基于等值反磁通原理的浅层瞬变电磁法

Opposing coils transient electromagnetic method for shallow subsurface detection

基于等值反磁通原理的瞬变电磁法是一种新的探测地下纯二次场的方法.该方法采用上下平行共轴的两个相同线圈通以反向电流作为发射源,且在该双线圈源合成的一次场零磁通平面上,测量对地中心耦合的纯二次场.理论计算和物理实验论证了该方法能够有效消除接收线圈本身的感应电动势,从而获得地下纯二次场的响应.理论推导和数值计算证明了该方法采用的双线圈源比传统瞬变电磁法采用的单线圈源对地中心耦合场能量更集中,因而有利于减少旁侧影响、提高探测的横向分辨率.实测试验表明该方法是浅层探测的一种有效方法.

Transient electromagnetic method（TEM）is a commonly-used,non-intrusive,geophysical method of measuring the secondary electromagnetic field induced by transient pulse sources.Because of the inherent mutual-induction between TX and RX coils,the measured field always contains the mutual-induction field,which becomes stronger when TX coil becomes smaller and closer to RX coil.It influences the near-surface TEM systems,which utilize small TX coils to effectively resolve the near surface explorations.The mixture of the mutual-induction field with the secondary field at early period leads to a blind region in the shallow surface in TEM exploration.To solve this problem,we applied two concentric and parallel current coils in the TEM system.They are opposing coils,which means the two coils are physically same,but the currents in them are of equal value and reverse directions.The RX coil is equidistant to the two opposing coils.In terms of this special arrangement,the mutual-induction field received at the RX coil is zero,which means that the mutual induction effect is efficiently eliminated.It is quite suitable for small TX coil TEM system aiming at shallow subsurface detection.