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doi:10.2204/iodp.proc.346.104.2015

Downhole measurements

Logging operations

Downhole logging measurements in Hole U1423B were made after completion of APC coring to a total depth of 249.1 m CSF-A. In preparation for logging, the hole was circulated and the pipe was pulled up to 80 m CSF-A. Two tool strings were deployed in Hole U1423B: the paleo combo and FMS-sonic (Fig. F39) (see “Downhole measurements” in the “Methods” chapter [Tada et al., 2015b]; for tool acronyms see Table T12 in the “Methods” chapter [Tada et al., 2015b]).

On 22 August 2013 at 0915 h UTC, the paleo combo tool string (comprising resistivity, density, NGR, and magnetic susceptibility tools) descended from the rig floor into the pipe. A downlog was taken at ~600 m/h and reached the base of the hole at ~251 m WSF (see “Downhole measurements” in the “Methods” chapter [Tada et al., 2015b]). The hole was logged up (Pass 1) to ~121 m WSF at 540 m/h. The tool string returned for a second pass (main pass) from the bottom of the hole to the seafloor.

The FMS-sonic was rigged at ~1200 h on 22 August. A downlog was taken at 850 m/h, from which we established the best settings for the source frequency and the automated picking of P-wave velocity from the sonic waveforms. The tool string reached the base of the hole at 250.7 m WSF. Two uphole passes of the FMS-sonic were run, Pass 1 to ~120 m WSF and Pass 2 to the seafloor, both at 550 m/h. Rig down was completed at 1855 h.

The seafloor depth was given by the step in the gamma logs. There is typically some variability in choosing the exact point because the seafloor can appear as a gradual change. The paleo combo downlog found the seafloor at 1794.5 m WRF and the uplog (Pass 2) found it at 1793.5 m WRF. The second pass of the FMS-sonic found the seafloor at 1795.2 m WRF, compared to the drillers mudline tagged at 1796.8 mbrf (Hole U1423B). Tides were negligible and seas were calm (maximum peak-to-peak heave of 20 cm), giving little contribution to the offset between the FMS-sonic and the paleo combo downlog and uplog seafloor depths. The remaining difference possibly comes from wireline stretching. A reasonably good depth match of the open hole NGR logs between logging runs and with the core data was achieved using a seafloor of 1794.6 m WRF for paleo combo Pass 2 and 1794.2 m WRF for the second pass of the FMS-sonic.

Logging data quality

Tool calibration was performed both before and after the logging runs to ensure quality control. In Hole U1423B, borehole conditions were good with a baseline diameter (12–13 inches) close to the bit size (9.875 inches) (Fig. F37). Only a short section larger than 13 inches, although not exceeding 14 inches, was found between ~157 and 167 mbsf. As a consequence of good borehole conditions and negligible heave during downhole logging data acquisition, log data quality is generally very good.

Agreement between physical properties data and logging data is excellent for the density and NGR logs along almost the entire borehole (Fig. F37, Columns 2 and 3). From ~125 mbsf to the pipe entrance, the uplog gamma ray data, however, deviates from the core NGR data to lower values and no longer overlaps with the downlog (Fig. F37, Column 2). The gamma ray and density data are corrected from borehole diameter, although as the caliper was closed over this interval, the data quality is no longer ensured. We related this shift to be the result of an artificial correction of the gamma ray signal from the borehole size, with the caliper reading a smaller hole diameter than the actual size. For the same reason, the density log shows slightly lower values than the core data over the same interval. The natural and spectral gamma ray and density downlogs should thus be preferred for interpretations over this interval (from ~125 mbsf to the pipe entrance). Also, 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 (Fig. F37). The resistivity curve worked well, except for some high-frequency noise on the uplog starting ~40 m below the pipe and increasing uphole. The downlog should be preferred for interpretations.

The Magnetic Susceptibility Sonde tool diagnostics were normal during the runs. However, because of the very low magnetic susceptibility signal in the sediment, we were possibly operating at the lower limits of detection in the tool. The sensor electronics are sensitive to borehole temperature, and the acquired magnetic susceptibility data were thus strongly affected by a nonlinear long-period temperature-related drift superimposed on low signal variability. Preliminary processing was completed offshore to remove the temperature drift by calculating a least-squares polynomial fit to the data and subtracting the calculated trend from the data set. The residual components from both the high-resolution and deep readings are plotted in Figure F37 (Column 5) and should be an indication of the magnetic signal variability in the formation. The drift-corrected high-resolution log correlates relatively well with the magnetic susceptibility measurements on cores, especially shallower than 150 mbsf and deeper than 205 mbsf. The corrected deep-reading low-resolution log is generally inversely correlated with the density log. Further processing remains necessary. The velocity log shows a downhole increasing trend with higher values than the P-wave velocities measured on cores, especially deeper than 180 mbsf (Fig. F37, Column 6). The FMS resistivity images were of excellent quality because of good contact with the borehole wall (Fig. F38).

