土壤電阻率與含水特性關係之探討

Abstract

非飽和土層其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之監測有其必要性。非飽和土層含水特性與電阻率具有高度相關性，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影像探測（Electrical Resistivity Tomography, ERT）量測地層含水特性分佈，前期研究提出結合時域反射技術（Time Domain Reflectometry, TDR）具同時量測土層含水特性及電阻率之特性，由TDR監測結果率定地文因子，但室內砂箱模擬試驗發現在模擬降雨與乾燥過程，含水量與電阻率具有遲滯現象，在相同含水量下，其電阻率隨降水和自然乾燥階段而有所不同。 本研究主要探討此一遲滯現象，以提高在電阻率轉換含水特性時之精確性，首先探討是否為TDR含水量與導電度量測在感測器側向具有不同的非均勻空間影響範圍及砂箱試驗配置所造成；結果顯示將TDR感測器垂直擺設可以有效減少遲滯圈範圍，但此現象依然明顯存在。因此回歸至更為基礎之研究，就溫度因子進行探討，並改以能產生較為均質試體且易控制含水量之方式進行含水量與電阻率量測試驗；結果顯示溫度影響電阻率轉換含水特性之精確性，本研究建立一土壤電阻率溫度補償模式，並將後續量測進行溫度修正。由不同乾溼循環速度之試驗結果，發現遲滯現象的產生與乾溼循環速度有關，在快速濕潤過程中產生類似砂箱降雨入滲模型試驗之遲滯現象；但當濕潤速度緩慢則此遲滯現象將會大幅減緩，推測遲滯現象的產生應為在乾溼過程中，其孔隙中水分及空氣分佈不同所造成。

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 variation of the soil moisture content. The water content characteristics of unsaturated soils has a close relation with soil electrical resistivity. 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 for on-site calibration. However, the sandbox physical model tests revealed that, during simulated rainfall infiltration and drying process, there exists a hysteresis between soil moisture content and resistivity, in which a resistivity value corresponds to different soil moisture content depending on whether it is in drying or wetting process.. In order to enhance the accuracy of soil moisture imaging based on ERT, this research was aimed to investigate the hysteresis phenomenon. First, experiments were conducted to show whether the hysteresis is due to non-uniform spatial sampling of TDR probes and configuration of the sandbox physical model. The results showed that the vertical installation of TDR probes can reduce the hysteresis loop slightly, but the hysteresis phenomenon is still significant. This research was then turned to more fundamental investigations, including temperature effect and experiments using devices that can produce more uniform samples and better control the testing soil moisture. Significant temperature effect on soil resistivity was observed. This study proposed a temperature compensation model and further measurements were temperature corrected. Experimental results from different wetting and drying speed showed that the hysteresis seems related to the wetting and drying speed. Similar hysteresis phenomenon was observed in quick wetting and drying process while there is no significant hysteresis in slow wetting and drying process. The hysteresis is attributed to different air and water distributions in soils during wetting and drying process.