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doi:10.2204/iodp.proc.320321.102.2010

Physical properties

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 Expedition 320/321 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 intervals of split sections using two different track systems, the STMSL and the WRMSL. Sections from intervals that overlapped with those cored in earlier holes were run through the STMSL immediately after being brought in from the catwalk to aid in preliminary determinations of depth offsets in the composite section (see "Stratigraphic correlation and composite section"). Following STMSL measurements and prior to being run through the WRMSL, sections were allowed to thermally equilibrate to ambient room temperature (i.e., 20°–22°C) to ensure thermal homogeneity for the physical property measurements. The STMSL incorporates a GRA bulk densitometer and a magnetic susceptibility sensor. The WRMSL incorporates a GRA bulk densitometer, a magnetic susceptibility sensor, a compressional P-wave velocity sensor (PWL), and a noncontact resistivity (NCR) detector. The NCR detector did not operate properly during Expedition 320, was switched off after scanning cores from Site U1332, and was not used during Expedition 321. Natural gamma radiation (NGR) and thermal conductivity also were measured on whole-round sections using the NGR logger and the needle-probe method, respectively. Compressional wave velocity was measured on split-core sections, and MAD measurements were made on discrete samples taken from the split cores. Bulk properties determined by MAD analyses include wet bulk density, dry bulk density, grain density, water content, and porosity. Color reflectance was measured on the split-core archive sections with the SHMSL. A comprehensive discussion of all methodologies and calculations used in the JOIDES Resolution physical properties laboratory is presented in Blum (1997).

Special Task Multisensor Logger measurements

The principal aim of acquiring STMSL data during Expedition 320/321 was to obtain rapid, medium- to high-resolution data sets of GRA and magnetic susceptibility to determine offsets in the composite section during the drilling of multiple-hole sites. The GRA bulk densitometer and magnetic susceptibility loop incorporated in the STMSL are essentially identical to those of the WRMSL (see "Whole-Round Multisensor Logger measurements"). The STMSL GRA densitometer was not operational during Expedition 320. The typical measurement interval for the STMSL was 5 cm.

Whole-Round Multisensor Logger measurements

Collection of high-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 had to be completed within a reasonable time frame without compromising the shipboard processing of recovered core. The quality of the WRMSL data is highly dependent on the condition of the core.

GRA bulk density, magnetic susceptibility, compressional wave velocity, and NCR were measured nondestructively with the WRMSL on all whole-round core sections. To optimize WRMSL performance, sampling intervals and measurement residence times were the same for all sensors for any one core. Sampling intervals were set at 2.5 cm so that a 9.5 m long core would take ~3 h to pass through the WRMSL with a residence time of 3 s for each measurement. These sampling intervals are common denominators of the distances between the sensors installed on the WRMSL (30–50 cm) and allow a combination of sequential and simultaneous measurements optimize the total measurement time.

Gamma ray attenuation bulk density

Bulk density reflects the combined effect of variations in porosity, grain density (dominant mineralogy), and coring disturbance. Porosity is mainly controlled by lithology and texture, compaction, and cementation (controlled by both mechanical and chemical processes).

The GRA densitometer uses a 10 mCi ¹³⁷Cs capsule as the gamma ray source (with the principal energy peak at 0.662 MeV) and a scintillation detector. The narrow collimated peak is attenuated as it passes through the center of the core. Incident photons are scattered by the electrons of the sediment material by Compton scattering.

The attenuation of the incident intensity (I₀) is directly related to the electron density in the sediment core of diameter (D), which can be related to bulk density given the average attenuation coefficient (in micrometers) of the sediment (Evans, 1965; Harms and Choquette, 1965). Because the attenuation coefficient is similar for most common minerals and aluminum, bulk density is obtained through direct calibration of the densitometer using aluminum rods of different diameters mounted in a core liner that is filled with distilled water. The GRA densitometer has a spatial resolution of <1 cm.

Magnetic susceptibility

Magnetic susceptibility is a measure of the degree to which a material can be magnetized by an external magnetic field. It provides information on the magnetic composition of the sediments that often can be related to mineralogical composition (e.g., terrigenous versus biogenic materials) and diagenetic overprinting. Magnetite and a few other iron oxides with ferromagnetic characteristics have a specific magnetic susceptibility several orders of magnitude higher than clay, which has paramagnetic properties. Carbonate, silica, water, and plastics (core liner) have small negative values of magnetic susceptibility. Sediments rich in biogenic carbonate and opal therefore have generally low to negative magnetic susceptibility if practically no clay or magnetite is present. In such cases, measured values approach the detection limit of magnetic susceptibility sensors.

