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Long-term borehole monitoring system


A deep-sea borehole strainmeter was designed by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for deep-sea borehole installation (Fig. F9). The principle of the strainmeter is similar to the Sacks-Evertson type of volumetric strainmeter that measures the volume change of an oil-filled cylindrical strain-sensing volume that responds to ground deformation. Ground deformation is transmitted to the volume through the cement filling the annulus between the formation and the strainmeter. This type of strainmeter is widely used in land boreholes. For example, the Japan Meteorological Agency has deployed more than 10 volumetric strainmeters in land boreholes to monitor a possible precursor event for the Tokai anticipated earthquake.

The deep-sea strainmeter is installed in NanTroSEIZE Hole C0002G. For compatibility with IODP boreholes (targeting 9⅝ to 10¾ inch hole installation), the deep-sea strainmeter was designed to have a 168.3 mm diameter sensing volume, which is much larger than conventional land-based strainmeters (~75 mm). The length of the deep-sea strainmeter is ~10 m (Table T1), including digitizer electronics and transducer housing, chambers, joints, and the sensing volume (~1.7 m length). It also has a cement pass line through it so that cement can bypass the instrument and pour into the bottom of the borehole to cement the strainmeter and other seismic instruments.

The deep-sea borehole strainmeter is designed to withstand high ambient pressures up to the 70 MPa that the instrument experiences as it passes through the water column and into the borehole. The strainmeter is very sensitive to deformation of the sensing volume, achieved by measuring small volume changes of the sensing volume by the bellows connected to it (Fig. F10). The full-scale range of the volume change measured by the bellows is ~1 cm3, corresponding to 3 × 10–5 strain change of the sensing volume. While lowering the instrument from the ship, it experiences much larger strain changes than it would tolerate within the full scale of measurement. Therefore, to protect the sensor, a valve bypassing the bellows is installed, and it is kept open while the instrument is deployed. After the instrument is cemented into position, the bypass valve is closed by a command from the CORK head on the seafloor to start strain measurements. The bypass valve is also opened when the instrument measures strain changes outside its full measurement range. By opening the valve, the bellows are recentered to zero to reset accumulated strain; the valve can then be closed to resume strain measurement. The bellows position (indicating strain change) is measured by a digitizer in the strainmeter, and the strain data are transmitted uphole by an electrical cable. A Paroscientific pressure gauge (8B7000-2) is also connected to the strain sensing volume, and its pressure reading is used to check the system status of components such as the bypass valve.

A controller inside the strainmeter controls the digitizing displacement of bellows and pressure and valve position (open and closed) and transmits data through an RS-422 serial link (57,600 bps) with the CORK head at the seafloor. The cable can also conduct power (24–30 V DC) to the strainmeter. The power consumption of the strainmeter is ~3 W during observation, which increases to ~4.5 W when operating the valve.

Broadband seismometer

A CMG3T borehole broadband seismometer (Guralp Systems, Ltd.) was installed in the instrument carrier to monitor broadband seismic signals in the borehole. The CMG3T seismometer measures ground velocity in three orthogonal axes in the frequency range between and 50 Hz. Three separate sensors measuring ground motion for each axis are housed in a titanium pressure housing (Fig. F11). The seismometer is mounted near the bottom of the instrument carrier and is cemented in the hole to assure good coupling to the formation. Each sensor has a motorized leveling mechanism so that the sensors are functional within 4.5° of tilt.

In each sensor, a proof mass is supported on a pivot and suspended by a leaf spring during operation. The pivot is purposely weakly designed to assure very high sensitivity to ground motion. To prevent the weak pivot from damage during transport, the proof mass must be motion-locked. Locking and unlocking the proof mass, leveling of each sensor, and digitizing the x, y, and z ground motion are performed by a DM24 digitizer, which is installed in the same pressure housing.

The three-component velocity seismic data along with the position of proof mass and inclination of sensor from the Micro-Electro-Mechanical Systems (MEMS) tiltmeter are encoded in a format called Guralp Compressed Format (GCF). The data are digitally uplinked through the borehole on a serial cable in RS-422 protocol. The three-component velocity data are digitized at 100 Hz in 24-bit resolution, whereas the mass position and tilt from the MEMS sensor are digitized at 4 samples/s.

