After coring had reached target depth at 570.2 mbsf, Hole 1095B was filled with viscous mud, reamed, and flushed of debris. We ran one full pass and a shorter repeat pass of the TC tool string (the hostile environment natural gamma-ray sonde [HNGS], accelerator porosity sonde [APS], HLDS, and dual induction tool [DIT]), two full passes of the GHMT string (natural gamma-ray tool [NGT], susceptibility magnetic sonde, and nuclear magnetic resonance sonde), and performed a velocity checkshot survey using the WST (see Fig. F39; also see "Downhole Measurements" in the "Explanatory Notes" chapter). Logging operations started at 1030 hr on 22 February and finished at 0030 hr on 24 February (Table T35).
During hole preparation, the sea state worsened to wave heights of 2-3 m combined with swells of 2.5-4 m (period = 10-14 s), which proved problematic for logging operations. All tool strings had to be lowered through the pipe slowly (~1000 m/hr instead of the usual 3000m/hr), especially in the upper part, to prevent cable slip when the ship heaved downward. The ship's heave was large enough to cause the wireline heave compensator to hit its limit switches (maximum extent = 6 m) within a few minutes of being switched on, so we decided to run the TC without heave compensation. At the start of the repeat TC run, the caliper arm of the HLDS broke off, and the DIT failed. Subsequently, the swell reduced enough for the GHMT and WST to be run with the heave compensator on.
Borehole caliper measurements showed that the hole was typically 16-18 in (40-45 cm) in diameter, with some zones of wider washout beyond the maximum caliper extent of 18.5 in (47 cm) (Fig. F40). 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 (e.g., 280-310 mbsf and 175-205 mbsf). Although some of the sections of excessive hole diameter are probably caused by drill bit rotation at the same depth for a length of time (e.g., in between taking cores), there is also a lithologic control. The deeper penetrating logs, such as medium resistivity and magnetic susceptibility, are much less affected by changing borehole diameter.
Because the hole fills with debris falling from the borehole wall over time, logging-tool runs vary in the depths that they reach. The TC, which was the first tool string to be run, reached the bottom of the hole, and the GHMT reached to within 13 m of bottom. The WST, the last logging run, reached 43 m from bottom.
The natural gamma logs from TC and GHMT runs were only partially repeatable. This is a result of the two different tools used to measure natural gamma: the HNGS on the TC corrects for borehole diameter and potassium in the borehole fluid, whereas the NGT on the GHMT does not. The HNGS logs are shown in Figure F41. However, the depth control on the GHMT runs was better than on the TC runs because of the heave compensation. The HNGS is the more sensitive of the two, hence its results are presented in Figures F40 and F41. The erroneous spikes in the HNGS logs were probably caused by measuring sediment previously activated by the neutron source in the porosity (APS) tool located below the HNGS during high heave.
The absolute values of density and density-derived porosity match well the index physical properties of the cores, apart from the anomalous log density lows at washouts (Fig. F40). The APS porosity measurements also match the index properties porosity values, except in the wide-diameter intervals.
The pattern in the magnetic susceptibility log matches well the MST susceptibility record (Fig. F42), but with a depth offset of between 4 and 6 m for the logged interval. The location of the seafloor is apparent in the natural gamma log of the second GHMT run, and it is unlikely that the wireline stretched 5 m between 110 and 0 mbsf. The origin of the depth mismatch probably lies in using the core seafloor depth obtained from the mudline in Hole 1095A and applying it to Hole 1095B, where core recovery started at only 83 mbsf. The correlation between Holes 1095A and 1095B over the 5 m of nominal overlap is unclear, and a 5-m offset is possible.
The sedimentary sequence could be divided into two units on the basis of changes in the character of the downhole logs (Figs. F40, F41).
The overall trend we expect in the logs is one of compaction: porosity decreasing downhole, and consequent increasing density, resistivity, and sonic velocity. However, in Unit 1, density and porosity have no overall downhole gradient. Only below 460 mbsf do the logs begin to show the more normal compaction trend. This could be a result of increasing downhole proportion of diatoms and radiolarians, whose skeletons maintain porosity. The resistivity actually decreases downhole, although the trend becomes similar to that of the porosity and density logs when increasing downhole temperature is considered (resistivity decreases with increasing temperature).
Within Unit 1, we can define two subunits on the basis of the susceptibility log. Both subunits show a downhole increase in the base level of susceptibility variations, with a boundary marked by a (downhole) step decrease at 325 mbsf. Toward the top of both subunits, the amplitude of susceptibility variation reaches a maximum. The lower subunit contains a pattern that repeats about three times. Each repetition is ~40 m thick and shows a steady uphole increase in resistivity, density, and natural gamma, topped by a sharp decrease; porosity and susceptibility behave in the opposite way.
The transition from Unit 1 to Unit 2 is marked by a step increase in density, resistivity, natural gamma, and susceptibility; porosity decreases to ~40%. Below the boundary, density and porosity maintain fairly steady values; in contrast, resistivity and susceptibility show a slight increase in variability.
The natural gamma and resistivity logs respond to variations in lithology. High natural gamma levels indicate higher clay, mica, and K-feldspar contents in the sediment. Susceptibility is governed principally by the concentration of magnetic minerals in the sediment, the most important of which is magnetite, which occurs mostly in the detrital fraction. From comparison of susceptibility with the core lithology (Fig. F43), the sediments that are richer in silt layers have higher susceptibility. In Figure F43, the natural gamma peaks seem to occur in about the same places as, but slightly higher up the log than, the susceptibility peaks, both marking intervals of increased terrigenous input.
The total magnetic field (MAGB) and magnetic susceptibility (RMGS) measurements from the GHMT tool string were analyzed in tandem to construct a logged magnetic polarity stratigraphy (see "Downhole Measurements" in the "Explanatory Notes" chapter). This GHMT polarity sequence matches well the polarity zones based on split-core inclination measurements (see "Paleomagnetism"), after the 5-m downward shift of the core depth scale is considered.
The GHMT polarity sequence enables the core polarities to be extended into the interval of very low core recovery below ~480 mbsf. The interval from 491 to 523 mbsf (486-518 mbsf in terms of core depths) is reversed polarity; from below that to the base of the logs (556 mbsf) is normal polarity with two or three shorter reversed polarity events. The short reversed polarity interval at 546-548 mbsf in Figure F44 probably corresponds to the reversed interval between C5n.1n and C5n.2n (9.88-9.92 Ma) (Cande and Kent, 1995). By extrapolation downward at the implied sedimentation rate (13.5 cm/ky), an age of 10.1 Ma is determined for the base of the cored hole (570.2 mbsf).
One-way sonic traveltimes from a Generator Injector (GI) gun (at the surface) to the WST (in the borehole) were recorded at 12 depth stations between 527 and 142 mbsf to give a depth-to-traveltime conversion and interval velocities. Although the station spacing was not close enough for the survey to be a full zero-offset vertical seismic profile, we could nevertheless identify strong reflectors below the logged interval. Details of the survey are given in "Seismic Stratigraphy".
The Lamont-Doherty temperature-logging tool recorded the temperature of the fluid in Hole 1095B during the first pass of the TC tool string (Fig. F45). These measurements underestimate the formation temperature, as the fluid did not have time to equilibrate to the formation temperature. A temperature of 19ºC was recorded at the bottom of the hole (570 mbsf); thus, the temperature gradient could be at least 33ºC/km. Both downhole and uphole curves show a constant offset of ~0.5ºC because the borehole continued to re-equilibrate during acquisition.