时间域航空电磁扩散特征和成像深度研究

2016-09-29殷长春邱长凯刘云鹤蔡晶

地球物理学报 2016年8期

关键词：电磁场深度

殷长春，邱长凯，刘云鹤，蔡晶

吉林大学地球探测科学与技术学院，长春　130026

时间域航空电磁扩散特征和成像深度研究

殷长春，邱长凯，刘云鹤，蔡晶

吉林大学地球探测科学与技术学院，长春130026

为了研究时间域航空电磁系统扩散特征，基于连续性边界条件将电磁场向下延拓得到地下介质中各点处的频率域响应，通过傅里叶变换将其变换到时间域.计算阶跃波形发射时垂直磁偶极子和水平磁偶极子在均匀半空间介质和两层介质中产生的电磁场，通过电流密度矢量图和随时间变化的电流密度等值线展示电磁场在地下介质中的扩散过程.电磁场在地下的扩散受电阻率结构的影响.在良导地区电磁场扩散慢、衰减快，而在高阻地区电磁场扩散快、衰减慢.垂直磁偶极子在地下产生的感应电流形成一个电流环随时间向下、向外扩散.水平磁偶极子在地下产生的感应电流形成两个互相叠加的电流环.时间域电磁场扩散代表着真正物理意义上的电磁扩散，因为它展示了地下电磁场空间分布随时间的变化和场强的衰减.基于对感应电流环的研究，我们探究了扩散深度和时间域航空电磁数据成像深度的关系.时间域电磁场扩散规律的研究不仅加深了对电磁扩散特征的理解，更有助于提高航空电磁数据解释水平.

航空电磁；电磁扩散；烟圈效应；成像深度

To study the EM diffusion of a time-domain AEM system,we first calculate the frequency-domain EM field inside the earth based on the continuity boundary condition and then transform it into time-domain via a Fourier transform.We calculate the EM fields induced by a step pulse for a homogeneous half-space and a two-layered earth model and display the EM diffusion in the earth as 2D vectors or 3D animated time-varying contours (like the "smoke ring").These time-domain current rings display the true EM diffusion,because they demonstrate the spatial variation and decay of underground EM fields with time.

Study of the EM current rings shows that EM diffusion is strongly influenced by the resistivity structure inside the earth.In a conductive earth,the current ring diffuses slowly but decays fast,while in a resistive earth it diffuses fast but decays slowly.The induced current for a vertical magnetic dipole forms a single current ring that propagates with time outward and downward,while for a horizontal magnetic dipole,the induced current forms two stacked current rings in the underground,diffusing with time into the deep earth.Based on the EM current ring,we find that the imaging depth for time-domain AEM data is about 0.55 times the EM diffusion depth.

By showing the underground induced current as vectors and 3D time-varying animated contours,the EM diffusion and smoke ring effect are clearly observed.An animated EM smoke ring offers more information than a static contour of current density.The relation between imaging depth and diffusion depth is further confirmed by the smoke rings.Study on EM smoke ring can not only offer an insight into EM diffusion in the earth,but can also assist in AEM data interpretation.