b),
in grams per cubic centimeter, is the density of the saturated sample,Shipboard measurements of physical properties are used for characterizing lithostratigraphic units, correlating cored material with downhole logging data, and interpreting seismic reflection profiles.
After recovery, the cores are allowed to come to room temperature (22°-23°C) before thermal conductivity, bulk density, magnetic susceptibility (MS), compressional wave velocity, and natural gamma radiation (NGR) are measured in a series of nondestructive tests. Additional measurements of P-wave velocity, vane shear strength, bulk density, porosity, and water content are made on split cores or discrete samples.
Four sets of measurements, MS, gamma-ray attenuation (GRA) bulk density, P-wave velocity, and NGR are made in sequence on whole-core sections on the MST. MST data are sampled at discrete intervals, with the sampling intervals and count times chosen to optimize the resolution of the data in the time available to run each core section through the device. During Leg 191, MS, GRA bulk density, and NGR were measured on all APC and XCB cores. RCB cores were not passed through the MST because only fragments of chert or basalt were recovered; none of the intervening sediment survived the coring process.
The MST includes a Bartington susceptibility meter (model MS2C) that has an 8-cm loop and operates at 0.565 kHz with a field intensity of 80 A/m. Volume susceptibility, k, is a dimensionless measure of the degree to which material can be magnetized in an external magnetic field:
where M is the magnetization induced in the material by an external field of strength H. Magnetic susceptibility is sensitive to variations of the type and content of magnetic grains in the sediment and is thus an indicator of compositional variations. During Leg 191, susceptibility was measured for 3 s on 2-cm intervals on all APC and XCB sections. Four-centimeter intervals at the top and bottom of each section were not sampled because of end effects.
GRA by Compton scattering is actually a measure of electron density. This method is useful for estimating the bulk densities of sediments and crystalline rocks because the ratio Z/A of atomic number/atomic mass of elements that make up the common rock-forming minerals is essentially constant (see Blum, 1997). Porosity is estimated from GRA bulk density by using an assumed grain density.
The GRA bulk densitometer measures the attenuation of a collimated beam of gamma-ray rays from a 137Cs source as it passes through a sample of known thickness (Boyce, 1976). Having a well-known path length is critical to acquiring reliable GRA bulk densities, so the method was restricted to APC and XCB cores, which usually fill the core liner. GRA bulk densitometer data were acquired during Leg 191 for 4 s at intervals of 2 cm, except for the top and bottom 2 cm of each section.
Measured P-wave velocities are useful for interpreting seismic reflection profiles and correlating lithology with downhole sonic logs. The P-wave logger on the MST transmits a 500-kHz ultrasonic pulse through the core. Velocities are measured perpendicular to the long axis of the core, and a pair of displacement transducers is used to measure the separation between the compressional-wave transducers. During Leg 191, P-wave velocities were measured at 2-cm intervals on APC and XCB cores.
In nature, gamma-ray emissions result from the decay of the unstable elements 40K, 232Th, and 238U. On the time scale of the measurements, these elements decay at constant rates, and the level of gamma-ray emissions depends on their concentration in the sediment. NGR emissions recorded in the laboratory can be correlated with the downhole measurements of natural gamma-ray emissions. The operating principles of the NGR system is described by Hoppie et al. (1994).
During Leg 191, NGR was measured for 60 s at intervals of 20 cm on all APC and XCB cores. The "area of influence" of the four NGR detectors is about ±10 cm along the core axis from the point of measurement. As gamma-ray emission is a random event, count times have to be sufficiently long to average out short-period variations. Averaging is achieved on the MST by the long area of influence of the sensors and by applying a moving-average window to smooth count rate variations and to achieve a statistically valid measure of gamma-ray emissions.
The NGR was calibrated using a thorium source. Results are output in counts per second, which can be qualitatively compared to the American Petroleum Institute (API) units obtained from borehole logging.
Thermal conductivity is measured by transient heating of a material with a known heating power generated from a source of known geometry and then measuring the temperature change with time, using the TK04 system described by Blum (1997). Thermal conductivity profiles of sediments and rock sections are used along with temperature measurements to estimate heat flow. Heat flow varies with the age of oceanic crust and fluid circulation processes at depth (Blum, 1997). Whole-round core sections are allowed to adjust to room temperature for at least 2 hr in preparation for thermal conductivity measurements. In the case of soft sediments, thermal conductivity is measured on whole-core sections. The thermal conductivity of hard materials is measured on split-core pieces (working half). The needle probe method is used in full-space configuration for soft sediments (von Herzen and Maxwell, 1959) and in half-space mode (Vacquier, 1985) for lithified sediment and hard-rock samples. Measurements were made at an interval of one per core (whole-round or split core).
In comparatively soft sediments, P-wave velocities are measured in three directions using the PWS1, PWS2, and PWS3 (Hamilton Frame) systems. The PWS3 is also used to measure velocities in hard-sediment split cores and in discrete samples (cut cores and cubes) of harder sediments or crystalline rocks.
