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doi:10.2204/iodp.proc.327.102.2011

Physical properties

Shipboard measurements of physical properties were performed to characterize the recovered material. Once cut to length, whole-round core sections were run through the WRMSL for measurement of gamma ray attenuation (GRA) density and magnetic susceptibility. The WRMSL also incorporates a compressional wave velocity sensor (P-wave logger [PWL]) for sedimentary sections. PWL measurements were omitted for all hard rock cores, which rarely fill the core liner, resulting in velocity measurements that tend to be far less reliable than discrete sample data. Hard rock sections longer than 50 cm were measured with the NGRL, which is primarily intended to measure gamma rays resulting from the decay of ²³⁸U, ²³²Th, and ⁴⁰K isotopes.

After measurements with the WRMSL and NGRL were completed, thermal conductivity was measured on whole-round sediment cores using the needle probe method (Von Herzen and Maxwell, 1959). Thermal conductivity measurements on hard rock cores were made on pieces from the working halves using the half-space needle probe method (Vacquier, 1985).

Lastly, two discrete samples were usually taken from the working half of each section. Discrete samples were used for triaxial P-wave velocity measurements and moisture and density (MAD) measurements, including wet bulk density, dry bulk density, grain density, water content, and porosity. A comprehensive discussion of the methodologies and calculations used in the physical properties laboratory is presented in Blum (1997).

Whole-Round Multisensor Logger

GRA bulk density, magnetic susceptibility, and P-wave velocity were measured nondestructively with the WRMSL. Sampling intervals were set at 1 cm for hard rock and 2 cm for sediment, with an integration time of 5 s for each measurement of both types, which was determined to maximize the number and quality of measurements taken without slowing core flow. Quality assurance/quality control (QA/QC) was monitored by passing a core liner filled with freshwater through the WRSML after every core.

The primary objective of Expedition 327 was the recovery of hard rock from upper basaltic basement. In general, WRMSL measurements are most effective on a liner completely filled with core that has suffered minimal drilling disturbance. As a result, the diameter of the core liner (66 mm) is assumed for hard rock density calculations, even though the liner is often <100% filled. In addition, hard rocks are often recovered in pieces rather than as a continuous core, which reduces the actual volume even more. Therefore, GRA bulk density and magnetic susceptibility measurements tend to underestimate actual values for hard rock cores. P-wave velocities measured by the PWL suffer similar limitations and were thus measured only on sediment cores.

Gamma ray attenuation bulk density

The GRA densitometer on the WRMSL operates by passing gamma rays from a ¹³⁷Cs source down through a whole-round core into a 75 mm × 75 mm sodium iodide detector located directly below the core. Gamma rays with an energy peak at 662 keV are attenuated by Compton scattering as they pass through the core. The resultant gamma ray count is proportional to bulk density. Calibration of the GRA densitometer was performed using core liners filled with seawater and aluminum density standards. Recalibration was performed as needed if the freshwater QA/QC standard after every core deviated significantly (more than a few percent) from 1 g/cm³. The spatial resolution of the GRA densitometer is <1 cm.

Magnetic susceptibility

Magnetic susceptibility, k, is a dimensionless measure of the degree to which a material can be magnetized by an external magnetic field:

where M is the magnetization induced in the material by an external field strength H. Magnetic susceptibility responds to variations in the type and concentration of magnetic grains, making it useful for identifying compositional variations and alteration in hard rock cores. In the case of sediments, magnetic susceptibility can often be related to mineralogical composition (e.g., terrigenous versus biogenic material) and diagenetic overprinting (e.g., clays from the alteration of igneous materials have a susceptibility orders of magnitude lower than the material’s original iron oxide constituents). Water and plastics (core liner) have a slightly negative magnetic susceptibility.

The WRMSL incorporates a Bartington Instruments MS2 meter coupled to an MS2C sensor coil with a diameter of 8.8 cm that operates at a frequency of 565 Hz. The sensor output can be set to centimeter-gram-second (cgs) units or SI units, with the IODP standard being the SI setting. The core diameter is smaller than the aperture through which it passes during measurement. Therefore, a volume-correction factor must be applied to the data off-line. Assuming a core diameter of 66 mm and using the coil aperture of 88 mm, the correction factor is found by multiplying the ×10^–5 SI units by a factor of 0.68 (Blum, 1997).

The MS2C coil is calibrated with a homogeneous mixture of magnetite and epoxy in a 40 cm long piece of core liner to an accuracy of ±5%. The resolution of the method is ±4 cm for continuous core section; therefore, magnetic susceptibility is underestimated for core material that is not continuous over an 8 cm interval.

