To test the hypotheses discussed above, we originally proposed three primary drill sites extending to depths of 400-700 mbsf. Preliminary analysis of the 3-D seismic data confirmed the rationale behind these three sites but led to minor modifications of site locations. It also led to the addition of seven shallow-penetration (<260 mbsf) sites to sample the massive hydrate at the summit and to determine changes along subsurface horizons that appeared to be fluid pathways feeding the summit vents. Four of these additional sites were primary sites and three were alternates as described in the Leg 204 Scientific Prospectus; all were approved for drilling by the Pollution Prevention and Safety Panel (PPSP). We drilled all sites except for alternate Site HR1c, for a total of nine sites. The locations of the sites drilled (see Table T2) are overlain on a map showing seafloor bathymetry in Figure F1C.
Leg 204 started with 3 weeks dedicated to LWD. For safety reasons, Site 1244 (proposed Site HR1a) was cored to 350 mbsf prior to LWD, to sample all seismic facies that were to be drilled. We had approval from the PPSP to proceed with LWD prior to coring at all additional sites, or until the time allocated to LWD was expended, if no safety issues were encountered while drilling at Site 1244. Our objective was to use the LWD data to determine where to deploy time-consuming special downhole tools. The LWD data were of excellent quality, and this strategy proved to be very useful.
Another novel aspect of this leg was the use of IR thermal imaging to systematically scan each core (from within and near the GHSZ) as soon as it was brought on board. Because gas hydrate dissociation is a strongly endothermic process, cold spots thus detected permitted us to quickly confirm what portions of the core contained significant amounts of gas hydrate. Experiments were designed to calibrate the temperature record obtained with the IR camera relative to concentration estimates obtained from PCS and pore water chloride concentration sampling.