Downhole measurements were made in Hole 801C during Leg 185 after completion of drilling with the RCB and before testing of the DCB. Three logging runs were performed consisting of one pass with the triple combo tool string and two passes with the FMS/sonic tool string. In all runs, the tool strings were lowered to within 70-90 m of the total hole depth cored by the RCB (926 mbsf) and were logged up to the bottom of the casing (483 mbsf) installed during Leg 129. Access to the lower reaches of Hole 801C by the tool strings was likely limited by soft fill that had washed back into the hole or by a bridge created by a breccia unit cored between 840 and 850 mbsf.
The triple combo tool
string was deployed first and measured porosity, density, resistivity, and
natural gamma-ray emissions to within 70 m of the total hole depth. The tool
string included the hostile environment natural gamma sonde (HNGS), hostile
environment lithodensity sonde, accelerator porosity sonde, dual laterolog
(DLL), and temperature acceleration pressure sensor (TAP). The dual induction
tool (DIT) was replaced by the DLL because of the potential for resistivity
values of the massive basalt flows to exceed the dynamic range of the DIT
(0.2-2000 
m).
The logging run went very smoothly, and continuous, high-quality data were
obtained even though the heave compensator on the take-up winch was not
functioning during this run. The problem was remedied in subsequent lowering of
the FMS/sonic tool string.
The FMS/sonic tool string was used to measure microresistivity (FMS), seismic velocity, magnetic field, and natural gamma-ray emissions to within 90 m of the total hole depth. The tool string included the natural-gamma spectrometry tool (NGT), dipole sonic imager (DSI), FMS, and general purpose inclinometer tool. Problems with the DSI prevented the measurement of seismic velocities during this lowering; however, the other tools provided continuous and high-quality data.
For the second FMS/sonic run, the DSI tool was replaced with the long spacing sonic sonde (LSS) to obtain seismic velocity measurements. The tool string was then lowered to within 90 m of the total hole depth. During this logging run, the tool string encountered two sections of the borehole that impeded progress on the uphaul. The first "sticky" section was at ~820 mbsf and appeared to be the result of debris and cuttings from above falling down on top of the tool string. The second sticky section occurred at ~480 mbsf and appeared to coincide with a rugose section of the borehole within the hydrothermal zone identified during drilling operations of Leg 129 and logging operations of Leg 144. Even though this logging run was not as smooth as the previous two, the resulting data were still of high quality and provided continuous coverage up the borehole.
All logging runs were registered relative to the cores and the bottom of the Leg 129 casing as described in the next section. For each of the logging runs, the location of the casing was easily identified by the sudden change in data recorded by various instruments (e.g., resistivity and caliper width).
The logs were correlated with both the cores and the cored intervals by matching various distinctive features in the logs to the same or related features in the cores. The specific cores used for these correlations are Cores 129-801C-3R and 5R and 185-801C-16R, all of which have distinctive features associated with bounding recovered hydrothermal units.
Core 129-801C-3R (Fig. F68) is a 0.95-m massive alkali basalt, which lies at or just above the top of the upper hydrothermal unit that is especially well defined by a pronounced change in logged values of resistivity and velocity. Core 129-801C-5R contains very altered tholeiitic basalt with a large increase in potassium content, especially in Sections 1 and 2. This potassium anomaly correlates with the pronounced potassium peak observed in the gamma-ray logs. Finally, Core 185-801C-16R is made up of several units that include the lower hydrothermal unit starting at the bottom of Section 2 and mainly recovered in Section 3 (Fig. F69). This hydrothermal unit correlates with a distinctive microresistivity signature in the FMS logs. The FMS signature of this hydrothermal unit (Fig. F69) is a distinctive 40-cm-thick unit of high resistivity. The upper and lower boundaries are both abrupt, suggesting a marked change in lithology at both levels. The upper boundary undulates, whereas the lower boundary is nearly horizontal. These dimensions and characteristics are very similar to those of the hydrothermal unit in the recovered core between intervals 185-801C-16R-2, 126-128 cm, and 16R-3, 0-42 cm (Fig. F69).
