Downhole logging was conducted in Hole 1125B. The drill string was placed at 96 mbsf as the logging tools were lowered to the bottom of the hole. During logging, the drill string was raised to 81 mbsf. The drill string had to be maintained at 81 mbsf to keep the upper hole wall from collapsing.
Operations began at 0600 hr on 6 October 1998 and ended at 1300 hr on the same day. Because of time constraints, logging operations were limited to one full pass of the triple combination tool string (Fig. F20) (see "Downhole Measurements" in the "Explanatory Notes" chapter for details). There was ~1-2 m of heave throughout operations, and the wireline heave compensator was used during all measurements. The hole conditions were generally good, with a relatively uniform borehole diameter (~12 in), except near the top where the hole widened to >18 in. As a result, the data quality for most of the hole is excellent. The principal results are shown in Figure F21.
The caliper reading from the lithodensity tool on the triple combination indicates that there are zones of fluctuating hole width (e.g., 250-275 mbsf). A plot of the caliper log with the occurrence of tephra beds shows that there is no systematic correlation between zones of increased hole width and tephra layers (Fig. F22). However, at 250-275 mbsf, there is a peak-to-peak correlation between the gamma-ray and caliper data, with lower gamma-ray values (sand-rich lithologies) corresponding to larger hole widths (Fig. F22).
If the gamma-ray data are separated into their constituent parts (K, Th, and U) (Fig. F23A), a good correlation can be seen between Th and K, but U concentrations often fluctuate independently of the other two radioactive elements (e.g., 100-200 mbsf). This is partly because U is soluble under oxidizing conditions and is, therefore, often leached from the sediment, and partly because U is often present within a different component of the sediment: Th and K are likely to be contained within the clay mineral fraction, whereas the U has probably been adsorbed by organic matter. However, a plot of total natural gamma radioactivity (HSGR) against just Th and K radioactivity (HCGR) (Fig. F23B) shows that the signal is dominated by Th and K, indicating that fluctuations in clay content control gamma-ray variability. Because clay content appears to be controlling the natural gamma ray, compelling cyclical deposition of the clay emerges, with distinct low-frequency cycles (Fig. F23) (see also "Logging Units"). Nevertheless, certain anomalously high spikes in natural gamma ray may well indicate the position of tephra horizons, as discussed at the end of this section.
The good hole conditions allow for confident interpretation of the bulk density and neutron porosity logs. Inversion of the bulk density into a density-based porosity log, using a matrix density of 2.71 g/cm3 (calcite) and fluid density of 1.03 g/cm3 (seawater) (see "Triple Combination Tool String" in "Tool String Configurations" in "Downhole Measurements" in the "Explanatory Notes" chapter) gives the most reliable proxy for downhole porosity variability. Comparison of the density porosity with the neutron porosity shows a tight linear relationship (Fig. F24). This linear relationship shows several factors. First, the data is reliable because the two logs agree. Second, the absence of any significant deviation between the two indicates that the clay content of Hole 1125B is low enough not to adversely affect the density porosity/neutron porosity correlation. Since clays have bound water in their composition, regions of high clay content would cause the neutron porosity to give anomalously high values relative to the density porosity. This is not the case anywhere in Hole 1125B.
An apparent deviation from the 1:1 linear relationship at high-neutron porosities could be inferred in Figure F24, with the regression slope becoming gentler. This may be a result of variable instrument response in high-porosity sediments. The overall slight deviation from a pure 1:1 relationship at lower porosities may be caused by slight errors in the inversion parameters for computing density-porosity, as well as a small bias in the lithodensity tool between measured densities and the true values.
The failure of the MAXIS depth recorder software that occurred during the logging operations of Hole 1123B did not recur. Therefore, the temperature tool data is reliable. The temperature data from the Lamont Temperature Tool are shown in Figure F25. No zones of hydrothermal input are seen in the data and the increase of temperature with depth reflects a normal geothermal temperature increase downhole.
In common with the results
from Site 1123, on the northeast margin of the Chatham Rise, the log data from
Hole 1125B show relatively low-amplitude fluctuations (Fig. F21).
