任意方向长导线源瞬变电磁一维正演模拟

One Dimensional Forward Modeling of Transient Electromagnetic with an Arbitrary Direction Wire Source

在野外开展瞬变电磁法探测时,由于环境影响,使得布设的长导线无法完全平行测线,对数据处理及解释产生一定的影响。为了解决此问题,本文首先采用层状介质格林函数实现了两个垂直方向有限长导线源产生的频率域电磁场,与半空间电偶极子源计算的解析解进行了验证,其次利用两个垂直方向的有限长导线源组合成任意方向的水平电性源,通过余弦变换折线逼近法,实现了任意方向电性源瞬变电磁一维正演。在层状介质模型中进行了不同偏转角度布设有限长导线源的瞬变电磁正演模拟,分析了其相对平行测线方向的源所产生的瞬变电场强度及感应电动势的异常响应变化特征,同时对多个任意方向电性源的层状介质模型进行了瞬变电磁正演模拟。通过正演模拟分析:对于电性源瞬变电磁法,由于野外地理环境所造成的源无法完全平行测线方向的布设会对观测数据产生一定的的影响,偏转角度越大,影响越大,特别是电场强度,需采用任意方向的电性源瞬变电磁进行正演计算。通过对单源与双模型的正演模拟对比得出,采用多方向布设双源探测不仅可以有效提高至少两倍观测信号强度,增大抗干扰能力,而且多方向布设源可以增强异常体的异常响应变化幅值,相对平行测线布设的源其异常响应最大增幅可超55%。该方法的实现及模拟对野外开展瞬变电磁法的探测具有一定的参考价值。

When conducting transient electromagnetic detection in the field,due to environmental influences,the long wires laid cannot be completely parallel to the survey line,which has a certain impact on data processing and interpretation.To solve this issue,firstly,the Green’s function of layered medium is used to realize the frequency domain electromagnetic field generated by two vertical finite length wire sources,verified with the analytical expression of half space electric dipole source calculation.Secondly,two vertical finite length wire sources are combined to form a horizontal electric source in any direction,and the one-dimensional forward modeling of transient electromagnetic of electrical source in any direction is realized through cosine transform broken line approximation method.Transient electromagnetic forward modeling was conducted in a layered medium model with finite length wire sources arranged at different deflection angles.The anomalous response characteristics of the transient electric field intensity and induced electromotive force generated by sources in parallel to the measurement line direction were analyzed.At the same time,transient electromagnetic forward modeling was conducted on a layered medium model with multiple arbitrary electrical sources in arbitrary direction.Forward simulation analysis indicates that for the transient electromagnetic method of electrical sources,the layout of sources not completely parallel to the measurement line direction due to the outdoor geographical environment has a certain impact on the observation data.The larger the deflection angle,the greater the impact,especially for the electric field strength.It is therefore necessary to use arbitrary direction of electrical source transient electromagnetic for forward calculation.By comparing the forward simulation of single source and multi-source models,the use of multi-directional deployment of multi-source detection not only can effectively increase the observation signal strength by at least twice and enhance anti-interference ability,but also enhance the amplitude of abnormal response changes of abnormal bodies.Compared to sources arranged in parallel lines,the maximum increase in abnormal response can exceed 55%.The implementation and simulation of this method has certain reference value for detecting transient electromagnetic methods in the field study.