A solution that brings the above three units in the cores and logs into coincidence is to place the bottom of the casing pipe at 483 mbsf (6168 meters below rig floor [mbrf]), as observed on the resistivity log (Fig. F68). This solution leaves unaffected the core depths recorded during Leg 129 and subsequently in the same sequence during Leg 185 (Figs. F68, F69). This is a relative downward shift of 2 m for the logs because it was thought during Leg 129 that the bottom of casing was at 481 mbsf (6166 mbrf). Also recorded during Leg 129 was the bottom of the "rathole" drilled to accept the casing string at 491 mbsf (6176 mbrf). However, the dual-caliper log on the FMS logs clearly shows only 9 m of open rathole below the bottom of casing, not the 10 m implied by the above estimates. Moreover, the open 9-m rathole is extremely smooth, suggesting that the 3-m expansion joint built into the casing string during Leg 129 is fully open, and thus the bottom of casing is actually at 482 mbsf (6167 mbsf). The additional 1 m of relative offset probably resulted from a bookkeeping error that placed the top of Core 129-801C-1R at least 1 m deeper than its actual depth. To correlate the logs to the cores, we prefer to lower the depths of the logs by 2 m and leave the cored intervals as they were recorded and published for Leg 129. This solution has the disadvantages of placing the bottom of casing 1 m lower than its probable actual depth and producing a 2-m mismatch with the logs taken during Leg 144 (Shipboard Scientific Party, 1993; Larson et al., 1993) because these logs were calibrated to the bottom of casing at 481 mbsf (6166 mbrf). We believe that these disadvantages are outweighed by the benefits from retaining the cored intervals from Legs 129 and 185 at their originally published and recorded levels, respectively.
Inspection of Figure F68 suggests that 2 m is the minimum shift between the originally recorded depths of the logs and cores that is necessary to bring the three features in question into coincidence. Another 1-2 m of offset in the same sense would not violate the previously described data in any of the three cores or logged intervals. However, we believe that the offset can be fixed at 2 m and, furthermore, that the cored intervals can be tied to that offset with the following observations and logic.
Cored intervals are more difficult to relate to logs than features within cores because the former requires complete core recovery of the interval in question, whereas the latter only requires identification of a common unit in both the cores and logs. If correct, the 2-m shift proposed here between the cores and logs implies that Core 185-801C-16R recovered the complete section from its top, down to the hydrothermal unit, because the proposed shift places the hydrothermal unit in the logs at exactly the same level as it is in the core. A larger relative shift would require that the excess difference was missed in the cored interval somewhere between the top of Core 185-801C-16R and the hydrothermal unit. Close inspection of the FMS logs reveals two low-resistivity units at 1.8-1.9 and 2.1-2.3 m above the top of the hydrothermal unit (Fig. F69). These appear to correlate with two large veins of calcareous precipitate in the core at intervals 185-801C-16R-1, 70-72 cm, and 27-37 cm. In the core, these veins are present at ~2.0 and 2.3-2.4 m above the top of the hydrothermal unit, respectively. Other basaltic units between the veins and hydrothermal units also can be generally identified in the FMS logs.
Thus, it appears that the complete section was recovered between the upper large vein at interval 185-801C-16R-1, 27-37 cm, and the hydrothermal unit (Fig. F69). If we make the fairly reasonable assumption that the full section also was recovered in the top 27 cm of this core, then complete recovery occurred between the top of the cored interval and the hydrothermal unit. This allows us to place the top of this cored interval of Core 185-801C-16R at 2.7 m above the top of the hydrothermal unit relative to the FMS logs after the 2-m relative shift proposed above (Fig. F69). This cored interval was recorded at 623.3 mbsf (6308.3 mbrf). We consider this correlation to be precise within ~10-20 cm.
