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doi:10.2204/iodp.proc.301.105.2005

Physical properties

This section outlines the procedures used to measure physical properties during Expedition 301. A comprehensive discussion of all methodologies and calculations used in the JOIDES Resolution physical properties laboratory can be found in ODP Technical Note 26 (Blum, 1997).

Shipboard measurements of physical properties are used to characterize lithologic units, correlate cored material with downhole logging data, and interpret seismic reflection profiles. After the cores equilibrated to ambient room temperature, physical properties were measured on whole-round sections, undisturbed parts of split cores, and discrete samples.

The suite of whole-core analyses performed on recovered cores varied according to the type of coring method used. For sediment core recovered by APC, the core was cut into 1.5 m sections and whole-round samples were collected for geochemical and microbiological analyses. Remaining APC sections were set aside to equilibrate to laboratory temperature (>6 h) and passed through the MST. The MST has a GRA bulk densitometer, a P-wave logger (PWL), a magnetic susceptibility (MS) meter, and an NGR sensor. During Expedition 301, only the GRA, MS, and NGR were used for the entire interval of sediment coring. The PWL was used on the first four cores but then generated unreliable data. It was not repaired in time for use during the rest of the expedition. Thermal conductivity measurements were performed before the sediment sections were split. Moisture and density (MAD) properties (bulk density, grain density, water content, and porosity) and P-wave velocity were measured on split sections. Shear strength measurements were generally made once per section, depending on lithology and recovery.

Hard rock basement was cored using the RCB system. Due to poor core recovery, the MST was only used to measure magnetic susceptibility. Measurements of thermal conductivity, P-wave velocity, and MAD properties were made approximately once per section. Additional samples were collected when there was a visible change in lithology or texture.

Multisensor track measurements

Measurement of wet bulk density by the GRA system is based on the principle that the attenuation (mainly by Compton scattering) of a collimated beam of gamma rays produced by a ¹³⁷Ce source passing through a known volume of sediment is related to material density (Evans, 1965). Calibration of the GRA system was completed using seawater/aluminum density standards. The minimum integration time for a statistically significant GRA measurement is 1 s, and routine Expedition 301 GRA measurements used a 3 s integration time. The measurement width of the GRA sensor is ~5 mm, with sample spacing generally set at 2.5 cm. A freshwater control standard was run with each section to measure instrument drift. GRA bulk density data are of highest quality when determined on APC cores in which the liner is completely filled with sediment. Occasionally, we recovered sandy intervals in which the core liner was incompletely filled. These intervals yielded poor-quality data. No corrections were made to the archived data to correct for partially filled core liners. Basement cores were not subjected to GRA measurements.

Whole-core MS was measured with the MST using a Bartington MS2 meter coupled to a MS2C sensor coil with a diameter of 8.8 cm operating at 565 Hz. The measurement resolution of the MS2C sensor is 4 cm, with a minimum statistically significant count time of 1 s. On sediment cores, MST magnetic susceptibility was routinely measured every 2.5 cm with a 1 s sampling interval. On hard rock cores, MS was measured every centimeter with a 1 s measurement period. Magnetic susceptibility data were archived as raw instrument units (SI) and not corrected for changes in volume, although a correction was made for instrument drift.

Transverse P-wave velocity was measured on the MST with the PWL for sediment Cores 301-U1301C-1H to 4H, at which time the PWL stopped working. The PWL transmits a 500 kHz compressional wave pulse through the core every 1 ms. The transmitting and receiving transducers are aligned perpendicular to the core axis, and a pair of displacement transducers monitors the separation between the compressional wave transducers. Variations in the outer diameter of the liner do not degrade the accuracy of the velocities, but the unconsolidated sediment or rock core must completely fill the liner for the PWL to provide acoustic coupling. When it was operational, measurements were made every 2.5 cm with 50 data acquisitions per site (1 s). Calibration of the displacement transducer and measurement of electronic delay within the PWL circuitry were conducted using a series of acrylic blocks of known thickness and P-wave traveltime. Repeated measurements of P-wave velocity through a core liner filled with distilled water were used to check calibration validity. The use of the PWL on basement cores was prevented by poor acoustic coupling between the sample and the core liner.

