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doi:10.2204/iodp.pr.328.2010

Introduction

Operations carried out during Integrated Ocean Drilling Program (IODP) Expedition 328 were devoted to the installation of a hydrologic observatory near Ocean Drilling Program (ODP) Site 889, the location chosen for a CORK (circulation obviation retrofit kit) observatory installation during ODP Leg 146 in 1992 (Westbrook, Carson, Musgrave, et al., 1994; Davis et al., 1995). In that early attempt, the relatively unstable sediments of the formation rapidly intruded through the perforations and bottom of an open-ended casing liner and prevented proper sealing of the hole. The new installation utilizes the Advanced CORK (ACORK) design, developed initially for installations at the Nankai subduction zone during ODP Leg 196 to permit pressure monitoring at multiple formation levels on the outside of a 10¾ inch casing string. The casing is sealed at the bottom, leaving the inside available for future installation of additional monitoring instruments.

A broad range of objectives will be addressed with monitoring over the decades to follow, some the same as those to have been addressed by the installation at Site 889, including documenting the average state of pressure and the vertical component of fluid flow from the consolidating sediments in the frontal part of the Cascadia accretionary prism and investigating the mode of formation of gas hydrates. Other objectives have been added subsequently as a result of the multiple discrete monitoring levels provided by the ACORK configuration, advances in measurement resolution, and knowledge gained from other monitoring experiments. These include determining the influence of gas hydrates and free gas on the mechanical properties of their host lithology, the response of the formation to seismic ground motion, and the magnitude of strain at the site caused by episodic seismic and aseismic slip in this subduction setting. Instrumentation deployed at the time of drilling includes autonomously recorded seafloor and formation pressure sensors and seafloor temperature sensors. Sensors planned for deployment inside the sealed casing at a later date will measure temperature, tilt, and seismic ground motion. It is planned that all instruments will soon be connected to the NEPTUNE-Canada fiber-optic cable for power and real-time communications from land. This will permit sensor sampling rates, time accuracy, and monitoring lifetime that are much greater than would be possible with autonomous operation using battery power and locally stored data. The program grew from an ODP Letter of Intent submitted by E. Davis and Roy Hyndman and IODP Ancillary Project Letter 734 by E. Davis and Keir Becker.

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