The principal objectives at Site 1188 were achieved by drilling at the Snowcap hydrothermal site. This site is characterized by extensive diffuse venting of low-temperature hydrothermal fluids. A vertical profile was delineated with mineralization and hydrothermal alteration patterns to a depth of ~400 meters below seafloor (mbsf). We found no significant occurrences of base metal sulfides or any geochemical evidence for precious metal enrichments. However, beneath an ~40-m-thick cap of unaltered dacite-rhyodacite, hydrothermal alteration was more intense and more pervasive than expected. Within this zone the rocks are very porous and highly fractured—characteristics that would facilitate diffuse fluid flow to the seafloor. The maximum borehole temperature measured was 313°C at 360 mbsf. Possible faults are indicated by uranium anomalies defined by wireline and resistivity-at-the-bit (RAB) logging. Whereas these uranium anomalies appear unrepresented in core samples, there is no evidence that they constitute major fluid pathways for the active hydrothermal system.
Using rotary core barrel (RCB) drilling, Hole 1188A was penetrated to 211.6 mbsf, although recovery was poor. While drilling, we intersected relatively unaltered vesicular dacite-rhyodacite, then at ~40 mbsf, a rapid transition to intensely altered volcanic rocks. These latter rocks continued with no further intercalations of unaltered rock to the end of Hole 1188A and then as deep as drilling penetrated in nearby Hole 1188F, which was cored with the advanced diamond core barrel (ADCB) from 218.0 to 386.7 mbsf. Recovery improved in the ADCB hole, although several significant gaps occurred in the returned core. Wireline logging and resistivity imaging with the Formation MicroScanner (FMS) of this hole from the base of casing at 195 mbsf to a blockage at ~360 mbsf provided valuable supplements to the core-based data. Logging while drilling (LWD) adjacent Hole 1188B with the RAB tool similarly aided the interpretation of structure and lithology in the top 72.0 m of the profile. An attempt to continue Hole 1188B as a cored hole with the ADCB failed because of bottom fill, but two wash samples collected at 66 mbsf were altered rocks comparable with those recovered at the same level in Hole 1188A. Uncored Holes 1188C, 1188D, and 1188E were unsuccessful attempts to spud a deep hole, which was eventually achieved with Hole 1188F. The difficulty in initializing Holes 1188C, 1188D, and 1188E suggests that the cap of unaltered dacite found in Hole 1188A is, in fact, quite extensive across Snowcap Knoll. This at least applies to areas with thin sediment mantles, which were the preferred sites for jetting in these holes rather than the locations where the vibration-isolated television (VIT) showed rock outcrops or presumed Fe-Mn oxide crusts.
The rate of penetration (ROP) for Hole 1188C increased from ~30 mbsf to where it terminated at 45 mbsf, indicating that it entered altered rocks at about the same depth as Hole 1188A. Accumulated cuttings from the running tool recovered after casing Hole 1188F and drilling its pilot hole to 218 mbsf were composed of both fresh rhyodacite and intensely altered rocks comparable with those profiled by Hole 1188A. Assuming this consistency at shallower subsurface levels below the Snowcap hydrothermal site continues at depth, it is reasonable to infer that the results of coring at Holes 1188A and 1188F may be combined into a single representative lithologic profile nearly 400 m deep, or 80% of the height of Pual Ridge.
A major conclusion from the shipboard studies of this profile is that to this depth (~400 mbsf) we are dealing with a sequence of felsic lavas and minor volcaniclastic horizons that originally erupted at or near the seafloor. Vesicles, microphenocrysts, microlitic groundmasses, perlitic and spherulitic textures, and flow banding have commonly been preserved in palimpsest despite the intensity of hydrothermal alteration. There is no textural evidence for the presence of slowly cooled intrusive units, but lacking identifiable contacts because of poor core recovery and alteration effects, we cannot rule out the possibility that subvolcanic dikes or sills are present. Whether any part of the drilled sequence might have formed as a high profile lava dome is also an open question. No useful limits can be placed on the number of lava flows in the sequence drilled. Only two unequivocal volcaniclastic horizons representing former seafloor positions were identified, although several more possible occurrences were noted and intersections of rocks with perlitic fabric also represent possible lava rinds. Seventy-two lithologic units were defined in core from the two holes combined. Being based on a mixture of igneous textural criteria and alteration characteristics, this figure by no means implies a succession of very thin flows. Dips of layering defined by flattened vesicles or flow banding vary randomly from horizontal to vertical, suggesting that both proximal and distal portions of flows were intersected.
The key geochemical indicator ratio Zr/TiO2 (Zr is measured in parts per million and TiO2 is measured in weight percent) varies between 120 and 300 in analyzed altered rocks at Site 1188. If this ratio did not change during alteration, it would suggest a dacitic (grading into rhyodacitic) parentage for the entire sequence cored. Fresh glassy and vesicular lavas near the seafloor contain scattered plagioclase and clinopyroxene microphenocrysts, and they bridge the dacite-rhyodacite compositional boundary. A content of 70.4% wt% SiO2 on a volatile-free basis in the shipboard analysis of one specimen is consistent with the refractive index (RI) measurements on glass in others.
Mineral assemblages arising from hydrothermal alteration vary with depth and are complicated by overprinting relationships. The profile commences with patchy films of opaline silica and smectitic clay with trace pyrite on fracture surfaces of otherwise unaltered dacite-rhyodacite. In a former perlitic glass at ~35 mbsf, this develops rapidly into pervasive alteration with an assemblage dominated by these same phases together with minor illite. Bleached lavas that follow downward to ~50 mbsf consist mainly of cristobalite and mixed-layer clays and contain minor disseminated pyrite. Anhydrite becomes a prominent phase together with pyrite as vesicle linings and in reticulate late veins as with associated pale cristobalite-pyrophyllite alteration halos. The latter create a concentrically zoned appearance to slightly darker wall rock kernels. From ~50 to ~125 mbsf, disseminated pyrite increases to a consistent several percent in former massive, vitric, vesicular, flow-banded, and brecciated lavas whose silicate component varies between two dominant assemblages. In both of these, there is a progressive downward decrease in cristobalite abundance accompanied by an increase in quartz. The first and apparently earlier or "background" assemblage, not obviously related to a recognizable veining event, is greenish clay with either cristobalite or quartz. Illite and pyrophyllite are the dominant phyllosilicates. The second assemblage is present in zones of conspicuous bleaching, which locally predominate over many meters. These are also composed of cristobalite or quartz together with whiter "clay" within which smectite, illite, chlorite, and pyrophyllite have been identified. The relative proportions of these phyllosilicates in the two alteration styles, and the role of anhydrite that also is present in places, require more quantitative studies than were possible on board. The rock products of both styles are characteristically very soft. Veins, stockworks, and vesicle fillings of anhydrite-cristobalite/quartz-pyrite are again common in this interval, and the same assemblage also is present in breccia matrices. Many smaller intervals of bleaching are clearly halos around anhydrite-dominated late-stage veins and are superimposed on the greenish silica-clay style of alteration (this being the chief evidence for relative timing).
