Cooperative acoustic emission locating with velocity tomography in true triaxial experiment

Abstract

During true triaxial experiments, it is impractical to directly affix sensors to the rock sample, which consequently gives rise to a heterogeneous velocity structure extending from the source of acoustic emissions to the sensors. In pursuit of ascertaining the travel path of acoustic emission waves under anisotropic conditions and comprehending the evolution of wave velocity during triaxial loading, we conducted cooperative event localization in conjunction with velocity tomography. Initially, a numerical test was executed to validate the precision and stability of the employed fast-marching tomography method. Subsequently, true triaxial loading was applied to a granite rock specimen. The algorithm's accuracy was corroborated by the congruence between the final locating results and the observable crack positions. The tomography inversion results indicated that the wave velocity field remained relatively stable when the maximum principal stress was below 16 MPa, despite the occurrence of numerous acoustic emissions during this stage. The P-wave velocity exhibited a discernible upward trend with increasing maximum principal stress, particularly evident after reaching the 40 MPa threshold. Nevertheless, it's important to note that stress escalation does not invariably correspond to fracturing during the elastic deformation phase under confining pressure. The inversion method employed here enables the visualization of variable velocities and provides insight into the rock's deformation physics.