Ten in situ temperature runs were made at this site: eight using the APCT tool and two using the DVTPP (Table T15; Fig. F27). APCT data were modeled using the software program TFIT (as described in "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter) on the basis of measured thermal conductivities (see "Physical Properties"). Temperatures were measured directly from the DVTPP data because the tool was deployed long enough to approach equilibrium.
The resulting temperature estimates are shown vs. depth below the seafloor in Figure F28A. In determining the temperature gradient from these measurements, we did not include the temperature of 11.5°C measured at 90 mbsf using DVTPP tip 3 because of uncertainties in the calibration of this tool (see "Downhole Tools and Pressure Coring" in the "Site 1244" chapter). The thermal gradient of 0.054°C/m derived from these data is similar to the thermal gradients at other sites and is not sensitive to whether the mudline temperature is included.
The scatter in the data, however, is greater than that at most other sites, especially in the depth range of 20-40 mbsf, where other observations indicate the presence of high concentrations of gas hydrate (see "Interstitial Water Geochemistry," "Organic Geochemistry," and "Downhole Logging"). This scatter may have several sources, including different calibration for APCT 11 and 12, the effect of gas hydrate on in situ thermal conductivity, and possible dissociation of gas hydrate in situ in response to probe insertion. To evaluate the importance of calibration effects, we added 0.51°C to temperatures derived from APCT 11, as indicated by the ice-bath tests discussed in "Downhole Tools and Pressure Coring" in the "Site 1246" chapter. This reduced the scatter in the data and led to a lower estimate for thermal gradient (Fig. F28B).
The possible effect of in situ gas hydrate on estimates of in situ temperature was considered because we observed that the misfit between the observed temperature decay and the decay predicted by the software program TFIT was larger than normal for data of this quality for several APCT tool runs. One possible explanation for this misfit is that the conductivity determined on board is not representative of the in situ conductivity. Because the thermal conductivity of gas hydrate is ~0.4 W/(m·K) (Ruppel, 2000), the in situ thermal conductivity will be quite different from the conductivity measured on board if a high concentration of hydrate is present in the subsurface. Figure F28C shows the results of a preliminary effort to determine whether in situ thermal conductivity as well as in situ temperature can be independently resolved. The equilibrium temperature was determined using TFIT for thermal conductivity ranging from 0.3 to 1.2 W/(m·K) in steps of 0.05 W/(m·K). For "typical" measurements, where in situ conductivities are in the range of 0.7-1.2 W/(m·K), equilibrium temperature and thermal conductivity cannot be independently resolved and an independent measurement of thermal conductivity is required (Core 204-1249F-12H in Fig. F28C). For "anomalous" measurements, however, the best fit to the data occurs for low thermal conductivity and the in situ temperature corresponding to the best solution for thermal conductivity is higher than that derived using the shipboard measurements (Core 204-1249-8H in Fig. F28C). If in situ temperatures at Site 1249 are recalculated, solving simultaneously for in situ temperature and in situ thermal conductivity (Table T15), in situ temperatures are generally higher and the fit of the data to a linear temperature gradient is improved. Additional analysis of this phenomenon and of other possible effects of the presence of gas hydrate on in situ temperature will be undertaken postcruise. For example, the frictional heat pulse resulting from probe insertion can raise the temperature above in situ hydrate stability for as long as 1 min.
In summary, although there is considerable uncertainty about the in situ temperature gradient at this site, it is clear that the temperature gradient is not significantly higher and may even be lower than those at other sites drilled during Leg 204. This implies that upward advection of warm fluid from greater depths must be relatively slow (probably <1 cm/yr) (Tréhu et al., 2003) and represents an additional constraint on models for the dynamics of hydrate formation at this summit. The low in situ thermal conductivities derived for some of the temperature measurements are consistent with other data, indicating large concentrations of gas hydrate in the subsurface at Site 1249.
Pressure data measured by the DVTPP are shown in Figure F27B. Analysis is pending.
