The primary goals of physical properties measurements during Leg 189 were to (1) calibrate the near-continuous records of physical properties that were used for hole-to-hole correlations, construction of complete stratigraphic sequences, and core-to-downhole log ties; (2) examine gradients in physical properties, such as porosity, natural gamma radiation, magnetic susceptibility, and compressional wave velocity, which are related to variations in sediment composition; and (3) provide data to aid the interpretation of seismic reflection and downhole geophysical logs. Initial measurements of physical properties are undertaken on the MST. These measurements are performed on unsplit, 1.5-m-long (and shorter) sections and are nondestructive. The MST incorporates a GRA bulk density device, a P-wave logger (PWL), a magnetic susceptibility meter (MSM), and a natural gamma sensor (NGR). The quality of the MST data is highly dependent upon the condition of the core. Bulk density and magnetic susceptibility measurements were taken at 2-cm intervals on the MST. The PWL measurements were taken at 2-cm intervals in APC cores only. Natural gamma data were not routinely measured. Thermal conductivity, using the needle-probe method, was also measured at discrete intervals in whole-round sections. Physical properties measurements made on split-core sections included undrained shear strength, and longitudinal, transverse, and orthogonal compressional wave velocity. Moisture and density (MAD) measurements determined for discrete samples included dry bulk density, grain density, porosity, and void ratio. One MAD sample per section was taken in the Pleistocene and Pliocene sections of the A holes; three samples per core were taken in Miocene through Paleogene sections.
The GRA bulk density device 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; Breitzke, 2000). Calibration of the system was conducted using a known seawater/aluminum density standard with four components of different average densities.
The PWL transmits a 500-kHz compressional wave pulse through the core at 1 kHz. The transmitting and receiving transducers are aligned perpendicular to the core axis. A pair of displacement transducers monitors the separation between the compressional wave transducers so that the variations in the outside diameter of the liner do not degrade the accuracy of the velocities. Where there is poor acoustic coupling between the sediment and the liner, the PWL does not provide accurate velocity values and therefore is used only on undisturbed APC cores. Calibration of the displacement transducer and measurement of electronic delay within the PWL circuitry were conducted using a series of acrylic blocks of known thickness and P-wave traveltime. Calibration validity was checked by measuring the P-wave velocity through a section filled with distilled water.
Whole-core magnetic susceptibility was measured at 2-cm intervals on a Bartington MS2C meter with an 80-mm (internal diameter) loop sensor using a 1-s integration time and averaging five readings. Susceptibility values were archived in raw instrument units (SI), which require multiplication by 6.6 × 10-6 to convert to volume-normalized SI units. The accuracy of the GRA, PWL, and MSM measurements are degraded in APC and XCB sections with gas voids, where the core does not fill the liner completely, or is disturbed. Nevertheless, in such cases the downhole trends are still useful for stratigraphic correlation.
Natural gamma-ray emission was not routinely measured because of time constraints imposed by the high recovery rate. NGR data were recorded on selected holes, mainly at 10- to 20-cm intervals along each section. The area of influence for the four NGR sensors is about 10 cm from each point of measurement along the core axis. The installation and operating principles of the NGR system used on the JOIDES Resolution are discussed by Hoppie et al. (1994). Data from 2048 energy channels were collected and archived. Counts were summed over the range from 200 to 3000 keV (in five windows), so as to be comparable with data collected during previous legs. This integration range also allows for a direct comparison with downhole logging data, which are collected over a similar integration range (Hoppie et al., 1994). Over the 200- to 3000-keV integration range, background counts (measured using a core liner filled with distilled water) averaged 18 per 30 s measurement period. No corrections were made to XCB core NGR data to account for inaccuracies resulting from core liners that were incompletely filled with sediment. Before taking measurements, the four sensor gains were adjusted so that the combined potassium peak was as sharp as the individual peaks when the other three were disabled. The multichannel analyzer was then calibrated by assigning certain channels to the characteristic energies of 40K and the main peak of 232Th (Blum, 1997).
