ERT在地工調查應用之問題評析與空間解析度探討

Abstract

直流電阻法在過去幾年來，由於地電阻儀器設備改進與反算方法發展成熟，探測方式從一維度進入二維與三維度，施測快速有效率。ERT施測可獲得大範圍二維電阻率影像剖面，呈現地層電阻率變化情形，因水的電阻率值與地質材料電阻率質差異甚大，而時常應用在環境水文之方面調查，例如地下水位與土壤污染探測，近年來ERT在地工調查案例也逐漸增加，如斷層調查、崩塌區調查、土壩滲漏調查與垃圾掩埋場滲漏監，但ERT之空間解析度仍存在許多疑慮，調查資料反算所得到之電阻率影像剖面受到雜訊與空間靈敏度之影響，常為真實電阻率剖面經某種程度模糊化之結果。本研究在不同地質構造條件下進行ERT施測調查地質構造，探討ERT之潛在問題，案例分析結果指出，ERT影像之色階呈現會影響資料判識，深層解析度較不理想，且邊界條件與三維效應也可能影響施測結果。本研究進而以Res2dmod軟體模擬五種地層模型之電探資料，以探討一維垂直解析度、一維側向解析度與二維空間解析度，由模擬結果可以定性暸解空間解析度如何受到深度、幾何變化與電阻率變化之影響。

In the last few years, the measurements from 1-D to 2-D or 3-D have been more rapid and effective due to the improvement of electrical instrument, and the development of inversion method is getting maturity. ERT provides the continuous image of the subsurface in 2-D and it gives an overview of the variations of the ground resistivity. As water is low resistivity material and the resistivity difference between water and soil material is large, ERT has been using in detecting ground water and environmental pollution. It also applies in finding fault, landslide zone, dam leakage and landfill leakage monitoring. However, the spatial resolution of ERT still has a lot of doubts. The inversion of the measurement data usually shows some cloudy true-resistivity affected by noise and spatial sensitivity. Four ERT investigating cases are studied to discuss the potential problem, as the results shows, the different color map of ERT results will affect the judgments. The deeper the depth is, the worse the sensitivity performing the change of the subsurface is. The boundary condition and 3D effect also affect the measurement data. So we use Res2dmod as the software to simulate five numerical earth models and study the relations between the earth model and 1D vertical, lateral resolution and 2D spatial resolution. It shows that spatial resolution qualitative change due to depth, geometry size and different resistivity.