時域反射量測技術改良及於水土混和物之應用

Abstract

時域反射法(Time domain reflectometry, TDR)為可量測物體的視介電度(Apparent dielectric constant)、導電度(Electrical conductivity)以及介電頻譜(Dielectric spectroscopy)特性之新穎技術，且具現地量測及多工(Multi-function)優勢，因此近來被廣泛應用至土壤或混擬土等材料性質量測。然而目前TDR視介電度及導電度量測方法有待釐清之處，且TDR的介電頻譜量測方法過於複雜，高頻量測結果可靠性不足，所以本研究的目的在於發展改良TDR量測方法，並提出TDR感測器製作原則，提供正確穩定的量測資料，以應用於土水混和物之電學性質探討。 TDR視介電度可由不同走時分析方法求得，但此一量測值缺乏實際物理意義，因此本研究採用數值模擬方法，探討視介電度及其等效對應頻率受材料導電度、介電頻散特性及纜線阻抗等因子影響，瞭解視介電度量測實務限制，進而提倡介電頻譜量測之重要性。本文並以實際量測與數值模擬，分析介電頻譜量測靈敏度與可靠度，探討介電頻譜量測誤差來源與可能改善方法。鑑於TDR介電頻譜高頻量測限制，本研究發展TDR頻率域相位速度分析方法，模擬分析結果顯示此一方法能有效提供材料高頻的相位速度，且於土壤含水量量測應用，不受導電度、土壤種類與纜線電阻影響，具有極大發展潛力。本研究另外針對TDR導電度量測，建立考慮纜線電阻之TDR導電度量測理論與系統誤差率定方法，配合模擬分析與實驗結果，並考量穩定反應所需之資料擷取時間長度，證實所提理論與率定方法可克服系統電阻以及系統誤差影響，提供材料正確導電度量測。 本研究另採用3D電磁波模擬分析工具，以及量測靈敏度理論推導，提供TDR感測器製作原則。為達到深層土壤（岩石）性質探討目的，研發之TDR圓錐貫入器 (TDR penetrometer)可同時提供材料之視介電度及導電度量測。配合此一TDR圓錐貫入器，於石門水庫進行水庫底層淤泥性質探討，基於底泥導電度量測結果，可推估底層淤泥含量與工程物理性質。由於近來台灣水庫因洪颱事件而產生大量入庫泥沙之危機，本研究進一步利用視介電度與感測器製作研究成果，發展新式走時分析方法以及TDR相位速度分析方法，研發高精度懸浮泥沙濃度量測技術，期以TDR多工多點的優勢，建置TDR自動化懸浮泥沙濃度量測。唯未來研究將建議持續現場量測驗證，以提供穩定量測資料。

Time domain reflectometry (TDR) can be used to measure apparent dielectric constant, electrical conductivity, and dielectric permittivity as a function of frequency. This relatively new technique is gaining popularity in characterization of engineering materials, such as suspended suspension, soil, concrete, etc, due to its versatility and applicability in field measurements. However, some disputes about the measurement methods for apparent dielectric constant and electrical conductivity (EC) have not been resolved. And dielectric spectroscopy remains relatively difficult in practice. The objectives of this study were to investigate and improve the TDR measurement techniques, provide guidelines for TDR probe design, and, as an application example, apply TDR to characterization of soil-water mixture. Since the apparent dielectric constant derived from various travel time analyses dose not have clear physical meanings, this study first investigated the influence factors, such as electrical conductivity, dielectric dispersion, and cable resistance, and associated effective frequencies. The applicability and limitations of travel time analyses are revealed with emphasis on the importance of dielectric spectroscopy. The dielectric spectrum, although more informative, is difficult to be reliably obtained. This study further examined the sensitivity and reliability of dielectric spectroscopy to identify the source of uncertainty and provide preferred guidelines. A novel approach to obtain dielectric permittivity at high frequencies was proposed based on the frequency domain phase velocity. The TDR EC measurement is more straightforward, but methods accounting for the cable resistance remain controversial; and the effect of TDR recording time has been underrated when long cables are used. A comprehensive full waveform model and the DC analysis were used to show the correct method for taking account of cable resistance and guideline for selecting proper recording time. In addition, a system error in typical TDR EC measurements was identified and a countermeasure was proposed, leading to a complete and accurate procedure for TDR EC measurements. Following the studies on TDR dielectric permittivity and EC measurements, this study further investigated the factors associated with probe designs for both types of measurements. The sensitivity of TDR measurements as affected by the probe parameters was discussed to provide guidelines for probe design. In addition, a penetrometer type of TDR probes was developed to allow simultaneous measurements of dielectric permittivity and electrical conductivity during cone penetration for measurements at depths. Although the aforementioned TDR measurement methodology was originally developed with soil applications in mind, the sediment problems in Shihmen Reservoir manifested by the Typhoon Aere in 2004 provides imperative opportunities for TDR applications. The TDR measurement techniques were adapted for characterization of soil-water mixtures. TDR penetrometer was integrated with the Marchetti dilatometer (DMT) and the TDR/DMT probe was pushed into the bottom mud to determine simultaneously, the solid concentration, stiffness and stress state of the bottom mud. A novel TDR probe and measurement procedure were further developed for accurate monitoring of suspended sediment concentration in fluvial and reservoir environment.