孔隙狀態對土壤電阻特性之影響研究

Abstract

地電阻量測在地球物理探勘中已被使用很長的一段時間，其通常被用來做為是地層調查的工具。因為地質材料的電阻不同，因此由個別地層之地質材料電阻間的相異，可以辨別出地下之成層系統。 在以往的研究中，發現礫石與砂等顆粒性材料之電阻與其孔隙率和飽和度相關。黏土的電阻則與其礦物組成、活性、以及其它和擴散雙層相關的因素有關。近年來，地電阻量測也被應用在對地下水水質的了解上。由地層中電阻的變化，可以推斷地下水水質的改變。例如火中含有鹽類則將使水的導電度提高，而使整個地層之電阻降低；反之，水中若含有有機污染物則將使水之導電度下降，而地層之整體電阻提高。利用這些變化，可以從地電阻的探測結果描繪出地下水中污染團的分佈。 本研究企圖利用將個別控制變因獨立改變，來探索電阻與地材料各種性質之間的關係。透過實驗，控制不同獨立變因，量測電阻率的變化來掌握地質材料的性質改變如何影響電阻的變化。此外，由於黏土的擴散雙層與表面傳導現象與孔隙液體化學性質密切相關，使得電阻與黏土性質之間的關係更為復雜，因此本研究將對於黏土電阻與孔隙液體間之關係做一研究。 本研究之試驗結果顯示，土壤的電阻率在固定孔隙比下隨著土體含水量的增加而降低，而固定含水量時隨著孔隙比的增加而增加，若控制相同夯實能量，則在過了最佳含水量後電阻率趨向一定值。另外，若孔隙液體為含金屬離子液體，則電阻率將明顯下降，而趨勢類同於孔隙水為去離子水之時。以相同濃度的KBr與CdCl2作為孔隙液體時，以CdCl2作為孔隙液體之試體電阻率較低。 綜合而言，透過這些基礎的研究，把地電阻與材料之礦物組成、孔隙率、飽和度、密度、孔隙液體成份等做出一較完整之函數分析。這些成果可以在未來對掩埋場黏土阻水層的有效性用電阻量測來評估，及早採取補救方法。對於地下阻水壁也可以做出有效性的評估，而不需要利用鑽探取樣來分析。此外，也可以使得地物專家與工程師在面對復雜地質狀況或地下水污染時，針對地電阻調查資料能做更準確的判斷與銓釋。

The electrical resistivity measurement in geophysical survey has been used for a long time. The electrical resistivity of geological formations can be correlated to the porosity and degree of saturation for granular materials such as gravel and sand. Recently, the resistivity survey has also been used to investigate the plumes of contaminated ground water. This application is besed on the variation of resistivity with the concentration of contaminants in pore fluids. For clays the resistivity is also related to the surface conductance of diffuse double layer. The purpose of this study is to set up a system for measuring the resistivity of geological materials and to understand the factors which affected the resistivity. An important part of the research is the study of effects of pore liquids on the resistivity of clays. Since the chemistry of pore liquid has a strong influence on the diffuse double layer and, in turn, the surface conductance of clay minerals, the relationship between resistivity of clay and the pore liquid is much more complicated than the condition in granular materials. Through rigorous laboratory tests, the study may be able to understand all the important factors that controls the resistivity. The results of the experiments show that at a particular water content, an increase in void ratio will result in a increase in resistivity. On the contrary, at a particular void ratio, an increase in water contrent will result in a decrease in resistivity. In the dry side of the compacted clay, compacted energy also plays an important role in determining resistivity. But in the wet side, the soil shows low anisotropy. The results also show that the pore fluid affects the results very much. The results of the study would allow engineers to investigate the contaminants migration in compacted clay liners in the landfill and in the cut-off wall in contaminated sites. Furthermore, the geophysicist and engineers can make much better judgment when examining resistivity data, especially when confronting the combinations of comples geological conditions and ground water contamination problems.