The ODP PCS was deployed seven times at Site 1249. Only three of these deployments were successful (i.e., a core under pressure was recovered). The ball valve failed to actuate during the other four deployments. 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 (0-80 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. All three cores (Cores 204-1249C-6P [33.5-34.5 mbsf] and 14P [71.4-72.4 mbsf] and 204-1249F-4P [13.5-14.5 mbsf]) were recovered from above the BSR at ~115 mbsf.
The time to degass the three PCS chambers ranged from 967 to 11,268 min (Table T16). Pressure was recorded during degassing experiments (Fig. F29). 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 T16), and samples were taken for analysis of physical properties (see "Physical Properties").
Gas was collected in 2- to 1300-mL increments. The measured incremental and cumulative volumes are plotted vs. time (Fig. F29). The cumulative volume of released gas varies from 4,800 (Core 204-1249F-14P) to 95,110 mL (Core 4P) (Table T16). The volume of the last gas splits varies from 2 (Core 204-1249C-6P) to 20 mL (Core 204-1249F-4P). This measurement 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 (0.6%-6.9% of gas mixtures) suggests that air was not always 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 (Fig. F30).
Sediments in cores recovered by the PCS have lithologies that are similar to sediments recovered by the APC and XCB at adjacent depths (see "Physical Properties"). Porosity values measured on samples from APC and XCB cores taken near the PCS were averaged and used to estimate the methane concentration in situ (Table T16).
The concentration of in situ methane was estimated based on data from the degassing experiments (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 215.7 to perhaps >5000 mM of methane in pore water (Table T16). 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. F30).
Preliminary analysis suggests that gas hydrates have been present in relatively high concentrations (perhaps >40% of pore volume) in the shallowest Core 204-1249F-4P recovered from ~14 mbsf. In addition to high gas concentrations, strong evidence of the presence of gas hydrate was found in the pressure record of core degassing (Fig. F29B). Gas hydrate concentration in Core 204-1249C-6P (~34 mbsf) is estimated to be >5% of pore volume. Unfortunately, the core was not maintained at 0°C during the degassing experiment, and no information about the presence of gas hydrate in the core can be obtained from the pressure record.
The wide range in the estimates of methane concentrations is related to the uncertainty about the lengths of the recovered core (Table T16). Full 1-m-long cores were recovered during successful deployments at Site 1249; therefore, the lowest methane concentration values presented in Table T16 are the most reliable. Relatively low gas hydrate concentrations (~2% of pore volume) are estimated in Core 204-1249F-14P (~72 mbsf). Interestingly, the pressure record suggests that only dissolved gas was present in the core (Fig. F29C).
The estimated decrease of gas hydrate saturation with increasing depth at Site 1249 is consistent with similar trends proposed based on well logging data (see "Downhole Logging") and Cl- anomalies (see "Interstitial Water Geochemistry"). Additional comparisons of measured methane concentrations with theoretical methane solubility both 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.
We deployed the HYACINTH pressure coring tools five times at Site 1249 near the summit of Hydrate Ridge (Table T17) (see "Operations"). During the middle of Leg 204, the FPC and HRC tools were each used in an effort to capture massive hydrate samples under pressure from 8 mbsf. This experiment was repeated at the very end of Leg 204, again using both the FPC and HRC at 13.5 mbsf. In addition, a further HRC core was taken deeper in more disseminated hydrate at 37.5 mbsf.
Core 204-1249D-2Y (FPC 5) recovered a core from 8 mbsf that was initially thought to be under in situ pressure. However, it transpired that the pressure had been released during the ascent, only to build up once the core was at the surface. These difficulties culminated in an exploding core that still contained some remnants of hydrate. Core 204-1249F-13Y (FPC 6) recovered a nearly full core from a depth of 70.4 mbsf. However, difficulties with the lower autoclave valve again prevented the core from being recovered at in situ pressures.
Core 204-1249F-2E (HRC 4) was taken at the same depth (8 mbsf) as Core 204-1249D-2Y (FPC 5). Despite some difficulties at the surface (see "HYACE Rotary Corer Operations"), a 80- to 90-cm-long core (as determined by core logging) was recovered under full pressure. It was cooled in the ice bath to maintain stability and successfully sheared and transferred into the HYACINTH logging chamber. It was subsequently logged repeatedly in the Geotek V-MSCL while being degassed over the following 2 days (see "Core Logging and Analysis").
