水平层状大地中任意方向偶极源激发的电磁场

目前已存在多种物理模型用于解释地震前兆电磁异常,不同机制下产生电磁波场的源可等效为不同类型偶极源（磁型/电型）的组合.针对不同偶极源激发的电磁响应特征的研究对地震孕育过程中观测的电磁信号的提取与解释具有重要意义.基于此,本文推导了一套正演算法,可用于研究任意方向（方位角和倾角）、大小和位置的一个或多个偶极源在地壳或岩石圈-大气圈-电离层（Lithosphere-Atmosphere-Ionosphere,LAI）模型中产生的电磁波场的传播特征.基于多源组合,该算法可进一步拓展到线电流源和面电流源的电磁响应的计算.本文给出了水平、垂直和倾斜电偶极源响应计算的实例,并针对不同类型的偶极源激发下的地中、地面以及空间电磁场响应特征进行了初步分析.模拟结果显示,相比于单纯水平或垂直的电偶极源,地中倾斜电偶极源激发的电磁响应在地表的各场量辐射花样虽然仍可反映偶极子的方位角和空间位置,但不再具有对称性,其模式更复杂.本文提出的正演算法可用于地中低频电磁辐射源的正演模拟和反演定位.

Electromagnetic fields excited by arbitrarily orientated dipoles in horizontally layered earth

Different excitation mechanisms have been used to explain the electromagnetic (EM) anomalies as earthquake precursors, whose origins can be regarded as the superposition of electric or magnetic dipoles. The study of characteristics of electromagnetic responses excited by different dipoles is significant in extracting and interpreting the electromagnetic signals observed in earthquake preparation. We derived the forward algorithm to study the propagation characteristics of electromagnetic wave field generated by one or more electric/magnetic dipoles with arbitrary directions (inclination and azimuth), positions, and size in the crust or Lithosphere-Atmosphere-Ionosphere (LAI) system. Based on the multi-source superposition, this algorithm can be further extended to calculate electromagnetic fields excited by line current and surface current sources. In this paper, the electromagnetic field responses induced by the horizontal, vertical, and tilted dipoles underground are calculated, and characteristics of responses at the ground surface, subsurface, and in the air are preliminarily analyzed for different types of dipoles. The simulated results show that the radiation patterns of the electromagnetic fields at the ground surface, excited by a tilted electric dipole underground, can basically reflect the azimuth and position of the dipole. However, compared to horizontal or vertical electric dipoles, these patterns exhibit greater complexity and asymmetry. The algorithm proposed in this paper can be used for forward modeling of the low-frequency electromagnetic responses of arbitrarily orientated dipoles underground and used for inversion and locating of the seismogenic electromagnetic radiation source.