Logging operations at Hole 1194B, which was drilled to a total depth (TD) of 427.1 mbsf, began at 0530 hr on 25 January 2001 with the deployment of the triple combination (triple combo) tool (natural gamma ray, density, porosity, and resistivity tool) that also carried the LDEO temperature tool on the bottom and the LDEO multisensor gamma ray tool (MGT) on the top (Table T13). In the first pass, data were collected with the triple combo and the temperature tool. For the second pass, power was switched to the downhole measurement lab to collect data with the MGT, which uses the same communication lines as the triple combo. To reach optimal resolution, the MGT requires a slower logging speed (700 ft/hr; 210 m/hr) (Table T13).
In the narrow but open hole, the tools reached a depth at 425 mbsf, which is only 2.1 m above the drilled TD of 427.1 mbsf. The seafloor was detected from the log signals at 385.5 mbrf. For the second run, the FMS and DSI combination was rigged up. Malfunctioning of the DSI tool was discovered while lowering the tool into the hole; later, this proved to be related to a hardware problem. Nevertheless, a first pass with the FMS was completed. For the third run, the DSI was replaced by the LSS tool. During the run, the borehole started to deteriorate, forming a tight spot at around 230 mbsf that was passed after several attempts. The last run was the check shot survey with the WST, using an 80-in3 water gun, as the air gun was leaking. The WST tool could not pass a tight spot at ~158 mbsf; thus, only three stations were measured at shallow depths of 143.2, 117.2, and 88.2 mbsf. The water gun signal was not clean because a low-frequency precursor interfered with the main peak. As a result, the first arrival of the P-wave signal/response could not be picked accurately. Logging operations ended at ~0030 hr on 26 January.
All logging data from the first run are of good quality. In the intervals between 197-211 mbsf and 260.5-272 mbsf, the caliper opened to its maximum of 17 in, indicating that the hole diameter was >17 in (43 cm). In these intervals, the eccentered tool string had no contact with the borehole wall, negatively affecting mainly the density and porosity measurements (Fig. F45). The FMS data from the second run were of excellent quality throughout the entire open-hole interval, with the exception of the above mentioned intervals of large hole diameter. Little evidence of sticking was detected in the data. In the third run, no FMS data could be recorded between 255.5 and 239.7 mbsf because the caliper arms had to be closed as a result of high tension from the small borehole diameter (<5 in). Because of good initial hole conditions, the FMS images show that after preliminary shipboard processing, most downhole changes are sensitive to resistivity, such as the degree of bioturbation, grain size, bedding character, and cementation. The velocity measurements from the third run are generally good, but some erroneous data were recorded when the FMS arms had problems passing narrow intervals and the tool string shifted in the hole.
The MGT and the Schlumberger hostile environment spectral gamma ray sonde (HNGS) were both run in Hole 1194B; overall, both data sets are of high quality and correlate well (Fig. F46). A slight offset of 4 gAPI of the MGT gamma ray can be observed that is due to the borehole size correction applied to HNGS data but not to MGT data. This comparison demonstrates the higher vertical resolution of the MGT vs. the HNGS, which according to the tool specification is ~10 cm vs. 45 cm (see "Downhole Measurements" in the "Explanatory Notes" chapter). The comparison of the MGT total counts with the core-derived natural gamma ray MST total counts shows that these two data curves also correlate well and that the measurements have approximately the same vertical resolution (Fig. F47). The comparison, however, is hampered by the bad recovery.
Compressional wave velocity, density, and natural gamma ray data obtained by borehole logging show good agreement with those data obtained from core samples, with the exception of the GRA bulk density, which was not corrected for core diameter (Fig. F48). Bulk density from MAD data are slightly higher than the log data. P-wave sensor (PWS) and log velocity data both record the downhole increase of velocity, but the discrete samples show higher values in thin beds (e.g., hardgrounds at 114.5 and 159.5 mbsf). In the lower part of the hole, PWS velocity values are generally lower than the log data. This shift is probably caused by the lack of overburden pressure in the core samples. Although the natural gamma ray pattern of the MGT log matches well with the HSGR (standard gamma ray) data taken on cores, absolute values of these two tools (MGT and MST) should not be compared because they are specific for each sensor (Fig. F47).
