DATA ACQUISITION AND PROCESSING

Mulitbeam Bathymetry

We collected multibeam bathymetric data over the Leg 190 Muroto Transect area of the Nankai Trough in June and July, 1999, with the Maurice Ewing's Hydrosweep system. Data processing with the Lamont-Doherty Earth Observatory MB-System software (Caress and Chayes, 1996) consisted of determination of the sound velocity profile (using Levitus data), ray tracing through the water column to determine the depth of each beam point, manual ping-by-ping editing to remove spurious depth values, along- and across-track filtering to remove artifacts, and gridding at 100-m intervals. The grid was then contoured to produce a bathymetric map (Fig. F3) and was fitted with a smooth surface and illuminated to produce a shaded relief image (Fig. F4). This image was then combined with data from the Japan Hydrologic Office (Okino and Kato, 1995) to produce a regional shaded relief image for regional interpretations (Fig. F5).

3-D Seismic Reflection Data

We also collected a 3-D multichannel seismic data set on the Maurice Ewing. This survey imaged an 8 km × 80 km area with 81 individual lines, each 80 km long with a cross-track spacing of 100 m (Fig. F2). The ship's position was located at 1-s intervals with a commercial differential Global Positioning System (DGPS) navigation system supplied by Fugro Geodetic. Acquisition used a source array of 14 air guns with a total volume of 70 L (4276 in3) that was fired at 50-m intervals. Seismic reflection returns were received by a 6000-m, 240-channel digital streamer with a group interval of 25 m. Streamer navigation used 20 compasses and periodic DGPS location of the streamer's tail buoy. We recorded the seismic reflection data at 2-ms sample intervals in SEG-D format on 3490E tapes with the Maurice Ewing's Syntrack 480-24 recording system. In addition to the 3-D seismic lines, we collected several segments of two-dimensional (2-D) data at each end of the 3-D survey.

Initial data processing was conducted on the Maurice Ewing and consisted of reading the SEG-D tapes, antialias filtering, resampling from 2 to 4 ms, and writing the data in SEG-Y format on digital linear tapes. We also edited each shot for bad/noisy traces during the cruise. The 2-D line segments were processed through a standard 2-D sequence of geometry (12.5-m common midpoint [CMP]) bins, velocity analysis, normal move-out (NMO) correction, top mute, CMP stack, and F-K poststack time migration. The 3-D data were sorted into bins that arc 25 m wide in the inline direction and 50 m wide in the crossline direction. Three-dimensional processing consisted of velocity analysis, NMO correction, inside and top mute, 3-D stack, and 3-D poststack migration.

Figure F6 is a composite line consisting of the seaward extension of a 2-D line and an "arbitrary" 3-D line that passes through each of the Muroto Transect drill sites. This line illustrates the main structural elements of the Muroto Transect.

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