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doi:10.2204/iodp.proc.310.107.2007

Petrophysics

Recovery at Tiarei inner ridge Site M0023, on the northeastern side of the island of Tahiti, was generally good (Hole M0023A = 77%; Fig. F10). Good recovery is mainly from the last deglacial sequence (Unit I). The older Pleistocene sequence (Unit II) has discontinuous (Hole M0023A; Fig. F10) to no data (Hole M0023B; Fig. F11). Cores 310-M0023A-14R, 15R, 16R, and 16R were left unsaturated and therefore have different data coverage and quality (see the “Methods” chapter for more details). Water depths are as follows: Hole M0023A = 67.98 mbsl and Hole M0023B = 67.58 mbsl.

Density and porosity

Bulk density at Tiarei inner ridge sites was computed from gamma ray attenuation (GRA) and moisture and density (MAD) measurements on discrete plug samples. In both holes, three intervals are recognized.

• Interval 1 corresponds to the top of the last deglacial sequence, from 0 to 11 mbsf (Cores 310-M0023A-1R through 7R). This interval shows a scattered, generally low density of ~2.0 g/cm³. Porosity is as high as 50% but is generally ~35%–40%.
• Interval 2 (Cores 310-M0023A-8R through 13R) is also located in the last deglacial sequence. The upper boundary (11 mbsf) shows a remarkable sharp change in density and porosity and coincides with a change in coral assemblages (see “Sedimentology and biological assemblages”). Densities are higher, ~2.2 g/cm³, and average porosity is ~30%. Toward the lower part of this unit, recovery drops and a slight decrease in density is observed.
• Interval 3 (24.5 mbsf to the bottom of the hole; Cores 310-M0023A-14R through 16R) corresponds to the older Pleistocene sequence and has poor recovery because of poor lithification. Sediments consist of coral gravels with densities of ~2.3 g/cm³ and highly scattered porosity values ranging from 15% to 30% with outliers to 50%. MAD density ranges between 1.84 and 2.64 g/cm³ for Unit I and increases to 2.69 to 2.80 g/cm³ for Unit II (e.g., 26 mbsf in Hole M0025A).

Grain density values average 2.77 g/cm³, range between 2.69 and 2.81 g/cm³, and do not show any clear downhole trends. Average grain density is not consistent with calcite mineralogy (2.71 g/cm³). It is thought that deviations occur because of the volcaniclastic input for which minerals have different matrix densities.

P-wave velocity

P-wave velocities were measured with the Geotek MSCL P-wave logger (PWL) on whole cores and the PWS3 contact sensor system on a modified Hamilton frame on ~2–4 cm long, 1 inch round discrete samples of semilithified and lithified sediments (see the “Methods” chapter). Velocities in one transverse (x) direction were measured on the plugs. Velocities follow the same subdivision into three intervals, as described in “Density and porosity.“ Interval 1 shows scattered observations around 3000 m/s with slower velocities centered around 1800 m/s. Simultaneously with the increase in density in Interval 2, the velocity profile is more continuous with an average velocity of 3900 m/s. Toward the lower part of Interval 2, density decreases slightly, and velocities follow this trend toward 3600 m/s. Interval 3 does not reveal any velocity measurements, as sections from this depth were left unsaturated because of the presence of (semi-) lithified gravels and sands.

Measurements on discrete samples confirm velocities measured with the MSCL. Discrete velocity measurements range from 3781 to 4830 m/s and generally represent the higher velocities observed in core measurements. A cross plot of velocity versus porosity for Tiarei inner ridge sites reveals a general inverse relationship (Fig. F12). For the time-average empirical equation of Wyllie et al. (1956) and Raymer et al. (1980), the traveltime of an acoustic signal through rock is a specific sum of the traveltime through the solid matrix and the fluid phase. Porosity and velocity data do not match the time-average equation but show large scatter around the general trend line. For a given density of 2.0 g/cm³, velocity may vary as much as 2000 m/s. Comparison of V_P MSCL data and downhole sonic logging data (Fig. F13) confirms scattered velocity data. Peaks in high velocity values correlate well. Sonic logging values are on average 500 m/s slower than V_P MST, attributable to scaling effects. Whereas the MSCL measures velocity directly only on matrix sediments, the sonic log provides an average over an interval of 1 ft (~31 cm) in which velocity is averaged over large primary pores containing seawater (~1535 m/s) and rock, always resulting in a lower average than direct measurements of matrix properties.

Magnetic susceptibility

Magnetic susceptibility at this site also follows the general subdivision into intervals (see “Density and porosity”). Interval 1 has highly variable magnetic susceptibilities varying from 0 to a maximum of 600 × 10^–5 SI units. Susceptibilities vary over short distances, and individual peaks from locations associated with a high influx of volcaniclastics correlate with the higher values. Interval 2 reveals a highly variable but generally high level of magnetic susceptibility. Values on average lie around 500 × 10^–5 SI units with lower limits around 300 × 10^–5 SI units and maxima up to 750 × 10^–5 SI units. Zone 3 has lower magnetic susceptibilities with a maximum of 220 × 10^–5 SI units but averages ~120 × 10^–5 SI units. High susceptibility values may be associated with a higher concentration of magnetic minerals because of the proximal location of the Tiarei River, which brings in a high influx of generally fine to coarse sand-size volcaniclastic material.

Resistivity

See “Resistivity” in the “Maraa western transect” chapter.

Diffuse color reflectance spectrophotometry

In the last deglacial sequence at Site M0023, color reflectance values range from 25 to 79 L* units (Fig. F14). No clear downhole trends can be observed in Hole M0023B. Highest values of L* in Unit I (up to 79 L* units) originate from measurements on massive Porites corals in Sections 310-M0023A-5R-1 and 6R-1 (6.7–9.9 mbsf). In Unit II (Cores 310-M0023A-15R through 16R; below 27.4 mbsf), color reflectance again has L* values up to 79 L* units. This interval corresponds to the coralgal framework that includes Acropora, Porites, and faviid corals.

Hole-to-hole correlation

Holes M0023A and M0023B are located several meters away from each other. Clear correlation of Intervals 1 and 2 is made based on density and magnetic susceptibility changes (Fig. F15).

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