Core 204-1249G-2E (HRC 7) was taken a little deeper (13.5 mbsf) than Core 204-1249F-2E (HRC 4) (see "HYACE Rotary Corer Operations"). A successful 75-cm core containing massive hydrate layers (see "Core Logging and Analysis") was recovered at full pressure and transferred into a HYACINTH storage chamber. It was subsequently frozen in He under pressure and successfully transferred into liquid nitrogen for preservation. It is probably the most pristine sample of natural gas hydrate ever recovered and preserved.
Core 204-1249H-2Y (FPC 10) was taken at the same depth as Core 204-1249G-2E (HRC 7) (13.5 mbsf). A successful (75 cm) core was recovered at full pressure, and a good GRA log was obtained from the core in the storage chamber showing massive hydrate layers. This core was designated as a "reference core" and companion to the APC and XCB cores that were taken and repressurized under methane. It was kept in the refrigerator ready for transportation to TAMU for further study.
Core 204-1249L-5E (HRC 8) was taken from deeper at this site (37.5 mbsf) in an attempt to recover pristine material under pressure from a region where the hydrate is present in lower concentrations and may be more disseminated. Some pressure was lost during disassembly (see "HYACE Rotary Corer Operations"), but it was rapidly repressurized to in situ pressures before being transferred to the logging chamber.
Three HRC deployments were made at Site 1249 (Table T17). Core 204-1249F-2E (HRC 4) was taken at Site 1249, which was near the summit of Hydrate Ridge at 777 meters below sea level (mbsl). It is well known that massive hydrate exists near the surface and core recovery using regular APC and XCB coring through the upper 50 m was particularly poor in the previous holes. The purpose of this pressure coring attempt was to recover an undisturbed sample from massive hydrate just below the seafloor. Although the sediments in this near-surface environment would normally be extremely soft and highly unsuited to pressure coring with the HRC, it was thought possible that the massive hydrate ice structure would be hard enough to lend itself to rotary coring.
Core 204-1249F-2E (HRC 4) was deployed at 8 mbsf after an APC core was shot at the surface. Drilling procedures at these shallow depths mean that in practice these are separate holes. The tool was prepared on the piperacker and lifted into the vertical position and lowered into the drill string as in previous deployments. The DSA tool was attached and run in the hole at 40 m/min. After landing and lowering the HRC to TD, 5 m of slack wire was payed out to ensure that no tension was accidentally applied to the tool. The bottom of pipe (BOP) was closed and pumping started at 50 gallons per minute (gpm), causing the pressure to rise to 620 psi, which indicated that the tool had been activated (started rotating). With almost no load on bit we pumped for 12 min with 80 gpm before pumping for 8 min with 120 gpm. The drill string was lifted about 4 m, followed by a further 1-min pumping at 80 gpm. Slowly the wireline was pulled at 8 m/min for the first 16 m, after which it was increased to 74 m/min.
At the surface the tool was disassembled on the pipe racker, which took ~35 min before the tool could be moved to the transfer area outside the downhole tools shop. While breaking out the accumulator it was clear that there was high pressure inside the autoclave, which was measured with the pressure gauge. While mounting the pressure gauge, a very short high pressure burst was heard. Pressure was measured at ~220 kbar, and the autoclave chamber was then cooled down in the ice trough where the pressure soon dropped to ~120 kbar. Once the core was stable it was transferred through the shear transfer chamber and into the logging chamber, which was in turn placed in the ice trough for cooling and was left there and monitored overnight. The following morning the temperature and pressure was stable at around 0°C and 110 kbar. It was transferred to the Geotek V-MSCL, where it was degassed and logged over the next 2 days (see below). It transpired that the core (~90 cm long; 90% recovery) contained ~40% methane hydrate, which accounted for the rapid pressure rise at the surface (presumably caused by the partial dissociation of hydrate). During disassembly of the tool we discovered that the burst disk (230-270 kbar) had failed. This presumably occurred on deck, but rapid expulsion of sediment had immediately blocked the small orifice and, hence, no significant pressure had been lost.
