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doi:10.2204/iodp.proc.311.102.2006

Physical properties

A suite of physical property measurements was made to complement other data sets taken on board and to support the scientific objectives of Expedition 311. Physical characteristics of the subsurface environment play an important role in determining the nature of fluid and gas migration, which, in turn, affects the nature of gas hydrate formation and microbial communities.

As soon as cores arrived on the catwalk, they were wiped to remove excess water or sediment film and then scanned using a track-mounted IR camera system. These scans provided the temperature of the outer surface of the butyrate liner for virtually all cores recovered. After sectioning, section-end IR images were taken of selected cores, primarily of cores dedicated to microbiological studies. Cold spots in the IR images, interpreted to represent partially or completely dissociated gas hydrate, were used to select a subset of the samples collected on the catwalk, mainly IW, HS, and gas hydrate samples. The IR images were also processed to provide downcore temperature images and plots as described below. The core sections were then moved into the laboratory to equilibrate to room temperature. Thermal equilibration, which was monitored with temperature probes in a subset of the cores, took ~4 h. Magnetic susceptibility, GRA density, noncontact electrical resistivity, and seismic P-wave velocity (VP) were measured on whole-round cores using the MST. Thermal conductivity was also measured on whole-round cores. The core sections were then split for measurements of contact resistivity, shear strength (handheld Torvane device or Giesa automated shear vane [AVS]), and VP with a Hamilton Frame velocimeter. Discrete samples were taken for measuring MAD properties (e.g., bulk density, porosity, and grain density). Selected cores were taken directly from the catwalk for special gas hydrate dissociation experiments and imaging using the MST and IR camera. This volume also presents methods used to determine in situ temperature and to monitor the temperature of cores during acquisition and recovery, including tests of a prototype next-generation downhole temperature tool.

Core temperature measurement and infrared thermal imaging

The temperature history of a core sample from in situ conditions beneath the seafloor to thermal equilibrium on the ship is complicated but needs to be known to understand the impact of this history on microbiology and gas hydrate studies. During the coring process, frictional heat is generated, warming the cores by a poorly known, variable amount. Frictional heat is also generated by rotary drilling, even though the bit is being cooled by seawater that is pumped downhole at near-bottom water temperatures (<6C). During core recovery, the sediments are first exposed to cooler temperatures, with the minimum occurring at the seafloor. Significant warming does not start until the cores pass through the ocean=s thermocline on the way to the surface, and continued warming occurs once a core arrives on deck. During Expedition 311, surface seawater and ambient air temperatures were ~14 and 15 3C, respectively. Because of these complications and others discussed below, temperatures of marine cores are not commonly measured. However, since the process of core retrieval is fairly uniform, the temperatures at which neighboring cores arrive on the catwalk should be relatively consistent unless there are additional heat sources or sinks. Dissociation of gas hydrate, which is an endothermic process, represents one such heat sink, resulting in anomalous cold spots in the core. Other processes that can lead to cold spots in cores include gas exsolution from pore water and adiabatic expansion of gas (Ussler et al., 2002).

A variety of approaches have been used to measure core temperatures to take advantage of the thermal impact of gas hydrate dissociation. One of the early systematic approaches, applied during ODP Leg 164, used an array of digitally recorded thermocouple probes (Paull, Matsumoto, Wallace, et al., 1996). Since those early efforts, two developments have markedly changed our ability to obtain useful information about gas hydrates from core temperature measurements. First, commercially available digital IR imaging cameras permit quantitative temperature estimates for each pixel in a recorded scene, and second, the advanced piston corer methane (APCM) tool, initially deployed during Leg 201, is now deployed on a routine basis, providing temperature, pressure, and conductivity information at the top of APC cores at 1 s intervals (Ussler et al., 2006). By combining these two developments, it is theoretically possible to forward-model the thermal behavior of individual cores, including the impact of gas hydrate dissociation, thus constraining gas hydrate concentrations in a manner not previously possible. Initial development of the IR imaging technique was accomplished during Leg 201, where thermal anomalies in IR images were associated with gas hydrate and voids (Ford et al., 2003). Systematic IR thermal imaging of the surface of the core liner was first fully implemented during ODP Leg 204 (Trhu, Bohrmann, Rack, Torres, et al., 2003; Trhu et al., 2004). For Expedition 311, an improved IR track system was used in which the IR camera traveled along the core on a skate guided by a rail/belt system. This system automatically stitches images and produces a single IR image and temperature array for each core.

