三针式TDR探头测试区域范围分析及试验验证

在时域反射法（TDR）测试系统中,三针式探头作为应用最广泛的探头形式,其测试区域范围的确定是工程应用中一个重要问题。通过对比分析麦克斯韦方程及地下水渗流方程,利用岩土工程中常用的地下水渗流软件（如：Geostudio软件中SEEP/W模块）对一典型的三针式TDR探头测试区域范围进行计算,并设计了专门的试验对计算结果进行验证。结果显示：①地下水渗流分析软件对三针式TDR探头测试区域范围的计算结果与试验结果有较好的一致性；②三针式TDR探头的测试区域近似长半轴为33.0 mm、短半轴为20.7 mm的椭圆,且该区域内50%的电场能量集中在中间探针周围长半轴为10.7 mm、短半轴为8.7 mm的椭圆内,显示出较明显的“趋肤效应”；③介质的电导率对三针式TDR探头测试范围基本无影响。通过该研究,提出了三针式TDR探头测试范围的计算方法,为工程或试验中该种探头的设计提供了指导。

Analysis and experimental verification of sampling area of three-rod time-domain reflectometry probe

In the measurement system of time domain reflectometry (TDR), three-rod probe is most widely used. The sampling area of this probe form is an important question in engineering application. In this paper, through comparative analysis of Maxwell equations and groundwater seepage equation, the software for groundwater seepage (e.g., the SEEP/W module in the Geostudio) is used to compute the sampling area of a typical three-rod probe. A special experiment is designed to verify the computed results. The results show that: (1) the results of sampling area are well consistent between the numerical computation using Geostudio and the experiment; (2) the sampling area of the three-rod probe approximates an ellipse with semi-major and semi-minor axes of 33.0 and 20.7 mm respectively. In this area, the media with 50% influencing the measured results concentrate around the central rod, and this area also approximates an ellipse with semi-major and semi-minor axes of 10.7 mm and 8.7 mm respectively. It shows the “skin effect” significantly; (3) the electrical conductivity of the media has insignificant effect on the sampling area of the three-rod probe. A computational method for the sampling area of three-rod probe is poposed, and it provides guidance for the design of three-rod TDR probe in engineering and experiments.