Technical Library

  • Water column corrections, joint water velocity inversion for 4D marine surveys.

    Author: Didier Lecerf, Mathieu Lange, Andrew Oates, Jyoti Kumar
    EAGE - 25 May 2022

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    Water column variations are an important source of non-repeatability in time-lapse marine surveys. In a deep water context, the physical property variations within the water layers can generate significant time-shifts between repeated time-lapse seismic data. We present a new methodology for estimating water velocity changes and correcting the 4D seismic datasets. Each sail line is migrated independently and Common Depth Point (CDP) gathers are produced for the overburden along a subsurface strip for both vintages. The 4D approach consists of performing cross-correlations using collocated CDPs for each sail lines pair. It creates water bottom “cross-image CDP gathers”. The cross-image CDP curvature along offsets is used to compute the water velocity difference between the two vintages. Once a water velocity is inverted for each sail line, the kinematic correction is performed on the pre-migrated datasets. The main difference with conventional approaches is the simultaneous usage of both 4D datasets for estimating the water velocity changes and therefore minimizing the seismic difference in the overburden. The methodology is significantly beneficial for deep-water 4D acquisitions. The correction for such small environmental variations improves the time-lapse data repeatability and is part of the effort for providing high resolution 4D images.

  • Low-Frequency Marine Seismic Source Considerations

    Author: Andrew Long
    Industry Insights - 13 December 2021

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    Driven largely by the significance of Full Waveform Inversion (FWI) in many seismic imaging workflows, several marine seismic source concepts have been developed over the years that share a common ambition of displacing a large volume of water (hundreds of liters) per cycle to yield high amplitudes in the 1-8 Hz frequency range where the output from traditional air guns decays rapidly. Most low-frequency source concepts are either large-volume pneumatic devices that variously operate at low or high pressure, or large-volume mechanical resonators or vibrators that displace the surrounding water with a flexible external surface. For reasons of practicality and to reduce cost, most low-frequency source concepts are likely to be used with sparse source lines and large ‘shot’ intervals. Nevertheless, it can be demonstrated that dense 3D spatial sampling of both the source and receiver wavefields will often be beneficial to multi-channel signal processing or wave equation-based imaging workflows, including FWI.

    I provide a simple framework to understand the comparative merits of marine seismic low-frequency source concepts recently published at EAGE 2021 and elsewhere. Overall, finding an efficient solution that generates high-amplitude low-frequency data remains a key historical challenge, but some recent progress is evident. I briefly consider the comparative elements of two low-frequency pneumatic source concepts (the Tuned Pulse Source concept of Sercel, and the Gemini concept of ION), the Wolfspar mechanical resonator of bp, and the relevance of the eSeismic method of PGS to acquire continuous wavefields from individually triggered air guns. I also consider methods to 'manufacture' additional low-frequency amplitude content using either ambient noise interferometry or some form of machine learning and conclude with a consideration of low-frequency source deployment factors that may in fact contaminate FWI efforts and present a challenge to model convergence.

  • Maximizing quality and efficiency with wide-tow multi-source configurations

    Author: Martin Widmaier, Carine Roalkvam, Julien Oukili, Rune Tønnessen
    First Break - 9 November 2021

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    The authors present the latest achievements in multi-sensor streamer acquisition with wide-tow sources and how these have optimized high-resolution imaging of the shallow subsurface.

  • EAGE 2021: Decarbonization the Catalyst for a New Geoscience Era?

    Author: Andrew Long
    Industry Insights - 26 October 2021

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    Held in Amsterdam on October 17-22, the annual EAGE conference provided a particularly interesting industry forum as the world moves into a lower carbon future. Carbon Capture and Storage (CCS) featured strongly in the program, and the proportional CCS content of future conferences is expected to grow substantially. I comment on short-term applications of surface seismic methods and geological paradigms to CCS and note that much R&D into both geophysical and geochemical aspects is necessary to support the likely scale of CCS for global net-zero goals.

    Synergies between towed streamer and ocean bottom node (OBN) acquisition had a high profile—with particular emphasis upon wide-tow multi-source developments, low-frequency seismic considerations, and various continuous wavefield source concepts. For seismic imaging, Full Waveform Inversion (FWI) has progressed beyond a velocity model building tool to now yield seismic interpretation deliverables of various sophistication. The most complete realization combines model building and full-wavefield least-squares migration into an abbreviated workflow for rapid project turnaround. Overall, it is evident that greater seismic acquisition and imaging effort, combined with better integration of geoscience and engineering methods, is necessary to solve long-standing conventional hydrocarbon discovery and recovery challenges, and to meet the unique subsurface resolution and characterization requirements for the transition to a net-zero carbon emissions. This may seem familiar, but a clear urgency exists to accelerate access to better data—augmented of course by machine learning and other automation platforms—and to throw everything at previously unassailable problems on quite grand scales.

    An elephant in the room is whether the challenge of decarbonizing the planet with sustainable, affordable and accessible energy sources can motivate a new generation and boost recruitment to the geosciences and engineering – and on what timescale. Several forum discussions attempted to address these challenges.

