Seasonal height change influence in GPS and gravimetric campaign data

Low motion rates, typical of intraplate settings, make it particularly difficult to isolate a tectonic signal in vertical displacements of the ground dominated by near-surface influences. Since the spring 2003, we have performed half-yearly GPS and gravimetric campaigns in NE Ardenne in order to evaluate the seasonal changes imposed to the ground height by groundwater variations. The GPS height data show an excellent negative correlation with a proxy for groundwater variations, based on rainfall in the 6 months before the survey, that allows a reliable correction of the measured height changes. During the 2003-2005 time span, the seasonal groundwater-dependent height changes have amounted to a maximum 7.5 mm. The gravimetric campaigns were able to detect reliably only gravity changes larger than 10 μgal, which corresponds to the upper limit of the gravity changes associated with the proposed groundwater-GPS model. No conclusive result may therefore be derived from the gravity observations.