DOWNHOLE MEASUREMENTS

Downhole measurements were used to determine in situ physical properties, geologic structures, and thermal structure, especially in sections where core recovery was poor. They also enabled us to evaluate borehole conditions such as hole geometry, stress orientation, and stability in the lower part of the borehole, where downhole instruments were expected to be installed. 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. Logs are also used to generate synthetic seismograms for comparison with field seismic records to correlate reflections with drilled geological boundaries.

Logging operations at Site 1150 are summarized in Table T21 and Figure F66. Five logging runs were made from 1 through 3 July 1999. An obstruction was encountered at 646 mbsf during the first run, so the triple combo was logged from 646 mbsf to the seafloor. The wireline heave compensator was used on all runs and performed well because maximum ship heave was ~3 m for most of the operations. Because another obstruction was encountered at 473 mbsf during the second run, the FMS/dipole shear sonic imager (DSI) string was logged from 472 to 140 mbsf. No shear velocity or crossed-dipole data could be acquired by the DSI tool, apparently because of malfunctioning modes of the tool. Therefore, the DSI was run in a mode that allowed the tool to automatically pick the traveltime and calculate the compressional velocity. Unfortunately, the complete wave forms cannot be saved in this mode. After the pipe was lowered to 745 mbsf, the third run went within 7 m of bottom without any problems. The triple combo was logged from 1174 to 660 mbsf. The FMS/long-spaced sonic imager (LSS) string, which replaced the DSI, was deployed in the fourth run in the interval of 1176 to 692 mbsf. Finally, the BHTV string was deployed in fifth run in the interval of 1174 to 731 mbsf.

The wireline logs recorded in Holes 1150B are generally of high quality except for the neutron porosity log due to excessive borehole enlargement (Fig. F18). Density, resistivity, sonic, and FMS data recorded in the drill pipe cannot be interpreted. The NGR logs are highly attenuated and should be interpreted only qualitatively.

Hole 1150B was drilled with an RCB bit with a diameter of 9 in (~25 cm). The hole diameter was generally >48 cm (i.e., maximum extension of the one-arm caliper) in the interval from 200 to 350 mbsf, possibly because of washouts within low-porosity zones. As a result, the FMS images in these sections are degraded. Nevertheless, the density log in this interval is of high quality because of good contact with the borehole wall regardless of washouts. The FMS calipers in the fourth descent (713-1174 mbsf) revealed that the hole generally has an oval shape with a short axis close to the bit size (~25 cm) and a long axis of ~30-38 cm. The image quality along the short axis is very good, whereas that along the long axis is poorer because of the rugosity of the borehole wall and poor pad contact with the borehole wall. The compressional velocity data from the LSS log is of higher quality than the DSI compressional velocity data. 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 or washouts. The overall quality of BHTV data is poorer than what was originally expected and difficult to interpret at this site. This is because of eccentric motion of the BHTV tool during logging, and because the formation/borehole impedance ratio (~2 at this site) is much lower in the diatomaceous claystone than the normal range (~10) for the tool.

Bulk Density

In situ bulk density measurements were collected from 113-638 and 740-1170 mbsf by the HLDS as a part of the triple combo string (Fig. F18). Downhole density measurements correlate closely with core measurements (Fig. F63). The downhole densities are generally of similar magnitude to the discrete core-based values, whereas they are higher than the semicontinuous GRA bulk density core-based values down to ~700 mbsf. The in situ bulk density is anticipated to be higher than the core-based measurements because of core decompaction and disturbances. It is assumed that the discrete samples tended to be more indurated than surrounding core sections and, therefore, that they were less affected by decompaction and disturbance.

Bulk density values in the lithologic Unit I decrease with depth from 1.7 to 1.4 g/cm³. A large excursion, which reaches values as high as 1.95 g/cm³, occurs at the interval of 137 to 138 mbsf. This peak coincides with the boundary between lithologic Subunits IA and IB and may be caused by pebble clasts observed in Section 186-1150A-16X-1 (see "Lithostratigraphy").

