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doi:10.2204/iodp.proc.314315316.123.2009

Core-log-seismic integration

Cores taken from Holes C0001B, C0001E, and C0001F were cross-correlated based on MSCL-W magnetic susceptibility data. Cores from Hole C0001B provide a more continuous record of the uppermost 40 m of sediment than those from Hole C0001E; Core 315-C0001E-3H was almost completely missing because of a jammed core liner. The fitted profiles and depth shifts applied to Hole C0001E cores are given in Figure F52. Uncertainty of the depth correlation is <10 cm. The lowermost core in Hole C0001E (315-C0001E-13H) overlaps the uppermost core in Hole C0001F (315-C0001F-1H). A distinctive pair of magnetic susceptibility peaks, corresponding to ash layers, was found at 110.0 and 110.4 m CSF in Core 315-C0001E-13H and at 112.9 and 113.1 m CSF in Core 315-C0001F-1H. This leads us to estimate an offset of 2.7 m between strata in Holes C0001E and C0001F.

Gamma ray logs show quasi-periodic oscillations of 10 m wavelengths in the interval from 50 to 120 m CSF (Fig. F53). Considering the sedimentation rates (see “Biostratigraphy” and “Paleomagnetism”), these may correspond to 100,000 y eccentricity cycles (see Fig. F22 in the “Expedition 315 methods” chapter). Independent of their interpretation, these regular oscillations provide a reliable basis for core-log correlation over this interval, which comprises cores from Holes C0001E and C0001F. In the interval from 120 to 200 m CSF, the gamma ray log alone does not provide unique correlation solutions, but additional constraints arise from the sand layers of lithologic Subunit IC. These are found, with intercalated mud intervals, between 196.76 and 206.93 m CSF-B (IODP Method B; see the “Expedition 315 methods” chapter) in cores and are well imaged as conductive layers in the resistivity-at-the-bit image between 192 and 199 m LSF. This match constrains the position of the unconformity between the accretionary prism and overlying slope sediment. It also constrains the stratigraphic offset between Holes C0001F and C0001D to 4.75 m at the top of the sand and 8 m at the base. Correlations in the 120–200 m interval were refined from gamma ray nearest peak correlations. A good fit of the MSCL and LWD gamma ray data is obtained in the lower part of Hole C0001D (from 212 to 230 m CSF) with offsets of 7 to 9 m. This suggests a lateral variation of sand thickness from 7 m in Hole C0001D to ~10 m in Hole C0001F.

Taking into account this 9 m shift of the base of Hole C0001F, it is logical to fit the magnetic susceptibility peaks within Unit II at 231.08 and 233.04 m CSF in Core 315-C0001H-1R with those located ~8 m deeper at 239.49 and 240.37 m CSF-B in Core 315-C0001F-21X (Fig. F52) and to infer an upward shift of ~1 m of the uppermost part of Hole C0001H with respect to the LWD hole. Further downhole, correlation between Holes C0001H and C0001D relies primarily on a limited number of cores with good recovery and distinctive gamma ray profiles: Cores 315-C0001H-4R (262 m CSF), 7R (295 m CSF), 11R (327 m CSF), 12R (332 m CSF), 14R (355 m CSF), and 16R (372–373 m CSF). There are no reliable constraints in natural gamma ray values from 373 m CSF to the bottom of the cored hole at 456.8 m CSF. The fit of the gamma ray data from Holes C0001E, C0001F, and C0001G is summarized in Table T24.

In Hole C0001H, sonic and resistivity logs can be compared with measurements on samples. Sample resistivity is computed from the mean of two complex conductivity measurements made in the horizontal plane at a 90° angle. A correction for in situ temperature (T, in degrees Celsius) is then applied, assuming the resistivity (R) of the samples varies in the same way as that of seawater (Bourlange et al., 2003):

R(T) = R(25°C)/[1 + 0.02(T – 25)].

Temperature at a given depth is computed based on heat flow determination (47 mW/m²) and thermal conductivity measurements on cores made during this expedition (see “Thermal conductivity”). LWD ring resistivity is compared with temperature-corrected sample resistivity in Figure F54. Data from measured samples follow the LWD trend, but values tend to be higher on average, notably near the top and bottom of the cored interval (220–300 and 439–460 m CSF). One possible explanation for the deviation in the upper interval could be clay surface conductivity, which depends less on temperature than seawater conductivity (Revil et al., 1998). The temperature extrapolated at the bottom of Hole C0001H is ~20°C, close to laboratory temperature (21.5° ± 0.2°C), so the deviation observed in the lowermost cores (Cores 315-C0001H-25R and 26R) probably requires another explanation.

P-wave velocity measured on samples is lower than in situ values of P-wave velocity from LWD logs but follows the same trends: increasing from 230 to 320 m CSF, nearly constant from 320 to 400 m CSF, and then increasing again to from 400 m CSF to the base of the borehole (456.8 m CSF) (Fig. F55). P-wave velocity is dependent on temperature, fluid pressure, and confining pressure. 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 m below the seafloor is applied to account for the effect of confining pressure. The corrected velocities closely follow the 10–50 m wavelength variations in the LWD data, except in the lower part of the borehole, where values measured on samples increase more rapidly than those of the LWD data. The same was observed for resistivity. Several explanations are feasible:

• Sampling bias, if only the more lithified (higher velocity and resistivity) layers were recovered in the last few cores,
• Increase of borehole diameter, or
• Fluid pressure greater than hydrostatic in the formation or in the borehole, causing dilatancy.

Caliper data from the Azimuthal Density Neutron tool and measurement-while-drilling annular pressure data do not indicate an increase of borehole diameter or of borehole fluid pressure above 450 m LSF. In fact, conditions in the LWD hole deteriorate only below 500 m LSF. Sampling bias and/or elevated formation fluid pressure remain possible explanations.

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