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doi:10.2204/iodp.sp.332.2010

Site C0002 riserless observatory

Site C0002 will be the location of the first CORK installation during NanTroSEIZE. The suite of sensors for the downhole portion of the observatory includes (1) pressure ports, (2) a volumetric strainmeter, (3) a broadband seismometer, (4) a tiltmeter, (5) three-component geophones, (6) three-component accelerometers, and (7) a thermometer array (Fig. F6). The set of sensors is designed to collect, as a whole, multiparameter observations in wide period range from months to 1/100 s and a wide dynamic range covering events from local microearthquakes, very low frequency earthquakes, to the largest, earthquake slips of the Tonankai plate boundary 6 km below the sensors. A 20 inch conductor pipe will be run to 36 mbsf, and then 9⅝ inch casing will be run to ~955 mbsf. To ensure good coupling of the strainmeter to the formation and to eliminate local fluid motion around seismic sensors and tiltmeter, the sensors will be cemented in the open hole section below the 9⅝ inch casing shoe in Unit III. One pressure port will be installed in Unit IV below the strainmeter to sample pore fluid pressure in the accretionary prism. Above the cemented sensors in the open hole, the 9⅝ inch casing has a screened interval to sample pore fluid pressure in Unit II below a swellable packer to isolate the interval from the seafloor. The downhole sensors are digitally connected to the seafloor where power is supplied and data are recovered, whereas pore fluid pressure is transmitted through hydraulic tubing to be recorded in the seafloor recorder. The observatory will be linked to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) submarine cable observatory network so that measurements can be observed in real time from a shore-based monitoring station, though it will be initially operated by seafloor installed batteries and data recorders, with data recovery by subsequent ROV operations.

As a fallback option in case of unforeseeable problems during installation of the long-term borehole monitoring system (LTBMS), a Baker-Hughes retrievable mechanical bridge plug instrumented with a GENIUSPlug (see "Site C0010 riserless observatory") is available to temporarily complete the hole.

Drilling strategy

The plan for riserless drilling at Site C0002 is to proceed in several steps (Table T1).

The uppermost 0–1401 mbsf at Site C0002 was logged with a comprehensive LWD program during Expedition 314 (Expedition 314 Scientists, 2009), and intervals 0–204 mbsf and 475–1057 mbsf were cored during Expedition 315 (Expedition 315 Scientists, 2009).

Logging/Downhole measurements strategy

Since Hole C0002A was previously drilled with LWD/MWD tools, no logging (other than with basic LWD) is planned for the tophole section of the proposed new hole (0–800 mbsf). Records from Expedition 314 LWD composite logging data and Expedition 315 core measurements can be detectable to the Unit II/III boundary at 830.4 mbsf and the Unit III/IV boundary at 935.6 mbsf.

The LTBMS will be set just above the Unit II/III boundary, and the strainmeter will be set just below the Unit III/IV boundary. Resistivity and gamma ray values at both the upper and lower boundaries showed sharp and clear higher to lower values to identify this section.

Logging while drilling/measuring while drilling

Drilling with LWD will be performed to monitor drilling conditions and define major lithologic and structural changes in real time, primarily to identify the best target depths for installation of the observatory components. Tools include the TeleScope MWD, located on the main body of the arcVISION LWD tool string (Fig. F7), which measures drilling parameters and transmits these data in real time and also stores data in onboard memory for later downloading on deck. Because of a gap of several meters from the top of the bit and sensors, there is a time delay between bit drilling and the sensors collecting data and transferring them to memory. During Expedition 314 LWD data, there was a 10–15 min delay, whereas during Expedition 332, we anticipate a much faster resistivity and natural gamma ray data update rate.

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