Logging units

The Hole U1423B logs change gradually downhole, with no major steps in base levels. The entire logged interval was thus assigned to one logging unit (LI; Fig. F37). At the scale of this unit, the upper part (from below pipe to ~123 mbsf) is characterized by higher gamma ray, density, and resistivity than the rest of the hole. The sonic velocity log increases downhole, reflecting low sediment compaction with depth. Resistivity has a negative downhole gradient rather than the normal increasing downhole compaction trend (see below). Density decreases in the upper part of the hole and remains relatively flat deeper in the hole. The above log trends and their correlation with in situ lithologies (see below) are generally in good agreement with the logging data acquired during Legs 127 and 128 at ODP Sites 794B, 795, 797, and 798, which show similar patterns.

The Hostile Environment Natural Gamma Ray Sonde signal ranges on average from 20 to 45 gAPI, with peak values reaching 70 gAPI. The signal shows moderately high amplitude variability on a several-meter to submeter scale, and given the sedimentological context (see “Lithostratigraphy”), it most likely tracks clay and organic matter content (silica and calcite contain no radioactive elements). The potassium and thorium curves are generally well correlated (Fig. F38). Uranium behaves differently from potassium and thorium because it is not chemically combined in the main terrigenous minerals. It does, however, show locally good correlations with thorium and potassium, particularly where prominent peaks are observed in the total gamma ray log. Uranium generally accounts for 25%–45% of the total gamma radiation signal, and locally for >50%. Uranium is usually associated with the organic matter–rich intervals in lithologic Unit I (above ~82.5 mbsf, see “Lithostratigraphy”), in which it shows the highest values (see below). The uranium content remains relatively high in lithologic Unit II (from ~82.5 mbsf to the bottom of the hole), which is less rich in organic matter.

Logging Unit LI has been divided into two subunits on the basis of changes in character of gamma ray and density logs.

Logging Subunit LIa: base of drill pipe (~80 mbsf) to ~124 mbsf

The upper logging subunit is characterized by moderate to high-amplitude swings in NGR (and its U, Th, and K components), with an overall decreasing downhole trend. The gamma ray signal correlates well with the bulk density log, which shows high-amplitude variations ranging from 1.25 to 1.55 g/cm3 (Fig. F37). The sonic curve is relatively flat, with values generally increasing downhole. Below ~103 mbsf, progressive decreases in U, Th, and K contents are observed, likely reflecting a decrease in organic matter and clay content. Slightly higher values in K content are, however, observed between ~107 and ~115 mbsf (Fig. F38). The depth of ~103 mbsf fits well with the transition between lithologic Unit I (primarily fine-grained material derived from terrigenous sources with color banding that is suggested to be related to variable content of organic matter and pyrite) and Unit II (dominantly composed of moderate to heavily bioturbated diatomaceous silty clay and clay and diatom ooze), placed in Section 346-U1423A-12H-1 at ~103.35 m CSF-A (see “Lithostratigraphy”) and reflecting a significant downhole increase in diatom content relative to terrigenous sediment.

The main changes in character of the downhole logs have been correlated with the base of logging Subunit LIa, which approximates the depth of the lithologic Subunit IIA/IIB boundary, placed at ~121 m CSF-A in Hole U1423A (Section 346-U1423A-14H-1) (see “Lithostratigraphy”). From a sedimentological point of view, however, no clear characteristic allowed straightforward division of lithologic Unit II into Subunits IIA and IIB. The relative decrease in NGR core and downhole log values was the basis for differentiating these two lithologic subunits (see also “Physical properties”).

