No phase distortion. Given many noise seismograms at relatively close spacing, the following method, originally by G. A. McMechan, allows indentification and picking of Rayleigh-wave phase velocity spectra. Low-pass filter records to maximum frequency of interest. Identify and pick trends of high ratio that show normal-mode dispersion by sloping to lower velocities at higher frequencies. Spectral-ratio plots computed as above from the airport noise records.
Warmer colors are higher spectral ratios.
The vertical axis is slowness increasing linearly down, so velocity increases non-linearly upwards as noted. Frequency increases linearly to the right. Slopes down to the left are record truncation, aliasing, and slantstack artifacts. Any spatial Fourier analysis would be aliased below the white dotted lines. The slantstack can follow velocity trends, with increasing error, into the aliased area, because more than two traces are used and surface waves arrive in groups and are not continuously harmonic.