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doi:10.2204/iodp.proc.343343T.102.2013

Physical properties

Physical properties measurements provide fundamental information required to characterize lithologic units and allow for the correlation of coring results with downhole logging data. A variety of techniques and methods were used to characterize Expedition 343 core samples. Prior to sampling and interpretation, X-ray CT images were captured for all cores. After the CT scans were completed, GRA density, magnetic susceptibility, NGR, PWV, and electrical resistivity were measured using the MSCL-W after thermal equilibration with room temperature (~20°C). After that, cores were split in two longitudinally: one archive half and one working half for sampling and analysis. Thermal conductivity measurements were made on working halves using the TeKa thermal conductivity meter in half-space mode. For unconsolidated sediment, electrical resistivity was measured by inserting a four-pin electrode into the surface of working halves. For consolidated sediment, PWV and electrical resistivity were made on discrete cube samples in the x-, y-, and z-directions to evaluate anisotropy of velocity and resistivity. In addition, unconfined compressive strength (UCS) tests were performed on 1.98 cm diameter minicores using a manual hydraulic press (Carver Inc., model 30-12). MAD properties were determined for discrete samples collected from working halves, using a custom-built motion-compensated shipboard balance system and a Quantachrome penta-pycnometer. Particle size distribution on unconsolidated sediment was analyzed by a multiwavelength laser particle analyzer (Beckman Coulter, model LS13320). Details and procedures for each physical properties measurement are described below.

MSCL-W

GRA density

Bulk density can be used to estimate the pore volume in sediment and evaluate the consolidation state of sediment. GRA density is an estimate of bulk density based on the attenuation of a gamma ray beam. The beam is produced by a ¹³⁷Cs gamma ray source at a radiation level of 370 MBq within a lead shield with a 5 mm collimator, which is directed through the WRC. The gamma ray detector on the opposite side of the core from the source includes a scintillator and an integral photomultiplier tube to record the gamma radiation that passes through the core. Bulk density (ρ_b) is determined from GRA by

where

• I_o = gamma ray source intensity,

• I = measured intensity of gamma rays passing through the sample,

• µ = Compton attenuation coefficient, and

• d = sample diameter.

The Compton attenuation coefficient (µ) and I_o are provided by the MSCL-W and are treated as constants, so ρ_b can be calculated from I.

The gamma ray detector is calibrated with sealed calibration cores (one standard core liner filled with distilled water and aluminum cylinders of various diameters). To establish the calibration curves, gamma ray counts were taken through each aluminum cylinder for 60 s. Each aluminum cylinder has a density of 2.7 g/cm³, and d is 1, 2, 3, 4, 5, or 6 cm. The relationship between I and µd is

where A, B, and C are coefficients determined from the calibration. GRA density measurements on the core samples were conducted every 4 cm for 4 s. The spatial resolution of each measurement is 5 mm.

Magnetic susceptibility

Magnetic susceptibility is the degree to which a material can be magnetized by an external magnetic field and therefore provides information about sediment composition. The measurements were made using a Bartington MS2C loop sensor with an 8 cm diameter. An oscillator circuit in the sensor produces a low-intensity (~140 A/m RMS), nonsaturating, alternating magnetic field (0.565 kHz). Bringing any material within the influence of this field will cause a change in oscillator frequency. The frequency information returned in pulse form to the susceptometer is converted into magnetic susceptibility. The spatial resolution of the loop sensor is 23–27 mm and is accurate to within 2%. Magnetic susceptibility data were collected every 4 cm along the core.

P-wave velocity

PWV data can be used to evaluate small-strain moduli, correlate between downhole logging and core data, and evaluate porosity and cementation. P-wave (compressional) velocity (V_P) is defined by the time required for a compressional wave to travel a specific distance:

where d is the path length of the wave across the core and t_core is the traveltime through the core.

