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doi:10.2204/iodp.proc.333.102.2012

Physical properties

Physical property measurements provide valuable constraints on bulk physical character to augment lithologic unit characterization and to facilitate correlation of seismic reflection data with discrete core measurements and core descriptions. Thus, these data provide information necessary for reliable core-seismic integration. Expedition 333 employed multiple approaches and methods to characterize the physical properties of cores. Prior to core physical property measurements, X-ray CT images were collected for all cores, and cores were equilibrated with room temperature (~20°C). After temperature equilibration, whole-round core sections were processed in the whole-round multisensor core logger (MSCL-W) to measure gamma ray attenuation (GRA) density, magnetic susceptibility, natural gamma radiation, P-wave (compressional) velocity, and electrical resistivity. For cores with soft, unconsolidated sediments, thermal conductivity was measured using a TeKa thermal conductivity meter with the VLQ full-space needle probe inserted in whole-round cores after MSCL-W measurements. Cores were then split into archive and working halves. For cores with lithified sediments, a needle probe could not be inserted, thermal conductivity measurements were made on working halves using the TeKa thermal conductivity meter in half-space mode (HLQ). Discrete samples were collected from the working halves for moisture and density (MAD) measurements. Specimen mass was taken with a BAL-2 motion-compensated shipboard balance system. The volume of solids was measured with a Quantachrome penta-pycnometer. For soft sediments, undrained shear strength at discrete intervals from the working halves was determined from vane shear measurements made with a Wykeham Farrance WF23544 vane apparatus and penetrometer measurements made with a Geotest E-284B penetrometer. Resistivity was measured on soft-sediment cores using a Wenner array of electrodes and an impedance meter. P-wave velocity and electrical resistivity were measured on discrete core samples in the x-, y-, and z-directions when sediments were hard enough to permit samples to be taken. Details and procedures for each physical property measurement are described below.

For basalt samples, modified procedures were used. If recovery allowed, one sample per core was cut to be used for P-wave velocity and electrical resistivity. After cutting, this sample was soaked in 35 g/L NaCl solution under vacuum for 24 h prior to measurement. After measurement, this sample was used for paleomagnetic analyses. A MAD sample was obtained from the same interval as the P-wave and electrical resistivity sample. It was soaked under vacuum for 24 h. After soaking, the outer surface of the sample was blotted to remove surficial water. It then followed typical MAD procedures described below. Because of the limited size of the available vacuum chamber, samples for thermal conductivity could not be soaked under vacuum prior to measurement. Instead, the sample was obtained as soon as practical after the core was split and soaked in 35 g/L NaCl solution for 15 min.

MSCL-W

GRA density

Bulk density can be used to evaluate pore volume in sediment, which provides information on the consolidation state of sediment. GRA density is based on the detection of a gamma ray beam produced by a cesium source. The beam, produced by a ¹³⁷Cs gamma ray source at a radiation level of 370 MBq within a lead shield with a 5 mm collimator, is directed through whole-round cores. The gamma ray detector includes a scintillator and an integral photomultiplier tube to record the gamma rays that pass through the whole-round core. GRA bulk density (ρ_b) is calculated as

where

• I₀ = 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 (µ) and source intensity (I₀) 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 pure water [Elix] and aluminum cylinders of various diameters). To establish the calibration curves, gamma ray counts were taken through each aluminum cylinder for 60 s. Each aluminum cylinder has a density of 2.7 g/cm³, and d is 1, 2, 3, 4, 5, or 6 cm. The relationship between I and µd is

where A and B are coefficients determined from the calibration experiment. GRA density measurements on core samples were conducted every 4 cm for 4 s. The spatial resolution was 5 mm.

