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Acoustic velocity, density, and porosity

Laboratory data

Cross-plots of discrete measurements of P-wave velocity versus porosity and density for shipboard and laboratory samples are shown in Figure F4. The Tahiti samples have a narrow range in velocities. P-wave velocity of the laboratory samples varies between 2900 and 5000 m/s, and S-wave velocity varies between 1700 and 2850 m/s (Table T1). The VP/VS ratio varies between 1.74 and 2.11, with an average of 1.92. Poisson’s ratio varies between 0.21 and 0.40, with an average of 0.31. Porosity ranges from 1% to 35%, and bulk density ranges between 1.94 and 2.54 g/cm3 for saturated samples. Grain density values are between 2.58 and 3.10 g/cm3 (mean = 2.88 g/cm3). Generally, velocities display a nearly linear decrease with increasing porosity and an increase with increasing density (Fig. F4). The scatter in velocity of samples with the same porosity can be as much as 1000 m/s for P-wave velocity and 600 m/s for S-wave velocity. The mean percentage deviation of the averaged orthogonal polarized S-waves for the samples is 1% (mean absolute deviation = 30 m/s, σ = 35 m/s). To assess the errors associated with measurement, the standard deviations of ultrasonic velocities were estimated by error propagation. The ends of the samples are flattened within 0.01 mm. When the length is repeatedly measured, no significant variability is observed, so we consider the length error free. The precision of the measured velocities is within 3%. We estimate the standard deviation of the density to be within 0.5%. The estimated standard deviations (σ) are as follows: σ (VP) = 9 m/s, σ (VS) = 5 m/s, and σ (ρw) = 0.006 g/cm3.

Core measurements and downhole logs

Physical properties as a function of depth for three holes are plotted in Figure F5A–F5C along with the same physical properties determined from the discrete shipboard (open triangles) and laboratory (open rectangles) measurements. What is most obvious is that discrete velocity measurements do not coincide with core measurements and downhole logs. Measurements on the core plugs are offset positively from core measurements, whereas the downhole logging data show lower velocities compared to core measurements. Density and porosity measurements of discrete samples are in good agreement with core measurements. Differences between discrete velocity measurements and core and downhole logging data are as much as 800 and 1200 m/s maximum for the lithologic Unit I samples, respectively. Discrete measurements of Unit II samples show much smaller offsets, as much as 400 and 800 m/s maximum, respectively, for core measurements and downhole derived data (Fig. F5C).

Figure F6 shows cross-plots of velocity and porosity from MSCL core logging data and discrete plug measurements for Holes M0005D, M0009B, M0009E, M0015A, and M0017A, along with the time-average and Raymer equations for a matrix velocity of calcite. Discrete laboratory measurements (open triangles) show specific porosity-velocity relationships. Hole M0005D samples vary as much as 1000 m/s at a given porosity and have a trend that falls on the Wyllie velocity transform. Data from Holes M0009B, M0009E, and M0015A show small variations (<500 m/s) and follow the predicted trend of the Raymer velocity transform. Finally, Hole M0017A and M0023B data follow the predicted trend of the Raymer transform below 20% porosity and are positively offset with respect to the velocity transforms over 20% porosity. In general, P-wave velocities of different samples show a nonlinear concave-upward trend similar to the velocity equations, rather than the linear relationship visible in MSCL data. MSCL data generally follow the predicted Wyllie trend for most boreholes, but the variation at a given porosity, locally >1.5 km/s, is not explained by the velocity transform. No clear difference is observed among the trends at the various boreholes. Finally, the discrete shipboard velocity measurements (open rectangles) are all positively offset from laboratory and MSCL data points. This offset increases toward higher porosity (as much as 1500 m/s).

Petrography and insoluble residue

The mixed siliciclastic-carbonate samples were divided into groups based on petrographic observations of textural factors (e.g., Vernik and Nur, 1992; Kenter et al., 2007) and geochemically segregated according to content of carbonate and noncarbonate fractions (Table T1). These petrophysical groups and their associated characteristics include: (1) Holocene volcaniclastic-poor carbonates (Maraa Site, lithologic Unit I), (2) Holocene volcaniclastic-rich carbonates (Tiarei Site, Unit I), and (3) Pleistocene reefal-volcaniclastic carbonates (Maraa Site, Unit II).

For the Unit I samples from Maraa, mean carbonate content is 95% (minimum = 90%; maximum = 96%). Samples are dominated by coralgal-microbialite framework with little contamination from volcaniclastic input. Unit I samples from Tiarei display mean carbonate content of 83% (minimum = 69%; maximum = 96%). The samples are also mostly composed of coralgal-microbialite framework but with variable amounts of volcaniclastic grains dispersed through the matrix. The fraction of noncarbonate content may be as much as 31%. Volcanic grains are predominantly silt- to medium sand–sized grains of olivine, pyroxene, and plagioclase. Organic matter and minerals containing hydroxyl groups (clays) are present in fractions up to 9%. Unit II samples from Maraa also show coralgal framework along with moderately cemented skeletal-volcaniclastic sand showing clear isopachous rims of early marine cement (~100 µm thick). Marine cements have bladed and fibrous morphologies. Mean carbonate content is 90% (minimum = 79%; maximum = 97%).