Bulk density values fluctuate mainly between 1.4 and 1.5 g/cm³ in lithologic Unit II. Large excursions up to 2.2 g/cm³ occur at the intervals of 440-441, 472-473, and 474-475 mbsf. The excursion in the interval of 440-441 mbsf can be correlated with a 12-cm-thick dolomite layer in Core 186-1150A-47X (see "Lithostratigraphy"). An increase in bulk density to 1.6 g/cm³ occurs at 598 mbsf, corresponding to the boundary between lithologic Units II and III.

Bulk density values in the logged section of lithologic Subunit IIIA fluctuate mainly between 1.4 and 1.6 g/cm³. A large excursion up to 2.2 g/cm³ occurs at the interval of 602-604 mbsf. An abrupt increase in bulk density from 1.5 to 1.7 g/cm³ occurs at 794 mbsf, corresponding to the boundary between lithologic Subunits IIIA and IIIB. Bulk density values in lithologic Subunits IIIB and IIIC fluctuate in a range of ±0.1 g/cm³ and decrease with depth from 1.65 to 1.55 g/cm³. A large decrease in bulk density occurs at 918 mbsf and can be correlated with the boundary between lithologic Subunits IIIB and IIIC. Large excursions up to 2.2 g/cm³ occur at 810 and 946 mbsf. The excursions can be correlated with a 5-cm-thick dolomite layer recovered in Core 186-1150B-12R and a 17-cm-thick dolomite layer recovered in Core 26R, respectively (see "Lithostratigraphy").

An increase in bulk density from 1.5 to 1.7 g/cm³ occurs at ~1050 mbsf, corresponding to the boundary between lithologic Units III and IV. The average value of bulk density is ~1.7 g/cm³ throughout the unit. A number of large excursions as high as 2.0 g/cm³ occur at the interval of 1055-1063 mbsf. One of the excursions can be correlated with an ~20-cm-thick carbonate-rich layer observed in Core 186-1150A-38R. A large excursion of low-density values of <1.2 g/cm³ is identified in the interval of 1066-1067 mbsf, which corresponds to a large washout identified by the caliper logs.

Sonic Velocity

In situ sonic velocity measurements were acquired by the DSI and LSS tools. The upper interval (152-458 mbsf) was logged by the DSI tool, and the lower part (746-1161 mbsf) was logged by the LSS tool. Borehole-compensated P-wave velocity in the upper part shows a gradual increase from 1600 to 1800 m/s with depth. In the upper part of the hole where the DSI sonic tool was used, the velocity values for dolomite layers are lower than expected, possibly because the tool was used in the "digital first motion detection" mode instead of the normal operation mode because of the malfunction of an electronic part.

In situ P-wave velocity values in the lower part show better quality than those in the upper part. Velocity values in lithologic Subunit IIIA are in the range of 1700 to 1800 m/s. A large excursion up to 2100 m/s occurs at the top of lithologic Subunit IIIB. Velocity values in lithologic Subunits IIIB and IIIC fluctuate between 1750 and 1850 m/s in the interval of 791-865 mbsf and between 1800 and 1900 m/s in the interval of 865-1050 mbsf. Velocity excursions as high as 2300 m/s occur at 808 and 3300 m/s at 943-944 mbsf. The excursions can be correlated with a 5-cm-thick dolomite layer recovered in Core 186-1150B-12R and a 17-cm-thick dolomite layer recovered in Core 26R, respectively (see "Lithostratigraphy"). Thin low-velocity excursions occur at 1045 and 1064 mbsf, which corresponds to a major washout identified by the caliper logs. P-wave velocity increases gradually from 1900 to 2000 m/s in the interval between two washouts. A thin high-velocity excursion can be identified at 1056 mbsf, which can be correlated with an ~20-cm-thick carbonate-rich layer observed in Core 186-1150A-38R (see "Lithostratigraphy"). The velocity values fluctuate around 2000 m/s below the two washouts. The P-wave velocity values acquired with the LSS tool are slightly higher and more tightly grouped than are the P-wave measurements from discrete core specimens (Fig. F63).

