Non-physical head wave identification in <i>P</i> -wave arrivals retrieved from seismic ambient noise at the Chémery underground gas storage (France)

SUMMARY This study aims to retrieve P waves from seismic ambient noise recorded by a dense local broad-band network at the Chémery underground gas storage site, where anticline deformation was previously identified through wells and seismic reflection survey. To this end, we propose a new approach for reconstructing P waves from ambient noise. We process the passive seismic data to reconstruct the body wave component of the empirical Green’s functions. The retrieved P-wave arrivals were identified and analysed, revealing that in this data set, the picked arrival times likely correspond to non-physical head waves rather than direct or diving P-wave arrivals between virtual sources and receivers. Numerical simulations support this idea of non-unique interpretation of the passively reconstructed P-wave arrivals. The simulations suggest the potential for mapping lateral heterogeneities above the critical refractor as a cumulative time-delay, although for this data set the anticline-induced time-delay is likely within the measurement uncertainties. It is found that the dominance of non-physical head waves over diving waves is primarily due to the large distance from the network to ambient noise sources.