PHYSICAL PROPERTIES

Introduction

Measurements of the following physical properties were recorded during Leg 176: natural gamma-ray emissions, bulk density (wet and dry), compressional wave velocity, magnetic susceptibility, thermal conductivity, and electrical resistivity.

Physical properties measurements were performed on the cores in three phases:

1. The whole-core sections (fully intact core in split liners), once equilibrated to laboratory temperatures (>18ºC), were run through the MST system. This measured three parameters: bulk density, magnetic susceptibility, and natural gamma-ray (NGR) emission. Compressional wave (P-wave) velocity was not recorded using the MST during this leg because of poor acoustic coupling between the liner and core.
2. Following core splitting, thermal conductivity measurements were performed on representative half-round core pieces.
3. Following sampling, the minicores were tested for P-wave velocity, resistivity, and index properties (wet and dry mass and dry volume).

Figure F9 shows a flow diagram of core flow for physical properties measurements for ODP hard-rock boreholes.

MST Measurements

Bulk Density (Gamma-Ray Attenuation)

The gamma-ray attenuation densiometer (GRAPE) allowed determination of wet bulk density. This was achieved by measuring the attenuation (Compton scattering) of gamma rays passing through the unsplit cores; the degree of attenuation being proportional to natural bulk density (Boyce, 1976; Gerland and Villinger, 1995). The system was calibrated during the cruise using an aluminum density standard. This consisted of four cylinders of varying diameters in a distilled-water-filled core liner; the four different diameters resulted in four different bulk densities. The GRAPE measurements are compromised where the core does not completely fill the core liner, as is the case for all hard-rock cores. Actual measurements were taken at a 4-cm interval, with an integration time of 10 s, and results were output in grams per cubic centimeter.

Magnetic Susceptibility

Whole-core magnetic susceptibility was measured using a Bartington MS2C meter with an 80-mm (internal diameter) loop sensor. Measurements were made at 4-cm intervals, with an integration time of 10 s. The principal controls for the rate of data acquisition of the MST system were the rate of arrival of new core and the acquisition rates of the NGR tool. Longer integration periods could not be chosen because of the limited time available for each measurement. For shipboard analysis, the magnetic susceptibility data were converted to nominal volume-normalized 10^-5 SI units based on the assumption that the geometric correction factor of 0.66 applied to sediment cores could be used to approximate the correction factor for hard-rock cores.

Natural Gamma-Ray Emission

Natural gamma-ray emission was routinely recorded for all core sections, both to monitor variations in radioactive counts of sample rocks and to provide a correlation with the geophysical logging. The NGR system records radioactive decay of ⁴⁰K, ²³²Th, and ²³⁸U, three long-period isotopes that decay at an essentially constant rate within measurable time scales. Both the total gamma-ray count and the individual counts from these three isotopes (and therefore their relative contents) were recorded. The installation and operating principles of the NGR system used on board JOIDES Resolution are discussed by Hoppie et al. (1994)

The area of influence for the four NGR sensors was about ±10 cm from the points of measurements along the core axis. As gamma-ray emission is a random event, count times have to be sufficiently large to average for short-period variation. This was achieved on the MST system by utilizing the long area of influence on the sensors and using a moving average window to smooth count rate variations and to achieve a statistically valid sample.

The NGR was calibrated in port against a thorium source. The measurements were made for 10 s every 4 cm, using a twofold moving average window. Results were output in GAPI units for ease of comparability with borehole logging.

Thermal Conductivity

Half-core specimens were nondestructively measured for thermal conductivity using a single-probe Teka (Berlin) TK-04 unit. A half-space needle probe, containing a heater wire and a calibrated thermistor, was clamped onto the flat surface of the half core. Good coupling with the needle probes was ensured by flattening and smoothing the core surface with 240 carbide grit on a glass plate. Thermal conductivity was further improved by the application of a thermally conductive compound between the needle and sample. The samples and needles were then immersed in seawater. This procedure has been used since Leg 140 (Shipboard Scientific Party, 1992), with the TK-04 unit first being used for hard-rock thermal conductivity measurements during Leg 169, and for which full procedures are reported (Shipboard Scientific Party, 1998).

