Fontaine d'Orbe, inversion model for Pan 5, 3 m electrode spacing

The study site lies on a karst system in the Pyrénées (France) that crosses the Urgonian limestones. Organic matter was dumped on the west bank of the River Vert d'Arette, which polluted a spring (the Fontaine d'Orbe) situated on the east bank. Electrical resistivity tomography was utilised in order to understand the geometry of this karst outlet. After tests with the pole-pole array, the ideal electrode spacing was found to be less than 3 m. A conduit, a little less than 10 m wide, was discovered, which proved to be the terminal conduit of the karst system. It was established that the conduit traverses both banks of the River Vert d'Arette, thus presenting a hydraulic link between the two sides. Moreover, this conduit, which is totally submerged, passes under the riverbed and goes upwards to the spring. In addition to the geophysical measurements, hydrogeological measurements and observations were made. Based on the characterisation of the geometry of the karst network's terminal conduit, and the fact that is completely full of water and inclined, it is concluded that the Fontaine d'Orbe spring is of the Vauclusian variety.