final gathers that were preprocessed by CGG were used as input to the 3-D PSDM (see the “Appendix”). The 3-D PSDM clearly images details of faults and small-scale structures that were not easily recognized in the PSTM data set (Fig. F5). However, because of the length of the streamers (4500 m) and the strong feathering, velocity resolution deeper than ~4500–5000 m is less accurate. Therefore, although imaging of the deeper section (i.e., the top of oceanic crust) is quite good, the depths are less accurate than in the shallow region. Resolution of the 3-D seismic dataIn order to conduct proper geologic interpretations of seismic reflection data, the resolution of the data must be known. Seismic resolution is generally defined in terms of the wavelength of the data (Sheriff, 1991): Limit of resolution = λ/4 = V/4f, where
For the near-surface sediments, P-wave velocity is generally <1600–1800 m/s and frequency content is ~60–80 Hz. At our maximum drilling depths of ~1400 m, velocities are ~2400 m/s and frequencies are ~40 Hz. For the deepest regions imaged by our data set, velocities increase to >4000 m/s and frequency decreases to ~8 Hz. These values yield the following results:
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