Time synchronization of the data is governed by the DM24 digitizer. The DM24 receives a time reference through the RS-422 downlink on the serial cable in a format called Streamsync. The same downlink serial connection is also used to send commands to the DM24 to control the CMG3T seismometer (e.g., level the sensor, lock, unlock, center the proof mass, and sensor calibration).

Electric power for the CMG3T broadband seismometer is supplied by the same cable used for the data link. The nominal functional voltage range is 24–36 V DC, and power consumption of the seismometer ranges from 2 to 3 W, although it increases to 6 W when operating the onboard motor to level, unlock, and lock the sensors. Because the 1 km borehole cable dissipates power because of resistance (~32 Ω), higher voltages (~30 V) need to be applied to the end of borehole cable so that the seismometer can receive sufficient power to run the motor. Table T2 summarizes the specifications of the CMG3T borehole sensor for this long-term borehole observatory installation.

The CMG3T broadband seismometer for this installation was modified from the original in that the sensor has a better supporting mechanism to reduce the impact of severe vibration and shock the sensor may encounter during deployment and installation into the bottom of the borehole. During Expedition 319, a sensor dummy run test was conducted at Site C0010, which revealed the possibility that sensors may experience more than 2 g of acceleration while being lowered to the seafloor borehole because of VIV caused by strong sea current (see Fig. F14 in the “Site C0010” chapter [Expedition 332 Scientists, 2011b]). Therefore, design of the sensor and the sensor carrier was revisited to reduce movement (and therefore damage) of components from vibration-induced stress. The design revision was confirmed through a series of vibration tests for each sensor before all the components were assembled. After assembly, each component of the system was checked for noise performance in a vault, subjected to vibration, and rechecked for noise performance again to make sure that vibration does not affect the performance of the seismometer. Upon delivery of the seismometer to Japan, we noise tested it in the Matsushiro vault of the Japan Meteorological Agency. We compared the seismometer with the reference sensors in the vault (Figs. F12, F13), and the results suggest that a good performance can be expected from the new design.

Tilt combo


The tilt combo is an integrated sensor module that combines sensors such as a tiltmeter, geophone, accelerometer, and stand-alone heat flow meter (SAHF) digitizer. A schematic diagram of the tilt combo is shown in Figure F14. The details are described as follows:

  • The three-component geophone (4.5 Hz GS-11D, OYO Geospace), the three-component accelerometer (JA-5H200, JAE), electrical boards, a CPU board, an A/D conversion board, a power supply board, and internal batteries, are installed in the tilt logger titanium cylinder.

  • Signal from the three-component geophones and accelerometers are digitized by the A/D converter (ADS1282, Texas Instruments) on the A/D conversion board at a 125 Hz sampling rate.

  • The tiltmeter (LILY, Applied Geomechanics) and the thermometer digitizer (SAHF, Kaiyo Denshi Co., Ltd.) have their own A/D converters in separate titanium housings and are connected to the tilt logger cylinder through an electrical cable. The LILY has a 5 Hz sampling rate, and the SAHF has a 0.5 Hz sampling rate.

  • All data from the sensors are merged and telemetered as RS-422 serial data to a recorder in the seafloor in WIN format. In stand-alone acquisition mode, all data may be stored to an SD memory card in the main tilt logger housing.

  • The tiltmeter, thermometer digitizer, and accelerometer can individually be powered by switching electrical relays on the power supply board.

  • Geophones may be calibrated by a circuit in the A/D board, which transmits an electrical pulse to the geophones.

  • The clock time can be synchronized to a GPS 1 pulse per second signal through the RS-422 serial link.

  • Power consumption is ~100 mA at 24.0 V when all the sensors are activated.

