Offshore Angola remains one of the most promising yet technically challenging exploration environments in West Africa. While the Lower Congo Basin hosts a proven petroleum system, vast areas remain underexplored due to the difficulty of imaging beneath complex salt. For operators, the challenge is clear: maximize subsurface understanding across large deepwater frontier acreage without the cost and complexity of ocean bottom-based acquisition methods.

A recent multi-client program by TGS across Blocks 33, 49 and 50 demonstrated how a seamless design-to-imaging execution combining one-sided wide-azimuth (WAZ) vessel configuration, Gemini Enhanced Frequency Sources (EFS), multisensor GeoStreamer and Elastic Dynamic Matching FWI (E-DMFWI) improved results while remaining aligned with deepwater exploration constraints.

A Basin Shaped by Salt Tectonics

The Lower Congo Basin is defined by extensive salt tectonics. Over time, the salt has formed diapirs, sutures and large sheets that complicate both structure and seismic response. These features affect how seismic energy travels through the subsurface. Wave paths become distorted; illumination is uneven, and areas beneath salt overhangs can be difficult to image.

The post-salt section is dominated by thick deepwater turbidite systems, consisting of stacked channels and basin-floor fans with strong lateral variability. Reliable imaging is needed to map these features with confidence and to understand how reservoirs are connected. Below the salt, the stakes increase. Detailed salt geometry mapping is critical to the understanding of shallow mature traps and of the underexplored deeper post- and pre-salt petroleum systems.

Seismic Acquisition Design and Data Conditioning

To address these challenges, TGS designed a fully integrated solution that combines acquisition, processing and imaging into an optimized workflow. At the acquisition stage, a multi-vessel one-sided WAZ configuration was deployed using deep tow multisensor GeoStreamer and a total of six Gemini sources (Figure 1). This configuration enabled the acquisition of azimuth-rich data for improved subsurface illumination. The six Gemini sources played a critical role by delivering extended low-frequency energy essential for stable full-waveform inversion and signal penetration through the salt, along with consistent source signature, spectral output and bandwidth for imaging. The deep tow GeoStreamer solution provided co-located pressure and particle velocity measurements, improving signal-to-noise ratio and enabling accurate wavefield separation.

The optimized pre-conditioning of the data included machine learning-driven denoising, inversion-based deblending and 4D regularization. These processes reduced incoherent noise and artifacts while improving spatial consistency across offset and azimuth domains.

E-DMFWI: Leveraging the Full Wavefield

In complex salt environments, acoustic imaging methods often fail to capture the full physics of wave propagation. TGS’ E-DMFWI addresses this limitation by incorporating elastic effects in the propagator, including shear waves and mode conversions, which are critical in areas of strong velocity contrast. This data-driven process progressively refines salt geometries, carbonates and sediment structures through improved velocity model building. By incorporating well calibration and elastic relationships, the resulting models remain grounded in geological reality while capturing detailed subsurface complexity.

Importantly, the combination of broadband GeoStreamer data, Gemini EFS input and wide-azimuth illumination improved convergence and stability of the inversion, ensuring that the full wavefield could be effectively utilized.

Imaging Improvements Above and Below Salt

The integrated workflow delivered measurable improvements in both post-salt and subsalt imaging. In the post-salt section, broadband data and advanced pre-processing improved gather quality and frequency content, resulting in better reflector continuity and amplitude preservation. This supports clearer delineation of channel systems, levees and basin-floor fans, improving confidence in reservoir connectivity and distribution.

In the subsalt domain, the addition of WAZ data increased illumination beneath complex salt geometries. The low-frequency content from Gemini sources supported recovery of long-wavelength velocity trends, while iterative E-DMFWI refined salt and carbonate geometries. These updates reduced overburden uncertainty and improved depth positioning. The resulting images show improved reflector continuity and focusing at depth, with more reliable structural definition beneath salt (Figure 2).

A Practical Option for Complex Frontier Exploration

Ocean floor-based acquisition methods, such as OBN, can provide strong subsalt imaging due to long offsets and improved wavefield sampling, but they introduce high cost and logistical complexity. The combination of one-sided WAZ acquisition, GeoStreamer, Gemini EFS and E-DMFWI provides a cost-effective alternative that improves illumination, bandwidth and velocity model building within a streamer-based framework. This approach is well-suited to large frontier surveys where regional coverage and cost control are key constraints.

As exploration targets become more complex, integrating acquisition design, source characteristics and advanced imaging become increasingly important. Workflows combining broadband data acquisition, azimuth-rich coverage and elastic full-waveform inversion are expected to play a larger role in improving velocity model accuracy and subsalt imaging in frontier exploration.

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