Modern seismic reflection profiling has contributed substantially to our understanding of deformation processes at actively convergent plate margins. Nonetheless, the complexity of most seismic profiles leads to great uncertainty about internal prism structures because interpretations of particular profiles can vary from one showing dominantly bedded but faulted sediments to another showing chaotic melange. The steep dips of bedding and faults form a complex three-dimensional geometry that causes widely recognized but poorly understood imaging problems. In this report, the specific effects on seismic wave propagation of roughness of a few hundred meters amplitude at the interface between the tectonized sediments of a prism and the overlying slope sediments are systematically analyzed through normal incidence and offset synthetic modeling. For a surface roughness and velocity contrast simlar to that between the prism and slope cover off Costa Rica, the modeling shows that there are important focusing and defocusing effects because of wave front distortion at the interface. The synthetic seismic profiles of simple continuous interfaces within a model prism produce discontinuous reflections beneath the interface with amplitudes reduced by a factor of ~2, and phase changes occurring for over 30% of the rays traced through the models. The significance of these effects increases with the depth and the dip of the reflecting interface. The distortions are greater for dip lines than for strike lines as the structure changes more rapidly in the dipdirection. The study demonstrates that whenever there is surface roughness comparable to that found off Costa Rica, the detection of even simple structures within the prism will be quite difficult with only two-dimensional reflection methods. Three-dimensional imaging techniques will be necessary at many convergent plate margins to differentiate between models of the prism accretion and deformation. ¿ American Geophysical Union 1989