地電阻影像探測在地工調查之應用與問題探討

Abstract

直流電阻法發展已將近一個世紀，近年來，技術已由一維、二維，延伸至真三維直流電阻法，但因施測難度受現場空間的限制，目前工程的應用主要仍以二維地電阻剖面影像法(2D Electrical Resistivity Tomography, 2D ERT)為主；2D ERT雖已廣泛應用於大地工程調查上，但生動的彩色地電阻率剖面背後隱藏了許多不確定性，若要將地電阻率剖面有效的應用在大地工程的尺度，必須提高施測的準確度及空間解析能力，或至少掌握這些不確定性，避免過份解讀施測結果。 本研究主要探討二維地電阻剖面的空間解析能力及潛在的施測問題，透過新竹斷層調查及新山水庫滲漏調查二案例，發現二維地電阻剖面影像法施測成果資料解讀確實對工程師而言常是一項很大的挑戰。本研究認為電阻率剖面圖判釋過程中常遭遇的問題，包含電阻率剖面空間解析能力的問題、電阻率剖面的可信度、邊界效應之影響、三維效應之影響等問題，致影響成果判釋之合理性。因此，本研究透過數值模擬方法，建立數種不同地質模型，包括：單一水平層面地層、水平夾層地層、單一垂直層面地層、垂直夾層層面、複合地層、傾斜層面地層及土石夾雜地層等簡化模型，定性探討不同地質狀況二維地電阻剖面空間解析能力的特性。另由於二維地電阻施測時常受到三維效應及邊界效應的影響，致二維地電阻剖面可能造成失真的現象，此效應目前尚未有詳細的評估，本研究亦透過數值模擬探討不同情境的可能3D效應、邊界效應，並建立新竹斷層、新山水庫案例的三維模型，探討此二效應造成的影響。 經由本研究的案例分析及數值模擬評估發現，三維及邊界效應確實會影響施測結果，造成判釋誤差；透過數種不同地質模型的模擬結果，發現地層變化對二維地電阻剖面影響的變化規則，本研究亦提出判讀上的建議，以提升未來在成果資料判釋的準確性，避免誤判及過度判讀。

Direct current (DC) electrical resistivity method has been developed for almost a century. It has evolved from the 1D, 2D, to more recently the true 3D method. However, field conditions often obstruct 3D surveys and 2D electrical resistivity tomography (ERT) remains to be the state of the practice in geotechnical investigation due to its simplicity in field works and less space requirement. The 2D ERT has been widely applied, but the uncertainty behind the obtained vivid resistivity image is not clear. Its accuracy, spatial resolution, and possible pitfalls should be understood to avoid misinterpretation. The objectives of this study are to investigate the spatial resolution of 2D ERT measurements and its potential limitations. The challenges for engineers to interpret the 2D ERT results are manifested via two case studies including the Hsinchu fault and Hsinsan reservoir leakage investigations. The spatial resolution, reliability of inverted resistivity section, boundary effect, and 3D effect are identified as unclear problems which may significantly affect the interpretation of 2D ERT results. The resolving ability of 2D ERT were qualitatively studied by numerical simulations of 2D ERT surveys in various geological conditions including single horizontal layer, horizontal sandwiched layer, single vertical layer, vertical sandwiched layer, inclined layer, and block-in-matrix structure. The results of 2D survey may be distorted in conditions which violate the assumption of 2D structure and infinitive boundary. This study further used 3D numerical modeling to assess how 3D effects might distort the 2D inversion in some typical scenarios. The Hsinchu fault and Hsinsan reservoir field cases were re-visited by 3D modeling to verify the results of 2D ERT under 3D conditions. From the case studies and numerical simulations, it was shown that 3D and boundary effects may significantly influence the results of 2D ERT resulting in false interpretations. The behavior and pattern of 3D and boundary effects are revealed by the 3D modeling. Suggestions are made accordingly to facilitate reasonable interpretations of 2D ERT and avoid false or over interpretations.