Magnetic susceptibility

Magnetic susceptibility is the degree to which a material can be magnetized by an external magnetic field. Therefore, magnetic susceptibility provides information on sediment composition. A Bartington loop sensor with an 8 cm diameter was used to measure magnetic susceptibility. An oscillator circuit in the sensor produces a low-intensity (~80 A/m RMS), nonsaturating, alternating magnetic field (0.565 kHz). This pulse frequency is converted into magnetic susceptibility. The spatial resolution of the loop sensor is 23–27 mm, and it is accurate to within 5%. Magnetic susceptibility data were collected every 4 cm along the core.

Natural gamma radiation

Natural gamma radiation measurements provide insights into sediment composition and thus can be used to identify lithology. Whole-round cores were monitored for natural gamma ray (NGR) emissions to obtain spatial variability in radioactivity and establish gamma ray logs of cores for correlation to downhole gamma ray logs. A lead-shielded counter, optically coupled to a photomultiplier tube and connected to a bias base that supplies high-voltage power and a signal preamplifier, is used. Two horizontal and two vertical sensors are mounted in a lead, cube-shaped housing. The NGR system records radioactive decay of long-period isotopes ⁴⁰K, ²³²Th, and ²³⁸U. NGR has a resolution of 120–170 mm and was measured every 16 cm with a count time of 30 s. Background radiation noise was determined by taking measurements on a water-filled calibration core.

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 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_delay = time delay related to transducer faces and electronic circuitry,

• t_pulse = delay related to the peak detection procedure,

• t_liner = transit time through the core liner, and

• t_core = traveltime through the sediment or rock.

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

where

• d_cl = measured diameter of core and liner,

• d_liner = liner wall thickness, and

• t_o = measured total traveltime.

Equation 25 assumes that the core completely fills the core liner.

Electrical resistivity

Within limits, electrical resistivity may be useful for estimating other sediment physical properties, including porosity, tortuosity, permeability, and thermal conductivity. Bulk electrical resistivity is controlled by solid grain resistivity, interstitial water resistivity, pore space distribution, and pore connectivity. Electrical resistivity (ρ) 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. The secondary magnetic field generated by this induced electrical current is measured by a receiver coil. 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.

Moisture and density measurements

Discrete samples from the working-half cores were used for determination of index properties (e.g., bulk density, grain density, dry density, water content, porosity, and void ratio). Index properties are determined from mass measurements on wet and dry specimens and volume measurements on dry specimens. Standard ODP/IODP practices, which include a salt correction, were used to determine interstitial water mass and volume, salt mass and volume, and solid grain mass and volume (Blum, 1997). Standard seawater density (1.024 g/cm³) and salinity (35‰) and a constant salt density (2.22 g/cm³) were assumed for all calculations.

In general, two discrete samples of homogeneous lithology for index properties were collected from each core section. Where whole-round samples were taken (e.g., interstitial water sample, community whole round, or individual sample requests), a MAD sample was taken adjacent to the whole-round sample as part of a cluster sample. Sample intervals and frequency were occasionally shifted to select minimally disturbed, homogeneous samples. Lithology was noted where it was visibly distinct from clay or silt, such as samples dominated by sand or ash.

Wet and dry masses were measured using a paired electronic balance system, which is designed to compensate for the ship’s heave. The sample mass was counterbalanced with a precisely known mass (10 g for sediment, 40 g for basalt). The sample mass was determined to a precision of ±0.01 g. The balance system was calibrated twice a day or more frequently during poor weather conditions. To minimize desiccation, MAD sample collection was followed immediately by measurement of wet sediment mass (M_wet). After M_wet measurements, samples were dried in a convection oven at 105° ± 5°C for 24 h. Dry samples were placed in a desiccator for at least 1 h to equilibrate to room temperature (~20°C), and then dry sediment mass (M_dry) and volume (V_dry) were measured. Two five-chamber Quantachrome penta-pycnometers were used to measure V_dry with a helium-displacement technique providing a precision of ±0.04 cm³. The five-chamber system allowed the measurement of four sample volumes and one calibration sphere. Each measured volume (V_dry) is the average of five volume measurements. The calibration sphere was rotated between all measurement chambers to monitor for errors in each chamber. The pycnometer was calibrated at least once per 24 h.

