5:15 PM - 6:30 PM
[SSS10-P02] Determination of the youngest active domain in major fault zones using CT number by medical X-ray CT
Keywords:medical X-ray CT, CT number, bulk density, effective atomic number, rock/protolith density ratio, classification of fault rocks
For faults without overlaying Quaternary sediments, determination of the most recent activity zone on a fault is significant for not only evaluation of fault activity, restoration of the latest stress field obtained from slip data, but also earthquake disaster prevention. Considering the exhumation of a fault zone, the most recent activity zone is supposed to correspond to an activity trace at the minimum depth of the fault, so the most vulnerable area, where rock/protolith density ratio is minimum, is assumed to be the most recent activity zone of a fault. However, since it is not easy to measure the density of fault rocks, we utilized medical X-ray CT, which is a non-destructive technique that enables materials to be observed and analyzed, and attempted to determine the most recent activity zone on a fault by using CT values, which are a function of density and effective atomic number.
X-ray CT is a non-destructive technique that allows the three-dimensional internal structure of materials to be observed and analyzed. The possibility of using X-ray CT in geoscience was recognized soon after its development as a medical imaging technique in 1972, resulting in a large number of studies starting in the 1980s (e.g., Wellington and Vinegar, 1987; Geet et al., 2000; Ueta et al., 2000).
A CT image is essentially a bitmap of each pixel’s CT number, but it contains various artifacts caused by X-ray photography and image reconstruction. Therefore, the effects of these artifacts, especially BH, must be eliminated or reduced to ensure the depiction of accurate CT numbers and allow accurate quantitative analysis.
Iwamori et al. (2020) investigated six kinds of mineral samples with known densities and effective atomic numbers using CT images taken by medical CT and reported a method for estimating density and effective atomic number using CT images taken with one tube voltage.
This study considered samples of fault rocks and protoliths derived from pelitic schist, tonalite, metabasalt, and granite at active faults (the MTL, Tsuruga Fault, and Yamada Fault) and an inactive fault (the MTL). The feature of density, porosity and effective atomic number were organized by fault and kind of protolith and the relationship among CT value, density and effective atomic number are investigated. Our major findings and conclusions are as follows.
The fault rock density decreases as it approaches the latest active fault plane and the porosity tends to increase by about 24% as the density decreases by 1 g / cm3 regardless of the kinds of fault rocks and protoliths. There is a unique positive correlation between the density and the effective atomic number for each fault and each rock species. The mode value of CT number, NCTM, calculated from a two-dimensional CT image (excluding the periphery where BH is significant and the influence of cracks and of minerals with a large effective atomic number) can be used to estimate the bulk density and the effective atomic number of rock samples (protolith and fault rocks). The latest active area of the fault rocks can be recognized as the area with the smallest CT value and rock/protolith density ratio.
References
Geet, M.V., Swennen, R., Wevers, M., 2000, Sediment. Geol., 132, 25-36
Iwamori, A., Takagi, H., Asahi, N., Sugimori, T., Nakata, E., Nohara, S., Ueta, K., 2020, Jap. Mag. Mineral. Petrol. Sci., 49, 101-117.
Ueta, K., Tani, K., Kato, T., 2000, J. Eng. Geol., 56, 197-210.
Wellington, S. L., and Vinegar, H. J., 1987, J. Pet. Technol., 39, 885-898.
X-ray CT is a non-destructive technique that allows the three-dimensional internal structure of materials to be observed and analyzed. The possibility of using X-ray CT in geoscience was recognized soon after its development as a medical imaging technique in 1972, resulting in a large number of studies starting in the 1980s (e.g., Wellington and Vinegar, 1987; Geet et al., 2000; Ueta et al., 2000).
A CT image is essentially a bitmap of each pixel’s CT number, but it contains various artifacts caused by X-ray photography and image reconstruction. Therefore, the effects of these artifacts, especially BH, must be eliminated or reduced to ensure the depiction of accurate CT numbers and allow accurate quantitative analysis.
Iwamori et al. (2020) investigated six kinds of mineral samples with known densities and effective atomic numbers using CT images taken by medical CT and reported a method for estimating density and effective atomic number using CT images taken with one tube voltage.
This study considered samples of fault rocks and protoliths derived from pelitic schist, tonalite, metabasalt, and granite at active faults (the MTL, Tsuruga Fault, and Yamada Fault) and an inactive fault (the MTL). The feature of density, porosity and effective atomic number were organized by fault and kind of protolith and the relationship among CT value, density and effective atomic number are investigated. Our major findings and conclusions are as follows.
The fault rock density decreases as it approaches the latest active fault plane and the porosity tends to increase by about 24% as the density decreases by 1 g / cm3 regardless of the kinds of fault rocks and protoliths. There is a unique positive correlation between the density and the effective atomic number for each fault and each rock species. The mode value of CT number, NCTM, calculated from a two-dimensional CT image (excluding the periphery where BH is significant and the influence of cracks and of minerals with a large effective atomic number) can be used to estimate the bulk density and the effective atomic number of rock samples (protolith and fault rocks). The latest active area of the fault rocks can be recognized as the area with the smallest CT value and rock/protolith density ratio.
References
Geet, M.V., Swennen, R., Wevers, M., 2000, Sediment. Geol., 132, 25-36
Iwamori, A., Takagi, H., Asahi, N., Sugimori, T., Nakata, E., Nohara, S., Ueta, K., 2020, Jap. Mag. Mineral. Petrol. Sci., 49, 101-117.
Ueta, K., Tani, K., Kato, T., 2000, J. Eng. Geol., 56, 197-210.
Wellington, S. L., and Vinegar, H. J., 1987, J. Pet. Technol., 39, 885-898.