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doi:10.2204/iodp.proc.323.107.2011

Downhole measurements

Logging operations

Downhole logging of Hole U1343E started after APC/XCB coring to a total depth of 743.8 m DSF (2711.3 m DRF) ended on 12 August 2009 at 0750 h (all times are ship local time, UTC – 11 h). In preparation for logging, the hole was conditioned with a ~50 bbl sweep of high-viscosity mud and displaced with 265 bbl of logging mud. The bit was then raised to the logging depth of 99.5 m DSF (2067 m DRF).

Two tool strings were deployed in Hole U1343E: 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 1315 h, and the string was run in hole (RIH) at 1410 h. The tool string reached the bottom of the hole (total depth = 2711 m wireline log depth below rig floor [WRF]), and a first short (~50 m) uphole logging pass started at 900 ft/h at 1625 h. After the pass was completed, the triple combo was sent back to total depth, and the main pass started at 1650 h at 900 ft/h. The pass ended at 1945 h when the tool string crossed the seafloor, marked by a drop in natural radioactivity at 1962.5 m WRF, ~5 m shallower than that detected by the driller. The triple combo reached the rig floor at 2100 h and was rigged down at 2150 h.

Overall, the caliper of the density sonde showed an irregular borehole with several enlarged intervals above 420 m WSF but very few large washouts and very good conditions in the lower section, indicating that deployment of the FMS-sonic tool string would provide worthwhile velocity and image data. The tool string was built up and RIH at 2230 h. It reached its maximum depth at 2710 m WRF at 0020 h, August 13, and the first pass started at 900 ft/h. The pass was completed at 0233 h with the bottom of the 35 m long tool string at 2110 m WRF. After the tool string returned to the bottom of the hole, the second pass started at 2710 m WRF at 0300 h and ended at 0535 h after the last velocity measurements were recorded immediately below the bit. The FMS-sonic was rigged down after reaching the surface at 0650 h, and the rig floor resumed normal operations at 0745 h in preparation for transit to the next site.

Downhole logging data quality

Figures F38 and F39 show a summary of the main logging data recorded in Hole U1343E. 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 Figures F38 and F39, respectively. Although both runs indicate an irregular hole, particularly above 430 m WSF, all of the calipers show that the tools were making contact with the formation over most of the interval logged, suggesting that the overall quality of the data is good.

Irregular hole size has an effect on measurements that require good contact with the formation, namely density and porosity. Although the HLDS caliper in Figure F38 suggests that the tools were making good contact between 300 and 360 m WSF and that the hole was even smaller than the nominal bit size in part of this interval, the density and neutron porosity data in this interval seem questionable. The anomalously low density readings between 307 and 322 m WSF and the very high neutron porosity values between 300 and 360 m WSF suggest that the tool was not properly measuring formation properties. A comparison with density measurements made with the GRA track sensor on cores recovered from Hole U1343E and MAD measurements made on samples from Site U1343 shows generally good agreement, except in this interval, where logging data are significantly lower than core measurements.

Because this interval includes the estimated depth of one of the main seismic reflectors at this site, possibly a BSR, it is necessary to carefully assess the quality of the data recorded in this key transition interval. Figure F40 shows a close-up comparison of hole size, gamma ray, and density data recorded over this interval by several passes and tool strings. The triple combo downlog was recorded routinely to guarantee that data were recorded even in the case of tool failure later in the deployment. The caliper was not open during this pass, and only the gamma ray and resistivity measurements taken while the tool was being lowered are generally considered to be reliable. However, the density sonde is active and can make valid measurements in places where the tool is actually making contact with the formation on its way down.

The caliper logs show that the smaller hole size between 307 and 322 m WSF was measured only during the triple combo uplog, and both FMS passes measured a larger hole in this interval. Comparison of the gamma ray logs shows that the triple combo uplog also recorded significantly higher values than any of the other passes in this same interval because it was in direct contact with the formation. The lower density values measured below 307 m WSF during both the uplog and downlog show that either the in situ density is significantly lower than the core density measured with the GRA track sensor and MAD measurements, which is unlikely, or the density sonde was not measuring the formation property despite being in apparent good contact. The good correlation between the resistivity and velocity logs and the GRA density data in this interval, which is expected under normal consolidation of sediments, suggests that the GRA track measurements are more reliable than the density log in this interval. One possible explanation for this apparent contradiction is that the narrow hole size measured by the caliper was an indication of shallower material falling off and building a temporary ledge of loose sediments whose density was measured by the HLDS.

Because the triple combo uplog was the only pass to measure elevated gamma radiation between 303 and 317 m WMSF and all other passes show very good repeatability (Fig. F40), we consider the high readings to be the anomalous consequence of temporary fill in the borehole. We use the gamma ray log recorded during the second pass of the FMS-sonic tool string in the following figures and discussion.