Below 125 mbsf in Hole 1188A and throughout cores from Hole 1188F, the predominant alteration assemblage is siliceous but with subtle variations that define five depth subdivisions. Shipboard petrographic and geochemical evidence suggests that, relative to presumed precursors resembling the "background" green clay assemblages higher in Hole 1188A, these rocks are consistently enriched in silica. Quartz is present as vesicle linings and amygdules, as siliceous halos surrounding thin quartz-pyrite veins, and as finer groundmass grains accompanying or overprinting phyllosilicates. These rocks are distinctively harder than those of the higher zones in Hole 1188A and vary from pale gray to dark gray. In the uppermost silicified zone of Hole 1188A, quartz is accompanied by illite with subsidiary chlorite. The middle silicified zone has numerous remnants of greenish clay alteration and zones of superimposed bleaching. Chlorite and illite are the principal phyllosilicates, and some samples with chlorite contain magnetite and remnant igneous plagioclase. In the lower silicified zone of Hole 1188A, corrensite is the dominant clay species in some samples, denoting relatively high-temperature alteration.
Wireline logging with the spectral gamma tool delineated a uranium anomaly in the uncored gap between Holes 1188A and 1188F. Two subdivisions of siliceous alteration were recognized in Hole 1188F. The upper subdivision, from 218 to 269 mbsf, is characterized by illite clay and almost no relict plagioclase is preserved. In the lower subdivision (from 269 mbsf to the end of the hole), illite is accompanied by chlorite, relict plagioclase is common, and there are several intervals with disseminated magnetite. Disseminated and amygdaloidal pyrite is ubiquitous in the altered rocks of Hole 1188F, but its overall abundance falls from the upper to the lower subdivision, starting at slightly lower contents than in siliceous rocks from the lower part of Hole 1188A. Quartz amygdules are generally completely filled, lending the rocks a spotted appearance. Crystalline anhydrite or books of chlorite tend to line any open vesicles present. Especially in the upper subdivision of Hole 1188F, anhydrite-pyrite ± quartz veins are common. These are again characterized by laminated or concentric alteration halos of bleached appearance, in which pyrophyllite and quartz appear to be the major phases.
The abundance of anhydrite in veins and associated alteration halos, as vesicle fill, and as breccia matrices is a noteworthy feature of Site 1188. It also is less abundant as disseminated grains associated with clays in the different alteration styles where, however, a genetic relationship with veining rather than pervasive alteration is usually evident. Chemical analyses were performed on board on 42 altered rocks from Site 1188—mostly on homogeneous rocks but also on some with anhydrite-dominated veins or breccia matrices. Some analyses of these anhydrite-bearing rocks display elevated contents of CaO. More generally, CaO is similar to or lower than its content in fresh dacites depending on whether relict igneous plagioclase has been preserved, and Na2O shows a similar behavior. The enrichment in MgO relative to its content in likely parents is a common phenomenon, particularly in chloritic rocks. Depending on the relative abundance of illite, K2O varies from severely depleted to modestly enriched. On a relative basis, MnO is always extremely depleted. Sulfur is distinctly enriched, reflecting the occurrence of pyrite or of pyrite and anhydrite. Both enrichment and depletion of FeO occur, but some pyritic samples have similar iron contents to those of unaltered dacites and rhyodacites, which indicates that pyrite has often formed by a sulfidation process. Immobile behavior is apparent for Al2O3, TiO2, Zr, Y, and, less certainly, P2O5.
Fracturing as well as veining is a conspicuous feature of the altered rocks underlying Site 1188. Core recovered by ADCB showed an unhelpful tendency to disintegrate because of the pressure relief on a myriad of fractures when extracted from the barrel. This and the fracture patterns evident in resistivity imagery (discussed below) probably explain the low core recovery achieved in RCB drilling. No preferred orientation is evident for either veining or fracturing in cores. Single veins, branching veins, arrays of multiple veins, and vein networks are all present. The greater majority of veins are <1 mm in width, and only ~4% by number are >1 cm. Anhydrite, pyrite, quartz, and cristobalite are the dominant vein-filling minerals. Cristobalite-bearing veins were encountered only in the upper 126 mbsf of Hole 1188A, which is consistent with the distribution of silica polymorphs in pervasively altered rocks. Magnetite-bearing veins were observed in the two intervals where this mineral is also present in wall rock at 146-184 and 322-379 mbsf. Local alteration halos around veins, and the superimposition of veins, are better developed deeper in Hole 1188F compared to Hole 1188A. Anhydrite-bearing veins are generally late relative to those not containing this mineral.
Pyrite is the only common sulfide mineral in the Site 1188 cores. Marcasite, sphalerite, and chalcopyrite are present locally in trace amounts, and rare pyrrhotite was identified as tiny inclusions in pyrite from a magnetite-bearing interval. Pyrite rarely exceeds 5% in abundance and is in two settings as (1) disseminations within altered rocks of all the styles described above and as (2) generally euhedral grains in anhydrite and/or silica veins or vesicle linings. A paragenetic sequence has been unraveled involving early formation of Fe oxide minerals (magnetite, spinel, ilmenite, and hematite) that are included in or replaced by the disseminated form of pyrite, which in turn may be overgrown by sphalerite or chalcopyrite. The second kind of pyrite, in anhydrite-bearing veins and vesicles, represents the final phase of sulfide deposition.
Magnetic susceptibility varies greatly over the length of the recovered core, with highs corresponding to fresh dacite and magnetite-bearing intervals. The average compressional wave velocity for all samples from this site is 4.1 km/s; however, the more massive volcanic rocks commonly have higher compressional wave velocities than the brecciated and flow-banded rocks. Thermal conductivity is higher in brecciated rocks (average >2 W/[m·K]) than in massive altered dacite (usually <2 W/[m·K]). Average measured solid density differs somewhat between Holes 1188A and 1188F (2.65 and 2.82 g/cm3, respectively). This may reflect a higher abundance of veins (anhydrite + pyrite) in samples from the lower part of the cored interval. Porosity is highly variable, from <1% to ~45%, but with an overall trend toward decreasing porosity with depth.
The top 35 m of the cored material is characterized by high magnetic susceptibility and remanent intensity. This depth corresponds to the relatively fresh dacite-rhyodacite section at the top of the core. The magnetic measurement values show a considerable drop from 35 to ~135 mbsf, representing the high alteration in Hole 1188A. A sudden increase in both the susceptibility and remanent intensity is present below 135 mbsf. This is coincident with an increase in magnetite content. Thermal demagnetization experiments indicate that the dominant magnetic carrier in the upper part of the section is titanomagnetite with variable degrees of alteration. Deeper in the core, magnetite and possibly maghemite may be the important sources of magnetization. One of the most notable features in the magnetic character of this section is a sharp rise in susceptibility values below 275 mbsf. The high magnetization intensity at the bottom of the hole is consistent with the presence of magnetite, which was identified in hand specimen, by X-ray diffraction (XRD), and with optical microscopy.
