PHYSICAL PROPERTIES

Shipboard measurements of physical properties provide information that assists in characterizing lithologic units, correlating cored material with downhole logging data, understanding the nature of consolidation, and interpreting seismic reflection profiles.

First, nondestructive measurements of bulk density, magnetic susceptibility, transverse compressional wave velocity, and NGR were made using the MST on whole-round sections after the core equilibrated to room temperature (~25°C). Next, thermal conductivity in soft sediment was measured on whole-core sections or on split-core pieces (working half) if the core material was too hard to be penetrated by the needle without excessive force. After the sections were split, compressional wave velocity was measured on all lithologies, whereas undrained shear strength was measured only on soft-sediment cores. Finally, discrete samples were selected for the measurement of index properties including wet bulk density, dry bulk density, grain density, water content, porosity, and void ratio as well as compressional wave velocity.

The MST included four physical properties sensors: a magnetic susceptibility meter (MSM), gamma-ray attenuation porosity evaluator (GRAPE), P-wave logger (PWL), and NGR detector. MST data were sampled at discrete intervals, with the sampling interval and count time chosen to optimize the resolution of the data and the time necessary to run each core section through the device. The MSM, GRAPE, and NGR were measured on all cores regardless of collection method, (i.e., APC, XCB, or RCB). The PWL was used only on APC-cored intervals because of the likelihood of discontinuous core, core disturbance, and/or a loss of coupling between the liner and the core with XCB and RCB drilling. MST measurements were also conducted on previously sampled half cores of igneous basement drilled during Leg 129 during the transit from Hong Kong to Site 801 (see "Background and Objectives" in the "Site 801" chapter).

Magnetic susceptibility is the degree to which material can be magnetized in an external magnetic field. The MSM aids in the detection of fine variations in magnetic intensity associated with magnetic reversals or lithologic changes (alteration or grain size). The quality of the results is degraded in RCB sections if the core liner is not completely filled and/or the core is disturbed. However, general downhole trends may still be used for laboratory to well log correlation. Magnetic susceptibility was determined on all sections at an interval of 4 cm with 4-s acquisition times using the 1.0-s integration time range on the Bartington meter (model MS2C), which has an 8-cm diameter loop.

GRAPE was used for the estimation of sediment bulk density. This measurement is based on the principle that the density of material is related to attenuation, mainly by Compton scattering, of a collimated beam of gamma rays (produced by a ¹³⁷Ce source) passing through a known volume of sediment (Boyce, 1976). GRAPE data were acquired every 4 cm for a 4-s period. GRAPE data are most reliable in undisturbed cores (i.e., APC) and offer the potential of direct correlation with downhole bulk density logs. For discontinuous and/or fragmented core (i.e., RCB) for which the core liner was not completely filled, GRAPE data are unreliable.

The measurement of P-wave velocity provides information that assists in the interpretation of seismic reflection profiles and the correlation of lithology with downhole logging data. The PWL on the MST transmits a 500-kHz compressional wave pulse through the core, where the transmitting and receiving transducers are aligned perpendicular to the long axis of the core. A pair of displacement transducers monitors the separation between the compressional wave transducers. P-wave velocity measurements were taken on APC cores every 4 cm for a 4-s period.

NGR emission was recorded for all core sections to measure variations in radioactive counts of sample rocks (alteration and pelagic clay) and to provide a correlation with the downhole measurements of NGR emissions. The NGR system records radioactive decay of ⁴⁰K, ²³²Th, and ²³⁸U, three long-period isotopes that decay at an essentially constant rate within measurable time scales. The installation and operating principles of the NGR system used during Leg 185 are discussed by Hoppie et al. (1994).

NGR emission was measured every 10 cm for a 20-s period. The area of influence for the four NGR sensors was about ±10 cm from the points of measurements along the core axis. As gamma-ray emission is a random event, count times have to be sufficiently long to average short-period variation. This was achieved on the MST system by utilizing the long area of influence on the sensors and using a moving average window to smooth count rate variations and to achieve a statistically valid sample.

The NGR instrument was calibrated with a thorium source during transit to the first drill site. Background counts measured using a core liner filled with distilled water averaged 11.75 cps over a 30-min measurement period. No corrections were made to account for sediment incompletely filling the core liner in XCB cores. Results were output in counts per second, which can be qualitatively compared to the API units obtained from borehole logging.

