重力及GPS基線網之自由基準及加權約制平差

Abstract

本文以自由基準及加權約制法平差相對重力觀測量及基線座標分量。當有可靠之已知重力值或座標值時，或低精度網需附著在已知之高精度網時，可以加權約制方式解算。而以自由基準方式解算時，由於不需固定任何點位即可求得唯一之相對重力值或座標值，因此適用於僅平差觀測量，以及偵測相對重力或座標值之間的變化。本文採用台灣相對之重力資料用5種不同的方式平差，以驗證所推導的平差模式，及所發展的平差程式之正確性。驗證的結果顯示，所用之重力資料無粗差存在，LCR G型重力儀(儀器型號838)的讀數精度約為0.02mgal，以及儀器漂移率為每月0.9mgal，率定函數之長波長項及週期部份之係數在統計上等於零。根據自由基準方式平差結果得知，只有水上重力點有顯著的變動，可能是受到集集地震之餘震的影響。上述兩種平差方式亦應用於台灣中部地區基本控制點變位監測。同時並運用多期基線向量發展平差程式以計算點位之速度。本文所發展之程式平差結果，包括座標及速度均與Bernese軟體解算的結果相符。其平差的結果顯示，集集地震後，車籠埔斷層的東側有很大的水平位移。

Weighted constraint and datum-free constraint model are used to adjust relative gravity measurements and GPS baseline vectors. The weighted constraint model is chosen when there exist reliable a priori gravity values or coordinates as additional data, or when it is requested that a lower-order network be attached to an existing network of a higher order. The datum-free model uniquely determines relative gravity values or coordinates among all stations without the need of a fixed station, thus it is suitable for adjusting observations only and for detecting relative changes of gravity or coordinate. A set of relative gravity data in Taiwan was used to test the adjustment models and the developed computer programs in five different cases. The results show that there is no outlier in the data, the estimated reading accuracy of the LaCoste and Romberg G meter (serial number 838) is about 0.02 mgal and the gravimeter drift rate is about 0.9 mgal/month. The coefficients of the long wavelength and periodic components of calibration function are statistically equal to zero. The result from the datum-free solution shows that only one station (Shui-Shang) experiences a significant gravity change due to an aftershock of the 1999 Chi-Chi earthquake. The two adjustment models are then applied to a GPS baseline vector network in the central Taiwan. A method and a computer program is also developed to compute station velocities using multi-campaign baseline vectors. The coordinates and velocities computed by the methods developed in this study are consistent with the results from the BERNES GPS package. The adjustment results show large horizontal motions occurred at the plate east of the Chelungpu Fault after the Chi-Chi earthquake.