时间域航空电磁法激电效应对电磁扩散的影响

由于激发极化效应的影响,时间域航空电磁晚期道信号经常会出现变号现象.基于电阻率的传统反演方法无法对变号数据进行正确反演,因此通常在数据处理中予以剔除.为深入了解极化介质的电磁扩散特征,认识航空瞬变电磁负响应的产生机理,本文研究时间域航空电磁系统的电磁扩散特征.我们以均匀极化、非极化半空间及层状介质模型为例,通过直接积分的方法求解频率域电场响应,并由欧姆定律得到电流响应,再经过汉克尔变换得到时间域电流响应.通过研究电流随时间在地下极化介质中的传播特征研究电磁扩散过程;通过对比不同激电参数对电磁扩散的影响,研究极化介质中感应电流与极化电流的扩散规律,从而合理地解释极化介质中负响应的产生机理.基于本文研究和分析结果,可加深对时间域航空电磁法中激电效应的认识.

The effect of induced polarization on time-domain airborne EM diffusion

Airborne transient electromagnetic (ATEM) data frequently shows sign reversal in the late-time channels duo to IP effect. Traditional inversion algorithms based on real resistivity cannot deliver reasonable results for such kind of data. They are usually deleted in the data processing. For in-depth understanding the characteristics of EM diffusion in a polarizable medium and acquainting the mechanism of negative responses caused by the polarizable medium, we study the IP effect on EM diffusion for an airborne EM system.We take as examples a polarizable and non-polarizable homogeneous half-space and a layered earth model. For the time-domain airborne EM system, we use direct integration to calculate the frequency-domain electric field and then obtain the frequency-domain current by using the Ohm's law. After that, we convert the frequency-domain responses to time-domain via a Hankel's transform. By introducing the time factor in the current responses, we can describe the dynamic process of EM diffusion in the underground. In the dynamic presentations (not shown in the paper but downloadable from proper website), we display the induced current, polarization current and total current diffusion in a polarizable medium as 3D animated contours.From the dynamic presentations of EM diffusion in a polarizable medium, we find that the induced current diffuses in the underground, forming the "smoking ring". However, the polarization current has sign reversal during the diffusion process. At the early time, the polarization current is heavily connected with the induced current and flows in the same direction as the induced current, the underground medium is charged until the time when the induced current can no longer maintain the charging, the polarization current begins to reverse its sign (flow direction). The sign reversal of polarization current can result in negative EM signal in airborne EM systems. We further study the influence of chargeability and the conductivity at infinite frequency on the EM diffusion and find that the greater the chargeability, the earlier the sign reverses and the larger the influence range is. For the same chargeability, the smaller the conductivity, the earlier the sign reverses and the larger the influence range is. In a polarizable and resistive region, one can easily measure the negative EM signal.From the research of this paper, we can draw the conclusion that depending on the resistivity and IP parameters of the earth, the induced polarization can heavily influence the EM diffusion process, resulting in the possible sign reversal in AEM survey signal. Special attention needs to be paid in resistive areas with high polarizations.