Browsing by Author "Cheng, Tze-Chiang"

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LAND SUBSIDENCE USING ABSOLUTE AND RELATIVE GRAVIMETRY: A CASE STUDY IN CENTRAL TAIWAN
(2010-01-01)
Hwang, Cheinway
;
Cheng, Tze-Chiang
;
Cheng, C. C.
;
Hung, W. C.
;
土木工程學系
We experiment with absolute and relative gravimetry, to determine land subsidence. A gravin, network in the Yunlin County of central Taiwan is established to determine gravity variations that are largely due to land subsidence. Every 6 months, gravity values at two absolute gravity stations were measured by a FG5 gravimeter and those at 7 relative stations by a Graviton-EG and a Scintrex CG-5 relative gravimeter A weighted constraint network adjustment was carried out by holding fixed gravity values at the absolute stations. Correction models for temporal gravity changes are developed and applied to raw gravity measurements. The adjusted gravity values oscillate, but in general increase with time, showing signature of land subsidence. An empirical gravity-to-height admittance factor is determined using gravity change (from FG5) and height change (from levelling) at the two absolute gravity stations. At most gravity stations, there is good agreement between subsidence rates from gravimetry and levelling. Some large discrepancies (> 1 cm/year) also exist and am, caused by uncertainties in relative gravity accuracy, hydrological effects and the admittance factor This study suggests that gravimetry has the potential to determine land subsidence to cm accuracy and is more efficient and economic than tools such as levelling and GPS.
Precise determination of ocean tide loading gravity effect for absolute gravity stations in coastal area of China: Effects of land-sea boundary and station coordinate
(2013-08-01)
Zhou, Jiangcun
;
Hwang, Cheinway
;
Sun, Heping
;
Xu, Jianqiao
;
Zhang, Weimin
Gravity effect due to ocean tide loading (OTL) is an important signal and correction in various gravimetric applications. In this paper, we assessed the OTL gravity effects at four absolute gravity (AG) stations in coastal China from several perspectives. The integrated Green's function of the Newtonian part was derived analytically and that of the elastic part was computed based on the PREM earth model. Ocean tide (OT) records near the four AG stations were used to enhance the accuracy of the global ocean tide model of NA099b and the regional model of NA099jb for the innermost field (<0.02 degrees). The high-resolution digital elevation model (DEM) from the Shuttle Radar Topography Mission (SRTM) was used to define the land-sea boundary in the optimized near field (<1.0 degrees). Results show that the high-resolution land-sea boundary is indispensible for OTL gravity modeling when the shoreline near a coastal station is complex. The SRTM-based OTL model outperforms the GMT-based model shoreline in terms of the agreement between modeled and observed gravity residuals at the four stations. The final gravity residuals, corrected for OIL, at the four stations are significantly smaller than those without OTL corrections. We give examples of accuracy requirements in coordinates at Qingdao for different station heights. At a station height of 80 m and to ensure a 0.1 mu gal accuracy in OTL modeling, the required accuracies in the horizontal and vertical coordinate are 2.5 and 1.3 m, respectively. For a new coastal station and an expected OTL accuracy, one should inspect the variation of OTL due to coordinate variation to find a best strategy to determine the required accuracy of coordinate. (C) 2013 Elsevier Ltd. All rights reserved.