Efficient reflectivity modelling for full wavefield FWI
Author: Tony Martin, Yang Yang, Norman Daniel Whitmore, Nizar Chemingui, Tiago Alcantara, Eric Frugier
First Break - 1 March 2021
The authors describe an efficient modeling of reflectivity to drive full wavefield full waveform inversion. Reformulating the variable density acoustic wave-equation in terms of vector-reflectivity, represents a step towards full automation, and enables a greater diversity of applications.
Seismic Processing - What is Required?
Author: Tony Martin, Øystein Korsmo, Nizar Chemingui
EAGE Workshop on Optimizing Project Turnaround Performance - 24 February 2021
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.
Can Data Analytics Help Reduce Seismic Processing Turnaround
Author: Tony Martin, Suhail Butt, Andrew Long
EAGE Workshop Optimizing Project Turnaround - 24 February 2021
Seismic data volumes are increasing and pressure is growing to accelerate the turnaround time of seismic processing projects. Testing, validation, and production administration are time-consuming components of a project. Testing is performed to optimize the parameters for each step in the processing sequence. It can be both computer and human resource intensive, as many testing phases require numerous repeat runs and significant interaction with the data. A Proof-of-Concept (POC) example illustrates that parameter testing can be helped by mining for parameters from a database.
Post-EAGE 2020 Comments on Full Waveform Inversion (FWI)
Author: Andrew Long
Industry Insights - 26 December 2020
The recent virtual EAGE 2020 conference included an interesting discussion on ‘High frequency FWI’. Whilst the short-term motivation is to deliver fast-track interpretation products with an abbreviated processing flow, the ambition of seismic inversion has always been to recover high-resolution elastic medium properties from the earth. After reviewing the traditional separation between low wavenumber background model building with FWI and high wavenumber reflectivity imaging, I consider the nature of the ‘reflectivity’ resolved by running traditional FWI to high frequencies. The issue of whether FWI is ‘imaging multiples’ by nature of being one form of a Least-Squares Reverse Time Migration (LS-RTM) is also considered. This discussion hopefully helps understood the products delivered by high frequency FWI.
What’s New in OBN Imaging at EAGE 2020?
Author: Andrew Long
Industry Insights - 6 December 2020
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.
Mixed Phase Seismic Wavelet Estimation Using the Bispectrum
Author: Maiza Bekara
EAGE - 1 December 2020
The ability to estimate a mixed phase wavelet is a useful tool for processing and quality control in seismic imaging. The wavelet is estimated using higher order statistics of the data. In practice, these methods tend to show some instability issues when the wavelet length is increased. To improve the stability of the solution, this abstract proposes a new formulation of the wavelet estimation problem that constrains the solution to be a finite duration, phase-only compensation applied to a known base wavelet. The proposed solution works in the frequency domain and consists of three steps. First, the bispectrum
of the data is deconvolved using the bispectrum of the base wavelet to increase its bandwidth. This helps to improve the sensitivity of third order statistics to phase information. Then, a phase-only wavelet is estimated from the deconvolved bispectrum using an iterative least-squares approach without phase unwrapping. Finally, the estimated phase-only wavelet is conditioned using a projection onto convex sets type algorithm to enforce the constraint of the finite time duration giving the user a control on the amount of phase deviation from the base wavelet. Test examples on synthetic and real data both show reliable results with robustness to noise contamination.
Continuous Wavefields Method - Results from a Shallow Water Field Test
Author: Stian Hegna, Tilman Klüver, Jostein Lima
EAGE - 1 December 2020
In the autumn of 2019, PGS acquired a small 2D test offshore Malaysia in water depths ranging mainly between 125 and 200m. The primary purpose of this test was to demonstrate the continuous wavefields method in shallow water areas. In addition, different source configurations and time intervals between consecutive actuations of airguns were tested. In this paper we discuss the different source configurations that were tested, and present preliminary results. The overall sound exposure levels (SEL) are very similar for the different source configurations. The small differences are mainly related to trigger time intervals of the individual air-guns, and to air-pressure. Preliminary results achieved so far indicate that the continuous wavefields method works well in shallow water, and with relatively small differences between the different triggering schemes and source configurations.
Full Waveform Inversion Using Wave Equation Reflectivity Modeling
Author: Yang Yang, Jaime Ramos-Martinez, Norman Dan Whitmore, Alejandro Valenciano, Nizar Chemingui
EAGE - 1 December 2020
Full Waveform Inversion (FWI) is routinely used to improve the accuracy and resolution of velocity models. However, utilizing reflections to produce low-wavenumber updates creates more operational challenges than using transmitted events. In order to simulate backscattered sensitivity kernels, FWI needs hard boundaries in the velocity/density models. Alternatively, one can apply the wave equation and first-order Born approximation to decompose the seismic wavefields into background and perturbation. Here, we derive an acoustic wave equation in terms of vector reflectivity to be used as the wave’s propagation engine of FWI. The new derivation results from the change of variables from impedance to reflectivity in the variable density wave equation. The main advantages of its insertion in our FWI algorithm are the following: it does not require the construction of density/hard boundaries in the velocity model to generate reflections; it allows the use of reflected events without the need of solving two different wave equations in the forward and backward propagation; it is more accurate than the method based on the first-order Born approximation to perturbation theory. We show synthetic and field data examples illustrating the advantages of the new algorithm.
Broadband 4D Seismic Provides New Level of Detail
Author: Yulia Biryaltseva, Tor Vegar Mårdalen, Per-Harald Saure-Thomassen, Marta Wierzchowska, Anastasiya Tantsereva, Julien Oukili
GeoExpro - 1 December 2020
Understanding new details in the 4D seismic signal uncovered by a broadband timelapse dataset at Gullfaks Main Field in the North Sea.
High-Resolution Angle Gathers from Iterative Least-Squares Migration
Author: Lian Duan, Norman Dan Whitmore, Nizar Chemingui, Elena Klochikhina
EAGE - 1 December 2020
Using wave-equation migration and demigration in the extended subsurface domain, we introduce a practical least-squares migration for the inversion of angle-domain common-image gathers. Through synthetic and field data examples, we demonstrate that least-squares migration provides high-resolution angle-domain common-image gathers with extended angle range, enhanced illumination, and balanced amplitudes. Similarly, the stacked images show better structural fidelity, improved resolution, balanced illumination, and reduced artifacts.