Downhole logging was performed in Hole 1203A after it had been drilled to a total depth of 914.6 mbsf. The upper 300 m of the hole had been drilled ahead, and coring started at this depth. After drilling and coring operations were completed in Hole 1203A, the borehole was conditioned with a mixture of sepiolite drilling mud and seawater and a wiper trip was conducted. A wiper trip involves pushing the pipe all the way back to the base of the hole and then pulling the bit back to the required depth. The base of the BHA was set at 206 mbsf.
Four tool string configurations were planned to be deployed in the 1203A borehole (see "Downhole Measurements" in the "Explanatory Notes" chapter), including the triple combo tool string, the FMS/sonic tool string, the GBM, and the susceptibility tool. The borehole proved to be in very good shape and gave no problems from start to finish. The wireline heave compensator was used during the runs to counter ship heave resulting from the mild sea conditions.
Logging operations began at 0830 hr on 23 July with the deployment of the triple combo tool string. It included the accelerator porosity sonde, the DLL, the hostile-environment natural gamma sonde, the high-temperature lithodensity tool, and the Lamont-Doherty Earth Observatory temperature/acceleration/pressure tool. The tool string was lowered from the bottom of the pipe to the bottom of the hole, coming out of the pipe without any difficulties. While going downhole, DLL and gamma ray counts were measured. The first triple combo pass was done from the bottom of the hole to the bottom of the drill pipe. During this first run, an interval from 396 to 321 mbsf was traversed where the density sensor voltage became unstable and provided unreliable density data. A repeat section was made in this interval to replace the density data on the main pass and to check the repeatability of the measurements.
For the second logging run, the FMS/sonic tool string including the FMS, the general purpose inclinometer tool (GPIT), the DSI, and the natural gamma ray tool was deployed. The tool string was lowered down to the bottom of the hole without difficulty and logging proceeded. The first pass recorded FMS and sonic data only in the basement section, from 914.6 to 420 mbsf. The second pass was done in the basement and in the sedimentary section. During the second logging run, the FMS/sonic tool string was lowered down to the bottom of the hole without difficulty and logging proceeded upward.
The third logging run consisted of the GBM tool. The tool required an additional weight (a standard Schlumberger weight) for safe deployment. The potential magnetic influence of this weight on the tool was tested on board and found to be minimal. The first pass was interrupted because the x- and z-components measured by the magnetometer were outside the sensing range (±50,000 nT). After adapting the tool to sense fields in the range of ±100,000 nT (an additional resistor was employed and an electronic circuit that automatically increased the sensitivity of the x-component was removed), the tool was rigged and lowered back to the bottom of the hole. During the second attempt, data were recorded while going down and up without further difficulty. The fourth logging run was devoted to the magnetic susceptibility tool. Just before the deployment the tool failed, and deployment was canceled. Logging operations ended at 0230 hr on 25 July.
Logging data recorded in Hole 1203A range in quality from poor to high. In the upper 450 mbsf, the calipers of the hostile environment lithodensity sonde (HLDS) and FMS reached their maximum aperture (18 in). Degraded borehole width affects measurements that require eccentralization and good contact between the tool and the borehole wall. Despite the large borehole size, most of the recorded parameters provide reliable results except for the DSI and the FMS data. In the sediment section shallower than 352 mbsf, the wide hole caused problems and the DSI data are unreliable. FMS data in the sedimentary section are of low quality because the FMS pads did not contact the wall of the enlarged borehole. Most of the time, only one or two pads recorded reliable data.
In the basement section, where the major objectives of Leg 197 were addressed, logging data are of good to excellent quality for most of the measured parameters. The HLDS and FMS calipers show that the borehole is enlarged in a few intervals that correspond to sedimentary or volcaniclastic interbeds (see "Results from Standard Downhole Measurements and FMS Images") (Fig. F78). In these intervals, FMS data quality is highly variable, ranging from poor to good. The DSI results in the basement are generally good with only a handful of anomalous spikes, which have been deleted from the plots (Fig. F78). For the basaltic basement, the HLDS and FMS calipers recorded a borehole diameter ranging from 10.5 to 14 in. The interlayered basalt-volcaniclastic succession is well recorded from most of the geophysical parameters (electrical resistivity, density, porosity, and seismic velocities [VP and VS]). Shipboard processing provides preliminary FMS images; these images captured most of the important features of the cores.
