Methods and systems to separate wavefields using pressure wavefield data

This disclosure is directed to wavefield separation methods and systems. In one aspect, methods and systems compute an approximate vertical particle velocity wavefield based on a measured pressure wavefield and knowledge of free-surface when the pressure wavefield was measured. The measured pressure wavefield is used to compute an approximate frozen free-surface profile. The approximate frozen free-surface profile and the measured pressure wavefield are used to compute an approximate vertical particle velocity wavefield. The approximate vertical particle velocity wavefield and measured pressure wavefield may be used to compute separate up-going and down-going pressure, or vertical particle velocity, wavefields.