1. A method for processing seismic data acquired in a survey conducted in the presence of a rough sea, comprising: receiving a first dataset derived from data acquired by seismic sensors of a first set of least one streamer generally towed at a first depth in the survey, the first dataset representing at least pressure measurements; receiving a second dataset derived from data acquired by seismic sensors of a second set of at least one streamer generally towed at a second depth different than the first depth in the survey, the second dataset representing at least crossline particle motion measurements containing a rough sea perturbation component; and determining crossline particle motion measurements in the presence of a relatively flat sea surface based at least in part on the crossline measurements represented by the second dataset and the pressure measurements, the determining comprising: applying a one way propagator to the pressure measurements represented by the first dataset to derive crossline article motion measurements at the second depth substantially containing the rough sea perturbation component; and combining the derived crossline particle motion measurements at the second depth with crossline measurements indicated by the first dataset to determine the crossline article motion measurements in the presence of a relatively flat sea surface.