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Long-term borehole monitoring system

All completion operations for the long-term observatory installation at Site C0002 were conducted during IODP Expedition 332, beginning with drilling the hole with LWD and followed by installation of 9⅝ inch casing with a screened section. We then prepared a completion string with all the sensors, cables, pressure ports, and hydraulic lines, a swellable packer (attached to 3½ inch tubing joints), and finally terminated the assembly with a CORK head mounting UMCs and hydraulic lines with valves and a pressure recorder (Fig. F9). The final assembly operation involved attaching the ROV platform to the string prior to lowering everything into the water. All of the string assembly was conducted in a LCA to reduce the chance of damage and interference with safe operations from the Kuroshio Current to the sensitive instruments and cables. The following comprised the most difficult operations for observatory installation at Site C0002: drifting and guiding the whole completion string into the borehole location without damage from the 3.5–4.5 kt Kuroshio Current in the uppermost 500 meters below sea level (mbsl). Suppression of current-induced vibration was successful (see “Vortex-induced vibration measurements”). Also, a series of communication tests with the individual borehole instruments were conducted to confirm the health of sensors and cables in the course of operations such as lowering, drifting, reentering the borehole, and landing before cementing of the instrument. A detailed sequence of operations is given in “Operations.” Here, we describe how the borehole and completion string were configured and assembled, including tests conducted to verify each step of observatory installation as well as the initial data obtained from the borehole.

Completion string

The main part of the completion string was the 3½ inch tubing, functioning as a central support for the observatory and as a cement delivery line for cementing the strainmeter and seismic instruments. The tubing supports the cables and hydraulic lines (flatpack with triple ¼ inch stainless steel tube) as well as the thermistor string in the lower 146 m section above the instrument carrier. Because the cables and the thermistor string are delicate and susceptible to damage from being scratched on the casing or open borehole, the cables were secured to the tubing with tie wraps spaced at intervals of ~1 m (Fig. F10). We also attached finned centralizers (Fig. F11) to the tubing to help prevent damage to the cables from brushing the casing and borehole wall. In the lower section, we inserted four centralizers per tubing joint, and two centralizers per tubing joint above the float collar depth. At each centralizer, the cables were covered with rubber protectors. Hydraulic tubes were attached to the tubing with stainless bands as illustrated in Figure F10.

Because there is a small clearance between the outside diameter of the 3½ inch tubing and the drift diameter of the 9⅝ inch casing (~8.6 inches or so), extra care was needed to prevent unintentionally increasing the diameter of the string after fastening the cables and wrapping with tie wraps, tape, and rubber protectors. In critical sections, such as just above the instrument carrier and at the CORK head landing point to the casing hanger, we also checked the clearance to the borehole using a ruler (Fig. F11).

The swellable packer is another point of small clearance to the 9⅝ inch casing. Swelling began as soon as the packer was attached to our system and lowered into the sea. We checked the swelling of the packer by ROV camera before running the string into the borehole (Fig. F12). The packer is designed to swell and seal the inside diameter of the 9⅝ inch casing within 2 weeks after installation, although the sealing time may be extended by cold water temperatures. Visual inspection of the swellable packer using the ROV confirmed measurable inflation after 4 days of installation. We did not encounter difficulties inserting the packer into the casing to the intended depth.

The orientation of the seismic and tilt sensors relative to each other is constrained by the design of the instrument carrier, and we tried to orient the sensors to face east by rotating the whole completion string during visual inspection by ROV before the string was entered into the borehole.

Layout of sensors

The location of the strainmeter, tiltmeter, seismometers, and pressure ports for pore fluid pressure monitoring were determined based on information obtained from a nearby borehole (Hole C0002A ~50 m east-northeast of Hole C0002G) drilled during a previous expedition as well as LWD data collected during the drilling of Hole C0002G for observatory installation. Gamma ray and resistivity data from LWD tools in Hole C0002G showed good correlation with the Expedition 314 Hole C0002A LWD data set, confirming the depths of major boundaries, with only a few meters difference in depth (see “Downhole measurements”).

