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doi:10.2204/iodp.proc.337.102.2013

Physical properties

Physical properties measurements provide fundamental information required to characterize lithostratigraphic units. A suite of measurements taken during Expedition 337 allowed correlation of recovered sedimentary cores and cuttings samples with downhole logging data and complemented other data sets taken on board the ship. After X-ray CT scanning, core sections wrapped with ESCAL bags were applied to the MSCL-W for measurements of gamma ray attenuation (GRA) density, magnetic susceptibility (MS), natural gamma radiation (NGR), P-wave velocity, and noncontact electrical resistivity. For LDC sections, a wide-diameter sensor track on the split core multisensor core logger (MSCL-S) was used. Thermal conductivity was measured basically on working halves and partially on WRCs. Discrete samples were taken from working halves for MAD analyses, discrete P-wave velocity, and electric resistivity analyses. Samples were also available from community WRC samples. MAD analyses provide water content, bulk density, porosity, void ratio, and grain density. Discrete samples for MAD analyses were taken in cubic form. Riser drilling cuttings were also subjected to MAD analyses. Details and procedures for each physical properties measurement are described below.

MSCL-W

Gamma ray attenuation density

GRA density is based on detection of a gamma ray beam produced by a cesium source and directed through the WRC. The beam is produced by a ¹³⁷Cs gamma ray source at a radiation level of 370 MBq within a lead shield with a 5 mm collimator. The gamma ray detector includes a scintillator and an integral photomultiplier tube to record the gamma rays that pass through the WRC. GRA bulk density (ρ_b) is defined by

where

• I_o = gamma ray source intensity,

• I = measured intensity of gamma rays passing through the sample,

• µ = Compton attenuation coefficient, and

• d = sample diameter.

The Compton attenuation coefficient and I_o are provided by the MSCL-W and are treated as constants, so ρ_b can be calculated from I.

The gamma ray detector is calibrated with a sealed calibration core (a standard core liner filled with distilled water and aluminum cylinders of various diameters). To establish the calibration curves, gamma ray counts are measured through a 7 cm diameter standard cylinder composed of aluminum with six different diameters (1–6 cm) (density = 2.7 g/cm³) filled with surrounding water. The relationship between I and µd is

where A, B, and C are coefficients determined during calibration. GRA density measurements were conducted on core samples every 4 cm for 4 s. GRA bulk density can be used to evaluate the sediment pore volume, which is used for evaluating the sediment consolidation state.

Magnetic susceptibility

MS is the degree to which a material can be magnetized by an external magnetic field. Therefore, MS provides fundamental information about sediment composition. A Bartington loop sensor (MS2C) with an 8 cm loop diameter was used to measure whole-round section MS. An oscillator circuit in the sensor produces a low-intensity (~80 A/m root mean square), nonsaturating, alternating magnetic field (0.565 kHz). Any material brought within the influence of this field will result in a change in the oscillating frequency. The frequency information (returned in pulse form to the susceptometer) was converted into MS. MS data were collected every 4 cm along the core. A reference piece with known MS was measured for condition calibration at least once a day.

Natural gamma radiation

NGR measurements provide insight into sediment composition and thus can be used to identify lithology. WRCs were monitored for NGR emissions to obtain spatial variability in radioactivity and to establish gamma ray logs of cores for correlation with downhole gamma ray logs. A lead-shielded counter, optically coupled to a photomultiplier tube and connected to a bias base that supplied the high-voltage power and a signal preamplifier, was used. Two horizontal and two vertical sensors were mounted in a lead cube-shaped housing. The NGR system records radioactive decay of long-lived radioisotopes ⁴⁰K, ²³²Th, and ²³⁸U. NGR has a resolution of 120–170 mm and was measured every 16 cm for 30 s. Background radiation noise was determined by making measurements on a water-filled calibration core. A granite reference material was measured for checking the calibration of the detector at least once a day.

P-wave velocity

P-wave data can be used to evaluate small strain moduli, to correlate between log and core data, and to evaluate pore structure and cementation. P-wave (compressional) velocity (V_P) is defined by the time required for a compressional wave to travel a set distance:

where

• d = path length of the wave across the core, and

• t_core = traveltime through the core.

