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doi:10.2204/iodp.proc.346.108.2015

Downhole measurements

Logging operations

Downhole logging measurements at Site U1427 were conducted in Hole U1427A after completion of APC/XCB coring to a total depth of 548.6 m CSF-A. The hole was reported to be in good condition. In preparation for logging, the hole was flushed with a 220 bbl sweep of high-viscosity mud, and the bit was pulled up to 80 mbsf. The paleo combo and FMS-sonic tool strings were deployed in Hole U1427A (Fig. F52) (see “Downhole measurements” in the “Methods” chapter [Tada et al., 2015b], see also Table T12 in the “Methods” chapter [Tada et al., 2015b] for tool acronyms). The Hostile Environment Natural Gamma Ray Sonde (HNGS) was included in the FMS-sonic tool string to increase precise depth match processing between logging strings and passes.

On 7 September 2013 at 2315 h (UTC), the paleo combo tool string (comprising resistivity, density, natural and spectral gamma radiation, and magnetic susceptibility tools) descended from the rig floor into the pipe. A downlog was taken at ~600 m/h to the bottom of the hole at 550.5 m WSF. The hole was logged up to ~463 m WSF at 540 m/h (Pass 1). The tool string returned for the main pass from the bottom of the hole to the seafloor.

The FMS-sonic tool string was rigged at ~0315 h on 8 September. A downlog was taken at 850 m/h. The tool string reached the bottom of Hole U1427A at ~548.5 m WSF. Two uphole passes of the FMS-sonic tool string were run, the first pass to ~461 m WSF and the main pass to the seafloor, both at 550 m/h. Rig down was completed at 0810 h.

The seafloor depth was indicated by the step in the gamma logs. The paleo combo tool string uplog (main pass) found the seafloor at 339.5 mbrf (see the “Methods” chapter [Tada et al., 2015b]), and the main pass of the FMS-sonic tool string found the seafloor at 338 mbrf. Heave was very low (<0.3 m peak-to-peak) during operations and the wireline heave compensator was not used (see “Downhole measurements” in the “Methods” chapter [Tada et al., 2015b]).

Logging data quality

Tool calibration was performed both before and after the logging runs to ensure quality control. In Hole U1427A, borehole conditions were very good with a baseline diameter (11.5–12.5 inches) close to the bit size (~11 inches) and few washouts (225, ~236, 316–340, and 474–478 mbsf). The borehole is slightly larger from the pipe entrance to 187 mbsf, although it does not exceed 14 inches in diameter (Fig. F53, Column 1). Because of good borehole conditions and negligible heave during downhole logging data acquisition, log data quality is generally very good.

There is good agreement between physical properties and logging data for the NGR and density logs along almost the entire borehole depth (Fig. F50, Columns 2 and 3). As a result of caliper closure (see “Downhole measurements” in the “Site U1423” chapter [Tada et al., 2015d]), from ~103 mbsf to the pipe entrance, the uplog gamma ray deviates from the core NGR data to lower values and no longer overlaps the downlog (Fig. F50, Column 2). For the same reason, the density log shows lower values than the core data over the same interval. Natural and spectral gamma ray data recorded shallower than 80 mbsf should only be used qualitatively because of the attenuation of the signal through the pipe (Figs. F50, F53). The resistivity curves are good, except for some high-frequency noise starting from ~157 mbsf to the pipe entrance and increasing uphole. Preprocessing has been applied to correct the logs of the Magnetic Susceptibility Sonde (MSS) from temperature drift (see “Downhole measurements” in the “Site U1423” chapter [Tada et al., 2015d]). The corrected deep-reading and high-resolution logs do not show any clear evidence for correlations with the magnetic susceptibility measurements on cores (Fig. F50, Column 5). This may reflect the very low magnetic susceptibility signal in the sediment, which may be lower than the detection limits of the tool. The step in the high-resolution MSS log observed at ~180 m probably reflects the borehole diameter enlargement, as the high-resolution response is affected by borehole size and borehole wall rugosity. The velocity log correlates relatively well with the density log and has a steadily increasing downhole trend. The FMS resistivity images were of very good quality, except for some short intervals where the borehole diameter was enlarged (225, ~236, 316–340, and 474–478 mbsf), resulting in very conductive intervals (Fig. F53, Column 6).

