PHYSICAL PROPERTIES

Shipboard measurements of physical properties provide quantitative information about the composition and lithology of core material and are used to characterize lithologic units and to correlate core data with downhole logging and seismic reflection data. All physical properties measurements were taken on cores after they equilibrated to room temperature (~25°C). Equilibration to room temperature takes 2-4 hr. Magnetic susceptibility, gamma ray attenuation bulk density, compressional wave (P-wave) velocity, and natural gamma radiation were measured on whole cores using the MST. Thermal conductivity was measured on each core, using the whole core where possible. After core splitting, undrained shear strength, index properties, and additional measurements of P-wave velocity were conducted on the working half.

The MST, which is described in detail by Blum (1997), consists of four sensors: the magnetic susceptibility logger, gamma ray attenuation densiometer (GRA), P-wave logger (PWL), and natural gamma ray detector (NGR). MST data were sampled at discrete intervals along the core. The sample interval and the data acquisition period for each sensor were set to optimize the resolution of data acquired within the sampling time available for each core. MST data are significantly degraded if the core liner is only partially filled or if the core is disturbed. When RCB or XCB drilling was used, the core diameter was less than the nominal 6.6-cm core diameter. The reduced core diameter required corrections of the values measured by the MST. The values in the database do not reflect these corrections, but the figures presented in the following chapters show corrected data.

Magnetic Susceptibility Logger

Magnetic susceptibility is the degree to which a material can be magnetized by an external magnetic field. If the ratio of magnetic susceptibility is expressed per unit of volume, volume susceptibility is defined as

= M/H,

where M = the volume magnetization induced in a material of susceptibility () by the applied external field (H). Volume susceptibility is a dimensionless quantity. It can be used to help detect changes in magnetic properties caused by variations in lithology or by alteration. Magnetic susceptibility was measured at 5-cm intervals along the core using a Bartington meter (model MS2C) with an 88-mm coil diameter and a 2-s integration period. The Bartington meter operates at a frequency of 0.565 kHz and creates a field intensity of 80 A/m (= 0.1 mT), significantly lower than the field intensity needed to change the field orientation of magnetite grains (~50 mT). The width of the instrument response to a thin layer of material with a high magnetic susceptibility is ~10 cm. For this reason, the first and last measurement of each core section was taken 4 cm from the core section ends.

Gamma Ray Attenuation Densiometer

The GRA densiometer estimates bulk density by measuring the attenuation of gamma rays traveling through the core from a ¹³⁷Cs source. The gamma rays are attenuated by Compton scattering as they pass through the sample. The transmission of gamma rays through the sample is related to the electron density of the sample by

where

Y_t = the transmitted flux,

Y_i = the incident flux on a scatterer of thickness d,

n = the number of scatterers per unit volume or the electron density, and

s = the cross-sectional area per electron.

The bulk density () of the material is related to the electron density (n) by

n = x N_AV x (Z/A),

where

Z = the atomic number or number of electrons,

A = the atomic mass of the material, and

N_AV = Avogadro's number.

Bulk density estimates are therefore accurate as long as the ratio Z/A of the constituent elements is approximately constant and corresponds to the ratio Z/A of the calibration standard. The GRA densiometer was calibrated to a standard consisting of varying amounts of water and aluminum so that the densities of sediments can be accurately determined. GRA density was measured using a 2-s integration period at 5-cm intervals along the core.

Compressional Wave (P-Wave) Logger

The compressional wave (P-wave) logger (PWL) measures the ultrasonic traveltime of a 500-kHz compressional wave pulse through the core and the core liner. A pair of displacement transducers monitors the separation between the P-wave transducers, and the distance is used to convert ultrasonic traveltime into velocity after correcting for the liner. Good coupling between the liner and the core is crucial to obtaining reliable measurements. The PWL is calibrated by placing a water core between the transducers. The PWL was set to take the mean of 1000 velocity measurements over a 2-s period at 5-cm intervals along the core.

Natural Gamma Ray Detector

The NGR measures the discrete decay of ⁴⁰K, ²³²Th, and ²³⁸U, three long-period isotopes that decay at essentially constant rates within measurable timescales. Minerals that include K, Th, and U are the primary source of natural gamma rays. These minerals are found in clays, arkosic silts and sandstones, potassium salts, bituminous and alunitic schists, phosphates, certain carbonates, some coals, and acid or intermediate igneous rocks (Serra, 1984). The operation of the NGR is outlined by Hoppie et al. (1994). The NGR system contains four scintillation counters arranged at 90º angles from each other in a plane orthogonal to the core track. The counters contain doped sodium iodide crystals and photomultipliers to produce countable pulses. The total response curve of the instrument is estimated to be ~40 cm and so integrates a relatively long length of core in comparison to the other instruments of the MST. Natural gamma ray emissions were measured over a 20-s period at 10-cm intervals. The NGR was calibrated in port against a thorium source and during Leg 195 by measuring sample standards at the end of operations at every site.

