4D FWI — Learnings from a North Sea OBN case study

In recent years the industry has seen a paradigm shift with the advent of Full Waveform Inversion (FWI) imaging in 3D seismic data processing projects. Fundamentally this is a fairly simple process as the image is merely a directional derivative of a velocity model, the resolution of which is largely controlled by the maximum frequency the FWI and of course the subsurface properties.As FWI has several potential advantages over conventional imaging. It is an iterative least squares solution of the full wavefield and thus has the ability to provide cleaner attributes as a result of the least squares nature of the process. As FWI uses the full wavefield (primary and multiples) it is possible to generate attributes over a larger area relative to area obtained from conventional 4D imaging. Finally, FWI imaging has also enabled turnaround time for projects to be significantly reduced.Initially there was some skepticism about the amplitude fidelity of such volumes, but through a series of examples, the industry has accepted the technique as either an alternative solution, or in some cases as the primary interpretative product.In this paper we extend FWI imaging into 4D, outline several approaches for 4D FWI, discuss the pro's & con's of each and demonstrate these by applying each approach to a dense OBN survey in the North Sea. We will compare the reflectivity and FWI derived differences from the various approaches and discuss how this technology may evolve in the future.