Previous   |    Next

doi:10.2204/iodp.proc.339.102.2013

Physical properties

High-resolution physical property measurements were made during Expedition 339 mainly to aid lithostratigraphic correlation and for tying borehole data to seismic profiles. In more detail, physical property data plays a major role in the following tasks:

• Hole-to-hole and site-to-site stratigraphic correlation for construction of composite stratigraphic sections;

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

• Obtaining information about differences in the composition and texture of sediment, especially regarding the distinction between contourite and turbidite deposits;

• Detection of orbital cycles and tuning to reference cores for stratigraphic purposes; and

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

First, physical properties were measured on whole-round core sections. Core sections are generally 1.5 m in length, so a typical coring length (stroke) of 9.5 m yields six sections plus a shorter seventh section. Two core logging systems were run on each section: (1) the STMSL was run on sections immediately after they were sectioned on the catwalk and (2) the WRMSL was run on core sections that had warmed to ambient laboratory temperature (20°–21°C). The linear track of the STMSL houses a gamma ray attenuation (GRA) bulk densitometer and a magnetic susceptibility sensor loop. The WRMSL employs, in order, a GRA bulk densitometer, a magnetic susceptibility sensor loop, and a compressional P-wave velocity sensor.

After WRMSL scanning, the whole-round sections were logged for NGR. Thermal conductivity was then measured on certain whole-round sections (Section 3 of each core in one hole per site, usually Hole A) by a needle probe inserted into the section through a small hole drilled through the plastic core liner close to the middle of the section.

At this phase, the sections were split into working and archive halves. Discrete samples were collected from the working halves of every second section to measure wet bulk density, dry bulk density, water content, porosity, and grain density with moisture and density (MAD) procedures. Compressional P-wave velocity measurements on split cores were made for every section for one hole (usually Hole A), employing the transducers oriented in x-axis direction. Archive halves were documented by the SHIL and analyzed with the SHMSL for color reflectance and magnetic susceptibility. A full discussion of all methodologies and calculations used aboard the JOIDES Resolution in the Physical Properties Laboratory is available in Blum (1997).

Special Task Multisensor Logger

The purpose of STMSL logging operations during Expedition 339 was to rapidly record medium- to high-resolution sets of GRA and magnetic susceptibility data. This information was needed at multihole sites to ensure that drilling depth offsets were set at different stratigraphic depths in each hole so that a complete composite stratigraphic section could be constructed. The GRA bulk densitometer and magnetic susceptibility loop incorporated in the STMSL are effectively identical to those in the WRMSL (see below). The spacing distance between STMSL measurements was typically 2.5 cm for GRA density and magnetic susceptibility measurements. Scanning time averaged 8 s per sample with one repeat for the magnetic susceptibility measurements. A core can therefore be passed in ~25 min through the STMSL. Comments about structural integrity and incomplete filling of liners were recorded.

Whole-Round Multisensor Logger

High-resolution WRMSL data, especially GRA bulk density and magnetic susceptibility, were gathered to advance shipboard core-to-core correlation between drill holes and the construction of composite stratigraphic sections. WRMSL data assembly had to be completed within a reasonable time to not encumber downstream core processing and sample collection. The quality of WRMSL data is highly dependent on the structural integrity of the sediment (cracks, voids, biscuiting, etc.) and whether any gaps between the sediment and the core liner are present.

GRA bulk density and magnetic susceptibility were measured nondestructively on all whole-round core sections. P-wave velocity was measured in sections undisturbed by gas expansion voids and cracks. To optimize WRMSL performance, the same sampling spacing, typically 2.5 cm, was set for all sensors. Measurement time, although somewhat different for the different sensors, averaged ~8 s per data, with one repeat of the magnetic susceptibility providing sufficient reproducibility. With handling and data initializing, a 1.5 m section took ~10 min to scan. A 9.5 m core therefore took ~1 h to pass through the WRMSL.

Gamma ray attenuation bulk density

Bulk density is a reflection of water-saturated porosity, grain density (dominant mineralogy), grain packing, and coring disturbance. To measure bulk density, the GRA densitometer uses a 10 mCi ¹³⁷Cs capsule as a gamma ray source (with the principal energy peak at 0.662 MeV) and a scintillation detector. The narrow collimated peak is attenuated as it passes through the center of the core. Incident photons are scattered by the electrons of the sediment by Compton scattering.

