Theoretical Analysis On Near-Field Propagation Of ELF Electromagnetic Field And Its Applica-Tions To Mixed Propagation Path

Extremely low frequency electromagnetic waves have important applications in submarine communication,subsea exploration,geological prospecting,seismic electromagnetic monitoring and many other fields.This motivates people to deepen their understanding of its propagation mechanism.However,there are few research materials in this field,especially the solution of the extremely low frequency electromagnetic wave propagation in the near region,which requires more research and attention.This essay attempts to discuss the solution of the extremely low frequency electromagnetic wave propagation in the near region,and its applications in lightning,submarine propagation,mixed path waveguide and other aspects are described and deduced.The main research works are summarized as follows:1.Firstly,the electromagnetic field component is estimated by quasi-static approximation method based on the general expression of Sommerfeld Integral.Then,the analytical approxi-mate expressions of the extremely low frequency electromagnetic wave in the near region under the excitation of a vertical electric dipole are derived,and the theoretical analysis results of the extremely low frequency electromagnetic wave propagation in the half-space along the interface between air and ground are given.2.The approximate expressions of the transient solution under the excitation of a vertical electric dipole are given furtherly.These transient solutions are based on the frequency domain solutions presented above this paper,which are suitable for the calculation of extremely low frequency electromagnetic wave in the near region.The corresponding results of the unit pulse are convolved with the dual-exponential source to obtain the transient analytical expressions of the dual exponential source excitation,which are used to simulate the time-frequency character-istics of the extremely low frequency pulse generated by the lightning signal in space.3.Based on the general expression of Sommerfeld Integral,the approximate solutions in the frequency domain of the extremely low frequency electromagnetic wave excited by a hori-zontal electric dipole are derived.In particular,the extremely low frequency electromagnetic waves excited by a horizontal electric dipole have very important application background in the submarine structure propagated along the interface between sea water and rock.By comparing the analytical approximate solutions given in this paper with the existing results,the correctness of the results is demonstrated.4.The approximate results of the extremely low frequency electromagnetic wave excited by a horizontal electric dipole along one-dimensional anisotropic medium surface are given fur-therly.Through the comparison with the measured data,we can see that the submarine litho-spheric structure regarded as anisotropic medium has better comparable role than as isotropic medium,and the numerical results are closer to the measured data than the isotropic medium.5.Examples are given for the application of extremely low frequency electromagnetic wave propagation in the near region.The propagation characteristics of extremely low frequen-cy electromagnetic wave excited by a lightning signal in the near region are studied,and the lightning process is regarded as the time domain response process of the extremely low fre-quency electromagnetic wave generated by a dual exponential function source in space.In addi-tion,the spatial distribution and the Poynting energy flow distribution of the extremely low fre-quency electromagnetic wave propagation in the near region at the seabed are also studied,which provide some theoretical help for the application of the extremely frequency electromag-netic wave in submarine exploration research.6.Finally,the calculation scheme of the near region of the extremely low frequency elec-tromagnetic wave propagation in the mixed path waveguide is studied.The analytical expres-sions of the extremely low frequency electromagnetic wave and the mode conversion coefficient propagation over the transition zone of land-sea are given.The numerical results can be used as a reference for the calculation of the near region of the extremely low frequency propagation in the mixed paths waveguide.