Method for imaging discontinuites in seismic data

Details Method for imaging discontinuites in seismic data Method for imaging discontinuites in seismic data

2001-04-06

2002-12-03

Lefkowitz, Edward (Department: 2862)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

C702S016000, C702S017000

Reexamination Certificate

active

06490528

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of geophysical prospecting. More particularly, the invention is a method of identifying structural and stratigraphic discontinuities in a three-dimensional (3-D) seismic data volume.
BACKGROUND OF THE INVENTION
As part of the hydrocarbon exploration and production work process, geoscience interpreters often need to recognize and map subsurface structural features, such as faults, and stratigraphic features, such as channel- or sand-body edges, in three-dimensional seismic data. However, identifying structural and stratigraphic features in 3-D seismic data can be a time consuming, subjective, and difficult process. There is a need to generate, in a computationally efficient matter, a derivative data volume (i.e., a data volume derived from the original seismic data volume), that displays clear sharply focussed structural and stratigraphic features that can be quickly recognized and exploited in the mapping process.
Several techniques have been used in the oil industry to enhance the interpretation of structural and stratigraphic features in 3-D seismic data. A well-known technique is to transform the original amplitude data into a coherence volume using a series of one-dimensional cross-correlation calculations. For every data sample in a volume, the cross-correlation calculation is performed using a user-defined vertical window with the equivalent portion of an adjacent trace. Typically, the vertical window is the target sample in question, plus 3-7 data samples above and below the target sample, depending upon the frequency of the data. This operation is repeated for all data samples and all traces, all in the same correlation direction. The correlation direction is generally in-line, cross-line, or either diagonal direction. The resulting coherence volume typically contains values normalized between −1 and +1. For adjacent traces that are very similar, the value of the coherence sample will be close to +1, since +1 represents high correlation. This similarity, and hence correlation, is expected for adjacent traces that do not straddle a structural or stratigraphic discontinuity. For adjacent traces that do straddle a discontinuity, lack of similarity is expected. Thus, their coherence value would be closer to 0, since 0 represents no correlation. A coherence value of −1 represents negative correlation, such as high correlation with phase reversal. This standard technique has limitations, though, because features perpendicular to the single correlation direction are highlighted, while features parallel to the correlation direction are poorly imaged.
Bahorich and Farmer received U.S. Pat. No. 5,563,949, “Method of Seismic Signal Processing and Exploration”, issued Oct. 8, 1996. This patent is commonly known as the “coherence cube” patent. Bahorich and Farmer also obtained a continuation of this patent in U.S. Pat. No. 5,838,564, “Apparatus for Seismic Signal Processing and Exploration”, issued Nov. 17, 1998.
Bahorich and Farmer's '949 patent describes a method for converting a fully processed 3-D seismic data volume into a cube of coherence measurements. According to their method, the 3-D data volume is divided into a plurality of horizontal slices, and each horizontal slice is further divided into a plurality of cells, each of which contains portions of at least three seismic data traces. As described in the '949 patent, these at least three traces in each cell comprise a reference trace, an in-line trace, and a cross-line trace. The in-line trace and the cross-line trace are each compared to the reference trace in each cell using a measure of coherency. Then the in-line and cross-line coherency measures are combined to obtain a single value that is representative of the coherence of the three seismic traces for each cell. This process is repeated for every cell, using every trace in the 3-D seismic volume as a reference trace, in order to obtain a 3-D cube of coherence measurements. Bahorich and Farmer's '564 patent describes the corresponding apparatus for carrying out the process of their '949 patent.
Bahorich and Farmer's patented technique combines information from more than one correlation direction at each data sample in the 3-D seismic data volume, thereby highlighting structural and stratigraphic information along multiple azimuths. According to Bahorich and Farmer, in their invention “the concept of cross-correlation is extended to two dimensions by taking the geometric means between the classical one dimensional cross-correlations” (U.S. Pat. No. 5,563,949, column 4, lines 17-20). This technique has limitations, however. Combining information from different correlation directions may effect the image clarity of the structural and stratigraphic features. This decrease in clarity can make it more difficult to extract structural and stratigraphic information in automated mapping processes. In addition, the computational complexity of this procedure is significantly greater than the traditional method using classical one-dimensional cross-correlations.
Higgs and Luo received U.S. Pat. No. 5,724,309 “Method for Geophysical Processing and Interpretation Using Instantaneous Phase and Its Derivatives and Their Derivatives”, issued Mar. 3, 1998. Higgs and Luo's '309 patent describes a related technique for interpretation of faults and stratigraphic features. The technique uses instantaneous phase and its spatial derivatives to determine values of spatial frequency, instantaneous frequency, dip magnitude and dip azimuth. These values are plotted to produce a derivative seismic volume that highlights subsurface changes. The main advantage is its computational speed. However, the instantaneous phase and frequency images tend to be of lower resolution than traditional cross-correlation images. A similar technique was also published by Hardage et al., 1998, “3-D Instantaneous Frequency used as a Coherency/Continuity Parameter to Interpret Reservoir Compartment Boundaries Across an Area of Complex Turbidite Deposition”, Geophysics, Vol. 63, No. 5, pp. 1520-1531. This technique uses instantaneous frequency images to define reservoir compartments by identifying facies boundaries.
Gersztenkom's International Patent Application No. PCT/US97100249, “Method and Apparatus for Seismic Signal Processing”, was published as International Publication No. WO 97/39367 on Oct. 23, 1997. This technique generates a covariance matrix for an ensemble of seismic traces and then estimates the degree of similarity between traces by estimating the largest eigenvalue of the covariance matrix. It identifies the maximum coherence component and therefore identifies structural and stratigraphic discontinuities in the data at all azimuths. The main disadvantage is that because this technique estimates eigenvalues of the covariance matrix for each time sample in the volume, it is computationally intensive.
Marfurt, Kirlin, Farmer, and Bahorich received U.S. Pat. No. 5,930,730 “Method and Apparatus for Seismic Signal Processing and Exploration”, issued Jul. 27, 1999. The '730 patent describes a method for identifying structural and stratigraphic features in three dimensions. After datumming is applied to remove a significant portion of the regional structural dip, a semblance calculation is applied as a function of time to multiple seismic traces in multiple directions to further estimate and correct for local dip. A maximum semblance cube is created that highlights structural and stratigraphic discontinuities, corrected for structural dips. Improved imaging is obtained in areas of higher structural dip and seismic noise. The main disadvantage of this method is that it is very computationally intensive.
Marfurt, Gersztenkorn, Nissen, Sudhaker, and Crawford published a paper in Geophysics, Vol. 64, No. 1, pp. 1040111, January-February 1999, “Coherency Calculations in the Presence of Structural Dip”. The technique described in this publication examines the simila

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