The archive MST (AMST 188 version) was used to make systematic measurements of the relative spectral reflectance of the sediment surfaces. The instrument consists of a computer-controlled motorized track assembly that advances a set of detectors over the cut sediment surface of a core half. The sensors include an LB1101 laser displacement transducer measuring the sediment micromorphology and a Minolta CM-2002 spectral photometer. The laser displacement transducer was run before every scan to obtain information on the drop-down distance for the spectrophotometer. The controller software allows manual configuration and calibration for all sensors and automatically runs scans of core sections.
The Minolta CM-2002 spectrophotometer uses a pulsed xenon arc lightbulb, which illuminates a round aperture 0.8 cm in diameter. The light reflection is focused on a silicon photodiode and measures the wavelength range between 400 and 700 nm (in 10-nm steps) and the L* a* b* (lightness, red/green, and yellow/blue chromaticity coordinate) values. These data are also converted into the Munsell color scheme. The Minolta CM-2002 spectrophotometer was calibrated every 24 hr by a zero-measurement (aperture open to infinity) and a white calibration using the standard white ceramic lid supplied by Minolta. Measurements were taken at 2-cm intervals on all APC, XCB, and RCB cores at each site. The Minolta CM-2002 spectrophotometer is designed to operate in direct contact with the sediment surface, and the offset was set to -0.1 units in relation to the laser-measured distance. In order to protect the aperture, the sediment was covered with Gladwrap brand clear plastic wrap. Dried sediment sections were moistened with distilled water.
Samples of ~10 cm3 were taken from the fresh core for determination of moisture and density (MAD) measurements. 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 with an error within 0.1% using a Scitech electronic balance. The balance was 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 generally were <1 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 at least three times, until the last two measurements had <0.01% standard deviation. 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. A purge time of 3 min was used before each run. The sample beakers used for discrete determinations of moisture and density 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 these properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1990).
In addition to the velocity measurements with the PWL, compressional wave velocity was measured on split-core sections with the digital sound velocimeter (PSW1 and PSW2) using two types of piezoelectric transducer pairs. The transducers were inserted into soft sediments along (z-direction) and orthogonal (y-direction) to the core axis. Velocity calculation is based on the fixed distance between the transducers (7 and 3.5 cm, respectively), measurement of the traveltime of an acoustic impulse, and a delay constant determined by measuring a water standard. Periodically, the separation was precisely evaluated by running a calibration procedure in distilled water. A value of the sound velocity in distilled water is determined (based on standard equations) for the measured temperature, with the computer calculating the transducer separation using the signal traveltime. Use of the PSW1 and PSW2 was stopped in more indurated sediments when the sediment started to crack during insertion of the transducers. The modified Hamilton frame velocimeter was also used, which measured the traveltime of a 500-kHz signal orthogonally across the split-core section and core liner (x-direction). Orientation of the x-, y-, and z-directions is indicated in Shipboard Scientific Party (1997; fig. 12). Sample thickness was measured directly from the velocimeter frame lead screw through a linear resistor output to a digital multimeter. Zero traveltimes for the velocity transducers were estimated by a linear regression of traveltime vs. distance for a series of aluminum and lucite standards. Velocity data recorded in the Janus database are uncorrected for in situ temperature and pressure. However, these corrections can be made using the relationships in Wyllie et al. (1956), Wilson (1960), and Mackenzie (1981).
The undrained shear strength (Su) of the sediment was determined using the ODP motorized miniature vane shear device following the procedures of Boyce (1977). The vane rotation rate was set to 90°/min. Measurements were made only in the fine-grained, soft to very stiff units. A range of springs of various strengths were available; the B-4 spring was used during this leg. The spring was calibrated before the start of the leg. The instrument measures the torque and strain at the vane shaft using a torque transducer and potentiometer, respectively. The shear strength reported is the peak strength determined from the torque vs. strain plot. In addition to the peak shear strength, the residual strength was determined from the same plot where the failure was not dominated by cracking of the sample (Pyle, 1984). In the analysis of vane tests, the assumption is made 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 to be undrained), and stick-slip behavior.
