Seismic Tomography for Anisotropic Velocity Estimation
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Solution Overview
Problem
Current methods for prestack depth migration in seismic data processing face inaccuracies when dealing with anisotropic seismic velocities, particularly in vertically transversely isotropic media, leading to incorrect positioning of seismic events and suboptimal energy focusing, which can impact the determination of subsurface formation boundaries.
Innovation Solution
A method is developed to estimate seismic velocities and anisotropy parameters by generating initial estimates of vertical interval velocity and normal moveout velocity, followed by iterative tomographic inversions to adjust layer depths and calculate anelliptic parameters, ensuring accurate travel time differentials and improved migration imaging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If anisotropy is ignored in migration processing, then processing complexity is reduced, but positioning accuracy of seismic events deteriorates
Solution Approach 1:
The patent applies parameter changes by transitioning from isotropic velocity models to anisotropic velocity models (specifically VTI and TTI parameters) to account for directional dependence of seismic wave propagation. This involves estimating and incorporating anisotropy parameters (ε, δ, θ) into the migration process, which changes the fundamental parameters of the velocity model to match the actual subsurface anisotropic conditions, thereby improving positioning accuracy without excessive complexity increase
Solution Approach 2:
The patent implements preliminary action by performing prestack depth migration with anisotropic velocity estimation before final imaging. The method estimates interval velocities and anisotropy parameters in advance using travel time tomography, then uses these pre-estimated parameters to guide the migration process, ensuring accurate positioning is achieved from the outset rather than requiring post-processing corrections
2Use of energy by moving object
If anisotropy is ignored in migration processing, then computational resources are reduced, but energy focusing quality deteriorates
Solution Approach 1:
The patent changes the velocity model parameters from isotropic to anisotropic (VTI/TTI), which accurately represents the subsurface medium. This parameter change ensures that seismic energy is correctly focused at subsurface events during migration, improving imaging quality and energy focusing accuracy while managing computational resources through efficient algorithms
3Measurement precision
If iterative tomographic inversions are performed, then velocity estimation accuracy is improved, but processing time increases
Solution Approach 1:
The patent performs preliminary velocity estimation using initial models and travel time tomography before conducting iterative inversions. This preliminary action provides a good starting point for the iterative process, reducing the number of iterations needed to converge to an accurate solution, thereby balancing velocity estimation accuracy with processing time efficiency
Solution Approach 2:
The patent implements feedback mechanisms through iterative tomographic inversions where velocity models are continuously refined based on residuals between observed and calculated travel times. Each iteration uses feedback from previous iterations to update the velocity model, progressively improving accuracy while monitoring convergence to avoid excessive processing time
Data Source
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AI summary
A method for estimating seismic velocities (V1-Vn) in vertically transversely isotropic media includes generating an initial estimate of vertical interval velocity and interval normal moveout velocity with respect to depth (d1-dn) from seismic data. An initial estimate is generated of a first anisotropy parameter with respect to depth (d1-dn). The first anisotropy parameter is related to the interval normal moveout velocity and the interval vertical velocity. An initial estimate is generated with respect to depth (d1-dn) of a second anisotropy parameter. The second anisotropy parameter is related to the first anisotropy parameter and an interval anelliptic parameter. A first tomographic inversion is performed with respect to the interval normal moveout velocity and the second anisotropy parameter at a constant value of the first anisotropy parameter until travel time differentials reach minimum values. Layer depths are adjusted with the initial estimate of vertical interval velocity. Using values of the second anisotropy parameter determined in the first tomographic inversion, a second tomographic inversion is performed of interval normal moveout velocity and the first anisotropy parameter with respect to depth. The adjusted layer depths, interval normal moveout velocities and interval vertical velocities are again adjusted and interval anelliptic parameters are calculated from the second tomographic inversion.