- T(t)
= (q/4
k)
· ln (t) + L(t), (3)
Shipboard physical properties determinations may be correlated with core lithology, downhole geophysical results, and regional seismic data. The principal objectives of the physical properties measurement program were as follows:
We measured physical properties on unsplit cores and on the undisturbed parts of split cores. We used the MST for nondestructive measurements of wet bulk density, compressional wave velocity, magnetic susceptibility, and natural gamma radiation in unsplit cores. Thermal conductivity was measured on unsplit soft-sediment cores and split lithified sediment and hard-rock cores and three-directional compressional wave (P-wave) velocities on both soft- and hard-rock cores. Portions of split cores that were undisturbed by drilling provided specimens for measurement and calculation of index properties (wet bulk density, grain density, dry bulk density, water content, void ratio, and porosity).
Measurements were made after cores had been allowed to stand for 2-4 hr to equilibrate approximately to ambient room temperature (i.e., 22°-24°C). All instruments and apparatuses used in the shipboard laboratory and the principles of the methods employed were described by Blum (1997). Below, we summarize each type of physical properties measurement made during Leg 192.
The MST includes four physical properties sensors (magnetic susceptibility meter, gamma ray attenuation (GRA) densitometer, P-wave logger, and natural gamma radiation detector). We placed individual, unsplit core sections on the MST, which automatically moves the section through the sensors on a fiberglass track. MST data were taken at discrete intervals chosen to optimize data resolution within the time limitations of running each core section through the device.
We determined magnetic susceptibility on all sections at 4-cm intervals using the 1.0 (1 s integration time) range on the Bartington meter (model MS2C), which has an 88-mm coil diameter. The MS2C meter measures relative susceptibilities, which are not corrected for the difference between core and coil diameters. Magnetic susceptibility helps detect variations in magnetic properties caused by lithologic changes or alteration. The quality of the data is degraded in RCB sections if the core liner is not completely filled or the core is disturbed. However, general downhole trends may still be used for correlation with well logs. During Leg 192, we also measured magnetic susceptibility with a point susceptibility meter on the AMST. We routinely compared data from the two instruments.
Bulk density was estimated for unsplit core sections as they passed through the GRA densitometer using a sampling period of 2 s every 4 cm on the MST. The gamma ray source was 137Cs. For each site, we compared the bulk density obtained from the densitometer and the bulk densities measured on discrete samples. GRA bulk density data are most reliable in undisturbed cores and offer the potential of direct correlation with downhole bulk density logs. We did not acquire GRA bulk density data on highly fragmented cores. Where cores were not filling the liners or were disturbed or fractured, we expect the GRA density to be generally lower.
The P-wave logger (PWL) operates simultaneously with the GRA and transmits a 500-kHz compressional wave pulse through the core at a frequency of 1 kHz. A pair of displacement transducers monitors the separation between the P-wave transducers. Data are collected at 4-cm intervals. MST measurement of P-wave velocities typically is not possible on RCB cores because of the loss of coupling between the liner and the core and thus was not done on most of the Leg 192 cores.
Measurement of natural gamma radiation depends on the random and discrete decay of radioactive atoms and is measured with scintillation detectors as outlined by Hoppie et al. (1994). During Leg 192 we measured natural gamma radiation (NGR) every 4-cm length of core. Results were output in counts per second. The NGR detector system was calibrated in port against a thorium source, and sample standards were measured at the end of operations at every site.
Thermal conductivity is the rate at which heat is transmitted by molecular conduction. It is an intrinsic material property that depends on the chemical composition, porosity, density, structure, and fabric of the material. Thermal conductivity profiles of sediment and rock sections are used mainly, along with temperature measurements, to calculate heat flow. Heat flow is not only characteristic of the material but also helps to indicate the age of ocean crust and fluid circulation processes at a range of depths (e.g., Blum, 1997). During Leg 192, we used the TK04 (TeKa, Berlin) system to acquire thermal conductivity data. This system employs a continuously heated single-needle probe (von Herzen and Maxwell, 1959) used in full-space configuration for soft sediments and in half-space mode for lithified sediment and hard-rock samples. We typically acquired data for every core. The thermal conductivity value reported for each sample is the average of three (full-space method) or four (half-space method) repeated measurements. Data are reported in units of watts per meter per degree kelvin (W/[m·K]).
