DOWNHOLE TOOLS AND PRESSURE CORING

During Leg 204, a suite of downhole tools was employed to measure in situ temperature and pore pressure, to retrieve cores under pressure, and to estimate the in situ concentration of methane and other natural gases. Temperature, pressure, and gas composition and concentration are the critical factors for determining the extent of the GHSZ and whether gas hydrate can form in that zone. In addition, temperature effects rates of sediment diagenesis and microbial activity. Pore pressure is important because fluid flow occurs if the pressure gradient differs from hydrostatic, thus transporting natural gas into the GHSZ, providing nutrients for microbes, and modifying the temperature and pressure field.

In situ sediment thermal measurements were made during Leg 204 using the APC temperature (APCT) tool and the Davis-Villinger Temperature Probe (DVTP) (Davis et al., 1997). Temperatures and pressures were measured using a DVTP modified to include a pressure port and sensor (Davis-Villinger Temperature-Pressure Probe [DVTPP]) that was previously used during Legs 190 and 201. Pressure was also measured during a trial run of the Fugro-McClelland piezoprobe, which operates on similar principles as the DVTPP.

Retrieval of cores at in situ pressure was a high priority during Leg 204. Natural gas in deep sediment may be present in three phases. If the concentration (molality) of gas in pore water is less than the solubility, the gas is dissolved. If the concentration of gas is greater than its solubility, gas is present as a free phase (bubbles) below the GHSZ and as solid hydrate within the GHSZ. Knowledge of the gas concentration in deep sediment is critical for understanding the dynamics of hydrate formation and the effect hydrates have on the physical properties of the sediment. However, reliable data on gas concentration are difficult to obtain. Because gas solubility decreases as pressure decreases and temperature increases, cores recovered from great depth often release a large volume of gas during recovery (Wallace et al., 2000; Paull and Ussler, 2001). The only way to determine true in situ concentrations of natural gas in the subseafloor is to retrieve cores in an autoclave that maintains in situ conditions. The original ODP pressure core sampler (PCS) has proven to be an essential tool that is very effective for estimating in situ gas concentrations (Dickens et al., 1997, 2000b) and was used extensively during Leg 204. However, it is less effective for studies of physical properties of gas hydrate-bearing sediments at in situ conditions.

The HYACINTH (deployment of Hydrate Autoclave Coring Equipment [HYACE] tools in new tests on hydrates) program, funded by the European Union (EU), is developing the next generation of pressure corers. Both HYACE coring systems were used during Leg 204. The Fugro Pressure Corer (FPC) is designed for sediments that are normally cored with the APC and XCB, and the HYACE Rotary Corer (HRC) is designed to drill more lithified sediments and rocks normally cored with the XCB and RCB. These pressure cores are contained in an inner plastic liner that can be transferred (under full pressure) from the autoclave into other pressure chambers. When transferred into a logging chamber, the pressurized cores can be logged using the V-MSCL. This was used to make measurements on cores collected by the HYACE coring tools and on standard ODP cores repressurized to in situ pressures. By measuring V_P , P-wave attenuation, and GRA density at in situ pressures and by pressure cycling, we anticipated being able to distinguish between hydrate and free gas while also measuring some in situ properties that would help to constrain models of hydrate and free gas distribution.

Advanced Piston Corer Temperature Tool

The APCT tool fits directly into the cutting shoe on the APC and can, therefore, be used to measure sediment temperatures during regular piston coring. The tool consists of electronic components, including battery packs, a data logger, and a platinum resistance-temperature device calibrated over a temperature range of 0°-30°C. Descriptions of the tool and of the principles behind analysis of the data it acquires can be found in Pribnow et al. (2000) and Graber et al. (2002) and the references therein. The thermal time constant of the cutting shoe assembly where the APCT tool is inserted is ~2-3 min. The only modification to normal APC procedures required to obtain temperature measurements is to hold the corer in place for ~10 min after cutting the core. During this time, the APCT tool logs temperature data on a microprocessor contained within the instrument as it approaches equilibrium with the in situ temperature of the sediments. Following deployment, the data are downloaded for processing. The tool can be preprogrammed to record temperatures at a range of sampling rates. Sampling rates of 10 s were used during Leg 204. A typical APCT measurement consists of a mudline temperature record lasting 10 min for the first deployment at each borehole and 2 min on subsequent runs. This is followed by a pulse of frictional heating when the piston is fired, a period of thermal decay that is monitored for 10 min or more, and a frictional pulse upon removal of the corer.

