二維地電阻法探測應用於壩體檢測之三維效應影響

Abstract

二維地電阻探測法具有應用於水庫壩體安全檢查之實用性，做為檢測工具有助於提早發現土石壩內部滲漏問題，基於經濟、效率與施測難易度的考量，主要於壩頂之平行壩軸測線進行探測，因測線兩側劇烈的地形變化、壩體內部的不同材料分區、以及不同水位高低造成不同程度的三維效應，導致二維電阻率剖面受到兩側不同程度的映射現象。為使二維地電阻法發揮確切之成效，本研究目的在探討二維地電阻法探測應用於壩頂檢測之三維效應影響。本論文以湖山大壩為例建立三維正算數值模型，研究內容分為三項，分別探討簡易2D堤壩幾何三維效應影響規律、湖山水庫大壩幾何三維效應影響因子、蓄水對湖山水庫壩體地物資料判釋影響評估。 簡易2D堤壩幾何三維效應影響規律以簡化壩體模型將幾何區分為壩寬度、壩高高度、壩體坡角等因子，結果顯示，三維效應的影響，主要為層狀電阻率上升，電阻率差異由表層往深層趨於顯著。此外，三維效應亦隨著坡角增加、壩頂寬度減小、壩高增加，影響更為顯著。瞭解三維效應影響規律後，本文以湖山水庫湖南壩為例，探討大壩幾何三維效應影響因子，分別以壩體幾何、地形特徵、內在分區參數與蓄水狀態進行情境模擬。研究顯示，內在分區參數之三維效應來源為濾層之電阻率500 Ohm.m影響最顯著，電阻率剖面偏差約150%，其次為蓄水滿庫情境，因左右兩側山脊與測線偏左山脊，導致電阻率剖面左側偏差約上升38%。最後探討蓄水對湖山水庫壩體地物資料判釋影響評估，蓄水滿庫情境湖南壩三維正算模型，將潛在異常區域分為不同發展時期、不同深度、不同規模與不同類型進行交叉模擬。結果顯示，潛在異常區域隨深度增加、規模縮小，電阻率差異趨於不顯著。此外，異常區發展後期較前期電阻率差異顯著，異常區掏空情境較滲漏情境顯著。最後以蓄水變動討論水位急洩降與緩洩降時，非相同蓄水條件之潛在異常區域進行時序分析會因蓄水位變動造成之三維效應而遮蔽潛在異常區域，故建議當進行時序分析時，勢必要以相同條件、相同水位進行分析，並清楚掌握水位變化，才能避免三維效應影響施測結果與異常現象的判讀

Two-Dimensional Electrical Resistivity Tomography (2D-ERT) is practical in dam safety inspection of reservoirs as an inspection tool for early detection of internal dam leakage problem. Based on integrated consideration of economy, efficiency and surveying difficulty, this 2D-ERT survey was mainly conducted along the crest of the dam. Three-Dimensional effects (3D effect) on 2D-ERT are induced by drastic topography on both dam sides, internal partition of different materials and reservoir water level variations, which led to different degrees of mapping phenomenon on 2D-ERT generated resistivity profile. In order to ensure data effectiveness of 2D-ERT, this study aimed to investigate 3D effect of 2D-ERT on dam crest investigation and established a 3D forward numerical model of Hushan dam. This research is divided into three aspects, investigation on general influence pattern of 2D dam geometric factor, geometrical 3D effect influence factor of Hushan dam, and impact assessment of water storage towards the geophysical exploration data interpretation of Hushan dam. This study simulated a simplified dam model to study general influence pattern of various 2D dam geometric factors, such as dam width, dam height and dam slope angle. Results shown that the 3D-effect mainly caused layered resistivity to rise and this resistivity difference is more obvious as depth increases. 3D effect is more significant due to increment of both the dam slope angle and dam height, and the decrement of dam crest width. Understanding the influence of 3D-effect and based on Hunan Dam of the Hushan Dams, this study simulated model scenarios by various dam geometry, topography features, internal partition parameters, and water storage condition, in order to study the dam geometrical 3D effect. Scenario simulation results indicated that the most significant 3D effect is induced by internal partition parameter of 500 Ohm-m filter layer, which causes 150% difference in resistivity profile. The next significant factor is when reservoir is maintained at full water level, the resistivity profile deviation increased to 38% due to ridges on both sides of the dam and slight shifting of survey line to the left. Impact assessment of geophysical exploration data interpretation hence simulated another 3D forward model of full reservoir at the highest water level. The model considered potential anomalous areas at various development periods, depths, magnitudes and defect type to cross-correlate their impacts. Simulated results demonstrated that the potential anomalies are more insignificant in terms of resistivity difference as the occurrence depth increases and as the anomaly magnitude decreases. Furthermore, the resistivity variance are more noticeable in later development period of anomalous area compared to early stages, while the internal erosion type of anomaly area had clearer impact on resistivity variation than dam seepage anomalous area. This study shows that regardless of the declination speed of reservoir water table, if different water table condition is assumed in time-lapse anomalous area analysis, 3D effect from water table variation may mask up the resistivity difference of potential anomalous area. Hence the author recommended that same water table conditions should be met during time-lapse inversions in order to avoid misinterpretation of results and anomalies from geophysical survey data due to 3D effect.