Noise-based Ballistic Wave Passive Seismic Monitoring – Part 2: Surface Waves

We develop a new method to monitor and locate seismic velocity changes in the subsurface  using seismic noise interferometry. We use the ballistic Rayleigh waves computed from 30  days records on a dense nodal array located above the Groningen gas field (the Netherlands). We infer the daily relative phase velocity dispersion changes as a function of frequency and  propagation distance with a cross-wavelet transform processing. Assuming a one-dimensional velocity change within the medium, the induced ballistic Rayleigh wave phase shift exhibits a linear trend as a function of the propagation distance.Measuring this trend for the fundamental mode and the first overtone of the Rayleigh waves for frequencies between 0.5 and 1 Hz enables us to invert for shear-wave daily velocity changes in the first 1.5 km of the subsurface. Most of the changes are observed in a carbonate layer below 800 m depth. Combined with  P-wave velocity changes observations from a companion study (Brenguier et al. 2019), we interpret the changes as caused by slight water saturation variations induced by deep fluid flows.