Mississippi Canyon Federal Lease Block 118 has been selected by the Gulf of Mexico Gas Hydrates Research Consortium for emplacement of a gas hydrates sea floor observatory. A high-resolution acoustic survey using an autonomous underwater vehicle (AUV) was conducted at the site to help Consortium members understand the site settingand aid in the installation of the observatory. The AUV was equipped with a 200 kHz swath bathymetric system; a dual frequency (120 and 240kHz) side scan sonar and a 2-10 kHz chirp sonar subbottom profiler. The survey was run across the entire block on east-west transects spaced 200 meters apart with two additional north-south transects near the block boundaries. The high-resolution bathymetry, side scan sonar images and chirp data obtained by this survey provide detailed geomorphic, stratigraphic and structural character of the seafloor at the observatory site. Special reprocessing of the swath bathymetry data by Alessandro Bosman (one of the authors) enhanced the detail of the images even further.
A three-dimensional projection of the reprocessed data (fig. 1) illustrates the gentle southeast slope across the block with the gas hydrate mound clearly visible in the south-central area. A regional submarine canyon cuts across the northeastern part of the block. Slope gradient maps of the site indicate the slope to be at most three to four degrees across the site with steeper slopes south of the hydrate mound. In the area of the mound, slope angles present a chaotic picture of localized ribbon patterns with slopes on the order of 10 to 12 degrees. The steepest slopes, however, are along the rim of the submarine canyon where locally they are as steep as 30 degrees.
A shaded relief map in gray scale (fig. 2) provides a clear, crisp image of the site. At least three semi-circular vent structures1 are present. A detailed view of the mound area (fig. 3) show that the conical vent structures are actually composed of several small craters. Placing sensor arrays and microbial incubators in this area of the site will clearly pose more challenges than in other areas of the mound. It is this area, however, that active venting, gas hydrates deposits, and bacterial mats have been documented during a manned submersible dive at the site2. The dive video (with audio) attests to the submersible operator's consternation with negotiating the tight spaces of the narrow and steep walled vents in attempts to retrieve hydrate samples.
The side scan data show high backscatter in areas of the mound that form roughly five separate semi-circular patterns, three of which align well with the vent structures identified at the site by Sassen1 (fig. 4). There are many processes associated with gas hydrate mounds that may be contributing to the textural contrast identified by the side scan and include biological and authigenic carbonate accumulation, gas hydrate formation, bacterial mats, and both surface and subsurface disturbances by stoping and venting fluids.