In general the core characteristics are supposed at least reliable for the cubic meter of rock centered on the core (which goes from its upper to its lower face). Therefore one porosity value refers to that cubic meter and is computed from meter to meter along the bore-hole.

The surface of the fractures are at least of one square meter. Such an hypothesis is of course somewhere biased because it's commonly known that quite often fractures stop one on the other (their real surface shape is a complex polygon one). But same it is commonly admitted that a relation (complex and highly changing from case to case) exist between the length and the aperture in the way of a concomitant increase of both parameters. Thus it means that the error on the length will affect preferably the"small" apertures fractures which length could be under a meter and less or not the "large" open fractures which length is certainly over a meter. Consequently the error on the porosity will be limited regarding that the contribution of small open fractures is minor when compared with the one of the "large" open one.

The calculation is done by the cumuli of all individual fractures porosity families. More further each fracture family porosity is computed from the above cumuli fracture frequency affected by a specific aperture.