Whitepapers

  • Full Waveform Inversion

    TechNote - 30 June 2016

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    Full Waveform Inversion (FWI) is a methodology that seeks to find a high-resolution, high fidelity velocity model of the subsurface capable of matching individual synthetic seismic waveforms with an original raw field dataset. This is achieved iteratively by determining and minimizing a residual; the difference between modeled and recorded data.

  • Complete Wavefield Imaging

    TechNote - 30 June 2016

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    PGS Complete Wavefield Imaging (CWI) is a processing flow developed by PGS which integrates advanced technology for high-resolution velocity model building and depth imaging using reflections, refractions and multiples.

  • 4D Using Non-repeated OBS Acquisition Systems on the Njord Field

    30 May 2016

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    Seismic monitoring has been challenging on the Njord field. The rather weak 4D responses related to production have been difficult to detect due to the noise level in the streamer 4D seismic data and due to dominant overpressure effect after injection. The streamer seismic data was replaced by Ocean Bottom Seismic (OBS) in 2010 and the first repeat was performed in 2014.  The 4D noise level is expected to improve using Ocean Bottom Seismic (OBS) due to repeated receiver positions and better coverage closer to installations. However the sensor technologies and the seismic source were not repeated and we show how we accommodated for this. In addition, we will show how the lack of shallow overburden illumination through the OBS acquisition was compensated for using streamer seismic data and imaging with multiples.

  • Simultaneous Q and Velocity Model Building - Incorporating Attenuation to Enhance Model Resolution

    30 May 2016

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    We present a new model building workflow for simultaneous building of high resolution Q and anisotropic velocity models. The tomography algorithm uses both travel times and spectral ratios from subsurface seismic data to build the Q model together with the anisotropic velocity model. A consistent spectral analysis is achieved by computing spectral ratios in windows that follow the moveout of events in common image gathers. These simultaneous updates ensure that both the Q- and velocity-models are updated in a kinematically consistent way. The jointly derived earth models are used to perform Q anisotropic pre-stack depth migration. The performance of the new workflow is illustrated with an example from a North Sea dataset with a shallow gas cloud. The results show that QVMB generates high quality anisotropic velocity and Q models that greatly enhance imaging below the gas anomalies.

  • A Robust Gradient for Long Wavelength FWI Updates

    30 May 2016

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    We present a robust method to produce long wavelength updates in gradient-based Full Waveform Inversion (FWI). The solution introduces dynamic weights in the velocity sensitivity kernel derived from impedance and velocity parameterization of the classical objective function. The new kernel implementation effectively eliminates the migration isochrones produced by the specular reflections and enhances the low wavenumber components in the gradient in heterogeneous media. It is able to deliver velocity updates beyond penetration depth of diving waves. We use synthetic examples to illustrate how this dynamic weighted FWI gradient successfully recovers the velocity from pre-critical reflections. We also show with dual sensor data from deep-water Gulf of Mexico how the dynamic weighted FWI gradient can combine both transmitted and reflected energy in a global FWI scheme.

  • De-coupling Anomalous Fluid and Lithology Resistivity Effects by Using the Complete Seismic Wavefield

    30 May 2016

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    We present a workflow that enables identification and characterization of leads and prospects by combining all recorded wavefields from dual-sensor streamer seismic data and the sub-surface resistivity derived from Towed Streamer EM data.  Deep tow, dual-sensor seismic acquisition does not only provide low-frequency rich, high signal-to-noise data, that is the foundation for high-resolution broadband imaging, but also serves as the ideal input for refraction based velocity estimation through full waveform inversion (FWI). In shallow water environments conventional 3D seismic imaging often struggles to provide accurate, high resolution imaging of the near surface mainly due to the lack of near offsets (angles) in typical 3D marine seismic data. However, dual-sensor seismic data also provide separated (up-and down-going) wavefields that we use to construct images and image gathers that span a complete range of incidence angles enabling amplitude versus angle (AVA) analysis to be carried out. We show that combining high resolution seismic velocity and resistivity models enables identification and isolation of background trends, and the correlation between anomalously high resistivity and low velocity zones to be investigated. This is a potentially useful precursory/complimentary activity to the robust AVA driven prospect and lead analysis enabled by the dual-sensor seismic data.

  • Deblending by Using Ghost

    30 May 2016

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    The source ghost introduced by the sea surface reflection is usually considered noise which needs to be removed before imaging. We propose to utilize the source ghost in deblending as a natural blending code such that the end result is both deblended and deghosted. This method is easy to combine with other temporal source codes and provides an interesting alternative to deal with the current depth distributed source for a broadband solution. In this abstract, we discuss how to use the source ghosts in the case of lateral blending and vertical blending to deblend and deghost with illustrations of simple synthetic models. We applied the method to field data where two sources are blended in the same lateral position but at different depths. The results obtained show that it is possible to deblend and deghost in one step in the variable depth source setting.

  • Focus on Operational Efficiency and Crew Safety - Introducing Advanced ROV Technology in Marine Towed Streamer Seismic

    30 May 2016

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    Barnacles have been hampering the seismic industry since the start of the offshore seismic era. They attach to the streamer and impact signal to noise. Particularly in the areas close to the equator, there is a constant fight against the barnacles. In-sea cleaning of streamers is typically performed from workboat, exposing the crews to risk.

    A method is presented that removes the human interaction in barnacle cleaning and at the same time significantly improves cleaning window and hence operational efficiency. The method is based on a recently developed Remotely Operated Vehicle (ROV). Unlike traditional ROV’s, typically operated close to zero forward speed, this ROV is launched and operated at seismic speed. The ROV is launched off the deck of support vessels capable of operating over the streamer spread. A typical operational mode for the ROV is to carry self-propelled streamer cleaners and place them on the streamer while at operational depth. The streamer cleaners then travels along the streamer while performing the cleaning duty and are finally recovered by the ROV after having reached the streamer tail end.

  • Full Waveform Inversion and Ambiguities Related to Strong Anisotropy in Exploration Areas – Case Study Barents Sea

    30 May 2016

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    In this case study from the Barents Sea, we have used refraction based Full Waveform Inversion (FWI) in an extreme anisotropy regime without the support from well information. We reveal our observations for decoupling the vertical and horizontal velocity, which enables us to achieve good data matching as well as flat gathers, focused images and a geological consistent model. Our frequency cascaded FWI flow results in a high resolution velocity model to the depth of interest, following the faulted crest in great detail, as well as low velocity zones correlating with the bright spots in the seismic image.

  • On Broadband Data and Rough Sea Surface Receiver Deghosting

    30 May 2016

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    Efficient removal of the ghosts is key to achieving broadband seismic data. Accurate deghosting on the receiver-side can only be performed when both pressure and vertical particle velocity information is available. We use acoustic reciprocity to derive a relationship that defines the total pressure wavefield as a function of the up-going pressure wavefield and the ghost function. Utilizing this relationship, and assuming the shape of the sea surface is known, we propose a pre-stack deghosting method based on integral inversion for a pressure-only dataset. Deghosting by spectral division with a flat sea surface or a statistical ghost function is shown to be special cases of this new inversion based method. The behavior of the pressure ghost function under rough sea condition is analysed in comparison to the flat and the statistical pressure ghost functions. The error of deghosting rough sea pressure-only data with a flat sea surface or a statistical ghost function is quantified using both synthetic and real seismic data.