Wireline logging operations were carried out in Hole 1201D, the pilot hole that was drilled prior to installation of the seismometer package. During logging, the base of the drill string was kept at 76.5 mbsf to prevent borehole collapse in the upper soft sediment section (Fig. F82). The depth to the seafloor was determined to be 5723 mbrf, based on changes in the density and porosity logs at that depth. Two tool strings were run during wireline logging operations: the triple combo (temperature, resistivity, density, porosity, and natural gamma) and the FMS/sonic combination (Formation MicroScanner, sonic wave velocity, and natural gamma ray) (Table T13).
The first downlog run with the triple combo tool string reached TD at 591 mbsf after encountering sticky intervals at ~107 mbsf, just below where the drill string was set, and between 417 and 467 mbsf (Table T13). There was no apparent geological reason from the recovered cores why this interval should have been sticky and difficult to penetrate. The hole was logged up from TD, past the basement contact at 512 mbsf and up to ~417 mbsf, where the caliper was closed to get through the deepest tight spot. When the caliper was closed, the current to the dual laterolog tool was lost and one of the telemetry modules had to be exchanged to restore power. Logging ceased at this point and restarted again just above the tight spot; as a result, there is a gap in the data over this interval (see Table T14; also see "Data Quality"). No problems were encountered logging from there up to 127 mbsf, the sticky interval below the pipe. Logging continued with the caliper open up to the seafloor at 5723 mbrf; however, no dual laterolog data were recorded in the uppermost interval (see "Data Quality"). An interval was repeated from 177 to 93 mbsf without any further difficulty.
The run with the FMS/sonic tool string was more routine than the first run with the triple combo. Initially, some difficulty was encountered getting the tool string out of the pipe, and applying pump pressure with circulation was necessary to wash material out of the end of the pipe. While the tool was lowered for the first logging pass, an obstruction that could not be passed was encountered at 352 mbsf, higher up than the tight interval found on the first run. On the first logging pass, the FMS calipers did not open fully until 250 mbsf. The hole was logged up to 117 mbsf, at which point the tool string was lowered again for a second pass. The second pass commenced 15 m deeper, at 366 mbsf, and was successfully logged up to the pipe depth at 82 mbsf.
Hole conditions were good in the basement, with the caliper measurements showing a hole diameter varying between 10 and 11.5 in. In the sediment section, the caliper measurement varied from 12 to 18.5 in, with occasional tight spots below bit size. There were several washed-out zones where the density and porosity tools lost contact with the borehole wall, resulting in degradation of the data. Resistivity logs are relatively insensitive to borehole size; however, the natural gamma ray and density logs were corrected for borehole size during acquisition.
During logging operations in Hole 1201D, a speed correction was applied automatically by the Schlumberger acquisition software. However, when the speed correction was removed and the data were restored to measured depth because of a software error, some of the high-resolution data were lost, resulting in straight-line sections on the high-resolution density and porosity curves. The FMS images and natural gamma ray measurements from the FMS/sonic tool string were particularly affected by this loss of data. There are many stretched or compressed intervals and spikes in the data (Fig. F83), and much of the textural detail on the images has been lost. Although it appears that the data loss is irretrievable, preliminary observation suggests that the conventional standard logs are still of reasonable quality.
The logging data recorded on the run down to TD are the only data without the default speed correction. The downlogs are recorded at a faster speed than the uplogs, too fast for reliable gamma ray measurements, and they are run with the caliper retracted so that no correction for borehole size can be applied to the gamma and density logs. Neutron porosity is not measured on the downlog because bombardment of the formation with fast neutrons on the way down would affect the gamma ray measurement on the subsequent uplog run. The resistivity data, however, are more reliable because they are not as adversely affected by borehole size and logging speed. These data were used for depth matching with the speed-corrected data from the uplog runs during onshore processing.
The data from the two uplog runs that were recorded above and below the tight spot were spliced at 396.5 mbsf, although some gaps in the gamma ray and porosity log data remain. The gaps are different for each tool because the tools are situated at different depths along the tool string. Table T14 shows the depths of the first and last readings and the data gap for each tool in the triple combo string. The readings on the gamma ray curve, measured just above the tight spot during the second uplog, may be slightly elevated because of prior activation of the formation by the neutron source (see Fig. F83). The density logs are unreliable over a short interval between 451 and 444 mbsf, where the long-spacing voltage became unstable. The short-spacing curves over this interval are unaffected. There is one erroneous spike on the shallow laterolog curve between 412 and 417 mbsf, caused by the splice with the downlog data over this interval, and a second feature at 297 mbsf that was not observed on the downlog. Despite these minor problems, the repeat measurements of all logs compare well and demonstrate the good quality of the standard resolution geophysical logs.
Two logging units have been identified (Figs. F83, F84). Logging Unit 1 corresponds to lithostratigraphic Unit II, and logging Unit 2 corresponds to the underlying basaltic basement. Lithostratigraphic Unit I was not logged because it only extends to ~60 mbsf, and the base of the pipe was set at 76.5 mbsf during logging. The log data show a good correlation with the physical properties measurements made on cores, particularly the discrete sample density and velocity measurements (Fig. F83). The porosity measurements from the logs are higher than the measurements made on discrete core samples because the neutron porosity log responds to the presence of hydrogen in the formation and also measures bound water and fracture porosity (Schlumberger, 1989; Rider, 1996). The logs show no obvious trends with depth other than a step change at the contact between the sediment and the basaltic basement, which is marked by a sharp increase in resistivity and density and a corresponding decrease in porosity. Both logging units appear to be relatively homogeneous but include subtle variations that reflect changes in lithology.
