Extraction of phase and group velocities from ambient surface noise in a patch-array configuration

We have investigated the use of ambient-noise data to extract phase and group velocities from surface-noise sources in a microseismic monitoring context. The data were continuously recorded on 44 patch arrays with an interpatch distance on the order of 1 km. Typically, a patch-array design consists of a few tens of patches, each containing 48 strings of 12 single-vertical-component geophones densely distributed within the patch area. The specificity of the patch-array design allows seismic analysis at two different scales. Within each patch, highly coherent signals at small distances provide phase information at high frequency (up to 10 Hz), from which surface-wave phase velocities can be extracted. Between the pairs of patches, surface-wave group-velocity maps can be built using correctly identified and localized surface-noise sources. The technique can be generalized to every patch pair using different noise sources identified at the surface. We note that the incoherent but localized noise sources accelerate the convergence of the noise-correlation functions. This opens the route to passive seismic monitoring of the near surface from repetitive inversion of phase-and group-velocity maps.