Marine seismic data acquired with towed pressure sensing streamers suffers from the interference of the upgoing wave with the downgoing wave reflected at the sea surface. The downgoing wave introduces time-variant perturbations due to the time-variant shape of the sea surface, especially in rough weather conditions, meaning that the sea-surface ghost becomes time variant. These problems can only partly be overcome if a single streamer is replaced by a pair of streamers such that one streamer is towed vertically above the other. The basic dilemma with twin-streamer data is that one has to make a data processing decision: either to attenuate the downgoing wave to reduce the rough-sea perturbations while introducing a pseudo ghost, still attenuating the signal energy at certain frequencies, or to estimate the upgoing wavefield, while leaving the rough-sea surface perturbations still affecting the output data. If one accepts to stay with pressure sensing towed streamers, the situation changes drastically if at least one further streamer is added vertically to the twin-streamer pair. With such an extended over/under streamer configuration one can derive an estimate of the upgoing wave at the flat sea surface, whatever the actual sea-surface state is.


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