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    Forward modeling method of 2D transient electromagnetic field induced by line source
    For more complex near-surface conditions such as mountain areas, a new improved ray tracing method based on shortest path assumption is studied, it has higher accuracy and faster computing efficiency than conventional ones .
    The result of calculation using total potential and anormalous potential were compared and analysed.
    In order to get high precision, the appropriate distance of boundary were determined compared result of FEM with exact solution and calculated large number of examples.
    Thequantic-boundary condition method made the 3-D resistivity modeling using FEM applied in practical.
    Using the finite element method under the quantic-boundary condition, forward the apparent resistivity in complex condition, such as tunnel and below water.
    The calculation of the partial derivatives is important in modeling IP and inversion resistivity.
    1. A program was developed to model 3D GPR responses using finite difference time domain method.
    1.利用时间域有限差分方法实现了探地雷达三维正演计算。 解决了目前探地雷达常见软件无法对模型介质加入干扰信号问题,为应用数值模拟技术开展介质不均匀性问题研究提供了有力工具。
    The main contents and achievements of the thesis are as follows:(1) The 2-D forward modeling formula of the resistivity and susceptibility as model parameters is deduced.
    (1) 推导了电阻率和磁化率同为参数时的二维大地电磁正演计算公式。
    By use of the finite difference method, which is based on transmission network proposed by Madden and Mackie, we completed forward calculation and demonstrated in detail the effect of resistivity and susceptibility on MT response. It is conformed that the susceptibility mainly affects the in-phase components of magnetic signal, but a little effect on the quadrature components of magnetic and electrical signal.
    Firstly, in order to carry out thetarget, forward-modelling computations are realized for traveltimes of waves and NMO velocitiesin TI media. Then, by means of the genetic algorithm, inversion is performed for parameters ofanisotropy and interfaces.
    The computational method ofeffective NMO velocities for multilayer TI media with dipping interfaces is given using thesolution to NMO-velocity surface, and the numerical computation is performed.
    The errors of NMO velocities determined by theactual survey data relative to NMO velocities computed by the analytic solution increase with theenlargement of the maximum offset.
    g horizontal gradient nodule、? gz horizontalgradient module、? gzz in 3D models and their combined models ,based on the gravityanomalies characteristic value, the boundary locating errors have been statisticallyanalyzed, and finally the applied conditions about each method of boundary locatinghave been summarized.
    (3) The parallel algorithm is introduced into the forward modeling.
    With the increasing of transmiting current and reducing of the minimaldiscriminable voltage, the influence of geological interference becomes more evident, so it is very necessary to introduce feasible mathematical methods to direct and inverse calculation of TEM, such as ANN, GA.
    At base of the theories analysis at the Rayleigh Wave and the work with people of the past, Rayleigh wave dispersion function in multi-layer medium is educed. And Knopoff fast algorithm is improved by normalized method and the method of thinning one layer. There, high frequency numeral overflowing, valid number losing, low upper limit frequency that existed in forward computing for Rayleigh Wave dispersion has been resolved, and it has increased the upper frequency to 800Hz.
    By forwardly calculating the theoretical models under the effluence of the distortion factors, the author draws some regular conclusions, which are conductive to study the impedance effluence from the distortion factors.
    That is, althought only one forward calculation is required in eachinversion iteration, however, it requires tens of inersion iterations and each iterationcrquries much hours, so the inversion is much more time-consuming.
    Experiments on the models show thatRayleigh wave in multi-layered media occurs dispersive, and the dispersion curvescan be theoretically computed by Knopoff method quickly and efficiently.


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