P-wave logger

The WRMSL also includes a PWL, which was used to determine P-wave velocities in whole-round sediment cores. Specifics on the PWL and on other methods utilized to determine P-wave velocities are discussed in detail in “P-wave velocity.”

Natural Gamma Radiation Logger

The NGRL was designed and built at IODP at Texas A&M University in order to augment geologic interpretations. Natural gamma rays occur primarily as a result of the decay of ²³⁸U, ²³²Th, and ⁴⁰K isotopes.

The main NGRL detector unit consists of 8 sodium iodide (NaI) scintillation detectors, 7 plastic scintillation detectors, 22 photomultipliers, and passive lead shielding. The NaI detectors are covered by 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI detectors. The lead shielding closest to the NaI detectors is composed of low-background lead, whereas the outer half is composed of regular (virgin) lead. In addition to passive lead shielding, the NGRL detector unit employs a plastic scintillator to suppress the high-energy gamma and muon components of cosmic radiation by producing a veto signal when these charged particles pass through the plastic scintillators. The NGRL detector unit was calibrated using ¹³⁷Cs and ⁶⁰Co sources and identifying the peaks at 662 keV (¹³⁷Cs) and 1330 keV (⁶⁰Co).

Counts were summed over the range of 100–3000 keV to be compatible with data collection from previous cruises and for direct comparison with downhole logging data. Background measurements of an empty core liner counted for 40,000 s (11 h) were made before measuring Hole U1362A cores. Over the 100–3000 keV integration range, background counts averaged 3 counts per second, which often accounted for half or more of the overall signal.

A single NGRL run consisted of two sets of measurements by eight sensors, each spaced 20 cm apart. The two sets of measurements were offset 10 cm, which yielded a total of 16 measurements equally spaced 10 cm apart over a 150 cm long section of core. Cores less than 50 cm in length were not run through the NGRL.

The quality of the energy spectrum measured in a core depends on the concentration of radionuclides in the sample but also on the counting time, with higher times yielding more clearly defined spectra. Because of the low overall core yield, we had the opportunity to count for longer times (5400 s for each position [3 h total]), yielding statistically significant energy spectra for many cores.

Thermal conductivity

Thermal conductivity is the coefficient of proportionality relating conductive heat flow to a thermal gradient (Blum, 1997). Thermal conductivity was measured during Expedition 327 using the transient needle probe method in whole- or half-space geometry (Von Herzen and Maxwell, 1959), using a Teka Bolin TK04 system.

Half-space measurements were made with a needle probe embedded in the surface of an epoxy block having a low thermal conductivity (Vacquier, 1985). Samples were smoothed to ensure adequate contact with the heating needle. Visible saw marks were removed when necessary by grinding and polishing the split face using 120–320 gauge silicon carbide. In addition, a MACOR ceramic standard with a certified thermal conductivity of 1.637 ± 0.033 W/(m·K) was run repeatedly to verify instrument performance.

Half-space samples were equilibrated to room temperature in a seawater saturator for 12 h, and the samples and sensor needle were equilibrated together in an insulated seawater bath for at least 15 min prior to measurement. Isolation of the sample and sensor needle eliminated the effect of rapid but small temperature changes introduced by air currents in the laboratory. The instrument measures drift internally and does not begin a heating run until sufficient thermal equilibrium is attained. Samples were selected at irregular intervals (nominally one measurement per section) depending on the availability of homogeneous and relatively vein- and crack-free pieces long enough to be measured without edge effects (i.e., longer than the instrument needle, or 8 cm). Measurements were made at room temperature and pressure and were not corrected for in situ conditions. In practice, the shipboard thermal conductivity system was found to be unreliable for half-space measurements of hard rock during Expedition 327 because of a combination of hardware instability and the inflexibility of the acquisition and processing software. After many hours of effort debugging and crafting work-arounds for the system, evaluation of hard rock thermal conductivity was abandoned after collection of reliable data from only three samples.

For sediment cores, thermal conductivity was measured prior to the cores being split using the needle probe method in full-space configuration for soft sediments (Von Herzen and Maxwell, 1959). The needle probe was inserted into the unconsolidated sediment through 2 mm holes drilled into the core liner roughly once per section, generally in locations determined using magnetic susceptibility data gathered on the WRMSL. In general, magnetic susceptibility is higher in sandier intervals and lower in clay-rich intervals. Both kinds of intervals were targeted in order to assess thermal conductivity within these end-members so that later analyses could assess thermal conductivity within intervals having a mixed lithology.

