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doi:10.2204/iodp.pr.332.2011

Site summaries

Site C0010 riserless observatory

• Hole C0010A = 32°12.5981′N, 136°41.1924′E
• Water depth = 2523.7 m
• Drilling depth = 555 mbsf (Table T1)

Site C0010 is one of the key observatory sites because it intersects a branch of the megasplay fault at ~410 mbsf (see Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010) (Figs. F3, F4). During Expedition 332, the site was revisited about 15 months after the hole was drilled and cased. The two major objectives this time were (1) to recover the SmartPlug temporary observatory installed in 2009 and (2) to replace it with a GeniusPlug temporary observatory. No drilling, LWD/MWD, or other operations were performed at this site, which—in the future—will be a location for a LTBMS.

A number of science objectives for Site C0010 were achieved during Expedition 319 in 2009, including drilling with LWD/MWD across the megasplay fault to TD of 560 mbsf, casing the borehole (with casing screens at the fault), and installation of a simple pore pressure and temperature monitoring system (SmartPlug). It was possible to (1) define major lithologic unit boundaries of the shallow megasplay fault zone and (2) determine the preferred placement of the screened joints interval within the fracture zone interval for the temporary observatory. Through comparison with previously drilled Site C0004 located only 3 km along strike (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009) these data also provide insights into along-strike differences in the architecture of the megasplay fault and hanging wall.

Although the SmartPlug is a relatively simple mini-CORK instrument, it marks the first observatory placement during the NanTroSEIZE project. The instrument contains a cylindrical steel hull with an internal metal frame that hosts the electronics and transducers (see Fig. F5). The inner section is cushioned against shock and is therefore suitable for installation in the Kuroshio Current and other high-current and violent VIV areas. The instrument package includes a data logger, a temperature sensor within the data logger housing, a self-contained temperature sensor, and two pressure gauges (one “upward looking” and one “downward looking”). These sensors monitor pressure (1) below the packer seal in a screened interval that is open to the fault zone and (2) above the packer seal to serve as a hydrostatic reference open to the overlying water column. Apart from the thermistors onboard the pressure transducers, two independent temperature sensors are included in the SmartPlug. These two temperature sensors are at a depth just below the packer in the upward-looking section of the casing packer. The retrieval of the SmartPlug allows the collection of the first observatory data (pressure and temperature) from the shallow section of the megasplay fault, as well as the evaluation of the observatory installation procedures in the rough Kuroshio Current. These data will be invaluable in planning for the permanent observatory, which is due for installation before 2013.

After reentry into Hole C0010A, the latching tool connected to the retrievable bridge plug without much effort, and the bridge plug and SmartPlug package was safely pulled out of the hole only a few hours after reentry (Fig. F6). On a long but careful recovery trip through the water column, the SmartPlug remained undamaged and was recovered safely on deck on 7 November (Fig. F6). Once the tack-welding points of the SmartPlug-to-bridge plug connection were cut, the SmartPlug was unscrewed; corrosion was found on the outside and inside of the instrument, most likely a result of the rusted inner casing of the borehole.

The SmartPlug sensor carrier with the pressure transducers, miniature temperature logger (MTL), and pressure housing containing the data logger was cleaned prior to hooking it to a computer via the AQW connector for data download. A full record of the deployment period (23 August 2009–7 November 2010) plus some extra time prior to that was recovered from all transducers and the MTL, attesting to the robust design of the instrument. Full-time series records are shown for the upward-looking (hydrostatic reference) and downward-looking (megasplay fault zone) pressure (Fig. F7) and the platinum chip and MTL temperature (Fig. F8). The full records clearly show an increase in pressure and decrease in temperature as the instrument entered the water and was lowered toward and into the seafloor. Thereafter, equilibration began, but both records suggest that the borehole had not fully equilibrated by the time we recovered the instrument (see Figs. F7, F8). At the same time, data also attest that the bridge plug effectively sealed the borehole because upon reentry of the drill string and latching onto the device during Expedition 332, the upward-looking pressure sensor shows a strong fluctuation owing to displacement of borehole fluid, whereas the downward-looking pressure sensor encounters no such interference and remains at a near-constant value (Fig. F9).

