Velocity Analysis Using VSP Primary and Multiple Reflections
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Solution Overview
Problem
Conventional vertical seismic profile (VSP) methods face challenges in obtaining accurate seismic wave velocity models for subsurface formations, particularly below downhole receivers, due to limited incident wave angles and inaccurate velocity models resulting in smearing of reflection events and mismatches between VSP and surface seismic images.
Innovation Solution
A method that uses primary and multiply-reflected signals from VSP data to construct and compare images of subsurface regions, adjusting the velocity model based on the cross-correlation functional until a desired level of match is achieved, employing techniques like adjoint-state and conjugate-gradient methods for gradient computation and optimization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional velocity analysis or reflection tomography is used for VSP data, then the velocity model above the borehole receivers can be improved, but the velocity model below the borehole receivers remains inaccurate due to limited incident wave angles
Solution Approach 1:
The patent transitions from conventional 2D velocity analysis (limited to above receivers) to 3D velocity modeling that incorporates both primary and multiple reflections. By adding the dimension of multiple reflection paths, the method enables velocity model improvement in regions previously inaccessible (below the receivers), effectively using an additional dimensional approach to solve the coverage limitation.
Solution Approach 2:
The patent uses multiple reflected signals as an intermediary to indirectly probe the velocity structure below the receivers. Since direct primary reflections from below the receivers are not accessible, the multiple reflections serve as a mediator that carries information about the subsurface velocity model in those regions, enabling indirect measurement and improvement of the velocity model where direct measurement is not possible.
2Productivity
If an inaccurate velocity model from surface seismic is used for VSP imaging, then the imaging process can proceed, but reflection events are smeared and mismatches occur between VSP and surface seismic images
Solution Approach 1:
The patent implements a feedback mechanism where the velocity model is iteratively refined by comparing VSP images with surface seismic images. The mismatch between the two imaging systems provides feedback information that drives velocity model updates, continuously improving the velocity model accuracy until the images are properly focused and matched, thereby resolving the smearing and mismatch problems.
Solution Approach 2:
The patent performs preliminary velocity analysis using available surface seismic data to obtain an initial velocity model before conducting VSP imaging. This preliminary action provides a starting point for the imaging process, allowing it to proceed despite the initial inaccuracy, and subsequent refinement steps then improve this preliminary model to achieve the desired image quality.
3Device complexity
If only primary reflected signals are used for velocity analysis, then the analysis process is simpler, but the velocity model below the downhole receivers cannot be accurately recovered
Solution Approach 1:
The patent merges primary reflected signals with multiple reflected signals in the velocity analysis process. By combining these different signal types, the method achieves comprehensive coverage of the subsurface velocity model, including regions below the receivers that are inaccessible using primary reflections alone. The merging of signal types provides complementary information that improves overall velocity model accuracy without requiring separate analysis processes.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in a more accurate seismic wave velocity model for both above and below downhole receivers, improving the resolution and coherence of subsurface images, thereby enhancing the reliability of subsurface characterization and reservoir analysis.
Implementation Method 1
a) providing a set of data comprising signals produced by at least one source and collected by at least one receiver in the borehole, wherein the data include at least one primary signal that has been reflected off of one of the reflective interfaces and at least one multiply-reflected signal that has been reflected off of at least two reflective interfaces
Data Source
AI summary
A method for providing a velocity profile for a subsurface region that includes the reflective interfaces, the method comprising the steps of: providing a set of data comprising signals transmitted by a transmitter and collected at a receiver, wherein the data include a primary signal that has been reflected off of one of the reflective interfaces and a multiply-reflected signal that has been reflected off of at least two of the reflective interfaces, providing a velocity model for the subsurface region, using the velocity model and the primary signal to construct a first image of the subsurface region, using the velocity model and the multiply-reflected signal to construct a second image of the subsurface region, determining a measure of match between the first and second images, adjusting the velocity model based on this measure, and repeating the steps until the measure of match attains a desired level.