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doi:10.2204/iodp.proc.338.102.2014

X-ray computed tomography

X-ray CT imaging provided information about structures and sedimentological features in cores and helped to assess sample locations and quality for whole-round samples. Our methods followed those in the measurement manual prepared by CDEX/JAMSTEC (3-D X-ray CT Scanning, Version 2.10, 3 July 2012) and used during previous expeditions (e.g., Expedition 337). The manual is based on GE Healthcare (2007), Mees et al. (2003), and Nakano et al. (2000).

The X-ray CT instrument on the Chikyu is a GE Yokogawa Medical Systems LightSpeed Ultra 16 capable of scanning a 1.5 m core sample in 5 min, generating 0.625 mm thick slice images. Data generated for each core consist of core-axis-normal planes of X-ray attenuation values with dimensions of 512 × 512 pixels. Data were stored as Digital Imaging and Communication in Medicine (DICOM) formatted files.

Background

The theory behind X-ray CT has been well established through medical research and is very briefly outlined here. X-ray intensity varies as a function of X-ray path length and the linear attenuation coefficient (LAC) of the target material as

where

• I = transmitted X-ray intensity,
• I₀ = initial X-ray intensity,
• η = LAC of the target material, and
• L = X-ray path length through the material.

LAC is a function of the chemical composition and density of the target material. The basic measure of attenuation, or radiodensity, is the CT number given in Hounsfield units (HU) and is defined as

where

• η_t = LAC for the target material, and
• η_w = LAC for water.

The distribution of attenuation values mapped to an individual slice comprises the raw data that are used for subsequent image processing. Successive 2-D slices yield a representation of attenuation values in 3-D pixels referred to as voxels.

Calibration standards used during Expedition 338 were air (CT number = –1000), water (CT number = 0), and aluminum (2477 < CT number < 2487) in an acrylic core mock-up. All three standards were run once daily after air calibration. For each standard analysis, the CT number was determined for a 24.85 mm² area at fixed coordinates near the center of the cylinder. A reference “core sample” for quality control was a three-layer sample: one section was filled with air and one section was filled with both water and a stepped piece of aluminum. This reference standard was used to calibrate CT numbers of air, water, and aluminum when the “Fast Calibration” CT numbers of these three references fell out of normal range.

X-ray CT scan data usage

X-ray CT scans were used during Expedition 338 to

• Examine 3-D features of deformation structures, bioturbation, and so on;
• Distinguish “natural” fracture/faults from drilling-induced fractures;
• Measure strike and dip angles of planar structures such as faults, bedding, veins, and so on;
• Provide an assessment of core and core liner integrity;
• Determine locations for whole-round samples; and
• Identify important structural or sedimentological features to be avoided by whole-round sampling.

X-ray CT scanning was performed immediately after core cutting for time-sensitive (e.g., anelastic strain and interstitial water) whole-round samples to finalize selection of the samples. All whole-round core sections were screened to avoid destructive testing of core samples that might contain critical structural features. This also ensured minimal drilling disturbance of whole-round samples and an assessment of heterogeneity (essential for postexpedition physical and mechanical property studies).

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