PHYSICAL PROPERTIES

The standard procedures for shipboard measurements of physical properties provide insights to variations in the core material characteristics. The general objectives for the physical properties group were to

1. Collect comprehensive physical properties data sets for constructing a complete stratigraphic section and determining lithologic units;
2. Determine physical and deformational properties of lithologic units for defining erosional unconformities, delineating the consolidation degree of cored sections, and estimating mass accumulation rates;
3. Cross-correlate and calibrate shipboard analyses of physical properties with other shipboard analyses. Physical properties samples were commonly taken next to shipboard carbonate and XRD samples;
4. Integrate core, downhole logging, and seismic reflection data. Bulk density, porosity, compressional velocity and natural gamma radiation (NGR) data from core sampling are all valuable for core-log integration; and
5. Cross-correlate results from Sites 1150 and 1151 with the DSDP Sites 438, 439, and 584 that also are located on the continental slope of the Japan Trench.

In addition, physical properties serve to address basic questions related to the underlying themes of this cruise. For example, how do physical properties of sediment and sedimentary rock differ for seismic and aseismic segments of the continental slope?

Except for data from Holes 1151C and 1151D, physical properties data were acquired after the cores had equilibrated to ambient laboratory room temperature (18°-25°C, measured in the top of the section). Nondestructive measurements were made on whole-round core sections with the MST, which acquires magnetic susceptibility, bulk density, compressional P-wave velocity, and NGR data, and with a needle probe that measures thermal conductivity. Destructive measurements were conducted on split-core sections with relatively unlithified sediments (compressional P-wave velocity and shear strength) and on discrete samples (index properties and compressional P-wave velocities). A detailed description of the principles for these techniques is given in the ODP physical properties handbook (Blum, 1997).

Multisensor Track

The MST incorporates the magnetic susceptibility, gamma-ray attenuation (GRA), P-wave logger (PWL), and NGR devices. Individual whole-round core sections are scanned by the four sensors and sampled at constant intervals and periods, from the top to bottom depth of the section. Each section was oriented so that the working half was facing up. This provided internal consistency for individual cores, but not between different cores because their orientation to north was not known. Raw data from MST measurements are stored in the Janus database, whereas the additional corrections of the data (see below) are not.

Magnetic susceptibility was measured with a 2-cm sampling interval on all cores recovered in Site 1150 and Hole 1151A, and with a 5-cm sampling interval on cores from Holes 1151C and 1151D. The 1.0 (1-s integration time) range on the Bartington meter (Model MS2C), which has an 8.8-cm diameter loop, was used and the sampling period was 4 s. Magnetic susceptibility aids in the detection of variations in the concentrations of magnetic minerals associated with lithologic changes. The quality of the data 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 core-log correlation. The precision and accuracy of magnetic susceptibility measurements are 2 × 10^-6 (SI) and 5%, respectively (Blum, 1997). The results of magnetic susceptibility measurements are included in the site chapters.

The GRA measured bulk density at 2-cm intervals (in Site 1150 and Hole 1151A) and at 5-cm intervals (in Holes 1151C and 1151D) during 4-s-long sampling periods. The attenuation of gamma rays through the cores are compared with attenuation through aluminum and water standards, and the GRA bulk density is calculated assuming a constant core diameter (Boyce, 1976). Gamma-ray attenuation data are most reliable in undisturbed cores, whereas bulk density tends to be underestimated in sections with incompletely filled core liner. In RCB cores, GRA bulk density measurements were corrected to account for their smaller core diameters (Blum, 1997):

r_c = r · d_c / d, (4)

where r_c is corrected bulk density, r is measured GRA bulk density, d_c is the corrected core diameter, and d is the assumed core diameter (66 mm). All density data were edited by removing values less than 1.0 g/cm³ (i.e., density of pure water). The precision of GRA measurements depends on the count rate, sampling period, and number of standard deviations for the normal distribution. Counting rates of 20,000 counts per second (cps) and sampling periods of 4 s correspond to a statistical error of less than 0.4% for a 68% confidence interval (Blum, 1997). The GRA sensor was generally calibrated every 2 or 3 days with a distilled-water standard.

The PWL transmits a 500-kHz compressional wave (P-wave) pulse through the core at a repetition rate of 1 kHz. The transmitting and receiving transducers are aligned normal to the core axis (horizontal direction). A pair of displacement transducers monitors the separation between the P-wave transducers. Measurements were taken at 2-cm intervals (in Site 1150) and at 5-cm intervals (in Site 1151) during a sampling period of 4 s. Only continuous cores that filled their core liners were measured (i.e., only APC cores). Corrections were made for the P-wave traveltime through the liner (2 · d_liner = 5.08 mm and v_liner = 1990 m/s). The velocity data were edited in two steps. First, data with a signal quality less than 40 were removed. The signal quality is a measure of how well the acoustic signal traveled through the core, and it can vary from 0 to 255. Second, velocity values less than 1450 m/s (i.e., the velocity of seawater at 0°C) were deleted. Velocities from sediment cores from below a few hundred meters are typically compatible with downhole log measurements within less than 3%, whereas shallower core measurements tend to be as much as 5% lower than the corresponding logs (Blum, 1997). The PWL sensor was calibrated approximately once per week.

