-
T(t) = (q/4xk) ln(4
t/Ba2),
Physical property measurements made during Leg 210 provided (1) sediment properties (density, porosity, natural gamma radiation [NGR], and magnetic susceptibility) to compare with composition, diagenesis, and consolidation history and to help constrain the location of unconformities, sediment fracturing, and fluid migration and expulsion; (2) compressional wave (P-wave) velocity and bulk density to calculate synthetic seismograms and integrated traveltime curves; and (3) near-continuous records of density, velocity, NGR, and magnetic susceptibility to help define lithologic composition and stratigraphic cyclicity.
Physical properties were measured on unsplit cores, undisturbed parts of split cores, rock cubes selected from the split cores, and cylinders prepared from the rock cubes. For rock cores (e.g., indurated sediment and basement samples), the liners were split and the curated lengths of the whole cores were determined prior to measurement on the MST. The MST was used on whole-core RCB sections for nondestructive measurements of wet bulk density, magnetic susceptibility, and NGR. Thermal conductivity measurements were conducted on split rock cores. Three-directional P-wave velocity was measured on rock cubes. Wet and dry mass and dry volume were measured from rock cylinders prepared from the rock cubes, and moisture content, bulk density, porosity, grain density, and void ratio were calculated.
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 densiometer, the P-wave logger (not used), and the NGR detector. MST sampling intervals and measurement times for each core section were selected so that physical properties could be accurately characterized in 20 min for a 1.5-m-long section without hindering the flow of core processing through the laboratories (Table T7). After MST analysis, the cores were split and thermal conductivity measurements were conducted on half-core samples.
The working half of each section was used for further physical property 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 using the moisture and density (MAD) method C for lithified sediment and rock samples. A summary of each of the physical property measurement procedures for Leg 210 is outlined below. Blum (1997) provides a detailed description of the physical principles underlying the methods.
In order to collect measurements, individual unsplit core sections were placed on the MST, which automatically moved 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 (in places) of whole-round sections immediately following core arrival on deck. The quality of these data is also degraded in RCB sections where the core may be undersized with respect to the liner diameter and/or disturbed. Nevertheless, the general downhole trends and peak amplitudes may be useful for stratigraphic correlations or comparison with logging data.
GRA bulk density, magnetic susceptibility, and NGR were measured on all cores that exceeded a minimum length of ~0.5 m. The top and bottom 5 cm of each section were not included in the MST measurements to avoid using data from disturbed parts of the core. P-wave velocity was not measured on the MST because of degraded core continuity and volume caused by RCB drilling.
High-amplitude magnetic susceptibility primarily indicates the presence and concentration of ferro- and ferrimagnetic minerals such as magnetite, hematite, goethite, and titanomagnetite in sediment. This material may be associated with the coarse sediment fraction of, for example, proximal turbidites and/or with single-domain magnetic material in the clay fraction. In contrast, low-amplitude magnetic susceptibility is induced by paramagnetic and diamagnetic minerals such as clays, evaporites, and various precipitates. Because magnetic susceptibility reflects changes in magnetic mineralogy, it is widely used as a proxy for lithologic variations and thus depositional processes. Sampling was conducted at 2.5-cm intervals with three data acquisitions per interval (Table T7). The technical details of the MSL are presented in "Magnetic Susceptibility Measurements and the Königsberger Ratio" in "Paleomagnetism".
The MSL is set to SI units, and data are stored in the Janus database in raw meter units (instrument units). To convert to true SI volume susceptibilities, magnetic susceptibility instrument units 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. No correction was applied to the magnetic susceptibility values for reduced core volume.
GRA bulk density was estimated for unsplit core sections using the GRA densiometer every 2.5 cm over a sampling period of 10 s (Table T7). The GRA bulk density is an independent estimate that has a shorter sampling interval than the bulk density determined using the 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). GRA bulk density is therefore volume dependent, and the quality of GRA bulk density measurements is compromised because of RCB-induced core disruption and size variation.
NGR emissions are a function of the random and discrete decay of radioactive isotopes, predominantly those of U, Th, and K, and they are measured through scintillation detectors arranged at 90° angles to each other and perpendicular to the core. NGR count typically correlates positively with increasing clay/mudrock content and/or diagenesis where uranium is sequestered from seawater and organic matter. Sand-prone units usually tend to be characterized by low NGR counts and vice versa (although this relationship can be reversed or confused by peculiarities of the eroding source region). Ideally, these relationships can be used to define the locations of mudrock- and sand-prone formations downhole. For siliciclastic units, mudrock- and sand-prone units relate primarily to changes in relative sea level or to individual turbidite sequences. In particular, upward-coarsening and upward-fining sedimentary sequences are expressed as overall decreasing and increasing NGR signatures, respectively. NGR was measured for 10 s at 2.5-cm intervals (Table T7). NGR calibration was performed at the beginning of the leg.
Thermal conductivity was measured during Leg 210 using the TK04 system described by Blum (1997). This system employs a single-needle probe (Von Herzen and Maxwell, 1959), heated continuously, in a 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 approximate an infinitely long, continuous medium; the temperature near the line source is measured as a function of time. If it is assumed that the sediment or rock sample to be measured can be represented as solids in a fluid medium, it is then possible to determine a relationship between thermal diffusivity and thermal conductivity. With this assumption, the change in temperature of the probe as a function of time is given to a good approximation by (Von Herzen and Maxwell, 1959)
t/Ba2),
where
= thermal diffusivity of the sample [m2/s],
This relationship is valid when t is large compared with a2/. A plot of T vs. ln(t) will then give a straight line, the slope of which determines k (Von Herzen and Maxwell, 1959).
