| Tunnel DC resistivity methods are widely used in tunnel advanced exploration with the advantages of being simple, cheaper and easy to interpretation, compared with others. However, due to the restriction of the survey configuration and working environment, it is impossible to avoid the interferences of tunnel cavity and the heterogeneous accumulations behind the tunnel face, for the conventional tunnel direct-current advanced prospecting. To solve this problem, Bai-Yao Ruan proposed a new exploration technique for tunnel advanced detection, i.e., DC focusing resistivity advanced detection method. It is based on the principles of repulsion and focusing like point sources. Supported by the National Natural Science Foundation of China (Project Number:40774057), this thesis focuses on the advanced detection measurements with vertical focusing arrangement on the tunnel face and tunnel wall, based on the idea of electric current focusing effect. As a new method of direct current advanced detection in tunnel, it is necessary for us to develop appropriate techniques to analyze and appraise the working principle, application conditions and exploration capability of the measurements.In this thesis, a numerical simulation method of finite element was used to evaluate the feasibility and effect of focus DC current advanced detection in tunnel for several models with2D axial-symmetric structure and3D full space structure. At first, the2D and3D target model is divided into finite rectangular elements and finite hexahedron elements with bilinear variation in conductivity respectively. Then linear interpolation was used to achieve the potential within each element for2D model. For3D model, we use bilinear interpolation for the conductivity and3-D linear interpolation for the potential as to get the anomalous potential, whereas it should improve the stability and accuracy in the numerical simulation. To save the computer memory, an iterative solver based on the splitting pre-conjugate gradient and row compressed storage scheme were employed to solve the linear system with less computation time. During the numerical modeling, the existence of cavities was considered, its information was directly included into the linear system during the process of solving the equation. Under the same conditions, the results gained with FEM are consistent with the result gained by the physic experiment.In the first two chapters, we give a detailed discussion and analysis of the field source distribution, exploration ability, applicability and anti-noise capability of tunnel face vertical focusing device and tunnel wall vertical focusing profile device. The response of anomalous body for three-electrode focus current array, five-electrode focus current array and night-electrode focus current array applied to tunnel face were modeled by multiple combination models. Finally, five-point focus current array is proved to be optimal in terms of exploration effects and practicability. Generally, tunnel wall focus current profile arrays are plentiful compared to tunnel face focus arrays.The measurement configuration mentioned in this paper, e.g., two-pole focus array, three-electrode focus array and five-electrode focus array, can be applied to potential and potential gradient measurement. Moreover, the bipole focus array can be extending to different survey configurations, e.g., center measurement method, unilateral measurement method and bilateral measurement method, according to different exploration purpose. The numerical results for several typical models, show that the vertical focus profile array design is effective and practical.Based on the concept of'PLB', we build the relationship of the current from the shielding electrodes and from the primary electrode. The model calculations show that we can control the restriction of shielding current on primary current in focusing field, by the parameter'PLB'After years'development and experiment in practice, conventional tunnel DC detection method has been proved to be effective. In this paper, focus current array, for the tunnel conventional DC electrodes arrangement device, was applied to profile prospecting and sounding in tunnel, and the numerical results show that the focus current array could achieve the corresponding exploration efficiency compared to conventional DC device. However, there are also some problems, e.g., the apparent resistivity appears negative, calculated form the resistivity calculation formulas for the homogeneous point-source device. This may result from the source distribution characteristics and anomaly response mechanism.Source distribution characteristics directly affect the interpretation of field data and the judgment of anomalous bodies.2D finite element numerical simulation tools based on MATLAB were used to modeling the distribution of abnormal potential and current density for different models and array types.
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