Numerical Modeling of Surface Waves Over Shallow Cavities
Résumé
A new stable and accurate, finite-difference 2D elastic seismic wave propagation modeling algorithm is developed, with a Perfectly Matched Layer (PML) absorbing boundary condition to avoid reflections from the edges of the numerical model. This is used to study the interaction of seismic surface waves with near-surface heterogeneities. The effects of different empty cavity shapes and depths and altered zones is evaluated from the direct and diffracted seismograms and corresponding Rayleigh wave dispersion images. Differential seismograms calculated for models both with and without a cavity show a complex, non-symmetrical, diffraction pattern and their dispersion images show welllocalized scattering from heterogeneities. Missing coherent energy appears in specific frequency bands, related to the cavity depth, shape and degree of modification of the surrounding medium. The cavity more strongly affects seismograms when it is shallower. A 2-m deep rectangular cavity generates more severe perturbations than a circular section, due to non-symmetrical backscattered patterns recognized in dispersion images. Furthermore, low velocity zones around and above the cavity, due to soil alteration, trap waves and lead to increased ground roll attenuation and strong footprints in dispersion images that can possibly mask the cavity signature. If the altered zone extends to the surface as a cone, trapping phenomena completely dominate seismograms. The detection of altered zones is a very useful indication in natural hazard assessment, making it possible to distinguish the signature of a 'safe' cavity from a potentially dangerous one.