The bottom-hole pressure port (PP1, Fig. F13) is configured to sit below the Unit III/IV boundary to sample pore fluid pressure in the accretionary complex beneath the Kumano Basin sediment. The depth of the strainmeter location was chosen carefully to position its sensing surface (1.7 m interval) into the zone without fracture in the lower Unit III mudstone layer. Pressure Port PP2 then occupies a depth just below the strainmeter sensing surface (Fig. F14). The strainmeter depth is further considered so that the other seismic instruments, connected just above the strainmeter, may also be situated in a fracture free zone. Above the strainmeter is the instrument carrier (Fig. F15), hosting a Guralp broadband seismometer and tilt combo package (tiltmeter, geophone, accelerometer, and thermometer string digitizer). Depths of each of the five thermometer nodes of the thermistor string were determined automatically from the depth of the instrument carrier where the bottom of the thermistor string is terminated at the digitizer. We tried to optimize the location of the thermometer node by cutting and molding the thermistor string after LWD drilling of Hole C0002G, which resulted in a total thermistor string length of 146 m, with the two uppermost nodes positioned in the screened casing interval, one below in the casing (but not cemented), and the remaining two nodes in the cemented section above the instrument carrier.

The 9⅝ inch casing string represents important equipment used to guide all sensors to the bottom open hole section safely, but the casing shoe needed to be located above the seismic sensor and cemented section to ensure that no casing motion from above is transmitted to the seismic sensors. In this hole, the casing shoe depth was set at 888 mbsf, above the top of instrument carrier, in an interval where there were no significant washouts. The depth of the screened casing was set in a mudstone layer in Unit II so that we can compare the pore fluid pressure in sedimentary layers with the accretionary prism. Finally, the swellable packer depth was chosen to be close to the screened casing interval to minimize the volume associated with the pressure measurement in pressure Port PP3, yet in accordance with 9⅝ inch casing configuration so that the packer sealing surface (~1.5 m) does not overlap with the casing joint. Table T2 and Figure F9 summarize the depth of major observatory items along with unit boundaries in Hole C0002G.

Cementing is another crucial part of the observatory. The strainmeter needs to be coupled to the formation in order to sense the change of ground strain. Fluid flow around the strainmeter and seismic sensors causes severe noise. Therefore, the strainmeter and seismic sensors mounted in the instrument carrier were cemented in the open hole section below the 9⅝ inch casing shoe to provide good coupling, eliminate free fluids in the vicinity of seismic sensors, minimize sensing volume (i.e., fluids) around the bottom pressure Port PP1, and provide isolation from the motion of the 9⅝ inch casing. Schlumberger “FlexSTONE” cement slurry with nonshrinking characteristics was used to cement the strainmeter and seismic instrument. The slurry was optimized and tested in a laboratory to give a Young’s modulus similar to that of the target formation to be as compatible as possible to the surrounding formation in the zone of strainmeter installation, resulting in a Young’s modulus of 5.5 GPa, Poisson’s ratio of 0.17, and specific gravity of 1.90 g/cm3.

Assembly and tests during installation

Prior to the assembly of the completion string, each sensor went through a series of testing. The strainmeter was tested in a pool in the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for background noise performance before shipment to Shimizu. It was also tested and set up for deployment in the Core-tech shop on the Chikyu before being made up with 3½ inch tubing on the rig floor. The tilt combo system and Guralp CMG3T seismometer were tested for function and performance in the Matsushiro vault before shipment to the Chikyu (see “Long-term borehole monitoring system” in the “Methods” chapter [Expedition 332 Scientists, 2011b]). These instruments also went through overnight test runs on the Chikyu and were setup for deployment in the Core laboratory and assembled onto the instrument carrier in the Core-tech shop. The software in the strainmeter and tilt combo was organized to go through an automated checking procedure upon power up so that a change in the condition of the instrument may be identified by comparison with previous test results.

