oa Regional velocity modelling methodology in the Gippsland Basin

A new velocity volume has been constructed across the Gippsland Basin to enable regional scale depth conversion of seismic interpretations. Average stacking velocities from seventeen seismic surveys (fourteen 3D and three 2D surveys) were used to build the 3D velocity volume. Check-shot velocity data was also used to constrain and guide the velocity distribution throughout the 3D grid. A total of 263 wells were used totalling ~14000 data points.

Seismic stacking velocities were broken up into eight separate intervals using time horizons derived from seismic interpretations. This provided a typical distribution of velocities from which to sample from during simulation in data-poor areas. A semi-variogram analysis was performed for each velocity interval to characterise the spatial variation of the velocity data. This process produced a search ellipse which facilitated distribution of the data. The ellipse was transformed to align with the seismic time surfaces so that the geometry of the ellipse was distorted to the same shape as stratigraphy (i.e. surfaces). Velocities were therefore distributed along stratigraphic horizons preserving geological integrity during the gridding process. Velocities were kriged close to data points, and simulated away from data points. The final step was to integrate check-shot velocities by kriging primary data (check-shot velocities) alongside secondary data (stacking velocities) using a locally varying mean approach.

This velocity model has applications not only for depth estimation, but also calculating layer thickness from interval velocities, density estimation, fluid overpressure analysis, assessing compaction / porosity and burial history.