Since 1986, several near-vertical seismic reflection profiles have been
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 crystalline
massifs, important elements for understanding the geodynamics of the
Alpine orogeny. The PNR-20 seismic line W1, located in the Rawil
depression 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
outcrop 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 seismic response of
two alternative down-dip projection models. To constrain the
interpretation in the southern part of the profile, `'true amplitude''
processing has provided information on the strength of the reflections.
Density and velocity measurements on core samples collected up-dip from
the region of the seismic line have been used to evaluate reflection
coefficients of typical lithologic boundaries in the region. The
cover-basement contact itself is not a source of strong reflections, but
strong reflections arise from within the overlaying metasedimentary
cover sequence, allowing the geometry of the top of the basement to be
determined on the basis of `'near-basement'' reflections. The 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 whose seismic
response shows reflection patterns very similar to the observed is
proposed.