Comparison of the time-domain electromagnetic field from an infinitesimal point charge and dipole source

An electromagnetic field is generated through the accelerating movement of two equal but opposite charges of a single dipole. An electromagnetic field can also be generated by a time-varying infinitesimal point charge. In this study, a comparison between the electromagnetic fields of an infinitesimal point charge and a dipole has been presented. First, the time-domain potential function of a point source in a 3D conductive medium is derived. Then the electric and magnetic fields in a 3D homogeneous lossless space are derived via the relation between the potential and field. The field differences between the infinitesimal point charge and the dipole in the step-off time, far-source, and near-source zones are analyzed, and the accuracy of the solutions from these sources is investigated. It is also shown that the field of the infinitesimal point charge in the near-source zone is different from that of the dipole, whereas the far-source zone fields of these two sources are identical. The comparison of real and simulated data shows that the infinitesimal point charge represents the real source better than the divole source.

三维数值计算中螺绕环近钻头电磁波电阻率测井方法加源方式的讨论

螺绕环式激励与电极型和线圈型聚焦激励相比是一种更适合近钻头电磁波电阻率测井的激励方式,本文通过COMSOL软件建模仿真计算,将复杂的螺绕环激励源分别等效为点电偶极子和面磁流源,利用3D有限元素法并结合电磁场理论对螺绕环式随钻电磁波测井仪器加源方式进行了仿真分析,为螺绕环近钻头电磁波电阻率测井方法选择合适的等效源。结果表明:面磁流激励源与点电偶极子激励源相比更能准确地等效螺绕环激励源;研究成果为螺绕环近钻头电磁波电阻率测井方法数值模拟提供了理论支撑。