1:45 PM - 3:15 PM
[O06-P71] Geotechnical Survey of Active Fault Zones using Radiation
Keywords:geology, radon, radiation, Senya Fault, Yokote River
Background.
In a study conducted on a river in Kyoto (Ishida et al., 2003), a relation between radon radiation and groundwater flow was investigated. Radon concentrations increased at the intersection of a fault line and a river. However, this study only suggested that this could be due to groundwater inflow near the fault. So far, no study has been conducted to determine whether faults are the direct cause of elevated radon concentrations, and the relationship between radon and faults is not clearly understood. Therefore, the purpose of this study was set to clarify the relationship between the characteristics of radon series and changes in radon content at faults.
Experimental Apparatus
In this experiment, a radon detector was used. By applying a voltage to the can in which the detector made of silicon is installed so that the detector side becomes a negative pole, polonium ions produced during radon decay can be collected, and by measuring the alpha decay of this polonium, radon can be observed indirectly. The signal from the detector is amplified and formatted, and processed by Arduino*.
*The mesurement was perfomed by Prof. Kazuo Tanaka of Waseda University.
Stone sampling location and measurement results
We set a basis point B, which is an intersection of Senya Fault and Yokote Basin East Rim Fault Zone in Yokote River. Senya Fault was caused by a major magnitude 7.2 earthquake of the Senya Earthquake in 1896.We also set point A at 519 m upstream from there, and 100 m downstream as point C. The size and weight of the sample stones were all different at A, B, and C. The measurement time was approximately the same for A, B, and C.
Peaks are found at 240ch, 280ch, 310ch, and 380ch from figureA,B,C. We cannot identify significant differences between sampling points due to insufficient statistics. And also, we cannot relate the ADC Value (channels) to the energy of the radiations due to the failure of the radon detector. However, the ratios of A and C are almost the same except for 310ch, whereas the ratio of B is sparse due to the small number of counts(table1).
Discussion
The fact that the same ADC peaks were observed at sites A and C and the ratios of counts were close at those two sites suggests that the composition ratios of radon from the thorium series and radon from the uranium series in the stones collected at sites A and C were close, i.e., the radiation materials in the stones were the same at sites A and C. On the other hand, the counts at site B were generally low, even taking into account the mass of the stones and the time of measurement. On the other hand, the counts at site B were generally lower than those at sites A and C, even taking into account the mass of the stones and the measurement time. This may be due to the difference in the ratio of thorium- and uranium-series compositions. Further data accumulation and verification are needed because of the differences from previous studies.
Outlook
We would like to explore the reason why the counts at Point B were generally low for all ADC Value by increasing the data until the position of the peak in the graph can be clearly determined in the measurement at Point B.
Since the distance between the three points is small compared to the scale of the fault, we would like to collect stones from a wider area and re-measure them. In addition, we would like to measure the data under various conditions such as type of stone, weight, volume, and measurement time so that we can make more accurate comparisons. We would like to find out which materials have more radioactive decay and present the results and discussion.
In a study conducted on a river in Kyoto (Ishida et al., 2003), a relation between radon radiation and groundwater flow was investigated. Radon concentrations increased at the intersection of a fault line and a river. However, this study only suggested that this could be due to groundwater inflow near the fault. So far, no study has been conducted to determine whether faults are the direct cause of elevated radon concentrations, and the relationship between radon and faults is not clearly understood. Therefore, the purpose of this study was set to clarify the relationship between the characteristics of radon series and changes in radon content at faults.
Experimental Apparatus
In this experiment, a radon detector was used. By applying a voltage to the can in which the detector made of silicon is installed so that the detector side becomes a negative pole, polonium ions produced during radon decay can be collected, and by measuring the alpha decay of this polonium, radon can be observed indirectly. The signal from the detector is amplified and formatted, and processed by Arduino*.
*The mesurement was perfomed by Prof. Kazuo Tanaka of Waseda University.
Stone sampling location and measurement results
We set a basis point B, which is an intersection of Senya Fault and Yokote Basin East Rim Fault Zone in Yokote River. Senya Fault was caused by a major magnitude 7.2 earthquake of the Senya Earthquake in 1896.We also set point A at 519 m upstream from there, and 100 m downstream as point C. The size and weight of the sample stones were all different at A, B, and C. The measurement time was approximately the same for A, B, and C.
Peaks are found at 240ch, 280ch, 310ch, and 380ch from figureA,B,C. We cannot identify significant differences between sampling points due to insufficient statistics. And also, we cannot relate the ADC Value (channels) to the energy of the radiations due to the failure of the radon detector. However, the ratios of A and C are almost the same except for 310ch, whereas the ratio of B is sparse due to the small number of counts(table1).
Discussion
The fact that the same ADC peaks were observed at sites A and C and the ratios of counts were close at those two sites suggests that the composition ratios of radon from the thorium series and radon from the uranium series in the stones collected at sites A and C were close, i.e., the radiation materials in the stones were the same at sites A and C. On the other hand, the counts at site B were generally low, even taking into account the mass of the stones and the time of measurement. On the other hand, the counts at site B were generally lower than those at sites A and C, even taking into account the mass of the stones and the measurement time. This may be due to the difference in the ratio of thorium- and uranium-series compositions. Further data accumulation and verification are needed because of the differences from previous studies.
Outlook
We would like to explore the reason why the counts at Point B were generally low for all ADC Value by increasing the data until the position of the peak in the graph can be clearly determined in the measurement at Point B.
Since the distance between the three points is small compared to the scale of the fault, we would like to collect stones from a wider area and re-measure them. In addition, we would like to measure the data under various conditions such as type of stone, weight, volume, and measurement time so that we can make more accurate comparisons. We would like to find out which materials have more radioactive decay and present the results and discussion.