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doi:10.2204/iodp.proc.319.104.2010

Discussion and conclusions

Architecture and along-strike variation of the megasplay fault

Although we drilled Site C0010 with only a limited suite of LWD/MWD tools, the resistivity and gamma ray data sets provide a useful basis for comparison with nearby Site C0004, located ~3.5 km along strike to the northeast. Based on the two penetrations of the thrust wedge, along with observations from 3-D seismic reflection data, it is clear that the character and physical properties of the megasplay fault system vary considerably along strike.

At Site C0010, both gamma ray and resistivity values are higher in the thrust wedge than in the slope sediment above and below (Figs. F5, F7, F28, F29). In contrast, at Site C0004, gamma ray values and resistivity within the thrust wedge are only very slightly higher than in the overlying and underlying units and are considerably lower than in the thrust wedge at Site C0010 (Kinoshita et al., 2008). Both gamma ray and resistivity logs are also characterized by large variations in the thrust wedge at Site C0010 that are not observed at Site C0004. The values for the minima in gamma ray and resistivity at Site C0010 are similar to those for the entire thrust wedge at Site C0004.

The base of the thrust wedge at Site C0010 is marked by a negative polarity seismic reflection (impedance decreases across the boundary). In contrast, the base of the thrust wedge at Site C0004 is marked by a positive polarity reflection consistent with an increase in impedance expected based on observed P-wave velocity and bulk density from LWD and core data (Kinoshita et al., 2008; Kimura et al., 2008). The thrust wedge in the vicinity of Site C0004 is seismically transparent in character, whereas at Site C0010 there are several reflectors that likely correlate with the variations in gamma ray and resistivity (Figs. F30, F31, F32). From both LWD azimuthal resistivity images and seismic data, the base of the thrust wedge is sharper at Site C0010 than at Site C0004, where coring documented a ~50 m thick fault-bounded package. This is consistent with the observation that at Site C0010 the mean borehole breakout orientation changes abruptly by ~20°–30° across the base of the thrust wedge (Fig. F16), whereas at Site C0004 the change is more gradual.

We interpret the higher gamma ray values and resistivity in the thrust wedge at Site C0010 to reflect increased compaction relative to the sediment above and below and relative to the thrust wedge at Site C0004, although it is also possible that these data could reflect a higher clay content. In the latter case, resistivity would be higher because of increased tortuosity associated with fine grain size and surface area. Similarly, the fluctuations in gamma ray and resistivity in the thrust wedge at Site C0004 could reflect variations in porosity or fracture density (with lower values associated with zones of increased fracturing or lower porosity), compositional layering, or a combination of the two.

The negative polarity reflection at the base of the thrust wedge at Site C0010 also suggests that the wedge has a lower porosity than the overridden slope sediments below. At Site C0010, the compaction trend for the slope sediments above and below the thrust wedge (inferred from resistivity data; Conin et al., 2008) suggests that the overridden slope sediments are not underconsolidated, as might be the case for compaction disequilibrium (Hart et al., 1995; Saffer, 2003). Thus, we conclude that the thrust wedge at Site C0010 is overcompacted, meaning that its porosity is anomalously low for its present burial depth (Fig. F22). This could result from increased mean effective stresses in the thrust wedge or uplift of the wedge along the megasplay from greater depth. In contrast, at Site C0004, the thrust wedge exhibits porosity similar to the slope sediments and no evidence for enhanced compaction. Downdip from Site C0004, the seismic reflection polarity at the base of the thrust wedge becomes negative, most likely indicating increased compaction of the thrust wedge relative to the footwall.

Taken together, these observations suggest that in the area of Site C0010 the thrust wedge comprises an overconsolidated package that probably originated at greater depth than the thrust wedge sampled at Site C0004. In contrast, the wedge at Site C0004 may be composed of reworked and deformed slope deposits that have never been deeply buried. This interpretation is also consistent with the location of Site C0010 on the flank of a lateral ramp on the megasplay fault (Fig. F30).

