Details of the OBS Shooting
The crest of the Blake Ridge is an ideal location for the proposed shear wave experiment, because of the abundant geochemical and geophysical data obtained during ODP Leg 164. The interpreted increase in hydrate concentration toward the crest of the ridge provides an ideal target for our proposed OBS line across Sites 994, 995, and 997. The orientation of the line along geologic strike will enable quantitative waveform inversion, which assumes one-dimensionality. Eight OBSs will be deployed at 2 km spacing, starting 2 km NE of Site 997. The resulting data will enable us to (1) map possible lateral changes in Vs in the hydrated zone and relate them to changes in hydrate concentration, (2) test if the observed P- to-S conversion points are laterally continuous horizons (e.g. at the top of the hydrate zone at ~200 mbsf), and (3) determine whether there are differences in Vs structure between areas with a continuous BSR those lacking a distinct BSR.
The "stringer" reflection is most prominent in SCS data around 31o 40' N, 75o 35' W, along CH-12-92 line 12 (Figure). We propose to deploy five OBSs in this area along a line of geologic strike with a 1 km spacing. The close spacing will ensure that a flat, continuous section of the "stringer" reflection is targeted, which is important for waveform inversion, and will allow an investigation of possible lateral velocity changes along this reflection. Three additional OBSs will be deployed within the 3-D data grid to provide P-wave velocity control for depth migration of the 3D reflection data. Source-to-OBS offsets of 20 km will provide wide-angle information from the water multiple, which yields densely sampled subsurface reflection points.
We plan to shoot the profiles twice, once with two GI guns (2 chambers, 105 cu.in. each, ca. 70 Hz) to provide higher resolution and a clean source wavelet, and once with a stronger, lower-frequency Bolt airgun (300 cu.in., ca. 40 Hz), which will provide higher signal levels and arrivals from deeper beneath the BSR. The OBSs will be modified to allow a 5 ms sampling rate (during the pilot study they were limited to a 10 ms sampling rate), which is necessary to properly record the G/I gun signal and enable waveform inversion. The OBS data will also be used to obtain large- scale P-velocity models in order to facilitate processing and interpretation of the 3-D data set.
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