Technical Library

  • Post-EAGE 2020 Comments on Full Waveform Inversion (FWI)

    Author: Andrew Long
    Industry Insights - 26 December 2020

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    The recent virtual EAGE 2020 conference included an interesting discussion on ‘High frequency FWI’. Whilst the short-term motivation is to deliver fast-track interpretation products with an abbreviated processing flow, the ambition of seismic inversion has always been to recover high-resolution elastic medium properties from the earth. After reviewing the traditional separation between low wavenumber background model building with FWI and high wavenumber reflectivity imaging, I consider the nature of the ‘reflectivity’ resolved by running traditional FWI to high frequencies. The issue of whether FWI is ‘imaging multiples’ by nature of being one form of a Least-Squares Reverse Time Migration (LS-RTM) is also considered. This discussion hopefully helps understood the products delivered by high frequency FWI.

  • What’s New in OBN Imaging at EAGE 2020?

    Author: Andrew Long
    Industry Insights - 6 December 2020

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    The source and wavefield spatial sampling of towed streamer acquisition exceeds that of Ocean Bottom Node (OBN) and Ocean Bottom Cable (OBC) acquisition, and the survey costs are considerably less. Nevertheless, the decoupled source and receiver locations, the flexibility of OBN deployment in certain environments, and the growing industry focus upon very low frequency signals and very long offsets for FWI applications are motivating a growing market for OBN. After reviewing some recent developments in the application of Full Wavefield Migration (FWM) and Least-Squares Migration (LSM) of OBN and OBC data, I share a rather contrary perspective on receiver-side spatial sampling, and note how FWM can exploit the decoupled nature of OBN receivers, and may enable sparser (and lower cost) acquisition than is typical. This consideration of sparse and/or irregular OBN acquisition geometry leads to a brief overview of several presentations that address the acoustic imaging of OBN data at the upcoming virtual EAGE conference in December.

  • Mixed Phase Seismic Wavelet Estimation Using the Bispectrum

    Author: Maiza Bekara
    EAGE - 1 December 2020

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    The ability to estimate a mixed phase wavelet is a useful tool for processing and quality control in seismic imaging. The wavelet is estimated using higher order statistics of the data. In practice, these methods tend to show some instability issues when the wavelet length is increased. To improve the stability of the solution, this abstract proposes a new formulation of the wavelet estimation problem that constrains the solution to be a finite duration, phase-only compensation applied to a known base wavelet. The proposed solution works in the frequency domain and consists of three steps. First, the bispectrum
    of the data is deconvolved using the bispectrum of the base wavelet to increase its bandwidth. This helps to improve the sensitivity of third order statistics to phase information. Then, a phase-only wavelet is estimated from the deconvolved bispectrum using an iterative least-squares approach without phase unwrapping. Finally, the estimated phase-only wavelet is conditioned using a projection onto convex sets type algorithm to enforce the constraint of the finite time duration giving the user a control on the amount of phase deviation from the base wavelet. Test examples on synthetic and real data both show reliable results with robustness to noise contamination.

  • Broadband 4D Seismic Provides New Level of Detail

    Author: Yulia Biryaltseva, Tor Vegar Mårdalen, Per-Harald Saure-Thomassen, Marta Wierzchowska, Anastasiya Tantsereva, Julien Oukili
    GeoExpro - 1 December 2020

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    Understanding new details in the 4D seismic signal uncovered by a broadband timelapse dataset at Gullfaks Main Field in the North Sea.

  • Full Waveform Inversion Using Wave Equation Reflectivity Modeling

    Author: Yang Yang, Jaime Ramos-Martinez, Norman Dan Whitmore, Alejandro Valenciano, Nizar Chemingui
    EAGE - 1 December 2020

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    Full Waveform Inversion (FWI) is routinely used to improve the accuracy and resolution of velocity models. However, utilizing reflections to produce low-wavenumber updates creates more operational challenges than using transmitted events. In order to simulate backscattered sensitivity kernels, FWI needs hard boundaries in the velocity/density models. Alternatively, one can apply the wave equation and first-order Born approximation to decompose the seismic wavefields into background and perturbation. Here, we derive an acoustic wave equation in terms of vector reflectivity to be used as the wave’s propagation engine of FWI. The new derivation results from the change of variables from impedance to reflectivity in the variable density wave equation. The main advantages of its insertion in our FWI algorithm are the following: it does not require the construction of density/hard boundaries in the velocity model to generate reflections; it allows the use of reflected events without the need of solving two different wave equations in the forward and backward propagation; it is more accurate than the method based on the first-order Born approximation to perturbation theory. We show synthetic and field data examples illustrating the advantages of the new algorithm.

