Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
1999-10-18
2002-09-03
Lefkowitz, Edward (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C703S005000
Reexamination Certificate
active
06446007
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of seismic prospecting and, more particularly, to imaging of seismic data. Specifically, the invention is a method for controlled-amplitude prestack time migration of seismic data.
BACKGROUND OF THE INVENTION
In the oil and gas industry, seismic prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. A seismic prospecting operation consists of three separate stages: data acquisition, data processing, and data interpretation. The success of a seismic prospecting operation is dependent on satisfactory completion of all three stages.
In the data acquisition stage, a seismic source is used to generate a physical impulse known as a “seismic signal” that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflected signals (known as “seismic reflections”) are detected and recorded by an array of seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The seismic energy recorded by each seismic receiver is known as a “seismic data trace.”
During the data processing stage, the raw seismic data traces recorded in the data acquisition stage are refined and enhanced using a variety of procedures that depend on the nature of the geologic structure being investigated and on the characteristics of the raw data traces themselves. In general, the purpose of the data processing stage is to produce an image of the subsurface geologic structure from the recorded seismic data for use during the data interpretation stage. The image is developed using theoretical and empirical models of the manner in which the seismic signals are transmitted into the earth, attenuated by the subsurface strata, and reflected from the geologic structures. The quality of the final product of the data processing stage is heavily dependent on the accuracy of the procedures used to process the data.
The purpose of the data interpretation stage is to determine information about the subsurface geology of the earth from the processed seismic data. For example, data interpretation may be used to determine the general geologic structure of a subsurface region, or to locate potential hydrocarbon reservoirs, or to guide the development of an already discovered reservoir. Obviously, the data interpretation stage cannot be successful unless the processed seismic data provide an accurate representation of the subsurface geology.
Typically, some form of seismic migration (also known as “imaging”) must be performed during the data processing stage in order to accurately position the subsurface seismic reflectors. The need for seismic migration arises because variable seismic velocities and dipping reflectors cause seismic reflections in unmigrated seismic images to appear at incorrect locations. Seismic migration is an inversion operation in which the seismic reflections are moved or “migrated” to their true subsurface positions.
There are many different seismic migration techniques. Some of these migration techniques are applied after common-midpoint (CMP) stacking of the data traces. (As is well known, CMP stacking is a data processing procedure in which a plurality of seismic data traces having the same source-receiver midpoint but different offsets are summed to form a stacked data trace that simulates a zero-offset data trace for the midpoint in question.) Such “poststack” migration can be done, for example, by integration along diffraction curves (known as “Kirchhoff” migration), by numerical finite difference or phase-shift downward-continuation of the wavefield, or by equivalent operations in frequency-wavenumber or other domains.
Conversely, other seismic migration techniques are applied before stacking of the seismic data traces. In other words, these “prestack” migration techniques are applied to the individual nonzero-offset data traces and the migrated results are then stacked to form the final image. Prestack migration typically produces better images than poststack migration. However, prestack migration is generally much more expensive than poststack migration. Accordingly, the use of prestack migration has typically been limited to situations where poststack migration does not provide an acceptable result, e.g., where the reflectors are steeply dipping.
In some cases, reflector dip can exceed 90 degrees. As is well known in the art, it may be possible to image these “overturned” reflectors using data from seismic “turning rays.” Prestack migration techniques must be used in order to obtain an accurate image of overturned reflectors from seismic turning ray data.
There are two general types of prestack migration, prestack time migration and prestack depth migration. The present invention relates to prestack time migration which is used in situations where the subsurface seismic velocity varies in the vertical direction, but can be regarded as approximately constant laterally. Prestack time migration is widely used in the petroleum industry; it is generally considered applicable to most (but not all) prospects.
In practice, prestack time migration is usually approximated by a composite procedure involving successive steps of normal moveout correction (denoted as “NMO”), followed by dip moveout correction (denoted as “DMO”), followed by zero-offset migration (referred to as “ZOM”). This sequence of processes is performed on collections of seismic data traces having the same or nearly the same source-receiver separation distance and the same or nearly the same azimuthal orientation (referred to as “common-offset, common-azimuth gathers”). It is known from experience that the combination of these imaging steps results in a good approximation to prestack time migration in most situations. The approximation tends to be less accurate in situations where there are dipping reflectors in a medium whose velocity varies significantly with depth. In such situations, the inaccuracies manifest themselves in mispositioned images of seismic reflectors, as well as in distorted amplitudes. A principal advantage of the decomposition of prestack time migration into the NMO/DMO/ZOM sequence is that the required algorithms execute rapidly on commonly available digital computers, which results in economical processing.
Kirchhoff prestack migration is the only commonly known method of performing prestack time migration which overcomes the inaccuracies of the NMO/DMO/ZOM decomposition mentioned in the previous paragraph, and which is applicable to common offset gathers. However, Kirchhoff prestack migration, as compared to the NMO/DMO/ZOM decomposition, is generally very expensive and time consuming. For this reason, the seismic data processing industry is continuing its efforts to develop techniques that can be used to produce accurate prestack migrations in an economical manner.
In Kirchhoff prestack migration, the data in a common-offset gather are summed over all source-receiver midpoints (denoted by {right arrow over (&xgr;)}). During Kirchhoff summation, the data are time shifted by an amount that depends on the assumed seismic velocity structure in the earth. The time shift is denoted by the symbol t
D
({right arrow over (&xgr;)}, {right arrow over (x)}), which depends on the midpoint location {right arrow over (&xgr;)} and the imaged point {right arrow over (x)}=(x,y,z). During summation, the data may be multiplied with a weight w that generally depends on {right arrow over (&xgr;)} and {right arrow over (x)}. As explained in Schleicher, J. et al., “3-D true-amplitude finite-offset migration,”
Geophysics,
volume 58, pp. 1112-1126 (1993), the weight w may be chosen in such a way that the migration processing preserves the seismic amplitudes. This is usually considered very desirable, since the amplitudes are widely used in interpreting the processed data.
The Kirchhoff prestack migration process
Finn Christopher J.
Winbow Graham A.
Bell Keith A.
ExxonMobil Upstream Research Company
Lefkowitz Edward
Taylor Victor J.
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