Analysis of the repeated absolute gravity measurements in the Czech Republic, Slovakia and Hungary from the period 1991-2010 considering instrumental and hydrological effects

Since August 2001, the absolute gravimeter FG5#215 has been used for the modernization of the national gravity networks of the Czech Republic, Slovakia, and Hungary. Altogether 43 absolute stations were measured, some of them repeatedly. Absolute gravity at 29 stations had already been determined in 1990s by other absolute gravimeters (FG5 or JILAg). Differences of repeated measurements at most of the stations show an unexpected decrease of gravity (up to 22 mu Gal) over the whole region. An uncertainty assessment of absolute measurements with a special emphasis put on hydrological effects shows a statistical significance of the detected gravity variations at many stations. In this manuscript, three possible reasons of such findings are discussed: (1) a regional geodynamic activity, (2) systematic instrumental errors (offsets), (3) hydrological effects. The analysis and statistics of the gravity differences in context of international comparisons of absolute gravimeters show offsets up to 9 mu Gal related to data of the JILAg-6 and FG5#107 gravimeters. Data collected in this study demonstrate that considering instrumental and hydrological effects on gravity are crucial for a correct interpretation of repeated absolute gravity measurements.