CORE PHYSICAL PROPERTIES

Physical properties measurements on Leg 194 provided

1. Near-continuous records for hole-to-hole correlation and construction of complete stratigraphic sequences and downhole log calibration;
2. Sediment properties (density, porosity, natural gamma radiation, magnetic susceptibility [MS], and color reflectance) for comparison with composition, diagenesis, and consolidation history and to help constrain the location of unconformities, sediment fracturing, and fluid migration and expulsion;
3. Data for the calculation of synthetic seismograms (i.e., P-wave velocity and bulk density) and integrated traveltime curves; and
4. Data for the calculation of local heat flow (downhole temperature and thermal conductivity).

Physical properties were measured on unsplit cores, on undisturbed parts of split cores, and on core plugs cut from the split cores. For rock cores (e.g., carbonate platform sediments and basement samples) the liners were split and the curated length of the whole cores was determined prior to MST measurement. The MST was used on whole cores and on some half-core sections (APC, XCB, and RCB) for nondestructive measurements of wet bulk density, P-wave velocity, MS, and natural gamma radiation. Because of their larger diameter relative to cores obtained with usual coring techniques, cores obtained using the ADCB were not analyzed with the MST. Thermal conductivity measurements were conducted on unsplit soft sediment cores and split rock cores. Three-directional P-wave velocities were measured on sediment and rock cores. Moisture content, wet and dry mass, and dry volume were measured from undisturbed samples of split cores, and bulk density, porosity, grain density, and void ratio were calculated. For samples of consolidated sediments, dry mass and wet and dry volume were determined to calculate the same suite of parameters.

To ensure thermal homogeneity for the physical properties measurements, the cores were allowed to come to thermal equilibrium at ambient room temperature (i.e., 22°-25°C). The first measurement station was the MST, which combines four sensors on an automated track. The sensors are the magnetic susceptibility loop (MSL), the gamma ray attenuation (GRA) bulk density, the P-wave logger (PWL), and the natural gamma radiation (NGR) detector. MST measurement intervals and periods for each core section were selected so that physical properties could be accurately characterized in 15 min for a 1.5-m-long section without hindering the flow of core processing through the laboratories. After MST analysis, thermal conductivity was measured on whole sediment cores. For rock cores, the cores were split prior to thermal conductivity measurements, and each measurement was conducted on half-core samples.

The working half of each section was used for further physical properties measurements of P-wave velocity, water content, and grain volume. Water content and grain volume were used to calculate bulk density, porosity, grain density, and related parameters. A summary of each of the physical properties measurement procedures for Leg 194 is outlined below. Blum (1997) provides a detailed description of the physical principles underlying the sampling methods.

MS, bulk density, and NGR were generally measured on all cores that exceeded a minimum length of ~0.5 m regardless of the coring method. P-wave velocity was measured on all APC-cored intervals and some XCB- and RCB-cored intervals using the PWL.

In order to collect measurements, individual unsplit core sections were placed on the MST, which automatically moves the core section through the four sensors on a fiberglass boat. MST data are not continuous as a function of depth because of incomplete recovery, drilling-disturbed intervals, and removal of whole-round sections immediately after coring. The quality of these data are degraded in XCB and RCB sections where the core may be undersized with respect to the liner diameter and/or disturbed. Nevertheless, the general downhole trends are useful for stratigraphic correlations.

High-amplitude MS is a function of the existence and concentration of ferro- and ferrimagnetic minerals such as magnetite, hematite, goethite, and titanomagnetite within sediment. The source of this material may be associated with the coarse sediment fraction of, for example, proximal turbidites and/or as single-domained magnetic material contained with the clay fraction. In the absence of ferro- and ferrimagnetic minerals, the MS is often of low amplitude induced by paramagnetic and diamagnetic minerals such as clays and evaporites. MS reflects changes in magnetic mineralogy and, as a result, is widely used as a proxy for lithologic variations. MS is also used as a proxy for changes in composition that can be linked to lithologic changes and depositional processes.

The MSL was set to measure MS of unsplit core sections using a 5-cm sampling interval and a 4-s sampling period. MS is measured using a Bartington MS2 meter coupled to a MS2C sensor coil with a diameter of 88 cm operating at 565 Hz. The sensor is set to SI units, and data are stored in the ODP database in raw meter units. The sensor coil is sensitive over an interval of ~4 cm, and the width of the sensing region corresponds to a volume of 166 cm³ of cored material. To convert to true SI volume susceptibilities, these values are multiplied first by 10^-5 and then by a correction factor to take into account the volume of material that passed through the susceptibility coils. Except for measurements near the end of each section, the correction factor for a standard full ODP core is ~0.66. The diamagnetic contribution of the carbonates recovered during this leg reduced the value of weak field susceptibility in magnetic analysis because it biases the susceptibility. No correction was applied to the MS values for reduced core volume.