Both direct bacterial counting and adenosine triphosphate (ATP) analysis established the presence of microbial biomass within core samples from ~34 and 49 mbsf, respectively, but not on a sample from 60 mbsf or other samples deeper in Holes 1188A and 1188F. Microbes were successfully cultivated at various temperatures under both aerobic and anaerobic conditions to 25° and 90°C from samples taken at 10 and 34 mbsf and at 25°C under anaerobic conditions from the 49 mbsf sample. Aerobic microbes were also cultivated at 25°C from very deep rock samples (222 and 225 mbsf), but these are believed to arise from seawater contamination.
Maximum borehole temperature during drilling at Hole 1188A was 4°C as recorded with the developmental Lamont-Doherty Earth Observatory (LDEO) core barrel temperature tool (CBTT). Borehole temperatures were also measured with the wireline logging temperature tool in Hole 1188F, registering 100°C a few hours after coring ended. Five days later the ultra-high-temperature multisensor memory (UHT-MSM) tool measured a maximum temperature of 304°C in the bottom of Hole 1188F. A final temperature run in Hole 1188F with the UHT-MSM tool on the last day of operations recorded a maximum of 313°C at 360 mbsf. Water samples were collected from Holes 1188B and 1188F. The sample from Hole 1188B was taken only a few meters below seafloor because of a blockage in the hole, and a concomitant temperature measurement was 6°C. Water samples also followed each UHT-MSM run in Hole 1188F, but difficulties in rapidly estimating the temperature gradient to avoid too high a temperature resulted in water samples taken at 12°C (107 mbsf) and 22°C (206 mbsf). Hole 1188B was drilled with the LWD/RAB bottom-hole assembly (BHA), and the data provide a 360° image of the borehole resistivity characteristics that might be used for lithologic correlation. Wireline logging in Hole 1188F indicated that the borehole had a much larger diameter (>17 in) than we expected from drilling with the 7.5-in diameter diamond bit. High U in spectral gamma-ray measurements was recorded between 197 and 209 mbsf, and a smaller peak was recorded between 239 and 245 mbsf.
Site 1188 is located on the Snowcap hydrothermal site, which straddles a low knoll on the crest of Pual Ridge. The location is characterized by scattered outcrops of intensely altered to fresh dacite-rhyodacite and intervening areas of sediment with patches of dark Fe-Mn oxide crust and areas covered by a white material, thought to be either microbial mat or clathrate. Previous gravity coring of the sediment showed it to be composed predominantly of disaggregated altered dacite, formed by bioturbation and/or hydrothermal fragmentation. Temperatures of 6°C were measured during submersible dives at one of the many shimmering water sites, most of which are close to the edge of rock outcrops.
The principal objectives of drilling at Site 1188 were to establish subsurface alteration and mineralization patterns, and their variation with depth, beneath this area of low-temperature diffuse venting and acid sulfate alteration at the seafloor. Other objectives included defining fluid pathway structures, testing the possible existence of "subhalative" massive sulfide layers, establishing the volcanic architecture if allowed by the alteration, and delineating the extent and characteristics of subsurface microbial life. These objectives required relatively deep penetration, 500 mbsf being aimed for with the possibility of even deeper drilling if conditions were suitable.
Leg 193 began 37 hr early when the first line was passed ashore to the Southwest Point fueling dock at 1700 hr on 7 November 2000. United States Territory of Guam Customs and Immigration authorities cleared the ship's complement that day, and we commenced bunkering activities. Refueling began at 2030 hr and was completed the following morning by 0700 hr. A total of 1398.5 T of fuel was loaded.
With tug boats secured fore and aft and the pilot on board, the last line was let go at 0750 hr on 8 November 2000 and the vessel moved to Hotel (H) Berth, Ammunition Dock. The first line ashore was at 0820 hr, which began the remainder of the Guam resupply activities. Port call activities aside from the normal loading and off-loading of freight included the rearranging of the forward drill collar rack to accommodate 6.75-in drill collars to be used with the ADCB. In addition, four additional free-fall funnels (FFFs) were put on board and three hard-rock reentry system (HRRS) cones were loaded. In the riser hold, the remainder of the HRRS hardware was loaded, including 9.5-in drill collars. The SDS Digger Tools fluid hammer and some remaining HRRS hardware were placed on the riser hatch.
There were two pacing items for the port call. One was the replacement of the number 4 generator (the last of the generators in the engine room being rebuilt) and the other was the replacement of all passive heave compensator (PHC) rod and piston seals, as well as changing out the aft piston rod because of the chrome plating deterioration. In addition, the PHC rods and cylinders were measured to document the current state of wear in the system.
Texas A&M University work items included machining HRRS parts at a local shop to accommodate the installation of a float valve. All of the HRRS and ADCB hardware that had been stored in Guam after Leg 191 was transferred to the ship for possible use during Leg 193. Hardware from Singapore and Salt Lake City, where the ADCB had recently undergone land drilling tests, was also loaded. Other Transocean Sedco Forex (TSF) work items included replacement of the forward drawworks motor and installation of a new motor on the spare cement pump. A spare motor for the top drive was also loaded on board the ship during the port call. The night before departure, Catermar hosted a barbecue on the dock for the entire ship's complement. Despite intermittent rain showers, the event was a great success and served as an outstanding icebreaker for the upcoming expedition.
Staffing for Leg 193 included several nonroutine personnel. Two Japanese engineers sailed to observe deployment of the ADCB. Three observers from Papua New Guinea (PNG) sailed as a part of the shipboard science party. An engineer from SDS Digger Tools sailed to support deployment of the hammer drill. The hammer drill engineer was replaced by another SDS employee on 14 December 2000 by helicopter transfer. Finally, a technician-in-training for the Japan Marine Science and Technology Center (JAMSTEC) sailed as a replacement of an Ocean Drilling Program (ODP) technician who could not make the cruise.
At 0800 hr on 14 November 2000, the last line was cast away. The pilot left the ship at 0836 hr, and the ship got under way at full speed for Site 1188. During the transit to Site 1188, we enjoyed fair weather the entire voyage, highlighted by an equator-crossing ceremony. During the trip the TSF crew completed the electrical installation of the forward drawworks motor and the spare cement pump motor. In addition, they continued to troubleshoot the active heave compensator (AHC) system. Also during the transit, efforts continued to resolve whether or not the ship would be required to officially clear customs and immigration into and out of PNG waters. The end result was, despite indications to the contrary, that the ship did need to clear into and out of the country, which effected, albeit not severely, our operations schedule.
Upon arrival at Site 1188, arrangements for customs and immigration clearance to PNG had not been finalized. The ship continued past the Global Positioning System (GPS) coordinates of the drill site toward a point 10 nmi off Rabaul in the hope that this would expedite the clearance process. Our continued discussions with agents in Rabaul indicated that, while we should not begin coring, operations initiated to stabilize the ship while awaiting arrival of the customs officers would be possible. Based on this information, we reversed course and returned to the GPS coordinates for Site 1188. On a side note, while en route to Site 1188 (on 16 November 2000), an earthquake (M8) was reported with the epicenter located within 20-30 nmi east of the Leg 193 drilling location.
