After reaching the target coring depth of 711 mbsf, Hole 1148A was filled with viscous mud, reamed, and flushed of debris. We first ran one full pass and a shorter repeat pass with the triple combo tool string, including the hostile environment natural gamma-ray sonde (HNGS), accelerator porosity sonde (APS), hostile environment lithodensity sonde (HLDS), and dual-induction tool (DIT; resistivity) (Fig. F33). (See the "Related Leg Data" contents list.) The Lamont-Doherty temperature/acceleration/pressure tool was not run because the attempts to start up the tool failed before its deployment.
Next, we ran two full passes with the FMS, long-spaced sonic (LSS), and natural gamma-ray spectrometry tool (NGT) string (Fig. F33). Before this descent, the pipe was lowered to 200.7 mbsf. Finally, we ran three full passes with the GHMT string, including the nuclear magnetic remanence sonde, susceptibility measurement sonde, and NGT (Fig. F33). Before the GHMT descent, another short wiper trip was made to knock out some of the swelling clays and to reset the end of pipe to 200 mbsf. The GHMT tool string encountered slight difficulty in passing through the interval of 395.5-444.5 mbsf.
The wireline heave compensator performed well. Sea heave was between 1.5 and 3.0 m for the duration of the logging. Logging operations started at 0950 hr on 4 April and finished at 1500 hr on 5 April (Table T18).
All three tool strings reached within several meters of the bottom of the hole, indicating that little debris fell from the borehole wall. The two passes for the three tool string runs showed generally excellent repeatability for all the log parameters. During the triple combo run, the hole was in excellent condition from total depth to ~344.5 mbsf. From 344.5 to 184.5 mbsf, the hole was generally 14 in or less, causing only occasional bogus spikes on the APS and HLDS. Above 184.5 mbsf, most of the hole was washed out beyond 18 in, making APS and HLDS data problematic. Even the HNGS was affected drastically in this upper interval because the caliper did not reflect the true hole size. The APS and HNGS hole-size corrections were performed as always (Fig. F34). The DIT ran a good log. Both the shallow (spherically focused log [SFL]) and medium (medium induction phasor-processed resistivity [IMPH]) resistivity were adversely affected by the washouts in the top part of the hole.
The FMS pad contact in this hole was the best of the leg, almost perfect in the lower interval of the hole. Tool heaves of ~0.3 m were present as usual. Pad orientation did change between the two passes at some intervals. The sonic run was excellent, except that it skipped in the interval from 394.5 to 399.5 mbsf because of a rapid change in the hole diameter caused by swelling clays. The second pass did better in this interval, with only one spike in the sonic velocity at 395.5 mbsf. Otherwise, the two P-wave velocity measurements are almost completely superimposed (Fig. F34).
Although the general-purpose inclinometer tool was not run at this site, the magnetic field data showed the same spiky quality as Sites 1144 and 1146 with even more large-amplitude spikes. After noting that the spiking was significantly less on the second pass, we decided to make a third pass. The third pass was of the same quality as the second. Interestingly, the spikes often repeat themselves on subsequent passes at the same depth but with different amplitudes. Thus, we infer that the spikes may be caused by an anisotropic environmental influence with their amplitude determined by tool orientation. These spikes were edited out in the data presented in this report. The MS data were mostly of good quality (Fig. F34).
In general, standard gamma ray (HSGR) and computed gamma ray (HCGR) from the HNGS tool in the triple combo run read 10% to 25% higher than spectroscopy gamma ray or computed gamma ray from the NGT in the FMS-LSS run, a difference easily accounted for by eccentricity and hole-size correction. Contrary to the NGT, the HNGS corrects for borehole diameter and potassium in the borehole fluid, although the corrections near individual large washouts were apparently inadequate. The HNGS is more sensitive than the NGT; hence, its results are presented in Figure F35.
The downhole logging and core sample data together suggest that Hole 1148A can be divided into six main intervals of distinct physical properties below the logging depth of 111.5 mbsf. The general dividing lines seem to be at ~180, ~420, ~450, ~475, and ~490 mbsf (Fig. F34).
