FIG. H-3. Head waves. (a) Time-distance plot showing head waves that extend as straight alignments to the right of θ1 and θ2, where they are tangent to the respective reflections. (b) First-arrival wavefronts for horizontally layered media. (c) Same as (b) except for addition of a high-velocity mass (salt dome). (d) A number of types of head waves are possible from an incident P-wave depending on relative velocities, but usually only the P2P1 type is observed.