Paper Title
Combined Use of Land Gravity Data and 3D Geological Model to Image Deep Geological Basin: Case of Region of La Broye, Switzerland

Abstract
Over the last decade, in Switzerland, geothermal projects target deeper structures in hope to find potential geothermal resources to produce electricity. Among the deep geological features, deep Permo-carboniferous basins could host large geothermal reservoirs. The region of La Broye, in the western molasse plateau in Switzerland, was investigated to confirm the existence of such deep structures. The aim of the study is to bring new insight on the origin of the gravity variations in the region of La Broye. Reprocessing of previous gravity data set has improved the resolution of gravity decrease associated to the basin of la Basse-Broye that is made of moraine and post glacial deposits. Over 100 km2, we surveyed 824 new gravity stations. Then, a 2 mGal gravity decrease long of 10km has been well defined, which is located over the molasse hills, between Neuch�tel Lake and the plain of Basse-Broye. A 2D sensitivity study using a 2D geological model from seismic atlas shows that observed gravity decrease over the hills cannot be associated to rock density variation from either shallow layer from post ice age period, or sedimentary layer from Mesozoic period. However, the 2D model was inconclusive for a deep structure source. The 3D forward gravity model computed using Swisstopo 3D geological model provide clear information that only a deep structure can generate the observed gravity decrease. Shape of this deep structure has been estimated through 3D forward gravity model, by doing a stripping of the gravity effect associated to each formation for geological layers from the Tertiary, Quaternary and Mesozoic periods. Finally, based on its location and extend, the source of the gravity variations over the hills can only be a deep Permo-carboniferous basin.