Practical Multi-Parameter FWI at the South Arne Field

The South Arne field, in the Danish North Sea, presents a velocity-model building challenge with a large gas cloud obscuring underlying chalk reservoirs in a structural anticline. In the near surface, lateral and vertical velocity (Vp) variations are significant, with a thin, shallow layer of strong anisotropy occupying an otherwise isotropic overburden. Because of the near-surface complexity, and strong attenuation in the gas cloud, the velocity model was built using multi-parameter FWI to solve for Vp, Q, and epsilon. Using surface seismic data, 3-parameter FWI is a challenging problem, and, for practical reasons, this was done with two passes of 2-parameter joint FWI: first Vp/epsilon, then Vp/Q. The inversions were run with combined OBN and towed-streamer input data, using a shot weighting strategy to balance the contributions to the FWI gradient and cost function. Results in the near surface are consistent with nearby borehole data, and produce good depth tie of imaged reflectors to lithological well tops without explicit depth calibration. The gas zone has low-Q, low-Vp pockets which, after migration, produce a seismic image with significant structural changes to chalk reservoirs below the gas when compared with an image produced from a model built mainly with reflection tomography.