Half-core rock specimens were measured for thermal conductivity using the half-space configuration. A needle probe was secured onto the flat surface of the half core. The samples and needles were then immersed in seawater for ~30 min prior to measurement. After immersion of the rock sample, the probe was heated at 3 W/m and the temperature rise was monitored. The optimal integration time for each conductivity measurement was calculated by an algorithm in the TK04 program.
The half-space method works well on unfractured sections of core that are at least ~5 cm long. In Leg 210 cores, mudrock sections were rarely preserved in unfractured sections longer than ~2 cm after core splitting, whereas sandstones often remained intact. Therefore, to avoid a sampling bias, a technique was devised for conducting thermal conductivity measurements on the mudrock lithologies with the half-space method. Several consecutive (i.e., well-fitting; separated by cracking rather than by drilling disturbance) pieces of mudrock from a given section were extracted and clamped together with a C-clamp to help close the fractures between the pieces and to simulate an unfractured sample. The probe was then secured to the sample, and the usual method was applied. A similar method of wrapping mudrock pieces in heat-wrap plastic was attempted, but the rock reconstruction was not as strong as that for the C-clamp approach. Perhaps more importantly, the thermal conductivity measurements on the heat-wrapped samples showed considerable scatter for each sample. Thus, the C-clamp technique was the preferred method for thermal conductivity measurements on shales and mudrocks.
In general, one thermal conductivity measurement was taken per core. The reported thermal conductivity measurement for each sample was the average of four repeated measurements using the half-space method. Data are reported in watts per meter degree Kelvin with a stated error of ~5% and precision of 2%.
Thermal conductivity typically varies with 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 (kbulk) on the solid-matrix grain thermal conductivity (kgrain) and the thermal conductivity of the interstitial fluid (kw) (Keen and Beaumont, 1990). This equation can be expressed as
x kgrain(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 T8 (Keen and Beaumont, 1990).
MAD quantities (water content, wet and dry bulk density, grain density, and porosity) were routinely measured from cylinders prepared from cubes (~8 cm3) that initially were cut from consolidated sediment and/or basement rocks. The cubes were prepared for triaxial P-wave velocity measurements. Routine measurements were made on one sample per section to characterize all significant lithologies in a core. This is particularly important if downhole logging proves problematic or is not possible. Particular care was taken to sample undisturbed parts of the core and to avoid drilling slurry from RCB cores, which frequently showed a biscuiting-type disturbance.
Immediately after the rock cubes were cut, the P-wave velocity measurements were made and then the cubes were recut into cylinders to fit into 10-mL beakers for measurement of wet and dry mass and dry volume. Wet sediment mass was measured. Dry sediment mass and dry sediment volume were measured for samples after they 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 20 min 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 assumptions and fundamental interrelationships between the various MAD properties, summarized below:
pw) of 1.024 g/cm3 and a salt density (salt) of 2.257 g/cm3. In order to determine the bulk and grain densities and porosity, 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 wet volume (Vwet) need to be defined.pw ,
salt,
wet), dry bulk density (dry), sediment grain (solid) density (solid), porosity (), and void ratio (e) are calculated fromwet = Mwet/Vwet,
dry = Msolid/Vwet or Msolid/Vbulk,
solid = Msolid/Vsolid,
= Vpw/Vwet, and
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) = 0 e –kz,
where (z) is the porosity as a function of depth z, 0 is the surface porosity, and k controls the rate of decay of porosity with depth. A linear regression of ln([z]) vs. z can be used to estimate the natural logarithm of 0 and k. Obtained results are compared with the predicted porosity-depth curve. Variations from the predicted curve are often diagnostic of grain size and composition, facies variations, and interstitial fluid pressure disequilibrium.
Velocity was measured using the P-wave sensor 3 (PWS3) modified Hamilton Frame velocimeter (Boyce, 1976). The PWS3 system uses a vertically oriented transducer pair that can be used with or without a liner correction. Using cube samples cut from cores, the P-wave velocity can be measured in each of the coordinate axis directions. An acoustic signal of 500 kHz was transmitted and received by the two transducers. Analog-to-digital transformation of the signal allows the seismic signal to be displayed on a digital oscilloscope so that the first-arrival waveform can be picked automatically and the velocity can be calculated. Cube thickness was measured directly by a digital caliper. If the received signal was weak, the transmitter voltage was manually increased. To improve the coupling between the transducer and sample, deionized water was applied to the transducer/receiver heads. Additionally, if the received signal waveform was complicated, the first arrival was manually selected and velocity was calculated.
Calibration of the system was performed in accordance with Blum (1997). The separation between transducers was calibrated with four polycarbonate standards with varying thicknesses (20–50 mm). The delay time was determined by a linear regression of traveltime vs. thickness (15.4–57.5 mm) of seawater.
Anisotropy was determined using the difference between the average horizontal (x- and y-directions) and vertical (z-direction) velocity using the following equation:
where VPt is the average (horizontal or traverse) P-wave velocity and VPl is the vertical or longitudinal velocity.
