Previous   |    Next

doi:10.2204/iodp.proc.303306.110.2006

Physical properties

Physical properties were measured on core material recovered during Expedition 306 to

1. Allow real-time stratigraphic correlation and feedback to the drillers;

2. Provide further data for the hole-to-hole core correlation of any given site and for the construction of composite stratigraphic sequences;

3. Detect changes in sediment properties that could be related to lithologic changes, diagenetic features, or consolidation history;

4. Provide the dry density records needed for computing mass accumulation rates;

5. Identify natural and/or coring-induced discontinuities; and

6. Provide data to aid interpretation of seismic reflection and downhole geophysical logs.

Magnetic susceptibility was first measured with the Geotek MSCL on whole-round core sections immediately after recovery. These measurements aided real-time stratigraphic correlation without being limited by the time constraints of MST measurements. After the cores reached room temperature, magnetic susceptibility, GRA bulk density, and NGR were measured on the whole-core MST. Noncontact resistivity was not measured.

Split-core measurements on the working half of the core included V_P with the IODP P-wave sensor number 3 (PWS3–measuring perpendicular to the core axis) and moisture and density. A comprehensive description of most of the methodologies and calculations used in the JOIDES Resolution physical properties laboratory can be found in Blum (1997).

Magnetic susceptibility MSCL sampling strategy

Rapid stratigraphic correlation of cores from adjacent holes and real-time feedback to drillers for recovering complete stratigraphic sections at all sites was important. An automated dedicated MSCL for rapid measurement of magnetic susceptibility was used to provide a nondestructive proxy of sediment variability to the stratigraphic correlators. Measurements were made as soon as whole-core sections became available. The MSCL system employs a Geotek track system with dual Bartington MS2C meters and two 80 mm inside diameter coils separated by 45 cm. Because of the proximity of the coils to each other, different frequencies were used to reduce interference: 0.621 kHz for the first and 0.513 kHz for the second. Distinct correction factors (1.099 and 0.908, respectively) must be used to synchronize data acquired from the two loops. Measurements were taken every 10 cm. The use of two loops allowed simultaneous interlaced measurements, decreasing the sampling interval to 5 cm. This was fast enough to keep up with core recovery under most circumstances and is significantly faster than the MST logging time. The Geotek uses a core pusher system that measures sections continuously, minimizing edge effects at section breaks. The susceptibility data collected with the MSCL is not appropriate for other uses because cores were measured prior to warming to room temperature and temperature drift corrections were not made.

The MST susceptibility meter (a Bartington MS2C meter with an 80 mm coil diameter driven at 0.565 kHz frequency) was set on SI units, and the output values were stored in the Janus database. The width at half-height of the response function of the susceptibility coil is ~4 cm (Blum, 1997), and the sensing region corresponds to a cored volume of 160. To convert the stored values to SI units of volume susceptibility, values should be multiplied by 10^–5 and by a correction factor to take into account the volume of material within the response function of the susceptibility coils. For a standard IODP core, this factor is ~6.8 × 10^–5 (Thomas et al., 2001), based on laboratory/ship comparisons for Leg 162 sediments. More recent estimates based on Leg 202 sediments suggest a factor of ~6 × 10^–5 SI units.

Multisensor track sampling strategy

Magnetic susceptibility, GRA bulk density, and NGR were measured nondestructively on all whole-round core sections with the MST. The noncontact resistivity and compressional wave velocity loggers were not working, and these parameters were not measured. To optimize MST performance, sampling intervals and measurement residence times were the same for all sensors for any one core. Sampling intervals were therefore set at 2.5 or 5 cm, depending on time constraints. These sampling intervals are common denominators of the distances between the sensors installed on the MST (30–50 cm) and allow truly simultaneous measurements and therefore optimal use of total measurement times.

We endeavored to optimize the trade-off between core logging times and depth resolution. Longer analysis times were selected if required to improve measurement precision. These times varied from 3 to 5 s, with most cores measured at 5 s per sample.

The total time availability for MST logging at a site was predicted based on the operational time estimate for the site, subsequent transit time, and any other time available before core was on deck at the subsequent site. Sampling parameters were then optimized to use the total available time (e.g., 2.5 cm and 5 s [~2 h/core]; 5 cm and 5 s [~1.2 h/core]; 5 cm and 3 s [fast logging, ~0.9 h/core]).

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/or 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 and negative values of magnetic susceptibility. Sediments rich in biogenic carbonate and opal therefore have generally low magnetic susceptibility, even negative values, if practically no clay or magnetite is present. In such cases, measured values approach the detection limit of magnetic susceptibility meters.

Magnetic susceptibility was measured with the Bartington Instruments MS2C system on both the MST and the MSCL. 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, as described above.