Following the success of the HRC at this site earlier in the leg (Core 204-1249F-2E [HRC 4]), a second HRC core (Core 204-1249G-2E [HRC 7]) was taken at 13.5 mbsf. This is the same depth as Core 204-1249H-2Y (FPC 7) but in a different hole. The tool was deployed as in previous deployments.
Once on the trestles it was clear that the valve had closed, and ice bags were placed along the length of the autoclave to keep the system cold during the breakdown of the tool. Once outside the downhole tools laboratory the autoclave was kept under ice bags while the pressure was measured at ~80 kbar (the same as in situ pressures). The core was successfully transferred from the autoclave into the shear transfer chamber and then into the storage chamber at full pressure before being placed in an ice bath prior to logging. The logging revealed that a core about 75 cm long was retrieved and contained significant amounts of hydrate (the lower 25 cm appeared to be water) (see "Core Logging and Analysis").
Having recovered good pressure cores at this site from near the top of the section where the hydrates were relatively massive (10 and 13.5 mbsf), there was interest in recovering a core from slightly lower in the section where the hydrates were thought to be more disseminated. Consequently, the final deployment of the HRC during this leg was at the bottom of Hole 1249L (Core 204-1249L-5E [HRC 9]). The only real issue was whether the nature of the sediment in this region would lend itself to being cored by the HRC and held by the core catcher. The deployment was run similarly to previous deployments with the DSA tool. It was run into the hole at 70 m/min while circulating and rotating. Pumping and rotation was stopped while the tool was landed, and 5 m of slack wire was payed out. The drill string was lowered to TD, followed by the wireline. Weight on bit was set at 7,000-10,000 klb. Again, the active heave was not working effectively (max heave = ~2 m), and, therefore, we relied only on the passive heave compensator. The BOP was closed and pumping began slowly. Coring continued after the first pressure peak (450 psi) for 20 min at 90 gpm and ~360 psi. A second spike was observed (indicating full stroke) after the drill string had been lifted 3 m above TD. Pumping continued at 100 gpm for 1 min before the tool was lifted on the wireline slowly at 7 m/min for the first 20 m and then continued at 70 m/min while circulating. Once the tool was broken out of the drill string and laid on the trestle, the flapper valve was observed to have closed and care was taken to ensure that the autoclave was cooled with ice bags during the disassembly. However, during the later part of the disassembly some pressure leaked away when the connecting rod was removed. The leak was occurring in the piston extension rod. Only 30 kbar remained when measured, and the autoclave was immediately pumped up to 80 kbar to stabilize any hydrate. The core was then transferred under pressure into the logging chamber and kept in the ice bath until logged in the V-MSCL (see "Core Logging and Analysis"). We found a 30-cm-long core without any significant hydrate layers. It is thought that these sediments were too soft to be retained by the type of sleeve catcher used.
Three FPC cores were taken at Site 1249 (Table T17). The first deployment of the FPC at Site 1249 was Core 204-1249D-2Y (FPC 5). The purpose of attempting a shallow core at this site was the same as for Core 204-1249F-2E (HRC 4), to recover massive hydrate from just below the seafloor at ~8 mbsf. This deployment proceeded smoothly with the same operational procedures being used as previously. Active heave compensation was used throughout. On recovery it was clear that a full stroke had been achieved and that the autoclave had sealed. However, when the onboard FPC data logger was analyzed, it was clear that it was not sealed and had depressurized coming to the surface. The autoclave had then sealed at the surface, allowing the pressure to rise to ~20 kbar.
The rapid rise in pressure to 20 kbar indicated that hydrate was present in Core 204-1249D-2Y (FPC 5). We connected the autoclave to the shear transfer chamber, pressurized the complete system to ~70 kbar (to stabilize the system and put any hydrate back into the stability field), and attempted to make the transfer. However, problems with the transfer (caused by expansion) meant that the core had to be depressurized and removed manually. Having done this, the liner then burst under internal pressure and some very soupy mud was collected from the floor in a plastic bag. It was clearly cold to the touch and even contained a few remnants of hydrate. Some optimism was gained from this deployment, as it was clear that the valve had seated properly during the handling of the tool at the surface allowing a build up of pressure to occur.