Natural gamma radiation activity was measured at 20-cm intervals in each section, with a sampling period of 20 s. Data from 2048 energy channels were collected and archived, and counts were summed over the range from 200 to 3000 keV. This integration range allows for comparison of the trends of NGR data with those of downhole logging data, although the two methods use different units (cps and gAPI [gamma-ray American Petroleum Institute], respectively). Before starting measurements, the four sensor gains were calibrated so that the combined thorium peak was as sharp as the individual peaks when the other three were disabled. The multichannel analyzer was calibrated by assigning certain channels to the characteristic energies of ⁴⁰K and the main peak of ²³²Th (Ocean Drilling Program, 1996). Furthermore, a background radiation of 12.27 cps was measured using a core liner filled with distilled water and a 30-s sampling period. The axial resolution is about 12 cm due to the geometry of the device, and the error of the system (estimated from reference values) varies from 3% to 7% (Blum, 1997). Corrections for sampling volume as proposed by Hoppie et al. (1994) were not made; thus, the data are reported in counts per second.

Thermal Conductivity

Thermal conductivity (k) is the rate at which heat is transmitted by molecular conduction. Thermal conductivity and temperature measurements of sediments and rock sections are used to determine heat flow. Heat flow is not only characteristic of the material, but also an indicator of type and age of ocean crust, and fluid circulation processes at shallow and great depth (Blum, 1997).

Thermal conductivity was only measured in soft sediments using the needle-probe method in full-space configuration (von Herzen and Maxwell, 1959). Data were typically acquired once in every core. The mean error associated with these determinations is estimated to ±0.2 W/(m·K), or less than 15% (Blum, 1997).

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 test, temperatures in the samples were monitored without applying current to the heating element to verify that temperature drift was <0.04 K/min. The heater was then turned on and the temperature rise in the probes recorded. After heating for about 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 fit to the following equation using the least-squares method (von Herzen and Maxwell, 1959):

T(t) = [q / (4 · p · k)] · ln(t) + L(t), (5)

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

L(t) = A · t + T_e, (6)

where A is rate of temperature change and T_e is equilibrium temperature. Thus, L(t) 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.

All measurements were made close to each other. Samples were taken from approximately the same depth interval of each section to provide a uniform sampling frequency. Disturbed core sections (i.e., sections with drilling biscuits and other drilling disturbances) were avoided.

Compressional (P-Wave) Velocity

P-wave velocity was measured using the PWS1, PWS2 and PWS3 systems. The choice of method and sampling frequency of discrete P-wave velocity measurements depends on the degree of induration of the sediment and the recovery. In soft sediments, P-wave velocity is measured by PWS1 and PWS2 systems, whereas only PWS3 is used in lithified sediments. Generally, P-wave velocity was measured once per section.

PWS1 and PWS2 consists of two pairs of digital sound velocimeters. The pairs are aligned parallel (PWS1; vertical) and normal (PWS2; horizontal) to the core axis. The transducer pairs of PWS1 has a fixed spacing of 7 cm, whereas PWS2 has a fixed spacing of 3.5 cm. An acoustic signal of 500 kHz is emitted and received by the transducers. The signal is digitized in an oscilloscope, from which the first arrival waveform can be picked and the P-wave velocity is obtained by dividing the spacing of the transducers with the first arrival time. The temperature of the sediment was measured adjacent to the PWS1 measurement in soft lithologies.

An improved Hamilton Frame system (PWS3) is used to measure P-wave velocity through the split core (horizontal direction), on indurated core pieces (horizontal direction) and cylindrical minicores (vertical and two orthogonal horizontal directions). Only coherent pieces that could be cut into minicores were selected for measurements. Hence, sampling tended to favor indurated and nonfractured sections. The PWS3 emits a 500 kHz P-wave pulse through the sediment. The sample thickness (and transducer separation) is measured with a digital caliper that is mounted on the transducers. The distance between the two transducers is decreased until a measurable waveform appears on the oscilloscope. To improve the coupling between transducer and sample, distilled water was applied between the sample and the transducer heads. Zero traveltimes were measured with a series of polycarbonate standards of known length. All measurements on split cores were corrected for the additional traveltime passing through the core-liner (d_liner = 2.54 mm and v_liner = 1990 m/s).