GRA bulk density was estimated for unsplit core sections and occasionally on half-core sections using the GRA sensor and a sampling period of 5 s every 5 cm. For each site, the GRA bulk density is an independent estimate at higher sampling resolution compared with the bulk density determined by the moisture and density (MAD) method. Measurement of GRA density 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).

The PWL measures P-wave velocity across the unsplit core sections using a sampling interval of 5 cm (4-s period). In order to determine the P-wave velocity, the PWL transmits 500-kHz P-wave pulses through the core at a frequency of 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 P-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 and RCB cores. Measuring P-wave velocities on XCB and RCB cores, therefore, is not recommended. However, if the coupling is adequate, the measured velocities often provide an upper limit of the core velocity. Data quality is improved when the coupling between the liner and transducers is maintained by wetting the outside of the liner, which was not achieved during Leg 194. Overall, the resulting PWL data during Leg 194 have been seriously compromised.

As with MS, the NGR amplitude is a function of the terrigenous clay content within sediment. NGR emissions 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 at 90° angles to each other and perpendicular to the core. NGR count typically correlates with increasing clay/shale content and/or diagenesis where uranium is sequestered from seawater and organic matter. Sand-prone and carbonate units usually tend to be characterized by low NGR counts. Ideally, these relationships can be used to define the location of shale-prone and sand-prone formations down the borehole. For siliciclastic units, shale and sand-prone units relate primarily to changes in relative sea level. In particular, coarsening-upward and fining-upward sedimentary sequences are expressed as overall decreasing and increasing NGR signatures, respectively. For carbonate units, lowstand systems tend to result in a decrease in carbonate production and deposition, thereby leading to sediment starvation and glauconite formation and deposition and the formation of hardgrounds.

NGR was measured for 5 s at 5-cm intervals. NGR calibration was performed at the beginning of the leg. For the interval at the top of the hole supported by the pipe, the maximum amplitude of the MST-NGR data was used to correct for the attenuation of the gamma ray wireline log data collected through the pipe.

Thermal conductivity was measured during Leg 194 using the TK04 system described by Blum (1997). This system employs a single-needle probe (Von Herzen and Maxwell, 1959), heated continuously, in full-space configuration for soft sediments and in half-space configuration for hard rock. The needle probe is a thin metal tube that contains a thermistor and a heater wire. The needle 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/4k) · ln(t) + C,

where T is the temperature, q is the heat input per unit length per unit time, k is the thermal conductivity, t is the time after the initiation of the heat, and C is a constant.

For full-space measurements, an aperture was drilled through the outer core liner and the 2 mm-diameter temperature probe inserted between the archive and working half of the core section. After insertion, the probe was heated at 3 W/m and the temperature rise was monitored. The optimal integration time for each conductivity measurement is calculated by an algorithm in the TK04 program.

Consolidated half-core rock specimens were measured for thermal conductivity using the half-space configuration. The needle probe was secured onto the flat surface of the half core. Good coupling with the needle probes was ensured by flattening and smoothing the core surface with carbide grit sandpaper. The samples and needles were then immersed in seawater for a minimum of 15 min prior to measurement.

The temperature analysis began once thermal stability of the sample had been established. Only when the background thermal drift was determined to be <0.04°C/min did the measurement process commence by activating the heater circuit and monitoring the temperature increase in the probe. This technique proved highly sensitive to small variations in ambient temperature. In order to minimize this affect in the case of the half-core measurements, immersion in seawater kept the samples saturated, improved the thermal contact between the needle and the sample, and reduced thermal drift during measurement.

Thermal conductivity measurements were taken at a minimum frequency of one per core (usually Section 2) and at increased frequencies when time allowed. The reported thermal conductivity measurement for each sample was the average of three repeated measurements for the full-space method and four repetitions for the half-space method. Data are reported in W/(m·K) with a stated error of about 5% and precision of 2%. Variations in thermal conductivity are consistent with those in bulk density and porosity. A direct inverse relationship should exist between porosity () and thermal conductivity because of the power law dependence of bulk thermal conductivity (K_bulk) on the solid matrix grain thermal conductivity (K_grain) and the thermal conductivity of the interstitial fluid (K_w) (Keen and Beaumont, 1990). This equation can be expressed as

K_bulk = K_w · K_grain^{(1 - )}.

The observed relationship between the thermal conductivity and porosity can be compared with calculated bulk thermal conductivity using the measured porosity values and grain thermal conductivity values summarized in Table T6 (Keen and Beaumont, 1990).

