After completion of drilling operations at Hole 1127B, the hole was prepared for logging (see "Operations"). As a result of high levels of H2S in the recovered core (see "Organic Geochemistry"), sepiolite mud was added to the hole to reduce the chance of gas exsolution into the borehole. A corrosion inhibitor (Marvel Mystery Oil) was applied to the wireline cable to prevent chemical reactions between H2S and steel, as this could result in stress cracking, reducing wireline cable ductility. Visual inspection of the wireline cable, tool casings, and tool joints during rig-down at the end of each tool run found no evidence to indicate that H2S had affected the cable or tools.
Three tool strings were run in Hole 1127B in the following order: (1) a modified triple combo including the Lamont-Doherty Earth Observatory high-resolution temperature/acceleration/pressure tool, (2) FMS/sonic, and (3) GHMT/general-purpose inclinometer tool (GPIT) (Table T16, also in ASCII format). The pipe was placed at 87 mbsf and raised by 10 m during the first and third logging runs to extend the open-hole logged interval. The triple combo was run without the radioactive source in the density tool (hostile environment lithodensity sonde [HLDS]) because of the increased risk of losing the tool string during logging as a result of high H2S concentrations. Thus, no density or photoelectric effect measurements were collected at this site with the HLDS, which essentially functioned as a caliper on this tool string. To reduce the exposure time of cable and tools to H2S, no repeat section was measured and the downhole pass of the triple combo acted as a quality control run. A single main pass was made uphole at 550 m/hr (twice the normal logging speed) with the modified triple combo from 510.7 mbsf to seafloor. Two passes of the FMS/sonic were made between 512.6 mbsf and the end of pipe. The FMS calipers were closed at 130 mbsf upon entering a washed out area below the pipe, whereas sonic velocity was logged into the pipe. A single pass with the GHMT/GPIT was run from 514.1 mbsf up into the pipe.
Conditions were excellent for logging at Hole 1127B. The wireline heave compensator was used during all logging runs and coped well with the moderate-to-light heave conditions. Borehole diameter as measured by the caliper log was remarkably uniform (~30-32 cm) from the base of the hole to 187 mbsf. Above 187 mbsf, the borehole diameter increased, with a maximum diameter of <40 cm.
Despite the fast wireline speed, the triple combo data appear to be of good quality. On the first FMS/sonic pass, the caliper arms failed to fully open, and contact with the borehole wall was poor. Consequently, the first pass is of poor quality, whereas the second pass produced high-quality FMS images. Excellent borehole conditions enabled the collection of a high-quality sonic log, reducing the need for the planned check-shot survey. The GHMT produced a good quality susceptibility log of the open-hole interval.
Downhole logging data at Site 1127 are essential to understanding sedimentation at this site, as the recovered core underwent extensive disturbance during exsolution of H2S and methane gases. Downhole trends in porosity, sonic velocity, and density at Site 1127 were dominated by compaction that results in gradual increases in velocity and density and decreases in porosity. Superimposed onto these trends are excursions that correlate to more indurated portions of the sedimentary sections. The most spectacular feature of the logs at Site 1127 is the cyclic nature of the NGR log. This cyclicity is likely to be a proxy of orbitally forced changes in sedimentation.
The entire section logged in Hole 1127B is characterized by a rather uniform appearance, both in geophysical logs and in the recovered core (Figs. F23, F24, F25; also see "Lithostratigraphy"). The logged section is therefore defined as one logging unit, divided into three subunits on the basis of variations in the character of the gamma-ray log (Fig. F24).
Subunit 1A was logged through pipe to 80 mbsf, and in open hole for the remaining interval. Variations of NGR (15-35 American Petroleum Institute [API] units) in Subunit 1A are dominated by changes in uranium concentrations (Fig. F24). Porosity readings free of pipe effects vary between 50% and 90%, with a distinct high-porosity interval between 175 and 188 mbsf (Fig. F25). This marked increase in porosity immediately above a lithified horizon at 187 mbsf is probably an artifact caused by increased borehole diameter (from 12 to 14 in) not compensated for by the porosity sonde. Sonic velocities increase linearly with depth from ~1.8 to 2.1 km/s, indicating a normal compaction profile with some peaks, possibly resulting from thin beds of more indurated sediment (Fig. F25). The combination of low gamma radiation, low porosity, and high velocity indicate that the boundary between logging Subunits 1A and 1B is a lithified horizon. This was not observed in the recovered core, possibly as a result of incomplete recovery at this depth (Figs. F24, F25; also see "Lithostratigraphy"). This conclusion is supported by FMS images that indicate the occurrence of a ~25-cm-thick lithified interval at the base of Subunit IA.
Subunit 1B is defined by a change to more high-frequency fluctuations in the cyclic character of the uranium gamma-ray log. The unit can be divided into 18-20 individual cycles, with cycle thickness decreasing downhole from 15-20 m in the top to ~10 m at the base of Subunit 1B (Figs. F24, F25). The top of logging Subunit 1B appears to correlate with Horizon A in seismic Sequence 2 (see "Seismic Stratigraphy"). Thus, Horizon A marks the initial change in cyclic character observed in logging Subunit 1B and may be useful for tracing the top of the cyclic package along the transect of Sites 1127, 1129, and 1131. The natural gamma cyclicity seen in Subunit 1B may result from changes in the organic matter content of the formation and/or variations in diagenetic concentration of uranium (e.g., associated with firmgrounds or "blackened grains"). Enrichments of diagenetic uranium are frequently associated with decreased porosity and increased density, sonic, and resistivity logs, and indicate the presence of hardgrounds that are also visible in FMS images as bright, resistive layers (Fig. F26). The FMS images from logging Subunit 1B show several horizons with highly conductive features 5-8 cm across that may be interpreted as either very large burrows or water-filled vugs in the borehole wall (Fig. F27). As a result of the large size of these features, our preferred interpretation is that they represent water-filled vugs caused by fluid (or gas) leaking into the borehole.
Subunit 1C is identified by a shift to lower gamma-ray values at the top of the subunit accompanied by a small decrease in resistivity and sonic velocity and an increase in MS. Subunit 1C displays relatively high sonic velocities (2.5 km/s), with porosity averaging ~40% (Fig. F25). The separation of shallow- and deep-resistivity logs in the lowermost part of Subunit 1C reflects fluid invasion as compared with Subunits 1A and 1B, where resistivity curves generally coincide (Fig. F25). The boundary between logging Subunits 1B and 1C at 470 mbsf coincides with a depositional hiatus at the Pleistocene/lower Pliocene boundary and corresponds to the boundary between lithostratigraphic Units II and III. Excessive borehole rugosity makes logging Subunit 1C appear chaotic on FMS images.
A change in downhole temperature gradient was seen at 370 mbsf, accompanied by high variability in the temperature profile below this level (Fig. F28). The reason for this offset is unclear. The increase in temperature readings between downhole and uphole passes (2°-3°C) is probably caused by heating of borehole fluids toward formation temperature.