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Results from temporary “mini-CORK” deployments

After reentry into Hole C0010A, the latching tool connected to the retrievable bridge plug without difficulty, and the bridge plug and SmartPlug package were safely pulled out of the hole only a few hours after reentry (see “Operations”). On a long, careful recovery trip through the water column, the SmartPlug remained undamaged and was recovered safely on deck, detached from the running tool and bridge plug, and cleaned for opening and data download. The data were successfully secured to a laptop and transferred to an external drive for backup and subsequent processing. The data indicate successful continuous monitoring of pore pressure and temperature at a 1 min sampling interval for each of the sensors, which included three temperature sensors and two pressure transducers. The MTL also worked without problems and provided a continuous temperature record at a 30 min sampling interval. The data are presented here to provide evidence for successful deployment and data recovery, give an overview, and show a few details in the time series data set.

Pressure data

The SmartPlug placement was designed to monitor pressure and temperature at a screened interval in the formation, as well as hydrostatic pressure to be used as a reference. The screened interval included the splay fault located at ~410 mbsf, which was hydraulically separated from the seafloor by an inflatable packer that surrounded the bridge plug. The SmartPlug and bridge plug configuration is shown in Figure F8 in the “Methods” chapter (Expedition 332 Scientists, 2011a).

An overview of the full pressure data set is shown in Figure F6. The pressure record surrounding deployment reflects drifting prior to entering Hole C0010A, as well as the disturbance associated with the installation (Fig. F7). After ~2 weeks, the formation pressure data suggest that the SmartPlug was successfully isolated from the seafloor, as indicated by a stable pressure record in excess of hydrostatic pressure. The tidal fluctuations during the rest of the SmartPlug deployment are consistent with expectations for a pressurized formation relative to hydrostatic pressure. Specifically, the tidal signature of the formation has a diminished amplitude and small phase shift when compared to the seafloor, or hydrostatic pressure (Fig. F8).

A cursory review of the data identified multiple pressure disturbances potentially related to seismic events, although further detrending and processing of the data are required to filter the tidal signal and resolve pressure anomalies. Despite the need for further processing, it was possible to identify the arrival of the tsunami wave that resulted from the Chile M 8.8 far-field earthquake on 27 February 2010, which arrived in southern Japan ~23.5 h after the earthquake (Fig. F9).

Data surrounding the instrument recovery provide additional confirmation that the packer had successfully isolated the formation. Figure F10 illustrates the disturbance in the hydrostatic pressure (seafloor) associated with the reentry of the drill string prior to removing the bridge plug. The relative quiescence of the formation data prior to removing the bridge plug indicates hydraulic isolation from the overlying disturbance.

Temperature data

The SmartPlug incorporates four different temperature sensors: one stand-alone MTL, one temperature sensor in the pressure housing (platinum chip), and one thermistor in each of the pressure transducers. An overview of the temperature data gathered during the 15 month period is given in Figure F11.

The initial deployment phase is marked by strong temperature variations, differing in amplitude as well as frequency. All temperature sensors show the same curve progression and almost the same temperature except for the pressure housing temperature, which is ~0.6°C higher. Afterward, all sensors simultaneously register a sudden increase in temperature, followed by steep incline to up to <2°C. A day later, the temperature starts rising, and at 13°C, the temperature data split apart again, indicating that the SmartPlug reached its final depth and was sealed by the bridge plug.

The following main deployment phase shows a curve progression typical for the onset of equilibration in the borehole. The beginning is marked by a strong increase of 0.07°C/day within the first 30 days after installation of the SmartPlug was completed. Afterward, the slope of the temperature curve decreases significantly until the temperature rise reaches one-hundredth of the starting rate 305 days after deployment. Data indicate that complete equilibrium was not reached.

Similar to the initial portion of the deployment phase, the final part of the recovery phase is clearly visible in the data, with the former trend of the temperature curve ending with an abrupt, strong decrease of overall temperature followed by a sudden increase.

When comparing data from the different temperature sensors, the sensors in the hydrostatic reference pressure transducer and the MTL show nearly identical values. The temperature measured by the sensor monitoring pressure in the formation (i.e., megasplay fault branch) is slightly elevated, which may be realistic. In contrast, the platinum chip mounted inside the pressure housing shows consistently higher temperatures than the other three thermistors, ranging ~0.6°C above the other curves (Fig. F11). A problem with calibration is most likely the cause for the different temperatures within the SmartPlug casing.

When taking a closer look at the main deployment phase (Fig. F12), a temperature decay is visible in the MTL data at ~182 days, which was not registered by the other temperature sensors. After 295 days, a negative peak lasting 1.5 days is visible in the MTL temperature, which is less prominent in the other data sets. The shift back to higher temperatures was registered by the thermistors in both pressure transducers, too, whereas the sensor in the platinum chip in the pressure housing experienced a drop in temperature. The reason for these anomalies remains unclear at this point.