Abstract:Based on approximation of weak reflectivity contrast presented by Aki Richards,the paper presented a new approximate formula of reflectivity after Taylor expansion of angle terms (incident and transmission) in reflectivity formula of P-SV wave described by Zoeppritz equation and after keeping second order terms related to density and velocity.The quantitative calculation results for 4 three layered oil bearing sand models showed that the approximate formula can better approach the precise solution of Zoeppritz equation within 40°,and has better effect than the Aki's,so that is suitable for AVO parameter inversion of long offset data and theoretical study.Meanwhile,the process of using the approximate formula for lithologic parameter inversion has been deeply discussed from three aspects separately:converted wave reflectivity in different representative forms,AVO weighted stack method for P-SV converted wave and AVO simultaneous inversion of PP wave and P-SV wave.