Pre-Show Focus | Improving Today For a Better Tomorrow

Andrew Long provides a preview of PGS technical talks in the context of the wider EAGE technical program. 

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Last Minute Covid Changes

The 2021 EAGE technical program contains about 200 oral sessions, ePoster sessions, workshop events, short courses, and special forums distributed over the days of October 17-22 in a hybrid mixture of live and pre-recorded content.

When the EAGE 2021 program was first published many weeks ago, I wrote a fairly detailed summary of the content. Since then the program has been quite significantly rearranged—most notably with respect to the number of Forum Sessions during program breaks, and regarding the Workshop content. The original 16 workshops are now 10, apparently because the EAGE organizers insisted that all speakers appear in person. This quite unreasonable expectation resulted in several cancellations, and one also wonders how many planned conference registrations were converted to virtual participation because of employer concerns about covid risks. As was the case at the recent IMAGE21 conference in Denver, I expect the clear message arising at EAGE 2021 is a clear need to embrace virtual participation for all future events. Even if the pandemic disappears, there have always been so many geoscientists unable to physically travel for financial and logistical reasons. Time for the major professional societies to get their act together and make virtual platforms accessible and highly functional for everyone.

Anyway, sanity did obviously prevail in some cases, and some workshops were converted to Dedicated Sessions that include online speaker participation. Now a highlight of the overall program, the Dedicated Session themes include Induced Seismicity, Innovations and Game Changers, Energy Transition, several topics related to Reservoir Management, Petroleum Systems of NW Europe, Depositional Processes, and two sessions dedicated to Low Frequency Seismic Data Acquisition and its Impact on Imaging and Inversion. I have a particular interest in the latter topic, and although no abstracts are available, I plan to write a dedicated summary after the EAGE 2021 event.

Preparing For Transition

Although the overall EAGE 2021 program does contain several sessions dedicated to energy transition / net-zero carbon topics such as CO2 storage, geothermal energy and mining, the technical content remains overwhelmingly focused upon the characterization and recovery of convenventional hydrocarbons. There are, however, a series of Forum Sessions in the main program breaks each day that address the ‘Great Career Challenge – the Changing Education and Opportunities for Tomorrow’s Energy Professionals’ (two sessions), ‘Role of Geoscience and Engineering in Meeting Decarbonization Goals’ (two sessions), and ‘How the Oil Industry is Addressing the Energymix to Meet the Goals of the Transition Era’ (2 sessions). Such themes will expectably be increasingly represented in future EAGE event technical programs.

As has traditionally been the case, a large component of the EAGE 2021 program is dedicated to reservoir management, with session themes such as Reservoir Engineering, Integrated Subsurface, Mining & Civil Engineering, SPE and Joint EAGE – SPE. 4D (time-lapse 3D) technical content has a heavy component related to CCS (carbon capture and storage), as well as applications to geothermal and energy storage in the subsurface.

I note that as CCS grows on a global scale, there is sometimes voiced an opinion that 4D monitor surveys will be done as cheaply and coarsely as necessary only to satisfy regulatory requirements. This cynical perspective should be revised in light of the presentation by Wierzchowska et al. of PGS and Equinor, titled ‘Broadband processing improves 4D repeatability and resolution at the Sleipner CO2 storage project, North Sea’. The Sleipner CO2 seismic monitoring program in the North Sea has evolved over several years, is the world’s longest-running CCS project, and has involved many contrasting seismic platforms for the monitoring. It is shown that broadband solutions recently helped to reduce uncertainties in 4D interpretation, and increased the resolution necessary to reveal new details of the CO2 plume movement. In contrast, previous 4D datasets have historically been difficult to interpret due to poor imaging. There is no room for compromise…

Seismic Acquisition Continues to Blossom

Several EAGE 2021 presentations address the increased use of OBN (ocean bottom node) surveys as a complement to towed streamer acquisition in marine seismic surveys. Much of the growth in OBN has been driven by the commoditization of FWI (full waveform inversion) for velocity model building, and the traditional dependence of FWI upon long offsets not historically afforded by streamer acquisition. More on FWI later, as the technology continues to change rapidly, but it can generally be observed that ‘hybrid streamer-OBN’ surveys are more common with ‘sparse’ OBN deployment driven by the fact that OBN acquisition remains expensive. Indeed, PGS recently completed the acquisition of a truly innovative combined wide-tow hexa-source towed streamer survey with the sources towed over the streamers, and over a thousand OBN sensors on a regional grid used to simultaneously record the entire survey. You can read about the exciting results in the recent IMAGE21 presentation by Dhelie et al. of Lundin Energy titled ‘Combining nodes and streamers to tackle the imaging challenges of salt basins in the Barents Sea’.

