(color online) (a) Synthetic Lamb wave signals with two modes of A₀ and S₀ excited by an ultrasonic pulse with a center frequency of 2.0 MHz in 1-mm-thickness aluminum plate. (b) Distance-traces mapping after a backward propagation calculation using the propagation function of S₀ mode, i.e., the signals from a Fourier transform of V(x,ω)F(ω) and U(x,ω)F(ω) on ω. (c) The signals from a Fourier transform of U(x,ω)F(ω) and (A_u/A_v)V(x,ω)F(ω). (d) Final S₀ distance-trace after mode separation and dispersion removal.