The tilt combo sensors were attached to the sensor carrier. We prepared at least two sets of each sensor for redundancy. Two geophone and accelerometer modules (1 and 2) were available. Three LILY tiltmeters (serial numbers N8035, N8068, and N8069) were also available, and two SAHF digitizers (1 and 2) were ready to be installed. Geophone and accelerometer Module 1, LILYN8068, LILY N8069, and SAHF Digitizer 1 were loaded on the Chikyu on 25 October 2010, whereas geophone and accelerometer Module 2, LILY N8035, and SAHF Digitizer 2 were loaded on the Chikyu during the Shimizu port call on 27 November 2010.

Performance test results

We conducted noise evaluation tests for the two sets of the tilt combo module at Matsushiro vault Seismological Observatory for confirming the noise level and long-term stability of the sensors. Matsushiro Seismological Observatory has an underground tunnel that is isolated from noise sources such as railroads, automobile traffic, and the sea coast. Temperature changes in the vault are small throughout the year. We also carried out a test at the JAMSTEC laboratory in Yokosuka, Japan, for performance in a noisy environment. An example of the time series data acquired in the Matsushiro tunnel is shown in Figure F15.

As a key result of the Matsushiro experiment, we obtained power spectral density (PSD) performance of the tilt combo sensors as shown in Figures F16, F17, and F18. Even in the low noise environment in Matsushiro, the microseism band peak of ~0.2 Hz can be clearly confirmed for the geophone, tiltmeter, and broadband seismometer as well as the reference broadband seismometer (CMG3T) deployed in the same Matsushiro vault.

The performance of the tilt combo sensors are summarized as follows:

  1. The geophone has almost the same noise level as the CMG3T from 0.15 to 20 Hz after correction for geophone amplitude response (Fig. F16). The PSD of the CMG3T and the geophone have the same peak of –115 dB in reference to (m/s2)2/Hz in the microseism band. In the high-frequency band, from 20 to 50 Hz, the PSD of the geophone is much higher than that of the CMG3T. It was probably caused by the difference in installation conditions in the vault. We confirmed that the geophone has the same sensitivity as the CMG3T to ground motion from 0.15 to 20 Hz in this test.

  2. The accelerometer PSD data presented in Figure F17 do not show the microseisms peak. In Matsushiro vault, the ground motion in the microseism band is quieter than –115 dB from CMG3T data. It is lower than the accelerometer noise in the Matsushiro vault (–110 to –85 dB at a frequency range of 0.001–50 Hz). Because the accelerometer is adjusted for strong motion of more than ±6 g, low-level background noise in Matsushiro cannot be resolved. In the JAMSTEC laboratory data, the PSD of the accelerometer shows good correlation to that of the tiltmeter from 0.001 to 1 Hz and the geophone from 0.2 to 50 Hz.

  3. The LILY tiltmeter has the same noise level in the microseism band from 0.13 to 1 Hz as the CMG3T (Fig. F18). From 0.01 to 0.13 Hz, the PSD of LILY data ranges from –140 to –120 dB and that of CMG3T data ranges from –180 to –140 dB. The difference is ~40 dB at 0.03 Hz. From 1 to 2.5 Hz, one of the LILY (N8068) sensors exhibits good noise level comparing to the CMG3T. However, the other LILY (N8069) sensor has a higher noise level than the CMG3T, showing 10 dB in this frequency band. Tiltmeters have different internal noise levels, especially at high frequencies. At 0.015 Hz, a remarkable peak is found in the LILY data record. This peak is probably caused by internal noise of the tiltmeter because this peak was found in the other tiltmeters in other tests and also appears in stand-alone acquisition by the LILY tiltmeter. The estimated noise level of the LILY tiltmeter is around –140 dB from 0.1 to 2.5 Hz in reference to ground acceleration ([m/s2]2/Hz).

Thermistor string

The thermistor string is designed for long-term monitoring temperature in the borehole. The string has five thermistors at different intervals along an electrical cable for array monitoring as shown in Figure F19. The electrical wires are covered by polyether-based material that has good hydrolytic stability. Thermistors and connectors are molded using the same material as the cable. The thermistor string is connected to the thermometer digitizer (Fig. F14). A/D converted data are merged to a WIN file on the CPU board in the main tilt combo cylinder. Thermistor cables were calibrated in a precise isothermal bath for temperatures ranging from 5° to 45°C. The calibration was conducted using the following empirical formula:

T = 1/{A + B * ln(R/2 – R0) +
C * [ln(R/2 – R0)]2} – 273.15,


  • T = temperature (degrees C) in the isothermal bath measured by high-precision quartz thermometer,

  • R = reading of the data logger (2× resistance in ohms),

  • R0 = resistance of lead cable (in this case up to 150 m two-way, depending on the position of the sensors), and

  • A, B, and C = coefficients determined for each thermistor.

