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doi:10.2204/iodp.proc.323.105.2011

Downhole measurements

Logging operations

Downhole logging of Hole U1341B started after APC/XCB coring to a total depth of 600 m DSF (2750 m DRF) and ended on 31 July 2009 at 1520 h (all times are ship local time, UTC – 11 h). In preparation for logging, the hole was conditioned with a ~200 bbl sweep of sea gel (attapulgite, ~9 ppg) and the bit was raised to the logging depth of 79 m DSF (2229 m DRF).

Two tool strings were deployed in Hole U1341B: 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 started at 1930 h and the string was run in hole (RIH) at 2200 h after troubleshooting and replacing one of the tools. The tool string reached the bottom of the hole (total depth = 2750 m wireline log depth below rig floor [WRF]), and a first uphole logging pass started at 0030 h on 1 August, at a speed of 900 ft/h. After a short (~50 m) repeat section was completed, the triple combo was sent back to total depth and the main pass started at 0100 h. On its way up, the tool encountered significant drag in many places above 2650 m WSF, proceeding in a stick/slip motion that was detrimental to the quality of the data. As a result, the logging speed was increased to 1200 ft/h for most of the section logged, which seemed to reduce the sticking of the tool against the formation. The logging pass ended at 0315 h when the tool string crossed the seafloor, marked by a drop in natural radioactivity at 2150 m WRF. The triple combo reached the rig floor at 0500 h and was rigged down at 0600 h.

Overall, the caliper of the density sonde showed a very irregular borehole, with many intervals having a hole diameter >20 inches. Although the quality of the FMS images was expected to be poor in some intervals, we decided that there was no risk for the tools and that the deployment of the FMS-sonic tool string would provide worthwhile velocity and image data. The tool string was built up and RIH at 0705 h. It reached the bottom of the hole at 2748 m WRF at 0935 h, and the first pass started at the logging speed of 900 ft/h. The tool drag observed during the triple combo run also affected the FMS-sonic tool, and the logging speed was increased to 1200 ft/h for the rest of operations after the tool was configured to prevent any consequence to the vertical resolution of the data. The pass was completed at 1055 h with the bottom of the 35 m long tool string at 2275 m WRF. After the tool string returned to the bottom of the hole, the second pass started from total depth at 1125 h and ended at 1305 h after the last velocity measurements were recorded immediately below the bit. The tool string was at the surface at 1420 h, and the rig floor was ready to begin operations in Hole U1341C at 1530 h.

Downhole log data quality

Figures F36 and F37 show a summary of the logging data acquired in Hole U1341B. 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 indicators 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 (HLDS) caliper during the triple combo run and by the FMS arms is shown in the first column of Figures F36 and F37, respectively. Although both runs indicate an enlarged and irregular hole, the readings of the two orthogonal FMS calipers suggest that the borehole section was not circular and was probably highly elliptical. One caliper read <10 inches over most of the lower half of the interval logged, whereas the other caliper reading remained close to ~14 inches, near the limit of its range. The fact that the curves display variability over most of the hole suggests that both sets of arms were making some kind of contact with the formation, possibly with only one pad in some places. The readings of the larger HLDS caliper show that this single-arm caliper was likely following the longest "axis" of the hole and that the stronger and narrower arm was actually pushing inside the formation.

The large hole size mostly affected the measurements that require good contact with the formation, namely density and porosity. The very high neutron porosity values above ~275 m WMSF indicate that porosity readings are invalid above this depth. Similarly, the anomalously low density values between 180 and 210 m WMSF are also indicative of bad tool contact and are not valid. Even if the FMS arms seem to have been in contact with the formation over most of the interval logged, this contact was likely only partial in places, resulting in blurry or featureless images in many intervals. It is still possible to identify many fine layers, mostly in the deeper part of the hole.

The quality of the logs can also be assessed by comparing the logs with the core measurements at the same site or by the repeatability of measurements acquired in different runs. Figure F36 shows a comparison of the gamma ray and density logs with the NGR and GRA track measurements on cores recovered from Hole U1341B and with MAD measurements made on samples from Site U1341. Except for the low density logging data between 180 and 210 m WMSF, all data sets are in good agreement, confirming that the logs are of generally good quality despite poor hole conditions. Comparison of the gamma ray logs measured during the main pass of the two runs in Figure F37 shows excellent repeatability between the two runs. All logs were referenced to the seafloor depth of 2150 m WRF identified during the main pass of the triple combo tool string.

The resistivity values measured by the electrode spherically focused resistivity (SFLU) measurement were lower than those recorded by induction measurements (e.g., medium induction phasor-processed resistivity [IMPH] and deep induction phasor-processed resistivity [IDPH] in Fig. F36), probably because of current loss at the electrodes and the eccentralization of the sonde. The higher induction resistivity values are more representative of the resistivity of the formation, but the higher resolution SFLU data are a good indication of the finer scale variability in the formation.

