Method for optimizing the exploitation of an underground medium containing fluids, on the basis of a flow model consisting of a first grid forming a set of cells discretizing said medium, wherein, during exploitation of said medium, production data and seismic data associated with a second grid are acquired, each cell of the first grid containing a set of cells of the second grid, characterized in that the following steps are carried out: - production data, and pressure and saturation values in each cell of said first grid, are simulated by means of a flow simulator; - a pressure value is attributed to each cell of said second grid; - a saturation value is attributed to each cell of said second grid by carrying out the following steps for each cell of said first grid: ∘ regions (Rn) consisting of a set of adjoining cells of said second grid having one same lithofacies value are identified; ∘ for each identified region, a saturation value is determined by solving a system composed of a first system of equations expressing a continuity in the flows of the fluids between neighbouring identified regions, and a second system of equations expressing a conservation of a porous volume in each identified region and a conservation of a volume of each fluid in said cell of said first grid; - seismic data are simulated on said second grid by means of the pressure and saturation values associated with each of the cells of said second grid; - the flow model is modified so as to minimize a difference between the measured production data and the simulated production data, and so as to minimize a difference between the measured seismic data and the simulated seismic data, said modification of said flow model consisting in modifying values of associated petrophysical properties in each cell of said second grid; - the exploitation of said underground medium is optimized by defining various schemes of exploitation of said underground medium, by predicting the production of said medium by means of said modified flow model and of said flow simulator for each of said schemes, and by determining an optimal scheme of exploitation of said underground medium, said optimal scheme of exploitation allowing said production to be maximized, and said underground medium is exploited using said optimal scheme. - production data, and pressure and saturation values in each cell of said first grid, are simulated by means of a flow simulator; - a pressure value is attributed to each cell of said second grid; - a saturation value is attributed to each cell of said second grid by carrying out the following steps for each cell of said first grid: ∘ regions (Rn) consisting of a set of adjoining cells of said second grid having one same lithofacies value are identified; ∘ for each identified region, a saturation value is determined by solving a system composed of a first system of equations expressing a continuity in the flows of the fluids between neighbouring identified regions, and a second system of equations expressing a conservation of a porous volume in each identified region and a conservation of a volume of each fluid in said cell of said first grid; ∘ regions (Rn) consisting of a set of adjoining cells of said second grid having one same lithofacies value are identified; ∘ for each identified region, a saturation value is determined by solving a system composed of a first system of equations expressing a continuity in the flows of the fluids between neighbouring identified regions, and a second system of equations expressing a conservation of a porous volume in each identified region and a conservation of a volume of each fluid in said cell of said first grid; - seismic data are simulated on said second grid by means of the pressure and saturation values associated with each of the cells of said second grid; - the flow model is modified so as to minimize a difference between the measured production data and the simulated production data, and so as to minimize a difference between the measured seismic data and the simulated seismic data, said modification of said flow model consisting in modifying values of associated petrophysical properties in each cell of said second grid; - the exploitation of said underground medium is optimized by defining various schemes of exploitation of said underground medium, by predicting the production of said medium by means of said modified flow model and of said flow simulator for each of said schemes, and by determining an optimal scheme of exploitation of said underground medium, said optimal scheme of exploitation allowing said production to be maximized, and said underground medium is exploited using said optimal scheme.