At 1730 hr on 18 November 2000, the ship was on location and we lowered thrusters and hydrophones. The precision depth recorder (PDR) depth reading for this site was 1652.4 meters below rig floor (mbrf) corrected to the rig floor dual elevator stool (DES). After spacing out the RCB and assembling the remaining 8.25-in BHA, the AHC was tested to ensure that it was performing as designed. Although vessel heave was slight to negligible, the system appeared to be functioning correctly.
During the pipe trip to the seafloor, the VIT/subsea camera system was deployed. A positioning beacon was attached to the VIT frame to allow precise placement of the beacon near our drilling target. At 0315 hr on 19 November 2000, we began a seafloor VIT survey. The beacon was released from the VIT sleeve at 0357 hr and the survey continued until 0630 hr. The VIT was recovered and operations were secured by 0730 hr, awaiting the arrival of the PNG customs officials.
At 0830 hr the helicopter arrived carrying two PNG customs agents, two PNG quarantine officers, and two logistics agents. Authorization to proceed with operations was granted at 0845 hr on 19 November 2000, and at 0930 hr the helicopter departed with the PNG officials.
The upper guide horn was pulled and the VIT/subsea TV camera was deployed once again. After picking up the top drive, an RCB was pumped down and another hour was spent viewing the seabed for a candidate spud location. At 1200 hr a 15-min jet-in test was conducted to 2.0 mbsf, indicating that there was some soft material at the seafloor. The bit was pulled clear of the seabed, and Hole 1188A (Table T1) was spudded at 1230 hr on 19 November 2000. Seafloor depth as measured by the drill string from the DES was 1651 mbrf. The bit was maintained at the seafloor while the VIT was recovered, and at 1315 hr, continuous RCB coring began.
Coring continued through Core 193-1188A-4R to a depth of 1684.6 mbrf (33.6 mbsf) before the driller experienced high torque and the pipe stuck. After working the pipe for 45 min, it was freed with 100 kilopounds (klb) of overpull. Coring resumed with Core 193-1188A-5R after reaming back to the bottom through 5.0 m of hard fill and continued until 0900 hr on 21 November 2000, when the pipe once again stuck in the borehole while recovering Core 193-1188A-23R (to 211.6 mbsf). All efforts to free the drill string, including releasing the bit, failed, which indicated that the hole had collapsed above the bit. A severing charge was rigged up and deployed, and at 2015 hr on 21 November 2000, the pipe was severed at ~1747 mbrf (96 mbsf). After the severing charge line was removed from the pipe, we determined that the pipe was still stuck, requiring deployment of a second severing charge. At 0415 hr on 22 November 2000, the second charge was successfully detonated, severing the drill string in the 5-in pipe at a depth of 1655 mbrf (4.0 mbsf). Hole 1188A ended at 0830 hr on 22 November 2000, when the severed drill pipe reached the rig floor. A total of 21.93 m of core was recovered for an overall average of 10.36% recovery (Table T1). Recovery was erratic because of the core jamming and the fractured nature of the formation.
Chemical contamination tracers for microbiological studies were injected into the number 2 mud pump suction line during deployment of Core 193-1188-11R. A Whirl-Pak plastic bag containing fluorescent microspheres (also used as a contamination tracer) was deployed with Core 193-1188A-3R. Finally, the LDEO drill string acceleration (DSA) tool housing was used to deploy maximum-reading thermometers on Core 193-1188A-4R. The unit was deployed on top of the core barrel in the sinker bar string, just as with the DSA deployments. The circulating temperature recorded was ~4°C.
The weather while drilling Hole 1188A was exceptionally calm. Day after day, the bridge log reported no roll, pitch, or heave. The ship was parked in the middle of the proverbial mill pond, with New Ireland and New Britain (including a sporadically erupting volcano at Rabaul [see cover photograph]) in sight throughout the leg. With the drill string recovered and the severed joint laid out, the crew went about assembling a new outer core barrel assembly, spacing out the RCB system, and threading 8.25-in drill collars. While this was taking place, the ship moved in dynamic positioning (DP) mode ~0.6 nmi to Site 1189 (see "Operations Summary" in "Introduction" in the "Site 1189" chapter). For a chronological summary of the ship operations, please see "Operations Summary" and Table T1, both in the "Leg Summary" chapter.
After the initial operations at Sites 1189 and 1190 (see "Operations Summary" in "Introduction" in the "Site 1189" chapter, and "Operations Summary" in "Introduction" in the "Site 1190" chapter), we made preparations to run the LWD/RAB tool and the ship was moved back to Site 1188. The RAB tool was run with a 9.875-in tricone drill bit and a standard three-stand 8.25-in BHA. The plan was to spud a hole with the RAB tool, drill a LWD hole to ~75 mbsf, deploy a FFF if the surface sediment allowed, and then reenter with the ADCB coring system. The depth of the LWD/RAB penetration was limited to 75 mbsf to keep the top of the 8.25-in drill collars above the seafloor. By doing this, we reduced the risk of getting stuck and losing another BHA.
The morning of 25 November 2000, the second rendezvous helicopter arrived bringing lithium batteries required for the RAB tool as well as an AHC service technician from Maritime Hydraulics. After a delay while the AHC was adjusted by an engineer and concluding a brief 1-hr VIT survey of the seafloor, Hole 1188B was spudded at 1500 hr in 1653 m of water. The VIT was recovered and drilling proceeded with the LWD/RAB assembly beginning at 1615 hr.
The RAB experiment required 13.0 hr to drill to a depth of 1725.0 mbrf. During the last 4 m of drilling, the ROP slowed considerably, so we decided to terminate the LWD hole at 72.0 mbsf. The drill string was brought to 17.0 mbsf, and a FFF was rigged up and released in 1.25 hr. The VIT was deployed, and we observed the bit pulling clear of the FFF at 0945 hr on 26 November 2000. The RAB tool was disassembled, and we began preparations for deploying the ADCB coring system. The 4.5-m core barrels were spaced out, the latches adjusted, and pressure/flow tests were conducted to ensure that the proper amount of flow was passing the core shoe. Six stands of 6.75-in drill collars were made up as part of the ADCB BHA along with a diamond-impregnated bit. This bit had an outside diameter of 7.25 in and an inside diameter of 3.345 in. With the entire BHA made up and hung off at the rotary table, two wiper plugs were pumped to clear any loose rust from the inside diameter of the drill collars. In addition, we deployed and recovered a core barrel to ensure that the ADCB system was functional. The pipe trip continued without incident, and at 0100 hr on 27 November 2000, the FFF for Hole 1188B was reentered after only 37 min of vessel maneuvering.