Magnetic susceptibility was measured with the Bartington Instruments MS2C system on the WRMSL and STMSL. The frequency at which the magnetic susceptibility loop operates is 621 Hz for the WRMSL and 513 Hz for the STMSL. The output of the magnetic susceptibility sensors can be set to centimeter-gram-second (cgs) units or SI units. The IODP standard is the SI setting. However, to actually obtain the dimensionless SI volume-specific magnetic susceptibility values, the instrument units stored in the IODP database must be multiplied by a correction factor to compensate for instrument scaling and the geometric ratio between core and loop dimensions.

Compressional wave velocity

The P-wave velocity sensor measures the ultrasonic P-wave velocity of the whole-round sample in the core liner. The PWL transmits a 500 kHz P-wave pulse through the core section at a specified repetition rate. This signal is coupled to the sample by the plastic pole pieces of the transducers and the pressure applied by the linear actuator. No water is used to improve coupling between the transducers and the liner. The pressure applied by the actuator is sufficient for reliable P-wave measurement. The transmitting and receiving ultrasonic transducers are aligned so that wave propagation is perpendicular to the section's long axis. Torque applied by the actuator can be set by the user to ensure good acoustic contact between the liner and the core material.

Traveltime is determined by signal processing software which automatically detects first arrival of the P-wave signal to a precision of 50 ns. It is a challenge for an automated routine to pick the first arrival of a potentially weak signal with significant background noise. The search method skips the first positive amplitude and finds the second positive amplitude using a detection threshold limit, typically set to 30% of the maximum amplitude of the signal. It then finds the preceding zero crossing and subtracts one period to determine the first arrival. To avoid extremely weak signals, minimum signal strength can be set (typically 0.02 V) and weaker signals are ignored. To avoid cross-talk signals at the beginning of the record from the receiver, a delay (typically 0.01 ms) can be set to force the amplitude search to begin in the quiet interval preceding the first arrival. In addition, a trigger (typically 4 V) is selected to initiate the arrival search process, and the number of waveforms to be stacked (typically five) can also be set. A linear voltage differential transformer is used to measure the separation of the transducer to derive a travel path length for the signal (i.e., the core diameter). After corrections for system propagation delay, liner thickness, and liner material velocity, the ultrasonic P-wave velocity is calculated.

P-wave velocity varies with lithology, porosity, and bulk density of material; state of stress; temperature; and fabric or degree of fracturing. In marine sediments and rocks, velocity is controlled by degree of consolidation and lithification, fracturing, and occurrence and abundance of free gas and gas hydrate. Together with bulk density, velocity data are used to derive porosities and to calculate acoustic impedance and reflection coefficients that can be used to construct synthetic seismic profiles and to estimate the depth of seismic horizons.

Noncontact resistivity

The NCR sensor measures the resistivity of the whole-core section. It can be used to investigate core properties in terms of the fluid/solid phases of the sediment or rock and type of porosity. The NCR induces a high-frequency magnetic field in the core from a transmitter coil that induces eddy currents in the core that are inversely proportional to the resistivity. A receiver coil measures very small magnetic fields that are regenerated by the eddy current. To accurately measure these very small magnetic fields, a difference technique compares the readings generated from the core measuring coils to the readings from an identical set of coils operating in the air. Resistivities between 0.1 and 10 Ωm can be measured using this sensor.

Factors affecting noncontact resistivity results include magnetic susceptibility, pore water salinity, porosity, and mineral composition. Noncontact resistivity measurements provide important information that can be used to understand sediment facies and geochemical processes. Combination logs of resistivity and density provide lithologic information that cannot be achieved with other nondestructive measurement methods, such as grain size, permeability, and tortuosity. Because resistivity is influenced by salinity, NCR data are valuable as a continuous log for geochemical interpretation.

The NCR detector was not performing during Expedition 320 and was switched off after scanning cores from Site U1332. The nature of the problem is unclear, but it could not be remedied by moving the detector closer to the core. The NCR detector was kept switched off during Expedition 321.

Natural gamma radiation

The NGR logger was designed and built at the IODP-USIO Texas A&M University facility from 2006 to 2008. The NGR logger measures gamma rays emitted from whole-round core sections. Gamma rays arise primarily as a result of the decay of U, Th, and K isotopes. Data generated from this instrument are used to augment geologic interpretations and fine tune stratigraphic correlations.

