| 摘要: | 地球物理探勘藉由非破壞性的方法調查地下介質之物理特性，藉此推測出地底下物質成分、組成，廣泛應用在地質調查、工程調查等領域。其中地電阻方法可以量測地下電性構造，已經廣泛運用在近地表地質調查、水文地質與地下水探查、隧道工程、以及地下管線調查等工作。結合地表及垂直鑽孔電極的佈設，可以進行地電阻斷面掃描，對地表電極與井孔內電極所涵蓋的空間進行掃描量測，再經由反演算推求出地層電阻率之空間分布，以繪製地層電性構造剖面。本研究曾在池上大坡國小地區進行地面-井下跨孔式地電阻量測，使用任意可量測之四極組成之陣列（Hybrid array）方法，於地表佈設20個電極並且連接大坡井2和大坡井6的井孔電極，在量測資料3129筆中去除雜訊後的運算資料點數2300筆，然而反演結果僅對地表淺部低阻構造有較佳的解析度，井孔周圍的資料受井孔影響品質較差，井孔間深部的資料也因缺少解析度而成像品質差。為了瞭解地電阻影像法的空間解析度與成像能力，特別是井下與地面的聯合施測時，需要在測線佈設前，加以量化評估電極陣列配合的方式，並調整電極佈設與施測的設定，方能有效避免產生人為的假象。本研究引用震測斷層層析方法所運用的棋盤模擬正反演方法，藉由AGI開發之地電阻影像處理軟體 EarthImager 2D進行地電阻斷層掃描正演模擬，以評估反演影像之空間解析度與成像結果。模擬電極陣列選用雙極排列法（Pole-Pole array）和雙偶極排列法（Dipole-Dipole array）發現利用不同陣列取得模擬成像結果上之差異，如：異常體最小解析能力、井孔間距與井深的解析能力關係，以及針對特定構造會產生的反演假象等，未來在進行跨孔式地電阻法施測以及資料處理，均應事先加以考量以避免收取並排除資料假象，以避免做出錯誤的解釋。;The non-destructive geophysical investigation methods are widely used in geological surveys, engineering surveys or the composition of the subsurface materials. Especially, because of the high sensitivity of electrical structure, the electrical resistivity tomography method （ERT） has been widely used in near-surface geological surveys, such as hydrogeology groundwater exploration, tunnel engineering, and underground pipelines surveys. Also, combining the surface-borehole electrical resistivity tomography, we will gain more detail of subsurface information from the measured resistivity data. We operated the cross-borehole ERT measurement with the electrode configuration “Hybrid array” at Dapo elementary school locates at Taiwan, Taitung, Chihshang. With 20 electrodes at the surface and the subsurface electrodes of Dapo#2 and Dapo#6. The total data is 3129 and leaves 2300 data after removing the noisy data. Although we get better information at shallow subsurface after the data processing, the information of deeper subsurface is still hard to identify. In order to understand the spatial resolution and resolving ability of ERT method, especially cross-borehole ERT measurement, we should quantify the electrode array and survey parameters before we go field working. So we could efficiently avoid producing the artificial image. This research follows the checkerboard analysis to evaluate the accuracy of inversion results. The forward modeling method and inversion are operated by the ERT software, EarthImager 2D. We choose the electrode configurations Pole-Pole array and Dipole –Dipole array to analyze the spatial resolution and inversion results of surface-borehole ERT. The controlling measure parameter includes electrode spacing, number of wells, the distance of two wells, the grid size of checkerboard, and the structure types. |