Thermal conductivity is the measure of the rate at which heat flows through a material. It is dependent on the composition, porosity, density, and structure of the material. Thermal conductivity profiles of sediments and rock sections are used, along with temperature measurements, to estimate heat flow. Thermal conductivity is measured through the transient heating of a core sample with a known geometry using a known heat source and recording the change in temperature with time, using the TK04 system described by Blum (1997). For soft sediment, thermal conductivity measurements are made using a needle probe (Von Herzen and Maxwell, 1959) on whole-core sections; the reported value is the mean of three repeated measurements. For materials too hard for the needle probe to penetrate, thermal conductivity measurements are made after core splitting, using the needle probe in a half-space configuration (Vacquier, 1985); the reported value is the mean of four repeated measurements. Thermal conductivity measurements were made at an interval of at least one per core unless variations in lithology required more frequent sampling.

The undrained and residual shear strength of sediments and serpentinite mud was measured using a Wykeham-Farrance motorized vane shear apparatus following procedures described by Boyce (1977). In making vane shear measurements, it is assumed that a cylinder of sediment is uniformly sheared around the axis of the vane in an undrained condition. The vane used for all measurements has a 1:1 length to diameter blade ratio with a dimension of 1.28 cm. A high vane rotation rate of 90°/min was used to minimize pore fluid expulsion while measurements take place. Torque and strain measurements at the vane shaft were made using a torque transducer and potentiometer. Undrained shear strength measurements were made at least once per core section unless variations in lithology required more frequent sampling.

Discrete P-wave velocity measurements were made in three directions in the sediments using two pairs of insertion transducers (PWS1 and PWS2) with fixed separations of 7 and 3.5 cm, respectively, and a pair of contact transducers (PWS3) in a modified Hamilton Frame. PWS1, PWS2, and PWS3 use a 500-kHz compressional wave pulse to measure ultrasonic traveltimes, which, when combined with transducer separation data, can be used to determine velocity. PWS1 and PWS2 were only used to measure velocity in soft sediments, where they were inserted into the face of the split core. PWS1 is aligned with the core axis (the z-direction), and PWS2 is aligned perpendicular to the core axis (the y-direction). PWS3 is mounted vertically with one transducer fixed and the other mounted onto a screw, allowing the transducer separation to be altered. PWS3 measures velocity in the x-direction in split cores but is also used to measure velocity in discrete samples of hard sediments or crystalline rock. Distilled water is applied to PWS3 to improve the acoustic coupling between the transducers and the sample. P-wave velocity measurements were made at least once per core section.

Minicore samples of ~10 cm³ were collected using a piston sampler in soft sediment or an electric drill in rocks. Samples were taken at least once per section. Sediment samples were placed in a 20-mL beaker and sealed to prevent moisture loss. Rock samples were soaked in seawater for 24 hr before determining the wet mass. Samples were then dried in an oven at 105° ± 5°C for 24 hr and allowed to cool in a desiccator before measuring dry weights and volumes (method C in Blum, 1997). Wet and dry sample masses and dry volumes were measured and used to calculate wet bulk density, dry density, grain density, water content, and porosity. Sample mass was determined using two Scientech electronic balances. The balances are equipped with a computerized averaging system that corrects for ship accelerations. The sample mass is counterbalanced by a known mass such that the mass differentials are generally <1 g. Sample volumes were measured at least three times, or until a consistent reading was obtained, using a helium-displacement Quantachrome penta-pycnometer. A standard reference volume was included with each group of samples during the measurements and rotated among the cells to check for instrument drift and systematic error; each time an error was detected in the measurement of the reference volume, the offending cell was calibrated. The following relationships can be computed from the two mass measurements and dry volume measurements (taken from Blum, 1997, pp. 2-2 to 2-3). When a beaker is used, its mass and volume are subtracted from the measured total mass and volume. This results in the following directly measured values:

M_b (bulk mass),

M_d (dry mass) = mass of solids (M_s) + mass of residual salt, and

V_d (dry volume) = volume of solids (V_s) + volume of evaporated salt (V_salt).

