The invention provides a method for measuring and locating a fluid communication pathway in a material behind a casing wall, wherein said material is disposed in an annulus between said casing and a geological formation, and said method comprising: measuring a set of parameters of the material behind the casing within a range of radius, depths and azimuthal angles; defining sections comprising a sub-set of parameters wherein said sub-set of parameters is taken in said set of parameter for a given range of radius, depths and azimuthal angles included in said range of radius, depths and azimuthal angles; defining for each section a first limit zone and a second limit zone in frontier of said given range; determining among said sections the ones that comprise a continuous fluid communication pathway from said first limit zone to said second limit zone, said sections being renamed in retained sections; determining from said continuous fluid communication pathway an area or a width of pathway versus depth for each of said retained sections; extracting a fluid communication index versus depth for the material behind the casing, wherein said fluid communication index versus depth: depends of said area or width for retained sections and, is equal to zero for non retained sections; deducing from said fluid communication index the existence and location of fluid communication pathway in said material behind said casing wall.