The central North Sea presents a number of difficulties for velocity model building and seismic imaging. In relatively shallow waters, acquisition geometries typically used for 3D seismic exploration leave little scope for reflection tomography to update the near surface velocity. Shallow gas pockets complicate both velocity estimation and imaging at depth, and anisotropy is difficult to estimate or validate without near-surface well control. Under these conditions Full Waveform Inversion (FWI) provides an option for velocity estimation. We present a 3-parameter workflow addressing velocity, absorption factor (Q), and epsilon. This is achieved with double application of 2-parameter joint FWI, solving first for Vp/Q then Vp/epsilon. This improves the imaging and positioning of complex injectite structures around 2.1 km depth. Beyond the needs of seismic imaging, we also seek an increased level of detail in the final velocity model for use as the low-frequency contribution to impedance inversion. This is achieved by altering the FWI to balance the contributions of reflections and refractions to the global gradient and cost function. By carefully increasing the weight of reflections, the level of velocity detail is increased at injectite level whilst maintaining the stability of the inversion and velocity estimate brought by refractions.


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