The primary benefits of using IR cameras include

  • Rapid identification of gas hydrate for sampling from temperature anomalies on the surface of the core liner,
  • Quantification of the relative proportions of different gas hydrate textures,
  • Assessment of the thermal structure of entire cores and the differences in thermal structure between APC and XCB cores,
  • Estimation of the cross-sectional temperature gradient in cores prior to sampling for microbiology and to support postcruise thermal modeling of gas hydrate abundance and dissociation kinetics, and
  • Estimation of gas hydrate volumes from processed images.

The IR camera is also quicker and simpler to use and has a much higher spatial resolution than an array of thermocouples. The resolution of thermal anomalies observed indicates that the camera can detect small volumes of gas hydrate if they are adjacent to the core liner. Determining quantitative estimates of gas hydrate in cores was an objective during Leg 204 that was accomplished only after extensive postcruise analysis of collected data (e.g., Trhu et al., 2004). For Expedition 311, many of the analyses of the IR data were completed shipboard and concatenated images of each core were made available as prime data for the cruise. Substantial postcruise analysis will also be required to fully exploit the IR data.

Methodology

During Expedition 311, two ThermaCam SC 2000 cameras and a ThermaCam SC 500 camera (all FLIR Systems) were used to map temperature variations along cores. The FLIR Systems cameras provide temperature-calibrated images over a temperature range from –40 to 1500C. For shipboard measurements, the cameras were set to record a more limited range of temperatures from –40 to 120C (Range 1). To perform the critical task of rapid identification of gas hydrate within the core on the catwalk, one of the IR cameras was mounted on a track above the catwalk and driven automatically by a stepper motor controlled by custom software. The camera track and software were provided by GeoTek, Ltd. The camera was mounted with the lens 33 cm above the core liner, providing a 15.5 cm field of view along the core. To minimize the effect of external IR radiation reflecting off the core-liner surface, the camera was enclosed by black felt (Fig. F11A, F11B). The catwalk was also shaded with 17 mm thick plywood. Images and data for each core were acquired immediately after the core liner was wiped nearly dry and positioned with the top of the core at a fixed zero point. The camera moved along the track in 13 cm increments, starting 6.5 cm from the top of the core. During the scan, images were saved in FLIR Systems proprietary format, as bitmap images, and as temperature arrays. Bitmap images and temperature arrays were automatically concatenated and output as single files for analysis. A physical property scientist, a Co-Chief Scientist, and other personnel on the catwalk observed the scan results by looking at one of four monitors connected to the computer controlling the scan. The locations of thermal anomalies were identified from the concatenated images on the catwalk monitors and whole-round samples (e.g., gas hydrate, IW, and microbiology samples) were collected as defined by the core-sampling plan for the hole.

The core liner–mounted IR camera was supplemented on most cores by discrete imaging using a second, handheld SC 2000 camera (Fig. F11C, F11D) to obtain section-end IR images as quickly as possible after sections were cut. These images were collected using a device to mate the camera lens and the core liner, providing a fixed focal length while minimizing stray IR radiation from the catwalk environment. The top and bottom of each core section selected for microbiological sampling was imaged and systematically recorded and uploaded to a shipboard data server. Additional images were acquired for further analysis of the thermal history of cores.

The IR images were internally calibrated to a thermocouple/temperature logger (Thermistor model 6001-075, Barnant Company, Barrington, IL, and YSI 709B, Yellow Springs Instrument Co.; temperature accuracy = 0.15C; time constant = 1.1 s) in the field of view of each IR image. A set of external, known emissivity standards was imaged at the "home" position for the IR camera just prior to each scan. The standards included imbedded temperature logger (HOBO Pendant, Onset Computer Corporation; see "Monitoring the catwalk environment" for specifications) and light intensity measurements once per minute. An additional HOBO Pendant logger was also deployed on the opposite side of the IR camera field of view from the YSI temperature probe.

