利用重力觀測網決定含水層比出水率參數: 應用於濁水溪沖積扇貯水研究

Abstract

比出水率是定義單位含水層，受重力作用下，每降單位水位時所排出水的體積。比出水率是用於評估含水層是否具開發價值，最重要的水文地質參數之一。抽水試驗是目前獲取比出水率最標準的方式，但由於試驗費用昂貴，且時常造成不準確的估計。不準確的比出水率參數，將大幅增加含水層儲量的估計不確定性。濁水溪沖積扇與名竹盆地皆具有非常高的地下含水量潛能，本計畫將在這兩個地區，利用絕對、相對重力儀建立重力觀測網，並採用與水位觀測井共站之方式，進行網狀重力重覆測量。我們將在重力變化最大的豐、枯水期進行重力觀測。重力量測數值將扣除環境重力效應後，以平差技術提升整體成果。重力量測結果將與水位變化一同求解比出水率參數。我們將利用ALOS衛星與InSAR技術判定濁水溪沖積扇與名竹盆地之地表變形，用以修正地表垂直變化造成的重力變化，並檢視地下水位變動與地表變形關係。我們將採用現地水位觀測資訊做為控制條件，以MODFLOW對濁水溪沖積扇與名竹盆地之地下水位變化進行模擬，進而推估比出水率參數。本計畫將提供濁水溪沖積扇與名竹盆地，高空間密度之比出水率參數，可以用於精進含水層的儲水量推估。

Specific yield (Sy) is defined as the quantity of water that can be derived from the saturated ground water per unit volume of aquifer under the force of gravity. Sy is one of the most critical hydrological parameters that decide the value of an aquifer for groundwater exploration. A standard method for Sy determination is pumping test, which is expensive and often results in incorrect Sy estimation. Poorly determined Sy values lead to a large uncertainty in estimating the water storage capacity of an aquifer. In this project, we will carry out repeat gravity measurements using absolute and relative gravimeters in a network of co-located gravity-groundwater stations in the Choshui River Alluvial Fan (CRAF) and Ming-Chu Basin (MCB). The potential water storage capacities in these two regions are very high. The gravity surveys will be carried out in the dry and wet seasons to detect the maximum possible gravity changes. The gravity measurements will be network adjusted and corrected for temporal gravity changes. The resulting gravity measurements will be used to determine Sy values in combination with the variations of groundwater at the stations. We will use ALOS satellite data and the INSAR technique to determine the ground deformation over CRAF and MCB, which is then used to explain the relation between vertical displacement and groundwater change, and to model the deformation-induced gravity corrections for gravity measurements. With the boundary values constrained by in situ groundwater observations, we will use MODFLOW to model the groundwater levels over CRAF and MCB, which, together with the gravity measurements, are then used to estimate Sy values. This project will result in a densified spatial coverage of Sy values that enables a refined estimate of the water storage capacity of CRAF and MCB.