m, intermediate (BM) measurements of 0.3–26.3 m, and shallow measurements (BS) of 0.3–36.6 m (Fig. F101). The BS curve shows the widest range of variability, likely due to changes in fracture intensity.
On 24 June the RAB-8 LWC system was deployed for the first time during an ODP hard rock leg. Initial operations were delayed owing to tool construction, as there was a 2-in error in the assembly drawing, specifically with a bearing coupling part. This error prevented coupling of the inner and outer core barrel. Shipboard parts fabrication resolved the misfit with little loss of operations time. Because we had very few spare parts for this system, the rig mechanics fabricated two spacer subs from an extra core barrel in order to build a second lower core barrel assembly. More than 15 hr was required to core to 20.8 mbsf, and the three core barrels recovered returned only a few small pieces of rock (<20 cm curated length). Based on poor recovery, LWC operations were terminated. In comparison, coring with the conventional RCB system a few meters away in Hole 1275D reached >17 m in just over 10 hr, and the three cores recovered from this interval averaged >30% recovery (5.62 m). In a postcruise evaluation, the core technician hypothesized that a design flaw in the one-way core flow valve (core catcher) contributed to poor recovery with the LWC system.
Ship heave during the RAB-8 tool deployment averaged 0.15 m with peaks of ~0.45 m. The first core was drilled using active heave compensation (AHC), whereas the second and third cores were drilled using the passive heave compensation (PHC) system. AHC deviations from heave at surface were consistently ~0.06 m, and PHC deviations from heave at surface were ~0.10 m with peaks up to 0.20 m. Therefore, heave was not a factor in depth control and image quality. The drillers kept between 5 and 15 kft·lb of weight on the bit (measured by AHC cylinder force when using the AHC and by hookload differential when using the PHC), ensuring constant contact between the bit and the bottom of the hole.
The RAB-8 tool records time vs. formation properties, and the Schlumberger surface acquisition system records time vs. depth. The downhole and surface files are merged in order to generate logs of formation properties as a function of depth. Depth is monitored using a monitor on the drawworks (drawworks encoder; DWE) and a hook and sensor.
While drilling Hole 1275C, the DWE measurement did not always correspond with actual bit movement. At times, the driller lowered the block but the weight was taken by pipe compression and flexing, in which case the bit experienced little or no downward-directed force. Similarly, raising the drill string did not necessarily lift the bit off bottom as the pipe unflexed and decompressed. Thus, the time-depth file generated by the surface system is flawed, and logs generated by merging it with the downhole data resulted in smeared resistivity images.
To correctly process the data, the time-depth file was modified to account for what the bit was doing at a given moment. Figure F97 illustrates the improvement of the smoothed image over the original. Our fundamental assumption in smoothing the data was that the bit was always on bottom except during core retrievals and one other brief interval. The assumption is supported by the surface weight on bit and AHC cylinder force data recorded by the TruVu rig instrumentation system.
The time vs. depth data were smoothed (37 vs. >1000 time-depth pairs in the raw data) but adjusted to match the original slope (i.e., rate of penetration) and points of known depth obtained from drillers depth, the only reference. Figure F98 is a graphical representation of the original and smoothed time-depth file. This depth-tracking problem is uncommon in the oil industry, for which the DWE system was designed. However, ODP operations (specifically LWC in Hole 1275C) are significantly different than typical oilfield operations. In Hole 1275C, we did not have a riser and we were coring in a hard rock formation with a light BHA, resulting in a slow penetration rate.
Only shallow- and deep-resistivity images were obtained during the RAB-8 deployment in order to conserve tool memory. Static and dynamic processing of deep-resistivity images shows a significant number of structures, and the static images show distinct delineations between resistive and conductive intervals (Fig. F99). Some of the most prominent features are the upper resistive layer and a distinct resistivity contrast interpreted to be a large fracture between 3.5 and 4 mbsf. Three-dimensional presentations of the dynamic RAB images show orientations of structural features that likely represent fractures dipping in various directions (Fig. F100). Electrical resistivity values show deep measurements (BD) of 0.3–24.1 m, intermediate (BM) measurements of 0.3–26.3 m, and shallow measurements (BS) of 0.3–36.6 m (Fig. F101). The BS curve shows the widest range of variability, likely due to changes in fracture intensity.
