DOWNHOLE MEASUREMENTS

Logging data were recorded in Hole 1179D during Leg 191. One logging run was performed with the triple-combo tool string including the newly developed LDEO multisensor gamma-ray tool (MGT). Only the upper part of the borehole was logged because of a borehole obstruction at ~260 mbsf. The triple-combo measurements are affected by the large hole size, particularly for the accelerator porosity sonde (APS) and high-temperature lithodensity logging tool (HLDT), so the density and porosity data should be treated cautiously. Despite the poor hole condition and an incomplete logged section, the natural radioactivity and resistivity measurements nicely recorded the lithologic changes described in the Hole 1179C sedimentary section (see "Sedimentology").

Logging operations were conducted in Hole 1179D after it had been drilled to a depth of 475 mbsf with a 9.75-in drill bit. The borehole was conditioned with sepiolite drilling mud mixed with seawater. The base of the BHA was set at 154.5 mbsf. Three tool-string runs were planned: triple combo, Formation MicroScanner sonic, and ultrasonic borehole imager. However, operational difficulties only allowed the triple-combo deployment.

The triple-combo tool string was deployed first. It included the APS, the hostile environment natural gamma sonde (HNGS), the HLDT, and the phasor dual induction-spherically focused resistivity tool (DIT). The new LDEO MGT was attached to the top of the tool string and the LDEO temperature/acceleration/pressure (TAP) tool was attached to the bottom of the tool string. The first triple-combo repeat pass was logged from 300 to 203 mbsf with the standard triple-combo probes. After this repeat pass, we attempted to lower the tool string but a bridge was encountered at ~260 mbsf. After several unsuccessful attempts to pass the tool string through the bridge, only the upper part of the borehole was logged and the remaining passes started from this point. The second and third passes were used for recording MGT data only and the fourth pass to run the standard triple combo. No further logging was conducted in Hole 1179D.

Degraded borehole width affects most measurements, particularly those that require eccentralization and good contact between the tool and the borehole wall (APS and HLDT). In Hole 1179D, the caliper data from 290 to 246 mbsf are very rough, ranging from 7 to 16.5 in (Fig. F63). From 246 mbsf to the pipe depth, the caliper remained fully opened (16.5 in, which is the full range of the tool). Poor borehole conditions, ranging from washed to constricted along the whole logged interval, resulted in logs of poor quality, especially for density and porosity. Furthermore, the HLDT developed a functioning problem with the far-spaced detector, resulting in erroneous spikes in the density and photoelectric (PEF) measurements. These poor-quality logs will not be discussed in the next section.

Despite the washed-out conditions, both gamma-ray (HNGS and MGT) and resistivity (DIT) from the upper part of the borehole are of good quality.

The results of both triple-combo passes have been spliced together to produce a composite and depth-corrected data set (see the "Related Leg Data" contents list).

Natural radioactivity was measured downhole by both the HNGS and MGT. Both tools have scintillation detectors that measure the natural gamma radiation emitted by the formation and resulting from radioactive decay. In general, the total gamma-ray record shows an overall increase in natural radioactivity with depth (Fig. F63). There is, however, a significant decrease in the log data at 246 mbsf; this correlates with similar tool responses at this depth for resistivity and PEF. The strong decrease in resistivity recorded at 245 mbsf is probably linked to the tool response as a result of the poor hole conditions at this depth (caliper jumping from 7 to 16.5 in).

A number of distinctive peaks in the gamma-ray measurements in the uppermost part of the logged section at depths of 182.9, 188.0, 195.7, and 242.2 mbsf may correspond to the ash layers recovered in the sediment cores (see "Sedimentology").

The downhole variations in potassium and thorium concentrations closely follow the pattern for the total gamma-ray record. The uranium log, however, shows a rather different pattern with a number of maxima and minima not identifiable on the total gamma-ray record. Peaks in the uranium data occur at depths of 195.7, 208.1, 217.7, 235.6, and 251.3 mbsf. A minimum in the uranium data at 200 mbsf mirrors peaks in the potassium and thorium logs and may correspond to a filled burrow located in the sediment core at this depth.

In general, the uranium concentrations measured by the MGT closely match HNGS data. Differences in potassium and thorium concentrations are caused by different calibration schemes for the tools. The average level of potassium measured by the MGT is more consistent with previous measurements in the similar environment (Leg 185, Site 1149) (Plank, Ludden, Escutia, et al., 2000).

The MGT data are well correlated with total NGR counts from HNGS and core data (MST) while presenting much higher vertical resolution and better defined layer boundaries (Fig. F64). All gamma-ray measurements indicate the presence of ash layers in the uppermost part of the logged section.

The electrical measurements were made with the DIT. This tool provides three measurements of the formation electrical resistivity—deep, medium, and shallow (SFLU)—on the basis of the penetration depth of the focused current into the formation. The different measurements along the whole logged section are shown in Figure F65. The resistivity data clearly delineate the main lithologic types observed in the sediment column at Hole 1179C.

The electrical resistivity in lithostratigraphic Unit I (diatom ooze) is low, with small increases in resistivity at 180, 188, 200, and 212 mbsf. These increases in resistivity can be associated with ash layers or burrows described from the core description (see "Sedimentology") and with the gamma-ray measurements as shown in Figure F65. The boundary between Units I and II (radiolarian ooze) is marked by a slight increase in resistivity at ~221.5 mbsf (only 0.01 m of difference between Units I and II). The DIT data clearly record the lithologic change between Units II and III (pelagic clays) at ~243 mbsf, marked by an increase in the electrical resistivity (SFLU increases from 0.25 to 0.35 m). The boundary between lithostratigraphic Units III and IV (cherts) at ~283 m is marked by a strong increase in resistivity (from 0.25 to 1.5 m). The chert-bearing formation consists of alternating high resistivity layers (nodules) and interbedded low resistivity layers (clay-rich layers).

The strong decrease in resistivity recorded at 245 mbsf is linked to the tool response because of the poor hole conditions at this depth (caliper jumping from 7 to 16.5 in).

The DSA tool operates as a memory tool recording the tool's acceleration, ambient pressure, and internal temperature. The tool uses two accelerometers: an axial (vertical) high-sensitivity accelerometer, and a three-axis low-sensitivity accelerometer (LSA).

The DSA tool was deployed on APC Cores 191-1179C-4H and 7H at 77.3 and 105.8 mbsf, respectively. Both deployments were successful: the tool started recording data at the predetermined depth (thus relieving pressure transducer concerns), and two full data sets were recovered without any problems (confirming proper functioning of the controller and memory card). The maximum pressure recorded by the tool was ~9000 psi, and shocks did not exceed 2.7 G. Plots of x- and y-axis acceleration data (Fig. F66) clearly show significant events during core barrel hole penetration and recovery, in particular, deviation of the core barrel from a vertical position at the beginning of coring, which could negatively affect core recovery. Figure F67 clearly shows time variations of amplitude and visible frequency of downhole heave recorded by the LSA. The uphole heave from the wireline heave compensator accelerometer was also recorded, making possible the direct comparison of uphole and downhole heave and evaluation of the effectiveness of the drill-string heave compensator. Overall, the tool provides valuable information for monitoring the quality of drilling and coring processes.