Marine vertical cable multiple attenuation beyond up/down separation

Abstract

Marine vertical cable acquisition is an emerging technology. It represents an alternative to surface seismic acquisition in areas congested by platforms or other obstacles. The vertical cable acquisition consists of recording pressure at several fixed vertical receiver arrays as for VSP. A small vessel carries only the source and is very flexible. The vertical cable receivers are in a quiet environment, which can explain the data quality. One main concern with vertical cable data is to remove the multiples, which are mainly the free-surface multiples and the receiver ghosts of multiples, preserving the primaries. Furthermore the emerging imaging algorithms tend to use the receiver ghosts of primaries instead of the primaries themselves. The classical approach to remove multiples in vertical cable data is to consider the primaries as up-going waves and the multiples as down-going waves. The current multiple attenuation methods used in the E&P industry are actually up/down wavefield separation. Different techniques can be used to achieve the wavefield separation. The common technique is the F-K dip filtering. This method suffers from the space sampling with the aliasing problem that controls the efficiency of the filtering. An interpolation method can be used to construct the needed traces to satisfy the sampling theorem and to remove the aliases. A wave equation based up/down separation can also be used for vertical cable surveys. This method has the advantage of being less influenced by the sampling problem, but it is dependent on the availability of vertical particle velocity data. This particle velocity data can be calculated from the pressure gradient, and a good cable design is required for this method. The up/down separation based multiple attenuation methods give only a partial answer to the multiple attenuation process. This technique ignores all the remaining free-surface multiples still contained in the up-going waves. So it fails in removing all the multiples. Another multiple attenuation method based on the inverse scattering theory has been proposed by Ikelle (2001) to preserve the primaries or the receiver ghosts of primaries for vertical cable data. It is efficient, but is dependent on the availability of surface seismic data. The method consists in combining the streamer data with the vertical cable data to predict the multiples and then to subtract them from the original vertical cable data after applying a scaling factor. The application of this technique on a synthetic model with the combining streamer and vertical cable data gives promising results, but in some cases the streamer data is not commonly available. It is then necessary to construct it from the vertical cable data with an extrapolation technique.