Previous   |    Next

doi:10.2204/iodp.proc.346.106.2015

Downhole measurements

Logging operations

Downhole logging measurements at Site U1425 were conducted in Hole U1425B after completion of APC/XCB coring to a total depth of 407.3 m CSF-A. Several hard dolomite layers were encountered during coring, but the hole was reported to be in good condition. Seawater was circulated to clean the hole, and the bit was pulled up to 80 m CSF-A. Two tool strings were deployed in Hole U1425B: the paleo combo and FMS-sonic (Fig. F61) (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 31 August 2013 at 0355 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. There was some difficulty descending past ~350 mbsf, but after a few attempts the tool passed that depth and reached the bottom of the hole at 403.2 m WSF. The hole was logged up to ~302 m WSF (Pass 1) at 540 m/h. The tool string returned for a main pass from the bottom of the hole (~404.2 m WSF) to the seafloor.

The FMS-sonic was rigged at ~0915 h on 31 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 bottom of Hole U1425B at ~396.5 m WSF rather than the total depth of ~404 mbsf. The hole was either collapsing or filling in by the time the FMS-sonic run was conducted. Two uphole passes of the FMS-sonic were run, the first pass to ~123 m WSF and the main pass 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. The paleo combo downlog found the seafloor at 1916.3 mbrf and the uplog (main pass) found it at 1918.9 mbrf. The main pass of the FMS-sonic found the seafloor at 1916.7 mbrf, compared to the drillers mudline tagged at 1919.1 mbrf (Hole U1425B). Heave was relatively high (~0.6 and ~1.8 m peak-to-peak) and is likely the cause of the offset between the FMS-sonic and the paleo combo downlog and uplog seafloor depths. Despite the use of the wireline heave compensator for each uplog pass (see “Downhole measurements” in the “Methods” chapter [Tada et al., 2015b]), depth matching may be required to correct for possible heave-related depth discrepancies.

Logging data quality

Tool calibration was performed both before and after the logging runs to ensure quality control. In Hole U1425B, the borehole conditions were medium with a diameter ranging from 11 to 16 inches (Fig. F60, Column 1) with a decreasing trend downhole. The hole was asymmetrical as evidenced from the FMS caliper, and the bottom of the hole was unstable. Deeper than 370 mbsf, the borehole diameter was ~12 inches when the paleo combo tool string was run, but when the FMS-sonic was run, the caliper subsequently read some values <5 inches. The gamma ray log (Fig. F60, Column 2) generally anticorrelates with the caliper log. This is partly because, despite borehole diameter corrections, less gamma radiation reaches the detector in a wider borehole. Also, there is lithologic control, with less clayey sediment being more easily washed out. Despite moderately good borehole conditions and high heave during downhole logging data acquisition, log data quality is generally good.

Agreement between physical properties and logging data is excellent for the density and NGR logs along almost the entire borehole (Fig. F59, Columns 2 and 3). As a result of caliper closure (see “Downhole measurements” in the “Site U1423” chapter [Tada et al., 2015d]) from ~115 mbsf to the pipe entrance, the gamma ray uplogs deviate from the core NGR data to lower values and no longer overlap the downlogs (Fig. F59, Column 2). For the same reason, the density log shows much lower values than the core data over the same interval. Deeper than ~370 mbsf, gamma ray logs from the FMS-sonic tool are also affected by borehole size, as the FMS caliper was not fully opened because of the very tight diameter of the bottom-hole section. 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. F59). The resistivity curve worked well, except for a few noisy peaks, starting ~50 m below the pipe and increasing uphole. Once corrected for temperature drift (see “Downhole measurements” in the “Site U1423” chapter [Tada et al., 2015d]), the Magnetic Susceptibility Sonde (MSS) (Fig. F59, Column 5) should be an indicator of the magnetic signal variability in the formation. The temperature drift–corrected deep-reading log is generally inversely correlated with the density and resistivity logs. The corrected high-resolution log does not correlate with magnetic susceptibility measurements on cores. This is possibly due to the very low magnetic susceptibility signal in the sediment, which may be lower than the detection limits of the tool. Further processing remains necessary. The velocity log increases downhole and shows little variability except in the lowest part of the hole (Fig. F59, Column 6). The increase in velocity observed deeper than ~360 mbsf is likely due to bad borehole conditions. The FMS resistivity images were of good quality despite the changes in borehole diameter (Fig. F60). Deeper than ~370 mbsf, the data should be used with caution because of the apparent poor borehole conditions.

