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Abstract:
We study acoustic emissions (AEs) associated with shear and tensile failures around a horizontal borehole in a sandstone sample subjected to triaxial stress. The aim is to relate the AE event rate to macroscopic observations of sample deformation and the percentage of isotropic and deviatoric components of the seismic moment tensors to the expected failure mechanisms. The horizontal hole interferes with the applied load and forms a strongly spatially dependent anisotropic stress field, focusing the crack initiation into both shear and tensile failures. The recorded AEs follows reasonably well existing damage models, but the elastic solution of hoop stress does not represent the onset of failure around the borehole. The focal mechanisms correlate with the orientation of macroscopic fractures in the sample. Events close to the borehole show a higher fraction of isotropic percentage in moment tensors compared to events occurring in the macroscopic fracture featuring higher double-couple percentages. The inhomogeneous stress field due to the borehole and the stress induced damage is strongly affecting the axial and radial velocities which in turn affect the waveforms of the recorded AEs and the resulting moment tensors. The VP/VS ratio obtained from the ratio of isotropic to compensated linear vector dipole components of the moment tensors is close to that obtained from ultrasonic velocity measurements.