P-wave velocity transducers mounted on the MSCL-W system measure total traveltime of the compressional wave between transducers. The wave travels horizontally across the whole core and core liner. The total traveltime observed is composed of t_core , t_delay , and t_liner , where

• t_delay = delay related to mechanical effects, and

• t_liner = transit time through the core liner.

The system is calibrated using a core liner filled with distilled water, which provides control for t_delay and t_liner. With these calibrations, core velocity (V_P) can be calculated on whole-round specimens in core liners:

where

• d_cl = measured diameter of core and liner,

• d_liner = liner wall thickness, and

• t_o = measured total traveltime.

Electrical resistivity

Electrical resistivity measurements are useful for estimating other physical properties, such as porosity, tortuosity, permeability, and thermal conductivity. Bulk electrical resistivity is controlled by solid grain resistivity, pore fluid resistivity, and pore space distribution and connectivity. Electrical resistivity (R_e) is defined by the electrical resistance and geometry of the core measured:

where

• R = electrical resistance,

• L = length of measurement, and

• A = cross-sectional area of the core.

The noncontact resistivity sensor on the MSCL-W system induces a high-frequency magnetic field in the core with a transmitter coil. This generates an electrical current in the bulk sediment that is inversely proportional to its resistivity. A receiver coil measures the secondary magnetic field generated by this induced electrical current. To measure this smaller magnetic field accurately, a differencing technique has been developed that compares readings from the sample core to readings from an identical set of coils operating in air. Electrical resistivity data were obtained at 4 cm intervals on the MSCL-W. For the MSCL measurements including MSCL-S mentioned below, wrapping in an ESCAL bag may impede tight contact between the sensors and the core liner; therefore, P-wave velocity data quality were potentially compromised. However, test measurements of standard pieces wrapped in ESCAL bags showed that this issue is minimal for noncontact measurements of GRA, NGR, and MS. Cores were run on the MSCL-W without waiting for thermal equilibrium to room temperature. Hence, temperature-dependent electrical resistivity results on the MSCL-W were treated as supplementary data.

MSCL-S

The MSCL-S was used for LDC whole-round section measurements. Core material removed from the aluminum core liner was placed on a semicylindrical plastic tray with a diameter of 100 mm. The LDC WRC section and the tray were wrapped, flushed with N₂, and vacuum sealed in an ESCAL bag in the same manner as IODP cores. As the upper half was not covered with plastic, core thickness was not measurable because of unevenness due to a partially ragged surface along the sections. The MSCL-S was equipped with the same sensors as the MSCL-W, except for the NGR and MS sensors, which were equipped with wider loop sensors (125 mm loop diameter for NGR and 120 mm for MS).

Thermal conductivity

Thermal conductivity is the rate at which heat flows through a material and is dependent on mineral and fluid compositions, porosity, and structure. Most of the measurements were conducted on the working-half of WRC samples using a half-space line source (mini-HLQ) probe (Vacquier, 1985), and very partially a needle type (standard VLQ) probe was applied for WRCs of soft sediments (Von Herzen and Maxwell, 1959). This approach approximates the heating element as an infinite line source (Blum, 1997). The measurements were performed three times at each measurement point, and the intermediate thermal conductivity value among the three results was selected as a representative value. For HLQ probe measurements, samples must be smooth to ensure adequate contact with the probe. Visible saw marks were removed, when necessary, by grinding and polishing the split surface. The measurement produces a scalar value in a plane perpendicular to each orientation of the line source of the HLQ and the needle probe of the VLQ. All measurements were made after the sediment cores had equilibrated to ambient laboratory temperature. In order to eliminate the effect of rapid but small temperature changes, the sample and the sensor probes were equilibrated together in an insulated Styrofoam box for at least 10 min prior to measurement. The instrument internally measures drift and does not begin a heating run until sufficient thermal equilibrium is attained. Cores were measured at irregular intervals (aiming for one sample per section) depending on the availability of homogeneous and relatively vein/crack-free pieces that were also long enough to be measured without edge effects. At the beginning of each measurement, temperature in the sample was monitored to ensure that the background thermal drift was <0.04°C/min. After the background thermal drift was determined as stable, the heater circuit was closed and the increase in the probe temperature was recorded. The condition of the probes was checked at least once every 24 h. The condition check was performed on a Macor sample (glass ceramic) of known thermal conductivity.

