Determination of gravity at MunGyung (Mungyeong) superconducting gravity observatory, Korea

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 μGal (νGal=10^-8 m•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 μGal• 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 μGal•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 μGal•volt^-1. The difference between the two coefficients is 0.012 μGal•volt ^-1, which lies within the standard error of the determined coefficient, 0.224 μGal•volt^-1. Therefore, a value of 64.548 μGal•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.