The main NGR detector unit consists of 8 NaI scintillator detectors, 7 plastic scintillator detectors, 22 photomultipliers, and passive lead shielding. The NaI detectors are covered by at least 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI detectors. Half of the lead shielding closest to the NaI detectors is composed of low-background lead, whereas the outer half is composed of regular (virgin) lead. In addition to this passive lead shielding, the NGR employs a plastic scintillator to suppress the high-energy gamma and muon components of cosmic radiation by producing a veto signal when charged particles from cosmic radiation pass through the plastic scintillator.

A measurement run consists of two positions on each core section counted for at least 5 min each for a total of 16 measurements per section. Complete spectra for each measurement are uploaded to the LIMS.

NGR measurements collected from Holes U1331B and U1331C and all holes at Sites U1332–U1334 were measured without the use of the active shielding feature. This was unintentional and was caused by a power failure early during the expedition. The shield was turned back on before the cores from the last two sites (U1335 and U1336) were measured. Lack of active shielding resulted in elevated background counts. Furthermore, because nobody was initially aware of this situation, the routine data reduction on the ship used background data acquired with the active shielding on. This resulted in background-corrected values that were generally too high by ~4 cps, and the edge effect correction created additional, clearly visible artifacts in the form of "spikes" at the section ends.

The incorrectly processed data were reprocessed postcruise using appropriate background data collected without active shielding before the expedition. The incorrectly processed data were replaced with the correctly processed data in the LIMS database and the comment "Active shielding OFF" was added to each replacement record.

Thermal conductivity

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). During Expedition 320/321 a foam insulating jacket was used to prevent temperature changes in the section being analyzed for thermal conductivity (a heating/ventilation/air conditioning vent is located directly over the thermal conductivity station in the core laboratory). The needle probe contains a heater wire and calibrated thermistor. Thermal transfer compound was used to improve the coupling between the needle and the sediment. The probe 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,

T(t) = (q/4πk) × ln(t) + C,

where

T = temperature (K),

q = heat input per unit length per unit time (J/m/s),

k = thermal conductivity (W/[m·K]),

t = time after initiation of heat (s), and

C = a constant.

Thermal conductivity was measured by inserting the needle into the unconsolidated sediment through a small, 2 mm hole drilled into the core liner. Five 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 (typically in Section 3 of Hole A cores), into which the TK04 needles could be inserted without risk of damage.

Moisture and density

Wet and dry bulk density, 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, usually from the middle of each core section, and placed in preweighed 16 mL Wheaton beakers. Stiff sediments drilled with XCB were sampled, where appropriate, by extracting ~10 cm³ blocks of in situ sections using a spatula and placed into a beaker as above. One sample was routinely collected in each undisturbed section from Hole A. Samples were taken in subsequent holes when gaps in Hole A needed to be spliced.

Sample mass was determined to a precision of 0.005 g using two Mettler Toledo electronic balances and a computer averaging system to compensate for the ship's motion. Sample volumes were determined using a hexapycnometer system that consists of six custom-configured Micromeritics AccuPyc 1330TC helium-displacement pycnometers with a precision of 1% of the nominal full-scale volume. Volume measurements were preceded by three purges of the sample chambers with helium warmed to ~25°C. Three acquisition cycles were used for each sample. A reference volume was included within each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. 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 h and allowed to cool in a desiccator. Procedures for the determination of these properties comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1990). Fundamental phase relationships and assumptions for the calculations of all physical property parameters are discussed by Blum (1997) and summarized in "Mass and volume calculation" and "Calculation of bulk properties."

Mass and volume calculation

Wet mass (M_wet), dry mass (M_dry), and dry volume (V_dry) are measured in the laboratory. The mass ratio (rm) is a computational constant of 0.965 (i.e., 0.965 g of freshwater per 1 g of seawater). Salt precipitated in sediment pores during the drying process is included in the M_dry and V_dry values. The mass of the evaporated water (M_water) and the salt (M_salt) in the sample are given by

M_water = M_wet – M_dry

and

M_salt = M_water [s/(1 – s)],

where s is the assumed saltwater salinity (0.035) corresponding to a pore water density (ρ_pw) of 1.024 g/cm³ and a salt density (ρ_salt) of 2.22 g/cm³. The corrected mass of pore water (M_pw), volume of pore water (V_pw), mass of solids excluding salt (M_solid), volume of salt (V_salt), volume of solids excluding salt (V_solid), and wet volume (V_wet) are, respectively,

M_pw = (M_wet – M_dry)/rm,

V_pw = M_pw/ρ_pw,

M_solid = M_wet – M_pw,

M_salt = M_pw – (M_wet – M_dry),

V_salt = M_salt/ρ_salt,

V_wet = V_dry – V_salt + V_pw,

and

V_solid = V_wet – V_pw.