The ADCB coring system was deployed to 1718.7 mbrf (65.7 mbsf) without much trouble. However, penetrating the 6.3 m of hard fill in the bottom of the hole proved problematic. Multiple core barrel deployments and 4.75 hr of reaming attempts with and without barrels installed resulted in no appreciable progress. Throughout the ordeal, circulating pump pressures were normal when off bottom, but ~100 psi low when on bottom. Eventually, efforts to get the ADCB system to total depth in a clean hole were abandoned. The hard rubble (identified by the recovery of two fist-sized chunks of altered volcanic rock in the ADCB wireline core barrels) prevented any viable test of the ADCB system's ability to recover core in this environment. The drill string was recovered, clearing the FFF at 1020 hr and arriving at the rig floor at 1400 hr on 27 November 2000, thus ending Hole 1188B.
While changing the BHA components after operations at Site 1191 (see "Operations Summary" in "Introduction" in the "Site 1191" chapter), the ship was offset in DP mode back to Site 1188. The next hole was planned as a reentry hole that would hopefully enable us to achieve the scientific target depth of 400-500 mbsf. There were several complications, however, to establishing a reentry structure on the seafloor. The terrain at the drill site, although relatively flat, contained patches of sediment interspersed with low outcrops of volcanic rock and platy crusts. To allow jetting in, the sediment patches were the preferred sites. Particularly for this deep hole, to have a chance at establishing a conventional reentry cone structure, we needed in excess of 3 m of sediment on the surface overlying the hard, fresh volcanic material that we had encountered near the surface at Holes 1188A and 1188B. Finally, it was obvious from early operations that drilling a hole deep enough to set surface casing was not going to be easy and might require several aborted attempts before achieving the goal. We expected to need at least 50-60 m of large-diameter casing in our initial deployment to offer the best chance of success for eventually casing to the approximate depth of Hole 1188A (211 mbsf). We only had two standard reentry cones and one each 16- and 10.75-in casing hangers, so we had only one opportunity for successful deployment of a conventional casing system.
Based on these limitations, we decided to deploy the reentry cone in an unconventional manner. We opted to free-fall deploy the standard reentry cone in the same manner as the FFF deployments. This deployment scenario, albeit unconventional, improved our chances for a successful installation. First, we could pick an acceptable drilling location (i.e., flat with sediment cover and not too close to any outcropping structures) using the subsea TV system. Then we could spud the hole with the camera down and conduct a jet-in test to determine if there was enough sediment to allow 3 m of 16-in casing to be installed beneath the reentry cone. Finally, the hole could be predrilled with a 14.75-in tricone bit to establish whether hole stability would remain acceptable and allow drilling to the required depth of ~200 mbsf (total depth of the cased interval of our planned reentry hole).
This improvisational strategy required some preparatory modification to the conventional reentry system. The reentry cone panels were assembled into two halves and were not installed on the cone base. The mud-skirt extension was also not installed. Prior to running in the hole, we moved the reentry cone base over the moonpool center and installed a 16-in casing hanger with 3 m of 16-in casing. The drill string was tripped to the seafloor through the interior of the reentry cone base/16-in casing assembly.
A 14.75-in drill bit and four-stand BHA was assembled and, tripped to the seafloor, and the subsea VIT was deployed. We conducted a brief 30-min survey of the seafloor before spudding Hole 1188C ~30 m west of Hole 1188A at 1715 hr on 29 November 2000. The seafloor was tagged at a drill pipe depth measured from the rig floor DES of 1654 mbrf, similar to the predicted 3.5-kHz PDR depth of 1652.4 mbrf. A jet-in test to a depth of 1659.0 mbrf (5.0 mbsf) was successful, indicating that there was more than adequate sediment in which to deploy the reentry cone with the attached 3 m of 16-in casing. The jet-in test, conducted at 50 strokes per min (spm) and a weight on bit (WOB) of 5 klb, was completed within 10 min. The VIT was recovered, and drilling was initiated at 1815 hr that same day.
The driller was unable to advance deeper than 44.0 mbsf, well short of our goal of 200 mbsf of large-diameter borehole. Each time the driller would stop to make a connection, he would lose 10 m of hole to fill. The VIT was deployed to observe pulling out of the hole, and at 0705 hr on 30 November 2000, the bit cleared the seafloor, ending Hole 1188C.
After surveying the seafloor for 45 min, we located another candidate drill site ~40 m north of Hole 1188C. Hole 1188D was spudded at 0815 hr on 30 November 2000, and the second jet-in test reached 3.0 mbsf (1645 mbrf). Once again the VIT was recovered, and drilling commenced at 0945 hr. In 3.5 hr, the hole was advanced to a mere 15.0 mbsf. High torque and multiple reaming operations characterized drilling parameters at this location. Ultimately, the pipe stuck and then was worked free with 200 klb of overpull. However, the circulating pressure was low and indicated a problem with the BHA. The seafloor was cleared at 1345 hr, the top drive was set back, and the pipe was tripped for inspection, arriving at the rotary table by 1630 hr on 30 November 2000, ending Hole 1188D. The mechanical bit release (MBR) had failed, leaving a 14.75-in tricone bit, a bit sub crossover, and a bit disconnect in the hole.
Another 14.75-in tricone bit and MBR was made up to the four-stand 8.25-in drill collar BHA, and the pipe was tripped to bottom. The VIT was deployed, and after a 30-min TV survey to ~30 m north of Hole 1188A, Hole 1188E was spudded at 2115 hr on 30 November 2000. The seafloor was tagged at a drill pipe depth of 1652.0 mbrf and another 3.0-m jet-in test was completed. The camera was recovered at 2200 hr, and drilling commenced. After 4.5 hr of drilling, the hole was advanced to 1668.0 mbrf (15.0 mbsf). Once again, the hole appeared to be collapsing when making connections, and drilling parameters were characterized by high torque and elevated pump pressure. The driller required 100 klb of overpull to free the drill string at depths of 15.0, 12.0, and 8.0 mbsf. Because this did not bode well for a deep hole, the bit was pulled clear of the seafloor, ending Hole 1188E at 0240 hr on 1 December 2000.
The VIT was deployed, and we spent 45 min attempting to locate another candidate drill site. At 0415 hr, we found another suitable location close to Hole 1188E, performed a 4.0-m jet-in test, and initiated Hole 1188F before recovering the VIT. At 0515 hr, drilling was initiated from a seafloor depth of 1653.0 mbrf.
From the start, the hole conditions appeared more promising than at all of the previous locations. By 2400 hr (<19 hr after starting) we had reached 78.0 mbsf, and the hole appeared stable with all drilling parameters consistent. The driller noted no excessive torque, overpull, or elevated pump pressures and had no indication of fill in the hole after making connections. By 0745 hr the following morning, the hole had been advanced to a depth of 1757.0 mbrf (104.0 mbsf; total possible depth without running the top of the BHA below seafloor). A wiper trip made it to within 15.0 mbsf and washed back to total depth. After cleaning the hole by flushing sepiolite mud, we prepared for deployment of the reentry cone. It was our opinion that the 100-m depth would be sufficient to deploy ~60 m of large-diameter casing.
