A total of 158 portions of whole-core samples were imaged with the portable CT scanner during Leg 210, and 23 2-D X-ray images were compared with digital images of the split surface with good correlation on the basis of cracks, lithologic boundaries, and characteristic sedimentary structures. Herein, we show results of this comparison for 17 samples divided into groups with characteristic sedimentary structures as below. Sampling horizons are shown using those in digital images.
Sample 210-1276A-21R-4, 108–117 cm (Fig. F1A, F1B), consists of horizontally laminated calcareous sandstone with intrabed folding structures in the upper part of the sample (see 110–111.5 cm in Fig. F1A). The X-ray image shows only several dark laminations thicker than ~1 mm. The half-core is broken at thick dark laminae horizons.
Sample 210-1276A-44R-5, 27–36 cm (Fig. F1C, F1D), consists of piled trough cross-laminated sandstone. The X-ray image shows laminae thicker than ~1 mm.
Sample 210-1276A-48R-3, 56–65 cm (Fig. F2A, F2B), consists of planar-trough cross-laminated sandstone. The X-ray image captured the clear appearance of trough cross-laminae thicker than ~1 mm.
Sample 210-1276A-48R-3, 65–74 cm (Fig. F2C, F2D), consists of trough cross-laminated and horizontally laminated sandstone in the upper part of the interval and faintly parallel laminated grainstone in the lower part. The X-ray image clearly captured the upper trough cross-laminae thicker than ~1 mm, whereas the X-ray image of the lower part of the sample is structureless despite coarse-grained mudstone fragments (maximum of ~1.5 mm length) concentrated in layers in the middle and basal parts of the grainstone.
Sample 210-1276A-55R-2, 122–131 cm (Fig. F3A, F3B), consists mainly of horizontally laminated sandstone. The lamination varies from ~2 to 0.5 mm thick. In the X-ray image, thin laminae (less than ~0.8 mm thick) cannot be recognized. Thin lamina bundles are recognized as thick laminae in the lower part of the sample (126–129 cm).
Sample 210-1276A-75R-6, 37–46 cm (Fig. F3C, F3D), consists of trough cross-laminated grainstone. Although the X-ray image was taken from the same direction with digital image, judging from the appearance of a crack, it does not show any lamination except for a lamina that formed the crack during core splitting.
Sample 210-1276A-89R-4, 103–111 cm (Fig. F4A, F4B), consists of parallel laminated sandstone. The laminae vary from 1 to 0.3 mm thick. The X-ray image faintly captures only two thick (8~4 mm) dark-colored beds.
Sample 210-1276A-96R-1, 65–74 cm (Fig. F4C, F4D), consists of parallel laminated calcareous sandstone. Discontinuous light-colored laminae and light-colored spots suggest recrystallization of the sandstone. The laminae vary from 2 to 0.5 mm thick. The X-ray image faintly captures several laminae thicker than ~1.5 mm.
Sample 210-1276A-53R-3, 0–9 cm (Fig. F5A, F5B), consists of laminated sandstone. The upper part of the sample shows trough cross-lamination and the lower part shows deformed lamination. The difference in the appearance of trough cross-laminae in the two images suggests that the X-ray image and split surface may be offset by 120°–150°. An obscure dark-colored band in the lower part (5–7 cm) of the X-ray image may correspond with thick deformed black laminae.
Sample 210-1276A-60R-1, 76–85 cm (Fig. F5C, F5D), consists of laminated siltstone. Dip angles of the parallel lamination in the lowermost part of the sample suggest that the X-ray image and split surface image are offset by 150°–180°. The lamination is deformed and partially subtle in the X-ray image.
Sample 210-1276A-61R-4, 55–64 cm (Fig. F6A, F6B), consists of laminated sandstone. Dip angles of the parallel lamination in the lower part of the sample and morphology of the intrabed folding suggests that the X-ray image and split surface are offset by ~45°. Soft-sediment deformation (intrabed folding) in the central part of the sample (57–61 cm) is seen clearly in both the DIS and X-ray CT images.
