Whitepapers

  • Towed Streamer EM - Survey Design and Acquisition

    TechNote - 4 July 2017

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    Towed Streamer EM delivers high density 3D CSEM data with exceptional acquisition
    efficiency. Daily acquisition rates in the order of 140 sq. km are frequently achieved with the current record at over 200 sq. km in a single day. PGS acquires 2D GeoStreamer® data simultaneously with EM data, or 3D EM data over existing or planned 3D seismic.

  • Towed Streamer EM - Feasibility Studies

    TechNote - 4 July 2017

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    PGS conducts EM feasibility studies in order to confirm that the Towed Streamer EM system will be sensitive to changes in target resistivity. A typical feasibility study involves forward modeling for a range of water depths, target depths, target sizes and resistivity scenarios. A feasibility study is not a pre-requisite to EM acquisition; rather it is a useful first step when looking at new areas or play types.

  • Towed Streamer EM - Processing and Inversion

    TechNote - 4 July 2017

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    Towed Streamer EM data is monitored on board the vessel in real time as it is acquired. Once a sail line is complete offline QC is conducted, including a 1D QC inversion of every shot acquired. The EM field deliverable is denoised, navigation merged data, and this is delivered to both the customer and PGS’ EM imaging team for 2.5D and 3D anisotropic inversion which is performed onshore.

  • High Fidelity Velocity Model Building, Imaging and Reflectivity Inversion – a Case Study Over the Viking Graben Area, Norwegian North Sea

    12 June 2017

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    A case study was carried out over the Viking Graben area with special focus on the Volund field, where a high fidelity velocity model was built to solve the complex velocity variations in the overburden resulting from a widespread presence of channels and numerous small-scale sand injectites. Significant improvements to the image quality were achieved with the use of the detailed velocity model and by incorporating a variable quality field to compensate for visco-elastic effects. Alternative imaging algorithms and reflectivity inversion techniques were explored to overcome limitations of the imaging system. This further improved the spatial resolution, enhanced the signal and helped the interpretation of the remobilized sand system over the Volund field.

  • Very Sparse Seabed Seismic Acquisition for 3D/4D Reservoir Imagng with High-order Multiples. Application to Jubarte PRM

    12 June 2017

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    A receiver decimation study has been performed for assessing the potential of using very sparse receiver grids in conjunction with high-order multiple imaging techniques for 4D reservoir monitoring. Reducing the density of receivers in a PRM (permanent reservoir monitoring) system will reduce the capital expenditure. For OBN (ocean bottom node) acquisition, cost saving can be achieve from more effective node deployment. The imaging process makes use of the multiple sea-surface reflections to significantly increase the fold, hence compensating for the reduced number of sensors. We will show that with the above imaging technology it is possible to use sparser seabed recording geometry without compromising the 3D or 4D image quality. Using the Jubarte PRM
    dataset acquired by Petrobras, the decimation test consists of taking out receiver included into circular patches within a diameter of 600 m. The number of sensors is then reduced by a factor two. The results demonstrate that it is possible to break some established limit in term of seabed sensor sparseness without compromising the resolution of 3D/4D imaging.

  • Optimising 4D Seismic with Evolving Technology over 20 Years of Reservoir Monitoring of the Gullfaks Field, North Sea

    12 June 2017

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    Seismic monitoring of the Gullfaks field has been in progress for over 20 years. Over that time changes in acquisition equipment and planning have led to improved 4D repeatability. In addition, upgrades in processing and imaging technology such as designature and demultiple have reduced 4D noise and consequently given us higher confidence in the interpretability of 4D signal. A monitor survey was acquired in 2016 with a multicomponent streamer solution and thus led to a 4D time-lapse processing project including older vintage surveys.
    This paper summarises the various changes in acquisition over the years and shows that continuous improvement in acquisition design alongside backward compatibility, rather than strictly repeating acquisition parameters, pays off with better quality 4D results. Finally, some intermediate results from the latest processing show that even the best repeated surveys can benefit from reprocessing when compared with previous processing results.

  • Rough Sea Surface Reflection Coefficient Estimation and Its Implication On Hydrophone-Only Pre-Stack Deghosting

    12 June 2017

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    Deghosting a hydrophone-only measurement requires accurate description of seismic wavefield scattering at the sea surface. In recent years, attempts have been made to deghost hydrophone-only data using a frequency and angle dependent sea surface reflection coefficient. Nevertheless, rough sea surface reflection coefficients have both specular and non-specular contributions. In pre-stack hydrophone-only deghosting, the non-specular contributions are often assumed to be negligible. In this paper, we characterize the sea surface scattering using numerical modelling and quantify the contributions of both specular and non-specular reflections from two different realistic sea states. The results show that the magnitude of the specular reflection coefficient decreases with frequency and increases with angle. Moreover, the rougher the sea surface, the higher the magnitude of the non-specular contributions.

  • Modelling of Time-varying Rough Sea Surface Ghosts and Source Deghosting by Integral Inversion

    12 June 2017

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    A major impediment to the full understanding of the data acquired in marine seismic is the restricting assumption of a flat and stationary sea surface used in certain pre-processing tools. A first step towards removing this assumption is to accurately account for the sea state (time varying free surface) in the deghosting process. On the receiver side, this is handled properly by using dual-sensor streamer. In this work, we present
    an integral approach to model the source side ghost effects from time-varying rough sea surfaces and show that the interaction with time-varying sea surfaces affect the subsurface reflections and may have a ignificant impact on seismic repeatability. We then continue with a theoretical derivation where we develop deghosting operator based on an integral inversion of the modeling operator. This formulation for source deghosting can account for the time-variation of rough sea surfaces.

  • Shot by Shot Source Wave Field Estimation

    12 June 2017

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    In this paper we discuss the behaviour of air gun source arrays in marine seismic acquisition. We comment the fact that the source configuration and depth are changing continually with the combined actions of surface waves, sea currents and general towing conditions. This has a direct effect on the emitted source pressure wave field and thus on the signature in the seismic data. We describe the method of backpropagation with relative motion that allows an efficient and robust estimation of notional and far-field signatures from near-field measurements at every shot point. The derived shot by shot signatures show very good correlation with seastate and sea-currents, as we would expect. We show that the variation of the signature can affect the quality of seismic data. We demonstrate that the estimated far-field signatures describe the real variation of the signature in the data and we show how the estimated shot by shot signatures can be used to mitigate the effect of signature variations and thereby improve the quality of the seismic data.

  • A Robust FWI Method for Model Updating in High Contrast Bodies

    12 June 2017

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    We present a robust method for Full Waveform Inversion (FWI), enabling the recovery of long-wavelength features of a velocity model. By using both transmitted and reflected waves the dynamically weighted FWI gradient enables high-resolution model building deeper than those achieved by diving waves alone. This reduces the dependency on long offset data acquisition.
    The FWI approach uses a sophisticated regularization scheme to stabilize the inversion space. This methodology, which forms an extra constraint on the objective function, overcomes some of the limitations of the inversion in the presence of high contrast bodies. The implementation uses the split Bregman method, making it efficient and accurate.
    We demonstrate the benefits of using the new gradient and regularization scheme by presenting the results on an intra-volcanic reservoir velocity model build from the Faroes-Shetland Basin.