Previous   |    Next

doi:10.2204/iodp.proc.318.107.2011

Downhole measurements

Logging operations

Downhole logging measurements in Hole U1359D were made after completion of RCB coring to a total depth of 602.2 mbsf (DSF). In preparation for logging, the hole was flushed with a 50 bbl sweep of sepiolite mud, and the bit was released at the bottom of the hole. The hole was then displaced with 191 bbl of barite-weighted mud (10.5 ppg), and the pipe was pulled up to 96.9 mbsf (DSF). No tight spots were encountered during the reaming. Three tool strings were deployed in Hole U1359D: the triple combo, FMS-sonic, and VSI (Fig. F65; see “Downhole logging” in the “Methods” chapter).

At 0045 h on 23 February 2010, the triple combo tool string (resistivity, density, porosity, and natural gamma ray tools) started down the pipe and logged down to the bottom of the hole at 606.5 mbsf (wireline depth below seafloor [WSF]). The hole was then logged up to seafloor. The tools provided continuous and good-quality log data. The hole was drilled with a 9⅞ inch diameter RCB bit and the borehole diameter ranged between 10 and 13 inches (25.4–33.0 cm) below 380 mbsf (WSF), increasing to 12–16 inches (30.5–40.6 cm) shallower than 380 mbsf, with a few thin washouts between 110 and 160 mbsf (WSF).

The seafloor depth of 3019.5 mbrf (wireline depth below rig floor) was determined from the step increase in gamma ray values at the sediment/water interface on both the downward and upward passes of the tool string. A driller’s seafloor depth of 3023 mbrf was established from the seafloor tag in Hole U1359D, and the top 150 m of the hole was washed down. But based on differences between the wireline and driller’s depths to the base of pipe and the base of hole and also on the offset in correlations between core and log data from Hole U1359D (e.g., Fig. F66), the actual seafloor depth is ~5 m shallower, at ~3018 mbrf (drilling depth below rig floor) at the location of Hole U1359D.

At 0800 h, the FMS-sonic tool string (FMS microresistivity imager, sonic, and natural gamma ray tools) was run into the pipe and recorded one downward pass and three upward passes in Hole U1359D. All passes reached the base of the hole at 606.5 mbsf. The opportunity was taken to run a third pass with the FMS-sonic tool string because a mis-wrap of the cable on the winch meant the cable had to be run back down and re-wrapped. The tools were back at the surface at 1855 h on 23 February.

The VSI check shot survey was delayed until the next day because of high winds, misty conditions, and the approach of nightfall. Marine mammal watch started at 0600 h on 24 February, and the VSI tool started down the pipe at 0730 h. The air gun was positioned on the port side of the R/V JOIDES Resolution at a water depth of ~7 mbsl and offset laterally ~30 m from the borehole. The soft-start of the Sercel G. Gun Parallel Cluster (composed of two 250 in³ air guns separated by 1 m) started at 0930 h. Whales entered the safety radius (940 m) once, so air gun firing was stopped until the whales left; we began the air gun soft-start again. It was planned to make check shot stations at 25 m intervals, but after the tool reached the bottom of the hole the caliper arm would not open to clamp the VSI’s geophone against the borehole wall. Waveforms were extremely noisy without clamping, and it was not possible to determine a check shot traveltime. However, with the tool resting on the infill at the bottom of the hole at 601.5 mbsf (WSF), noise was sufficiently reduced and it was possible to get four reliable waveforms. These waveforms were stacked to yield a one-way traveltime of 2.3867 s. The VSI tool string was brought back to the surface and disassembled, concluding logging operations at Site U1359 at 1400 h.

Logging units

Hole U1359D was divided into two units on the basis of the logs (Fig. F10).

Logging Unit 1: base of drill pipe to 260 mbsf

The upper logging unit is characterized by high-amplitude fluctuations in bulk density, NGR, and resistivity values on the scale of 1–5 m. It is also characterized by a lack of a compaction trend with depth; the mean bulk density value remains quite constant, whereas the resistivity values decrease with depth. The transition to the unit below is gradual.

Logging Unit 2: 260–606 mbsf

Logging Unit 2 is characterized by generally lower amplitude bulk density and resistivity variations than the unit above, but the several meter–scale alternations are still clearly defined. A normal compaction trend resumes in bulk density and sonic velocity. NGR continues to show quite high variability, and several large drops in NGR values are observed between 350 and 450 mbsf. Near the base of the hole at 574–580 mbsf, a 6 m interval of higher bulk density and resistivity occurs, indicating a cemented bed or series of cemented beds.

