Method of estimating the fracture density in a rock medium

The invention is a method of constructing a three-dimensional fracture density log of a porous formation traversed by fractures and a borehole. A number N of intersections between the fractures and the borehole is measured over a section of the borehole of length L located at a depth d, from observations of the wall of this section. A conditional probability law of the three-dimensional fracture density is then estimated, knowing the number N of intersections. The value of the three-dimensional fracture density at depth d is determined by calculating the mean of this conditional probability law. Finally, the three-dimensional fracture density log is constructed by repeating the previous stages for different depths. A three-dimensional fracture density uncertainty log can also be associated by calculating quantiles of the conditional probability law.