Temperature differences between measured and calculated values (ΔT) were primarily attributed to actual temperature fluctuations in the isothermal bath. The absolute accuracy was estimated as ~10 mK. After calibration of the thermometer string, we conducted a continuous recording test in Matsushiro with other sensors of the tilt combo modules.

Temperature data from the Matsushiro experiment are shown in Figure F20. Two MTLs were deployed for comparison to the thermistor string data. In this experiment, the thermistor string was coiled and installed in almost the same position. We confirmed that MTLs and thermistors show the same trend and good correlation. However, there are some differences between thermistors and MTLs that reach 40 mK maximum, with the MTL delivering the higher values. It seems that the difference is caused by using the coefficients that have some margin of error due to limitation of precision of our isothermal bath used for the calibration. However, we confirmed that the thermistor string has a resolution of 1–2 mK. Afterward, two sets of thermistor strings were prepared for Expedition 332. One of them, String 1, was already adjusted to the total length of 150 m, whose thermometer sensor distribution was determined from the existing Expedition 314 LWD data set. String 1 was loaded on the Chikyu on 25 October 2010 after calibration and pressure tank testing. String 2 was prepared for an optimum length of 146 m after having obtained LWD data from Hole C0002G. Calibration and pressure tank tests of String 2 were conducted prior to loading it onto the Chikyu during the Shimizu port call on 27 November 2010.

Observatory hydrogeologic (pressure) unit

For the LTBMS deployed during Expedition 332, a pressure unit was developed for multilevel monitoring of pore pressure transients in the formation (as a proxy for strain) and comparison to seafloor reference pressure. The pressure unit was equipped with four Paroscientific Digiquartz pressure transducers, a RTC-PPC, a 24-bit/channel A/D converter and data logger, and associated “Paroscientific Intelligent Module” A/D converters. The Paroscientific gauges, which are also used in the temporary mini-CORKs (see “SmartPlug/GeniusPlug”), have proven to be accurate and very reliable, with accuracy within 0.01% of the full-scale range and pressure resolution to within 1 part per million of full scale (Becker and Davis, 2005). Each transducer connects to ¼ inch stainless tubing that terminates at three measurement points in the subseafloor, as well as a hydrostatic line at the seafloor. The three ⅛ inch hydraulic lines are housed in a urethane-coated flatpack umbilical that connects the monitored intervals to the CORK head (see Fig. F9 in the “Site C0002” chapter [Expedition 332 Scientists, 2011a]). A swellable packer set at 746 mbsf inside the casing isolates the screened intervals to enable pore pressure monitoring of in situ formation pressures once the response to drilling and open hole operations has dissipated.

The lowermost monitoring interval is located in the accretionary prism (Unit IV) at the bottom of the open borehole, below the strainmeter (see Fig. F9 in the “Site C0002” chapter [Expedition 332 Scientists, 2011a]). Pressure monitoring is achieved using three 1 inch diameter “miniscreens” plumbed to a single manifold that connects to a ⅛ inch stainless steel tube within the flatpack umbilical. This configuration maximizes azimuthal coverage of the borehole while minimizing the impact of screen obstruction. The second hydraulic line terminates within the cemented interval to act as a simple strainmeter, and the third monitors the screened interval from 757 to 780 mbsf, within the mudstones of the forearc basin fill.

When finally assembled, the pressure unit filled Bay 1 of the LTBMS CORK head (see Fig. F21). In its lower portion, a metal shield was mounted to facilitate remotely operated vehicle (ROV) operations and protect some delicate parts from collision or other damage. The data logger within the pressure unit can be accessed via an ODI Teledyne UMC with the same specifications as the set of three UMCs of the other CORK instruments mounted in Bay 2 (i.e., seismometer, strainmeter, and tilt combo; see “CORK head”). The ODI connector has a pin layout that is compatible to its three counterparts and hence allows the ROV pilot to use the same interface for communication and data download.

