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doi:10.2204/iodp.proc.309312.102.2006

Digital imaging

During Expedition 309/312, external surfaces of whole-round basalt, dike, and gabbro 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 archive-half cores were imaged in slabbed mode with the Geotek DIS. Core imaging during this expedition had four main objectives:

1. To provide a comprehensive suite of digital core images, including both unrolled 360° and slabbed images, to aid petrological interpretation;
2. To identify and measure planar features on unrolled images for comparison with core structural analysis and integration with structures measured on geographically oriented FMS and Ultrasonic Borehole Imager (UBI) images (see “High-resolution electrical images” and “Ultrasonic borehole images”);
3. To correlate core images with FMS and UBI images of the borehole wall derived from downhole logging measurements to determine true core depth as opposed to curated depth in intervals with <100% recovery; and
4. To match structures observed on core images with FMS and UBI images, as well as to reorient core pieces and associated structural data to magnetic north obtained from the General Purpose Inclinometer Tool (GPIT) on the FMS and UBI tool strings.

Core orientation is particularly important for Expedition 309/312 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. Site 1256 is sufficiently close (<10 km) to the magnetic 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 expedition objectives, estimating the true, rather than the relative, paleomagnetic direction is critical and can only be accomplished if the core is oriented.

DMT CoreScan system

The DMT Color CoreScan system (Fig. F18), developed by Deutsche Montan Technologie, is a portable core imaging device that was previously used on board the JOIDES Resolution during ODP Legs 173 (Whitmarsh, Beslier, Wallace, et al., 1998), 176 (Dick, Natland, Miller, et al., 1999), 197 (Tarduno, Duncan, Scholl, et al., 2002), and 206 (Wilson, Teagle, Acton, et al., 2003). Images are recorded on whole-round outer core surfaces using a 3 pixel × 1728 pixel, 24 bit/pixel, three-color (red, green, and blue) charge-coupled device line-scan camera that has a spectral response between 400 and 700 nm, positioned to have a resolution of 5.0 pixel/mm (DMT GeoTec, 1996; DMT GmbH, 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 1.5 m long, are recorded in 30–34 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 per meter of core (DMT GeoTec, 1996). The scanned images were then integrated for each core using the Core Recovery Quality Control program (DMT GmbH, 2000). The images are plotted on a depth scale according to their IODP curated depths and then edited and saved in Adobe Illustrator format with a corresponding slab image of split archive-half cores obtained by Geotek DIS.

Methodology

During Expedition 309/312, all core pieces that could be rotated cleanly through 360° were scanned in unrolled mode. Conventional IODP core piece numbers, associated curated depths, and piece lengths were entered into a spreadsheet (see “Digital imaging” in “Expedition 309” and “Digital imaging” in “Expedition 312” in the “Site 1256” chapter and SCANLOG.XLS in “Supplementary material”). The lengths of intervals of unscanned pieces 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 GeoTec, 1996; DMT GmbH, 2000). During Expedition 312, some pieces that were not fully cylindrical or oriented were scanned because subparallel horizontal fractures and vein features on the core surface may prove to help finalize depth-matching between whole core images and FMS logs.

On each core piece, a vertical red line was drawn with a red grease pencil to define the core split. Convention is such that, with the core upright, the archive half is to the left of this line and the working half is to the right. When the core 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 or by the UBI logging tools (see “Ultrasonic borehole images”). 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.

Funding for the DMT 360° core scanner on Expedition 309/312 was provided by Natural Environment Research Council (U.K.) Integrated Ocean Drilling Program Directed Science Programme Urgency Grant (NE/D001277/1 to Teagle/Brewer) and the generous support of the School of Ocean and Earth Science, University of Southampton; the Department of Geology, University of Leicester; and the U.S. National Science Foundation, through the Integrated Ocean Drilling Program (Texas A&M University) and the United States Science Support Program (Joint Oceanographic Institutions, Inc.).

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