TDR脈衝反射係數法之實驗評估及於底泥探測之應用

Abstract

由於國內各主要河川於相關法令公告後，出現底泥調查與量測需求，除現有規定之相關取樣標準作業可能受限於取樣空間代表性以及其作業效率，國內外研究計畫均初步建議配合地球物理調查方法，以可大範圍輔助底泥範圍、厚度，甚至污染潛勢等相關特性之調查，但目前地球物理探測方法受限於探測深度、深度轉換(透地雷達、超音波)、解析度不足(地電阻影像探測法)以及實際量化(Quantification)方法等問題而有待改進。 可同時量測材料介電性質(可提供電磁波速)以及導電率(電阻率之倒數)的時域反射儀技術為極具潛力之輔助工具，然而其目前仍須克服貫入深度、於高導電度環境施測分析以及高空間解析度(公分等級)之問題。本研究的目的在於透過對TDR反射脈衝係數法之室內試驗評估，相較過往採用走時分析獲得介電度之方法改採以反射係數進行，提出可具高空間解析度並可在高導電度環境下量測分析之介電度分析方法，而後配合此分析方法，發展一現地可直接貫入底泥之感測桿，使該量測系統具備容易組裝及操作的特性，有助於探討底泥厚度與對應之電學特性。 根據室內試驗之結果顯示，正規化後之TDR反射脈衝係數與視電阻率具有線性關係，且土壤種類與水導電度對視介電度估計的影響約±2。此結果顯示透過正規化後之TDR反射脈衝係數確實可進行視介電度的推估，如此即使在高導電度短感測器長度的環境與設計條件下亦可達到進行視介電度量測的目的。新設計之高空間解析度底泥貫入器透過室內全尺度試驗結果顯示，應用正規化後之TDR反射脈衝係數可有效辨識底泥厚度，但在視介電度的估算上，對於低視介電度(<35)的量測靈敏度較差，此現象與感測器型式有關，建議未來進一步改善。

According to recent regulations, sediment properties of major rivers should be conducted at least once during wet season and dry season to establish the sediment quality data base and update the baseline for each major river. Current standard practice of sediment sampling may be limited in terms of spatial representativeness and working efficiency. Geophysical methods are often suggested to complement sampling for investigation of sediment coverage, thickness, and even pollution potential in large scale. However, current geophysical practice has limited investigation depth and lacks quantification approach. Time domain Reflectomerty(TDR) penetrometer, which simultaneously provides apparent dielectric constant and electrical conductivity estimation, has the high potential to counter above problems. However, penetrating depth, measurement in highly conductive condition and space resolution are the problems should be considered before applied in field. The objective of this study is to develop a field measurement penetrometer based on TDR technique that is easy to assemble and operate for measuring sediment thickness and profiling electrical properties. TDR pulse reflection coefficient method, instead of travel time analysis, is alternative method proposed to deal with the difficulties when conducting TDR measurement in highly conductive condition with high space resolution requirement. According to the results of laboratory experiments, TDR pulse reflection is proportional to the apparent dielectric constant reduced from travel time analysis. The influence of the soil type and electrical conductivity of water is small. Only 2% error would be produced if neglecting these two factors when applying the proposed method. Based on it, the high space resolution TDR penetrometer was designed. With verification in full scale experiment, the designed probe was proved to be an efficient tool in measuring thickness of sediment. However, sensitivity would be lower for measuring apparent dielectric below 35. Further modification is suggested to optimize the TDR penetrometer.