1. E. V. Nikol’skii, “On solution of direct and inverse seismic problems for a one-dimensional inhomogeneous medium in the case of a normally incident plane wave,” in Seismic Exploration Methods (Nauka, Moscow, 1965), pp. 190–205 [in Russian].
2. A. S. Blagoveshchenskii, “An inverse problem in seismic wave propagation theory,” in Problems in Mathematical Physics (Leningr. Gos. Univ., Leningrad, 1966), pp. 68–81 [in Russian].
3. B. S. Pariiskii, “An inverse problem for the wave equation with a depth effect,” in Direct and Inverse Problems in Seismology (Nauka, Moscow, 1968), pp. 25–40 [in Russian].
4. V. G. Romanov, Inverse Problems of Mathematical Physics (Nauka, Moscow, 1984; VNU Science, Utrecht, 1987).
5. M. I. Belishev and A. S. Blagoveshchenskii, Dynamic Inverse Problems in Wave Theory (S.-Peterburg. Univ., St. Petersburg, 1999) [in Russian].