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doi:10.2204/iodp.proc.319.102.2010

X-ray computed tomography

X-ray CT 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 "cookbook" prepared by Center for Deep Earth Exploration (CDEX)/Japan Agency for Marine-Earth Science and Technology (JAMSTEC) (X-ray CT Scanner, version 3.00, 31 July 2007) and used during previous expeditions (e.g., IODP Expedition 315). The cookbook is based on GE Healthcare (2006), Mees et al. (2003), and Nakano et al. (2000).

The X-ray CT instrument on the D/V Chikyu is a GE Yokogawa Medical Systems LightSpeed Ultra 16 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 (Table T1). 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

I = I0 × e–ηL, (1)

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

CT number = [(ηt – ηw)/ηw] × 1000, (2)

where

  • ηt = LAC for the target material, and

  • ηw = 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 twodimensional (2-D) slices yield a representation of attenuation values in three-dimensional (3-D) pixels referred to as voxels.

Analytical standards used during Expedition 319 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.

X-ray CT scan data usage

X-ray CT scans were used during Expedition 319 to

  • 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.

X-ray CT scanning was done 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 on 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).