MAD measurements (water content, wet and dry bulk density, grain density, and porosity) were routinely measured on unconsolidated sediment in 10-mL beakers and/or ~9.5-cm³ cubes cut from consolidated sediment and basement rocks. Both were sampled at a frequency of one per section. Sampling frequency was adjusted as needed to characterize all significant lithologies within a core. In XCB cores, which frequently showed a biscuiting-type disturbance, particular care was taken to sample undisturbed parts of the core and to avoid drilling slurry. Immediately after the beaker samples were collected, wet sediment mass (M_wet) was measured. For the sediment cubes, length, width, and height were measured with a SPI2000 caliper with an accuracy of 0.05 mm in order to calculate the wet bulk volume (V_bulk), which is equivalent to V_wet for the beaker calculation; V_bulk is the product of cube length, width, and height.

Dry sediment mass (M_dry) and dry sediment volume (V_dry) were measured after the beaker and sediment cube samples had dried in a convection oven for 24 hr at a temperature of 105° ± 5°C. After drying and prior to measuring dry mass and volume, the samples were stored in a desiccator for at least 1 hr to cool. Sample mass was determined to a precision of 0.01 g using two Scientech 202 electronic balances to compensate for the ship's motion. Grain volumes were determined using a helium Quantachrome Penta-Pycnometer with a precision of 0.02 cm³. The determination of water content followed the methods of the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1980). Blum (1997) discusses the fundamental phase relations and assumptions for the calculations of all relevant phase relationships summarized below:

1. Unconsolidated (beaker) sediment samples. The mass of the evaporated water (M_water) and the salt (M_salt) in the sample is 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 (_pw) of 1.024 g/cm³ and a salt density (_salt) of 2.257 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 the wet volume (V_wet) are, respectively,

    

   M_pw = M_water + M_salt = M_water/(1- s),

   V_pw = M_pw/_pw ,

   M_solid = M_dry - M_salt ,

   V_salt = M_salt/_salt ,

   V_solid = V_dry - V_salt = V_dry - M_salt/_salt , and

   V_wet = V_solid + V_pw ,

    

   where M_dry and V_dry are the dry mass and volume that include the salt precipitated in the pores during the drying process.

2. Consolidated sediment cubes. For the consolidated sediment cubes, wet volume (V_wet) and wet mass (M_wet) are determined from

    

   V_pw - V_water = V_bulk - V_dry ,

   M_pw = V_{pw pw} ,

   M_salt = M_pw s,

   V_salt = M_salt/_salt ,

   V_solid = V_dry - V_salt ,

   M_water = M_pw [(1 - s)/s],

   V_wet - V_bulk , and

   M_wet = M_dry + M_water .

    

3. Calculation of MAD parameters. For all sediment samples (beakers and cubes), wet water content (w_wet) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass, and the dry water content (w_dry) is the ratio of the mass of pore water to the mass of solids (excluding salt):

    

   w_wet = M_pw/M_wet and

   w_dry = M_pw/M_solid .

In turn, bulk density (_wet), dry bulk density (_dry), sediment grain (solid) density (_solid), porosity (), and void ratio (e) are calculated from

_wet = M_wet/V_wet ,

_dry = M_solid/V_wet or M_solid/V_bulk ,

_solid = M_solid/V_solid ,

= V_pw/V_wet , and

e = V_pw/V_solid .

Porosity behavior as a function of depth is often described in terms of Athy's law (Athy, 1930). This empirical relationship presumes a negative exponential relationship between depth and porosity:

(z) = _o e^-kz,

where (z) is the porosity as a function of depth z, _o is the surface porosity, and k controls the rate of decay of porosity with depth. A least-squares fit to this equation can be applied to estimate the surface porosity _o and the rate of decay of porosity. Variations from the general curve are often diagnostic of grain size and composition, facies variations, and interstitial fluid pressure disequilibrium.

Velocity was measured using three pairs of perpendicularly oriented P-wave sensors (PWS1, PWS2, and PWS3). The method chosen for P-wave velocity measurements (V_P) was dependent on the degree of sediment consolidation. For unconsolidated sediments, the PWS1 (z-direction) and PWS2 (y-direction) insertion probe systems were used to measure the transverse (across the core axis) and longitudinal (along the core axis) P-wave velocity and seismic anisotropy. PWS1 and PWS2 transducer pairs, which have a fixed spacing of 7 cm (vertical) and 3.5 cm (horizontal), were inserted into the soft-sediment split cores. The PWS3 system uses a vertically oriented transducer pair that can be used with or without a liner correction. Alternatively, sample cubes or cylinders can also be measured. An acoustic signal of 500 kHz was transmitted and received by the two transducers. Analog to digital transformation of the signal allowed the seismic signal to be displayed on a digital oscilloscope so that the first-arrival waveform could be manually picked and velocity calculated. Zero traveltimes for the velocity transducers were measured using water (assumed to be at ambient temperature). To improve the coupling between the transducer and sample, distilled water was applied to the transducer/receiver heads. Measurements were corrected for the additional traveltime required to pass through the core liner.

