Technical Library

  • Separated Wavefield Imaging for Ocean Bottom Surveys - Feasibility Study of Receiver Decimation on a North Sea Dataset

    Author: Dennis van der Burg, Tony Martin, A. Asnaashari, Stephane Perrier, J. Fasterling, J. Musser
    EAGE - 4 June 2018

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    We present a method for imaging with separated wavefields from an ocean bottom cable acquisition to deliver images of the subsurface with enhanced illumination and angular diversity. Using this data, we investigate whether it is feasible to decimate the sources or receivers without degrading the structural image from separated wavefield imaging. We conclude by determining if this would be beneficial to reduce acquisition cost and to provide illumination below obstructions like a platform hole.

  • Making the Transition from Discrete Shot Records to Continuous Wavefields – Real Data Application

    Author: Tilman Klüver, Stian Hegna, Jostein Lima
    EAGE - 4 June 2018

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    In this paper, we present real data applications of the acquisition and processing method outlined in the companion paper (Hegna, S., Klüver, T., and Lima J., 2018, Making the transition from discrete shot records to continuous
    wavefields – Methodology: Expanded abstract submitted to the EAGE annual meeting). We discuss the design of sources that emit continuous wavefields approaching the properties of white noise using air-guns. The design
    minimizes correlation between sources and allows a six-source configuration with a vessel towing six strings of airguns, thereby increasing efficiency or cross-line sampling. The environmental benefit of spreading source energy in time will be demonstrated. We illustrate the processing method based on continuous source and receiver wavefields on seismic data acquired on top of data acquired previously based on discrete shot records. Both data sets will be compared at image level.

  • Intra-Chalk Porosity Variations in Norway-Danish Central Graben: Integrated Mapping Using Broadband Elastic Attributes

    Author: Noémie Pernin, Tim Bird, Cyrille Reiser
    EAGE - 4 June 2018

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    Understanding the influence of porosity on elastic properties through rock physics analysis is essential in seismic reservoir characterisation and key in the context of the chalk play. It is known that acoustic impedance is strongly correlated with porosity in carbonates but also that the chalk properties can vary widely creating intra-chalk layers of varying reservoir quality. This integrated study shows how reliable broadband relative elastic attributes tie at the wells and are able to map lateral and vertical changes of porosity within the Chalk interval in the Norwegian and Danish North Sea Central Graben.

  • Shot to Shot Source Signature Variation Correction

    Author: Hocine Tabti, Jens Fredrik Wisløff, Torben Høy, Martin Widmaier
    EAGE - 4 June 2018

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    In this paper we discuss the ability to correct for shot to shot source signature variations and show how this could impact current survey source specifications. Contractual survey specifications define acceptability limits as to which
    changes can be tolerated based on source wavelet attribute changes. Parts of two acquisition sequences have been acquired with deliberate survey source out-of-spec configurations. The out-of-spec tests included both source dropouts and pressure drop in separate sequences. Shot by shot source signatures have been generated from near field measurements. These were used to correct the seismic data for source wavelet changes including those caused by the out-ofspec pressure and source dropouts. We have demonstrated in this work that changes in the source signature, including changes beyond out-of-spec limits, can be estimated at every shot point and can efficiently be corrected for in processing. Current source out-of-spec limits are possibly too rigid if source variations
    can be estimated at each shot point and corrected for in processing.

  • 4D Broadband Towed-Streamer Assessment, West Africa Deep Water Case Study

    Author: Didier Lecerf, David Raistrick, Bertrand Caselitz, M. Wingham, J. Bradley, B. Moseley
    EAGE - 4 June 2018

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    A 4D broadband assessment has been performed in a deep offshore West Africa environment using repeated dualsensor streamer acquisitions. Three sail lines have been re-acquired only a few weeks after the original acquisition with standard 4D acquisition consideration. The study has compared the data  repeatability metric NRMS along the processing sequences using, on one side, a reconstructed band limited 4D data corresponding to a single hydrophone and on the other side an extended bandwidth 4D data using the up-going wavefield only. Because the datasets have been recorded using the same acquisition, the main differences come from the bandwidth discrepancy and the variable sea-state. The 4D “Up-going on Up-going” preserves, for all processing steps, around 1% NRMS benefit against the 4D hydrophone-only. Because the NRMS comparison is biased by the dominant frequency discrepancy, the 1% gain does not fully reflect the detectability advantage of the 4D broadband. The NRMS analysis on the low frequency part has demonstrated the clear improvement on 4D broadband and a
    qualitative evaluation on the 4D differences has highlighted the negative effect of the sea-state on the hydrophoneonly 4D results.

  • A Method to Suppress Salt-related Converted Wave Using 3D Acoustic Modelling

    Author: Jyoti Kumar, Mamdouh Salem, D.E. Cegani
    EAGE - 4 June 2018

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    Converted waves can be recorded in a marine environment in the presence of large velocity contrasts, like a salt body. Salt-related converted waves can significantly contaminate P-wave image. The most noteworthy converted wave recorded in a salt province is generated from the base of salt. In this paper a method has been proposed to attenuate this energy before migration to improve sub-salt imaging. A clean converted wave model is generated using 3D acoustic forward modelling. This is then adaptively subtracted from the input dataset. Application of the proposed method is demonstrated in a complex deep water salt environment data set from the Mediterranean Sea.