The reentry cone panels (prebolted and welded into two semicircle halves) were moved onto the reentry cone base and bolted and welded together. Two snatch block sheaves were shackled on opposite sides of the cone, 180° apart. A wire rope was attached to the starboard forward tugger, run through the sheaves, and shackled to the hang-off pad eye underneath the rotary table on the aft port side. This technique was the same as that used for deploying a FFF, only we used wire rope rather than manila soft line. The cone was lowered through the splash zone below the lower guide horn, and an acetylene torch was used to cut through the wire rope. The cone was released without incident and tracked on the 3.5-kHz PDR. The cone descent was readily apparent because of the resonant ringing as it passed each tool joint. The total deployment time was just short of 9 min, and the cone was tracked at a free fall rate of 187 m/min, or ~7 mph.
Once the cone was released, the VIT was deployed and run to bottom. The cone was resting nicely on the seafloor and appeared to have much the same orientation as other reentry cones deployed in the more conventional manner.
As we expected, some loose material at the surface had fallen in on top of the bit because of the cone impact with the seafloor. However, with some overpull and back reaming, the pipe was eventually freed up enough to pull to a depth of 1659.0 mbrf (6.0 mbsf). The 14.75-in bit passed easily through the 16-in casing shoe and up into the throat of the reentry cone; however, when lowered, the bit would consistently hang up at ~2.0 m below the casing hanger shoe.
The VIT was recovered so that we could use rotation to work/ream the bit through the trouble spot. We spent the next 8.5 hr washing/reaming the 14.75-in hole to a depth of 87.0 mbsf before retrieving the drill string, which cleared the reentry cone at 0300 hr on 3 December 2000. Although the VIT was deployed to observe the pipe withdrawal, the visibility was poor because of the turbid water from the circulated cuttings and drilling mud. The VIT was recovered and the bit cleared the rig floor at 0630 hr. The tricone bit was in excellent condition with regard to the cutting structure and bearings. However, the bit was missing two of three grease reservoirs and the third reservoir fell out on deck during handling because of the damage to the upper bit shank.
After some discussion, we decided that we would increase our chances of a successful deployment of a cased reentry hole to ~200 mbsf if the borehole diameter was 14.75 in all the way to the bottom. Another 14.75-in tricone bit was made up with a long enough BHA to allow for drilling to >200 mbsf without placing the top of the BHA below seafloor. We reentered Hole 1188F at 1315 hr on 3 December 2000 and used the elongated drilling assembly to wash and ream to a depth of 91.0 mbsf. Several tight spots were identified by high pump pressure and high drilling torque. Repeated wiper trips eventually cleaned the hole to 104.0 mbsf.
At 0230 hr on 4 December 2000, drilling began on a new 14.75-in hole and continued for 17 hr until the hole depth reached 195.0 mbsf. Two 20-bbl sepiolite mud sweeps were used to flush the borehole and the drill string was pulled. The pipe cleared the reentry cone at 2140 hr, the top drive set back, and the trip continued until the BHA reached the rig floor.
Considering the difficulties we experienced cleaning the borehole, we decided to deploy the fluid hammer with a 14.75-in dual cam underreamer bit in advance of the casing to help clear any blockage. It is important to note this is not the original deployment design for the hammer, but an innovative application choreographed as a fit-to-mission strategy. The rig crew went to work assembling five joints of 13.375-in flush joint casing. A casing shoe was not part of the assembly because the available shoes were under gauge and would not have allowed passage of the bit. The casing joints were tack welded and the entire assembly was hung in the moonpool with the casing elevators on C-plates. The first deployment challenge arose when the running tool would not slip all of the way into the 13.375-in casing hanger. After inspection and measuring, we determined that the centralizing bushing below the running tool was too big to fit in the casing hanger bore. The bushing was removed, and the running tool threaded into the casing hanger. The remaining BHA was assembled and run in the hole. The subsea TV was deployed at 2000 hr on 5 December 2000, and we reentered Hole 1188F after only 10 min of vessel maneuvering.
The hammer-assisted installation of the casing string proceeded well with the fluid hammer periodically firing as the casing encountered bridges or restrictions in the 14.75-in hole. It was a tight fit with occasional overpull and very slow progress at various trouble spots in the hole. Much to our relief, at 2330 hr, with the end of the 13.375-in casing at 61.6 mbsf, the hanger landed out and 20 klb of overpull were applied to confirm latch ring engagement.
Unfortunately, all efforts to retrieve the 14.75-in underreamer bit back through the bore of the casing proved futile. The underreamer arms on the bit were supposed to fold back into the body, leaving a 12.35-in maximum diameter to pull back through the casing. After several hours of working the pipe and attempting to close the arms, the bit eventually stuck just inside the end of the casing. With no ability to rotate or circulate (without releasing from the casing), we had no choice at that point but to pull harder. At 150 klb overpull, the bit finally started to move up; however, after ~2.5 m of upward travel, the casing came free and the entire string began to pull out of the ground along with the bit. We surmised that the casing hanger latch ring had not fully engaged because the reentry cone was left at the seafloor. The drill string and casing assembly was tripped back to the ship, and by 1100 hr on 6 December 2000, the casing hanger was disengaged from the running tool at the rig floor. Because the bit would not move up through the casing, the 13.375-in casing joints had to be disassembled, stripped over the drill pipe, and laid out on the pipe racker (necessitating cutting the welds on the casing). Upon recovery of the shoe joint, we could see a deep, spiral-shaped groove inside the lower several inches of the casing as if it were threaded. After several inches, the groove geometry changed from spiral to axial grooves (Fig. F1). Our interpretation is that this change in groove patterns marked the point where the bit began to travel up into the casing before becoming stuck.
Undeterred by this setback, we replaced the 14.75-in dual cam underreamer-style pilot bit with a standard 12.25-in hammer bit, believing that this deployment would be sufficient to pilot the casing into the hole. The entire casing assembly was rebuilt and run into the hole, this time with a casing shoe that had been machined on board the JOIDES Resolution. The casing string washed in until it encountered an obstruction at 21 mbsf; however, after 5 hr, 15 min, the casing would not advance beyond 25 mbsf. Because it was clear that no further progress was likely, we pulled the casing assembly back to the rig to deploy a reaming bit and clean out the hole again. To save time and effort, the casing was hung off with slings and placed forward in the moonpool. Each of the braided slings used in the hanging operation was rated to 8 T.
At 2210 hr on 7 December 2000, we reentered Hole 1188F with a 14.75-in tricone drilling assembly. By 1000 hr on 8 December 2000 we had washed, reamed, and cleaned the hole to a depth of 75 mbsf, well below the length of the 61-m-long 13.375-in casing string. We tripped the cleaned-out bit, rebuilt the casing running assembly (including the 12.25-in hammer bit), and ran the casing back to the bottom of the hole.
Our third attempt at deploying the casing went well at first, but progress became agonizingly slow, and the fluid hammer functioned continuously to a depth of 58.9 mbsf. At that point, the hammer ceased to function (likely because of the skin friction holding the casing and open hole beneath the hammer), and all further progress of the casing string ended ~2.5 m from the required landing point.
