Iterative modification produces a best fit model that refines the definition of the subsurface geometry o1' 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 lb'st-order lithologic contrasts. This study also demonstrates not only the importance of threedimensional effects (sideswipe) in interpreting seismic data, but also that these effects can be considered quantitatively through three-dimensional modeling.