Surface geological mapping, laboratory measurements of rock properties,
and seismic reflection data are integrated through three-dimensional
seismic modeling to determine the likely cause of upper crustal reflections
and to elucidate the deep structure of the Penninic Alps in eastern
Switzerland. Results indicate that the principal upper crustal reflections
recorded on the south end of Swiss seismic line NFP20-EAST can be
explained by the subsurface geometry of stacked basement nappes.
In addition, modeling results provide improvements to structural
maps based solely on surface trends and suggest the presence of previously
unrecognized rock units in the subsurface. Construction of the initial
model is based upon extrapolation of plunging surface. structures;
velocities and densities are established by laboratory measurements
of corresponding rock units. Iterative modification produces a best
fit model that refines the definition of the subsurface geometry
of major structures. We conclude that most reflections from the upper
20 km can be ascribed to the presence of sedimentary cover rocks
(especially carbonates) and ophiolites juxtaposed against crystalline
basement nappes. Thus, in this area, reflections appear to be principally
due to first-order lithologic contrasts. This study also demonstrates
not only the importance of three-dimensional effects (sideswipe)
in interpreting seismic data, but also that these effects can be
considered quantitatively through three-dimensional modeling.