Shipboard measurements of physical properties provide important information that assists in characterization of lithologic units, correlation of lithology with downhole geophysical logging data, assessment of the nature of consolidation, and interpretation of seismic reflection profiles. The primary objectives of the Leg 198 physical properties program were to collect high-resolution data to (1) facilitate hole-to-hole correlation and construction of composite stratigraphic sections, (2) allow correlation between sites, (3) enable postcruise cyclostratigraphy studies, (4) facilitate construction of synthetic seismic profiles, and (5)investigate the characteristics of major seismic reflectors.
Several types of physical properties measurements were initially performed on unsplit whole-round 1.5-m (and shorter) sections. Nondestructive measurements of wet bulk density, magnetic susceptibility, transverse compressional wave (P-wave) velocity and natural gamma radiation were made using the MST. The MST incorporates a GRA bulk density device, a P-wave logger (PWL), a magnetic susceptibility meter (MSM) and a natural gamma radiation (NGR) sensor. The quality of the MST data is highly dependent on the condition of the core. Thermal conductivity, using the needle-probe method, was measured at discrete intervals in whole-round sections. By comparison, discrete transverse compressional wave velocity and index properties measurements were made on split-core sections. Index properties determined for discrete samples included wet bulk density, dry bulk density, grain density, water content, porosity, and void ratio. A comprehensive discussion of all methodologies and calculations used in the JOIDES Resolution physical properties laboratory can be found in Blum (1997).
The principal aim of MST data acquisition during Leg 198 was to obtain high sampling resolution data sets, especially of GRA bulk density and magnetic susceptibility, to facilitate shipboard core-to-core correlation and the construction of composite stratigraphic sections. This objective had to be completed within a reasonable time frame without compromising the shipboard processing of recovered core. It should be noted that GRA, MSM, and PWL measurements are degraded in sections that contain gas voids, where the sediment does not fill the liner completely and where sediment is disturbed. Nevertheless, in such cases, the downhole data trends can still be useful for stratigraphic correlation.
The GRA bulk density device allows estimation of wet bulk density by measuring the attenuation of gamma rays that have passed through the cores, where the degree of attenuation is proportional to density (Boyce, 1976). Calibration of the GRA system was completed using known seawater/aluminum density standards. GRA bulk density data are of highest quality when determined on APC cores because the liner is generally completely filled with sediment. In XCB, MDCB, and RCB cores, however, GRA measurements are of lower quality and cannot be used to reliably determine bulk density on their own. The measurement width of the GRA sensor is ~5 mm, with sample spacing generally set at 3.0 cm (2.5 cm for Sites 1207 and 1208) for Leg 198 cores. The minimum integration time for a statistically significant GRA measurement is 1 s, and routine Leg 198 GRA measurements used either a 5- or 3-s integration time. A freshwater control was run with each section to measure instrument drift.
Whole-core magnetic susceptibility was measured with the MST using a Bartington MS2C meter with an 8-cm (internal diameter) loop. The measurement resolution of the MSM sensor is 4 cm, with a minimum statistically significant count time of 1 s. During Leg 198, MST magnetic susceptibility was routinely measured at a spacing of 3.0 cm (2.5 cm for Sites 1207 and 1208), with five data acquisitions. Magnetic susceptibility data were archived as raw instrument units and not corrected for changes in sediment volume, although a correction was made for instrument drift. To obtain SI units these raw instrument data need to be multiplied by 0.68 x 10-5 (Blum, 1997).
Transverse P-wave velocity was measured on the MST track with the PWL for all APC cores. The PWL was not used on XCB, MDCB, or RCB cores because the core liner/sediment interface is usually poor, resulting in poor acoustic coupling between the sediment and the liner and, thus, in inaccurate velocity determinations. 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 variations in the outside diameter of the liner do not degrade the accuracy of the measured velocities. 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. Repeated measurement of P-wave velocity through a core liner filled with distilled water was used to check calibration validity. The measurement width of the PWL sensor is ~1 mm, with sample spacing routinely set at 2.5-10 cm for Leg 198 APC cores; analyses were completed in <1 s.
NGR was measured using the NGR sensor on the MST. The installation and operating principles of the NGR system used on the JOIDES Resolution are discussed by Hoppie et al. (1994). Data from 256 energy channels were collected and archived. For presentation purposes, the counts were summed over the range of 200-3000 keV, so as to be comparable with data collected during previous legs. This integration range also allows direct comparison with downhole logging data, which were 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 30 during a 1-hr measurement period. Before taking measurements, each of the four NGR amplifiers were adjusted so that the thorium peak was at the highest resolution possible when the other three amplifiers 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 measurement width of the NGR is ~15 cm, with a statistically significant count time of at least 5 s, depending on lithology. Because of the long time required for NGR measurements, sample spacing and count time for NGR measurements varied depending on the age and lithology of the sediment recovered. Natural gamma radiation data were not collected for all of the Leg 198 holes and sites. No corrections were made to NGR data obtained from XCB, MDCB, or RCB cores to account for sediment incompletely filling the core liner.
During Leg 198 activities some problems were encountered with the MST, GRA, and PWL sensors, resulting in significant and erroneous offsets in some data sets. These limitations to the Leg 198 MST-derived physical properties data are discussed in the relevant site chapters.
