瞬变电磁横磁场响应特征与探测能力分析

瞬变电磁法是重建地下电阻率等电性结构的重要方法.传统磁性源和电性源瞬变电磁法主要观测横电极化场，横电极化场仅对良导目标敏感，对高阻目标的分辨能力有限.横磁极化场对高阻目标具有较强的分辨能力，但未得到有效利用.双线源瞬变电磁法可以增强观测电场中横磁极化场的占比，但对该方法的响应特征和分辨能力缺乏系统性的研究.为此，本文以双线源为例开展瞬变电磁横磁场响应特征与分辨能力分析.双线源瞬变电磁水平电场的响应强度要小于传统接地导线源，在发射源的中垂线上，层状大地模型的响应为零，观测的水平电场响应只能由地下的三维目标体产生.分别提取双线源和传统接地导线源激发电磁场中的横磁场和横电场，双线源瞬变电磁场中的横磁场占比要大于传统接地导线源，特别是在中晚期，横磁场远大于横电场，横磁场占比得到明显增强.通过均方根差和三维数值模拟的计算，双线源瞬变电磁水平电场显示出相较于传统接地导线源更强的高阻目标分辨能力，特别是在赤道向，水平电场对高阻目标分辨能力的增强效果更加明显.

Analysis on response characteristics and detectability of transient electromagnetic transverse magnetic field

Transient electromagnetic method (TEM) is an important method to recover the underground geo-electrical structure. The traditional loop-source and grounded-wire TEM mainly observe the transverse electric field, which is only sensitive to the conductive target and not sensitive to the resistive target. The transverse magnetic field has a high resolution for high-resistivity targets, but has not been effectively used. Double-line grounded-wire TEM can enhance the proportion of transverse magnetic field in the observed electric field. However, there is a lack of systematic research on the response characteristics and resolution of this method. Therefore, the response characteristics and detectability of double-line grounded-wire TEM are analyzed in this paper. The response amplitude of the horizontal electric field of the double-line source is smaller than that of the traditional single-line source. On the perpendicular line of the double-line source, the observed horizontal electric field can only be generated by the 3D structure, and the response on surface of the layered earth is zero. The transverse magnetic field and the transverse electric field of the double-line and the traditional single-line sources are extracted respectively. The proportion of the TM field in horizontal electric field of the double-line source is much larger than that of the traditional single-line source. The proportion of the TM field is obviously enhanced and much larger than that of TE field at late time. The horizontal electric field of double-line TEM shows a higher resolution to resistive targets than the traditional single-line source based on root mean square (RMS) misfit and 3D numerical simulation. The resolution of horizontal electric field to resistive target is improved obvious in the equatorial direction.