Section Half Multisensor Logger

Magnetic susceptibility

Magnetic susceptibility was measured with a Bartington Instruments MS2E point sensor on the SHMSL. Because the SHMSL demands flush contact between the magnetic susceptibility point sensor and the split core, measurements were made on the archive halves of split cores that were covered with clear plastic wrap. A built-in laser surface analyzer aided in the recognition of irregularities in the split-core surface (e.g., cracks and voids), and data from this tool were recorded to provide an independent check on the fidelity of SHMSL measurements (e.g., Expedition 301 Scientists, 2005b).

Color reflectance

Reflectance spectroscopy and colorimetry data were collected using an OceanOptics spectrophotometer (model USB4000) to provide a high-resolution stratigraphic record of color variations at Site U1363. These data are displayed in the Site U1363 visual core descriptions in “Core descriptions.”

The SHMSL skips empty intervals and intervals where the core surface is well below the level of the core liner, but it does not recognize relatively small cracks or disturbed areas of core. Thus, SHMSL data may contain spurious measurements that should, to the extent possible, be edited out of the data set before use. The OceanOptics spectrophotometer measures the spectra from 380 to 900 nm at 2 nm bins. Data were captured at a measurement spacing of 1 cm using the L*a*b* color system based on a CIE D65 Standard Illuminant and 10-2 degree Standard Observer.

Moisture and density

Several basic physical properties of interest (bulk density, dry density, grain density, porosity, and void ratio) are found most accurately through mass and volume determinations on discrete samples. MAD data are also used for comparison with GRA bulk density data from the WRMSL. The shipboard MAD facility consists of a vacuum water saturator (for hard rock), a dual balance system for mass measurements, and a pycnometer for volume measurements.

In soft sediments, ~10 cm³ samples were extracted—usually from the same locations as thermal conductivity measurements—and placed in preweighed 16 mL Wheaton beakers. Stiff sediments drilled with the XCB were sampled, where appropriate, by extracting ~10 cm³ blocks using a spatula and placing the blocks into beakers, as above. One sample was routinely collected from each undisturbed section, and three to four samples per core were collected where recovery was good and sedimentation rates were high. Additional samples were taken where major changes in lithology were observed.

In hard rock sections, discrete samples were cut from the working halves of split cores at a nominal frequency of two samples per section. Cube-shaped samples, which were also used to measure P-wave velocities in three orthogonal directions, were extracted from oriented pieces. Cylinder-shaped samples were extracted from unoriented pieces. These ~7 cm³ samples are intended to represent the general variation and lithologies of the core. Therefore, samples were collected when there was a visible change in lithology or texture.

Vacuum water saturator

Determination of a precise and accurate wet mass of sparsely porous material requires that the pore space of the sample be completely saturated. Hard rock samples were saturated prior to measurement using a vacuum pump system. The system consisted of a plastic chamber filled with seawater into which samples were placed. A vacuum pump removed air from the chamber, forcing seawater into the sample pore spaces. The samples were kept under vacuum for at least 12 h, after which the vacuum was checked at 2–3 h intervals to ensure a stable underpressure condition. After removal from the saturator, the cubes were stored in sample containers filled with seawater to prevent the evaporation of pore water. After P-wave velocity measurements, the cube surfaces were lightly patted with a paper towel to remove water clinging to the outer surfaces, and wet mass was immediately determined using the dual balance system.

Dual balance system

The dual balance system was used to measure both wet and dry masses. Two Mettler-Toledo XS204 analytical balances compensated for ship motion; one acted as a reference, and the other measured the unknown sample. A standard weight similar to that of the sample was placed on the reference balance to increase accuracy. The default setting of the balances is 300 measurements (taking ~1.5 min to measure), which was deemed sufficient because no measurements were taken in transit.

Pycnometer system

The pycnometer system was used to measure dry sample volume using pressurized helium-filled chambers. At the start of the expedition and whenever the helium gas tank was changed, shipboard technicians performed a calibration using stainless steel spheres of known volume. A batch of samples consisted of four cells with unknowns and one cell with two stainless steel spheres (3 and 7 cm³). The spheres were cycled through the cells to identify any systematic error or instrument drift. Spheres are assumed to be known within 1% of their total volume. Individual volume measurements were preceded by three purges of the sample chambers with research-grade (99.995% or better) helium heated to 25°C.

Measurement of wet mass and dry mass by the dual balance system, along with dry volume measured by the pycnometer, allows for the determination of a number of MAD properties. The MAD Method C in Blum (1997) was used to determine dry volumes by gas pycnometry.

