| The rapid development of science and technology makes wireless communication well-known to people, and various high-tech wireless communication products are affecting nearly every aspect of our daily life. Similarly, in geological engineering and some other related fields, the wireless communication technology is gradually adopted and applied to some relevant products, which makes the geological engineering more informationalized and intelligent. In the drilling area, the borehole trajectory offset like hole incline and bend often occurs, which brings great trouble to the whole work. If the underground drilling parameters can be transmitted to ground in real time, the borehole trajectory can be corrected timely, and the happening of the borehole trajectory deviates from the scheduled one can be significantly prevented. So the measurement while drilling (MWD) technology emerges as the times require, whose key problem is the drilling signals through the earth communication. Compared with other forms of through the earth communication, the electromagnetic wave signals wireless through the earth communication has the characteristics of fast transmission speed, high stability and no need of exclusive channel etc, so it is the best choice of the drilling signals through the earth transmission. The technology takes the electromagnetic wave as information carrier, and can realize the information wireless transmission through the earth in real time between underground and ground by using the electromagnetic wave transmitting and receiving devices.Except the wireless electromagnetic measurement while drilling (EM-MWD) technology used in drilling area, there are also a wide range of applications of the drilling electromagnetic wave signals through the earth transmission technology in other areas of geological engineering. In mining area, the mine through the earth communication system by using electromagnetic wave signals through the earth transmission technology can make the ground staff carry on the supervision of downhole operation personnel at any time and carry out evacuation and rescue when danger occurs. In trenchless area, the electromagnetic wave signal generator in the avigraph installed near the bit can transmit various drilling information to ground in order to achieve the purpose of monitoring and guiding the borehole trajectory. In geotechnical engineering exploration area, the stratum information collected by the probe of the static penetrometer can be transmitted to ground in real time by using the electromagnetic wave wireless through the earth transmission technology, which is not only stable and reliable but also economic.Compared with the traditional wireless communication form such as radio and mobile phone, the electromagnetic wave wireless through the earth communication system uses the lossy earth as the propagation medium, whose condition is more complicated. Because of the serious attenuation of signals in lossy earth medium, the super low frequency (SLF) or extremely low frequency (ELF) signals have to be used most of the time, so the antenna size can be used in practice belongs to electrically small size compared with the wavelength, and the working area of the electrically small antenna belongs to the near field zone of the antenna. There is no lack of examples that study the wireless electromagnetic wave signals through the earth transmission system at home and abroad, but the technology is not mature nowadays because of the complexity of the earth medium and the uncertainty of the electrically small antenna near field distribution. The exact analytical solution is difficult to be obtained by using strict analytic method because of the complex boundary conditions in electromagnetic wave wireless through the earth transmission area. With the development of the computer technology, to solve the electromagnetic field problem in electromagnetic wave signals through the earth transmission by numerical analysis has become a new research trend. The electrically small antenna immersed in the earth medium is taken as the study object in this paper. The near field distribution of the antenna and the law of transmitting electromagnetic wave signals through the earth of the antenna are analyzed and discussed, and the application of the electromagnetic wave signals through the earth transmission to engineering examples like electromagnetic wave measurement while drilling is made based on the need of practice. In the theoretical analysis, the quasi-static theory is used to compute the Sommerfeld integrals encountered in the analysis according to the characteristic of the working frequency used in the electromagnetic wave signals through the earth transmission system, which simplifies the calculation process under the condition of ensuring the accuracy. The similarity theory of electromagnetic is used in the experimental study. Through the study of the electromagnetic scale model of the electrically small antenna, the electromagnetic wave signals through the earth transmission situation of the antenna in the prototype system is known.According to the research status and actual need of the drilling electromagnetic wave signals wireless through the earth transmission system, the detailed theoretical analysis and experimental study of the electromagnetic wave signals through the earth transmission law of the electrically small antenna immersed in the earth medium are done from the basic theory of the electromagnetic wave propagation and antenna. This paper is divided into seven chapters, each chapter is as follows.Chapter one is the introduction. The research background, purpose and significance of the drilling electromagnetic wave signals through the earth transmission are discussed in this chapter. According to the analysis of the research status of the electromagnetic wave through the earth communication, the antenna immersed in lossy medium and the near field of the antenna at home and abroad, the existing problems and the development trend of the electromagnetic wave signals through the earth transmission are presented in this chapter.The basic theories of electromagnetic wave like Maxwell’s equations, the constitutive relation between medium, wave equations of the electromagnetic physical quantities, potential function theory and the boundary conditions of the field in the media interface are introduced and the study on the electromagnetic characteristic of the earth medium is made in chapter two. Based on these work, the propagation characteristic of the plane electromagnetic wave in the earth and the attenuation law of the SLF or ELF electromagnetic wave in the earth are studied afterwards.The Green function and the magnetic vector potential solution of the point source in unbounded space are introduced firstly in chapter three. Based on them, the boundary conditions of the magnetic vector potential in the media interface and the method of images for solving electromagnetic field problems are analyzed. Using these theories and methods, the magnetic vector potentials of the vertical electric dipoleand horizontal electric dipole in semi-infinite lossy medium are studied as well as the field analytic formula of the source in semi-infinite lossy medium. As a further study, the magnetic vector potentials of the vertical electric dipole and horizontal electric dipole in three layered media and multi-layered media are analyzed, and the field analytic formula of the source in three layered media are obtained as well as the field calculation method of the source in multi-layered media.The methods for wire antenna analysis including method of moment (MOM) are introduced and the electric field integral equation of the wire antenna is deduced firstly in chapter four. Then the approximation of the Sommerfeld integrals existing in the integral equation is made according to the characteristic of the working frequency used of the antenna. On the basis of the above analysis, the numerical solutions of the vertical wire antenna and horizontal wire antenna in the earth medium are done, which includes the current distribution on the antenna surface, the electric field distribution on the ground of the antenna and the voltage received distribution on the ground. As the application example of wire antenna in geological engineering, an EM-MWD model is established to analyze the current distribution on the antenna surface, the electric field distribution on the ground and the voltage received distribution on the ground of the antenna in this model.The influence factors of the wire antenna in the earth medium is mainly studied in chapter five, which includes the factors from the antenna itself like antenna length and the factors from the external environment like the electromagnetic parameters of the medium. Through the analysis of the vertical wire antenna and the horizontal wire antenna in the earth medium, the results that the antenna length, working frequency and conductivity of the medium have a great influence on the antenna field while the relative permittivity only has a slight influence on the field are obtained. In additional, the research about the antenna field variation with the location depth of the vertical wire antenna and the horizontal wire antenna is done as well as the research about the current distribution with different feed position on the antenna.The principle of the scale model experiment and the significance and feasibility of the use of scale model experiment are introduced firstly in chapter six. Then the experimental equipments and measurement devices as well as the experiment process are introduced. Finally, the experimental data collected are analyzed. Through the study of the scale model experiment, the electromagnetic wave signals through the earth transmission situation of the antenna in the corresponding proportion prototype system is obtained.The achievements and innovations as well as the problems and suggestions of the study are summarized in chapter seven. The study and application prospects of the drilling electromagnetic wave signals through the earth transmission technology are also mentioned in this chapter. |
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