m within the sediment (minimum = 0.26 m), increasing to average values of 21.09 m (maximum = 48.33 m) in the basement.
The downhole logging program during Leg 203 was comprised of measurements from Holes 1243A and 1243B. Measurements in Hole 1243B were performed first, using the triple combo tool string, the FMS-sonic tool string, and the WST. Two runs were performed with each of the two tool strings and one for the WST. After logging had been completed in Hole 1243B, the ship moved back to Hole 1243A. Logging in Hole 1243A comprised measurements of hole deviation with the General Purpose Inclinometer Tool (GPIT) and cement bond quality with the cement bonding tool (CBT). Although the GPIT was lowered for only one run, the CBT was deployed three times.
The first part of the Leg 203 logging program was performed in Hole 1243B, after it had been drilled to a total depth of 195.2 mbsf. Pack-off (PO) zones had been detected during drilling and coring at depths of 142, 159, and below 175 mbsf. Below the last PO zone (175 mbsf), we experienced problems because the lower portion of the hole filled with cuttings. Drilling/coring was stopped because of concern over a possible hole collapse. The bottom of hole (BOH) was established at 184 mbsf. After drilling and coring operations were stopped in Hole 1243B, 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 rotary core barrel bit was released at the BOH, and the base of the bottom-hole assembly was pulled up to 79 mbsf (drilling depth). The hole was then ready for logging.
Logging in Hole 1243B started at 1600 hr on 23 June, with wireline rig up. The first tool string (triple combo) was lowered at 1830 hr, passed the end of pipe (EOP) at 82 mbsf (logging depth), and reached the BOH at 2040 hr without difficulties. The depth of the borehole was determined to be 181 mbsf (logging depth; 4049.0 meters below rig floor [mbrf]). The first run was performed logging upward at a speed of 900 ft/hr to the seafloor. The tool was then lowered to the BOH, and a second run was performed logging upward at a speed of 900 ft/hr. Upon completion of the second run, logging with the triple combo tool ended, and the tool string was pulled up, reaching the rig floor at 2345 hr. The tool string was rigged down at 0045 hr on 24 June.
The second tool string (FMS-sonic) was rigged up and lowered downhole at 0145 hr on 24 June. The BOH was reached at 0330 hr, and the tool string was prepared for the first run. The previously determined the BOH logging depth (181 mbsf) was confirmed by this run. The tool string was pulled up at a constant speed of 900 ft/hr until it reached seafloor. The tool was then lowered to the BOH, and a second run was performed at a speed of 900 ft/hr. Logging was completed when the tool string reached the depth of the seafloor. The tool string was then pulled up and reached the rig floor at 0630 hr. The tool string was rigged down by 0715 hr.
The WST was the next tool deployed in Hole 1243B. The ODP generator-injector (GI) air gun (Sodera 210-in3 Harmonic) was used as the source. The air gun was lowered ~2 m into the water at a 55.8 m offset from the hole, and two hydrophones were attached (one at the source and one below the source). The WST was rigged up and lowered downhole at 0800 hr. It reached the BOH at 1120 hr. Because of spikes on the caliper arms and the small inner diameter of the drill pipe, the WST required a slower speed and closing of the caliper arms many times during descent. Several test measurements (~30-40 shots) were conducted in the drill pipe (at ~1150 mbrf) as well as at the BOH. During these tests, the lower hydrophone gave a bad response, whereas the upper hydrophone gave a very good break signal. Full measurements were performed at station intervals of 10 m beginning at the BOH; each station comprised 10 shots on average. Eight stations within the basement section gave excellent results with high-quality data. The last station of the basement section was at 3985 mbrf (117 mbsf, close to the sediment/basement interface). One test shot within the sediment section at 3975 mbrf (107 mbsf) was attempted but yielded poor results because of poor tool contact caused by large borehole diameter. The wireline tension indicated that the tool was slipping downward during attempts to couple the tool to the borehole wall within the sediment section. Consequently, the WST survey was terminated, and the tool was pulled up to the rig floor and rigged down by 1600 hr.
The GPIT was the first tool deployed in Hole 1243A. The tool was rigged up and lowered at 2130 hr. It passed the EOP at a depth of 3967 mbrf (drillers depth = 84.6 mbsf) and reached the BOH at 2300 hr. The logging depth to the BOH was determined to be 4088 mbrf (205.6 mbsf). One measurement uphole was performed at a speed of 3600 ft/hr and completed at 3911 mbrf (28.6 mbsf). The tool was pulled up to the rig floor and rigged down by 0045 hr on 25 June.
The CBT was deployed in Hole 1243A to measure cement bond quality. The tool was rigged up and lowered at 0115 hr. The tool reached the BOH at 0330 hr and was pulled up at a speed of 3000 ft/hr. It passed the EOP at a logging depth of 3968.5 mbrf (86.1 mbsf; determined by an increase in gamma ray activity and an increase of transit time from pipe to casing). A second run was performed at the same speed after the tool had been lowered to the BOH. After the second run was completed and the tool was lowered to the BOH, a third run was performed uphole at a speed of 2000 ft/hr and stopped at the EOP. All three runs confirmed the BOH at 4088 mbrf (205.6 mbsf). The tool was then pulled up to the rig floor and rigged down by 0600 hr, at which time logging was completed. The wireline was rigged down at 0800 hr 25 June.