Discrete sample measurements

For discrete samples, 8 cm³ cubic samples were cut from the working halves of split cores at an average frequency of four samples per core. However, sampling frequency changed depending on lithology variation. Discrete samples were selected to best represent the general variation and lithologies of the core. These samples were measured for P-wave velocity, electrical resistivity, and MAD (discussed below). When a cubic sample could not be taken from the core, sediment blocks ~10 cm³ in total were used for MAD measurements. In that case, P-wave velocity and electrical resistivity measurements were omitted. Samples for MAD measurements were also taken from community WRC samples.

Cuttings sample measurements

Cuttings samples used for physical properties measurements were washed using freshwater before separation into four categories based on the size fraction (>>4 mm, >4 mm, 1–4 mm, and <1 mm) by sieving. Large-sized cuttings >10 mm on each side axis were collected and pressed into cubic shapes (5–10 cm³ of bulk volume) for measuring P-wave velocity and electrical resistivity in addition to porosity. The methodology is identical to that of discrete samples, even though sample orientations were unknown.

MAD measurements

MAD data were obtained through mass and volume determinations on discrete samples. MAD data allow for calculation of several basic quantities: water content, bulk density, dry density, porosity, and void ratio. For the measurements, a dual-balance system and a pentapycnometer in the core laboratory were used. Each discrete sample taken from a working half, as well as cuttings from each depth, was treated in a beaker of known mass and volume during MAD measurements. Cuttings samples were separated into four categories: one represents the original bulk cuttings samples that were washed with seawater, and the other categories represent cuttings samples that were sieved into >4.0 mm, 1.0–4.0 mm, and 0.25–1.0 mm particle size fractions.

Dual-balance system

A motion-compensated shipboard balance system, a so-called dual-balance system, was used to measure both wet and dry masses. The two analytical balances were used to compensate for ship motion, one acting as a reference and the other for measurement of the unknown. A standard mass of similar value to the sample was placed on the reference balance to increase accuracy. The dual-balance system was calibrated at least once every 24 h. The calibration was performed with a standard mass of 40 g on both balances; an accuracy of ±0.005 g was required for calibration.

Pentapycnometer system

The pentapycnometer system (Quantachrome) measures dry sample volume at room temperature using pressurized, helium-filled chambers. A five-chamber Quantachrome pentapycnometer system allowed the measurement of four sample volumes and one calibration sphere. The medium volume sample chamber was selected during Expedition 337. Each measured volume is the average of five volume measurements. The stainless steel sphere (28.9583 cm³ in volume) used for calibration was rotated between all measurement chambers to monitor for errors in each chamber. Spheres are assumed to be between 28.90 and 29.02 cm³ in volume. Individual volume measurements were preceded by five helium purges of the sample chambers, followed by five data acquisition cycles. The pentapycnometer was calibrated with the stainless steel sphere at least once every 24 h.

Wet and dry mass measurements

Discrete samples taken from the working half and cuttings samples were measured for the determination of wet sediment mass (M_w). Cuttings and core samples were soaked in 35‰ NaCl solution for a few hours before measurement. After soaking, the wet cuttings samples were wiped to remove excess water. Dry sediment mass (M_d) and volume (V_d) were measured after drying the samples in a convection oven for >24 h at 105° ± 5°C. Dried samples were then cooled in a desiccator for >1 h before the dry mass was measured. Dry volume was measured using a helium-displacement pycnometer with a nominal precision of ±0.04 cm³. Each reported value consists of an average of five measurements. A reference volume (calibrated sphere) was run with each group of four samples, and the sphere was rotated between cells to check for systematic error.

