Regional velocity modelling methodology in the Gippsland Basin

Abstract

A new velocity volume has been constructed across the Gippsland Basin to enable regional scale depth conversion of seismic interpretations. The velocity model covers a region 210km x 150km and a vertical extent of 5500ms, and has cell dimensions of 500m x 500m x 10ms. Average stacking velocities from 17 seismic surveys (14 3D surveys and 3 2D) were used to inform the 3D velocity volume. These datasets collectively totalled ~12.2 million separate velocity points. 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 8 separate intervals using time horizons derived from seismic interpretation. This provided a typical distribution of velocities 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, and determine how far velocities can be statistically interpolated. This process produced a search ellipse which facilitated distribution of the data. The ellipse was UVW transformed with the seismic time surfaces so that the geometry of the ellipse was aligned to stratigraphy. Velocities were therefore distributed along stratigraphic horizons preserving the 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 into the velocity volume. This was done 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, assessing compaction / porosity and burial history. The model will be freely available on the Department of Primary Industries' online store.