Prior to the beginning of wireline operations on 30 June, a 30-bbl sepiolite sweep was done during the last core, the bit was released at the bottom of the hole, 60 bbl of sepiolite mud was pumped after releasing the bit, and the pipe was brought up to a logging depth of 1582 meters below rig floor (mbrf). The first deployment consisted of the triple combo tool string, which contained the Hostile Environment Gamma Ray Sonde (HNGS), the Accelerator Porosity Sonde (APS), the Hostile Environment Litho-Density Sonde (HLDS), the Dual Induction–Phasor Resistivity Tool (DIT-E), and the Lamont Doherty Earth Observatory Temperature/Pressure/Acceleration tool (TAP). The APS was included in this deployment after the previously documented problems at Site 1272 were resolved. After an 11-min temperature equilibration station at seafloor, we began lowering the tool string at 200 m/hr until a hole obstruction was encountered at ~104.1 mbsf. Several attempts to get past the obstruction failed; therefore, we did a 5-min temperature station at the bottom of the hole and then began logging upward at 274 m/hr. The HLDS caliper arm did not open until reaching 67 mbsf. Once the tool string reached the bottom of the pipe, the HLDS caliper would not close; therefore, the tool was slowly brought inside the pipe and a second open hole pass was aborted. The maximum heave during this run was ~0.82 m.
The second tool string deployment consisted of the Scintillation Gamma Ray Tool (SGT), the General Purpose Inclinometry Tool (GPIT), and the FMS. Attempts to pass through the obstruction at 104 mbsf also failed, and two full passes above the obstruction were completed. After the first pass, the tool string was brought completely inside the pipe and the BHA was raised 10 m. The tool string was then lowered to the bottom of the hole and the second pass recorded images up to 21 mbsf. After the second deployment had concluded, the wireline and the sheaves were removed and the pipe was lowered to 104 mbsf in an attempt to break through the obstruction. After circulating and putting >20,000 lb of weight on the BHA, all other attempts to bypass the obstruction were terminated.
The wireline heave compensator (WHC) began to move erratically during the first SGT-GPIT-FMS pass, and it was turned off for both passes. After examining the WHC for ~1 hr, sudden changes in pressure were observed, suggesting that either the linear positioning transducer or air in the lines from a previous oil and filter change could have been contributing to the problem. However, the problem was not solved and all subsequent logging runs were done without the WHC. The heave varied from 0.75 to 0.87 m during the first SGT-GPIT-FMS pass and from 0.95 to 1.06 m during the second pass. The logging speed for both runs was 274 m/hr.
A third tool string deployment consisted of the SGT, GPIT, and Dipole Sonic Imager (DSI). Two passes were made at 540 m/hr with this tool string, although problems with the tool's isolation joint produced intermittent low voltage and hardware failure error messages throughout the deployment. Postcruise waveform processing and assessment of the shipboard time coherence analyses corrected for these problems.
The HLDS and FMS calipers show a relatively good borehole with dimensions ranging 8.6–17.4 in (mean = 10.8 in). There are several intervals with large washouts including sections at 41–42 mbsf, 77–78 mbsf, and 94–98 mbsf. The widest portion of the borehole measured with the HLDS caliper was at 41–42 mbsf, with a maximum dimension of 17.4 in. The maximum aperture of the FMS calipers is 15.5 in, and this was recorded in the bottommost washout interval.
Deep measurements (IDPH) of electrical resistivity range 8.7–45.4 m, and the shallow resistivity (SFLU) varies between 5.9 and 98.4 m (Fig. F102). The SFLU curve shows the widest range of variability, likely due to changes in fracture intensity and alteration. Several zones with higher resistivity values occur over the same interval as changes in density, photoelectric effect, and porosity (Figs. F102, F103).
Porosity measurements are relatively high, especially for the interval 34–52 mbsf that corresponds to Cores 209-1275D-7R to 11R (Fig. F102). The porosities range 3%–80% (mean = 20.1%). The highest porosity value was recorded in a washout zone located between 41 and 42 mbsf. Four discrete samples from the high-porosity interval (34–52 mbsf) do not have high porosity relative to samples from the rest of the core. However, the interval in the core between 25 and 45 mbsf is more intensely altered than the rest of the core (see "Hydrothermal Alteration" in "Hole 1275D" in "Metamorphic Petrology").
High-resolution density measurements show values of 1.6–3.8 g/cm3 (mean = 2.7 g/cm3 for the logged interval). Densities are lower in the depth interval 34–52 mbsf, where the highest porosities are also encountered (Fig. F103). In keeping with the HLDS measurements, which show a washout at 41–42 mbsf, the density values in this interval are low. The photoelectric effect (PEF) values range 1.3–11.0 barns/e– (mean = 4.1 barns/e– for the logged interval) (Fig. F103). The relatively high PEF values are consistent with minerals such as amphiboles, pyroxenes, Fe-Ti oxides, and calcite found in these rocks (see Table T13 in the "Explanatory Notes" chapter).
