Left: B-spline functions used in the expansion of the supergranule model SG2. The specific choice of knots might be important for the inversion to converge to the correct model. The density of knots with depth is reflective of the degree of stratification. The B-splines are not normalized by the corresponding coefficients. Middle: supergranule model from Eq. (3) (gray solid line) and the smoothing spline approximation to this model (black circles). Right: error in the spline approximation with depth.