Reservoir depth map is often produced by the time structure map and vertical velocity estimated based on seismic(stacking)velocity analysis.Although seismic velocity is generally the only available velocity information away from well locations,it may not match with the velocity estimated at well locations.VTI (transverse isotropy with vertical symmetry axis)is widely recognized as a typical source of difference between the seismic and vertical velocities and hence the misties in time-to-depth conversion.However, small-scale lateral heterogeneity(LH),often associated with velocity lenses such as channels in the overburden,can cause more significant errors in both time-and depth-domain velocity analyses.I discuss the influence of a thin laterally heterogeneous layer on the normal moveout(NMO)velocity for stratified media using an analytic description.The analytic result shows that distortion of the seiemic velocity is caused primarily by lateral velocity curvature(i.e.,by lateral velocity variation with the scale smaller than the maximum offset).The heterogeneity-related distortion increases with the maginitude of velocity contrast and with the target depth.Furthermore,application of the Dix differentiation(used for conversion from NMO velocity to interval velocity)can significantly amplify the lens-induced distortion.The analytic results also indicate that small velocity errors can increase with depth in migration velocity analysis as well.A field example shows that lens-induced 20-30 ms time distortion(pull-up or push-down anomalies)leads to a few hundred m/s errors in the seismic velocity,which are close to the results estimated by the analytic method.The presented analytic approach should help quick evaluation of the seismic velocity analysis and prevent from significant errors in reservoir depth estimation.
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