【作者】 杨静；

【导师】 陈小斌；

【作者基本信息】 中国地震局地质研究所 ， 固体地球物理学， 2011， 硕士

【摘要】 人工源极低频(CSELF)电磁波技术是用人工方法产生大功率交变电磁场的新技术,是无线电通信领域和地球物理领域巧妙结合的新技术,其信号强、覆盖范围广、探测深度大,可广泛用于地震监测、矿产资源探查、核废料处理,同时还可用于海陆通信及空间电离层结构探测等。目前国内对于极低频的研究才刚刚起步,特别是理论研究更为贫乏。大功率人工源极低频电磁波几乎覆盖了空间所有区段,根据传播特征可以将极低频电磁波的传播区域可以划分为近区、远区、波导区三个区域。电磁波在近区、远区的传播主要表现为传导电流的流动和感应,位移电流及电离层和地球形状的影响可忽略不计,电磁场主要是感应场(似稳场),其传播特征可用似稳场理论描述,与经典电磁测深理论一致。电磁波在波导区主要依靠位移电流进行传播,同时还须考虑电离层以及地球球体形状的影响,电磁场主要是辐射场,其传播理论与经典电磁测深理论完全不同。极低频电磁波的场强在近区、远区、波导区是如何分布,三个区域之间是如何衔接,各自的范围有多大,对于极低频电磁波的理论和应用十分重要,但目前还未开展全面系统的研究。本论文首先从经典电磁测深理论出发,归纳整理了近区、远区场的计算公式,对比分析了近区、远区场的分布特征及其衔接情况,然后借鉴通信领域波导区的近似解公式,对比研究了远区与波导区场的分布和衔接转换特征,并对影响波导区场强的多种因素做了重点分析讨论。此外,本文还在球形坐标系下,开展了极低频电磁波精确解(同时包括近区、远区、波导区)的求解工作,力求建立更完善的人工源极低频电磁波传播理论。本论文通过研究所取得的主要认识如下:1)近区、远区极低频电磁场的计算。对经典电磁测深领域中水平电偶极子场的计算公式进行整理,修正了部分电磁场原有的计算公式,给出了以地心为原点的空间坐标系下场的计算公式,在此基础上,计算绘制了三种主要坐标系(直角坐标系、柱坐标系、球坐标系)下似稳场的场强空间分布辐射图。结果表明,在直角坐标系下,水平分量有两条零线场呈四个象限分布状态;垂向分量场仅有一条零线,场为半平面分布状态。在柱坐标和球坐标系下,所有场分量都只有一条零线,场为半平面分布状态。场的这些分布特征对于野外地震台址选择以及数据的处理分析具有指导意义。同时,本文对近区、远区各自的适用范围以近似解与精确解的差值百分比为标准进行了量化分析,在近区、远区之间存在有一定的过渡区。2)将理论结果与观测资料进行了对比。对比结果表明两者具有较高的一致性,场强的衰减曲线能够较好的吻合。从对比结果发现磁场受外界干扰的影响小,电场受外界干扰大。在对比过程中还发现理论计算的场强曲线随距离的变化并非都是平稳光滑的,受方位角影响,在靠近零线方位,理论曲线会有抖动,呈折线变化状态,而实测数据也有相同的变化趋势。根据磁场的对比结果,本文推断了源区的大地电阻率,与大地电磁测深得到的结果一致性较好。在理论与实测数据对比时,发展了时间域、频率域场的标定技术。对于时间域数据,首先采用自适应滤波技术获得单频时间序列信号,然后时间域幅值标定,得到了时间域场值;在时间域场值标定的基础上,采用自动拟合的滑动标定技术,将频率域观测的谱信号转变为场值。场值的标定技术使得观测值与理论计算值之间的对比验证成为可能,尤其是使得频率域观测结果不再受困于积分时间长度的限制。3)对远区、波导区之间电磁波的衔接转换问题进行了量化的分析。远区场强衰减较快,波导区场强衰减很慢,体现了感应场和辐射场不同的空间分布和传播特征。在远区和波导区之间的转换过程中,电场和磁场的传播特征是不同的。电场较早进入波导区传播,而磁场较晚进入波导区传播。在相对高频端(50-100Hz)波导区与远区的划分结果与美国科学家的划分结果相接近。论文还对影响波导区场强的各种因素做了分析,发射电流,大地电阻率,发射频率都与波导区场强正相关,而电离层高度则与波导区场强反相关。4)对同时适用于近区、远区、波导区的精确解的求解。从地球-空气-电离层耦合的三层球状波导模型出发,在普适性的电磁场连续性条件下,对水平电偶极源问题进行了求解,得到了地球层、空气层以及电离层的电磁场精确解表达式。初步计算结果展示了CSELF场的主要特征,并能给出近似解所不能表现的一些特征,如场的干涉增强和震荡现象等,在一定程度上验证了求解公式得正确性。对于计算中的难点:高阶、大宗量Bessel函数计算困难、场的求和级数收敛慢等,研究寻求稳定的加速计算技术,给出了一整套求解思路和相应的求解公式,为近区、远区、波导区精确解的计算建立了坚实的基础。5)极低频电磁场的计算软件。利用面向对象技术,采用Delphi语言编程,编写了可视化的极低频电磁场的计算软件。实现了极低频近区、远区的近似解与精确解的计算以及波导区的近似求解,波导区的精确求解技术正在发展中,尚未完全实现。该软件的出现大大方便了我们后续的研究,在进一步完善之后,将可望成为人工源极低频电磁场研究的重要工具。更多还原

【Abstract】 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.更多还原