The display of the high coherence in sonic waveforms used to derive the compressional and shear velocities suggests that despite the enlarged hole and the closeness of the formation velocity to the sound velocity in the borehole fluid (~1500 m/s), the Dipole Sonic Imager (DSI) was able to capture distinct wave arrivals and measure reliable VP and VS profiles. Additional postcruise processing will be required to refine these profiles and will likely reduce the variability of VP and VS in some intervals.

Logging stratigraphy and correlation

The combined analysis of gamma ray, resistivity, density, and velocity logs allowed for the definition of several logging units characterized by specific trends. Variations in the sediment content of the three radioactive elements contributing to the natural radioactivity of the formation (K, U, Th; Fig. F38) were also used to define these units.

Logging Unit 1 (80–220 m WMSF) is characterized mainly by decreasing trends with depth in gamma radiation and resistivity, accompanied by several high peaks in these measurements. It coincides mostly with lithologic Unit I, which is composed of diatom ooze and diatom silt (see "Lithostratigraphy"). Figure F38 shows that most of the peaks in gamma radiation are related to high uranium content. The coincidence of these higher uranium values with higher resistivity and, to some extent, higher density is an indication that they are caused by authigenic carbonate, which was observed at many of these depths (Sections 323-U1341B-17H-2 and 17H-3; see Fig. F39A for a comparison with FMS images). One of the gamma ray peaks is associated with a prominent high in thorium at ~135 m WMSF, which could be related to the ash layers observed at the same depth (Section 323-U1341B-15H-2; see "Lithostratigraphy").

Logging Unit 2 (220–350 mbsf) is defined by slightly increasing trends with depth in gamma radiation and density, whereas resistivity mostly decreases. The close similarity between the gamma ray and potassium profiles over this unit (Fig. F38) suggest enrichment in potassium-rich clay minerals with depth. Several peaks in gamma radiation can be observed in this unit as well, again generally due to higher uranium content and often associated with authigenic carbonate observed in the core (Sections 323-U1341B-32H-3 and 39H-4; see Fig. F39B for a comparison with FMS images)

The top of logging Unit 3 (350–425 m WMSF) is defined by a sharp drop in resistivity at ~350 m WMSF and by similar changes in gamma radiation and density. Because VP does not display any significant change at this depth, the change in density is likely responsible for the strong reflector that can be observed in seismic Line Stk5-1 at 3340 ms two-way traveltime (Sakamoto et al., 2005). This reflector can be reproduced, although with a lower amplitude, by a synthetic seismogram generated from density and VP logs (Fig. F40). The wavelet used to produce the synthetic seismogram was extracted from the seafloor reflection identified from the traces adjacent to Shotpoint 670 and Site U1341 in Line Stk5-1. This logging unit is characterized by an increase in gamma radiation and resistivity with depth and generally by higher uranium content, as shown in Figure F38 by the consistent offset between total and computed gamma radiation. This can be associated with higher organic matter content, in agreement with higher TOC measured in this interval (see "Geochemistry and microbiology").

The top of logging Unit 4 (425–600 m WMSF) is defined by a drop in resistivity, which decreases with depth over the entire unit. The top of Unit 4 also coincides with an inflection in the overall increase with depth of shear velocity (Fig. F37) and, to a lesser extent, compressional velocity. Gamma ray values are generally lower than in the overlying logging unit, but they increase slightly with depth and again display several peaks due to higher uranium content associated with subtle resistivity peaks and the occurrence of authigenic carbonate (Cores 323-U1341B-51H and 62X). Some of the resistivity peaks in this unit, prominent in the FMS electrical images, can also be associated with dolostones (Section 323-U1341B-58X-3; Fig. F39C).

Temperature measurements

The third-generation advanced piston corer temperature tool (APCT-3) was successfully deployed three times in Hole U1341A. The measured temperatures range from 4.68°C at 41.0 m DSF to 11.12°C at 136.0 m DSF and closely fit a linear geothermal gradient of 67.8°C/km (Fig. F41). The temperature at the seafloor was 1.95°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 (0.825 W/[m·K]; see "Physical properties"), which gives a value of 55.9 mW/m2, within the range of previous measurements in the area (the global heat flow database of the International Heat Flow Commission can be found at www.heatflow.und.edu/index.html).

Considering the variations in thermal conductivity with depth, a more accurate measure of heat flow in a conductive regime can be given by a "Bullard plot." The thermal resistance of an interval is calculated by integrating the inverse of thermal conductivity over depth. If the thermal regime is purely conductive, the heat flow will be the slope of the temperature versus thermal resistance profiles (Bullard, 1939). Thermal resistance calculated over the intervals overlying the APCT-3 measurements is shown in Table T19, and the resulting linear fit of the temperature gives a slightly higher heat flow value of 56.2 mW/m2.