Nine measurements of in situ temperatures were made at this site: five with the APCT tool and four with the DVTPP (Tables T17, T18, T19). Four of the APCT tool runs resulted in temperature histories that showed clear penetration and extraction pulses and smooth temperature decay (see "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter). A fifth APCT tool run turned on prematurely and ran out of memory prior to recovery but recorded long enough to provide a good temperature measurement (Core 204-1251D-20H). Raw data are shown in Figure F34. Only data from the immediate time period before, during, and after tool insertion is shown. APCT data were modeled using the software program TFIT (as described in "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter) measured thermal conductivities (see "Physical Properties"). Uncertainty in the extrapolated value of in situ temperature resulting from subjective analyst picking of tp, ti, and tf is <0.02°C for these high-quality records. Uncertainty resulting from uncertainty in values of thermal conductivity is ~0.02°C. Additional uncertainty results from uncertainty in instrument calibration (see "Downhole Tools and Pressure Coring" in the "Site 1246" chapter).
The DVTPP temperature data do not show the "textbook" response observed during the first deployment (Fig. F34B). Moreover, the run at 155.6 mbsf was noisy and showed an unrealistic value of 6.0°C for the mudline temperature (measured at the end of the run), probably reflecting a calibration error for DVTPP tool 3. The DVTPP measurement at 198.6 mbsf yielded an anomalously low temperature value compared to the other data (Fig. F35). These two data points were not included in the determination of temperature gradient. The other two DVTPP deployments yielded temperatures consistent with the APCT data, suggesting that good-quality temperature measurements can be taken in spite of poor-quality pressure records. Moreover, the data suggest that there is no significant change in temperature gradient at the BSR, although this is not well constrained.
Least-square linear fit temperature gradients were calculated for different subsets of the data, excluding the two outliers. The solution is not sensitive to the inclusion or exclusion of the DVTPP data or to the mudline temperature estimate. The solution shown in Fig. F35 is for the combined data set. This temperature gradient predicts that the BSR should be at 202 mbsf, which is not significantly different from the BSR depth of 193 mbsf indicated by seismic data, given uncertainties in the velocity used to obtain the estimate of BSR depth.
Four in situ pressure measurements were attempted using the DVTPP. The signals from these measurements do not follow the expected decay patterns, suggesting problems with insertion of the probe. Interpretation of these measurements will be the object of postcruise research.
The PCS was deployed nine times at Site 1251 (Table T18). Eight of these deployments were successful (i.e., a core under pressure was recovered). Only water was recovered during the other deployment because the tool actuated prematurely. The main objectives of the deployments were (1) to construct a detailed profile of concentration and composition of natural gases in the upper part of the section (0-300 mbsf) and (2) to identify the presence/absence and concentration of gas hydrate within the GHSZ.
Specific depth intervals were targeted for deployment of the PCS. Six cores (Cores 204-1251B-12P [104.1-105.1 mbsf] and 18P [153.6-154.6 mbsf]; 204-1251D-6P [45.9-46.9 mbsf], 10P [76.4-77.4 mbsf], and 21P [173.4-174.4 mbsf]; and 204-1251G-2P [20-21 mbsf]) were recovered from above the BSR at ~193 mbsf. Successful retrieval and degassing of Core 204-1251G-2P suggests that the PCS can be deployed at shallow subseafloor depths. The other two cores (Cores 204-1251B-35P [290.6-291.6 mbsf] and 204-1251D-29P [227.5-228.5 mbsf]) were recovered from below the BSR.
The time to degas the PCS chambers ranged from 672 to 1567 min (Table T18). Pressure was recorded during degassing experiments (Fig. F36). Gas was collected in a series of sample increments (splits), and most were analyzed for molecular composition (see "Organic Geochemistry"). In addition, gas splits were subsampled for onshore analyses. After degassing, the PCS chambers were disassembled. The lengths of the cores were measured (Table T18), and samples were taken for analysis of physical properties (see "Physical Properties").
Gas was collected in 5- to 720-mL increments. The measured incremental and cumulative volumes are plotted vs. time (Fig. F36). The cumulative volume of released gas varies from 1320 (Core 204-1251G-2P) to 3365 mL (Core 204-1251B-12P) (Table T18). The volume of the last gas splits varies from 5 (Core 204-1251B-12P) to 20 mL (Core 204-1251D-6P). This observation suggests that almost all gas present in the cores was collected.
Gases released from the PCS are mixtures of air (N2 and O2), CH4, CO2, and C2+ hydrocarbon gases (see "Gas Hydrate and Pressure Cores" in "Organic Geochemistry"). The abundance of air components in the PCS gas samples (2.7%-53.2% of gas mixtures) suggests that air was not properly displaced from the PCS by seawater during deployments. Methane is the dominant natural gas present in collected gas splits. The molecular composition of gases from the PCS is similar to the composition of gas voids at adjacent depths (see Fig. F21).
Sediments in cores recovered with the PCS have lithologies that are similar to sediments recovered by the APC at adjacent depths (see "Physical Properties"). Porosity values measured on samples from APC cores taken near the PCS were used to estimate the methane concentration in situ (Table T18).
