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doi:10.2204/iodp.proc.339.105.2013

Downhole measurements

Logging operations

Downhole logging measurements in Hole U1387C were made after completion of RCB coring to a total depth of 870 m drillers depth below seafloor (DSF). In preparation for logging, a wiper trip was run (28 m of soft fill was found at the bottom of the hole), the hole was flushed with a 50 bbl sweep of high-viscosity mud, and the RCB bit was released. The hole was then displaced with 290 bbl of barite-weighted mud (10.5 ppg), and the pipe was pulled up to 103.8 m DSF. Three tool strings were deployed in Hole U1387C: the triple combo, VSI, and FMS-sonic tool strings (Fig. F40; see also “Downhole measurements” and Table T6 in the “Methods” chapter [Expedition 339 Scientists, 2013b] for tool definitions).

At 0230 h UTC on 17 December 2011, the triple combo tool string (resistivity, density, porosity, and natural gamma radiation tools) descended from the rig floor into the pipe (Fig. F41). A downlog was taken at ~900 m/h to ~649 mbsf, where it was blocked from further downhole progress by a bridge in the hole. During the wiper trip, a tight spot from 649 to 653 mbsf had already been noted and worked during hole preparation. The hole was logged up to seafloor at 275 m/h.

The VSI tool string was run next, because running it after the FMS-sonic tool string would not have left enough daylight hours to complete the VSP. Marine mammal watch started at 0930 h. The soft-start of the Sercel G. Gun parallel cluster (two 250 in³ air guns separated by 1 m) begun at 1100 h. The air gun cluster was positioned on the port side of the R/V JOIDES Resolution at a water depth of ~7 mbsl with a borehole offset of ~45 m. Before reaching the bottom of the hole, a short uplog was taken to ensure depth accuracy by comparing the shape of the VSI gamma ray curve to those of the other logging runs. The VSI tool string reached a blocked zone at 255 mbsf. The shallow penetration for this tool run indicated that the hole was closing quickly with time. Obtaining a good clamp was often difficult with the VSI caliper arm because of the large borehole diameter and soft formation. Consequently, the received sonic waveforms were noisy. Station depths were chosen where the borehole was smooth and in gauge based on the examination of the triple combo resistivity and caliper curves. Five of the seven stations yielded satisfactory first arrival times.

The FMS-sonic tool string was rigged up at ~1540 h on 17 December. A downlog was taken at 1100 m/h. Standard (high)-frequency Dipole Sonic Imager transmitter settings were used, with P-wave window settings of 130–190 µs/ft. The tool string was blocked from further downhole progress by a bridge at 334 mbsf. The first uphole pass of the FMS-sonic tool string (Pass 1) was run to the seafloor at 550 m/h. A second uphole pass was run from 278 mbsf to the base of the pipe. Rig down was completed at 2300 h.

The seafloor depth was given by the step in the gamma ray logs. The triple combo downlog found seafloor at 568.5 mbrf, and the uplog (main) pass found the seafloor at 568 mbsf. The FMS-sonic Pass 1 found seafloor at 569 mbrf, compared to the drillers mudline tagged at 569.8 mbrf (Hole U1387C). Tides account for only part of this difference; sea level was at +0.9 m for the triple combo main pass, +0.6 m for FMS Pass 1, and +0.3 m for the drillers mudline in Hole U1386A (see Fig. F48 in the “Site U1386” chapter [Expedition 339 Scientists, 2013d]). Seas were calm, with a maximum peak-to-peak heave of ~0.6 m, giving little contribution to the offset. Further difference comes from the wireline heave compensator being centered slightly differently for each seafloor depth determination.

Log data quality

Log data quality was reduced by the range in diameter of the borehole, which often exceeded the 18 inch limit of the Hostile Environment Litho-Density Sonde caliper arm (Fig. F42). Numerous thin, narrow sections with diameters less than the bit size (9.875 inches) were also found below 320 mbsf. The VSI and FMS-sonic tool strings were blocked by two of the narrow sections found during the triple combo run at 256 and 335 mbsf. The NGR log strongly anticorrelates with the caliper log. In large part, this is because less gamma radiation reaches the detector in a wider borehole and more reaches the detector when the borehole is closed in. Additionally, the anticorrelation is partly lithologically controlled; sediments lacking clays are more easily washed out. We also analyzed the NGR logs after correction for borehole diameter (by the Schlumberger engineer). This results in smaller amplitude for the peaks associated with narrow parts of the hole and higher values for the wider parts of the hole. However, even small discrepancies in depth between the caliper and NGR logs introduces new errors into the corrected NGR logs, so here we choose to present the original (uncorrected) NGR logs, noting that features associated with very wide or very narrow borehole diameters should be treated with caution because they are likely to have a lesser amplitude on correction.

