A vibroseis sweep and a dispersive geological volume act to negate each other: the sweep delays the arrival of higher frequencies and the dispersive medium hurries them along. We derive a simple analysis procedure in which the time-frequency spectra of a programmed sweep and the uncorrelated measurement are compared in order to estimate the frequency-dependence of the phase velocity. The approach is developed with a synthetic example and applied to the a 3C VSP data set. Geophysically reasonable phase velocity estimates - though ones which must still be independently verified - are derived.
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