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doi:10.2204/iodp.proc.317.102.2011

Physical properties

Shipboard physical property measurements were made during Expedition 317 to characterize lithologic units and aid in the following: correlating lithology with downhole geophysical logging data, assessing the lithologic dependence of porosity, assessing consolidation history, and interpreting seismic reflection profiles. The primary objectives of the Expedition 317 physical properties program were to collect medium-resolution data that would

1. Provide porosity information for the decompaction algorithms used in the backstripping calculations used to estimate eustatic magnitudes;

2. Facilitate hole-to-hole and site-to-site correlation and the construction of composite stratigraphic sections;

3. Provide a framework for possible postcruise climatic cyclostratigraphy studies;

4. Facilitate the construction of synthetic seismograms;

5. Investigate the characteristics of major seismic reflectors; and

6. Promote a better understanding of the strength of hemipelagic sediments in the context of sea level change, sequence stratigraphy, and possible changes of strength across unconformities.

Physical properties were measured on whole-round sections using the WRMSL. After cores were brought in from the catwalk, they were allowed to equilibrate to ambient room temperature (20°–22°C) to ensure thermal homogeneity in order to minimize temperature effects on physical property measurements and to protect the sensors from damage. The WRMSL incorporates a gamma ray attenuation (GRA) densitometer, an MSL, and a compressional P-wave velocity logger (PWL). NGR was also measured on whole-round sections using the NGRL. MSP measurements and color measurements (reflectance spectroscopy and colorimetry [RSC]) were taken on the archive halves of split cores using the SHMSL. High-resolution digital color images were captured using the Section Half Imaging Logger (SHIL). Moisture and density (MAD) measurements were made on discrete samples taken from section halves, often adjacent to samples used for XRD analyses. Bulk properties determined by MAD analyses include wet bulk density, dry bulk density, grain density, water content, and porosity. Shear strength was measured on working halves with the automated vane shear (AVS) system and with the fall cone penetrometer (FCP), which is a third-party tool provided by the Center for Marine Environmental Sciences (MARUM), University of Bremen, Germany.

To help illustrate key trends, the often noisy magnetic susceptibility, NGR, and color data were processed on board using the Igor Pro 5 software. An efficient approach was to use a built-in Gaussian filter. The binomial smoothing algorithm used (based on Pascal's triangle) is described by Marchand and Marmet (1983). The frequency response of the binomial smoothing algorithm is expressed as a percentage of the sampling frequency (= n passes). This method should be improved postcruise by cleaning the data from caved intervals of the uppermost and/or lowermost parts of the cores and by using more appropriate filtering methods. Processed data are available in FILT_DATA in "Supplementary material."

Whole-Round 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 core condition. To optimize WRMSL performance, sampling intervals and measurement residence times were the same for all sensors for any one core. Sampling intervals were initially set at 5 cm so that a 9.5 m long core would take ~1.2 h to pass through the WRMSL with a residence time of 3 s for each measurement and 14 s for all three measurements. During the course of the expedition, it became apparent that the sampling interval could be reduced without constraining the workflow. Therefore, the sampling resolution for all three measurements was increased to 2.5 cm to provide a more detailed record of the lowermost cores from Hole U1352C (Section 317-U1352C-87R-1 [1361.4 m; total depth]) and all cores from Sites U1353 and U1354. These sampling intervals are common denominators of the distances between the sensors installed on the WRMSL (30–50 cm), which allowed for a combination of sequential measurements that optimized total measurement time.

Section Half Multisensor Logger measurements

The SHMSL employs multiple sensors to measure bulk physical properties in a motorized and computer-controlled section-half logging machine. The SHMSL includes sensors for magnetic susceptibility, RSC, and a laser surface analyzer. The sampling interval was set at 5 cm for magnetic susceptibility and RSC. The SHIL captured continuous high-resolution images of the surface of the archive section halves.

Gamma ray attenuation bulk density

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

The GRA densitometer on the WRMSL uses a 10 mCi ¹³⁷Cs capsule as the gamma ray source (principal energy peak = 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 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). Because the attenuation coefficient is similar for most common minerals and aluminum, bulk density was obtained through a 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 sediments, which can often 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. Diamagnetic carbonate, silica, water, and plastics (core liner) have small negative values of magnetic susceptibility. Sediments rich in biogenic carbonate and opal therefore have generally low to negative magnetic susceptibility values because practically no clay or magnetite is present. In such cases, measured values approach the detection limit of the magnetic susceptibility sensors.

