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doi:10.2204/iodp.proc.323.103.2011

Downhole measurements

Logging operations

Downhole logging of Hole U1339D started after APC coring to a total depth of 200 m DSF ended on 20 July 2009 at 0930 h (all times are ship local time, UTC – 11 h). In preparation for logging, the hole was conditioned with a ~50 bbl sweep of sea gel (attapulgite, ~9 ppg), a go-devil was pumped through the drill string to open and lock the lockable flapper valve, and the bit was raised to the logging depth of 84 m DSF (1962 m DRF).

Two tool strings were deployed in Hole U1339D: the triple combo and the FMS-sonic combination (for tool and measurement acronyms, see "Downhole measurements" in the "Methods" chapter). Assembly of the triple combo tool string began at 1210 h, and the string was run in hole (RIH) at 1330 h. After some testing of the wireline heave compensator (WHC), the tool string reached the bottom of the hole (2080 m wireline log depth below rig floor [WRF]) and a first uphole logging pass started at 1550 h at a speed of 900 ft/h. This pass was completed when the bottom of the string reached 2000 m WRF at 1610 h. The caliper arm would not close while the tool was being lowered to record the main pass, presumably because of debris underneath it. Some time was spent attempting to close the caliper arm by reentering the pipe to force the closing mechanism; however, the arm closed by itself before it was forced shut. After establishing that there was no apparent damage to the arm, the tool string was lowered back to the bottom of the hole and a second uphole pass started at 1647 h. This second pass ended when the tool string crossed the seafloor, marked by a drop in natural radioactivity at 1876 m WRF, 2.4 m shallower than the drillers seafloor depth at 1878.4 m DRF. The triple combo tool string reached the rig floor at 1848 h and was rigged down at 1935 h.

The FMS-sonic tool string was then rigged up and RIH at 2010 h. It reached the bottom of the hole at 2080 m WRF at 2130 h, and the first pass started at the logging speed of 900 ft/h. The pass was completed at 2150 h, with the bottom of the 35 m long tool string at 2005 m WRF. After the tool string returned to the bottom of the hole, the second pass started at 2200 h and ended at 2250 h after the seafloor was detected at 1875 m WRF. The tool string was back on the rig floor at 0000 h 21 July, and rig-down was complete at 0110 h.

Downhole log data quality

Figures F31 and F32 show a summary of the downhole logging data acquired in Hole U1339D. These data were processed and converted to depth below seafloor and matched to depths between different logging runs. The resulting depth scale is wireline log matched depth below seafloor (WMSF) (see "Downhole measurements" in the "Methods" chapter).

The first indications of the overall quality of the logs are the size and shape of the borehole measured by the calipers. The hole size measured by the Hostile Environment Litho-Density Sonde caliper during the triple combo run and by the FMS arms is shown in the first column of Figures F31 and F32, respectively. For safety, the caliper and the FMS arms were closed before the top of the tool string reached the bottom of the pipe, but the data recorded below this depth indicate good hole conditions and only minor excursions from the nominal size of the drill bit.

The quality of the logs can also be assessed by comparing them with core measurements in the same hole or by looking at the repeatability of the measurements acquired in different runs. Figure F31 shows a comparison of the gamma ray and density logs with the NGR and GRA measurements on Hole U1339D cores and with MAD measurements made on samples from Site U1339. All data are in good agreement, which should allow for reliable core-log integration, but there is an apparent depth offset of ~3 m between the track and the log data, likely a result of different identifications of the seafloor. All logs were referenced to the seafloor depth of 1875 m WRF identified during the last pass of the FMS-sonic tool string. Comparison of the gamma ray logs measured during the main pass of the two runs shows excellent repeatability (Fig. F32).

Resistivity values measured by the spherically focused resistivity (SFLU) tool are lower than those recorded by induction measurements (e.g., medium and deep induction phasor-processed resistivity [IMPH and IDPH] in Fig. F31), probably because of current loss at the electrodes. The higher induction resistivities are closer to values typically measured in deep-sea sediments.

The high coherence in sonic waveforms used to derive compressional velocity suggests that, despite the closeness of the formation velocity to the sound velocity in the borehole fluid (~1500 m/h), the Dipole Sonic Imager was able to capture compressional wave arrivals and measure a reliable V_P profile over the entire open interval logged. Additional postcruise processing will, however, be necessary to derive V_S logs from the recorded dipole waveforms.

Finally, the examples of FMS images in Figure F32 show that some of the buttons on one arm were not working properly, generating a continuous streak along most of the interval logged. When the FMS-sonic tool string returned to the surface, these buttons appeared to be covered with an insulating substance, which was then cleaned off. The overall quality of the images was not significantly affected by the faulty buttons, and the two passes provide a high-resolution image of the borehole wall.

Logging stratigraphy and correlation

The downhole log measurements of bulk density, porosity, and electrical resistivity in Hole U1339D correlate very well (Fig. F31). Without significant variations in the overall composition of the mineral matrix, changes in sediment composition result in variations of porosity that affect bulk density and resistivity in a similar manner. These measurements in Hole U1339D also mostly correlate with the gamma ray logs over the entire interval logged, with low variability reflecting the mostly uniform nature of the formation logged. Despite this uniformity, some trends express changes in the sedimentation history at this site, such as the parallel decrease with depth in gamma radiation, density, and resistivity from 86 to 102 m WMSF, which is typical of a retrograding fining-upward sequence. Similar sequences, although less clearly defined, seem to define the general trend of the logs downhole.

The downhole variations of gamma ray radioactivity (Fig. F33) are controlled by the sediment content of naturally occurring radioactive elements (K, U, and Th). Computed gamma ray (or gamma ray without uranium) is a more accurate measure of clay content than total gamma ray (Rider, 1996), which can be influenced by factors such as organic matter or detrital minerals. The most significant feature in the gamma ray logs is the increase at ~142 m WMSF, which is associated with an increase in the three radioactive components and a peak in uranium. The peak in uranium can be related to the occurrence of dolostones observed in the cores in this interval.

Temperature measurements

Downhole temperature measurements at Site U1339 include one third-generation advanced piston corer temperature tool (APCT-3) deployment in Hole U1339A and four APCT-3 deployments in Hole U1339B (Table T20). During the deployment in Hole U1339A, the APCT-3 failed to couple properly with the formation and the recorded data could not be used. The measured temperatures range from 3.65°C at 23.0 m DSF to 12.83°C at 158.0 m DSF and closely fit a linear geothermal gradient of 68.0°C/km (Fig. F34). The record for Core 323-U1339B-13H displays some irregularities in temperature decay after penetration and was not used to calculate this gradient. The temperature at the seafloor was 2.1°C based on the average of the measurements at the mudline during all APCT-3 deployments. A simple estimate of heat flow can be obtained from the product of the geothermal gradient by the average thermal conductivity (0.80 W/[m·K]; see "Physical properties"), which gives a value of 54.4 mW/m², within the range of previous measurement in the area (the global heat flow database of the international heat flow commission can be found at www.heatflow.und.edu/index.html).

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