Identification of microfossil-rich intervals from natural gamma radiation logs

Spectral NGR logs were obtained using the Hostile Environment Natural Gamma Ray Sonde tool on all six passes of the triple combo and FMS-sonic tool strings. The triple combo main (upward) pass has the best statistics because it was run at the slowest speed (300 m/h rather than 600 m/h as used on the other passes). The NGR signal is strong (average = ~70 gAPI) and is dominated by the radioactivity of potassium and thorium, with uranium contributing a relatively minor component. Potassium and thorium concentrations co-vary closely (Fig. F66), suggesting they are present in the same fraction of the sediment. Both of these elements are found in clay minerals, and the sediments at Site U1359 are clay rich, so at first order the NGR signal is probably tracking clay content. Less abundant minerals like potassium feldspar also contribute to the NGR signal.

The NGR downhole logs contain an abundance of distinctive peaks and troughs that can be easily correlated to the NGR data measured on cores (Figs. F66, F67). In terms of lithology, intervals of low NGR values correspond to diatom-rich layers in the core, because diatoms are not radioactive and they dilute the NGR signal from K, Th, and U in the clays and terrigenous minerals that make up the balance of the sediment. The NGR logs promise to be a useful method for identifying diatom-rich and diatom-bearing zones in the core (where they are not always apparent to the eye) and complete the stratigraphy in unrecovered intervals.

The density log also helps to identify diatom-rich zones. Relatively low density values result from the intragranular porosity contained in the diatom shells and the low grain density of the opal that forms the diatom shells (2.1–2.2 g/cm³ compared to 2.6–2.75 g/cm³ for the other major sediment minerals). Shallower than 350 mbsf, the resistivity and sonic velocity logs also follow the pattern of the NGR and density logs because the higher porosity in the diatom-rich intervals leads to low resistivity and low velocity. However, deeper than 350 mbsf, the opposite relation holds: low NGR values often correspond to higher resistivity and sonic velocity (Fig. F10). One possible explanation is that the diatom- and nannofossil-rich intervals are more easily cemented than the clay-rich sediments that enclose them.

Formation MicroScanner resistivity images

FMS resistivity images reveal stratigraphic information including layered bedding, inclined bedding at the base of the hole, and dropstones and IRD larger than ~1 cm. The dropstones appear as resistive (light colored) spots in the image (Fig. F68A, F68B), and it will be possible to map their occurrence from 105 to 600 mbsf. The dipping beds between 575 to 579 mbsf appear as light-colored sinusoids in the image and are the most resistive sediment in the stratigraphy. The steepness of dip is highly variable (up to ~45°), and the beds dip to the east and west (Fig. F68C), indicating slumps.

Sonic velocity

The Dipole Sonic Imager tool recorded four passes of P- and S-wave velocity logs in Hole U1359D (Fig. F65). Below 250 mbsf, both P- and S-wave velocity logs repeat very well in shape and amplitude, giving confidence in the measured values (Fig. F69). However, shallower than 250 mbsf, P-wave velocities could not be determined from the initial slowness time coherence picks from the monopole source data by the Schlumberger data acquisition software. A substitute P-wave velocity log for 100–250 m was provided by the dipole source, usually used solely for S-wave velocities, which gave a noisy but generally reliable P-wave arrival (Fig. F69). The P-wave velocity logs will be critical to constructing a synthetic seismogram for this site.

Sonic check shots were successful at only one station, at the bottom of the hole at 601.5 mbsf, where a one-way traveltime of 2386.7 ms was found from the first arrival of four stacked waveforms. This puts the base of the hole at the bottom of the package of low-amplitude reflectors in the WEGA-26 seismic section. Given a seafloor two-way traveltime of 4060 ms (Escutia et al., 2008), an average P-wave velocity of ~1686 m/s can be calculated for Hole U1359D. When this average is converted to a linearly increasing velocity with depth, it matches the values from the core and log velocity measurements quite well (Fig. F69).

Heat flow

Heat flow at Site U1359 was determined according to the procedure of Pribnow et al. (2000). Four advanced piston corer temperature tool (APCT-3) measurements in Hole U1359A ranged from 2.05°C at 29.1 mbsf (DSF) to 7.17°C at 119.6 mbsf (DSF) (Table T17), giving a geothermal gradient of 62.1°C/km (Fig. F67). The seafloor temperature was –0.29°C based on the average of the four minimum temperature values in the APCT-3 temperature profiles (Fig. F70A). The thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndmann et al. (1974) (see “Physical properties” in the “Methods” chapter). The calculated in situ values are ~2% lower than the measured laboratory values. Thermal resistance was then calculated by cumulatively adding the inverse of the in situ thermal conductivity values over depth intervals downhole (Fig. F70B). Heat flow was obtained from the linear fit between temperature and thermal resistance (Fig. F70C). The heat flow estimate for Site U1359 is 62.4 mW/m², which is a typical value for the ocean floor.

Top of page   |    Previous   |    Next