1. A computer implemented method of developing an underground formation including constructing a fracture density log of the underground formation traversed by a series of fractures and at least one borehole, comprising: a) measuring a number of intersections between fractures and the at least one borehole over a section of the at least one borehole of a length located at a depth from observations of a wall of the section; b) expressing a conditional probability law of a three-dimensional fracture density with knowledge of the number of intersections and the three-dimensional fracture density corresponding to a sum of fracture surface areas per volume unit; c) determining with a computer a three-dimensional fracture density value at the depth calculated from the expression of the conditional probability law; d) constructing a three-dimensional fracture density log by repeating a), b) and c) for different depths; e) constructing a three-dimensional fracture density log and at least one three-dimensional fracture density uncertainty log; f) constructing a three-dimensional fracture density map and a map of uncertainties of fracture density values from at least one of the logs; and g) optimizing with a computer development of the underground formation by accounting for the density map and the map of the uncertainties to extract flows between the fractures; and wherein at least one three-dimensional fracture density uncertainty log is associated with the three-dimensional fracture density log by calculating quantiles of the conditional probability law.