結合地電阻法與TDR於土層含水特性之監測

Abstract

非飽和土層含水特性常影響邊坡穩定性與土壤污染傳輸特性，因此對非飽和土層進行含水特性監測有其必要性。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。利用時域反射法（Time Domain Reflectometry, TDR）可同時量測感測器周圍土層的含水特性及電阻率，因此本研究提出結合TDR與ERT量測非飽和土層含水特行之技術，利用TDR量測結果率定電阻率與含水特性關係中之地文及水文因子，進而以ERT之電阻率分佈推估地層之含水特性分佈。 為驗證上述構想的可行性，本研究嘗試利用TDR量測不同含水量之試體探討三種體積含水量與導電度的關係，並從中選取較適合之函數，再進一步進行砂箱物理模型降入滲試驗，以模擬現地含水特性率定關係之建立，進而獲得土層含水特性分佈；此外，藉由砂箱物理模型針對地電阻影像剖面解析度與TDR儀器埋設方式進行研究探討。 利用文獻中三種不同建議的導電度與體積含水量的函數關係，迴歸TDR所量測到之體積含水量與導電度，結果不同方法之R2均大於0.95以上，顯示資料迴歸結果相關性高，但以廣義Archie’s law迴歸所得之RMSE值最小，因此本研究選定以廣義Archie’s law的函數關係做為後續砂箱導電度與體積含水量的分析。而室內砂箱降雨入滲試驗中，TDR所得結果顯示TDR量測所造成之延遲現象已有所改善，但量測結果中乾側與溼側的體積含水量與導電度之率定關係明顯不同，其原因可能係降雨入滲時，雨水分佈不均勻造成含水量分佈並非均質，而TDR導電度與體積含水量之空間解析度亦非相同，因此造成乾溼兩側迴歸參數之差異。

The slope stability and transportation of underground pollution in unsaturated soils much depend on the soil moisture content, thus, there is a demand for monitoring spatial and temporal changes of the soil moisture content. Electrical resistivity tomography (ERT）method, which can be used to investigate and monitor 2D or 3D resistivity distribution, has a great potential to serve such a purpose since resistivity is highly related to the soil moisture 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. This study proposed a new monitoring scheme by integrating ERT with time domain reflectometry (TDR) technique. TDR probes are used to simultaneously measure the soil moisture and electric resistivity at some locations to establish the local relationship between the soil moisture and electrical resistivity. Using TDR to calibrate the hydrological and geological factors, resistivity distribution monitored by ERT can then be transformed to soil moisture distribution. In order to verify the feasibility of the proposed method, this study first evaluate three common formula between bulk soil moisture and electrical resistivity by TDR experiments on soil samples with various moisture contents and water salinities. The formula that best describe the relationship between soil moisture and electrical resistivity was selected for next stud phase conducted in a sand box, in which experiments simulated the field conditions of rainfall and drying and the proposed method was carried out. Among the three functions (Rhoades et al.,1976; Lin,1999; and Shan et al.,2005） that describe the relationship between electrical resistivity and soil moisture, the generalized Archie’s law proposed by Shan et al. (2005) has lowest RMSE although all methods show R2 greater than 0.95. The model tests conducted in a sandbox show that there is an apparent hysteresis in the resistivity-water content relationship during wetting and drying. This may be attributed to non-uniform distribution of soil moisture and difference in TDR sampling volume for soil moisture and electrical resistivity. Lessons learned from the model tests leads to suggestion of improved TDR installation for further investigations.