Snapshots of the radial (a) and azimuthal (b) velocity perturbations across the bottom boundary of the domain at a time t = 805 s in the shifted wave driver simulation WD2. The signal for the velocity perturbation v_y (after converting to Cartesian coordinates) is shown in panel c for a slice taken along x = 7 Mm, where the shifted drivers (WD2 and WD3) are positioned. The resulting Fourier transform (d) of the signal in panel c allows us to compute the fast mode reflection height z_refl at this moment in space and time.