Extracting high-resolution P-wave reflectivity of the shallow subsurface by seismic interferometry based on autocorrelation of blast mining signals

Body-wave reflections are sensitive to sharp velocity contrasts, making them useful for lithological imaging. We analysed seismic data from natural earthquake, ambient noise and mine blasts to map P-wave reflection profiles at the Hishikari mine area by autocorrelation analysis. Because fissure-filled gold veins are dominant in this area, we evaluated the potential of autocorrelation analysis for investigating the shallow subsurface, including the ore deposits. Seismic interferometry is commonly performed based on the autocorrelation of ambient noise or natural earthquake signals; here, we instead used blasting in the mine because blast events include high-frequency signals that boost the spatial resolution of the imaging. To effectively extract P-wave reflections from seismic signals including blast events, we applied Gaussian smoothing and spectral whitening to remove source effects and then investigated the optimum frequency band. We successfully obtained auto-correlograms showing high-resolution seismic reflectors at shallow formation depths. These reflections are interpreted to be lithological boundaries shallower than 500 m. A comparison with profiles obtained from ambient noise and earthquake data showed that blasting signals yielded highly spatially consistent reflections that would not be achievable with natural or ambient seismic sources. This study highlights the potential of using blast autocorrelation seismic analysis during short survey periods. By using single-blast shots and dense seismic station spacings, we successfully achieved higher resolution 3D reflection images of lithological interfaces, possibly including ore veins.