The disclosed method includes, from a topographic map showing, for a set of points {x} located in a midplane of the fracture surface, a height of the fracture surface h relative to the midplane: a step of determining, for each point x on the topographic map, a quantity ωε representative of an average difference in height)IδxI≤ε between the height h of the fracture surface at point x in question and the height h of the fracture surface at one or more points {x+δx} located inside a circle of radius ε centered on point x in question, a step of determining, according to a test distance δr, a spatial correlation function Cε representing a spatial correlation between points {x} and points {x+δx} such as IδxI=δr, and a step of determining a correlation length ξ from the spatial correlation function Cε.