P-wave velocity is measured using the contact probe systems (PWS1, PWS2, and PWS3) (Hamilton Frame). The choice of method and sampling frequency of discrete compressional wave velocity (V_p) measurements depends on the hardness of the sediment and rock.

PWS1 and PWS2 Insertion Probe Systems for Soft Sediment

Compressional wave velocity was measured using two pairs of the digital sound velocimeters for soft sediments. One pair was aligned along the core axis (PWS1; z direction), and the other one across the core (PWS2; y direction). The transducer pairs had a fixed spacing of 7 cm (z direction) and 3.5 cm (y direction) and were inserted into the split cores of soft sediment. An acoustic signal of 500 kHz was emitted and received by the two transducers. This signal was then digitized by an oscilloscope so that the first arrival waveform could be picked manually and velocity calculated.

Additionally, the PWS3 contact probe system (Hamilton Frame velocimeter) was conducted through the split core (x direction) using vertically oriented transducer pairs (500 kHz), with the upper transducer pressed against the split surface and the lower transducer pressed against the core liner. These velocities were measured at an interval of one or two per split-core section (working half).

PWS3 Contact Probe System for Lithified Sediment and Hard Rock

Compressional velocity on sedimentary and igneous rocks was measured using the PWS3 contact probe system. The PWS3 data were measured through the split core (x direction) as described in the previous paragraph. Selected igneous rocks and sedimentary rocks were sawed into oriented cubes (minicores) and velocities were measured in three mutually perpendicular directions (x, y, and z). The magnitude of velocity anisotropy was estimated according to the relationship

where, V_z (direction along the core), V_x (direction into the split core, perpendicular to core axis), V_y (direction across the split core), and V_max and V_min are the maximum and minimum velocities (among V_x, V_y, and V_z).

Both split-core and discrete-sample thickness were measured by a digital caliper that was directly mounted on the transducer pair. Zero traveltimes for the velocity transducers were measured with a series of polycarbonate standards of known length. The pressure cell that monitors the axial pressure applied between sample and transducer was not operational. Pressure was applied until a measurable waveform appeared on the oscilloscope. To improve the coupling between the transducer and sample, distilled water was applied to the top and bottom of the sample and transducer heads. Measurements were corrected for the additional traveltime passing through the core liner.

The peak undrained and residual shear strength of soft sediment was measured at an interval of one per split-core section (working half), using a Wykeham-Farrance motorized vane shear apparatus following the procedures of Boyce (1977). The vane rotation rate was set to 90°/min, and the vane used for all measurements had a 1:1 blade ratio with a dimension of 1.28 cm. This instrument measures the torque and strain at the vane shaft using a torque transducer and potentiometer, respectively. Output for torque and strain were recorded in volts on a Hewlett-Packard X-Y recorder. The shear strength reported was the peak strength determined from the torque vs. strain plot.

In the interpretation of shear vane measurements, it is assumed that a cylinder of sediment is uniformly sheared around the axis of the vane in an undrained condition, with cohesion as the principal contributor to shear strength. Departures from this assumption include progressive cracking within and outside of the failing specimen, uplift of the failing core cylinder, drainage of local pore pressures, and stick-slip behavior.

Samples of ~2-4 cm³ (5-15 g) were taken from each section adjacent to cut cubes and minicores and were used for velocity determination. Additional samples were taken to characterize the core where sawing cubes was not possible. All basalt and interpillow material samples were soaked in seawater for 24 hr before determination of index properties, whereas index properties of chert, chalk, marl, and pelagic clay were made directly after sampling. Bulk density, grain density, water content, porosity, and dry density were calculated from wet and dry sample weights and dry volumes. Sample mass was determined to a precision of ±0.001 g using two Scitech electronic balances. The balances were equipped with a computer averaging system that corrected for ship accelerations. The sample mass on one balance was counterbalanced by a known mass on an adjacent balance such that the mass differentials generally were <2 g. Sample volumes were determined using a Quantachrome Penta-Pycnometer, a helium-displacement pycnometer with a nominal precision of ±0.01 cm³. Sample volumes were determined at least three times and then averaged. Three of the five measurement cells were operational during Leg 185 (cells 1, 2, and 4). A standard reference volume sphere was run in each of three operating cells after nine consecutive measurements. The cell volume was recalibrated if the measured volume was not within 0.02 cm³ of the calibration sphere volume. The sample beakers used for discrete determinations of index properties of sediment were calibrated carefully before the cruise. Dry weight and volume measurements were performed after the samples were oven dried at 105° ± 5°C for 24 hr and allowed to cool in a desiccator. The main problem with this drying temperature is that chemically bound water in some clay minerals can be lost in addition to interstitial water.

