1 ) A method intended for direct updating, by dynamic data, of a geologic model discretized by a fine grid pattern representative of the distribution, in an underground reservoir, of a physical quantity characteristic of the subsoil structure, characterized in that it comprises: parameterization of the fine geologic model by a parameterization factor (&thgr;) in order to obtain the distribution of the physical quantity in this geologic model; upscaling so as to determine the distribution of said physical quantity in a simulation model discretized by a rough grid pattern; solution, via the simulation model, of the fluid flow equations so as to obtain simulated dynamic data; and determination of the analytical relations connecting the variations of the simulated dynamic data and the corresponding variations of parameterization factor (&thgr;).