In order to illustrate some of these findings, we provide a few preliminary data examples of pressure and temperature. Figure F10 provides an example of the tidal forcing seen in the upward- and downward-looking pressure transducers. As is consistent with the expectation from a pressurized formation, the tidal signature of the formation has a diminished amplitude and small phase shift when compared to the seafloor, or hydrostatic pressure.

A cursory review of the data identified multiple pressure and—to a lesser extent—temperature excursions that may be related to seismic events, although further detrending and processing of the data are required to filter the tidal signal and resolve pressure anomalies. Nevertheless, some of the prominent earthquakes that took place during the deployment period were imaged by the SmartPlug despite its rather low temporal resolution during the recording (i.e., 60 s sampling interval). Figure F11 shows the sinoidal records of the seafloor and formation pressure data, with the M 8.8 Maule (Chile) event imaged on the left. About 24 h later, the associated tsunami had crossed the Pacific Ocean and loaded the formation at Site C0010 for several days. Many similar examples are found in the time series record (A. Kopf et al., unpubl. data). In contrast to the pressure responses recorded, temperature signals are less pronounced and, thus, more ambiguous. There are numerous examples where temperature remains unchanged while pressure shows excursions related to seismic or wave loading. On the other hand, significant changes in temperature were recorded on one occasion on 29 May 2010 (Fig. F12). Interestingly, no significant change in the pressure data was observed around that time. Also, it remains unresolved why three thermistors show an increase in temperature while the platinum chip temperature drops slightly. At this point, neither a seismic event nor any other natural explanation for the observed shifts in temperature can be provided. Also, artifacts such as settling of the instrument or violent fluid circulation may help explain the temperature changes but would have ultimately caused the pressure to vary as well (which is not the case).

The SmartPlug record contains a number of events seen in both the pressure and temperature data, which will be studied in more detail after Expedition 332. At first glance, however, the following summary statements can be made:

1. The plug apparently settled about 50 cm over the first 2 months, or some of the thread grease or anticorrosion additive to the borehole fluid was mucking the inside of the instrument (and hence the upward-looking pressure transducer) and loaded the seafloor sensor.

2. The formation appears to be very weakly overpressured and is still recovering at the end of the record.

3. The temperature records are well resolved but show yet-to-be-explained offsets, which may either have something to do with a change in the thermal insulation around the sensors or the instrument pressure case, the changing nature of heat dissipation, or hydrogeologic events such as earthquake loading of the formation. Postcruise research may shed light on the observed problem of opposite temperature trends from the different sensors during the same event (Fig. F12).

4. The pressure records show abundant earthquake events and associated Rayleigh and tsunami waves. The tsunami waveform related to the M 8.8 Maule, Chile, earthquake is similar to existing records from, for example, the NEPTUNE seafloor cabled network (E. Davis, pers. comm., 2010) but has larger amplitude and a more persistent pressure signal lasting 3–4 days (A. Kopf et al., unpubl. data).

5. The loading efficiency at the tsunami frequency is very similar to that at tidal frequency. Rayleigh waves are much larger in the formation relative to the water column, whereas in the tidal signal the formation has the lower magnitude signal when compared to the seafloor reference port (Fig. F10).

More information regarding both the instrument and the data set and the interpretation of the results can be found in a manuscript included in the Expedition 332 Proceedings (A. Kopf et al., unpubl. data).

One additional data record collected during the SmartPlug deployment at Site C0010 is triaxial accelerometer data monitoring drill string VIV. This measurement was taken after the problems associated with some of the observatory installation procedures tested during Expedition 319 (for details see Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). In 2009, an instrument carrier was damaged and two borehole instruments were lost while another's internal circuitry was destroyed by drill string VIV-induced shocks. As a result, from fall 2009 to spring 2010 Japan Agency for Marine-Earth Science and Technology tested various techniques on land to minimize/reduce VIV and demonstrated that VIV related to strong current and wave action can be efficiently reduced by attaching ropes along the axis of the drill string. This was tested in the field during a Chikyu shakedown cruise in March 2010, so this technique was made a routine part of observatory operations in the high-velocity regions of the Kuroshio Current.

