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Downhole measurements

Logging operations

Downhole logging measurements in Hole U1331A were made after completion of APC/XCB coring to a total depth of 190.6 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 during the reaming. The deployment of two tool strings (modified triple combination [triple combo] and FMS-sonic) was planned for Hole U1331A; however, because of difficulties with the wireline winch unit during deployment of the first tool string, the second tool string (FMS-sonic) was not deployed.

On 18 March 2009, the modified triple combo tool string (magnetic susceptibility, density, and NGR) (Fig. F34) was lowered and logged to 191 m wireline log depth below seafloor (WSF) and reached the bottom of the hole. The hole was then logged to seafloor. The tools provided continuous and good quality log data. The borehole diameter was 10 inches (25.4 cm) near the base of the hole, increasing to >18 inches (~45.7 cm) above 131 m WSF. The depth of the chert-rich interval was identified in the logs, which allowed the coring strategy in Holes U1331B and U1331C to be adjusted to maximize recovery of the target sediment interval beneath the chert.

Both the wireline winch and the wireline heave compensator (WHC) had problems during the logging operation. The WHC was initialized just after entering the open hole, but the flying head on the piston/pulley system moved up to its maximum extent and stayed there, rather than oscillating about the middle point as it would under normal operation. Initial troubleshooting failed to fix the WHC, and given the relatively calm sea state (<1 m heave), the hole was logged without heave compensation. As a result of the fully extended WHC piston, apparent logging depths (wireline log depth below rig floor [WRF]) for this hole are ~6–7 m deeper than expected. The cause of the WHC problem was later found to be the Vickers valve, which switches the hydraulic system to alternately move the piston up or down.

On the upward logging pass, logging speed became erratic because the winch motor and clutch was slipping under the weight of the cable and tool string. The upward pass was completed, although cable speed remained erratic. At one point it was difficult to raise the tool string at all. Logging operations were terminated because of the risk to the second tool string (FMS-sonic).

The wireline depth to seafloor was determined from the step increase in gamma ray values at the sediment/water interface to be at 5133.5 m WRF; the drillers mudline depth used for establishing core depth was 5127.3 m DRF.

Logging units

Hole U1331A was divided into three units on the basis of the logs (Fig. F35). The uplog was used as the reference to establish the wireline log matched depth below seafloor (WMSF) depth scale.

Logging Unit 1: base of drill pipe to 157 m WMSF

Unit 1 is characterized by low gamma ray values (between 2 and 6 gAPI), low and uniform density values (~1.25 g/cm3), low magnetic susceptibility, and a wide hole diameter. Logged density values are slightly higher than MAD bulk density measurements, probably because of core expansion (Fig. F35). NGR values were logged through the pipe, and these data were corrected by a factor of 3.5 to account for attenuation by the thickness of the drill pipe (Figs. F35, F36). Gamma ray values are similar to the open hole except in the top 1 m of the hole, where values reach 54 gAPI.

Logging Unit 2: 157–175 m WMSF

Unit 2 is characterized by a series of peaks to higher gamma, density, magnetic susceptibility, and photo-electric factor (PEF) values than those found in Unit 1 (Fig. F33). This is consistent with harder and less porous material than is found in Units 1 and 3. Drilling and coring data from this section indicate it is a porcellanite-rich interval (See "Lithostratigraphy").

Logging Unit 3: 175–189 m WMSF

Unit 3 is similar to Unit 1 both in the absolute density, gamma radiation, and susceptibility values and in the relatively uniform log response. Susceptibility and conductivity logs increase in the lowermost 2 m of the hole and probably represent a different lithology; they may also be affected by the underlying basaltic basement.

Heat flow

Heat flow at Site U1331 was determined according to the procedure of Pribnow et al. (2000). Five APCT-3 temperature measurements in Hole U1331B ranged from 1.88°C at 19.6 m to 2.86°C at 95.6 m, giving a geothermal gradient of 13.4°C/km (Fig. F37; Table T41). Seafloor temperature was ~1.5°C. 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). Calculated in situ values are ~2% lower than 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. F37). Heat flow was obtained from the linear fit between temperature and thermal resistance (Fig. F37). The heat flow estimate for Site U1331 is 10.3 mW/m2, which is lower than heat flow values from nearby Sites 1218, 1219, and 1220 but within the range of the lower values in the global heat flow data set for the eastern Pacific (Pollack et al., 1993).