Data acquired using Walkaway VSP techniques are used to image subsurface structure. The migration techniques involved require some prior knowledge of the structure and velocity distribution to generate a background model. Although VSP surveys provide accurate interval velocities along the borehole we cannot always assume to have a good estimate of layer velocities away from the well, even in relatively smoothly varying structure. Velocity inaccuracies can arise in the presence of transverse isotropy, laterally varying velocities or incorrect estimates of overburden dip. Velocity perturbations are applied to a background model in order to examine the sensitivity of the imaging of a fault structure. Using a case study we show that inaccurate estimates of velocities produce significant time shifts but do not have much effect on the lateral positioning of a fault edge.


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