Browsing by Author "Kim, Jeong Woo"

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Absolute gravity change in Taiwan: Present result of geodynamic process investigation
(2017-12-01)
Kao, Ricky
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Hwang, Cheinway
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Kim, Jeong Woo
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Ching, Kuo-En
;
Masson, Frederic
Gravity values at 24 sites over 2004-2016 measured with absolute gravimeters are used to study geodynamic processes in Taiwan. We model rain-induced gravity effects and other temporal effects of non-geodynamic origins to obtain residual gravity, which cannot be fully explained by GPS-derived vertical displacements. We explain the gravity changes associated with deposited debris, earthquake, volcanism and Moho deepening. Gravity changes of 53.37 and 23.38 mu Gal near Sinwulyu and Laonong Rivers are caused by typhoon Morakot, leading to estimated volumes of 6.0 x 10(5) and 3.6 x 10(5)m(3) in deposited debris. The observed co-seismic gravity change near the epicenter of the M 6.9 Pingtung earthquake (26 December 2006) is 3.12 +/- 0.99 mu Gal, consistent with a dislocation-based gravity change at the mu Gal level, thereby supplying a gravity constraint on the modeled fault parameters. The AG record at the Tatun Volcano Group is the longest, but large temporal gravity effects here has led to a current gravity signal-to-noise ratio of less than one, which cannot convince a sinking magma chamber, but supply an error bound for gravity detections of long-term or transient magma movements. The gravity values at Ludao and Lanyu decline steadily at the rates of -2.20 and -0.50 mu Gal yr(-1), consistent with the expected magma states of the two extinct volcanoes. The gravity rates at an uplifting site in central Taiwan and three subsiding sites in eastern Taiwan are negative, and are potentially caused by Moho deepening at a rate of -3.34 cm yr(-1) and a combined Moho deepening and plate subduction at the rates of -0.18, -2.03, and -1.34 cm yr(-1).
Bathymetry enhancement by altimetry-derived gravity anomalies in the East Sea (Sea of Japan)
(2010-12-01)
Kim, Kwang Bae
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Hsiao, Yu-Shen
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Kim, Jeong Woo
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Lee, Bang Yong
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Kwon, Yi Kyun
The gravity-geologic method (GGM) was used to enhance the bathymetry of the East Sea (Sea of Japan) with satellite altimetry-derived free-air gravity anomalies and shipborne depth measurements. By comparison with the bathymetry model of Smith and Sandwell's (SAS) approach (1994), GGM was found to have an advantage with short wavelength (<= 12 km) components, while SAS better predicts longer wavelength (>= 25 km) components, despite its dependency on density contrast. To mitigate this limitation, a tuning density contrast of 10.25 g/cm(3) between seawater and the seafloor was primarily estimated by the downward continuation method and then validated by the check points method with GGM. Similarly, SAS is limited by the "A" value in low-pass part of the Wiener filter, which defines the effective range of the wavelength components on bathymetry. As a final result, we present an enhanced GGM bathymetry model by integrating all available data.
Bathymetry Estimation Using the Gravity-Geologic Method: An Investigation of Density Contrast Predicted by the Downward Continuation Method
(2011-06-01)
Hsiao, Yu-Shen
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Kim, Jeong Woo
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Kim, Kwang Bae
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Lee, Bang Yong
;
Hwang, Cheinway
The downward continuation (DWC) method was used to determine the density contrast between the seawater and the ocean bottom topographic mass to estimate accurate bathymetry using the gravity-geologic method (GGM) in two study areas, which are located south of Greenland (Test Area #1: 40 - 50 degrees W and 50 - 60 degrees N) and south of Alaska (Test Area #2: 140 - 150 degrees W and 45 - 55 degrees N). The data used in this study include altimetry-derived gravity anomalies, shipborne depths and gravity anomalies. Density contrasts of 1.47 and 1.30 g cm(-1) were estimated by DWC for the two test areas. The considerations of predicted density contrasts can enhance the accuracy of 3 similar to 4 m for GGM. The GGM model provided results closer to the NGDC (National Geophysical Data Center) model than the ETOPO1 (Earth topographical database I) model. The differences along the shipborne tracks between the GGM and NGDC models for Test Areas #1 and #2 were 35.8 and 50.4 m in standard deviation, respectively. Furthermore, these differences were more strongly correlated with gravity anomalies than bathymetry in the test areas. It is shown that an accuracy of under 40 m can be obtained with comparisons to shipborne depths only in Test Area #1.
Determination of gravity at MunGyung (Mungyeong) superconducting gravity observatory, Korea
(2009-06-01)
Kim, Ki-Dong
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Kim, Jeong Woo
;
Neumeyer, Juergen
;
Kao, Ricky
;
Hwang, Cheinway
Absolute gravity measurements were made to calibrate Korea's first superconducting gravimeter (SG) at MunGyung (MG, Mungyeong) Observatory. A calibration coefficient (CC) of the MG SG was determined by a parallel registration with an FG5 absolute gravimeter. A total of 8,541 drops were measured over a period of 37 hours between October 8(th) and 10(th), 2007. We first determined the absolute gravity value to be 979,859,179.3 +/- 88.481 mu Gal (nu Gal=10(-8) m center dot s(-2)) after atmospheric pressure, Earth tide and ocean loading corrections. In a linear regression analysis between the FG5 recordings and the raw SG data, a CC of 64.548 +/- 0.224 mu Gal center dot volt(-1) was determined, having previously removed invalid drops and outliers from the data sets. Together with the absolute measurements, a vertical gravity gradient of 2.72 mu Gal center dot cm(-1) was calculated using a Graviton-EG spring gravimeter to take the absolute gravity value down to the SG observatory platform level. The validity of the CC was additionally tested by a comparison between the recorded SG data and the theoretical tides (HW95 and Wahr-Dehant models) as reference. Gravity variations induced by atmospheric pressure and ocean loading were added to the theoretical Earth tides. The CC based on the theoretical tide was determined to be 64.560 mu Gal center dot volt(-1). The difference between the two coefficients is 0.012 mu Gal center dot volt(-1), which lies within the standard error of the determined coefficient, 0.224 mu Gal center dot volt(-1). Therefore, a value of 64.548 mu Gal center dot volt(-1), determined by the parallel registration with the absolute gravimeter, was accepted as the CC of the SG (GWR Instrument Inc. #045) installed at MG Observatory. During the gravity measurements, the other gravity values and heights such as normal gravity and the gravity gradient, orthometric and dynamic heights were also calculated.