Instrument carrier

The broadband seismometer (CMG3T) and tilt combo, which consists of a geophone, accelerometer and tilt logger, SAHF digitizer, and a LILY tiltmeter, were mounted to the instrument carrier attached above the strainmeter. A H-beam shaped instrument carrier was newly developed to withstand the strong vibrations associated with high current and hold the high precision and sensitive sensors (see schematic drawing in Fig. F22). The instrument carrier secures borehole sensors, electrical cables, a thermistor string, and the hydraulic lines to the carrier and protects them from damage from contacting the casing and open hole walls while lowering the package into the hole. This instrument carrier was tested for safe operation under high-current velocities with dummy sensors during the March 2010 Chikyu CK10-01 cruise. The dimensions and weight of the instrument carrier as well as the sensors, which were housed in the instrument carrier, are summarized in Table T1.

A 190 mm diameter and 30 mm thick flange was adapted at both connection ends to ensure sufficient strength against bending and tension loading. Cables, thermistor string, and hydraulic lines pass through six slits. Six M16 high-tension bolts with self-locking nuts were used at each flange connection. Cement pipe with 48.6 mm OD and 34.4 mm ID was installed along the instrument carrier for cementing the sensors.

Before being loaded on the Chikyu, a test of sensor attachment and cable routing was performed on the instrument carrier to confirm the attachment procedures for the cables, thermistor string, and hydraulic line routing. Two instrument carriers (one for backup) were loaded onto the Chikyu on 27 October 2010 and stored in the Core Tech workshop and middle pipe rack.

The broadband seismometer, sensors consisting tilt combo (tilt logger, SAHF digitizer, tiltmeter) are installed on the instrument carrier using a band-type attachment tool. Isolation between sensor casings and instrument carrier was achieved with vinyl tape and fiberglass reinforced plastic. The electrical cables connected to the seismometer and tilt combo have a molded part (40 mm diameter). These were fastened inside the carrier (Fig. F23). The tiltmeter and thermometer digitizer were connected to the tilt-logger by the three-phase cable shown in Figure F24. The thermistor string was connected to the thermometer digitizer and placed on the top of the carrier. The electrical cable for the strainmeter and two ⅛ inch hydraulic lines were mounted and attached to the carrier in order to pass them through from bottom to top. These cables, lines, and strings were attached by cable ties and steel bands. Specifications of the sensors and instrument carrier are summarized in Figure F22. The orientation of the broadband seismometer, geophones, accelerometer, and the tiltmeter, all mounted on the instrument carrier, is illustrated in Figure F25.

Electrical cable

Three electrical cables of 21.3 mm diameter were connected to borehole sensors (tilt logger, broadband seismometer, and strainmeter) and placed in the BHA. The buoyancy of the cables was designed to be almost neutral to lower risks such as cable slack during long-term operation after installation. The weight of the cable in seawater is about –4.6 kg/km, and the minimum bending radius is 426 mm under tension. Seacon MINK-CCP connectors were molded at the bottom end of each cable before being loaded onto the Chikyu for sensor connections. A schematic drawing of the bottom ends are shown in Figure F23. These cables are attached by tie wraps and duct tape to 3½ inch tubing from the BHA to the CORK head, which is installed on the wellhead. The upper ends of the cables were terminated with ODI Teledyne UMCs on board the Chikyu.

3½ inch tubing and centralizers

The sensor tree mandrel used 3½ inch 12.7 lb/ft tubing. The CORK head was connected to the top joint of the tubing. Three electrical cables (21.3 mm diameter), a flatpack (12 mm × 28 mm, containing three strands of ⅛ inch diameter tubing), and a thermistor string (6 mm diameter) were attached tightly on 3½ inch tubing from the instrument carrier to the CORK head by cable ties and steel bands. Cable drums equipped with a compressed air braking system were used for the operation.