PWV transducers on the MSCL-W system measure total traveltime of the compressional wave between transducers. The wave travels horizontally across the whole core and core liner. The total traveltime observed is composed of

• t_delay = time delay related to transducer faces and electronic circuitry,

• t_pulse = delay related to the peak detection procedure,

• t_liner = transit time through the core liner, and

• t_core = traveltime through the sediment or rock.

The system is calibrated using a core liner filled with distilled water, which provides control for t_delay, t_pulse, and t_liner. From these calibrations, V_P can be calculated for the whole-round specimens in core liners as follows:

where

• d_cl = measured diameter of core and liner,

• d_liner = liner wall thickness, and

• t_o = measured total traveltime.

Equation 29 assumes that the core completely fills the core liner.

Electrical resistivity

Within limits, electrical resistivity measurements may be useful for estimating other sediment physical properties, such as porosity, tortuosity, permeability, and thermal conductivity. Bulk electrical resistivity is controlled by solid grain resistivity, pore fluid resistivity, and pore space distribution and connectivity. Electrical resistivity (R) is defined by the electrical resistance and geometry of the core measured:

where

• Γ = electrical resistance,

• L = length of measurement, and

• A = cross-sectional area of the core.

The NCR sensor on the MSCL-W system induces a high-frequency magnetic field in the core with a transmitter coil. This induces an electrical current in the bulk sediment that is inversely proportional to its resistivity. A receiver coil measures the secondary magnetic field generated by this induced electrical current. To measure this smaller magnetic field accurately, a differencing technique has been developed that compares readings from the sample core to readings from an identical set of coils operating in air. Electrical resistivity data were obtained at 4 cm intervals on the MSCL-W.

Natural gamma radiation

NGR measurements provide insight into sediment composition and thus can be used to describe lithology. WRCs were monitored for NGR emissions to obtain spatial variability in radioactivity and establish profiles that can be correlated with downhole gamma ray logs. The measurement device utilizes a lead-shielded detector counter optically coupled to a photomultiplier tube and connected to a bias base with a high-voltage power supply and signal preamplifier. Two horizontal and two vertical sensors are mounted in a lead, cube-shaped housing. The NGR system records radioactive decay of long-period isotopes ⁴⁰K, ²³²Th, and ²³⁸U. NGR has a resolution of 120–170 mm and was measured every 16 cm with a count time of 30 s. Background radiation noise was determined by making measurements on a water-filled calibration core.

Thermal conductivity

Thermal conductivity was measured on the surface of working halves using the half-space line source (Vacquier, 1985), which approximates an infinite line source. The half-space probe was placed directly on the split core and saturated with seawater to provide good contact. The measurements produce a scalar thermal conductivity value in the plane perpendicular to the orientation of the probe. All measurements were made after the cores had equilibrated with room temperature. At the beginning of each measurement, temperature in the sediment was monitored to ensure that thermal drift was <0.4 mK/min (typically within 1–2 min). After it was established that the temperature was near equilibrium, a calibrated heat source was applied and the rise in temperature was recorded for ~80 s. Values of thermal conductivity were based on the observed rise in temperature for a given quantity of heat. The half-space line source probe was calibrated at least once every 24 h. The calibration was performed on one of two Macor samples of known thermal conductivity (1.652 ± 2% W/[m·K]).

Unconfined compressive strength measurements

For stiff indurated sediment, UCS tests were performed on minicores 1.4–2.3 cm in length and 1.98 cm in diameter. These tests provide valuable strength data for sediment when vane shear or penetrometer devices are unsuitable. Samples were drilled from the working half perpendicular to the core axis (in the x-direction with respect to the core reference model; Fig. F20). Care was taken to ensure that the cores were free of defects and the core end surfaces were parallel and planar. Samples were then placed in a manual hydraulic press (Carver, Inc., model 30-12) with a load capacity of 30 tons and aligned so that the load from the press was applied vertically along the minicore axis. In order to measure force (F), a load cell with a capacity of 300 kN (TEAC Corp., model KR300KN) was placed directly beneath the sample. The sample was loaded to failure manually at a slow rate, and the maximum value registered by the load cell (F_max) was recorded. The UCS of the sample is calculated as

where A is the cross-sectional area of the sample. The reading resolution of the load cell is 10 N. For a sample 2.0 cm in diameter, this corresponds to a resolution of 0.3 MPa.

