3D numerical simulation of ambient seismic noise for arbitrary surface source distributions: application to the study of horizontal-to-vertical spectral ratios and their variability

Summary We present a comprehensive numerical investigation of the sensitivity of horizontaltovertical spectral ratio (H/V) measurements of ambient seismic noise to the spatial distribution of noise sources and to strong lateral structural contrasts. Our approach relies on the computation of realistic 3D viscoelastic Green’s functions between a dense set of potential surface sources and a few surface receivers, allowing the synthesis of ambient noise wavefields corresponding to arbitrary spatiotemporal source distributions. The study focuses on a simplified 3D representation of the Mygdonian Basin (northern Greece), where significant temporal variations of H/V amplitude, polarisation, and peak frequency have been observed at a site located near a blind fault. Comparisons between the synthetic H/V and the predictions of the elastic Diffuse Field Theory (DFT) show good agreement under isotropic source conditions, confirming the usefulness of DFT as a reference framework in diffuse or quasidiffuse regimes. However, attenuation, lateral heterogeneities, and uneven source distributions lead to substantial deviations from DFT predictions. We find that sourcerelated effects can dominate both amplitude and polarisation patterns, sometimes masking the structural signature of the fault. Nevertheless, the simulations reveal robust spatial trends of H/V polarisation relative to the fault geometry, suggesting that polarisation mapping may help identify lateral heterogeneities when combined with dense, longduration measurements. Our framework provides a physically consistent way to test the limits of the diffuse field assumption and to explore the applicability of ambient noise–based observables in complex geological environments.