  • Continuous Wavefields Method - Results from a Shallow Water Field Test

    Author: Stian Hegna, Tilman Klüver, Jostein Lima
    EAGE - 1 December 2020

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    In the autumn of 2019, PGS acquired a small 2D test offshore Malaysia in water depths ranging mainly between 125 and 200m. The primary purpose of this test was to demonstrate the continuous wavefields method in shallow water areas. In addition, different source configurations and time intervals between consecutive actuations of airguns were tested. In this paper we discuss the different source configurations that were tested, and present preliminary results. The overall sound exposure levels (SEL) are very similar for the different source configurations. The small differences are mainly related to trigger time intervals of the individual air-guns, and to air-pressure. Preliminary results achieved so far indicate that the continuous wavefields method works well in shallow water, and with relatively small differences between the different triggering schemes and source configurations.

  • Recent advances with wide-tow multi-sources in marine seismic streamer acquisition and imaging

    Author: Martin Widmaier, Rune Tønnessen, Julien Oukili, Carine Roalkvam
    First Break - 1 December 2020

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    The authors share progress made with towing marine sources wider to enable better acquisition efficiency and improved near offset coverage for more accurate seismic images.

  • Redefining High Resolution Multi-Azimuth Towed Streamer Acquisition on the Norwegian Continental Shelf

    Author: David O'Dowd, Martin Widmaier, Carine Roalkvam, Marielle Ciotoli, Luca Limonta
    EAGE - 1 December 2020

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    The introduction of wide-tow multi-sources to seismic streamer vessels enables improved near offset sampling and thus the recording of near incidence angle information needed for shallow target AVO analysis. Long offset streamer tails can be added to the high-resolution acquisition set up, delivering the offsets required for FWI driven velocity model building. Combining the benefits of wide-tow sources with extra-long streamers in addition to acquiring in multiple azimuths can solve exploration challenges in areas like the North Sea. We have chosen the Viking Graben Offshore Norway for a novel high-resolution multi-azimuth towed streamer acquisition project starting in 2019. The imaging challenges in the Viking Graben include isolated cemented injectites that have historically resulted in shadow zones at target levels. This case study includes the key technical advances in towed streamer acquisition and how survey design has incorporated these to solve the challenges. The application of the latest technology on a single seismic vessel enables advanced multi-azimuth acquisition at a much lower cost compared to OBN operations. We will present the acquisition concept and the imaging results from the 2019 exploration project.

  • Quantifying 4D Repeatability Improvements with Evolutionary Acquisition on the Gullfaks Field, North Sea

    Author: Marta Wierzchowska, D. Anderson, Julien Oukili, Y. Biryaltseva, D. Fischer, E. Sadikhov, B. King
    EAGE - 1 December 2020

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    This article presents quantitative measures of geometrical repeatability versus time-lapse data quality, showing the advantages of the deep-tow multisensor streamer acquisition for 4D projects. Authors demonstrate that the multisensor streamer enabled the optimization of the acquisition turnaround together with the improvement of the 4D data quality. They propose to look at both the NRMS and the fNRMS attributes (NRMS value calculated per
    single frequency) to evaluate 4D repeatability and therefore data quality improvements with evolving acquisition technology over the last 34 years of the reservoir monitoring program over the Gullfaks field, North Sea.

  • Can we reduce significantly the number of OBN by using Full Wavefield Migration?

    Author: Didier Lecerf, Amir Asnaashari, Abderrahim Lafram, Rasaki Salami
    EAGE - 1 December 2020

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    This paper analyses the ability of Full Wavefield Migration (FWM) algorithms for providing an extended illumination for imaging without the OBN positioning constrain. Using deep-water OBN datasets, we investigate the effect of minimal sensor distributions on the resolution of the seismic image. Unusual OBN acquisition parametrization such as very limited aperture, extensive separation distance and innovative “donut” layout designs are investigated and discussed. The results demonstrate that the Full Wavefield Migration algorithm allows the reduction of OBN density, enabling a large distance between receivers and providing more geometry flexibility without compromising the image quality. An innovative OBN acquisition with a “donut” design is proposed for combining velocity modeling using refracted and diving waves (FWI) and imaging with Full Wavefield Migration. Reducing the number of nodes deployed for the same final image quality can
    positively influence the economics of an OBN acquisition survey.