Pre-stack interpolation: an alternative to costly acquisition
Authors: Spitz S. , Huard I. 
Published: October 2008

The last decade has seen a dramatic increase in the number of streamers and sources in offshore seismic acquisition. In particular, multi-source shooting aims at decreasing the bin size by reducing the distance between acquisition lines. Unfortunately, the coarse sampling of the offset variable can result in spatial aliasing. In regions with strong multiple reflections (for instance generated in the water layer) this aliasing leads to strong acquisition imprints. It seems that in such areas acquisition has to revert to traditional high fold shooting geometries. This paper illustrates a processing method that compensates for the quality degradation related to modem acquisition designs.

Introduction

Reducing the cost of offshore acquisition has been (and still is) a major preoccupation of the oil exploration industry. Lower cost and faster turnaround not only have a major financial impact, but also contribute to the speed of decision making. 3-D marine acquisition techniques have been revolutionized over the last decade by the increase in the number of streamers and sources used. One of the main advantages of using multi-streamer and multi-source configurations has been the reduction in the bin size and, in particular, the decrease in the distance between acquisition lines. The other incentive that has accelerated this tendency has been a financial one, as acquisition performance has been increased by a factor of five to ten.

The design of a 3-D survey is the best compromise between cost and the resolution of the final seismic sections. Cost optimization requires the acquisition of the widest possibleswath in one passage of the vessel. The number of towed streamers defines therefore the cross-line spatial interval. At the same time, resolution requires a uniform data distribution in the two directions. If the optimized design involves a con- figuration with streamers as far apart as possible, then a multisource decreases the bin size cross-line, without increasing the number of recorded traces. But since the total number of acquired traces remains constant, the multi-source shooting leads to a reduction in coverage (by a factor of two for 'flip-flop'shooting) with respect to a traditional acquisition configuration.

Unfortunately, sparse shooting geometries lead to departures from the ideal symmetric sampling of the shot and receivercoordinates advocated by Vermee1.. In regions characterized by a low signal-to-noise ratio due to the presence of strong multiple reflections they can lead to spatial aliasing of the multiple wave field. This spatial aliasing of the multiple events becomes stronger as the exploration moves to deep waters and as the design implies longer offsets. Sometimesthis undersampling can be so strong that multiple energy, otherwise attenuated by the stacking process, can leak into the stack itself with dramatic consequences (Figs. 1 and 2). Ref. 2 proposes several means to remove this spurious energy from the stacked sections. However, such techniques leave most of the coherent noise in the data, with adverse consequences for pre-stack analysis techniques, such as AVO.