用扩散电场法估算CHTEM-I系统的探测深度

时间域航空电磁系统的探测深度是一项关键技术参数，在系统研制设计和资料解释中都有重要作用.当目标层与围岩电性差异不大或目标层引起的异常响应不明显时，常规的估算探测深度方法不再适用，为克服这种缺陷，本文提出了一种系统探测深度的估算方法，该方法通过模拟电场在均匀半空间模型和层状模型中的扩散过程，记录电场最大幅值在地下的瞬时位置，将系统的探测深度定义为观测到的响应值等于给定的噪声水平时，对应时刻的地下介质中感应电场幅度的最大值对应的深度.论文以我国自主研制的直升机时间域航空电磁系统CHTEM-I为例，根据对不同条件下的计算结果的分析，给出了探测深度与模型电导率、飞行高度和噪声水平之间的关系曲线，并总结出一些能够提高系统探测深度的建议.结果表明半空间模型电导率在0.000295~0.0422 S/m范围内，CHTEM-I系统可以在噪声水平与t^－0.5成正比条件下达到300 m的探测深度.本文方法的估算结果不受模型层厚薄，或目标层与围岩电性差异小等因素影响，因而具有较高的实用性.文中的结论对时间域航空电磁系统设计有理论指导意义，也可用于其它时间域电磁勘探系统探测深度的估算.

The estimated prospecting depth of CHTEM-I system by the method of diffusion electric field

The Prospecting Depth of the System (PDOS) is one of the key technology indicators of the Airborne Time Domain Electromagnetic (ATEM) system, which plays an important role in the system design and data interpretation. If the difference of the conductivity between the target layer and the host rock is small or the anomalous response caused by the target layer is unobvious, the routine estimation method of PDOS is no longer effective. To overcome this drawback, a new estimation method of PDOS is presented in the paper. Based on simulating the diffusion process of the electric field in a uniform half-space model or a layered model and recording the location of the maximum value of the electric field from the subsurface, the prospecting depth of the system is defined as the corresponding depth of the maximum value of induced electric field amplitude in the earth medium in the depth where the observed response value is equal to the noise of the system. In this paper, a helicopter time domain electromagnetic system named CHTEM-I (China Helicopter Time Domain Electromagnetic System-I) developed independently by Chinese is used as an example, according to the analyses results under different conditions, the relationship curves between the prospecting depth and the conductivity of the models, the height of the loop and the noise level are given, and the suggestion about how to improve the prospecting depth is also included in the paper. The results in this paper show that the CHTEM-I system can realize the prospecting depth of 300m at a noise level being proportional to t^－0.5 if the conductivity of half-space model is in the range of 0.000295~0.0422 S/m. The estimation depth value from the method given in this paper is unaffected by the factors of the thickness of the target layer and the resistivity difference between the target layer and the host rock, so it is a practicable method. The conclusion in this paper has the theoretical significance for the airborne time domain electromagnetic system design, and the research ideas can also be applied to estimate the prospecting depth of other time domain electromagnetic systems.