Previous   |    Next

doi:10.2204/iodp.proc.310.203.2009

Methods and materials

The data set analyzed in this paper is composed of downhole logging data, core logging data, and discrete measurements on core plugs from the following holes: M0005D, M0015A, and M0017A, informally called Site Maraa, and M0009B, M0009E, and M0023B, informally called Site Tiarei. Downhole logging and core logging were carried out during the offshore phase of the expedition. Core description and sampling took place during the onshore scientific party held at the University of Bremen (Germany) in February and March 2006. Discrete samples were analyzed at the Vrije Universiteit in Amsterdam (The Netherlands). All depth measurements are reported in meters below seafloor or noted otherwise.

Velocity, density, and porosity

Acoustic velocity, density, and porosity were measured on 79 samples drilled from the Tahiti cores. In the core laboratory 1.5 inch diameter cylindrical samples were drilled using a water-cooled diamond coring drill. Samples were taken in the horizontal direction at an approximate resolution of one per 1.5 m core section. Although sufficient for our purposes, the sampling was not ideal because of the small core diameter (6.7 cm) with respect to the sample diameter (3.81 cm), which limited sampling locations. Therefore, the downhole representation of physical properties might be limited. Sample ends were ground flat and parallel to within 0.01 mm. Following these measurements, samples were saturated with deaired brine (35‰ NaCl) by storing them in a vacuum for 72 h. Ultrasonic P-wave (V_P) and S-wave (V_S) velocities were measured as a function of pressure. A single P-wave and two S-wave velocities were measured with a transducer arrangement (Verde Geoscience, Vermont, USA) that propagated the P-wave and two independent and orthogonally polarized S-waves (V_S 1 and V_S 2) along the core axis. The experimental procedure for obtaining acoustic velocities involves measuring the one-way traveltime along the sample axis and dividing by the sample length. In the experiment, a source and receiver pair of similar crystals is selected through an ultrasonic signal selector switch. The source crystal is excited by a fast rise–time electrical voltage pulse, which produces an ultrasonic pulse with a ~1 MHz frequency. The arrival time is picked when the signal exceeds a threshold voltage equal to 3% of the overall peak to peak amplitude of the first three half-cycles of the signal. Precision of the velocity measurement in low-porosity carbonates is within ~0.5% (Kenter et al., 1997a). Ultrasonic measurements were conducted at five effective stresses (Pe) that ranged from 0 to 10 MPa. Common values for confining pressures were 0, 2.5, 5, 7.5, and 10 MPa. Because pore pressure is kept constant at 0 MPa, confining pressures represent effective pressures. Following the acoustic measurements, dry (ρ_d) and wet (ρ_w) bulk densities were calculated from dry and wet weights and measured cylinder volumes. Grain densities (ρ_g) were measured using a Micromeritics AccuPyc 1330 helium pycnometer. Total porosity (ϕ) was calculated from dry and grain density. More detailed procedures are described by Kenter and Ivanov (1995).

Shipboard discrete measurements of P-wave velocity were made using a P-wave sensor (PWS3) on a modified Hamilton frame velocimeter (Boyce, 1976). The PWS3 system uses a vertically oriented transducer pair capable of measuring sample cubes or cylinders. An acoustic signal of 500 kHz was transmitted and received by the two transducers. First-arrival waveforms from samples in which the signal was weak were manually picked. Measurements were performed on brine-saturated samples (35‰ NaCl) at ambient pressure.

Core acoustic velocity, gamma density, and magnetic susceptibility were measured using a multisensor core logger (MSCL; Geotek Ltd.). Transverse P-wave velocity was measured using two P-wave transducers aligned perpendicular to the core axis with P-waves passing through the core horizontally (in whole-core setup). A P-wave pulse centered on 320 kHz frequency is transmitted through the core. The traveltime of the acoustic signal divided by the core thickness presents the acoustic velocity. Sensors were calibrated against a tube filled with synthetic brine (35‰ NaCl). Gamma density is measured by determining the attenuation of gamma rays (from a small ¹³⁷Cs source [10 millicurie; 0.662 MeV energy]) that pass through the cores and is used to estimate bulk density. The degree of attenuation is proportional to the electron density in the gamma path. Attenuation measurements were calibrated against an aluminum cylinder with varying thicknesses of known density. Porosity is converted from the density curve by making use of a constant grain density for carbonate of 2.718 g/cm³ (Mavko et al., 1998). Whole-core magnetic susceptibility was measured on the MSCL using a Bartington MS2 meter coupled to a MS2C sensor coil with a coil diameter of 88 mm, operating at a frequency of 565 Hz. Measurement spacing interval was set to 1 cm for all three sensors.

Sonic velocity was measured with a 2PSA-1000 sonic probe (Mountsopris Ltd.). The tool was calibrated by the manufacturer. The measurement spacing interval was set to 0.15 m. No nuclear tools for measuring density were deployed during Expedition 310 because of environmental constraints. Further information regarding shipboard measurements of discrete samples, downhole logging, and core logging are described in detail in the “Expedition 310 summary” chapter.

Mineralogic content

Mineralogic content was measured using thermogravimetric analysis (TGA) using a TGA-601. The thermogravimetric analyzer measures weight loss as a function of temperature. Weight loss of 1–2 g of sample material is measured under increasing temperature. Temperature, temperature rate, and atmosphere type are selected and monitored during five continuous weight-loss steps. In the first stage, under an oxygen atmosphere, the decomposition of organic matter is detected. In the second stage, under a nitrogen atmosphere, the decomposition of carbonate is measured. Temperature steps included 25°–105°C (moisture), 105°–480°C (organic matter), 480°–600°C (hydroxyl groups), 600°–875°C, and 875°–1000°C (carbonate). The leftover ash residue represents the mineral fraction that is not combusted below 1000°C. Precision is within 3% depending on sediment origin (Konert, unpubl. data).

Petrography

For petrographic study, all plug ends were selected to reveal the sediment fabric and diagenetic features. Plug ends were impregnated (Dickson, 1965) with epoxy, and thin sections were cut parallel to the longitudinal axis. Thin sections were studied under plane-polarized light on a Zeiss optical microscope.

Top of page   |    Previous   |    Next