Walter Rietveld

Quantifying and increasing the value of multi-azimuth seismic

Conventional marine exploration offshore Nile Delta is challenged by shallow heterogeneities and a deep, complex anhydrite layer called the Messinian. These subsurface complexities cause variable illumination and strong diffracted multiples resulting in imaging challenges below the anhydrite on conventional narrow-azimuth towed streamer acquisition because the recording antenna is too small (Figure 1a). Additionally, it has proved difficult to build the detailed depth migration velocity model required to correctly image below the Messinian layer. Multi-azimuth (MAZ) acquisition using time-domain processing however, has been very successful in addressing both the noise and illumination problems in the Nile Delta. The increased azimuthal and crossline offset coverage—bigger antenna—is achieved by acquiring six conventional marine surveys over the same area at 30° sail-line increments (Figure 1b).

Multi-Azimuth towed streamer 3D Seismic in the Nile Delta, Egypt

A thin but complex layer of partially eroded anhydrite and other facies lie at a depth of around 3km across large areas of the Nile Delta in the Mediterranean. Wavefield distortion, attenuation and the generation of complex multiple diffraction noise cause the quality of the underlying seismic image to be highly variable. (Figure 1) In this paper we describe the problem and then demonstrate how multi-azimuth seismic is able to improve the PreMessinian image.

Multi-azimuth 3D provides robust improvements in Nile Delta seismic imaging

Since gas was first discovered in the Nile Delta in the late 60s, most exploration programmes have focussed on shallow Pliocene reservoirs, where gas can clearly be seen as bright events on excellent quality seismic data and where exploration success has been very high. The petroleum geology of the deeper pre-Pliocene section is fundamentally no different to that seen in the Pliocene, where potential reservoirs consist of sand prone channel systems originating from the Nile. Why then, have we been deterred from exploring in the deeper section? The problem is two fold: 1. Deeper burial and harder rocks mean that reservoir sands and hydrocarbons will be less visible on our seismic data. 2. Pre-Pliocene seismic quality is highly variable and often very poor. (Figure 1) Poor imaging being the result of wavefield distortion though the Messinian anhydrite layer, attenuation, and the presence of complex multiple diffraction noise.

Multi-azimuth towed streamer 3D seismic in the Nile Delta, Egypt – processing solutions

Multi-azimuth or wide azimuth seismic is not a new technology, and has been with us for many years in the form of land, and ocean bottom surveys. The literature is rich with examples of how high-fold multi-azimuth data can produce stunning improvements over their single azimuth 3D equivalents (Rogno et al., 1999, Keggin et al., 2002, Gaus and Hegna, 2003, Arntsen and Thompson, 2003, Riou et al., 2005, Manley et al., 2006, Michell et al., 2006). We know from theory and case histories that multi-azimuth data will lead to improved signal to noise, improved multiple attenuation and improved illumination. However, because of approximations in current processing technology, the processing of multi-azimuth data will leave errors in the final imaged results. Simple stacking of the data, though surprisingly robust makes assumptions about data consistency between surveys and will likely not result in the most optimal image. This paper shows how multi-azimuth (MAZ) towed streamer data is processed in the Nile Delta, looks at some of the issues highlighted above and discusses the initial processing sequence to improve the combined subsurface image.

The key practical aspects of 3D tomography: Data picking and model representation

There are a wide variety of approaches to building velocity models with prestack depth migration. The most common pitfalls we see in them are extracting reliable yet detailed information about velocity errors from migrated data and then the inability to produce geologically reasonable velocity models from these error estimates. We extract detailed error estimates along horizons from finely sampled migrated data, edit them statistically, and QC the edits in map view. Tomography converts these error estimates into new velocity models or velocity model updates using a flexible model building system.

The effect of 3-D prestack seismic migration on seismic coherence and amplitude variability

We compare the effect of 3-D poststack versus 3-D prestack imaging on seismic coherence, seismic amplitude, and seismic amplitude variation. We find that the improved resolution and amplitude preservation of the prestack imaging result in more sharply defined terminations and hence better delineation by coherence and amplitude gradients even though the (macro) velocity models used in both imaging approaches are laterally invariant [v(z)].

Leveraging the Value of Multi-azimuth (MAZ) Seismic through MAZ-stack

The growing demand for energy requires that the search for hydrocarbons must extend into more challenging settings and become more reliant on technology, exposing the limitations of conventional 3D marine seismic. This decade has seen a major response to these challenges in the marine setting, in the form of Wide Azimuth seismic acquisition, like Multi-Azimuth, Wide Azimuth Towed Streamer, Nodes and OBC. These methods illuminate the sub-surface more completely, and sample problematic 3D noise across azimuth as well as offset for better attenuation during stack. These surveys are much more expensive to acquire and process, so it is thus important to lever all the value from the data. This paper discusses two seismic processing options to lever additional value from MAZ data. The first, MAZ-Stack, has been developed to weight up signal in areas of poor illumination, in other words, to favour strong signal over weak/absent signal. This approach has the effect of reducing fold and so decreases random noise suppression. The second technique, 3D warping, extends an existing method to align sub-surface seismic volumes, and so minimize registration errors between the azimuth stacks which result from imperfect knowledge of the sub-surface velocity field.

Depth migration combined with controlled illumination

Introduction Stacking ol common receivergathers at the surface products plane wave responses of the subsurface, Tancr (1976). Since these plant wave responses are physical experiments, wave qualion based extrapolation techniques @restack) can be used to migrate or rcdatum these responsesHowever, due to the inhomogeneitics of the subsurface the wave front may bc (seriously) distortcd when arriving at the levelof interestthe target zone (Fig 1). thesedistortions hould bc lakcn into account in the migration process (Bcrkhout, 1992).

Experience with towed streamer multi-azimuth processing and acquisition

Multi-azimuth towed streamer acquisition is a technique whereby conventional marine 3D seismic surveys are acquired in several distinct acquisition directions and then combined in some way to produce an improved image. The method as described here has produced some impressive comparisons in the literature, e.g., Keggin et al. (2006), Page et al. (2006), and is drawing increasing interest especially for areas of poor signalto-noise ratio. Over the years that this method has been pioneered, some standard acquisition and processing procedures have been established which will be discussed here with examples from BP’s six azimuth Raven survey from the Nile Delta, courtesy BP Egypt.

Building representative velocity and density models for a finite-difference modeling study in offshore Nile-Delta, Egypt

Summary Full scale 3D finite-difference modeling studies improved our understanding of the influence of acquisition geometry in complex geological situations. In order to develop an efficient acquisition geometry using finite-difference modeling studies we need to use velocity and density models representative of the subsurface. In this paper, we present an innovative methodology for building the velocity and density model for the complex geological setting of offshore Nile-Delta in Egypt. Numerical data generated with these models represent the various features observed in field data.