本研究的目的為應用地電阻影像法評估墩基深度之可行性，由於常用於基墩檢測的力學非破壞性檢測方法，易受到基礎本身的結構和承載土層的干擾，而地電阻影像法主要透過電流在地層中的流動，量測電位，測得視電阻率，再反算獲得真實地層的電阻率剖面圖，依電阻率的差異來判斷墩基的深度。
使用三種電極排列方法為Wenner、Schlumberger和Dipole-Dipole排列方法進行下淡水溪鐵橋第21號墩基的探測，由Wenner排列方式的地電阻影像圖，圖中顯示墩基所在位置會有類似長方形的高電阻率區域的存在，而其於兩種電極排列方法則沒有明顯的基墩存在，並以Wenner排列方法進行五根墩基深度的量測，探測出此五根墩基第三層的平均長度為3.5公尺，再以音波回音法為輔助方法，驗證地電阻影像法探測的結果，探測出第三層的平均長度為3.79公尺，此兩種方法的結果相近，增加地電阻影像法探測基墩深度的可靠性。

The purpose of this study is to use electrical resistivity tomography (ERT) to evaluate of the pier depth. Non-destructive test method is generally used in examination of foundation depth. The method is limited to be used by foundation structure and the soil bearing layer. The ERT method uses electric current to penetrate into soil to obtain the electric potential, apparent resistivity and earth resistivity tomography. This study tries to use the ERT method to determine the depth of pier with the difference of earth resistivity.
Three electrode arrays are used in this study to evaluate the pier depth of Xia-Dan-Shui-Xi bridge. They are Wenner, Schlumberger and the dipole-dipole array. The earth resistance tomography shows high resistivity rectangular region in the position of pillar base in the result of the Wenner array. However, there are not found the obvious phenomenon of existence of foundation piers in the Schlumberger and the dipole-dipole array. Five piers are tested using the Wenner array. The results show that the average depth is 3.5 meters. The result is checked by using sonic echo method, and found the depth is about 3.79 meters. Both results are similar, which will verify the reliability in the evaluation of the pier depth with using the ERT.

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