Monitoring the catwalk environment

A total of eight temperature measurement devices were deployed on the catwalk, including the three discussed above. Catwalk data were collected during Expedition 311 to ensure that any temperature gradient along the catwalk could be corrected for if necessary. HOBO Pendant temperature loggers were deployed at five locations along the core rack and provided a record of temperature and light intensity as a function of time and position on the catwalk (temperature accuracy = 0.47C; temperature resolution = 0.1C; light intensity = 0–320,000 lux, with an equivalent response >40% of the human eye from 400 to 1130 nm wavelength). The HOBO Pendant loggers were positioned at –0.3, 2.35, 3.9, 6.2, and 7.7 m along the catwalk relative to the top of the core rack and at ~5 cm below the lower edge of the core liner. In addition, a single point midway on the catwalk was monitored for both temperature and humidity (HOBO Pro RH/Temp H08-32-08; temperature accuracy = approximately 0.2C; temperature resolution = approximately 0.05C; relative humidity range = 0%–100%; drift = <3%/y, except for when humidity is >70%, in which case drift can be >3%/y). This device was attached to the plywood sun shield, 5 m horizontally from the top of the core rack and ~75 cm above the core. Prior to processing the first cores and again near the end of Expedition 311, an ice bath calibration check was performed on most temperature loggers used on the catwalk or for core temperature measurements. All tested devices returned values that were within manufacturer=s specifications.

Monitoring core equilibration by direct contact measurement of core temperature

Direct measurement of core temperature was made on a routine basis using weatherproof temperature loggers (HOBO H08-008-4) and stainless steel-sheathed thermocouples (TMC6-HC; temperature accuracy = 0.5C; temperature precision = 0.41C; response time = 3 min in air, 15 s in stirred water). The probes were inserted ~3 cm into the center of the bottom of three to four sections per core. Temperature measurement was started after sections were brought into the core laboratory for thermal equilibration. Thermocouples were checked for accuracy in ice-water baths prior to the beginning of coring and once again during the last week of the expedition. Full thermal equilibration of cores typically took as much as 4 h, and temperature probes were left in the cores until the temperature was greater than ~17C. It was not possible to collect probe temperatures on all cores because of the degree of induration of accretionary wedge sediments and because of limitations in the number of available temperature probes. Direct-contact temperature measurements were not routinely done on the catwalk because of the difficulty of handling temperature probes in the catwalk environment and because of the quality of the IR images. However, as a cross check, selected cores were monitored with direct-contact temperature probes inserted 10 cm into the bottom of cores immediately after the first IR scan. These probes were left in place as long as possible, usually ~10 min, while the core was being sampled and cut into sections. The typical temperature probe arrangement was one probe in the center, two probes directly beneath the liner on opposites sides of the core, and one probe between the center and liner probes.

Infrared image processing and extraction of thermal anomaly data

Following initial image concatenation and creation of temperature arrays, temperature images for each core were combined to make montages of downhole temperature anomalies. Temperature arrays for each core were processed in a spreadsheet by averaging cross-core temperatures at a given pixel depth after removing the outer edges and central portion of the array to eliminate thermal artifacts along the sides of the core and a central reflection from the IR camera. The resulting averaged temperatures at each depth were then concatenated into a downcore array and plotted. Thermal anomalies were identified from the downcore temperature profiles. An analysis of thermal data on board showed that T values indicative of gas hydrate were relatively insensitive to ambient catwalk temperature and illumination conditions. The T values provide an approximate measure of gas hydrate abundance, albeit influenced by the proximity of gas hydrate to the core liner. Gas hydrate undergoing dissociation and directly in contact with the core liner produces a larger T than gas hydrate insulated from the liner by sediment. It is important to note that depth measurements from the IR scans are relative to uncut core liners, prior to sectioning and removal of gas voids. Depth assignments of IR temperature anomalies must be adjusted to precisely match the curated depths of core sections.

Correlation of infrared thermal anomalies with interstitial water chlorinity anomalies and headspace gas composition

Selection of catwalk samples for IW chemical analyses was, in part, based on IR anomalies. For selected samples with varying T values, IR images and visible digital images were taken to identify internal parts of the sample that were still cold. Subsampling could then be more selective for parts of the sample formerly containing gas hydrate. For most of the samples, gas hydrate had already dissociated, but the thermal signal was almost always obvious, allowing designation of the sediment type (sand, silt, or clay) hosting the gas hydrate in situ.

Special experiments

Selected sections with and without gas hydrate were removed from the catwalk immediately after sectioning and taken to the core laboratory. Sections were immediately run through the MST and then instrumented with temperature probes similar to those used for direct measurement of core temperatures (see above). Time-lapse IR images were obtained using a FLIR SC-500 IR camera. After ~15 min, the section was returned to the MST, and the process was repeated until thermal equilibrium was achieved (~2–4 h).