Two types of centralizers were used for accurate centralization and cable protection. Bowspring centralizers were attached below the strainmeter and above the cement port to allow uniform cement flow around the instruments. Further above, four rigid centralizers with cable protectors were attached to each joint of 3½ inch tubing to prevent cable and thermistor string damage caused by contact with the casing wall (Fig. F26). Cables and the thermistor string were secured by tie wraps and steel bands and were also protected by sheaths of rubber hose in areas where sharp edges or a change in diameter of the string was encountered.

Cementing equipment

Sensors installed at the bottom of the hole were cemented for coupling with the surrounding rock. The observatory system includes cementing system components that were embedded as a part of the hole completion operations. They consist of a plug landing collar, cement port, and top and bottom special combination plugs. A float collar was used as a landing point for the cementing plugs at the planned depth of the top of the cement column in Hole C0002G. The cement port was set to the planned depth of the bottom of the cement column at 937 mbsf. After the bottom plug was dropped, 39 bbl of cement was pumped downward using a rig floor pump. The top plug was behind the cement slurry. Seawater was pumped until plug bumping occurred at the collar. The bottom plug was bumped and ruptured with 2500 pounds per square inch (psi), whereas the top plug was bumped with 1000 psi.

Swellable packer

The swellable packer is a nonmechanical borehole seal that utilizes the swelling properties of rubber. The packer materials swell when in contact with water, which results in swelling of the element of up to 350% of its original volume (depending on pressure and temperature conditions).We installed a swellable packer from Haliburton at the target depth of 746 mbsf at Site C0002 to isolate a predefined section of the borehole for pore pressure measurements. A photograph of the swellable packer is shown in Figure F27. The initial OD is 7.89 inches and the active length is 1.5 m. The 9⅝ inch casing ID is 8.68 inches, so the initial clearance between the packer and the casing is 10 mm (20 mm total). The anti-extrusion end rings with the same OD as the packer were attached to each end of the packer. The end ring flaps expand radially against the casing ID, so that the packer seal elements are not able to come out over the end rings. The packer mandrel with 3½ inch OD and 4.5 m length was connected to the 3½ inch tubing, and the packer was covered with the diffusion barrier (type 8L), which is a material that slows water migration into the packer through reduced permeability. In the case of the 7.89 inch OD packer with diffusion barrier 8L, the OD is estimated as 8.09 inches after 1 week and 8.17 inches after 2 weeks at 4°C and also as 8.12 inches after 1 week and 8.19 inches after 2 weeks at 40°C by the swell simulation. From former laboratory test results at JAMSTEC, it was determined that the packer swelling rate is slightly lower at low temperatures.

In order to communicate with a series of sensors and perform the pore pressure measurements, the electrical cables (21.3 mm OD) and hydraulic flatpack (28 mm × 12 mm) were fed through the swellable packer on the working cart in the moonpool while lowering the sensor assembly. Cable installation tools for each electrical cable and the hydraulic flatpack were used for the operation. The clearance around the cable was set to be 2.5 mm (5 mm total). The clearance is needed to reduce the risk of cable damage due to the cable pull force and slack during lowering the packer into the borehole. The packer had to be cut four times along its length (each slit 90° apart) to allow for feed-though of the three electrical cables and the flatpack. After the cable installation, the anti-extrusion end rings with the cable slit cover were tightened, and the cables were fixed on the 3½ inch tubing independent of the packer.

CORK head

The CORK head (30 inch effective diameter and 24 ft, 2⅞ inch length) is the top of the sensor string mounted on a 9⅝ inch casing hanger inside the SG-5 riserless wellhead. Figure F28 shows a photograph of the CORK head with the pressure unit being installed. The ODs of the inner and outer mandrels are 4½ inches and 9⅝ inches, respectively. Three bays are found in the central part of the CORK head (Fig. F29): Bay 1 for a pressure data logger and valves, Bay 2 with three ODI Teledyne UMCs for downhole sensors and cable coiling space, and Bay 3 for extra cable being attached.