Index properties (wet and dry bulk density, grain density, water content, porosity, and void ratio) were determined from measurements of wet and dry sediment mass and dry sediment volume. Discrete samples of ~10 cm3 were taken from soft sediments and placed in 10-mL beakers. One sample was routinely collected in each section of Hole A at each site and in each section of Holes B and C when these cored intervals were deeper than the base of Hole A.
Sample mass was determined with a reproducibility of ±3% standard deviation using a Scitech electronic balance. The balance was equipped with a computer averaging system that corrected for ship acceleration. The sample mass was counterbalanced by a known mass so that the mass differentials generally were <1 g. Sample volumes were determined using a Quantachrome Penta-Pycnometer, a helium-displacement pycnometer. Volume measurements were repeated at least three times, until the last two measurements exhibited <0.01% standard deviation. A reference volume was included within each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. A purge time of 3-5 min was used before each run. The sample beakers used for discrete determination of moisture and density were calibrated before the cruise.
Individual index properties were calculated using measurements of wet and dry sediment mass and dry sediment volume; the latter two parameters were measured after samples had been oven dried at 105° ± 5°C for 24 hr and allowed to cool in a desiccator (see Blum, 1997, for relevant procedures and equations). This analytical method is preferred because volume measurements of wet samples are less accurate than dry volume samples. The procedures for the determination of these index properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1990).
The thermal conductivity of core material was measured in whole-core sections from Holes 1207A, 1208A, and 1209A with the TK04 (Teka Bolin), using the needle-probe method in full-space configuration for soft sediments (von Herzen and Maxwell, 1959). The full-space needle, containing a heater wire and 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 sampling location. At the beginning of each test, a self-test that 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). Measurement errors were 5%-10%. No correction was attempted for in situ temperature or pressure effects. Thermal conductivity was measured only in soft sediments, into which the TK04 needles could be inserted without risk of damage. Measurements of thermal conductivity were not conducted at all Leg 198 sites.
In addition to velocity measurements made with the PWL, compressional wave velocity was measured on split-core sections using the modified Hamilton frame velocimeter (PWS3). The digital sonic velocimeter (DSV) was not used during Leg 198, so the disturbance of sediment cores was minimized. The PWS3 measures the traveltime of a 500-kHz signal directed orthogonally (x-direction) across the split-core section and core liner between two piezoelectric transducers located at opposite sides of the split-sediment core. Sample thickness was measured directly from the velocimeter frame lead screw through a linear resistor output to a digital multimeter. The PWS3 transducers were calibrated using the linear regression of travel time vs. distance for a set of Lucite standards. Split-core velocities were measured perpendicular to the length of the core and were corrected for the presence of the liner. Routine sampling frequency for P-wave measurements was one per section, with "exotic" lithologies (e.g., black shale, radiolarite, chert, basalt, and porcellanite) sampled where relevant; the positions of PWS3 P-wave measurements were next to those for index properties whenever possible. Deionized water was added to the contact between the transducers and sample to improve acoustic coupling. The velocity data stored in the ODP Janus database are uncorrected for in situ temperature and pressure. However, these corrections can be made using the relationships outlined in Wyllie et al. (1956), Wilson (1960), and Mackenzie (1981).
Downhole temperature measurements were taken at Site 1209 to determine the heat flow on Shatsky Rise. In situ temperature measurements were made using the Adara temperature tool as part of regular APC coring operations. The components of the Adara tool are contained in an annulus in the coring shoe of the APC string and include a platinum temperature sensor and a data logger. The platinum resistance-temperature device is calibrated over a range of 0° to 100°C, with a resolution of 0.01°C. During operation, the coring shoe is attached to a core barrel and lowered down the pipe by wireline. The tool is typically held for 5-10 min at the mudline to equilibrate with bottom-water temperatures then lowered to the end of the drill string. The standard APC coring technique is subsequently used, with the core barrel fired through the drill bit using hydraulic pressure. The Adara tool is left in the sediment for 10-15 min to obtain a temperature record. These data provide a sufficiently long transient record for reliable extrapolation of the steady-state temperature. The nominal accuracy of the Adara temperature measurement is ±0.1°C. These data were combined with measurements of thermal conductivity obtained from whole-core samples to calculate heat flow values.
Depths to significant horizons at the Leg 198 sites were converted to traveltime so that horizons could be identified with reflectors in the seismic section. Three different methods of depth-traveltime conversion were employed to provide a gauge of the range of uncertainty in the results because each method carries a different set of assumptions:
All three methods gave consistent depth-traveltime conversions, to within ~0.01 s two-way traveltime per 100 m of depth. The traveltimes from PWS velocities were the longest at most sites, the traveltimes from the Carlson et al. (1986) empirical formula were the shortest, and the tie points from the synthetic seismograms fell in between. The average of the PWS and empirical traveltimes was used to plot the borehole on the seismic section (see Figs. F15, F19, F25, and F29, all in the "Leg 198 Summary" chapter). The exceptions to this pattern are Sites 1207 and 1213 (Figs. F11 and F35, both in the "Leg 198 Summary" chapter), which penetrate Lower Cretaceous formations containing chert and lithified sediments, where the synthetic seismogram tie points and the traveltimes based on core/log velocities have shorter traveltimes than the Carlson et al. (1986) empirical conversion. For Site 1207, the traveltimes from synthetic seismogram tie points were used to plot the borehole on the seismic section (see Figs. F61 and F62, both in the "Site 1207" chapter).