二維地電阻影像探測之3D效應研究

Abstract

本研究將利用數值模擬及現地試驗探討不同3D條件下，二維電探可能造成的失真現象。前半部將利用AGI公司(2003)所發展之正算（EarthImager3DMOD）與反算（EarthImager3DINV）的分析軟體，將每種不同的排列、適用情形，利用數值模型做一明顯的介紹與區別。後半部再利用現地施測成果做比對分析，以了解當地層具有3D變化時可能造成的影響範圍。 根據本研究數值模擬，在測線與斷層走向非平行或垂直時，會有3D效應產生，其影響主要是在高電阻率側，隨著所夾角度增加，3D效應的影響程度會下降，當角度大於45度後，3D效應可予以忽略。若測線與斷層走向平行時，當測線在低電阻率側時，不論n值，3D效應之影響範圍至多為3公尺，而當測線在高電阻率側時，3D效應之影響範圍至多為6公尺。

Numerical simulation and case study are performed in different 3D earth structures to investigate the possible errors caused by 3D effect. Commercial softwares (EarthImager 3D MOD and EarthImager 3D INV) developed by AGI Ltd. are aimed to simulate the data acquisition and to invert the acquired data. At first, earth models with different situations and alignments were simulated to examine the possible influence caused by 3D effect and to figure out the possible influencing range. Further, a real case was presented to verify the finding from simulations. According to the results of numerical simulations, 3D effect can be observed when the survey line is not perpendicular or parallel to the strike of faults. The main influence is in the ground with high resistivity. With increasing the angle between survey line and the strike of a fault, the influence of 3D effect decreases and can be ignore if the angle is larger than 45 degrees. When the survey line is parallel to the strike of faults, the influencing distance of 3D effect is no more than 3 meters if the survey line is in the ground with lower resistivity, while it is no more than 6 meters if the survey line is in the ground with higher resistivity.