A NEW APPROACH FOR THE PREDICTION OF UNEXPOSED FRACTURED RESERVOIRS -A CASE-STUDY IN MILLAS GRANITE (FRENCH PYRENEES)

A fracture map is an essential aid in the study of fissured aquifers e ven though it often includes covered areas where the fractures are not visible. Is it possible to predict fracturing or permeable zones unde r the cover? This problem is dealt with using a stochastic approach ba sed on fracture density. Classical methods of fracture density assessm ent involve several biases and are not adapted to hydrogeology. A prop osed ''fracture density index'' (FDI) eliminates these biases and give s weight to two important parameters influencing the hydraulic conduct ivity of a zone: length and proximity of long fractures. The FDI is a regionalized variable and allows one to make predictions by kriging ov er covered zones. The proposed method has been applied to the Campouss y site (Pyrenees Orientales, France). In contrast to previous methods, the FDI predictions were in good agreement with the apparent hydrauli c transmissivity and electrical anisotropy. The results were promising but the method needs to be tested on other sites before being conside red a useful and efficient hydrogeological tool.