m,
and magnetic susceptibility shows less than a twofold increase downhole (Fig. F44A,
F44B). Magnetic
susceptibility values appear to show very little long-term variation, except for
a sharp increase at the base of the log (Fig. F44B).Downhole logging was performed in Hole 1123B. The drill string was placed at 100 mbsf as the logging tools were lowered to the bottom of the hole. Before logging, the drill string was raised to 86 mbsf. The drill string had to be maintained at 86 mbsf to keep the upper hole wall from collapsing.
Three tool string configurations were run in Hole 1123B, in the following order: the triple combination, the FMS-sonic, and the GHMT (see "Downhole Measurements" in the "Explanatory Notes" chapter). A repeat interval was measured with the triple combination, and two full passes were made with the FMS-sonic. Logging operations are summarized in Figure F43. Logging operations began at 1100 hr on 18 September 1998 and finished at 0700 hr on 19 September 1998. There was between 3 and 4 m of heave, and the wireline heave compensator was used during all measurements. With the exception of the uppermost 65 m, the hole conditions were good, with a relatively uniform borehole diameter (~13.5 in) throughout (Fig. F44A). The NMRS sonde on the GHMT again failed to work.
The quality of the data is excellent, with very few erroneous readings caused by hole washouts. The only section of the hole to yield relatively poor-quality data was the uppermost 65 m (86-150 mbsf), where the borehole was regularly greater than 15 in and occasional more than 18 in diameter. Because the FMS pads only extend to 15 in, readings cannot be made where the borehole exceeds this value. Where the borehole is greater than 18 in, the data from the HLDS and the APS are unreliable. The upper section of the sonic log contained around 5 cycle skips, which have been edited out (Fig. F44B).
A comparison of log-based lithodensity with index bulk density determined from discrete samples shows general agreement, although log-based lithodensity values are higher below 150 mbsf (Fig. F45). At 150 mbsf, XCB coring began, indicating that the change in coring techniques could be partially responsible for the discrepancy between log and core-based results.
The downhole bulk density log was converted into a density-porosity log, using a matrix density of 2.71 g/cm3 and a fluid density of 1.03 g/cm3 for seawater (Fig. F46). In the above calculation, the matrix density value was determined by plotting a histogram of all the index-based matrix densities. This plot exhibited a single dominant peak at 2.71 g/cm3.
Overall, neutron porosity correlates well with the other porosity proxies (density and sonic velocity) (Fig. F44A). There are, however, a few regions of disagreement (see "Preliminary Interpretation"). Most notably, between 245 and 267 mbsf, neutron porosity shows a dramatic increase, but bulk density only shows a narrow zone of decrease, and there is no appreciable change in sonic velocity (Figs. F46, F44B). The FMS shows this section of the hole to be a conductive region, but the FMS shows increased conduction starting at 233.5 mbsf. The caliper is steady throughout and the resistivity data fail to show any change in this region (Fig. F44A).
The top of the log also shows deviation between the neutron porosity log and the lithodensity/sonic logs (Fig. F44A, F44B). Some of the lack of agreement could be caused by fluctuating caliper values and, in some places, by a caliper too wide for the neutron porosity and lithodensity tools to record usable values, but much of the record appears to be reliable.
Between 150 and 460 mbsf, XCB coring produced ~50%-60% recovery in some cores, which were frequently "biscuity." Downhole magnetic susceptibility was correlated with core-based magnetic susceptibility in an effort to place the drilling biscuits at their proper depths within the cored interval. The results are consistent with the biscuits representing an even sampling of the entire 9.5-m cored interval, compressed into the lower part of the core liner (Fig. F47) (see also "Paleomagnetism").
Sonic data show steadily increasing velocity downhole. Velocity at the top of the hole is 1.6 km/s and around 2.2 km/s in the lithified zone at the base of the hole (see "Lithostratigraphy"). The sonic velocity data were converted into a downhole integrated traveltime, for the purpose of assessing the depth of reflectors evident on the site survey seismic profile.
