Bore Thierry, Wagner Norman, Lesoille Sylvie Delepine, Taillade Frederic, Six Gonzague, Daout Franck, Placko Dominique
SATIE, ENS Cachan, CNRS, Paris-Saclay University, Cachan 94230, France.
Institute of Materials Research and Testing at the Bauhaus-University, Weimar 99423, Germany.
Sensors (Basel). 2016 Apr 18;16(4):554. doi: 10.3390/s16040554.
Broadband electromagnetic frequency or time domain sensor techniques present high potential for quantitative water content monitoring in porous media. Prior to in situ application, the impact of the relationship between the broadband electromagnetic properties of the porous material (clay-rock) and the water content on the frequency or time domain sensor response is required. For this purpose, dielectric properties of intact clay rock samples experimental determined in the frequency range from 1 MHz to 10 GHz were used as input data in 3-D numerical frequency domain finite element field calculations to model the one port broadband frequency or time domain transfer function for a three rods based sensor embedded in the clay-rock. The sensor response in terms of the reflection factor was analyzed in time domain with classical travel time analysis in combination with an empirical model according to Topp equation, as well as the theoretical Lichtenecker and Rother model (LRM) to estimate the volumetric water content. The mixture equation considering the appropriate porosity of the investigated material provide a practical and efficient approach for water content estimation based on classical travel time analysis with the onset-method. The inflection method is not recommended for water content estimation in electrical dispersive and absorptive material. Moreover, the results clearly indicate that effects due to coupling of the sensor to the material cannot be neglected. Coupling problems caused by an air gap lead to dramatic effects on water content estimation, even for submillimeter gaps. Thus, the quantitative determination of the in situ water content requires careful sensor installation in order to reach a perfect probe clay rock coupling.
宽带电磁频率或时域传感器技术在多孔介质含水量定量监测方面具有很高的潜力。在进行现场应用之前，需要了解多孔材料（黏土岩）的宽带电磁特性与含水量之间的关系对频率或时域传感器响应的影响。为此，将在1 MHz至10 GHz频率范围内通过实验测定的完整黏土岩样品的介电特性作为输入数据，用于三维数值频域有限元场计算，以模拟嵌入黏土岩中的三棒式传感器的单端口宽带频率或时域传递函数。根据Topp方程，结合经验模型，通过经典走时分析在时域中分析了反射系数方面的传感器响应，同时还采用了理论Lichtenecker和Rother模型（LRM）来估算体积含水量。考虑到所研究材料的适当孔隙率的混合方程为基于经典走时分析和起始法的含水量估算提供了一种实用且有效的方法。不建议使用拐点法来估算电色散和吸收性材料中的含水量。此外，结果清楚地表明，传感器与材料之间的耦合效应不可忽略。气隙引起的耦合问题对含水量估算有显著影响，即使对于亚毫米级的气隙也是如此。因此，为了实现完美的探头与黏土岩耦合，原位含水量的定量测定需要仔细安装传感器。