Because of the size limitations imposed by the geometry of the working half of the core, our samples have a length/diameter ratio (l/d) close to 1. The uniaxial strength of geologic materials is known to depend inversely on this ratio and asymptotically approaches a steady value at l/d = ~3 (Paterson and Wong, 2005). Based on these data, we use a quadratic function to calculate a correction factor (C_f) from the aspect ratio of the sample (l/d):

We consider this correction to be valid for l/d ≤ 2.5.

P-wave velocity and electrical resistivity measurements

Cubic samples (~2 cm × 2 cm × 2 cm) were cut from working halves with a diamond blade saw in order to measure electrical resistivity and P-wave velocity for discrete samples (PWVD). For soft sediment, where cubic samples could not be made, only electrical resistivity was measured using the bridge method with a four-pin electrode. All cubes were cut to be consistently oriented with respect to the core reference model (Fig. F20). All samples were immersed in a 3.5% NaCl solution for a few seconds prior to measurement. Carefully controlling the sample orientation during preparation allowed first-order measurements of both PWVD and electrical resistivity, as well as anisotropy of PWVD and electrical resistivity.

PWVD along a given direction was measured using a P-wave logger for discrete samples (PWL-D). The PWL-D is equipped with two 230 kHz transducers, one used as a transmitter and one as a receiver. Sample length (L) was measured with a laser distance sensor. During measurement, the sample was placed between the transducers and held in place with a weight of 2.5 kg. The transmitter was connected to a pulse generator, and the receiver was connected to an oscilloscope synchronized with the pulse generator. P-wave total traveltime (t) for the first arrival was picked and logged from the digitally displayed oscilloscope signal. The velocity in any direction (e.g., V_Px) is calculated from the sample length (e.g., L_x), total traveltime (t_x), and system-calibrated delay time (t_delay):

The traveltime delay and laser distance sensor were calibrated daily. Calibration of traveltime delay was determined by placing the transmitter and receiver in direct contact and measuring traveltime. Similarly, the laser distance was calibrated by placing the transmitter and the receiver in direct contact and also by spacing them using a 2.5 cm long reference specimen. Routine QC measurements were made by measuring velocity on glass and acrylic standards with known lengths and velocities.

Measurements of P-wave and S-wave velocity were also performed at elevated pressure. Sample size was the same as the PWVD measurements under atmospheric conditions (~2 cm × 2 cm × 2 cm); however, these samples were vacuum-dried at room temperature for ~24 h prior to testing. Piezoelectric transducers (2 MHz) are attached with an adhesive gel cement to opposing faces of the sample, and the remaining sample surfaces are coated in silicone grease to isolate it from the confining medium. The sample is then set inside a pressure vessel, where P-wave and S-wave velocities are initially calculated according to Equation 33 under atmospheric conditions. Updated velocities are automatically calculated as isotropic confining pressure is applied. The sample length (L) of the dried sample is measured prior to being encased in silicone and loaded in the pressure vessel. Because decreases in sample length caused by increasing pressure during measurement are not recorded, these measurements should be considered the upper bounds on wave velocity.

