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.

  • 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.

  • Separated Wavefield Imaging for Shallow Geohazard Analysis - An Ocean Bottom Study on a North Sea Dataset

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

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    We present a method using separated wavefield imaging to mitigate risk associated with well drilling. In shallow water environments the process of geohazard  assessment is challenging. Conventional acquisition processed for shallow hazard assessment is deficient in shallow reflectivity. This leads to appraisal difficulties. High resolution site surveys are often acquired and then processed, but incur additional costs. Imaging with separated wavefields uses up-going and down-going wavefields at the surface to deliver high-resolution images of the subsurface. It takes advantage of the extended illumination provided by surface-multiple energy. We present a feasibility study for using this technology on a shallow water ocean bottom cable acquisition, demonstrating the suitability of this technology to mitigate geohazard assessment.

  • Removal of Time-varying Sea Surface Effects from Seismic Data

    Author: Elsa Cecconello, Endrias Asgedom, Walter Söllner
    EAGE - 4 June 2018

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    Removal of the sea surface effects is an important step for subsurface imaging. In this abstract, we investigate source deghosting and demultiple considering time-varying rough sea surfaces. We derive, based on Rayleigh's reciprocity theorem, integral equations for modelling the source ghost and surface related multiples from timevarying sea-surfaces. From the derived theory, we show that source deghosting fails in the presence of free-surface multiples. This observation is supported by synthetic data examples. However, if the time-varying sea surface interactions are solely in the input wavefields, the inversion is successful. Thus, by applying source deghosting and demultiple by integral inversion, all effects from the time-varying sea surface are removed in one step.

  • Bandwidth Enhancement by Constrained Convex Optimisation of Spectral Scalars

    Author: Maiza Bekara, Chris Davison, Peter Stroia-Williams
    EAGE - 4 June 2018

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    Bandwidth enhancement is a solution that is used to increase the vertical resolution of seismic images. The objective is to improve the interpretation of thin geological structures. The enhancement is achieved by increasing the temporal frequency content of the seismic data using approaches such as de-convolution and spectral scaling.
    The main problem with these approaches is that they also tend to boost the level of noise. In this abstract, we propose a window-based spectral enhancement solution that has a minimal side effect of noise boosting. The solution first determines, from the data, the useful signal bandwidth that needs to be extended. Then, spectral scalars are computed using a constrained convex optimisation process to obtain a flat output spectrum. The method is computationally cheap, works with few parameters and does not produce any spectral boosting when the data has a poor SNR.