The prescribed motion to be used as base excitation for shear wave propagation studies in soils is not well defined in the literature. This paper utilizes two methods to investigate the concept of utilizing one-half the rock surface velocity as the incident velocity at a transmitting boundary at the soil base. A range of material properties, thicknesses, and frequencies are used to explore the variation of the ratio: incident velocity to rock surface velocity. Although the ratio varies over a broad range recommendations are made to assist the analyst in selecting a conservative ratio suitable for engineering studies.