A second source of uncertainty in these data is possible temporal change of the bottom-water temperature resulting from tides, seasons, and longer-term climate change. Evidence for short-term changes in this region is seen in data from a near-bottom current meter that was deployed for 6 months at a water depth of 800 m in the saddle between the northern and southern summits of Hydrate Ridge (R. Collier, pers. comm., 2000). These data show peak-to-peak tidal variations of up to 0.3°C, a superimposed variation with a timescale of 2 months and peak-to-peak amplitude of 0.04°C, and an apparent seasonal variation of 0.3°C. These multiple sources of bottom-water temperature variation, which occur on a timescale that will not be felt at subseafloor depths greater than a few meters lead to significant temporal variability in the mudline temperature. Because of these observations, it may, in general, be inappropriate to include the mudline temperature when determining the subsurface temperature gradient from downhole temperature data. Mudline temperatures, however, are reported in the data tables because they can provide a useful data point for postcruise studies.

Davis-Villinger Temperature Probe

The temperature measurement aspects of the DVTP are described in detail by Davis et al. (1997) and summarized by Pribnow et al. (2000) and Graber et al. (2002). The probe is conical and has two thermistors; the first is located 1 cm from the tip of the probe and the other 12 cm above the tip. A third thermistor, referred to as the internal thermistor, is in the electronics package. Thermistor sensitivity is 1 mK in an operating range of -5° to 20°C, and the total operating range is -5° to 100°C. The thermistors were calibrated at the factory and on the laboratory bench before installation in the probe. In addition to the thermistors, the probe contains an accelerometer sensitive to 0.98 m/s². Both peak and mean acceleration are recorded by the logger. The accelerometer data are used to track disturbances to the instrument package during the equilibration interval. In a DVTP deployment, mudline temperatures (within the drill pipe) are measured for 10 min during the first run within each hole and for 2 min during subsequent runs, before descent into the hole for a 10-min equilibration time series at the measurement depth in the subseafloor. The time constants for the sensors are ~1 min for the probe-tip thermistor and ~2 min for the thermistor 12 cm from the tip. Only data from the probe tip thermistor were used for estimation of in situ temperatures.

In Situ Temperature Data Reduction

The transient thermal-decay curves for sediment thermal probes are a function of the geometry of the probes and the thermal properties of the probe and sediments (Bullard, 1954; Horai and Von Herzen, 1985). Data analysis requires fitting the measurements to predicted temperature decay curves calculated based on tool geometry and the thermal properties of the sediment. Pribnow et al. (2000) discuss data analysis procedures and uncertainties. For the APCT tool, the software program TFIT, developed by K. Becker and J. Craig, was used. The DVTP and DVTPP data can be analyzed using the software program CONEFIT, developed by Davis et al. (1997).

However, during Leg 204, DVTP and DVTPP data were generally noisy and CONEFIT did not yield stable solutions. In some cases, we assumed that the temperature had reached equilibrium. In other cases, no estimate of in situ temperatures was attempted on board.