Water content

Water content (W_c) was determined following the American Society for Testing and Materials (ASTM) standard D2216 (ASTM International, 1990). Corrections are required for salt when measuring the water content of marine samples. In addition to the water content calculation in ASTM D2216 (i.e., the ratio of interstitial water mass to dry sediment mass; W_c[dry]), we also calculated the ratio of interstitial water mass to total sample mass (W_c[wet]). The equations for water content are

and

where

• M_wet = total mass of the discrete sample,

• M_dry = mass of the dry sample, and

• s = salinity (assumed constant at 0.035).

Bulk density

Bulk density is the density of the discrete core sample (ρ_b = M_wet/V_t). Total sample volume assuming 100% saturation (V_t = V_g + V_IW) was determined from the pycnometer measurement of grain volume (V_g) and calculated volume of interstitial water (V_IW). Solid grain and interstitial water volume are determined as

and

where

• V_dry = dry volume,

• ρ_salt = salt density, and

• ρ_IW = standard seawater density.

Porosity and void ratio

Porosity (ϕ) relates the volume of the pores to the total sample volume; void ratio (e) relates the pore volume to the volume of the solid grains. They are calculated as

and

Grain density

Grain density (ρ_g) was determined from measurements of dry mass and dry volume made with the dual-balance system and the pycnometer, respectively. Mass and volume were corrected for salt, yielding

where

• M_salt = mass of salt.

Shear strength measurements

The undrained shear strength of soft sediments in the working half of the core was measured using an analog vane shear device (Wykeham Farrance, model WF23544) and a pocket penetrometer (Geotest E-284B). Measurements were made at discrete locations on the working halves above the depth where sediments behaved in a brittle manner, which corresponded to measurable shear strength <180 kPa. Where possible, the measurements were made near MAD samples. Care was taken to conduct tests within minimally disturbed, homogeneous sediments. Measurements were made on the working half of split cores with vane rotation axis and penetrometer penetration direction perpendicular to the y-z plane of the core.

Vane shear strength (S_u[v]) can be determined by the torque required to cause failure (T) and a vane constant (K_v):

All vane shear strength measurements were obtained using a vane with a height of 12.7 mm and a blade length of 6.35 mm. Failure torque was determined by measuring the rotation of a torsional spring using a spring-specific relation between rotation angle and torque. Vane shear strength results were only reliable for samples with vane shear strength <180 kPa. When cracking or core separation occurred, measurements were discarded. Below the depth where strength exceeded 180 kPa, no measurements were made.

The pocket penetrometer provides a measure of unconfined compressive strength (q_u), with units of mass per area), which can be related to undrained shear strength (S_u[penet]):

where

• g = acceleration due to gravity.

Unconfined compressive strength is calculated from the penetration resistance generated by pushing a cylindrical probe with a diameter of 6.4 mm into the y-z plane of the core. The average of three adjacent penetrometer tests in intact, homogeneous sediment is the recorded unconfined compressive strength. After shear strength exceeded ~180 kPa, no penetrometer measurements were made.

P-wave velocity

P-wave velocity and electrical resistivity measurements were performed on cubic samples cut from rock cores with a diamond blade saw. Samples for P-wave velocity and electrical resistivity measurements were ~20 mm × 20 mm × 20 mm. All cubes were cut with faces orthogonal to the x-, y-, and z-axes of the core reference. This three-component measurement plan enables first-order measurement of both P-wave velocity and electrical resistivity anisotropies.