All the logging data have been depth shifted. Usually, the mudline is located with the gamma ray data to obtain the depth of the seafloor. In Hole 1203A, the seafloor could not be detected by the natural gamma ray gradient because of the extremely low natural radioactivity of the nannofossil ooze. However, both the base of pipe and the basement contact can clearly be identified, and both were ~4 m deeper than the equivalent driller's/core depths. Logs have been shifted to bring them in line with the sediment/basement interface in the core and base of pipe.
High recovery and good downhole measurement quality allow correlation between the logging results and the core observations. In sections with low recovery, the combination of standard downhole measurements and FMS images allows us to calculate true bed and flow thicknesses. Because of the good correlation observed between the core lithology and the logging data (Fig. F78) and to make comparison easier between the cores and the logging data, we will adopt the core lithology nomenclature (see "Physical Volcanology and Igneous Petrology"). From standard logging data and FMS images, we identified most of the 31 lithologic units of the basement section, which are based on recovered core. Furthermore, the recovered cores have undergone physical properties measurements, including density, porosity, and seismic velocity (VP). Plotting log data next to core data shows good correlation between these measurements (Fig. F78).
High contrast in petrophysical properties between basalt and sediment/volcaniclastic units allows a fairly good identification of these different lithologies. From FMS images as well as from the logging data, at least five main lithologies can be distinguished: (1) nonvolcanic sediment (from the base of the pipe to 467 mbsf), (2) volcaniclastic sediment (interbedded in basement), (3) pillow basalt, (4) massive basalt, and (5) basaltic breccia.
This sedimentary section presents fairly constant and low gamma ray values (<15 gAPI). In the upper 450 m of the section, these low values are linked to the low abundance of clay minerals in the nannofossil ooze. An exception is the interval between 388 and 400 mbsf, where total natural gamma ray counts reach 27 gAPI, mainly associated with a significant increase in Th (up to 3 ppm). At the same depth, a strong increase in both P- and S-wave velocities is recorded. These results are correlated with a so-called "388-mbsf reflector" in seismic profiles and can also be seen in the core, corresponding to high CaCO3 content (see "Lithostratigraphy"), which is related to high abundance of authigenic carbonate minerals. The high Th content measured in this zone is probably linked to the occurrence of volcanic ash (see "Lithostratigraphy").
The gradual increase in density and decrease in porosity recorded in the sedimentary section (Fig. F78) is linked to expected consolidation and compaction effects. Because the borehole diameter exceeds the maximum extent of the tool pads in these units, information can only rarely be extracted from FMS images. From standard downhole measurements and FMS images, we can easily distinguish the boundary between sediment and basement at 467 mbsf. This boundary is marked by a strong increase in resistivity, density, and natural gamma ray and a decrease in porosity (Figs. F78, F79).
Strong changes or shifts in the log data are observed for all measured geophysical parameters (Figs. F78, F79) from 457 to 915 mbsf. These variations in the downhole measurements nicely reflect the complex lithology encountered in Hole 1203A where volcaniclastic sediment is interlayered with basalt. In general, the basaltic rocks are characterized by high electrical resistivity (up to 10 
m), low porosity (<0.5%), high density (up to 2.5 g/cm3), low natural gamma ray (<20 gAPI), and high P- and S-wave velocities (up to 4 and 2 km/s, respectively). In contrast, sediment and volcaniclastic interbeds are characterized by low resistivity (~1 m), low density (<2 g/cm3), high porosity (up to 40%), and low P- and S-wave velocities (~2 and 1 km/s, respectively).
