Suppression of water-column multiples by combining components of OBS surveys

Yan Yan, R. James Brown


In ocean-bottom seismic data, multiples can be catalogued into pure water-column reverberations, receiver-side multiples, and source-side multiples. The multiples belonging to pure water-column reverberations and receiver-side reflections at the free surface are contained in the downgoing wavefield, while the primary reflections and pure source-side multiples are contained in the upgoing wavefield. Multiples containing bounces on both the source and receiver sides are considered receiver-side multiples for our purposes, since they arrive travelling downwards. Since the primary reflections are contained only in the upgoing wavefield, it is natural to consider a wavefield-separation technique.

The essence of wavefield decomposition techniques is to combine the pressure, the horizontal and vertical velocity components in proper proportions to obtain the upgoing and downgoing wavefields. With wavefield-separation techniques, the upgoing wavefield, without any receiver-side multiples, can be successfully extracted. After removal of the downgoing wavefield, however, source-side multiple energy remains as part of the upgoing wavefield. In laterally homogeneous cases, source-side multiples will have raypaths that are equivalent in length to those of corresponding receiver-side multiples. So the two types of multiple are recorded simultaneously. We exploit this circumstance to devise a method, based on cross-correlation, to further attenuate the source-side multiple energy.