The objectives of physical properties measurements are (1) to obtain the basic data for the correlation between adjacent holes at a given site and construct complete stratigraphic sequences, (2) to detect changes in lithology and diagenetic zones within the sediment, (3) to identify potential stratigraphic hiatuses that are presented as inflections, discontinuities, or reversals in physical properties, and (4) to determine the sediment's consolidation characteristics, such as shear strength and porosity.
Compressional wave velocity (P-wave), wet-bulk density, magnetic susceptibility, and natural gamma-ray emission were measured continuously and nondestructively on whole-round core sections on the MST.
P-wave velocity varies with the lithology, the degree of consolidation, and the occurrence of gassy sediment and hydrates. Together with GRAPE (gamma-ray attenuation porosity evaluator) density, P-wave velocity is used to calculate acoustic impedance and the reflection coefficient, which are then used to estimate the depth of reflectors observed in seismic profiles.
P-wave velocity was measured, orthogonal to the core axis, using the P-wave logger (PWL) mounted on the MST. The PWL transmits a 500-kHz compressional wave pulse through the core at a repetition rate of 1 kHz. P-wave velocity was determined by picking the first arrival of wavelets. The travel time of the acoustic signal was measured to an accuracy of 50 ns. This corresponds to an instrument resolution of 1.5 m/s. The accuracy of velocity, however, was about 5 m/s because of the liner wall thickness (Schultheiss and McPhail, 1989).
The PWL system requires two types of calibration, which are performed at the beginning of each core section measurement. The first calibration is carried out for the displacement of the transducers, using the nominal diameter of an aluminum core and a standard core liner. The displacement of the transducer measures the core thickness and uses this measurement to calculate compressional wave velocity in the sediment. The distance traveled is measured with an accuracy of 0.1 mm. The second calibration, P-wave offset (time delay), adjusts to give the expected theoretical velocity in water at laboratory temperature. The P-wave offset is made by running the water standard core to compensate for sound propagation through the core liner and the electronic delay. P-wave velocity was only measured in hydraulic piston cores in a period of 4 s at 4-cm intervals.
All P-wave velocities were corrected to 20° C temperature values according to the following equation:
where V20 = velocity at 20° C, and Ts = the sediment temperature at which the measurement is made (° C). The measured velocity (VTm/s) at temperature Ts is defined as
where d is the diameter of the inner core liner, CDD is core diameter deviation from the standard nominal core, W is the wall thickness of the core liner, ts is the travel time through the sediment core and core liner, and X is the P-wave offset.
Note: Because of poor quality of the PWL results, all P-wave velocity measured by the PWL are not presented in the Leg 181 Initial Reports but are available from the ODP database.
GRAPE density provides a precise and high-resolution record of lithology and porosity changes. Together with P-wave velocity, GRAPE density can provide acoustic impedance and reflection coefficients to construct synthetic seismograms.
Estimates of wet-bulk density have been obtained from continuous logging of whole-round core sections with the GRAPE mounted on the MST. A 10-mCi Cesium-137 capsule was used as the gamma-ray source. A narrow gamma-ray beam with energy principally at 0.662 MeV was attenuated on passing through the diameter of the core. For most sediments, the incident photons are scattered by the electrons with a partial energy loss at this energy level (Boyce, 1976). The primary mechanism for the attenuation of gamma rays is by Compton scattering. The Compton attenuation is therefore directly related to the number of electrons encountered by the gamma-ray beam. This device measures the count rate per second of gamma-ray attenuation, which is related logarithmically to density times the thickness of sediment. Using a calibration based on aluminum standards of different thicknesses with distilled water inside the core liner, and ascertaining the slope of regression, the GRAPE density was obtained. Sediment was sampled at 2-s and 4-cm intervals. Longer counting times would allow greater precision. To maintain satisfactory precision of density measurement, aluminum standards have to be run with each core section to have calibration parameters ready for density calculation.
Magnetic susceptibility is used mostly as a proxy indicator of changes in composition that can be linked to paleoclimate-controlled depositional processes. This measurement also is extremely useful for correlation among adjacent holes at a given site.
Magnetic susceptibility was measured with the Bartington Instruments MS2C system using an 8-cm diameter loop, an inducing field frequency of 0.565 kHz, and a low sensitivity setting of 1.0. Sediment was measured at 4-s and 4-cm intervals. The susceptibility instrument was set on SI units and the raw values are stored in the JANUS database. To convert to true SI volume susceptibilities, these values should be multiplied by 10-5 and then multiplied by a correction factor (0.66) to take into account the volume of material that passed through the susceptibility coils.
Natural gamma-ray measurements are used for three purposes: (1) correlation of core and downhole data in multiple holes, (2) evaluation of the clay/shale content of a formation, and (3) abundance estimates for K, U, and Th.
