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doi:10.2204/iodp.proc.309312.102.2006

Physical properties

Shipboard measurements of physical properties were undertaken to provide preliminary information on variations in the recovered core material. After equilibrating to room temperature, measurements of physical properties were made on whole-round sections, undisturbed parts of split cores, and discrete samples. Nondestructive measurements of bulk density, magnetic susceptibility, and NGR were made on whole-round sections using the MST. Thermal conductivity measurements were made on split cores. Measurements of P-wave velocity and moisture and density properties were made on discrete sample cubes (~9 cm3). A comprehensive discussion of all methodologies and calculations used in the JOIDES Resolution physical properties laboratory can be found in Blum (1997).

Multisensor track measurements

The MST consists of four physical property sensors on an automated track that measure, in order, magnetic susceptibility, gamma ray attenuation (GRA) bulk density, compressional P-wave velocity, and NGR emissions on whole-round core sections. During Expedition 309/312, only magnetic susceptibility, GRA bulk density, and NGR were measured on cores. The P-wave logger, which requires full-diameter core and adequate coupling to the liner for velocity to be measured effectively, was not used.

Magnetic susceptibility

The degree to which a sample can be magnetized by an external magnetic field is magnetic susceptibility. Whole-core magnetic susceptibility was measured using a Bartington MS2 meter coupled to a MS2C sensor coil with a diameter of 8.8 cm operating at 565 Hz. The measurement resolution of the MS2C sensor is 4 cm, with a minimum statistically significant count time of 1 s. During Expedition 309/312, magnetic susceptibility was measured every 2.5 cm along cores, with five data acquisitions at each interval. Magnetic susceptibility data were archived as raw instrument units (SI). Raw susceptibility measurements can be converted to SI volume units by multiplying by 10–5 and then multiplying by a correction factor to account for the volume of material that passed through the susceptibility coils. This factor is typically ~0.7 for IODP cores (Blum, 1997); however, none of these data were corrected for volume.

Typically, the center of a long piece will have higher magnetic susceptibility than the ends because of the incomplete volume of rock on the ends. However, anomalously low values of magnetic susceptibility were observed in long sections of intact core for which GRA bulk density values were high. For example, both Cores 312-1256D-187R and 189R consist of long rock pieces with high density, but only the former exhibits anomalously low values (Fig. F16). These low values are interpreted as a simple truncation of the digit in the ten-thousand position. Any value which exceeds 10,000 SI is reported as the value minus 10,000. When we ran the archive half of one of the problem cores through the MS2C loop, we received values that were greater than the values for the whole core and about half of the sum of the reported number plus 10,000 SI. Technical services at Bartington Instruments confirmed our interpretation. We plotted and reviewed all magnetic susceptibility values recorded during Expedition 309/312 using graphs similar to Figure F16. However, it is possible that in some cores the data truncation problem was missed in our visual review of magnetic susceptibility plots. Further examination of magnetic susceptibility or magnetic mineralogy is necessary to confirm the validity of other low values. For observations that we judged to be in error, we altered the values in the Janus database by adding 10,000. The green line in Figure F16 shows the new interpretation. Altered magnetic susceptibility data can be identified easily as the only values >10,000 SI.

GRA bulk density

Measurement of bulk density by the GRA densiometer is based on the principle that the attenuation, mainly by Compton scattering, of a collimated beam of gamma rays produced by a 137Ce source passing through a known volume is related to material density (Evans, 1965). Calibration of the GRA densiometer was completed using known seawater/aluminum density standards. A freshwater control standard was run with each section to measure instrument drift.

The measurement width of the GRA sensor is ~5 mm, with sample spacing set at 2.5 cm for Expedition 309/312. The minimum integration time for a statistically significant GRA measurement is 1 s, and routine Expedition 309/312 GRA measurements used a 5 s integration time.

GRA density output is based on the assumption that core is 66 mm in diameter. However, the average diameter of intact cores is ~58 mm. Thus, GRA density values would be more accurate if scaled by a factor of 66/58. Further corrections to data would need to be made to account for irregular shapes and broken pieces. GRA density values in the Janus database and those reported here have not been volume corrected, unless otherwise indicated.

