Cone-shaped source characteristics and inductance effect of transient electromagnetic method

瞬变电磁法圆锥型场源特征与电感效应（英文）

Small multi-turn coil devices are used with the transient electromagnetic method （TEM） in areas with limited space, particularly in underground environments such as coal mines roadways and engineering tunnels, and for detecting shallow geological targets in environmental and engineering fields. However, the equipment involved has strong mutual inductance coupling, which causes a lengthy turn-off time and a deep “blind zone”. This study proposes a new transmitter device with a conical-shape source and derives the radius formula of each coil and the mutual inductance coefficient of the cone. According to primary field characteristics, results of the two fields created, calculation of the conical-shaped source in a uniform medium using theoretical analysis, and a comparison of the inductance of the new device with that of the multi-turn coil, show that inductance of the multi-turn coil is nine times greater than that of the conical source with the same equivalent magnetic moment of 926.1 A·m2. This indicates that the new source leads to a much shallower “blind zone.” Furthermore, increasing the bottom radius and turn of the cone creates a larger mutual inductance but increasing the cone height results in a lower mutual inductance. Using the superposition principle, the primary and secondary magnetic fields for a conical source in a homogeneous medium are calculated; results indicate that the magnetic behavior of the cone is the same as that of the multi-turn coils, but the transient responses of the secondary field and the total field are more stronger than those of the multi-turn coils. To study the transient response characteristics using a cone-shaped source in a layered earth, a numerical filtering algorithm is then developed using the fast Hankel transform and the improved cosine transform, again using the superposition principle. During development, an average apparent resistivity inverted from the induced electromotive force using each coil is defined to represent the comprehensive resistivity of the conical source. To verify the forward calculation method, the transient responses of H type models and KH type models are calculated, and data are inverted using a “smoke ring” inversion. The results of inversion have good agreement with original models and show that the forward calculation method is effective. The results of this study provide an option for solving the problem of a deep “blind zone” and also provide a theoretical indicator for further research.

应用于煤矿巷道、工程隧道等地下有限空间以及城市和工程浅层探测的瞬变电磁法常采用多匝小线圈装置,但是多匝线圈装置的互感耦合大,关断时间长,使探测的浅部"盲区"增大。为此,提出了一种圆锥型场源装置,推导了该装置内各匝线圈半径的计算公式。采用理论解析式对均匀介质中圆锥型场源一次场、二次场特征进行了计算分析,对圆锥型场源与多匝线圈互感系数进行了比较;应用快速汉克尔变换和改进的余弦变换数值滤波法,基于叠加原理的思想讨论了圆锥型场源下瞬变电磁正演方法,并以H型和KH型地电模型为例对正演结果进行了"烟圈"反演分析。研究表明:对于等效磁矩为926.1A·m^2的多匝回线和圆锥型场源,前者的互感约为后者的9倍;增大圆锥型场源的高度可减小互感系数,但其底半径的进一步变化对互感影响有限。圆锥型场源一、二次场特征与多匝线圈相似,但其二次场和总场暂态响应强于多匝线圈。所提出的综合视电阻率方法适用于圆锥型场源计算,正演结果和"烟圈"反演结果与初始模型的吻合性良好。本文研究成果为减小瞬变电磁探测"盲区"提供一种选择,也为装置组合形式和非线性反演技术等后续研究提供理论参考。