IODP expedition 328: Early results of cascadia subduction zone ACORK observatory

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 328 was devoted to the installation of an "Advanced CORK" (Circulation Obviation Retrofit Kit) in the Cascadia subduction zone accretionary prism to observe the physical state and properties of the formation as they are influenced by long-term and episodic deformation and by gas hydrate accumulation. Pressures are monitored at four levels on the outside of a standard 10 3/4-inch casing string, two above and two below the base of the gas-hydrate stability zone at 230 mbsf (m below seafloor). The casing was sealed at the bottom, leaving the inside open down to 302 mbsf for installation of a tilt meter, seismometer, and thermistor cable (scheduled for 2013). The initial data, recovered in July 2011, document an initially smooth recovery from the drilling perturbation followed by what may be a sequence of hole-collapse events. Pressure at the deepest screen is roughly 40 kPa above hydrostatic; higher pressures (80 kPa) are observed at the two screens close to the level of hydrate stability. Tidal variations at the deepest screen are in phase with ocean tides, and define a loading efficiency of 0.6, which is reasonable in light of the consolidation state of the for-mation (porosity ~0.5). Tidal signals near the level of gas hydrate stability display large phase lags, probably as a consequence of hydraulic diffusion stimulated by the large contrast in interstitial fluid compressibility at the gas-hydrate boundary. The degree of isolation among the screens, the anticipated good coupling, and the estimated strain-to-pressure conversion efficiency (~5 kPa μstrain -1) indicate that this installation will serve well to host a variety of hydrologic, seismic, and geodynamic experiments.