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 = M_pw/M_wet.

Wet bulk density (ρ_wet), dry bulk density (ρ_dry), sediment grain density (ρ_solid), porosity (ϕ), and void ratio (VR) are calculated from

ρ_wet = M_wet/V_wet,

ρ_dry = M_solid/V_wet,

ρ_solid = M_solid/V_solid,

ϕ = V_pw/V_wet,

and

VR = V_pw/V_solid.

P-wave velocity

The Section Half Velocity Gantry measures the ultrasonic velocity of materials placed between its transducers in the x-, y-, and z-directions on undisturbed working-half split-core sections from Hole A. Routine sampling frequency for P-wave measurements on the x-, y-, and z-axes was one per section, positioned next to MAD discrete samples. Velocity is measured in the x-direction using a caliper velocimeter (the contact probe) and in the y-, and z-directions using pairs of piezoelectric transducers (the insertion probe). The method of measurement is to derive an estimate of the traveltime and accurately measure the path length of the ultrasonic P-wave through the sample, with velocity (m/s) = path length (m)/traveltime (s) = dS/dt.

Traveltime measurement is estimated by an algorithm for graphical first arrival pick. An ultrasonic pulser generates a high-impulse voltage which is applied to the ultrasonic transmitter to induce oscillation of the crystal element at the required frequency. A trigger pulse from the pulser is applied to the oscilloscope to start the recording of the waveform from the receiving transducer. By recognizing unique features of the waveform, an estimate of the time for the ultrasonic pulse to propagate through the system is derived. This time includes traveltime through the sample and propagation system delays introduced by various components. A system delay time is calculated during the calibration process. Subtraction of the system delay time (and liner material propagation time, if required) from the total traveltime gives the traveltime for the ultrasonic pulse through the sample. Precise lengths for the sample are derived from a linear variable displacement transducer. The transducer converts changes in physical position into an alternating current electrical output and requires calibration, which is performed at the same time as the system delay is derived. The chisel transducers are fixed at 8.2 cm for the z (downhole)-axis and 3.44 cm for the y-axis (IODP axis designation). The x-axis separation is derived from a linear differential voltage transformer and calibration constants.

Section-Half Multisensor Logger measurements

The sensors included in the SHMSL are reflectance spectroscopy and colorimetry and magnetic susceptibility. Magnetic susceptibility data were not captured on this system during Expedition 320/321 because the magnetic properties of the brackets upon which the track is mounted cause artifacts in the data that cannot be corrected automatically using the software currently available. Not collecting susceptibility data allowed the collection of color reflectance data at a higher resolution than would otherwise have been possible in the time available. Two spectrophotometers were used during Expedition 320/321. A Minolta spectrophotometer (model CM-202) was used at Sites U1331–U1334, and an Ocean Optics spectrophotometer (model USB4000) was used at Sites U1335–U1338. Both spectrophotometers provide a high-resolution stratigraphic record of color variations. At each measurement depth the Minolta spectrophotometer records a spectra of 31 determinations at 10 nm intervals over the 400 to 700 nm wavelength band. The Ocean Optics spectrophotometer records a spectra with 3840 determinations spaced at ~0.2 nm between 177 and 925 nm at each measurement depth. The switch to the Ocean Optics spectrophotometer occurred after initial concerns about the linearity of the Ocean Optics data were alleviated.

Measurements were made on the archive halves of freshly split cores that were covered with clear plastic wrap. The SHMSL skips empty intervals and intervals where the core surface is well below the level of the core liner but does not recognize relatively small cracks or disturbed areas of core. Thus, SHMSL data may contain spurious measurements that should, to the extent possible, be edited out of the data set before use. Data are generated using the L*a*b* color system (Fig. F13), which describes colors by their position along three axes: (1) L*, which represents the white to black content (high to low values); (2) a*, which ranges from blue (negative) to yellow (positive); and (3) b*, which ranges from green (negative) to red (positive). During Expedition 320/321, data were captured at a minimum sample spacing of 2.5 cm, but through critical intervals this spacing was reduced to achieve higher resolution data (1 cm).

Percentage reflectance and color spectral data can support lithology description (Balsam et al., 1997; Balsam and Damuth, 2000). Color parameters may be used to obtain a detailed time series of relative changes in composition that can be used to correlate sections and analyze cyclicity. Spectral data can be used to estimate the abundances of certain compounds: visible range provides a semiquantitative estimate of hematite and goethite, whereas near-infrared and near-ultraviolet ranges allow estimation of carbonate, opal, organic matter, chlorite, and some combinations of clay minerals.

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