The TK04 (Teka Bolin) was used for thermal conductivity measurements. The full-space needle probe, containing a heater wire and a calibrated thermistor, was inserted into the unconsolidated sediment through a small hole drilled into the core liner. Three measuring cycles were automatically performed at each location. 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 (for details see Kristiansen, 1982, and Blum, 1997). Errors are between 5% and 10%. Corrections were not attempted for in situ temperature or pressure effects.
In situ temperature measurements were made with the Adara tool and the Davis Villinger Temperature Probe (DVTP). The Adara tool is housed entirely inside the coring shoe of the APC. In a normal deployment, the tool first stops briefly at the mudline before entering the borehole and thermally equilibrates with the bottom water. After the APC penetrates to the bottom sediments, it is held there for ~10 min and records the temperature of the cutting shoe every 5 s. As a result of the frictional heating caused by the APC penetration, the temperature rises instantaneously but decreases gradually as the heat dissipates to the surrounding sediments. One can theoretically extrapolate the equilibrium temperature of the sediment by applying a mathematical heat conduction model to the temperature decay record (Horai and Von Herzen, 1985). More technical instrument information can be found in Initial Reports Volumes 139 and 150 (Shipboard Scientific Party, 1992, 1994).
In a normal deployment in deep water (>2000 m), the mudline stop can be very short (<5 min) because the thermal gradient of the bottom water is near zero and the APC is already approaching thermal equilibrium during the long descent. The time between the surface deployment and the mudline stop is very short. In addition, the water circulated within the drill pipe can be several degrees warmer than the ambient bottom water, although the circulation pump is usually shut off just before the mudline stop. Therefore, it takes ~10 min before the APC shoe thermally equilibrates with the bottom water at the mudline.
The DVTP tool is used in semilithified sediments, which the APC cannot penetrate, and unlike the Adara, the DVTP requires a separate wireline run. This tool measures formation temperature using a probe that is pushed into the top of the sediment section. The probe is conical with two thermistors, one located 1 cm from the tip of the probe and the other 12 cm above the tip. A third thermistor, referred to as the internal thermistor, is located in the electronics package. Thermistor sensitivity is 1 mK in an operating range from -5° to 20°C, and the total operating range is -5° to 100°C. The thermistors were calibrated at the factory and on the laboratory bench before installation in the probe. In addition to the thermistors, the probe contains an accelerometer sensitive to 0.98 m/s2. Both peak and mean acceleration are recorded by the logger. The accelerometer data are used to track disturbances to the instrument package during the equilibration interval. In a DVTP deployment, mudline temperatures are measured for 10 min on the first run within each hole and for 2 min for subsequent runs before descent into the hole for a 10-min equilibration interval in the bottom. Mudline temperatures are also collected for at least 2 min on ascent. Data from the probe tip thermistor are used for estimation of in situ temperatures.
Data reduction procedures are similar for both temperature tools. The synthetic thermal decay curves for the Adara tool and DVTP are a function of the geometry and thermal properties of the probe and the sediments (Bullard, 1954; Horai and von Herzen, 1985). However, it is never possible to obtain a perfect match between the synthetic curves and the data because (1) the probe never reaches thermal equilibrium during the penetration period; (2) contrary to theory, the frictional pulse upon insertion is never instantaneous; and (3) temperature data are sampled at discrete intervals, meaning that the exact time of penetration is always uncertain. Thus, both the effective penetration time and equilibrium temperature must be estimated by applying a fitting procedure, which involves shifting the synthetic curves in time to obtain a match with the recorded data. The data collected >20-50 s after penetration usually provide a reliable estimate of equilibrium temperature. However, where the APC has not achieved a full stroke, leakage of drilling fluid into the formation may occur and results are not considered reliable.