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We demonstrate a novel workflow using reflections, refractions and multiples for building highly accurate PSDM velocity models for a complex geological setting. By combining wavelet shift tomography, full waveform inversion and separated wavefield imaging, we are able to produce high-resolution velocity models that are ideally suited for imaging of broadband data. Leveraging dual sensor streamer technology and the wavefield separation that comes with it, we are using up-and down-going wavefields in imaging and tomography to improve resolution and illumination. Further, we utilize the refracted, low-frequency energy for FWI. As the streamer is towed deep, we preserve the low frequencies that are so important for the success of FWI, but without sacrificing a broadband signal that is key for producing high-resolution reflection images of the shallow overburden and deep reservoir sections.

A case study was carried out over the Viking Graben area with special focus on the Volund field, where a high fidelity velocity model was built to solve the complex velocity variations in the overburden resulting from a widespread presence of channels and numerous small-scale sand injectites. Significant improvements to the image quality were achieved with the use of the detailed velocity model and by incorporating a variable quality field to compensate for visco-elastic effects. Alternative imaging algorithms and reflectivity inversion techniques were explored to overcome limitations of the imaging system. This further improved the spatial resolution, enhanced the signal and helped the interpretation of the remobilized sand system over the Volund field.

Successful application of shallow water full-waveform inversion (FWI) requires a good starting model, the right data, and an optimal algorithm. Our implementation is based on a pseudo-analytic (PA) method that solves the two-way acoustic wave equation in VTI media. The non-linear inversion uses a regularization operator that combines smoothing and wavenumber filtering in order to minimize the shallow water acquisition footprint. We show, using a 3D dual sensors streamer survey from the North Sea, that the inversion of refracted and diving waves yields a high-resolution velocity model for the shallow sediments. The small-scale velocity variations obtained by FWI in this area correlate very well with the geological features in the migrated images. The FWI model yields better migrated images than the starting model from reflection tomography. Examining both the flatness of common-image gathers and the match between modelled and recorded data corroborates the accuracy of the FWI velocity model.