Welcome to RokBites, your bi-weekly guide to brushing up on RokDoc techniques. 👩💻💡
This week, we're diving into how to navigate efficiently RokDoc's Blocky AVO module.
Hit play on the video below or jump to the summary below to start!
Welcome to Blocky AVO Reimagined
Blocky AVO Modelling is grounded in the principles of seismic wave reflection and transmission at seismic interfaces. The core theory involves understanding how seismic waves interact with different rock layers, which is crucial for interpreting subsurface structures.
Zoeppritz Equations
The Zoeppritz equations, formulated in 1919, are fundamental to AVO (Amplitude Versus Offset) analysis. These equations describe how seismic waves reflect and transmit at an interface between two different media. They account for changes in wave amplitude with varying angles of incidence, providing a comprehensive model of wave behavior at boundaries.
However, the Zoeppritz equations are complex and nonlinear, making them challenging to invert directly for practical applications. To address this, several linear approximations have been developed, which simplify the equations for small angles and contrasts. These approximations, such as those by Aki and Richards (1980) and others, are widely used in AVO analysis for their computational efficiency and ease of interpretation. All approximations are available in RokDoc.
Linear Approximations
Linear approximations to the Zoeppritz equations allow for the estimation of key seismic attributes like intercept and gradient. These attributes help in identifying fluid content and lithology changes across interfaces. The intercept represents the normal incidence reflection coefficient, while the gradient indicates how this coefficient changes with angle.
Elastic Impedance
Elastic impedance extends the concept of acoustic impedance to account for angle-dependent reflectivity. It provides a framework for analyzing seismic data at non-normal incidence angles, enhancing the ability to differentiate between lithologies and fluid types.
Getting Started with Lithologies
Begin by loading at least two lithologies, which can be imported from averages, probability density functions, or depth trend analysis sessions. These lithologies form the foundation for creating interfaces, essential for modelling various scenarios such as fluid cases involving water, hydrocarbon or even CO2. Remember, consistent naming and color coding of lithologies and interfaces enhance a fast scenario identification.
Plotting and Analysis
The plot area is central to analyzing AVO models, offering multiple charts and graphs that can be viewed simultaneously or individually, suchas:
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AVO Plot: Displays reflection coefficients versus angle of incidence, allowing for detailed analysis of interface responses.
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Contrast Analysis Plot: A bar chart showing contrasts in elastic properties between lithologies, aiding in understanding interface characteristics.
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AVO Cross-Plot: Visualizes intercept-gradient data or elastic properties, with options for weighted-stack line analysis to easily find your optimal Chi angle (EEI based feasibility study).
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AVO Weighted-Stack Plot: Complements the cross-plot by applying weighted-stack equations to interfaces or lithologies, useful for histogram analysis and to identify your Chi angle.
The user can customize the reflectivity calculation methods and intercept-gradient options to suit your analysis needs. AVO response sets from pre-stack seismic data can be loaded and scaled as well, ensuring easy comparison between modelled and seismic responses.
Advanced Features and Anisotropy
Blocky AVO Modelling supports anisotropic AVO modelling, allowing you to explore scenarios where anisotropy effects are significant. This feature is particularly useful when there is a mismatch between modelled and seismic data at far angles. The user is able to choose between Vertical Transverse Isotropy (VTI) and Horizontal Transverse Isotropy (HTI) to explore directional velocity variations and their impact on reflectivity. By enabling anisotropy, you can explore the effects of anisotropy on the AVO character and use any prior knowledge on Thomsen parameters to perform a more accurate analysis.
Monte Carlo Simulation
Monte Carlo simulation in AVO modelling introduces variability in lithological properties, allowing for the assessment of uncertainty in seismic responses. By simulating numerous realizations, it provides a statistical framework to evaluate the range of possible outcomes, enhancing the robustness of interpretations.
Rock Physics and Forward Modelling
Rock physics modelling is integral to forward modelling the AVO response. By simulating variations in rock properties, it provides a predictive framework for understanding rock frame behavior and how that behavior is translated in the elastic space. Certain Rock Physics models, such as Patchy Cement, might also help in understanding how pressure variations in the reservoir might affect the AVO signature and eventually lead to a change of AVO class. This approach is invaluable for reservoir monitoring studies and enhances the interpretation of subsurface conditions.
Conclusion
Our re-imagined Blocky AVO Modelling is a powerful tool for the initial stages of feasibility studies. When combined with rock physics modelling, it offers an incredibly robust framework for seismic analysis, helping geoscientists unlock the mysteries beneath the Earth's surface with confidence. There's many other things you can achieve with it - so open RokDoc and see where your analysis might lead you!
We hope you found this post insightful. Feel free share your feedback and propose any topics you would like us to explore in future posts. Your input helps us create content that truly resonates with our community.
Thank you for being part of our journey - see you next post! ☕🍪
Special thanks to the co-author of this post, Amaury Toloza, Customer Success Geoscientist at Ikon Science.