Natural gamma radiation

NGR emissions of rocks are a function of the random and discrete decay of radioactive isotopes, predominantly those of uranium, thorium, and potassium, and are measured through scintillation detectors arranged at 90° to each other and perpendicular to the core. The installation and operating principles of the NGR system used on the JOIDES Resolution are discussed in Hoppie et al. (1994). Data from 256 energy channels were collected and archived. For presentation purposes, the counts were summed over the range of 200–3000 keV, so as to be comparable with data collected during previous cruises. This integration range also allows direct comparison with downhole logging data, which are collected over a similar integration range (Hoppie et al., 1994). Over the 200–3000 keV integration range, background counts, measured using a core liner filled with distilled water, averaged 30 during a 1 h measurement period. Before taking measurements, each of the four NGR amplifiers was adjusted so that the thorium peak was at the highest resolution possible when the other three amplifiers were disabled. The multichannel analyzer was then calibrated by assigning certain channels to the characteristic energies of 40K and the main peak of 232Th (Blum, 1997).

The measurement width of the NGR is ~15 cm, with a minimum statistically significant count time of 5 s, depending on lithology. During Expedition 309/312, NGR measurements were made every 5 cm along cores for 20 s each. NGR values were recorded in units of gAPI (Blum, 1997). No corrections were made to NGR data to account for core diameters of <66 mm.

Thermal conductivity

Thermal conductivity is the rate at which heat flows through a material and is dependent on composition, porosity, and structure. Thermal conductivity was measured by transient heating of the core with a known heating power and a known geometry and recording the change in temperature with time using the TK04 system (Blum, 1997). The temperature of the superconductive probe has a linear relationship with the natural logarithm of the time after the initiation of heat:

T(t) = (q/4k) ln(t) + C,

where

  • T = temperature,
  • q = heat input per unit length per unit time,
  • k = thermal conductivity,
  • t = time after the initiation of heat, and
  • C = a constant.

Thermal conductivity was measured on the archive half of the split core, with the probe in half-space mode (Vacquier, 1985). These half-space determinations were made with a needle probe embedded in the surface of an epoxy block with a low thermal conductivity (Vacquier, 1985). Samples needed to be quite smooth to ensure adequate contact with the heating needle. Visible saw marks were removed by grinding and polishing the split face using 120 gauge silicon carbide powder.

Samples were allowed to equilibrate to room temperature for at least 4 h, and then a sample and the sensor needle were equilibrated together in a room-temperature seawater bath enclosed within a cooler for at least 15 min prior to measurement. Isolation of the samples and sensor needle eliminated the effect of rapid but small temperature changes introduced by air currents in the laboratory. The instrument internally measures drift and does not begin a heating run until sufficient thermal equilibrium is attained.

Cores were measured at irregular intervals (at least one sample every other core), depending on the availability of pieces long enough to be measured without edge effects (>7 cm) and on the degree of lithologic variability. Measurements were made at room pressure and temperature and were not corrected for in situ conditions. Results were reported in units of watts per meter degree Kelvin.

Compressional wave velocity

Compressional wave velocity was measured using P-wave sensor 3 (PWS3), a modified and updated version of the classic Hamilton Frame velocimeter designed with one transducer fixed and the other mounted on an adjustable screw. The compressional wave velocity calculation is based on the accurate measurement of the delay time of a 500 kHz square wave signal traveling between the pair of transducers. Transducer separation was measured by a digital caliper attached to the transducers.

Samples of ~9 cm3 were collected at a frequency of one every other section, depending on the availability of relatively homogeneous pieces without cracks. The PWS3 is used to measure velocity in each direction (x, y, and z) of the cubes, which are marked during sampling with an arrow pointing upcore in the z-direction on the x-direction face (Fig. F17). The PWS3 is mounted vertically, and samples were manually rotated to measure each direction. Deionized water was added to the contact between the transducers and sample to improve acoustic coupling. The sample was placed on the lower transducer, and the upper transducer was slowly adjusted until direct contact with the upper surface transducer pair was made with the sample.

Core temperature is recorded at the time velocity is measured; however, velocity data stored in the Janus database are uncorrected for in situ temperature and pressure. These corrections can be made using the relationships outlined in Wyllie et al. (1956), Wilson (1960), and Mackenzie (1981).

