Radon concentrations in soil gas, considering radioactive equilibrium conditions with application to estimating fault-zone geometry
Radon concentrations in soil gas, considering radioactive equilibrium conditions with application to estimating fault-zone geometry
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DOI：
10.1007/s00254-008-1252-x
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发表时间：
2009-02-01
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影响因子：
--
通讯作者：
Asaue, Hisafumi
中科院分区：
文献类型：
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作者：
Koike, Katsuaki;Yoshinaga, Tohru;Asaue, Hisafumi
本文提出了一种测定测量开始时Rn同位素及其子体含量的计算方法，作为估算土壤气中Rn初始浓度的一种更为准确的方法。CRAS利用Rn-222和Rn-220同位素与子产物Po-218和Po-216之间的衰变规律，适用于α闪烁计数器测量。由于Rn是惰性的且化学稳定的，因此它可用于基于土壤气体地球化学的断层调查。然而，由Rn衰变发射的α粒子的总数通常被认为与初始Rn浓度成比例，而不考虑相对于放射性平衡的气体条件。CRAS方法是有效的，以获得土壤气体的Rn浓度在两个非平衡条件下，其中衰变的总数随时间的推移而增加，和平衡条件下，这是典型的正常土壤下的低气体通量。将CRAS方法与有限差分法模拟相结合应用于日本两个活动断层区的分析，结果表明，这种组合可以检测到与断层相关的Rn-222浓度的急剧上升。该方法还允许确定故障的几何形状附近的表面的基础上的Rn浓度分布的不对称变化时，再加上一个数值模拟的Rn-222运输。新方法应用于两个案例研究的结果与地质调查收集的数据一致。这意味着CRAS方法适用于研究断裂系统和间隙气体通过裂缝的活动性。目前的分析还表明，高Rn浓度需要最近和重复积累的Rn-222父母（Th-230和Ra-226）在断层泥通过深层气体释放在断层运动。
A calculation method for determining the amount of Rn isotopes and daughter products at the start of measurement (CRAS) is proposed as a more accurate means of estimating the initial Rn concentration in soil gas. The CRAS utilizes the decay law between Rn-222 and Rn-220 isotopes and the daughter products Po-218 and Po-216, and is applicable to alpha-scintillation counter measurements. As Rn is both inert and chemically stable, it is useful for fault investigation based on the soil gas geochemistry. However, the total number of alpha particles emitted by the decay of Rn has generally been considered to be proportional to the initial Rn concentration, without considering the gas condition with respect to radioactive equilibrium. The CRAS method is shown to be effective to derive Rn concentration for soil gases under both nonequilibrium conditions, in which the total number of decays increases with time, and equilibrium conditions, which are typical of normal soil under low gas flux. The CRAS method in conjunction with finite difference method simulation is applied to the analysis of two active fault areas in Japan, and it is demonstrated that this combination could detect the sharp rises in Rn-222 concentrations associated with faults. The method also allows the determination of fault geometry near the surface based on the asymmetry variation of the Rn concentration distribution when coupled with a numerical simulation of Rn-222 transport. The results for the new method as applied to the two case studies are consistent with the data collected from the geological survey. It implies that the CRAS method is suitable for investigating the fault system and interstitial gas mobility through fractures. The present analyses have also demonstrated that high Rn concentrations require the recent and repeated accumulation of Rn-222 parents (Th-230 and Ra-226) in fault gouges through deep gas release during fault movement.