瞬变电磁法浅层探测技术

Time domain transient electromagnetic shallow prospecting

从理论上推导发射电流关断时间对瞬变电磁法勘探结果的影响,给出均匀半空间大地模型重叠回线互阻抗计算方法,分析阶跃关断与线性关断早期瞬变响应的区别,讨论了线性非零关断瞬变电磁法发射信号对浅层探测结果的影响。利用IGBT(绝缘栅双极型晶体管)实现瞬变电磁法发射电路,减小发射机关断时间,使之接近理想的阶跃函数,消除非零关断的影响,解决了加大发射电流与快关断的矛盾。实验结果表明,减小发射电流关断时间,可以提高瞬变电磁浅层勘探结果分辨率,减小瞬变电磁法勘探盲区。

The effect of turn-off time of transmitting current on transient electromagnetic exploration is derived theoretically, the mutual-impedance of coincident loop in uniform half-space is evaluated, the difference effect of step turn-off and linear turn-off on early transient signal is analyzed, and the effect of linear nonzero turn-off of transmitting current on shallow subsurface sounding is discussed. A insulated-gate bipolar transistor （IGBT） is used in transient electromagnetic transmitter system to shorten turn-off time. The turn-off waveform is designed to approach ideal step function to eliminate the effect of nonzero turn off and solve the conflict between strengthening transmitter and rapid switching. The experimental results indicate that shortening turn-off time can enhance resolution of transient electromagnetic for shallow subsurface sounding and reduce the blind zone of transient electromagnetic sounding.