Shipboard measurements of physical properties provide preliminary information on variations in the recovered core material, which can be used to characterize lithologic units, correlate with downhole geophysical logging data, and interpret seismic reflection data. After the cores had equilibrated to ambient room temperature, physical properties were measured on whole-round sections, undisturbed parts of split cores, and discrete samples.
Nondestructive measurements of wet bulk density, magnetic susceptibility, transverse compressional wave (P-wave) velocity, and NGR were made on whole-round sections using the MST. The MST has a gamma ray attenuation (GRA) bulk densitometer, a P-wave logger (PWL), a magnetic susceptibility meter, and an NGR sensor. Thermal conductivity measurements were also made on sediment whole cores and hard rock split cores. P-wave velocity measurements were made on sediment and hard rock cores and on discrete sample cubes (minicubes) of hard rock. Bulk properties of discrete samples were determined by moisture and density (MAD) analysis, which included measurements of wet bulk density, dry bulk density, grain density, water content, and porosity. A comprehensive discussion of all methodologies and calculations used in the JOIDES Resolution Physical Properties laboratory can be found in Blum (1997).
Visible and near-infrared spectroscopy (VNIS) analyses were performed on all MAD sediment samples as well as an additional sample per section to determine semiquantitative mineralogy. On hard rock core, VNIS analyses were performed directly on the archive split-core half. VNIS analysis was conducted for the determination of alteration and physical property variations and the effect of crustal age and extrusive style on alteration.
The MST consists of four physical property sensors on an automated track that measures magnetic susceptibility, bulk density, compressional wave velocity, and NGR emissions on whole-round core sections. During Leg 206, MS, GRA bulk density, and NGR were measured on both soft-sediment cores and hard rock cores; compressional wave velocities were measured using the PWL on APC cores only.
The measurement of wet bulk density by the GRA system is based on the principle that the attenuation, mainly by Compton scattering, of a collimated beam of gamma rays produced by a 137Ce source passing through a known volume of sediment is related to material density (Evans, 1965). Calibration of the GRA system was completed using known seawater/aluminum density standards. GRA bulk density data are of highest quality when determined on APC cores because the liner is generally completely filled with sediment. In XCB and RCB cores, GRA measurements are of lower quality and typically cannot be used to reliably determine bulk density on their own. The measurement width of the GRA sensor is ~5 mm, with sample spacing generally set at 2.5 cm for Leg 206 cores. The minimum integration time for a statistically significant GRA measurement is 1 s, and routine Leg 206 GRA measurements used either a 3- or 5-s integration time. A freshwater control standard was run with each section to measure instrument drift.
Whole-core magnetic susceptibility was measured with the MST using a Bartington MS2 meter coupled to a MS2C sensor coil with a diameter of 8.8 cm operating at 565 Hz. The measurement resolution of the MS2C sensor is 4 cm, with a minimum statistically significant count time of 1 s. During Leg 206, MST magnetic susceptibility was routinely measured every 2.5 cm, with five data acquisitions at each interval. Magnetic susceptibility data were archived as raw instrument units (SI) and not corrected for changes in volume, although a correction was made for instrument drift. The raw susceptibility measurements can be converted to SI volume units as described in "Susceptibility" in "Instruments and Measurements" in "Paleomagnetism" (see also Blum, 1997).
Transverse P-wave velocity was measured on the MST with the PWL for all APC cores. The use of the PWL on XCB and RCB cores was limited by poor acoustic coupling between the sediment and the core liner. The PWL transmits a 500-kHz compressional wave pulse through the core every 1 ms. The transmitting and receiving transducers are aligned perpendicular to the core axis, and a pair of displacement transducers monitors the separation between the compressional wave transducers. Variations in the outer diameter of the liner do not degrade the accuracy of the velocities, but the unconsolidated sediment or rock core must completely fill the liner for the PWL to provide acoustic coupling. Calibration of the displacement transducer 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 measurement of P-wave velocity through a core liner filled with distilled water was used to check calibration validity. The measurement width of the PWL sensor is ~1 mm, with sample spacing routinely set at 5 cm for Leg 206 APC cores.
NGR emissions of sediments are a function of the random and discrete decay of radioactive isotopes, predominantly those of uranium, thorium, and potassium, and are measured through scintillation detectors arranged at 90° to each other and perpendicular to the core. The installation and operating principles of the NGR system used on the JOIDES Resolution are discussed in Hoppie et al. (1994). Data from 256 energy channels were collected and archived. For presentation purposes, the counts were summed over the range of 200-3000 keV, so as to be comparable with data collected during previous legs. This integration range also allows direct comparison with downhole logging data, which are collected over a similar integration range (Hoppie et al., 1994). Over the 200- to 3000-keV integration range, background counts, measured using a core liner filled with distilled water, averaged 30 during a 1-hr measurement period. Before taking measurements, each of the four NGR amplifiers were adjusted so that the thorium peak was at the highest resolution possible when the other three amplifiers were disabled. The multichannel analyzer was then calibrated by assigning certain channels to the characteristic energies of 40K and the main peak of 232Th (Blum, 1997). The measurement width of the NGR is ~15 cm, with a statistically significant count time of at least 5 s, depending on lithology. The sample spacing of the NGR measurements was set at 5 cm for 6 s on the sediment cores and 5 cm for 20 s on the hard rock cores. No corrections were made to NGR data obtained from XCB and RCB cores to account for incomplete filling of the core liner.
