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

摘要：

文中对人工源极低频(CSELF)电磁波的空间传播特征进行了较为细致的研究。CSELF电磁波的空间传播区域可划分为近区、 远区和波导区。在近区和远区, CSELF电磁波的传播理论与CSAMT相似, 文中整理、 验证了已有文献中的场强计算公式; 在波导区, 借鉴无线电通信技术成果, 给出了地球-大气层-电离层球形谐振腔模型的CSELF电磁波近似计算公式, 并在此基础上设计了可视化软件, 实现了3种坐标系下CSELF电磁波场的计算; 此外, 依据计算结果分析了CSELF在近区、 远区、 波导区的空间传播特征。研究表明: CSELF电磁场在近区和远区衰减很快, 而在波导区衰减较慢; 电场比磁场更早进入波导区; 在地球模型下, 在场源对极点波导区场强存在局部极大值, 显示了与水平层状模型完全不同的电磁波传播特征。同时, 在基于水平电偶极子源的频率域电磁测深中, 远区测深主要依赖于磁场而非电场。文中研究为CSELF的应用提供了理论和计算方面的支持。

关键词: CSELF, 感应场, 辐射场, 波场分布, 电磁测深