After completion of drilling operations at Hole 1126D, the borehole was prepared for logging (see "Operations"). The lower limit of the BHA was placed below a chert horizon at 117 mbsf in an effort to avoid chert ledges against which the cable could get trapped and severed by pipe heave. All logging runs started at the base of the hole, which became progressively shallower during logging operations because of fill accumulation. The WHC was used throughout logging but had difficulty coping with the complex cross-heave sea state. Three logging strings were deployed in the following order: (1) triple combo, including the Lamont-Doherty Earth Observatory high-resolution temperature/acceleration/pressure tool (LDEO-TAP); (2) sonic/GHMT combination; and (3) the WST (Table T18; also see "Downhole Measurements" in the "Explanatory Notes" chapter).
The main pass with the triple combo and LDEO-TAP was run from 443 mbsf to the mudline. Before the main run a quality control pass was made from 443 to 380 mbsf with the accelerator porosity sonde (APS) switched off to prevent neutron charging of the formation. The WHC did not function in the intervals between 320 and 303 mbsf and between 197 mbsf and the seafloor after reaching the maximum limit of heave correction. The caliper on the hostile environment lithodensity sonde was closed at 150 mbsf and logging speed briefly increased between 175 and 165 mbsf to protect the tools from violent downheave.
The FMS tool was not included on the second tool string because of the risk of damaging the FMS caliper arms in the complex heave conditions. Because only the susceptibility magnetic sonde in the GHMT was functioning, the GHMT could be combined with the sonic tool on the second tool string, dispensing with the need for a dedicated GHMT run. The general-purpose inclinometer tool (GPIT) was also included on the second string to provide data on the effects of heave on tool motion. The sonic/GHMT was run from 430 mbsf to the seafloor. The WST recorded nine check-shot stations (stacks of sever or more shots) spaced ~30 m apart, adjacent to significant boundaries as indicated by the acquired logs and seismic stratigraphy (see "Seismic Stratigraphy").
Although caliper measurements identified some borehole rugosity caused by washouts and caving at Hole 1126D, variations in borehole diameter fell within the range necessary for accurate measurement using eccentralized tools (e.g., porosity and density). The largest borehole diameters, approaching 45 cm (18 in), occurred between 140 and 150 mbsf and below 404 mbsf (Fig. F26). During logging, changes in cable tension and tool speed resulting from swell-induced heave degraded data collected in some intervals by causing repeated measurements. During the main triple combo run, neutron loading of the formation by the APS tool caused the gamma-ray log to record an artificial peak during downheave. These spurious measurements were later removed by processing. In general, triple combo data appear to be of reasonable quality, even in the interval in which the WHC was not operating. Reduced data quality for porosity and density occurs above 139 mbsf, where the caliper arm on the triple combo was closed approaching the pipe (Fig. F26).
The sonic/GHMT pass produced quality data, even in the interval above 165 mbsf in which the WHC was not operating. Orientation measurements made by the GPIT were corrupted by the close proximity of other metallic tools, although the acceleration data should still provide useful information about how the tool string behaved in the complex heave conditions.
The check-shot survey was affected by borehole and/or the wireline noise, which increased uphole and was dominated by peak frequencies between 75 and 150 Hz. A few stations were abandoned because it was not possible to get a clear first-arrival signal. Despite this, enough wavelets showed a clear first break suitable for check-shot purposes (see "Seismic Stratigraphy"). Damage to the cable may have affected data quality at the final station.
All logs are plotted on a linear scale chosen according to maximum excursions of individual data sets (Figs. F26, F27, F28, F29). Porosity and density logs are plotted on compatible scales using the following relationship between density and porosity in a clean water-filled limestone:
where p = bulk density, ø = porosity, 2.71 = density of calcite (g/cm3), and 1.0 = density of (fresh) water (g/cm3). On this scale, porosity and density curves will coincide for clean, water-filled limestones, whereas mixing with other lithologies will be seen as either negative or positive separations between the curves (Rider, 1996).
Low core recovery within the open-hole logged interval at Site 1126 limited the correlation of downhole logs to the sedimentary section. Except for the basal 25 m, photoelectric effect (PEF) values (averaging ~4 barn/e-) and the minimal neutron-density separations indicate calcium carbonate-rich sediments throughout the open-hole logged section. Porosity-density cross-overs and shallow resistivity peaks indicate the presence of many chert horizons. The succession was divided into six logging units on the basis of trends in the measured data sets. These units are described below.
This unit is characterized by relatively high gamma-ray values averaging 7 American Petroleum Institute (API) units measured through the pipe, with uranium contributing ~75% of the radioactivity (Fig. F27). Four gamma-ray cycles may be discerned in this unit, with maximum excursions ranging between 11 and 3 API units. The high levels of uranium indicate an increased abundance of organic matter and/or aragonite (see "Inorganic Geochemistry"). High gamma-ray values also appear to correlate with portions of the core with increased amounts of blackened grains. These grains may contain elevated uranium concentrations (see "Lithostratigraphy"). The base of Unit 1 is defined by an abrupt decrease in gamma-ray values, mainly caused by a decrease in uranium content (Fig. F27). Logging Unit 1 corresponds to PP Unit 1 and lithostratigraphic Subunit IA (see "Lithostratigraphy" and "Physical Properties"). The base of logging Unit 1 correlates to the base of seismic Sequence 2 and to the Pliocene/Pleistocene boundary (see "Seismic Stratigraphy").
