This study summarizes the findings from the observations of two SG (serial no. 48 and 49) three FG5 (serial no. 224, 228, and 231) and a regional GPS network around Taiwan. The main objectives of this study are:
1. The introductions of SG and AG in Taiwan, including individual devices and the procedures of operation.
2. The analyses of the SG’s critical parameters and the potential applications to related studies, such as solid earth tide, ocean tide loading, atmospheric effect, hydrologic effect, typhoon and earthquake.
3. The preliminary results from the AGTO project in connection to the result of GPS.
The major findings of this study are listed below:
1. Establishing procedures to determine many different types of detided gravity enabling to analyze solid earth tide, ocean tide loading, atmospheric effect, hydrological effect, polar motion effect and other gravity changes caused by environmental changes.
2. The drifting rate of AG and T48 are 2.2±0.3 μgal year-1 and 1.3±0.1 μgal year-1 from the AG and T48 measurements from 2006 to 2011. Based on the parallel observations of AG and SG, the CF and the different rate between AG and T48 were -76.087 ± 0.067 μgal voltage-1 and -0.8 ± 1.3 μgal year-1, respectively. The observed drifting rate of T48 is smaller than the nominal drift-rate of 6 μgal year-1 reported by the SG manufacturer.
3. The north area of HS is a groundwater-rich alluvium with evident gravity variations. A vertical site motion of -0.22±0.97 cm year-1 at HS was found and not directly related to the movement of the Hsinchu Fault.
4. The gravity-atmosphere admittances during typhoons are 30% larger than the mean value. In modeling gravity change due to atmosphere, the global atmospheric pressure model is suitable over a large area, especially for the long period variation. Short period atmosphere-induced gravity change is only well predicted by high frequent atmospheric data.
5. The relative errors between the gravimetric amplitude factors determined with T48 (corrected by NAO.99b) and the model factors of Dehant et al.
(1999) range from 99.95 to 96.07%. The Newtonian part contributes a significant portion to ocean tide loading gravity effects (about 20% for M2 at HS) at HS. The largest Newtonian gravity effect of ocean tide is found around Matzu.
6. The gravity offset generated by the earthquake of September 6, 2007 reached 0.6 μgal, which contributes 0.3 μgal year-1 to the gravity rate at HS over 2006-2011.
7. The mean gravity and vertical rates at AGTO sites are -1.39±4.21 μgal year-1 and 0.50±0.94 cm year-1. The sediment thicknesses changes near AG6 and AG3 due to landslides caused by Typhoon Morakot are 2.45 m and 1.25 m.
8. Using repeated absolute gravimetric measurements to determine gravity changes at islandwide (Taiwan) stations, and the preliminary causes of such changes are identified with the help of GPS and other data. The average rate of gravity change from 2005 to 2008 is -0.58 μgal year-1 in Taiwan.
In this study, the SG and AG gravimeter measurements have been applied to tidal analysis, geodynamics, typhoon and atmosphere. Potential applications of these instruments of methodologies are listed below.
1. The SG gravimeter, T49, is currently at Hsinchu, but it can be deployed in a new location (other than Hsinchu) that has a different geodynamic feature.
With two SG at two locations, many research opportunities can be explored.
For example, a proposed new site for T49 is Mt. Yangming (see Fig. 6-4, YMSG), where possible volcano eruptions can occur and hydrological changes will induce large mass and gravity changes. Records from two SG can be used to study gravity changes in sea level ocean circulation
associated with the Kuroshio Current.
2. Use of gravimetry to study groundwater in central Taiwan. Since 90 % of all unfrozen fresh water is hidden underground, it is difficult to determine its volume. The volume can be estimated with a network of AG sites around central Taiwan.
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