Logging unit

The Hole U1427A logs do not show major steps in the base levels. The entire logged interval was thus assigned to one logging unit (LI, Fig. F50) correlating with lithologic Unit A (see “Lithostratigraphy”). Logging Unit LI is characterized by moderate-amplitude variability in NGR (and its U, Th, and K components) on a decameter to submeter scale (Fig. F53). The HNGS signal ranges on average from 40 to 80 gAPI and most likely tracks clay content. The K and Th curves are generally well correlated. Uranium behaves differently and often shows inverse correlation to K and Th. The U content varies from 1 to 3.5 ppm. The gamma ray signal correlates well with the bulk density log, which shows high-amplitude variations ranging from 1.60 to 1.9 g/cm³ (Fig. F50, Column 2) with a very slight increasing trend downhole. Resistivity logs have a negative trend downhole, reflecting compaction, and show pronounced oscillations of varied amplitude and frequency, which are well correlated with the density logs. Sonic velocity ranges from 1.5 to 2.0 km/s, with an increasing downhole trend reflecting downhole compaction.

Terrigenous clays have high K and Th contents and have relatively higher density and lower porosity than diatom-rich sediment. As a consequence, intervals with high gamma ray values, high density, and high resistivity generally reflect an increase in terrigenous clay content relative to diatom-rich intervals. Such intervals likely correspond to glacial stages characterized by lowered productivity (see “Lithostratigraphy”). Inversely, the intervals with gamma ray, density, and resistivity low values correlate in cores with diatom-rich intervals (e.g., 246–256 mbsf, diatom ooze, Cores 346-U1427B-31H through 33H, see “Lithostratigraphy”).

FMS images

In Hole U1427A, the FMS resistivity data quality was very good, allowing the borehole formation resistivity to be interpreted at several scales. The FMS images reveal numerous resistive and conductive intervals, with thicknesses ranging from several tens of centimeters to a few meters. At the scale of the borehole, the interval shallower than ~200 mbsf is characterized by medium to high conductivity (orange and dark colors in the FMS image in Fig. F53). Higher resistivity (light colors in the FMS image) are observed at deeper depths, from ~200 to ~340 mbsf. The interval deeper than ~340 mbsf is dominated by medium conductivity with intercalated higher resistivity intervals.

Conductive intervals (dark color in the FMS image in Figs. F53, F54) generally correlate with diatomaceous-rich intervals characterized by low gamma ray, low density, and low resistivity logs. Conversely, more resistive intervals (light colors in the FMS images) generally correlate with clay-rich intervals with higher values in the gamma ray, bulk density, and resistivity logs. This is particularly clear for three, several meters thick, very conductive intervals from 388 to 433 mbsf (Fig. F54) and consisting of diatomaceous ooze (see “Lithostratigraphy”).

In situ temperature and heat flow

APCT-3 downhole temperature measurements were performed in Hole U1427A at five depths, including the mudline. Additional APCT-3 measurements were performed in Hole U1427C at three depths, including the mudline. In situ temperatures range from 3.13°C at 27.0 m CSF-A to 9.10°C at 115.8 m CSF-A (Table T20). Deeper than 30.3 mbsf, temperatures increase linearly with depth. A linear fit of temperature versus depth gives a geothermal gradient of 69°C/km, which is much lower than was measured at previous Expedition 346 sites by tens of degrees. A heat flow of 71 mW/m² was obtained from the slope of the linear fit between in situ temperature and calculated in situ thermal resistance (Fig. F55) (Pribnow et al., 2000). These very low values, compared to other sites, reflect the location of Site U1427 above continental crust, rather than in the open, deep marginal basin.

The trend line of the in situ temperature measurements intersects the seafloor at 1.08°C (Fig. F55A). This is much lower than the average mudline temperature in the four APCT-3 measurements (7.31°C). In a stable area, a temperature-depth plot should converge with the average annual seafloor temperature. The bottom water temperature was difficult to determine accurately from the Hole U1427A APCT-3 temperature profiles, as the measured temperature trend did not stabilize when the APCT-3 was held at the mudline. In Hole U1427C, the measurement duration at the mudline was increased from 5 to 10 min, but the bottom water temperature measurement remained inaccurate. The seafloor temperature at Site U1427 was expected to be slightly warmer than at previous sites because of the shallower water depth and the presence of the warm Tsushima Current. However, the hole was drilled below the thermocline (which is at ~200 m water depth), and it is likely that the temperature estimate from the mudline measurements was overestimated. A measurement performed into the pipe at 30 mbsf in Hole U1427C before running Core 346-U1427C-5H yielded a temperature of 7.5°C, which is essentially the same value as when the mudline temperature was measured (7.31°C, also through the pipe). The measurement at 30 mbsf in Hole U1427C is ~4.4°C warmer than the in situ temperature measured in Hole U1427A at approximately the same depth. We suggest that because of the shallow water depth, the drill pipe may not have time to fully equilibrate with the ambient bottom water and borehole temperatures compared to the very warm air temperature, which lead to overestimated in-pipe measurements.

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