Microfocus X-ray CT imaging was conducted on a granite core, 50 mm in diameter and 100 mm in length, containing a single fracture in the longitudinal direction. The three-dimensional geometry and the aperture distribution of the fracture were evaluated by analyzing the CT data. A very notable artifact called beam hardening was found in the obtained CT images because of the high X-ray absorption coefficient of the material compared with the source intensity of the X-rays. Therefore, the CT image was corrected using a simulation image of the beam hardening of granite. As a result, the spatially averaged aperture thickness and the contact ratio of the fracture asperities under unconfined conditions were estimated to be 0.39 mm and 2.0%, respectively. The result was compared with the aperture distribution that was measured using a laser beam profiler system.
Precise measurement of the geometric characteristics of rock fractures such as the elevation distribution of fracture walls, aperture distribution, and contacting asperities within the fracture, is essential because of their significant influence on the mechanical and hydrological behaviors of rock fracture. On a laboratory scale, the elevation distribution of fracture walls can be measured with micrometer accuracy using a system that combines a laser displacement sensor and a high-precision automatic positioning stage (e.g., [1]). However, aperture distribution and contacting area within the fracture are rather difficult to measure experimentally. In previous researches, various techniques have been proposed such as 1) the surface topography approach, in which the topography of a pair of fracture surfaces is measured separately by a laser beam profiler, and the aperture is computed indirectly as the distance between the two fracture surfaces [2,3]; 2) the injection approach, in which the specimen containing the fracture is cut into slices after some resin has been injected, and the aperture is measured as the thickness of the injected resin [4,5]; and 3) the casting approach, in which replicas of the fracture apertures are made by casting [6]. X-ray computed tomography (CT) is a useful technique for visualizing the inner structure of rock samples in a noninvasive and nondestructive manner. It has also been used to measure fracture aperture and to detect contact areas (e.g., [11,13–15]). This study used a microfocus X-ray CT scanner to take CT images of a granite core containing a single fracture in the longitudinal direction. Based on the result of the CT imaging, we evaluated the three-dimensional geometry and the aperture thickness distribution of the fracture. The result was compared with the aperture distribution that was measured using a laser beam profiler system.
The specimen (Fig.1) is a granite core of 50 mm in diameter and 100 mm in length containing a single fracture in a longitudinal direction, which was sampled in Mizunami, Gifu Prefecture, Japan at the depth of 180 to 200 m. The bulk density was 2.58 g/cm3 and the porosity was 1.00%. Based on X-ray diffraction results, the major mineral components were quartz, feldspar and biotite.