The GPIT device recorded only the borehole deviation reliably. Because of the magnetization of the drill pipe and the casing, the magnetometer-derived inclinometry measurements were recorded but may be inaccurate.
The casing bond log (CBL) as well as the variable density log (VDL) showed excellent results within the entire cased hole. The data can be used to determine the depth to the BOH, the top of cement, and, to a certain extent, the bond quality. The starting depths of the three CBT runs were 3908.5 mbrf (26.1 mbsf) for the first run, 3971.4 mbrf (89.0 mbsf) for the second run, and 3970.5 mbrf (88.1 mbsf) for the third run.
The deployment of the triple combo, the FMS-sonic, and the WST recorded downhole data of excellent quality and very good repeatability. Despite hole filling at the BOH and breakout zones between 158 mbsf and the BOH, the borehole proved to be in very good shape and gave absolutely no problems from start to finish of the logging operations. The deployment of the Lamont-Doherty Earth Observatory Temperature/Acceleration/Pressure tool yielded no results, probably because of an internal loss of electrical contact between the sensors and the recording devices. Loss of electrical contact could have occurred at any time between tool closure and opening.
In general, logging data recorded in Hole 1243B range from poor to high quality. In the upper 114.5 mbsf (sediment section), the calipers of the Hostile Environment Litho-Density Sonde (HLDS) and FMS reached their maximum aperture (18 in). Degraded borehole width affects measurements, which 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 Dipole Sonic Imager (DSI) and the FMS data. In the sediment section, the wide hole caused problems and the DSI data are unreliable. The DSI recorded low-frequency and high-frequency dipole shear in addition to compressional (P)- and shear (S)-wave monopole. First motion detection (FMD) was attempted, but no data were obtained for unknown reasons. It appears that the P- and S-wave monopole and dipole high-frequency tools provided adequate response in the basement section, but the sediment section did not yield any good results. There appeared to be a response in this sediment zone, but it was most likely a tool wave phenomenon observed only at higher gains. The response was simply too rapid to be realistic in such a slow sediment, and the caliper data show that the hole was too large. The high-frequency dipole and P- and S-wave modes were logged, and the data compared well to the first pass. FMD was again attempted, but no usable data were obtained. 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, the HLDS and FMS calipers confirmed that the borehole is enlarged in a few intervals (see "Results of Log Interpretation"). In these intervals, FMS data quality is highly variable, ranging from poor to good. In the basaltic basement, the HLDS and FMS calipers recorded a borehole diameter ranging from 9.875 to 12.5 in. Shipboard processing provided preliminary FMS images; these images captured most of the important features of the cores.
HLDS density measurement clearly detected the seafloor at 3868.0 mbrf. DSI measurements indicated seafloor at 3866 mbrf (Fig. F36), 2 m higher than that detected by the HLDS tool. Above 3866 mbrf, the response from the monopole transmitter-receiver pairs show a fast waveform with a slowness of ~50-55 µs/ft, probably caused by the drill pipe itself. The next significant amplitude recorded exhibited a slowness of ~160-170 µs/ft and is most likely the S-wave traveling in the pipe. The slowest wave, with a range of ~200-210 µs/ft, ends at 3866 mbrf, which is most likely the seafloor interface. This very slow waveform, with the velocity of water or mud, could be the response from the seawater behind the drill pipe. As soon as the environment changes to formation around the pipe, this waveform disappears.
A possible mismatch in depth determination of seafloor between the different logging runs has an impact on the measurements. Differences in depth determinations can be related to tool setups as well as to variable cable elongations. To detect these depth mismatches, data from the gamma ray tools (Hostile Environment Gamma Ray Sonde and Natural Gamma Ray Spectrometry Tool) are used to match the various runs with each other. For a first rough estimation, the FMS-sonic data of both runs have been shifted downward, using a constant value of 0.8 m.
Logging in Hole 1243A comprised measurements of hole deviation (GPIT) and cement bond quality (CBT) (Fig. F37). The GPIT recorded a hole deviation that does not exceed 1° from the vertical. All three CBL runs indicated a bond/no bond interface at 4037 mbrf (154.6 mbsf) and confirmed the BOH at 4088 mbrf (205.6 mbsf). Bond quality is indicated by the amplitude response of the CBT—lower amplitudes are consistent with good cementing quality. However, low amplitudes can also be produced by other effects, such as tool decentralization or casing collars/joints. Good bond, as seen in relation to no/poor bond areas, is indicated in the following depth ranges: 4040-4045 (157.6-162.6 mbsf), 4058-4063 (175.6-180.6 mbsf), and 4071-4079 mbrf (188.6-196.6 mbsf). Because higher-amplitude readings can be observed periodically, these areas might also be a result of the influence of the casing joints. Thus, the quality estimations have to be regarded with care. Observations from VDL readings, which "look" deeper into the formation, might help to distinguish between artificial effects (e.g., casing joints) and "real" bond quality. Readings below 4083 mbrf (200.6 mbsf), although indicating a good bond zone, have to be interpreted with caution because the tool starts recording reliably a few meters above the lowermost logging depth.
