Conductivity-depth Imaging Algorithm for Central-loop Helicopter TEM
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摘要: 以理论上的中心回线式直升机航空瞬变电磁系统为例,提出一种快速的电导率-深度成像方法,将感生电动势瞬变响应数据变换为地下介质电导率-深度断面。该方法针对成像计算中视电导率非唯一性问题和成像深度难以确定的困难,首先给出一个关于均匀半空间模型电导率σ的数据函数β(σ)的变换公式,由此建立数据表σ-β来查询视电导率值,可以得到可靠的视电导率值。其次用均匀半空间中的感应电场最大幅值对应深度值来定义视深度,建立视电导率-视深度的数据表,并在2个相邻时间道期间的最大电场深度变化的基础上定义成像深度,从而导出CDI结果。该方法的特点是理论简单、物理意义明确,且利用了数据表查询技术大大提高了计算速度,使得实时CDI处理成为可能。最后用理论模型进行试算,结果表明,电导率值的查询范围宽至10-4~102S/m时,高阻薄层、低阻薄层及二维目标体模型均有较好的成像效果。Abstract: A Conductivity-Depth Imaging(CDI) algorithm is presented for a central-loop helicopter airborne system as an example in this paper,to transform the transient induced electromotive force response data into a conductivity-depth section very quickly.The method aimed at the non-uniqueness problem of the apparent conductivity and the difficulties to obtain the imaging depth in the imaging method,and is successfully developed according to the following steps:firstly,a transformation formula of the data function β(σ)about the conductivity of homogeneous half-space model σ is presented,and then a table of the data function relationship with the conductivity is built to search the apparent conductivity value.This method can overcome the problem of the non-unique value of the apparent conductivity.Secondly,the apparent depth is approximated using the depth of the maximum induced electric field in the homogeneous half-space earth,and a data table about apparent conductivity and apparent depth is established to accelerate computing.Finally,the imaging depth is determined based on the depth change of the maximum electric field at the two adjacent delay time and then the CDI result is obtained consequently.The advantage of the method is that its high-efficiency of the table lookup technique can make the real-time CDI possible.Using a table lookup with a large conductivity range from 10-4 S/m to 102 S/m,tests on synthetic data demonstrate that the algorithm does a good job of solving conductive or resistive thin layers and 2D object.
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