航空瞬变电磁法对地下典型目标体的探测能力研究

航空电磁法的探测能力受飞行高度、发射波形、发射磁矩和发射基频等因素的影响,致使不同分量间的勘探能力存在差异.航空电磁如对所有磁场和磁感应分量、on-和off-time数据进行观测和解释,不仅数据量大、耗时长,而且出现大量冗余数据.目前国内针对此问题尚无系统解决方法.本文针对吊舱式直升机航空电磁系统,采用积分方程法求解频率域响应,经汉克尔变换转换到时间域,计算了地下三维目标体的B和dB/dt时间域响应.利用异常体响应与背景场响应作比值,并通过设定响应阀值定义最大勘探深度,进而分析不同发射波形、不同分量以及on-和off-time期间的航空电磁系统的探测能力.基于本文分析手段,可根据实际勘探目标,确定一套探测能力较强的航空电磁最佳参数组合,为野外测量和数据处理提供技术指导,高效完成勘探任务.

Exploration capability of airborne TEM systems for typical targets in the subsurface

For airborne EM (AEM) systems, flight altitude, transmitting waveforms, transmitting dipole moment and base frequency can strongly affect the depth of exploration. In an AEM survey, if all the magnetic field and magnetic induction components are measured, the EM dataset will be huge,resulting in costly data processing. In this paper, we investigate the exploration capability of an AEM system to different targets in the subsurface. We try to optimize parameter combinations of the airborne system (e.g. the waveform, field components, on- and off-time etc.) to balance the survey cost and resolution of targets.#br#For the towed-bird AEM system, we use an integral equation algorithm to calculate the frequency-domain EM field responses and convert them into the time domain via a Hankel's transform. The on- and off-time magnetic fields B and magnetic induction dB/dt are calculated for 3D targets of a plate or a prism embedded in a conductive earth. We propose a response ratio to define the largest exploration depth, based on which we calculate the exploration depth to different underground targets for different transmitting waves, field components and on- and off-time signals.#br#We study and analyze the influence of transmitting waveforms, field components, the on- and off-time signals on the target exploration capability. We find that the off-time B_x, dB_x/dt, and dB_z/dt for a trapezoid transmitting wave is the best parameter combination for resolving an underground conductive prism target. For a vertical thin plate, however, the off-time B_x, dB_x/dt and on-time dB_z/dt for a half-sine transmitting wave perform better. The combination of off-time B_x, dB_x/dt and on-time B_z, dB_z/dt for a trapezoid waveform can make deeper exploration for a horizontal thick plate.#br#From the research in this paper, we draw the conclusion that depending on the exploration target, there exists an optimal parameter combination of the AEM system that can achieve the maximum exploration capability. This will guide AEM survey design and data processing for an effective and efficient exploration. This research further lays the foundation for establishment of AEM standards and regulations.