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doi:10.2204/iodp.proc.335.102.2012

Core section image analysis

During Expedition 335, external surfaces of whole-round core sections were scanned using an experimental system involving multiple passes through the IODP Section Half Imaging Logger (SHIL). In addition, all archive section halves were imaged with the same SHIL, following a long tradition using film photography or digital imaging. Core imaging during this expedition had four main objectives:

1. To provide a comprehensive suite of digital core images, including both unrolled 360° and section half surface images, to aid petrological interpretation;

2. To identify and measure planar features on unrolled 360° images for comparison with core structural analysis and integration with structures measured on geographically oriented FMS and UBI images (see “Formation MicroScanner” and “Ultrasonic borehole images”);

3. To correlate core images with FMS and UBI images of the borehole wall 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 GPIT on the FMS and UBI tool strings.

Core orientation is particularly important for Expedition 335 because Site 1256 is at 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.

SHIL core scanning system

To avoid the expense of renting a dedicated 360° core scanner (e.g., the DMT Color CoreScan system deployed during Leg 206 and Expedition 309/312, among other expeditions), an experimental system was developed at IODP-USIO to use the existing, higher resolution SHIL generally used for section half surfaces to image the outer surface of the core. The main element of this system is an aluminum frame that can simultaneously hold the cylindrical pieces of a single core section and rotate them in 90° increments (Fig. F29). The frame consists of four aluminum strips ~155 cm × 4 cm in dimension, all of which latch at each end into a pair of rotatable spindles. Each strip is milled with a concave surface that rests against the core pieces. Four images of the core surface are later processed to simulate a continuous image of the unrolled 360° surface.

Methodology

On each core piece, a vertical red line was drawn with a wax 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 (Fig. F2). The split line therefore corresponds to the 90° direction (+y) in the core-reference coordinates used for structural geology and paleomagnetism. When the core images are processed to a simulated unrolled image, nonhorizontal planar structures (e.g., veins, faults, or fractures) should 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. 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 (e.g., MacLeod et al., 1994; Morris et al., 2009).

In detail, the process for generating the simulated unrolled image includes the following steps:

1. The vertically oriented pieces for a single section are placed at their curated relative depths within the aluminum frame, with two aluminum strips in place to hold the core and two removed for access.

2. After the frame is moved to the SHIL, three aluminum strips secure the core surface and one is removed to allow imaging.

3. After imaging one surface, one aluminum strip is replaced, the section is turned 90°, and the next strip is removed to allow imaging.

This process is repeated to generate four images. These images are combined in a simulated unrolled image using a computer program written in C language that uses core diameter as the parameter to relocate pixels, stretching each image for the conversion from orthogonal to unrolled view and assembling them into a composite image with the image width scaled to the core circumference. The source code for this program is provided in WHOLE_ROUND_IMAGE_CODE in PHYSPROP in “Supplementary material.”

For matching core-surface features to borehole features, one generally must assume the features are planar to extrapolate across the material destroyed by the drill bit. An obvious point to consider is that the vertical extent of a planar feature will scale with the ratio of the borehole diameter to the core diameter, generally between 4 and 5 cm. A less obvious point is that the usual presentation of images looking inward toward the core surface but outward toward the borehole wall results in mirroring geometric relations, with clockwise downcore (increasing angle in IODP coordinates) left-to-right in the logging image but right-to-left in the core surface image. Generally at least three planar features in a large piece from a high-recovery core will be required for unambiguous orientation. Unfortunately, neither FMS nor UBI tools were successfully deployed during Expedition 335, so orientation of pieces below ~1410 mbsf from Expeditions 312 and 335 requires future logging.

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