Tilt Observation and Modelling of Deformation Induced by a Confined Aquifer

Within the framework of a hydrological research program (ANR ECCO). In April 2006, on the site of Ploemeur (Morbihan), two long base tiltmeter were installed in a tunnel at 10 m depth. We have one tiltmeter on 33 m long base in N80°E direction and a tiltmeter on a 7.5 m long base in330°E direction. Our object is to study at the same time the long term hydrological signal and to compare with the piezometric observation. We want also to study the behavior of the aquifer requested by a faster pumping. on May 10 2006, a stop of pumping was executed during 41 hours, that corresponds to a volume of water of 4500 m3. The objective is to bring an additional constraint on the hydrological parameters of the aquifer (porosity, compressibility, permeability). A multi-parameter recording was made for this experiment during 2 days: tiltmetric measurements, relative and absolute gravimetric measurements, GPS and levelling measurements. Only the long base tiltmeters could observe the strain induced by an elastic inflating of the aquifer (confined behavior). Other pendular tiltmeter also gave information on the expanse of the strain on the surface. The amplitude of the signal corresponds to a relative vertical displacement lower than 0.3 mm on a base of 200 m. the resumption of pumpings was done on 3 different drillings. On each resumption, we observed a relative vertical displacements lower than 0.03 mm. From the evolution of the azimuth of the maximal tilt, we could underline a North-South flow of feed for the aquifer. We also try to underline this observation by modelling the tilt deformation by integrating piezometric pressure measurements.