地層電學性質與含水特性關係之現地尺度探討

Abstract

地層含水特性與邊坡穩定性有密切的關係，因此對於地層進行含水特性監測有其必要性。而地電阻法（Electrical Resistivity Tomography, ERT）具有探測或監測地層2D或3D電阻率空間分佈的能力，且地層地電阻與土層含水特性有高度相關係，但電阻率又同時受到地文及水文因子的影響，因此難以單獨利用地電阻法量測地層含水特性的空間分佈。前期研究提出結合時域反射法（Time Domain Reflectometry, TDR）與ERT，藉由TDR可同時量測土層的含水特性及電阻率，建立電阻率與含水量率定關係。 為驗證上述構想的可行性，並克服室內試驗邊界效應影響，本研究進行現地尺度試驗，探討地層電阻率與土壤含水量之關係。為量測現地不同深度之含水量與電阻率，本研究設計一貫入式TDR感測桿，裝設於現地，擷取現地經過因降雨之濕潤與乾燥過程資料，藉以建立現地含水量與導電度率定關係，進而利用ERT電阻量測，以獲得大範圍土層含水特性分布。由現地乾溼過程的觀測結果發現濕潤過程的快慢會影響導電度與含水量關係之斜率，但可能由於現場乾燥過程因消散水分緩慢，並無如室內實驗顯示明顯的遲滯現象，濕潤過後乾燥的路徑會沿著濕潤的路徑乾燥。而ERT與TDR量測到之電阻率，可能因為取樣空間不同的關係，在不同濕潤與乾燥過程，亦存在不同的相關趨勢，此部分尚須後續進一步之探討，以評估利用ERT影像估計含水量分佈之可靠度。

Since the soil water characteristic is essential for slope stability, its monitoring is critical and necessary for related stability evaluation. Electrical Resistivity Tomography(ERT) has the ability to monitor two dimensional or three dimensional distribution of soil resistivity, which is closely related to soil water content. Hence, it is possible to use Electrical Resistivity Tomography for large-scale monitoring of soil water content. However, electrical resistivity depends not only on soil moisture content, but also on the groundwater characteristics and geological factors. Therefore, it is difficult to monitor soil moisture distribution by ERT alone. Earlier study proposed to integrate electrical resistivity tomography (ERT) with Time Domain Reflectometry (TDR), which can monitor soil moisture content and resistivity simultaneously in situ. To verify the feasibility and overcome the boundary effect in the laboratory testing, this study was aimed to perform full scale testing in the field to investigate the relationship between electrical resistivity and soil water content. A penetration-type TDR probe was designed to enable simultaneous measurement of resistivity and water content at different depths. The probe was installed in a test site to collect data to establish the relation between resistivity and soil water content. The results obtained during different stages of wetting and drying show that the relationship between resistivity and soil water content is affected by the speed of wetting. But unlike what has been observed in the laboratory testing, no apparent hysteresis in wetting-drying cycle was observed in the field, perhaps due to the fact that the drying process in the field is quite slow. Furthermore, the relation between ERT-measured resistivity and TDR-measured resistivity also depends on different wetting and drying conditions. This may be attributed to different spatial sampling of the two measurements. Further study is required to evaluate the reliability of estimation of water content distribution from resistivity imaging.