應用大地比電阻法於淺層地層分界判釋之研究

Abstract

台灣因地質環境及氣候條件的特殊性，崩塌為坡地常見災害之一，尤其以淺層崩塌發生頻率較高。淺層崩塌定義為基岩面上的土體崩壞現象，因此淺層崩塌的規模及範圍會受到土壤厚度及基岩面位置等地形、地質條件影響，但因地表下未可視，除了從當地肉眼可視之地質環境進行判斷外，進行地下物水文地質分布概況之探測尤為重要。一般調查地表下地層或水文地質環境方法多採鑽探法，或輔以震波法、電氣法等地球物理探測來判斷地表深層的表現，較少研究討論淺層部分。為探討地表淺層之土壤厚度及基岩面位置，本研究使用大地比電阻法（Electrical Resistivity Tomography, ERT）、簡易貫入試驗（Cone Penetration Test, CPT）及鑽探法（Geological Boring），於南投臺大實驗林內茅埔營林區100-20號造林地（一處淺層崩塌樣區及一處新植造林樣區）進行試驗，對該處水文地質狀況、淺層地層分界（基岩面位置）進行探討。並嘗試三種分層方法（基岩面之比電阻值分布範圍、比電阻值及N_h變化曲線、比電阻值垂直變動幅度）施用在樣區資料上，討論其分層效果。 大地比電阻基礎測試中，可依實驗目的或需求進行垂直或水平面向的分段施測，具有機動性；而連續記測方式及時間序列反算法（Time-lapse inversion）可以瞭解大地比電阻儀器之收集資料的穩定性及應用性。 現地結果顯示，淺層崩塌樣區之鑽探岩芯資料證明簡易貫入試驗對於土壤硬度垂直結構表現、土壤厚度、基岩面位置是有效的。使用大地比電阻法能表現出現地的水文地質特性及環境表現，並推測地層分層位置（土壤與岩層界面），透過調整不同電極排列法、電極間距大小或數量可以獲得更好的淺層資料。從淺層崩塌樣區較精細的施測方式，推廣至新植造林樣區較大的施測規模，亦具有良好表現，但大地比電阻法有時會出現不易判斷的情況，若輔助簡易貫入試驗結果，在各個樣區都可以有效地判釋淺層地層分界。 應用大地比電阻法及簡易貫入試驗所作的三種分層結果顯示， “基岩面之比電阻值分布範圍” 可獲得整個樣區之基岩面比電阻值，Wenner法之25-75 %落在34~182 ohm.m，Dipole-Dipole法之25-75 %落在17~57 ohm.m，未來可應用在基岩邊緣偵測法的研究。“比電阻值與N_h值變化曲線” 繪出比電阻值（ρ）與貫入阻抗值（N_h）的垂直深度變化圖，發現當比電阻值隨深度迅速變化而數值線彎曲處，N_h值也會有起伏，而兩者深度誤差幾乎在20 cm內，顯示比電阻值跟N_h值具有相關性；“比電阻值垂直變動幅度” 中兩種電極排列法皆可對樣區進行地層分界，在一號樣區Dipole-Dipole法推測的地層分界與貫入點位置較符合，準確率73 %，淺層崩塌樣區Wenner法推測的地層分界與貫入點位置較符合，準確率56 %。

In Taiwan, slope failure is one of the common sediment disasters due to specific geological condition, environment, and climate. To be more specific, shallow slope failure has higher frequency of occurrence. Shallow slope failure is defined as the collapse of soil layers above bedrock surface. The size and range of a shallow slope failure depend on topographic and geological conditions, such as soil depth and bedrock topography. Since soil and geological conditions are difficult to be observed directly above ground surface, it’s important to detect underground conditions and distribution by direct or indirect methods. In general, people use geological boring, seismic methods or electrical methods to determine the geological stratification. However, shallow stratigraphy is seldom being focused. Therefore, in order to understand the soil-bedrock interface, electrical resistivity tomography（ERT）,cone penetration test （CPT）, and geological boring were conducted. The above methods were performed in order to explore the hydrogeological conditions and shallow stratigraphic boundary （soil-bedrock interface） at the following two sites in the Nei-Mou-Pu Tract of the Experimental Forest of National Taiwan University ─ a new planting hillslope and a hillslope with shallow slope failure. Three methods were attempted in this research in order to determine the soil-bedrock interface by using ERT and CPT data. Results show that from the fundamental tests of ERT, vertical or horizontal segmented survey could be conducted in accordance with the purposes or necessities of a research. Therefore, the test could be rather flexible. The continuous measurement method and time-lapse inversion method show the stability and applicability of ERT. Based on the result of boring cores, it was confirmed that the vertical distribution of soil resistance, soil depth, and the location of soil-bedrock interface could be effectively detected by CPT. Besides, ERT data could reflect the hydrogeological characteristics and soil-bedrock interface at most locations, but it showed inconsistencies at some locations within the sampled sites. Better shallow tomography could be gained by adjusting different electrode array method, electrode spacing and electrode number. At shallow slope failure plot, ERT results were more applicable to this research. As the method being extended to a larger scale at the new planting reforestation plot, ERT results were more applicable respectively. However, the utilization of ERT still has its limits at some plots. Therefore, in addition to ERT data, the combination of ERT and CPT data would improve the effectiveness of soil-bedrock interface detection in every plot. The method of “Electrical resistivity of soil-bedrock interface” shows the resistivity value of soil-bedrock interface. The Wenner array is in 34~182 ohm.m（25-75%） and the Dipole-Dipole array is in 17~57 ohm.m（25-75%）. The method of “Electrical resistivity and penetration resistance’s curve” shows that the values of both electrical resistivity and penetration resistance data significantly changed at the depth around the soil-bedrock interface with an error in 20 cm. Moreover, it indicates that electrical resistivity and penetration resistance rate are greatly correlated. The method of “Vertical variations of electrical resistivity” shows both two types of electrode array are able to detect the soil-bedrock interface, while the Dipole-Dipole array is 73 % accuracy rate at new planting hillslope, and the Wenner array is 56 % accuracy rate at the slope failure hillslope.