4D Seismic Velocity Estimation Using Non-Zero Offset Ray Tracing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current 4D seismic data processing methods face challenges in accurately estimating time shifts and velocity changes in subsurface volumes, particularly in regions like the overburden and underburden, due to weak signal amplitudes and non-linear inversion complexities, and often rely on zero-offset assumptions that discard direction-dependent information.
Innovation Solution
A method that uses non-zero offset pre-stack seismic data and ray tracing to determine seismic signal paths, allowing for the estimation of velocity changes without processing the data to zero-offset, and employs a linear tomographic system to link offset-dependent time shifts to model parameters, eliminating the need for zero-offset assumptions and enhancing the accuracy of time-lapse velocity anomaly detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If zero-offset assumptions are used in 4D seismic processing, then the processing complexity is reduced, but direction-dependent information is lost and measurement precision deteriorates
Solution Approach 1:
Instead of converting non-zero offset data to zero-offset data (conventional approach), the invention inverts the approach by directly processing non-zero offset pre-stack data using ray tracing and tomographic inversion. This preserves direction-dependent information while avoiding the complexity of zero-offset conversion
Solution Approach 2:
The invention adds the offset dimension back into the processing by using non-zero offset pre-stack data instead of stacked zero-offset data. Ray tracing operations incorporate offset-dependent time shifts, utilizing the additional dimensional information to improve measurement precision
2Measurement precision
If non-zero offset pre-stack data is used with ray tracing, then direction-dependent information is retained and measurement precision improves, but computational complexity increases
Solution Approach 1:
Ray paths are determined once using ray tracing before the inversion process. These pre-computed ray paths are then reused in the tomographic inversion, avoiding the need to re-trace rays during each iteration and significantly reducing computational complexity
Solution Approach 2:
The invention introduces a linear tomographic system as an intermediary between the non-zero offset pre-stack data and the velocity model updates. This linear system simplifies the inversion mathematics and reduces computational burden compared to fully non-linear inversion approaches
3Device complexity
If conventional 4D inversion is used, then processing is simpler, but accuracy in overburden and underburden regions deteriorates due to weak signal amplitudes
Solution Approach 1:
The invention applies local quality by treating different subsurface regions (overburden, reservoir, underburden) with appropriate weighting in the objective function. This allows the processing to be more sensitive to weak signals in specific regions while maintaining overall processing efficiency
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 enables more precise characterization of subsurface volume evolution over time by retaining direction-dependent information and improving the estimation of time shifts and velocity changes, even in structurally complex regions, without the computational expense of re-tracing ray paths for each iteration.
Implementation Method 1
one or several sources emit elastic waves in the form of pressure or ground motion modulation from specific locations (wavefield), at or below the land or sea surface or in a borehole. This wavefield propagates away from the source(s) through the subsurface.
Implementation Method 2
Along with this propagation, a fraction of the incident wavefield is reflected from the heterogeneities in the elastic material properties of the subsurface (such as acoustic impedance).
Data Source
Figure 1(a)~1(b)
Figure 2(a)~2(b)
Figure 3
AI summary
< > 26 <<06/01/2015>> Abstract Disclosed is a method for characterising the evolution of a subsurface volume over time. Themethod comprises providing firstand secondsurveysof the subsurface volume. Each survey comprises seismic data acquired by transmitting seismic signals into thesubsurface volume and subsequently detecting some or all of the seismic signals after reflection within the subsurface.The first seismic data of the first survey correspondsto a first timeand thesecondseismic data of the second survey corresponds to a secondtime. At least some of the first seismic data and the second seismic data isobtained with a non-zero offset. An inversionis performedto obtain estimates of changes having occurred between the first time and the second time in terms of at least one model parameter;wherein for the inversion: the first seismic data and the second seismic data is not processed to be equivalent to zero-offset data prior to the inversion; andit is assumed that the path taken by each received seismic signal between its transmission and reception is the same for the first survey and the second survey