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YAN SHU 1) CHEN MING-SHENG 2) FU JUN MEI 1) 1) Microwave and Optical Communication Institute, School of Electronic and Information Engineering, Xi'an Jiaotong University, Xi'an 710049, China 2) Xi'an Branch, CCRI, Xi'an 710054, Chin  
In order to understand exploration principle, a numerical analysis is made in time domain for two dimensional transient field excited by a negative step pulse. The method used in this paper is to derive the homogeneous diffusion equation of time domain electric field from Maxwell's equations and to do the difference discretion in the investigated region. The source is joined as initial condition and the boundary in the air is dealt with by quasi static approximation, then the time stepping is carried out. Hence could be shown the entire process of transient electromagnetic fields diffusing with time in the earth. The distribution patterns of transient electromagnetic fields at different instants in the earth and the corresponding changes of electromotive force on the earth are calculated, which could enunciate the conductor's concentrating effect on induced eddy current, a decrease in diffusing velocity for conductive overburden and delay effect of transient fields. Therefore, transient electromagnetic method is sensitive to conductors, and a longer time is needed to detecting the same depth where conductive overburden exists. Because of delay effect the bodies embedded in upper subsurface could be detected in late time. [
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