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doi:10.2204/iodp.proc.318.102.2011

Physical properties

Expedition 318 physical property shipboard measurements were made in order to characterize lithologic units and detect changes in sediment properties that could be related to lithologic changes, diagenetic features, or consolidation history. Furthermore, these measurements aid to identify natural and/or coring induced discontinuities and the interpretation of seismic reflection and downhole geophysical logs. The primary objectives of the Expedition 318 physical property program were to collect medium-resolution data for

• Examining variations in physical properties related to the variations in sediment composition (and therefore depositional history) on the Wilkes Land margin;

• Providing estimates of sediment properties, which can be used to reconstruct glacial and interglacial/proglacial depositional processes;

• More fully understanding the strength of hemipelagic sediments in the context of sea level change, sequence stratigraphy, and possible changes of physical properties across unconformities;

• Investigating data sets to aid in the interpretation of seismic reflection and downhole geophysical measurements, including the construction of synthetic seismograms;

• Providing porosity information to construct quantitative algorithms used to decompact the sediments recovered by the backstripping method to resolve any possible eustatic signal recovered; and

• Facilitating hole to hole correlation, thereby allowing real-time stratigraphic correlation and feedback to the drillers, and the construction of a composite stratigraphic section for multiply cored sites.

Physical properties were measured on whole-round sections with the WRMSL. After being brought in from the catwalk, sections were allowed to equilibrate to ambient room temperature (i.e., 20°–22°C) to ensure thermal homogeneity in order to minimize temperature effects on physical property measurements and to protect sensors from damage. The WRMSL incorporates a GRA density meter, a magnetic susceptibility loop sensor, and a compressional P-wave velocity logger (PWL).

For Sites U1357 and U1359, where multiple holes were drilled, we also used the STMSL. The purpose of the STMSL during Expedition 318 was to rapidly record medium- to high-resolution sets of GRA density and magnetic susceptibility data. The GRA bulk density meter and magnetic susceptibility loop incorporated in the STMSL are effectively identical to those of the WRMSL. The spacing distance between STMSL measurements was typically 5 cm for GRA density and magnetic susceptibility measurements. Scanning time averaged 8 s per measurement.

Natural gamma radiation (NGR) was also measured on whole-round sections using the NGRL. Moisture and density (MAD) measurements were made on discrete samples taken from the section halves. Bulk properties determined by MAD analyses include wet bulk density, dry bulk density, grain density, water content, and porosity (Blum, 1997).

Whole-Round Multisensor Logger and Special Task Multisensor Logger measurements

GRA bulk density, magnetic susceptibility, and compressional P-wave velocity were measured nondestructively with the WRMSL on all whole-round core sections. The quality of the WRMSL data is highly dependent on the condition of the core. To optimize WRMSL performance, sampling intervals and measurement residence times were the same for all sensors for any one core. Sampling intervals were set at 2.5 cm for density and magnetic susceptibility and 5 cm for P-wave velocity, and integration times were set for 10 s for GRA density and 2 s for magnetic susceptibility. These intervals and integration times were set so that a 9.5 m long core would take ~1.2 h to pass through the WRMSL, taking ~14 s per point of measurement. These sampling intervals are common denominators of the distances between the sensors installed on the WRMSL (30–50 cm) and allowed a combination of sequential measurements that optimized the total measurement time. On the STMSL the sampling intervals for the two sensors were set at 5 cm so that a 9.5 m long core took <40 min.

Gamma ray attenuation bulk density

Bulk density reflects the combined effect of the lithology, variations in porosity, grain density (dominant mineralogy), and coring disturbance. Porosity is mainly controlled by lithology, texture, compaction, and cementation (controlled by both mechanical and chemical processes).

Because the attenuation coefficient is similar for most common minerals and aluminum, bulk density was obtained through direct calibration of the density meter using aluminum rods of different diameters mounted in a core liner that is filled with distilled water. The GRA density meter has a spatial resolution of <1 cm. The GRA density meter uses a 10 mCi ¹³⁷Cs capsule as the 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 material by Compton scattering.

