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. Values picked at circles inverted below.
Velocities are apparent velocities, computed along a straight array.
We pick along the lower edge of the trends to get closer to the true velocity. Energy concentrates near the true velocity due to the non-linearity of the cosine azimuth factor. Noise record dispersion picks and synthetic dispersion at Reno Airport. We compute synthetic dispersion using a code of Saito modified by Yuehua Zeng. Shallow velocity model yielding synthetic dispersion above.