Evolution of P-wave anisotropy as an indicator of pre-failure deformation in shale

Detecting the early stages of damage in clay-rich rocks is essential for understanding fault mechanics and geological engineering processes. We present coupled uniaxial compression experiments performed inside an environmental scanning electron microscope (ESEM), combining microscale digital image correlation (DIC) with simultaneous ultrasonic P-wave velocity measurements. Sample preparation was optimized using cryogenic broad ion beam polishing to preserve the natural microstructure. During the initial stages of loading, deformation starts with compaction and crack closure, followed by shear along pre-existing, inclined clay-rich bands inherited from diagenesis. These heterogeneities control strain localization, dilatancy onset, and the evolution of elastic anisotropy. Although clay particle reorientation occurs, its effect remains secondary to the inherited texture. P-wave velocities thus provide a robust proxy for tracking microstructural anisotropy during deformation and offer new perspectives for anticipating damage nucleation in clay-rich faults and engineered barriers.