DOWNHOLE MEASUREMENTS

Logging Operations

Following the completion of APC and XCB coring operations (bit size = 117/16 in) in Hole 1265A, the hole was conditioned with a wiper trip and was displaced with 142 bbl of 8.9-lb/gal sepiolite mud. The drill pipe was withdrawn to 69 mbsf in preparation for logging. All three scheduled tool strings were run: the triple combo tool string with the LDEO high-resolution MGT on top and the Temperature/Acceleration/Pressure (TAP) tool on bottom, the FMS-Sonic tool string, and the three-component WST (WST-3) for a checkshot survey (see "Downhole Measurements" in the "Explanatory Notes" chapter). A summary of the logging operation, including tools used, is provided in Figure F36 and Table T14. The heave conditions were good, typically <2 m throughout the logging operation. However, the wireline heave compensator stroked out once during the repeat run of the triple combo tool string in Schlumberger mode. A total of four main and two short repeat passes were made with no bridges encountered, and the bottom of the hole was reached on all passes (Fig. F36). The tool string rig-up began at 1200 hr on 10 April, and the full operation was completed by 1000 hr on 11 April.

The triple combo tool string was run first and was successfully lowered into the bottom of the hole at 321 mbsf logging depth. The triple combo caliper readings suggested that the upper section of the hole was enlarged to 18 in (i.e., wider than the maximum caliper extension). Upon completion of the main pass, a short repeat pass was performed at the bottom of the hole with the neutron source (Accelerator Porosity Sonde [APS]) turned off to prevent activation of gamma radiation before the MGT run. Control of the triple combo tool string was then transferred to the downhole measurements laboratory, and the tool string was again lowered to the bottom of the hole. The formation was logged uphole for one main pass, with the tool string in "Lamont mode" for MGT data acquisition. At the end of this pass, the tool string was retrieved to the rig floor. No TAP tool data were recorded during this run.

The FMS-sonic tool string was rigged up and run to the bottom of the hole. To save time, the main pass was followed by a short pass in the bottom of the hole because the interval of interest was the lower part of the borehole. The FMS images are good in the bottom part of the hole and progressively degrade uphole as the pad contact was on only one axis for most of the hole. The other axis was larger than the 15-in caliper of the FMS-sonic tool string. The Dipole Sonic Imager (DSI) was run in primary (compressional [P]) and secondary (shear [S]) modes, P and S monopole and dipole shear modes, and also in first motion detection (FMD) mode. As a result of the slowness of the formation and the large hole size, the P-wave velocity logs of the FMD mode are incomplete and the S-wave log is not usable. The FMS-sonic tool string was retrieved from the rig floor at 0500 hr on 11 April.

The final tool string, the WST-3, was successfully run to the bottom of the hole. A total of six checkshot stations were acquired with a spacing conditioned by previously defined lithologic changes and hole condition. In summary, three tool strings were run during the logging operation, with four main logging passes. All main passes were from total depth into pipe, providing a logged section of 252 m (Fig. F36).

Data Quality

The triple combo caliper indicated that the hole conditions were good in the lower-middle part of the borehole (from total depth, 321 to 210 mbsf). Above 210 mbsf and up to 160 mbsf, the caliper indicated minor localized washouts (Fig. F37A). The upper section of the hole, logged with the caliper (160–89 mbsf), is characterized by a hole diameter exceeding the maximum extension of the caliper (18 in). A hole diameter of this magnitude normally would degrade the quality of the data acquired with tools that require contact with the borehole wall (e.g., the density or porosity tools). Remarkably, with the exception of a malfunction of the porosity (APS) sonde at the end of the main pass (from 140 mbsf to pipe depth [69 mbsf]), little evidence of deterioration for data quality exists uphole in these logs.