Towed streamer technology is also improving, and wide-tow multi-source acquisition has revolutionized the high-resolution imaging of shallow geology during high-efficiency towed streamer surveys. In addition, the use of long streamer ‘tails’ now enables much deeper FWI model updates; and where desired, multi-azimuth multisensor acquisition combines dense subsurface illumination and AVO-compliant imaging at much higher efficiency than OBN surveys. There is still much scope to explore with towed streamer survey designs, and the collective platform at the heart of several PGS presentations at EAGE 2021 is known as ‘GeoStreamer X’.

Oukili et al. use ‘High resolution meets high efficiency with an ultra-wide-tow penta source solution in the Barents Sea’ to showcase how an efficient ultra-wide-tow penta source set-up improved trace density and near offset coverage in a Barents Sea survey with various near-surface challenges. The CMP grid was 6.25 x 5.625 m, and the shot interval was only 7.5 m, with demonstrable benefits for higher fidelity imaging workflows.

In a related presentation by Limonta et al. titled ‘Novel acquisition design to improve illumination for velocity estimation and imaging North Sea case study’, a novel marine multi-azimuth acquisition solution that included long streamer ‘tails’ enabled better illumination below and within complex velocity structures in the Viking Graben area of the North Sea, with demonstrable benefits for deep and high-resolution FWI model building, improved multiple removal, and consistent image quality from the seafloor to basement.

Reiser and Bird also use ‘Multi-azimuth quantitative Interpretation: A case study from the South Viking Graben, Norway’ to emphasize how elastic attributes derived from multi-azimuth data enabled the computation of better reservoir attributes, identified several untested (likely) oil accumulations, and overall, yielded much richer subsurface information. An interactive rock-physics modeling platform named rockAVO was used to assess the variation and sensitivity of both elastic properties and pre-stack seismic responses to changes in reservoir properties. The multi-azimuth GeoStreamer X data is shown to delineate and map the various known fields and discoveries at all stratigraphic levels in a superior manner, as well as highlighting new near-field exploration leads and opportunities.

PGS continues to also deliver new paradigms in marine seismic technology. Hegna uses ‘Continuous Wavefields Method - The acoustic wavefield generated by the seismic vessel' to show how to image the subsurface without an active source: the acoustic signals associated with the seismic vessel may instead provide a viable seismic solution in the most environmentally restrictive settings. Note that this application is an extension of the ‘Continuous shooting and recording methodology’ known as ‘eSeismic’, and is another reminder that continuous source wavefields in various forms may soon play a more prominent role in marine seismic.

As noted earlier, OBN acquisition is growing, but OBN processing presents many unique challenges—one of which is the long-standing topic of solving for cold-water statics. Bekara et al. present ‘Parametric inversion of water column velocity for cold water statics correction in Ocean bottom seismic surveys’, which describes a methodology to compute time-varying, depth-dependent water column interval velocity profiles from OBS data. Signal processing challenges related to wavelet phase are always fundamental to successful imaging, and Bekara uses ‘Mixed phase seismic wavelet estimation using the Bispectrum’ to demonstrate how the stable estimation of mixed phase wavelets can be improved, particularly when the wavelet length is increased—also relevant to the low frequency topics below.

Moving Beyond the Dogma of Cascaded Model Building and Migration

Seismic processing and imaging has historically been divided into two broad categories: everything associated with many ‘preconditioning’ steps such as designature, denoise/demultiple and data regularization prior to migration; and the migration step itself which comes in many flavors. The long-awaited commoditization of FWI has changed this traditional viewpoint, as

  1. FWI is applied to field gathers with minimal pre-processing
  2. FWI itself uses a powerful depth imaging kernel that includes the modeling of multiples (reverse time migration; RTM)
  3. FWI has in recent years been uses as a proxy for traditional migration—via spatial derivatives of the velocity model to yield an ‘FWI image’. The latter product circumvents traditional cascaded processing flows of many stages by yielding an interpretation product ‘in one step’—at high computational cost, and inclusive of several approximations and assumptions

So it is understandable that FWI again has several dedicated oral and ePoster sessions, and plays a part in several workshops, including ‘WS09: High Resolution Full Waveform Inversion: Is It Only Cosmetics, or Is There Any Value for Imaging and Interpretation?’.