Overall, the postprocessing of the data produced continuous and very high quality records of downhole formation properties. These data are in very good agreement between the various tools, separate logging runs, core observations, and previously recorded logging data. The following is a summary of the initial observations from the logging data.
The size and shape of the borehole and its deviation with respect to north are recorded by the calipers of the FMS/sonic tool string and by the slim-hole lithodensity logging tool (HLDT) section of the triple combo tool string. These data are important for postprocessing other logging data (e.g., HNGS, seismic velocity, and magnetic field data). In addition, the uniformity and smoothness of a borehole can often be an indicator of the quality of data collected, as well as the integrity and rock type comprising the borehole walls.
For the majority of the borehole, the walls are relatively smooth and uniform with a typical diameter of ~11 in (~28 cm) (Figs. F70, F71, F72). These uniform portions correspond to the upper (III) and lower (VI) massive flow sequences, respectively. More rugose sections of the borehole are located within the upper hydrothermal zone (510-530 mbsf), at the interface between Sequence III flows and Sequence IV pillows and flows (580-600 mbsf), and within the Sequence IV pillows and flows below the lower hydrothermal unit (625-715 mbsf). There are several locations within the upper hydrothermal zone where the diameter of the borehole exceeds the limits of both the FMS and HLDT calipers (>16 in [>40 cm]), and in these locations data quality is expected to be very low. Within the remainder of the borehole, the data are continuous and appear to be of very high quality.
On each logging run, natural radioactivity was measured continuously with either the HNGS or NGT. Both tools utilize scintillation detectors to determine the gamma radiation emitted by the decay of radioactive elements within the formation. Spectral processing of the measured gamma radiation identifies characteristic radiation peaks that are used to determine the concentrations of potassium (K, in weight percent), thorium (Th, in parts per million) and uranium (U, in parts per million). These values are combined to provide a measure of the total gamma-ray counts and uranium-free or computed gamma-ray counts. Corrections to the HNGS are made to account for variability in borehole size and borehole-fluid potassium concentrations.
The overall character of the total gamma-ray exhibits a general, but not monotonic, decrease in radioactivity with depth (Fig. F70), and these data are similar to the data collected during Leg 144 in the region of data overlap (483-560 mbsf). The highest gamma-ray counts are associated with Sequence I alkali basalts (480-510 mbsf) and the highly altered green tholeiitic basalts at the top of the Sequence III flows (~530 mbsf) cored during Leg 129 (Shipboard Scientific Party, 1990). Another region of elevated gamma-ray counts is near the top of the upper pillows and flows section (580-640 mbsf) and appears to be associated with a high frequency of pillow basalts, breccias, or interbedded siliceous sediments.
The potassium concentration closely mimics the total gamma-ray pattern, but the uranium and thorium concentrations are significantly different. Both the uranium and thorium data exhibit elevated concentrations within the alkali basalt section (480-510 mbsf); however, the uranium concentration is elevated at two distinctive peaks (~630 and 640 mbsf). These uranium peaks may be associated with the lower hydrothermal unit or sediments bounding the hydrothermal unit. Other increases in uranium occur at the top (725-750 mbsf) and at the base (830-845 mbsf) of Sequence VI flows.
The thorium data exhibit relatively high concentrations within the minimally altered tholeiitic basalts of Sequence III flows (535-580 mbsf). The source of these high thorium values is not well understood because geochemical analyses of discrete samples from this region (Castillo et al., 1992) exhibit extremely low values typical of MORB (<0.2 ppm Th). A similar pattern of thorium concentration is also observed in the Leg 144 logging data (Shipboard Scientific Party, 1993). This inconsistency probably reflects an artifact from the spectral processing of the HNGS and NGT tools.
The electrical resistivity of the formation was measured with the DLL tool and provides a rough estimate of the porosity of the formation. The DLL provides two resistivity measurements labeled "deep" and "shallow" on the basis of respective horizontal depths of penetration of the current into the formation. Both measurements are virtually identical throughout the logged interval (Fig. F71), suggesting the absence of a preferred fracture direction within the formation (Pezard and Anderson, 1989).
