True 3D—wide azimuth seismic comes of age onshore as well as offshore

Abstract

The advantages of recording a well-sampled, wider range of azimuths on seismic surveys are being seen in different geological settings worldwide. The success of wide-azimuth surveys has been well documented for sub-salt illumination in the deep water plays of the Gulf of Mexico. Multi-azimuth surveys have similarly shown improved S/N beneath the Messinian unconformity offshore Egypt. More recently, the benefits of improved sampling (and hence attenuation) of noise by dense, wide-azimuth surveys have been demonstrated onshore Oman. Although the successful marine results have highlighted the benefits of true 3D or wide-azimuth surveys, land surveys have often been recorded with a wide range of azimuths for many years. The step-change occurring onshore is the dramatic increase in the trace density on wide-azimuth surveys. We are learning that one of the reasons that conventional land seismic data have often been viewed as noisy is that coherent energy, particularly near surface events, is grossly under-sampled. Modern land recording techniques such as overlapping vibrator sweeps, single vibrator (point source) recording, multiple vibrator fleets and high channel systems allow us to record data much more efficiently. In turn, these enabling technologies allow us to obtain much improved seismic data quality via greatly increased sampling density. Moreover, recording geometries on this new generation of land supercrews provide trace densities that are many times higher than on marine surveys. The next step for marine wide-azimuth surveys may require a similar step-change in recording density in order to address difficult geologies that cannot be imaged with current recording techniques.