1. Vinnik, L. P. and Dashkov, G. G.: 1970, Phys. Sol. Earth (Eng. trans.) 4–9, Jan 1970.
2. Kelly, E. J. and Lacoss, R. T.: 1969, Tech. Note 1969 - 41 Lincoln Laboratories, MIT, Cambridge, Mass.
3. Ringdal, F.: 1975, Bull. Seism. Soc. Am. 65, 1631–1642.
4. Ringdal, F.: 1976, Bull. Seism. Soc. Am. 66, 789–802.
5. Christoffersson, A.: 1978, NTNF/NORSAR Sci. Rep. No. 1–77/ 78, Kjeller, Norway.