The thermal conductivity was measured with the TK04 (Teka Bolin) system, using the needle-probe method in full-space configuration for soft sediments (von Herzen and Maxwell, 1959). The needle probe contains a heater wire and calibrated thermistor. It is assumed to be a perfect conductor because it is much more conductive than unconsolidated sediments. With this assumption, the temperature of the superconductive probe has a linear relationship with the natural logarithm of the time after the initiation of the heat:
where,
Thermal conductivity was measured on unconsolidated sediment and rock samples using the TK04 system as described by Blum (1997). These measurements are used, along with in situ temperature measurements, to estimate heat flow. The system uses a single-needle probe (Von Herzen and Maxwell, 1959) heated continuously in full-space mode for soft sediments and in half-space configuration for hard rock samples (Vacquier, 1985). A self-test, including a drift study, was conducted at the beginning of each cycle. Once the samples were thermally equilibrated, the heater circuit was closed and the temperature rise in the probes was recorded. Thermal conductivities were calculated from the rate of temperature rise while the heater current was flowing. Temperatures measured during the first 150 s of the heating cycle were fitted to an approximate solution of a constantly heated line source (for details see Kristiansen, 1982; Blum, 1997). For full-core soft-sediment sections, a hole was drilled in the outer core liner and a 2-mm-diameter temperature probe was inserted into the working half of the core section. For hard rock samples, a half-space needle probe was strapped onto ~5-cm-long split-core sections that had been immersed in a water bath for at least 15 min. The thermal conductivity measurement for each sample was the average of three repeated measurements for the full-space method and of four repeated measurements for the half-space method. All results are in units of watts per meter degree Kelvin. Thermal conductivity measurements were taken with a frequency of one per core except for instances when downhole temperature measurements were taken, in which case thermal conductivity was taken three times per core.
Samples of ~10 cm3 for sediments and <10 cm3 for hard rock were collected at a frequency one per section for sediment cores and one every other section or one per section for the hard rock cores (depending on the variability) to allow for determination of moisture and density. Samples were taken from undisturbed parts of the core where possible. Wet sediment mass was measured immediately after the samples were collected. Dry mass and volume were measured after samples were heated in an oven at 105°C ± 5°C for 24 hr and allowed to cool in a desiccator. Hard rock samples were soaked in seawater for 24 hr, and then moisture and density properties were measured using the same procedure as for the sediment sections.
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 volumes were determined using a Quantachrome penta-pycnometer, a helium-displacement pycnometer with a precision of 0.02 cm3. Volume measurements were repeated five times. All cells were calibrated after three sample runs to check for instrument drift and systematic error. A purge time of 3-5 min was used before each run. The procedures for the determination of these properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1990). Blum (1997) discusses the fundamental phase relations and assumptions for the calculations of all relevant phase relationships, summarized below.
Wet mass (Mwet), dry mass (Mdry), and dry volume (Vdry) are measured in the laboratory. Salt precipitated in sediment pores during the drying process is included in the dry mass and dry volume values. The mass of the evaporated water (Mwater) and the salt (Msalt) in the sample are given by
where s = assumed saltwater salinity (0.035) corresponding to a pore water density (
pw) of 1.024 g/cm3 and a salt density (salt) of 2.257 g/cm3. The corrected mass of pore water (Mpw), volume of pore water (Vpw), mass of solids excluding salt (Msolid), volume of salt (Vsalt), volume of solids excluding salt (Vsolid), and wet volume (Vwet) are, respectively,
pw ,
salt ,
salt , and
For all sediment samples, water content (w) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass:
Wet-bulk density (wet), dry-bulk density (dry), sediment grain (solid) density (solid), and porosity (
) are calculated from, respectively,
wet = Mwet/Vwet ,
dry = Msolid/Vwet ,
solid = Msolid/Vsolid , and
= Vpw/Vwet .
For the sediment cores, VNIS studies were conducted on the MAD sample powdered samples as well as an additional sample per section. The ~10-cm3 samples were heated in an oven at 105° ± 5°C for 24 hr and allowed to cool in a desiccator. Samples need to be dry or water will completely dominate the spectral signature. Samples were then crushed to reduce the variability in repeat total reflectance measurements. Light reflectance, at a bandwidth of 350 to 2500 nm, was found for each sample using the FieldSpec Pro FR portable spectroradiometer. Semiquantitative mineral concentrations were then calculated from the collected spectra, assuming a three-component system: calcite, opal, and smectite. For a complete description of the VNIS technique and calibration methods, refer to Vanden Berg and Jarrard (2002).
For sediment sections, velocity determinations were made using the Hamilton Frame PWS3 contact probe system. Using this system, P-wave velocities were measured at a frequency of one per section on all cores. For hard rock samples, x-, y-, and z- directional velocities were measured on the 2-cm x 2-cm x 2-cm minicubes that were also used for shipboard paleomagnetic measurements. The compressional wave velocity calculation is based on the accurate measurement of the delay time of a 500-kHz square wave signal traveling between a pair of piezoelectric transducers. The transducer pair for PWS3 is adjusted to the thickness of the core half or discrete sample. The separation between the fixed lower PWS3 transducer and the movable upper transducer is measured by a linear voltage displacement transducer. Deionized water was added to the contact between the transducers and sample to improve acoustic coupling.
The core temperature was recorded at the time velocity was measured; however, the velocity data stored in the Janus database are uncorrected for in situ temperature and pressure. These corrections can be made using the relationships outlined in Wyllie et al. (1956), Wilson (1960), and Mackenzie (1981).
