During Leg 206 the external surfaces of whole-round basalt cores were scanned using the DMT Digital Color CoreScan system after they had been run through the MST but prior to splitting. In addition, all split archive-half cores were imaged in slabbed mode with the Geotek camera system. Core imaging during this leg had four main objectives:
Core orientation is particularly important for Leg 206 because Site 1256 has a low paleolatitude, which means the paleomagnetic inclination will be nearly horizontal and the magnetic polarity will be indeterminate from azimuthally unoriented cores. Similarly, without a known polarity, the paleomagnetic declination cannot be used to orient the core for structural analyses or for the determination of anisotropy of physical properties. The drill Site 1256 is sufficiently close (<10 km) to the Anomaly 5Bn/5Br boundary that the polarity cannot simply be assumed. In order to determine the source of marine magnetic anomalies, which is one of the leg objectives, estimating the true rather than the relative paleomagnetic direction is critical and can only be accomplished if the core is oriented.
The DMT Color CoreScan system (Fig. F18), developed by Deutsche Montan Technologie (DMT), is a portable core imaging device that was previously used on board the JOIDES Resolution during Leg 173 (Whitmarsh, Beslier, Wallace, et al., 1998), Leg 176 (Dick, Natland, Miller, et al., 1999), and Leg 197 (Tarduno, Duncan, Scholl, et al., 2002). Images are recorded on whole-round outer core surfaces using a 24-bit, three-color (red, green, and blue) charge-coupled device line-scan camera that has a resolution of 5184 pixels/m (131 pixels/in) and a spectral response of between 400 and 700 nm (DMT, 1996, 2000).
The whole-round core is rotated 360° around its cylindrical axis with the line-scan camera positioned parallel to the axis of rotation. The unrolled images, up to 1 m long, are recorded in 33-cm sections that are integrated and light calibrated using the DigiCore software provided with the DMT CoreScan System. Whole-round cores are scanned in the unrolled mode at a rate of ~1.2 min/m, creating a ~14-MB bitmap file (DMT, 1996).
During Leg 206, all core pieces that could be rotated cleanly through 360° were scanned in unrolled mode. The conventional ODP core piece numbers, associated curated depths, and piece lengths were entered into a spreadsheet (see "Digital Imaging" in "Basement Formed at Superfast Spreading Rate (Holes 1256C and 1256C)" and Table T45 both in the "Site 1256" chapter). Pieces that were not fully cylindrical or intervals of unoriented drilling rubble were not imaged, but the lengths of these intervals were measured and recorded in the spreadsheet so that allowance could be made for them when integrated into core barrel lengths using the DMT CoreLog software (DMT, 1996, 2000). The vertical line marked on the core with a red grease pencil allows an initial reorientation of the core images back to the ODP reference frame. When the cores images are unrolled, nonhorizontal planar structures (e.g., veins, faults, or fractures) produce sinusoidal-shaped curves. These can be matched to similar shaped features imaged along the borehole wall by the four pads of the FMS logging tool or by the Ultrasonic Borehole Imager (UBI) (see "Ultrasonic Borehole Images" in "Tool String Configurations and Geophysical Measurements" in "Downhole Measurements"). Other distinct petrological features or structures that are imaged on the outer surface of the core and the borehole wall can be similarly matched to determine the depth of the core in the borehole and reorient the core azimuth (Haggas et al., 2001). Initial comparisons with FMS and UBI images and structural analyses were performed onboard, but detailed structural analysis, core-log integration, and core reorientation work will be done postcruise.