L-curve for the best forward model (see Table 1) from each model set, with the regularisation parameter ranging from 10³ to 10¹⁰. The x-axis is the goodness of fit, given by the χ_obs². The y-axis shows the RLS penalty term, which is the integral of the square of the derivative of the profile. The optimal value for the regularisation according to the L-curve criterion should be in the hook of the L shapes in the profiles.