Observatory installations

Dummy run

The dummy run test in Hole C0010A successfully evaluated a subset of planned operations for future permanent borehole observatory installation. The planned permanent observatory consists of three major parts: bottom-hole instruments, tubing to support downhole cables and hydraulic lines, and the circulation obviation retrofit kit (CORK) head, which suspends everything below and seals the hole. In this test, we confirmed procedures to make up the bottom-hole instruments and lower them into the water. There was concern that the weak surface of the strainmeter might hit the guide funnel below the rotary table as the tubing below the strainmeter drifted in the ocean current, but by experimenting and adjusting the length of tubing prior to assembly we were able to find the optimum length to operate safely. We found that the bottom-hole instrument string was subjected to significant vibration from the drill string when the Chikyu drifted for reentry into the hole because of the Kuroshio Current. Before the experiment, vibration on the drill string was suggested but not emphasized as a major concern. Structurally, the tubing and instrument string were much weaker than the drill pipes; therefore, vibration was amplified in these weak sections through resonance. The effects were sufficiently severe that the delicate internal workings of the instruments (such as hinges and pivots in the seismometers) would not survive to record data after installation in the hole. Modifications to the design of the instrument carrier are thus required to ensure it maintains integrity during installation. In addition, we have not tested the complete instrument string, which will have the CORK head and 500 m (or more) of tubing attached with soft cables and hydraulic lines. The experience during Expedition 319 also highlights the need for more complete evaluation of overall observatory design and installation. Evaluation and treatment are necessary to address (1) sensor integrity under vibration, (2) resonance effects from drill string vibration, (3) tolerance of the downhole cable and hydraulic lines to stress and vibration, and (4) the ability of other components, such as swellable packers and CORK head, to withstand the vibration. Acceleration data from the dummy run test in this expedition are valuable for such evaluations.

There are also other options for installing long-term observatories. The smart plug installed during this expedition is an encouraging option for intermediate- to long-term borehole observatory emplacement. An extended version of the smart plug that includes a seismic component may also be an option. In such a system, continuous seismic observations for a period of 2 y, in conjunction with pressure and temperature observation, would be possible. We still have to evaluate effects of the vibration on seismometers and pressure gauges because the smart plug is, as with the bottom-hole instrument string for the dummy run test, also deployed by drill pipe, and thus subject to vibration in the current.

As another option, by separately lowering the sensor into the hole after lowering the observatory to the seafloor, it may be possible to reduce effects from the current-induced vibration while lowering the observatory to the seafloor. An observatory package that houses a bottom-hole sensor and downhole cable wound in a winch would be landed on the reentry funnel of the borehole without severe vibration or stress on sensors and cables. The sensors could then be lowered to the bottom of the hole by wireline. Such an observation system was developed for logging (Amitani et al., 2002) but has not been deployed in boreholes. Installation of a cabled borehole sensor into a borehole via a controlled wireline has also been tested (Stephen et al., 2003). In this case, the cable that would be in the borehole is lowered beneath the ship before reentry. In these cases, there are risks of damage to the borehole cable caused by the ship's heave while the cable is being lowered into the borehole.

Temporary monitoring system

After LWD, casing, and the dummy run test at Site C0010, we suspended the hole by installing a smart plug sensor package attached beneath a retrievable casing packer (Figs. F45, F46, F47, F48). The smart plug is a robust retrievable stand-alone instrument package designed with a relatively short lead-time, in order to make use of suspended boreholes prior to final observatory installation. Although it is relatively simple, the smart plug at Site C0010 represents the first long-term monitoring in the NanTroSEIZE project and the first observatory element installed by the Chikyu. If successful, it will provide another tool for long-term hydrologic and/or thermal monitoring in scientific ocean boreholes.

For installation at Site C0010, the retrievable casing packer was placed above two screened casing joints that provide hydraulic communication with the megasplay fault zone (Fig. F45). In this configuration, the smart plug will monitor pore pressure and temperature within the megasplay fault and also record the hydrostatic pressure open to the overlying ocean as a reference (Fig. F45; see also Fig. F28 in the "Methods" chapter). Hole completion relies on cement at the base of the casing shoe and in the annulus (a planned top of cement at ~40 m above the casing shoe) and on the collapse of soft sediment and thrust wedge material against the casing over the ~400 m of annulus above the screens to achieve hydraulic isolation from the sediments above and below, respectively. Upon recovery of the instruments (anticipated for Site C0010 in 2010 or 2011), we will assess the efficacy of hydraulic isolation by comparison of the fault zone and hydrostatic pore pressure time series and the response of the fault zone pore pressure to tidal loading (e.g., Wang and Davis, 1996). We also anticipate conducting a cement-bond log as part of future operations to define the top of cement and evaluate the extent of formation collapse against the casing above the screens.

Despite strong ocean currents (up to 4.5 kt and persisting to a depth of several hundred meters below sea level), the smart plug was successfully run to the wellhead and set inside the casing. However, even for installation of this relatively simple and short sensor package, we encountered problems associated with the Kuroshio Current; upon running the drill pipe to the surface, the running tool sheared off from the drill pipe at a 3½ inch tubing connection, presumably as a result of vibration in the water column. One key difference between the smart plug and many previous hydrologic observatory installations in Ocean Drilling Program (ODP) (e.g., Becker and Davis, 2005) is that data cannot be downloaded from the sensor package until it is retrieved with the packer using a drillship. Thus, we cannot assess whether damage to the electronics or pressure sensors was sustained during running to the wellhead or hole reentry prior to instrument recovery.

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