Chroma Q Color Force 48
Gisberto Letter
Physical properties were measured on core material recovered during Leg 202 in order to (1) provide data for the correlation of cores among holes at any given site for the construction of complete stratigraphic sequences; (2) detect changes in sediment properties that could be related to lithologic changes, diagenetic features, or consolidation history; (3) provide the dry density records needed for computing mass accumulation rates; (4) identify natural and/or coring-induced discontinuities (e.g., gaps and hiatuses); and (5) provide data to aid interpretation of seismic reflection and downhole geophysical logs.
Magnetic susceptibility (MS), GRA bulk density, compressional wave velocity (VP), and NGR were measured on the whole-core MST. Thermal conductivity (TC) was also measured on whole-round cores. Split-core measurements on the working half of core included VP with the ODP P-wave sensor number 3 (PWS3) and moisture and density (MAD). Color reflectance (CR), magnetic susceptibility point sensor (MSP) measurements, and digital imaging were performed on the archive-half cores.
Magnetic susceptibility was also measured with the OSUS Fast Track, installed during Leg 202 specifically for the fast logging of whole-round core sections immediately after recovery. These measurements aided real-time stratigraphic correlation without being limited by the time constraints of MST measurements.
The shipboard party decided to integrate sedimentological and physical properties observations into the "Lithostratigraphy" sections of the site chapters. This was compatible with and thought to promote the primary objective of the physical properties program, to aid lithologic characterization of sediment sections.
Magnetic susceptibility and GRA bulk density were measured nondestructively on all whole-round core sections with the MST. NGR was measured on most core sections, but the sensor had to be turned off to save core logging time for some intervals. Compressional wave velocity was measured on most core sections, but the sensor was turned off when acoustic coupling proved insufficient (gassy sediment) or when core disturbance was too pervasive to give a reliable signal.
Sampling intervals and periods were the same for all sensors for any one core in order to optimize MST performance. At some sites, comparison with downhole Formation MicroScanner (FMS) data shows that bulk density variability in the sediment could potentially be resolved at subcentimeter resolution. However, the limits of instrument performance combined with the time constraints from coring operations did not allow sampling at 1 cm or less. Sampling intervals were therefore set at 2.5 or 5 cm, depending on time availability, with most cores measured at 5-cm intervals. These particular intervals are common denominators of the distances between the instruments installed on the MST (30-50 cm) and allow truly simultaneous measurements and therefore optimal use of total measurement times.
Sampling periods varied from 3 to 5 s, depending on total time available, with most cores measured at 5 s per sample. Longer sampling times would have been desirable to improve the NGR and MS signal but were not possible given the rate of core recovery.
Two instruments were mounted on the AMST: the Minolta photospectrometer measuring diffuse color reflectance and an MSP. CR was measured at 2.5 cm throughout Leg 202 cores. MSP measurements were taken only at the first two sites (1232 and 1233), when it became clear that the measurement was too slow to be accommodated in the time available. The present AMST configuration requires that CR and MSP runs must be taken one after the other. A measurement system that allows simultaneous measurement of CR and MSP would be needed to make MSP measurements feasible.
Magnetic susceptibility is the degree to which a material can be magnetized in 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 vs. biogenic materials) and/or diagenetic overprinting (e.g., Thompson and Oldfield, 1986). Magnetite and a few other iron oxides with ferromagnetic characteristics have a specific MS several orders of magnitude higher than clay, which has paramagnetic properties. Carbonate, silica, water, and plastics (core liner) have small negative values of MS. Sediments rich in biogenic carbonate and opal therefore have generally low MS, even negative values, if practically no clay or magnetite is present. In such cases, measured values approach the detection limit of MS meters.
The output of the MS meters can be set to centimeter-gram-second (cgs) units or International System (Systme International [SI]) units, and the ODP standard is the SI setting. However, to actually obtain the dimensionless SI volume-specific magnetic susceptibility values, the instrument units stored in the ODP database must be multiplied by a correction factor to compensate for instrument scaling and the geometric ratio between core and loop dimensions. For a standard APC core diameter of 66 mm and loop diameters of 88 mm (MST) and 108 mm (OSUS Fast Track), these correction factors are 1.46 10-6 and 0.79 10-6, respectively, as read from a graph in the Bartington operation manual.
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 measurements. The OSUS track did not have this option and was mostly run with 10-s integration time.
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 (e.g., clay, biogenic silica, and carbonate content, and grain size and sorting) and compaction and cementation. In homogeneous pelagic and hemipelagic sediments drilled during Leg 202, bulk density was often a function of the relative amount of calcareous nannofossils and diatoms, which resulted in significant fabric and thus porosity variations as well as grain density variations.
The GRA densitometer consists of 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). Since the attenuation coefficient is similar for most common minerals and aluminum for practical purposes, bulk density is obtained through direct calibration 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
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 Leg 202, with the MST-mounted P-wave logger (PWL) on whole-round cores (Schultheiss and McPhail, 1989) and with the PWS3 on split cores. The P-wave sensors number 1 and 2 (PWS1 and PWS2), transducer pairs built into pairs of knifelike probes that are inserted into soft sediment, were not used during Leg 202. All ODP 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 (DTL), 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 (MSS) 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, and the number of waveforms to be stacked (typically 5) can also be set. Length of the travel path is determined by linear voltage differential transducers.
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.
In cases of bad acoustic coupling between the sediment and the liner, the PWL generally does not provide accurate velocity values. The system is, therefore, most useful in undisturbed APC cores, and values become highly questionable when gas is present in the sediment. A correction for the core liner, which is made of Tenite with a sonic velocity of 1987 m/s, is applied routinely. This is accomplished by subtracting the total liner thickness (2.54 mm for split cores and 5.12 mm for whole cores) from the transducer displacement measurement and subtracting the calculated transit time based on the sonic velocity in Tenite from the measured transit time.
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