Estimation of Dispersion Curves for Rayleigh Waves Complicated by Multipath Effects

Dispersed surface waves that arrive at a seismograph by traveling other than a great-circle path, or have been delayed by reflection along the path, interfere with the first arriving great circle waves. The problem of finding the correct group velocity dispersion in the presence of such multipath interference can be simplified by the use of phase equalization filters based on assumed dispersion curves. Group velocity dispersion curves calculated using zero crossing, multiple-filter, or moving-window methods can be perturbed slightly with each perturbation yielding a different phase equalization filter. Cross-correlation of the chirp implied by the filter with the dispersed wave train produces a time function whose real spectrum can be computed. The process is repeated with different trial chirps until the integral of the real part of the spectrum is maximized. The associated dispersion curve is then considered to be a best estimate of the true curve. Application of the technique to synthetic seismograms has shown a resolution significantly greater than existing methods when multipath effects are present. The use of this method is demonstrated with teleseismic Rayleigh waves recorded at LASA. Author