During Expedition 332, a set of four ropes was attached in the moonpool area when the drill string was passing from the rig floor into the water (Fig. F13). The attachment was carried out in the upper 500 m portion of the drill string where the current velocities are the highest. Figure F14 demonstrates how the ropes serve to reduce VIV along the drill string where the sensitive observatory instruments are located. VIV in the upper part increased slightly when compared to a deployment free of ropes; however, such vibrations are tolerable given that no electronics or other sensitive parts are housed there. During drifting for SmartPlug retrieval at Site C0010 (see Fig. F14), we encountered accelerations of ~0.5 g or less. This value was even lower during slower drift speeds at Site C0002, where 0.2 g was not exceeded. The angle between drifting direction and sea current was ~45°, which also helps reduce VIVs. This finding has important repercussions for safe future observatory installations.

Once the SmartPlug and accelerometer instruments were safely recovered, the final step of the temporary instrumentation operations was the first deployment of a GeniusPlug (Fig. F15) in IODP history. In essence, the observatory is similar to the SmartPlug except that the instrument is ~30 cm longer. This extension includes the added OsmoSampler for fluid storage over time as well as the osmotically driven microbiology chambers of the flow-through osmotic colonization system (FLOCS) unit and also means that final placement needs to be much closer to the active fault zone than the SmartPlug. Therefore, the bridge plug assembly was adjusted to include two joints of 3.5 inch tubing, which were added between the instrument and the bridge plug (Fig. F4). The GeniusPlug was set up to be capable of sampling fluids for up to 24 months and was deployed after a short drifting period with ropes but no accelerometer instruments attached to the drill string. The wellhead was protected using a corrosion cap and will be revisited in 2011 or 2012 for instrument recovery.

Site C0002 riserless observatory

• Hole C0002G = 33°18.0126′N, 136°38.1488′E
• Water depth = 1937.5 m
• Drilling depth = 980 mbsf (Table T1)

Site C0002 is the centerpiece of the NanTroSEIZE project, intended to access the plate interface fault system at a location where it is believed to be capable of seismogenic locking and in a zone thought to have slipped coseismically in the 1944 Tonankai earthquake. The primary targets include both the basal décollement at ~6200 mbsf and the megasplay fault (Tobin and Kinoshita, 2006b). The megasplay fault reflector lies at an estimated depth of 5000~5200 mbsf, and the top of the subducting basement is estimated to lie at 6800~7000 mbsf (Figs. F1, F2).

At Site C0002, a large number of scientific objectives have been covered during previous expeditions, during which a total of six holes have been drilled within a 100 m radius (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). Operations included drilling and logging with a full suite of LWD/MWD to 1400 mbsf during Expedition 314 and coring two intervals from 0 to 204 mbsf and 475 to 1057 mbsf during Expedition 315 (Fig. F3). The succession explored comprises four units, the upper three of which were different lithofacies in the Kumano forearc basin. From ~920 to 936 mbsf, the transition zone to the underlying accreted strata was found. The primary objective at Site C0002 during Expedition 332 was to install the first permanent borehole observatory system during the NanTroSEIZE project into the basal forearc basin (Unit III, dominated by condensed mudstone) and the upper accretionary prism (Unit IV, containing interbeds of mudstone, siltstone, and sandstone) (Fig. F16).

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. F16). For practical reasons, downhole components 4–6 are incorporated into one pressure housing, so the observatory includes

• The hydrogeological component with a pressure unit at the CORK head and a flatpack umbilical with three stainless steel hydraulic lines running into the hole and terminated at a certain depth,

• The strainmeter,

• The broadband seismometer,

• The tilt-combo instrument, and

• A thermistor string.

The set of sensors is designed to collect, as a whole, multiparameter observations in a wide period ranging 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. The hydrogeologic component of the sensor set is further laid out to monitor formation pressures at three downhole levels and further records data from a seafloor pressure port for reference.