The tool string accelerometer data from the MGT indicate that stick-slip of the tool remained at low levels through most of the hole except in four short (<1 m) intervals at ~255 mbsf (Fig. F37B). As a further check, a wavelet analysis of the accelerometer data was undertaken. The wavelet transform analysis of the acceleration data allows the multiscale components of the tool acceleration to be deciphered (Fig. F37C). The analyzed record (90–250 mbsf) is characterized by acceleration/deceleration of generally <0.6 m, indicating almost perfect heave compensation during this run. No intervals of intermediate scale (~4 to 7 m), usually attributed to localized stick-slip displacement over washout, were detected. Even without acceleration data for the triple combo tool string in Schlumberger mode, similar results are expected for this tool string.

In the upper part of the hole (99–135 mbsf; calipers closed at 99 mbsf), one axis of the FMS-sonic caliper readings clearly indicates a narrower diameter than that indicated by the triple combo caliper (Fig. F37D). This observation indicates that the hole is elliptical in shape and that hole conditions were better than suggested by the triple combo tool string data. As for the triple combo tool string, vertical acceleration of the FMS-sonic tool string remained at levels characteristic of no stick-slip displacement (Fig. F37E, F37F). Rapid rotation of the FMS-sonic tool string may impair the quality of the microresistivity images. As a quality check, a wavelet analysis of the horizontal acceleration record was undertaken (Fig. F37G). The depth-scale representation is characterized by a progressive decrease in rotation wavelength/smoothness uphole, decreasing from >20 m in the upper part of the hole, to 8 m in the middle interval with washouts (170–210 mbsf), to 3 m above (Fig. F37H). The interval above 140 mbsf shows the highest horizontal acceleration of the record, as only one axis of the FMS-sonic tool string was in contact with the formation. Below 240 mbsf, localized and rapid variations in horizontal acceleration are rare and of low amplitude and do not impair data quality. As a result, the tool orientation is smoothly guided by the change in hole azimuth (Fig. F37I) and the quality of the microresistivity images is good in the bottom part of the hole and progressively degrades uphole.

Because of the low natural gamma radiation (NGR) level of the penetrated formation, the seafloor could not be detected through the pipe. Consequently, the original triple combo logs (main pass) were depth shifted (–3075.0 m) to the seafloor using the upper part (70–120 mbsf) of the total gamma radiation data from the Hostile Environment Natural Gamma Ray Sonde (HNGS) and the NGR data acquired with the whole-core MST (Fig. F38B). The logging data seafloor depth differs by 4.0 m from the seafloor depth determined by the driller. Using this reference scale, the pipe depth was placed at 69 mbsf. Data from the triple combo tool string are good, with excellent repeatability and only minor depth mismatches between the main pass and the short repeat section (Fig. F38C). This earlier run served as a reference by which the features in the equivalent logs of subsequent runs (gamma radiation data from the MGT and environmentally corrected gamma radiation from the Scintillation Gamma Ray Tool [SGT] on the FMS-sonic tool string) were matched (Fig. F38D, F38F). In detail, gamma ray data from the HNGS, MGT, and SGT are also well matched, providing easy depth shifting for the MGT and FMS-sonic logging runs (Fig. F38G, F38H). The depth adjustments required to match the gamma radiation log were subsequently applied to all the other logs from the same tool string.

Logging Stratigraphy

The logged section is characterized by subtle variations around very low mean values. For example, total gamma ray counts do not exceed 25 API and are mostly centered at 6 API (Fig. F39A, F39B, F39C). The spherically focused resistivity, medium-induction phasor-processed resistivity, and deep-induction phasor-processed resistivity are commonly plotted on a logarithmic scale because the values typically range over a few orders of magnitude. Here, resistivities are between 0.5 and 1.5 m and are plotted on a linear scale (Fig. F39D). Highest variabilities are recorded in porosity and density logs (Fig. F39E, F39F) and their associated parameters, the capture cross section (f) and the photoelectric factor (PEF). When reliable, the sonic log is correlated with density and shows an increase in compressional velocity downhole (Fig. F39G).

Based on the homogeneity of the formation, one log unit and four subunits have been defined (Fig. F39H). These subunits are differentiated by density, porosity, and resistivity logs.