Recognizing that the geometry of our casing assembly (out of the throat of our reentry cone and sticking up ~2.5 m above the casing hanger) was identical in concept to the design of our HRRS, a quick adaptation to the FFF and centralizer guide was all that was required to create a nested funnel system. An HRRS cone was modified slightly by grinding on the internal landing shoulders and guide dogs to ensure passage over the running tool. At 0930 hr on 9 December 2000, the HRRS cone was deployed via free fall. A single tethered glass ball float was attached to the HRRS cone to allow recovery if should it become necessary. The subsea TV was deployed, and we verified that the HRRS cone had landed properly on top of the 13.375-in casing hanger (Fig. F2). The running tool was released, and the hammer was advanced 1 m out ahead of the casing shoe to verify that everything associated with the installation was properly seated. We then recovered the drill string, which cleared the HRRS cone at 1110 hr and the rig floor at 1645 hr on 9 December 2000.
We reentered Hole 1188F at 0000 hr on 10 December 2000 after ~30 min of vessel maneuvering. Although there were several intervals of hole cleaning required, we eventually reached our target depth of 215 mbsf. A wiper trip back up inside the 13.375-in casing shoe showed no indication of hole blockage, torque, or elevated pump pressure. During what was supposed to be a final trip back to the bottom of the hole to check the status of the borehole, the drill string stuck fast. High torque and high pump pressure accompanied by high overpull indicated that the hole had started to collapse. After some tense moments, the driller was able to work the pipe free and eventually clean the hole. To convince ourselves that the hole was indeed clean and ready for deployment of our second casing string, the bit was pulled back into the 13.375-in casing shoe and lowered without rotation and minimal circulation to the bottom of the hole.
We recovered the drilling assembly and began preparations for running the 10.75-in casing string at 2315 hr on 10 December 2000. A standard cementing float shoe was attached to the shoe joint, and 13 joints of 10.75-in buttress thread casing were assembled with a standard 10.75-in casing hanger. The casing string was hung off of C-plates in the moonpool, and the subsea release assembly and 10.75-in running tool were included as part of the BHA. By 0400 hr on 11 December 2000 we were running the casing string to the seafloor.
After all our trials and tribulations to this point, the 10.75-in casing string went into the hole flawlessly. The casing shoe was cemented with 15 bbl of 15.8-lb/gal cement slurry. This is less cement than we might normally emplace, but we had experienced problems earlier in our operations when pumping mud pills that were >10 bbl. We released the pipe from the casing assembly, and the pipe cleared the rig floor at 1600 hr on 11 December 2000.
On the ensuing pipe trip, only 1 min, 7 s of maneuvering time was required to reenter the borehole. In fact, our reentry was so quick that we had to wait an additional 4.75 hr for the cement to cure before we could drill it out. It took ~4 hr to drill out the cement assembly, and by 0715 hr on 12 December 2000, we had cleaned the hole to our previous depth. To provide some initial piloting and stabilization for the ADCB coring system, we advanced the hole another 3.0 m to 218.0 mbsf and thoroughly cleaned and circulated fluid through the hole. The drill string cleared the rig floor at 1205 hr on 12 December 2000.
On our initial attempt with diamond coring, we chose to employ the 15-ft (nominal 4.5 m) ADCB for two reasons. First, it simplified core and core barrel handling (the concept of extricating 10 m of large-diameter core from a barrel without a liner seemed daunting). Additionally, the optimal operation strategy for diamond coring is to trip the core barrel whenever the throat of the bit is jammed. We expected core jamming to be a problem prior to coring even a few meters (much less for 10 m). At 1645 hr we initiated ADCB coring with slow circulation rates while the drillers and engineers monitored the drilling progress.
We intentionally kept the coring rate slow initially because the ADCB and associated 6.75-in drill collars were not supported in the larger-diameter hole. Coring parameters for the first core were 5 klb WOB, 50 rpm, 20 spm, and 500 psi. Conservative coring parameters were used for the first nine cores, or 20.3 m of advancement. Except for two empty core barrels, recovery ranged from 63% to 92% over this interval and averaged 75%. The average ROP over this period was 1.8 m/hr. Core blockages requiring barrels to be pulled early in the coring cycle caused the net coring rate to be even lower with the first 20 m of advancement, requiring a total of 28.5 hr. After establishing the ADCB-cored borehole, our emphasis was to optimize the penetration rate.
In many formations it is probable that rapid penetration and high recovery with the ADCB can be achieved simultaneously. During the short time we had available during Leg 193 for this operation, we did not have time to optimize both, so we sacrificed high recovery to improve the ROP. Over the next 22 cores, drilling parameters were varied as follows: 5-10 klb WOB, 60-120 rpm, 30-50 spm, and pump pressures of 700-1240 psi. Over this period we were able to nearly double the ROP to an average of 3.5 m/hr; however, core recovery suffered accordingly, dropping to only 10%. Of the first 31 cores recovered, there were nine with no recovery. Because most of the material we recovered was highly fragmented, it is likely that many of the empty barrels lost material on recovery. The lost materials could not be retained by the collet-style core catcher designed to capture full-diameter core. Alternative design core catchers did not appear strong enough to survive drilling through the rock we were recovering.
Continuous ADCB coring proceeded until reaching a depth of 327.2 mbsf. After deploying the core barrel for Core 193-1188F-32Z, the driller noted that the circulation pump pressures were too low, indicating that the barrel had not landed. Several attempts at deploying a core barrel were unsuccessful, so we decided to trip the pipe to inspect the BHA. We were nearly to our maximum penetration depth while still protecting the top of the BHA, so a pipe trip was pending in any event. The HRRS reentry cone was cleared at 1525 hr, and the bit cleared the rig floor at 1900 hr on 16 December 2000. Our inspection of the coring assembly indicated the landing ring had dislodged preventing the core barrel from seating. The inside and outside cutting structures of the bit were intact, but most of the cutting structure was severely worn. Several stabilizer pads were loose and/or missing from the drill collar stabilizer, which may have contributed to the bit failure.
A new BHA was assembled using all the available 6.75-in drill collars (341.7 m). We tripped the drill string and reentered the Hole 1188F HRRS reentry cone at 0211 hr after maneuvering the ship for 15 min. Once again we had difficulty in landing a core barrel, and after several more attempts, the ADCB landing seat was found wedged in the throat of the core lifter case.
Without a landing seat in the BHA, we had no choice but to pull the drill string, clearing the rig floor at 1545 hr on 17 December 2000. After repairs, we tripped the pipe, reentered Hole 1188F, and resumed coring. ADCB coring then continued through Core 193-1188F-47Z to a depth of 2039.7 mbrf (386.7 mbsf). Further penetration was prevented at that point by an apparent bit failure. A steadily decreasing ROP (<1.0 m/hr) and no recovery for the last three cores prompted us to terminate the ADCB coring. During the second ADCB bit run, recovery ranged from 17% to 48% and averaged 25.7%. The ROP ranged from 1.7 to 4.5 m/hr and averaged 3.0 m/hr. For the 15 cores cut with the second bit, the coring parameters were varied as follows: 5-8 klb WOB, 60-130 rpm, 30- 40 spm, and pump pressures of 700-1240 psi. Upon recovery of the second diamond bit, it proved to be totally worn and devoid of diamonds.
