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doi:10.2204/iodp.proc.318.109.2011 Downhole measurementsLogging operationsDownhole logging operations started after a successful reentry of Hole U1361A, which had been left temporarily to allow an iceberg to pass. ACP/XCB coring had reached a total depth of 388.0 mbsf (drillers depth below seafloor [DSF]). In preparation for logging, the hole was flushed with a 50 bbl sweep of sepiolite mud and then displaced with 179 bbl of 10.5 ppg barite-weighted mud. The pipe was pulled up to 103.2 mbsf (DSF). The drillers reported no problems with the hole. The logging plan for Hole U1361A was to run the triple combo tool string followed by the FMS-sonic tool string.
At 0725 h on 1 March 2010, the triple combo tool string (resistivity, density, porosity, and natural gamma ray sensors), with a go-devil at the bottom, was run into the hole and logged down to the bottom at 390 mbsf (wireline depth below seafloor). The hole was then logged up to seafloor. The tools provided continuous and good-quality log data. The hole was drilled with an 11 At 1725 h, rig-up of the FMS-sonic tool string (FMS microresistivity imager, sonic, and NGR tools) was completed and the tool string was run down to the bottom of the hole (Fig. F28). Two upward passes were logged at 550 and 275 m/h, respectively. At the top of the second upward log, as the base of the tool string was entering the pipe the cable tension increased and the tool string had to be stopped and lowered again. After an hour of retrying to enter the pipe, initially without pumping water down and then with pumping, the tool string cleared the obstruction and passed back up fully into the pipe. The FMS-sonic tool string was then pulled up the pipe slowly, because large tension swings caused by ship heave (as much as 3 m) made it impossible to increase the speed. The tools were back on the rig floor at 0405 h on 2 March. Logging unitsDownhole logs in Hole U1361A have high-amplitude, 1–5 m variability superimposed on a downhole compaction trend. The character of the logs changes gradually downhole, with no major steps in the base levels, so the entire logged interval was assigned to one logging unit (Fig. F29). The overall character and values of the logs are similar to those at Site U1359, 50 km to the south. NGR varies between 35 and 80 gAPI, with higher amplitude swings from 225 mbsf up to the pipe. Below 250 mbsf, there is a slight trend to lower NGR values downhole, but the 1–5 m variability remains. As at Site U1359, the NGR signal comes mostly from the radioactive isotopes of potassium and thorium (Fig. F30). The density and resistivity logs also have high-amplitude variability in the interval from 225 mbsf up to the pipe. Density and sonic velocity have an increasing downhole trend, reflecting sediment compaction with depth. Resistivity, similar to that at Site U1359, shows the opposite trend (decreasing slightly downhole), possibly because of increasing temperature with depth. Resistivity would normally be expected to increase with depth. Compressional wave velocity has good repeatability between the two passes below 170 mbsf, but from 170 mbsf to the pipe, velocities could not be determined from the real-time slowness-time-coherence analyses. In general, the logged velocities have higher values than those measured on the XCB cores (P-wave caliper; x-direction), possibly because of the slightly disturbed nature of the XCB cores and log measurement under in situ conditions. Correlations between the downhole logsA change in the relationship between the natural gamma log and the physical properties logs occurs at ~265 mbsf (Fig. F31). From the pipe to 265 mbsf, all logs co-vary, with low NGR corresponding to low-density and low-resistivity values. As at Site U1359, this is most probably because the diatoms retain intragranular porosity because the opal they are made of has a low density and because the diatoms dilute the more radioactive clays and other terrigenous minerals that make up the balance of the sediment. In contrast, below 265 mbsf the resistivity log anticorrelates with the NGR log, and the density log has some features in common with both NGR and resistivity (Fig. F31). One possible explanation for this is that cementation is taking place preferentially in the diatom-rich layers, blocking the pore throats that allow electrical conduction and slightly increasing the bulk density but leaving the clay and terrigenous content of the layer unchanged. The appearance of calcareous nannofossils below 313 mbsf (see “Lithostratigraphy”) could also have an additional influence, because calcite has a higher density (~2.7 g/cm3) than opal (2.1–2.2 g/cm3), but this would not explain the higher resistivity values in the microfossil-rich layers. The implication is that levels of cementation increase in the microfossil-rich layers below 265 mbsf. CyclicityFigure F31 illustrates the cyclic nature of the sediment sequence, which alternates between high and low log values at intervals of 1–5 m. As a first rough estimate of the average duration of these alternations, the number of cycles in both of the intervals shown in Figure F31 was counted. For the 130–180 mbsf interval, there are ~15 alternations and the duration is, very approximately, 2 m.y. (see “Biostratigraphy”). This gives an average duration of 133 k.y. for each cycle, which seems to be in the ballpark of the orbital eccentricity Milankovitch cycle (96 and 125 k.y.). The 300–350 mbsf interval also contains ~15 alternations, laid down over perhaps 1 m.y., giving an average duration of 67 k.y. for each cycle. Given the uncertainties in the initial age estimates, the probability that all cycles are not recorded equally well in the sediment record, the possibility of multiple cyclicities influencing the sediment record, and the subjective nature of counting cycles, this early estimate requires further verification. It is certainly reasonable, however, that Milankovitch band variability at eccentricity and possibly obliquity periods is recorded at Site U1361. Formation MicroScanner resistivity imagesFMS resistivity images reveal stratigraphic information at a finer spatial resolution than the standard resistivity logs (Fig. F32A), including both gradual and sharp transitions between the alternations of resistive and conductive beds (Fig. F32B) and dropstones and ice-rafted debris larger than ~0.5 cm. The dropstones appear as resistive (light colored) spots in the image (Fig. F32C), and it will be possible to map their occurrence from 105 to 390 mbsf. Temperature measurementsDownhole measurements at Site U1361 included four APCT-3 deployments in Hole U1361A (Table T10). During deployment in Cores 318-U1361A-4H and 13H, the APCT-3 failed to couple properly with the formation and the recorded data could not be used. The measured temperatures ranged from 4.37°C at 66.0 mbsf to 5.58°C at 94.5 mbsf (Fig. F33A) and provide a linear geothermal gradient of 64.5°C/km (Fig. F33B). The temperature at the seafloor was –0.3°C, based on the average of the measurements at the mudline during all APCT-3 deployments. The thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndmann et al. (1974) (see “Physical properties” in the “Methods” chapter). Considering the variations in thermal conductivity with depth, a measure of the heat flow in a conductive regime can be given by a “Bullard plot.” Thermal resistance is then calculated by cumulatively adding the inverse of the in situ thermal conductivity values over depth intervals downhole. 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 T10, and the resulting linear fit of the temperature gives the heat flow value of 58.2 mW/m2 (Fig. F33C). |
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inch diameter APC/XCB bit and the diameter recorded by the caliper ranged from 12 to 15 inches (30.5–38.1 cm), with several thin washouts >16 inches shallower than 155 mbsf. The seafloor was identified at 3469.5 mbrf (wireline depth below rig floor) from an increase in gamma ray values at the sediment/water interface (Fig.