Impact of velocity evolution on AE event location and fracture process interpretation

The ability to reliably locate microseismic events is crucial for monitoring the fracture process in the subsurface. However, changes in rock properties during fracturing alter seismic velocity, impacting event localization and potentially misinterpreting damage evolution. We investigated the impact of seismic velocity changes on the reconstruction of acoustic emission (AE, i.e., lab-scale microseismicity) evolution during the deformation of sandstone samples under controlled laboratory conditions. We localized the AE events using a novel approach that integrates double difference (DD) with real-time velocity updates, accounting for rock damage. This approach significantly improves the reliability of the reconstructed spatio-temporal AE distribution, as validated against CT imaging of the main fracture. By quantifying how AE characteristics—distribution and event rate—evolve across stress stages from pre-failure to post-failure, we demonstrate their correlation with measurable velocity changes, underscoring the importance of incorporating dynamic velocity in AE localization. Neglecting velocity variations can cause systematic location bias, misrepresenting damage progression and crack coalescence into a major fracture—highlighting the need for dynamic velocity models. By integrating insights from rock mechanics and rock physics into seismological methods—specifically the DD approach— we establish a more reliable framework for tracking damage progression. This integrated approach enhances passive seismic monitoring and holds broad applications for subsurface reservoir management, including CO 2 geological storage and Enhanced Geothermal Systems, where reliable tracking of fracture evolution is critical.