Velocity changes in the shallow and mid-depth subsurface induced by hydraulic fracturing at a shale play site in southwestern China

ABSTRACT Through the use of continuous ambient noise data from a broadband seismic array in the Weiyuan shale region of the Sichuan Basin, subsurface velocity changes at shallow (within approximately 100 m) and mid-depth (approximately 400–2000 m) levels during hydraulic fracturing from 29 October to 10 November 2014 (excluding 31 October) were examined using seismic ambient noise tomography and autocorrelation function extraction. In the 2–3 Hz range, corresponding to shallow depth, seismic ambient noise tomography results and velocity changes obtained by the autocorrelation method indicated a significant positive velocity anomaly in the shallow projection area of the well trajectory post-fracturing. In the 0.2–0.4 Hz range, corresponding to mid-depth, the velocity changes from the autocorrelation method reached a minimum on the 7th day post-fracturing, showing a slow recovery, and the velocity reduction inflection point coincided with the onset of fracturing. Moreover, across the three fracturing stages, the shallow (2–3 Hz) and mid-depth (0.2–0.4 Hz) regions exhibited consistent velocity change patterns relative to the distance from stations to the fracturing site: the influence of distance peaked in the second stage and was minimal in the final stage, with strain states in both regions corresponding to each other. Hydraulic fracturing affected deeper velocities more significantly, likely due to tensile strain from fluid injection, whereas shallow velocity changes were influenced by compressive strain and various other factors, such as precipitation. Seismic interferometry offers a novel approach for future monitoring of hydraulic fracturing and wastewater injection.