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doi:10.2204/iodp.proc.346.110.2015

Downhole measurements

Logging operations

Downhole logging measurements at Site U1430 were conducted in Hole U1430B after completion of APC/XCB coring to a total depth of 275 m CSF-A. In preparation for logging, the hole was flushed with a 114 bbl sweep of high-viscosity mud, and the bit was pulled up to 80 mbsf. The wireline logging tool strings were modified to maximize data acquisition in the lowest part of the hole. Two short modified paleo combo and Formation MicroScanner (FMS)-sonic tool strings were deployed to the bottom of Hole U1430B (Fig. F48) (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).

On 19 September 2013 at 1830 h (UTC), the paleo combo tool string (comprising natural and spectral gamma radiation, density, 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 271.6 m WSF. The hole was logged up to ~191 m WSF at 500 m/h (Pass 1). The tool string returned for a main pass from the bottom of the hole to the seafloor.

The second tool string included resistivity at the top and natural and spectral gamma radiation at the bottom. Data are uncorrected from borehole diameter. The tool string started down the pipe at 2250 h. The downlog was run at a logging speed of 500 m/h to the bottom of the hole at 271.5 m WSF. The repeat uplog (Pass 1) was done at a logging speed of 500 m/h up to ~170.5 m WSF. The tool string returned for a main pass from the bottom of the hole to the seafloor.

The FMS-sonic was rigged at ~0200 h on 20 September. A downlog was taken at 600 m/h. The tool string reached the bottom of Hole U1430B at ~271.7 m WSF. Two uphole passes of the FMS-sonic were run, the first pass to ~75 m WSF and the main pass to the seafloor, both at 550 m/h. The first pass was conducted normally up to ~114 m WSF, at which point the surface system indicated a telemetry drop-out. There is a small interval on the first pass from 113.8 to 111.3 m WSF where the FMS readings are not valid. The main pass was conducted without any disruption, providing data over the entire interval of interest without any gaps. The FMS-sonic tool string was back to the surface at 0525 h. Rig down was finished by 0700 h, concluding logging operations at Site U1430.

The seafloor depth was indicated by the step in the gamma logs. The paleo combo uplogs (main passes) found the seafloor at 1081.30 and 1081.55 mbrf, and the main pass of the FMS-sonic found the seafloor at 1081.60 mbrf. Heave was very low (<0.2 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 U1430B, the borehole conditions were of medium quality with a diameter <12.5 inches from 100 to 152 mbsf and deeper than 250 mbsf (Fig. F49, Column 1). The borehole is slightly larger from the pipe entrance to 100 mbsf, although it did not exceed ~14 inches in diameter. The diameter is noticeably enlarged from ~168 to 257 mbsf, which impacted FMS image quality, especially from ~168 to ~190 mbsf. The borehole diameter is also asymmetrical, as evidenced from FMS calipers (Fig. F50, Column 1). Heave was negligible during downhole logging data acquisition, and despite the moderate borehole quality, 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. F49, Columns 2 and 3). As a result of caliper closure (see “Downhole measurements” in the “Site U1423” chapter [Tada et al., 2015d]), the uplog gamma rays from the first tool string and the FMS-sonic deviate from the core NGR data to lower values from ~85 and ~103 mbsf to the pipe entrance (Fig. F49, Column 2). For the same reason, the density log shows lower values than the core data over the same interval (shallower than ~85 mbsf). The second tool string was run centralized in the borehole, so the gamma ray uplog is not affected by this phenomenon, although in the absence of the caliper measurements the data are not corrected from borehole diameter. As the total spectral gamma ray (HSGR) tool was located at the bottom of the tool string during the second run, the recorded data cover a longer interval (Fig. F50, Column 2). For the three tool strings, 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. F49, F50).

The resistivity curves are good, except for some high-frequency noise starting from ~120 mbsf to the pipe entrance. Preprocessing has been applied to correct the logs of the Magnetic Susceptibility Sonde from temperature drift (see “Downhole measurements” in the “Site U1423” chapter [Tada et al., 2015d]). The corrected deep-reading, low-resolution log is generally inversely correlated with the density log (Fig. F49, Column 5). The drift-corrected, high-resolution log correlates relatively well with the magnetic susceptibility measurements on cores, except where affected by borehole size and borehole wall rugosity. Further processing remains necessary. The velocity log has a steadily increasing downhole trend and correlates well with the density log deeper than ~245 mbsf. The sonic quality may have been impacted by the enlarged borehole diameter shallower than ~105 mbsf and from ~200 to ~245 mbsf. The FMS resistivity images were of very good quality, except for some short intervals where the borehole diameter was enlarged with high-frequency changes in diameter (Fig. F50, Column 6).