While building up the instruments with tubing joints and cables to form the completion string, we performed a function check of these three instruments at each major step where the condition of the instruments could have been affected:

  1. Upon connection to the borehole cable,

  2. Before inserting the cable to the swellable packer,

  3. After inserting the cable to the swellable packer,

  4. Before cutting cable for termination to the ODI connectors,

  5. After cable termination,

  6. After mounting the terminated connectors on the CORK head,

  7. After running the string in water and when the CORK head is at 350 mbsl,

  8. After drifting and before running the string into the borehole, and

  9. When the CORK head landed on to the reentry funnel.

For each of these tests, we performed function checks for the points listed in Table T3. After landing the completion string onto the wellhead, we checked the condition of the strainmeter and tilt combo systems by ROV communication and set the valve positions of the pressure system to prepare for circulation and cementing of the borehole instruments. After cementing, the drill string was released from the completion string at the CORK head. Finally, we initialized the observatory by operating valves to connect the borehole pressure ports to the pressure logger as well as to recover initial borehole pressure data by ROV communication.

These tests produced more or less the same results each time, proving the healthy condition of each instrument, except that one of the strainmeter valve status indicators showed erratic values after the test prior to cable termination. This will have little effect on the function of the instrument and continued installation because the valve indicator is for an optional valve that is usually not used, and we also confirmed that the corresponding valve as well as others can be operated without any problem. During Test 8 (after drifting and before running the string into the borehole), the ROV pulled the ODI UMC for the Guralp CMG3T seismometer too hard and separated the connector from the mounting plate, but the cable remained undamaged and there was no problem with the function of the seismometer. We took extra care when connecting to other ODI connectors for the strainmeter and tilt combo because the mounting structure was similar for all three instruments. No similar problems were encountered with the strainmeter and tilt combo and we confirmed that these instruments were functioning normally after landing the whole completion string in the borehole. Still, we decided to postpone another check on all three instruments after cementing until we visit the observatory using the JAMSTEC R/V Kaiyo and ROV Hyper dolphin. Prior to another attempt to download data, it is planned to additionally secure the connectors for safer ROV operation.

Observatory status and initial data recovery

Before lowering the completion string into the water, all the hydraulic tubes were opened to seawater pressure by opening two-way valves on Bay 1 of the CORK head, allowing fluid to enter the tube and displace trapped air. All of the pressure transducers were connected to ocean by three-way valves to protect the transducer from excess pressures and damage. After landing on the borehole but prior to cementing, all two-way valves were closed to inhibit borehole fluid flow in the hydraulic tubes and were kept closed until after cementing operations. When cementing was completed and the completion string was released using the running tool, we switched the three-way valves to connect three pressure transducers to start observation of borehole pressure. By ROV communication, we obtained a record of ~15 h since observation started as well as all pressure readings prior to switching the valves that record the pressure of seawater at the depth of the pressure recorder. The record (Fig. F16) clearly shows tidal variations with the pressure disturbance during circulation and cementing before the valves were switched. After switching the valves, we observed a shift in pressure and different tidal response from the ocean, especially for the bottom pressure port (PP1) below the cemented section near the strainmeter. At first glance, this suggests the cement column successfully isolated the bottom section from the upper sections.

We obtained temperature data from bottom-hole instruments and thermometer string as shown in Table T4. These temperature readings were consistent, except that the strainmeter temperature was lower by 2°~3°C. One possible explanation may be that the thermometer is enclosed in the pressure cylinder and we kept lowering the string until just before the measurement was taken. Communication with the Guralp CMG3T seismometer was confirmed before lowering the instrument into the borehole and will be checked during the next visit to the observatory by ROV connection, along with the other instruments.

Before leaving the observatory, we made sure the valve configurations and the ODI connectors for the bottom-hole instruments were protected by a dust cover to prepare for the next visit to initialize the observatory for long-term observation. Figures F17, F18, and F19 illustrate the arrangement of the seafloor part of the observatory (CORK head and ROV platform), showing orientation of the pressure recorder and ODI Teledyne UMCs.