Multiparameter full-waveform inversion of 3D ocean-bottom cable data from the Valhall field

Full-waveform inversion (FWI) applications on 3D oceanbottom cable (OBC) data from the Valhall oil field in the North Sea have demonstrated the importance of appropriately accounting for attenuation. The Valhall field contains unconsolidated shallow sediments and a low-velocity anomaly in its center - indicative of gas clouds - which have a significant attenuation imprint on the data. Our challenge is to perform time-domain viscoacoustic 3D FWI, which requires more sophisticated tools than in the frequency domain wherein attenuation can be incorporated in a straightforward manner. The benefit of using a viscoacoustic, instead of a purely acoustic, modeling engine is illustrated. We have determined that, in the frequency band used (2.5-7.0 Hz), it is better to reconstruct the velocity only keeping the attenuation lixed because simultaneous inversion of the velocity and quality factor Q does not provide reliable Q updates. We develop an efficient time-domain workflow combining a random source decimation algorithm, modeling using standard linear solid mechanisms, and wavefield preconditioning. Our results are similar to those obtained from state-of-the-art frequency-domain algorithms, at a lower computational cost compared to conventional check-pointing techniques. We clearly illustrate the improvement in terms of imaging and data fit achieved when accounting for attenuation.