Figure F15A shows a typical temperature history recorded by the APCT tool. Various stages in the tool-deployment history are marked. Mudline temperature is determined from the time the tool is held near the seafloor prior to penetration of the APC. Initial APC penetration is marked by a temperature pulse resulting from friction. A second pulse is observed when the tool is extracted from the sediment. The best fitting time of penetration and in situ temperature are calculated from data delimited by three points that are picked by the shipboard analyst. The thermal conductivity of the sediment must also be specified. Thermal conductivities measured from the core interval closest to the APCT measurement were used (see "Physical Properties"). The estimated uncertainty of the derived in situ temperature for good-quality measurements is 0.1°C (Pribnow et al., 2000), although the uncertainty may be considerably larger for poor-quality measurements. Because of problems with instrument calibration that became evident during the leg (see "Downhole Tools and Pressure Coring" in the "Site 1244" chapter), temperature gradients may be better resolved than absolute values of temperature, provided the same tool was used to make all measurements at a given site. Additional analysis will be performed postcruise to decrease uncertainty resulting from instrument calibration.

Davis-Villinger Temperature-Pressure Probe

Simultaneous measurement of formation temperature and pressure was achieved using a modified DVTP. The probe has a tip that incorporates both a single thermistor in an oil-filled needle and ports to allow hydraulic transmission of formation fluid pressures to a precision Paroscientific pressure gauge inside. A standard data logger was modified to accept the pressure signal instead of the second thermistor signal in the normal DVTP described above. Thermistor sensitivity of the modified tool is reduced to 0.02 K in an operating range of -5° to 20°C. A typical deployment of the tool is shown in Figure F15B. It consists of lowering the tool by wireline to the mudline where there is a 10-min pause to collect data. Subsequently, it is lowered to the base of the hole and latched in at the bottom of the drill string, with the end of the tool extending 1.1 m below the drill bit. The extended probe is pushed into the sediment below the bottom of the hole and pressure is recorded for ~40 min. If smooth pressure decay curves are recorded after penetration, then theoretical extrapolations to in situ pore pressures are possible.

Temperature data from the DVTPP were treated as discussed for the DVTP. For both the DVTPP and the piezoprobe (discussed below), the pressure response is qualitatively similar to, but slower, than the thermal response. The decay time is a function of the sediment permeability and the magnitude of the initial pulse, which is a function of the taper angle and diameter of the tool (Whittle et al., 2001; Heeseman, 2002). Analysis of in situ pressure will be done postcruise.

Fugro-McClelland Piezoprobe

In April 2001, a proposal was submitted to the U.S. Department of Energy to modify and implement the use of the Fugro-McClelland piezoprobe tool on the JOIDES Resolution during ODP Leg 204. The piezoprobe has been tested and proven (e.g., Ostermeier et al., 2000; Whittle et al., 2001) on numerous geotechnical cruises that measured pressure and temperature, but it had not been adapted for ODP until Leg 204. To adapt it on the JOIDES Resolution for testing and use with the APC/XCB bottom-hole assembly (BHA) required modifications prior to the leg. The modifications made by Fugro-McClelland and ODP were designed to (1) adapt the piezoprobe for a Schlumberger wireline, (2) increase the landing ring size, (3) implement a stabilizer sleeve to prevent bending, (4) shorten the bit to minimize risk of bending, and (5) extend pawls for the four-cone APC bit used on the JOIDES Resolution.

The piezoprobe works within the borehole and measures pressure through a transducer at its tip, which is similar to the pop-up pore pressure instrument (PUPPI) (see Schultheiss and McPhail, 1986). The probe is lowered through the drill pipe, measures hydrostatic pressure, and is pushed into the sediment ~1 m beyond the base of the borehole, where pressure is again measured. The resultant pressure vs. time curves for multiple experiments provide estimates of in situ pressure as a function of depth. The pressure decay can be used to evaluate the permeability and coefficient of consolidation (e.g., Elsworth et al., 1998; Schnaid et al., 1997), two parameters that are necessary to describe fluid flow and deformation within the shallow subsurface. The narrow taper of the piezoprobe allows a pressure decay to be measured in low-permeability sediments within an hour, a time frame that is reasonable for use on the JOIDES Resolution. The piezoprobe also records temperature data during each measurement. Similar to the APCT tool and the DVTP tool, the temperature decay can be used to estimate in situ temperature.