Electric Resistivity

In situ electric resistivity measurements were collected from the seafloor to 646 mbsf and 747-1170 mbsf by the dual induction tool as part of the triple combo string. The deep-, medium-, and shallow-resistivity logs in Figure F18 show similar trends throughout the logged intervals, indicating good hole conditions. However, the shallow resistivity log in lithologic Unit I and Subunit IIA shows a smaller amplitude as a result of borehole enlargement indicated by the caliper log.

Deep resistivity values in lithologic Unit I decrease with depth from 1.2 to 0.7 m. Significant drops in resistivity from 1.2 to 0.9, 1.1 to 0.8, and 0.9 to 0.7 m occur at 118, 139, and 175 mbsf, respectively. The resistivity increases in short intervals between these three depths. The same trend can be observed in the bulk density log. Deep resistivity shows cyclic changes between 0.65 and 0.85 m and a gradual decrease with depth in lithologic Subunit IIA. The resistivity abruptly increases from 0.65 to 0.8 m at the top of lithologic Subunit IIB. The resistivity values fluctuate between 0.7 and 0.8 m and show several major cycles in lithologic Subunit IIB. Excursions up to 1.4 and 2.1 m occur in the intervals of 440-441 and 472-475 mbsf, respectively. The excursion in the interval of 440-441 mbsf can be correlated with a 12-cm-thick dolomite layer in Core 186-1150A-47X (see "Lithostratigraphy"). The resistivity value abruptly increases from 0.8 to 1.1 m at the top of lithologic Unit III. The resistivity fluctuates between 0.9 and 1.0 m in lithologic Subunit IIIA. An abrupt change in resistivity from 1.0 to 1.3 m occurs at 792 mbsf, corresponding to the boundary between lithologic Subunits IIIA and IIIB. Resistivity values in lithologic Subunits IIIB and IIIC fluctuate in a range of ±0.2 m and decrease with depth from 1.2 to 1.0 m. Large excursions up to 1.9 m and 1.7 m occur at 810 and 945 mbsf, respectively. The excursions can be correlated with a 5-cm-thick dolomite layer recovered in Core 186-1150B-12R and a 17-cm-thick dolomite layer recovered in Core 26R, respectively (see "Lithostratigraphy").

Low frequency cycles (50- to 100-m scale) and high frequency cycles (5- to 10-m scale) can be identified in lithologic Subunits IIIB and IIIC. An increase in resistivity from 1.1 to 1.4 m occurs at ~1050 mbsf, corresponding to the boundary between lithologic Units III and IV.

Resistivity values in lithologic Unit IV fluctuate in a range of ±0.1 m and decrease with depth from 1.4 to 1.1 m. A large excursion up to 1.9 m occurs at 1057 mbsf, which can be correlated with an ~20-cm-thick carbonate-rich layer observed in Core 186-1150A-38R (see "Lithostratigraphy"). Negative excursions in shallow resistivity down to 1.0 m are identified at 1046 and 1066 mbsf, which corresponds to large washouts.

Natural Gamma Radiation

In situ NGR measurements were collected from the seafloor to 623 mbsf and 745-1165 mbsf by the hostile environment natural gamma-ray sonde (Fig. F18). The log responds to mineral composition and therefore indicates change in lithology. Potassium and thorium values show a similar trend throughout the logged section, although potassium values are more sensitive and variable with depth. The potassium values show a decreasing trend from 115 to 140 mbsf and an increasing trend from 140 to 175 mbsf. A large decrease from 0.012 to 0.007 wt% occurs from 175 mbsf, and values decrease down to 0.005 wt% with depth in lithologic Unit I. Lithologic Subunits IIA and IIB are characterized by regular oscillations of the range of ±0.005 wt% in 10- to 20-m intervals. Relatively high K (0.006-0.010 wt%) zones were identified in the intervals of 290-372 and 425-528 mbsf and low K (0.004-0.008 wt%) zones were identified in the intervals of 215-290, 372-425, and 528-598 mbsf. Uranium values show regular oscillations between 1 and 2 ppm with ~70-m cycles throughout lithologic Unit I and Subunit IIA. Several excursions in Th and K correspond to those of resistivity and density curves, suggesting volcanic ash layers in lithologic Subunit IIB. The averaged potassium value yields 0.007 with relatively small fluctuations in lithologic Subunit IIIA. A large increase in K (from 0.008 to 0.013 wt%) and Th (from 2.5 to 4 ppm) occurs at the boundary between lithologic Subunits IIIA and IIIB. In lithologic Subunit IIIB, potassium values show regular oscillations between 0.007 and 0.012 wt% in ~5-m intervals. In lithologic Subunit IIIB, the average potassium value yields 0.008 wt%. In the interval of 964-1030 mbsf, potassium values show regular oscillation between 0.005 and 0.012 wt% in 10-m intervals. An increase in K (from 0.008 to 0.012 wt%) and Th (from 3 to 5 ppm) occurs at the boundary between lithologic Units III and IV. Two excursions as high as 0.015 wt% in potassium values occur in the intervals of 1047-1150 and 1150-1145 mbsf.

