Percolation networks are useful tools to investigate transport properties at various scales in heterogeneous media. They consist of conductances distributed over a geometrical network. Transport properties depend on the topology, on the type of distribution functions and on the connectivity of these networks. Two examples of applications are presented at two different scales: the first one is the pressure dependence of permeability in rocks and the second one is the fracture density variation through the crust. The model suggest that the existence of fractal networks of fracture could result from the fact that natural media are close to a threshold.