Of all the logs, resistivity and sonic velocity are the most robust to variable hole diameter. Density and porosity are highly affected, in washouts giving density values close to water (Fig. F42) and porosity close to 100%. The photoelectric effect log shows anomalously high values, especially below ~580 mbsf, because barium in the logging mud swamped the signal. However, barite-weighted mud was a necessity to keep the borehole open. The FMS resistivity images are also dominated by poor contact with the borehole wall in the wide areas, although some intervals of good images are present.

Logging units

The Hole U1387C logs change gradually downhole, with no major steps in the base levels. The entire logged interval was thus assigned to one logging unit (Fig. F42). At the scale of this unit, the NGR signal generally ranges from 30 to 60 gAPI, with peak values reaching 85 gAPI. The signal shows moderately high amplitude variability on a several-meter to submeter scale, and given the sedimentological context (see “Lithostratigraphy”), is primarily tracking clay content (quartz and calcite contain no radioactive elements). The signal is dominated by the radioactivity of potassium and thorium, and both curves are generally closely correlated (Fig. F43). Uranium generally contributes a moderate component, except in intervals with concentrations >2.5 ppm, in which it can account for as much as 35% of the total NGR signal. Uranium generally behaves as an independent constituent compared to potassium and thorium because it is not chemically combined in the main rock-forming minerals. The sonic velocity log increases downhole (Fig. F42), reflecting sediment compaction with depth. It generally co-varies with the NGR log.

Logging Unit 1 is divided into four subunits on the basis of changes in mineralogy inferred from the NGR logs and changes in character of the resistivity and sonic logs.

Logging Subunit 1A: base of drill pipe to 211 mbsf

The upper logging subunit is characterized by medium-amplitude cyclic swings in NGR (including uranium, thorium, and potassium components) and sonic velocity values (Fig. F42). This cyclic pattern resembles the one observed in logging Subunit 1A at Site U1386 (see Fig. F49 in the “Site U1386” chapter [Expedition 339 Scientists, 2013d]). Several orders of cycles are observed, varying from few meters to several tens of meters in thickness (Fig. F44). The potassium and thorium concentrations co-vary closely, suggesting that clay content controls these logs. Although in this subunit there is a separation between the potassium and thorium curves similar to the character of logging Subunit 1B in Hole U1386C, the two intervals are of different ages (see “Biostratigraphy”) and therefore do not correlate. The relatively higher potassium levels indicate an increased level of potassium-bearing minerals (e.g., K-feldspar and mica) in addition to clay minerals (Fig. F43). The uranium concentrations are locally relatively well correlated to potassium (e.g., 110–140 and 165–210 mbsf) but vary independently elsewhere in the subunit.

As expected from downhole compaction, the sonic velocity log displays an increasing downhole trend, with slightly lower values from ~140 to 170 mbsf. The general trend of the resistivity curve correlates well with the sonic velocity, except from ~130 to 160 mbsf, where resistivity is artificially noisy. The base of logging Subunit 1A is close to the base of the physical properties Unit II fixed at 220 mbsf based on the good positive correlation between GRA density, NGR, magnetic susceptibility, and spectral reflectance components a* and L* (see “Physical properties”). The transition between logging Subunits 1A and 1B does not correlate with any major changes in the sediment record (see “Lithostratigraphy”).

Logging Subunit 1B: 211–453 mbsf

The upper limit of logging Subunit 1B is characterized by a step decrease in the sonic, resistivity, and NGR logs (Fig. F42). This subunit is also distinguished from the subunit above by its lower potassium and thorium contents (Fig. F43). Subunit 1B is also characterized by less-regular alternations in the NGR logs compared to the subunit above, with some intervals of low-amplitude variability (e.g., 211–234 and 338–362 mbsf) alternating with some intervals containing high amounts of potassium (>2%) forming thin pronounced peaks in the logs (e.g., ~255, 322, 395, and 424 mbsf). These intervals generally correlate with high resistivity values. The base of Subunit 1B corresponds to the lower limit of lithostratigraphic Unit I at ~454 mbsf in Hole U1387C (see “Lithostratigraphy”).