The measured velocities can be compared with the time-average equation of Wyllie et al. (1956), which states that the traveltime of an acoustic signal through rock is the sum of the traveltime through the solid matrix and the fluid phase:

1/V_rock = (1 - )/V_matrix + /V_fluid.

In general, the matrix is assumed to be calcite (V_matrix = 6530 m/s), and the pore fluid is assumed to be seawater (V_fluid = 1500 m/s). Often, the time-average equation provides a lower envelope for carbonate sediments. Deviations from the time-average equation are explained by different kinds of pore types (Anselmetti and Eberli, 1993). Moldic porosity shows a positive deviation from the time-average equation because the pores are integrated in a rigid framework. This type of porosity is common in the platform carbonates of the Leg 194 sites.

Anisotropy is determined using the difference between the average horizontal and vertical velocity using the following equation:

Anisotropy = 2(V_Pt - V_Pl)/(V_Pt + V_Pl),

where V_Pt is the average transverse P-wave velocity and V_Pl is the longitudinal velocity. The velocity meter was calibrated by measuring V_P in distilled water. In unconsolidated sediments, at least one P-wave velocity measurement per section was made using one or both of the PWS1 and PWS2 systems.

Color was measured on split-core surfaces (of the archive half) using diffuse-reflected spectrophotometry. Light reflected from the material is collected in an integration sphere, normalized to the source light of the reflectance, and calibrated with the measurement of a pure white standard (100% reflection) and a black box (zero reflection) over the entire wavelength spectrum of visible light. Reflectance spectra are related to color using established international conventions. Shipboard reflectance was measured using an automated Minolta Photospectrometer (CM-2002) that measures the spectral reflectance of surfaces with a diameter of >8 mm. The instrument combines measurement, data processing, and display functions in a single unit. To ensure accuracy, the CM-2002 uses a double-beam feedback system, monitoring the illumination on the specimen at the time of measurement and automatically compensating for any changes in the intensity or spectral distribution of the light.

Color reflectance was measured on the archive halves of split cores to provide quantitative descriptions of sediment color. The color and variations in color can be used to interpret variations that occur within one core or between cores. A quantitative description allows the data to be analyzed in a formal manner and correlated with other data sets (e.g., NGR, carbonate content).

The two most common uses of color reflectance data are (1) color components such as L*a*b*, which provide detailed time series of relative changes in the composition of the bulk material and are frequently used to correlate sections from core to core or hole to hole and to analyze the cyclicity of lithologic changes, and (2) spectral data, which can be used to estimate the abundance of certain minerals. The first type of investigation, referred to as colorimetry, is simple and straightforward and is used for empirical correlations with other physical properties data and/or lithologic observations.

Color is reported using the L*a*b* system. It can be visualized as a cylindrical coordinate system in which the axis of the cylinder is the lightness component L*, ranging from 0% to 100%, and the radii are the chromaticity components a* and b*. Component a* is the green (negative) to red (positive) axis, and component b* is the blue (negative) to yellow (positive) axis. For Leg 194, the lightness (L*) component was used as a proxy for carbonate vs. terrigenous content, and the green-red component (a*) was used as a proxy for clay and glauconite concentration/variation within sediments.

In situ temperature measurements were made using either an advanced piston core temperature (APCT) tool or a Davis-Villinger temperature probe (DVTP). The APCT tool fits directly into the coring shoe of the APC and consists of a battery pack, a data logger, and a platinum resistance temperature device calibrated over a temperature range from 0° to 30°C. Before entering the borehole, the tool was first briefly stopped at the mudline to thermally equilibrate with bottom water. After the APCT penetrated the sediment, it was held in place for 10 min and the APCT instrument recorded the temperature of the cutting shoe every 10 s. Initially, there was an instantaneous temperature rise due to frictional heating caused by APCT tool penetration, which gradually dissipates into the surrounding sediments. An equilibrium sediment temperature was then estimated by applying a mathematical heat-conduction model to the temperature decay record (Horai and Von Herzen, 1985). Additional information on the APCT tool can be found in Fisher and Becker (1993).

The DVTP tool is used in semilithified sediments, which the APCT tool cannot penetrate, and unlike the APCT, 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. The thermistor 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.

For shallow-water sites, a longer mudline stop was required to ensure that the temperature tools had sufficient time to equilibrate to bottom-water temperatures. At deeper sites, this time was reduced as the tools were able to thermally equilibrate during descent through deeper waters with very low thermal gradients. This problem is less serious with the DVTP because it has a lower heat capacity and a thermal time constant that is less than that of the APCT tool.

Data reduction procedures are similar for both temperature tools. The synthetic thermal decay curves for the APCT 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 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, 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.