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[MIS23-P07] An attempt to evaluate lithology and fracture properties of accretionary complexes by X-ray CT image analysis of borehole cores
Keywords:borehole core, x-ray CT, Neogene sedimentary rock, accretionary complex, ImageJ
X-ray CT images of borehole cores obtained from a deep borehole drilling in an accretionary complex were analyzed to quantitatively evaluate changes in density, diameter, fracture frequency, fracture and deformation structures of the cores with depth. The geology of the borehole is Neogene sedimentary rocks (Hayama Group), an accretionary complex belonging to the most recent age exposed on land (e.g. Takahashi, 2008). Visual observation of the borehole core revealed that the core is mainly mudstone containing tuffs in patchy and massive form, with small-scale fault gouge and faults with fault angular conglomerate, but no structures that can be judged as clear stratigraphic boundaries were observed, and the core was monotonous with little change in character. A multi-slice helical X-ray CT scanner (Aquilion precision, Canon Medical Systems) owned by Central Research Institute of Electric Power Industry was used for CT imaging of the borehole core. The imaging conditions were tube voltage 140 kVp, tube current 200 mA and detector settings of 0.25 mm slice thickness x 160 rows. Drilling cores were removed from the core box, placed on a dedicated table and imaged one meter at a time. The CT images of the cores were taken one month after the cores were drilled, due to the schedule for CT imaging, and the observation of the internal structure of the cores by CT imaging confirmed the development of mosaic-like fractures without apertures in the mudstone at all depths. The CT image analysis using ImageJ quantitatively determined (1) the density, (2) the diameter and (3) the fracture characteristics in the core in the depth direction. The analysis method and results are reported below.
(1) Variation in core density
Changes in brightness values (≈density) in the depth direction were observed for slice images perpendicular to the body axis direction of the core; CT images were extracted at 1 mm intervals, the center 40 mm of the core was cut out to avoid the effect of beam hardening observed on the slice images and the brightness values were averaged within one slice image. The results showed that the core density tended to change at certain intervals, regardless of the lithology type. In comparison with the results of analyses using other methods, this may reflect the degree of deformation, consolidation, clayification.
(2) Changes in core diameter
As mentioned above, the CT images were taken one month after excavation. The cores visually showed a change in diameter due to expansion at different depths. In order to quantitatively indicate the interval of expansion, the core diameters were measured continuously at 1 cm intervals in the direction of depth by CT image analysis. The borehole core diameters averaged approximately 83 mm. In comparison with this value, the core diameter tended to increase continuously from 140 m to 310 m depth. Locally, core diameters increased from 4 mm to a maximum of 7 mm at some depths, and at depths where core diameters increased, smectite and illite were identified from XRD analysis. The cores after drilling were regularly sprayed with tap water to prevent drying, which is thought to have caused the expansion of clay minerals.
(3) Variation in fracture characteristics
From CT images, it was observed that mudstones in the Hayama Group have four types of fracture characteristics in terms of fracture frequency and fracture shape: ‘hard’, ‘microfracture’, ‘dilated’ and ‘wrinkled’, and the depth at which cores with such characteristics are continuously produced (Hamada & Oyama, 2021; 2024). In order to quantitatively evaluate these changes in fracture characteristics, only fractures were extracted by binarization, and fractal dimensional analysis was carried out using the box-counting method, with the fractures being linearized. The results showed that the fractal dimension tended to increase with an increase in the number of extracted cracks and short length of cracks in the ‘microfracture’, ‘dilated’ and ‘wrinkled’ properties.
The results of (1)-(3) show that even core samples that appear monotonous in visual observation of cores can be quantitatively evaluated in terms of density, expansion, deformation and fracture structures by CT images, and are useful for discriminating geological structures at the core scale (cm to m scale) and for estimating constituent minerals from post-excavation changes.
(1) Variation in core density
Changes in brightness values (≈density) in the depth direction were observed for slice images perpendicular to the body axis direction of the core; CT images were extracted at 1 mm intervals, the center 40 mm of the core was cut out to avoid the effect of beam hardening observed on the slice images and the brightness values were averaged within one slice image. The results showed that the core density tended to change at certain intervals, regardless of the lithology type. In comparison with the results of analyses using other methods, this may reflect the degree of deformation, consolidation, clayification.
(2) Changes in core diameter
As mentioned above, the CT images were taken one month after excavation. The cores visually showed a change in diameter due to expansion at different depths. In order to quantitatively indicate the interval of expansion, the core diameters were measured continuously at 1 cm intervals in the direction of depth by CT image analysis. The borehole core diameters averaged approximately 83 mm. In comparison with this value, the core diameter tended to increase continuously from 140 m to 310 m depth. Locally, core diameters increased from 4 mm to a maximum of 7 mm at some depths, and at depths where core diameters increased, smectite and illite were identified from XRD analysis. The cores after drilling were regularly sprayed with tap water to prevent drying, which is thought to have caused the expansion of clay minerals.
(3) Variation in fracture characteristics
From CT images, it was observed that mudstones in the Hayama Group have four types of fracture characteristics in terms of fracture frequency and fracture shape: ‘hard’, ‘microfracture’, ‘dilated’ and ‘wrinkled’, and the depth at which cores with such characteristics are continuously produced (Hamada & Oyama, 2021; 2024). In order to quantitatively evaluate these changes in fracture characteristics, only fractures were extracted by binarization, and fractal dimensional analysis was carried out using the box-counting method, with the fractures being linearized. The results showed that the fractal dimension tended to increase with an increase in the number of extracted cracks and short length of cracks in the ‘microfracture’, ‘dilated’ and ‘wrinkled’ properties.
The results of (1)-(3) show that even core samples that appear monotonous in visual observation of cores can be quantitatively evaluated in terms of density, expansion, deformation and fracture structures by CT images, and are useful for discriminating geological structures at the core scale (cm to m scale) and for estimating constituent minerals from post-excavation changes.