The concentration of methane in situ was estimated based on data from the degassing experiment (i.e., total volume of methane) and core examination (i.e., length of recovered core and the porosity of sediments). The calculation yields equivalent concentrations varying from 46.9 to 157.6 mM of methane in pore water. These concentrations have been compared with the theoretical methane-solubility curve extrapolated from values calculated for higher pressures (depths) (Handa, 1990; Duan et al., 1992) (Fig. F37).
Preliminary analysis of gas concentrations suggests that gas hydrate may have been present in small concentrations (<1% of pore volume) in Cores 204-1251B-12P and 204-1251D-6P. In addition to relatively high gas concentrations, evidence of the presence of gas hydrate was found in the pressure record of core degassing (Fig. F36). However, other cores retrieved from the GHSZ, including Core 204-1251D-21P recovered from ~173.9 mbsf, just above the gas hydrate-bearing intervals inferred from IR thermal anomalies in Cores 22X and 24X at depths from 175 to 191 mbsf, suggest that only dissolved methane is present in some intervals within the GHSZ. A high concentration of methane in Core 204-1251B-35P may indicate the presence of free gas in the deep subsurface at ~291.1 mbsf (~100 m below the BSR), but only dissolved methane appears to be present in Core 204-1251D-29P, recovered from ~32 m below the BSR. Additional comparison of measured methane concentrations with theoretical methane solubility above and below the BSR will be performed on shore to better estimate if methane was present in situ in solution, in free phase, or as gas hydrate.
Five deployments of the HYACINTH pressure coring tools were made at Site 1251: three with the FPC and two with the HRC (Table T19). These were the first deployments made during Leg 204 and were primarily considered to be engineering tests. One of the most important successes was the improvement in the handling of the tools and the downhole procedures compared with tests that had been conducted during previous legs (Legs 194 and 201). As a result, the total rig time used per deployment was limited to about 1.5 hr for each tool at this water depth (1224 m). A number of minor technical and operational issues arose with each tool during the tests, which were addressed at each stage during the testing program. The final outcome at the end of Site 1251 was that the HRC had recovered a short 22-cm-long core (22% recovery) under pressure (but lower than in situ pressures) and the FPC had recovered good cores, 70-80 cm in length (70%-80% recovery), but without retaining pressure. Unfortunately, the DSA tool failed to provide any downhole data during the coring operations, which limited the ability for downhole performance analysis. However, the rig floor data will prove valuable in analyzing some aspects of the behavior during each deployment. The transfer chamber system was tested, but expanding cores and coring-related problems prevented a satisfactory transfer.
The first two HRC deployments, Cores 204-1251B-48E (HRC 1) and 204-1251D-30E (HRC 2), were made at Site 1251, where the BSR is at 196 mbsf and which coincided with the transition between the use of the APC and XCB (194.6 mbsf) in Hole 1251B (see Table T19). This is relevant because it has been generally proposed that the type of material that is best suited for the HRC is at the upper limit of shear strengths that can be cored using the APC.
Core 204-1251B-48E (HRC 1) was collected in Hole 1251B at 396.9 mbsf, where the clay sediments are well indurated and the mechanical properties were considered well suited for the "dry auger bit" used by the HRC. Shear strengths in recovered cores from this depth are well in excess of the maximum shear strength that can be measured with the hand-held Torvane (250 kPa). Although large amounts of gas expansion were occurring in shallower parts of the hole, by this depth these expansion effects had almost disappeared and recovery with the XCB had been generally very good.
The tool was assembled for the first time on the pipe racker in 3 hr and fitted with the new strongbacks (one of these had been slightly shortened for easier handling on the rig floor). It was raised into a vertical position, and the strongbacks were removed as it was lowered into the shuck where the DSA tool was attached. This handling procedure (using the strongbacks) only took about 20 min and was a significant improvement compared to the last engineering tests of the tool during Leg 194, when it was assembled in a vertical position. It was then lifted into the drill pipe and lowered on the wireline while circulating and rotating. The tool was lowered onto the landing shoulder when the drill string rotation and circulation had been stopped, and slack wire was payed out. Problems with cleaning the hole resulted in lifting the tool out of the landing position at one time during the procedure. Pumping began slowly to build up pressure to activate the tool and continued at 93 gallons per minute (gpm), which causes the core barrel to rotate and cut core. However, a final pressure spike was not observed (even after 18 min) that would have indicated that the full stroke had been reached. Pumping was stopped while the drill string was lifted and continued for 2 min afterward to ensure full stroke. The tool was then raised on the wireline (slowly for the first 16 m) to the surface. The DSA tool was removed, and the HRC was broken into three vertical sections vertically and then taken aft to the pipe racker and laid on the trestles. On examination of the autoclave chamber, it was found that the core liner had broken (no core recovered) and that the by-pass port, which gives the indication of end-stroke, had not opened. The evidence suggests that the tool may have been activated while it was still in the hanging position prior to landing at TD.