Logging Subunit 1a

Depth: base of pipe (69 mbsf) to 170 mbsf

Logging Subunit 1a is characterized by a downhole increase in density (from 1.8 to 1.9 g/cm3) concomitant with a decrease in porosity and high-frequency fluctuations in these properties. Resistivity values are low (~0.8 m) (Fig. F39). Logging Subunit 1a correlates with lithostratigraphic Subunit IIA, containing lower Oligocene to upper Miocene nannofossil ooze and foraminifer-bearing nannofossil ooze (see "Subunit IIA" in "Unit II" in "Description of Lithostratigraphic Units" in "Lithostratigraphy").

Logging Subunit 1b

Depth: 170–235 mbsf

Overall, this subunit is characterized by cyclic fluctuations of intermediate frequency in the porosity log and by high variability in PEF, which correlates with resistivity (Fig. F39). The lower boundary of logging Subunit 1b is defined by the minimal value in the gamma ray log and a step in the sonic log. Density fluctuates around a mean value of 1.9 g/cm3. Logging Subunit 1b correlates mainly with lithostratigraphic Subunit IIB, containing lower to upper Eocene nannofossil ooze and foraminifer-bearing nannofossil ooze, and the upper part (~10 m) of lithostratigraphic Subunit IIC, containing Paleocene to lower Eocene nannofossil ooze and foraminifer-bearing nannofossil ooze (see "Subunit IIB" and "Subunit IIC" both in "Unit II" in "Description of Lithostratigraphic Units" in "Lithostratigraphy").

Logging Subunit 1c

Depth: 235–289 mbsf

Logging Subunit 1c is characterized by low-frequency cycles in porosity. Four well-defined high-amplitude cycles are clearly visible in this log. Subtle increases downhole in NGR from thorium and potassium and in resistivity mark the upper limit of this subunit. Logging Subunit 1c correlates with the bottom part of lithostratigraphic Subunit IIC, containing Paleocene to lower Eocene nannofossil ooze and foraminifer-bearing nannofossil ooze (see "Subunit IIC" in "Unit II" in "Description of Lithostratigraphic Units" in "Lithostratigraphy").

Logging Subunit 1d

Depth: 289 mbsf to the bottom of the hole (321 mbsf)

High-frequency variations in the porosity values characterize logging Subunit 1d. This subunit also shows an increase in the variability of the resistivity values associated with chert layers. The density and velocity continuously increase downhole following the gradient of logging Subunit 1c. Logging Subunit 1d correlates with lithostratigraphic Subunit IIC, containing Paleocene to lower Eocene nannofossil ooze and foraminifer-bearing nannofossil ooze (see "Subunit IIC" in "Unit II" in "Description of Lithostratigraphic Units" in "Lithostratigraphy").

Discussion

Comparison between continuous in situ logs and the whole-core MST records (NGR and bulk density) and discrete sample measurements of physical properties (density, porosity, and sonic velocity) provides the basis for depth matching the core-derived mcd scale to the logging depth, thus creating a logging equivalent depth scale—a necessary step to correct for the expansion of the cores and obtain correct sedimentation and mass accumulation rates (see "Age Model and Mass Accumulation Rates" in the "Explanatory Notes" chapter). To account for slightly different compression ratios observed for the different subunits of Site 1265, features recognized both in the whole-core NGR data (vs. mcd) and in the equivalent downhole logging data (vs. logging depth) were carefully mapped using 41 tie points picked between 71.45 and 292 mbsf logging depth (Table T15; Fig. F40). Above (pipe) and below (poor core recovery) this interval, no correlation was possible because data were lacking; a linear extrapolation was used. The extrapolation was based on the mean mapping ratio of 1.15, which is in very good agreement with the core-derived composite depth growth rate (see "Composite Depth"). The lower parts of logging Subunits 1a and 1b (mean density = 1.9 g/cm3; almost constant within this interval) are characterized by a lower core expansion rate.

