f High resolution multi-channel seismic reflection imaging of the Rhone delta, Lake Geneva (Switzerland)

Lake Geneva, located in western Switzerland along the French border, is one of the biggest European peri-alpine lakes. The Rhone River contributes 80% of its sediment influx from the alpine valleys, and its sublacustrine delta is 10 km long. This lake formed in a glacial trough that was carved in an Oligocene-Miocene molasse basement. The deposits left by the retreating glacier and the fluvio-lacustrine sediments are up to about 500 m thick. Past seismic studies in this area, mostly single-channel surveys, were unable to provide sufficient information on the deep internal structure of the delta. The aim of the present work was to test more efficient and powerful methods of water-borne high-resolution seismic reflection that would allow a deeper penetration. During our 1998 acquisition campaign, we recorded 24 km of data that were divided into 4 lines (Fig. 1). The following instruments were used: a double-chamber 30 cu. in. airgun (bubble-suppressor Mini GI gun), a 48-channel streamer array, a Bison seismograph and a differential GPS positioning system. The air gun covered a frequency spectrum of up to 400 Hz, and the survey design allowed a 9-fold data coverage. GeovecteurPlus software from the Companie Générale de Géophysique was used to process the data. Conventional processing was applied: trace editing, geometry assignment, spherical divergence correction, spiking deconvolution, bandpass filter (40-56-350-494 Hz), AGC gain (30 ms window) and coherency filter. The 9-fold coverage provides a better signal/noise ratio than single-fold coverage and allows different seismofacies to be more clearly distinguished (compare Figs. 2a and 2b with 2c). Migration corrected reflection geometry and collapsed numerous diffractions associated with complex structures (compare Figs. 2a and 2b). Currently, attempts are being made to attenuate an intermittent but strong water-bottom multiple that may obliterate some deep reflections (Figs. 2a and 3). In many areas, the profiling penetration reached more than 100 ms beneath the water bottom which is equivalent to a sediment thickness of about 100 m. The high resolution of the data allows us to identify several typical sedimentary features within the delta. Figure 2 shows a prograding facies (facies A) with foreset (dipping NW), bottomset beds and the underwater Rhone channel with its natural levees. This facies is interpreted as prograding sediments originating from the actual channel and downlapping onto horizontaly bedded deposits (facies B). These deeper layers may have formed when the mouth of the Rhone River was further east of its present location. The top of this facies can be identified on both lines (event D). A third, relatively hummocky and faulted facies (facies C) is identified underlying facies B. Some of the folds are indicated in Figs. 2a and 2b. All three facies belong to the Quaternary fluvio-lacustrine sequence of the Rhone delta. In places, the absence of seismic reflection may be caused by the presence of gas within the upper sedimentary layers (Fig. 3). Gas occurrence has already been observed elsewhere in Lake Geneva.