Waveguides formed between the earth's free-surface and sharp subsurface velocity discontinuities, such as the watertable, may have a significant impact on high-resolution seismic refraction and reflection data. Energy trapped in such waveguides may propagate over long distances with little loss due to geometrical spreading. The dispersive nature of the guided waves results in pronounced shingling of the first arrivals. This narrows the ''optimum reflection window'' between the first arrivals and the ground roll, and may lead to mispicking of first breaks and difficulties in identifying and processing shallow reflections. ¿ American Geophysical Union 1996