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doi:10.2204/iodp.proc.323.108.2011

Downhole measurements

Logging operations

Downhole logging of Hole U1344A started after APC/XCB coring to a total depth of 745.0 m DSF (3928.4 m DRF) and was completed at 1020 h on 18 August 2009 (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 logging mud. The bit was then raised to the logging depth of 99.6 m DSF (3283 m DRF).

Two tool strings were deployed in Hole U1344A: 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 1515 h, and the string was run in hole (RIH) at 1600 h. The tool string reached the bottom of the hole (total depth = 3928 m wireline log depth below rig floor [WRF]) and a first short (~50 m) uphole logging pass started at 1830 h at a speed of 900 ft/h. After the pass was completed, the triple combo was sent back to total depth, and the main pass started at 1905 h at the logging speed of 900 ft/h. The pass ended at 2155 h when the tool string crossed the seafloor, marked by a drop in natural radioactivity at 3184.5 m WRF, ~1 m deeper than that detected by the driller. The triple combo reached the rig floor at 0030 h, 19 August, and was rigged down at 0125 h.

Overall, the caliper of the density sonde showed an irregular borehole, with a particularly large interval between 170 and 260 m WSF but very good conditions in the lower section, indicating 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 0215 h. At 0400 h it could not pass below 3910 m WRF (725.5 m WSF), and the first pass started at the logging speed of 1500 ft/h. The pass was complete at 0600 h with the bottom of the 35 m long tool string at 3320 m WRF. While the tool was descending to the bottom of the hole for a second pass, a quick preliminary processing of the FMS electrical images established that the data quality and resolution were not diminished by the faster than normal (900 ft/h) logging speed, and the second pass started at 0630 h at 3910 m WRF at the same logging speed of 1500 ft/h. It ended at 0805 h after the last velocity measurements were recorded immediately below the bit. The FMS-sonic tool string was rigged down immediately after reaching the surface at 0940 h, and the rig floor resumed normal operations at 1110 h to pull out of the hole and begin coring Hole U1344B.

Downhole log data quality

Figures F41 and F42 show a summary of the main logging data recorded in Hole U1344A. These data were processed and converted to depth below seafloor and matched to depths between different logging runs. The resulting depth scale is 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 HLDS caliper during the triple combo run and by the FMS arms is shown in the lefthand columns of Figures F41 and F42, respectively. Both sets of calipers indicate a large hole with an average diameter of >14 inches, and the HLDS shows that the hole was particularly large between ~170 and 270 m WSF. Deeper in the borehole, small enlargements regularly spaced every ~9.5 m indicate where the bit was sitting when a core was recovered. However, all of the calipers show that the tools made at least partial 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. The anomalously low density values between 230 and 250 m WSF, which is within the 100 m interval with the largest hole size, are probably erroneous, as are most of the neutron porosity measurements in this entire interval.

The quality of the logs can also be assessed by comparing them with core measurements made at the same site or by the repeatability of measurements acquired in different runs. Figure F41 shows a comparison of the gamma ray and density logs with the NGR and GRA track data and the MAD measurements made on cores recovered from Hole U1344A. The trends in gamma radiation are very similar between the track and log data, and the lower log gamma ray readings between 170 and 270 mbsf are likely a consequence of the large hole diameter rather than a change in lithology. Considering the scatter inherent in the GRA track measurements, the density log should be compared with the upper envelope of the GRA data. Except for two short intervals with lower density logging data (230–250 m WMSF and 420–430 m WSF), all density data sets are in good agreement, confirming good data quality overall despite the enlarged hole. Comparison of the gamma ray logs measured during the main passes of the two runs (Fig. F42) shows excellent repeatability between the two runs. All logs were referenced to the seafloor depth of 3184.5 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. F41), 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 the finer scale variability in the formation.

The display in Figure F42 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 V_P to the sound velocity in the borehole fluid (~1500 m/h), the Dipole Sonic Imager (DSI) was able to capture compressional and flexural wave arrivals. However, the erratic V_P curve derived during the second pass of the tool string shows that the acquisition algorithm could not consistently differentiate between the two most coherent arrivals. It mistakenly identified the stronger direct fluid wave as the compressional arrival in many places. This will be corrected by a careful reprocessing, which will also likely reduce the variability of V_P and V_S in some intervals. However, the V_P profile that was routinely recorded while the tool was being lowered provided a more robust V_P profile for preliminary interpretation and will be used in the following discussion.

