Surface-wave imaging of inversely dispersive media: A permafrost example

Surface waves are effective in identifying inversely dispersive media, but because dispersion-curve retrieval is susceptible to ambiguities, commonly-used inversion methods often become inapplicable. Here we highlight a full-wavefield approach that uses dispersion spectra instead of dispersion curves to invert for shear-wave velocity (V_S) structures. With a case study of permafrost imaging, we demonstrate that this method is able to achieve optimal data fits without requiring good starting models. This makes the spectrum-fitting inversion superior to waveform-fitting approaches when little prior knowledge is available. This study also takes advantage of particle swarm optimization (PSO), a derivative-free global search method that is suitable for but not yet widely used in solving nonlinear inverse problems in geophysics.