The attenuation of the incident intensity (I₀) is directly related to the electron density in the sediment core of diameter D, which can be related to bulk density given the average attenuation coefficient (in micrometers) of the sediment (Evans, 1965; Harms and Choquette, 1965).

Magnetic susceptibility

Magnetic susceptibility provides information on the magnetic composition of the sediments that can often be related to mineralogical composition (e.g., terrigenous versus biogenic materials) and diagenetic overprinting. Sediments rich in biogenic opal have generally low to negative magnetic susceptibility because practically no clay or magnetite is present. In such cases, measured values approach the detection limit of magnetic susceptibility sensors.

Magnetic susceptibility was measured with the Bartington Instruments MS2C system with a loop sensor on the WRMSL. The magnetic susceptibility loop operates at a frequency of 621 Hz. The output of the magnetic susceptibility sensors was set to the IODP standard SI unit setting. However, to actually obtain the 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, including core disturbance, between core and loop dimensions. This correction was not undertaken during Expedition 318, and all magnetic susceptibility values are therefore relative (instrument units).

P-wave velocity

P-wave velocity varies with lithology, porosity, and bulk density of material; state of stress; temperature; and fabric or degree of fracturing. In marine sediments and rocks, velocity is controlled by degree of consolidation and lithification, fracturing, and occurrence and abundance of free gas and gas hydrate. Microscopic and macroscopic fracturing may completely attenuate the signal so that it is not possible to obtain data from the PWL. Together with bulk density, velocity data are used to derive porosities and to calculate acoustic impedance and reflection coefficients that can be used to construct synthetic seismograms and to estimate the depths of seismic horizons.

P-wave velocities 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 PWV and PWB measure the P-wave velocity in a Cartesian coordinate system on section halves. The PWC measures the P-wave velocity vertically to the sectional plane of the working half (x-axis), whereas the PWB measures the cross-section (y-axis) and the long axis (z-axis) of the core.

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 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 known to be sufficient for reliable P-wave measurements. The transmitting and receiving ultrasonic transducers are aligned so that wave propagation is perpendicular to the section’s long axis.

Traveltime is determined by signal processing software that automatically detects first arrival of the P-wave signal to a precision of 50 ns. It is a challenge for an automated routine to pick the first arrival of a potentially weak signal with significant background noise. The search method skips the first positive amplitude and finds the second positive amplitude using a detection threshold limit, typically set to 30% of the maximum amplitude of the signal. It then finds the preceding zero crossing and subtracts one period to determine the first arrival. To avoid extremely weak signals, minimum signal strength can be set (typically 0.02 V) and weaker signals are ignored. To avoid cross-talk signals from the receiver at the beginning of the record, a delay (typically 0.01 ms) can be set to force the amplitude search to begin in the quiet interval preceding the first arrival. In addition, a trigger (typically 4 V) to initiate the arrival search process and the number of waveforms to be stacked (typically five) can also be set. A linear voltage differential transformer is used to measure the separation of the transducer to derive a travel path length for the signal (i.e., the slightly compressed core diameter). After corrections for system propagation delay, liner thickness, and liner material velocity, the ultrasonic P-wave velocity is calculated.

Natural gamma radiation

The NGRL measures gamma rays emitted from whole-round core sections. Gamma rays arise primarily as a result of the decay of uranium, thorium, and potassium isotopes. Data generated from this instrument are used to augment geologic interpretations and fine tune stratigraphic correlations. NGR logging is also quite useful as representing the same parameter, which could also be measured in downhole logs.

The main NGR detector unit consists of 8 NaI scintillator detectors, 7 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. The inner half of 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 this passive lead shielding, the NGR employs a plastic scintillator to suppress the high-energy gamma (γ) and muon (µ) components of cosmic radiation by producing a veto signal when charged particles from cosmic radiation pass through the plastic scintillator.