P-wave velocity

P-wave velocity varies with the material’s lithology, porosity, bulk density, state of stress, temperature, and fabric or degree of fracturing. Together with bulk density, velocity data are used to derive porosities and to calculate acoustic impedance, which can be used to construct synthetic seismograms and estimate the depths of seismic horizons.

Sediment

P-wave velocities of sediments were measured with the PWL on the WRMSL and with the P-wave caliper (PWC) and P-wave bayonets (PWB). The PWL measures the ultrasonic P-wave velocity of the whole-round sample in the core liner. The PWC and PWB measure P-wave velocity in a Cartesian coordinate system on section halves. The PWC measures P-wave velocity vertically to the sectional plane of the working half (horizontal direction; x-axis), whereas the PWB measures the cross section (horizontal direction; y-axis) and long axis (vertical direction; z-axis) of the core. We used the PWC only for lithified sediments because the cracks induced by the bayonets on the PWB precluded the acquisition of reliable velocities.

All tools transmit a 500 kHz P-wave pulse through the core section at a specified repetition rate. This signal is coupled to the sample by the plastic pole pieces of the transducers and by the pressure applied by the linear actuator. In contrast to the PWC and PWB, no water is used to improve coupling between the transducers of the PWL and the liner because the pressure applied by the actuator is sufficient for reliable P-wave measurement. In the PWL measurement, the wave propagation direction is perpendicular to the section’s long axis (horizontal direction).

Traveltime was determined by signal-processing software that automatically detects the arrival of the P-wave signal to a precision of 50 ns. A linear voltage differential transformer was used to measure the separation of the transducer to derive a travel path length for the signal (i.e., the slightly compressed core diameter). Ultrasonic P-wave velocity was then calculated after corrections were made for system propagation delay, liner thickness, and liner material velocity.

Hard rock

P-wave velocity measurements of basaltic samples were performed on the same discrete samples (cube and cylinder samples) used for MAD measurements. Because discontinuous samples completely attenuate the signal, it is not possible to obtain continuous data from the PWL on the WRMSL. Sample preparation included cutting cubes and cylinders with flat and parallel sides. Using the Buehler Petrothin thin section system (240 grit), all surfaces of the samples were polished to ensure good contact between the sample and transducer.

P-wave velocity measurements were performed on seawater-saturated samples directly before wet mass determinations because P-wave velocity is very sensitive to the degree of saturation (Knight and Nolen-Hoeksema, 1990). Measurements used the x-axis caliper-type contact probe transducers (PWC). Oriented cube samples were rotated manually to measure y- and z-axis velocities with the same instrument. Deionized water was applied to the contact between the transducers and sample in order to improve acoustic coupling. The system uses Panametrics-NDT Microscan delay line transducers, which transmit a 500 kHz P-wave pulse. Although 500 kHz pulse generator and transducers were used, waveforms with ~110 kHz dominant frequency were obtained.

The estimated P-wave velocities are significantly higher than the expected velocity of basalt. Therefore, all saturated samples were measured, and the velocity of calibration standards was checked after every measurement. When the acrylic standard velocity calculated during every measurement was not within tolerances (2750 ± 20 m/s), the measurement device was calibrated with a series of acrylic cylinders of differing thicknesses until the values converged. The velocity measurement device was usually calibrated after roughly every four velocity measurements. Because the estimated P-wave velocities were not stable, the velocities were measured four times on each axis by rotating every 90° and averaging the results.

After all velocity measurements were completed, velocities were recalculated for several discrete samples. Even though the measurement device was frequently recalibrated, the velocities differed from each other (±100 m/s difference). To check the accuracy of the velocity values, P-wave velocities were further calculated in dry conditions. P-wave velocities in dry conditions are usually slower than those in saturated conditions.

In the automatic picking procedure, the first positive peak was picked for traveltime determination. However, because the arrival time of the first positive peak is affected by sample attenuation and reflections, the first arrival (first break) of the waveforms was also picked. To pick the first arrival of the P-wave signal, waveforms were recorded for all samples in saturated and dry conditions. A reference waveform (i.e., waveform without a sample) was also calculated for every sample measurement. Traveltime (time lag between recorded waveform and reference waveform) was then obtained by displaying and correlating these waveforms (Fig. F9). Although the P-wave velocity collected by this method is a little slower (~150 m/s) than the velocity estimated via the conventional automatic picking method, the overall velocity trends derived from the two methods are consistent.

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