Inspection of the electrical resistivity data (Fig. F71) suggests that the measured interval may be separated into four main sections based on the variability and magnitude of the measured resistivity: top (483-590 mbsf), shallow (590-715 mbsf), intermediate (715-765 mbsf), and deep (765-850 mbsf). The top zone is characterized by long-wavelength, large-amplitude resistivity variations and coincides with the alkali basalt and hydrothermal and tholeiitic basalt sections cored during Leg 129. The highest and lowest resistivity values for the entire borehole are measured within this zone and correspond to Sequence III flows and Sequence II hydrothermal unit, respectively. The shallow zone has intermediate resistivity values that oscillate at a very high frequency. This portion of the borehole coincides with Sequence IV characterized by alternating layers of pillow basalts, thin flows, breccias, and interbedded silicic sediments. The intermediate zone has relatively high resistivity values with a longer wavelength variability. This section corresponds to the top of Sequence VI characterized by relatively thick (>2 m) flow units that are separated by less frequent pillow basalts and breccias. The deep section has fairly constant intermediate resistivity values, except in the region of a massive basalt flow between 810 and 825 mbsf where resistivity reaches a local maximum.
The HLDT uses the detection of scattered gamma rays from a radioactive cesium source to determine the bulk density of rock units. These measurements are very sensitive to the integrity and smoothness of the borehole walls; therefore, the bulk density values along the more rugose sections of the borehole may be of lower quality than the data obtained along the more uniform sections.
The bulk density for the majority of the borehole is fairly constant with an average value of ~2.7 g/cm3 (Fig. F71). The most notable exception is in the region of the upper hydrothermal unit (i.e., 510 to ~530 mbsf) where the bulk density approaches 1 g/cm3. This anomalous section as well as the increased amplitude of the variability in the more rugose section of the upper pillows and flows section may be an artifact resulting from poor contact of the sensors with the rugose borehole walls. Overall, the variability of the bulk density closely mimics the general character of the resistivity measurements. In addition, the bulk density values are very similar to the density measurements made on discrete samples presented in "Index Properties".
The LSS uses two acoustic transmitters and two receivers to record the full waveform of sound waves that travel along the borehole wall. Compressional wave velocity (Vp) is determined through the depth-derived compensation principle, whereas acoustic travel times recorded at one depth are combined with a second set of readings at another given depth.
The compressional wave velocities above and below ~580 mbsf exhibit a very different character (Fig. F71). Above 580 mbsf, extreme velocity values are observed and correspond to the upper hydrothermal unit (Vp = ~2 km/s) and the upper massive flow section (Vp = >6 km/s). This high-velocity unit corresponds very well with velocity measurements obtained from discrete samples described in "Compressional Wave Velocity Measurements". The alkali basalt section has more intermediate velocity values of 5 km/s. Below 580 mbsf, the compressional wave velocities increase slightly with depth and exhibit a high frequency variability. A notable increase in velocity at 740-765 mbsf is coincident with massive basalts cored within the Sequence VI flows.
Downhole magnetic field measurements were made with a three-axis fluxgate magnetometer that enabled the calculation of the horizontal and vertical magnetic intensity, inclination, and declination. Generally, the magnetic field directions are used to orient the FMS traces with respect to magnetic north and to calculate the deviation of the hole from true vertical if the NRM of the formation is negligible. However, within a borehole with rocks having a strong NRM different from the present-day magnetic field, such as oceanic basalt, this magnetic orientation is distorted. In this case, we acquire valuable information on the NRM directions and intensities in those rocks.
