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doi:10.2204/iodp.proc.314315316.132.2009

X-ray computed tomography

X-ray CT imaging provided real-time information for Expedition 316 core logging and sampling strategies. The following explanation of our methods was derived from the “cookbook” prepared by CDEX/​Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for onboard use (X-ray CT Scanner, version 3.00, 31 July 2007) and practical experience. The cookbook is based on GE Healthcare (2006), 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 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 (see Table T1 for operating parameters). As a result of this high scanning rate, 1.5 m sections of core were typically imaged in <3 min. 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,
• µ = 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

• µ_t = LAC for the target material and
• µ_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 two-dimensional slices yield a representation of attenuation values in three-dimensional (3-D) pixels referred to as voxels. Visualizations are generally accomplished by varying the manner in which the CT number distribution is assigned to 256 shades of gray or a color pallet. Onboard analysis of CT images was made using both the Advantage Workstation (Lightspeed Ultra 16, General Electric Medical Systems) and the OsiriX 32-bit DICOM viewer (version 2.7.5) running on a G5 Macintosh (OS 10.4).

Analytical standards used during Expedition 316 were air (CT number = –1000), water (CT number = 0), and aluminum (2467 < CT number < 2487) in an acrylic core mock-up. All three standards were run once daily after air calibration, and once each week the three standards were run both before and 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 routinely during Expedition 316 to

• Provide an assessment of core recovery and core-liner integrity for drilling operations,
• Determine the appropriateness of whole-round sampling,
• Identify the location of subtle features that warrant detailed study and special handling during VCD and sampling, and
• Determine 3-D geometry, cross-cutting and other spatial relations, and orientation of primary and secondary structures.

X-ray CT scans provided timely information on the integrity of the cores. Cores that experienced gas expulsion during depressurization could be readily identified, as could cores in which extensive drilling-induced disturbance had occurred (Fig. F4A). During rotary drilling, CT scans were used to determine sections of the core that underwent differential rotation and helped determine sections of the core that are contiguous with respect to orientation (Fig. F4B). The amount of core recovery and nature of drilling-induced damage may depend on drilling technique, and information from CT scans was reported to CDEX for drilling operations (Fig. F5).

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 (essential for microbiology as well as physical and mechanical properties) and an assessment of whole-round homogeneity. X-ray CT scanning was done immediately after core cutting so that time-sensitive whole-round samples (e.g., those for interstitial water and microbiological analyses) could be included in this screening process.

The value of X-ray CT scans to identify sedimentary and tectonic features prior to VCD was quickly recognized (Fig. F6). Three-dimensional structure orientation in the core reference frame could easily be determined from CT scan sections, whereas performing the same measurement on the cores generally requires cutting orthogonal faces. Furthermore, structures could be classified as a function of their CT contrast, which is presumably related to porosity changes or chemical alteration within shear zones. Structural and stratigraphic CT observations are incorporated into structural geology and lithologic sections.

X-ray CT scan data have multiple uses, from early assessment of cores to description and synthesis. For this reason, several hundred gigabytes of scan data (~825 Mb/m) were stored on a local database at the OsiriX interpretation station. These data are archived to tape and stored on terabyte disks.

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