Gradient spectra of fBm maps generated with ζ = 3, S/N = ∞, enhanced on scale of l_f = 15 pix and for different inverse brightness contrast parameters W = 0.25,1,1.75 and 2 (full, dotted, dashed, and dot-dashed lines, respectively). The vertical dashed line indicates the upper limit to the scale where the Δ-variance slope can be measured reliably. Above that scale, the spectrum is strongly affected by boundary effects of the assumed finite map.