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doi:10.2204/iodp.proc.320321.104.2010

Downhole measurements

Logging operations

Downhole logging measurements in Hole U1332A were made after completion of APC/XCB coring to a total depth of 152.9 m DSF. In preparation for logging, the hole was flushed with a 65 bbl sweep of high viscosity mud and the go-devil was dropped to open the lockable flapper valve. The hole was then displaced with 80 bbl of mud, and the bit was pulled up to ~80 m DSF. No tight spots were encountered while raising the drill string. The deployment of two tool strings (modified triple combo and FMS-sonic) was planned for Hole U1332A.

On 24 March 2009, the modified triple combo tool string (magnetic susceptibility, density, and NGR) was lowered and logged down to ~150 m WSF, almost to the bottom of the hole. Two upward logging passes were made up to the base of the pipe (Fig. F37). The tools provided continuous and good quality log data, but they are affected by ship heave (typically 2 m peak to peak) because the wireline heave compensator (WHC) was not working. The borehole diameter ranged from ~33 cm (13 inches) near the base of the hole to >50 cm (20 inches) above 127 m WMSF. Logging results gave information on the porcellanite-bearing sediment interval below 136 m WMSF that was only partially recovered in the cores.

At the end of the second upward pass we encountered difficulties when attempting to pull the tool string back into the pipe. Four attempts were made to enter the pipe, and each time increasing cable tension indicated that the head of the tool was obstructed at the base of the pipe, likely near the LFV. The pipe was raised 5 m and then another 5 m, and four more unsuccessful attempts were made to enter the pipe. The pipe was rotated, and then seawater was pumped down to attempt to remove any obstructions and push open the LFV. During this procedure communications with the tool string were partially lost, and shortly after that the wireline lost ~800 lb of weight, corresponding to the weight of the tool string. At 0600 h (HST) on 25 March, the wireline was retrieved and it was confirmed that the tool string was severed from the wireline. The end of the wireline had suffered an apparently clean cut, making the most likely culprit the LFV. Fishing attempts were made to retrieve the tool string over ~18 h, using two kinds of grapple on the end of an APC core barrel; however, these were unsuccessful. Hole U1332A was cemented and abandoned on 26 March.

Logging units

Hole U1332A was divided into three units on the basis of the logs (Fig. F38).

Logging Unit 1: base of drill pipe to 136 m WMSF

Unit 1 is characterized by mostly low gamma ray values (between 3 and 9 gAPI), low density values varying between 1.25 and 1.5 g/cm³, and low magnetic susceptibility. Unit 1 has been divided into seven subunits (1A–1G). Subunits 1A, 1C, and 1E are characterized by low gamma, photoelectric effect (PEF), and density, with slightly higher magnetic susceptibility and electrical conductivity than the surrounding subunits. The low bulk density (~1.3 g/cm³) of these subunits is consistent with the lithostratigraphy of high-porosity radiolarian ooze (radiolarian opal has a density of ~2.15 g/cm³, whereas calcite and most clays have densities around 2.7 g/cm³). In comparison, Subunits 1B and 1D have higher density and PEF, most likely indicative of higher carbonate content. Electrical conductivity is lower, indicating lower porosity, and magnetic susceptibility is also low, indicating probably lower terrigenous content than in the radiolarian oozes.

Subunit 1F is characterized by high density (~1.8 g/cm³) and high PEF (~3 capture units), consistent with the recovered porcellanite in this zone (see "Lithostratigraphy"). Magnetic susceptibility is higher in Subunit 1F, and conductivity is low.

The lowermost subunit (1G) includes a peak in total gamma ray to 45 gAPI units (Fig. F39), which is mostly made up of contributions from uranium and potassium. Its origin is unclear at the moment. It seems likely that an increased proportion of clays would account for the potassium but not the uranium.

Logging Unit 2: 136–146 m WMSF

Unit 2 is characterized by a series of peaks reaching densities of 2.0 g/cm³ and lower electrical conductivity, indicating harder and less porous sediment overall (Fig. F38). PEF values higher than 4 capture units indicate that high PEF elements such as Mn or Fe may also be present in addition to calcium carbonate (PEF = ~5). The corresponding lithostratigraphy only partially recovered in this interval, however, is a mixture of radiolarian oozes and porcellanites.

Logging Unit 3: 146–151 m WMSF

The only log data for Unit 3 is magnetic susceptibility and conductivity measured at the base of the tool string. Magnetic susceptibility sharply increases to higher values at the top of the unit. These are identified in the lithostratigraphy as dark brown zeolitic clays.

Heat flow

Seven APCT-3 temperature measurements in Holes U1332B and U1332C ranged from 1.77°C at 11.6 m to 9.11°C at 100.6 m (Table T36), giving a geothermal gradient of 75.0°C/km (Fig. F40). The seafloor temperature was 1.46°C, based on the average temperature minima of the eight temperature profiles (one APCT-3 deployment, on Core 320-U1332B-5H, did not result in a valid in situ temperature). Thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndman et al. (1974) (see "Physical properties" in the "Methods" chapter). The calculated in situ values are within 2.2% of the measured laboratory values. Thermal resistance was then calculated by cumulatively adding the inverse of the in situ thermal conductivity values over depth intervals downhole (Fig. F40). Heat flow was obtained from the linear fit between temperature and thermal resistance (Fig. F40) (Pribnow et al., 2000). The heat flow estimate for Site 1332 is 70.7 mW/m², which is similar to heat flow values from nearby Sites 1218 and 1219.

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