-
T(t) = (q/4
k) x ln(t) + C,
Shipboard measurements of physical properties provide information that assists in characterization of lithologic units, correlation of lithology with downhole geophysical logging data, assessment of the consolidation history, and interpretation of seismic reflection profiles. The primary objectives of the Leg 199 physical properties program were to collect high-resolution data to: (1) provide bulk density data for determination of mass accumulation rates, (2) facilitate hole-to-hole and site-to-site correlation and construction of composite stratigraphic sections, (3) enable postcruise cyclostratigraphy studies, (4) facilitate construction of synthetic seismic profiles, and (5) investigate the characteristics of major seismic reflectors.
Physical properties were measured on whole-round sections and undisturbed parts of split cores. Nondestructive measurements of wet bulk density, MS, transverse compressional wave (P-wave) velocity, and natural gamma radiation were made on whole-round sections using the MST. The MST incorporates a GRA bulk density device, a P-wave logger (PWL), a MS meter, and a natural gamma radiation sensor. Thermal conductivity, using the needle-probe method, was measured on whole-round sections. Compressional wave velocity and moisture and density (MAD) measurements were made on split-core sections. Bulk properties determined by MAD analyses included wet bulk density, DBD, grain density, water content, and porosity. Light absorption spectroscopy (LAS) analyses were performed on all MAD samples as well as an additional one sample per section. Prior to the whole-round measurements, the cores were allowed to come to thermal equilibrium at ambient room temperature (i.e., 20°-22°C) to ensure thermal homogeneity for the physical properties measurements. A comprehensive discussion of all methodologies and calculations used in the JOIDES Resolution physical properties laboratory can be found in Blum (1997).
In situ temperature measurements were made using the Adara temperature tool as part of regular APC coring operations. These data were combined with measurements of thermal conductivity to calculate heat flow.
The principal aim of MST data acquisition during Leg 199 was to obtain high sampling resolution data sets, especially of GRA bulk density and MS, 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. The quality of the MST data is highly dependent on the condition of the core.
The measurement of wet bulk density by the GRA system is based on the principle that the attenuation, mainly by Compton scattering, of a collimated beam of gamma rays produced by a 137Ce source passing through a known volume of sediment is related to material density (Evans, 1965). 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 cores, GRA measurements are of lower quality and typically 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 4.0 cm for Leg 199 cores. The minimum integration time for a statistically significant GRA measurement is 1 s, and routine Leg 199 GRA measurements used either a 3- or 5-s integration time. A freshwater control was run with each section to measure instrument drift.
Whole-core MS was measured with the MST using a Bartington MS2 meter coupled to a MS2C sensor coil with an internal diameter of 8.8 cm operating at 565 Hz. The measurement resolution of the magnetic susceptibility motion sensor is 4 cm, with a minimum statistically significant count time of 1 s. During Leg 199, MST MS was routinely measured at a spacing of 2.0 cm, with five data acquisitions. MS data were archived and are displayed as raw instrument units (SI) and are not corrected for changes in sediment volume (for details see Blum, 1997) although a correction was made for instrument drift.
Transverse P-wave velocity was measured on the MST track with the PWL for all APC cores. The use of the PWL on XCB cores was limited by poor acoustic coupling between the sediment and the core 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, and a pair of displacement transducers monitors the separation between the compressional wave transducers. Variations in the outer diameter of the liner do not degrade the accuracy of the velocities, but the unconsolidated sediment or rock core must completely fill the liner for the PWL to provide acoustic coupling, which is often not the case with XCB 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. 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 2.0 cm for Leg 199 APC cores.
NGR emissions of sediments are a function of the random and discrete decay of radioactive isotopes, predominantly those of U, Th, and K, and are measured through scintillating detectors arranged 90° to each other and perpendicular to the core. 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 cps during a 1-hr measurement period. Before taking measurements, each of the four NGR amplifiers were adjusted so that the Th 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. No corrections were made to NGR data obtained from XCB cores to account for sediment incompletely filling the core liner.
The thermal conductivity was measured with the TK04 (Teka Bolin) system using the needle-probe method in full-space configuration for soft sediments (Von Herzen and Maxwell, 1959). The needle probe contains a heater wire and calibrated thermistor. It is assumed to be a perfect conductor because it is much more conductive than unconsolidated sediments. With this assumption, the temperature of the superconductive probe has a linear relationship with the natural logarithm of the time after the initiation of the heat:
k) x ln(t) + C,
where,
The thermal conductivity was measured by inserting the needle into the unconsolidated sediment through a small hole drilled into the core liner. Three measuring cycles were automatically performed at each sampling location and used to calculate an average conductivity. A self-test, which included a drift study, was conducted at the beginning of each cycle. 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, Blum, 1997). Measurement errors were 5%-10%. At sites where in situ temperatures were measured, thermal conductivity was corrected for in situ temperature and pressure as part of the calculation of heat flow. Thermal conductivity measurements were taken with a frequency of one per core in soft sediments, into which the TK04 needles could be inserted without risk of damage.
