| With the decreasing of the surface mineral resources as well as the implementation of the strategy of national deep prospecting, traditional electromagnetic prospecting methods can no longer satisfy the actual needs. Because of its great exploration depth, high resolution and strong anti-jamming, the transient electromagnetic method with grounded line source has got more and more attention. At present, it has been widely used in metal mineral exploration, reservoir dynamic monitoring, the groundwater resources exploration, the prediction of natural disasters and environmental monitoring, etc. However, the development of the transient method with grounded line source has been greatly hindered owing to the backward data processing and interpretation techniques, the non-uniqueness of single resistivity inversion. The phenomenon of conduction and polarization are in real earth media under the action of electromagnetic. In order to detect the propagation law of electrical source tem field, improve the level of its data processing and interpretation, and increase the reliability of inversion results; it is necessary to consider the mathematical and physical description and calculation of transient electromagnetic under the three-dimensional polarized media.This paper applies the method of the staggered grid time domain finite difference algorithm to calculate the forward modeling of complex medium model of three-dimensional transient electromagnetic field. The electromagnetic field equations were derived from the Maxwell equations in time domain, which were discrete by using the DuFort-Frankel, and then we realized three dimensional forward modeling of the transient electromagnetic field with grounded line. IP model is added to forward modeling in time domain, and we realized three dimensional forward modeling of the transient electromagnetic field with grounded line based on induced polarization medium.On the basis of predecessor’s study, A-comprehensive research has made for the forward modeling of induced polarization media of three-dimensional transient electromagnetic field. The main research content of this thesis is as follows:(1) We used D.Guptasarma algorithms to deduce Cole-Cole model, Dias model and GEMT1P model with step current, and studied their temporal amplitude characteristics in time domain. Respectively, Analytical solution of Cole-Cole model and GEMTIP model was deduced based on inverse Laplace transforme theory when the frequency correlation coefficient is equal to 1 and 0.5. Results of numerical calculations performed filtering algorithm comparative validation, and validated IP model expression in time domain. It provides theoretical support for the time domain induced polarization information.(2) We deduced transient electromagnetic field expressions excited by electric dipole source. Use the relationship between Bessel functions and Hankel transform to solve the frequency domain electromagnetic field, and then use the G-S transform to get the time domain electromagnetic field. The electromagnetic field by dipole excitation in time domain is mainly used to verify the correctness of staggered grid finite-difference time-domain numerical results and a three-dimensional forward modeling of the initial field.(3) One-dimensional forward modeling of transient electromagnetic field based on induced polarization medium. We can get the real resistivity with IP when we put the IP model into forward modeling in time domain. Then calculate its electromagnetic field in frequency domain, and use the G-S transform to get the time domain electromagnetic field with induced polarization, which realized one dimensional forward modeling of the transient electromagnetic field with grounded line based on induced polarization medium.(4) Three-dimensional forward modeling of transient electromagnetic field based on the staggered grid time domain finite difference algorithm. The electromagnetic field equations were derived from the Maxwell equations in time domain, which were discreted by using the DuFort-Frankel. The key technology such as numerical stability, boundary conditions, the method of space subdivision, the initial time and IP information are discussed. Finally, we simulated the low resistance polarization and high resistance polarization.This paper has obtained the following innovative results:(1) We can derive out the analytic expression with Mathematic when electric dipole spread over the surface in homogeneous half space, which is based on Somerfield formula and Fork formula. This result helps us to avoid Hankel transform effectively when we calculate the initial field, accelerating the calculation of initial field,so far as accelerate the speed of three dimension forward simulation.(2) We first derive out the analytical expression of Cole-Cole model and GEMTIP model by using an inverse Laplace transformed under step-current excitation when the frequency correlation coefficient is equal to 1 and 0.5. It provides possiblity for adding induced polarization information in time domain. And it is benefitcial for studying the time domain electromagnetic response based on induced polarization medium.(3) We first introduced GEMTIP which is based on generalized effective-medium theory by Zhdanov to the forward modeling of transient electromagnetic field with ground line source, and we realized forward modeling of transient electromagnetic field based on induced polarization media.This paper has achieved three-dimensional forward modeling of transient electromagnetic field based on induced polarization medium. But I think that we need further study for the efficiency of computation, the stability of FDTD method, the accuracy of the calculation results and boundary condition. And I hope that this research will provide help and reference for subsequent study. |
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