In general, core physical property data show a higher degree of variation with higher amplitudes. Reasons for these discrepancies include (1) a difference in resolution, centimeter scale on core vs. decimeter scale in logs; (2) variability in core physical properties due to sample disturbance and core diameter variations (see "Core Physical Properties"); and (3) laboratory vs. in situ conditions. Data from logs and core samples, however, provide similar physical values for the subsurface strata, despite certain limitations of both methods. This good correlation gives confidence for supplementing one data set with the other as it is done when calculating synthetic seismograms (e.g., at Site 1193) (see "Seismic Stratigraphy" in the "Site 1193" chapter).
Downhole measurements from Hole 1194B retrieved a continuous geophysical record from sediments at 84 mbsf to basement at 425 mbsf (Fig. F45). The dominant lithologies are carbonates with small admixtures of siliciclastics that show overall low NGR (see "Lithostratigraphy and Sedimentology"). Density and velocity display a general increasing downhole trend, whereas porosity values decrease as a result of compaction. However, large excursions and inversions interrupt these trends. These deviations are typical in carbonates in which early diagenesis can preserve high porosity intervals; alternatively, marine burial diagenesis can generate secondary porosity (Longman, 1981; Anselmetti and Eberli, 1993; Melim et al., 1995).
Three intervals (logging Units I-III) with characteristic log signature are observed within the logged section (Fig. F49). In addition, five horizons within these units also mar prominent changes in the log signature. These changes correlate well with the boundaries of the lithologic units, indicating that facies changes across unit boundaries produce a distinct petrophysical signal. The logs also provide information about the location of hardgrounds and facies that were not recovered. In particular, the FMS images help reconstruct a more complete succession of facies types calibrated with the incompletely recovered drilled section, as it is possible to distinguish highly bioturbated beds rich in skeletal material, bedding planes, and cemented zones. In addition, grain size differences are apparent in the coarser sediments. In the following, the character of the intervals and changes in log signature are described and correlated to the described lithology. All depths are from the log data, which differ from the curated depths used for the core.
Logging Unit I is characterized by low values in density, velocity, resistivity, and natural gamma ray values, with corresponding high porosity values (Fig. F49). This log unit corresponds to the basal 30 m of lithologic Unit II and seismic Megasequence D. The sediments consist of unlithified mudstones/wackestones (see "Lithostratigraphy and Sedimentology" and "Seismic Stratigraphy").
Logging Unit 2 is characterized by sharp and large variations in all logs and has four internal log signature changes (Fig. F49). The most dramatic of these variations marks the upper unit boundary, where all logs show a distinct peak. For example, velocity increases from 1.7 to 3.7 km/s, and density changes from 1.8 to 2.55 g/cm3. HSGR, mostly derived from uranium, increases from 15 to over 60 gAPI. This thin peak corresponds to a 1-m-thick, tightly cemented hardground bed with a reddish brown crust marking the top of lithologic Subunit IIIA and the upper boundary of seismic Megasequence B (Figs. F49, F50A) (see "Lithostratigraphy and Sedimentology" and "Seismic Stratigraphy"). At 159.5 mbsf, a similar peak in the logs occurs, indicating the presence of a second hardground bed that was not fully recovered in the core (Fig. F49). Pieces of limestone in Core 194-1194B-7R (~160 mbsf) that have high velocities of up to 4 km/s are probably from this hardground bed (Fig. F48). In the FMS images, both beds show high resistivity with a mottled structure that is indicative of intense cementation and partial dolomitization (Fig. F50). The second inferred hardground bed, however, is less resistive and seems to be slightly thicker, extending from 159.5 to 161.2 mbsf (Fig. F50B).
At 177 mbsf, a downhole decrease in velocity and density and an abrupt increase of HSGR values and increased porosity followed by large troughs with either increased or decreased values in all logs indicate another significant change in lithology. On the FMS log, this change is imaged by a cemented layer above thin-bedded layers separated by conductive intercalations with higher clay content (Fig. F51). This interpretation of increased clay content is supported by increased potassium and thorium and decreased velocity values. The top of these alternations might correlate with the recovered firmground in Core 194-1194B-9R-1 that marks the boundary between lithologic Subunits IIIB and IVA (Figs. F49, F51), where the lithology changes from coarse skeletal packstone to fine-grained carbonate richer in clay (see "Lithostratigraphy and Sedimentology"). Subunit IVA has low overall core recovery, but the logs provide evidence for changes in sedimentation. For example, at 197 mbsf the log values, in particular velocity and density, become nearly constant down to a depth of 235 mbsf. At this depth, density shifts to slightly higher values and porosity shifts to lower values downhole. A negative velocity peak and a high gamma ray peak accompany this change, indicating the influx of siliciclastic material. Another, but less pronounced, gamma ray peak and low velocity values from 247 to 254 mbsf also might be related to increased siliciclastic input at the bottom of logging Unit 2 (Fig. F49). The increased gamma ray values are in concert with the observed general increase in siliciclastic material in Subunit IVA, in particular at the bottom and the top of this subunit (see "Lithostratigraphy and Sedimentology"). The occurrence of well-cemented beds at the bottom of the subunit, as indicated by high-velocity peaks, might correspond to bryozoan-rich intervals, one of which was recovered at 230 mbsf (curated depth).
