DOWNHOLE MEASUREMENTS

The only downhole measurement program planned for Leg 190 was at Site 1173 because a full suite of logging while drilling (LWD) is planned for key Leg 190 sites during Leg 196 in 2001. However, there is at present no LWD capability for sonic velocity measurement in slow, poorly consolidated formations, nor for FMS imaging. Velocity in particular is a key desirable parameter because it is required to convert three-dimensional (3-D) seismic data to accurate depths, as well as to interpret porosity and other physical properties from seismic data. Additionally, the capability to record shear wave traveltimes with new logging instruments permits calculation of V_P/V_S and Poisson's ratio, which are useful for interpreting petrophysical properties from the logs. Consequently, velocity and FMS imaging were the highest priority objectives for logging at Site 1173. As expected, logging Site 1173 was technically challenging; several logging passes were completed from 0 to 440 mbsf with difficulty, but no deeper logs were obtained.

Downhole measurement operations in Hole 1173A consisted of runs with both the triple-combo (spectral gamma ray, dual-induction resistivity, lithodensity, and neutron porosity tools) and FMS-sonic (FMS and dipole shear sonic imager) tool strings (see Fig. F13 in the "Explanatory Notes" chapter). Downhole conditions prevented the entire 734-m-long drilled interval from being logged. The triple-combo and temperature/acceleration/pressure tool were run in two stages because of hole bridging; logs were collected from 97 to 338 and then from 358 to 440 mbsf after the pipe was lowered to span the bridge (Table T24). Log data quality is generally good from 97 to 338 mbsf, but poor in the deeper interval as a result of large and highly variable hole diameter. A wiper trip was then made to try to open the hole to the bottom before we attempted to collect the FMS-sonic velocity logs, and the hole was filled with heavy barite mud because of deteriorating conditions and ODP hole abandonment requirements. The FMS-sonic logging string was run in to the hole but could not be lowered beyond a bridge at 380 mbsf. The interval from 65 to 373 mbsf was then successfully logged twice. High-quality FMS images and compressional and shear-wave velocity data were acquired. During the second pass, a new low-frequency (<1 kHz) dipole sonic energy source was used for the first time in an ODP hole, producing excellent shear and compressional waveforms despite very low formation velocity.

A summary of the logging results is presented in Figure F43, and the complete logging data are available (see the "Related Leg Data" contents list). The caliper log shows that the hole was in generally fair to good condition down to 332 mbsf, although it is consistently 14-18 in wide from 100 to 220 mbsf. The hole was drilled with a 9.875-in bit. Logs from this interval are of good quality. Below 350 mbsf, the hole is wide and highly rugose, considerably degrading the log data. Spectral gamma-ray data (Fig. F44) are consistent with the homogeneous silty clay lithology in the logged interval, staying in a narrow range of ~50-80 API units throughout. No strong lithologic variations are detected. Hostile environment standard gamma ray (HSGR) is the borehole-compensated log generated on the triple-combo string, and it can be considered a more accurate measurement of true formation natural gamma-ray values than the standard gamma-ray (SGR) tool on the FMS-sonic string, which is not corrected for borehole diameter effects. The SGR is primarily used for depth registration of the logging runs.

Resistivity is low overall, ranging only between ~0.4 and 0.7 m (Fig. F43). The interval from 70 to 336 mbsf has an overall trend of decreasing resistivity, which is an unusual pattern. We attribute this reversed trend to the strong downhole increase in porosity over the same interval because resistivity is primarily sensitive to the saline pore fluids. In the deeper logged interval, resistivity is higher, again consistent with the lower core porosity. Numerous narrow high-resistivity spikes, especially well exhibited by the high-resolution spherically-focused induction log, appear to correlate well with ash layers identified in the cores and in the FMS microresistivity log.

Density data indicate an unusually low density zone from 93 to 336 mbsf, with a trend of slightly decreasing density with depth from 93 to ~130 mbsf, then nearly constant density of ~1.63-1.66 g/cm³ to 320 mbsf. Several higher density intervals at 172, 265, and 325 mbsf do not clearly correlate with specific features in the cores. Log densities are significantly higher in the 358- to 440-mbsf interval than above, although the scatter is much higher because of poor hole conditions. Trends in these data are well matched by core density measurements, although the latter are systematically lower above 336 mbsf, probably as a result of elastic rebound effects in the cores. Because they are in situ measurements, the log values are more likely to be close to the true formation density. Below 358 mbsf, core-based density measurements are higher than log values, reversing the difference observed in the shallower interval. The quality of the density measured by the high-temperature lithodensity sonde tool is sensitive to borehole diameter and smoothness; therefore, the noisy caliper log in this interval suggests the log data are suspect. However, the upper bound of the log values ought to be representative of the formation density; the small discrepancy between this upper bound and the core-based data remains unexplained.