In order to set the stage for the LTBMS installation, Hole C0002G was drilled without coring but with three LWD runs (once with a 12¼ inch assembly and twice with a 10⅝ inch assembly). The speed was limited to a maximum rate of penetration of 20–30 m/h in order to achieve a high data resolution in order to identify the most promising locations for LTBMS instrument locations. Both resistivity and natural gamma ray were measured to confirm the anticipated depths of lithologic boundaries identified during drilling of nearby Hole C0002A (Expedition 314; see Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). Changes in composition and texture were identified by variations in natural gamma values that coincided with amplitude changes in the electrical resistivity data. LWD data confirm that both the depth and character of the transition were broadly consistent with previous logging. In particular, a marked decrease in the resistivity fluctuations coupled with an increase in natural gamma values defined the Unit II/III boundary. The lower boundary of Unit III was identified in the same manner in order to position the strainmeter in a relatively continuous zone of elevated gamma ray and resistivity values.

A 20 inch conductor pipe was jetted in to 41 mbsf, and then 9 inch casing was run to 887 mbsf. To ensure good coupling of the strainmeter to the formation and to eliminate local fluid motion around the seismic sensors and the tiltmeter, the sensors were cemented in the open-hole section below the 9 inch casing shoe in Unit III. One pressure port was installed in Unit IV below the strainmeter to sample pore fluid pressure in the accretionary prism. This pressure port is protected by miniscreens so that cement or clay-rich material does not hamper conductivity of the hydraulic tubing (Fig. F17). Above the cemented sensors in the open hole, the 9 inch casing has screened casing joints from 757 to 780 mbsf (Fig. F16) to sample pore fluid pressure in Unit II below a swellable packer to isolate the interval from the seafloor (Fig. F18). 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 (Fig. F19).

The nonhydraulic components of the observatory are all located in the deeper portion of the CORK assembly string (Fig. F16). The location of the strainmeter, tiltmeter, seismometers, and pressure ports for pore fluid pressure monitoring are determined based on LWD information. In the lowermost portion, above the bullnose, the strainmeter is implemented in a fracture-free zone. It is a volumetric system with a small sensing volume (Fig. F20). Above the strainmeter is the instrument carrier (Fig. F21) mounting 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 thermistor string is terminated at the digitizer. Figure F22 shows one thermometer node protected by polyurethane and tie-wrapped to the 3.5 inch tubing. The thermistor has a total length of 146 m, with five nodes irregularly spaced over its length. It is terminated at the bottom by a digitizing unit that is mounted to the top of the instrument carrier.

In total, three cables plus the flatpack umbilical were guided upward from the instrument carrier together with the thermistor. Apart from the latter, everything was fed through four slits in the swellable packer's rubber compound mantle. Further above, centralizers, tie wraps, and steel bands secure the cables and umbilical to the string and feed all lines into the CORK head (Fig. F23).

Figures F23, F24, and F25 illustrate the arrangement of seafloor part of the observatory (CORK head and ROV platform), showing the orientation of the pressure recorder and Teledyne Ocean Design, Inc. (ODI) connectors. Before lowering the completion string into water, all the hydraulic tubes were equilibrated to seawater pressure by opening two-way valves on Bay 1 of the CORK head to allow fluid to enter the tube and displace trapped air (see also Fig. F19). All 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 they remained closed after the cementing operation. When the cementing was completed and the completion string was released at the running tool, we switched the three-way valves to connect the three pressure transducers to start observation of borehole formation pressure. By ROV communication, we obtained the record of the initial ~15 h after observation had started as well as all pressure readings prior to switching the valves from the seawater port (i.e., hydrostatic pressure) to the lines from the pressure gauge to the ports at depth. The record obtained (Fig. F26) clearly shows tidal variations with the pressure disturbance during circulation and cementing before the valves were closed. After switching the valves after cementing was finished, we observed a shift in pressure and different tidal response from ocean especially for the bottom pressure port (P1) below the cemented section near the strainmeter, suggesting that the cement column successfully isolates the bottom section from the upper sections.

Before the ROV left the CORK head, the observatory was set to standby mode until it is visited again by ROV in March 2011. One exception is the CORK head–mounted pressure sensor unit, which was set to record prior to deployment into the seafloor, so that by March 2011 a 4 month record at four levels (3× formation pressure plus seafloor reference) will be available. In summary, the health of all observatory components installed with the LTBMS can be confirmed. In March 2011, we will download pressure data and start long-term observation with all instruments by data recorder installed in the seafloor. The data recorder has a stack of batteries that can power the whole system for 1 y. We plan to connect the observatory to the undersea 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 during the next visit to the Site C0002 observatory in November 2011 or later.

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