瞬变电磁法的探测深度问题

The probing depth of transient electromagnetic field method

用解析分析、时域有限差分、时-频分析的方法,以地面中心回线装置和阶跃脉冲激励源为例,分析讨论了瞬变电磁测深法的勘探深度问题,以便为野外勘探工作设计提供依据,达到预期的探测目的.解析计算证实了瞬变场在地下以有限速度传播,数值模拟表示出了准静态条件下瞬变场的反射.研究结果表明,由于时间域电磁场遵循因果律,瞬变电磁法的探测深度主要由观测时间决定.瞬变电磁场的初始传播速度与大地电阻率无关,继后在大地色散作用下,阶跃脉冲前沿逐渐变得平缓,各频率分量的传播速度与电阻率有关,在低阻地层中探测同样的深度需要较长的观测时间.最大探测深度是在给定时间内电磁波往返地下某一深度的单程距离,最小探测深度受仪器性能的限制,但是埋藏较浅的异常体也有可能在晚时段被观测到.从时-频密度谱中可得到瞬变电磁场信号时间与频率的关系.

Taking central loop configuration excited by a step pulse on the ground as an example, we study the depth of investigation with analytical method, finite difference time domain and time-frequency method. The aim is to provide a basis for field work design in order to reach the expected target. Analytical calculation proves that transient electromagnetic fields propagate in the earth at a limited velocity, and numerical simulation exhibits reflection phenomenon of the fields under quasi-static conditions. The results show that the depth of investigation in transient electromagnetic exploration is mainly determined by measuring time because time-domain electromagnetic fields follow the law of causality. The initial propagation velocity of transient electromagnetic fields is not related to earth resistivity, later due to earth dispersion the front edge of the impulse becomes smoothed gradually and every frequency component has its velocity that concerns resistivity, and longer measuring time will be required for the same depth in lower resistivity stratum. The maximum depth of investigation is the distance that an electromagnetic wave goes to and back from some depth in the earth in given time. The minimum depth is limited by apparatus% performance, however, the bodies embedded in upper subsurface may be detected in late time. The relations between time and frequency can be obtained from time-frequency density spectrum of transient field signals.