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We present an application of 3D frequency-domain full waveform inversion (FWI) on ocean-bottom cable data from the North Sea. Frequency-domain seismic modeling is performed in the visco-acoustic VTI approximation with a sparse direct solver based on the multifrontal method. The computational cost of the multifrontal LU factorization is efficiently reduced with a block-low rank (BLR) approximation of the dense frontal matrices. A multiscale frequency-domain FWI is applied by successive inversions of 11 discrete frequencies in the 3.5Hz-10Hz frequency band. The velocity model built by FWI reveals short-scale features such as channels, scrapes left by drifting icebergs on the paleo-seafloor, fractures and deep reflectors below the reservoir level, although the presence of gas in the overburden. The quality of the FWI results is controlled by time-domain modeling and source wavelet estimation. Next step is the application of multi-parameter FWI with second-order optimization algorithms, which can be efficiently implemented with our frequency-domain modeling engine.