Time Preserving Tomography - Poststack Tomographic Depthing for Systematically Improving Misties
Featured Technology: Explorer™
One of the common challenges in time-to-depth conversion is the existence of depth misties in scaled seismic depth horizons and well markers. The existing procedures for handling this issue are often based on local manipulation of the velocity model (i.e. vertical adjustment only around well areas). These prove inefficient when propagating the adjustment layer by layer. However, it is critically important to convert the time domain seismic data into depth while maintaining the structural integrity.
Time Preserving Tomography (TPT) is an alternative depthing tool that updates the subsurface velocity model in a more accurate, efficient and flexible way. TPT makes use of the concepts embodied in seismic reflection tomography, in which the velocity is updated for all layers at once, and takes into account the normal incidence rays to achieve accuracy, rather than an empirical vertical stretch/squeeze methodology. Both velocity and anisotropy (VTI & TTI) can be updated, and the flatness of the original depth gathers are preserved. The value of this methodology for the interpreter is that the workflow functions on the poststack seismic velocity volume do not need prestack gathers as input, but still maintain the benefits and flexibilities contained within seismic reflection tomography.
This tomographic-depthing approach is an excellent solution for interpreters carrying out final depth adjustments of horizon maps and PSDM volumes to improve seismic-to-well misties. Examples of mistie correction for different scenarios will be presented, together with a discussion of how to use TPT to correctly handle common pull-down/up issues in the presence of shallow gas and near-surface channels.
|(Top left) Original, unreconciled velocity, interpretation and markers. (Top right) Updated velocity and interpretation after application of TPT shows much better formational constraint and tie between different data types. (Bottom) Before and after “Horizon 6” mistie maps showing improvement.