The natural gamma-ray system consists of four shielded scintillation counters arranged at 90° from each other in a plane orthogonal to the core track. The scintillation counters contain doped sodium iodide crystals and photomultipliers to produce countable pulses (Blum, 1997). Natural gamma-ray measurements were taken for a period of 20 s at 15-cm intervals.
Thermal conductivity was measured immediately after the MST measurements using whole-round core from intervals where downhole temperature measurements were taken. The needle probe method was used in full space configuration. The probes were calibrated based on three standards: red rubber, macor ceramic, and black rubber. Thermal conductivities of these three standards are 0.96 W/(m·K), 1.61 W/(m·K), and 0.54 W/(m·K), respectively.
For each measurement, a needle probe was inserted into whole-round core. While the needle was heated, the temperature (T) was measured as a function of time (t). The temperature is related to the thermal conductivity of the sediment by
where q is heat input per unit length of wire per unit time (W/m·s), k is the apparent thermal conductivity in W/(m·K), and L(t) is a linear change in temperature with time, including temperature drift during measurement as well as instrumental errors.
All measurements were corrected for a linear offset between measured and true thermal conductivities. That is, the least-squares fit of the linear regression was used to obtain the true thermal conductivity based on a measured probe reading.
Water content, grain density (to calculate bulk density, porosity, and related properties), and vane shear strength measurements were obtained from split cores.
An additional estimate of wet-bulk density was determined by measuring water content and grain density to check the accuracy of the density measured on the MST. Water content is determined by weighing a sample of sediment and then drying the sample in an oven at a temperature of 105°-110°C for a period of 24 hr and re-weighing. Water content as a percentage of water mass over dry mass is determined by
where Wdry = dry water content (%), Mw is water mass, Md is dry mass, and Msalt is the salt in the fluid:
where s is salinity (0.035).
Wet-bulk density is then defined as the ratio of mass to volume of wet sediment:
where g = grain density (see below) and f is fluid density (1.025 g/cm3).
Porosity (ø) is defined as the ratio of the water volume to the total volume and was determined using the following relationship:
Grain density is defined as the mass of the grains divided by the volume of the grains:
where Vd = dry volume and salt is the density of salt (2.257 g/cm3).
Dry volumes were determined using a helium-displacement Quantachrome Penta-Pycnometer by employing Archimedes' principle of fluid displacement. The displaced fluid is a helium gas that can penetrate pores as fine as one Angstrom (10-10 m). The pycnometer measures the volume of each sample to an approximate precision of ±0.02 cm3. Determinations were made five times, or until 0.01 of the standard deviation was achieved, and then averaged.
Undrained shear strength was determined using the Wykeham-Farrence motorized miniature vane shear apparatus, following ASTM D4648-94 procedure (ASTM, 1995). A four-bladed vane was inserted into the split-core and rotated to determine the torque required to cause a cylindrical surface to be sheared by the vane. Torque at failure was converted through a calibrated spring into a measure of the sediment's undrained shear strength given in units of kPa. The results of a series of undrained shear tests were represented by measuring the value of cohesion.
The vane used for all measurements has a 1:1 blade ratio with a dimension of 12.7 mm. A rotation rate of 90° /min was used. The vane shear strength of sediment can be calculated from the equation:
where Su = undrained shear strength in kN/m2 or kPa, T is torque at failure to shear the sediment in Nm, K is a constant (in m3) depending on overall vane width, d, and vane length, h:
where d and h are equal to 12.7 mm; therefore, K = 4.29 × 10-6 m3.
The springs used to measure torque must be calibrated to the angles of rotation. ODP personnel are responsible for this occasional calibration. The spring constant, B, is defined as
where T is the torque (provided in kg·cm by the manufacturer) and is the corresponding deflection angle. ODP personnel enter the data into a calibration utility that converts the data to N·m and determines the regression slope that corresponds to B. The conversion is
The calibration constant for spring no. 4 that was used during Leg 181 is 0.045146.
In addition to the measurement by the P-wave logger mounted on the MST, P-wave velocity was also measured on each split core section on the DSV.
Two types of sound propagation measurements were obtained. First, two piezoelectric transducer pairs, PW1 and PW2, were inserted into soft sediment, parallel and orthogonal to the core axis, respectively. Sound was propagated through the sediment without the effect of the core liner. Transducers are separated by about 7 cm and firmly fixed at one end to a steel plate. Secondly, a modified Hamilton frame velocimeter, PW3, was used to measure sound speed orthogonal to the split core section, with transducers at the top and the bottom of the core section. In well-compacted or indurated sediment, discrete samples are required for good transducer contact. The DSV measurements were made at the same intervals where index properties and vane shear strength were taken. Calibration and calculation of the DSV are similar to those of the P-wave logger on the MST. A correction for the core liner, when present, is made.