Logging units

The Hole U1425B logs change gradually downhole, with no major steps in base levels. The lowermost part of the hole (from ~338 mbsf to the bottom) is characterized by higher gamma ray, density, and resistivity values than the rest of the hole. Density has a positive trend downhole when resistivity is relatively flat, increasing in the lower part of the hole deeper than ~338 mbsf. The sonic velocity log shows an increasing downhole trend, with values ranging from ~1.5 to 1.8 km/s, reflecting compaction. The HNGS signal ranges on average from 20 to 90 gAPI with one maximum value reaching 100 gAPI. The signal shows moderately high amplitude variability on a several-meter to submeter scale and most likely tracks clay and organic matter content (see “Lithostratigraphy”). The K and Th curves are generally well correlated (Fig. F60). Uranium behaves differently than K and Th because it is not chemically combined with the main terrigenous minerals. Deeper than ~132 mbsf in Hole U1425B, however, U is also generally well correlated to U and K. Uranium generally accounts for 40%–50% of the total gamma radiation signal and locally for >60%. The highest values (>4 ppm) are observed from 255 to 290 mbsf and from 341 to 390 mbsf in lithologic Unit III (see “Lithostratigraphy”). Three logging units have been identified on the basis of changes in character of gamma ray, resistivity, and density logs (logging Units LI–LIII; Fig. F59).

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

Logging Unit LI has been divided into two subunits (LIa and LIb; Fig. F59).

Logging Subunit LIa: base of drill pipe (~80 mbsf) to ~132 mbsf

The upper logging subunit presents strong analogies with logging Subunit LIa identified in Hole U1423B. It is characterized by NGR > 40 gAPI and moderate- to high-amplitude swings in U, Th, and K. The gamma ray signal correlates well with the bulk density log, which shows high-amplitude variations ranging from 1.30 to 1.47 g/cm³ (Figs. F59 [Column 1], F60). The peak values >1.50 g/cm³ (~106 and ~117 mbsf) correlate with intervals of low core recovery in Hole U1425B and dolomite layers in Section 346-U1425B-13X-1 and Core 16X, respectively. These dolomite layers are also well expressed in the resistivity logs with values exceeding 1 Ωm (Fig. F59, Column 4). The sonic curve is smooth. The resistivity curves are characterized by high-frequency oscillations. The base of logging Subunit Lia at ~132 mbsf fits well with the transition between lithologic Subunits IIA (dominantly diatom-bearing and diatom-rich clay) and IIB (diatom ooze with few clay intervals) placed in Section 20H-1 at ~131.90 m CSF-A (see “Lithostratigraphy”). This depth fits with a marked step in the NGR, density, and resistivity curves, which likely reflects the significant downhole increase in diatom content relative to terrigenous sediment.

Logging Subunit LIb: ~132 to 244 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”). A downhole increasing trend is also observed, with moderate- to high-amplitude swings (Fig. F60). Lower densities than those in logging Subunit LIa are observed from ~132 to ~187 mbsf (<1.40 g/cm³), possibly reflecting an increased abundance of diatoms within lithologic Subunit IIB (diatom ooze, see “Lithostratigraphy”). Density slightly increases deeper than ~187 mbsf and shows higher amplitude oscillations. At the scale of logging Subunit LIb, the density log shows a positive downhole trend, reflecting compaction and/or a decrease in diatom content (Fig. F59, Column 3). Resistivity is also lower than within logging Subunit LIa, and the curve is relatively flat all along the logging subunit (Fig. F59, Column 4). Some distinct peaks in resistivity >0.6 Ωm seem to correlate in cores either with ash layers, dolomite intervals, or nodules (Section 346-U1425B-24H-7 and empty Core 31H). The sonic curve shows small-amplitude variations with few peaks at ~200 mbsf not correlating with clear lithologic features in the sediment. The prominent peaks in U, Th, and K observed at ~210 mbsf correspond to a prominent ash layer, which is >10 cm thick (interval 346-U1425D-25H-4, 44–74 cm). This ash layer is also clearly associated with a peak in density, magnetic susceptibility, and resistivity logs. It also correlates in the FMS images with an ~30 cm thick resistive horizon (light color, Fig. F60, Column 5).