Logging Subunit LIb: ~124–250 mbsf

Logging Subunit LIb is distinguished from logging Subunit LIa by lower values in total and spectral NGR, likely reflecting the abundance of nonradioactive elements within lithologic Subunit IIB (diatoms and other siliceous components, see “Lithostratigraphy”). Logging Subunit LIb is also characterized by moderate-amplitude swings in NGR and its U, Th, and K components. Two prominent peaks in at least two of these components are observed at ~142 and ~193 mbsf and correlate well with the NGR data measured on cores. The peak observed at ~193 mbsf does not correlate with anything obvious in the cored sediment. The peak at ~142 mbsf may correspond to a prominent ash layer, which is >10 cm thick (interval 346-U1423B-16H-6, 84–96 cm). This ash layer is also clearly associated with a peak in the density log. Another peak in density at ~238 mbsf also correlates with a thick ash layer in interval 346-U1423B-27H-6, 23–33 cm, and a prominent peak in the NGR data measured on the core. Logging Subunit LIb is also distinguished from logging Subunit LIa by lower values in bulk density, showing a relatively uniform trend, with values <1.45 g/cm3 associated with lower amplitude oscillations compared to Subunit LIa. Sonic velocity increases downhole. At the borehole scale, the resistivity curves show an opposite trend to the sonic curve, with mean values decreasing downhole from 0.9 to 0.4 Ωm, although this gradient becomes gentler in logging Subunit LIb. Similar downhole decreasing resistivity patterns have also been observed at Sites 794, 795, 797, and 798 (Ingle, Suyehiro, von Breymann, et al., 1990; Tamaki, Pisciotto, Allan, et al., 1990) and in the upper 300 m of the sediment column. This negative gradient is likely due to the increasing temperature with depth and the high porosities (see “Physical properties”), making the physical properties of the pore water dominate the resistivity log response. The prominent peak in magnetic susceptibility observed at ~206 mbsf on both core and log data correlate well with a dark ash layer found in interval 346-U1423B-24H-4, 129–131 cm.

FMS images

Because of the good borehole conditions in Hole U1423B, the FMS resistivity data quality allows the borehole formation resistivity to be interpreted at several scales. At the scale of the borehole, the interval above ~206 mbsf is characterized by relatively high conductivities (dark colored upper interval in the FMS image in Fig. F38) and high resistivities below (light color in the FMS image). This change does not correlate in core with any major change in lithology (see “Lithostratigraphy”). At a finer scale, the FMS images reveal numerous resistive and conductive intervals, with thicknesses ranging from several tens of centimeters to a few meters. As an example, the conductive intervals observed from 110 to 126 mbsf on the FMS images generally correlate with lower values in the gamma ray, bulk density, and resistivity logs (Fig. F40) and seem to correspond in cores, although not always, to brownish intervals. These intervals contrast with lighter, more grayish intervals that are less conductive on the FMS images and correlate with higher values in the NGR, bulk density, and resistivity logs. Transitions between alternations are mainly gradual, although some sharp contacts are observed locally. Inclined bedding (appearing as sinusoids) at slight angles is observed at some depths. It will be possible to map their occurrence and measure dip directions and angle. As shown in Figure F41, decimetric to centimetric resistive and conductive intervals are also clearly observed in the bottom part of the hole (234–243 mbsf) in an interval dominated by homogeneous lithology (diatomaceous silty clay and clay and diatomite ooze, see “Lithostratigraphy”). The conductive layers correlate in cores with intervals characterized by low gamma ray and high L* values, possibly reflecting an increase in biosiliceous fraction content. These initial correlations need to be further examined by shore-based research. FMS resistivity images also reveal stratigraphic information at a finer spatial resolution than the standard logs. In Figure F41, the highly resistive layers around 236.4 and 238 mbsf (with thicknesses ~0.1 m), correlate with two thick ash layers observed in intervals 346-U1423B-27H-5, 0–10 cm, and 27H-6, 23–33 cm. Other ash layers are also observed at other depths on the FMS images (e.g., Fig. F40).

In situ temperature and heat flow

APCT-3 downhole temperature measurements were performed in Hole U1423A at five depths, including the mudline. In situ temperatures range from 5.50°C at 35.8 m CSF-A to 17.45°C at 121.3 m CSF-A (Table T17), with a linear downhole increase indicating that the gradient is uniform with depth (Fig. F42). A linear fit of temperature versus depth gives a geothermal gradient of 140°C/km. This value is higher than was measured at Site U1422 (134°C/km). The bottom water temperature at this site is estimated to be 0.47°C, based on the average mudline temperature in the four APCT-3 measurements. A heat flow of 133 mW/m2 was obtained from the slope of the linear fit between in situ temperature and calculated in situ thermal resistance (Pribnow et al., 2000). This value is also higher than the one calculated at Site U1422 (120 mW/m2).