  • Least-squares Inversion for Imaging the Full Seismic Wavefield

    Author: Shaoping Lu, Faqi Liu, Nizar Chemingui, Mikhail Orlovich
    EAGE - 4 June 2018

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    We present a Least-Squares inversion solution for depth migration of the full seismic wavefield including both primaries and high-order reflections. Conventional seismic processing uses only primary reflections for imaging. It
    treats high-order reflected events as noise and removes them before migration. Separated Wavefield IMaging (SWIM) takes advantage of the extended  illumination provided by surface-multiple energy and delivers highresolution
    images of the subsurface. Primary and high-order reflections are complementary, and can augment the overall imaging results when properly combined. However, imaging the full wavefield of both primaries and multiples in a standard migration has been challenging for balancing the contribution of each component. In
    addition, imaging of the full wavefield includes a blending process, which roduces crosstalk. Inversion automatically weighs the primary and multiple reflection energy and directly estimates the earth reflectivity that explains boundary data without matching the crosstalk. Successful applications to both synthetic and field data examples demonstrate that Least-Square Full Wavefield Migration (LS-FWM) resolves the crosstalk issue in FWM and greatly improves the imaging illumination and resolution compared to conventional migration.

  • Imaging Using PP Multiples: Practical Applications for Shallow Imaging and 4D Monitoring on the Snorre Field, Norway

    Author: Julien Oukili, Andrey Pankov
    EAGE - 4 June 2018

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    Multiples are traditionally considered as noise for conventional data processing. However, it can be used for imaging in order to extend the illumination recovery. In this paper, we present the results of imaging using multiples on PRM datasets from the North Sea including two vintage surveys and minimal data pre-conditioning. For 4D analysis, the up-going wavefield is used as it is less affected by water layer variations. The images using multiples show very little footprint in the shallow overburden and, to some extent, an interpretable image in the large
    obstruction zones. We show that the images using multiples also contain the expected 4D effects, though the results seem to be contaminated with crosstalks. Finally, we propose to test the method on different wavefields recorded by the multicomponent system to demonstrate the robustness of the deconvolution imaging conditions and observe the difference in illumination provided by different components.

  • Newly-Observed Post-Variscan Extensional Mini-Basins: The Key to the Prospectivity of the Under-Explored Platform Areas

    Author: Stefano Patruno, Verena Lampart
    EAGE - 4 June 2018

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    Thick Paleozoic successions are buried under the underexplored platform areas flanking the main North Sea depocentres. Here, we focus on the tectono-stratigraphy and prospectivity of the East Shetland Platform and Norwegian-Danish Basin, on the U.K. and Norwegian-Danish Continental Shelf respectively. These areas have been extensively covered with 3D broadband dual-sensor towed streamer (GeoStreamer®) seismic surveys, enabling improved imaging and play understanding. Although differences are apparent between the study areas (e.g., Zechstein salt thickness and mobility), they all underwent a similar post-Iapetus poly-phase tectonic evolution, and consist of alternations between Paleozoic highs with long-lasting hiatuses and post-Variscan intra-platform basins, with a near-complete succession and maximum sedimentary thickness of 5-7 seconds two-way-time.
    Numerous high effective porosity reservoir units are highlighted (including Devonian-Permian sandstones), and large Paleozoic structural traps are potentially present below intra-platform basins. Hydrocarbon charge occurs via
    lateral migration from Jurassic kitchens and/or vertical/lateral migration from deeper Devono-Carboniferous source intervals. Evidence of active hydrocarbon generation are present at even c. 100 km from the closest Jurassic source kitchen (e.g., seeps, pipes, shallow gas).
    This study argues that post-Variscan intra-platform basins play a crucial and often under-appreciated role in North Sea platform prospectivity, potentially enabling both Paleozoic source maturation and pre-Cretaceous trapping.

  • Spread Spectrum Sweep Strategy to Improve Operational Efficiency with Marine Vibrators

    Author: Rune Tenghamn, Andrew Long, Christian Strand, Manuel Beitz
    EAGE - 4 June 2018

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    A spread spectrum sweep strategy is described that enables efficient and flexible opportunities for towed marine vibrator seismic surveys capable of generating arbitrary signals with full phase control over the frequency of interest to seismic imaging. Continuous source operation with orthogonal signal sequences enable the use of many simultaneous marine vibrator units whilst towing at efficient vessel speed. Post-survey, the desired shot point interval for the optimized imaging of a particular depth range can be optimized by combining varying numbers of sub-sequences: shallow targets are imaged with short sub-sweeps and therefore short shot point interval, and deeper targets are imaged with longer sub-sweeps, longer shot point interval, and higher signal amplitude by virtue of longer sweeps. Overall, this strategy may enable less marine vibrator units by comparison to that required to achieve a given sound pressure level (SPL) using conventional sweep strategies, have significantly lower environmental impact than any air gun source design, and enable greatly improved operational efficiency.