Model-based interval velocity analysis

Abstract

A velocity analysis process, SIVA, is described which combines the information in CMP gathered trace data with interpreted seismic data (e.g. time maps) to create a model of structure and P wave interval velocity. The technique works one depth interval at a time, in a top down fashion. Raytracing is used to estimate the shape of the reflecting surface and determine a family of non-hyperbolic moveout trajectories associated with a range of trial interval velocities. The trace data are stacked along the computed moveout curves, and semblance techniques are used to determine the best-fit velocity for the interval being analyzed. The analysis is performed for a number of gathers to yield a spatially varying interval velocity field. This field is then used to re-determine the shape and depth of the reflector, the model is updated with the velocity field and structural information, and analysis continues to the next lower interpreted horizon. The resulting depth/velocity model is fully consistent with all the information in the seismic trace data, as well as the interpreted time picks, and is fully compensated for overburden structure as well as structure and reflection-point smear on the reflector. SIVA analysis may be performed to produce a 2D model from a single line, or to create a 3D model for input data from multiple crossing lines or a 3D survey.