WIRELINE LOGGING

After reaching the target coring depth of 400 mbsf, Hole 1143A was filled with viscous mud, reamed, and flushed of debris. We ran one full pass and a shorter repeat pass with the triple combo tool suite, including the hostile environment natural gamma-ray sonde (HNGS), accelerator porosity sonde, hostile environment lithodensity sonde (HLDS), dual-induction tool (DIT; resistivity), and Lamont-Doherty Earth Observatory temperature/acceleration/pressure tool (LDEO TAP); and one pass with the FMS, long-spaced sonic (LSS), and natural gamma-ray spectrometry tool (NGT) tool string (Fig. F33; also see "Wireline Logging" in the "Explanatory Notes" chapter). (The raw data are given on the "Related Leg Data" contents list.) The GHMT string was not run because of deteriorating hole conditions. Since the computer driving the LDEO TAP tool crashed during the first pass, the temperature data could not be off-loaded at the end of the triple combo run. Logging operations started at 0015 hr on 6 March and finished at 2315 hr on 6 March (Table T15). Because the sea state was stable, the triple combo was run without the wireline heave compensator (WHC). The WHC was used only for the FMS because it acquires data every 2.5 mm and thus requires greater stability of the tool string.

Clay swelling was observed on the logs during the triple combo run (the hole was as small as 10 cm in places). Subsequently, a pipe trip was made before the next descent. The first run with the FMS-dipole sonic imager (DSI)-NGT tool string started off with a failure of the DSI, which had to be replaced by the LSS. Running this new tool string, we encountered an obstruction <20 m below the end of the pipe. After several attempts to pass it, we used the pumps to try to push it through, since the top of the tool was still in the sealbore. This gained us only about 3 m; upon coming up, a moderate overpull was necessary to free the tool. After coming out and laying aside the tools and wireline, the pipe was short tripped to a depth of 163 mbsf and then pulled back and set at 134 mbsf. After the tool string reached open hole, well over an hour was required for it to work through the clays. Finally, however, it reached a depth of 378 mbsf. The hole was then logged up to pipe with no repeat section.

Because the hole fills with debris falling from the borehole wall over time, logging-tool runs vary in the maximum depth that they reach. The triple combo, which was the first tool-string run, reached the bottom of the hole, whereas the FMS-LSS reached within 23 m of the bottom. The two triple combo runs showed excellent repeatability for all the log parameters.

The hole was generally in good shape below 200 mbsf. Borehole caliper measurements showed that the lower part of the hole was typically 25 cm in diameter, yielding good data. The upper part of the hole, however, showed zones of washout reaching 45 cm in diameter (Fig. F34), alternating with narrow ledges caused by swollen clays. The washed-out zones resulted in poor contact with the borehole wall and hence negative spikes in the density log and positive spikes in the porosity log. Although some of the sections with excessive hole diameter were probably caused by the drill bit rotating at the same depth for a length of time (e.g., between taking cores), there is also a lithologic control. The deeper penetrating logs, such as medium resistivity, are much less affected by changing borehole diameter: whereas the upper part of the resistivity values from the spherically focused log (SFL) is characterized by many spikes, the medium and deep resistivity values from the DIT are undisturbed (Fig. F34). Despite this, the three resistivity curves show excellent agreement throughout the entire logged interval.

The LSS log is of good quality from total depth (366 m) up to ~158 mbsf; the two P-wave velocity measurements are almost completely superimposed (Fig. F34). Above this depth, the recorded velocity seems too slow, probably indicating fluid velocity. The FMS measurements were also of good quality, with one apparent dead button on the right side of pad 3. The WHC was charged with nitrogen and run. Stick and slip on the log was on the order of .25 m, which is typical even when the WHC is functioning at its best. The effect on the data was corrected during the processing of the FMS images with the Geoframe software. Again, above 200 mbsf, image quality is poor because of rugosity of the hole.

In general, standard gamma ray (HSGR) and computed gamma ray (HCGR) from the HNGS tool in the triple combo run read 10% to 25% higher than spectroscopy gamma ray (SGR) or computed gamma ray (CGR) from the NGT tool in the FMS-LSS run, a difference easily accounted for by eccentering and hole-size correction. Contrary to the NGT, the HNGS corrects for borehole diameter and potassium in the borehole fluid. The HNGS is the more sensitive of the two; hence, its results are presented in Figures F34 (column 1) and F35.

