Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2000-04-24
2001-11-27
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06324478
ABSTRACT:
BACKGROUND
This invention relates to geophysical prospecting using seismic signals, and in particular to methods of calculating traveltimes for geophysical data processing.
Effectively searching for oil and gas reservoirs often requires imaging the reservoirs using three-dimensional (3-D) seismic data. Seismic data is recorded at the earth's surface or in wells, and an accurate model of the underlying geologic structure is constructed by processing the data. Imaging 3-D seismic data is perhaps the most computationally intensive task facing the oil and gas industry today. The size of typical 3-D seismic surveys can be in the range of hundreds of gigabytes to tens of terabytes of data. Processing such large amounts of data often poses serious computational challenges.
Obtaining high-quality earth images necessary for contemporary reservoir development and exploration is particularly difficult in areas with complex geologic structures. In such regions, conventional seismic technology may either incorrectly reconstruct the position of geological features or create no usable image at all. Moreover, as old oil fields are depleted, the search for hydrocarbons has moved to smaller reservoirs and increasingly hostile environments, where drilling is more expensive. Advanced imaging techniques capable of providing improved knowledge of the subsurface detail in areas with complex geologic structures are becoming increasingly important.
Many imaging techniques, such as techniques based on Kirchhoff migration, require computing traveltimes for the region of interest. Efficiently applying such imaging techniques to 3-D seismic information requires fast, robust, and accurate methods to compute traveltimes.
Commonly used traveltime computation techniques face a number of challenges. Ray tracing methods, while relatively accurate, often suffer from considerable complexity and inability to illuminate shadow zones. Finite-difference schemes are typically simpler computationally, but often suffer from stability and accuracy issues. In particular, currently available finite-difference schemes often fail to adequately handle complex propagation effects in fields where complex geology and associated velocity variations are present.
A fast, accurate and unconditionally stable 3-D traveltime computation method would be an important tool in the arsenal of the seismic imaging geophysicist. A robust traveltime computation technique could be useful in many seismic data processing methods, including migration, datuming, modeling, and data acquisition design. Such a technique would allow improved use of three-dimensional (3-D) seismic data to characterize and delineate reservoirs and to monitor enhanced oil recovery (EOR) processes. A fast and robust traveltime computation method would be particularly useful for characterizing extremely complicated geological conditions such as those that exist below layers of salt in the Gulf of Mexico and in the overthrust regions of the Western United States. Better seismic images of complex subsurface geology can reduce development costs, as well as increase the amount of hydrocarbons recovered and the amount of national oil reserves.
In U.S. Pat. No. 6,018,499, Sethian et al. describe seismic data processing methods which include computing traveltimes by advancing a traveltime front selectively at the minimum-traveltime gridpoint along the front. The traveltimes along the front are stored as a heap, and are thus easily sorted. The Sethian et al. patent focuses on a first-order traveltime computation operator. Such a first-order operator is computationally efficient, but its local accuracy can depend on the local orientation of the traveltime front relative to the grid direction.
SUMMARY
The present invention provides a method of accurately and efficiently processing seismic signals for seismic exploration volumes or regions having complex geological structures. The method uses a high-order (second- or higher-order) traveltime computation operator when enough upwind traveltimes are available, and a first-order traveltime computation operator otherwise. For typical data sets, the method computes the vast majority of traveltimes using a high-order operator, and reverts to first-order for a limited number of exceptional points, typically around discontinuities and sharp velocity variations. Attempting to use a high-order operator in such exceptional regions can be computationally challenging. Selectively reverting to a first-order operator only in the presence of discontinuities or sharp velocity variations provides increased accuracy at most points without adding inordinate computational complexity.
In the preferred embodiment, the traveltime computation method includes providing a set of accepted traveltimes for an accepted grid region in the volume, providing a set of tentative traveltimes for a set of trial grid points arranged in a narrow band around the accepted grid region, and arranging the set of tentative traveltimes on a heap. The minimum traveltime in the heap is selected and added to the set of accepted traveltimes. The grid point corresponding to the minimum traveltime (the minimum-traveltime gridpoint) is implicitly added to the accepted grid region. The neighbors of the minimum-traveltime grid point which are not in the accepted grid region are added to the set of trial grid points. Tentative traveltimes for the non-accepted neighbors are computed/recomputed and put on the heap. The process continues by point-wise addition of the minimum traveltime in the heap at each position of the traveltime front, to advance the front until accepted traveltimes are computed for the entire grid region of interest.
The tentative traveltimes are preferably computed using a high-order (second- or higher-order) or first-order approximation to the eikonal equation. The traveltime computation includes solving the eikonal equation in all the points of a 3-D grid. For the grid points for which enough accepted neighboring traveltimes are available, a high-order approximation is employed. For the points for which enough neighboring accepted traveltimes are not available, a first-order method is used.
REFERENCES:
patent: 5513150 (1996-04-01), Sicking et al.
patent: 6018499 (2000-01-01), Sethian et al.
patent: 6081482 (2000-06-01), Bevc
Bevc, “Imaging Complex Structures with First-Arrival Traveltimes,” Technical Program, p. 1189-1192, Society of Exploration Geophysicists (SEG) International Exposition and 65thAnnual Meeting, Oct. 8-13, 1995, Houston.
Cao et al., “Finite Difference Solution of the Eikonal Equation using an Efficient, First-Arrival, Wavefront Tracking Scheme,”Geophysics59(4):632-643 (Apr. 1994).
Cerveny, “Ray Tracing Algorithms in Three-Dimensional Laterally-Varying Layered Structures,”Seismic Tomography(ed. G. Nolet), p. 99-133, Reidel Publishing Company, 1987.
Dellinger et al., “Anisotropic Finite-Difference Traveltimes using a Hamilton-Jacobi Solver,” Technical Program, p. 1786-1789, Society of Exploration Geophysicists (SEG) International Exposition and 66thAnnual Meeting, Nov. 10-15, 1996, Denver.
Fei et al., “Finite-Difference Solutions to the 3-D Eikonal Equation,” Technical Program, p. 1129-1132, Society of Exploration Geophysicists (SEG) International Exposition and 65thAnnual Meeting, Oct. 8-13, 1995, Houston.
Fei et al., “Depth Migration Artifacts Associated with First-Arrival Traveltimes,” Technical Program, p. 499-500, Society of Exploration Geophysicists (SEG) International Exposition and 66thAnnual Meeting, Nov. 10-15, 1996, Denver.
Fowler, “Finite-Difference Solution of the 3-D Eikonal Equation in Spherical Coordinates,” Technical Program, p. 1394-1397, Society of Exploration Geophysicists (SEG) International Exposition and 65thAnnual Meeting, Oct. 8-13, 1995, Houston.
Gray, “Efficient Traveltime Calculations for Kirchhoff Migration,”Geophysics51(8):1685-1688, Aug. 1986.
Gray et al., “Kirchhoff Migration using Eikonal Equation Traveltimes,”Geophysics59(5): p. 810-817, May, 1994.
Kästner, “Accurate Finite-Difference Calculations of Traveltimes an
Popovici Alexander M.
Sethian James A.
3D Geo Development, Inc.
McElheny Jr. Donald E.
Popovici Andrei D.
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