A common operational problem with the Bartington meter is that 1 s measurements are rapid but not precise enough for biogenic-rich sediments and the 10 s measurements are much more precise but take a prohibitively long time to measure at the desired sampling interval of 2.5 to 5 cm. The MST program was therefore equipped with the option to average any number of 1 s measurements, and we usually averaged five such measurements.

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, texture (e.g., clay, biogenic silica, and carbonate content and grain size and sorting), compaction, and cementation.

The GRA densitometer comprises 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 (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.

Natural gamma radiation

Terrigenous sediment is often characterized by NGR from K, Th, and U, which are present mostly in clays but can also originate from heavy minerals or lithic grains. Uranium often dominates the NGR in carbonate-rich sediments with little terrigenous input. Uranium concentration is largely controlled by organic matter flux to the seafloor and the existing redox conditions there. It is also mobile and can migrate to certain layers and diagenetic horizons.

The NGR system consists of four shielded scintillation counters with 3 inch × 3 inch doped sodium iodide crystals, arranged at 90° from each other in a plane orthogonal to the core track. The NGR response curve is ~17 cm (full-width half-maximum), an interval that could be considered a reasonable sampling interval. Measurement precision is a direct function (inverse square root) of the total counts (N) accumulated for one measurement, according to Poisson’s law of random counting error. N is the product of the intrinsic activity of the material measured and the total measurement time. Therefore, one seeks to maximize the measurement time to improve data quality. Unfortunately, time constraints on core logging during the cruise do not permit long counting times. Furthermore, NGR is measured every 2.5 or 5 cm (same sampling interval as other MST measurements for optimal MST efficiency) and, therefore, only for a relatively short time (typically 5 s). It may be necessary, particularly in low-activity material, to integrate (or smooth) several adjacent measurements to reduce the counting error to an acceptable level. Five-point smoothing is a reasonable data reduction in view of the relatively wide response curve of the sensors. (See Blum, 1997, for more detailed discussions).

P-wave velocity

P-wave velocity in marine sediments varies with the lithology, porosity or bulk density, state of stress such as lithostatic pressure, fabric or degree of fracturing, degree of consolidation and lithification, occurrence and abundance of free gas and gas hydrate, and other properties. P-wave velocity was measured with two systems during Expedition 306, with the MST-mounted P-wave logger on whole-round cores (Schultheiss and McPhail, 1989) and with the PWS3 on every section of the split cores. All IODP P-wave piezoelectric transducers transmit a 500 kHz compressional wave pulse through the core at a repetition rate of 1 kHz.

Traveltime is determined by the software, which automatically picks the arrival of the first wavelet to a precision of 50 ns. It is difficult for an automated routine to pick the first arrival of a potentially weak signal with significant background noise. The search method applied 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. Then it 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 to 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 set to 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.The number of waveforms to be stacked (typically five) can also be set. Linear voltage differential transducers determine length of the travel path.

The P-wave velocity systems require two types of calibration, one for the displacement of the transducers and one for the time offset. For the displacement calibration, five acrylic standards of different thickness are measured and the linear voltage-distance relationship determined using least-squares analyses. For the time offset calibration, room-temperature water in a plastic bag is measured multiple times with different transducer displacements. The inverse of the regression slope is equal to the velocity of sound in water, and the intercept represents the delay in the transducers.

Moisture content and density

Samples of 10 cm³ were taken from the working half sections with a piston minicorer and transferred into previously calibrated 10 mL glass vials. Usually one sample from the top of the first section and the base of the sixth section were taken. Sampling location was coordinated for P-wave velocity as well as for subsequent extraction of specimens for carbonate. To minimize the destruction of the available sediment on the specific sampling zones, another 2 cm³ sample was collected as close as possible to the physical properties sample for carbonate analysis.

Wet and dry weights were determined with twin Scientech 202 electronic balances, which allow for compensation of the ship's motion effect on the balance and give a precision better than 1%.

The samples were dried in a convection oven at a temperature of 105° ± 5°C for a period of 24 h. Dry volume was measured in a He-displacement five-chambered pycnometer with an uncertainty of 0.02 cm³. This equipment allows the simultaneous analysis of four different samples and a calibration sphere and took ~15–20 min. Three measurements were averaged per sample. The calibration sphere was cycled from cell to cell of the pycnometer during each batch, so that all cells could be checked for accuracy at least once every five runs.

Archive multisensor track sampling strategy

Two instruments were mounted on the AMST, the Minolta spectrophotometer measuring diffuse color reflectance and a point magnetic susceptibility meter, which was not used during this cruise. Color reflectance was measured at 2.0 cm throughout Expedition 306 cores (for color reflectance see “Lithostratigraphy”).

Top of page   |    Previous   |    Next