  • Broadband processing improves 4D repeatability and resolution at the Sleipner CO2 storage project, North Sea

    Author: Marta Wierzchowska, H. Alnes, Julien Oukili, Christian Otterbein
    EAGE - 1 October 2021

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    The Sleipner natural gas field situated in the Norwegian sector of the North Sea is the world’s longest-running industrial-scale CO2 storage project. The CO2 injection commenced in 1996, inserting almost one million tonnes (1MT) of CO2 per year into the Utsira Fm. By 2020, over 18 MT of CO2 had been securely stored. The acquisition and processing used for Sleipner CO2 seismic monitoring program has evolved over several years in a successful and cost-effective monitoring program. Employing up-to-date processing technologies, including broadband solutions and 3D demultiple, has recently helped to reduce uncertainties in 4D interpretation and increased the resolution needed to reveal new details of the CO2 plume movement. Within the Utsira Fm., it is now possible to track some thin shale layers that can be important for predicting future growth of the CO2 plume. The deeper layers of CO2 are more well-defined. These have been historically difficult to interpret due to poor imaging in the previous 4D datasets.

  • Parametric inversion of water column velocity for cold water statics correction in ocean bottom seismic surveys

    Author: Maiza Bekara, Chris Davidson, Ramzi Djebbi
    EAGE - 1 October 2021

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    The abstract describes a methodology to compute time-varying, depth-dependent water column interval velocity profiles from Ocean bottom seismic data. The objective is to use these profiles to correct for cold water statics. They are inverted over time slots using direct arrival picks. The inversion is an integrated part of a flow that jointly inverts also for the nodes’ positions and their clock drift. The main innovation in this work is the use of a second-order polynomial approximation to model the velocity profile as function of depth. The rationale is to relax the need for an accurate initial velocity profile which is required by the standard methods and to improve the fitting for a more accurate velocity estimation. The parameters of the polynomial model are constrained to give a
    physically sensible velocity profile. The method is tested on synthetic and real data in comparison with a standard method. It performs quite well in terms of fitting the direct arrival picks and gives an overall better velocity estimation in terms of accuracy and time resolution. For the real data, the most visible uplift was for the far offsets (outer lines), where the standard method is known to produce biased velocity values.

  • Marine Seismic Technology Workshop: Highlights and Summary

    Author: Andrew Long
    Industry Insights - 23 September 2021

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    The SEG Advances in Marine Seismic Data Acquisition Workshop was held virtually on September 7-9, attracting about 80 registered participants. 24 presentations were spread over three half-day timeframes and six sessions. I summarize key elements of the workshop content below.

    Topics covered included increased near-surface seismic resolution, time-lapse 3D (4D) seismic, towed streamer and ocean bottom node (OBN) sensor considerations, distributed acoustic sensor (DAS) fiber optics, reducing towed streamer survey cost by pragmatic compromises in survey design, reducing OBN survey cost with autonomous node designs, fundamentally-different approaches to spatial and/or spectral wavefield sampling, multi-source towed streamer case studies, and marine vibrator insights. Overall, it is clear that significant capital efforts are inevitably required over long timeframes to bring ‘new acquisition concepts’ to fruition—more so when the concepts represent a fundamental departure from traditional hardware platforms. Correspondingly, creative methods that repurpose traditional acquisition hardware and survey design templates can offer significant data improvements without necessitating expensive engineering development.

  • New Shallow Seismic Insights

    Author: Andrew Long
    Industry Insights - 31 May 2021

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    We will continue to use high-resolution seismic to understand the subsurface risks and opportunities in marine areas as we transition to renewables and lower carbon energy sources as a complement to fossil fuels. CO2 storage in aquifers must be monitored for shallow seal integrity and offshore wind farms require geotechnical ground models and seismic characterization of the near surface. This article addresses the question of whether we will necessarily require entirely new seismic acquisition solutions, or whether existing methods can be adapted to provide new value and insights.

    Towed-streamer seismic using tailored acquisition configurations provide an efficient method of accessing high-resolution datasets for several purposes, and some examples are given below. I also show how full wavefield imaging solutions can high-grade existing seismic data to significantly enhance the resolution and quality of shallow images.

     

  • Seismic Processing - What is Required?

    Author: Tony Martin, Øystein Korsmo, Nizar Chemingui
    EAGE Workshop on Optimizing Project Turnaround Performance - 24 February 2021

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    In a turnaround and cost-conscious environment, do we really need to apply all the algorithmic processes in a seismic processing sequence? Full wavefield migration may be one way to eliminate certain processes. If we treat the full wavefield migrated image as part of an inverse problem in a least-squares migration, we may exclude more steps. Least-squares full wavefield migration (Lu et al., 2018) uses the raw seismic data, and many processing steps in both the data and image domain can be excluded, potentially reducing turnaround whilst maintaining, or improving, image quality for the entire data record.

  • 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.