The primary goals of physical properties measurements during Leg 188 were (1) to examine variations in physical properties related to the variations in sediment composition (thus, depositional history) on the continental rise, slope, and shelf of Prydz Bay; (2) to provide data sets to aid in the interpretation of seismic reflection and downhole geophysical measurements; and (3) to determine a preliminary stress history. Initial measurements of physical properties were undertaken on the MST after the cores had equilibrated to ambient temperature 3-4 hr after recovery. These nondestructive measurements were performed on unsplit, 1.5-m-long sections. The MST combines four sensors on an automated track to measure bulk density by gamma-ray attenuation, P-wave velocity, magnetic susceptibility, and natural gamma-ray emission. The MST provides a nearly continuous physical properties record; however, the quality of the data is highly dependent upon the condition of the core. Where possible, thermal conductivity was measured at intervals of one or two per core on whole-round sections. The cores were then split, and undrained shear strength and longitudinal and transverse P-wave velocity were measured on the working half. The moisture and density (MAD) measurements determined on discrete samples were dry density, bulk density, grain density, water content, porosity, and void ratio. Usually, one to two samples per section were taken from the same position as the discrete velocity measurements except where lithology or time dictated otherwise. Thermal conductivity was measured on pieces of split core in cases where measurements had not been possible on the whole-round core and where there was an intact piece of sufficient length. Physical properties data were transferred to the ODP database from the computer systems controlling the MST and index properties sensors.
Samples of ~10 cm3 volume were taken for determination of MAD. Bulk density, grain density, water content, porosity, and dry density were calculated from wet and dry sample weights and dry volumes. Masses were measured with a Scitech electronic balance (precision = 0.1%). The balance was connected to a computer with weight-averaging software that corrected for ship accelerations. The sample mass was counterbalanced by a reference mass such that the mass differences were generally <2 g. Sample volumes were determined using a Quantachrome Penta-Pycnometer, a helium-displacement pycnometer with a precision of ±0.04 cm3. Sample volumes were determined as many as three times. A standard reference volume was run with each group of samples during the measurements and rotated among the measurement cells to check for instrument drift and systematic error. A purge time of 3 min was used before each run. The sample beakers used for discrete determinations of index properties were calibrated 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. Water content, bulk density, porosity, grain density, dry density, and void ratio were determined following the procedures and equations outlined in Blum (1997). The procedures for the determination of water content comply with the American Society for Testing and Materials (ASTM) designation D2216 (ASTM, 1990). Bulk density, grain density, and porosity were computed from the wet and dry masses and the dry volume of the sample using Method C of Blum (1997).
The GRA bulk densiometer allows an estimation of wet bulk density by measuring the attenuation of gamma rays passing through the cores, where the degree of attenuation is proportional to density (Boyce, 1976; Gerland and Villinger, 1995). Calibration of the system was carried out using a known freshwater/aluminum density standard with four components of different average densities. Density measurements were taken at 2- to 4-cm intervals with counting times of 2 s.
The P-wave logger (PWL) transmits a 500-kHz compressional wave pulse through the core at a rate of 1 kHz. The transmitting and receiving transducers are aligned perpendicular to the core axis. An average of 50 traveltime determinations was taken. A displacement transducer monitors the separation between the compressional wave transducers, so that variations in the outside diameter of the liner do not degrade the accuracy of the velocities. The PWL does not provide accurate velocity values if there is no acoustic coupling between the sediment and the liner and is therefore most useful in undisturbed APC cores. Measurements were taken at 2- to 4-cm intervals. Calibration of the displacement transducer and measurement of electronic delay within the PWL circuitry was carried out using a series of acrylic blocks of known thickness and P-wave traveltime. The validity of the calibration was checked by measuring the P-wave velocity through a section filled with distilled water.
Whole-core magnetic susceptibility was measured at 4-cm intervals on a Bartington MS2C meter with an 80-mm (internal diameter) loop sensor using the 1.0 (1 s integration time) range and averaging five readings. Susceptibility values were archived in raw instrument units, which require multiplication by a factor of 6.6 × 10-6 to convert to volume-normalized SI units.
The area of influence for the four natural gamma-ray (NGR) sensors is ~±10 cm from the points of measurements along the core axis. The installation and operating principles of the NGR system are discussed by Blum (1997). Data from 256 energy channels were collected and archived. Total counts have been summed up over the range from 200 to 3000 keV to be comparable with data collected during previous legs. Measurements were made at 12-cm intervals. No corrections were made to account for incompletely filled core liners. The calibration procedure consisted of tuning all four scintillation counters to the same signal level for a particular emission energy, using the potassium peak; measuring background radiation caused by impurities in the system; and making an energy calibration by measuring standards with characteristic emission peaks at known energies (see Blum, 1997).
