Relative L² error between the FEM solution and the analytical solution for the piecewise homogeneous layered medium (Sect. 6.6 and Fig. 3) for non-axial incidence of a plane wave at frequency ω/ 2π = 2 mHz. The error is plotted as a function of the maximum mesh size h for different polynomial orders p. The observed rate of convergence is consistent with the theoretical expectation, h^{(p + 1)}.