A borehole airgun source reveals depth-dependent seismic velocity changes at the Beishan underground research laboratory

Abstract Monitoring subtle change of seismic velocity helps understand subsurface property changes and the underlying physical mechanisms. We deployed a borehole airgun at a depth of 95 m below the surface in the Beishan Desert, Gansu, China, to detect daily seismic velocity changes. The airgun fired every ten minutes for 1 week at a pressure of 10 MPa, generating 50–80 Hz repeating signals that propagated up to 2 km. We used water-level deconvolution to eliminate source effects and clock errors in the excitation time caused by mechanic delays, and applied moving-window cross-spectral analysis to compute seismic velocity changes over one to two periods of waves around the strongest S wave arrivals. We observed that seismic velocity increased up to approximately 0.2% in the late afternoon local time, lagging behind the temperature change by about four hours. In-phase velocity changes at both surface stations and borehole stations, along with thermoelastic strain and stress modeling, suggest that the velocity variations at the subsurface are likely attributed to thermally induced stress changes. Seismic velocity changes measured at different depths of the borehole suggested a depth sensitivity of up to 300 m. Seismic velocity changes decrease with depth. Our results demonstrate that the airgun is a useful tool for detecting depth-dependent velocity variations at a scale of several hundreds of meters. Graphical Abstract