We observed a general trend of increasing P-wave velocities below 250 mbsf in both the split-core samples and logs (Fig. F36; also see "Physical Properties"). Some velocity peaks of the log curve are not seen in the core data. These discrepancies might have resulted from the lower sampling rate on the split cores.

The absolute values of gamma-ray attenuation bulk density data from cores and the downhole log generally agree, apart from the anomalous log density lows at washouts in the upper part (Fig. F36). At ~190 mbsf, core data from MST measurements show an offset not visible in the log data and are somewhat lower (0.1 g/cm³) than the logging densities in the lower part of the hole. This suggests that the offset seen in core-derived data is a result of changing coring from APC to XCB.

Natural gamma-ray data from core and downhole logging show similar general trends on a first-order approximation (Fig. F37) but are offset in value. As for the density values, reduced core diameter and lower average density as a result of remolding during XCB coring caused lower gamma radiation values from the cores below 190 mbsf (see "Physical Properties").

Downhole neutron porosity and porosity calculated from the moisture content of core samples also show similar general trends apart from the intervals with large hole diameter (Fig. F37). A major turbidite (around 338 mbsf) is visible on both downhole and core data but with an offset of ~5 m (Figs. F34, F37, F38).

The sedimentary sequence could be divided into two intervals based on changes in the character of some downhole logs (Figs. F34, F35, F38).

Interval 1 (Base of Pipe at 85.5 mbsf to a Depth
of 200 mbsf)

This interval is characterized by bad hole conditions. Spikes resulting from the washouts can be seen on the logs, especially on HCGR, bulk density, porosity, and SFL resistivity logs (Fig. F34).

In interval 1, the photoelectric effect (PEF) log has an average value of 2.6 barn/e^-. This interval generally shows high gamma-ray and porosity values as well as low density values (Figs. F34, F38). These conditions are compatible with a nannofossil clay (see "Lithostratigraphy").

Within interval 1, we can define two different features based on P-wave velocity and PEF logs (Figs. F34, F38). The first extends to 190 mbsf, nearly where the dark ash layer in Core 184-1143A-20H affects the logs. This feature shows a slight downhole increase in the base level of density and resistivity variations, whereas gamma ray and porosity behave in the opposite way. The second feature is marked by a downhole step increase in P-wave velocity and in PEF values (Figs. F34, F38). This increase is related to the second increase of carbonate content measured on the core at ~190 mbsf (Figs. F34, F38; see "Organic Geochemistry").

Interval 2 (200 mbsf to Bottom of Hole at 400 mbsf)

The transition from interval 1 to interval 2 is marked by smaller increasing trends in density, smaller decreasing trends in natural gamma and porosity, and a jump in the PEF curve. In interval 2, PEF log values lie mostly between 2.5 and 3.4 barn/e^- (Fig. F38), closer to the carbonate PEF value. Generally lower gamma ray and porosity together with higher density and resistivity also indicate higher carbonate and less clay content. This interval contains a major anomaly from 220 to 300 mbsf, where a change in the increasing trend of resistivity can be seen; at 220 mbsf, we noticed a steady downhole decrease in resistivity to ~0.7 m. P-wave velocity keeps increasing steadily, whereas porosity keeps decreasing gently. Resistivity and gamma ray show a slight increase in variability (Fig. F34). Near 320 mbsf, we distinguished small cycles between low gamma ray-high resistivity layers and high gamma ray-low resistivity layers that are easily seen in the FMS images (Fig. F39). This alternation between dark and light layers suggests cycles with more or less clay, respectively. The FMS images also show the particular slump seen in Core 184-1143A-36X (Fig. F39).

All the major turbidite layers described in the cores are clearly distinguishable both in logging data and FMS images (Fig. F34). The base of the turbidites (sandy part) is characterized by lower gamma-ray, density, and resistivity and higher porosity and P-wave velocity values; the opposite is true for the top (clayey part). The major turbidite layer (Cores 184-1143A-37X and 38X) seen in the FMS images is presented in Figure F40. Although usually darker intervals in FMS images are conductive (clay layer) and lighter ones are resistive (sand layer), turbidites look darker at the base and lighter at the top. Here, the resistivity is much lower than the clayey part because the sandy base has higher grain size, reflecting less matrix and more fluid. This is why the color scale is reversed for a turbidite. By looking at both the standard logs and FMS images, we are able to count the number of turbidite layers present in the lower part of Hole 1143A.