p p u03b4 u2062 u2062 p a h s u2062 0 3 the first stage calculates the density of the wet rock u03c1 b by means of the formula as follows where u03c1 gr designates the grain density defined for each facies and u03c6 the porosity a constant per facies or a value per pixel the downscaling stage allows calculation of the fluid density u03c1 f by taking account of the pressure effects u03c1 f u03c1 o p o s o u03c1 w p w s w u03c1 g p g s g 0 5 where u03c1 o u03c1 w u03c1 s s o s w s s et p o p w p s are the phase densities saturations and pressures the phase densities are deduced from pressure density tables calculation of the incompressibility modulus of the wet rock k is carried out by means of the gassmann equations 21 k k m 1 k m k gr u03d5 k f 1 u03d5 k gr k m k gr 2 u2062 u2062 and u2062 u2062 1 k f s o k o s w k w s g k g 0 6 where k gr designates the incompressibility modulus of the grain a constant per facies depending on the composition of the rock k f the incompressibility modulus of the fluid k m the incompressibility modulus of the porous matrix and k o k w k g the moduli for each phase from pressure incompressibility modulus tables incompressibility modulus k m is selected by the user a constant.