Variations in pore water salinity (s) and density (_pw) that typically occur in marine sediments do not affect the calculations significantly, and standard seawater values under laboratory conditions are used:

s = 0.035 wt% and

_pw = 1.024 g/cm³.

Pore water mass (M_pw), mass of solids (M_s), and pore water volume (V_pw) can then be calculated:

V_pw = M_pw /_pw = (M_b - M_d)/[(1 - s) x _pw].

Additional parameters required are the mass and volume of salt (M_salt and V_salt, respectively) to account for the phase change of pore water salt during drying. It should be kept in mind that for practical purposes, the mass of salt is the same in solution and as a precipitate, whereas the volume of salt in solution is negligible. Thus,

V_salt = M_salt /_salt = {[(M_b - M_d) x s]/(1 - s)}/_salt,

where the salt density (_salt = 2.20 g/cm³) is a calculated value for average seawater salt.

Moisture content is the pore water mass expressed either as percentage of wet bulk mass or as a percentage of the mass of salt-corrected solids:

Calculations of the volume of solids and bulk volume are as follows:

Bulk density (_b), density of solids or grain density (_s), dry density (_d), porosity (P), and void ratio (e) are then calculated according to the following equations:

_b = M_b/V_b,

_s = M_s/V_s,

_d = M_s/V_b,

The electrical resistivity of the sediment was measured using a four-electrode configuration. The instrument used was modified at the University of California, Santa Cruz, from the design of Andrews and Bennett (1981) and was built at the University of Hawaii. The electrodes consisted of four stainless steel pins that are 2 mm in diameter, 15 mm in length, and spaced 13 mm apart. A 20-kHz square-wave current was applied on the outer electrodes, and the difference in potential between the two inner electrodes was measured. The size of the current (typically 50 mA) was measured over a resistor in the outer circuit.

The main purpose of measuring sediment resistivity was to determine the formation factor, defined as the ratio of the resistivity of sediment with included pore water divided by the resistivity of the pore water alone. In practice, the formation factor is approximated by measuring the apparent resistivity of the sediment in the split core liner and dividing that value by the apparent resistivity of seawater of similar salinity and the same temperature in a 30-cm length of split core liner. Using the same configuration for the measurement of the apparent resistivities removes the effects of geometry from the determination of the formation factor.

The hydraulic conductivity and specific storage of the serpentinite mud was measured during a consolidation test. In this test, an axial surface load is applied to a laterally constrained sample. The axial load produces an excess pore fluid pressure along the length of the core. The bottom of the sample is drained so that the excess pore fluid pressure at that point is zero. The loads and boundary conditions are applied by a Manheim squeezer, and the amount of fluid displaced is measured as a function of time. Figure F10 is a cartoon of the apparatus and the boundary conditions. Also shown is the pressure profile along the length of the sample at various times. The assumption of incompressible mineral grains and water, common to soil mechanics (Wang, 2000), allows the volume of water discharged from the sample to be converted to axial displacement using the cross-sectional area of the sample. Because the frame, not the mineral grains or the water, is compressed, we can calculate the axial displacement using the following equation:

w = volume of water discharged/cross-sectional area of sample,

where w = the axial displacement. We then use the relationship for displacement in an infinite length cylinder as a function of time (Wang, 2000):

,

to determine the lumped product of constants (on the right hand side of the following equation) by plotting the slope of the displacement over the square root of time

,

where

c_m = the vertical compressibility,

= the loading efficiency,

_z = the axial load, and

D = the hydraulic diffusivity.

Figure F11 compares experimentally determined displacements with calculated displacements as a function of time. Only the early time portion of the plot is used to determine the lumped product of constants. Early in the experiment, the decrease in pore pressure has not yet diffused to the end of the sample and so the approximation of an infinite cylinder is still valid. To determine the hydraulic diffusivity (D) from the lumped product, we need to determine the other unknown factors, c_m and .

The vertical compressibility is defined by

.

Because all the components of the equation above, axial displacement (w), axial stress (_z), and sample length (w_o) are measured, it is possible to calculate the vertical compressibility. Also, because the pore pressure throughout the entire length of the sample returns to zero at very long times, the boundary condition of no change in pore pressure (P_pore = 0) is met.

The loading efficiency is defined as

,

where P_pore = the pore fluid pressure. The assumption of incompressible grains and pore fluid leads to a value = 1 (Wang, 2000).

The specific storage (S_s) is related to the vertical compressibility under the assumptions of incompressible grains and pore fluid by

S_s = c_m x _f x g,

where

_f = the fluid density, and

g = the acceleration of gravity.

Finally, we can determine the hydraulic conductivity from the hydraulic diffusivity and specific storage using