A note on the angular correction applied to fracture intensity profiles along drill core

We derive the angular correction that has to be applied to borehole fracture intensities to recover the actual three-dimensional distribution when the fracture networks have a length distribution. We assume that the fracture intensity is calculated from the fractures that fully transect the core. Because of the length distribution, the classical Terzaghi correction, which involves the cosine of fracture dip, is no longer valid. Solutions have been calculated in the specific case of fractal fracture networks with power law length distribution. We show that the Terzaghi correction may significantly overestimate the frequency of fractures subparallel to the borehole. The correction procedure proposed here was tested on a fracture database recorded at a study site for a repository of spent nuclear fuel located in SE Sweden, with three outcrop maps and three boreholes. A consistency between the dip distributions of outcrops and boreholes was achieved when applying the angular correcting term calculated with the power law length distribution deduced from outcrop maps.