Poroelasticity: observations of anomalous near surface tilt induced by ground water pumping

Abstract

Ground tilt signals observed at three different test sites in Germany confirm earlier observations of surface deformation occurences caused by the withdrawal of ground water from nearby wells. The strength of the signals presented here ranges from a few μrad to 230 μrad, whereby ground tilt was measured in shallow boreholes, at depths between 2 and 4 m. Analysis of the tilt signals suggests that local inhomogeneities in the subsoil can have significant impact on several signal parameters, namely its maximum amplitude, its variation in time, and its strike with respect to the direction of the productive well. The observed data emphasize the likelihood of various causes for deviations from the ‘normal’ tilt response that holds for an isotropic poroelastic half-space. In particular, at one site, a reverse well level change, also known as ‘Noordbergum effect’ in hydrogeology, is seen together with a respective response in the tilt signal. The study concludes that near surface deformation in the vicinity of pumped wells bears a wealth of information that may be useful to constrain the conditions of fluid flow at depth; and that there is a need for model calculations to fully understand the involved phenomena.