This unit was logged through pipe to 117 mbsf. The open-hole interval shows nearly constant values for all logs, with low gamma radiation (6 API units), high porosity (70%), low sonic velocities (1.9 km/s), and PEF values of ~3.4 barn/e-, all characteristic of a poorly lithified carbonate (Fig. F26). The base of logging Unit 2 corresponds to the base of seismic Sequence 3 and the base of PP Subunit 2B and is within 5 m of the base of lithostratigraphic Unit III (see "Lithostratigraphy," "Seismic Stratigraphy," and "Physical Properties").
Logging Unit 3 is characterized by a marked change to higher amplitude variations in porosity, density, and sonic velocity (Figs. F26, F28). There is a significant separation of shallow and deep resistivity curves within this unit, which indicates borehole fluid invasion into the formation (Fig. F26). Sonic velocity generally increases throughout Unit 3 from 2.1 to 2.4 km/s, whereas MS decreases from 380 to 320 ppm (Fig. F26). Low-porosity excursions that cross over the bulk-density curve correlate with peaks in the shallow-resistivity and sonic velocity logs and indicate the occurrence of lithified layers (Fig. F26). These features most likely correspond to chert horizons described from recovered sediments. The sonic log indicates that these harder layers are interbedded with much less indurated sediments, as confirmed by calcareous oozes recovered in the cores (see "Lithostratigraphy").
Logging Unit 4 is characterized by a gradual decrease in MS from 330 to 290 ppm and a linear increase in sonic velocity from 2.3 to 2.5 km/s over the same interval. Less-pronounced peaks in porosity and resistivity indicate the occurrence of probable chert bands near the top of Unit 4 (Fig. F26). At the base of Unit 4 there is a small peak in the otherwise uniform gamma-ray log coinciding with a marked low-porosity peak, a decrease in MS, and a small increase in resistivity, which likely indicates a hardground surface (Fig. F26).
Within logging Unit 5 porosity decreases steadily from 60% to 55% and density increases from 1.9 to 2.1 g/cm3 (Fig. F26). Sonic velocities increase to the base of the unit from 2.4 to 2.7 km/s and MS decreases from 300 to 220 ppm. Convergence of porosity and density logs downhole in this unit indicate an increasingly pure limestone toward the base. This finding is supported by the decrease in MS (Fig. F26).
Below 390 m in Unit 5, gamma-ray values increase from ~10 to 35 API units (Fig. F26). This increase is dominated by Th and K, indicating greater amounts of clay in this interval (Fig. F28). This conclusion is supported by the increase in MS from 220 to 1300 ppm at the logging Unit 5/Unit 6 boundary (Fig. F26). Throughout Unit 5 there is a downhole increase in resistivity with a peak at the base of the unit. Sonic velocity is constant at ~2.6 km/s (Fig. F26). A velocity inversion (from ~2.2 to 1.75 km/s) is seen in the sonic log at the base of the unit, although this may be an artifact of an increased borehole diameter (Fig. F26). The base of logging Unit 5 correlates with the base of lithostratigraphic Unit IV, PP Unit 4 (see "Lithostratigraphy" and "Physical Properties"), and the hiatus between Cretaceous siliciclastic deposits and Tertiary carbonates (see "Biostratigraphy").
The top of logging Unit 6 is marked by significant inflections in all logs, especially MS (300 to 1300 ppm) and gamma radiation (30 and 70 API units) (Figs. F26, F28). A high gamma-ray signal results from significant contributions of Th and K (Fig. F28). Within Unit 6 the caliper indicates major washouts and caving. The PEF values in Unit 6 are highly variable with generally lower values being characteristic of siliciclastic sediments (2-3 barn/e-) and higher values indicating the presence of iron minerals (Fig. F26). This conclusion is supported by corresponding peaks in MS approaching 1500 ppm (Fig. F26).
Acoustic impedance was calculated using downhole velocity and density data in the following equation:
where Vp = sonic velocity (m/s) and p = bulk density (kg/m3) (Fig. F29). After calculating impedance, data were smoothed using a 20-point running average, which gave an ~3-m resolution. This resolution is 2-3 times higher than the resolution of the site-survey seismic data (see "Seismic Stratigraphy"). Numerous peaks in impedance are observed with the most prominent at 168, 187, 225, and 350 mbsf (Fig. F29). An impedance peak at 168 mbsf correlates well with the base of seismic Sequence 3 (see "Seismic Stratigraphy") and the base of logging Unit 2, whereas the peak at 225 mbsf correlates with the top of seismic Sequence 6A.