The strong differences in petrophysical properties between these lithologies allow us to distinguish the 31 basement units based on descriptions of the recovered cores (see "Physical Volcanology and Igneous Petrology"). In particular, the electrical microresistivity data record high-resolution images of the internal structure of the volcaniclastic sediments, pillow basalt, and more massive breccia units. In sections with low recovery, the combination of standard downhole measurements and FMS images allows us to estimate true bed and flow thicknesses (Fig. F80; Table T17). Basement unit numbers used in the next paragraphs are those assigned from visual core descriptions (see "Physical Volcanology and Igneous Petrology").
Nine intervals are identified as volcaniclastic sediment units from the downhole measurements (Fig. F78). Volcaniclastic sediment interbedded with basaltic units show intermediate physical properties (except for the natural radioactivity) between basalt and sediment from the uppermost part of the borehole. This results from greater consolidation and differences in lithology compared with sediment overlying basement. High gamma ray values are recorded in these volcaniclastic sediment intervals and can be attributed to alteration and/or clay-rich layers (Fig. F79). Volcaniclastic sediment units are homogeneous with respect to physical properties but cause large differences in the FMS images. Usually, pad contact is better than in the upper sediment section, and, consequently, data quality is higher, which allows us to identify the internal organization of the volcaniclastic sediment layers. Layered bedding is evident in some intervals (Fig. F81). In other intervals a patchy texture is observed. These heterogeneities are clearly related to the wide range of particle size and sorting (lapilli tuff, lapillistone, basaltic tuff, volcaniclastic siltstones, and sandstones) (see "Physical Volcanology and Igneous Petrology"). Generally, high gamma ray values are recorded in these intervals and can be attributed to alteration and/or clay-rich layers (Fig. F79).
Pillow lobes are easily recognized on the FMS images. Pillow basalt appears as circular to elliptical images of varying sizes (10-150 cm diameter). The rims are regions of enhanced conductivity compared with those of the central part of the pillows. One contrasting example is recorded between 667 and 701 mbsf (Unit 19), where interpillow material appears to be highly resistive compared to the rims and may correspond to calcite. Fractures and vesicles occur mainly as conductive features. Pillows often exhibit fractures and vesicle concentration along cooling rims as well as in the center part of the pillows (Fig. F82). Variable vesicle contents are observed along the whole section, from absent (or too small to be recorded) to high, particularly between 758 and 819 mbsf (in Unit 23, a thick pahoehoe lava flow with many cooling lobes).
A small proportion of pillow basalt appears to be highly fractured or even brecciated, showing up with angular pillow rims and with more conductive colors. So far, the occurrence of such highly fractured rocks is related only to few parts of the pillow basalt (e.g., 572-583 mbsf [part of Unit 8], 617-626 mbsf [Unit 14], or 654-666 mbsf [Unit 18]).
From 658 mbsf to the bottom of the hole, there is a clear decrease in porosity, density, resistivity, and VP (Figs. F78, F80) (from Units 19 to 31), which can be related to a higher degree of alteration.
Massive basalt mainly occurs as single lava flows in the borehole or as massive parts of pahoehoe flows, usually occurring near the base (e.g., Units 11 and 16). Flow thicknesses can range between 4 and 12 m. A typical internal structure of a massive flow, as can be inferred from the FMS images, is as follows (Fig. F83): a highly porous zone at the top of the flow caused by horizontal fracturing as well as vesicle layers followed by a massive part in the flow center. This massive part is accompanied by large vertical fractures several centimeters in width and up to 1.5 m in length. The flow bottom appears as a thin layer with a similar appearance to the top. From the standard downhole measurements, these massive units are characterized by a sharp transition in physical properties relative to the surrounding formations. Furthermore, most of the measured geophysical parameters appear to be fairly constant. Compared to pillowed units, these massive units exhibit higher electrical resistivity (reaching 1000 m) and lower natural gamma ray counts.
The interval from 610 to 616 mbsf (part of Unit 13) has been identified as a volcanic breccia (Fig. F84). This interval consists of highly heterogeneous material, with resistive material (basaltic glassy clasts) cemented in a conductive matrix (altered glass). This unit does not exhibit any fractures but shows a random distribution of variable resistivity material. From visual core descriptions, other intervals have been described as volcanic breccia (Units 17 and 28), but, unfortunately, image quality in these intervals appears to be poor.
