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doi:10.2204/iodp.proc.342.102.2014

Physical properties

High-resolution physical property measurements were made during Expedition 342 mainly to aid lithostratigraphic correlations and interpretations and to help document the Cenozoic behavior of the carbonate compensation depth (CCD) in the North Atlantic. More specifically, physical property data are used for

• Hole-to-hole stratigraphic correlations for the construction of composite depth scales and sampling splices at each site (see “Stratigraphic correlation”),

• Site-to-site stratigraphic correlations,

• Detection of discontinuities and lateral inhomogeneities caused either naturally or by the drilling process,

• Collection of information about differences in the composition and texture of sediment,

• Correlation with orbital cycles and tuning to reference cores for stratigraphic purposes, and

• Identification of major seismic reflectors and construction of synthetic seismic profiles.

Measured physical properties include gamma ray attenuation (GRA) bulk density, magnetic susceptibility with loop (MSL) and point (MSP) sensors, NGR with the NGRL, thermal conductivity, color reflectance, MAD, and compressional P-wave velocity.

The measurement sequence included several core logging and discrete sample measurement systems designed and built at IODP-TAMU for specific shipboard workflow requirements. First, physical properties were measured using three whole-round core logging systems:

1. The STMSL was run on selected sections immediately after cores were sectioned on the catwalk to obtain data that could guide coring operations (see “Stratigraphic correlation”).

2. All core sections were run on the WRMSL after they had warmed to ambient laboratory temperature (20°–21°C).

3. All core sections were also run on the NGRL.

Both the STMSL and WRMSL include a GRA bulk densitometer and an MSL system. In addition, the WRMSL employs a compressional P-wave velocity logger (PWL).

After the whole-round sections were logged, thermal conductivity was measured on selected whole-round sections (typically Section 3 of each core in one hole per site, usually Hole A). The sections were subsequently split into working and archive halves. The archive half was processed through the SHIL to acquire high-resolution core images, followed by the SHMSL for color reflectance and magnetic susceptibility measurements. The working half was placed on the Section Half Measurement Gantry (SHMG), on which instruments are set up to measure compressional P-wave velocity using a caliper-type system (PWC). P-wave measurements on section halves are often of better quality than those on whole-round sections because of better coupling between those sensors and the sediment. PWL measurements have the advantage of being of much higher spatial resolution than those produced by the PWC. Discrete samples were collected from the working halves of every section for MAD analysis.

The following sections describe the measurement methods and systems in more detail. A full discussion of all methodologies and calculations used aboard the JOIDES Resolution in the Physical Properties Laboratory is available in Blum (1997).

Gamma ray attenuation bulk density

The measurement of GRA can be directly related to the bulk density of sediment (Evans, 1965). The GRA densitometers on the STMSL and WRMSL operate by passing gamma radiation from a Cesium-137 (¹³⁷Cs) source through a whole-round core into a 75 mm × 75 mm sodium iodide detector situated directly below the core. The gamma ray peak has a principal energy of 662 KeV and is attenuated as it passes through the core. The attenuation of gamma radiation, mainly by Compton scattering, through a known sample thickness is proportional to the bulk density. The GRA densitometer has a spatial resolution of <1 cm. Because of this high spatial resolution, the quality of GRA data is highly dependent on the structural integrity of the sediment (i.e., the measurements are significantly affected by cracks, voids, biscuiting, etc.). The absolute values will be lower if the sediment does not completely fill the core liner (i.e., if a gap between the sediment and the core liner or cracks in the core exist). Bulk density can also be affected by vertical compaction during the collection of APC cores.

GRA precision is proportional to the square root of the counts measured, as gamma ray emission is subject to Poisson statistics. Measurements with the present system typically have count rates of 10,000 (dense rock) to 20,000 cps (soft mud). If measured for 4 s, the statistical error is therefore <40,000 ± 200, or 0.5% (i.e., the high flux of the ¹³⁷Cs source does not require excessive counting times. Calibration of the densitometer was performed using core liners filled with distilled water and aluminum blocks. GRA density measurement intervals on both the STMSL and WRMSL were 2.5 cm.

Magnetic susceptibility

Magnetic susceptibility is the degree to which a material can be magnetized in an external magnetic field.

Both the STMSL and WRMSL incorporate a Bartington (United Kingdom) MS2C meter. The MS2C meter in the STMSL had a coil with an internal diameter of 90 mm and an operating frequency of 565 Hz. The modified MS2C in the WRMSL had a coil with an internal diameter of 80 mm and ran at an operating frequency of 513 Hz. On both instruments, measurements were taken at intervals of 2.5 cm.