Borehole Geometries and Images

Electrical microresistivity images were recorded in the intervals of 140-472 and 692-1176 mbsf. FMS caliper data document borehole geometries (Fig. F18). In the upper part, Caliper 1 (C1) and Caliper 2 (C2) logs show similar values. The hole geometry above 400 mbsf was enlarged to more than 40 cm. C1 and C2 logs indicate different borehole diameters in the lower part. The azimuth of C1 and C2 is roughly north-northwest-south-southeast and east-northeast-west-southwest, respectively, throughout the lower part. The C1 log shows constant values of 25 ± 1 cm, which is close to the bit size. On the other hand, the C2 log changes from 30 to 38 cm, suggesting the borehole shape is elongated, roughly in the east-northeast-west-southwest direction.

FMS images clearly show five sharp resistive layers (at 440.01-440.70, 793.43-793.55, 810.69-811.03, 945.81-946.05, and 1058.04-1058.52 mbsf). A typical image of a sharp resistive layer from 943.3 to 944.4 mbsf is shown in Figure F67. The sharp resistive layer in the interval of 442.0-422.7 mbsf can be correlated with a 12-cm-thick dolomite layer in Core 186-1150A-47X (see "Lithostratigraphy"). A sharp resistive layer in the intervals of 808.1-808.5 and 943.3-944.4 mbsf can be correlated with a 5-cm-thick dolomite layer recovered in Core 186-1150B-12R and a 17-cm-thick dolomite layer recovered in Core 26R, respectively (see "Lithostratigraphy"). The sharp resistive layer at the interval of 1055.7-1056.0 mbsf can be correlated with an ~20-cm-thick carbonate-rich layer observed in Core 186-1150A-38R (see "Lithostratigraphy").

The microresistivity images also demonstrate patterns that coincide with the variation in NGR log (Fig. F68). In the intervals of 820-870, 895-920, and 960-1030 mbsf, striped patterns can be identified clearly with ~5-, ~5-, and 10-m frequency, respectively. These cyclic changes in microresistivity correspond to changes in resistivity, density, and NGR logs. Resistive zones correspond to high density and high NGR zones.

Conductive and resistive fractures were imaged by FMS. Preliminary shipboard analyses of fracture orientations from lithologic Units III and IV are shown in Figure F69. Fractures are developed in the east-northeast and west-southwest direction. These azimuthal data agree well with the magnetic-oriented core analysis (see "Structural Geology"). The fracture dips mainly range from 20°-50° for conductive fractures to 40°-60° for resistive fractures. Fracture distribution in azimuth imaged by FMS coincides with the direction of borehole elongation indicated by FMS calipers.

Temperature Measurements

Equilibrium temperatures obtained from the APC temperature tool and the DVTP are shown in Table T22 and Figure F70 and are also shown as a function of depth in Figure F71. The errors were determined subjectively, based on the stability of the equilibration record and tool performance. The geothermal gradient at Hole 1150A is 29°C/km (R = 0.997) in the interval from 0 to 164.4 mbsf. Extrapolation of this gradient suggests an in situ temperature of ~35°C at the instrument depth (~1200 mbsf). Heat flow is estimated from the in situ equilibrium temperature measurements and the thermal conductivity values measured on board (Fig. F56). The inverse of the slope of the temperature vs. the cumulative thermal resistance (Fig. F71) yields a direct measure of heat flow. The heat flow is 20 mW/m² (R = 0.996) in the interval from 0 to 154.8 mbsf.