A major change in the log signature and trend occurs at 260 mbsf, where porosity shifts downhole back to higher values from 30% to 60% and density rapidly decreases from 2.18 g/cm3 across the boundary to a low of 1.7 g/cm3 at 267 mbsf. Resistivity shifts back to lower values and HSGR displays smaller fluctuations from this point on downhole (Fig. F49). This major shift in log signature corresponds to the lithologic Subunit IVA/IVB boundary and represents a change from dolomitized clay-rich packstone to partly dolomitized grainstone without clay (see "Lithostratigraphy and Sedimentology"). Below the initial large excursions at 260 mbsf, the log curves again show the expected trends caused by increased overburden pressure; porosity decreases, whereas velocity and density increase. A small negative velocity peak at 325 mbsf and a small increase in HSGR are the log expressions of the boundary from lithologic Subunit IVB to Unit V (Fig. F49). Log changes of similar amplitude, however, occur in several places in Unit 3 between 260 and 371 mbsf.
At 371 mbsf, a major change in the HSGR and velocity logs occurs that corresponds to the boundary between lithologic Subunits VA and VB. From this point to the bottom of the hole, the HSGR log decreases and displays large and regularly spaced fluctuations, indicating a cyclic repetition of facies within the measured strata. The FMS images these cycles as 5- to 10-m-thick units, each consisting of thin alternating beds of high and low resistivity in the lower part and more homogeneous bioturbated beds in the upper part (Fig. F52A). As a result of low recovery, no distinct cycles were observed in the cored sediments. The FMS signature suggests that these cycles might contain a laminated basal portion with increased clay content and a more carbonate-rich, heavily bioturbated upper part.
Only the FMS tool, mounted at bottom of the logging string (see "Downhole Measurements" in the "Explanatory Notes" chapter), penetrated 0.5 m into the basement. The top of the basement, indicated by a layer of high resistivity at 424.5 mbsf, has a steep dip of ~70° (Fig. F52B).
In summary, logs from Hole 1194B can be grouped into three logging units that correspond to the lithologic and seismic units adjacent to the NMP. Logging Unit 1 is an interval with low log values and corresponding high porosity values that correlates to the sediments of Megasequence D. Logging Unit 2 (114.5-260 mbsf) is characterized by large variations, which roughly coincide with the high-amplitude inclined reflections on the upper slope of the NMP. This variability is most likely the combined result of changes in sedimentation rates, siliciclastic content, and cementation, as is expected in pulsed, proximal-slope sedimentation. Logging Unit 3, from 260 mbsf to the bottom of the sedimentary strata, contains log signatures of low but regular variability typical of distal cyclic shelf sedimentation. Changes in log signatures in Hole 1194B correlate well with lithologic unit boundaries, indicating that facies changes across unit boundaries produce a distinct petrophysical signal. In low-recovery zones, such as in lithologic Subunit IVA, the logs document the existence of sedimentary subdivisions that were not easily discernible from the recovered sediments. For example, the lithologic Subunit IIIA/IIIB boundary was placed at 158 mbsf based on changes in the benthic foraminifer content. Log data indicate that lithologic Unit III consists of two sedimentary successions, each capped with a hardground top, equivalent to lithologic Subunits IIIA and IIIB. The base of Subunit IIIA, interpreted to be a platform facies, thus overlies the lower hardground surface.
Temperature measurements were made from 423 mbsf to the seafloor (Fig. F53). The maximum temperature at the bottom of the hole is 29°C. This is probably not the true formation temperature, because pumping warm surface water downhole during drilling probably disturbed the in situ temperature regime. The measured temperature profile translates into a geothermal gradient of 22.5°C/km, if the borehole and the geological formation did indeed come into thermal equilibrium. This geothermal gradient is lower than the one estimated by the extrapolation of the temperature points measured with the APC temperature tool (see "Core Physical Properties").