几种典型近地表有限导体瞬变电磁特征响应研究

Modeling and analysis of transient electromagnetic responses of finite conductors in the near-surface

近地表有限导体的瞬变电磁响应中包含其形状、结构特征,如何建立这些特征与其响应之间的关系是近地表有限导体探测与识别的核心问题.本文建立了几种典型有限导体响应的三维感应偶极子模型,通过有限导体早期幂函数衰减的幅度与速率、晚期指数衰减的幅度与时间常数共四个模型参数表征有限导体的形状和结构特征,利用最小二乘拟合算法提取这些模型参数,揭示有限导体的形状、结构特征与模型参数之间的内在联系.理论与实验研究结果表明:具有绕球心旋转对称性的球形目标体,其三维感应偶极子响应完全相同,早期响应呈幂函数衰减,晚期呈e指数衰减,信号幅度和时间常数均能很好反映球体半径与材料特性;具有绕轴旋转对称性目标体,其三维感应偶极子响应部分相同,早期幂函数衰减的幅度与速率主要体现目标体的长度特征,晚期e指数衰减的时间常数体现了目标体的外径与壁厚特征;非轴对称性目标体,其三维感应偶极子响应均不相同,各个感应偶极子参数差异较大.本文的研究对提高瞬变电磁系统区分轴对称目标体与金属碎片能力,降低系统探测虚警率具有重要意义.

It is a key problem in near-surface conductor detection to identify real targets from the data containing numerous interferences depending on the shape and structure characters of targets reflected in the responses. According to the results of theoretical calculation and experimental verification, this study focuses on the influence of target shape and structure on its electromagnetic transient responses. Firstly, a group of induced dipoles with coincident central and orthogonal directions are established to replace finite conductors, then parametric response models are built to describe the induced responses of dipoles, and the parameters of these responses are picked up to analyze the influences of target shapes. Studies have shown that the characters of induced dipole responses are determined by the symmetry of the target shape and structure. The difference between the dipole responses is remarkable for an asymmetric target; all the dipole responses are identical for a sphere target, and dipole responses of an axial symmetry target can be divided into two types： vertical to the axis and axially. The axially symmetric target is the main object in near-surface detection. The amplitude and decay rate of early response depend on the length of the target. The longer the length is, the greater the amplitude and the slower the attenuation will be. The decay rate of the later exponential response depends on the diameter and thickness of the target. The larger the diameter and the wall thickness are, the slower the decay will be. This study is of significance to improve the distinguishing ability between axial symmetry targets and metal fragments, and reduce the false alarm ratio of the system.