Convergence of the preconditioned CG solver applied to the simulated data, as described in Sect. 5.2. Different line styles correspond to different sizes of the problem, while different colours correspond to the three different types of preconditioners: red for the standard block preconditioner M_BD, and green and blue for the two-level preconditioners, M_2lvl,S and M_2lvl,D, respectively. In the legend, K corresponds to the number of stationary intervals in the time domain data and dim(Z) denotes the number of columns in the deflation subspace operator, Z. For the a posteriori preconditioner, M_(2lvl,D), dim(Z) is the number of eigenvalues, which fulfill the condition Re | λ_i | <ϵ_tol = 0.2.