At the beginning of each test, temperatures in the samples were monitored automatically, without applying a heater current, until the background thermal drift was determined to be less than 0.04ºC/min. The heater circuit was then closed, and the temperature rise in the probe was recorded.

This technique proved highly sensitive to small variations in ambient temperature. Core samples and monitor needles were therefore equilibrated to a constant temperature by immersion in a covered tank of sea-water for at least 1 hr before measurements were taken. In addition, immersion in seawater kept the samples saturated, improved the thermal contact between the needle and the sample, and reduced thermal drift during the tests. Adding a sample or needle to the seawater while the test sample was equilibrating was enough to distort the measurements. Therefore, after any disturbance of the needle or sample, it was left to further equilibrate for at least 15 min to prevent acquisition of erroneous data.

Thermal conductivities were calculated from the rate of temperature rise while the heater current was flowing. The meter was calibrated during the cruise against four standards of known thermal conductivity: black rubber, red rubber, basalt, and Macor, which have a similar conductivity range to the tested samples. Temperatures were recorded during a time interval of 80 s, and data were reported in units of W/(m·K).

Compressional (P-) Wave Velocities

The pulse transmission method was employed to determine compressional wave velocity. This utilized piezoelectric transducers as sources and detectors in a screw-press modified Hamilton Frame, described by Boyce (1976). All measurements were made on seawater-saturated minicores cut perpendicular to the axis of the core (diameter = 2.54 cm; approximate length = 2 cm) at a confining pressure of zero. A small number of minicores cut parallel to the core axis were also tested.

Minicores for P-wave velocity measurements were taken approximately once per core section. The minicores were resaturated with seawater in a vacuum for 24 hr before measurement. Flat ends of the minicores were polished with 240 carbide grit on a glass plate to ensure parallel faces. The length of each minicore was checked using a caliper along its circumference, and grinding continued until all length measurements were within 0.02 mm. Before measurement, the grit was removed by thoroughly cleaning the samples in an ultrasonic bath. Distilled water was used to improve the acoustic contact between the sample and the transducers.

Calibration measurements were performed during the cruise using polycarbonate standard minicores of varying lengths to determine the zero-displacement time delay inherent in the measuring system. Results were recorded in meters per second.

Electrical Resistivity

The electrical resistivity of hard-rock samples was measured on seawater-saturated minicores at room temperature and atmospheric pressure using a two-electrode cell to measure resistance. The method used for derivation of resistivity on board JOIDES Resolution was described during Leg 158 (Shipboard Scientific Party, 1995b).

The cell consisted of a nylon holder and spring-loaded aluminum holders. The holder was designed to have the same diameter as the minicores to minimize leakage along the sides of the sample. The measurements were performed with the cylindrical surfaces of the minicores wrapped in Teflon tape to prevent a short circuit between the two ends.

The samples were resaturated in seawater under a vacuum for 24 hr before measurement. The minicore faces were cut smooth and parallel to allow good contact with the electrodes, and seawater was used to ensure good electrical coupling between the electrical contacts and the minicores. The apparatus was calibrated during the cruise against aluminum standard minicores. Because of time constraints, resistivity measurements were made only on a subset of available minicores. Measurements were made at 5 V AC and 50 kHz frequency and were recorded in ohm-meters.

Index Properties

Index properties, as defined for ODP shipboard procedures, were calculated from measurements of wet and dry masses and dry volumes. The sample mass was counterbalanced by a known mass, such that only mass differences of less than 2 g were measured.

Wet mass measurements were made on minicores that were resaturated with seawater in a vacuum for 24 hr. Samples were then oven dried for 24 hr before measurement of dry mass. Volumes were determined using a Quantachrome Penta-Pycnometer, a helium-displacement instrument. The pycnometer measures volumes to a precision of about ±0.005 cm³, which corresponds to a volume error of 1%. Sample volumes were repeated until the last two measurements had standard deviations smaller than 0.01%.

Water content, bulk density, grain density, dry density, porosity, and void ratio were determined following the procedures outlined in Blum (1997). The pycnometer was calibrated during each measurement run against a sphere of known volume. Mass data were output in grams for convenience, and density data were output as grams per cubic centimeter.