Low-Frequency Marine Seismic Source Considerations
Author: Andrew Long
Industry Insights - 13 December 2021
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.
Simultaneous inversion of velocity and reflectivity
Author: Yang Yang, Jaime Ramos-Martinez, Dan Whitmore, Guanghui Huang, Nizar Chemingui
First Break - 6 December 2021
The authors describe a new seismic inversion workflow to simultaneously invert for velocity and reflectivity. PGS Ultima.
Development of a rock physics atlas in the Talara-Progreso Basin, Peru
Author: Roberto Ruiz, Cyrille Reiser, Anna Roubíčková, Neelofer Banglawala
EAGE Rock Physics Workshop - 26 November 2021
The geological provinces on the Pacific Coast of Peru have a long and complex geological history. The petroleum system has undergone more than a hundred years of exploration and exploitation (de Souza et at.), however its full potential remains to be understood. Integration of seismic and well data is key for building a consistent geological framework that can be used to explore its prospectivity. A case study with a robust petrophysics and rock physics workflow implemented in eleven wells from the North-West Pacific region of Peru is presented here. This workflow allowed us to predict the in-situ elastic response of the well logs, as well as investigate, in real-time, how potential geological scenarios, such as changes in porosity, mineral volume and fluid properties, can affect the response in elastic well logs and by extension in the seismic amplitudes.
Maximizing quality and efficiency with wide-tow multi-source configurations
Author: Martin Widmaier, Carine Roalkvam, Julien Oukili, Rune Tønnessen
First Break - 9 November 2021
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.
Rare Tales from the Deep Seas
Author: Andrew Long
Industry Insights - 8 November 2021
The history of deep-sea mining is rich with tales of international subterfuge, espionage and growing alarm at the imbalance of global access to critical metals and rare-earth elements—increasingly precious commodities. I briefly discuss the seabed distribution, elemental composition, and common uses of rare-earth elements. Other strategic seafloor mineral deposits such as cobalt crusts and seafloor massive sulfide (SMS) deposits, or shallow methane gas hydrates, will be considered in a future article.
Quantitative Interpretation Reveals Prospectivity of the Kwanza Shelf, Offshore Angola
Author: Roberto Ruiz, Cyrille Reiser, Avril Burrell, Jean Afonso Colsoul, Naire Judith Ricardo Cahumba Quenge
GeoExpro - 4 November 2021
PGS in partnership with Agência Nacional de Petróleo, Gás e Biocombustíveis (ANPG) has recently acquired 8,304 sq. km of new multisensor GeoStreamer data over the Kwanza Shelf, 30 km off the central coast of Angola. This new 3D survey provides an unprecedented uplift in seismic imaging over the vintage 2D data. Modern processing workflows have been tailored to the unique imaging challenges of the presence of shallow water and subsurface salt. The new data unveils previously undetected pre-salt basins with many direct hydrocarbon indicators (DHIs) in both the syn-rift and post-rift sections, suggesting the presence of a working petroleum system in this underexplored area.
EAGE 2021: Decarbonization the Catalyst for a New Geoscience Era?
Author: Andrew Long
Industry Insights - 26 October 2021
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.
Simultaneous velocity and reflectivity inversion: FWI + LSRTM
Author: Yang Yang, Jaime Ramos-Martínez, Guanghui Huang, Dan Whitmore, Nizar Chemingui,
EAGE - 1 October 2021
We present an iterative non-linear inversion method to simultaneously estimate both velocity and reflectivity. The core of the inversion workflow is a full acoustic wavefield modeling relation parameterized in terms of velocity and vector reflectivity. A key aspect is the separation of the lowand high-wavenumber components of the gradient based on inverse scattering theory, enabling the sensitivity kernels to update the velocity and the vector reflectivity, respectively. The estimation problem is formulated as a multi-parameter adjoint-state inversion where the trade-off between velocity and reflectivity is minimized through scale separation. Our approach is equivalent to performing Full Waveform Inversion (FWI) and Least-Squares Reverse Time Migration (LSRTM) in
a single framework using the full wavefield. The output of the inversion is a detailed velocity model together with an accurate estimate of the earth reflectivity with compensation for incomplete acquisition, poor illumination, and multiple crosstalk. The new approach reduces the turnaround time of imaging projects by combining velocity model building (FWI) and imaging (LSRTM) into a single inversion process with minimal data pre-processing.
Least-squares Kirchoff PSDM with a local based inversion approach and compensation for limitations in modeling
Author: Øystein Korsmo, Sean Crawley, Chaoguang Zhou, S. Lee, Elena Klochikhina, Nizar Chemingui
EAGE - 1 October 2021
Reliable seismic amplitudes are crucial for the estimation of rock properties. In conventional depth imaging, amplitudes and resolution will be influenced by propagation effects in the imaging model. These limitations origin from the formulation of the migration operator, implemented as the adjoint rather than the inverse of modeling. Least-squares migration (LSM) tries to eliminate these effects and resolve the real reflectivity model. In this study, we make use of a newly developed local calibrated image-domain Kirchhoff least-squares migration to deconvolve the system response from the depth migrated gathers. We demonstrate how the inversion de-blurs the image and adjusts the prestack amplitude response, following better the expected response from well synthetic. The method is demonstrated on a North Sea dataset from the Viking Graben area, covering the Verdandi/Lille Prinsen discovery.
Extended domain FWI via time warping
Author: Guanghui Huang, Jaime Ramos-Martínez, Yang Yang, Ramzi Djebbi, Nizar Chemingui
EAGE - 1 October 2021
Overcoming cycle-skipping in Full Waveform Inversion (FWI) is a significant step toward enabling automation in velocity model building. This reduces the demand of acquiring very low-frequency data and/or starting the inversion procedure from kinematically accurate models. We present a new FWI method that uses time-warping as the extension domain to overcome cycle-skipping. The warping function dynamically transports the recorded field data to the modeled data and is imposed to represent the actual physical time. Thus, the derived objective function allows the inversion of the two parameters involved, model and time-warping extension, in a single optimization problem, whose solution is provided by the Alternate Direction Method (ADM). The novel FWI objective function enables automatic transition from a pure time-shift problem to a conventional least-squares one. We successfully apply the new FWI method to both synthetic and field data sets to demonstrate its effectiveness starting from inaccurate initial models. Results show the benefits of the new FWI approach in reducing the turnaround time for building high-resolution models from very simple initial velocity models.