2D projection of the double edge with exterior wedge angles θ_w1,2 and finite width d_w, extending the geometry of the single wedge in Figure 1. The source (blue cross) and receiver (red circle) are arbitrarily assigned to one of the edges and are located at the distances d_s1 and d_r2 at angles θ_s1 and θ_r2 with respect to edge 1 and 2 respectively, so that θ_s1 < θ_r1 The light grey area shows the “single” SZ and the dark grey area the “double” SZ.