Water-levels fluctuations in a borehole at the Royal Observatory of Belgium: Effects on local gravity, Earth tidal, and barometric responses

A borehole was drilled at the Royal Observatory of Belgium in Brussels, next to the superconducting gravimeter. Three aquifers belong to the 'multiple aquifer-aquitard system'. Continuously since Jun. 1984, various kinds of water levels variations are registered: long term and short term variations and, in the intermediate and deep wells, periodic tidal oscillations. Moreover, the water table and the pressure heads respond to the atmospheric pressure variations. The water levels variations perturbing effects in the superconducting gravimeter registrations are estimated and the hydraulic problem and the consolidation problem are studied. The classical Bouguer's formula in theories is enlarged, taking into account the nature of the layers, the expansions or compressions of each layer, the global land surface displacement and the various hydrostatic occurrences of phases on porous media. The Bouguer's theory is also extended to the case of a finite thickness layer, by a numerical integration. The water levels variations in the intermediate and deep aquifers (at long term, at short term, and the barometric and tidal responses) are only inducing negligible perturbing effects on the superconducting gravimeter registrations but the effect of the water table drift is at the limit of the actual precision of the gravimeter registrations. On the other hand, the barometric and tidal responses of the well-aquifer systems are used to estimate the in situ parameters of both the aquifers and the aquifer system (porosity, specific storage, vertical compressibility, and permeability). The estimated values are in good agreement with those deduced by using hydrogeological, soil, and rock mechanics considerations. The research presented can be applied to any hydrogeological configuration with any complexity.