Logging unit

The Hole U1430B logs change gradually downhole, with no major steps in base levels. The entire logged interval was divided in two logging units (LI and LII; Fig. F49). The upper part of the hole (from below the pipe to ~244 mbsf) is characterized by lower gamma ray, density, and resistivity than the bottom part of the hole. Hole U1430B is characterized by moderate-amplitude variability in NGR (and its U, Th, and K components) on a decameter to submeter scale (Fig. F50). The Hostile Environment Natural Gamma Ray Sonde (HNGS) signal ranges on average from 20 to 90 gAPI and most likely tracks clay content (Fig. F49). The U, Th, and K curves are generally well correlated (Fig. F50). The U content varies from 1 to 10 ppm on the data set uncorrected from borehole diameter (Fig. F50, Column 5). The gamma ray signal correlates well with the bulk density log, which shows high-amplitude variations ranging from 1.20 to 2.1 g/cm³ (Fig. F50, Column 2). Resistivity and sonic logs are relatively flat, except deeper than ~244 mbsf where they show pronounced oscillations of varied amplitude and frequency, which are well correlated with the density logs. Sonic velocity ranges from 1.5 to 2.3 km/s.

Logging Unit LI: base of drill pipe (~80 mbsf) to ~244 mbsf

Logging Unit LI is characterized by moderate-amplitude variability in NGR (and its U, Th, and K components) (Fig. F50). Terrigenous clay has high K and Th contents and has 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 (see “Lithostratigraphy”). Conversely, the intervals with gamma ray, density, and resistivity low values correlate in cores with diatom-rich intervals (see “Lithostratigraphy”). Logging Unit LI has been divided into two subunits on the basis of changes in character of the downhole logs (logging Subunits LIa and LIb; Figs. F49, F50).

Logging Subunit LIa: base of drill pipe (~80 mbsf) to ~200 mbsf

Logging Subunit LIa is characterized by NGR >30 gAPI and moderate- to high-amplitude swings in U, Th, and K. A slight downhole increasing trend is also observed, which likely reflects a downhole decrease in diatom content relative to terrigenous sediment (Fig. F50). Two low value intervals are observed at ~94 and ~98 mbsf, with the deepest approximating the depth of the lithologic Unit II/III boundary, placed at 98.10 m CSF-A in Hole U1430A (Section 346-U1430A-11H-7) (see “Lithostratigraphy”). The gamma ray signal correlates well with the bulk density log, which shows high-amplitude variations ranging from 1.20 to 1.50 g/cm³ (Figs. F49 [Column 1], F50). The peak value >1.60 g/cm³ (~142 mbsf) does not correlate with anything obvious in the cored sediment. The sonic and resistivity curve shows low-amplitude variations. Logging Subunit LIa extends over the lower part of lithologic Subunit IIB (diatom ooze with a few silty clay intervals) and part of Subunit IIIA (alternating layers of heavily bioturbated diatomaceous ooze and diatom-rich/bearing silty clay).

The transition from logging Subunit LIa to LIb is placed at ~200 mbsf. No noticeable change in lithology was observed at this depth (see “Lithostratigraphy”).

Logging Subunit LIb: ~200 to ~244 mbsf

Logging Subunit LIb is distinguished from logging Subunit LIa by a decrease in resistivity values. This subunit is also characterized by higher frequency oscillations in total and spectral natural gamma radiation compared to logging Subunit LIa (Fig. F50). A downhole decreasing trend in the total gamma ray counts (and in U, Th, and K) is also observed to ~230 mbsf, which is the depth at which the lowest values at the scale of the borehole are reached. A downhole increasing trend is observed deeper than this depth. The density curve follows the same trends with values ranging from 1.2 to 1.5 g/cm³ (Fig. F49, Column 3), with a minimum density observed at ~231 mbsf. This low density, low gamma ray interval is ~4 m thick and correlates with a thick laminated diatom ooze–rich interval in Core 346-U1430B-25H (see “Lithostratigraphy”).

Logging Subunit LIb extends over the lower part of lithologic Subunit IIIA. The base of logging Subunit LIb (~244 mbsf) fits the depth of the lithologic Subunit IIIA/IIIB boundary placed at ~244 m CSF-A in Section 346-U1430B-28H-1 (see “Lithostratigraphy”).

