Details of the 3D Shooting
The 3D MCS data grid will form a 15 km by 17 km rectangle, located in order to (1) include all three drillsites, so that we can incorporate stratigraphic, lithological, and seismological information from Leg 164; (2) cover the flank and crest of the ridge; (3) include the area where the "stringer" apparently joins or crosses the BSR; (4) tie in to both OBS profiles; and (5) cover an area where reconnaissance SCS data show strong lateral variations in gas distribution, including the change from a strong BSR to a weak or absent BSR.
The 3D data will be acquired using the Ewing's MCS system, which includes a 160-channel, 4-km streamer. Shooting at 25 m (10 s) interval, this will provide nominally 80-fold stacked data (20-fold after binning to 25 m line spacing). The sound source will be an array of two generator-injector (GI) guns, each configured as 105/105 in3. The bubble-pulse suppression ability of the GI gun provides a superior, high-resolution source wavelet. Our 1995 survey showed that a single 105/105 GI gun penetrates well beneath the BSR (Figure) ; on the OBS, the GI gun produces vertical-incidence reflections from 3.5 s beneath the seafloor (Figure) corresponding to about 7.0 s two- way traveltime on the SCS sections. We anticipate a gain in signal to noise of 69 over the 1995 data from stacking. Shooting will be at 25 m spacing (~every 10 s at 5 knots); record length will be 6 s with a 3 s deep-water delay, to allow recording of the first water-bottom multiple, which is useful for calculating vertical-incidence reflection coefficients . Shot times will be randomized to render previous-shot multiples incoherent.
The 3D grid covers a total area of 255 sq km, which can be shot in 19 days, assuming 5.0 knots, 100 m line spacing, and 2.5 hrs per turn. Line spacing will be 100 m, with data shot in the dip direction (NW-SE). Assuming a modest streamer feather angle of 3 degrees, which should be common in the eddies of the Gulf Stream, this line spacing will enable us to bin our cross-line data at 25 m spacing and interpolate to 12.5 m. For a dominant source frequency of 50 Hz, this will enable us to migrate cross-line dips of up to 43 degrees without aliasing. Alternatively, we could bin cross-line data at 50 m to achieve higher fold (40), which would (after interpolation) still enable migration of dips up to 17 degrees without aliasing, covering the dip angles of all stratigraphic horizons in the strike direction (Fig. 6). In the dip-line direction, the CMP spacing of 12.5 m will allow dips up to 43 degrees to be properly migrated; interpolation to 6.25 m will allow all dips to be properly migrated. The final image size will be reduced by the half-width of the migration operator, which is about 500 m for the velocity structure on our lines.
Navigation will be accomplished using differential GPS (DGPS) with four reference stations in the eastern U.S., using a STARFIX-MN8 Inmarsat Virtual Base Station Unit provided by John E. Chance and Associates. Streamer groups will be positioned using GPS fixes on the tail buoy, magnetic compasses on 12 Digicon Birds, and the DGPS position of the ship. This system will provide real-time navigation accuracy to 23 m and a computer log of positions. This system will be identical to the one that will have been used during G. Kent's Fall 1997 3D MCS experiment on the East Pacific Rise; indeed, two of the personnel participating in our cruise (Henkart and van Avendonk) will sail with Kent and gain important experience in 3D operations on the Ewing.
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