The attenuation of the incident intensity (I₀) is directly related to the electron density in the sediment core of diameter D that can be related to bulk density given the average attenuation coefficient (in micrometers) of the sediment (Evans, 1965; Harms and Choquette, 1965). Because the attenuation coefficient is similar for most common minerals and aluminum, bulk density is obtained through direct calibration of the densitometer using aluminum rods of different diameters mounted in a core liner filled with distilled water. The GRA densitometer has a spatial resolution of <1 cm.

Magnetic susceptibility

Magnetic susceptibility is a measure of the degree to which a material can be magnetized by an external magnetic field. It provides information on the magnetic composition of the sediment that commonly can be related to mineralogical composition (e.g., terrigenous versus biogenic materials) and diagenetic overprinting. Magnetite and a few other iron oxides with ferromagnetic characteristics have a specific magnetic susceptibility several orders of magnitude higher than clay, which has paramagnetic properties. Carbonate layers, opal, water, and plastic (core liner) have small negative values of magnetic susceptibility. Calcareous and biogenic deposits with low clay and iron-bearing mineral debris content thus have values approaching the detection limit of magnetic susceptibility meters.

Magnetic susceptibility was measured on the WRMSL and STMSL with the Bartington Instruments MS2C system. The frequency at which the magnetic susceptibility loop operates is 565 Hz for the WRMSL and STMSL (Blum, 1997). The output of the magnetic susceptibility sensors can be set to centimeter-gram-second (cgs) units or SI units, the IODP standard. However, to obtain dimensionless SI volume-specific magnetic susceptibility values, the instrument units stored in the IODP database must be multiplied by a correction factor (0.68) to compensate for instrument scaling and the geometric ratio between core and loop dimensions.

Compressional P-wave velocity

P-wave velocity varies with the material’s lithology, porosity, and bulk density, as well as state of stress, temperature, and fabric or degree of fracturing. In sediment and rock, velocity is controlled by the degree of consolidation and lithification, fracturing, and 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 P-wave velocity sensor measures the ultrasonic P-wave velocity of the whole-round sample residing in the core liner. The P-wave logger transmits a 500 kHz P-wave pulse across the core section at a specified repetition rate.

Traveltime is determined by signal processing software that automatically detects the first arrival of the P-wave signal to a precision of 50 ns. Ultrasonic P-wave velocity is calculated after correcting for system propagation delay, liner thickness, and liner material velocity.

Natural Gamma Radiation Logger

The NGRL was designed and built at the Texas A&M University IODP-USIO facility and measures gamma rays emitted from whole-round core sections. Gamma rays detected by the logger arise primarily from the decay of mineral-hosted uranium, thorium, and potassium isotopes. In general, high counts identify fine-grained deposits containing K-rich clay minerals and their absorbed U and Th atoms. The NGR data reveals stratigraphic details that aid in core-to-core correlations.

The main NGR detector unit consists of 8 sodium iodide (NaI) scintillator detectors surrounding the lower half of the section, 7 shielding plastic scintillator detectors, 22 photomultipliers, and passive lead shielding. The NaI detectors are covered by at least 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI detectors. Half of the lead shielding closest to the NaI detectors is composed of low-background lead, whereas the outer half is composed of common (virgin) lead. In addition to this passive lead shielding, the overlying plastic scintillators detect incoming high-energy gamma and muon cosmic radiation and cancel this signal from the total counted by the NaI detectors.

A measurement run generally consisted of counting one position on each core section for 7 min for a total of 8 measurements per 150 cm section. For analyses, “Position 2” was commonly chosen to avoid analyzing the disturbed uppermost part of the first section of each core. Therefore, NGR logging required ~50 min measurement time and another ~10 min for core, software, and data handling. In some cases, when time permitted, the two-position configuration (10 cm spacing) was chosen, doubling measurement time.

Thermal conductivity

After NGR measurements were completed, thermal conductivity was measured with the TK04 (Teka Bolin) system using the needle-probe method in full-space configuration for whole-round sediment cores (Von Herzen and Maxwell, 1959). The needle probe contains a heater wire and calibrated thermistor. The probe was inserted into a 2 mm hole drilled through the liner along one of the lines that later guided the splitting of the core. To avoid interference from airflow in the laboratory, an insulating jacket of foam rubber was placed over the core section during measurement of thermal conductivity.

Because the probe is much more conductive than unconsolidated sediment, the probe is assumed to be a perfect conductor. Under this assumption, the temperature of the superconductive probe has a linear relationship with the natural logarithm of the time after the initiation of the heat:

where

• T = temperature (K),

• q = heat input per unit length per unit time (J/m/s),

• k = thermal conductivity (W/[m·K]),

• t = time after the initiation of the heat (s), and

• C = instrumental constant.

