The seismic wave is generated by a source at the surface. In a homogeneous medium the wave will propagate spherically away from the source as shown in the figure below. A surface wave travels along the interface between the ground and the air. Each semicircular line is a wavefront that represents the location of the wave at a given time. The white wavefronts depict the wave traveling away from the source, while the red wavefronts depict reflections from the interface.
Figure 5 Wavefronts and Raypaths (Brown, 2001)
The straight lines are raypaths, which are imaginary lines drawn perpendicular to wavefront as it travels through the earth. While wavefronts depict how the seismic signal propagates through the earth, raypaths simplify diagrams and improve understanding.