The determination of water content as a fraction of total mass or as a ratio of water mass to solid mass followed the methods of the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1989). Total mass (M_t) and dry mass (M_d) were measured using the electronic balance and the difference was taken as the uncorrected water mass. Measurements were corrected for salt assuming a pore-water salinity (r) of 0.35%, following the discussion by Boyce (1976). The equations for the two water content (W) calculations are

where M_t and M_d are measured in grams.

Bulk density (_bulk) is the density of the total sample including the pore fluid (i.e., _bulk = M_t / V_b, where V_b is the bulk volume (in cubic centimeters).

Grain density (_g) was determined from the dry mass and dry volume measurements. Both mass and volume must be corrected for salt, based on the following equation:

_g = (M_d - M_s) / [V_d - (M_s / _s )], (6)

where M_d is the dry mass (in grams) and _s is the density of salt (2.257 g/cm³), V_d is dry volume, M_s = r M_w (in grams) is the mass of salt in the pore fluid, M_w (in grams) is the salt-corrected mass of the seawater:

Porosity (), represents the ratio of pore-water volume to total volume. The following relationship using calculated grain density and bulk density was employed:

= [(_g - _bulk) / (_g - _w)] × 100, (8)

where _g represents the grain density, _bulk is the bulk density, and _w is the density of seawater.

The dry density (_d) is the ratio of the dry mass (M_d) to the total volume (V_t). The dry density was calculated using the corrected water content (W_d) and porosity () as follows:

_d = ( / W_d) × _w. (9)

A complete description of density and porosity calculations is given by Blum (1997).

Thermal conductivity is measured by transient heating of a material with a known heating power generated from a source of known geometry and then measuring the temperature change with time, using the TK04 system described by Blum (1997). Thermal conductivity profiles of sediments and rock sections are mainly used, along with temperature measurements, to determine heat flow. Heat flow is not only characteristic of the material, but an indicator of type and age of oceanic crust and fluid circulation processes at shallow and great depth (Blum, 1997). Whole-round core sections were allowed to adjust to room temperature for at least 2 hr in preparation for thermal conductivity measurements. In the case of soft sediment, thermal conductivity was measured on whole-core sections. However, if the core material was too hard to be penetrated by the needle without excessive force, thermal conductivity was measured on split-core pieces (working half). The needle-probe method was used in full-space configuration for soft sediments (von Herzen and Maxwell, 1959), and in half-space mode (Vacquier, 1985) for lithified sediment and hard-rock samples. Measurements were made at an interval of one per core (whole-round core or split core).

Full-Space Determinations for Soft Sediment

A needle probe, containing a heater wire and a calibrated thermistor, was inserted into the sediment through a small hole drilled in the core liner before the sections were split. At the beginning of each measurement, temperatures in the samples were monitored without applying current to the heating element to verify that temperature drift was <0.04°C/min. The heater was then turned on and the temperature rise in the probes recorded. After heating for ~60 s, the needle probe behaves nearly as a line source with constant heat generation per unit length. Temperatures recorded between 60 and 240 s were fitted to the following equation using a least-squares method (von Herzen and Maxwell, 1959):

T(t) = (q / 4k) × ln (t) + L(t), (10)

where k is the apparent thermal conductivity (W/[m·K]), T is temperature (°C), t is time (s), and q is the heat input per unit length of wire (W/m). The term L(t) corrects for temperature drift, described by the following equation:

where A represents the rate of temperature change, and T_e is the equilibrium temperature. L(t) therefore corrects for the background temperature drift, systematic instrumental errors, probe response, and sample geometry. The best fit to the data determines the unknown terms k and A.

Half-Space Determinations for Lithified Sediment and
Hard Rock

Half-space determinations were made on selected lithified sediments and basaltic rock samples after the cores were split and their faces polished. The needle probe encased in a grooved epoxy block with relatively low conductivity (Sass et al., 1984; Vacquier, 1985) was placed onto the polished surface. Half-space measurements were conducted in a water bath to keep the samples saturated, to improve the thermal contact between the needle and the sample, and to reduce thermal drift. Data collection and reduction procedures for half-space tests were similar to those for full-space tests except for a multiplicative constant in Equation 4, which accounted for the different experimental geometry.