Disclosed is a method of determining front propagation within a subsurface volume such as a reservoir. The subsurface volume includes a plurality of cells and at least one geological fault. The method includes performing a fast marching algorithm so as to determine the front propagation in terms of the time of arrival of the front at a particular cell from one or more neighboring cells which make up the neighborhood of the particular cell. For each faulted cell that is adjacent a geological fault, the neighborhood of the faulted cell is defined as including only its geometric neighbors, where the geometric neighbors are those cells that are in contact with the faulted cell in a geometric sense, regardless of stratification.