Neutron porosity logging resulted in very high values of 70% to 90% porosity throughout most of the logged interval. This log is uncorrected for the effect of the clay mineral hydrogen content, which has a significant effect on neutron absorption (Schlumberger Corp., 1989). The highly clay-rich lithologies at Site 1173 suggest that this log should be considered unreliable without correction. The highly scattered neutron log exhibits a weak compaction trend with depth, with a sharp break to lower porosity at ~193 mbsf. Although there is a local washout at this depth, the reason for the downhole shift in the neutron log is not clear. The core- and density log-derived porosity (essentially the same data set as presented in the density log column) are shown for comparison (Fig. F43). We are not confident that even the general trends in the neutron log can be attributed to real porosity variations without correction for quantitative clay content, and we consider the porosity calculated from the density log to be more reliable.

Both compressional and shear-wave slowness (inverse of velocity) were measured with the new low-frequency, high-power dipole source transducer in the DSI. Traveltimes were picked through automated semblance coherence analysis of the arrivals at 16 receivers and converted to P- and S-wave velocity. Both logs appear to be of good quality over the logged interval (90-360 mbsf), which is especially remarkable given the very low shear-wave velocities of 300-600 m/s. P-wave velocities range from 1575 to 1675 m/s and show no trend down to 220-230 mbsf, where there is a well-defined change to an increasing trend with depth (Fig. F45). P-wave velocity reaches values of ~1850 m/s at ~345 mbsf. Velocities appear to decrease back to values of ~1700 m/s in the short logged interval below 345 mbsf but return to ~1800 m/s at the very bottom of the log. The overall trend is closely followed by the core-based measurements of P-wave velocity (except for the sharp decrease below 340 mbsf), although core values are 50 to 100 m/s lower, again attributed to the removal of in situ stress from the cores. A crossplot of velocity and density-derived porosity (Fig. F46) illustrates the decoupling between velocity and porosity. Taking the break in slope of the velocity-depth curve at ~230 mbsf as a dividing point, the zones above and below this depth plot in different fields, with little overlap. Porosity remains in the same range across this boundary, but velocity changes markedly.

In Figure F45, P-wave velocities are compared to the two-ship, split-spread, seismic experiment interval velocity values determined by Stoffa et al. (1992) near this site. They reported a velocity of 1965 m/s between 133 and 388 mbsf. There is an unexplained discrepancy of 100 to >300 m/s between this value and the log data throughout this interval.

Shear-wave velocity follows a similar trend to that of the P-wave velocity log, varying from just over 300 to >650 m/s. There is a change in slope of the shear-wave velocity curve at the same ~220- to 230-mbsf depth as that exhibited by the P-wave velocity. The shear-wave velocity has near-constant values of ~320 m/s above 220 mbsf, then an increasing trend with depth to a peak value of ~650 m/s at 345 mbsf, the depth of the Unit II/III boundary. Shear-wave velocity declines sharply to 330 m/s below this level to the bottom of the logged interval. Both P- and S-wave velocities exhibit a sharp local peak value at ~265 mbsf that coincides with resistivity and density peaks. This zone corresponds to an interval with a high concentration of ash layers in the cores and FMS logs.

The volcanic ash beds and other features observed in the cores were well imaged by both FMS passes. The two passes followed essentially the same azimuthal track over the entire logged interval, so no increase in borehole wall coverage was obtained. The second pass did confirm that even subtle variations in microresistivity were reproducible and hence image real features of the borehole wall. Both passes of the FMS data are presented in their entirety on the accompanying Log and Core Data CD.

Much of the logged interval exhibits horizontal or near-horizontal bands of high resistivity, which we interpret as individual ash layers identified in the cores (Fig. F47). These bands typically have sharp bases above conductive intervals then grade upward back to background resisitivity values. In the intervening hemipelagic silty clay between the ash bands, subtle features are imaged in the FMS microresistivity log, including narrow bands interpreted as Zoophycos trace fossils and mottled intervals interpreted as bioturbation and diagenetic sulfide mineralization, again based on correlation with core observations (Fig. F48). The Unit II/III boundary at ~343 mbsf was logged with FMS, and a change in resistivity character can be seen across this interval (Fig. F49). Above the boundary, bedding and ash layers are more apparent, whereas below it, the sediment has a more uniformly bioturbated appearance.

Overall, the logging data expand upon core-based observations and provide in situ data at Site 1173. The resistivity and density profiles document the anomalous maintenance of high porosity in lithostratigraphic Unit II as well as the abrupt shift to lower porosity below the Unit II/III boundary. In the upper part of this interval, above 225 mbsf, the sonic velocities remain low, suggesting possible compaction disequilibrium but a normal (still compaction dominated) relationship between velocity and porosity. The deviation of the two velocity-log trends from the density-log trend between 225 and 345 mbsf (Fig. F46) is more unusual. The increased formation velocity over this interval coupled with the near-constant density (or porosity) may be an indicator of cementation increasing the rigidity (bulk and/or shear modulus) of sediments in this interval, even though porosity is unchanged. The sharp increase in density seen in the core physical properties data and the abrupt decrease in P- and S-wave velocity at 345 mbsf would then mark a front below which this cementation is absent. Reasons for both the abrupt onset of this pattern at ~225 mbsf and its abrupt end at 345 mbsf remain unclear, although core-based evidence suggests the latter represents a diagenetic front (see "Lithostratigraphy").