Slowness (s) is a quantity introduced in Seismology which is the reciprocal of velocity. Thus travel time of a wave is the distance that the wave travels times the slowness of the medium ( in seismology, it refers to different layers of Earth exhibiting different densities) Thus,
Slowness = 1/Velocity
If we divide the path followed by the ray into N segments numbered i = 1, 2 .. N the total travel time of the wave is
Therefore, the residual travel time is due to departures in slowness experienced by the wave along the raypath.
Sonic well logs
Measurements of interval transit time, designated