Study On Three-dimensional Transient Electromagnetic Responses Characteristics Of Small Rectangle Loop And Dam Leakage Detection

Transient electromagnetic method as an important branch of electrical prospecting. It is suitable for engineering geophysics for its unique characteristics. In this paper, the author use a small rectangular loop device to improve the transient electromagnetic method of dam leakage channel detection. By adopting the method of non-uniform FDTD method, This method simulate dam leakage channel model of response characteristics and distribution of tem field. With the study of different leakage structure response and regularity of tem field, the method was applied to dam leakage detection test. The main research work and results are as follows:(1) By deriving from the expression of the Analytical solutions in the homogeneous half space and digital filter solutions in the layered media, the author get the expression of each component in the time domain electromagnetic field which was excited by rectangular loop source in homogeneous half space. With the summary on the finite difference method which is suitable for the calculation of inhomogeneous composite grid, The FDTD forward method is realized though analyzing the limit of iterative step size, boundary conditions and the application of the excitation source. On the basis of ensuring the accuracy of the calculation, the efficiency of calculation is improved.(2) In order to verify the accuracy of the 3D finite difference method based on the non-uniform subdivision, the author compared the analytical solution and the FDTD solutions of the homogeneous half space model. It is found that the current length of time does not affect early accuracy of the solution by the error analysis, but the longer energizing, the longer the time to get the exact solution. Based on comparing with the digital filtering solution with FDTD solution of the common layered model, this paper found that the algorithm has large error in the early and late time relatively consistent well, which further test the reliability of the algorithm.(3) By comparing the transient electromagnetic response of rectangular and square loop with the same equivalent area, a rectangle small loop device is proposed for the transient electromagnetic dam detection. In this model, the transient electromagnetic response of a homogeneous dam is simulated by using the way of non-uniform FDTD solution. By setting different shapes and types of columnar in the dam, such as the abnormal body type column, slab, complex leakage, the analysis shows that the simulation results with the model set is consistent, which indicate that the forward modeling algorithm and program can simulate the complex model containing multiple abnormal leakage.(4)The small loop source transient electromagnetic leakage detection method was used to detect the leakage of the reservoir dams and cofferdams, the detection results show the right direction for the leakage hidden trouble for the processing. The results were verified and show that the small rectangular loop source transient electromagnetic leakage detection method is feasible...