台灣大屯山群地區之時變重力觀測及增益相對重力網精度之研究

Abstract

本研究區的時變重力觀測網的設置以陽明山衛星追蹤站(YMSG)為中心，在2012年間分成四期的監測時期，測量規劃以32個相對重力點搭配5個絕對重力點的加密方式來進行重力網形監測工作，並考量儀器的漂移因素及絕對重力點的位置來分配成五條主要監測路線，監測儀器主要採用兩台相對重力儀Scintrex CG-5來進行相對重力點監測和一台絕對重力儀Micro-g LaCoste FG5來對絕對重力點進行監測，並經過重力網平差程式(gravnet)平差處理及進行以下增益精度方法：第一方面，在相對重力值經過海潮改正模式(SGOTL模式)的改正後能使重力網平差精度平均提升約1.4%~6.2%；第二方面，運用-0.35μgal/hPa的大氣壓力導納係數(gravity-atmosphere admittance factor)來進行氣壓改正能使在第二期監測時期的平差精度平均可提升1 μgal；第三方面，運用穩定的CG-5原始觀測數據，研究以有效的資料篩選處理方法，經過資料篩選處理後能使平差後的重力網平差精度提升約11.1%~53.8%。第四方面，設計6種的平差模式測試，運用有效提高平差精度之平差模式設定，使本研究區之平差後的共四期標準差平均逹7.8 μgal；最後應用於研究區之時變重力監測，經過以第一期為基準時間，通過第二、三、四期各自與第一期間產生的重力差值((∆g)1^2,(∆g)1^3,(∆g)1^4)比較，經過95%信心水平的顯著測試後，配合本研究區所用之水井及雨量計資料，並分析和討論各水井水位和累積雨量的相關性及各時期的變化趨勢。

The study area of the time-lapse, network-type microgravity is located at the Tatun Volcano Group in northern Taiwan. The gravity observation station at the GPS tracking station of Yangming Mountain (YMSG) is the center of gravity network. The gravity network consists of 32 gravity stations, where absolute and relative gravity observations are measured in four campaigns in 2012. The 32 gravity stations are visited by two Scintrex CG-5 relative gravimeters, and one absolute gravimeter (the Micro-g LaCoste FG5 absolute gravimeter) in each campaign. The relative gravimeter measurements in the network is least- squares adjusted using the software gravnet. The enhancement of gravity accuracy is achieved by (1) correction of ocean tide loading effect by the SGOTL model, obtaining improvement percentages of 1.4%-6.2%, (2) use of an gravity-atmosphere admittance factor of -0.35 μgal/hPa from the T49 supercounducting gravimeter, enhancing the gravity accuracy by 1 μgal in the second campaign, (3) use of only stable gravity readings of the raw CG-5 readings, enhancing the gravity accuracy by 11.1% to 53.8%, (4) use of the best adjustment model (selected from six models), achieving a mean gravity accuracy of 7.8 μgal. We computed gravity differences (〖∆g〗_1^2,〖∆g〗_1^3,〖∆g〗_1^4) from data in the four campaigns using the first campaign as the reference epoch. Significances of gravity changes are tested at the 95% confidence level. The trends of the gravity changes and their correlations with groundwater levels and precipitations are discussed.