Building Regionally Consistent 3D Velocity Models: A Case Study from NW Shelf, Australia

A regionally consistent seismic velocity model has been built over 400,000 km² of the North West Shelf, Australia, using 49 3D surveys, 83 2D surveys, 128 well calibrations and horizons.

Within and between 2D surveys, velocities were levelled to correct for mismatches by applying time-varying scaling functions calculated at intersections and interpolated in a structurally consistent manner. For 3D surveys, an average time varying scaling function calculated from overlapping regions was applied, followed by smooth blending of velocities across the overlap.

The levelled regional velocity model was calibrated to well data using geostatistical trend scaling to reduce depth conversion errors. Errors were calculated using the levelled velocities and well time-depth pairs, distributed in 3D using geostatistics and used to scale the velocity model. The geostatistical constraints comprised geologically meaningful trend terms that influence velocity. The trend scaled model removed a depth under prediction and narrowed the range of depth errors.

The regionally consistent 3D velocity model honours geological and geophysical constraints, smoothly blends across overlapping 2D and 3D surveys, reduces depth conversion errors, decreases uncertainties in quantitative interpretation and is easily updatable. The velocity model provides reliable starting points for building depth models for tomographic analysis and full-waveform inversion.