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doi:10.2204/iodp.proc.327.104.2011

Borehole observatories

The operational objectives for Expedition 327 included replacing the CORK sensor string that was originally installed in Hole U1301B during Expedition 301 (Expedition 301 Scientists, 2005). The sensor string could not be replaced as planned during 2009 operations using the Atlantis and the Alvin, despite pulling more than 5000 lb. We expected that the JOIDES Resolution would be more effective at pulling the sensor string, albeit with some risk of damaging the instrumentation originally installed during Expedition 301. Indeed, as described in “Operations” >10,000 lb of overpull was required before a portion of the Expedition 301 string broke free. The sensor string parted at the splice between the Spectra strength member and the middle plug at ~377 mbsf, and only the top plug and upper Spectra segment were recovered.

The recovered Spectra segment included five temperature loggers from the upper basement section, but nine other temperature loggers and all of the Expedition 301 OsmoSamplers were left in the hole. Of the five temperature loggers recovered (Table T1), only one had sufficient battery power to transfer the 6 y temperature record, which is not surprising given that the batteries were designed for 3–4 y deployments. Past experience suggests that the manufacturers should be able to recover the data files from nonvolatile memory in most of the nonresponsive units when they are returned for servicing. Given that most of the lower section of the casing and open hole was obstructed, the replacement sensor string was scaled back to ~54 m total length and included three Antares temperature loggers (temperature range = 0°–80°C) set for 2 h sampling intervals (Table T2) and a 200 lb (in water) sinker bar. These were positioned primarily to assess whether the hole was successfully sealed during Expedition 321T cementing operations by monitoring for the convex-upward gradient that would be diagnostic of upward flow in an unsealed, overpressured hole producing formation fluids.

Figure F5 shows the last year recorded by the Antares unit deployed at ~267 mbsf (~2 meters subbasement [msb]). The 2004 Antares electronics were designed to optimize resolution at expected basement temperatures of >60°C using a lower limit of ~40.5°C for the recording range; the first 5 y of the record was below this lower limit. The record shows a long-term recovery toward formation temperatures beginning shortly after the Expedition 321T cementing operations in June 2009, indicating that those operations reduced or ended cold-water flow down the hole. The most recent data are consistent with the temperatures recorded at the same depth during the temperature log conducted as part of the depth check before installing the replacement sensor string (see “Downhole measurements”). Thermal recovery of the borehole is incomplete, but this is not surprising given that the downhole flow lasted nearly 5 y, whereas the recovery period has lasted little more than a year thus far.

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