关键词: CSELF, 感应场, 辐射场, 波场分布, 电磁测深

Abstract: This paper has carried out a more detailed study on the spatial propagation characteristics of the electromagnetic (EM) of controlled-source extremely low frequency (CSELF). The CSELF EM waves cover almost all sections of space which can be divided into near, far and waveguide zones. The propagation theory of CSELF EM wave is similar to CSAMT in the near and far Zones, and we collated and verified the field strength calculation formulas in the existing literature. While in the waveguide zone, learned the achievements of communication technology, this paper presents the approximate calculation formula of CSELF EM wave of the earth-air-ionosphere spherical cavity model. Based on the above, this paper has complied a piece of visualized software for calculation of the CSELF EM field in three coordinate systems (Cartesian, cylindrical and spherical coordinates). Finally, according to the calculation results, the spatial propagation characteristics of CSELF in the near area, far area and waveguide area are analyzed.The results show that the decay of CSELF EM field intensity is rapid in the near and far zone, but slightly slow in the far zone, and the electric field enters the waveguide zone earlier than the magnetic field. Under the earth model, there is an increase in the field strength in the waveguide area near the antipole of the dipole source which shows completely different EM waves propagation characteristics in horizontal formation model. In addition, it is also recognized that in the frequency domain EM sounding based on the horizontal electric dipole source, the far-field sounding mainly depends on the magnetic field rather than the electric field. The research in this paper provides theoretical and computational support for the application of CSELF.

Key words: CSELF, Induction field, Radiation field, Wave field distribution, Electromagnetic sounding