During Leg 204, the piezoprobe was deployed twice, with the second run being successful (see "Downhole Tools and Pressure Coring" in the "Site 1244" chapter).

Comparison between the Piezoprobe and the Davis-Villinger Temperature-Pressure Probe

The DVTPP and the piezoprobe both provide the ability to make estimates of in situ temperature and pressure in low-permeability strata at a relatively quick rate (i.e., multiple measurements per hole and dozens of measurements per cruise). The basic operational procedure for each is similar to that for the temperature tools: (1) insert probe at the base of the borehole, (2) monitor pressure disturbance from probe insertion, and (3) record pressure decay and extrapolate out to infinite time for estimate of in situ pressure. The decay time is a function of the sediment permeability and the size of the initial pulse. The magnitude of the pressure pulse is a function of the taper angle and diameter of the tool (Whittle et al., 2001). The piezoprobe has a narrower diameter (6.4 mm) and smaller taper angle (<2°) than the DVTPP (diameter = 8 mm and taper = 2.5°) and therefore produces a smaller pressure disturbance. Whittle et al. (2001) have demonstrated that it is beneficial to monitor the pressure decay long enough so that a significant proportion of the pulse has dissipated before recovery of the tool; with the piezoprobe, this takes ~2 hr in low-permeability strata (Whittle et al., 2001), longer than is generally allowed for the DVTPP during ODP legs.

The PCS is a downhole tool designed to recover a 1-m-long sediment core with a diameter of 4.32 cm at in situ pressure up to a maximum of 10,000 psi (Pettigrew, 1992; Graber et al., 2002). It consists of the inner core barrel and a detachable sample chamber (Fig. F16). When its valves seal properly, controlled release of pressure from the PCS through a manifold permits collection of gases that would otherwise escape on the wireline trip. The PCS currently provides the only proven means to determine in situ gas abundance in deep-sea sediments where gas concentrations at depth exceed saturation at atmospheric pressure and room temperature (Dickens et al., 1997). The analysis of recorded data (e.g., time series of pressure and the volume of released gas) may also help to determine if gas hydrate is present in the cored interval (Dickens et al., 2000b).

After retrieval, the PCS is placed into an ice bath to keep the inside temperature at ~0°C. A manifold is connected to the PCS to decrease pressure by releasing gas under manual control. Only a small volume of gas (~100-150 mL) should be collected during the first gas release. This is because it has been empirically determined that the first gas sample thus obtained is contaminated by air. Additional gas releases should lead to immediate pressure drops. Ideally, the pressure in the PCS should then increase with time as gas exsolves from pore water or from decomposing gas hydrate. Gas should be released when pressure does not increase significantly over a 10- to 15-min time interval, and the process should be repeated. Sometimes gas may be released before the pressure has built up because of constraints with operational logistics. At the end of the experiment, ice should be removed from around the PCS and the PCS should be warmed up to release all gas remaining in the core. Splits of gases are collected into a 1-L bubbling chamber that consists of an inverted graduated cylinder placed in a plexiglass tube filled with a saturated NaCl solution. After measuring the volume of collected gas, gas aliquots are sampled from a valve at the top of the cylinder using a syringe.

Prior to Leg 204, the PCS was successfully used to study in situ gases during ODP Leg 164 on the gas hydrate-bearing Blake Ridge (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., 1997) and during Leg 201 at sites along the gas-rich Peru margin (Dickens et al., 2003). One of the objectives of PCS use during Leg 201 was to test the coring capabilities in a variety of lithologic conditions. Several modifications to the PCS were made prior to Leg 201 (Dickens et al., 2003), including the addition of an optional cutting shoe for rotary coring and the construction of a new gas manifold (see "Downhole Tools" in Dickens et al., 2003). The PCS was deployed 17 times during Leg 201. Dickens et al. (2003) concluded that (1) the tool performed better during Leg 201 than on Leg 164, (2) the PCS can operate successfully in a variety of submarine environments, and (3) cores collected at shallow sediment depth can be degassed to generate gas concentration profiles.

