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Discusses use of simultaneous-source acquisition for sparse (ROV-deployed) OBN surveys, using time and motion studies to quantify impact on acquisition time. Also describes new technique for sim-src crosstalk noise attenuation using modified Radon operators. Finally, application to Gorgon OBN data for baseline and monitor, with PSDM images and 4D differences. Concludes with a 10 % time saving for sim-src on the Gorgon OBN acquisition, and an additional NRMS penalty on the 4D difference of 6 % after crosstalk attenuation. Without special processing, the crosstalk adds 70 % NRMS to the final images.

Multiple timelapse realisations produced using wavefield separation, data selection, or diverging processing flows are combined using powerful noise-reducing data weights formulated using the 3D images as well as the 4D differences. The result separates 4D signal from incoherent 4D noise and also 4D noise that is coherent across multiple realisations of 4D difference but not repeated in both baseline and monitor 3D images (e.g. residual multiple in the monitor). Examples using Broadseis base and monitor from the North Sea, and towed-streamer base and OBN monitor from deep water offshore Angola, show considerable uplift to the final 4D difference.

In the context of WAZ data the VTI hypothesis is not always sufficient for insuring focusing of time migration. We propose an extension of non-linear slope tomography for time imaging to the orthorhombic case. We use a model of orthorhombic anisotropy parameterized by five effective parameters and where the effective velocity and eta vary according to the azimuth of the migrated trace. In our approach the five parameters are updated jointly, allowing extending to the orthorhombic case the advantages of non-linear slope tomography for time velocity model building, i.e. and improved accuracy and turn-around.

The seismic image represents the spatially variable reflectivity of the medium where migration effectively rotates the wavelet to be normal to the imaged reflectors. While this is the general case, it is often disregarded, and one-dimensional spectral analysis of the vertical coordinate is commonly used. We show that spectral processing of post-imaging data in the direction normal to the reflectors provides accurate results for steep and complex structures. We introduce the concept of Orthogonal-Image-Gathers (OIGs) which facilitate this approach while providing a platform for handling spatially variable spectral distortions due to the velocity field and other medium-related properties.

Estimation of primaries by sparse inversion (EPSI) is an iterative method that effectively separates primaries and surface-related multiples, especially in shallow water. Multiple attenuation in shallow water is challenging, mainly because of acquisition limitations. We propose a strategy for EPSI with the following objectives: (1) creates an alternating picking method for the first iteration of EPSI that correctly separates primaries and multiples and also expedites the convergence in some cases and (2) picks only strong shallow reflectors to alleviate the cost while attacking most of multiples generated by those reflectors . We applied our method to two synthetic data sets. Then, we tested the EPSI method on a complex field data set and demonstrated that it can effectively attenuate multiples.

The Point Thomson field, located on the North Slope of Alaska, covers the transition zone from onshore to a frozen lagoon. This complexity in the near-surface poses many challenges to proper imaging of the reservoir. Unground ice on the lagoon causes very poor signal-to-noise ratio (S/N) over a critical portion of the field. The rapid lateral variation in permafrost thickness introduces strong lateral velocity gradients in the near-surface. Onshore, the presence of ice lakes create highly localized anomalies. We present a workflow that we used to address these near-surface issues during the recent reprocessing of the vintage low-fold data. Aided by modern imaging technology for noise attenuation, statics, and velocity model building, we were able to address many of these issues and produce a significantly improved image.

Although wavefield extrapolation techniques are well developed for P-wave seismic imaging, ray based migration algorithms are still the workhorse for converted-wave (PS-wave) depth imaging. Full (exact) elastic-wave reverse-time anisotropic migration (RTM) has not been widely adopted for reasons of computational and workflow efficiency, despite its potential to deliver accurate sub-surface images in complex geological settings by directly solving the elastic wave equation. Even (approximate) converted-wave RTM in anisotropic media, using separate finite-difference propagators for quasi-P and quasi-S waves, is limited in applicability for algorithmic reasons. Here, we introduce an alternative converted-wave anisotropic RTM, using a low-rank decomposition of mixed-domain space-wavenumber propagators for quasi-P and quasi-S waves. These operators are formal integral solutions of the pure-mode wave equations which guarantee stable and dispersion-free time extrapolation for coarse time steps in anisotropic, heterogeneous media. The pure-mode extrapolators are attractive for both PS-wave structural imaging and velocity analysis. An ocean bottom cable synthetic example illustrates the effectiveness of low-rank PS-wave RTM when compared against state-of-the-art Gaussian beam and finite difference RTM algorithms.

4D seismic processing is designed to minimize non-repeatable noise while preserving real 4D signal. Coherent noise, such as from surface and interbed multiples, has different character from one vintage to another. Mitigating for these unwanted effects in a vintage-independent manner leads to suboptimal 4D results. In Zabihi et al., 2012, a method for reducing residual 4D multiple leakage was proposed. Khalil et al., 2013 expanded its applicability and put it in a feasible workflow. We extend this workflow to a multi-vintage setting including legacy and modern 4D acquisitions with very different 4D noise characteristics.

A precise knowledge of the seismic source far-field signature is required for accurate source de-signature. Near-field hydrophone data can be used to provide good quality signatures but are not always available. We describe a method of extracting the far-field signature from inversion of direct arrivals in the seismic data. We present the application of the methodology to a deep water survey offshore Gabon. The method can be applied to legacy surveys where near-field hydrophone data is unavailable for a more broadband result.