oa 2D-Macro-Velocity/Depth Model Building Through Multi-Offset Reflection Time Inversion

For pre-stack depth migration, a macro-velocity/depth model is required which reflects the general subsurface structure and contains the velocity distribution which correctly predicts wave propagation times. Such a macro- model can be determined by non-linear inversion of multi-offset reflection times belonging to seismic horizons picked on CDP gathers along a seismic line. In the inversion scheme, in a 2D earth model, the depths of reflecting interfaces and the laterally and vertically varying velocities within the layer above the interface are solved simultaneously.<br>Provided accurate multi-offset reflection times, two dimensional velocity anomalies in<br>single-layers in the shallow section (i.e. in the range 0-2000 m depending on the maximum offset available in seismic data) can be resolved. In the optimization of depth models containing macro-layers with two dimensional velocity distribution, taking the layer boundaries and vertical velocity trends from available sonic log will help in the inversion process in achieving a globally optimum solution. If a sonic log is not available, it is necessary to divide the macro-layers into thinner macro-layers with lateral velocity changes only in order to solve the velocity variation with depth.