Abstract:The anisotropy,porosity,fluid saturation and effective pressure are the significant factors which effect on amplitude variation with offset(AVO) of seismic wave in the medium.Based on the we11 known expressions of reflectivity of plane wave and after some reasonable simplification,the approximate expressions of reflectivity in weak transversely isotropic medium are deduced in this paper.With the same accuracy as the well known expressions,our expressions are more simple(explicit function of amplitude vs offset or incident angle) and have definite physical meaning.The paper also studied the effects of porosity,fluid saturation in diphasic medium(water-gas and oil-gas) and change of overburden pressure on reflection wave AVO.Considering that fluid saturation has strong effects on density of medium,P-wave and S-wave velocity and reflection amplitude,we attempt to express the variation of reflectivity with different saturation by an explicit function of velocity and density(the updated expression of saturation),which quantitatively expresses the effect of saturation on velocity,density and reflectivity that people qualitatively analyzed right up to now.The updated expression of saturation can be suitable for different effective pressure and fluid type.