All ADCB coring in Hole 1188F was conducted without core liners using the 15-ft core barrels. Core catchers consisted of the standard collet and spring finger types. No appreciable difference in recovery was noted, although the failure of one of the finger-type core catchers may have led to the premature bit failure. Two core barrels were recovered while reaming out the borehole with no advancement and were curated as core type "G." The recovery from these cores was not used in the accounting of recovery rates. One of the primary lessons we took from this operation was how difficult it can be to optimize bit and core catcher design as well as coring parameters in variable lithologies. Further details of the ADCB deployment in Hole 1188F can be found in the operations engineer's report for Leg 193 available from ODP.
Our first ADCB diamond bit, because it was completely worn and had no value, was converted to be used as an ADCB wireline logging clean-out bit. The remaining diamond pads on the bit interior and exterior were removed with a plasma torch, leaving the bit dimensions at ~7.125 in outside diameter and 4.00 in inside diameter. The ADCB 6.75-in drill collars and fabricated ADCB logging clean-out bit were used for the wireline logging BHA in case the pipe had to be lowered to the bottom of the hole to cool the hole for logging. For details of logging runs, the reader is referred to "Downhole Measurements". One item of note, however, is that despite drilling the ADCB portion of Hole 1188F with a small-diameter bit, the calibrated calipers on the logging tools suggest the borehole diameter is in excess of 17 in throughout the logged interval. Once the logging tools and sheaves were rigged down, the drill string was pulled out of the hole, clearing the Hole 1188F HRRS reentry cone at 2115 hr on 21 December 2000.
Because the temperature measurements during the logging run in Hole 1188F indicated a high geothermal gradient (see "Downhole Measurements"), we decided to move to nearby Hole 1188B and attempt a wireline temperature measurement and borehole water sample. This hole had been undisturbed since 27 November 2000, and we hoped that we might be able to collect at least a mixture of high-temperature fluid and borehole water as well as a temperature profile that might be repeated near the end of the leg. To ensure that the borehole temperature had not exceeded the maximum rated temperature of the batteries (65°C) and electronics (70°C) of the water-sampling temperature probe (WSTP), the temperature tool was deployed first. However, we wanted to minimize the potential for disturbance of the water column in the borehole, so the UHT-MSM temperature probe was deployed and kept just inside the end of the drill pipe prior to reentry. At 2327 hr on 21 December 2000, we reentered Hole 1188B and lowered the pipe to ~4 mbsf. The end of the pipe was unable to pass that point and the UHT-MSM tool would not pass deeper than 7 mbsf, frustrating our intentions. After recovering the temperature tool, the WSTP was deployed and a fluid sample (6°C) was taken at 3 mbsf. The drill string was pulled clear of the FFF at 0315 hr and was back on board the ship by 0745 hr on 22 December 2000.
After logging operations at Hole 1189B had concluded (at ~1700 hr on 26 December 2000), we moved back to Hole 1188F (undisturbed since 21 December 2000) to run a temperature log and attempt to sample high-temperature fluids from this hole. We experienced some difficulty in rapidly converting the time/temperature data from the UHT-MSM tool, but recognized the temperature at the bottom of the hole was much higher (>300°C) than when we had measured while wireline logging (~100°C). A quick estimate projected the depth at which we expected to exceed the maximum temperature rating of the WSTP (65°C) at only 105 mbsf (well into the casing shoe). We collected a water sample (12°C) at that depth, but the disparity between our initial temperature gradient and the measured temperature at our sampling station initiated another temperature run with the wireline logging tool to see if we could better determine a maximum deployment depth (within its temperature limit) of the WSTP. The wireline tool returned a temperature of just over 100°C, with an erratic signal deep in the hole. The tool returned with the temperature sensor damaged, but the maximum reading thermometers in the cable head registered temperatures just higher than their calibration range (260°C). The differences in these readings made us disinclined to attempt another water sample (potentially jeopardizing the electronics of the WSTP) until we could determine with certainty a reproducible temperature profile. Later processing indicated that the data with the UHT-MSM tool indicated a similar gradient to the wireline tool to ~250 mbsf (see "Downhole Measurements").
Upon completing the logging operations in Hole 1189C, we attempted a second temperature measurement and water sample in Hole 1188F. Whereas the maximum temperature recorded was close to that measured in our last temperature log (>300°C), the depth vs. time file from the wireline was corrupted, making the estimation of a depth to sample within the operational limits of the WSTP challenging. We deployed the water sampler to 204 mbsf (~14 m below the casing shoe), where we expected the temperature not to exceed 60°C. A water sample was collected but the temperature measured with the WSTP was only 22°C. Because of an unexpected suspension of activities to clear a TSF employee through PNG customs for emergency departure (requiring a transit to the port of Rabaul), this would turn out to be the final operation of Leg 193.
While waiting for the JOIDES Resolution to be cleared by customs and receive permission to drill in PNG waters, we took the opportunity to conduct a VIT survey of the crest of Snowcap Knoll (Site 1188). Initially, the drill bit and VIT were deployed precisely on target, and we saw the expected patches of bright microbial mat and dark crusts on sediments. After hauling off bottom to fill the pipe, the bit was lowered to the same point then moved under DP in a cruciform pattern 60 m east, 40 m west, and 20 m north and south from the target point (Fig. F3). The traverse delineated a belt of flat dark crusts with bright microbial mat patches, trending west from the target point and to the immediate northwest of the target, a zone some 20 m across of outcropping rocks. Areas of pale sediment with only occasional patches of mat or crust were also seen. The displayed camera depth and sonar altitude were recorded manually at various points, and these indicated that the proposed site possessed two parallel northwest-trending crests a few meters high. The VIT was withdrawn to the surface prior to the customs inspection.
The sediment patches were considered preferable by the operations manager for siting a hole. We decided to lower the VIT again after drilling operations were approved, to observe a jet-in test. This was conducted near the margin of a sediment patch ~13 m southwest of the original target. The test confirmed the suitability of the site for attempting to jet-in a modified reentry cone, if necessary, after first attempting a bare-rock spud with the RCB. After raising the VIT, Hole 1188A was spudded at this location. Additional seafloor cover information was added on subsequent camera surveys.
The bit and VIT were lowered to the nominal position of Site 1188A on Snowcap Knoll, landing a little north on dark crusts. A traverse south crossed quickly into sediments and within 10 m passed a large crater (severing of Hole 1188A), then a conical depression in sediment taken to be the hole location. After a farther traverse south for 10 m across sediments with scattered crusts and mat, the bit was moved 10 m west over sediments and back again.
Additional 30- to 45-min VIT surveys were performed prior to initiation of subsequent holes. These surveys generally covered terrain mapped during the surveys for Holes 1188A and 1188B; however, in each case some new area was surveyed and the surface lithology data were added to our survey map (Fig. F3).
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