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Surface-wave Inversion Limitations from Laser-Doppler Physical Modeling

¹ Laboratoire Central des Ponts et Chaussées, Reconnaissance et Géophysique, Nantes, France Email: ludovic.bodet{at}lcpc.fr, odile.abraham{at}lcpc.fr, philippe.cote{at}lcpc.fr
² Physical Acoustics Laboratory, Department of Geophysics, Colorado School of Mines, Golden, CO, USA Email: kasper{at}acoustics.mines.edu
³ Bureau de Recherches Géologiques et Minières, Aménagement et Risques Naturels, Orléans, France Email: a.bitri{at}brgm.fr, g.grandjean{at}brgm.fr
⁴ Institut de Physique du Globe, Géomatériaux - Hautes Pressions, Paris, France Email: leparoux{at}ipgp.jussieu.fr

Surface-wave dispersion inversion is growing in popularity for geotechnical applications, due to its noninvasive character, relative straightforward field procedures and interpretation, especially when the subsurface structure is locally assumed to be one-dimensional (1D). Here, laser-Doppler physical modeling of surface-wave propagation is used to address issues of surface-wave depth penetration, the presence of dipping layers, and the associated limitations and systematic errors propagated in conventional 1D surface-wave inversion. Flat-layered models show that, with an active source and linear spread, the maximum resolvable wavelength of the Rayleigh-wave fundamental mode is on the order of 40% of the spread length. Linearised inversions confirm the rule of thumb that the depth penetration is 20–25% of the spread length, and that correct a priori layer interface depths from refraction analysis allow more accurate results. However, even under optimal conditions, failing to account for a dominant higher mode at low frequency when a stiff shallow layer is present, causes an overestimate of deeper layer shear-wave velocity. Moreover, a layer dip of only a few degrees can significantly bias the surface-wave inversion. If the incorrect a priori information from a single-shot refraction analysis is incorporated in the inverse problem, estimated interface depth depends on the shot position and deeper layer shear-wave velocity is underestimated. Even if correct a priori constraints are used, an underestimate of half-space shear-wave velocity of up to 25% remains.