3Lanczos C.An iteration method for the solution of the eigenvalue problem of linear differential and integral operators .J Res N B S,1950, 45,255 - 282.
4Sassa Koichi. Suggested method for seismic testing and between boreholes,Part 4:Suggested method for seismic tomography.International Journal of Rock Mechanics and Minig Sciences &Geomechanics Abstracts,1988,25,447-472.
5Langan R T, et al. Tracing of rays through hoterogeneous media:An accurate and efficient procedure .Geophysics . 1985,50, 1456- 1465.