After coring had reached maximum depth at 607.7 mbsf, Hole 1096C was filled with viscous mud, and the pipe pulled to 97 mbsf. We ran the IPLT (natural gamma, porosity, density) and the GHMT (natural gamma, magnetic susceptibility, and total magnetic field) strings (see Fig. F42; also "Downhole Measurements" in the "Explanatory Notes" chapter). Logging operations started at 0030 hr on 4 March and finished at 0930 hr on 5 March (Table T29). The wireline heave compensator was used for all passes.
During the first run of the IPLT, we encountered a hole blockage at 353 mbsf that was not passable with the tool string. The hole was then logged up to the pipe, with a repeat section from 179 mbsf to seafloor. The hole conditions were unstable, and further constrictions were met at 170 and 240 mbsf. After a wiper trip, the end of the pipe was positioned below the blockage of the upper run and the IPLT was lowered downhole, but it was stopped 50 m above the bottom. For the two GHMT passes, the tool string could not pass below 514 mbsf because of continued hole fill.
While logging Hole 1096C, we contended with several tool malfunctions. Before the IPLT reached the borehole on the first attempted run, the run had to be aborted and the tool returned to the ship from the seafloor to have its telemetry unit replaced. The long-spaced gamma-ray detector on the hostile environment lithodensity sonde (HLDS) could not lock into its control signal for the upper IPLT run because the hole was excessively wide; thus, the density and photoelectric effect (PEF) logs were invalid for this pass. The caliper measurements saturated at 14.5 in (37 cm), whereas the maximum caliper extent measured on the surface was 18.5 in (47 cm); the Schlumberger engineer concluded that the caliper had been returning a systematically low reading and reprocessed the gamma-ray and porosity measurements accordingly. The dual induction tool remained unusable after its failure at Site 1095.
Hole 1096C was very wide: it exceeded the maximum 18.5 in (47 cm) of the caliper for ~80% of the logged interval. Thus, the density and porosity logs should be regarded with caution, although neither displays the wide spikes that are characteristic of bad contact of the tool with the borehole wall. The deeper penetrating logs, such as magnetic susceptibility, are much less affected by borehole diameter.
The open hole natural gamma logs contain apparently anomalously high values through the bridged zones at 170-180 and 234-246 mbsf, partly caused by the sediment being adjacent to the detector. Cores from these intervals show corresponding highs in the natural gamma measurements obtained using the MST, which implies some lithologic control. However, the natural gamma emissions measured through the pipe by the natural gamma-ray tool (NGT) during the GHMT run were not anomalous. Two different tools were used to measure natural gamma emission: the hostile environment natural gamma-ray sonde (HNGS) on the IPLT (more accurate) and the NGT on the GHMT. Both logs are shown in Figures F43 and F44 . The NGT logs are slightly noisier than the HNGS logs because of the downhole processing that the HNGS performs. The match between core and log natural gamma is patchy.
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 in intervals of excessive hole diameter (Fig. F43). The array porosity (APLC) porosity estimate is high compared to the index properties porosity values because of clay-bound water. The pattern in the susceptibility log matches closely the MST susceptibility record but is ~5 m deeper than the core depth scale.
Because of the interrupted logged depth intervals, we did not attempt to divide the formation into units on the basis of the logs alone.
Several logs directly represent lithologic variation (Table T30). Figure F45 illustrates that the logs can be rich in variability even in the moderately homogenous lithostratigraphic Unit III (see "Lithostratigraphy").
Computed gamma ray (the natural gamma resulting from K and Th), is typically used as an estimate of the sediment's clay content because K and Th are present in the clays. However, both K-feldspar and micas also contain K and both are present at Site 1096, so simple interpretation with respect to clays is not possible here. Uranium is precipitated under reducing conditions and can occur associated with organic matter. The uranium log (~3-4.5 Ma) has a strongly cyclic character, with a periodicity of ~40 k.y.
The ratio Th/K can be taken to indicate the abundance of mixed-layer clay compared with illite (higher Th/K indicates relatively more mixed-layer clay). The presence of K-feldspar decreases the Th/K ration.
Each element, and hence each mineral, has a characteristic PEF (absorption of low-energy gamma rays) (Table T30). The Hole 1096C PEF log values mostly lie between 3.2 (illite) and 2 barns/e- (montmorillonite). The presence of Mg chlorite (1.39 barns/e-) will lower the overall PEF, and Fe chlorite (12.36 barns/e-) will raise the PEF. The low PEF log indicates that Fe chlorite is not present in significant amounts in Hole 1096C; this observation is compatible with the average grain density of 2.76 g/cm3 (see "Physical Properties"), which is much less than that of Fe-chlorite (3.42 g/cm3) (Fig. F46).
The neutron-capture cross section (
f), like the PEF, is characteristic for each element. However, its use for lithologic purposes is limited by the very high f of chlorine, which is present in the seawater that fills the borehole. The average log value is shifted upward to 35 capture units (c.u.) by the dominance of chlorine.
Magnetic susceptibility is governed mainly by the concentration of magnetic minerals in the sediment, the most important of which is magnetite, which occurs mostly in the nonclay detrital fraction of the sediment. In example A of Figure F45, the small trough in the K log indicates a reduction in illite abundance at this level, which is also partly responsible for the low in PEF and high in the Th/K ratio. An associated U low is of uncertain origin, and no clear magnetic susceptibility anomaly is present at this level.
Natural gamma was logged through the pipe twice at Hole 1096C. The gamma rays are attenuated in proportion to the thickness of the pipe and bottom-hole assembly (BHA): the lowermost 64 m of the BHA is ~5.1 cm thick, and the pipe above is 1.75 cm thick, with thicker pipe joints spaced every 9.5 m. The through-pipe HSGR and SGR (the [standard] total gamma-ray measurements of the HNGS and NGT, respectively) logs were normalized to give the same amplitude as the open-hole HSGR at the same depth. The two modified logs have similar long- and short-wavelength features, which indicates that the normalization was successful (Fig. F44). The original HSGR and SGR logs are shown in Figure F43.
Total magnetic field and magnetic susceptibility (RMGS) measurements from the GHMT tool string were used to construct a downhole magnetic polarity stratigraphy (see "Downhole Measurements" in the "Explanatory Notes" chapter). This GHMT-derived polarity sequence matches quite well the polarities derived from half-core inclination measurements (see "Paleomagnetism"), after the 5-m downward shift of the core depth scale is considered (Fig. F47).
The Lamont-Doherty temperature-logging tool recorded the temperature of the fluid in Hole 1096C during the upper and lower passes of the IPLT tool string. The results of the lower pass are presented here. The uphole curve has a quite constant temperature gradient of ~24ºC/km (Fig. F48). The downhole and uphole curves show a constant offset of ~1.5ºC because the borehole continued to re-equilibrate during acquisition. These temperatures are not considered to represent in situ formation temperatures, although the temperature tool rested in the fill at the bottom of the hole before logging. Thus, some equilibration would have occurred. The temperature measured at bottom (16.7ºC) is therefore a lower bound on the true formation temperature (see also Fig. F41.
