Summary A high-resolution three-dimensional (3-D) seismic reflection survey has been conducted in Lake Geneva near the city of Lausanne, Switzerland, where thrust faults in the molasse basement (Tertiary sandstones) are overlain by complex Quaternary sedimentary structures. Using a single 48-channel streamer, an area of 1200 m x 600 m was surveyed in 10 days. With a 5-m shot spacing and a receiver spacing of 2.5 m in the inline direction and 7.5 m in the crossline direction, a 12-fold data coverage was achieved. Preliminary results show a maximum penetration depth of ~200 m with a 15 cu. in. water gun source operated at 140 bars. A particularly clean image of the fault zone and the overlying sediments is obtained with the multi-channel data, and they allow the determination of an accurate velocity field for eventual 3-D migration. A line repeated with a 30/30 cu. in. double-chamber air gun source, operated at 100 bars, shows a maximum penetration depth of ~450 m.