Stress induced elastic anisotropy of the Etnean basalt: Theoretical and laboratory examination

A theory, the stress-sensitivity approach, has been developed, which relates the elastic moduli of anisotropic rocks to the stress tensor and pore pressure for an arbitrary
symmetry of the applied load. The theory explains the stress-induced changes of seismic velocities in terms of
stress-induced changes of the pore space geometry. The stress dependent anisotropy is described in terms of Thomsen’s anisotropy parameters, g and d. To test the
theory we analyze the laboratory (high frequency) results of
deformation of an isotropically crack damaged dry lava
flow basalt from Mt. Etna volcano. The theory states that,
under an anisotropic (i.e. axisymmetric triaxial) load and in
the case of an initially isotropic rock, (1) the anisotropy
parameters are linear functions of the stress exponentials
(i. e. exponential functions of principal stress components)
and (2) the ratio of these anisotropy parameters as a function
of the stress is constant. In order to verify these
relationships, the stress exponentials and the anisotropy
parameters based on the measured velocities are computed
as well as the expected ratio of the Thomsen’s parameters.
Our experimental results are in very good agreement with
the theoretically predicted relations.