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 followed up on 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 installed during Expedition 319 at IODP Site C0010, which penetrates the shallow “megasplay” fault in the midforearc, and installation of a new suite of temporary sensors; (2) deployment of an upgraded temporary observatory 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 few weeks, the focus was on exchanging the SmartPlug temporary observatory with an upgraded 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. Times 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 proves the effective seal 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 equally self-contained but with an addendum that hosts an OsmoSampler for collecting fluids for geochemical analysis and a flow-through osmotic colonization system for microbiological study. The system was installed at a depth that placed the addendum in the center of the 22 m wider screened cased section across the megasplay fault.

        At Site C0002, somewhat 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. The monitoring system comprises a circulation obviation refrofit kit (CORK) assembly with a hydrogeological unit measuring pressure at four depth levels as well as a broadband seismometer, volumetric strainmeter, tiltmeter, geophones, 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 transition zone, and temperature and pressure monitoring in the overlying Kumano Basin sediments of Unit II. The string of the CORK assembly had a total length of 965 m and was carefully secured during deployment with centralizers, bands, and straps to withstand the strong current. VIV was minimized using ropes, and acceleration was monitored during deployment. The lower portion of the assembly is isolated against the overlying ocean body by a swellable packer 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 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 spring 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).

        For more information: http://www.jamstec.go.jp/chikyu/exp332/e/expedition.html