NGR emissions of sediments are a function of the random and discrete decay of radioactive isotopes, predominantly those of uranium, thorium, and potassium, and are measured through scintillation detectors arranged at 90° to each other and perpendicular to the core. The installation and operating principles of the NGR system are discussed in Hoppie et al. (1994). Data from 256 energy channels were collected and archived. For presentation purposes, counts were summed over the range of 200–3000 keV, so as to be comparable with data collected during previous ODP legs. The measurement width of the NGR is ~15 cm, with a statistically significant count time of at least 5 s, depending on lithology. The sample spacing of the NGR measurements was set at 15 cm, and integration time was 20 s.

Thermal conductivity

Thermal conductivity was measured with the TK04 (Teka, Berlin, Germany) system, using the needle-probe method (Von Herzen and Maxwell, 1959). The needle probe contains a heater wire and calibrated thermistor. It is assumed to be a perfect conductor because it is much more conductive than unconsolidated sediments. With this assumption, the temperature of the probe has a linear relationship with the natural logarithm of the time after the initiation of heating:

where

T = temperature;

q = heat input per unit length per unit time;

k = thermal conductivity;

t = time after the initiation of the heat; and

C = a constant.

Thermal conductivity was measured on unconsolidated sediment and rock samples using the TK04 system as described by Blum (1997). These measurements are used, along with in situ temperature measurements, to estimate heat flow. The system uses a single-needle probe heated continuously in full-space mode for soft sediments (Von Herzen and Maxwell, 1959) and in half-space configuration for hard rock samples (Vacquier, 1985). A small amount of thermal joint compound was used to ensure good contact between the needle and hard rock half cores. A self-test, including a drift study, was conducted at the beginning of each cycle. Once the samples were thermally equilibrated, the heater circuit was closed and the temperature rise in the probes was recorded. Thermal conductivities were calculated from the rate of temperature rise while the heater current was flowing. In full-space mode (sediment cores), temperatures measured during the first 150 s of the heating cycle were fitted to the equation shown above. In half-space mode, the heating cycle was adjusted to 80 s. For full-core soft-sediment sections, a hole was drilled in the outer core liner and a 2 mm diameter temperature probe was inserted into the working half of the core section. For hard rock samples, a half-space needle probe was attached to ~6 cm long split-core pieces. The face of the split core was sanded and/or polished to ensure complete thermal coupling of the probe to the sample. The core pieces were resaturated in seawater under vacuum for a minimum of 2 h and allowed to equilibrate thermally in a water bath for at least 20 min. The thermal conductivity measurement for each sample was the average of three repeated measurements for the full-space method and three to six repeated measurements for the half-space method. Individual measurements were evaluated and deleted (not archived) if the reported value was based on poor statistics and/or if the value was highly inconsistent with other values from the same sample. Thermal conductivity measurements were made once per section for sediment and one or two per core for hard rock samples. In order to check instrumental drift, thermal conductivity of two standard materials (ceramic and a water/gelatin mixture) was measured during coring operations. For basalt samples (Hole U1301B), we based the drift calculations on the macor standard because its properties are closest to the basalt core samples. Measured values of the standard showed no significant trend but suggested a small bias tending to measure values slightly higher than the known macor conductivity (mean measured value = 1.67 ± 0.01 W/m·K) (Fig. F15A). Because the measured value of the standard was within the uncertainty of the published value (1.61 ± 0.08 W/m·K), we did not apply a correction to the sample data. The gelatin and water standard (0.62 ± 0.06 W/m·K, 24°C) was used for measuring thermal conductivities in sediments. To prevent convection, 18% gelatin was added to the water. Measured values of the water/gelatin mixture showed no significant trend but suggested a small bias tending to measure slightly low (mean value = 0.58 ± 0.00 W/m·K) (Fig. F15B). No correction was applied to the samples because the calibration standard was below the thermal conductivity of the samples and it was not possible to determine a correction.

Moisture and density properties

Samples of ~10 cm³ for sediments and hard rock were collected at a frequency of one per section (depending on core recovery and lithology) to determine MAD properties.

Samples were taken from undisturbed parts of the core where possible. Wet sediment mass was measured immediately after the samples were collected. 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. Hard rock samples were resaturated in seawater for 24 h, and then moisture content and density were measured using the same procedure as for the sediment sections.

Sample mass was determined to a precision of 0.01 g using two Scientech 202 electronic balances and a computer averaging system to compensate for the ship's motion. Sample volumes were determined using a Quantachrome helium-displacement pycnometer with a precision of 0.02 cm³. Volume measurements were repeated five times. All cells were calibrated after three sample runs to check for instrument drift and systematic error. A purge time of 3–5 min was used before each run. The procedures for the determination of these properties comply with the American Society for Testing and Materials designation (D) 2216 (ASTM, 1990).