Two significant modifications were made between Legs 201 and 204 in order to better address the scientific objectives of Leg 204. First, a methane tool was installed inside the PCS to measure temperature, pressure, and conductivity during the PCS recovery (see below). Second, pressure transducers that permit continuous monitoring of pressure both on the manifold and inside the PCS were installed. Pressure is recorded on a personal computer every 5 s and is presented as a graph during the experiment. An ASCII file of the data is preserved at the end of the experiment. These modifications should permit better monitoring of pressure and temperature inside the PCS after the core is retrieved from the subsurface.

The APCM and the PCS methane tool (PCSM) continuously record the temperature, pressure, and electrical conductivity changes in the core headspace from the time the core is cut through its ascent to the rig floor. The APCM sensors are mounted in a special piston head on the standard ODP APC piston, and the data acquisition electronics are embedded within the piston. The PCSM is a slimmed-down version of the APCM, which is mounted on the top of the PCS manifold mandrel. Both tools operate passively and require little shipboard attention. Variations in the relative amounts of gas stored in different types of sediment can be determined by establishing families of ascent curves composed of data from successive cores. Models indicate that these data will also provide information on whether gas hydrate was present in the sediment before core retrieval. The methane tools are being developed jointly by ODP and Monterrey Bay Aquarium Research Institute (MBARI). They are derivatives of MBARI's Temperature-Pressure-Conductivity (TPC) tool.

Both tools are very similar in construction, the only difference being that the APCM replaces the piston-rod snubber in the APC coring system and therefore has a seal package on its exterior. The tools consist of an instrumented sensor head with the electronics and battery pack housed in a sealed case. The three sensors (temperature, pressure, and conductivity) and a data port are packaged in the face of the 2³/₈-in-diameter sensor head. The temperature sensor is a ±0.05°C accuracy thermistor installed in a ³/₁₆-in-diameter x -in-long probe. The pressure sensor is a 0- to 10,000-psi "Downhole Series" transducer with a ±0.15% full-scale accuracy that is especially designed for temperature stability. The electrical conductivity sensor is a three-pin bulkhead connector with an inconel body and gold-plated 0.040-in-diameter Kovar pins. The data port is a three-pin keyed bulkhead connector for RS-232 communication. The electronics consists of two boards, an analog to digital (A/D) board and a commercial microcontroller board. The microcontroller board plugs directly into the A/D board, and the A/D board is mounted on an aluminum backbone. The microcontroller includes a Motorola 68338 processor, a DOS-like operating system, and 48 MB of flash memory. The A/D board is an ODP/MBARI-designed board with one A/D device for the pressure transducer and one for the thermistor and conductivity sensors. The battery pack consists of an assembly of two double-C lithium/thionyl chloride batteries in series and an integral hard-mounted nine-pin connector. The 1-in-diameter x 9-in-long battery pack provides 7.3 V, with a 100-mA rating. The APCM is installed on the APC piston after the APC piston-rod snubber and piston-head body is removed from the lower piston rod. The connection at the lower piston rod consists of a threaded connection with a transverse spring pin running through the thread relief. The spring pin prevents the connection from unscrewing as a result of vibration. After the spring pin is punched out, the piston-rod snubber is removed and replaced with the APCM. This swap-out operation takes <3 min. The PCSM replaces the accumulator on the PCS and threads onto the top of the PCS manifold mandrel.

The APCM and PCSM tools were successfully deployed 107 times during Leg 204 (see Table T3 in the "Leg 204 Summary" chapter), but all data analysis was deferred until postcruise. Data and results will be presented in the Leg 204 Scientific Results volume.