Magnetic susceptibility was measured using the non-contact pass-through "loop" sensor (MSL) on the WRMSL and the magnetic susceptibility "point" sensor (MSP) on the SHMSL. The MSL demands flush contact with the split core. A laser surface analyzer aids in the recognition of irregularities in the split-core surface (e.g., cracks and voids), and data from this tool were recorded to provide an independent check on the fidelity of SHMSL measurements.

The frequency at which the MSL operates is 621 Hz. The output of the magnetic susceptibility sensors can be set to centimeter-gram-second (cgs) units or SI units, which are the standard units used at IODP. However, to actually 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, including core disturbance between core and loop dimensions. This correction was not undertaken during Expedition 317, and all magnetic susceptibility values are reported as instrument units.

Natural gamma radiation

The NGRL was designed and built at the Texas A&M University IODP-USIO facility from 2006 to 2008. 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.

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. In addition to this passive lead shielding, the NGRL 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 consisted of counting two positions on each core section for at least 5 min each for a total of 16 measurements per section. Complete spectra for each measurement as well as computed total counts were uploaded to LIMS.

P-wave velocities

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

P-wave velocities were measured on whole-round sections with the P-wave logger (PWL) on the WRMSL and with the P-wave caliper (PWC) and P-wave bayonets (PWB) on split cores relative to a Cartesian coordinate system (x-, y-, and z-directions). The PWC measures P-wave velocity vertically to the sectional plane of the working half (x-axis), whereas the PWB measures the cross section (y-axis) and 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 by 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 measurement. 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 the first arrival of the P-wave signal to a precision of 50 ns. It is challenging 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, a minimum signal strength can be set (typically 0.02 V) and weaker signals will be 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). The ultrasonic P-wave velocity is then calculated after corrections have been made for system propagation delay, liner thickness, and liner material velocity.

Digital color imaging

The SHIL was used shortly after core splitting in an effort to avoid time-dependent color changes resulting from sediment drying and oxidation. The shipboard system uses a commercial line-scan camera lens (AF Micro Nikkor, 60 mm, 1:2.8 D) with illumination provided by a custom assembly of three pairs of light emitting diode (LED) strip lights that provide constant illumination over a range of surface elevations. Each pair of lights has a color temperature of 6500 K and emits 90,000 lux at 3 inches. The resolution of the line-scan camera was set at 10 pixels per millimeter. Users set a crop rectangle for each image to remove extraneous information. Images were saved in high-resolution TIFF format. Available files include the original high-resolution image with gray scale and ruler, as well as reduced JPEG images cropped to show only the section-half surface.

Spectrophotometry and colorimetry

Reflectance spectroscopy (spectrophotometry) was carried out using an Ocean Optics USB4000 spectrophotometer, which measures the reflectance spectra of the split core from the ultraviolet to near-infrared range. Colorimetric information from split cores was also recorded by this instrument in the L*a*b* color space system, which expresses color as a function of lightness (L*) and color values a* and b*, where a* reflects the balance between red (positive a*) and green (negative a*) and b* reflects the balance between yellow (positive b*) and blue (negative b*). When color values a* and b* are zero, there is no color and L* determines gray scale.

Accurate spectrophotometry using the SHMSL demands flush contact between the instrument sensors and the split core. A built-in laser surface analyzer aids in the recognition of irregularities in the split-core surface (e.g., cracks and voids), and data from this tool were recorded in order to provide an independent check on the fidelity of SHMSL measurements.

Moisture and density

Wet and dry bulk density, grain density, water content, and porosity were determined from measurements of wet sediment mass and volume and/or dry sediment mass and 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 drilled with the XCB were sampled, where appropriate, by extracting ~10 cm³ blocks using a spatula and placing the blocks into a beaker, as above. Indurated sediments drilled with the RCB system were sampled by drilled cylinders ~1.8 cm in diameter and 2–2.5 cm in height. When the cylinders were regular in shape (no breakage, etc.) they were measured with an electronic digital caliper having a precision of 0.001 cm. Method C was applied to soft sediments and rock samples using the measurements of wet and dry mass and dry volume determined by gas pycnometry. In addition, Method D was applied to rock cylinder samples using wet volume determined with the caliper and dry mass and volume. One sample was routinely collected from each undisturbed section, and 3–4 samples per core were collected where recovery was good and sedimentation rates were high. Additional samples were taken where major changes in lithology were observed. Finally, for Holes U1353A and U1353B, one sample was taken by syringe on the catwalk during the initial core splitting in order to determine whether the WRMSL measurements and core-splitting procedures affected porosity results.

