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doi:10.2204/iodp.proc.323.102.2011

Physical properties

Shipboard measurements of physical properties provide information that assists in the characterization of lithologic units, correlation of lithostratigraphy with downhole geophysical logging data, assessment of consolidation history, and interpretation of seismic reflection profiles. The primary objectives of the Expedition 323 physical properties program were to collect downhole high-resolution data to facilitate (1) calculation of mass accumulation rates (MARs), (2) hole-to-hole and site-to-site stratigraphic correlation needed for the construction of composite stratigraphic sections, (3) detection of cyclostratigraphic sequences, and (4) identification of major seismic reflectors and construction of synthetic seismic profiles.

Physical properties were first 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 data logging systems were run on each section: (1) the "fast track" STMSL was run on sections immediately after they were sectioned on the catwalk and (2) the "slow track" WRMSL was run on core sections that had warmed to ambient laboratory temperature (19°–20°C). The linear track of the STMSL houses a GRA bulk densitometer and a magnetic susceptibility sensor. The WRMSL employs, in this order, a GRA bulk densitometer, a magnetic susceptibility sensor, 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 (one section per core) by a needle probe inserted into the section through a small hole drilled through the plastic core liner.

At this phase of data gathering, the sections were split into working and archive halves. Discrete samples were collected from the working halves to measure wet bulk density, dry bulk density, water content, porosity, and grain density with moisture and density (MAD) procedures. A full discussion of all methodologies and calculations used in the R/V JOIDES Resolution physical properties laboratory can be found in Blum (1997).

Special Task Multisensor Logger

The purpose of STMSL logging operations during Expedition 323 was to rapidly record medium- to high-resolution sets of GRA and magnetic susceptibility data. This information was urgently needed to identify offsets in the composite section during the drilling of multiple holes. The GRA bulk densitometer and magnetic susceptibility loop incorporated in the STMSL are effectively identical to those of the WRMSL (see "Whole-Round Multisensor Logger"). The spacing distance between STMSL measurements was typically 5 cm for GRA measurements and 10 cm for magnetic susceptibility measurements. Scanning time averaged 8 s. STMSL scans were also run for cores collected from dedicated microbiological holes.

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 and sediment fullness of the core sections. WRMSL scans were not run for cores collected from dedicated microbiological holes.

GRA bulk density and magnetic susceptibility were measured nondestructively on all whole-round core sections. Compressional wave (P-wave) velocity was measured in sections undisturbed by gas expansion voids and cracks. To optimize WRMSL performance, the same sampling spacing, typically 5 cm, was set for all sensors. Measurement time, although somewhat different for the different sensors, averaged ~8 s. With handling, data initializing, and track movement, a 1.5 m section took ~10 min to scan. A 9.5 m core thus 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.

GRA and magnetic susceptibility calibration issues occurred during the cruise. When these were noticed, GRA calibration standards were run. Problems with bias changes in magnetic susceptibility data will be addressed on shore (see "Physical properties calibration issues" in the "Appendix").

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 siliceous (diatomaceous) 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 621 Hz for the WRMSL and 513 Hz for the STMSL. 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 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 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

The NGRL was designed and built at the Texas A&M University IODP-USIO facility. The NGRL 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 NGRL maps fine stratigraphic details that importantly 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 a 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 consists of counting two positions on each core section for at least 5 min each for a total of 16 measurements per 150 cm section. Typically, core-section logging required ~15 min; thus, with handling and data management, the processing of a 9.5 m core required ~1.5 h.

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. Because an air conditioning vent was located directly over the thermal conductivity station, an insulating jacket of foam rubber was placed over the core section during measurement of thermal conductivity. A thermal transfer compound was used to improve the coupling between the needle and the sediment.

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.

Two 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%. At sites where in situ temperatures were measured, thermal conductivity was corrected for in situ temperature and pressure as part of the calculation of heat flow. Thermal conductivity measurements were routinely taken in one section per core, typically Section 2 because Section 1 was often disturbed. Thermal conductivity measurements were not taken for holes dedicated to microbiological sampling.

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 whose diameter fit that of the glass vial. An attempt was made to sample two or three sections per core at the 29–31 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. Lithified sediments—volcaniclastic siltstone, sandstone, and coarser breccia beds—were recovered from basement rock cored at Site U1342. No shipboard MAD properties were measured on these basement cores. 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. Periodically, a solid metallic reference volume 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 in "Mass and volume calculation" and "Calculation of bulk properties."

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 M_water = M_wet – M_dry and M_salt = M_water [s/(1 – s)], respectively, where s = 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,

and

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 follows:

and

Moisture and density properties reported and plotted in the "Physical properties" sections of all site reports were calculated with shipboard-prepared Excel spreadsheets.

Formation factor

Formation factor (F) was determined from electrical conductivity measurements taken every 10 or 20 cm on the split-core sediments from the top three to five cores from Holes U1339A, U1342A, U1343E, U1344A, and U1345A. Two in-line electrodes (1.5 cm in length and spaced 1 cm apart) mounted on a block of nonconducting grade PTFE and attached to a Metrohm 712 conductometer were inserted into the split-core sediments. This method is based on McDuff and Ellis (1979).

At each sampling location, measurements of sediment conductivity (the inverse of resistivity [R_core]) were made. Measurements of conductivity for room-temperature seawater (R_sw) were made regularly so that formation factor, F = R_core/R_sw , could be calculated. Both sediment and seawater were equilibrated to ambient laboratory temperature. Temperature measurements on both were made with each conductivity measurement.

This simple method for determining formation factor does not take into account surface conductivity effects of the sediment matrix. However, this is not of concern in high-porosity sediments where the conductive pathways depend dominantly on intergranular porosity and pore connectivity, even where the sediment matrix contains significant clays.

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