基于“双烟圈”理论的瞬变电磁全期视电阻率计算

The calculation of full time apparent resistivity for transient electromagnetic survey based on "double smoking ring" theory

由于受发射线圈电感和电容的影响,瞬变电磁发射电流类似斜阶跃关断,采用阶跃关断"单烟圈"理论进行视电阻率解释,在早、中期产生失真。根据斜阶跃关断二次场感应电压表达式,提出采用"双烟圈"理论进行解释:将完全关断后二次感应电压等效为大小相等、方向相反、先后发射的两个"单烟圈"二次磁感应强度之和。分别采用前向递推和后向递推两种方法,计算出每个烟圈二次磁感应强度,再利用"单烟圈"二次磁感应强度的单调性,计算斜阶跃关断全期视电阻率,消除了失真。最后通过数值模型,验证了方法的正确性,为全程瞬变电磁探测及解释提供理论支持。

Owing to the impact of inductor and capacitor of transmit coil, the transmit current of transient electromagnetic turns off like the ramp current. If we use the ＂single smoke ring＂ theory which fits for step turning off transmit current to make interpretation, the distortion will occur during the early and middle stage. According to the expression of second field induced voltage for ramp current, this paper presents interpretation with the＂double smoke ring＂ theory, i.e., the secondary field induced voltage is made to be equiva-lent to the summation of secondary magnetic induction excited by two equal, opposite and successive＂smoke ring＂ after the current is completely turned off. The secondary magnetic induction excited by every＂single smoke ring＂ is calculated with forward and backward recurrence respectively. After that, the monotony of secondary magnetic induction excited by ＂smoke ring＂ is utilized to calculate the full-time apparent resistivity of ramp current and remove the distortion. At last, the numerical simulation model is used to confirm the correctness. The results obtained by the authors provide theoretical support for the full-time transient electromagnetic exploration and ex-planation.