Background

Although the PCS was successful during Leg 164, there were a number of aspects worthy of improvement as described by Dickens et al. (2000a). A proposal submitted to the EU resulted in HYACE, which was a 3-yr project aimed at developing new wireline pressure coring tools that would address a wide range of scientific problems. The HYACE project resulted in the development of two new pressure coring tools. These tools underwent only limited testing on land and at sea during ODP Legs 194 and 201 (Leg 201 was after the end of the HYACE project and at the beginning of the HYACINTH project). The current HYACINTH project is a continuation of the HYACE project and is also funded by the EU. It is designed to bring these new coring tools into operational use and to develop new techniques of subsampling and analyzing cores under pressure. Leg 204 provided the opportunity for further testing and use of these new coring tools. Other important objectives of Leg 204 were to test and use the HYACINTH family of pressure chambers and the core-transfer mechanisms and to measure the physical properties of cores at in situ pressures.

The design and operation of the HYACE tools differs in two significant respects from that of the existing PCS. First, the HYACE tools penetrate the seabed using downhole driving mechanisms powered by fluid circulation rather than by top-driven rotation with the drill string. This allows the drill string to hang stationary in the hole while core is being cut, which should improve core quality and recovery. Second, the HYACE tools recover lined cores, which enables them to be transferred under pressure into a family of chambers, allowing cores to be preserved and studied under pressure.

Two different coring tools have been developed in order to accommodate a wide range of lithologies, a "percussion" corer and a "rotary" corer. Both tools have been designed for use with the same ODP BHA as the PCS (i.e., the APC/XCB BHA). The FPC is designed for recovering unlithified sediment ranging from clay to sand and gravel. When used in a gas hydrate-bearing environment, it is considered to be most applicable where any hydrate present has not significantly cemented the sedimentary particles. The core barrel is driven into the sediment by a hammer mechanism that is driven by fluid circulation. In soft sediments, the core barrel strokes out quickly so that in these lithologies the FPC essentially behaves like a push core.

The HRC is designed to cut a rotary core in more lithified sediment formations and incorporates a downhole mud motor. A dry auger-type of bit, extending beyond the reach of the circulating seawater, is used to cut the core, providing as contamination-free a core as is possible with rotary coring. It is designed, primarily, to recover cores in well-lithified sediments and rocks that can be obtained with the XCB and RCB. The phase II PCS development proposed by Pettigrew (1992) is similar to the approach used in the HRC. However, this was not pursued by ODP because of insufficient funds.

Both the FPC and the HRC use specially designed but different flapper valves to seal the tool's pressure chamber (autoclave), where the core is contained on recovery. This enables larger cores to be cut than with the PCS, which uses a ball valve as the sealing mechanism. The FPC cuts a 58-mm-diameter core, and the HRC cuts a 50-mm-diameter core. Like the PCS, both cores are ~1 m in length. Pressures up to 250 kbar (3625 psi) can be maintained in the present design.

After initial testing on land, the FPC and HRC underwent their first sea trials on the JOIDES Resolution at the start of ODP Leg 194. The FPC had limited success in recovering a core under pressure, whereas the HRC encountered significant problems because of its failure to latch properly in the BHA (Rack, 2001). A core was finally cut but was not retrieved under pressure. The FPC had further trials during Leg 201, but hole conditions are thought to have been unfavorable, which prevented the recovery of a pressure core. Valuable lessons were learned during both of these engineering trials of the FPC and the HRC (Rack, 2001), and a number of significant modifications were made to the tools and to the handling procedures prior to the start of Leg 204.

Tool Operations

Both the HRC and the FPC were prepared and assembled on tool trestles located on the port side of the piperacker. Stands of drill pipe normally used from the port side were moved to the starboard side to prevent disruption to the tool preparations as much as possible. The only time the piperacker could not be used was when we added pipe stands during drilling, tripping pipe, and wireline logging. This was particularly helpful in ensuring that the operations went as efficiently as practically possible. The space afforded by using the piperacker was particularly important in view of the fact that the three PCS tools and logging tools were being assembled above the core tech shop.

