| The electromagnetic (EM) method using controlled-source extremely low-frequency (CSELF) waves is a new technology that is based on the large-power alternating electromagnetic field generated by an artificial procedure. As a fine combination of radio communication and geophysics, it is characterized by strong signal, broad coverage and large exploration depth. 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 the research on the CSELF is still at a starting stage, particularly on its theory. 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, are mainly appeared 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 EM field is primarily a induced field (quasi-stable field). The propagation characteristics can be described by the theory of quasi-stable field which is analogous to that of the classical theory of EM sounding. While in the waveguide zone, EM waves run in a way completely different from what the classic theory describe. It is an important but yet open issue that how the CSELF EM waves are distributed in the near, far and waveguide zones, how these three zones link, and what their sizes are, respectively. Starting from the classic EM theory, this thesis reviews the calculation formulas for descriptions of EM wave distributions in the near and far zones as well as their linkage. Then, Learned the approximate solution of the communication field, the linkage between the far and waveguide zones is studied, and the factors affecting the field intensity of the waveguide zone are analyzed. In addition, in the spherical coordinates, this thesis thus also attempts to perform some calculation of the exact solution for the propagation of extremely low-frequency EM waves in the space including the near, far and waveguide zones simultaneously.(1) Calculation of the CSELF EM field in near and far zones. By examination of the calculation formulas for the horizontal electric dipole field in the classic theory of EM sounding, this work corrects the partial equations for the EM field, and suggests the calculation formula for the EM field in a spatial coordinate system with the origin of the Earth's center. Then the distributions of the intensities of quasi-stable fields are calculated and plotted for the three coordinate systems (Cartesian, cylindrical and spherical coordinates). The result shows 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 the field component fields have merely one zero line and are characterized by half-plane distribution. These features are of significance for site choice of seismic observational stations and EM data processing and Analysis. Meanwhile, using the difference percentage between approximate and exact solutions, we make a quantitative analysis of the applicable ranges of near zone and far zone. The result shows that there exists a transition zone between the near zone and far zone.(2) Comparison of theoretical results and observational data. The comparison shows that both are well consistent and attenuation curves of the field intensities coincide fairly well. It also shows that the effect from external interference on the magnetic field is relatively little while that on the electric field is large. From the theoretical calculation, the variations of the field intensity with distance are not stable and smooth, instead they are related to azimuths. When nearby the azimuth of the zero line, the theoretical curve swings up and down like a broken line. And the curve from real measurement data has the same tendency of changes. Based on comparison of the magnetic field, this work infers the terrestrial resistivity of the source zone, which is in good agreement with the real magnetotelluric sounding.When comparing the theoretical result and observational data, this work develops the calibration methods of the fields in the time and frequency domains. For the data of the time domain, this work first adopts the adaptive filtering to obtain the time sequence of single frequency signals. Then it makes calibration to the amplitude values in the time domain, yielding the field values in this domain. Based on this calibration, it utilizes the sliding calibration of automatic fitting to transform the spectral signals observed from the frequency domain into the field values. The calibration method for the field values makes it possible to compare and validate the theoretical and observational data. In particular, it allows the observations in the frequency domain to be no longer limited by the length of integral time.(3) A detailed and quantified analysis to conversion of CSELF EM waves in the far and waveguide zones and the linkage between these zones. The decay of field intensity is rapid in the far zone, and becomes very little in the waveguide zone, respectively, which characterizes the varied spatial distributions and propagation of the induction and radiation fields. During the conversion between the far and waveguide regions, the electric and magnetic fields have different propagation features. Specifically, the electric field enters the waveguide region earlier, while the magnetic field enters the waveguide region later. The division between the waveguide and far regions at relatively high frequencies (50-100Hz) is close to that made by American researchers. This thesis also analyzes the various factors that affect the field intensity of the waveguide region. It is found that the emission current, earth resistivity, and emission frequency are all positively correlated with the field intensity of the waveguide region. The altitude of the ionosphere is negatively correlated with the intensity of the waveguide zone. ?(4) Exact solutions simultaneously appropriate for the near, far and waveguide zones. Based on a three-layer spherical waveguide model with coupling between the earth, air and ionosphere, in the general condition of continuation of the EM field, this work attempts to solve the problem of a horizontal dipole source, resulting in the exact expressions of the EM field for these three layers. The preliminary calculation result shows the primary characters of the CSELF field and reveals some features that the approximate solutions cannot express, such as interference enhancement and oscillation of the field. It testified the correctness of the solution formulas to some extent. To the difficulties in calculation, for example, calculation of high-order and large-amount Bessel functions, slow convergence of the summation series for the field, and how to find a stable accelerated approach for calculation, this thesis suggests a set of ideas and equations for solutions which provide a solid basis for exact calculations of the near, far and waveguide zones.(5) Calculation software for the CSELF EM field. Using the plane image to image technology and the Delphi language for programming, this work has complied a piece of visualized calculation software for calculation of the CSELF EM field. It realizes approximate and exact calculations of the field in the near and far zones as well as the approximate solution in the waveguide zone. And the exact solution in the waveguide zone is undergoing. This software would greatly make the followed research more convenient. After further improvement, it would become an important tool for the research of the CSELF EM field. |
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