The raw temperature data recorded with the Lamont-Doherty Earth Observatory temperature/acceleration/pressure tool are shown in Figure F72. The geothermal gradient gradually decreases with depth in the upper interval (115-646 mbsf), whereas it gradually increases with depth in the lower interval (660-1174 mbsf).

Cyclic Changes in Sediment Composition

The cores recovered from Site 1150 show quite monotonic sedimentary facies except for occasional ash and dolomite layers. Although several peaks in thorium indicate volcanic ash layers, NGR logs were not generally useful for detecting ash layers. Carbonate/dolomite layers typically have weak negative peaks in gamma ray. However, NGR logs are useful in determining the lithologic change such as the diatom/clay ratio, which is a primary factor of the change in sedimentary composition throughout the whole section at Site 1150.

The composite profile made from NGR logs and MST NGR counts indicate the composition changes in three different modes: (1) first-order cycles, which include high gamma-ray activity in the interval from 0 to ~200 mbsf, low gamma-ray activity in the interval from ~200 to ~800 mbsf, and high gamma-ray activity in the interval from ~800 mbsf to the bottom; (2) second-order cycles, which include medium scale fluctuations in 50- to 100-m intervals, which basically correspond to lithologic units and subunits; and (3) third-order cycles, which include regular oscillations in 5- to 20-m intervals. The first- and second-order cycles coincide well with the vertical variations in the clay and opal concentrations shown by XRD analyses throughout the whole section (see "Lithostratigraphy"). NGR logs contain third-order cycles, which record high-resolution compositional changes in sediments, mainly in the lithologic Unit II and Subunits IIIB and IIIC. The third-order cycles can be observed in MST measurements (GRA bulk density and NGR) in lithologic Unit II. Low core recovery prevented identification of the third-order oscillation in cores from lithologic Subunits IIIB and IIIC. However, several complete cycles were identified in Cores 186-1150B-22R, 28R to 30R, and 32R and correlated well with the gamma-ray logs.

Resistivity logs recorded dewatering, consolidation, and lithification processes. The resistivity values increase discontinuously with depth at ~400, ~600, ~800, and ~1050 mbsf and correspond to the degree of lithification defined by the VCD (see "Lithostratigraphy"). Dolomite layers can be identified at most of the discontinuities of resistivity values. Dolomite can be easily identified as large excursions in resistivity values and can be correlated well with core descriptions and XRD analyses. Although the resistivity values increase abruptly at ~400 and ~600 mbsf, bulk density values do not change very much. These two depths of changes in resistivity can be correlated with two major changes in salinity in interstitial water (see "Geochemistry"). At ~800 and ~1050 mbsf, the bulk density values increase with depth corresponding to the increases in resistivity values. However, no change in pore-water chemistry can be identified at the two discontinuities. The different responses in resistivity, bulk density, and pore-water chemistry suggest that the hydrologic property may produce large effects on the electromagnetic property in 200-600 mbsf, and pore-space reduction affects the electromagnetic property of the formation from 600 mbsf to the bottom of the hole.

Figure F73 shows a crossplot of shallow resistivity vs. total spectral gamma ray by lithologic units. All data are divided into three sets: (1) conductive and low gamma ray, (2) intermediate, and (3) resistive and high gamma ray. The conductive set consists of lithologic Subunits IB, IIA, and IIB and is characterized by less variation of resistivity values with larger variation of gamma-ray values than in other units. This trend suggests that the electromagnetic property in sediments does not change because of a change in the diatom/clay ratio. The resistive set is characterized by a steep slope in the linear relationship between resistivity and gamma ray in lithologic Subunits IIIB and IIIC and large variations in resistivity in lithologic Unit IV. This trend suggests that the electromagnetic property is sensitive to change in the opal/clay ratio in lithologic Subunits IIIB and IIIC. Changes in the opal/clay ratio, however, correlate poorly with resistivity changes in lithologic Unit IV. This poorer correlation between resistivity and gamma ray in the deeper part may correspond to the occurrence of opal-CT in the sediments (see "Lithostratigraphy"). The intermediate set shows a transitional trend between conductive and resistive sets.

