VicGCS Report 9 - A new regional velocity model for the Gippsland Basin
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Product description:Executive Summary A new 3D velocity volume has been constructed across the Gippsland Basin to enable regional scale depth conversion of seismic time interpretations. Final pre-stack time migration (PSTM) velocities from eighteen seismic surveys (fourteen 3D and four 2D surveys) were used to build the velocity volume. The 3D velocity model covers a region of 210km x 150km and a vertical extent of 5500ms. Each cell has dimensions of 500m x 500m x 10ms. Check-shot / Vertical Seismic Profiles (VSP) velocity data was also used to constrain and guide the velocity distribution throughout the 3D grid. Two hundred and sixty-three wells were used totalling around 14,000 input time – velocity data points. Eight regional seismic surfaces interpreted by Keetley et al. (2010) and Blevin et al. (2013) were used to sub-divide the seismic velocities into discrete intervals and guide the distribution of the velocities along stratigraphic surfaces interpolated between the mapped horizons. This process preserved the integrity of the geology during the gridding process and allowed better velocity estimation within poorly sampled 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 the stratigraphy (i.e. bounding seismic surfaces for the interval). 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. This initial PSTM velocity model was then constrained to tie the well check-shot and VSP information by co-kriging primary data (check-shot/VSP velocities) alongside secondary data (initial PSTM velocities) using a locally varying mean approach. The well calibrated 3D velocity model has applications not only for depth estimation but also for calculating layer thickness from interval velocities, seismic inversion, initial model building for pre-stack depth migration (PSDM), density estimation, fluid overpressure analysis, assessing compaction/porosity and burial histories. Download The downloadable version of this report is supplied in PDF format (9.5MB). Bibliographic Reference McLEAN, M.A., & BLACKBURN, G.J., 2013. A new regional velocity model for the Gippsland Basin. VicGCS Report 9, Department of Primary Industries. Related products:
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