PHYSICAL PROPERTIES

Shipboard measurements of physical properties provide information useful in the characterization of lithostratigraphic 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 207 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 cyclostratigraphic studies to help constrain events across critical intervals,
  4. Facilitate construction of synthetic seismograms, and
  5. Investigate the characteristics of major seismic reflectors.

Several types of physical property measurements were performed on unsplit sections. Nondestructive measurements of bulk density, magnetic susceptibility, transverse compressional (P)-wave velocity, NGR, and resistivity were performed on the MST. The MST incorporates a GRA densitometer, a P-wave logger (PWL), a magnetic susceptibility meter, an NGR sensor, and an NCR sensor. Thermal conductivity, using the needle-probe method, was also measured at discrete intervals in whole-round sections from the upper four to five cores from two sites (Sites 1257 and 1260).

Measurements on split-core sections included discrete transverse and axial P-wave velocity and moisture and density (index) properties. Measured index properties of discrete samples (namely wet and dry mass and dry volume) were used to determine 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).

Multisensor Track Measurements

The principal aim of MST data acquisition during Leg 207 was to obtain high-resolution data sets to facilitate shipboard core-to-core correlation, to allow the construction of composite stratigraphic sections, and to provide for cyclostratigraphic analyses across critical boundaries and intervals. These objectives had to be completed within a reasonable time frame without compromising the shipboard processing of recovered core. As the IMS software used to control the MST was only capable of handling four instruments at a time, it was necessary to strategically employ certain types of measurements for different sites and holes. The PWL and NCR and NGR sensors were all taken off-line at times. The site, hole, and sections measured with each sensor are listed in Table T11.

The GRA densitometer measures the attenuation of gamma rays that pass through the core material, 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 and RCB cores where the cores do not completely fill the full inner diameter of the liner, 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 ~0.5 cm, with sample spacing generally set at 2.5 cm for Leg 207 cores. The minimum integration time for a statistically significant GRA measurement is 1 s, and routine Leg 207 GRA measurements used either a 3- or 4-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 susceptibility sensor is 5 cm on either side of the loop, with a minimum statistically significant count time of 1 s. During Leg 207, MST magnetic susceptibility was routinely measured at a spacing of 2.5 cm with a single data acquisition. Magnetic susceptibility data were archived as raw instrument units and not corrected for changes in sediment volume. To obtain SI units, these raw instrument values need to be multiplied by 0.68 x 10–5 (Blum, 1997).

Transverse P-wave velocity was measured on the MST with the PWL. All APC cores where coupling of the sediment and liner existed and saturation was high enough to transmit the pulse were measured (Table T11). The PWL was not used on XCB or RCB cores because the weak core liner/sediment interface results in poor acoustic coupling between the sediment and the liner. 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. The displacement transducers were calibrated using a series of acrylic blocks of known thickness, while the P-wave traveltime and measurement of electronic delay in the PWL circuitry was calibrated using a plastic bag filled with distilled water. The bag was manipulated to sit between the transducers at varying distances. Repeated measurement of P-wave velocity through a core liner filled with distilled water was used to check the calibration validity. The measurement width of the PWL sensor is ~0.1 cm, with sample spacing routinely set at 2.5 cm for Leg 207 APC cores.

Electrical resistivity of sediment cores was measured using the NCR sensor on the MST. NCR measurements are made by using a high-frequency magnetic field to induce an electrical current in the core. Magnetic fields, regenerated by the electrical current, are measured on a receiver coil and normalized with a set of coils operating in air. Calibration of the NCR is achieved by measuring a variety of core-sized standards containing seawater of varying but known salinity. The salinity of each standard was measured in the chemistry laboratory using a refractometer. The relationship between NCR millivolt output and true resistance is determined by plotting the sensor output for each standard against the empirically determined resistivities of the solutions. During Leg 207, five standards were constructed during transit using core liner cut into 15-cm pieces and filled with diluted solutions of seawater. One standard remained undiluted. The resistivity of each of these standards was calculated from the salinities using a power-curve relationship derived from the salinities of known NaCl solutions found in the CRC Handbook of Physical Properties of Rocks (Carmichael, 1982) and compared to the resistances derived from the calibration curve determined during the NCR sensor installation during Leg 204 (see Trhu, Bohrmann, Rack, Torres, et al., 2003). Excellent agreement existed between the two methods for determining the resistance of the various standards between 18.5 and 35.5 psu, illustrating that the relationship between the NCR millivolt output and true resistance (derived during Leg 204) remained accurate and applicable (Table T12).

Extensive RCB drilling during Leg 207 introduced a complication into the acquisition and interpretation of the resistivity data from the MST. RCB cores tend to have a substantial and variable gap between the core and the liner. Although resistivity was routinely measured on all cores and was occasionally found to be in good agreement with the GRA density, it was not until the end of drilling operations that an attempt was made to measure the response of the sensor to variable distances from the core. A sequence of measurements was made on a single seawater standard with the sensor placed at variable distances from the outside of the core liner (Table T13). With increasing distance from the water standard, one would expect the resistance to increase; however, it was not possible to define this relationship based upon the results from this test. As illustrated in Table T13, there does not appear to be a readily definable relationship between distance and sensor response. Without better constrained core diameters, normalization of the NCR resistivity data for most of the RCB cores and thus for most of the material recovered during Leg 207 does not appear possible. Resistivity data collected during Leg 207 should be interpreted with utmost caution.

NGR was measured on the MST, and the operating principles of the 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 in order 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 was adjusted so that the main thorium peaks (2615 keV) for each sensor were exactly aligned. 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 sensor 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 for NGR measurements varied between 7.5 and 15 cm, depending on the age and lithology of the sediment recovered (Table T11). No corrections were made to NGR data obtained from XCB or RCB cores to account for sediment incompletely filling the core liner.

