b) is the density of the saturated sample
Shipboard measurements of physical properties are used to for characterizing lithologic units, for correlating cored material with downhole logging data, and for interpreting seismic reflection profiles.
After recovery, the cores were allowed to come to room temperature (22°-23°C), then thermal conductivity, bulk density, and MS were measured in a series of nondestructive tests. Additional measurements of P-wave velocity, bulk density, porosity, and water content were made on right circular cylinders of basalt cut from the split core. The same samples were used for paleomagnetic measurements.
Four sets of measurements (MS, gamma ray attenuation [GRA] density, P-wave velocity, and natural gamma radiation [NGR]) can be made in sequence on whole-core sections on the MST. MST data are sampled at discrete intervals, with 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 203, no APC or XCB sediment cores were taken; consequently, only MS and GRA measurements were made on the basalt RCB cores
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,
k = M/H,where M is the magnetization induced in the material by an external field of strength H. MS is sensitive to variations of the type and content of magnetic grains in the sediment and, thus, is an indicator of compositional variations. During Leg 203, we sampled RCB basalt cores for 10 s at 2-cm intervals.
GRA by Compton scattering is actually a measure of electron density. The method is useful for estimating the bulk densities of sediments and crystalline rocks because the ratio Z/A of atomic number to atomic mass of elements that make up the common rock-forming minerals is essentially constant (see Blum, 1997). Porosity is estimated from GRA densities by using an assumed grain density.
The GRA densitometer measures the attenuation of a collimated beam of gamma rays from a 137Ce 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. RCB basalt cores were sampled for 10 s at 2-cm intervals. Path-length corrections must be made before these data can be used.
The P-wave logger was not used on RCB basalt cores.
The natural gamma radiation detector was not used on the RCB basalt cores recovered during Leg 203.
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, with temperature measurements, to estimate heat flow. The needle probe method is used in full-space (von Herzen and Maxwell, 1959) configuration for soft sediments and in half-space mode (Vacquier, 1985) for lithified sediment and hard rock samples. The thermal conductivity of hard materials is measured on split-core pieces (working half). Measurements were made at an interval of one per core.
The PWS3 (Hamilton Frame) was used to measure velocities in discrete samples (minicores) of basalt. The PWS3 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 (perpendicular to the core axis) by placing the sample 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, and the transducer separation is recorded by a digital caliper. During Leg 203, we measured P-wave velocities in discrete samples of basalt recovered from basement.
The minicores of basalt used for the velocity measurements were also used to estimate bulk density, grain density, and porosity from the volumes and wet and dry weights of the samples. Volumes were calculated from the dimensions of the samples. Sample mass is determined to a precision of ances 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 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. Dry weight and volume measurements were made after the samples were oven dried at 105 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 in addition to IW.
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 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) are given by
Wd (% dry mass) = [(Mt - Md)/(Md-rMt)] x 100 andwhere Mt is the mass of the saturated sample.
Bulk density (b) is the density of the saturated sample
b = Mt/Vt ,where Vt is the total sample volume, which is estimated from the dimensions of the minicores or from the volume of the dry sample (Vd) and the volume of the pore fluid (Vw) (see below):
Vt = Vd + Vw .
Grain density (g) 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 Md is the mass of salt in the pore fluid and s is the density of salt (2.257 g/cm3).
Since Ms = rMw, Mw, the salt-corrected mass of the seawater, is given by,
Mw = (Mt - Md)/(1-r).The volume of pore water is
Vw = (Mt - Md)/(1-r)w .Porosity (f) is the ratio of pore water volume to total volume and can be calculated from fluid density, grain density, and bulk density of the material,
f = [(g - b)/(g-w)] x 100,
where g is the grain density, b is the bulk density, and w is the density of 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 (f),
d = (f/Wd) x w .