The biomechanical materials are among the most complex mechanical systems. Most often, their micro-structure are complex and random. This is the case for the human cortical bones which are considered in this paper. For such a system, the micro-structure can be altered near its interface with the marrow (osteoporosis). A gradient of porosity is then observed in the thickness direction but, in this case, none of the usual theories of porous materials can be applied. For this reason, we present a simplified model with gradient for the elasticity tensor. The elasticity tensor is modelled by a random field. In this paper, the parameters of this probabilistic model are identified with experimental observations in ultrasonic range.