Hydrological experiments around the superconducting gravimeter at Moxa Observatory

Besides barometric pressure variations hydrological changes are known to cause significant disturbances in time-dependent gravity observations. For many geodynamic studies their effect, which can range from some hours up to years needs to be removed in the gravity data. Due to Newton's law, especially the different hydrological contributions in the immediate vicinity of a gravimeter must be understood. This can prove additionally difficult when the station is located in an area with clefted bedrock and/or hills. In our study we focus on these effects especially in the near vicinity of the instrument. At Moxa Observatory three principal areas can be identified from which hydrological effects may originate: the hilly area above the gravimeter level, the valley bottom and its vicinity below the instrument, and the roof area directly above the gravimeter. From the observed water movements a main flow path running from the hill top through the weathering layer and clefts downwards below the observatory can be assumed. The different contributing areas lead to hydrological model predictions more complex than e.g. for an observatory on a plain. This makes Moxa interesting for hydrogeologists as the local situation allows to study whether observations of gravity changes could provide a mean to validate and parameterise hydrological models especially for hilly areas. For this and for the development of reliable correction algorithms, experiments have been carried out in which a defined amount of water was added to specific areas in the gravimeter vicinity. The resulting gravity effect ranges from some nm/s(2) to more than 10 mn/s(2) depending on the area. The gravity changes due to the propagation and dispersion of the water volume can be successfully described by time-dependent changes in the water filled pore volume. (c) 2005 Elsevier Ltd. All right reserved.