Electrical resistivity was measured with an Agilent 4294A component analyzer using the bridge method with a two-terminal circuit for cubic samples. The oriented specimen cube was placed between two stainless steel electrodes covered with seawater-saturated filter paper. The magnitude (|Z|) and phase (θ) of the complex impedance were measured at 25 kHz between opposite cube faces. The cube was rotated to measure impedance in the x-, y-, and z-directions (Fig. F20). Electrical resistivity for each direction (e.g., R_x) was computed from the complex impedance measured along each direction (e.g., x) and sample dimensions defined by face lengths (L):

With P-wave velocity (V_Px, V_Py, and V_Pz) and electrical resistivity measured in the x-, y-, and z-directions, anisotropy is calculated following the approach of Carlson and Christensen (1977). Potential causes of anisotropy include (1) alignment of pores during compaction, (2) fabric development, and (3) microstructures such as microfractures and microcracks. Horizontal and vertical P-wave velocity anisotropy (α_VPhor, α_VPvert) and electrical resistivity anisotropy (α_Rhor, α_Rvert) are calculated by comparing the horizontal (x and y) and vertical (z) components of the measurement expressed as a percentage of the mean (e.g., Shipboard Scientific Party, 2001):

and

Moisture and density measurements

Discrete samples from the working-half cores were used for determination of index properties (e.g., bulk density, grain density, dry density, water content, porosity, and void ratio). Index properties are determined from phase relations, mass measurements on wet and dry specimens, volume measurements on dry specimens, and corrections for salinity. In general, one discrete sample for index properties was collected from each core section. Sample intervals were occasionally shifted to select minimally disturbed, homogeneous samples. Each discrete index property sample was ~5 cm³.

Wet and dry masses were measured using a paired electronic balance system, which is designed to compensate for the ship’s heave. The sample mass was counterbalanced with a precisely known mass (10 or 40 g) that was within 3–20 g or 20–60 g of the sample mass, respectively. The sample mass was determined to a precision of ±0.005 g. The balance system was calibrated at least once per 24 h.

To minimize desiccation, MAD sample collection was immediately followed by measurement of wet sediment mass (M_wet). After M_wet measurements, samples were dried in a convection oven at 105° ± 5°C for 24 h. Dry samples were placed in a desiccator for at least 1 h to equilibrate to room temperature (~20°C), and then dry sediment mass (M_dry) and dry sediment volume (V_dry) were measured. A five-chamber Quantachrome pentapycnometer was used to measure V_dry by helium-displacement technique with a precision of ±0.04 cm³. The five-chamber system allowed the measurement of four sample volumes and one calibration sphere. Each measured volume is the average of five volume measurements. The calibration sphere was rotated between all measurement chambers to monitor for errors in each chamber. The pycnometer was calibrated at least once per 24 h.

Standard ODP/IODP practices were used to determine pore water mass and volume, salt mass and volume, and solid grain mass and volume (Blum, 1997). From these data, bulk density, dry density, grain density, porosity, and void ratio were calculated (Blum, 1997). Standard seawater density (1.024 g/cm³) and salinity (35 parts per thousand [ppt]) and a constant salt density (2.22 g/cm³) were assumed for all calculations.

Water content

Water content (W_c) was determined following the American Society for Testing and Materials (ASTM) standard D2216 (ASTM International, 1990). Corrections are required for salt when measuring the water content of marine samples. In addition to the water content calculation in ASTM D2216 (i.e., the ratio of pore fluid mass to dry sediment mass; W_c[dry]), we also calculated the ratio of pore fluid mass to total sample mass (W_c[wet]). The equations for water content are

and

where

• M_wet = total mass of the discrete sample,

• M_d = mass of the dry sample, and

• s = salinity (assumed constant at 0.035).

Bulk density

Bulk density is the density of the discrete core sample (ρ_b = M_wet/V_t). Total wet sample mass (M_wet) was measured immediately after collecting each discrete sample using the dual-balance system. Total sample volume assuming 100% saturation (V_t = V_g + V_pw) was determined from the pycnometer measurement of grain volume (V_g) and calculated volume of pore water (V_pw). Solid grain and pore water volume are determined as

and

where

• V_d = dry volume,

• ρ_salt = salt density, and

• ρ_sw = standard seawater density.