Logging Unit LII: ~244 to ~272 mbsf

Logging Unit LII starts with a sharp increase in density, resistivity, and sonic log values (Fig. F49). High-amplitude swings are observed in these logs at a scale varying from one to few meters in thickness, reflecting the interbedded gravel, sandstone, diatom ooze, and tephras observed within lithologic Subunit IIIB and Unit IV (see “Lithostratigraphy”). Logging Unit LII is also distinguished from logging Unit LI by higher values in total NGR and especially in components Th and K. The high K concentrations reflect the presence of glauconite in the sediment.

FMS images

In Hole U1430B, as observed at previous Expedition 346 sites, conductive intervals (dark color in the FMS image in Fig. F50) generally correlate with low gamma ray, low density, and low resistivity logs. Conversely, more resistive intervals (light color in the FMS image in figure) generally correlate with higher values in the gamma ray, bulk density, and resistivity logs. This relationship can be interpreted in terms of the relative abundance of clay/diatom in the sediment, with clay having higher K and Th contents and relatively greater density than diatom-rich sediment. With the exception of ash, gravel, and indurated layers, conductive intervals in the FMS images tend to reflect intervals enriched in diatoms, whereas resistive intervals reflect relatively high terrigenous clay content. The good FMS resistivity data quality allows the borehole formation resistivity to be interpreted at several scales.

At the scale of the borehole, the interval shallower than ~200 mbsf (logging Subunit LIa) is characterized by (relatively) medium conductivity (orange-colored upper interval in the FMS image in Fig. F50). Higher conductivity (dark color in the FMS image) is observed deeper, within logging Subunit LIb. This is interpreted as reflecting a possible increase in diatom content within the lower part of lithologic Subunit IIIA (see “Lithostratigraphy”) compared to the upper part of the subunit. Deeper than ~240 mbsf, the FMS images are characterized by high resistivity (light colors in the FMS image in figure), which reflects the increase in clay content in lithologic Subunit IIIB and Unit IV (see “Lithostratigraphy”) as well as the presence of cemented intervals evidenced by high density, velocity, and resistivity values (Fig. F49, Columns 3, 4, and 6).

At a finer scale, the FMS images also reveal numerous resistive and conductive intervals, with thicknesses ranging from several tens of centimeters to a few meters (Figs. F51, F52). Transitions between alternations are mainly gradual, although some sharp contacts are observed locally. Figure F51 illustrates an apparent cyclic nature of some resistive and conductive intervals on the FMS images from 125 to 135 mbsf. The cyclic pattern consists of ~0.4–1 m thick resistive intervals (light color in the FMS image in Fig. F51), generally correlating with higher values in the gamma ray, bulk density, and resistivity logs. We interpret this as reflecting relatively clay rich intervals. These resistive intervals alternate with conductive intervals (dark color in the FMS image), generally correlating to laminated (diatomite or carbonate rich) intervals characterized by lower values in the gamma ray, bulk density, and resistivity logs. These very preliminary observations, however, need to be further examined by postexpedition shore-based research.

The resistive–conductive cycles described above are only slightly visible across certain intervals characterized by irregular enlarged borehole diameter, especially from ~168 to 190 mbsf.

In the lower part of the borehole, deeper than 244 mbsf (logging Unit LII), FMS images are characterized by highly resistive layers (in white in the FMS images) alternating with lower resistivity layers (Fig. F50). Core recovery across logging Unit LII is low. A first attempt of core-log integration suggests that some of the high-resistivity layers correlate to tephra and tuff layers in Sections 346-U1430B-32X-1 and 32X-CC. These preliminary observations, however, need to be further examined by postexpedition shore-based research. It should be possible to assess the lithologic successions within the core gaps by combining the density, resistivity, and gamma ray logs with the FMS images.

In situ temperature and heat flow

APCT-3 downhole temperature measurements were performed in Hole U1430A at five depths including the mudline. In situ temperatures range from 3.36°C at 32.1 m CSF-A to 12.11°C at 117.6 m CSF-A (Table T17). The temperatures increase linearly with depth deeper than 32.1 m CSF-A. The trend line of the in situ temperature measurements intersects the seafloor at 0.1°C (Fig. F52A). This is slightly lower than the average of the lowest mudline temperature in the four APCT-3 measurements (0.58°C). A linear fit of temperature versus depth gives a geothermal gradient of 103°C/km. A heat flow of 93 mW/m² was obtained from the slope of the linear fit between in situ temperature and the calculated in situ thermal resistance (Fig. F52) (Pribnow et al., 2000).

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