Three measuring cycles were automatically performed at each probe location to calculate average conductivity. A self-test, which included a drift study, was conducted at the beginning of each measurement cycle. Once the probe temperature stabilized, the heater circuit was closed and the temperature rise in the probe was recorded. Thermal conductivity was calculated from the rate of temperature rise while the heater current was flowing. Temperatures measured during the first 150 s of the heating cycle were fitted to an approximate solution of a constantly heated line source (for details, see Kristiansen, 1982; Blum, 1997). Measurement errors were 5%–10%. Thermal conductivity measurements were routinely taken in one hole per site and in one section per core, typically Section 3. Cores retrieved by XCB and RCB usually yielded no results for thermal conductivity measurements because cracks in the sediment caused bad coupling of the probe to the sediment. In these cases the self-test during the measurement procedure failed and no results were reported to the data base.

Moisture and density

After the completion of thermal conductivity measurements, whole-round cores were split into working and archive halves. The working halves were placed on the sampling table for the collection of discrete samples to determine wet and dry bulk density, grain density, water content, and porosity. In soft sediment, ~10 cm³ samples were collected with a plastic syringe, the diameter of which fit that of the glass vial. An attempt was made to sample every other section per core at the 59–60 cm position. Samples were placed in numbered, preweighed 16 mL Wheaton glass vials for wet and dry sediment weighing, drying, and wet and dry volume measurements. The weights of wet and dry sample masses were 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.

Dry sample volume was determined using a hexapycnometer system of a six-celled, custom-configured Micromeritics AccuPyc 1330TC helium-displacement pycnometer. The precision of each cell 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 weight against the average density of the vial glass. Dry mass and volume were measured after samples were heated in an oven at 105° ± 5°C for 24 h and allowed to cool in a desiccator. The procedures for the determination of these physical properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM International, 1990). The fundamental relation and assumptions for the calculations of all physical property parameters are discussed by Blum (1997) and summarized below.

Mass and volume calculation

Wet mass (M_wet), dry mass (M_dry), and dry volume (V_dry) were measured in the laboratory. The ratio of mass (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, respectively,

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, respectively,

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

Moisture and density properties reported and plotted in the “Physical properties” sections of all site chapters were calculated with the MADMax shipboard program.

Section Half Measurement Gantry

P-wave velocity measurements on split cores were performed by using the x-axis caliper-type contact probe transducers on the Section Half Measurement Gantry with one analysis per section, usually in Hole A. Measurements were usually taken at ~75 cm section depth; however, if this interval provided no good sediment/transducer coupling (e.g., caused by high amounts of sand or cracks), different positions were chosen to generate viable data. Cores drilled with XCB or RCB generally did not provide usable data because of bad sediment/liner contact and disturbed sediments. In some cases in which sediment was sufficiently indurated, P-wave measurements were carried out on discrete pieces taken from the liner.

The system uses Panametrics-NDT Microscan delay line transducers, which transmit at 0.5 MHz. The signal received through the sample was recorded by the computer attached to the system, and the peak (P-wave arrival) was chosen with autopicking software. In case of a weak signal, the first arrival was manually picked. The distance between transducers was measured with a built-in linear voltage displacement transformer (LDVT).

Calibration was performed with a series of acrylic cylinders of differing 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 (calculated by LDVT in meters) was divided by the traveltime (in seconds) to calculate P-wave velocity in meters per second.

Section Half Multisensor Logger

Spectrophotometry and magnetic susceptibility of the archive halves were measured with the SHMSL. The SHMSL also takes measurements on empty intervals and in places where the core surface is well below the level of the core liner. Spurious measurements can also result from small cracks, sediment disturbance caused by the drilling process, or plastic section dividers. This data should be edited out of the data set by the user. Additional detailed information about measurement and interpretation of spectral data can be found in Balsam et al. (1997, 1998) and Balsam and Damuth (2000).

Spectrophotometry

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 soft cores were covered with clear plastic wrap and placed on the SHMSL. Measurements were taken at 2–5 cm spacing to provide a high-resolution stratigraphic record of color variations for visible wavelengths. Each measurement was recorded in 2 nm wide spectral bands from 400 to 900 nm.

Magnetic susceptibility

Magnetic susceptibility was measured with a Bartington Instruments MS2E point sensor (high-resolution surface scanning 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. Measurements were taken at 2–5 cm spacing and integrate 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. For conversion of the instrument units stored in the IODP database, a correction factor (67/80) must be employed to correct for the relation of the sensor diameter and sediment thickness.

Top of page   |    Previous   |    Next