Porosity and void ratio

Porosity (ϕ) relates the volume of pores to total sample volume; void ratio (e) relates the pore volume to solid grain volume. They are calculated as

and

Grain density

Grain density (ρ_g) was determined from measurements of dry mass and dry volume made with the dual-balance system and the pycnometer, respectively. Mass and volume were corrected for salt, yielding

where M_salt is the mass of salt and ρ_salt is the density of salt (assumed to be constant at 2.22 g/cm³).

Particle size analysis

Particle size analysis is a tool used to characterize fine-grained sediment by grain size. It can be used to identify different lithologies and locate repeating sections of sediment by comparison of grain size spectra on series of samples. We used the multiwavelength laser particle analyzer (LS; Beckman Coulter, model LS13320) to measure grain size spectra of sediment samples taken from working halves. Laser particle analysis is based on the principle that particles of a given size diffract light at a given angle, which increases with decreasing particle size. A parallel beam of monochromatic light with a 750 nm wavelength is passed through a suspension of sample material, and the diffracted light is focused onto a multielement ring detector. The detector senses the angular distribution of scattered light intensity (Syvitski, 1991; McCave et al., 1995). In addition to this beam, a polarization intensity differential scattering (PIDS) assembly is used to characterize smaller scale particles (<0.4 µm). This assembly illuminates the particles with vertically and horizontally polarized lights of three different wavelengths (450, 600, and 900 nm), and the differential scattering patterns of each wavelength is used to determine the particle size. The laser particle analyzer can provide a rapid, automated, and precise measurement of sediment grain size ranging from 0.375 µm to 2 mm in ~10 min, including a rinse stage. The LS used is equipped with a microvolume module (MVM) system, and the autosampler allows the user to load a run of 30 samples, including two control samples, to be analyzed automatically. The accompanying software calculates the grain size distribution of the sample and appropriate statistics. Data may be exported to external programs.

During this expedition, an aliquot of a 0.5 g sample was used for grain size analysis. One sample per section was taken from the remainder of cube samples used for PWVD and resistivity measurements. Because organic particles may have shapes, geometries, and other surface properties that differ from routine laser particle size analysis of siliciclastic sediment, organic-rich samples were soaked in a 10% v/v H₂O₂ solution for 12 h to oxidize and remove organic matter. Sediment was then dispersed in an aqueous solution using both chemical and mechanical means. Chemical dispersion was performed using a dilute alkaline solution of sodium polyphosphates (NaPO₃)₆. Samples were then mechanically dispersed by turbulent mixing using a shaker.

Color spectrometry

Color spectrometry is an IODP standard measurement that is used to quantify visually observed changes in the split core using the MSCL-C. This system is equipped with a color spectrophotometer (Konica-Minolta, CM-2600d) and an aluminum frame that allows operators to load up to seven core sections. The sensor unit, which includes the spectrophotometer and distance laser measuring system, traverses each section and settles at each measurement point on the archive core surface.

Reflected light from the sample surface is collected by the color spectrophotometer’s integration sphere. In this case, specular component was excluded (SCE setting) in order to exclude glare; this setting is considered suitable for sediment. The light is divided into wavelengths at a 10 nm pitch (400–700 nm), and the spectral sensors in the sphere convert the light to electrical currents proportional to the light intensity. The color spectrum of the sample is then normalized by the source light of the reflectance. The obtained spectrum is calibrated with the measurement of a pure white standard and with a black box (zero calibration). Measurements can be calculated based on the 2° or 10° standard observer and any of 11 illuminants. The measured color spectrum is described by the parameters L*, a*, and b*. The parameter L* describes lightness on a white-to-black scale, a* quantifies color on a green-to-red scale, and b* quantifies color on a yellow-to-blue scale. These measurements may be used to recognize relative changes in bulk composition and are therefore useful for correlation between different recovered core sections and to analyze lithologic changes.

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