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 consisting 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, Methods C and D in "Chapter 2: moisture and density") and summarized in "Mass and volume calculation" and "Calculation of bulk properties," below.

Mass and volume calculation

Method C (sediments and hard rock)

Wet mass (M_wet), dry mass (M_dry), and dry volume (V_dry) were 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 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) were measured in the laboratory. Total mass, including freshwater in the pores, was calculated (using a density of water of 1 g/cm³) by

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

Finally, the mass of 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 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

Sediment strength

Shear strength describes the maximum strength of soil or sediment at which point a significant structural failure occurs in response to an applied shear stress. Sediment shear strength can be measured by various instruments in the laboratory, including direct simple shear, triaxial shear, vane shear, and fall cone devices. Only fall cone and vane devices are suited for measuring the shear strength of very soft to stiff marine sediments. Fall cone and vane tests are useful for determining the undrained shear strength of undisturbed clay- or silt-rich samples. The FCP is a third-party tool used during Expedition 317 for the first time, whereas the vane shear device is routinely available. These shear strength tests are not suitable for coarser grained sediments or sediments containing silt or sand laminations. Because additional forces from the ship's motion add uncertainty to stress tests, both FCP and AVS tests were conducted so as to minimize the limitations of performing a single test. The sampling rate was one measurement per core section until the sediment became too firm for cone penetration or vane insertion.

Fall cone tests

The FCP test is a rapid, simple, accurate method for determining the undrained shear strength of fine-grained soils or sediments. Undrained shear strength is critical in evaluating sediment compaction as well as processes such as sliding, slope failure, and erosion.

FCP tests were conducted with a "Strassentest" apparatus Type 318H, which conforms to the British Standard BS 1377. The weight of the cone was 80 g with a 30° cone angle. The cone was carefully lowered to the surface of the sediment so that the tip of the cone touched but did not penetrate the sediment surface. The cone was then allowed to fall freely and penetrate the sediment under its own weight. After 5 s, a magnetic clamp stopped the free fall of the cone. The final cone penetration depth was measured with a dial gauge. This destructive measurement was done on the working halves of cores, with the fall direction parallel to the bedding plane.

Hansbo (1957) showed that (dynamic) undrained shear strength (s_u) is related to the final depth of penetration (h_f) of the cone and can be expressed as

where

• W = cone weight (g),

• K = fall cone factor (a constant), and

• h_f = final penetration depth (mm).

Fall cone factor is a constant that is influenced by cone geometry, cone roughness, surrounding soil or sediment, and dynamic effects (Houlsby, 1982; Wood, 1985; Koumoto and Houlsby, 2001; Mahajan and Budhu, 2009). In empirical fall cone experiments performed with cones of different angles, Wood (1985) showed that the average cone factor value for the 30° cone is K = 0.85. Koumoto and Houlsby (2001) suggested that undrained shear strength in the fall cone test is dynamic shear strength, which is higher than static undrained shear strength because of the higher strain rates in the fall cone test. They proposed an empirically determined adjustment factor, K, to determine static undrained shear strength. For the British cone, the theoretically determined value of the constant is K = 1.33 (Dolinar and Trauner, 2005). In this study, we used Wood's factor to calculate undrained shear strength, yielding results in terms of Newtons (SI units) of kN/m².

Vane shear tests

The AVS test was conducted using the "Giesa Automated Vane System." The Giesa system consists of a controller and a gantry for shear vane insertion. A four-bladed miniature vane (diameter = height = 12.7 mm) was pushed carefully into the sediment of the working halves until the top of the vane was level with the sediment surface. The vane was then rotated at a constant rate of 90°/min to determine the torque required to cause a cylindrical surface to be sheared by the vane. This destructive measurement was done with the rotation axis parallel to the bedding plane. The torque required to shear the sediment along the vertical and horizontal edges of the vane is a relatively direct measurement of shear strength. Undrained shear strength, s_u, is given as a function of pressure in SI units of Pascals (kPa = kN/m²).

Both sediment strength tests were performed on working section halves at a resolution of one measurement per section. The tests were conducted in parallel to minimize the limitations of employing a single test. Samples were generally taken in undisturbed fine-grained sediments. XCB cores were sampled when core quality allowed, and measurements were made between "biscuits" and as far as possible from other drilling disturbances (e.g., cracks, gaps, or soupy core parts). Potential measurement failures in XCB cores due to low penetration depth of the fall cone and the formation of shear gaps during the insertion of the vane blades should be taken into consideration. No samples were taken from RCB cores because of the lithified nature of these sediments.

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