Acoustic Velocity Anisotropies in Cordoba Cream Limestone During Different Deformational Stress Paths

ABSTRACT: Ultrasonic compressional (P) and shear (S) waves were transmitted through the axial and lateral directions of cylindrical samples of a high porosity rock formation subjected to triaxial, hydrostatic and uniaxial strain deformational paths. Triaxial tests which terminate in a brittle shear fracture generate a P-wave anisotropy and a lateral shear wave birefringence in both the elastic and dilatant phases of loading. Post-failure sliding along the shear fracture does not result in changes in anisotropy. Triaxial tests at high confining pressure generate ductile yielding of the limestone. In the initial elastic phase no anisotropies are evident. At yielding significant P-wave and S-wave anisotropies develop which continue to increase until termination of the tests. Uniaxial strain (k_o) tests develop initial acoustic anisotropies at the point of ductile yielding. These anisotropies continue to increase after yielding, for about 1 to 2% volumetric strain, at which point the acoustic velocities begin to increase and the velocity anisotropies remain constant for the duration of the test. Hydrostatic compaction tests evidence small acoustic anisotropies which develop during the initial (elastic) phase but begin to decrease after hydrostatic yielding.