Numerical simulation for migration rule of fault gas radon in different overburden
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摘要: 断层气氡浓度是探测断层位置与断层活动性的一种有效手段. 该文基于Abdoh和Pilkington提出的氡迁移二维偏微分方程与相应的边界条件， 建立内部含裂隙、 裂隙系-断层带和非均质等3种覆盖层物理模型， 在Matlab平台上运用偏微分工具箱(pdetool)与非线性求解函数(pdenonlin), 对模型求解以及模拟覆盖层中氡迁移. 通过对3种模型的模拟结果分析， 分别解释了地表氡异常点与断层带位置不同步现象, 覆盖层厚度对氡浓度曲线形状的影响, 以及土壤结构性质对氡异常强度和异常形态的影响.Abstract: Measuring the concentration of the fault gas radon is an effective method to explore fault location and fault activity. Based on Abdoh and Pilkington’s two-dimensional partial differential equation and corresponding boundary conditions of radon migration, we establish three physical models of the overburden: internal fracture, fracture-fault and heterogeneous overburden. Then we solve these models and simulate radon migration in the overburden using partial differential toolbox (pdetool) and non-linear solution function (pdenonlin) on the Matlab platform. Finally, we analyze the simulation results of the three models, which explain the phenomena that the radon abnormal points on overburden surface are out of sync with the fault location. It is also interpreted the effect of thickness of overburden on the shape of the radon concentration curve, and impactions of the soil structural properties on the radon anomaly intensities and shapes.
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Keywords:
- fault gas /
- radon /
- migration rule /
- numerical simulation /
- physical model /
- Matlab
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图 1 含氡源断层带上方覆盖层的物理模型(引自Abdoh， Pilkington， 1989)
Figure 1. The physical model of fault zone that contains a radon source and is covered by overburden (after Abdoh, Pilkington, 1989)
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图 2 断层带上方含裂隙覆盖层的物理模型 (a) 裂隙倾角变化模型; (b) 裂隙偏移断裂带模型
Figure 2. (a) The model with variable dip-angle of the fracture; (b) The model with variable fracture deviation from fault zone
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图 3 裂隙倾角(t)变化对氡迁移的影响
Figure 3. Effect of the changes in dip-angle(t) of the fracture on radon migration in overburden
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图 4 裂隙偏移断裂带的距离(W3)变化对氡迁移的影响
Figure 4. Effect of the distance that the fracture deviates from the fault zone on radon migration in overburden
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图 5 裂隙系-断层带上方覆盖层的物理模型
Figure 5. The model of fracture system-fault zone covered by overburden
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图 6 裂隙系-断层带上方覆盖层中氡迁移与断层位置关系图
(a) 氡迁移规律模拟三维立体图; (b) 氡迁移浓度分布剖面图
Figure 6. Plot of radon migration with fault position in the overburden on the fracture system-fault zone
(a) 3D numerical simulation of radon migration； (b) Distribution of radon concentration
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图 7 断层-裂隙系上方的覆盖层厚度(b)与地表氡浓度峰值关系图(氡浓度单位： Bq/m3)
Figure 7. Relationship of thickness (b) of the overburden and radon concentration peak on surface above the fracture system-fault zone (radon concentration units: Bq/m3)
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图 8 断层上方非均质覆盖层物理模型
Figure 8. The model of the fault zone covered by nonhomogeneous overburden
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图 9 非均质覆盖层(a)与均质覆盖层(b)氡迁移规律模拟三维立体图
Figure 9. 3D plot of numerical simulation on radon migration in nonhomogeneous (a) and homogeneous overburdens (b)
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图 10 非均质覆盖层与均质覆盖层氡迁移浓度分布等值线对比图
Figure 10. The contour plots of radon concentration distribution in nonhomogeneous and homogeneous overburdens
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图 11 断层带上方地表下2 m 处的氡浓度曲线
Figure 11. Radon concentration curve 2 m beneath the ground across the fault zone
表 1 氡气在不同土壤性质下的有效扩散系数(D*)、 扩散系数(D)和孔隙度(e)三者之间的关系(据吴慧山等， 1995)
Table 1 The relationship among effective diffusion coefficient (D*), diffusion coefficient (D) and porosity (e) of radon in different types of soils (Wu et al, 1995)
土壤性质 孔隙度e 扩散系数D/(10-2 cm2·s-1) 有效扩散系数D*/(cm2·s-1) 砂子 40% 4.5—7.0 0.11—0.175 疏松沉积物 20% 2.0—2.5 0.1—0.125 白黏土 59.3% 1.53 0.023 砂质黏土 10.8% 1.09 0.1 
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