Seismic surface wave focal spot imaging: numerical resolution experiments

Bruno Giammarinaro (1) , Christina Tsarsitalidou (1) , Gregor Hillers (1) , Julien de Rosny (2) , Léonard Seydoux (3) , Stefan Catheline (4) , Michel Campillo (5) , Philippe Roux (5)

1 Helsingin yliopisto = Helsingfors universitet = University of Helsinki
2 IL - Institut Langevin - Ondes et Images (UMR7587)
3 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
4 INSERM - Institut National de la Santé et de la Recherche Médicale
5 ISTerre - Institut des Sciences de la Terre

Numerical experiments of seismic wave propagation in a laterally homogeneous layered medium explore subsurface imaging at subwavelength distances for dense seismic arrays. We choose a time-reversal approach to simulate fundamental mode Rayleigh surface wavefields that are equivalent to the cross-correlation results of three-component ambient seismic field records. We demonstrate that the synthesized 2-D spatial autocorrelation fields in the time domain support local or so-called focal spot imaging. Systematic tests involving clean isotropic surface wavefields but also interfering body wave components and anisotropic incidence assess the accuracy of the phase velocity and dispersion estimates obtained from focal spot properties. The results suggest that data collected within half a wavelength around the origin is usually sufficient to constrain the used Bessel functions models. Generally, the cleaner the surface wavefield the smaller the fitting distances that can be used to accurately estimate the local Rayleigh wave speed. Using models based on isotropic surface wave propagation we find that phase velocity estimates from vertical-radial component data are less biased by P-wave energy compared to estimates obtained from vertical-vertical component data, that even strong anisotropic surface wave incidence yields phase velocity estimates with an accuracy of 1 per cent or better, and that dispersion can be studied in the presence of noise. Estimates using a model to resolve potential medium anisotropy are significantly biased by anisotropic surface wave incidence. The overall accurate results obtained from near-field measurements using isotropic medium assumptions imply that dense array seismic Rayleigh wave focal spot imaging can increase the depth sensitivity compared to ambient noise surface wave tomography. The analogy to elastography focal spot medical imaging implies that a high station density and clean surface wavefields support subwavelength resolution of lateral medium variations.

Mots clés: Numerical modelling; Body waves; Seismic interferometry; Seismic noise; Surface waves and free oscillations; Wave propagation; Refocusing; Focal spot

Domaines: Planète et Univers [physics]

Bruno Giammarinaro, Christina Tsarsitalidou, Gregor Hillers, Julien de Rosny, Léonard Seydoux, et al.. Seismic surface wave focal spot imaging: numerical resolution experiments. Geophysical Journal International, 2023, 232, pp.201-222. ⟨10.1093/gji/ggac247⟩. ⟨insu-03839231⟩