11. Computed Tomography Scan Image Analysis of Sediments1

Three-dimensional high-resolution images of density distributions were obtained on unsplit cores using a medical X-ray computed tomography (CT) scanner. The resolution of CT scanning is about 0.6 mm2 and allows a 5-mm depth for beam width. Electrical resistivity measurements were also conducted to know porosity at high depth resolution after core splitting. Wholeround core samples for this study were collected from Sites 948 and 949 in the northern Barbados ridge accretionary prism during Ocean Drilling Program Leg 156. On the basis of the relationship between densities and linear attenuation coefficients of standard samples, densities derived from CT scans are consistent with densities by the mass/volume method of all the samples irrespective of their compositions. Sample disturbance is easily detected in CT-scan images by convolutional texture in CT images perpendicular to core axes. This nondestructive technique provides reliable densities at high depth resolutions for evaluating disturbances by drilling or core splitting. INTRODUCTION A large amount of physical property data has been collected worldwide by Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) cruises. During Leg 156, a logging-while-drilling (LWD) tool acquired density and formation conductivity profiles (Shipley, Ogawa, Blum, et al., 1995). This logging technique is very effective, especially in the accretionary prism where boreholes are unstable. Moore et al. (1995) reported abnormally low density in the décollement zone, suggesting the existence of high pore-pressure zones. Continuous or dense shipboard measurements of physical properties are required for correlation with logging data and studies of high-resolution sedimentary environments. ODP shipboard measurements of sediment density are usually made on cores at intervals of several decimeters. The gamma-ray attenuation porosity evaluator (GRAPE) has provided bulk density data (Evans, 1965) at 2-cm depth intervals. Without considering core condition, however, bulk densities derived from GRAPE can be misleading or incorrect due to original and/or artificial fracturing, or gas expansion (Westbrook, Carson, Musgrave, et al., 1994; Shipley, Ogawa, Blum, et al., 1995). The X-ray computed tomography (CT) technique has the advantage of being nondestructive and providing high-resolution density data and images. Use of the CT technique for geological samples has increased recently (e.g., Vinegar, 1986; Kenter, 1989; Kimura et al., 1989; Holler and Kögler, 1990; Orsi, 1994; Orsi et al., 1994; Boespflug et al., 1995). CT studies of internal structures and density determinations were also done for deep-sea drilling samples (Soh et al.,1993; Rutledge et al., 1995; Ashi, 1995). During Leg 156, eight whole-round and one half-round core samples were collected from the toe of the northern Barbados accretionary prism (Fig. 1, Table 1). All samples consist of semiconsolidated gray, light brownish gray, or pale yellowish claystone. In this study, CT and electrical resistivity are employed to evaluate small-scale density and porosity variations, respectively. Both results are calibrated using discrete physical property data. 1Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results, Vol. 156: College Station, TX (Ocean Drilling Program). 2Geological Institute, University of Tokyo, 7-3-1 Bunkyo Hongo, Tokyo, 113 Japan. [email protected] 3UHYLRXV &KDSWHU 7DEOH RI & METHODS Computed Tomography Scanning Two types of commercial medical X-ray CT scanners (Toshiba TCT-700S and Xforce) were used to acquire the CT images. In these systems, the X-ray source produces a fan-shaped collimated beam, and the beam rotates around the sample, with a detector pad at the opposite side. The CT scan image is mathematically reconstructed using the intensities of the transmitted X-ray beam collected at regular increments of rotation around the sample (Iwai, 1979; Morgan, 1983). The intensity of the transmitted X-ray beam is usually expressed as the CT value, i.e., the ratio of the linear attenuation coefficient μ of the material to that of pure water. A two-dimensional, black-and-white image corresponding to CT values is derived by irradiating a slice of finite thickness. Two different procedures were adopted for each scanner. In the TCT-700S type, samples were turned on the core axes and slice images were taken at 15° intervals. Blackand-white images from the CT scan were constructed using histogram equalization, which determines brightness windows and ranges, on the image processing system (Inazaki and Nakano, 1993). Histogram equalization provides contrastive images for structure observations. In the Xforce type, 2-mm slice profiles were taken sequentially from the top to the bottom of each whole-round core, and processed to make three-dimensional images by the core analysis system at Technology Research Center, Japan National Oil Corporation (JNOC). As a result, three-dimensional images derived from the Xforce scanner are helpful in the understanding of internal core structures. Reconstruction using 2-mm slices, however, results in a slight degradation of the images. CT scan data using TCT-700S are mainly discussed below. The unit volume for which the CT value is acquired depends on the slice width of scanning. This is changeable within 1, 2, 5, and 10 mm, although the resolution in a profile image is ~0.6 mm2. Using a 5-mm slice width, the unit volume corresponds to a 0.6 mm × 0.6 mm × 5.0 mm cube. CT values were calibrated for density using six bentonite standards with different densities regulated by void ratio (Ashi, 1995). The linear regression is density = CT value/1250 + 1.0 (R2 = 0.902). Linear attenuation coefficients depend on material compositions (e.g., Hounsfield, 1973). This is a question to be considered later. 151 RQWHQWV 1H[W &KDSWHU