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doi:10.2204/iodp.proc.345.102.2014

Core section image analysis

During Expedition 345, external surfaces of both archive-half and whole-round core sections were scanned using an experimental system involving multiple passes through the SHIL. 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 features on unrolled 360° images for comparison with core structural analysis;

3. To correlate core images with Formation MicroScanner (FMS) images of the borehole wall to determine true core depth as opposed to curated depth calculations in intervals with <100% recovery; and

4. To match structures observed on core images with FMS images to permit reoriention of core pieces and associated structural data to magnetic north obtained from the General Purpose Inclinometer Tool (GPIT) on the FMS tool.

Core orientation was particularly important for Expedition 345 because Hess Deep is at low paleolatitude, which means the expected paleomagnetic inclination would be nearly horizontal and the magnetic polarity would 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.

Section Half Imaging Logger core scanning system

A system was developed at IODP-Texas A&M University 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. F6). 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 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. The split line therefore corresponds to the 90° direction (+y) in the core-reference coordinates used for structural geology and paleomagnetism (Fig. F2). 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 similarly shaped features imaged along the borehole wall by the four pads of the FMS logging tool. 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 of the aluminum strips in place to hold the core and two removed for access.

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

3. After imaging one surface, the 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. The individual images are automatically uploaded to the LIMS database. The imaging specialist then downloads the images, aligns them visually, labels them by section, and makes them available to the science party.

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. 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, clockwise downcore (increasing angle in IODP coordinates) left-to-right in the logging image but right-to-left in the core surface image.

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