| Active fault activities can cause a wide range of natural disasters, in which the earthquake causes the most serious harm to human. The occurrence of the earthquake has a close relationship with fault activities. Great urban earthquake disaster is mainly caused by suddenly rapid dislocation of active fault located in the urban underground. The earthquake caused by active faults near town may also evoke activities of active fault and aggravate the building damage and ground disaster along fault-line. Along with the rapid economic development, urban construction speeds, urban population highly concentrate, and the destruction and loss caused by urban earthquake disaster is on a marked upward trend. Governments and scientists are well aware of the importance and urgency of active fault detection and study. China is a country where active faults widely distribute, also a country where the seismic activity is of high frequency and intensity, and earthquake disasters are serious. Urban active fault detection is of great practical significance for urban planning, earthquake-proof and reduction of earthquake damage to city facilities.Radon is a radioactive gas, whose matrix is deep underground radioactive element radium. In the nature, there are three radioisotopes: 219Rn, 220Rn and 222Rn. The half-lives are: 3.96s, 55.65s and 3.825d. 219Rn and 220Rn have too short half-lives to be tracer elements. 222Rn, which is stable in chemistry, has a strong capability of migration. It is often used as a tracer applied in various fields. The upward migrationof Radon from the deep underground is a complicated process, which is often subject to the control of many factors. The migration is a combined result of a variety of mechanisms. Study found that in the process that radon migrates from deep underground to active fault, radon's migration is its matrix uranium and radium's migration first, because of the restriction of radon's short half-life. Uranium and radium migrate from the internal earth to the earth's surface. In the broken zone of active fault, the cementation is poor, the effective porosity and permeability are high. 222Rn and its parent uranium and radium are enriched near fault and form radon source. This is the long-distance migration of 222Rn. Then in the Quaternary overburden above active fault, Radon migrates upward to the surface by diffusion and convection. This is the short-distance migration of 222Rn. Thus in the Quaternary overburden above the fault will create high concentrations of radon anomalies. Therefore, the location and the activity of the active fault can be evaluated by radon surveys.Through the analysis of the migration mechanism of radon, it is thought that diffusion and convection are the main transport mechanisms in the Quaternary soil cover. And based on this, the physical and mathematical models for the transport of radon in a stable condition in the homogeneous soil cover are established. The two-dimensional equation for Radon transport in the homogeneous soil cover is derived. And the numerical solution of radon concentration distribution in the soil overburden above active fault can be got by the finite difference method. Then radon concentration distributions with the different soil parameters and fault parameters are calculated, showing the characteristics of migration of radon and the distribution of radon concentration distribution in the overburden above active fault. If the background radon concentrations of both sides of the fault are of equal value, the radon concentration distribution is symmetrical. In the two-dimensional cross-section map, the contour curves arecircular arc, forming an oval radon halo anomaly above the fault zone. In the profile crossing the active fault in a certain depth, a high value anomaly is formed above the fault zone.By simulating radon concentration distributions with different soil parameters and fault parameters, the influences of the parameters to the migration of radon and radon concentration distribution is analyzed.The results are showed as follows:The thickness'variation of the overburden mainly influences radon concentration distribution in the vertical direction. With the thickness of the overburden increases, the migration length of radon gets longer, which makes the surface radon concentration decreases. The abnormal form is more and more unobvious. The thickness'increase of the overburden hinders the upward migration of radon to the surface.The width's variation of the fault has an impact on the concentration distribution of radon in both horizontal and vertical directions. The increasing width of the fault directly leads to the scope of radon source increases. This accelerates transport of radon in the vertical direction in the mean time of making a broader radon concentration distribution in the horizontal direction. Therefore, the radon halo formed in the soil cover in the two directions is expanding. And the surface radon concentration and the width and the peak of the abnormity increase with the fault width.With the effective diffusion coefficient increased, the near-surface radon concentration decreases. But the radon halo in soil continues to expand in the horizontal direction. The migration of radon in horizontal direction has been strengthened. So that the migration of radon in the horizontal direction in the overburden is largely due to the diffusion.With the convection velocity increases, migration of radon has been enhanced in vertical direction. Also, radon halo is growing in vertical direction. The radon concentration of a certain depth increases with theconvection velocity. And the abnormity above the fault has become increasingly evident. This shows the vertical convection is the important mechanism which makes radon migrate upward.Taking into account the heterogeneity of the overburden, two non-homogeneous overburdens: the vertical uneven overburden and the horizontal uneven overburden are used to simulate the radon migration in non-homogeneous overburden, which makes the radon concentration distribution features clear in the non-uniform soil.Radon concentration distribution in non-homogeneous and homogeneous overburden is somewhat similar. But all the curves are distorted or deformed compared to the homogeneous condition.Specifically, from the two-dimensional cross-section, radon halo with the bigger effective diffusion coefficient turns out to be more orbicular. Radon movement in the vertical direction is not very favorable; in the larger convection velocity overburden, radon halo is long in vertical direction, and radon movement in the vertical direction is significantly stronger. Radon concentration changes in the interface of the overburdens, and so do the two-dimensional contours and the one-dimensional profile curve. If the parameters of the two overburdens differ a lot, the curve will obviously, even causing abnormity. But with the increase of the depth, such deformation or abnormity will be gradually reduced or even disappear, unlikely to affect the identification of the fault abnormity.To validate the correctness of the model, Changchun Nanhu fault is forward simulated and surveyed. It turns out that the forward curve and the measurements curve match well. It proves that the model is correct.In summary, in this paper, the finite difference method is used to solve the radon migration equation in Quaternary overburden, which obtains the numerical solution of radon concentration distribution. The migration and the distribution of radon in Quaternary overburdenabove fault are simulated, which describes the invisible process that radon transports upward in soil visually. Finally, the successful forward of Nanhu fault proves the correctness of the model. The research work of radon migration and radon concentration distribution in the Quaternary overburden above active fault opens up a new way of thinking, having important theoretical significance.
|
PDF Full Text Request