Multi Azimuth Research Articles

Combining orthogonal polarization for elongated target detection with GPR

For an accurate imaging of ground penetrating radar data the polarization characteristics of the propagating electromagnetic (EM) wavefield and wave amplitude variations with antenna pattern orientation must be taken into account. For objects that show some directionality feature and cylindrical shape any misalignment between transmitter and target can strongly modify the polarization state of the backscattered wavefield, thus conditioning the detection capability of the system. Hints on the depolarization can be used to design the optimal GPR antenna survey to avoid omissions and pitfalls during data processing. This research addresses the issue of elongated target detection through a multi azimuth (or multi polarization) approach based on the combination of mutually orthogonal GPR data. Results from the analysis of the formal scattering problem demonstrate how this strategy can reach a scalar formulation of the scattering matrix and achieve a rotational invariant quantity. The effectiveness of the algorithm is then evaluated with a detailed field example showing results closely proximal to those obtained under the optimal alignment condition: detection is significantly improved and the risk of target missing is reduced.

Optimal stacking for multi-azimuth pre-stack seismic data

SummaryOffshore exploration for hydrocarbons in increasingly challenging environments often requires more advanced acquisition methods than conventional 3D narrowazimuth towed streamer to better image the sub-surface for AVO analysis and reservoir characterization. Multi- Azimuth (MAZ), Wide-Azimuth (WAZ) or Full-Azimuth (FAZ) seismic acquisition overcomes the limitations of the conventional acquisition in better illuminating the sub-surface, suppressing the multiple and enhancing signal to noise (S/N) ratio. Nevertheless, to realize the added value of multi-azimuth data, the data need to be combined in a way that will overcome the issues such as time-shift and amplitude difference due to varied illumination between the surveys.This paper describes a method that can be used for combining MAZ pre-stack data to generate AVOpreserving common image gathers (CIGs) in the presence of poor illumination. Based on the concept of crosscorrelation, MAZ CIGs are first flattened to account for any inaccuracy in the velocity model and imaging process so as to align the events to a pilot. Repeating the crosscorrelation process, weights are then derived from the correlation coefficients and applied to individual offsets that take into account the AVO behaviour. With this post-migration processing, any anomaly in AVO resulted from poor illumination can be mitigated. Applying it on MAZ post-stack data, the method can also provide optimal stacking for obtaining higher S/N images.We demonstrate, through synthetic and real data examples, that clearer images with high AVO fidelity can be obtained from MAZ data using our optimal stacking method.

Prestack depth imaging of multi-azimuth seismic data in the presence of orthorhombic anisotropy

The presence of orthorhombic anisotropy can severely affect the imaging of multi- and wide-azimuth data which is rapidly developing with the benefit of better illumination, better imaging, and better multiple elimination. Analysis of multi-azimuth (MAZ) data often reveals noticeable fluctuations in moveout between different acquisition directions, preventing constructive summation of MAZ images. Vertical transverse isotropy (VTI) effects can also co-exist causing well misties and higher order moveout. We have developed an approach for imaging in the presence of orthorhombic anisotropy. In this paper, we first describe our approach, including a newly developed orthorhombic imaging method and a newly developed practical method for orthorhombic anisotropy model building. We then demonstrate with both synthetic and real data from offshore Australia that our approach can successfully take into account the coexisting HTI/VTI effects, reduce the structural discrepancies between seismic images built for different azimuths, thereby producing constructive summation of MAZ datasets and resolving well misties to match with geology. The combined effect of these improvements is a step-change in the final seismic image quality.

Surface Inspection Technique for Steel Strip Using Relative Intensity Characteristics of Multi Azimuth Polarized Images

In steel strip surface inspection, high discrimination capability of extracting harmful defects by suppressing excessive detection such as harmless oil patterns is required. Firstly we estimate the relative intensity profiles along azimuth angle of polarized light reflection of surface objects. Secondly we execute experiments with rotating analyzer, finding that defects and oil patterns have different polarity of intensity to the normal surface in polarized images. Finally we bring the technique into practice as a real-time on-line inspection equipment and prove the effective function discriminating excessive-detection matters from real defects.

Multi-azimuth 3D survey in the Barents Sea

Michele Buia, Carmine Cirone, Johan Leutscher, Steve Tarran and Bruce Webb describe the acquisition and processing of a seismic survey over the Goliat area in the Barents Sea, north of Norway, the first multi-azimuth (MAZ) 3D survey in the area.

Using modern processing technology to improve signal to noise ratio, a Perth basin 3D land case study

The processing of two 3D land Perth basin seismic survey gave us the opportunity to apply and test some of the modern processing technologies available. The technologies we explore through those examples take place in the early steps of processing, later in the sequence before imaging and at the imaging level itself. For each of the steps we performed comparisons with more conventional technologies. Especially with land data, often acquired with poor signal to noise ratio, it is important to remove noise in the early phases of the processing sequence. Groundroll in particular needs special attention as the characteristic features of Groundroll (low frequency, highly energy) noise in the data can hide near offset reflection data that contains important signal. In this article we review a new adaptive method for Groundroll attenuation that could potentially better attenuate noise and at the same time better preserve signal. 3D land datasets are also often characterized by a wide range of azimuths being present in the data. This differs from most 3D marine acquisitions that are (with the exception if multi and wide azimuth surveys) mostly acquired in a narrow azimuth fashion behind the boat even with the presence of a wide array of streamers. Those wide azimuths represent a challenge for conventionally sorted data regularization prior to imaging. In this article, we analyze the results of using offset vector tile (OVT) gathers regularization and OVT gathers in the imaging process against the more conventional offset volume method. Finally, the use of Controlled Beam Migration (CBM) is compared to a more conventional Kirchhoff imaging process. Controlled Beam migration has often been used to image steep dips and provide multi arrivals in extremely complex environments. However in relatively low signal to noise ratio environment like the Perth basin we have seen in the second case study that this imaging technique could potentially bring better results than the conventional Kirchhoff.

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 – 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.

Extending the life of the Varg Field, a tail-end production opportunity

The Varg Field is located in the Norwegian North Sea, by the UK border c . 240 km west of Stavanger. The Varg Field (Licence PL 038) is unique on the Norwegian shelf because it is the first successful tail-end production by a new and small operator, Pertra AS. Since the acquisition of the licence, Pertra has achieved higher production levels and better reservoir management in the Varg Field due to careful re-evaluation of all available data and close collaboration between different disciplines to improve the understanding of the subsurface. This paper outlines how an improved understanding of the subsurface has been achieved through a range of specialized geological and geophysical techniques, such as: ichnofabric analysis, dipmeter and wellbore image analyses, re-evaluation of the bio- stratigraphical data, trace inversion, forward seismic modelling and fluid substitution modelling. As part of the Pertra work programme, a new multi azimuth seismic dataset was acquired. After pre-stack depth migrating, this became a new and valuable dataset for reservoir description. The enhanced oil recovery analysis, utilizing all the above re-evaluated data, has yielded positive results in terms of new reserves and increased production levels. From an economics point of view the cost of more geo-manpower is negligible when compared to the cost of the development. This has been the most important lesson learnt.