Comparisons with data recorded during Leg 144 (Ito et al., 1995) show that higher intensities were recorded in the vertical-field component during Leg 185, although the horizontal-field component is essentially identical for both legs. Initial qualitative comparisons with the Leg 144 vertical-field data suggest a constant offset of ~2900 nT. More quantitative comparisons will be made postcruise to verify the nature of this offset. The source of the additional vertical-field component could be the sonic tools run with the FMS string during Leg 185. During Leg 144 the FMS was run separately. A similar vertical-magnetization enhancement (although an order of magnitude less) was attributed to the permanent magnetization of the logging cable during Leg 148 in Hole 504B using a German magnetometer (Worm et al., 1996).
At least four distinct regions can be identified in the total magnetic intensity data (Fig. F72). The shallowest portion of the hole (483-550 mbsf) has magnetic intensity values that are remarkably uniform and monotonous and correspond to low magnetization values measured on Cores 129-801C-1R to 6R. Below this region, short-wavelength, small-amplitude variations in the intensity are present until ~595 mbsf. At this point, a large-amplitude, long-wavelength variation occurs in the uppermost portion of Sequence IV (595-620 mbsf). The source of this large-amplitude variation is difficult to determine because the core recovery in this portion of the borehole is very low. This large-amplitude variation also marks the upper limit of a long-wavelength variation in total magnetic intensity that continues until ~780 mbsf. Below 780 mbsf, the total magnetic intensity is relatively large and fairly uniform.
For the majority of the borehole, the horizontal and vertical components of magnetic intensity are generally in phase and exhibit similar patterns of variability, with the vertical component having a magnitude that is about half of the horizontal component. In three regions, however, the vertical and horizontal components appear to be out of phase (640-670, 710-745, and 760-780 mbsf). The in-phase relations suggest original magnetization in the Southern Hemisphere during the Middle Jurassic, whereas the out-of-phase relations suggest subsequent remagnetization in the Northern Hemisphere during the Cretaceous, as described by Ito et al. (1995). Additional detailed analysis and measurements are required to verify and quantify these initial, tentative observations.
The FMS/sonic tool string also contained three orthogonally oriented accelerometers that were used to estimate the angular deviation from true vertical of the borehole. This estimate is distorted by the shape of the borehole within the upper hydrothermal unit but appears very consistent elsewhere. Hole deviation averages ~1° off true vertical from the bottom of the casing pipe down to about the base of the upper pillows and flows at ~720 mbsf. Below that level, hole deviation averages ~1.5° to the bottom of the logged interval at 840 mbsf.
The TAP memory temperature tool was mounted on the bottom of the triple combo tool string to provide a continuous record of the borehole temperature. Because of the large amount of drilling mud circulated during drilling, the temperature in the borehole immediately after completion of coring is almost uniform and close to the seafloor temperature (Bullard, 1947; Lachenbruch and Brewer, 1959). The few hours elapsed between the end of mud circulation and logging do not allow the borehole to equilibrate to the undisturbed surrounding basement temperature, and the recorded data are typically lower than the basement temperature. Because the logging string was lowered twice to the bottom of the hole, temperatures were recorded three times along the way—twice on the way down and once on the way up—between the two logging runs (Fig. F73). The difference between the various passes measured only hours apart shows generally a progressive return to thermal equilibrium, but the comparison with temperature measured during Leg 144 (seven years after Leg 129 drilling) and with discrete temperature measurements made with the WSTP before drilling during Leg 185 at 490 and 540 mbsf (see Fig. F73 and "Temperature") indicate the actual extent of the necessary recovery. If the data do not provide the temperature of adjacent lithologic units, they can, however, indicate intervals with distinct thermal and hydraulic properties in the borehole.
