Evaluation of the Vertical Gravity Gradient in the Mass Standard Laboratory of Korea Research Institute of Standards and Science

To rectify standard weights by using the kilogram prototype, the gravity effects on the masses should be carefully considered. Since there exists approximately 1cm height difference between centers of mass of the kilogram prototype and standard weights during mass comparison at KRISS (Korea Research Institute of Standards and Science), the researchers of mass group have corrected the effect of vertical gravity gradient (VGG) by using the value of 0.3086 mgal/m which is commonly used by geophysicists. The value of VGG, however, is affected by local geology and building structures at the measurement site. We have measured precise VGG values by using a tripod to move gravimeter up and down at several sites in the mass laboratory, the vibration-free building, which is under construction, and the absolute gravity station in the force laboratory and one exterior site. The observed VGG values differ greatly with sites and we have interpret the difference by simplified model studies considering the effects of underground geology and surrounding building structures.