均匀半空间有限长接地导线源地面电场响应特征

Characteristics of the Electric Field Generated by a Finite-Length-Ground Wire Source in Homogeneous Half-Space

本文分析了导线对可控源音频大地电磁勘探中电场的影响。在分析过程中,将有限长直接地导线源在层状介质中激发的电场的x分量分解为接地电极贡献的传到部分和通电长导线形成的感应部分,分别计算出在均匀半空间条件下,两部分及总电场随频率和收发距的变化规律。通过对计算结果的分析可见:在小收发距时,电场实部在低频区完全由接地电极部分贡献,在高频区由通电导线部分决定;电场虚部在低频区完全由通电导线部分决定,在高频区由接地电极部分和通电导线部分共同决定,且数值近似相等。在大收发距时,电场实部在低频区完全是由接地电极部分贡献,在高频区由接地电极部分和通电导线部分共同决定,且数值近似相等;电场虚部在低频区由通电导线贡献,在高频区由接地电极部分和通电导线部分共同决定,且数值近似相等。从上述分析可以看出,作为大收发距电磁法代表的可控源音频大地电磁法,其观测数据是由接地两端的传导分量和通电导线感应分量组成,并且感应分量贡献与传导贡献相当。由于导线敷设的非直线形,而引发与直导线理论不一致的结果。因此在数据采集工作中,必须考虑导线形状。同时要注意选择合适的收发距和工作频率,尽量把通电导线的影响降到最低。

This paper presents an analysis on the affect of ground-wire on the electric field in CSAMT exploration. The x component of the electric field generated by finite-long-straight ground wire in a stratified medium is decomposed into electrode part and wire part. In homogeneous half-space,we calculate changes of these two parts and the total electric field with frequency and receive-transmitter distance. Results show that when the receive-transmitter distance is small,the real part of the electric field is completely contributed by the electrode at low frequency and decided by wire at high frequency;while the imaginary part of the electric field is completely controlled by wire at low frequency and co-determined by wire and electrode at high frequency with largely same values. When the receiver-transmitter distance is large,the real part of the electric field is absolutely contributed by electrode at low frequency,and determined jointly by wire and electrode at high frequency with approximately equal values;while the imaginary part is contributed by wire at low frequency and co-contributed by electrode and wire at high frequency with approximately equal values. According these results,CSAMT as representative of the great receiver-transmitter distance electromagnetic methods,data by CSAMT consist of two parts:the conduction(electrode part)and induction(wire part)components,and both have largely the same contributions. However,as the wire is non-straight line when it is laid on the surface,the real results may be different from the straight line theory. Therefore,in the actual work,the line-current shape must be considered. At the same time,we should select proper receive-transmitter distances to lower the effect of the wire shape as much as possible.