For deployment, both tools followed similar operational procedures on the rig floor. They were initially transferred from the piperacker working area, where they had been prepared on trestles, into the vertical position. To do this, a tugger line from the derrick was attached to the upper end of the tool while the base of the tool was lowered onto the piperacker skate using the port side racker crane. The tool was then hauled into a vertical position using the tugger line and lowered into the rig floor shuck as the strongbacks were removed. A tugger line supported the heavier strongbacks on the HRC. The tool was deployed in the open drill string and the Drill String Acceleration (DSA) tool was fitted above. When the drill string was closed, the tools were lowered on the wireline while pumping and rotating.

On retrieval from the pipe, both the FPC and the HRC followed a reverse procedure back to the trestles on the piperacker, including replacing the strongbacks. During an early deployment, a problem was encountered whereby the HRC was split into the three main subassemblies as it was removed from the drill string in the vertical position. Although this alleviated the need to reattach the strongbacks when placing the tool subassemblies back to the horizontal position, it took significantly longer and, hence, was subsequently avoided. After disassembly on the trestles, the autoclave was carried to the platform outside the downhole tools laboratory for examination and connection to the pressure transfer system.

Logging Cores at In Situ Pressure

The other components that make up the HYACINTH system used during Leg 204 are the transfer system, the shear mechanism, and the pressure chambers that are used to store and log the cores under pressure. The HYACE transfer mechanism, which consists of a manipulator chamber and a shear mechanism, is used to extract the core under pressure from either the HRC or FPC autoclave and then transfer it into a storage chamber or logging chamber. The shear mechanism cuts the core at the top, removing the "technical part" of the core (piston assembly, etc.) from the core liner containing the sample prior to inserting it into the other chambers. The manipulator can be used to subsequently transfer the core between the logging and storage chambers if and when required.

The specially adapted Geotek V-MSCL was used to measure gamma density and P-wave parameters while the cores were under pressure in the HYACINTH logging chambers. It was also used to log regular APC cores that had been repressurized in specially designed logging chambers. The cores were logged vertically to help control the process of degassing during pressure cycling and final pressure release.

As with the PCS, gases exsolved from solution or released by dissociation of gas hydrate were collected into a 1-L bubbling chamber to determine the in situ abundance of gas in the cores. An analysis of the data recorded during the degassing process should help to determine the relative amounts of free gas and gas hydrate present in the cored interval.

Pressurized core logging is unlike normal core logging with the ODP MST or a standard Geotek Multi Sensor Core Logger (MSCL) in that there are two core liners to consider: (1) the thin plastic core liner (the inner liner) and (2) the thicker glass-reinforced plastic (GRP) pressure tube (the outer liner). To calibrate for measurements of V_P and gamma density, similar techniques are used to those developed for the MST and MSCL, which use distilled water and aluminum as standards. In this mode of operation, the inner liner is assumed to have a constant diameter because it cannot be directly measured under pressure. The outer GRP liner was accurately calibrated to account for small variations in diameter and wall thickness along its length. The manufacturing technique necessitates that a change in the internal diameter of ~1 mm occurs along the 1.5 m length. To ensure consistency, the outer liner was always oriented to ensure that the small circumferential variations were effectively negated.

To calibrate V_P , the variations in the total P-wave traveltime along the length of the GRP tube were measured when both the inner liner and the GRP were filled with water of known velocity. All data are subsequently corrected as a function of position in the GRP tube. Changes in traveltime as a function of pressure were also measured (up to 200 bar). The measured variation in V_P with pressure is close to the theoretical variation for water. We therefore conclude that the traveltimes in the liner material are essentially constant with changing pressure. In practice, however, P-wave data for sediment cores were much harder to interpret than initially thought because of the interference of ultrasonic signals that propagate around the cylindrical GRP liner.

To calibrate the gamma density system, we used the same type of "standard section" as is used with the MST (see "Physical Properties"). During this step, graduated aluminum and water standards are placed in the GRP tube and logged at 2-mm intervals along the core. Consideration is given to the variation in GRP tube diameter by logging the complete tube filled with water and filled with air. We confirmed that there are no pressure effects on the measurements by repeating the experiment at pressures up to 200 bar.