This unit extends from 76.5 mbsf, the position of the pipe during logging, to the basement contact at 512 mbsf. The logs are consistent over this entire interval, which demonstrates the fairly homogenous nature of the reworked volcaniclastic sediments that dominate the unit. Recovery in the RCB hole was high (>80%), which facilitates detailed core-log correlation studies. The sediments consist of a series of low- to high-energy turbidite sequences characterized by alternating claystone-rich and coarser sandstone-rich or breccia intervals (see "Lithostratigraphy"). The logging data show small variations in resistivity, velocity, density, porosity, and gamma ray values, reflecting changes in lithology, porosity, and grain size. In logging Unit 1, four intervals that correspond to thicker, coarser turbidite packages can be recognized in the logging data (see gray shaded intervals a-d present at 90-124, 155-190, 242-257, and 305-322 mbsf in Figs. F83 and F84). These intervals are generally characterized by higher and more consistent velocity, density, and resistivity values and slightly lower and more consistent natural gamma ray values. They correspond to more massive sandstone or breccia intervals. but smaller-scale grain-size variations can also be identified in the log response. Between these intervals, the logs corresponding to the interbedded finer-grained units show more rapid fluctuations, with generally lower velocity, density, and resistivity values and slightly higher and more variable gamma ray values.
The FMS images in Figure F85 are from a coarse turbidite-claystone sequence between 240 and 320 mbsf. Despite the fact that the images have been adversely affected by the default speed correction, the resistivity difference between the coarser massive units and the finer interbedded sequences is immediately obvious on the static normalized image. Individual bed boundaries can be delineated and used for core-log integration studies, assuming that a good depth match to the triple combo data can be achieved.
Logging Unit 2 extends from 512 mbsf down to a total logging depth of 591 mbsf and is marked by a significant increase in density, resistivity, and velocity values and a corresponding decrease in porosity values. The resistivity values, although significantly higher than those recorded in logging Unit 1, are not as high as expected for a massive basalt lava sequence. Instead, they are comparable to values that have been reported from pillow lavas with interpillow breccia. Core recovery in basement was relatively poor (<23.1%) and included small pieces (<30 cm) of highly veined basalt with glassy pillow margins and interpillow material, including highly altered hyaloclastite. This is consistent with the interpretation of this unit as a pillow sequence with some interpillow breccia as opposed to massive basalt flows (see "Igneous Petrology"). The basement section is relatively homogenous and contains no obvious boundaries, although there appears to be a slight increase in resistivity and a decrease in gamma ray values with depth, which may indicate a trend toward slightly less altered material deeper in the basement section.
The Lamont-Doherty Earth Observatory TAP tool recorded the temperature of the fluid in Hole 1201D as part of the triple combo tool string (Fig. F86). Temperature and pressure are plotted against elapsed time because the computer that records the depth-time relationship failed during acquisition, resulting in a loss of the depth information. The temperature measurement lags behind the pressure measurement, reflecting the slow response of the thermistor. The measurement is an underestimate of the true formation temperature, as the borehole fluid did not have time to equilibrate to the formation temperature during the logging operation. The pressure and temperature curves illustrate well the different stages of the logging operation in Hole 1201D. Recording started ~300 m above the seafloor, and a temperature of ~5°C was recorded at the seafloor (Fig. F86). Pressure and temperature both increased steadily during downlog acquisition, and a maximum temperature of 16.4°C was recorded at the bottom of the hole (595 mbsf). The return path to the seafloor highlights the two stages of uplog acquisition, and the perturbations on the pressure curve correspond to intervals where tool sticking was experienced. During the repeat run, the pressure and temperature both increased slightly and then decreased again. The final pressure-temperature peak on the graph corresponds to an attempt that was made to lower the tool string again to assess whether further sticking would be encountered if the FMS/sonic tool was run.
Despite the problems that hampered logging operations at Site 1201, good data were obtained over the entire open hole interval (76.5-591 mbsf) with the triple combo tool string and reliable sonic data were recorded down to 366 mbsf during the second run with the FMS/sonic tool string. The logging data correlate well with core physical properties measurements. Although lithostratigraphic Unit I was not present within the logged interval, the boundary between lithostratigraphic Unit II (logging Unit 1) and the underlying basement (logging Unit 2) can be clearly delineated. The upper section (logging Unit 1) is characterized by a sequence of turbidites with varying proportions of sandstone or breccia and silty claystone intervals. No obvious trend in density, porosity, resistivity, or velocity with depth is apparent. However, the downhole logs show subtle but significant variations within this turbiditic sedimentary sequence, and fining-upward units that correlate well with sedimentological units described in the cores can be identified. The basement section (logging Unit 2) is relatively homogenous, although there may be a trend toward slightly less altered material with depth.