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doi:10.2204/iodp.proc.327.104.2011

Downhole measurements

A temperature log was collected inside the 4½ inch CORK casing in Hole U1301B following the recovery of part of the instrument string deployed during Expedition 301 (see “Borehole observatories” for a discussion of the recovery of this string and the deployment of a new string). A new autonomous temperature logger prepared for deployment on a CORK string during Expedition 327 was used as a logging tool. The logger was programmed to record temperature every 5 s and was placed inside one of the perforated steel carriers used to run pressure gauges below a go-devil during packer testing. The carrier was fitted to the front end of a sinker bar, and the complete assembly was run into the pipe on coring line to determine the temperature inside the CORK casing and the depth of open hole available for new instrumentation.

The temperature tool was run to within 5 m of the seafloor and was then stopped for 5 min to determine the temperature of bottom water. The bottom water temperature was found to be 1.79°C, consistent with earlier regional surveys and measurements made with the Hole U1301B CORK pressure logger. The temperature tool was lowered and held stationary for 5 min at 5 m increments from the seafloor to 50 mbsf. We continued lowering the tool and taking 5 min readings at 25 m increments until an obstruction was encountered in the CORK casing, with the final temperature measurement being made at 364 mbsf.

The primary data recovered from the tool comprise a record of temperature versus time (Fig. F3). Data were converted to pairs of temperature/depth values by averaging the most stable 3–4 min of temperatures measured when the tool was held stationary inside the CORK and merging these values with depth records from the wireline (Fig. F4). Temperatures at ~100 m into basement approach 63.5°C, somewhat higher than the value estimated for uppermost basement from extrapolation of the sediment thermal gradient in Hole U1301C (Expedition 301 Scientists, 2005), but conditions are considerably cooler within the uppermost 25–50 m of basement (Fig. F4). The thermal gradient indicated by temperatures measured in the CORK casing through the sediment section is also somewhat lower than that determined from in situ measurements in sediments during Expedition 301.

These data suggest that Hole U1301B was sealed successfully by cementing in summer 2009 during Expedition 321T and that thermal conditions within the borehole have rebounded considerably within the last year after the flow of cold bottom water down the annular gap between the 10¾ and 16 inch casings ended. The data are also consistent with the top of the primary crustal aquifer being located below uppermost basement.

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