Time–distance map of modeled m = 0 mode waves for three different cases. (a) TD map with the amplitudes of A_0, 1 = 2.25, A_0, 2 = 2.0, and A_0, 3 = 0.5 in units of km s⁻¹. (b) TD map with the amplitudes of A_0, 1 = 1.5, A_0, 2 = 2.25 and A_0, 3 = 1.5 in units of km s⁻¹. (c) TD map with the amplitudes of A_0, 1 = 0.5, A_0, 2 = 2.0 and A_0, 3 = 2.25 in units of km s⁻¹. All of them have the phase differences of t_0, 1 = 0, t_0, 2 = −20 and t_0, 3 = −40 in seconds. The solid lines represent the gradients of the ridges, and their values are shown in each panel.