The velocity of waves through subsurface media down to the nth interface, such that V_rms = (Σ‎V_int²t_i/Σ‎t_i)^Z\\x, where V_int and t_i are the interval velocity and single-travel time through the ith interval. V_rms is usually derived from normal moveout measurements from a T²–X² graph, and is commonly several per cent larger than the corresponding average velocity.