The propensity of geological volumes, particularly in the near surface, to cause seismic body waves to propagate dispersively is a serious impediment to FWI. Anelastic parameters, like Q P and Q S , can appear as unknowns in tractable seismic inverse algorithms, provided the associated dispersion laws are known. However, general attenuation factors with arbitrary variability in space and frequency cannot be solved for. If a FWI scheme solves for elastic and anelastic properties given an accurate prior knowledge of the dispersion law, we will require a flexible and unconstrained procedure for determining this law; one which occurs relatively early in the processing flow. An example of such a procedure is illustrated in this paper, based on picked arrival times from the Gabor spectra of uncorrelated vibe sweeps measured in a walkaway VSP experiment. Estimating phase velocity as the ratio of the source/receiver ray path length to the departure/arrival time difference of each frequency in the sweep, a remarkably consistent set of V P (f) estimates are derived for 20 depth levels between 76m and 116m depth. Calibrating these to match group velocities measure with standard time domain slopes, a range between roughly 500m/s and 2000m/s, from 10Hz to 70Hz is calculated. The curves are sigmoidal, with a negative curvature that does not match with standard logarithmic models.
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