Full cores of unconsolidated sediment were measured for thermal conductivity by inserting a full-space single-needle probe (containing a heater wire and a calibrated thermistor) into the sediment through a small hole drilled in the core liner before the sections were split. Temperatures, recorded for times between 60 and 240 s, were fitted to the following equation using the least-squares method (von Herzen and Maxwell, 1959):
k)
· ln (t) + L(t), (3)
where
The term L(t) corrects for temperature drift, as described in the following equation:
where A represents the rate of temperature change and Te is the equilibrium temperature. L(t) therefore corrects for the background temperature drift, systematic instrumental errors, probe response, and sample geometry. The best fit to the data determines the values of k and A.
We made half-space determinations on selected lithified sediments and basaltic rock samples after the cores were split and their faces were polished. The half-space needle probe was secured to the polished face. The needle probe rested between the polished surface and a grooved epoxy block with relatively low thermal conductivity (Sass et al., 1984; Vacquier, 1985). We conducted half-space measurements in a water bath to keep the samples saturated, to improve the thermal contact between the needle and the sample, and to reduce thermal drift. Data collection and reduction procedures for half-space tests are similar to those for full-space tests except for a multiplicative constant in Equation 4 that accounts for the different experimental geometry.
We extracted one ~10-cm3 sample from each section of freshly cut core to determine index properties. We calculated bulk density, grain density, water content, porosity, and dry density from wet and dry sample mass and dry volumes. Sample mass was determined using two Scitech electronic balances equipped with a computerized averaging system that corrected for ship accelerations. The sample mass was counterbalanced by a known mass, such that the mass differentials generally were <1 g. Using a helium-displacement Quantachrome Penta-Pycnometer, we determined sample volumes at least three times, until consistent readings were obtained. A standard reference volume was included with each group of samples during the measurements and rotated among the cells to check for instrument drift and systematic error. After the samples were oven dried at 105° ± 5°C for 24 hr and allowed to cool in a desiccator, we measured dry weights and volumes (method C of Blum, 1997). The following relationships were computed from two mass measurements and dry volume measurements (taken from Blum, 1997, pp. 2-2 to 2-3). First, we subtracted the beaker mass and volume (determined periodically and stored in the program's "look-up" table) from the measured total mass and volume, yielding the following directly measured values:
Variations in pore-water
salinity (s) and density (
pw
) that typically occur in marine sediments do not affect the calculations
significantly, and standard seawater values at laboratory conditions (Boyce,
1976) are used:
pw
= 1.024 g/cm3.
Pore-water mass (Mpw), mass of solids (Ms), and pore-water volume (Vpw) are then calculated as follows:
pw
= (Mb - Md)/(1 - s)pw
. (7)
To account for the phase change of pore-water salt during drying, the mass and volume of salt (Msalt and Vsalt , respectively) are required. For practical purposes, the mass of salt is the same in solution and as a precipitate, whereas the volume of the salt in solution is negligible.
salt
= [(Mb - Md)s/(1 - s)]/salt
, (9)
where the salt density salt
= 2.20 g/cm3 is calculated for average sea salt. Moisture content is
the pore-water mass expressed either as percentage of wet bulk mass (Wb
) or as percentage of the mass of salt-corrected solids (Ws ):
Calculation of the volume of solids and bulk volume is as follows:
Bulk density (b),
density of solids or grain density (s),
dry density (d),
porosity (P), and void ratio (e) are then calculated according
to the following equations:
b
= Mb/Vb , (14)
s
= Ms/Vs , (15)
d
= Ms/Vb, (16)
For discrete P-wave velocity measurements in split cores, we used the PWS3 contact probe system with a signal frequency of 500 kHz. We measured P-wave velocity (500 kHz) once or twice per section and in more than one direction of the core where possible. Distilled water constituted the coupling fluid at the transducer/core interface. Sediments were measured in half liners or in discrete samples taken from the core. We sampled crystalline and basaltic rocks as minicores (2.54 cm in diameter) drilled perpendicular to the axis of the core (x-direction) or sawed as oriented cubes. The ends of the minicores were trimmed parallel to each other with a rock saw, and we measured traveltimes and distances along the axis of the minicore. For cubes, velocities were determined in two or three mutually perpendicular directions, Vz (along the core), Vx (into the split core, perpendicular to core axis), and Vy (across the split core). Velocity anisotropy follows the relationship
where Vmax and Vmin are the maximum and minimum velocities among Vx, Vy, and Vz. During Leg 192 we made velocity determinations either adjacent to paleomagnetic minicores or directly on the minicores to save core material. We calculated velocities using the corrected traveltime and path length. Velocity data are reported in raw form. In intervals where discrete sampling was sparse and if time allowed, we made additional measurements on split pieces of hard rocks. Velocities from these measurements may have slightly lower values because the contact surface tended to be comparatively irregular. Nevertheless, velocities determined from hard-rock pieces should exhibit downhole trends similar to those determined from discrete samples.