Downhole and core logging data as well as the graphic lithostratigraphy are presented at the equivalent logging depth scale in Figure F41. On this scale, with the exception of porosity and sonic velocity, all core and logging data are in good agreement with regard to depth and magnitude. As previously mentioned, the APS malfunctioned in the upper part of the hole. Discrepancy between porosity measured on cores and downhole in the intermediate and lower intervals is without doubt due to a calibration problem, as the neutron log is plotted on a standardized arithmetic scale of neutron (or limestone) porosity units. This value represents real porosity only under standard conditions in clean limestones, and to find the real porosity in other lithologies, the neutron log value must be converted by using tables or empirical calibrations. Even if absolute values are biased, the relative variations in this record are still indicative of lithologic changes in the formation. Discrepancy between P-wave velocity measured on the core and downhole is discussed below.

Given the data, only the microresistivity (FMS; vertical resolution ~5 cm) images can theoretically resolve high-frequency cycles seen, for example, in color reflectance data obtained from the cores. In the section logged from 99 to 320 mbsf with the FMS-sonic tool string, the static-normalized microresistivity image confirms the increase in resistivity downhole as seen in the standard resistivity logs. Because of the enlargement in the upper part of the hole, only the intermediate and lower parts of the section can be used for cyclostratigraphic studies. In spite of the washouts in the middle segment (195–255 mbsf) of the hole and the low resistivity values of the formation, numerous submeter (<0.25 m) cycles can be resolved in the dynamically normalized FMS images (Fig. F42).

Microresistivity FMS images also allow mapping of chert layers that were poorly recovered in cores. Two resistive intervals associated with cherts at ~287 and 300 mbsf are marked by light color in the static image (Fig. F43). Dynamic normalization of the previous image (Fig. F43B) and zoom (Fig. F44) reveals that these layers are not massive but are associated with gradual and cyclic changes in lithologic composition. Resistive values still are extremely low.

As a preliminary step in the comparison between downhole logging and core properties, Figure F45 shows the relationship between (1) core density vs. in situ densities, (2) sonic velocity in cores vs. in situ, velocity and density measured (3) on core and (4) in situ, (5) P-wave velocity in cores vs. in situ density, and (6) in situ velocity vs. core density; all data are referenced to the equivalent logging depth. For the 90 samples available over the logged interval, all the previous relationships show, as expected, a linear increase in density and velocity with depth. This trend in velocity is corroborated by the sonic log and the checkshot survey (Fig. F46).

Assuming that the velocity in seawater is equal to 1525 m/s, the mean velocity of the formation is 1650 m/s, according to the lowermost checkshot. This is a typical value for soft sediment. The P/E boundary recovered in Hole 1265A at 275 m equivalent logging depth is associated with a reflector identified on the seismic line GeoB 01-048 (CDP = 3850) at ~4385 ms TWT (2192.5 ms one-way traveltime) (see "Introduction"). This is in excellent agreement with the traveltime from checkshots 3 and 4 at 265 and 285 mbsf, respectively (Table T16). In the lower part of the borehole (170–320 mbsf), where checkshot measurements are available every 15–25 m, interval velocities are between 1980 and 2300 m/s. These values are consistent with the in situ velocity measured by the DSI in that section, except for a few short intervals where the P-wave velocity labeling algorithm in the Schlumberger software had difficulty in identifying the P-wave velocity of the formation (Fig. F47). The mean interval velocity of the lower 145 m of the borehole (~2100 m/s) from the checkshots exceeds the velocity values measured on cores by >30%. Low velocity values of the core can be explained by their decompression and/or fabric modification before they are measured on board.

Interval velocities deduced from the checkshot survey result from a clear picking of arrival times of the P-wave at the WST-3 geophones, and high confidence can be attributed to these measurements. Indeed, they are in agreement with (1) the seafloor time and (2) the P/E reflector identified on the GeoB 01-048 seismic line. However, having a mean formation velocity equal to ~1650 m/s and a high velocity of ~2100 m/s in the lower part of the formation implies that the upper part (0–170 mbsf) has an interval velocity of <1400 m/s. This value is lower than that for wave propagation in seawater (~1520 m/s) and is therefore not realistic. Such inconsistent results have been reported by other acousticians working in poorly consolidated sediments.

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