Determination of the spatial TDR-sensor characteristics in strong dispersive subsoil using 3D-FEM frequency domain simulations in combination with microwave dielectric spectroscopy
Determination of the spatial TDR-sensor characteristics in strong dispersive subsoil using 3D-FEM frequency domain simulations in combination with microwave dielectric spectroscopy
复制标题
使用 3D-FEM 频域模拟结合微波介电谱确定强色散底土中的空间 TDR 传感器特性
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发表时间：
2006
期刊：
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通讯作者：
K. Kupfer
中科院分区：
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作者：
N. Wagner;E. Trinks;K. Kupfer
用高频结构模拟器HFSS有限元模型模拟了6 cm TDR扁平带电缆传感器在以下条件下的空间传感器特性：(I)与周围物质(空气、不同盐度的水、不同的合成和天然土壤(沙-粉-粘土混合物))直接接触；(Ii)考虑不同大小的规定间隙，填充空气或水；(Iii)电缆传感器受压在井壁上。用HP8720D网络分析仪在50 MHz~20 GHz频率范围内测量了饱和和非饱和土壤的复介电常数ε⋆(ω，τi或复电导率σ⋆(ω，τi=iωε⋆(ω，τi)。在所研究的频率-温度-压力范围内，假定了三个土壤特有的松弛过程：一个主要的α过程(主要的水松弛)和两个由于粘土-水-离子相互作用(束缚水松弛和麦克斯韦-瓦格纳效应)的次要的(α‘，β)过程。用一个简单的分数松弛模型描述了每个过程的介电松弛行为。采用基于λ/3的自适应网格加密算法，在1 MHz、10 MHz、0.1 GHz、1 GHz和12.5 GHz的求解频率下进行了三维有限元模拟。研究了其电磁场分布、S参数和阶跃响应。模拟结果较好地再现了电场和磁场的时空分布。随着重力含水率和容重的增加，高损耗土壤会导致TDR信号上升时间的增加以及对多次反射的强烈吸收。空气或水的缝隙起准波导的作用，即强烈地减小了周围介质的影响。可以量化适当的TDR旅行时间失真。
The spatial sensor characteristics of a 6 cm TDR flat band cable sensor section was simulated with finite element modelling (high frequency structure simulator—HFSS) under certain conditions: (i) in direct contact with the surrounding material (air, water of different salinities, different synthetic and natural soils (sand–silt–clay mixtures)), (ii) with consideration of a defined gap of different size filled with air or water and (iii) the cable sensor pressed at a borehole-wall. The complex dielectric permittivity ε⋆(ω, τi) or complex electrical conductivity σ⋆(ω, τi) = iωε⋆(ω, τi) of the investigated saturated and unsaturated soils was examined in the frequency range 50 MHz–20 GHz at room temperature and atmospheric pressure with a HP8720D-network analyser. Three soil-specific relaxation processes are assumed to act in the investigated frequency–temperature–pressure range: one primary α-process (main water relaxation) and two secondary (α′, β)-processes due to clay–water–ion interactions (bound water relaxation and the Maxwell–Wagner effect). The dielectric relaxation behaviour of every process is described with the use of a simple fractional relaxation model. 3D finite element simulation is performed with a λ/3 based adaptive mesh refinement at a solution frequency of 1 MHz, 10 MHz, 0.1 GHz, 1 GHz and 12.5 GHz. The electromagnetic field distribution, S-parameter and step responses were examined. The simulation adequately reproduces the spatial and temporal electrical and magnetic field distribution. High-lossy soils cause, as a function of increasing gravimetric water content and bulk density, an increase in TDR signal rise time as well as a strong absorption of multiple reflections. An air or water gap works as a quasi-waveguide, i.e. the influence of the surrounding medium is strongly reduced. Appropriate TDR-travel-time distortions can be quantified.