Spatial distribution of an assumed mock sample of 20 000 SNRs (points coloured by log(v_*/MHz) using the colour bar code). Absorbers (open circles) are distributed along the Galactic spiral arms taken from the Hou & Han (2014) model (solid black curves). Each absorber is assumed to have a constant low density n_e = 2.0 cm⁻³, a temperature T_e = 5000 K, and radius r drawn from a Maxwell-Boltzmann probability density distribution with a median of 50 pc. The scale of each open circle has been expanded by an arbitrary factor of 3 in order to make the plot clearer. The central plus symbol indicates the position of the Galactic centre, the solar symbol shows the Sun’s position, and the dotted lines the Galactic quadrants. The coloured points do not show any strong dependence on heliocentric distance or Galactic longitude (see Figs. 10 and 11).