Two tools were used to log magnetic data in Hole 1203A. The first one is the standard log in combination with the FMS/sonic tool string (GPIT), and the second one is the GBM. From the GPIT measurements (three-axis fluxgate magnetometer and three-axis accelerometer), the borehole deviation and the magnetic field are determined. These measurements are summarized in Figure F85. In Hole 1203A, the borehole deviation appears to be <1.5°.
In the upper sedimentary section (200-460 mbsf), the magnetic inclination derived from magnetic logging is ~63° (angle measured below the horizontal), which differs from the 67.9° expected from the International Geomagnetic Reference Field at this latitude. In the underlying basement, strong variations of the magnetic field were detected that correlate well with lithology. Both magnetic logs of the GPIT and GBM agree well with respect to the sequence of magnetic variations (Fig. F86). Only in a few depth intervals do both logs reveal a different shape (e.g., 580 mbsf) or show a distinct difference in magnitudes (e.g., 830 mbsf). Both logs were heave compensated, but differences of up to 2 m are occasionally seen. The z-components have a small offset from each other, which might be due to the magnetized housing and caliper arms of the tool string of the GPIT above the magnetic sensors. The logging speed of the GPIT was ~275 m/hr, and the GBM logging speed was ~300 m/hr.
The GBM tool collected data during the downhole and uphole run. This tool does not allow direct measurement of depth but records the magnetic field vs. time. The depth was obtained by comparison with the Schlumberger depth measurement. The magnetic raw data are shown in Figure F87. All data from this run are of good quality. Because of the strong magnetization of the pipe, the magnitude of the z-component of the magnetometer became saturated. Therefore, only the log data from the open hole are presented. The GPIT did not collect magnetic data in the pipe. Because of tool rotation in the borehole during lowering and raising, the horizontal components oscillate about the zero line. The anomalous magnetic field variations are obtained by subtracting the ambient geomagnetic main field (23,000 nT for the horizontal and 44,000 nT for the vertical component). The intensity of the horizontal field is derived from both horizontal components and plotted against depth (Fig. F88). The logs of the downhole and uphole run almost overlap, as can be seen by comparison of each horizontal and vertical component.
In addition to the fluxgate sensors, the angular rate around the vertical spin axis was measured using a fiber-optic gyro. The rotation history of the tool is determined by the accumulation of the angular rate during a log run. This is the first time such a sensor has been used to monitor instrument rotation in a borehole. Between the rig floor and hole bottom, the tool rotated almost 60 times around its vertical body axis (Fig. F89). On its run back to the rig floor the tool followed nearly the same rotation history as on the downhole run. A check of the direction indicated by the fiber-optic gyro after the run revealed only minor deviations from the initial orientation.
At 200 mbsf the tool emerged from the drill pipe into the open hole. Between the end of the pipe and 460 mbsf, only minor variations of the magnetic field with depth were recorded. These variations can be attributed to weakly magnetized sediments (Fig. F88). In the basement below, strongly magnetized layers were detected, which correlate well with sequences of massive basalt and pillows. These stronger magnetizations are interrupted by intervals of volcaniclastic sediment. The anomalous field variations of the vertical component always point toward negative values, which indicate a general normal magnetic polarity as represented by the present geomagnetic field. The strength of the recorded vertical or z-component of the anomalous field (relative to the horizontal anomalous field) suggests that in the basalt section the combined remanent and induced magnetic field has an inclination >45°.
In general, the magnetometer tool rotated uniformly during the run but increased its rotation rate with increasing cable length. Because of the friction on the borehole wall, this long-wave rotation is modified by short-wave variations. Strong deviations from the uniform spinning occurred in the pipe at 2300 and 2700 meters below rig floor. Before starting the log run, the magnetic components of the tool were aligned with the ship axis on the rig floor. After the tool returned to the rig floor, it was aligned again to double check the orientation with the initial orientation. This check is required to take into account changes of the heading of the ship and the Earth's rotation, which also adds to the angular rate (see "Downhole Measurements" in the "Explanatory Notes" chapter). The difference between the initial and final orientation value is <5°.