For calculation of sediment bulk density, dry density, grain density, porosity, and void ratio, conventional ODP methods were used (e.g., Shipboard Scientific Party, 1996). Water content, porosity, and void ratio were defined by the mass or volume of extracted water before and after the removal of interstitial water through the drying process. Standard seawater density (1.024 g/cm³) was assumed as the pore water density.

Water content

Water content (W_c) was determined using the methods of the American Society for Testing and Materials (ASTM) designation D2216 (ASTM International, 1990). Corrections are required for salt when measuring the water content of marine samples. In addition to the recommended water content calculation in ASTM D2216 (i.e., the ratio of pore fluid mass to dry sediment mass [percent dry weight]), we also calculated the ratio of pore fluid mass to total sample mass (percent wet weight). The equations for water content were

and

where

• M_t = total mass of the saturated sample,

• M_d = mass of the dried sample, and

• r = salinity.

Bulk density

Bulk density (ρ) is the density of the saturated samples, with ρ = M_t/V_t (V_t = total volume of the saturated sample). The mass (M_t) was measured using the balance, and V_t was determined from the pycnometer grain volume measurements and the calculated volumes of pore fluid (V_pore) and salt (V_s):

and

where

• ρ_w = pore fluid density, and

• ρ_salt = salt density (2.257 g/cm³).

Porosity

Porosity (ϕ) was calculated using

where

• ρ = measured bulk density,

• ρ_w = density of the pore fluid, and

• W_c = water content expressed as a decimal ratio of percent dry weight.

Grain density

Grain density (ρ_grain) was determined from measurements of dry mass and dry volume made in the balance and in the pycnometer, respectively. Mass and volume were corrected for salt content using

where

• s = salt content (in grams), and

• ρ_salt = salt density (2.257 g/cm³).

P-wave velocity

Discrete P-wave velocity measurements were obtained on cubic discrete sediment samples using the P-wave logger for discrete samples (GeoTek). The cubic samples were soaked in 35‰ NaCl solution prior to P-wave measurement. The measurements used caliper-type contact probe transducers on the P-wave velocity gantry. Oriented samples were rotated manually to measure x-, y-, and z-axis velocities with the same instrument. The system uses delay line transducers, which transmit compressional wave at 230 kHz. To maximize contact with the transducers, deionized water was applied to sample surfaces. The signal received through the sample was recorded by the computer attached to the system, and the peak of the initial arrival was chosen. The distance between transducers was measured with a thickness sensor. Before measurements were made, calibration was performed every 24 h with acrylic and glass cylinders with known P-wave velocities of 2722 ± 33 m/s and 5507 ± 138 m/s, respectively. The determined system time delay from calibration was subtracted from the picked arrival time to yield a traveltime of the P-wave through the sample. The sample thickness was divided by the traveltime (in seconds) to calculate a P-wave velocity in meters per second.

Electrical impedance

Electrical impedance was measured on discrete cubic sediment samples using the Agilent 4294A Precision Impedance Analyzer. The cubic samples were soaked in 35‰ NaCl solution prior to the measurements. The samples are held between two stainless steel electrodes. For the measurements, paper filters soaked in NaCl solution allow better contact between the electrodes and the cubic sample on both top and bottom sides. Oriented samples were rotated manually to measure electrical impedance along x-, y-, and z-axes with the same instrument. The frequency of electric transmission is 25 kHz.

Electrical impedance is defined as the ratio of the voltage and the current in an alternating current circuit. By the measurements, magnitude of electrical impedance (|Z|) (in Ω), phase angle (θ) (in degrees), and sample length (L) are first given. Electrical resistivity on the x-axis (R_x) is described as

where |Z_f|cosθ_f is the resistance of the paper filters and L_x, L_y, and L_z are the lengths of the triaxial directions. Other resistivity values on the y- and z-axes (R_y and R_z) are described by the same equation.

Horizontal anisotropy of electrical resistivity (A_h) and vertical anisotropy of electrical resistivity (A_v) were calculated using the following equations:

and

where R_x, R_y, and R_z are electrical resistivity in each axial direction.

