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doi:10.2204/iodp.proc.348.102.2015

X-ray computed tomography

XRCT imaging provided information about structures and sedimentological features in the core and helped to assess sample locations and quality for whole-round samples. Our methods followed those in the measurement manual prepared by the Center for Deep Earth Exploration (X-ray CT scanning, version 1.00; 26 Dec 2008) and used on previous expeditions (e.g., Expeditions 315, 316, 319, 322, and 331). The manual is based on GE Healthcare (2006), Mees et al. (2003), and Nakano et al. (2000).

The XRCT scanner on the Chikyu is a LightSpeed Ultra 16 (GE Yokogawa Medical Systems, Ltd.), capable of generating sixteen 0.625 mm thick slice images every 0.5 s (the time for one revolution of the X-ray source around the sample). 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

XRCT has been well established through medical research and is very briefly outlined here. CT images are built up from spatial distribution of the “CT value”:

where µ_t is the coefficient of absorption at scanning point, µ_w is the coefficient for absorption of water, and K is a constant (Hounsfield value) (Otani et al., 2010).

Analytical standards used during Expedition 348 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.

XRCT scan data usage

XRCT scans were used during Expedition 348 to

• Examine 3-D features of deformation structures, bioturbation, and so on;
• Distinguish “natural” fractures or faults and drilling-induced fractures;
• Measure dip angles of 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 and sedimentological features to be avoided by whole-round sampling.

XRCT scanning was done immediately after core cutting for selection of time-sensitive (interstitial water, microbiology, and organic geochemistry) samples. All whole-round core sections were screened to avoid destructive testing on intervals that might contain interesting structural or sedimentological features. This also facilitated identifying intervals with minimal drilling disturbance for whole-round sampling and for assessing heterogeneity (essential for postexpedition studies of frictional, geotechnical, and hydrogeological properties).

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