The two multichannel
seismic profiles we used for this study (Fig.
2), are not located exactly on the three holes, but these data are the
closest MCS data available collected along the Blake Ridge BSR transect (the
tomographic method requires MCS data). The average distance between the MCS
profiles and the three sites is about one marine mile, and we projected the site
positions orthogonal to the MCS strike. We assume that the lithology, elastic
properties of the sediments, and the abundances of clathrates and free gas
remain the same as those revealed along the BSR transect. The southern line is
oriented north-south, and the northern line is oriented approximately north 25º
east. Seismic data were shot by the Teledyne Exploration Company in the 1970s
(Gilbert, 1977), using an air-gun source with volume 4 
540 in3 (shot spacing 50 m) to yield 48-fold coverage, a nonlinear
analog streamer 3.6 km long (50 m spacing between traces 48 through 25, 75 m
spacing between trace 25 and 24, 100 m spacing between traces 24 through 1). The
sampling interval was 2 ms. Spherical divergence correction has been applied on
field data.
The seismic profiles cross the Blake Ridge crest orthogonally. The three holes are located at water depths of 2799 m (Site 994), 2777 m (Site 995), and 2770 m (Site 997). Sites 995 and 997 are characterized by a strong BSR, whereas Site 994 does not show any evidence of BSR. The nature of the sedimentary section in this area is very uniform, and consists primarily of homogeneous nannofossil-rich clays, with lithologic variations occurring only within the uppermost 150 m (Paull, Matsumoto, Wallace, et al., 1996). The profiles show a thin-bedded sequence of reflectors in the upper 0.13 s below the seafloor. Lithologic variations mainly related to calcium carbonate content can account for these shallow reflections. Some bottom current-related features (sediment waves) on the northeastern flank of the Blake Ridge are seen.
The Blake Ridge BSR is a well-defined, high-amplitude reflection with a single symmetrical pulse; it crosses the seismic horizons that reflect the position and orientations of sedimentary layers, and it must therefore be younger and superimposed upon the acoustic sedimentary structure. Its seismic character is variable through the section; it is strong at Sites 995 and 997, and (from Sites 995 to 994) disappears gradually. The low reflectance seen on the seismic profile above the BSR—that is, the "blanking" effect known in literature (Shipley et al., 1979; Lee et al., 1993)—can be explained as the naturally low background reflectance caused by the uniform sediments (Holbrook et al., 1996).
