  Frequency-domain Full Waveform Inversion (FWI) is potentially amenable to
efficient processing of full-azimuth long-offset stationary-recording seabed
acquisition carried out with sparse layout of ocean bottom nodes (OBNs) and
broadband sources because the inversion can be performed with a few discrete
frequencies. However, computing efficiently the solution of the forward
(boundary-value) problem in the frequency domain with linear algebra solvers
remains a challenge for large computational domains involving tens to hundreds
of millions of parameters. We illustrate the feasibility of 3D frequency-domain
FWI with the 2015/16 Gorgon OBN case study in the NorthWestern shelf,
Australia. We solve the forward problem with the massively-parallel
multifrontal direct solver MUMPS, which includes four key features to reach
high computational efficiency: An efficient parallelism combining
message-passing interface and multithreading, block low-rank compression, mixed
precision arithmetic and efficient processing of sparse sources. The Gorgon
subdataset involves 650 OBNs that are processed as reciprocal sources and
400,000 sources. Mono-parameter FWI for vertical wavespeed is performed in the
visco-acoustic VTI approximation with a classical frequency continuation
approach proceeding from a starting frequency of 1.7 Hz to a final frequency of
13 Hz. The target covers an area ranging from 260 km2 (frequency > 8.5 Hz) to
705 km2 (frequency < 8.5 Hz) for a maximum depth of 8 km. Compared to the
starting model, FWI dramatically improves the reconstruction of the bounding
faults of the Gorgon horst at reservoir depths as well as several intra-horst
faults and several horizons of the Mungaroo formation down to a depth of 7 km.
