The Finnish Geodetic Institute has been operating a Superconducting Gravimeter (SG) since 1994. According to Newton's law of universal gravitation, every point mass in the universe attracts every other point mass. Depending on the differences in scale between masses and their distances from each other, the gravity effects of independent sources and phenomena differ greatly from each other. The research scope of this study was to define and model the effect of local hydrology influencing the gravity g at Metsähovi. The study was conducted in two separate phases. In the first one, local hydrogeological phenomena and sources influencing g were identified and characterized. Moreover, the range of their theoretical impact on g was calculated and modelled. As a result of this thesis, four different factors influencing g were named. They are the hydraulic conductivity difference of non-homogeneous soils and bedrock, the water flow in the fractures in the bedrock above the groundwater table below the gravity station, the variation in the porosity of the bedrock within the range of the groundwater level, and the time-varying amount of surface and soil water at the study site. According to this study, the gravity effect of all the factors that were identified is so significant that without accounting for them and their modelling the behavior of g cannot be explained precisely. The second phase of the study was performed after a sufficiently long time series of soil moisture and groundwater observations had been collected; a soil model of the study site was built that included the topography of the study site. As a result of this work, a 3D time-lapse model describing the gravity impact of the local hydrology was constructed. Moreover, the significance of the modelling of the immediate vicinity of the SG, the phasing of the modelling parameters, and the dense sampling interval for the modelling of the gravity impact of the local hydrology were understood. The gravity model based on real hydrogeological data was also compared to the g gained when the global and regional gravity effects were removed from the SG data. This result showed that the gravity effect calculated with the hydrogeological model that was constructed does not explain well the residual gravity. This is explained by the inaccuracy of a local, regional and/or global model. According to the results, is clear that the local model must be based on denser sampling and more accurate modelling, especially in the vicinity of the SG, than those the 3D time-lapse model constructed in the present work is based on.
|Publication status||Published - 2016|
|MoE publication type||G5 Doctoral dissertation (article)|
- superconducting gravimeter, soil moisture, 3D time-lapse model, hydrogeological modelling, gravity effect, Metsähovi