To measure P-wave velocity in a given direction, a P-wave logger for discrete samples (PWL-D) was used. The PWL-D has two 230 kHz transducers, one used as a transmitter and one as a receiver, and a laser distance sensor. A sample was placed between the transducers. The transmitter was connected to a pulse generator, and the receiver was connected to an oscilloscope synchronized with the pulse generator. The oscilloscope signal was displayed digitally and the P-wave total traveltime (t) for the first arrival was picked and recorded. The laser distance sensor provided the sample length (L). The velocity in any direction (e.g., V_Px) was defined by the sample length (e.g., L_x), total traveltime (t_x), and system-calibrated delay time (t_delay):

Calibration of the traveltime delay and laser distance sensor was conducted daily. Traveltime delay was determined by placing the transmitter and receiver in direct contact and measuring traveltime. The laser distance sensor was calibrated by placing the transmitter and the receiver in direct contact and then spacing them using a 2.5 cm long reference specimen. Routine quality control measurements were made by measuring velocity on glass and acrylic standards with known lengths and acoustic velocities.

Electrical resistivity

On cores of soft sediment, an Agilent 4294A precision impedance analyzer was used to measure electrical resistivity using the four-electrode method. The measurement array consisted of four 1 cm long gold-plated electrodes spaced 7.5 mm apart. The array was inserted into the formation for the measurement. During measurement, the two outer electrodes inject an alternating current into the sample, and the two inner electrodes measure the resulting potential difference. The magnitude (|Z|) and phase (θ) of the complex impedance were measured at 2 kHz across the array. Electrical conductivity of the formation (σ_f) is computed from the complex impedance:

where

• d = geometric factor specific to the electrode array.

d was determined by comparing the measured |Z| and θ of a 35 g/L NaCl solution with the known resistivity of the solution measured with a resistivity meter. A new value of d was computed every 24 h, and the temperature was recorded. This accounted for changes in electrode array geometry because of slight bending of the electrodes and corrosion. Resistivity of the formation was then computed as 1/σ_f calculated from Equation 37. The ambient air temperature was recorded at each measurement; the formation temperature was assumed to be the same as the ambient air temperature because the cores were thermally equilibrated prior to measurement.

On rock cores, resistivity was measured on discrete samples. A cube of known dimensions is held between two electrodes and impedance is measured at 2 kHz with the Agilent 4294A impedance analyzer. Sample dimensions are obtained during P-wave velocity measurement. The dimensions of the samples being larger than the maximum spacing between the electrodes allowed by the built-in stand, it appeared necessary to remove the upper electrode holder from the stand. It is unknown if the procedure was used during Expedition 322 to circumvent this problem. Contact between the sample and each stainless steel electrode is obtained through a filter soaked in 35 g/L NaCl solution. The impedance of the sample is obtained by subtracting the impedance of the filters from the measured impedance. The impedance of the filters is measured by stacking the two filters between the electrodes immediately after measuring the sample impedance. Sample complex conductivity for example in the x-direction (σ_x) is computed from measured impedance R_x and X_x by

where L is the length and R₀ and X₀ refer to the measured impedance of the filter. Conductivity in the y- and z-directions is obtained by substitution in Equation 38.

With P-wave velocity and electrical conductivity measured in the x-, y-, and z-directions, the anisotropy is calculated following the approach of Carlson and Christensen (1977). Some sources for anisotropy include (1) alignment of pores during consolidation, (2) fabric development due to alignment of mineral grains, and (3) microstructures such as microfractures and microcracks. The horizontal-plane anisotropy (α_VPhor, α_ρhor) and vertical-plane anisotropy (α_VPvert, α_ρvert) calculation compares the horizontal (x and y) and vertical (z) components of P-wave velocity and conductivity expressed as a percentage of the mean:

and

In a truly transversely isotropic medium, the ratio of α_hor and α_vert for either P-wave velocity or resistivity is a function of the dip of foliation in the sample.