Thermal Conductivity

The thermal conductivity of core material was measured in suitable whole-core sections using the needle-probe method in full-space configuration for soft sediments (Von Herzen and Maxwell, 1959). Sediment soft enough to be measured with the insertion probe was restricted to Cores 1–5 in Holes 1257A and 1257B and Cores 1–4 in Hole 1260B.

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 the 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; 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.

Index Properties

Index (moisture and density; MAD) properties (bulk density, 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 ~10-cm3 samples were taken from soft sediments and placed in 10-mL beakers. Samples were collected at a frequency of one per section in Hole A at each site and in Holes B and C when these cored intervals were not recovered in Hole A. One sample per core was collected in intervals already recovered from other holes. Sampling was minimal through the Cretaceous black shale sequence and across critical boundaries.

Sample mass was determined with a reproducibility of 3% standard deviation using an electronic balance (Scientech). The balance is equipped with a computer averaging system that corrects for ship acceleration. The sample mass is counterbalanced by a known mass so that the mass differentials are generally <1 g. Sample volumes were determined using a helium-displacement pycnometer (Quantachrome penta-pycnometer). Volume measurements were repeated at least five times, until the last two measurements exhibited <0.01% standard deviation. A reference volume was included in each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. A purge time of 2 min was used before each run. The sample beakers used for discrete determination of moisture and density were calibrated before the cruise.

Each MAD property was 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 5C for 24 hr and allowed to cool in a desiccator. This analytical method is preferred because volume measurements of wet samples are less accurate than dry samples when measuring in the pycnometer. The procedures for the determination of these properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1990).

Wet mass (Mwet), dry mass (Mdry), and dry volume (Vdry) are measured in the laboratory. Salt precipitated in sediment pores during the drying process is included in the Mdry and Vdry values. The mass of the evaporated water (Mwater) and the salt (Msalt) in the sample are given by

Mwater = MwetMdry and
Msalt = Mwater [s/(1 – s)],

where s is the assumed seawater salinity (0.035) corresponding to a pore water density (pw) of 1.024 g/cm3 and a salt density (salt) of 2.257 g/cm3. The corrected mass of pore water (Mpw), volume of pore water (Vpw), mass of solids excluding salt (Msolid), volume of salt (Vsalt), volume of solids excluding salt (Vsolid), and the wet volume (Vwet) are, respectively,

Mpw = Mwater + Msalt = Mwater/(1 – s),
Vpw = Mpw/pw,
Msolid = MdryMsalt,
Vsalt = Msalt/salt,
Vsolid = VdryVsalt = VdryMsalt/salt, and
Vwet = Vsolid + Vpw.

Calculation of Bulk Properties

For all sediment samples, water content (w) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass,

w = Mpw/Mwet.

Bulk density (wet), dry bulk density (dry), sediment grain density (solid), and porosity () are calculated from

wet = Mwet/Vwet,
dry = Msolid/Vwet,
solid = Msolid/Vsolid, and
= Vpw/Vwet.

Compressional Wave Velocity

Velocity was measured on split-core sections using the PWS1 and PWS2 insertion probe system in soft sediments and the PWS3 contact probe system in consolidated sediments and rocks. The insertion probe system allows measurement of the longitudinal (perpendicular to bedding) P-wave velocity (PWS1) and the transverse P-wave velocity (PWS2). The contact probe system (PWS3) measures the transverse velocity across the split-core section and core liner or across samples taken from the cores. In both systems, the P-wave velocity calculation is based on the accurate measurement of the delay time of a 500-kHz acoustic square wave signal traveling between a pair of piezoelectric transducers. Transducer separations of PWS1 and PWS2 are fixed at 6.96 and 3.48 cm, respectively. The transducer pair for PWS3 is adjusted to the thickness of the core half or extracted sample. A linear voltage-displacement transducer measures the separation of the fixed lower PWS3 transducer and the movable upper transducer. Prior to measuring velocity on samples from each hole, the PWS1 and PWS2 transducers were calibrated by inserting the probes in a container of distilled water of known temperature and measuring the acoustic traveltime. The PWS3 transducers were calibrated using the linear regression of traveltime vs. distance across a plastic bag filled with water measured at a variety of thickness.

Routine sampling frequency for P-wave measurements was one per section while using the PWS1 and PWS2 sensors. When the PWS3 sensor was utilized, the sampling frequency was three measurements per section on split cores in Hole A at each site. Measurements of transverse (x- and y-direction) and longitudinal (z-direction) P-wave velocity were conducted on cube samples in Hole B at each site, allowing the calculation of velocity anisotropy using the following equation:

200 (VptVpl)/(Vpt + Vpl),

where,

Vpt = average transverse P-wave velocity and
Vpl = average longitudinal P-wave velocity.

The positions of the P-wave measurements are next to those for MAD analyses. Deionized water was added to the contact between the transducers and sample to improve acoustic coupling when needed. Data reported herein and those stored in the Janus database are uncorrected for in situ temperature and pressure. These corrections can be made using the relationships outlined in Wyllie et al. (1956), Wilson (1960), and Mackenzie (1981) and as applied in Boyce (1976).

In Situ Temperature Measurements and Heat Flow Calculations

A single in situ temperature measurement was made in Hole 1257A in the upper 40 m, which is the only Leg 207 APC-cored interval. The components of the Adara temperature 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 0–100C, with a resolution of 0.01C. 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. Data from the tool provide a sufficiently long transient record for reliable extrapolation of the steady-state temperature. The nominal accuracy of the Adara temperature measurement is 0.1C.

NEXT