François Marillier

Imaging Molasse and Quaternary Sediments in Lake Geneva, Switzerland, with 3-D High-Resolution Seismic Reflection Methods: A Case Study

A high-resolution three-dimensional (3-D) seismic reflection survey was conducted in Lake Geneva, near the city of Lausanne, Switzerland, as part of a project for developing such seismic techniques. Using a single 48-channel streamer, the 3-D site with an area of 1200 m x 600 m was surveyed in 10 days. A variety of complex geologic structures (e.g. thrusts, folds, channel-fill) up to —150 m below the water bottom were obtained with a 15 in.3 water gun. The 3-D data allowed the construction of an accurate velocity model and the distinction of five major seismic facies within the Lower Freshwater Molasse (Aquitanian) and the Quaternary sedimentary units. Additionally, the Plateau Molasse (PM) and Subalpine Molasse (SM) erosional surface, “La Paudeze” thrust fault (PM-SM boundary) and the thickness of Quaternary sediments were accurately delineated in 3-D.

Initial results of a high-resolution 3-D seismic reflection survey in north-central Lake Geneva, Switzerland

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.

3D very high resolution data: From acquisition to 3D prestack depth imaging

Very high resolution reflection seismics also known as shallow seismics is commonly used for land engineering purposes, when the target zone does not exceed a few hundreds of meters. As the target gets closer to the surface, data pre-processing becomes more and more difficult and final processing usually does not go further than DMO or post-stack time migration. In this work, we present first results of a 3D pre-stack depth migration applied to 3D lacustrine VHR data. The main idea was to test on the capacity of a 3D imaging algorithm, usually devoted to “oil-industry targets”, to handle the change of scales and the associated damaging of the reflected signal. In the geological frame of Lake Geneva and a thrust fault zone that runs beneath it, 3D pre-stack depth imaging emphasis the benefit of a recently conceived 3D VHR acquisition system.