A second FPC deployment (Core 204-1249F-13Y [FPC 6]) was made at 70.4 mbsf. At this depth, the hydrate was less massive and a core with virtually full recovery was achieved. Everything worked perfectly other than the sealing valve that had not fully closed. It would appear from an analysis that the sleeve might be coming down too quickly on top of the valve, causing it to jam on the edge of the sleeve in the "nearly closed" position. It was thought that removing the spring above the sleeve and lifting the pipe string more slowly after coring might help prevent this jamming.
Following the success of the FPC at Site 1244 (Core 204-1244E-8Y [FPC 9]), a third FPC core (Core 204-1249H-2Y [FPC 10]) was taken at a depth of 13.5 mbsf in Hole 1249H. This is the same depth as Core 204-1249G-2E (HPC-7) but in a different hole. The tool was deployed as in previous deployments. With the tool at TD (801 mbsl), the pressure was increased to 700 psi, and, after shearing, the pressure was raised to 800 psi and the tool hammered smoothly for ~6 min. At the end of the run the pressure was increased briefly to 850 psi, but this caused the hammering to become irregular. The same lifting procedure was followed as with the successful Core 204-1244E-8Y (FPC 9). However, to assist in the closing of the lower valve the FPC was stopped as it came out off the landing shoulder. We then pumped and rotated to provide some string vibration, which may have helped the valve to seat and seal properly. Half a minute later we continued to lift the FPC out of the drill string in the normal manner. Once on the trestle, it was clear that the valve had closed. Analysis of the data logger showed we had full in situ pressure (~80 kbar) so the autoclave was attached to the shear transfer chamber and manipulator. After considerable difficulty, caused by the tight tolerances in the ball valves, the core was transferred, cut, and moved into a HYACINTH storage chamber, where it was logged showing that a core ~75 cm long was retrieved containing significant amounts of hydrate (the lower 25 cm appeared to be water).
Core 204-1249F-2E (HRC 4) was stored within the logging chamber in the refrigerator at 5°C overnight before being loaded into the V-MSCL. The core was first logged at low resolution to get a quick impression of what type of core had been recovered. It was immediately clear that a nearly full core (85-90 cm) had been recovered with relatively low average densities. Interpretation of VP continued to prove difficult (see "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter). Over the next few hours, more detailed logs were run while the core warmed up slowly and increased in pressure. Temperature measurements were taken by inserting a probe in the ball-valve spindle. This provided only an approximation of the core temperature, even though the logging chamber was insulated by foam whenever possible and the chamber ends were insulated with "bubble wrap" as far as practically possible. When the temperature and pressure had increased to 16°C and 160 kbar, respectively, the complete chamber was again returned to the refrigerator to stabilize overnight. In the morning (at 5°C), the pressure had dropped to 85 kbar (close to in situ conditions again).
Over the next 2 days, the GRA density logs were obtained repeatedly during the process of degassing (Fig. F31). Gas was collected using an inverted measuring cylinder in water as was used to degas the PCS (see "Downhole Tools and Pressure Coring" in the "Explanatory Notes"). Temperature was allowed to rise slowly while the pressure was varied in an effort to control the rate at which gas was released during the process of depressurization and hydrate dissociation. At the end of day 2, the chamber was again left in the refrigerator to stabilize overnight. Throughout day 3, the core was completely degassed, warmed to room temperature, and completely depressurized.
