1. A method using a computing apparatus to develop an underground medium from a flow simulation grid implemented in a computer comprising a set of cells discretizing the underground medium and from dynamic data measurements wherein a fault network is associated with the flow simulation grid comprising: a) constructing the fault network by: i. constructing a multifractal density map by a multiplicative cascading; ii. constructing a fault center map by performing a random draw in the multifractal density map; and iii. generating a fault for each fault center; b) carrying out history matching of the grid implemented in a computer by deforming the fault network through continuous deformation of the multifractal density map by defining weights from at least one parameter and by modifying the at least one parameter which causes a modification of the multifractal density map and then reiterating ii and iii to reproduce with a flow simulation, using the modified multifractal density map, the dynamic data; and c) developing the underground medium according to a development scheme defined from the history matched grid.