Dewatering, consolidation, and lithification processes estimated from core and logs are (1) normal dewatering and consolidation from 0 to ~100 mbsf, (2) pore water preventing the consolidation from ~100 to ~600 mbsf because of high pore pressure and/or the diatomaceous nature of the sediment, (3) dewatering and consolidation proceeding from ~600 to 1050 mbsf, and (4) lithification from ~1050 mbsf downward.

A synthetic seismogram was constructed at Site 1150 (Fig. F74) for a correlation between the in situ properties of sediments and rocks at this site and the seismic reflections observed on field seismic data collected near the site. The field seismic trace is from shot gather no. 1171 from the seismic cruise KH96-3, Line 1, which has a CMP 2341. Hole 1150B was drilled near CMP 2337, which is ~100 m east of CMP 2341. Because the stacked seismic data was not available in digital format on board, CMP 2341 was selected for log-seismic correlation because it has a higher signal/noise ratio than the adjacent traces. A portion of Line 1 that is crossing Site 1150, together with the synthetic seismogram, is shown in Figure F75. From this comparison, the lithologic units from core descriptions (see "Lithostratigraphy") and the major seismic stratigraphy (SS) units are identified.

The density-depth profile shown in Figure F74 is a combination of in situ bulk density logs (HLDS) and laboratory GRA bulk density data measured on whole-round core (see "Physical Properties"). The bulk density and P-wave velocity logs in the logged intervals are generally of good quality. However the DSI responses for hard formations like dolomite underestimate the expected in situ values, because the DSI tool had an electronic problem as described in "Operations". For the intervals without logging, from the seafloor to 113 mbsf and from 638 to 745 mbsf, GRA bulk density values from MST measurements were used. Anomalous values caused by measurement errors were removed, and corrections were made for elastic rebound using the density logs from the intervals where MST and logging densities overlap. For the middle depth interval without logging, the average shift applied to the GRA bulk density data is ~0.1-0.2 g/cm³. For the two intervals without logging, velocity profiles were derived from the corrected GRA bulk density (e.g., Sun, in press).

Using these log-core combined density and velocity (GRA-HLDS density and DSI or LSS) profiles, a reflection coefficient series was calculated and then convolved with an estimated Ricker wavelet with a center frequency of 55 Hz to obtain the synthetic seismogram. The center frequency for the source wavelet was determined from spectral analysis of the field seismic record CMP 2341. Finally, both synthetic and field seismograms were converted to depth from two-way traveltime using integrated velocity profiles from log and core data. Some mismatches are caused by poor core recovery in the intervals without logging and by off-plane seismic reflections and other noises in the field record for the logged intervals. The effect of spherical divergence was not considered in generating the synthetic seismogram; however, the energy decay caused by first-order multiple backscattering was taken into account. Comparison of the synthetic and field record shows that the overall amplitude-depth dependence can be explained by considering this simple mechanism of seismic energy decay.

The overall quality of correlation between the synthetic and field records is quite good. All the major seismic stratigraphy units on the two-dimensional seismic section (cruise KH96-3, Line 1) can be identified on downhole log responses using the synthetic seismogram (Fig. F75). We determined 16 SS units on the field seismic section for the drilled-depth interval. All the lithologic boundaries coincide with SS boundaries within the resolution of the seismic record (10-20 m) (see "Lithostratigraphy"). Although the dolomite layers are typically ~<0.5 m thick (see "Lithostratigraphy"), some of them can be identified on both synthetic and field seismic traces because their large impedance contrasts with the adjacent hemipelagic layers generate strong amplitude reflections. On the other hand, because of the presence of thin layers of dolomite and their strong reflection amplitudes, the seismic reflections from the lithologic boundaries between hemipelagic formations appear to be relatively weak.