Over the course of the measurement period, >101.5 L of gas was collected, and 24 GRA density logs were obtained along the length of the core. The gas was subsampled during the degassing process for compositional analysis in the onboard chemistry laboratory (see "Interstitial Water Geochemistry"). The GRA density logs clearly showed how the physical structures within the core changed during the measurement period. Characteristic features interpreted as gas, hydrate, sediment, and water could be correlated and traced between logs. It was clear from these logs where gas was forming and where and when it was escaping from the core. Methane hydrate and water are difficult to distinguish via density alone, but when hydrate dissociates, the gas layers generated are easily apparent in the GRA density logs. Figure F31 shows three density logs at different stages of the experiment. Run 1 was collected prior to the degassing process at full pressure, showing the in situ density structure, which reveals low densities throughout with some layers interpreted as massive hydrate. Run 8 was in the early stages of degassing, showing distinct gas layers developing and the complete core structure being expanded by gas. Run 15 was in the middle of the degassing process (43 L of gas collected), showing how the complete structure had changed with sediment falling from the top and a water interval in the center of the core. By the time all the gas had been removed at the end of the process, the sediment had completely liquefied and most of it was removed from the chamber by bleeding it as a slurry through the pipework. It was collected, allowed to settle, and dried, and the total weight enabled a mass balance calculation to be performed. It was calculated that the core consisted of ~40% by volume of methane hydrate and 60% by volume of fine-grained sediment with an average bulk density of 1.3 g/cm3 (67% porosity). If hydrate is expressed as a percentage of pore volume, as is done elsewhere in this volume, then this core contained ~50% hydrate by pore volume.
Core 204-1249G-2E (HRC 7) was logged inside the storage chamber after having been stabilized at ~6°C. The density log is shown in Figure F32 (run 1). The log shows that ~25 cm of material may have fallen out of the end of the core, but the remainder shows detailed structure indicating several layers of massive hydrate above a water interval. It was decided to attempt to preserve this core as a "pristine example" in liquid nitrogen for study in the laboratory. This could then be used as a reference core to compare with samples taken without pressure preservation and as a complementary reference core to Core 204-1249H-2Y (FPC 10) (see below). The problem was, of course, how to remove the core from the pressure vessel without the gas in solution instantly coming out of solution and without the hydrate starting to dissociate. We decided to freeze the core. First, the core was cooled in the storage chamber to 0°C in an ice bath for several hours. Following this, the seawater pressurizing fluid was replaced by He using a high-pressure regulator after the pressure inside the chamber was first reduced to 63 kbar (maximum regulator pressure). Despite some difficulties caused by small amounts of sediment in the pipes, the operation was successful and the storage chamber with core was then rapidly logged as a check (see Fig. F32) (run 2), which showed that the sediment section had not moved and that the water section still remained in the core. The chamber was then placed in the freezer at -10°C for more than 24 hr before being rapidly logged again. Run 3 (Fig. F32) showed that some of the water had escaped and some had frozen. This caused some concern because it left a gas pocket inside the liner. To remove the core, the chamber was connected to manipulator and wrapped in ice bags. Hot water and rags were applied to the manipulator end of the chamber to unfreeze the contact between the core and the inside of the chamber. Hot water was also applied at the ball-valve end to free the bleed pipes and pressure gauge, which was frozen, and did not read the inside gas pressure correctly. Once clear, the pressure inside the chamber was measured at ~62 kbar. This pressure was then released slowly (~5 min) so as to allow time for the pressure inside the core liner to escape. When the pressure was zero, the ball valve was opened and the manipulator extended slowly to push the core from the chamber. Rapid gas expansion shot some frozen core containing massive hydrate from the chamber. However, this was easily collected and placed directly in liquid nitrogen. The core was further extruded with care (surrounded by steel tube), and further core sections were cut off and placed in liquid nitrogen. It was clear that a substantial part of this core contained massive hydrate, as could be clearly seen from the core ends and through the core liner during its brief exposure.
Core 204-1249H-2Y (FPC 10) was logged inside a HYACINTH storage chamber, after having been stabilized at ~6°C. The density log is shown in Figure F33 and shows a 75-cm-long core above a water interval of ~25 cm, indicating that some material may have fallen out of the end of the core. However, the sediment section shows detailed structure indicating several layers of massive hydrate. A thicker layer of hydrate (60- to 80-cm logging depth) shows a small interval where the average density is only ~0.75 g/cm3, clearly demonstrating that free gas does exist in situ inside the massive hydrate structure.
Core 204-1249L-5E (HRC 8) was logged inside a HYACINTH logging chamber. The GRA density log revealed a short core at the top of the liner (~30 cm), with a column of water beneath. There were no apparent signs of low-density layers that would be interpreted as hydrate. Consequently, the core was depressurized and the gas collected (total volume = 2.325 L). Two samples were collected and analyzed in the onboard chemistry laboratory.