Seismic Imaging Velocity Calibration via Mis-tie Volume
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Solution Overview
Problem
Isotropic velocity analysis followed by depth migration often results in large errors in seismic imaging, especially in anisotropic earth conditions, leading to mis-ties that are ambiguous due to sparse well locations, making it challenging to estimate calibration velocities far from nearest wells.
Innovation Solution
A method is developed to adjust isotropic depth images using a mis-tie volume or calibration velocity obtained from a mis-tie volume, which involves generating mis-tie values, assigning uncertainties, and applying regularization to achieve a target goodness of fit, thereby obtaining a calibration velocity for accurate seismic imaging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If isotropic velocity analysis is used for depth migration, then the imaging process is simple and fast, but large errors occur in seismic depth images where the earth is anisotropic
Solution Approach 1:
The patent changes the velocity model parameters from isotropic to anisotropic by introducing directional velocity variations. The anisotropic velocity model incorporates parameters such as vertical transverse isotropy (VTI) with theta-dependent velocity equations, allowing the imaging process to account for direction-dependent wave propagation speeds in the earth subsurface.
Solution Approach 2:
The patent inverts the conventional approach by using depth migration first to obtain an initial depth image, then performing time migration on the inverted polarity data, and finally combining the results. This inverted sequence allows the method to correct anisotropic errors while maintaining computational efficiency.
2Device complexity
If well locations are sparse, then the calibration process is simpler, but the calibration becomes ambiguous and unreliable far from wells
Solution Approach 1:
The patent implements feedback mechanisms by using the mis-tie volume to iteratively update the calibration velocity model. The mis-tie values from well locations are used to generate a three-dimensional mis-tie volume, which feeds back into the calibration process to improve velocity estimates in areas far from wells. This iterative feedback loop allows the system to propagate calibration information from sparse well locations across the entire survey area.
Solution Approach 2:
The patent transitions from two-dimensional calibration (at well locations only) to three-dimensional calibration by creating a mis-tie volume that interpolates and extrapolates calibration information throughout the entire subsurface volume. This dimensional extension allows reliable calibration estimates to be obtained far from well locations by utilizing spatial correlations in the three-dimensional mis-tie volume.
3Measurement precision
If isotropic images are adjusted using calibration velocity, then depth accuracy improves near wells, but the method fails to provide accurate velocities far from nearest well
Solution Approach 1:
The patent introduces the mis-tie volume as an intermediary that bridges the gap between sparse well location data and the need for continuous velocity calibration across the entire survey area. The mis-tie volume serves as a mediator that captures velocity errors in three dimensions and enables the derivation of calibration velocities at locations far from wells by spatial interpolation and correlation analysis.
Data Source
AI summary
A method for adjusting an isotropic depth image based on a mis-tie volume is provided. The method generally includes obtaining an isotropic velocity volume for a geophysical volume, obtaining an isotropic depth image of the geophysical volume, obtaining time-depth pairs at downhole locations in the geophysical volume, generating mis-tie values based on the time-depth pairs and the isotropic velocity volume, assigning uncertainties to the mis-tie values, generating a smoothest mis-tie volume that satisfies a target goodness of fit with the mis-tie values. Adjustment of the isotropic depth image may be achieved based on the mis-tie volume or a calibration velocity obtained from the mis-tie volume.