Natural gamma-ray values range between 22 and 79 API, and shallow-resistivity
values vary only between 0.73 and 2.21 
m.
In fact, shallow-resistivity values appear to be relatively constant (0.86 ±
0.05 m) in all but the
bottom 130 m of the hole (Fig. F21).
Nevertheless, distinct logging units can be assigned to this hole, mainly on the
basis of changes in the character and rhythmicity of the data.
Within this unit cyclical fluctuations in the natural gamma are particularly pronounced. Two main frequencies can be recognized: a low-frequency cyclicity, with a wavelength of ~100 m; and a high-frequency cyclicity with a wave length of ~10 m (Fig. F26). The neutron-porosity values record a slight compaction trend in the upper ~40 m of this unit. The base of log Unit 1 corresponds to the boundary between lithostratigraphic Subunits IB and IIA (Fig. F21).
Below 245 mbsf, the character of the gamma-ray curve changes (Fig. F21). Between 245-360 m, low-frequency cycles are less apparent, because of a decrease in amplitude, and they appear to have a shorter wavelength (~60 m). Below 360 m (to the base of the log), the low-frequency gamma-ray cycles increase in amplitude again, but show a further reduction in wavelength to ~40 m. However, caution must be employed in comparing the nature of the gamma-ray curves above and below 360 m: it is at this depth that sedimentation rates decrease dramatically (see "Age Models and Sedimentation Rates"). Within log Unit 2, resistivity, photoelectric effect, density, and neutron porosity values all remain relatively constant (Fig. F21).
Resistivity values begin
to rise within log Unit 3, from ~0.85 m
at the top to ~1.58 m at
the base. The increase in resistivity reflects an increase in compaction and
lithification of the sediments. This compaction trend is also recorded in an
increase in the neutron porosity and a decrease in the density, from the top to
the bottom of log Unit 3.
The contact between log Units 3 and 4 is characterized by a sharp increase in resistivity and density, and a sharp decrease in neutron porosity. These log responses are again thought to be a response to increased compaction and induration. A concomitant increase in the photoelectric effect value at the top of log Unit 4 may reflect increased calcite cementation.
As shown above, the low- and high-frequency cycles in the natural gamma results are indicative of a rhythmically fluctuating input of terrigenous clays at Hole 1125B. Post-cruise analysis of the natural gamma results from Hole 1125B will show if this cyclicity can be related to astronomical (Milankovitch) forcing. Cyclic sedimentation was also recorded in the reflectance data from the core (see "Lithostratigraphy") and was attributed to changes in the burial flux of the terrigenous and calcareous biogenic components of the sediment.
It is perhaps surprising that the numerous tephra layers sampled at this site, and at Sites 1123 and 1124, do not have a more distinctive signature in the logs. Rhyolitic volcanism from the North Island, New Zealand, could be expected to produce tephras that are relatively rich in radioactive Th, K, and U (Nelson et al., 1986), which should be evident in the natural gamma results. The relatively poor correlation between tephra horizons seen in the core, and spikes in the gamma log (Fig. F22) may be, in part, a result of the thinness of the tephra horizons (typically <0.1 m) and the relatively low resolution of the HNGS (~0.45 m). However, closer inspection of all the log data from Hole 1125B seems to indicate that some of the tephra horizons can be identified, especially considering there is likely to be a depth discrepancy between the core and log depths. Figure F27 shows selected logging results from 200-250 mbsf and 350-400 mbsf. Tephra horizons at ~222, 365, 372, and 374 mbsf can be identified in the logs by increases in the natural gamma, particularly the potassium radioactivity, and decreases in the photoelectric effect and the density. A tephra horizon seen in the core at ~ 386 mbsf is not recorded in the logs. There is, however, strong evidence from the logs that a tephra horizon exists at ~245 mbsf, even though no tephra was recovered in the core at this depth. Although core recovery is recorded as 100% at this point, it is still possible that a tephra horizon could have been preferentially washed out, especially considering the "tephra horizon" seen in the logs at ~245 mbsf falls in between Cores 181-1125B-26X and 27X, at the boundary of a change in lithology and in an area where core recovery was biscuity (see "Lithostratigraphy").