To reconcile the difference between the logging depth and the core depth, the core derived lithology has been shifted 6.3 m downward.
The sediment/basement interface was clearly detected at 114.5 mbsf (3982.5 mbrf) in most of the wireline logs. Porosity, density, and resistivity data show the expected changes as soon as the logging tools pass the sediment/basement interface (Figs. F38, F39). In general, the porosity decreases from average values of 89.2% (±9.6%; maximum = 100%) in the sediment section to average values of 14.8% (±4.6%; minimum = 6%) in the basement. In turn, density and resistivity increase. Density increases from average values of 1.50 g/cm3 (±0.10 g/cm3; minimum = 1.11 g/cm3) in the sediments to average values of 2.59 g/cm3 (±0.25 g/cm3; maximum = 3.07 g/cm3) in the basement. Resistivity shows very low average values of 0.59 m within the sediment (minimum = 0.26 m), increasing to average values of 21.09 m (maximum = 48.33 m) in the basement.
No abrupt transition from the sediment to the basement section is evident in the total gamma ray profile (Fig. F38). Within a depth range from 82 to 140 mbsf, gamma ray measurements with an average response of 5.73 American Petroleum Institute gamma ray units (gAPI) only showed small variations of ±2.96 gAPI. The maximum value in this section of 11.1 gAPI was reached at 130 mbsf. This transition points to formations that are not affected by alteration. This was confirmed by core observations (see "Petrology and Geochemistry"). The gamma ray response appears to be controlled mainly by the potassium content in the formation. Thorium and uranium show minor variations and are roughly anticorrelated.
In the transition from the sediment to the basement section, the amplitude of the sonic waveform (Fig. F40) in dipole acquisition mode (DAM) shows a decrease in slowness from 180-200 to 140-70 µs/ft in the basement. The sonic waveforms, as inferred from the monopole acquisition mode (MAM) show only minor response, with low coherency in the sediments and often in the basement as well. S-waves recorded in this mode yielded poor data in general. The end of the mud wave, from the sediment into the basement, can be regarded as the only qualitative indicator of the beginning of the basement. The first qualitative estimate of P-wave velocity was obtained at a depth of 124 mbsf (3992 mbrf).
The detection by core lithology of a thin sediment layer below the first basalt unit (see "Sedimentology") at depths of 116-117 mbsf (logging depth) is supported by the downhole porosity and density data. Within this zone, porosity increases slightly to maximum values of 28.9% and density decreases to a lower average of 1.96 g/cm3 (minimum = 1.73 g/cm3) compared to the surrounding basalts. Again, gamma ray response does not seem to be affected by this interlayer. Images derived from FMS data (Fig. F41) at a depth of 115.4-116.7 mbsf show a structure that apparently differs from the typical shapes of pillow basalts. This "cloudy" structure suggests the presence of a sediment interlayer.
Below 141.5 mbsf (Fig. F42), the gamma ray response increases rapidly up to average values of 18.9 gAPI (±1.6 gAPI; maximum = 23.0 gAPI). This change correlates with a unit change in core lithology from aphyric (Unit 3) to sparsely olivine-plagioclase phyric basalts (Unit 4) (see "Petrology and Geochemistry"). A small offset in shear wave velocity (DAM) and in mud wave (MAM) at this depth (Fig. F40) indicates the beginning of a new unit. In contrast, the P-wave as well as the S-wave (MAM) velocities show a continuing incoherent response of poor quality. Only the P-wave recording begins with a more or less coherent reading, after a small offset in amplitude between 138.8 and 140.7 mbsf (4006.8 and 4008.7 mbrf). FMS images of this transition zone (Fig. F43) reveal pillow basalts that are highly fractured and partly vesicular (top of Unit 4), allowing for the detection of a change in lithologic unit.
The increase in total gamma ray at 141.5 mbrf correlates with strong increases in potassium and thorium (Fig. F44). This change can be related to the beginning Unit 4. Uranium remains low (~0.1 ppm). The subsequent decrease of potassium (<0.6 wt%) at 144.5 mbsf correlates with an increase in uranium (~0.7 ppm), whereas thorium appears to experience a slightly retarded decrease. Potassium remains low until 155 mbsf, with values of ±0.6 wt%. In that zone, thorium and uranium, partly alternating with each other, are the inferred elements in the total gamma ray reading. Below 155 mbsf, the uranium response appears to drop below the minimum detector threshold, whereas potassium increases again, reaching its maximum of 1.2 wt% at 159.2 mbsf. However, these effects are most likely related to secondary changes within the basalt succession.
