Technical Paper: Velocity profile prediction using walkaway VSP data

Society: SEG
Paper Number: 2370259
Presentation Date: 2006


A velocity estimation procedure is described for walkaway VSP data. Ray tracing through a heterogeneous 2D model is iterated to optimize a functional based on the coherency along predicted moveout curves for reflected P events. The model is held fixed down to some chosen boundary at or below the receiver array while the velocities in conformable layers are allowed to vary below the boundary. This boundary could, for example, be the base of salt in a deep water offshore borehole seismic survey. Travel time inversion of direct travel times ensures that the overburden model has been calibrated, leaving the velocity profile in the layers below the boundary (e.g. base of salt) free. We use conventional travel time tomography to solve for velocity updates but map coherency into travel time so that an anti-coherency functional is minimized rather than residuals with respect to picked reflection times.This velocity inversion technique is aimed at the important problem of predicting velocities ahead of an intermediate drilling depth for input to pore pressure and fracture gradient models or for migration.

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