g) can be calculated from the measurements of dry mass (Md) and dry volume (Vd). Both of these values need to be corrected to take into account the salt content of the pore fluid,
Shipboard measurements of physical properties can be used to provide an initial look at variations in the recovered core material, which may be used to characterize lithologic units, correlate with downhole geophysical logging data, and interpret seismic reflection data. After the cores had attained room temperature, nondestructive tests of the whole-round (unsplit) core sections were made with the MST. After splitting the cores, additional measurements were made of P-wave velocity on split cores of soft sediments and on discrete samples of hard rock. Bulk density, grain density, porosity, and water content were calculated from moisture and density measurements on discrete samples. Thermal conductivity measurements were also made on whole sediment cores and split hard rock cores. The instruments and apparatus used during Leg 200 are discussed in Blum (1997) and are outlined below.
The MST consists of four physical property sensors on an automated track that measures magnetic susceptibility, bulk density, compressional wave velocity, and natural gamma ray emissions on whole-round core sections. During Leg 200, magnetic susceptibility, gamma ray attenuation (GRA) bulk density, and natural gamma ray were measured on both soft sediment cores and hard rock cores; compressional wave velocities were measured using the P-wave logger (PWL) on APC cores.
Magnetic susceptibility was measured on all sections at 1- or 2.5-cm intervals using the 1.0 range (1-s integration time) on the Bartington meter (model MS2C), which has an 88-mm coil diameter. The magnetic susceptibility data aid in detecting variations in magnetic properties caused by lithologic changes or alteration. The quality of the magnetic susceptibility measurement is somewhat limited in sedimentary cores if they are disturbed. However, general trends may still be useful for correlation with geophysical logging data. The units for the stored data and instrument sensitivity are given in "Paleomagnetism".
GRA measurements estimate bulk densities on unsplit core sections using a sampling period of 5 s every 1 or 2.5 cm. GRA data are most reliable in undisturbed cores and can often be directly correlated with the downhole density logs. In disturbed soft sediment cores, GRA density would be expected to give values lower than the true values for undisturbed sediments.
Natural gamma ray (NGR) emissions result from the decay of radioactive atoms and were measured in the laboratory by scintillation detectors as described by Hoppie et al. (1994). During Leg 200, NGR measurements were made at intervals of 10 cm for a period of 20 s on soft sediment and for a period of 60 s on hard rock cores. Results were output in counts per second, which can then be compared qualitatively with the downhole logging data. The NGR was calibrated using a thorium and potassium source.
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). For full-core soft sediment sections, a hole was drilled in the outer core liner and a 2-mm temperature probe was inserted into the working half of the core section. For hard rock samples, a half-space needle probe was secured on ~5-cm 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 repeat measurements for the full-space method and of four repeat measurements for the half-space method. All results are in units of watts per meter degree Kelvin.
Samples of ~10 cm3 for sediments and <10 cm3 for hard rock samples were collected at a frequency of one per section to allow for determination of moisture and density. Samples were taken from undisturbed parts of the core if possible. Wet sediment mass was measured immediately after the samples were collected. Dry sediment mass and dry sediment volume were determined after the samples had been dried in a convection oven for 24 hr at a temperature of 100°-110°C. Wet and dry masses were measured with two Scientech 202 electronic balances that compensate for the ship's motion; dry volume was determined with a helium-displacement Quantachrome penta-pycnometer. For hard rock sections, the samples were soaked in seawater for 24 hr, then moisture and density properties were measured using the same procedure as for the sediment sections.
Grain density, moisture content, bulk density, and porosity were calculated from wet and dry mass and dry volume as discussed by Blum (1997), who also gives values of seawater density, seawater salt density, and seawater salinity used in the calculation. Grain density (g) can be calculated from the measurements of dry mass (Md) and dry volume (Vd). Both of these values need to be corrected to take into account the salt content of the pore fluid,
g = (Md - Ms)/(Vd - [Ms/s]),
where s is salt density (2.257 g/cm3), and Ms is the mass of salt in the pore fluid.
The uncorrected water mass is taken as the difference between the total (water saturated) mass (Mt) and the dry mass (Md). The measured wet and dry masses are corrected for salt content using a pore water salinity (r) of 0.35% (Boyce, 1976). The wet and dry moisture contents are calculated by
The bulk density (b) is the density of the saturated sample,
b = Mt/Vt,
where Vt is the total sample volume.
Porosity (
) can be calculated from fluid density, grain density, and bulk density of the sample and is the ratio of pore water volume to total volume,
= [(g - b)/(g - w)] · 100,
where w is the density of the pore fluid (seawater).
For sediment sections, velocity determinations were made by the PWS3 contact probe system. Using this system, P-wave velocities on the x-, y-, and z- directions were measured on minicubes that were also used for shipboard paleomagnetic measurements.
