The aim of this thesis is to extract the gravitational constant G from relative gravity measurements underneath two artificial reservoirs in Vianden (Luxembourg). If one wants to determine the Newtonian constant G from measurements many variables play a role, like water level, density and temperature. All of them are measured, while the gravimeter is measuring the gravitational attraction. To extract the gravitational constant G it is important to know the gravitational attraction of each point inside the reservoir. For this calculation the 3D models of the lakes are filled with tiny cubes, from which the exact integral to calculate their gravitational attraction to the gravimeter is known. Calculated values are compared to the measured ones which makes it possible to extract the searched gravitational constant G.