We present an application of 3D VTI visco-acoustic frequency-domain full waveform inversion (FWI) based on sparse direct solver on wide-azimuth OBC data from the Valhall oil field. Mono-parameter inversion for the vertical wavespeed is applied to the hydrophone component in the 3.5-10Hz passband. Compared to reflection traveltime tomography, we show the significant resolution improvement provided by FWI in the velocity reconstruction down to the base cretaceous reflector located below the reservoir level. This improvement was achieved although the presence of gas in the overburden. The FWI models are assessed by frequency-domain and time-domain modeling. We show an excellent match between recorded and modeled monochromatic gathers up to 10Hz. Time-domain modeling highlights the fit of the main diving and reflection arrivals interpreted in the data. We discuss the pros and cons of the frequency-domain formulation of FWI in terms of computational efficiency compared to time-domain FWI. In particular, we show the efficiency of the frequency-domain approach to process a large number of shots collected by fixed-spread acquisitions with a limited number of nodes of parallel computers. Perspectives are the use of block low-rank version of the direct solver to push inversion at higher frequencies and multi-parameter reconstruction including density, attenuation and epsilon.