关键词: CSELF, 感应场, 辐射场, 波场分布, 电磁测深

Abstract:

The electromagnetic(EM)method using controlled-source extremely low-frequency(CSELF)waves is a new technology based on the large-power alternating electromagnetic field generated by an artificial procedure. The biggest advantage of this technology is that it has a long transmitting antenna(tens to hundreds of kilometers)and a large transmitting current(hundreds of amps)and can emit strong and stable electromagnetic waves, covering millions of square kilometers. It can be applied to earthquake monitoring, surveys for mineral resources and treatment of waste nuclear material as well as marine and land communication and detection to ionospheric structure in space. At present, domestic theoretical research on CSELF is not mature enough. 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 large-power CSELF EM waves cover almost all sections of space which can be divided into near, far and waveguide zones according to their propagation characteristics. The propagation of electromagnetic waves in the near and far zone is mainly manifested as the distribution and induction of the conductive currents, and the displacement current and effects of the ionosphere and spheric structure of the Earth can be neglected. The propagation theory of CSELF EM wave is similar to CSAMT in the near and far zones, and it can be described by the theory of quasi-stable field which is analogous to that of the classical theory of EM sounding. In this paper, we collated and verified the field strength calculation formulas in the existing literature. While in the waveguide zone, EM waves appear mainly as the displacement current, and the displacement current and effects of the ionosphere and spheric structure of the Earth must also be considered. The electromagnetic field is mainly the radiation field, and it runs in a way completely different from what the classic theory describes. Using the achievements of communication technology for reference, this paper presents the approximate calculation formula of CSELF EM wave of the earth-air-ionosphere spherical cavity model. Based on the field strength calculation formulas of the three regions obtained above, this paper has designed 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, which reflects the spatial distribution characteristics of the induced field in the lossy medium and the radiation field in the dielectric medium. 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. According to the calculation results of the CSELF EM field in near and far zones under the three coordinate systems, it is found that in the Cartesian coordinate system, the horizontal components have two zero lines and are distributed in four quadrants. While the vertical component field has only one zero line and are distributed in two half planes. In the cylindrical and spherical coordinate systems, all field components have merely one zero line and are characterized by half-plane distribution. The location of the zero line should be avoided as much as possible in the layout of field observation stations. We can choose different coordinate systems to solve this problem. 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. Furthermore, it is recognized that in the frequency domain electromagnetic sounding method based on the horizontal electric dipole, the horizontal component of the electric field in the near zone is proportional to the resistivity of the medium, and has nothing to do with the frequency; the vertical component is proportional to the frequency and has nothing to do with the dielectric resistivity; the magnetic field has no relationship with the frequency and the dielectric conductivity. In the far zone, the horizontal component of the electric field is basically independent of frequency, and the vertical component of the electric field is related to both frequency and earth conductivity. However, due to the difficulty of observation, it is generally not used in the actual sounding. The three components of magnetic field in the far zone are all related to the frequency and the earth’s conductivity, so the far-field sounding mainly depends on the magnetic field rather than the electric field.

Since CSELF antennas are generally very long(tens to hundreds of kilometers), the antenna can no longer be regarded as an electric dipole when measuring in the near and far zones, but should be regarded as a long wire source composed of multiple electric dipoles. In this paper, the electric dipole theory is still used for analysis, which has certain limitations that need to be overcome by further in-depth research.

Key words: CSELF, induction field, radiation field, wave field distribution, electromagnetic sounding