Multisensor track

The MST has five physical property sensors mounted on an automated track that sequentially measure magnetic susceptibility (MS), GRA density, VP , natural gamma ray emissions, and noncontact electrical resistivity (NCR). During Expedition 311, we recorded MS, GRA, VP , and NCR data. Whole-core MST measurements are nondestructive to sediment fabric and can be used as proxies for other data as well as for facilitating core-to-core correlation between adjacent holes at the same site or among different sites. Data quality is a function of both core quality and sensor precision. Optimal MST measurements require a completely filled core liner with minimal drilling disturbance. Precision is a function of measurement time for MS and GRA density but not for VP . The spatial interval used for all sensors was 2.5 cm.

Magnetic susceptibility

Magnetic susceptibility was measured using a Bartington MS2 meter with a 88 mm diameter sensor coil at a 565 Hz frequency and an alternating field of 80 A/m (0.1 mT) with the sensitivity range set to 1.0 Hz. Data were archived as raw instrument units (SI) and not corrected for changes in sediment volume or drift during the course of a run. The data are reported as "uncorrected volume susceptibility."

Gamma ray attenuation density

GRA density was determined using the GRA densitometer. This sensor system measures the attenuation (mainly by Compton scattering) of a gamma beam caused by the average electron density in the gamma path. A well-collimated gamma beam (primary photon energy = 662 keV) is produced from a small (370 MBq; ~1994) 137Cs source (half life = 30.2 y) and passes through an assumed known thickness of sediment (internal diameter of core liner).

The measurements are empirically related to the bulk density of the material, and hence, the data are often referred to as "wet bulk density." However, we believe this to be confusing and prefer to refer to this data set as "gamma density," or GRA density, which we then compare to wet bulk density measured by MAD gravimetric techniques. Although the empirical calibration procedure for GRA is based on bulk density measurements (i.e., of a known graduated aluminum and water standard), the measurements will vary from true gravimetric bulk density because of variations in mineralogy. Gamma attenuation coefficients for different materials vary as a function of atomic number. Fortunately, most earth-forming minerals have similar and low atomic numbers (similar to aluminum). Consequently, the correlation of GRA density and bulk density is usually very good. In summary, GRA density should be considered to be the density of sediment and rocks as determined from GRA measurements using aluminum and water as reference materials.

The gamma source collimator is 5 mm, which produces an effective downcore spatial resolution of ~1 cm. Following our detailed setup and calibration procedure, we logged the graduated aluminum and water standard as a check to confirm accurate calibration. These data indicate an excellent calibration and illustrate the downcore spatial resolution (Fig. F12). The minimum integration time for a statistically significant GRA density measurement is 1 s. During many ODP legs, a count time of 2 s was used; however, we considered this count period to be too short and used a 5 s count time throughout Expedition 311 to improve precision. A freshwater control was run with each section to measure instrument drift. GRA data are of highest quality when measured on nongassy APC cores because the liner is generally completely filled with sediment. In XCB cores, GRA measurements can often be unreliable (unless the sample points are very carefully chosen) because of the disturbance caused by the mixing of drilling slurry and core biscuits.

Noncontact resistivity

The NCR system installed during Leg 204 rapidly measures sediment resistivity on a whole core in the plastic liner. The NCR technique operates by inducing a high-frequency magnetic field in the core from a transmitter coil, which in turn induces electrical currents in the core that are inversely proportional to the resistivity. A receiver coil measures very small magnetic fields that are regenerated by the electrical current. To measure these very small magnetic fields accurately, a difference technique has been developed that compares the readings generated from the measuring coils to the readings from an identical set of coils operating in air. As with other parameters, the measurements are sensitive to core temperature and should be obtained in a stable temperature environment for best results. The NCR calibration is shown in Figure F13.

Compressional wave velocity

Transverse VP was measured on the MST with the P-wave logger (PWL). The PWL transmits a 500 kHz P-wave pulse through the core at a specified repetition rate (50 pulses/s). The transmitting and receiving ultrasonic transducers are aligned so that wave propagation is perpendicular to the core axis. Core diameter is measured using two displacement transducers that are mechanically linked to the ultrasonic transducers. The recorded velocity is the average of the user-defined number of acquisitions per location (10 during Expedition 311). Calibrations of the displacement transducers and measurement of electronic delay within the PWL circuitry were conducted using a series of acrylic blocks of known thickness and P-wave traveltime. Repeated measurements of VP through a core liner filled with distilled water at a known temperature were used to check calibration validity. The PWL was generally not used when cores were full of gas expansion cracks or when they were taken with the XCB, as the poor core quality precluded reliable velocity measurements.

