对瞬变电磁测深几个问题的思考（四）——从不同角度看瞬变电磁场法的探测深度

从几个方面阐述了瞬变电磁（TEM）场法的探测深度，引用“烟圈效应”的等效电流环向下扩散深度，推导平面波场极大值扩散深度，并通过时频（T-F）分析阐明时间域、频率域探测深度的联系与区别。TEM场的扩散深度主要取决于延迟时间，地面实际探测到同样深度的信息所需时间至少是单向扩散深度时间的2倍，即双程时；根据电磁感应、电磁波的反射、频率测深曲线的假极值现象以及拟地震转换波的慢度等几个方面论述了此结论。论述的TEM探测深度的理论可为其应用扩展思路。 

Detecting depth of transient electromagnetic method from different points of view

In this paper, the detection depth of transient electromagnetic(TEM) field method is described in several aspects. The “smoke ring effect” of the equivalent electric current ring downward extension depth was cited. The diffusion depth of the maximum plane wave field was derived. And through the analysis of the time-frequency(T-F) the relation and distinction between the penetration depth of time domain and frequency domain was explained. Ground actual detection time needed for the same depth of information is at least twice as much as one-way diffusion depth, namely double travel time. According to several aspects of electromagnetic induction, reflection of electromagnetic waves, the frequency sounding curve of false phenomenon of extreme value and quasi seismic converted wave slowness, etc., this conclusion is discussed.