By assuming the anisotropy of resistivity, formation factor on the x-axis (F_x) is described as

where R_f is the resistivity of pore fluid represented by standard seawater at room temperature (°C) as mentioned below. The relationship between R_f and temperature (T) is explained by (Shipley, Ogawa, Blum, et al., 1995):

Other formation factors on the y- and z-axes (F_y and F_z) are described by the same relation.

Formation factor of bulk rock (F_bulk) is defined as

where R_bulk is the mean value of triaxial resistivity described as

and R_fluid is the resistivity of standard seawater (Shipley, Ogawa, Blum, et al., 1995).

Calibration was required prior to measurement and every 24 h when using the instrument continuously. For calibration, measurements on both open and short states were performed. A standard disk attachment was applied to the calibration with the nonconductive cap on in an open state and also without the cap at short state. Saturated filter papers were applied for better contact between the electrodes and the cubic sample, and a characterized tuff cubic sample with filter papers was measured for quality control.

Anelastic strain recovery analysis

The anelastic strain recovery (ASR) technique is a core-based stress measurement that can evaluate both orientation and magnitude of 3-D principal stress on rock at present. The ASR technique principally measures the anelastic strain change by releasing the stress soon after core recovery. The methodology used for the ASR measurement during Expedition 337 is based on Matsuki (1991), following the guideline described by Lin et al. (2007). After the X-ray CT scan and MSCL-W measurement, a 17 cm long fresh WRC was immediately taken to the QA/QC laboratory for ASR determination because the analysis is time-sensitive. First, the outer surface of the core was washed in seawater to remove drilling mud. Before starting the ASR measurement, an elliptical section of core sample was measured by a 2-D measurement sensor (Keyence Corporation, TM-065) on the rotary table (Fig. F19). Diameter measurement of the LDC core was not undertaken because the core diameter was out of range for the sensor. The anelastic strain of the specimen was measured in nine directions, including six independent directions, using 18-wire strain gauges. During the treatment of homogeneous shales and also alternating sandstone with silt, fractures developed parallel to the bedding plane, so glue was placed on the fractures to prevent splitting. The attachment of 14 strain gauges took 1–2 h to achieve, and the total elapsed time from the recovery of core on deck was 3–5 h until the time when strain data (core diameter) could be recorded. Strain data were collected every 10 min for up to a maximum of 21 days. The core samples were double-bagged (i.e., with plastic and aluminum) and submerged in a thermostatic chamber, where temperature changes were controlled to within less than ±0.1°C during measurement.

Deviation of stress is the most important parameter for understanding stress state at depth. Using the data obtained from 2-D measurement, the relationship between the maximum horizontal stress (S_Hmax) and the minimum horizontal stress (S_Hmin) is estimated by the following relationship:

where

• d_max = maximum diameter,

• d_min = minimum diameter,

• d₀ = initial diameter before unloading of core samples,

• E = Young’s modulus, and

• ν = Poisson’s ratio.

By assuming that d₀ replaces the d_min and E and ν are the same among core samples, we can roughly expect S_Hmax – S_Hmin to increase with depth. Generally, even though Young’s modulus of sedimentary rocks increases with depth due to sediment consolidation, Young’s modulus E as well as ν varies among lithologies.

Vitrinite reflectance analysis

Vitrinite reflectance (R_o) is a major indicator of the thermal maturity of organic sedimentary materials. During Expedition 337, R_o values of coal fragments were measured on board the ship using the vitrinite reflectance analyzer for small coal particles designed by Sakaguchi et al. (2011) (Fig. F20). Coal fragments were extracted by rough crushing with mortar and separation in heavy liquid using a sodium polytungstate solution (density = 1.8 g/cm³). The sediment fraction was sieved through 75 µm mesh, and large coal particles were collected. The coaly fragments were then polished using SiC abrasive papers (i.e., 800, 1200, and 2400 grit) and an alumina burnishing cloth with two alumina suspensions in 1 and 0.3 µm particle size. A random mean R_o value was obtained by measuring R_o for 100 randomly oriented fragments under an oil immersion microscope with a microspotlighting system. Five standard samples were used for R_o value calibration.

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