Thermal conductivity

Thermal conductivity was measured on sediment and rock samples using either the full-space needle probe (Von Herzen and Maxwell, 1959) or the half-space line source (Vacquier, 1985), which approximates an infinite line source. In soft sediments where a probe could be inserted into the core without fracturing the sediment, the full-space probe was inserted into whole-core sections through a hole drilled in the working-half side of the core liner. When sediment strength precluded use of the full-space probe or good contact between the probe and sediment was not possible, the half-space probe was used on the working half of the split core. For uncemented samples, the half-space probe was placed directly on the split core with seawater used to provide good contact. Lithified samples were placed in a seawater bath for at least 15 min before measurement with the half-space probe. Both full- and half-space measurements produce a scalar thermal conductivity value in the plane perpendicular to the orientation of the probe.

All measurements of thermal conductivity were made after the cores had equilibrated to room temperature. At the beginning of each measurement, temperature in the sediment was monitored to ensure that thermal drift was <0.4 mK/min (typically within 1–2 min). After it was established that the temperature was near equilibrium, a calibrated heat source was applied and the rise in temperature was recorded for ~80 s. Values of thermal conductivity were based on the observed rise in temperature for a given quantity of heat.

The full-space needle probe was calibrated at least once every 24 h. The calibration was performed on one of two Macor samples of known thermal conductivity (1.611 ± 2% W/[m·K], 1.623 ± 2% W/[m·K]). Calibration procedures for the half-space system are similar to those for the full-space needle probe. Half-space calibrations were made on a Macor sample of known thermal conductivity (1.652 ± 2% W/[m·K]). In addition, thermal conductivity of a water sample (0.59 W/[m·K]) was measured using a small content of gelatin mixed with water to prevent convective heat transfer, providing another calibration point for thermal conductivity values obtained by the TK04 instrument.

In situ temperature measurements

In situ temperature measurements were carried out using the advanced piston corer temperature tool (APCT-3). The APCT-3 is the third-generation tool of its kind and is used with the HPCS. The APCT-3 consists of three components: sensor logger, APCT-3 HPCS shoe, and computer software. During Expedition 333, in situ temperature measurements were taken for approximately every fourth core at Site C0018 and every second to fourth core at Sites C0011 and C0012 during HPCS coring. The temperature sensors were calibrated for a working range of 0°–55°C.

Prior to entering the hole, each instrument was held at the approximate mudline for ~10 min to equilibrate with bottom water temperature. However, the core winch depth meter was not calibrated, and site variability in measured bottom water temperature is likely due to uncertainties in depth. After bottom water temperature equilibration, the tools were lowered down the hole and penetrated the formation. The penetration of each tool into the formation causes a rise in temperature because of frictional heating. Following the initial rise in temperature, temperature decreases along a decay curve to near equilibrium. During this decay phase, it is important that the temperature tool is not disturbed. A second rise in temperature is due to frictional heating as the tool is pulled out of the formation. Temperature was measured as a time series sampled every 1 s and logged onto a microprocessor within the downhole tool; when the tool was retrieved, data were downloaded to the computer. The formation equilibrium temperature is determined based on fitting the temperature decay curve using the program TP-Fit, which runs on Matlab (M. Heeseman, pers. comm., 2007). In cases where the duration of temperature recording is short, the extrapolated formation temperature may have a large error. Therefore, a careful examination of recorded temperature time series is very important to obtain reliable data of heat flow by the method employed during this expedition.

Determination of heat flow

If heat transfer is by conduction and heat flow is constant, the thermal gradient will be inversely proportional to thermal conductivity, according to Fourier’s law. This relationship can be linearized by plotting temperature as a function of cumulative thermal resistance (Bullard, 1939):

(43)

where

• T = temperature,

• z = depth,

• T_o = bottom water temperature,

• q = heat flow,

• = thermal resistance, and

• N = number of thermal conductivity measurements.

In practice, q and T_o are estimated by plotting T(z) against cumulative thermal resistance. By using the plot of temperature versus cumulative thermal resistance, we can make an assessment of the consistency of heat flow with depth.

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