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doi:10.2204/iodp.proc.329.106.2011

Downhole logging

Downhole logging data obtained from Hole U1368F included NGR, spectral gamma radiation, density, photoelectric factor, electrical resistivity measurements, and FMS images of the borehole wall. Interpretations of NGR downhole logs were used to identify two logging units in Hole U1368F, with one in the sediment sequence and one in the basaltic basement.

Wireline operations

A wiper trip was completed throughout the open hole before the start of the wireline logging operations. The drill pipe was set at 34 m wireline matched depth below seafloor (WMSF; 3592.5 mbrf), which is ~17 m below the sediment/basement interface. The hole was circulated with seawater. Downhole logging operations lasted 18 h, beginning at 1330 h on 17 November 2010. The wireline logging operations consisted of two tool string deployments and testing of the wireline heave compensator (WHC). Logging operations in Hole U1368F took place in good sea conditions with ship heave between 0.4 and 2 m, peak-to-peak.

Tool string deployment

HNGS-HLDS-GPIT-DIT

The wireline tool string deployment consisted of a 23.22 m long triple combo tool string that included a logging equipment cable head (LEH-QT), digital telemetry cartridge (DTC-H), Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Natural Gamma Ray Cartridge (HNGC), Litho-Density Sonde Cartridge (LDSC), Hostile Environment Litho-Density Sonde (HLDS), digital telemetry adapter (DTA-A), General Purpose Inclinometry Tool (GPIT), and the Digital Dual Induction Tool model E (DIT-E). Downhole logs were recorded in three passes:

1. A downlog from seafloor to 104.5 m WMSF,

2. An uplog from 104.5 to 53 m WMSF, and

3. An uplog from 104.5 m WMSF to seafloor.

After the downlog was stopped and prior to starting Pass 1, ~1 h was spent assessing downhole tool motion and optimizing the efficiency of the WHC for the water depth at Hole U1368F and heave conditions at the time of the logging operations. Once the best possible WHC parameters were chosen for the prevailing heave conditions, the tool string was lowered to 104.5 m WMSF to begin the first uplog.

HNGS-GPIT-FMS

The second wireline tool string deployment consisted of a 14.27 m long FMS-HNGS that included a LEH-QT, DTC-H, HNGS, DTA-A, GPIT, and FMS. Downhole logs were recorded in three passes:

1. A downlog from seafloor to 104.5 m WMSF,

2. An uplog from 104.5 to ~53 m WMSF, and

3. A second uplog from 104.5 m WMSF to seafloor.

Data processing

Logging data were recorded onboard the JOIDES Resolution by Schlumberger and archived in digital log information standard (DLIS) format. Data were sent by satellite transfer to the Borehole Research Group of the Lamont-Doherty Earth Observatory, processed there, and transferred back to the ship for archiving in the shipboard database. Processing and data quality notes are given below.

Depth shifting

In general, depth shifts that are applied to logging data by selecting a reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed) and features in equivalent logs from other passes are aligned by eye. The logging data seafloor depth could not be determined by the step in gamma ray values because the signal was affected by the presence of the drill collars. As a result, the seafloor depth given by the drillers (3752 m drilling depth below rig floor) was used for depth shift.

Data quality

The quality of wireline logging data was assessed by evaluating whether logged values are reasonable for the lithologies encountered and by checking consistency between different passes of the same tool. Gamma ray logs recorded through the BHA should be used only qualitatively because of the attenuation of the incoming signal. The thick-walled BHA attenuates the signal more than the thinner walled drill pipe.

A wide (>30.5 cm) and/or irregular borehole affects most recordings, particularly those like the HLDS that require eccentralization and a good contact with the borehole wall. The density log roughly correlates with the resistivity logs but is largely affected by hole conditions. Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL), which shows a very irregular borehole with intervals exceeding the maximum caliper aperture. Good repeatability was observed between Passes 1 and 2, particularly for measurements of electrical resistivity, gamma radiation, and density.

The FMS images are generally of good quality because of the relatively good hole conditions (hole size <35.6 cm). The irregular and possibly elliptical shape of the borehole occasionally prevented some FMS pads from being in direct contact with the formation, resulting in poor resolution or dark images.

Preliminary results

Electrical resistivity measurements

A total of three electrical resistivity curves were obtained with the DIT-E. The spherically focused resistivity (SFLU), medium induction phasor-processed resistivity (IMPH), and deep induction phasor-processed resistivity (IDPH) profiles represent different depths of investigation into the formation (64, 76, and 152 cm, respectively) and different vertical resolutions (76, 152, and 213 cm, respectively). Downhole open-hole electrical resistivity measurements covered 74 m of the basement lithologic units (Fig. F48). The DIT-E was the only tool that reached the bottom of the logged interval in Hole U1368F because it was the lowermost tool in the logging tool string (see Fig. F12 in the “Methods” chapter [Expedition 329 Scientists, 2011a]). IDPH measurements in the basaltic basement range from 3.0 to 84 Ωm, IMPH measurements range from 1.4 to 3.16 Ωm, and SFLU values range from 1.9 to 202.7 Ωm (Fig. F48).

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole U1368F with the HNGS. Downhole gamma ray measurements covered 95 m of the lithostratigraphic sequence in Hole U1368F. NGR measurements in the sediment sequence are attenuated by the BHA but show consistently lower values for total gamma, K, Th, and U than the underlying basement. The sediment/basement interface is clearly identifiable in total gamma ray and potassium measurements at 17 m WMSF.

Total gamma ray measurements in the basaltic basement are low, with basement values ranging between 3.5 and 14.1 gAPI (Fig. F49).

Potassium concentrations in the basaltic basement range between 0.08 and 0.65 wt%. Uranium concentrations range between 0.0 and 0.68 ppm. Thorium concentration ranges from 0.0 to 0.92 ppm).

Comparison between NGR measurements made on whole-round core sections and downhole logs shows good agreement between the two (Fig. F50). Total gamma ray data from the downhole logs have been corrected by a factor of 3.5 for the attenuation caused by the BHA between seafloor and 34 m WMSF. The measurements made on whole cores were used to provide a check on this correction.

Density

Density values range from 1.1 and 3.4 g/cm³ in the basement section. Comparison with caliper data indicate that the data are affected by hole size, particularly where the hole is washed out (Fig. F51). Caliper data indicate the hole diameter varies between 33 cm and the maximum extension of the caliper arm, 50.8 cm.

Formation MicroScanner images

FMS images were obtained for the open-hole interval between 39 and 103 m WMSF. The diameter of hole from the FMS calipers varied between 18.8 and 35.1 cm. High-quality FMS images were obtained in much of the borehole; only intervals that were washed out show poor-quality images. FMS images from the basement section show well-defined pillow structures, some of which are between 50 and 100 cm in size (Fig. F52). The most massive pillows show good recovery in core samples, and core photographs from Core 329-U1368F-5R are compared to FMS images from the same interval in Figure F53.

Lithostratigraphic correlations

Preliminary interpretation of the downhole log data divides Hole U1368F into two logging units (Figs. F48, F49). Unit I covers the sediment sequences logged through the BHA. Unit II covers the basaltic basement sequence.

Logging Unit I was identified in the section covered by the BHA between 0 and 17 m WMSF (Figs. F48, F49) and corresponds to the carbonate oozes and metalliferous clays described in the sediment sequence (see “Lithostratigraphy”).

Logging Unit II extends from the sediment/basement interface at 17 m WMSF to the logging total depth of 104.5 m WMSF. This unit covers a sequence of basaltic pillow lavas.

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