THE COVER BASEMENT CONTACT BENEATH THE RAWIL AXIAL DEPRESSION (WESTERN ALPS) - TRUE AMPLITUDE SEISMIC PROCESSING, PETROPHYSICAL PROPERTIES,AND MODELING

Since 1986, several near-vertical seismic reflection profiles have bee n recorded in Switzerland in order to map the deep geologic structure of the Alps. One objective of this endeavour has been to determine the geometries of the autochthonous basement and of the external crystall ine massifs, important elements for understanding the geodynamics of t he Alpine orogeny. The PNR-20 seismic line W1, located in the Rawil de pression of the western Swiss Alps, provides important information on this subject. It extends northward from the ''Penninic front'' across the Helvetic nappes to the Prealps. The crystalline massifs do not out crop along this profile. Thus, the interpretation of ''near-basement'' reflections has to be constrained by down-dip projections of surface geology, ''true amplitude'' processing, rock physical property studies and modelling. 3-D seismic modelling has been used to evaluate the se ismic response of two alternative down-dip projection models. To const rain the interpretation in the southern part of the profile, ''true am plitude'' processing has provided information on the strength of the r eflections. Density and velocity measurements on core samples collecte d up-dip from the region of the seismic line have been used to evaluat e reflection coefficients of typical lithologic boundaries in the regi on. The cover-basement contact itself is not a source of strong reflec tions, but strong reflections arise from within the overlaying metased imentary cover sequence, allowing the geometry of the top of the basem ent to be determined on the basis of ''near-basement'' reflections. Th e front of the external crystalline massifs is shown to extend beneath the Prealps, about 6 km north of the expected position. A 2-D model w hose seismic response shows reflection patterns very similar to the ob served is proposed.