“…In order to identify the frequency content of the data required to recover the low-frequency velocity model, as in the previous example, we analyse the SVD of the linearized modelling operator. Since the background is inhomogeneous in this case, the Green functions in equation (5) are computed using the iterative forward modelling algorithm in the frequency domain with a semi-analytical preconditioner proposed in Neklyudov et al (2010) (see Appendix B). We consider a 10 × 10 m grid in the target area and obtain matrix approximations of operatorsL(c 0 , ω i ) using equation (5), for the frequencies 3Hz, 2 Hz and 1 Hz.…”