Expedition 332 summary

Abstract

One primary objective of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) complex drilling project is to drill and instrument a series of holes across the seismogenic subduction system offshore the Kii Peninsula, Japan. Integrated Ocean Drilling Program (IODP) Expedition 332 continued initial observatory operations begun during IODP Expedition 319 in 2009. This expedition focused mainly on engineering work, including (1) retrieval of a temporary observatory instrument (“SmartPlug”) installed during Expedition 319 at IODP Site C0010, which penetrates the shallow “megasplay” fault in the mid-forearc; (2) deployment of an upgraded temporary observatory known as a “GeniusPlug” at Site C0010; and (3) installation of a permanent observatory at IODP Site C0002 in the outer Kumano Basin, at the location of planned future deep riser drilling. Expedition 332 began on 25 October 2010 and ended on 11 December 2010. During the first half of the expedition, the focus was on exchanging the SmartPlug temporary observatory with a GeniusPlug, both attached to a retrievable casing packer above the screened megasplay fault zone at Site C0010. The SmartPlug recovery was successful despite the strong Kuroshio Current, which can be attributed to an efficient reduction of vortex-induced vibration (VIV) on the drill string by attaching ropes. Time series data recovered from the self-contained instrument include seafloor and formation pressure as well as four independent temperature records from the fault zone and the overlying seafloor reference. Tentative analysis of the data documents effective sealing of the bridge plug; dampened pressure amplitudes in the tight, slightly overpressured formation; and identification of prominent earthquake and tsunami events in the 15 month record (23 August 2009–7 November 2010). The SmartPlug was replaced with a GeniusPlug, which is similar in geometry and also self-contained but with the addition of an OsmoSampler for collecting fluids for geochemical analysis and a flow-through osmo colonization system (FLOCS) for microbiological study. The system was installed at a depth that placed the OsmoSampler and FLOCS units in the center of the 22 m screened cased section across the megasplay fault. At Site C0002, located 11 km upslope of Site C0010, a new hole was drilled with logging while drilling (LWD) and cased for placement of a long-term borehole monitoring system (LTBMS). The monitoring system comprises a circulation obviation retrofit kit (CORK) assembly with a hydrogeological unit measuring pressure at four depth levels as well as a volumetric strainmeter, broadband seismometer, tiltmeter, geophones, accelerometers, and a thermistor string. The key goals include pore pressure monitoring in the upper accretionary prism (Unit IV), a series of measurements in the homogeneous sediments of Unit III (strain, tilt, seismicity, and pressure) in the sediments at the base of the Kumano Basin fill, directly overlying the accretionary prism below, and temperature and pressure monitoring in the overlying Kumano Basin sediments of Unit II. The lower portion of the assembly is hydraulically isolated from the overlying ocean by a swellable packer placed at 746 meters below seafloor (mbsf). Part of the instrument string below was cemented (~780–935 mbsf) to couple the strainmeter and seismometer to the formation/casing. The CORK head was revisited prior to and after cementing for system tests of the borehole instruments using the remotely operated vehicle (ROV), and all of these experiments were successful. The CORK was left with minimum battery power after the expedition and will be revisited in 2011 by ROV to connect an additional seafloor unit for power and data storage. In winter 2011/2012, the unit is anticipated to be connected to the real-time seafloor cabled network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET).