Abstract: This paper starts with a review related to the tomography for acoustic / clastic waves and then proceeds to the formula derivation for medium parameters from observed wave-fields. By using the Back-propaga tion algorithm, parameters including the velocity and the density arc able to be extracted for different frequencies based on the wave equation involving varying density parameters. For clastic waves, there arc four scattering wave modes (P-P, P-S, S-P and S-S) which arc exploitable for the recovery of clastie parameters.