Moisture and density analysis

MAD analyses measure wet mass, dry mass, and dry volume to determine moisture content, grain density, bulk density, porosity, and void ratio, as described in Blum (1997). Push-core samples of ~10 cm3 were placed in 10 mL beakers. Care was taken to sample undisturbed parts of the core and to avoid drilling slurry. Immediately after the samples were collected, wet sediment mass was measured. Dry mass and volume were measured after samples were heated in an oven at 105 5C for 24 h and allowed to cool in a desiccator. Sample mass was determined to a precision of 0.01 g using two Scientech 202 electronic balances and a computer averaging system to compensate for the ship=s motion. Sample volume was determined using a helium-displacement pentapycnometer with a precision of 0.02 cm3. Volume measurements were repeated five times, until the last two measurements exhibited <0.01% standard deviation. A reference volume was included within each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. Sampling frequency was one or two per section. The mass and volume of the evaporated pore water salts were calculated for standard seawater salinity and density at laboratory conditions (1.024 g/cm3), and an average seawater salt density of 2.20 g/cm3.

Thermal conductivity

Thermal conductivity measurements on whole-core samples were made using the TK04 (Teka Bolin) system described by Blum (1997). Measurements were generally made on every second core. The measurement system employs a single-needle probe (Von Herzen and Maxwell, 1959) heated continuously in "full-space configuration." At the beginning of each measurement, temperature in the sample was monitored automatically, without applying a heater current, until the background thermal drift was <0.04C/min. Once the sample equilibrated, the heater circuit was closed and the temperature rise in the probe was recorded. The needle probe contains a heater wire and calibrated thermistor. The probe is assumed to be a perfect conductor because of its high conductance relative to the core sediments. With this assumption, the temperature of the probe has a linear relationship with the natural logarithm of the time after the initiation of heating:

T(t) = (q/4k)ln(t) + C, (3)

where

  • T = temperature,
  • q = heat input per unit length per unit time,
  • k = thermal conductivity,
  • t = time after the initiation of the heat, and
  • C = constant.

The thermal conductivity was determined using Equation 3 by fitting the temperatures measured during the first 150 s of each heating experiment (for details see Kristiansen, 1982; Blum, 1997).

The reported thermal conductivity value for each sample is the average of three repeated measurements. Data are reported in watts per meter degree Kelvin (W/[mK]), with measurement errors of 5%–10% in high-quality cores. Unless otherwise specified, no corrections for in situ temperature or pressure conditions were made to thermal conductivity values reported in tables and figures (Pribnow et al., 2000). Throughout Expedition 311, we compared the thermal conductivity data to regional trends defined by Davis et al. (1990), who reported the relation

T(d) = 1.07 + 0.000586 x d – 0.000000317 x d2, (4)

where the depth (d) is in meters.

Contact electrical resistivity measurements

Within the physical properties laboratory, electrical resistivity was the first of the contact measurements to be made on the working half of split cores to reduce evaporation of pore water, which affects the measurements. The split cores were wrapped in cellophane after cutting to further decrease water loss if they could not be processed within an hour or two after being split. The instrument used during Expedition 311 was constructed by Randolph Enkin (Geological Survey of Canada). The experimental apparatus uses a four-pin Wenner electrode array. Each electrode is 3 mm in length and spacing between electrodes is 2.5 mm (Fig. F14). Gold-plated electrodes minimize corrosion. The probe was pushed into the sediment and a 90 Hz square wave of 18 V amplitude and 10 k resistance (i.e., 1.8 mA current) was sent between the outer two electrodes. Because the resistance of the sediment (<2 ) is negligible compared to the resistance of the circuit generating the current, the voltage between the inner two electrodes should be proportional to the sediment resistivity. The sediment resistivity was derived by measuring the voltage between the two inner electrodes. Alternating current was used rather than direct current (DC) to prevent charge buildup around the electrodes and unwanted electrochemical effects. The temperature of the sediment was also recorded, and the resistivity of the sediment was corrected to the resistivity at 20C.