The SHMSL included a Bartington Instruments MS2E MSP, which is a high-resolution surface scanning sensor with an operating frequency of 2 kHz. This sensor integrates a volume of 10.5 mm × 3.8 mm × 4 mm, where 10.5 mm is the length perpendicular to the core axis, 3.8 mm is the width in the core axis, and 4 mm is the depth. Because the MS2E demands flush contact between the probe and the section-half surface, the archive halves were covered with clear plastic wrap to avoid contamination. Measurements were taken at 2.5 cm when time permitted; the interval was increased to 5 cm when required because of high core recovery rates.

Magnetic susceptibility from all three instruments is reported in instrument units. To obtain results in dimensionless SI units, the instrument units need to be multiplied by a geometric correction factor that is a function of the probe type, core diameter, and loop size (see “Paleomagnetism” for parameter examples). Because we are not measuring the core diameter, application of a correction factor has no benefit over reporting instrument units. If normalized magnetic susceptibility values are needed, the best method is to measure discrete samples with good accuracy and precision using a Kappabridge to calibrate the logs (see “Paleomagnetism” for shipboard Kappabridge measurements).

Compressional P-wave velocity

Compressional P-wave velocity is the distance an ultrasonic P-wave travels through a sample per unit time. P-wave velocity varies with material composition, porosity, and bulk density, as well as state of stress, temperature, and fabric or degree of fracturing. In soft sediment, velocity is also controlled by the degree of consolidation and lithification and the occurrence and abundance of free gas and gas hydrate. Together with bulk density, velocity data are used to calculate acoustic impedance and reflection coefficients in order to construct synthetic seismic profiles and to estimate the depth of specific seismic horizons.

The PWL system on the WRMSL transmits a 500 kHz P-wave pulse across the whole-round core section at a specified repetition rate. Pulser and receiver are mounted on a caliper-type device and are aligned so that wave propagation is perpendicular to the section’s long axis. Core diameter is measured using a linear variable differential transducer. Torque applied by the actuator to clamp the pulser and receiver against the core liner ensures good acoustic contact. The first arrival of the wave at the receiver is picked by signal processing software with 50 ns precision and assessed against coefficients derived during calibration. The precision of the velocity on standard materials (e.g., water and acrylic calibration standards) is ±20 m/s.

The PWC system on the SHMG also uses a caliper-type configuration for the pulser and receiver. Typically, two measurements were performed per section (at ~50 and 100 cm). The system uses Panametrics-NDT Microscan delay line transducers that transmit at 0.5 MHz. The user has the option to override the automated pulse arrival and, particularly in the case of a weak signal, pick the first arrival manually. The distance between transducers was measured with a built-in linear voltage displacement transformer (LDVT).

Calibration of both the PWL and the PWC systems was performed with a series of acrylic cylinders of varying thicknesses and a known P-wave velocity of 2750 ± 20 m/s. The determined system time delay from calibration was subtracted from the picked arrival time to give a traveltime of the P-wave through the sample. The thickness of the sample, corrected for liner thickness, was divided by the traveltime to calculate P-wave velocity in meters per second.

Natural gamma radiation

Gamma radiation in geological formations arise primarily from the decay of mineral-hosted uranium (U), thorium (Th), and potassium (K) isotopes. In general, high counts identify fine-grained deposits containing K-rich clay minerals and their absorbed U and Th atoms. The NGR data reveal stratigraphic details that aid in stratigraphic correlations and formation characterization and can be compared directly against downhole logs of NGR for core-log integration.

The NGRL designed and built at the TAMU IODP-USIO facility from 2006 to 2008 (Vasiliev et al., 2011) measures gamma rays emitted from whole-round core sections. The main NGR detector unit consists of 8 sodium iodide (NaI) scintillation detectors, spaced at ~20 cm intervals along the core axis, surrounding the lower half of the core section to be measured; 7 active shield plastic scintillation detectors; 22 photomultipliers; and passive lead shielding.

A measurement run generally consists of counting one position in each core section for 5 min for a total of 8 measurements per 150 cm section and a second position for another 5 min for another 8 measurements, yielding a depth resolution of one data point per ~10 cm.

Thermal conductivity

Thermal conductivity is the rate at which heat flows through a material and is dependent on chemical composition, porosity, density, and fabric of material (e.g., Jumikis, 1966). It is used in conjunction with downhole temperature measurements to calculate heat flow.