The PWS1 and PWS2 systems measure P-wave velocities using two pairs of digital sound velocimeters that are inserted in soft sediments. One pair (PWS1) is aligned with the core axis (the z-direction, normal to bedding), and the other (PWS2) is aligned perpendicular to the core axis (in the y-direction, parallel to bedding). The transducer pairs have fixed separations of 7 cm (vertical) and 3.5 cm (horizontal), respectively. The received 500-kHz ultrasonic signal is digitized by an oscilloscope; the first arrival is picked automatically or manually using a threshold criterion, and the velocity is calculated from the transducer separation and the propagation time.
The PWS3 system is a modified and updated version of the classic Hamilton Frame velocimeter, in which one transducer is fixed and the other is mounted on a screw. The PWS3 is mounted vertically to measure velocities in the x-direction (i.e., perpendicular to both the core axis and PWS2). The PWS3 can be used to measure P-wave velocities in either discrete samples or split cores by placing the sample or the core liner on the lower transducer and bringing the upper transducer into direct contact with the upper surface. To improve the coupling (i.e., the impedance match) between the transducer and the sample, water is commonly applied to the top and bottom of the sample and transducer heads. Traveltimes are picked manually or automatically by the threshold method as described above, and the transducer separation is recorded by a digital caliper. Measurements on split cores are corrected for the additional path length and traveltime of the core liner. During Leg 191, P-wave velocities were measured on each APC and XCB split-core section and in a number of discrete samples of basalt recovered from basement.
The peak undrained and residual shear strength of soft sediment was measured at an interval of one per split-core section using a Wykeham-Farrance motorized vane shear apparatus following procedures described by Boyce (1977). The vane rotation rate was set to 90°/min, and the vane used for all measurements had a 1:1 blade ratio with a dimension of 1.28 cm. The vane shear instrument measures the torque and strain at the vane shaft using a torque transducer and potentiometer. The reported shear strength is the peak strength determined from the torque vs. strain analysis.
Interpretations of vane shear measurements assume that a cylinder of sediment is uniformly sheared around the axis of the vane in an undrained condition, with cohesion as the principal contributor to shear strength. Departures from this assumption include progressive cracking within and outside of the failing specimen, uplift of the failing core cylinder, drainage of local pore pressures, and stick-slip behavior.
Samples of ~10 cm3 were taken from each APC and XCB sediment section to measure the index properties. Bulk density, grain density, water content, porosity, and dry density were usually calculated from wet and dry sample weights and dry volumes. In a few cores, density and porosity were estimated from the wet and dry sample weights and the wet sample volume. Sample mass is determined to a precision of ±0.001 g using two Scitech electronic balances. The balances are equipped with a computer averaging system that compensates for the motion of the ship. The sample mass on one balance is counterbalanced by a known mass on the adjacent balance. Sample volumes are determined using a five-cell Quantachrome Penta-Pycnometer helium-displacement pycnometer with a nominal precision of ±0.01 cm3. Sample volumes are measured at least three times and then averaged. A standard reference sphere is run sequentially in each of the five operating cells to maintain calibration. The cell volume is recalibrated if the measured volume of the standard is not within 0.02 cm3 of the known volume of the standard. The beakers that are used for soft sediment samples were calibrated before the cruise. Dry weight and volume measurements were performed after the samples were oven dried at 105° ± 5°C for 24 hr and allowed to cool in a desiccator. A potential problem with this drying temperature is that chemically bound water in some clay minerals can be lost along with interstitial water.
Water content as a fraction of total mass or as a ratio of water mass to solid mass is determined by standard methods of the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1989). The total (water saturated) mass (Mt) and dry mass (Md) are measured using the electronic balance as described above, and the difference is taken as the uncorrected water mass. Measured wet and dry masses are corrected for salt assuming a pore-water salinity (r) of 0.35% (Boyce, 1976). The wet and dry water contents (Wd and Ww [in percent]) are given, respectively, by
Bulk density (b),
in grams per cubic centimeter, is the density of the saturated sample,
b
= Mt
/Vt,
where
Grain density (g),
in grams per cubic centimeter, is determined from the dry mass and dry volume
measurements. Both mass and volume must be corrected for the salt content of the
pore fluid:
g
= (Md
- Ms)
/ [Vd
- (Ms
/s)],
where
s
= the density of salt (2.257 g/cm3),
Porosity (
)
is the ratio of pore-water volume to total volume and can be calculated from the
fluid density, grain density, and bulk density of the material:
= [(g
- b)
/ (g
- w)]
× 100,
where
g
= grain density,
b
= bulk density, and
w
= density the pore fluid, which is assumed to be seawater.
The dry density (d)
is the ratio of the dry mass (Md)
to the total volume (Vt).
The dry density is calculated from the corrected water content (Wd)
and porosity ():
d
= (
/ Wd)
× w.