Electrical resistivity was measured along the split core every 10 cm in the uppermost ~10 m and at 20 to 30 cm intervals below this depth, avoiding drilling mud and cracks in the sediment fabric. The probe was set up so that it was perpendicular to the bedding (i.e., parallel to the core). In selected places, the probe was rotated 90° to be parallel to the bedding, and measurements were repeated with the probe in this position to allow detection of possible anisotropy.

The probe was cleaned regularly to prevent buildup of sediment residue on the electrodes, which can lead to inaccurate results. When results became erratic, the electrodes were replaced. The probe electrodes were also washed in distilled water and dried before calibrating. The probe was calibrated every two cores while in use, using standard mean ocean water.

Directional P-wave measurement

P-wave velocities were measured in three orthogonal directions on the split core (Fig. F15): PWS1 is the velocity along the axis of the core, PWS2 is perpendicular to the core in the plane along which it was split, and PWS3 is perpendicular to the core and to PWS2. PWS1 and PWS2 transducers were mounted onto a set of stainless steel blades that were sunk into the sediment. The PWS1 transducers were separated by 69.65 mm, and the PWS2 transducers were separated by 34.85 mm. The PWS3 transducer separation is determined by the size of the core. One transducer in each pair emits a 500 kHz, 100 V, peak-to-peak square wave. The traveltime between the transducers can either be picked by hand or picked automatically. Velocity was calculated by dividing the transducer separation distance by the traveltime. Temperature affects the results and was therefore measured and recorded as the velocity measurements were being made. During Expedition 311, temperature in the laboratory was reasonably constant at ~22°C, and cores had equibrated to room temperature when VP measurements were made.

The PWS1 and PWS2 transducers were calibrated using distilled water with a known velocity, which was corrected for temperature. PWS3 transducers were calibrated using a variety of different standards followed by a series of velocity measurements through distilled water. The transducers were calibrated for every core.

P-wave measurements can be made as frequently as needed whenever cores are undisturbed enough to yield a meaningful result. The measurement should be made in an undisturbed part of the core. P-wave velocities are generally more reliable in shallow core sections. Cores from deeper in the subsurface often contain many cracks and voids caused by gas expansion, causing the instruments to give unreliable results. During Expedition 311, velocity measurements were generally made every 20 cm in the upper 5–10 m of each hole.

Shear strength

Shear strength measurements were made on the split sections of cores after the contact electrical resistivity and P-wave velocity measurements. There are two devices that can measure shear strength: an AVS and a handheld Torvane. When using the AVS, a vane with four blades is inserted into the sediment. The vane is generally rotated 90/min. The torque applied to the vane is recorded before, during, and after sediment failure. The maximum torque recorded is the shear strength of the sediment. The Torvane comes in three sizes (19, 25, and 48 mm), which measures a maximum shear stress up to 20, 100, and 250 kPa, respectively. Each size records on a continuous scale of 0–10 units, and measurements are multiplied by 2, 10, and 25, respectively, to obtain shear stress in kilopascals.

One to three shear strength measurements were made per core section. Unfractured segments of the split core that were generally at least 5 cm away from places that had been disturbed by the P-wave measurements were chosen to make shear strength measurements. Measurements were made regularly throughout all holes at each site. Lower in the cored interval, gas expansion during recovery had usually strongly modified the sediment fabric. When making measurements on deeper cores, an effort was made to identify those sections of the core that were least disturbed.

In situ temperature

Four different downhole temperature tools were used during Expedition 311. The advanced piston corer temperature (APCT) tool fits into the cutting shoe of the APC and measures temperature during regular piston coring. We also tested a prototype for a new generation of this tool (see "Third-generation advanced piston corer tool") that is being developed by A. Fisher and H. Villinger (see Heesemann et al., this volume). In more indurated sediments where piston coring is not possible, we used the Davis-Villinger Temperature Probe (DVTP) or the Davis-Villinger Temperature-Pressure Probe (DVTPP). When deploying the DVTPP, we only attempted to acquire temperature measurements because the ship=s heave was generally too large for pressure measurements.

Advanced piston corer temperature tool

The APCT tool consists of electronic components, including battery packs, a data logger, and a platinum resistance-temperature device calibrated over a temperature range of 0–30C. Descriptions of the tool and data analysis principles can be found in Pribnow et al. (2000) and Graber et al. (2002) and 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 5–10 min near the seafloor to record bottom water temperatures and to hold it for ~10 min in the hole 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. The tool can be preprogrammed to record temperatures at a range of sampling rates. A sampling rate of 10 s was used during Expedition 311.

