PHYSICAL PROPERTIES

Shipboard measurements of physical properties can be used to provide an initial look at variations in the recovered core material that 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 unsplit core sections were made with the MST. Next, whole-round hard rock cores were scanned using the Deutsche Montan Technologie (DMT) Digital Color CoreScan. After splitting the cores, additional measurements were made of P-wave velocity on split cores of soft sediment samples and on discrete samples of hard rock. Bulk density, grain density, porosity, and water content were calculated from index properties measurements on discrete samples. Thermal conductivity measurements were also made on unsplit sediment cores and split hard rock cores. The instruments and apparatus used during Leg 197 are discussed in Blum (1997) and are outlined below.

Multisensor Track Measurements

The MST consists of four physical properties sensors on an automated track that measure magnetic susceptibility, bulk density, compressional wave velocity, and natural gamma ray emissions on whole-round core sections. During Leg 197, magnetic susceptibility and gamma ray attenuation (GRA) bulk density were measured on soft sediment cores and natural gamma ray measurements were made on both sediment and hard rock cores; the compressional wave logger was not used, as the rotary core barrel cores did not fill the core liner.

Magnetic Susceptibility

Magnetic susceptibility was measured on most sediment sections at 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. Magnetic susceptibility was not measured on hard rock cores. 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.

Gamma Ray Attenuation

The GRA densitometer estimates bulk densities on unsplit core sections using a sampling period of 5 s every 5 cm. GRA data are most reliable in undisturbed cores and can often be directly correlated with the downhole density logs. In hard rock core sections, GRA acquisition was turned off. In disturbed soft sediment cores, GRA density would be expected to have lower values.

Natural Gamma Ray Emissions

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 197, 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 source.

Core Imaging

The whole-round basalt cores were scanned using the DMT Digital Color CoreScan after they had been run through the MST. This had three main objectives:

  1. To provide a comprehensive suite of 360 unrolled digital core images,
  2. To identify and measure planar features on the unrolled images for comparison with core structural analysis and integration with geographically orientated FMS images, and
  3. To correlate and ultimately reorient core images and structural data to true geographic north using the general purpose inclinometer tool (GPIT) data on the FMS tool string.

These data offer the potential to derive paleomagnetic declination data from the otherwise azimuthally unoriented basalt cores. Such declination data could contribute significantly to the tests of hotspot fixity central to Leg 197.

The DMT Color CoreScan system is a portable core imaging device that was previously used on board the JOIDES Resolution during Legs 173 (Whitmarsh, Beslier, Wallace, et al., 1998) and 176 (Dick, Natland, Miller, et al., 1999). Images are recorded on whole-round outer core surfaces using a charge coupled device line-scan camera, which has a resolution of 5184 pixels/m and a spectral response of between 400 and 700 nm (DMT, 1996).

The whole-round core is rotated 360 around its cylindrical axis with the line-scan camera positioned parallel to the axis of rotation. The unrolled images are recorded in 33-cm sections that can be integrated into 1-m sections using the DigiCore software. The whole-round cores are scanned at a rate of ~1.20 min/m, creating a ~14-MB bitmap file (DMT, 1996).

During Leg 197, all core pieces that could be rotated cleanly through 360 were scanned in unrolled mode. Pieces that were not fully cylindrical or intervals of drilling breccia were not imaged, but the lengths of these intervals were measured so that allowance could be made for them when integrated into core barrel lengths using the DMT CoreLog software (DMT, 1996). The vertical line marked on the core with a red grease pencil allows an initial reorientation of the core images back to the ODP reference frame. Initial structural analyses were performed; however, the majority of the structural analysis, core-log integration, and core reorientation work was done postcruise.

Thermal Conductivity

Thermal conductivities were measured on unconsolidated sediment and rock samples using the TK04 system as described by Blum (1997). These measurements are used along with 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 sediment samples 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 ~10-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 four repeat measurements for the half-space method. All results are in units of watts per meter per degree Kelvin.

Index Properties Measurements

Samples of ~10 cm3 were collected from the fresh sediment cores at a frequency of one per section to allow for determination of index properties. 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 100-110C. Wet and dry masses were measured with two Scientech electronic balances that compensate for the ship's motion; dry volume was determined with a helium-displacement Quantachrome penta-pycnometer. For hard rock sections, rubble fragments and chips left after cutting of paleomagnetism minicore samples were collected at a frequency of approximately one per core. The samples were soaked in seawater for 24 hr, then index 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 = salt density (2.257 g/cm3) and
Ms = 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 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

Wd (% dry mass) = [(Mt - Md)/(Md - rMt)] x 100, and
Ww (% wet mass) = [(Mt - Md)/(1 - r)Mt] x 100.

The bulk density (b) is the density of the saturated sample is

b = Mt/Vt,

where Vt = 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)] x 100,

where w = the density of the pore fluid (seawater).

Velocity Determinations

For sediment sections, velocity determinations were made by the PWS3 contact probe system. The system was used to measure P-wave velocities in the x-direction of each sample (i.e., perpendicular to the core axis) (see "Paleomagnetism and Rock Magnetism" for a detailed description of sampling coordinates). In hard rock, velocity determinations in the x-direction were made on the minicores drilled for paleomagnetic measurements. P-wave velocities in the y- and z-directions were also measured on selected minicores.

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