應用地電阻法於崩積層含水特性調查與監測之初探

Abstract

由於崩積層具高度不均質性，其含水特性影響崩積邊坡之穩定性甚鉅，在洪颱期間常引致重大災害與經濟損失，故實有對崩積層進行含水特性監測的必要性。崩積層含水特性與地電阻法（Electrical Resistivity Tomography, ERT）所得電阻率參數有高度相關性，因此本研究嘗試以地電阻探測法調查及監測崩積地層的含水特性分佈；然而崩積層具高度非均值特性，除了影響地電阻解析可靠度，利用取樣進行含水特性與電阻率率定相當困難，故本研究進一步提出結合時域反射技術（Time Domain Reflectometry, TDR）與張力計，以室內砂箱實驗模擬現地電阻率與含水特性率定關係之建立，以期進一步推估土壤基質吸力之可行性。 本研究首先利用馬賽克數值模形來模擬崩積層複雜地質型態，以對地電阻法量測解析度進行數值分析探討；結果顯示當電極間距愈小、馬賽克方格距地表愈近、背景與方格電阻率對比度愈高，或方格分佈愈離散，地電阻法量測結果可得到較佳的解析度，但幾何形狀與電阻值有相當程度模糊化，且地電阻量測剖面底部與兩側因邊界效應而有異常電阻率分佈，不可過度解讀。室內砂箱實驗模擬現地長時間的降雨與乾燥行為，透過TDR含水量與導電度，以及張力計量測結果顯示，地電阻法結合TDR與張力計，應可有效地建立崩積地層含水特性（含水量與基質吸力）與電阻率關係，但因儀器反應延遲與感應空間範圍不同所造成的遲滯現象尚須進一步探討。

The stability of the heterogeneous colluvium much depends on the soil moisture and soil matric suction properties, especially during typhoon or rainstorm, thus it is neccesary to monitor spatial and temporal changes in these two parameters. Because of high correlative relationship between the soil moisture and the resistivity measured by Electrical Resistivity Tomography (ERT), this study considers investigating the distribution of soil moisture of the colluvium by using ERT. However, the inhomogeneity of the colluvium not only challenges the resolution of ERT. It is also difficult to calibrate the field relationship between soil moisture and the resistivity through sampling and laboratory calibration. For aforementioned consideration, the objective of this study was aimed at evaluating the utilization of Time Domain Reflectometry (TDR), matric suction sensor ,and ERT to monitor spatial and temporal changes in the soil moisture and soil matric suction by laboratory sandbox experiment, possible calibration establishment of the resistivity and those two moisture parameters is investigated meanwhile. First of all, the ERT forward model with mosaic arrangement was used to simulate the complex geology of the colluvium to gain insight on the resolution of the inverted ERT. The results show that with smaller electrode spacing, the smaller distance between mosaic model blocks and goundsurface, the higher contrast of the resistivity between model background and blocks, and the more dispersive of the mosaic blocks, there would be higher resolution for inverted resisitivity tomography. However, the geometry of mosaic block and estimated resistivity value are not absolutely identical to the ground truth, and there is also some anomalistic resistivity on both sides and bottom of the tomograms due to the boundary effect. Based on the laboratory sandbox experiment which simulates field longterm condition of rainfall and drying, the relationship between resistivity and moisture characteristics, including soil moisture and soil matric suction, is effectively established from TDR, matric suction sensor, and ERT monitoring. But the hysteresis between wetting and drying process due to instrument time delay and sample volume requires further investigations.