Figure 5 - uploaded by Zhiyuan Wei
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Section view of a source-over-spread NATS data set in the Barents Sea. (a) 100 Hz Kirchhoff image with the 15 Hz TLFWI model. (b) 25 Hz FWI image. (c) 50 Hz FWI image. (d) 100 Hz FWI image. (e)-(h) The zoomed-in displays of the white dashed rectangles in (a)-(d), respectively. High-frequency FWI imaging provides well-focused geologic details (Wei et al., 2021a).
Source publication
Although the resolution of a seismic image is ultimately bound by the spatial and temporal sampling of the acquired seismic data, the seismic images obtained through conventional imaging methods normally fall very short of this limit. Conventional seismic imaging methods take a piecemeal approach to imaging problems with many steps designed in prep...
Citations
Full waveform inversion (FWI) has proven itself as an essential tool for velocity model building using seismic data. In recent years, the geophysical community has made good progress in developing FWI to overcome some long-standing limitations, such as handling cycle skipping, better utilizing reflection energy, including more physics in the inversion algorithms, and increasing inverted frequencies to achieve ever higher frequencies. Not only targeted for larger and mid-scale model updates, more and more FWI examples were presented with increased resolution which allows for extraction of FWI derived reflectivity. Interrogating the FWI kernel with given geology and acquisition geometry can provide critical information on how the acquisition design should be optimized to provide FWI a better opportunity to update the velocity model at target depth even in the deep part of the model. When the acquisition geometry is different in terms of offsets, minimum frequencies, and azimuthal coverage, it can be analyzed to design workflows to enable FWI to optimally update the model parameters. Obviously, recent ocean-bottom node (OBN) acquisitions that record long offsets and low frequencies made FWI shine in the seismic industry. Modifying and reshaping a complex salt geometry is one of the ultimate goals of FWI. Besides the salt boundary being an issue, there is a potential cycle-skipping problem associated with uncertainties of large salt body missing or misplaced even though the frequencies used to start FWI are getting lower due to the new acquisition advancement. Furthermore, if the FWI-predicted data is simulated with an acoustic engine, it could pose amplitude discrepancies at high velocity contrast interfaces. Elastic FWI (EFWI) along has been proposed as a means of overcoming those previously mentioned challenges associated with salt and regions with high velocity contrast. We demonstrate the FWI progress of the last decade with latest examples from different acquisition geometries.
The Atlantis Field has gone through more than two decades of continuous seismic imaging efforts, during which time many innovative technologies were incubated, the most recent one being the successful application of full-waveform inversion (FWI) in salt environments. This technique led to a significant improvement in the subsalt image. However, imaging challenges remain for the Atlantis reservoirs, primarily due to the complex overburden salt geometries and the highly compartmentalized reservoir. Even with an improved velocity model from FWI, the conventional reverse time migration (RTM) images still suffer from illumination issues and contain strong migration swings that hinder the subsalt imaging and subsequent interpretations. Furthermore, early versions of FWI employed an acoustic assumption, leading to visible salt halos at the salt boundaries in the velocity model, which adversely impacted the reservoir imaging. In the last 12 months, elastic time-lag FWI (TLFWI) and FWI-derived reflectivity (FDR) imaging using long-offset ocean-bottom node data have minimized these imaging issues at Atlantis, providing another step change in subsalt understanding. Although the 3D RTM images using the elastic FWI velocity model are similar overall to their acoustic counterparts, the 4D time-lapse RTM images at Atlantis show noticeable improvements. Furthermore, FDR images derived from elastic FWI velocities show obvious benefits over the acoustic ones. With a more accurate modeling engine that allows for better match between synthetic and real data, FDR imaging shows improved illumination, higher signal-to-noise ratio, and better reservoir details over acoustic FDR imaging. This recent advancement in using elastic TLFWI has had immediate positive effects in facilitating the Atlantis Field's current and future development.
