[Society of Exploration Geophysicists and Environment and Engineering Geophysical Society Symposium on the Application of Geophysics to Engineering and Environmental Problems 2014 - Boston, Massachusetts (16 March to 20 March)] Symposium on the Application of Geophysics to Engineering and Environmental Problems 2014 - 3D ERT IMAGING OF THE FRACTURED-KARST AQUIFER UNDERLYING THE EXPERIMENTAL SITE OF POITIERS (FRANCE): COMPARING WENNER-SCHLUMBERGER, POLE-DIPOLE AND HYBRID ARRAYS