A typical temperature history recorded by the APCT tool is shown in Figure F16A. It consists of a mudline temperature record lasting 5 min. 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, a frictional pulse upon removal of the corer, and a second mudline temperature measurement for 5 min. The in situ temperature is determined by extrapolating from the thermal decay that follows the frictional pulse when the piston is fired. Details of this process and the associated uncertainties are discussed in the individual site chapters.

During the LWD phase of Expedition 311, we undertook a series of bench and tank tests to calibrate the APCT tool relative to the third-generation advanced piston corer temperature (APCT-3) tool prototype temperature tool, which had been calibrated to an accuracy of better than 0.01C three days before the start of Expedition 311 (see "Third-generation advanced piston corer temperature tool"). For these tests, we transferred the tools rapidly from a cold-water bath that had reached a stable temperature in the reefer to a room-temperature bath located just outside the reefer. As a result of these tests, we selected APCT16, which had a response time that was similar to the APCT-3 tool and an offset relative to the APCT-3 tool that was large (~0.97C) but relatively stable over a large range of temperatures. This offset was confirmed by observations of bottom water temperatures made with both tools.

Third-generation advanced piston corer temperature tool

The APCT-3 tool was designed to replace the APCT tool, which is no longer supported by the manufacturer. Like the APCT tool, the APCT-3 tool fits into the cutting shoe of the APC. Data are recorded in solid-state memory. The larger memory capacity, compared to the APCT tool, allows for a finer sampling rate (up to 1 Hz). Expedition 311 was the first field test for this new tool. For additional tool information, see Heesemann et al. (this volume).

Davis-Villinger Temperature Probe

The DVTP is 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 second is 12 cm above the tip. A third thermistor is in the electronics package. Thermistor sensitivity is 1 mK in an operating range from –5 to 20C. In addition to the thermistors, the probe contains an accelerometer sensitive to 0.98 m/s2. The accelerometer data are used to track disturbances to the instrument package during the equilibration interval. Data were recorded at a sampling rate of 3 s. A typical deployment of the DVTP showing the two temperature records from the probe is shown in Figure F16.

Unlike the APCT and APCT-3 tools, the DVTP requires a dedicated tool run, which consists of lowering the tool by wireline to the mudline, where there is a 5–10 min pause to collect temperature data within the drill pipe. 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 temperature is recorded for 10–20 min. Upon retrieval, a second stop of 5–10 min is made at the mudline. For discussion of ad hoc calibration of the DVTP relative to the APCT-3 and APCT tools, see "Physical properties" in the "Site U1329" chapter.

Davis-Villinger Temperature-Pressure Probe

Simultaneous measurement of formation temperature and pressure can be achieved using a modified DVTPP. 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 from –5 to 20C. A typical deployment of the tool 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. Unfortunately, excessive ship heave and time limitations did not allow for optimum deployment of this tool for obtaining in situ pressure. For temperature measurements only, the tool is operationally similar to the DVTP.

Thermal data reduction

Similar data reduction procedures were used for all temperature tools. Because equilibration to in situ temperatures takes much longer than the 10 min during which the instrument records subseafloor temperature, extrapolation based on the theoretical impulse response of the tools is required. The transient thermal decay curves for sediment thermal probes are known to be a function of the geometry of the probes and the thermal properties of the probe and the sediments (Bullard, 1954; Horai and Von Herzen, 1985). Analysis of data requires fitting the measurements to model decay curves calculated based on tool geometry, sampling interval, and tool and sediment thermal properties. For the APCT and APCT-3 tools, decay curves based on the model of Horai and Von Herzen (1985) were used, as implemented in TFIT. A new, more accurate numerical model for the impulse response of the APCT-3 tool is currently under development by M. Heesemann. For the DVTP and DVTPP, the impulse response of Davis et al. (1997), as implemented in CONEFIT, was used.

It is generally not possible to obtain a perfect match between the model temperature decay curves and the data because

  • The probe does not reach thermal equilibrium during the penetration period;
  • Contrary to ideal theory, the frictional pulse upon insertion is not instantaneous; and
  • Temperature data are sampled at discrete intervals so that the exact time of penetration is uncertain.

