PGS Access Westralia: PrSDM Velocity Model Building
The PGS Access Westralia Study provides a unique perspective on the geological evolution of the Westralian Superbasin by providing accurate PSDM images for 11 776 km across the North West Shelf.
Unlike PSTM (time imaging), PSDM provides un-distorted structural images by compensating for the varying travel times associated with natural velocity variations. The PSDM velocity models have also been calibrated to a regional grid of well data to ensure geological accuracy and consistency.
Figure 1: Structure in TWT, and Depth
The challenges facing a successful regional 2D PSDM project come predominantly from recorded out-of-plane energy and apparent azimuthal velocity variations. PSDM velocity model building is dependent on residual move-out information picked on PSDM gathers which are then used to improve the velocity model through successive tomographic velocity model updates. When the sea floor is rugose, or when subsurface geology is complex, a significant amount of reflected energy (primary and multiple) may emanate from outside of the 2D plane. The residual move-out characteristics of this energy are different to the in-plane data, which can cause spurious results during tomographic velocity updating.
The inclusion of anisotropy in a PSDM velocity model provides the ability to adjust the background velocity to match the orientation and velocity profile of the underlying geological structures. 2D PSDM velocity model building is limited to VTI anisotropic solutions because of the inability to derive 3D dip and ray-path information. Velocity variation based on directivity is not captured in the VTI model, so optimal imaging velocities will generally differ between lines shot in different directions. Without careful handling of the velocity models, this may lead to line tie challenges and the inability to produce a coherent regional velocity model.
The PGS Access Westralia PSDM velocity model building strategy addressed the challenges described above by using a relatively simple workflow that incorporated regional geological interpretations and a regional grid of well data. For the initial PSDM velocity models, the PSTM stacking velocities (time-RMS) were converted to depth-interval velocities, and then careful smoothing was applied to remove anomalies while preserving the correlation to geology. Two tomographic velocity model updates were then applied, focusing on medium-to-long wavelength solutions for the top 4 – 6 km of data. The tomography was run using conservative parameterization that provided significant improvement to the velocity models while maintaining stable results. Where possible, the PSDM velocity models were verified against velocity data obtained from proximal well logs.Figure 2: PSDM Velocity QCTo ensure a regionally consistent and geologically accurate velocity model throughout the deep section, a velocity flood derived from regional interpretations and existing velocities (seismic and well data) was incorporated into the modelling. VTI anisotropy was also introduced at this stage, with parameterisation being derived from seismic-to-well ties, local knowledge of 3D PSDM models in the area, and confirmation tests throughout the region.
Additional tomographic velocity model updating targeting the mid-to-deep section was then performed using PGS Beam “wide dipscan” data. This provided PSDM gathers with more coherent deep events and better signal-to-noise properties, allowing the tomographic velocity model update approach to be extended to greater depths than may otherwise have been possible. For the PGS Access Westralia Study, the tomographic velocity model update applied to the deep data reintroduced the medium-to-large wavelength velocity variations associated with the deep structure, but with more stability than in earlier models.
Figure 3: CDP Gathers - PGS Beam, and PGS Beam “wide dipscan”
Several different migration algorithms were utilized for final VTI anisotropic migration including Kirchhoff, RTM and PGS Beam migration. All of the PGS Access Westralia PSDM products show superior imaging compared to the legacy processing and are suitable for regional interpretation and evaluation, but the individual migration algorithms also provide additional and unique insight into some of the more subtle structural elements. Advantages of the algorithms include:
- Kirchhoff migration – high-frequency imaging using a robust and well-understood algorithm
- RTM – more accurate structural imaging beneath the complex areas of overburden
- PGS Beam migration – enhanced coherency and reduced background noise
Figure 4: NWS07-10 Legacy processing, PGS Beam Migration
The last stage of the PSDM processing was the final line tying which used a regional vertical velocity model to apply residual depth corrections. Line ties and well ties were already fairly accurate due to the robust velocity model update approach and the vertical velocity component already included in the VTI velocity models. To generate the regional vertical velocity model, the individual velocity models were tied at line intersections using spatially and depth varying averaging techniques. The Base Tertiary and Callovian Unconformity were then reinterpreted and used for the final well calibration as the main reference horizons. These horizons were chosen as they could be mapped across the entire region and provided suitable layer thicknesses for applying the calibration corrections. The final vertical velocity model was then used to recalibrate the PSDM datasets, resulting in a regionally consistent grid of high-quality data that is truly representative of the subsurface.
By paying careful attention to velocity model building methodology and application, PGS has created a detailed and comprehensive image of the Westralian Superbasin, spanning the Northern Carnarvon, Offshore Canning, Browse and Bonaparte Basins. The accurate depth images provide enhanced structural understanding, and will lead to new insights in structural formation and prospectivity of the North West Shelf.