利用地表地电阻方法协助建立台湾中部鼻子头隘口区域浅层地下水位分布变化之研究

Impact Using 2D Electrical Resistivity Imaging for Cha-racterize the Shallow Groundwater Surface across the Bizetou Pass of Bagua-Douliu Hills in Central Taiwan

摘要: 本研究尝试以二维地电阻方法调查浅层地下水面，于台湾浊水溪冲积扇顶，跨越八卦–斗六丘陵之鼻子头隘口处进行施测，协助决定浅层地下水位面之分布。根据地电阻施测结果，比对分析邻近观测井资料后，我们发现浅层自由含水层的地下水位面，均与一个连续性良好之电阻率由高转低之接口一致。因此可以运用地电阻施测之结果，作为浅层自由含水层地下水位面分布调查之用。由地电阻推测的地下水位分布看来，在鼻子头隘口东西侧之地下水位有明显之变化，水力坡降在隘口附近突然增高约两倍，通过隘口后又逐渐降低至小于3‰以下。另外，越靠近浊水溪主流河道，浅层地下水位面深度有逐渐变浅之趋势。造成等水位线形状呈现以隘口为根部，向两侧急降之舌状分布。此现象显示在隘口处应存在一导水系数较低之水文地质构造，并且暗示在隘口附近之浅层地下水补注来源主要为浊水溪之入渗，因此地下水位在通过隘口后有快速向四周辐射低降之情形。

Abstract: In the study, we tried to use the electrical resistivity method for characterizing the groundwater surface along the Choushui River near the Bizetou Pass across the Baqua-Douliu Hills. Comparing the results from the electrical resistivity Imaging (ERI) with the groundwater level measured in the nearby observation wells, we concluded that the groundwater surface in the unconfined aquifer is consistent to a horizontal boundary with a sudden resistivity change. Hence we are able to use the ERI results to estimate the groundwater level at where no observation wells are available in this area. We found that the groundwater level is getting deeper away from the Choushui River and forming a tongue-like structure for the groundwater surface in the west of the Pass. In addition, the hydraulic gradient suddenly increases for about twice near the Pass. And the hydraulic gradient then decreases to less than 3‰ in the west of the Pass. The findings suggest that the Choushui River is a hanging river and also the source for the groundwater in the unconfined aquifer, and there is a hydraulic boundary existed near the Bizetou Pass.