Sample 210-1276A-61R-2, 47–55 cm (Fig. F7A, F7B), consists of calcareous claystone with lenticular concretions. Two concretions (47 and 50 cm) visible in the X-ray image cannot be seen in the digital image. The upper lenticular concretion would have chipped out during core splitting, and the lower concretion is not exposed on the split surface.
Sample 210-1276A-61R-2, 56–64 cm (Fig. F7C, F7D), consists of calcareous claystone with a lenticular concretion and thin light-colored laminae. A 0.5-mm-thick lamina 2 mm upcore from the concretion (60.3 cm) is recognized on the X-ray image.
Sample 210-1276A-79R-2, 99–108 cm (Fig. F8A, F8B), consists of massive sandstone. The X-ray image shows a sparse distribution of light-colored mottles. A hydrochloric acid test on the splitting surface suggested that these mottles may consist of concreted carbonate minerals.
Sample 210-1276A-91R-3, 13–22 cm (Fig. F8C, F8D), consists of calcareous mudstone. The X-ray image captured a lenticular concretion and a water escape structure that cannot be seen on the digital image of the split surface. The water escape structure may also be filled with calcite.
Sample 210-1276A-1R-2, 90–99 cm (Fig. F9A, F9B), consists of bioturbated massive mudstone. A spiral stringlike structure in the lower part of the X-ray image was difficult to correlate with a corresponding structure on the split surface. The string is possibly a burrow filled with calcite or other minerals. It is notable that horizontal cracks developing on the split surface were not formed on the whole core observed by X-ray imaging. This means that the cracks formed during the core splitting and shows a significant advantage of X-ray observation with a CT scanner.
Sample 210-1276A-89R-6, 95–104 cm (Fig. F9C, F9D), consists of sandstone with convolute lamination and entrained and stretched intraclasts. The X-ray image captured yellowish gray intraclasts and lamination. These structures seem to include pyrite, judging from its color.
Descriptive features, size (thickness) of the structure in the digital images and in the 2-D X-ray images, lithologic composition, degree of deformation, and estimated offset between the digital and X-ray images are shown in Table T1.
Three-dimensional imaging with a medical CT scanner is time consuming and process intensive. However, it has some merits: (1) structures can be observed from any desired angle, and (2) observation can be limited by selecting a range of CT values. Selecting a range of values increases contrast, making sedimentary structures clearly visible.
Three-dimensional CT images of half-core Sample 210-1276A-44R-5, 27–36 cm, were taken from various directions around the axis of the core at CMCR, and processed in a selected CT value range of 1200–900. This sample corresponds to the sample shown in Figure F1C and F1D. As the core axis varies, some lamina sets drastically change in visibility. The changes are practically conspicuous in trough cross-laminated portions. Trough cross-lamination in the middle part of the sample is clearly visible in side view (Fig. F10A–F10C, F10H, F10I). The same structure in Figure F10D–F10F, F10J, and F10K is vague and the lamination is not clear. The 2-D X-ray image shown in Figure F1D is roughly equivalent to the images shown in Figure F10I or F10J.
Figure F11 shows 3-D CT images of Sample 210-1276A-48R-3, 56–65 cm, which corresponds with Figure F2A and F2B processed in a CT value range of 2100–1800. Visibility of cross-laminae sets changes according to changes in observation direction. Trough cross-lamination in the lower part of the sample is easily distinguished in side view in Figure F11A, F11B, F11G, and F11H. The same structure in Figure F11C–F11E, F11J, and F11K is not clear. Lamination in the upper part of the sample appears clearly in Figure F11E, F11F, F11K, and F11L but is not captured clearly in the Figure F11B–F11D and F11H–F11J. The 2-D X-ray image shown in Figure F2B is roughly equivalent to the one shown in Figure F11B, which was taken from a 30° oblique direction.