Angola Imaging

Revealing Shallow Geology in Shallow Waters of the Kwanza Shelf with Enhanced Processing Flows

To optimize seismic data quality and accuracy over target areas, TGS’ data library utilizes high-end imaging solutions to enhance the resolution of the Mesozoic sedimentary architecture.

On the Kwanza Shelf, shallow water environments present an imaging challenge that can be resolved by enhanced processing flows. Multisensor recording enables high-resolution velocity model building and depth imaging using reflections, refractions, and multiples. 

As recorded by multi-sensor streamers, primary reflections are used to resolve the long-wavelength velocity trend and shallow gradients using reflection tomography. The resulting velocity model is then used by Full Waveform Inversion (FWI) for further refinement. TGS FWI uses the full wavefield to overcome the restrictions posed by conventional reflection tomography and, as such, can resolve small-scale velocity features successfully.

The rich low frequencies of multi-sensor recordings help mitigate cycle-skipping in FWI, enabling a high-resolution velocity model to be built using refractions and reflections. 

Due to the lack of near offsets and angular illumination in shallow water areas, conventional depth-migrated gathers and stacks are unsuitable for validating the shallow overburden model provided by FWI. SWIM creates high-resolution stack images and well-sampled angle gathers using sea-surface reflections to complement primary reflection imaging. By using the separated up-going and down-going wavefields, uniquely recorded by multi-sensor streamers, SWIM provides enhanced subsurface illumination and improved angular coverage for shallow data.

Tackling Post-Salt Sediments with Innovative Velocity Model Building

TGS’ wavelet shift tomography (hyperTomo) is used with FWI to optimize pre-salt velocity model building. This results in improved imaging and a greater understanding of geological complexity in areas such as Block 16. Wavelet shift tomography enables quick cycle velocity updates using wavelet attributes and semi-automated inversion loops. TGS’ FWI algorithm inverts for the full wavefield, using refractions and reflections to update the velocity model. TGS has devised a modified Inverse Scattering Imaging Condition (ISIC) for FWI. It is based upon the ISIC in TGS RTM, and the migration isochron from the velocity kernel was removed. This prevents reflectivity leakage in the velocity model when updating sing deep reflections. FWI testing completed in parallel to hyperTomo updates allows for a seamless transition in the model-building workflow.

Salt interpretation for velocity model building can be time-consuming and costly. In areas such as Angola, this involves multiple RTMs and periods for interpretation. Adding cross-departmental expertise and semi-automation of auxiliary information helps reduce the project turnaround.

The final RTM using TGS’ ISIC excludes the backscatter energy, producing an image using only the migration isochron. This removes low wavenumber noise that often contaminates RTM images, minimizing post-processing and reducing the project turnaround. RTM is not affected by ray-based limitations, so top and base salt imaging is clearer, and uncertainty in the underlying structure is reduced.