Mass and volume calculation

Wet mass (M_wet), dry mass (M_dry), and dry volume (V_dry) are measured in the laboratory. Salt precipitated in sediment pores during the drying process is included in the dry mass and dry volume values. The mass of the evaporated water (M_water) and the salt (M_salt) in the sample are given by

and

where s = 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 2.2 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 the mass of pore water to the wet sediment (total) mass:

Wet bulk density (ρ_wet), dry bulk density (ρ_dry), sediment grain (solid) density (ρ_solid), and porosity (ϕ) are calculated from, respectively,

and

Velocity

For sediment sections, velocity determinations in the x-, y-, and z-directions were made using the Hamilton frame PWS3 contact probe system. Using this system, P-wave velocities were generally measured at a frequency of once per section on all cores except where changes in lithology required extra measurements. Hard rock samples were taken every section where there was sufficient oriented core. Sample preparation included cutting cubes with flat and parallel sides, followed by sanding and polishing the cubes to ensure good contact between sample and transducer, placing the samples in an ultrasonic bath to remove the polishing grit, and, finally, resaturating the samples under vacuum in a seawater bath for at least 2 h. The compressional wave velocity calculation is based on the accurate measurement of the delay time of a 500 kHz square wave signal traveling between a pair of piezoelectric transducers. The transducer pair for PWS3 is adjusted to the thickness of the half core or discrete sample. The separation between the fixed lower PWS3 transducer and the movable upper transducer is measured by a linear voltage displacement transducer. Deionized water was added to the contact between the transducers and sample to improve acoustic coupling. The core temperature was recorded at the time velocity was measured; however, the velocity data stored in the Janus database are uncorrected for in situ temperature and pressure. These corrections can be made using the relationships outlined in Wyllie et al. (1956), Wilson (1960), and Mackenzie (1981).

Shear strength

Sediment shear strength, or shear resistance, is an important aspect of slope stability. However, shear strength values measured at sea provide information only on the relative strength profile. For clay-rich marine sediments, the stress-strain behavior is greatly dependent upon the stress history of the sample. The stress history can be estimated in a semiquantitative way by the ratio of measured shear strength (s_u) to in situ overburden stress, σ_ov:

For normally consolidated, fine-grained, cohesive soils, h has a value of ~0.25. Larger values indicate overconsolidation; smaller values indicate underconsolidation. Marine sediments are typically overconsolidated in the uppermost few to several meters and lightly or strongly underconsolidated in the next 100–200 m and deeper.

Direct shear and triaxial tests are the most common laboratory shear strength tests. Additional special tests are for direct simple shear, ring shear, plain strain, and true triaxial test. These tests allow independent control and measurement of at least the principal stresses, σ₁ and σ₃, and changes in void ratio and pore pressure. The results can be analyzed by a σ-τ diagram (Mohr circle), a p-q diagram (stress path), or other methods (e.g., Lambe and Whitman, 1979; Holtz and Kovacs, 1981).

IODP provides two rapid and simple tests that can be applied at sea, the vane shear test and the penetrometer test. These tests should be used only as a guide because they provide rough estimates of sediment properties (e.g., Lambe and Whitman, 1979). Particularly, the influence of pore pressure changes during the undrained experiment cannot be estimated. More sophisticated tests must be performed in a shore-based laboratory.

Undrained shear strength is determined using a vane shear instrument that is inserted into soft sediment and rotated until the sediment fails. The torque, T, required to shear the sediment along the vertical and horizontal edges of the vane is a relatively direct measure of the shear strength. It must be normalized to the vane constant, K, which is a function of the vane size and geometry:

where τ_f is shear strength and s_u is a common notation for the vane shear strength (e.g., Lambe and Whitman, 1979). Shear strength has the units of pascals (= Newtons per square meter [N/m²]), torque has the units of Newton·meters (N·m), and K has the units of cubic meters (m³). The handheld Torvane instrument used during Expedition 301 returns a measure of shear strength from calibrated springs.

The penetrometer is a flat-footed, cylindrical probe that is pushed 6.4 mm deep below the split-core surface. The resulting resistance is the unconfined compressive strength of 2s_u. The mechanical scale is in units of kilogram per square centimeter (kg/cm²), which are converted to units of kilopascals by

The maximum τ_f that can be measured with the pocket penetrometer is 220 kPa.

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