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 seismic survey, the first dataset being indicative of 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 seismic survey, the second dataset being indicative of at least crossline particle motion measurements; and determining crossline particle motion measurements in the presence of a relatively flat sea surface based at least in part on the crossline measurements indicated by the second dataset and the pressure measurements indicated by the first dataset.