To test the hypotheses discussed above, we propose six primary drill sites (HR1a, HR2alt, HR3a, HR4a, HR4b, and HR4), extending to depths of 60700 mbsf (Table T1, T2). Six alternate sites (HR1b, HR5a, HR6, HR1c, HR2, and HR2altB) were identified that will be drilled if problems are encountered at the primary sites or if time allows (Table T3). Locations of the sites are overlain on a map showing seafloor topography in Figure F1C.
The leg will start with a dedicated LWD effort. To recover the section prior to LWD, proposed Site HR1a will be cored to 350 mbsf depth. We will then employ LWD at proposed Sites HR1a (to 350 mbsf), HR3a (to 350 mbsf), HR2a (to 350 mbsf), and HR4a (to 100 mbsf), HR4b (to 60 mbsf), and HR4c (to 240 mbsf) (Table T2). If time allows, alternative proposed Sites HR1b, HR1c, HR5, and HR6 will also be drilled with the LWD (Table T3). The cruise includes a scheduled port stop to offload the LWD equipment and personnel.
Following this port call, we will continue coring operations. At each site, we will core with the advanced piston corer (APC). After APC refusal, we will core with the extended core barrel (XCB) and then the rotary core barrel (RCB) as necessary (Table T2). Two cored holes are planned for each site. Because of the ephemeral nature of gas hydrate, the drilling plan emphasizes downhole measurements and in situ sampling strategies. These include extensive use of the ODP and HYACE/HYACINTH systems to acquire cores under in situ pressure and tools such as the Davis-Villinger temperature probe (DVTP) to measure in situ temperature and pressure. This strategy should allow reconstruction of the distribution and concentration of gas hydrate and free gas stored in the sediment.
A novel aspect of this leg will be the use of infrared thermal imaging to scan each core (at least those from within and near the hydrate stability zone) immediately as it is brought on board. Because hydrate dissociation is a strongly endothermic process, cold spots thus detected should permit us to quickly identify portions of the core containing both massive and disseminated hydrate. Multiple scans may permit calculation of the original amount of hydrate present based on the change in temperature with time.
We will record pressure and acceleration at the bit during some of the APC drilling using the drill string acceleration tool to provide information on the time and intensity of impact of the APC. During this time, OBSs will be deployed nearby by the Ewing to record "background" seismic noise. We will test whether the APC generates a detectable signal on the OBS. If so, we will collect reverse-VSP data, with the APC acting as a downhole source. This source has the potential to excite shear waves, whereas the conventional air gun/water gun sources generate only P-waves.
Next Section | Table of Contents