The lowest part of logging Subunit LIb (from ~236 to 244 mbsf) is characterized by a slight decrease in gamma ray, density, and resistivity logs (Fig. F59). The base of logging Subunit LIb (~244 mbsf) approximates the depth of the lithologic Unit II/III boundary, placed at ~253.3 mbsf in Hole U1425B (Section 346-U1425B-35H-1) (see “Lithostratigraphy”).

Logging Subunit LII: ~244 to ~338 mbsf

Logging Unit LII starts with a small step in gamma ray, density, and resistivity logs (Fig. F59). Logging Unit LII is also distinguished from logging Unit LI by higher values in total NGR (between 40 and 85 gAPI) and its components U, Th, and K, possibly reflecting the increase in terrigenous content in lithologic Unit III compared to lithologic Unit II (see “Lithostratigraphy”). Regular high-amplitude cyclic swings are observed in both the gamma ray and density logs, with several orders of cycles varying from one to several meters in thickness (Fig. F60). The cyclic nature of the sediment record at intervals of ~8–10 m is especially well expressed (see also Fig. F62 and FMS images in Fig. F63). U, Th, and K concentrations co-vary, suggesting that clay content controls these logs (Fig. F60). The low gamma ray and low density intervals correlate in cores with either diatom-rich or carbonate-rich intervals in lithologic Unit III (alternating layers of bioturbated diatom ooze and diatom-rich clay, see “Lithostratigraphy” and Fig. F62). The resistivity logs display higher values (generally between 0.55 and 0.65 Ωm) than in logging Subunit LIb and show regular cyclic swings correlating with the density log (Fig. F59, Column 4). The prominent double peaks in density (2.4 and 3.2 g/cm³) and resistivity (>0.7 Ωm) observed at ~297 mbsf do not correlate with any lithologic feature in the cored sediment. The high values in the U log observed from ~255 to ~270 and ~278 to ~290 mbsf suggest organic-rich intervals. In logging Unit LII, the sonic logs show slightly higher amplitude variability compared to logging Unit LI.

The base of logging Unit LII (~338 mbsf) approximates the depth of the lithologic Subunit IIIA/IIIB boundary, placed at ~341.3 mbsf in Hole U1425B (Section 346-U1425B-53H-1) (see “Lithostratigraphy”).

Logging Unit LIII: ~338 mbsf to the bottom of the hole

Logging Unit LIII starts with a step in gamma ray, density, sonic, and resistivity logs (Fig. F59). The gamma ray and its component continue to show high-amplitude cyclic variability (Fig. F60). The values increase downhole to ~350 mbsf and decrease further at depth. The resistivity log has a positive downhole trend, with true resistivities ranging from 0.65 to 0.90 Ωm (Fig. F59, Column 4). Density increases from 1.4 to 1.6 g/cm³ over the upper 5 m (~338–343 mbsf) and remains relatively constant below with values exceeding 1.6 g/cm³. The three negative peaks observed from 350 to 356 mbsf are possibly related to borehole conditions presenting a diameter of 17 inches at these depths. The shift toward higher density and resistivity characterizing logging Unit LIII corresponds to the transition to lithologic Subunit IIIB (gray siliceous claystone) placed at 341.3 mbsf (see “Lithostratigraphy”) and the diagenetic boundary from biogenic opal-A to opal-CT, starting at ~342 mbsf (see “Lithostratigraphy,” Fig. F22, and “Geochemistry”). The increased gamma ray values suggest a relative increase in clay content, although XRD measurements show evidence of a decrease in clay minerals deeper than ~370 mbsf (Fig. F8). The high values in the U log observed from ~350 to ~387 mbsf probably characterize an interval enriched in organic matter.

FMS images

In Hole U1425B, the FMS resistivity data quality allows the borehole formation resistivity to be interpreted at several scales. Conductive intervals (dark color in the FMS image in Fig. F60) correlate with low gamma ray, low density, and low resistivity log values (Fig. F59, Columns 1–3). Conversely, more resistive intervals 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. As evidenced during Leg 128 at ODP Site 798 (deMenocal et al., 1992), clay has high K and Th contents and has relatively higher density and lower porosity than diatom-rich sediment. As a consequence, with the exception of ash and dolomite layers and intervals affected by major changes in density, conductive intervals in the FMS images tend to reflect intervals enriched in diatoms, whereas resistive intervals reflect relative high-terrigenous clay content. Such a relationship is also true at a smaller scale (see below).