Formation natural radioactivity was measured during each run with two different tools. The SGT measured total gamma counts, whereas the HNGS provided spectral measurements. The different gamma ray tools produced similar response curves. The high-resolution environmentally corrected gamma ray (ECGR) total counts curve from the SGT and spectral data from the HNGS are shown in Figure F103. The SGT total count curve ranges 3.5–24.5 gAPI. The spectral gamma ray measured values of Th (
0.35–3.4 ppm), U (–0.2–0.9 ppm), and K (0.03–0.2 wt%) (Fig. F103). In some instances the spectral values fall below the tool detection limits of Th (0.7 ppm), U (0.35 ppm), and K (0.18 wt%). The spectral data for K are low in the same interval (34–52 mbsf) as the low-density, high-porosity, high-alteration interval. PEF in this interval is low as well. A positive Th anomaly occurs between 46 and 48 mbsf (Fig. F103). Laboratory NGR total count measurements obtained using the shipboard MST are compared with logging data in Figure F103.
All modes of the two passes (monopole and upper and lower dipole) were reprocessed postcruise. Overall, the waveforms were noisy and a slowness/time coherence algorithm was applied in a "multishot" mode instead of the full array mode. Coherence was calculated between all combinations of three adjacent receivers at each depth, and the best coherence result was then used. This provided an improvement over the full array coherence processing, although all the logs are noisy.
Compressional wave velocities obtained from the first pass range 2.2–6.5 km/s (mean = 4.3 km/s). In general, the mean compressional wave velocity is lower than expected for lower oceanic crust or upper mantle lithologies and may reflect the intensity of alteration and/or high fracture density present in the rocks. Compressional wave velocities are generally in good agreement with laboratory measurements made at ambient pressure (Fig. F103). Shear wave velocities from the first pass range 1.0–5.7 km/s (mean = 2.2 km/s). These values are generally low, and in some small intervals anomalies such as higher shear wave velocities than compressional wave velocities are present even after reprocessing (Fig. F103).
The GPIT was run in conjunction with the FMS and DSI to provide information on the intensity and direction of magnetization in the formation (see "Downhole Measurements" in the "Explanatory Notes" chapter). The pad 1 azimuth shows different orientations for both FMS passes, increasing the borehole coverage obtained from the microresistivity images (Fig. F104). The magnetic field logs show variations in the horizontal components between both passes, although the total field measurements (FNOR) remain constant (Fig. F104). The magnetic inclination measurements (FINC) show average values of 36.9°, which is steep when compared to the expected value of ~28° for this area (see "Paleomagnetism"). The FNOR and FINC curves also show a pipe effect that extends for ~15 m and anomalies at 72–75 and 88–92 mbsf (Fig. F104). Maximum hole deviation is 4.5° and hole azimuth is ~230° (Fig. F104).
The TAP tool downgoing temperature profile ranges 5.59°–5.55°C and the temperature gradient is 0.002°C/m (Fig. F105A). The pressure profile shows hydrostatic conditions throughout the entire hole (Fig. F105A). The TAP tool was held stationary for 5.4 min at a depth of 104 mbsf after all attempts to pass the obstruction failed. These measurements show no significant temperature changes during this time interval (Fig. F105B).
FMS images show many zones that are interpreted to represent high fracture density and deformation. The interval between 21 and 48.8 mbsf is characterized by mostly conductive features that occur in the same interval as high porosity and low density logging data, PEF, and potassium measurements. An apparent contact at 48.8 mbsf showing a distinct change from a conductive to resistive formation suggests a lithologic change or a transition to a less altered formation (Fig. F106). Below this contact, many conductive features interpreted to be fractures are also present, although the overall resistive nature of the formation is still apparent. We interpret the combination of steep- and shallow-dipping conductive features in the FMS images between 63 and 66.6 mbsf (Fig. F107) to be an intensely fractured interval. A speckled appearance in the FMS image between 69.9 and 71 mbsf (Fig. F108) suggests the presence of disseminated Fe-Ti oxides. Thin subvertical features resemble veins, large steep fractures, and resistivity contrasts, suggesting lithologic contacts are also present throughout the entire logged interval (Fig. F108).