A TeKa (Germany) TK04 system measures and records the changes in temperature with time after an initial heating of a needle probe inserted into the whole-round section through a small hole drilled through the plastic core liner. The temperature of the superconductive probe has a linear relationship with the natural logarithm of the time after the initiation of heat. Core sections and needle probes were equilibrated together and subsequently measured in a Styrofoam sleeve to minimize 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.

We attempted to conduct thermal conductivity measurements but were ultimately unsuccessful for all J-Anomaly Ridge sites and for SENR Site U1407. The instrument was placed in a shielded box to minimize external environmental effects. Although we obtained good results on an equilibrated standard and a core from Site U1402, we were unable to obtain stable temperature readings on cores from the sites listed above even after (in one case) the core was measured continually for 48 h. However, we successfully measured thermal conductivity in several core sections from Sites U1408–U1411.

Moisture and density

Procedure

Working section halves were sampled for MAD analysis using plastic syringes with a diameter of only slightly less than the diameter of the preweighed 16 mL Wheaton glass vials used to process and store the samples of ~10 cm³ volume. In some cases, where sediment was too indurated to be sampled with a syringe, a spatula was used to take several pieces. Typically one sample per section (at ~35 cm from the top of the section) was collected.

First, the mass of wet samples was determined to a precision of 0.005 g using two Mettler Toledo electronic balances and a computer averaging system to compensate for the ship’s motion. The samples were then heated in an oven at 105° ± 5°C for 24 h and allowed to cool in a desiccator. The mass of the dry sample was then determined with the same balance system.

Dry sample volume was determined using a six-celled, custom-configured Micromeritics AccuPyc 1330TC helium-displacement pycnometer system. The precision of each cell volume is 1% of the full-scale volume. Volume measurement was preceded by three purges of the sample chamber with helium warmed to ~28°C. Three measurement cycles were run for each sample. A reference volume (calibration sphere) was placed sequentially in one of the chambers to check for instrument drift and systematic error. The volumes of the numbered Wheaton vials were calculated before the cruise by multiplying each vial’s mass against the average density of the vial glass.

The procedures for the determination of physical properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM International, 1990). The phase relationships and assumptions for the calculations of all MAD parameters are discussed by Blum (1997) and summarized below in “Mass and volume calculation” and “Calculation of bulk properties.” The method applicable to saturated, fine-grained sediments is called “method C.” Method C is based on the measurement of wet mass, dry mass, and volume. It is not reliable or adapted for coarse-grained sediments in which water can be easily lost during the sampling (e.g., in foraminifer sands often found at the top of the hole).

Mass and volume calculation

Wet mass (M_wet), dry mass (M_dry), and dry volume (V_dry) were measured in the laboratory. The mass ratio (rm) is a computational constant of 0.965 (i.e., 0.965 g of freshwater per 1 g of seawater). Salt precipitated in sediment pores during the drying process is included in the M_dry and V_dry values. The mass of the evaporated water (M_water) and salt (M_salt) in the sample are given by

where s is the assumed saltwater salinity (0.035) corresponding to a pore water density (ρ_pw) of 1.024 g/cm³ and a salt density (ρ_salt) of 2.22 g/cm³. The corrected mass of pore water (M_pw), volume of pore water (V_pw), mass of solids excluding salt (M_solid), volume of salt (V_salt), volume of solids excluding salt (V_solid), and wet volume (V_wet) are

Calculation of bulk properties

For all sediment samples, water content (w) is expressed as the ratio of mass of pore water to wet sediment (total) mass,

Wet bulk density (ρ_wet), dry bulk density (ρ_dry), sediment grain density (ρ_solid), porosity (ϕ), and void ratio (VR) are calculated as

Color reflectance

Reflectance of visible light from the archive halves of sediment cores was measured using an Ocean Optics USB4000 spectrophotometer mounted on the automated SHMSL. Freshly split cores were covered with clear plastic wrap and placed on the SHMSL. Measurements were taken at 2.5 cm intervals when time permitted and at 5 cm intervals when the rate of core recovery required faster processing to provide a high-resolution stratigraphic record of color variation for visible wavelengths. Each measurement was recorded in 2 nm wide spectral bands from 400 to 900 nm.

Spurious measurements can result from small cracks, sediment disturbance caused by the drilling process, or plastic section dividers.

Additional detailed information about measurement and interpretation of spectral data can be found in Balsam et al. (1997, 1998) and Balsam and Damuth (2000).

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