The pressure data logger with four high-precision quartz pressure sensors (Paroscientific, Inc.) were installed on pressure monitoring Bay 1 and equipped with two-way and three-way valves to access the bottom hole pressure ports and an UMC attachment at the top of the pressure logger (Fig. F28). Three ¼ inch hydraulic lines from the bottom-hole pressure ports were connected to the valves. Additional protection was developed to shield the pressure data logger during ROV operations, and handles were added to assist the ROV in grabbing to stabilize while connecting to the UMCs. A pressure test of the hydraulic lines was performed to check for any leaks by hand pump and further modifications on valve indicators, pressure logger base plate, and the pressure logger recovering hook were made on the CORK head prior to deployment.

The top ends of the electrical cables were terminated and attached with UMCs on board the Chikyu. After the termination, the three electrical cables were securely attached to the split chain links welded on the bay panels and the outer mandrel using tie wraps and duct tape to prevent any damage during drifting through the high-current area and reentry. Three UMCs were mounted on the attachment plate. An UMC mounting plate was attached on Bay 2 using M16 bolts with self-locking nuts. The height of the mounting plate was set to be a minimum of 920 mm from the top of the ROV platform by considering the bending radius of the cable and allowing clearance for ROV operations. The attachment tools were designed to clear the valves and pressure logger installed in the next bay.

The ROV platform with 8 ft radius was attached for ROV access before lowering the CORK head into the moonpool. The final assembly, including the platform prior to launching it into the water, is shown in Figure F30.

Seafloor recording and submarine cable network connection

The long-term borehole observatory system has three separate cables for the downhole sensors: (1) strainmeter, (2) broadband seismometer, and (3) tilt combo (tiltmeter, geophones, accelerometer, and thermometer array). Pore fluid pressure through hydraulic lines from three different depths is recorded at the seafloor by the pressure recorder mounted on the CORK head. These sensors are all separate and can be regarded as four separate observatories sharing the same borehole. The electrical connection specifications are designed the same so as to give flexibility in a series of operations to connect to them.

ROV connection

During installation, the condition of the downhole sensors was inspected by cable connection to the connectors on the CORK head from an ROV. An interface circuit attached to the ROV Magnum on the Chikyu was used to switch power and RS-422 data connection (see Fig. F21 in the “Site C0002” chapter [Expedition Scientists, 2011a]). By receiving 24 V power from the ROV, the interface circuit can supply 17, 24, or 30 V of power to the borehole instrument as well as measure supplied current and voltage to the borehole sensor. The interface circuit also converts the data format from the borehole sensors (RS-422) to a format supported by the ROV Magnum (RS-232C). The ROV Magnum supported up to 115,200 bps data speed in two-way communication to the surface. We used 57,600 bps as a standard speed to communicate with our borehole sensors.

Long-term observatory operation plans

During Expedition 332, we did not install a long-term data recorder on the CORK head, but we inspected the condition of the sensors installed and obtained the short period of initial data through the ROV connection. To start long-term observation with the borehole sensors, a recording system will be installed following a visit to the observatory in Hole C0002G by the ROV Hyper Dolphin deployed by the R/V Kaiyo (JAMSTEC). The recording system consists of a repeater and a data recorder with batteries and connects via UMCs to the borehole sensors (strainmeter, broadband seismometer, and tilt combo) as well to the data recorder. The repeater exchanges data between the data recorder and the borehole sensors as well controls the power supply to each borehole sensor. The data recorder houses lithium batteries and data memory to support up to 1 y of continuous operation of these borehole sensors. The data recorder has another underwater connector for control and online data recovery from a cable connection from the ROV. Time synchronization of the entire observatory is made through a GPS-referenced time signal sent from the ROV through the cable connection. The same ROV can also connect to the pressure recorder on the CORK head for pressure data recovery.

After a period of ~1 y, the long-term data recording system will be recovered for system evaluation, and a cable connection from land will be implemented by replacing the repeater with an interface box for a submarine cable network. This network, called the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET), was laid out in this area during January–March 2010 (Fig. F31). The DONET junction point is within 10 km of the observatory. Upon connection to DONET, all the borehole sensors will receive power from the cable and the data will be received in real time on land. The timing of the borehole data will also be governed by precision time references from DONET.