[摘要] Integrated Ocean Drilling Program (IODP) Expedition328 was devoted to the installation of an "Advanced CORK"(Circulation Obviation Retrofit Kit) in the Cascadia subductionzone accretionary prism to observe the physical stateand properties of the formation as they are influenced bylong-term and episodic deformation and by gas hydrate accumulation.Pressures are monitored at four levels on theoutside of a standard 10 3/4-inch casing string, two above andtwo below the base of the gas-hydrate stability zone at230 mbsf (m below seafloor). The casing was sealed at thebottom, leaving the inside open down to 302 mbsf for installationof a tilt meter, seismometer, and thermistor cable(scheduled for 2013). The initial data, recovered in July2011, document an initially smooth recovery from the drillingperturbation followed by what may be a sequence ofhole-collapse events. Pressure at the deepest screen isroughly 40 kPa above hydrostatic; higher pressures (80 kPa) are observed at the two screens close to the level of hydratestability. Tidal variations at the deepest screen are in phasewith ocean tides, and define a loading efficiency of 0.6,which is reasonable in light of the consolidation state ofthe for-mation (porosity ~0.5). Tidal signals near the level ofgas hydrate stability display large phase lags, probably asa consequence of hydraulic diffusion stimulated by thelarge contrast in interstitial fluid compressibility at thegas-hydrate boundary. The degree of isolation among thescreens, the anticipated good coupling, and the estimatedstrain-to-pressure conversion efficiency (~5 kPa μstrain
−1)indicate that this installation will serve well to host a varietyof hydrologic, seismic, and geodynamic experiments.
doi:
10.2204/iodp.sd.13.02.2011