Wet and DBD, grain density, water content, and porosity were determined from measurements of wet sediment mass, dry sediment mass, and dry sediment volume. In soft sediments, samples of ~10 cm3 were extracted and placed in 10-mL beakers. Stiff sediments drilled with the XCB were cut into cubes using the dual-bladed diamond saw and placed in 50-mL beakers or processed without beakers. One sample was routinely collected in each section from Hole A. Samples were taken in subsequent holes when gaps in Hole A needed to be filled.
Sample mass was determined to a precision of 0.01 g using two Scientech 202 electronic balances and a computer averaging system to compensate for the ship's motion. Sample volumes were determined using a Quantachrome penta-pycnometer (a helium-displacement pycnometer) with a precision of 0.02 cm3. Volume measurements were repeated five 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. Sample beakers used for discrete determination of moisture and density were calibrated before the cruise. Dry mass and volume were measured after samples were heated in an oven at 105° ± 5°C for 24 hr and allowed to cool in a desiccator. The procedures for the determination of these properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1990). Blum (1997) discusses the fundamental phase relations and assumptions for the calculations of all relevant phase relationships summarized below.
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
where s = the assumed saltwater 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,
pwt ,
salt,
salt, and
For all sediment samples, water content (w) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass,
Wet bulk density (wet), DBD (dry), sediment grain (solid) density (solid), and porosity (
) are calculated from
wet = Mwet/Vwet,
dry = Msolid/Vwet,
solid = Msolid/Vsolid, and
= Vpw/Vwet.
LAS studies were conducted on the MAD sample residues as well as an additional one sample per section (located ~50 cm from the MAD sample). The ~10-cm3 samples were heated in an oven at 105° ± 5°C for 24 hr and allowed to cool in a desiccator. Samples need to be dry, otherwise water completely dominates the spectral signature. Samples were then crushed for improved analyses. Light reflectance, at a bandwidth of 350-2500 nm, was found for each sample using the FieldSpec Pro FR portable spectroradiometer. Semiquantitative mineral concentrations were then calculated from the collected spectra, assuming a four-component system: calcite, opal, smectite, and illite. For a complete description of the LAS technique and calibration methods, refer to Vanden Berg and Jarrard, this volume.
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 firm 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 compressional wave velocity calculation is based on the accurate measurement of the delay time of a 500-kHz 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. The separation of the fixed lower PWS3 transducer and the movable, upper transducer is measured by a linear voltage-displacement 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 traveltime. The PWS3 transducers were calibrated using the linear regression of traveltime vs. distance for a set of Lucite standards.
Routine sampling frequency for P-wave measurements was one per section. 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. The core temperature was recorded at the time velocity was measured; however, the velocity data 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).
Velocity anisotropy was calculated where both longitudinal and transverse compressional wave velocity were measured. Anisotropy is determined from the difference between the average horizontal and vertical velocity using the following equation:
where,
The Adara tool was used to obtain in situ sediment temperature measurements during 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 is calibrated over a range of 0°-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 and is 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.
Data reduction for the Adara tool estimates the steady-state, bottom-hole temperature by forward modeling the recorded transient temperature curve as a function of time. The shape of the transient temperature curve is determined by the response function of the tool and the thermal properties of the bottom-hole sediment (Bullard, 1954; Horai and Von Herzen, 1985). A synthetic curve is constructed based on the tool geometry, sampling interval, and the properties of the tool and surrounding sediments. It is difficult 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, which means that the exact time of penetration is always uncertain. As a result, both the effective penetration time and equilibrium temperature must be estimated by applying a fitting procedure that involves shifting the synthetic curves in time to obtain a match with the recorded data.
Heat flow was determined by the Bullard method, as outlined by Pribnow et al. (2000). This method assumes a linear relation between the temperature (T) and thermal resistance (
) of the sediments:
(z),
where,
with zi and zi - 1 bottom and top depths, respectively, of a horizontal layer with thermal conductivity (
i). The number of layers between the surface and depth (z) is I. Heat flow is calculated by plotting (z) vs. T(z) (the Bullard plot) and using linear regression to determine q from the slope of the best-fit line. Determination of thermal resistance depends on the downhole variation in thermal conductivity, which varies as a function of lithology at the Leg 199 sites. Radiolarian-rich sediments are characterized by nearly constant thermal conductivity, and an average value was used for determination of thermal resistance. Clays and nannofossil oozes typically display a downhole decrease in porosity and increase in thermal conductivity, which was modeled using the assumption of a linear increase in conductivity with depth. Because thermal conductivity is dependent on temperature and pressure, laboratory conductivity measurements were corrected for the appropriate in situ conditions using the correction of Hyndman et al. (1974):
P,T(z) = lab x {1 + (zw + z)/1829 x 100 + [T(z) - Tlab]/4 x 100},
where,
P,T(z) = in situ thermal conductivity at depth z (mbsf),
lab = thermal conductivity measured in the lab,
= mean sediment density (in grams per cubic centimeter),