A measurement run consists of two positions on each core section counted for at least 5 min each (10 min for Site U1357 cores) for a total of 16 measurements per section. Complete spectra for each measurement are uploaded to LIMS. Background measurement measures the signal generated when an acrylic liner containing a small amount of low-background lead is in the Ti boat inside the NGRL.

Moisture and density

Wet and dry bulk density, grain density, water content, and porosity were determined from measurements of wet sediment mass, dry sediment mass, and dry sediment volume. In soft sediments, ~10 cm³ samples were extracted, usually from the middle of each core section, and placed in preweighed 16 mL Wheaton beakers. Stiff sediments were sampled, where appropriate, by extracting ~10 cm³ blocks using a spatula and placed into a beaker as above. Indurated sediments were sampled by cut cubes that are ~2.5 cm in height, width, and depth. One sample was routinely collected in each undisturbed section where recovery was good and sedimentation rates were high.

Sample mass was determined to a precision of 0.01 g using two Mettler Toledo electronic balances and a computer averaging system to compensate for the ship’s motion. Dry sample volumes were determined using a hexapycnometer system that consists of six custom-configured Micromeritics AccuPyc 1330TC helium-displacement pycnometers with a precision of 1% of the nominal full-scale volume. Volume measurements were preceded by three purges of the sample chambers with helium warmed to ~25°C. Three acquisition cycles were used for each sample. A reference volume was included within each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. Sample beakers used for discrete determination of moisture and density were calibrated before the cruise. 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. Procedures for the determination of these properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM International, 1990). Fundamental phase relationships and assumptions for the calculations of all physical property parameters are discussed by Blum (1997) and summarized in “Mass and volume calculation” and “Calculation of bulk properties.”

Mass and volume calculation

Method C, sediments and hard rock

Wet mass (M_wet), dry mass (M_dry), and dry volume (V_dry) are measured in the laboratory. The mass ratio (r_m) 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 the salt (M_salt) in the sample are given by

and

where s is the assumed saltwater salinity (0.035) corresponding to a pore water density (ρ_pw) of 1.024 g/cm³ (from experimental and empirical relations between salinity and density at laboratory conditions; Blum 1997) 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), mass of salt (M_salt), volume of salt (V_salt), wet volume (V_wet), and volume of solids excluding salt (V_solid) are, respectively,

and

Method D, hard rock and measured volume of soft sediment

Wet (or total) volume (V_t), dry mass (M_dry), and dry volume (V_dry) are measured in the laboratory. The total mass including the fresh water that was in the pores is calculated using (density of water of 1 g/cm³) by

Assuming a pore water density of 1.024, the volume of the pore water is calculated by

Finally, the mass of the pore water is

Calculation of bulk properties

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

and

Thermal conductivity

Thermal conductivity is the measure of a material’s ability to transmit heat by molecular conduction. Thermal conductivity and temperature measurements of sediments and rock sections are used to determine heat flow. Heat flow is not only characteristic of the material but is also an indicator of type and age of ocean crust and fluid circulation processes at shallow and great depths. Thermal conductivity was measured on one section of unsplit soft-sediment core (usually at 75 cm) using the TK04 measurement system (see Blum, 1997), which employs the transient linear heat source method in full-space configuration (Von Herzen and Maxwell, 1959), after the core equilibrated to ambient temperature. The full-space needle, containing a heater wire and calibrated thermistor, was inserted into the sediment through a small hole drilled into the core liner. The TK04 system uses an automated routine to find the conductivity by least-squares fitting to the measured temperature time series. No calibration was required for this system because each probe is calibrated prior to leaving the factory. However, Macor standard samples are provided to assure quality of the needles regularly. Measurements were taken after the core temperature was equilibrated to laboratory temperature. Special care was taken to minimize measurement drift by covering the needle point of entry and insulating it from external temperature shifts. Measurement errors are 5%–10%.

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