PGS has three significant contributions to the FWI journey. The first two FWI presentations build upon important PGS contributions in recent years. Martinez et al. pioneered the use of a weighted ‘inverse scattering’ imaging condition in 2016 to remove the high-wavenumber migration isochrones from the velocity kernel in FWI, thereby enabling accurate low-wavenumber model building to large depths with both transmitted and reflected wavefields; and Whitmore et al. later developed a new version of the acoustic wave equation known as ‘vector reflectivity’ to accurately initiate reflections during forward modeling—without any use of a density model, and even when the model is smooth and immature.

At EAGE 2021, Korsmo et al. will present a joint paper with BP authors titled ‘FWI to full bandwidth with Vector Reflectivity and Inverse Scattering Imaging Condition, Clair field OBN’. FWI run to 60 Hz on a high-density OBN dataset benefitted from the ability to solve for the background model without artifacts, before inverting to high frequencies. The FWI model correspondingly captured significant high-resolution details at a large range of depths.

Huang et al. will present ‘Extended domain FWI via time warping’, which describes a new FWI method that uses time-warping as the extension domain to overcome cycle-skipping in an extremely robust manner. Dependence upon accurate starting models and/or the acquisition of low-frequency data is reduced, and the retrieval of high-resolution velocity models from simple initial models is faster and requires minimal pre-processing.

Before I mention the third new PGS contribution to FWI, I note that the classic imaging workflow includes two sequential tasks based upon scale separation: building a long wavelength velocity model, and imaging the subsurface reflectivity associated with higher wavenumber geological components. In recent years, the best-practice imaging platform has been least-squares migration (LSM) to optimize the spatial resolution and amplitude illumination of migrated images. This year, Korsmo et al. will present ‘Least-squares Kirchhoff PSDM with a local based inversion approach and compensation for limitations in modeling’ which uses a newly-developed local calibrated image-domain Kirchhoff LSM to optimize pre-stack image gathers for quantitative interpretation (QI) applications to a dataset covering the Verdandi/Lille Prinsen discovery in the Viking Graben.

LSM can be implemented in many ways, but any version will improve the reflectivity and spatial wavenumber content of migrated data without changing the velocity model. The grand ambition for several decades has been to simultaneously invert for an optimized velocity model and an optimized reflectivity image. The first-ever commercial realization of this dream will be presented by Yang et al. as ‘Simultaneous velocity and reflectivity inversion: FWI + LSRTM’. FWI is combined with LSM into a single simultaneous inversion framework, with a significant reduction in turnaround time for a model building and imaging project. Beyond the PGS contributions to EAGE2021, if you would like to learn more about this solution, please register for the webinar titled ‘PGS Ultima Live’.

More on Very-Low frequencies

PGS has two contributions to the two dedicated sessions titled ‘Low Frequency Seismic Data Acquisition and its Impact on Imaging and Inversion’.

Reiser demonstrates how to apply stable rock-physics transforms to FWI models in ‘Additional low frequencies in broadband seismic deliver increased confidence in prestack inversion and prospect de-risking’; thereby building elastic low-frequency models that overcome the traditional ‘low frequency gap’ that confronted quantitatively accurate pre-stack simultaneous AVA inversion. Note that any such application of FWI must implicitly be able to use data lacking the same low frequencies confronting the inversion of elastic attributes. In other words, the capacity of the PGS FWI solution to avoid cycle-skipping effects when using multisensor GeoStreamer data—naturally richer in low frequency content—is validated by the quantitative accuracy of the case study shown.

Hegna et al. will continue their development of the continuous shooting and recording methodology known as ‘eSeismic’ by presenting ‘Continuous Wavefields Method – Low frequency considerations’. Applicable also to towed marine vibrators, or even to the vessel noise wavefield, the eSeismic method most commonly involves the triggering of individual air-gun, and the iterative deconvolution of common receiver gathers from the entire sail line of acquisition recorded as a single continuous record. A variety of other exotic source concepts will be presented in the same Dedicated Session, and I look forward to writing the summary.