The temperature tool was run at the base of the triple combination. Because of a failure of the MAXIS depth recording software, conversion of the data from temperature vs. time to temperature vs. depth could not be made.
Excellent data quality
from Hole 1123B has enabled the results to be subdivided confidently into
distinct logging units (Fig. F44A,
F44B). The majority of data
acquired have a rhythmically varying response to the sediments in the borehole
wall, though the overall range of fluctuations in the data is slight. For
example, resistivity values only vary between ~0.6 and 2.1 m,
and magnetic susceptibility shows less than a twofold increase downhole (Fig. F44A,
F44B). Magnetic
susceptibility values appear to show very little long-term variation, except for
a sharp increase at the base of the log (Fig. F44B).
The base of log Subunit 2B correlates with the bottom of lithostratigraphic Unit I, and the base of log Unit 3 correlates with the bottom of lithostratigraphic Unit II (Fig. F44A, F44B). The log units identified in Hole 1123B are outlined below (Fig. F44A, F44B).
This unit is characterized
by relatively constant, low resistivity values (mean = 0.757 ± 0.035 m),
high neutron porosities (0.79 ± 0.078), and slow sonic travel times (179.5 ±
8.75 µs/ft).
At 145 mbsf the logs show a distinct change in character: resistivity, density, and photoelectric effect all increase sharply at this point, and porosities and sonic traveltimes decrease. Throughout Subunit 2A, resistivity shows abrupt and periodic increases that can be correlated with increases in bulk density and photoelectric effect and with decreases in porosity.
In this subunit, the resistivity values remain at a relatively constant level, in line with the minimum values recorded in Subunit 2A. Sonic velocities fluctuate, and the photoelectric effect decreases toward the base of this subunit. Neutron porosity values show a sharp increase within this subunit, in conjunction with a slight decrease in density.
The top of Unit 3 is characterized by a slight rise in resistivities and photoelectric effect and a decrease in sonic traveltimes. With the exception of natural gamma and magnetic susceptibility, all of the log data within this unit show small-scale fluctuations. An overall compaction trend with depth is reflected in a gradual increase in density and a decrease in porosity and sonic velocity.
Resistivity values within this subunit increase steadily with depth. This trend is accompanied by a more acute increase in density and decrease in porosity. The top of this unit is also marked by a noticeable decrease in sonic traveltime.
Magnetic susceptibility and resistivities show a sharp rise and reach their maximum values at the top of this subunit. Toward the base of this subunit, the resistivity and magnetic susceptibility values begin to decrease.
The spectral gamma-ray results show that downhole variations in thorium and potassium correlate (Fig. F48). Uranium values, however, appear to fluctuate independently of both thorium and potassium. Changes in the thorium and potassium values may be indicative of variations in the terrestrial clay content, whereas fluctuations in the uranium may be controlled by variations in organic material and/or redox potentials at the time of deposition.
The increase in neutron porosity vs. density porosity at 245-267 mbsf (Fig. F46) could be a result of bound water in clays or micas. The neutron porosity log is based on hydrogen in the formation. In clean formations, all the hydrogen is assumed to be in water and so the neutron log is a good indicator of porosity. Clays and micas, however, contain bound water in their molecular structure and the neutron log is unable to distinguish between different carriers of hydrogen. For this reason the discrepancies seen here could reflect an increase in clay or mica content. However, an increase in clay content would also be reflected in an increase in the natural gamma-ray; this is not observed at 245-267 mbsf (Fig. F44A). The reason for the sharp increase in neutron porosity within log Subunit 2B requires further postcruise investigation.
Particularly interesting results were obtained from Subunit 2A. Regular, correlative fluctuations in resistivity, density, and photoelectric effect can be seen throughout this unit (Fig. F44A, F44B). These responses in the log data, which are most clearly seen in the resistivity results, may reflect cycles of sedimentation. Looking upsection, resistivity values increase sharply, then gradually decrease back to their original value (Fig. F49).