A second deployment of the HRC was planned at the bottom of Hole 1251B, but the hole was abandoned after poor hole conditions developed at around 445 mbsf. The HRC was again deployed at this site, Core 204-1251D-30E (HRC 2), at a depth of 229.5 mbsf. Although the lithology was softer at this depth compared with the material at ~400 mbsf, it was still considered useful (although not ideal) as a test for the HRC rotary cutter. The test proceeded smoothly with the handling on the rig floor being the same as for the previous deployment. This time there were no hole cleaning problems and pumping began after lowering to TD with the wireline slack. Active heave compensation was used throughout. Pumping was continued at 100 gpm for 15 min without observing any significant pressure spikes. The drill string was raised off bottom and a further 2 min of pumping (92 gpm) should have ensured full stroke of the coring barrel. The tool was then raised on the wireline slowly for the first 16 m and then continued at 110 m/min before being broken out of the drill string at the rig floor and placed in the adjacent shuck. The DSA tool was removed and the strongbacks attached before the tool was transferred to the trestles on the pipe-racker rather than being disassembled vertically. This procedure saves about 40 min compared with the vertical disassembly procedure.
This time the barrel had fully retracted, the flapper valve had closed, and the pressure was measured at ~20 kbar (much less than in situ pressure). However, the core got stuck during the transfer process and had to be removed manually. A 22-cm-long core (recovery = 22%) was recovered, but sediment smeared on the inside of the liner indicated that the majority of the cored material had not been retained in the barrel during retrieval, indicating that this type of catcher is not suitable for these soft formations. A leak was found later in the upper part of the autoclave section, which explained the loss of high pressure inside the chamber.
The first three FPC deployments, Cores 204-1251B-21Y (FPC 1) and 40Y (FPC 2) and 204-1251D-28Y (FPC 3) were made at Site 1251 in a water depth of 1224 m. Core 204-1251B-21Y (FPC 1), at 171.7 mbsf, was in lithologies that were still suitable for the APC (XCB coring began at 194.6 mbsf) (see Table T19) and where large amounts of gas expansion were occurring. Shear strengths measured with the hand-held Torvane in Core 204-1251B-22H were ~160 kPa (see "Physical Properties"), although it should be noted that in situ strengths are likely to be higher, perhaps by up to a factor of 1.5. Operationally, the deployment ran smoothly with the active heave compensator being used throughout (including pull-out). The total time taken for the complete operation was 1.5 hr.
On recovery, it was apparent that the coring mechanism had undergone a full stroke but had not fully retracted into the autoclave. However, a good-quality 0.71-m-long core had been cut (recovery = 71%). Under normal circumstances of autoclave recovery, the core would have been cut free from the piston under full pressure in the shear transfer chamber. We attempted to remove this core from the piston with a hacksaw; however, after one cut the core liner exploded violently, leaving a pile of shattered liner and sediment on the deck. Obviously the liner was under significant stress from the gas expansion, with the piston and the lower part of the core providing good seals. As the liner was first cut, a stress concentration probably occurred in the liner, allowing the catastrophic failure to propagate. An analysis indicated that a latching mechanism may not have been operating smoothly, which prevented the core from fully retracting into the autoclave. This was modified prior to the next deployment. Both the FPC and the DSA data loggers stopped prematurely and failed to capture data during the operations on the bottom.
A second deployment of the FPC was made in Hole 1251B at 329.6 mbsf, Core 204-1251B-40Y (FPC 2). At this depth, the sediments had not changed significantly in character apart from being more indurated. Shear strengths were outside the range of the hand-held Torvane device (>250 kPa). The operational procedures were the same as for Core 204-1251B-21Y (FPC 1) and again went smoothly, taking only ~1.5 hr. On recovery, it was apparent that this time the coring mechanism had fully retracted into the autoclave. However, a visual examination revealed that the lower flapper valve (which seals the lower end of the autoclave) had not fully closed, and hence, the autoclave was not pressurized. Subsequent investigations revealed that the flapper valve was unable to seal because of debris that fell out of the retracted liner onto the valve seating. We also discovered that the core liner had imploded. This was probably caused on pull out as a result of not fully stroking into the sediments. Unfortunately, both the FPC and the DSA data loggers again failed to capture the test interval and, hence, could not provide any diagnostic information about the tool's behavior.
The final FPC deployment at this site, Core 204-1251D-28Y (FPC 3), was made at the bottom of Hole 1251D at 226.5 mbsf. All operations were as for previous deployments except that the driller observed a 100-psi increase in pressure indicating an "end of stroke." After setting the tool on the trestles, we found that a good core had been taken (full stroke) but retraction into the autoclave had been prevented because one of the upper seals had been dislodged. The seal was caught by the holes inside the core barrel during retrieval and jammed the liner in the core barrel. This made it impossible to pull the core out of the autoclave. This was rectified prior to the next deployment. Care was taken when releasing the pressure inside the liner by drilling small holes and then cutting rather than simply using a hacksaw as with Core 204-1251B-21Y (FPC 1). The core was removed for logging and curation. During this deployment, the FPC pressure and temperature logger worked well but the DSA tool only collected data from the very beginning part of the test.