Physical properties were measured (1) to contribute near-continuous records of physical properties for hole-to-hole correlations, construction of complete stratigraphic sequences, and core-to-downhole log ties; (2) to examine gradients in physical properties, such as porosity, natural gamma radiation, magnetic susceptibility and compressional wave velocity, which are related to variations in sediment composition; and (3) to provide data to aid the interpretation of seismic reflection and downhole geophysical logs.
The first measurement station was the MST. Unsplit core sections were run through the MST after equilibration to laboratory temperature (>18°C). Four sensors on an automated track measured non-destructively bulk density, magnetic susceptibility, natural gamma-ray emission, and ultrasonic compressional wave velocity on whole-core sections. Next thermal conductivity was measured on the whole-core sections. Then the cores were split, and discrete P-wave velocity and vane-shear values were obtained. The working half was sampled for further physical measurements, including water content and grain density, to calculate bulk density, porosity, and related properties. The methods used to measure and calculate these properties are described in the following sections (also see Blum, 1997). The accuracy of gamma-ray emission, P-wave velocity, and magnetic susceptibility measurements degrades considerably in APC and XCB sections with gas voids or where the core otherwise does not fill the liner completely or is disturbed.
The GRAPE allows determination of wet bulk density by measuring the attenuation (Compton scattering) of gamma rays passing through the unsplit cores. The degree of attenuation is proportional to natural bulk density (Boyce, 1976; Gerland and Villinger, 1995) and was calibrated using a density standard consisting of aluminum samples of varying thickness in a seawater-filled core liner. Four different diameters of the aluminum result in four different average densities. Measurements were taken at 2-, 4-, or 10-cm intervals, with an integration time of 5 s, depending on sub-bottom depth and time constraints.
The P-wave logger (PWL) on the MST track continuously measures ultrasonic compressional wave velocities orthogonal to the core axis. The logger transmits a 500-kHz compressional wave pulse through the core at a repetition rate of 1 kHz. A pair of transducers ("displacement transducers") monitor the separation between the compressional wave transducers, so that variations in the outside diameter of the liner do not degrade the accuracy of the velocities. In cases of bad acoustic coupling between the sediment and the liner, the PWL generally does not provide accurate velocity values. The system is, therefore, most useful in undisturbed APC cores, and values become highly questionable when gas is present in the sediment. Usually, a pulse transmitting interval of 2 or 4 cm with an integration time of 5 s was chosen. Calibration of the displacement transducer and measurement of the electronic delay within the PWL circuits were carried out using a series of acrylic cylinders of known thickness and P-wave traveltime. The validity of the calibration was checked by measuring the P-wave velocity through a section of liner filled with distilled water with an error estimation of ~0.1% (Shipboard Scientific Party, 1994).
In addition to the continuous velocity measurements with the PWL on the MST track, ultrasonic compressional wave velocities were measured on split-core sections with digital sound velocimeter DSV1 and the modified Hamilton Frame. A piezoelectric transducer pair was inserted into soft sediments along (z-direction) the core axis, with the x-direction assigned to be vertical (modified Hamilton Frame). The orientation of x-, y-, and z-directions is shown in Shipboard Scientific Party (1996b). The velocity calculation is based on the fixed distance between the transducers (7 and 3.5 cm, respectively), the measurement of the traveltime of an impulsive ultrasonic signal (500 kHz), and a delay constant that can be determined by pulse-transmitting a distilled water standard. Periodically, the separation was precisely evaluated by running this calibration procedure on the standard sample. The value of ultrasonic velocity in distilled water can be determined for the measured laboratory temperature, based on standard equations (Wilson, 1960; MacKenzie, 1981). Use of the DSV1 was disregarded in more indurated sediments when the sediment started to crack during insertion of the transducers. In this case, a modified Hamilton Frame velocimeter was used to measure the traveltime of a 500-kHz signal orthogonally across the split core section and core liner (x-direction). Transducer distance was measured directly from the velocimeter-frame lead screw through a linear resistor output into a digital multimeter. Zero traveltimes for the velocity transducers were estimated by linear regression of traveltime vs. distance for a series of polycarbonate standards. Velocity data might be corrected for in situ temperature and pressure conditions, which could be made using the relationships from Wylie et al. (1956), Wilson (1960), and Mackenzie (1981).
