Modelling gravity gradient variation due to water mass fluctuations

Two case studies were considered where gravity field changes were detected resulting from water mass variations. The first case is an urban water reservoir where the maximum daily change of water is 40000 m(3). The 3D model of the water mass allowed us to build an accurate polyhedral model of the variation of mass changes of the water. This mass density variation model made it possible to compute and compare variations of various gravitational field functionals. Gravity and full gravity gradient tensor changes were computed on a regular grid for the model area. Gravity changes were also compared with actual gravity field measurements made with two LaCoste & Romberg (LCR) gravimeters. The measured gravity change was nearly 30 mu Gal relative to 5000 m(3) water mass. A good agreement was found between the computed and measured changes. The second case is the water level fluctuations of the Danube River in Budapest during the great flood in 2002. In this case also modelling and measurements were compared. We found the gravitational change to be very sensitive to the actual distance of the point from the river bank.