Three logging runs were made in Hole 900A using two different tool strings that obtained data from only part of the total depth of the hole. Total penetration in Hole 900A was 805.0 mbsf (5853.5 mbrf). The wiper trip made in preparation for logging encountered drag during the upward trip in the intervals 770-763, 753, and 600-543 mbsf. The subsequent wiper trip downward had to ream through the interval from 563 to 568 mbsf and a tight spot at 588 mbsf.
The geophysical combination was run first. Data were recorded on two passes, the first upward from a bridge encountered in the hole at 238 mbsf to the drill pipe at 137 mbsf. The pipe then was lowered through this bridge to 330 mbsf, and a second pass run upward from a lower bridge at 451 mbsf to the drill pipe at 330 mbsf.
The Formation Microscanner (FMS) combination was used for the third and final run. We also used the conical side-entry sub (CSES) for this run because of the bridges previously encountered in the hole. The CSES was assembled on the drill string so as to allow logging up to 220 mbsf. The pipe was then worked down to 753 mbsf. FMS data were acquired upward from 785 to 712 mbsf. A bridge at 731 mbsf prevented lowering the tool for a repeat run. A second pass from the bridge at 731 mbsf upward ended at 646 mbsf, when the tool became stuck. We freed the tool by lowering the drill string down over it. We raised the drill string above this bad spot in the hole and attempted again to obtain FMS data, but it became apparent that the cable had been damaged when we freed the tool at 646 mbsf. The tool became stuck again during these attempts and, again, was freed by lowering the pipe over the tool. Cable problems and deteriorating hole conditions forced us to abandon further efforts to log this hole.
The Lamont-Doherty temperature tool was not used in this hole. A "hole finder" was substituted at the bottom of the geophysical combination to try to increase the possibility of passing hole constrictions. The temperature tool was omitted from the FMS combination because the CSES was deployed. Following is a summary of the logging runs.
Run 1
Pass 1: Geophysical combination; drill-pipe depth, 137 mbsf (5185.4 mbrf).
Logged interval: 137-238 mbsf; speed, 600 ft/hr (190 m/hr).
Tools: natural gamma-ray/shear sonic/resistivity.
Pass 2: Geophysical combination; drill-pipe depth, 330 mbsf (5378.7 mbrf).
Logged interval: 330-451 mbsf; speed, 600 ft/hr (190 m/hr).
Tools: natural gamma-ray/shear sonic/resistivity.
Run 2
FMS combination; drill-pipe depth, variable (using CSES). Logged intervals: 646-731 and 712-785 mbsf; speed, 600 ft/hr (190 m/hr).
Tools: natural gamma-ray /FMS.
The wire-line heave compensator was used during all logging runs; sea conditions were calm with minimal swell.
The quality of the sonic waveforms from the monopole transmitters for both passes generally was good, except for the shear waves. Real-time slowness-time coherence processing, however, proved problematic for all waveforms, resulting in unreliable P-wave velocity computations in both sections. This was observed as the "skipping" in delta T (velocities) in Figure 45 and Figure 46; shore-based processing will be required to improve these data. The amplitude of the recorded signals from the dipole transmitter in both logged sections was very low, thereby rendering the shear-wave data in the upper, "softer," logged interval useless. Shear-wave data from the lower section will require careful processing to obtain useful shear-wave velocities. Resistivity data from the induction phaser tool are good, and the correlation among shallow, medium, and deep resistivity over the interval also is good.
The quality of FMS data was poor in the lower pass, because the tool had problems with fully opening its arms and contacting the borehole wall. Good data were obtained from the second run between 646 and 731 mbsf. Good natural gamma-ray data were acquired from both passes and have been combined in Figure 47.
No intervals of the hole were logged with more than one tool string, so depth shifting of the logs is not necessary.
Interval at 137-238 mbsf
This section has natural gamma-ray values that range from 35 to 70 API units. The section has a uniform resistivity that averages 1.5 Ωm and tentative estimates of P-wave interval transit time are on the order of 160 to 175 µs/ft (1.7-1.9 km/s), all of which are typical of a sequence of relatively unconsolidated sediments. A slight peak in the natural gamma-ray log at 180 mbsf corresponds to the boundary between Units I and II in Core 149-900A-21R. The DSI sonic tool indicates a poor shear-wave response from the dipole source, which indicates both weak dipole source energy from the DSI and a poorly consolidated sedimentary formation.
Interval at 330-451 mbsf
This interval has constant values of natural gamma-ray and resistivity (a mean of 70-75 API and 1.5 Ωm, respectively). Tentative first estimates of P-wave interval transit time are on the order of 150 µs/ft (2.0 km/s). The log data suggest that the lithology in this section is similar to that of the upper section. A slightly higher natural gamma-ray value suggests a small change in lithology, although resistivity appears unchanged.
Interval at 646-785 mbsf
Natural gamma-ray data over this interval show a marked decrease from a high average of 80 API units above 710 mbsf, through a transitional zone (710-740 mbsf) to a low average of 15 API units below 740 mbsf. This corresponds to the change from the basal sedimentary Subunit IIB to acoustic basement; the contact is located some where between 740 and 750 mbsf. Given the nature of the acoustic basement, this sediment/basement contact probably is a sharp contact at the very base of the transitional zone. The transitional zone is likely to be the result of the change from the dominant clayey lithology of Subunit IIB to the sandy basal sediments, as observed in the cores. FMS data show that the hole is deviated (from vertical) by 12° through the basement section (azimuth 272°). In the section immediately above, the basement deviation is seen to decrease relatively steadily from 12° to 8° (azimuth 272°-273°) at the top of the logged interval.