Logging stratigraphy and correlation

The combined analysis of the 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 correlation with seismic stratigraphy, it was the primary guide in the delineation of 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. F43) were also used for the characterization of these units.

Logging Unit 1 (100–330 m WMSF) is characterized mainly by a steady increase with depth in V_P and V_S, whereas the other logging data remain mostly uniform despite some variability, such as in gamma radiation. The bottom of this unit is defined by a noticeable drop in V_P , V_S, gamma radiation, density, and resistivity immediately above a sharp peak in these measurements, particularly in V_S and resistivity, indicating a fine stiff layer. This sequence corresponds to a core with poor recovery (Core 323-U1344A-35X). The low gamma radiation suggests a more sandy layer, which is often prone to poor recovery. The fine hard layer does not seem to have been recovered either, because nothing was observed in Core 323-U1344A-36X that would match these readings. Using the preliminary V_P downhole log and the density log we were able to match this interval with a strong reflector observed at 4.67 s two-way traveltime (Fig. F44) on seismic Line Stk3-7 (Sakamoto et al., 2005) crossing Site U1344. A section with distinctive black sulfide speckles (Sections 323-U1344A-18H-3 and 18H-4), which occur in several places in this unit and are recognizable as black conductive features in the FMS images, is shown in Figure F45A.

Logging Unit 2 (330–460 m WMSF) is almost uniquely defined by the V_P and V_S log, both of which increase steadily throughout the unit. Gamma radiation and density also increase with depth in this unit in a more subdued manner. A subtle drop in V_P and V_S at 380 m WMSF can be correlated in Figure F44 with a reflector at 4.73 s two-way traveltime, which can be extended updip (to the east; see "Background and objectives") to high amplitudes that have been interpreted as free gas trapped at the bottom of the gas hydrate stability field. As at Site U1343, 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, suggest that some amount of gas hydrate may be present. A layer of authigenic carbonates and fine sand from this unit can be seen in the FMS images in Figure F45B.

The top of logging Unit 3 (460–620 m WMSF) is defined by an inflection in the velocity profiles, which, combined with a decreasing trend in density, generates the strong reflector at 4.83 s two-way traveltime (Fig. F44). In addition to the density logs, the resistivity and gamma ray logs also follow a slightly decreasing trend with depth in this unit. The variability with depth in gamma radiation and in most logs displays a cyclicity that is more clearly defined than in the upper units. A section (323-U1344A-61X-4) with distinctive white carbonate pebbles that occur in several places in this unit and are recognizable as bright resistive features in the FMS images is shown in Figure F45C.

Finally, the top of logging Unit 4 (620–745 m WMSF) is defined by a sharp increase in V_P , V_S, gamma radiation, and density, as well as by a significant change in the trends of all the logs. As in the deepest unit of Site U1343, the gamma ray, potassium, thorium, density, resistivity, V_P , and V_S logs all display a variability with depth of wider amplitude and lower frequency than in the upper units, suggesting a significant change in deposition history and rates. Figure F45D shows FMS images that were recorded over a 3 m section of the 20 m interval at the top of logging Unit 4 that was not recovered between ~620 and 640 m DSF. The image shows conductive (dark) layers dipping steeply to the north and overlaying more conductive speckles similar to the sulfide speckles observed in several places in the hole.

Temperature measurements

The third-generation advanced piston corer temperature tool (APCT-3) was successfully deployed three times in Hole U1344A. The measured temperatures range from 4.51°C at 47.1 m DSF to 9.57°C at 142.1 m DSF and closely fit a linear geothermal gradient of 53.3°C/km (Fig. F46). The temperature at the seafloor was 1.65°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 (0.911 W/[m·K]; see "Physical properties"), which gives a value of 48.5 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).

Considering the variations in thermal conductivity with depth, a more accurate measure of the 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 profiles (Bullard, 1939). The thermal resistance calculated over the intervals overlying the APCT-3 measurements is shown in Table T22, and the resulting linear fit of the temperature gives a slightly lower heat flow value of 48.2 mW/m².

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