Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2003-01-21
2004-08-17
McElheny, Jr., Donald (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06778909
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to geophysical prospecting using seismic signals, and in particular to systems and methods for performing seismic data processing.
BACKGROUND
Effectively searching for oil and gas reservoirs often requires imaging the reservoirs using three-dimensional (3-D) seismic data. Seismic data are recorded at the earth's surface or in wells, and an accurate model of the underlying geologic structure is constructed by processing the data. 3-D seismic imaging 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.
In a typical seismic survey, elastic (seismic) waves are propagated into the earth region of interest. The elastic waves may be generated by various types of sources such as dynamite, air guns, and hydraulic vibrators, situated along the earth's surface. As these waves propagate downward through the earth, portions of their energy are sent back to the earth's surface by reflection and refraction which occur whenever abrupt changes in impedance are encountered. The reflected and/or refracted seismic waves are recorded at the earth's surface or in wellbores by an array of receivers such as geophones, hydrophones, or other similar devices. The underlying earth structure can be imaged by appropriate processing of the signals returned to the receivers.
Raw seismic data as recorded are generally not readily interpretable. While such data show the existence of formation interfaces, raw data do not accurately inform the interpreter as to the location of these interfaces. The process of migration, also called imaging, repositions the seismic data so that a more accurate picture of subsurface reflectors is given. In order to perform migration calculations, the seismic velocities of the subsurface at a multitude of points are first determined, commonly by performing migration velocity analysis (MVA). A two- or three-dimensional spatial distribution of subsurface velocity forms a velocity model for the subsurface region of interest. A large-scale velocity model covering the extent of the seismic data acquisition volume can be a complicated structure with vertically and laterally varying velocity. The velocity model is used to compute a set of traveltimes for the volume of interest. A traveltime is the amount of time a seismic signal takes to travel from a source to a subsurface reflection point and back to a receiver. The migration process employs the computed traveltimes to generate an accurate image of the volume of interest.
Known migration approaches include Kirchhoff migration and wave-equation migration. Kirchhoff migration algorithms have been widely used for seismic imaging. While Kirchhoff integral equation methods are generally considered practical and efficient, such methods have several shortcomings. Wave-equation methods that downward continue the wavefield have been recently shown to produce good imaging results in many synthetic and real data cases. Wave-equation migration methods can yield improved images relative to Kirchhoff migration methods. At the same time, conventional wave-equation migration methods can suffer from suboptimal computational efficiency and undesired imaging artifacts.
SUMMARY
In the preferred embodiment, the present invention provides a computer-implemented common azimuth migration seismic data processing method comprising: providing a common-azimuth input data set for a geophysical data processing volume of interest; providing a velocity model for the geophysical data processing volume; applying an offset antialiasing operator to the input data set; and performing a recursive downward-continuation of the common-azimuth input data set to a plurality of successive common-azimuth surfaces to generate an image of the volume of interest. In the preferred embodiment, the present invention further provides for selecting a depth dependence of an offset range employed in the downward continuation; selecting a frequency-dependence of a depth step size employed in the downward continuation; selecting a frequency dependence of a cutoff depth employed in the downward continuation; and adding reciprocal traces to the data around zero offset, for reducing imaging artifacts introduced by data edge effects. Similar methods can be applied with a narrow-azimuth downward continuation operator.
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Bevc Dimitri
Biondi Biondo
Crawley Sean E.
Popovici Alexander M.
3DGeo Development, Inc.
McElheny Jr. Donald
Popovici Andrei D.
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