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.

  • 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.

  • 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.

  • Seismic Noise Attenuation Using Curvelet Transform and Dip Map Data Structure

    12 June 2017

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    The curvelet transform is a known tool used in the attenuation of coherent and incoherent noise in seismic data. It utilises the fact that signal and noise are usually better separated in the curvelet domain than in the timespace (TX) domain. Coefficients of the transform are not independent and neighbouring coefficients are strongly correlated, which existing curvelet-based noise attenuation algorithms do not fully utilise. In this work we propose to use a data structure called a ‘dip map’ to describe dip information in seismic data. This information
    links local curvelet coefficients together in adaptive thresholding or subtraction of curvelet coefficients in seismic denoising algorithms. We used the dip map to improve curvelet multiple subtraction algorithm and the results show significant improvement over traditional methods with real data.

  • A New Look at the Geology and Prospectivity of a North Sea Frontier Area with Modern Seismic - The East Shetland Platform

    12 June 2017

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    The greater East Shetland Platform (ESP) area is characterized by numerous proven Devonian-Eocene fields, but is still one of the few remaining underexplored regions of the UK Continental Shelf.
    In particular, Permo-Carboniferous carbonates and sandstones, Devonian sandstones and fractured basement in prominent structural traps have been successful, proving these units to be viable reservoirs. Some of these are large producing fields in nearby areas (e.g., Buchan, Clair, Auk, with >100 MMboe).
    The improved imaging of the 3D broadband dual-sensor towed streamer data has allowed the interpretation of the Paleozoic horizons over the entire outer ESP region (17,500 km2), enabling interpreters to spread new light on its evolution.
    Possible Paleozoic structural traps are often sealed by the overlying Cretaceous strata. Source rocks could be provided by the ‘classical’ upper Jurassic Kimmeridge Clay (lateral migration from the basin depocentres lying further east). Potential additional oil-prone sources may be provided by middle Devonian shales (c.f., Beatrice Field).
    Burial history modelling indicates that the best case scenario (late generation) did occur over parts of the study area. This supports previous geochemical analyses in analogue areas, which suggested a contribution from the Devonian source for the oils of large fields (e.g., Claymore and Clair fields).

  • Intelligent Adaptive Subtraction for Multiple Attenuation

    12 June 2017

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    A lot of improvements have been seen in the last decade in 3D multiple modelling techniques, such as 3D Surface Related Multiple Elimination (SRME) and 3D wavefield modelling. These processes create accurate multiple models in terms of kinematics and amplitude. The main challenge now is to improve the subtraction of multiples, with a reduction in both residual multiple and primary leakage. This paper describes a new approach for multiple adaptive subtraction, which combines different techniques (least squares filtering in the time-space domain and
    curvelet-based subtraction) and improves primary preservation by using true multiple adaptive subtraction. The ultimate aim is to automatically get the best subtraction according to the complexity of the geology with minimum testing for the geophysicist.