m,
and background values change between 0.75 and 1.15 m.
Large excursions occur at ~110, 275, 648, 658, and 830 mbsf, which can be
correlated to dolomite layers (see Table T5).Downhole measurements were used to determine in situ physical properties, geologic structure, and thermal structure, especially in sections where core recovery was poor. Logs also provide high-resolution records that will be used to study the paleoceanographic history in the western Pacific region and the nature of compaction, lithification, and deformation processes in the drilled forearc region.
Hole 1151D was drilled with an XCB bit with a diameter (bit size) of 29 cm (117/16 in). Logging operations at Site 1151 are summarized in Table T17. Two logging runs were performed on 11 and 12 August 1999. The FMS/long-spaced sonic imager (LSS) string was run from 871 mbsf to the mudline, and the triple combo tool string was run from 868 to 49 mbsf. The bottom of the drill pipe was at 105 mbsf.
The wireline logs recorded in Hole 1151D are shown in Figure F38; they are generally of high quality except for the neutron porosity log. Density, resistivity, sonic, and FMS data recorded in the drill pipe are not interpretable. The NGR logs in the drill pipe are highly attenuated and should be interpreted only qualitatively.
The hole diameter was elongated in the interval of 105-320 mbsf with a short axis of 27-30 cm and a log axis of 30-38 cm. The elongation may be stress-induced and/or be washouts within high-porosity zones. The FMS calipers revealed that the hole generally has a circular shape with a 30-cm diameter in the interval of 320-868 mbsf. The downhole variation of the calipers shows spikes in the borehole diameter each nine meters. This is especially apparent for caliper C2 in the upper 320 m of the hole. These spikes are caused by the pipe-tripping operation. The compressional velocity data from the LSS log are of high quality. A minor amount of cycle skipping is present in the raw data, but shipboard processing of the traveltime eliminated these excursions. Thin intervals of especially abrupt velocity changes corresponded to hard, thin layers.
In situ NGR measurements were collected from 67 to 844 mbsf by the hostile environment spectral natural gamma-ray sonde (Fig. F38). Potassium and thorium values show similar trends throughout the logged section. Potassium values typically fluctuate in the range of 0.002-0.005 wt% and change between 0.003 and 0.015 wt%. Vertical variations of potassium and thorium are similar to that of density.
In situ electric
resistivity measurements were collected from 108 to 865 mbsf by the dual
induction tool as part of the triple combo string. The deep-, medium-, and
shallow-resistivity logs in Figure F38
show similar trends throughout the logged intervals, indicating good hole
conditions. Resistivity values typically fluctuate in the range of 0.1-0.2 m,
and background values change between 0.75 and 1.15 m.
Large excursions occur at ~110, 275, 648, 658, and 830 mbsf, which can be
correlated to dolomite layers (see Table T5).
In situ sonic velocity was acquired from 107 to 856 mbsf in two passes with the LSS tool, which is part of the FMS/sonic tool string (Fig. F38). The downhole trend of log and core P-wave velocity measurements are similar, but the log measurements are generally ~100 m/s higher and less scattered than the discrete core measurements. P-wave velocity generally increases from 1540 to 1850 m/s across the logged interval of Hole 1151D, although higher values are commonly observed across dolomite layers, for example at 273 and 648 mbsf. The maximum P-wave velocity (3800 m/s) of Hole 1151D is measured at 648 mbsf. The width of scatter in P-wave velocity increases from ~600 mbsf, which probably reflects the increasing number of brittle structures in the logged sequence.
In situ bulk density was measured from 111 to 858 mbsf with the HLDS, which is part of the triple combo string (Fig. F38). The downhole trends of log and core density measurements are closely correlated, but log measurements are generally ~0.1-0.2 g/cm3 higher than the discrete core measurements in the upper part of the hole with soft lithologies. This discrepancy is expected because of core decompaction and drilling disturbance. Consequently, the difference between log and core measurements decreases gradually with depth as sediments become more indurated, and log and core values have similar magnitudes from ~700 mbsf.
Bulk density generally ranges from 1.4 to 1.7 g/cm3 in Hole 1151D, but spike values up to 2.5 g/cm3 are occasionally measured (Fig. F38). Bulk density decreases with depth across lithologic Unit I, from about 1.6 g/cm3 at the base of the drill pipe to 1.4 g/cm3 at 296 mbsf. A small excursion from this trend (i.e., slightly increasing values) occurs between 200 and 240 mbsf. A shift to rather constant values occurs across lithologic Unit II (296-407 mbsf), and bulk density generally ranges from 1.4 to 1.6 g/cm3. Infrequent higher values (<1.8 g/cm3) are measured in the upper section of the unit.
Bulk density values in lithologic Subunit IIIA generally fluctuate between 1.5 and 1.6 g/cm3. A significant excursion with bulk density ranging between 1.4 and 1.8 g/cm3 occurs from 452 to 467 mbsf. The width of the scatter in bulk density increases from about 1.5 to 1.6 g/cm3 in this subunit. At 647 mbsf, the maximum bulk density (2.5 g/cm3) of Hole 1151D is measured in a dolomite layer. Bulk density values fluctuate around 1.6 g/cm3 in lithologic Subunit IIIC (719-817 mbsf) with a range of ±0.1 g/cm3. This interval also includes a spike value in bulk density (<1.9 g/cm3) at 768 mbsf. Only the upper 40 m of lithologic Unit IV was logged, and bulk density values generally decrease from 1.7 to 1.5 g/cm3 in this interval.
Equilibrium temperatures obtained from the Adara temperature tool and the Davis-Villinger temperature probe are shown in Table T18 and Figure F39 and are also shown as a function of depth in Figure F40. The errors were determined subjectively, based on the stability of the equilibration record and tool performance. The geothermal gradients at Holes 1151A and 1151D are 28°C/km in the interval from 0 to 372.9 mbsf. Extrapolation of this gradient suggests an in situ temperature of ~34°C at the instrument depth (~1200 mbsf).