In Figure F73, two 10- to 20-m-thick intervals centered approximately at 520 and 710 mbsf display negative temperature anomalies that suggest a slower return to equilibrium. In the same figure, the resistivity log and the FMS images show that these intervals have very low resistivity. The shallower of the intervals corresponds to a fossilized hydrothermal zone where a packer experiment during Leg 144 had suggested a high permeability (Larson et al., 1993) but failed to measure the actual value. The deeper of the intervals corresponds to a dramatic change in the lithologic character of the formation (as indicated in the caliper log) and to the approximate location of the igneous Sequences IV-VI boundary. The present temperature anomalies indicate that the two intervals could be high-permeability sections that have been invaded by the cold drilling fluids, have been drawn to lower temperatures by the invasion, and are consequently recovering more slowly from the drilling process. The fact that the anomalies are limited to the vicinity of these intervals shows, however, that they do not correspond to active hydrothermal conduits where significant pressure gradients would generate borehole-scale temperature disturbances.
The FMS produces high-resolution borehole images that mimic visual geologic features in or just behind the borehole wall. Initial processing of the raw FMS data into electrical images was performed aboard ship using proprietary Schlumberger software. The FMS imagery is provided in two formats—static and dynamic displays. The static display (e.g., Fig. F69) shows resistivity variations over long sections of data and allows the identification of major lithologic units. The dynamic display adjusts the resistivity variations over much shorter sections of data and can be used to study individual features within major lithologic units. Initial analysis of these data indicates that high-quality data were obtained for the vast majority of the borehole. A sample of these data is shown in Figure F69 where the FMS data are compared to Core 185-801C-16R. Another qualitative comparison of the FMS imagery data with high-recovery core sections (e.g., Core 185-801C-37R) shows a very good correspondence with fractures, veins, and regions of low microresistivity.
Continuous, high-quality downhole measurements in Hole 801C were made during Leg 185 after completion of drilling with the RCB and before testing of the DCB. Three logging runs were performed, consisting of one pass with the triple combo tool string and two passes of the FMS/sonic tool string. In all three logging runs, the tool strings were lowered to within 70-90 m of the total hole depth cored by the RCB (926 mbsf) and were logged up to the bottom of the casing pipe (483 mbsf) installed during Leg 129.
Comparison of the logging data (e.g., FMS and resistivity) with the recovered cores identified a 2-m discrepancy between the depths of the cores and the logs within Hole 801C. Although the recorded depths of both the cores and the logs are in error, we have chosen to lower the depths of the logs by 2 m and leave the depths of the cored intervals as they were recorded and published for Leg 129 and in this volume.
The general character of the various logging data correlate very well with the lithologies recovered from drilling. High natural gamma radiation values associated with K, U, and Th are present within the alkali basalt section (483-510 mbsf), the highly altered tholeiitic basalt (525-530 mbsf) below the upper hydrothermal unit, and the upper half of Sequence IV pillows and flows (590-640 mbsf). Other notable, but enigmatic increases in gamma radiation are associated with uranium (730-750 and 820-840 mbsf) and thorium (530-570 mbsf).
Downhole measurements obtained with the triple combo tool string exhibit very good internal agreement with formation lithologies. In particular, relatively high resistivity, bulk density, and seismic velocity are associated with the more massive flow units (530-595 and 715-760 mbsf). Anomalously low values for all three measurements are observed within the upper hydrothermal unit (510-530 mbsf). The relative smoothness and rugosity of the borehole wall, measured with the calipers during each of the logging runs, correlates very well with the cored lithologies. The more uniform and massive flow units have much smoother walls than the sections of the borehole characterized by interlayers of pillows, flows, sediments, and breccias.
Initial analysis of the FMS microresistivity data indicates that high-quality data were obtained for the vast majority of the borehole. A qualitative comparison of the FMS imagery data with high-recovery core sections (e.g., Core 185-801C-37R) shows a very good correspondence with fractures, veins, and regions of low microresistivity. The FMS images were of sufficient quality to provide evidence for depth adjustments of the core depths with a precision of 10-20 cm.
The magnetic data are difficult to interpret without additional analyses, but the downhole magnetic intensity measurements exhibit an interesting large-amplitude variation at the top of Sequence III (595-630 mbsf). This larger amplitude variation also marks the top of a long wavelength change in intensity that continues until ~780 mbsf.