A significant offset occurs at ~1255 mbsf between compressional velocities measured on minicubes during Expedition 309/312 (see Fig. F330 in the “Site 1256” chapter). We attribute the jump to a change in instrumentation calibration. To verify this, both physical properties specialists on Expedition 312 measured three axes of velocity on six samples obtained during Expedition 309 after rehydrating the samples in a vacuum for 36 h (see Table T48 in the “Site 1256” chapter). The mean difference between all Expedition 309 measurements and all Expedition 312 measurements is 0.42 km/s with a standard deviation of 0.07 km/s (N = 28), whereas the mean difference between the two observers on Expedition 312 is <0.01 km/s ± 0.04 km/s (N = 14) (see Fig. F330 in the “Site 1256” chapter). Removal of water from sample porosity can reduce velocity, so we also tested the possibility that drying and storage of the Expedition 309 samples decreased their velocity. After Expedition 312, the Hamilton Frame was moved from the JOIDES Resolution to the IODP core repository in Bremen, Germany. In March 2006, the velocities of five samples from Expedition 312 were measured after rehydration for 36 h (see Table T49 in the “Site 1256” chapter). There is no significant difference between velocities of Expedition 312 samples measured during Expedition 312 and those measured after the expedition (see Fig. F330 in the “Site 1256” chapter), indicating that the drying process used during measurement of density did not affect velocity. These results convince us that the offset does not reflect a change in rock properties. Velocities of most Expedition 309 samples tested exceed sonic log velocities, consistent with the occurrence of crack porosity on length scales greater than a few centimeters. In contrast, Expedition 312 minicube velocities are lower than sonic log velocities, so the figures in this section use Expedition 312 VP to which 0.42 km/s has been added, but the results in the Janus database remain unchanged.

Moisture and density properties

Samples used for velocity measurements were also used to calculate bulk density, grain density, and porosity from wet weight, dry weight, and dry volume. First, samples were soaked in seawater, and then wet mass (Mw) was determined. Samples were then desiccated so that dry mass (Md) and dry volume (Vd) could be measured. During Expedition 309, moisture and density properties were computed with a spreadsheet, whereas IODP computer programs were used during Expedition 312; both methods are based on the calculations below.

Mass and volume

To prepare samples for wet mass determination, they were placed in a vacuum within open vials of seawater for 24 h. Next, excess water was wiped from the samples and wet mass was determined to a precision of 0.01 g using two Scientech 202 electronic balances and a computer averaging system to compensate for the ship’s motion. Samples were lightly wiped with deionized water, placed in an oven at 105° ± 5°C for 24 h, and then allowed to cool in a desiccator. Dry mass was determined using the Scientech 202 electronic balances, and dry volume was determined using a five-chambered Pentapycnometer, which is a helium-displacement pycnometer with a precision of 0.02 cm3. Calibration was maintained by including a standard reference sphere in one of the operating cells for each run and cycling it sequentially between the cells for successive runs. The cell volumes were recalibrated if the measured volume of the standard was not within 0.02 cm3 of the known volume of the standard. During Expedition 309, measurements were repeated five times for each sample, whereas only one measurement was made on each sample during Expedition 312. All cells were calibrated after four sample runs to check for instrument drift and systematic error. A purge time of 1 min was used before each run.

Bulk density calculation

For density calculations, both mass and volume are first corrected for the salt content of the pore fluid:

Ms = (s[MwMd])/(1 – s),

where

  • s = pore water salinity (0.0355, from the fluid sampled at ~713 m in the hole),
  • Ms = mass of salt,
  • Md = dry mass of the sample, and
  • Mw = wet mass of the sample.

Grain density (ρg) is determined from the dry mass and dry volume measurements:

ρg = (MdMs)/(Vd – [Ms – ρs]),

where ρs = density of salt (2.257 g/cm3). The salt-corrected mass of the pore water (Mpw) is calculated as

Mpw = (MwMd)/(1 – s).

Then, the volume of pore water (Vpw) is

Vpw = Mpwpw = (MwMd)/([1 – spw),

where the density of the pore fluid (ρpw) = 1.024 g/cm3 (from calculations of fluid sampled at ~713 m in the hole). To calculate sample bulk density (ρb), first compute the bulk volume:

Vb = Vd + Vpw ,

and then

ρb = Mw/Vb.

Porosity calculation

Porosity (ϕ) is simply the ratio of pore water volume to the total sample volume and can be calculated from the two volume parameters above:

ϕ = Vpw/Vb.