超導重力評估水文地質及斷層模式

Abstract

新竹超導站於2006年成立，其後2012年於陽明山衛星追蹤站增置一超導重力儀(SG)，超導重力儀具有高精度、高穩定度、並可長期觀測之特性，而超導重力資料扣除地球固體潮、海潮負載、大氣壓力和極移之後，其殘值主要受水文效應的影響最大。由於在估計水文效應時，水文參數以及地質資訊之正確性難以驗證，因此本研究利用長期之超導重力資料，觀測長短周期之水文效應，由觀測之重力資料驗證模式估計之水文效應，進而推求水文參數，並確定地質資訊；本研究所用之水文模式，是由水循環的觀點，配合土壤物理學、水文地質、水力方法模擬非飽和土壤含水量與飽和帶的地下水質量分布變化，再利用重力積分方法計算水質量對超導重力站造成的牛頓吸引力。 本論文首先敘述新竹與陽明山之超導重力資料處理與分析過程，發現了半年、一年與兩年的水文週期；接著提出應用於新竹超導站之水文動態模式，分別在未飽和及飽和水帶利用Richard’s equation以及地下水流方程式(藉由MODFLOW軟體)模擬水質量之變化，並與新竹超導站之SG T48相比較，得到良好的結果，在未飽和土壤層主要為短週期之效應，而飽和地下水帶主要為長周期之效應；再來以新竹超導站為例，以觀測之超導重力資料驗證MODFLOW模擬之水文效應，可用以(1)最佳化新竹超導站之水文地質參數，有導水係數、逕流係數、比儲水率、比出水率等，(2)亦可判斷受壓與非受壓含水層，(3)並利用斷層活動過會產生阻水性質，使斷層核心有極低的導水係數，提供新竹斷層存在的證據，更藉由調整斷層位置與深度，得到將新竹斷層位置從中央地調所提供地點往北移100公尺的重力與水文模擬之結果較佳，以及新竹斷層上一次活動在店子湖層沉積完成的年代；然後以陽明山超導站為例，藉由河流與觀測井作為邊界條件，模擬陽明山研究區域之地下水位面，以絕對重力(FG5)、相對重力(CG5)及超導重力資料鑑別山區之地下水文效應，發現在陽明山超導站以東的高山地區地下水相互連通，而山區與平原之地下水流應有不同的模式；最後利用複合式水桶模型模擬陽明山超導站的水文效應，以精度而言，此方法適合用於外業之重力資料之改正，僅需要有雨量與地下水位資料即可。

Two superconducting gravimeters (SG), T48 and T49, are installed in a tunnel of Mt. 18-peak, Hsinchu. T48 contributes SG data to the Global Geodynamic Project (GGP) since 2006. In April 2012, SG T49 was moved to the permanent GPS station of Mt. Yangming, Taipei. A superconducting gravimeter (SG) collects continuous, long-term gravity changes at a fixed station at a high sampling rate (i.e. up to one HZ). Residual gravity is obtained by removing gravity effects due to solid earth tides, ocean tidal loading, air pressure loading and polar motion from the raw gravity, and reflects hydrological variations of local and global origins. Evaluating the hydrological gravity effect requires geological information and soil parameters, which cannot be easily obtained and validated. Based on soil mechanic and geological knowledge, we develop a hydrogeological model around Hsinchu to predict groundwater level variations, which produce gravity changes that can be assessed by SG gravity observations. This work describes the SG data processing and analysis in Hsinchu and Mt. Yangming, and determines the semi-annual, annual and biennial oscillations caused by water storage change. We present a hydrogeological model in Hsinchu SG station for estimating water storage change in the unsaturated and saturated zones by Richard’s equation and the governing groundwater equation, respectively. The period of water storage change in the unsaturated zone is short, while that of the variation in the saturated zone is long. By assessing MODFLOW-modeled hydrological effect by observed SG gravity residual at Hsinchu, we (1) optimize hydraulic conductivity, runoff coefficient, specific storage, and specific yield, (2) identify confined and unconfined aquifers, and (3) show that the Hsinchu Fault contains a core of low hydraulic conductivity that resists groundwater passing through this fault. By adjusting the location and depth of the Hsinchu Fault, we find that (1) the modeled gravity agrees the best with the observed gravity if the Hsinchu Fault is situated 100 meters north of the location provided by CGS, (2) the latest activity of Hsinchu Fault occured when the Tientzehu formation had been formed. At the Mt. Yangming SG station, the boundary conditions of the groundwater table are provided by the elevations of rivers and measurements at monitoring wells. The groundwater effect is assessed by measurements by absolute gravimeters (FG5) and relative gravimeters (CG-5 and SG T49). In the eastern part of Mt Yangming where gravity and groundwater obervations are available, we verify the connectivity of groundwater flows of regional aquifers. The type of groundwater flow at Mt. Yangming is different from that in Hsinchu, which is over an alluvial plain. The hydrological modeling result using a complex tank model at the Mt. Yangming SG station concludes that the complex tank model is suitable for correcting hydrological gravity effects around Mt. Yangming. The tank model requires only rainfall and groundwater table data to predict hydrological gravity effect with sufficient accuracy.