Above 180 mbsf (interval 1), the caliper frequently saturated, indicating severe hole washouts. Correspondingly, neutron porosity and bulk density from downhole logging are spiky, and the shallow resistivity (SFL) is less than the medium (IMPH) and deep (deep induction phasor-processed resistivity [IDPH]) resistivities (Fig. F34). Gamma ray (reflecting the combined effects of potassium, thorium, and uranium), porosity, and density from downhole logging are noisy from 111.5 to 180.0 mbsf because of poor hole condition. Resistivity values increase with depth in this interval (Figs. F34, F35, F36).
Caliper measurements show that the hole condition in this interval is variable with some swelling clays between 285 and 310 mbsf (Fig. F34). Correspondingly, neutron porosity and bulk density logs are still somewhat spiky. In this interval, several physical properties indicators increase with depth, including bulk density, electric resistivity, P-wave velocity, and photoelectric effect (PEF) (Figs. F34, F37). Neutron porosity decreases with depth. Magnetic susceptibility reaches maxima at 260, 370, and 400 mbsf and minima at 330 mbsf (Figs. F34, F37). Gamma ray first increases with depth from 180 to 320 mbsf and then decreases from 320 to 420 mbsf (Fig. F35). The decrease in magnetic field with depth in this interval is caused by the proximity of the drill pipes (Fig. F36).
Caliper data show excellent hole condition in this interval (Fig. F34). Bulk density, resistivity, P-wave velocity, and PEF increase with depth, while magnetic susceptibility remains constant. Neutron porosity and magnetic field decrease slightly with depth (Figs. F34, F37, F36).
Hole condition is also excellent in this interval (Fig. F34). The top and bottom of this interval are associated with major discontinuities in P-wave velocity and thus likely correspond to the prominent double reflectors seen in seismic reflection profiles. The average P-wave velocity of this interval is 2.3 km/s, which is substantially greater than the values of 2.1 and 1.9 km/s at the top and bottom of this interval, respectively. Bulk density is also the greatest in this interval with a major discontinuity at the bottom. Resistivity and PEF also reach maxima at this interval with a major discontinuity again at the base of the interval (Figs. F34, F37). Neutron porosity reaches minima in this layer and again with a discontinuity at the interval base. Magnetic susceptibility decreases slightly with depth with a small maximum at 465 mbsf.
The hole condition is still excellent in this interval except for some small rugosity at 480 mbsf as shown by the caliper data (Fig. F34). This interval is characterized by relatively low bulk density, resistivity, magnetic susceptibility, P-wave velocity, and PEF, together with relatively high neutron porosity (Figs. F34, F37, F36). The top of this interval at 475 mbsf is the strongest discontinuity in the entire hole for many log parameters.
Caliper data show that this interval also has excellent hole condition except for some small rugosity at 680 mbsf (Fig. F34). The trends of bulk density, P-wave velocity, and PEF first increase with depth until ~620 mbsf and then decrease slightly toward the bottom of the hole (Figs. F34, F37). Correspondingly, the neutron porosity trend first decreases and then increases with a transition depth at 620 mbsf. The gamma-ray trend increases with depth, whereas those of resistivity and magnetic susceptibility remain more or less constant (Figs. F34, F35).
The trend in magnetic susceptibility from downhole logging is in good agreement with that from whole-core MST measurements (Fig. F37). The trends of thorium and PEF agree well between 490 and 640 mbsf. CaCO3 measurements of core samples and PEF from downhole logging show similar trends. Downhole logging gamma-ray and MST measurements of core samples agree in their general trends as well (Fig. F36).
The trend of bulk density from downhole logging agrees with that from both MAD measurements of core samples and core logging below 300 mbsf (Fig. F37). Above 300 mbsf, however, the bulk density from downhole logging is very spiky (possibly because of poor hole condition) and is systematically less than that of the core measurements. Similarly, porosity calculated from downhole bulk density agrees better with that calculated from the moisture content of core samples below 300 mbsf. Above 300 mbsf, the logging porosity is very spiky, presumably also because of poor hole condition (Fig. F36). Finally, P-wave velocity from core sample measurements is ~0.2 km/s smaller than that from downhole logging above 450 mbsf, 0.3 km/s smaller at 450-475 mbsf, and 0.1-0.2 km/s smaller below 475 mbsf. The FMS data revealed alternating layers of relatively high and low conductive layers (Fig. F38).