Anisotropic 3D full-waveform inversion

Abstract

We have developed and implemented a robust and practical scheme for anisotropic 3D acoustic full-waveform inversion (FWI). We demonstrate this scheme on a field data set, applying it to a 4C ocean-bottom survey over the Tommeliten Alpha field in the North Sea. This shallow-water data set provides good azimuthal coverage to offsets of 7 km, with reduced coverage to a maximum offset of about 11 km. The reservoir lies at the crest of a high-velocity antiformal chalk section, overlain by about 3000 m of clastics within which a low-velocity gas cloud produces a seismic obscured area. We inverted only the hydrophone data, and we retained free-surface multiples and ghosts within the field data. We invert in six narrow frequency bands, in the range 3 to 6.5 Hz. At each iteration, we selected only a subset of sources, using a different subset at each iteration; this strategy is more efficient than inverting all the data every iteration. Our starting velocity model was obtained using standard PSDM model building including anisotropic reflection tomography, and contained epsilon values as high as 20%. The final FWI velocity model shows a network of shallow high-velocity channels that match similar features in the reflection data. Deeper in the section, the FWI velocity model reveals a sharper and more-intense low-velocity region associated with the gas cloud in which low-velocity fingers match the location of gas-filled faults visible in the reflection data. The resulting velocity model provides a better match to well logs, and better flattens common-image gathers, than does the starting model. Reverse-time migration, using the FWI velocity model, provides significant uplift to the migrated image, simplifying the planform of the reservoir section at depth. The workflows, inversion strategy, and algorithms that we have used have broad application to invert a wide-range of analogous data sets.

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

“…Initial applications of FWI were performed in the time domain, but were limited given the high computational demand of the method (Kolb et al 1986). Three decades later and FWI is still performed based on these underlying principles, with modern codes capable of performing FWI in either the time or frequency domain, in two or three dimensions, approximating either the acoustic or elastic wave equation, and can include the effects of seismic attenuation and anisotropy (e.g., Pratt 1999; Brossier et al 2009;Warner et al 2013). It has also been shown that the maximum achievable resolution using these codes is on the order of half the seismic wavelength, making it superior to travel-time tomography .…”

“…In this study we perform a 2D time-domain, acoustic, isotropic FWI, using the codes of Warner et al (2013). In this code, synthetic traces are calculated through a starting model using a finite difference method and are subsequently scaled so that their RMS amplitude matches that of their corresponding observed trace.…”

“…Three-dimensional FWI has yielded impressive results on marine seismic datasets, producing high resolution velocity models which can be used directly for geological interpretation or for the migration of reflection seismic data to produce detailed images (e.g. Sirgue et al 2010;Ratcliffe et al 2011;Warner et al 2013;Mispel et al 2013;Jones et al 2013;Mothi et al 2013). The vast majority of such studies have utilised seismic data recorded on either hydrophone streamers or ocean bottom cables (OBC), in relatively shallow marine environments (water depth < 1,000 m).…”

Acceleration strategies for elastic full waveform inversion workflows in 2D and 3D

“…-Composite shot lists: This last strategy [27,49] simply distributes the complete shot list in a subset of shots which are run sequentially at each gradient iteration. In this way, for a fixed number of gradient iterations, the total amount of shots simulated is smaller, reducing the computational resources needed.…”

Full-waveform inversion in acoustic orthorhombic media and application to a North Sea data set

“…Over the past three decades, most FWI techniques have been designed to recover only a P-wave velocity model because of the high computational cost (e.g., Gauthier et al, 1986;Pratt, 1999). The recent progress in high-performance computing and the improvement in data acquisition resulted in the extension of FWI to 2D and 3D acoustic and elastic transverse isotropic media with a vertical axis of symmetry (VTI) (e.g., Warner et al, 2013;Operto et al, 2014;Wu and Alkhalifah, 2016;Kamath et al, 2017). This extension has resulted in challenges related to the trade-offs between the model parameters in our inversion process.…”