The accuracy of GRA bulk density, PWL, and magnetic susceptibility measurements degrades considerably in APC and XCB sections with gas voids or where the core otherwise does not fill the liner completely or is disturbed.
In addition to the velocity measurements with the PWL, compressional wave velocity, Vp, was measured on split-core sections with the digital sound velocimeter using two types of piezoelectric transducer pairs (P-wave sensors 1 and 2 [PWS1 and PWS2]). The transducers were inserted into soft sediments along (z-direction [PWS1]) and orthogonal (y-direction [PWS2]) to the core axis. Velocity calculation is based on the fixed distance between the transducers (7.0 and 3.5 cm, respectively), measurement of the traveltime of an acoustic impulse, and a delay constant determined by measuring a water standard. The velocity meter was calibrated by measuring Vp in distilled water. In indurated sediments, a modified Hamilton frame velocimeter (PWS3) was used, which measured the traveltime of a 500-kHz signal orthogonally across the split-core section and core liner (x-direction) (Blum, 1997). In addition to traveltimes, sample thickness was measured using a digital micrometer that is zeroed periodically. In cases where the core was sufficiently indurated, block samples were trimmed, and velocities were measured in the x-, y-, and z-directions using PWS3.
Velocity data recorded in the Janus database are uncorrected for in situ temperature and pressure. These corrections can be made using the relationships given in Wylie et al. (1956), Wilson (1960), and Mackenzie (1981).
The undrained shear strength, Cu, of the sediment was determined using three methods: the ODP motorized miniature vane shear device, a pocket penetrometer, and a fall-cone device provided for Leg 188 by one of the shipboard scientists.
The motorized miniature vane shear device was run in soft sediments following the procedures of Boyce (1977). The instrument measures the torque and rotation at the vane shaft using a torque transducer and potentiometer, respectively. The shear strength reported is the peak strength determined from the torque vs. rotation plot. In addition to the peak shear strength, the residual strength was determined from the same plot if the failure was not dominated by cracking of the sample (Pyle, 1984). In the analysis of vane tests, 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 the failing specimen, uplift of the failing core cylinder, drainage of local pore pressures (i.e., the test can no longer be considered undrained), and stick-slip behavior. The pocket penetrometer measurements were converted from kilograms per square centimeter to kilopascals and then divided by two, as the penetrometer was calibrated as an unconfined compression test (for the ideal clay) equal to twice the undrained shear strength (Holtz and Kovacs, 1981). A small adapter point was provided by one of the shipboard scientific party for sediments with strengths in the range of 200 to 900 kPa.
The fall-cone device (Skempton and Bishop, 1950) provides a rapid and simple method for determination of undrained shear strength for undisturbed (as well as remolded) clays. A cone of known weight and apex angle is lowered to touch the sediment surface. After release, it penetrates into the sediment only by its own weight. Based on empirical relationships, the penetration can be directly converted to undrained shear strength in kilopascals. The results are usually the average of three measurements. Four different cones were used: 10 g/30°, 60 g/60°, 100 g/30°, and 400 g/30°, covering the shear strength intervals of 0.1-1.5 kPa, 2.2-9.0 kPa, 18-88 kPa, and 55-370 kPa, respectively. Fall-cone measurements affect a smaller volume of sediment during the measurement and are therefore less affected by sand- and gravel-sized material than the vane shear measurements. Fracturing of the sediment, which is the main cause of error in the vane shear measurements, is also avoided using the fall-cone device.
The TK04 (Teka, Berlin) was used for thermal conductivity measurements. A full-space needle probe was used for unconsolidated sediments, and a half-space needle was used for lithified sediments. The full-space needle probe, containing a heater wire and a calibrated thermistor, was inserted into the sediment through a small hole drilled into the core liner. The half-space needle was attached to a section of split core, and the assembly was immersed in seawater. Initially, Velcro straps were used to attach the needle, but it was later found that elastic bands provided a more secure attachment. Three measuring cycles were automatically performed at each location with the full-space needle and four cycles with the half-space needle. At the beginning of each test, a self-test, which included a drift study, was conducted. Once the samples were 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. Temperatures measured during the first 150 s of the heating cycle were fitted to an approximate solution of a constantly heated line source (Kristiansen, 1982; see Blum, 1997, for details). Errors are between 5% and 10%. Corrections were not attempted for in situ temperature or pressure effects.