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doi:10.2204/iodp.proc.314315316.124.2009

Core-log-seismic integration

Hole C0002B is located 60 m south-southwest of Hole C0002A, a LWD hole drilled during Expedition 314. Three stratigraphic sequences are seen on the seismic profile (Inline 2529) in Figure F33. The forearc basin sequence from Hole C0002B toward Hole C0002A has a northwest dip of 6.5° to 7.0° in the lower half of lithologic Unit II, so a stratigraphic offset of 6.0 to 7.5 m is expected between the boreholes. Dips are nearly constant down to an unconformity within lithologic Unit III observed at 890 mbsf (2827 mbsl), well within lithologic Unit III. A strong reflector at ~950 mbsf (2887.5 mbsl) marks the main angular unconformity between sandy accreted sediments below and muddy slope or forearc-basin deposits above. The equivalent boundary between logging Units III and IV is positioned at 935.6 m LSF. Seismic reflectors dipping 30°–40° are observed below the main unconformity. Dips cannot be determined from the seismic image in the interval between the unconformities.

Correlation between natural gamma ray intensity from LWD and MSCL-W is limited by overall poor recovery and core quality in the turbidite sequences constituting lithologic Unit II and around the main unconformity (Fig. F34). Intervals where sand layers are thicker and more abundant can be identified from strong short-wavelength cycles in the gamma ray logs and correlate with intervals of very low core recovery at 418–435 and 773–825 m CSF. Oscillation of LWD gamma ray intensity with a wavelength of ~60 m may tentatively be correlated with the core logs, but only broad maxima around 550 and 735 m CSF are well identified in the core data. Lithologic Unit III provides core logs of sufficient quality to allow peak-to-peak correlations of shorter wavelengths, but the proposed solution is not unique (Fig. F34). Overall, these observations are consistent with a systematic deepening of forearc basin strata of 5 to 15 m from Hole C0002B to Hole C0002A (Table T20).

In Hole C0002B, density, sonic, and resistivity logs can be compared with measurements on samples. In Figure F35, LWD bit resistivity is compared with temperature-corrected sample resistivity and LWD gamma ray density is compared with sample density from moisture and density measurements.

For an accurate comparison with LWD bit resistivity, which averages horizontal and vertical conductivities over a rock volume, an average sample resistivity is computed as the geometric average of resistivities measured in horizontal plane and along the vertical axis. Horizontal plane electrical resistivity is computed from the mean of two complex conductivity measurements made in the horizontal plane at a 90° angle. As at Site C0001, a correction for in situ temperature is applied assuming the resistivity of the samples varies in the same way as that of seawater (Bourlange et al., 2003):

R (T,°C) = R (25°C)/[1 + 0.02(T – 25)].

The temperature at a given depth is computed based on the heat flow determination (40 mW/m²) and thermal conductivity measurements on cores made during this expedition (see “Physical properties”).

P-wave velocity measured on samples is lower than in situ P-wave velocity from LWD but follows the same trends (Fig. F36). The same corrections are applied for in situ conditions as at Site C0001; a 2.4% correction is applied to account for the variation of the P-wave velocity of seawater between the sea surface and 2200 m depth (Fofonoff, 1985) and an additional empirical correction of 1% per 100 mbsf is applied to account for the effect of confining pressure. Agreement between corrected P-wave velocity measured along the core axis and LWD P-wave velocity is satisfactory.

The zone around the lower unconformity is characterized by strong heterogeneity of physical properties according to the logs. Two intervals, presumably sand dominant, display both low natural gamma ray intensity and low resistivity on the logs, in part because of hole caving. These inferred sandy intervals were not recovered in cores. Cores 315-C0002B-51R and 52R (938.50–949.24 m CSF) likely correspond to a higher natural gamma ray and resistivity interval between 950 and 960 m LSF. Below this level, Cores 315-C0002X-53R and 54R were empty and, unfortunately, correspond to the depth of the reflector marking the main seismic unconformity (Fig. F37). Recovery remained low, with highly fractured and disturbed cores through Core 315-C0002B-59R. Brecciation of lithified materials (one sandstone sample from Core 312-C0002B-57R has a P-wave velocity of 5.5 km/s) should cause strong local impedance contrasts and seismic reflectivity.

In the lower part of Unit II, in addition to the correlation of gamma ray values, electrical resistivity and porosity suggest deepening from Hole C0002B to Hole C0002A. Notably, a V-shaped minimum in resistivity and density is found at 914 m CSF in the cores and at 925 m LSF in the logs and is consistent with an 11–12 m offset inferred independently from the gamma ray data alone (Fig. F35).

A trend of decreasing density and resistivity with depth is observed on samples from 1010.9 to 1042.7 m CSF (0.0055 Ωm/m). This trend may correlate with a similar trend in LWD resistivity (0.008 Ωm/m) observed between 980 and 1005 m LSF. A stratigraphic offset of 30–40 m between the boreholes would be compatible with the observed dip of reflectors within the accreted sediments, as well as with the dips observed in the cores. No strong arguments against this interpretation can be found in the natural gamma ray intensity data (Fig. F35).

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