At the scale of the borehole, the interval shallower than ~132 mbsf (logging Subunit LIa) is characterized by (relatively) medium conductivity (orange-colored upper interval in the FMS image in Fig. F60). Lower resistivity (dark color in the FMS image) is observed below, within logging Subunit LIb. This is interpreted as reflecting the significant downhole increase in diatom content within lithologic Subunit IIA compared to lithologic Subunit IIB (see “Lithostratigraphy”). Deeper than ~244 mbsf, within logging Units LII and LIII, the FMS images are characterized by higher resistivity, which reflects the increase in clay content in lithologic Unit III (see “Lithostratigraphy”). The change at ~244 mbsf appears ~10 m above the lithologic Unit II/III boundary placed at ~253 mbsf (see “Lithostratigraphy”). Deeper than 339 mbsf, logging Unit LIII is characterized by high resistivity (light color in the FMS image in Fig. F60), in agreement with the high density and resistivity observed within this unit (Fig. F59, Columns 3 and 4).

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. F62, F63). Figure F64 illustrates an apparent cyclic nature of some resistive and conductive intervals on the FMS images from 290 to 325 mbsf. The cyclic pattern consists of ~3–5 m thick resistive intervals (light color in the FMS image), correlating with higher values in the gamma ray, bulk density, and resistivity logs (Type A intervals) and interpreted as reflecting relatively clay-rich intervals. These resistive intervals alternate with conductive intervals (dark color in the FMS image), generally correlating in core with laminated (diatomite or carbonate rich) intervals characterized by lower values in the gamma ray, bulk density, and resistivity logs (Type C intervals). These conductive intervals often contain higher frequency, smaller scale interbedded layers. The transition between Type A and Type C intervals is often marked by an intermediate level (Type B intervals), characterized by medium conductivity in the FMS images and medium density, gamma ray, and resistivity in logs. This pattern appears to be symmetrical (Types A, B, C, B, and A) or asymmetrical (Types A, B, C, A, B, and C), possibly indicating cyclic, more or less progressive, changes in productivity conditions. These very preliminary observations, however, need to be further examined by postexpedition shore-based research. The resistive–conductive cycles described above do not to occur shallower than ~245 mbsf. The higher frequency resistive–conductive cycles (<1 m in thickness) are also essentially observed deeper than ~245 mbsf (Fig. F63).

At a finer spatial resolution, some highly resistive layers (in white in the FMS images) are observed and most likely reflect either dolomite or ash layers. As an example, the ~30 cm thick resistive horizon observed on the FMS images at ~210 mbsf (light color, Fig. F60, Column 5) corresponds to a prominent ash layer in interval 346-U1425D-25H-4, 44–74 cm. This ash layer is also clearly associated with a maximum in the density, magnetic susceptibility, and resistivity logs and with a prominent maximum in U, Th, and K. The two resistive layers observed at ~104.5 and 105 mbsf on the images correlate with a low core recovery interval and a dolomite layer in interval 346-U1425B-13X-1, 0–9 cm.

In situ temperature and heat flow

APCT-3 downhole temperature measurements were performed in Hole U1425B at four depths, including the mudline. In situ temperatures range from 4.25°C at 37.3 m CSF-A to 10.2°C at 94.3 m CSF-A (Table T19), with a linear downhole increase indicating that the gradient is uniform with depth (Fig. F64). A linear fit of temperature versus depth gives a geothermal gradient of 104°C/km. This value is higher than was measured at Site 799 during Leg 128 (98°C/km; Ingle, Suyehiro, von Breymann, et al., 1990) and generally higher than what was expected in the Yamato Rise area (Langseth and Tamaki, 1992). This discrepancy may be explained by the fact that the geothermal gradient at Site U1425 was calculated from only three in situ measurements in the sediment as the result of coring difficulties deeper than 110 mbsf (see “Operations”). The bottom water temperature at this site is estimated to be 0.41°C, based on average mudline temperature in the three APCT-3 measurements. A heat flow of 96 mW/m² was obtained from the slope of the linear fit between in situ temperature and calculated in situ thermal resistance (Fig. F64) (Pribnow et al., 2000). This value is higher than the one calculated for Site 799 (92 mW/m²).

Top of page   |    Previous   |    Next