多接地源半航空瞬变电磁响应规律研究

Analysis of multi-source semi-airborne transient electromagnetic response

接地源半航空瞬变电磁法一般采用单一线源激发,发射电流幅值很大程度受接地电阻影响,且复杂地形条件下大长度线源布设难度较大.采用多个场源代替单一线源是解决这一问题的途径.本文基于Crank-Nicolson-Cycle-Sweep-Uniform FDTD(CNCSUFDTD)隐式差分算法对不同发射源布置形式的多接地源半航空瞬变电磁模型进行三维正演模拟,并对比分析了不同模型的瞬变电磁响应特征,进而探讨采用多个场源代替单一线源的可行性.数值试验结果表明:相比仅采用单一线源激发,使用不同源间距的平行双辐射源可同时增加早期与晚期感应电动势衰减曲线幅值,且随着源间距的增加晚期幅值逐渐增大;平行发射源的数量与感应电动势衰减曲线幅值呈正相关关系,且多个小电流平行辐射源与单一大电流线源的勘探效果基本一致,采用此方式可在接地电阻较大时仍获得大发射电流,提高采集数据质量;单一长导线源与多个共线短源、多个平行短源衰减曲线早期存在轻微差异,而在晚期则基本重合,三者勘探效果基本一致;发射源处的山峰、山谷地形对全部计算时间范围内的衰减曲线均存在显著影响,山峰地形会显著降低晚期幅值,山谷地形则会显著降低早期幅值.本文的研究结果为采用多个场源代替单一线源提供理论基础,为降低复杂地形条件下半航空瞬变电磁发射源的野外布置工作难度提供参考.

Semi-airborne transient electromagnetic with galvanic source generally uses a single line source for excitation.The amplitude of the emission current is largely affected by the ground resistance,and it is difficult to deploy a large length line source in complex terrain condition.Adopting multi-source instead of a single line source is the way to solve the problem.In this paper,based on the Crank-Nicolson-Cycle-Sweep-Uniform FDTD(CNCSU-FDTD)which is an implicit difference algorithm,the 3D forward modeling of multi-source SATEM models with different emission source arrangements is conducted.The transient electromagnetic response characteristics of different models are compared and analyzed,and then the feasibility of using multi-sources instead of a single line source is explored.Numerical study revealed that the use of parallel dual sources with different source spacing can simultaneously increase induced electromotive force amplitude of all time compared to the use of only a single line source excitation,and the late-time amplitude gradually increases with the increase of the source distance.The number of parallel sources and the amplitude of the induced electromotive force decay curveshow a positive correlation.And multiple small-current parallel sources have basically the same exploration effect with a single high-current line source,using this method can obtain large emission current even when the ground resistance is large which can improve the quality of data.There is a slight difference between the decay curve of single long line source and multiple short common line sources and multiple short parallel sources in early time.But in late time,they basically overlap well,and the exploration effect of the three is basically the same.The topography of the peaks and valleys at the emission sources have a significant effect on the decay curve in the whole calculation time range.The peak topography significantly reduces late-time amplitude,while valley topography significantly reduces early-time amplitude.The results of this paper provide a theoretical basis for using multi-sources instead of a single line source,and provide a reference for reducing the difficulty of field setting of SATEM emission sources under complex terrain conditions.