Whole-core magnetic susceptibility was measured on a Bartington MS2C meter with an 80-mm (internal diameter) loop sensor at 2- or 4-cm intervals, with an integration time of 5 s. Longer periods could not be chosen because of the time constraints. Susceptibility values are archived in the JANUS database and are presented in the "Physical Properties" section of each site chapter (this volume) in raw instrument units, which require multiplication by a factor of 6.6 x 10-6 to convert to volume-normalized SI units.
Natural gamma-ray emission (NGR) was recorded at intervals between 4 and 50 cm in some core sections at selected holes. The area of influence for the four NGR sensors is about ±10 cm from the points of measurements along the core axis. The installation and operating principles of the NGR system used on the JOIDES Resolution are discussed by Hoppie et al. (1994).
Spectral gamma-ray analysis was conducted on data from 2048 energy channels that were collected and archived. Counts have been summed over the range 200 to 3000 keV (in 5 windows) to be comparable with data collected during previous legs. This integration range also allows direct comparison with downhole logging data, which are counted 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 18 counts per 30-s measurement period. No corrections were made on XCB core NGR data to account for sediment incompletely filling the core liner.
Before starting measurements, the four sensor gains were adjusted so that the combined potassium peak was as sharp as the individual peaks when the other three sensors were disabled. The multichannel analyzer was calibrated by assigning certain channels (a total of 2048 channels) to the characteristic energies of 40K and the main peak of 232Th.
Thermal conductivity is the measure of a material's ability to transmit heat by molecular conduction. This type of measurement is required for geothermal heat-flow determinations. Thermal conductivity was measured using needle probes in full-space configuration (von Herzen and Maxwell, 1959). Measurements were taken using a single-probe TeKa (Berlin) TK-04 unit after the cores had equilibrated to ambient temperature, about 3-4 hours after recovery.
Data are reported in W/(m·K) with an estimated error of 5% to 10%. Although the needle was heated, temperature T was measured with elapsed time t and related to the thermal conductivity of the sediment by
where k is the apparent thermal conductivity (in W/[m·K]), and q is the heat input per unit time and unit length in W/m2 (Shipboard Scientific Party, 1994). The term L(t) describes a linear change in temperature with time and includes the background temperature drift and any nonlinearity that results from instrumental errors and geometrical inadequacies of the experiment. These boundary conditions include the finite length of the probe and sample.
Index properties, as defined for ODP shipboard procedures, were calculated from measurements of wet and dry masses and dry volumes. Samples of ~10 cm3 were taken for determination of index properties. Usually 1-2 index samples per section were taken at the core location of the discrete velocity measurements.
Sample mass was determined using a Scitech electronic balance, resulting in a mass error within 0.1%. The sample mass was counterbalanced by a known mass so that only mass differences ~<5 g were measured. Volumes were determined using a Quantachrome Penta-Pycnometer, a helium-displacement pycnometer. The pycnometer measures volumes to a precision of about ±0.005 cm3, which equates with a volume error of 1%. Sample volumes were repeated until the last two measurements had standard deviations <0.01%.
Water content; bulk, grain, and dry densities; porosity; and void ratio were determined following the procedures outlined in Blum (1997). The procedures for determining these properties complies with the American Society for Testing and Materials (ASTM) (D) 2216 guidelines (ASTM, 1989). Bulk and grain densities and porosity are computed from the wet and dry masses of the sample and from dry volume, which was determined using Method C of Blum (1997).
The undrained vane shear strength was determined using a motorized miniature vane-shear device and following the procedures of Boyce (1977). The vane rotation rate was set to 90°/min. Measurements were made only in the fine-grained, soft-to-stiff units. A range of previously calibrated springs of various strengths were available. The instrument measures the torque and strain at the vane shaft using a torque transducer and potentiometer, respectively. The shear strength reported is the peak strength determined from the torque vs. strain plot. The residual strength was not routinely determined.
In the analyses of vane tests, the assumption is made that a cylinder of sediment is uniformly sheared about the axis of the vane in an undrained condition, with cohesion as the principal contributor to shear strength. This assumption is violated when progressive cracking within and outside the failing specimen, uplift of the failing core cylinder, drainage of local pore pressures (i.e., the test can no longer be considered to be undrained), or stick-slip behavior occurs. Evidence of cracking was noted in the comments section of the results file. When this condition occurred, a pocket penetrometer was used. The initial penetrometer measurements were converted from kilograms per square centimeter (kg/cm2) to kPa (1 kg/cm2 = 98.07 kPa) and then divided by 2 because the penetrometer is calibrated as an unconfined compression test, which (for the ideal clay) is equal to twice the undrained shear strength (Holtz and Kovacs, 1981).