多通道瞬变电磁法2D有限元模拟

2DFEM modeling on the multi-channel transient electromagnetic method

多通道瞬变电磁法的反演工作是在大地脉冲响应的基础上进行的,因此本文用2D有限元正演来求取大地脉冲响应,实现多道瞬变电磁法的正演模拟研究.研究工作首先在有源大地电磁法2D有限元模拟的基础上进行,原有2D有限元法的频带相对于多通道瞬变电磁法的频带是窄频的,为此采用校正法将其扩展到所需的宽度,实现了频率域宽频带有源电磁勘探方法的正演模拟;然后通过频时变换变换到时间域,得到瞬变电磁法的阶跃响应;最后通过求取阶跃响应的时间导数,得到大地脉冲响应.针对多通道瞬变电磁方法的装置特点,用有一定埋深的顺层成矿模型分析了大地脉冲响应特征,发现对有一定埋深的顺层产出的矿脉模型来说,频率域电磁场结果以及时间域大地脉冲响应结果均需移除背景场才能突出异常体的存在,然而,在实际工作中,未知的背景场限制了其实用性.为此,本文利用多通道瞬变电磁方法数据量大的特点,提出用不同偏移距的等时曲线与等时断面来展示脉冲响应模拟结果,事实表明这两种展示方式可以清晰地分辨出矿体引起的异常,且能准确定位矿体的中心在地面的投影位置,从而说明多通道瞬变电磁方法相对于频率域有源电磁方法,对有一定埋深的顺层矿床有更高的分辨能力.

The impulse response is studied using 2D finite element method because it is a basis for data interpretation of the multi-channel transient EM method(MTEM).In practice,the impulse response is obtained through cosin transformation from the results in the frequency domain,which are calculated by 2D finite element method(FEM)successfully used in the CSAMT modeling.However,the frequency band used in MTEM is wider than in CSAMT,and the modeling results at higher and lower frequencies deviate from the analytic results.We correct the differences by using far field and near field analytic formulas,and the correction results can be used in the cosin transformation to get the impulse response.In order to test the effect of the MTEM method,we design a three-layer model with a horizontal ore body limited-extending along the strata in the second layer.If one source is used,both in the frequency and time domain,the anomaly caused by the ore body cannot be distinguished clearly except for taking away the background field.However,the background is unknown in the field work;so it is not workable to be take away from the total field.We propose to use the isochronic curve or isochronic section of same offset to show the modeling results.In fact,from the two kinds of figures,not only the anomaly but also the horizontal location of the ore body can be distinguished clearly.It indicates that the MTEM method can distinguish the anomaly more clearly than the frequency method for in-layer deposits buried at some depth.