Logging Subunit 1C: 453–510 mbsf

Logging Subunit 1C is distinguished from the subunit above by the absence of high peaks of potassium and thorium (Fig. F43). Close to the upper limit of this subunit, a probable hiatus spanning from 1.8 to at least 3.19 Ma was detected between Cores 339-U1387C-18R and 19R (452.62–458.55 mbsf; see “Biostratigraphy”). On the logs, a double peak of high resistivity values is observed just below, at 461.1 and 462.6 m wireline depth below seafloor, and the highest uranium content of the entire hole (close to 3 ppm) is observed between the peaks (Figs. F42, F43). These high-resistivity horizons are caused by two well-consolidated dolostone beds (59 and 12 cm thick) of fine-grained dolomite that were identified near the top of lithologic Unit II at 457.3–458.0 and 462.7–462.8 mbsf (Cores 339-U1387C-19R and 20R; see “Lithostratigraphy”). The transition between logging Subunits 1C and 1D does not correlate with any major changes in lithology and is located close to the base of physical properties Unit IV, the lower boundary of which was placed at ~500 mbsf on the basis of the notable increase from decimeter- to meter-scale cycles, high fluctuations in L*, and very low and stable magnetic susceptibility values (see “Physical properties”).

Logging Subunit 1D: 510–650 mbsf

Logging Subunit 1D is distinguished from the subunit above by the presence of ~10 regularly spaced peaks in potassium and thorium alternating with intervals of lower content at ~12 m intervals (Fig. F44). The amplitude of the potassium and thorium log variability decreases below 590 mbsf, where good correlation with the uranium log is observed (Fig. F43). The large peaks appear at bridged sections of the hole, so their amplitude will be reduced (but not erased) when hole-size correction is taken into account (see “Log data quality”). At first view, the cyclic pattern observed on NGR logs in Subunit 1D seems to reflect lithologic cycles observed in this interval. Although some depth adjustments may be locally required, high NGR values appear to correlate relatively well with thick intervals of very dark greenish gray nannofossil mud (see “Lithostratigraphy”), probably because of high clay content in this lithology. For example, in Figure F44 the dark greenish layers observed at ~536.5, 539, and 542 mbsf possibly tie with logged gamma peaks centered at 535, 539, and 542 mbsf, whereas the dark layer at 526 mbsf could correlate with the peak at 525 mbsf. These distinctive peaks can also be correlated to the NGR data measured on cores.

Formation MicroScanner resistivity images

Despite the rugosity of the borehole wall associated with the presence of washout intervals, FMS resistivity images reveal locally some gradual and sharp transitions between alternations of resistive and conductive beds thicker than ~5 cm. Some of these alternations correlate well with the resistivity curves from the triple combination tool string (Fig. F45). Where silty sand and silty mud layers correlate with lower values in the NGR log (i.e, approximately 216–218, 221–223, and 232–234 mbsf in Fig. F45), the FMS resistivity images illustrate locally small-scale changes in the sediment electrical properties that may be related to subtle changes in clay content or sediment hardness. The FMS resistivity images, however, remain dominated by poor contact with the borehole wall in the wide areas. FMS caliper logs also show that new tight points developed since the triple combination tool string was run, indicating that the hole was closing quickly with time.

Vertical seismic profile and sonic velocity

One objective of the expedition is to establish the age and lithologic origin of the seismic reflections previously identified in seismic sections. Three ways of correlating between stratigraphy and seismic sections are by

1. VSP check shot traveltime,

2. Traveltime derived from the sonic velocity log, and

3. Synthetic seismogram.

In the VSP, five of the seven stations yielded check shots ranging from 0.8934 s two-way traveltime (TWT) at 131.0 mbsf to 1.0274 s TWT at the deepest station at 252.1 mbsf (Fig. F46; Table T20). Many of the seismic waveforms were noisy, and it was difficult to clamp the tool’s caliper arm firmly in the often wide borehole, but the waveform stacks from the five successful stations appear to be good. Sonic velocities increase downhole with a linear trend of ~0.1 km/s per 100 m between 104 and 318 mbsf. The similarity of the resistivity logs to the sonic velocity log (Fig. F42) offers the potential for a “pseudosonic” log to be constructed from the resistivity data to 650 mbsf and used as input for a synthetic seismogram.

Heat flow

One APCT-3 downhole temperature measurement was made at this site, on Core 339-U1387B-4H. The temperature was 14.65°C. The shallow change to XCB drilling at ~47 mbsf prevented more measurements from being obtained. Geothermal gradient and heat flow could not be determined from this one measurement, but it is similar to (slightly higher than) the temperature and an equivalent depth at Site U1386 (see “Downhole measurements” in the “Site U1386” chapter [Expedition 339 Scientists, 2013d]).

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