Except in the interval discussed above, Figure F38 shows that density and gamma ray measurements made downhole and on the recovered cores are in good agreement; however, they display a depth offset of ~5 m with the core data, indicating deeper depth for the same measurement. This offset was adjusted in the core measurement and recovery data shown in Figure F40 for a more precise comparison. All logs are referenced to the seafloor depth of 1962.5 m WRF, which was identified where the gamma ray tool stopped detecting any natural radioactivity at the end of the triple combo run. This depth is 5 m shallower than the drillers estimation.

Above 370 m WSF, 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. F38), probably because of current loss at the electrodes and 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 finer scale variability in the formation.

The display in Figure F39 of the high coherence in sonic waveforms used to derive compressional and shear velocities suggests that, despite the enlarged hole and the closeness of formation V_P to sound velocity in the borehole fluid (~1500 m/h), the Dipole Sonic Imager (DSI) was able to capture distinct wave arrivals and measure reliable V_P and V_S profiles. Additional postcruise processing will be required only to refine these profiles and will likely reduce the variability of V_P and V_S in some intervals.

Logging stratigraphy and correlation

The combined analysis of gamma ray, resistivity, density, and velocity logs allows for the identification of several logging units defined by characteristic trends. Because of the uniformity of the sediments at this site (see "Lithostratigraphy"), these units are mostly defined by subtle changes in trends and correlations rather than indications of significant changes in the formation. Because the V_P log also allows for correlation with seismic stratigraphy, it was the primary guide in delimiting the units. The variations in the content of the three radioactive elements contributing to the natural radioactivity of the formation (K, U, and Th; Fig. F41) were also used for the definition of these units.

Logging Unit 1 (100–330 m WMSF) is characterized mainly by a steady increase with depth in velocity, whereas the other logging data remain mostly uniform despite some variability, such as in gamma radiation. Using the preliminary V_P logs and a density profile composite made of the density log and the envelope of GRA bulk density between 310 and 360 m WMSF, we were able to reproduce the main reflectors observed in seismic Line Stk-1 (Sakamoto et al., 2005) crossing Site U1343. The velocity increase at the bottom of logging Unit 1 is likely responsible for the strong reflector that can be observed at 2860 ms two-way traveltime (Fig. F42). Although we speculated that this reflector might be a BSR that indicates the existence of gas hydrate overlying free gas, no conclusive indication from the logs supports the occurrence of gas hydrate. However, slightly higher velocity and resistivity trends and lower dipole waveform amplitudes above the reflector, as well as lower chlorinity values measured on several pore water samples (see "Geochemistry and microbiology"), suggest that some amount of gas hydrate might be present.

One of the ash layers (Section 323-U1343E-33H-3) recovered in this unit can be recognized in the FMS images (Fig. F43A).

Logging Unit 2 (330–510 m WMSF) is defined by slightly decreasing trends with depth in resistivity and slightly higher gamma ray values than those in shallower and deeper units. In the upper part of this unit (330–360 m WSF), the density values measured on the shipboard tracks were the highest recorded in this hole and were not matched by any of the density logs (Figs. F38, F40). These values were used in the synthetic seismogram (Fig. F42) and seem to be associated with the broad reflector at ~2880 ms two-way traveltime. One of the sections with the highest density readings (Section 323-U1343E-41H-6) can be associated with a resistive mottled area in the FMS images (Fig. F43B).

The top of logging Unit 3 (510–745 m WMSF) is defined by a drop in gamma radiation, an increase in V_P , and a change in the trends of all the logs. The gamma ray, potassium, thorium, density, resistivity, V_P, and V_S logs all display variability with depth of wider amplitude and lower frequency than in the upper units, suggesting a significant change in deposition history and rates. A dolostone recovered in this unit can be recognized in the FMS images (Section 323-U1343E-78X-6; Fig. F43C).

Temperature measurements

The APCT-3 tool was successfully deployed three times in Hole U1343A. The measured temperatures range from 4.34°C at 43.5 m DSF to 8.53°C at 129.0 m DSF and closely fit a linear geothermal gradient of 49.0°C/km (Fig. F44). The temperature at the seafloor was 2.05°C based on the average of the measurements at the mudline during all APCT-3 deployments. A simple estimate of the heat flow can be obtained from the product of the geothermal gradient by the average thermal conductivity (0.985 W/[m·K]; see "Physical properties"), which gives a value of 48.2 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 the thermal resistance profile (Bullard, 1939). The thermal resistance calculated over the intervals overlying the APCT-3 measurements is shown in Table T20, and the resulting linear fit of the temperature gives a slightly lower heat flow value of 46.6 mW/m².

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