The approach to measure P-wave velocity in two or three directions on minicores provides a measure of the acoustic anisotropy (A₁₂) of the sediments (Carlson and Christensen, 1977):

A₁₂ = 2 · (V₁ - V₂) / (V₁ + V₂), (7)

where, V₁ is V_X or V_Y, and V₂ is V_X, V_Y, or V_Z. Horizontal velocity is measured in two orthogonal directions (V_X and V_Y) perpendicular to the core axis, and vertical velocity (V_Z) is measured parallel to the core axis. The spatial relationships between the x, y, and z axes are illustrated in figure 1-1 of Blum (1997).

An estimate of the azimuthal influence on the magnitude of horizontal anisotropy (A_XY) was obtained from paleomagnetic declination data (see "Paleomagnetism"). Directional paleomagnetic data were acquired from all archive half cores with a sampling frequency of 2- to 5-cm after AF demagnetization of 30 mT. The mean declination and 95% confidence limit (₉₅) of the core section from which the minicore was taken were calculated with the program Direction (a stereographic calculation and plot program). The 95% confidence limit (₉₅) of Fisher statistics is a measure of the precision with which the true mean direction has been estimated and analogous to a 95% probability level of the estimated standard error of the mean of Gaussian statistics. The orientations of horizontal velocities were placed into geographical coordinates by a rotation equivalent to the observed declination (e.g., V_X^azimuth = 50°N and V_Y^azimuth = 140°N for a declination of 50°N, and V_X^azimuth = 100°N and V_Y^azimuth = 10°N for a declination of 280°N).

Undrained Shear Strength

The peak undrained shear strength (S_u) generally was measured once per split core section, 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 are recorded on a Hewlett-Packard X-Y recorder. The undrained shear strength (i.e., the peak strength) is determined from the torque vs. strain plot and reported in kilopascals (kPa).

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. Consequently, shear vane measurements are only conducted in soft and clay-rich sediments.

A pocket penetrometer is used for measurements in stiffer sediments. The pocket penetrometer is a small, flat-footed cylindrical probe that is pushed into the split core to a depth of 6.4 mm. The resulting resistance is the unconfined compressive strength, or 2 × S_u. A scale directly reads out in units of kilograms per cubic centimeter. The value of unconfined compression were converted to values of S_u and reported in units of kilopascals.

Index Properties

Samples of approximately 10 cm³ were taken from the fresh split core for determination of index properties. For Leg 186, measurements of the wet and dry mass and the dry volume of a sample were used to determine the index properties (i.e., Method C; Blum, 1997). Sample mass was determined with an error within 0.1% using two Scitech electronic balances. The balance is equipped with a computer averaging system that corrected for ship accelerations. The sample mass was counterbalanced by a known mass such that the mass differentials were less than about 10 g. 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 clay minerals is largely lost in addition to interstitial water. Dry volume of samples were determined using a Quantachrome Penta-Pycnometer, which is a helium-displacement pycnometer. Sample volumes were determined at least five times, until readings were consistent (i.e., standard deviation = < 0.01%). A standard reference volume was run with each group of samples during the measurements and rotated among the cells to check for instrument drift and systematic error. This exercise demonstrated that the measured volumes had a precision of about 0.02 cm³. The sample beakers used for discrete determinations of index properties were calibrated before the cruise.

The following index properties were calculated: water content (based on total wet mass and mass of solids), bulk density, dry density, grain density, porosity, and void ratio. Table T5 lists the measured parameters, assumptions, and relationships of index properties calculations, which were corrected for salinity and density of the pore water following Boyce (1976). The determination of water content followed the methods of the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1989). In situ measurements of the salinity of pore water (see "Geochemistry" in the "Site 1150" chapter and "Geochemistry" in the "Site 1151" chapter) showed that the salinity was significantly lower than standard salinity used for automatic calculations (S = 0.035). Therefore, index properties were recalculated using in situ values of salinity and density of the pore water. The pore water density is a function of temperature, salinity, and pressure (Blum, 1997). At laboratory conditions (20°C, 1 bar), we derived the following relationship between pore-water density (_pw) and pore-water salinity (S) by fitting the data to a line (see fig. 2-1 of Blum, 1997):

_pw = 0.998 + 7.7143 · S. (8)

To allow cross-examination of the data for internal consistency, values of porosity, dry density, and void ratio were calculated indirectly from the other index properties (see Table T5 for definition of terms):

= 100 · [(_g /_pw - _b) / (_g /_pw - _pw)], (9)

= W_s · _b /[(1 + W_s /100) · _pw], (10)

_d = _pw + /W_s , and (11)

e = _g · W_s /(_pw · 100). (12)

The magnitude of total vertical stress (_v) and effective vertical stress (_v´) from the sediment section was calculated by integrating bulk density (_b), water density (_pw), and porosity () data with depth from core and log measurements:

_v = g · (_b · z) dz, and (13)

_v´ = g · ([_b - _pw · /100] · z) dz, (14)

where g = the acceleration of gravity at the Earth's surface (9.802 m/s²) and z = depth.