Seismic Velocity Changes During Hydraulic Fracturing Revealed by Ambient Noise Cross-correlation Analysis

Seismic data collected over 12 days before and after hydraulic fracturing at Well 205, located near Heijiabao Town, Yanchang County, Yan’an City, Shaanxi Province, China, were analyzed using ambient noise cross-correlation and cross-correlation imaging techniques to assess the impact of hydraulic fracturing on subsurface seismic wave velocities. Coda wave signals were extracted, and velocity changes were quantified through interferometric measurements. Findings indicate pronounced directional variations in seismic wave velocity following fracturing. In the azimuth range of 60 to 70 degrees, velocity increased by approximately 0.2%, whereas in the perpendicular direction, spanning 150 to 160 degrees, velocity decreased by about 0.3%. The direction of maximum velocity increase closely corresponds to the orientation of induced fractures, suggesting a connection to fracture-induced strain. Cross-correlation imaging techniques produced velocity change maps that confirmed the heterogeneous distribution of these changes, with a significant velocity increase observed in the northeast direction. These observations underscore the complex alterations in subsurface physical properties, likely driven by changes in pore pressure and local stress redistribution caused by fracturing. Exploration of shallow subsurface velocity changes provides novel approaches and insights for quantitatively evaluating the dynamic responses of subsurface media to hydraulic fracturing, offering valuable perspectives on the interplay between fracturing processes and seismic wave propagation in diverse geological settings.