The original proposal for Expedition 311 included seven sites. Part of the proposed drilling program consisted of five sites to be drilled in a transect perpendicular to the continental margin across the North Cascadia accretionary prism. Located at the western edge of the transect, proposed Site CAS-04B is situated in the deep Cascadia Basin. The next site, CAS-03B, is located on top of the westernmost uplifted ridge, followed by Site CAS-02C, located in the first slope sediment basin. The fourth, Site CAS-01B, is located near Site 889. The fifth transect site (CAS-05D) represents the eastern limit of the gas hydrate occurrence on the margin. One additional site was included in the original proposal, CAS-06A, located at a cold seafloor vent. An alternate site for this cold-vent location, Site CAS-06B, was proposed, which is an inactive vent location. In addition to the proposed transect and cold vent drilling, a two-hole site (CAS-01C) for a hydrogeologic cross-hole experiment with A-CORKs was also included in the original proposal. As discussed in "Introduction," the proposed two-hole (Site CAS-01C) hydrogeologic cross-hole experiment has been dropped from this initial North Cascadia drilling program. The focus of this expedition will be the proposed multisite transect and the identified cold vent site.
For this expedition, we plan to carry out logging-while-drilling (LWD) and measurement-while-drilling (MWD) operations prior to coring each site (pending Environmental Pollution and Safety Panel [EPSP] approval). There is a strong scientific need to obtain LWD/MWD data prior to coring. The coring operations will be complemented by frequent deployment of the PCS/HYACINTH pressure core systems. It is essential to know before deploying the available pressure core systems what the gas hydrate concentrations and vertical distribution are to choose the optimum depths for pressure coring operations. The coring operations are also complemented by frequent sampling for interstitial water, headspace gas, and microbiological analyses. Although those samples will be taken at relatively regular depths (e.g., every 5 m), the sampling frequency can be adjusted if gas hydrate concentrations and distribution can be forward predicted through the analysis of the LWD/MWD precore logging surveys. Without prior knowledge of this distribution, subsampling may easily undersample (or miss completely) important sedimentary sections.
After a relatively short transit from Astoria, Oregon, LWD/MWD tools will be first deployed at proposed Site CAS-03B at the far western downslope end of the proposed multisite transect running from east to west down the continental margin (Table T1). The LWD/MWD hole at Site CAS-03B will be advanced to a depth of 300 mbsf. Site CAS-4B is now considered an alternate drill site. The next hole to be drilled and LWD/MWD logged will be at proposed Site CAS-02C, which will also be drilled to a depth of 300 mbsf. The third LWD/MWD hole will be drilled at proposed Site CAS-01B to a depth of 380 mbsf. The fourth LWD/MWD hole will be drilled at the cold vent proposed Site CAS-06A to a depth of 300 mbsf. The final LWD/MWD hole (proposed Site CAS-05D) will be drilled to a depth of 220 mbsf at the upslope limit of the planned multisite transect.
After completing the LWD/MWD logging program, the expedition will switch to conventional and pressure-controlled coring operations at each of the sites drilled during the LWD/MWD campaign. Each core hole will also be wireline logged (triple combination [triple combo] and Formation MicroScanner [FMS]-sonic tool strings). A single zero-offset VSP survey will be conducted at proposed Site CAS-01B. A single core hole is planned for each site. The standard core hole will consist of APC coring to an expected "refusal" depth of ~200 mbsf; each hole will be advanced to their proposed total depth (TD) by XCB coring. Interspersed with the APC/XCB cores in each hole will be approximately six PCS/HYACINTH pressure cores. Each of the APC core runs will also be monitored with the APC-methane (APCM) tool, which in turn will be used to analyze and modify gas hydrate core-handling operations. The coring program will start at proposed Site CAS-05D immediately following LWD/MWD operations. The core hole sequence will proceed down the continental slope to proposed Sites CAS-06A, CAS-01B, CAS-02C, and finishing at CAS-03B (Table T1).Proposed Downhole Measurement Strategy
Within the coring program special effort will be made to obtain geothermal gradient data to accurately predict the depth to the base of the gas hydrate stability zone at each site drilled during Expedition 311. Downhole temperature tool deployment will include multiple runs of the APC-temperature (APCT) tool (minimum of four runs per hole) and Davis-Villinger Temperature-Pressure Probe (DVTPP) (minimum of two runs per hole) systems both above and below the depth of the BSR.
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