The application of reflection tomography and interval velocity analysis to achieve accurate depth conversion of subtle structures: a case study

Reflection tomography and interval velocity analysis (IVA) is applied to a 2D seismic line in the Surat Basin - an area where the primary goal is achieving reliable depth conversion of low relief structures, in the presence of near surface velocity anomalies and subtle lateral velocity gradients.

The technique, using commercially available velocity analysis and Pre-Stack-Depth-Migration (PSDM) software, represents a potential solution to problems which have challenged previous workers. We demonstrate reflection tomography can resolve shallow velocity anomalies by using as input the moveout errors of deeper horizons from an initial pass of PSDM. Significantly, reflection energy from the near-surface layer is not required to resolve its velocity. When the shallow velocity model is determined, the interval velocity analysis of subsequent layers becomes stable and a more refined velocity model may be constructed. Several iterations of PSDM and reflection tomography are generally required before running a final pass of PSDM. The example seismic line contains three wells which are used to judge the quality of the final depth migration.

The results demonstrate that it is possible to extract accurate lateral velocity information inherent in high fold CMP gathers from land data containing shallow velocity anomalies or residual static errors. A lateral velocity gradient causing an apparent pull up of 26 metres between Wells B and C is successfully resolved. The example demonstrates the predicted formation tops of Well C would have been accurate to around +/- 5m if reflection tomography and interval velocity analysis had been applied in place of a regional velocity function derived from nearby well data.