Elastic anisotropy of porous and fractured rocks under stress

Abstract

Elastic properties of rocks are functions of tectonic stresses. Using the theory of poroelasticity we analyze mechanisms of these dependencies related to changes of the rock‐pore‐space geometry. We develop a formalism describing elastic moduli and anisotropy of rocks as non‐linear functions of confining stresses and pore pressure. It is the stress dependent geometry of the pore space that fully controls the stress‐induced changes of elastic moduli and seismic velocities. Changes of the compliant porosity with differential stress explain the often observed exponential behaviour of elastic moduli. We introduce a tensor quantity defining the sensitivity of elastic moduli of rocks to the differential stress and call it the stress‐sensitivity tensor. We show how anisotropy parameters depend on the stress sensitivity. Laboratory observations on rocks (e.g., porous basalts) are in a very good agreement with our formalism.