Additional uncertainty in the in situ temperature occurs because of tradeoffs between sediment thermal conductivity, penetration time, and temperature and because of poorly understood effects related to the presence of gas hydrate (Hartman and Villinger, 2002; Trhu, 2006). During Expedition 311, both the effective penetration time and equilibrium temperature were estimated by applying a least-squares fitting procedure, which involves shifting the synthetic curves in time to obtain a match with the recorded data. Generally, data within ~704 s after the apparent penetration time were not used. Laboratory thermal conductivity measurements were not corrected for in situ conditions because the correction would be small at the shallow depths drilled during Expedition 311. Postcruise processing will be necessary to better quantify and understand the uncertainties in the data and the effects of gas hydrate.

Advanced piston corer methane tool and pressure core sampler methane tool

The APCM tool and the pressure core sampler methane (PCSM) tool continuously record temperature, pressure, and electrical conductivity changes in the core headspace from the time the core is cut through its ascent to the rig floor. Both are derivatives of Monterey Bay Aquarium Research Institution's (MBARI's) Temperature-Pressure-Conductivity tool. The APCM sensors are mounted in a special piston head on the standard APC piston, and the data acquisition electronics are embedded within the piston. The PCSM tool is a slimmed-down version of the APCM tool and 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 in situ gas and gas hydrate can be determined from the pressure and temperature behavior during core recovery (Ussler et al., 2006).

Both tools are very similar in construction, the only difference being that the APCM tool 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 2 inch diameter (5.08 cm) sensor head. The temperature sensor has an accuracy of 0.05C. The pressure sensor is a 0 to 10,000 psi (0–68.95 MPa) "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 0.04 in (10 mm) diameter Kovar pins. The data port is a three-pin keyed bulkhead connector for RS-232 serial 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 battery pack provides 7.3 V, with a 100 mA rating. The APCM tool is installed on the APC piston after the APC piston-rod snubber and piston head body are 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 tool. This swap-out operation takes <3 min. The PCSM tool replaces the accumulator on the PCS and threads onto the top of the PCS manifold mandrel.

The APCM tool was deployed successfully once and the PCSM tool was deployed successfully several times during Expedition 311 (see Table T3 in the "Expedition 311 summary" chapter and the "Operations," "Physical properties," and "Pressure coring" sections in each site chapter). Examples of data are shown in the "Site U1328" chapter, but data interpretation was deferred until postcruise.

Paleomagnetism

Note: This section was contributed by Jennifer Henderson and Katerina Petronotis (Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station TX 77845, USA).

No scientist sailed as a paleomagnetist for Expedition 311 because of limited space. However, routine measurements of the remanent magnetization of archive-half sections before and after alternating-field (AF) demagnetization were made on cores.

Instrumentation and measurement procedures

Nonmagnetic core barrels were used, but the Tensor tool was not deployed.

Paleomagnetic analysis involved routine measurements of remanent magnetization using a pass-through cryogenic magnetometer equipped with a DC-superconducting quantum interference device (DC-SQUID; 2G Enterprises model 760-R). An AF demagnetizer aligned along axis with the magnetometer and set within the magnetometer=s mu-metal shielding allowed uniform demagnetization of the cores so that the remanent magnetization could be measured before and after demagnetization. An automated sample-handling system moved the core sections through the AF coils and magnetometer sensor region. The standard IODP magnetic coordinate system was followed, in which +x is vertical upward from the split surface of archive halves, +y is the left split surface when looking upcore, and +z is downcore.

The remanent magnetization was measured in 10 cm increments along each archive-half section before and after AF demagnetization. AF demagnetization was applied at 10 and 20 mT to remove the drill string magnetic overprint. Disturbed intervals and voids were manually noted on the "cryomag log sheets" and entered in LongCore. At the beginning of each shift, a measurement of the background tray magnetization was taken and subtracted from all measurements. The tray was demagnetized between cores. All data were stored using the standard IODP file format.

Discrete samples for magnetic analyses

Discrete samples were collected from working halves of core sections in round plastic tubes marked with arrows pointing upcore on the face that represents the split surface of the core. Sampling intervals were every 50 cm, but areas of deformation were avoided. The samples were stored in sample bags and flushed with nitrogen before being sealed and stored at 4C. Discrete samples will be analyzed postcruise by Randolph Enkin (Geological Survey of Canada).