Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Patent
1996-06-18
1997-09-02
Lobo, Ian J.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
367 51, G01V 136
Patent
active
056639287
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
In petroleum exploration, a seismic reflection survey is a common method for obtaining a seismic image of the subsurface. In this method, using appropriate energy sources, called emitters, acoustic waves are transmitted, travel in the subsurface to be explored, and are reflected on the different reflectors which it contains. The reflected waves are recorded, as a function of time, on adapted receivers disposed on the ground surface or in the water. Each recording (or trace) given by a receiver is then assigned to the location of the point which is situated at the middle of the segment connecting the source to the receiver. This operation is referred to as common midpoint gather.
A seismic prospecting technique, which is now conventional, is multiple coverage. In this technique, the sources or emitters and receivers are disposed on the ground surface in such a way that a given midpoint gathers several recordings. The series of recordings associated with the same midpoint forms what is generally called a common midpoint gather of recordings or traces. The set of gathers is associated with a series of different midpoints preferably located along the same line at the surface. Based on these gathers, seismic processing serves to obtain a seismic image in the vertical plane passing through all these midpoints. The arrival time of a recorded wave varies with the angle of incidence .theta., which is the angle between the normal to the reflector at the reflection point, called the mirror point, and the direction of the incident (descending) wave. For a given gather and a given mirror point, this angle varies for each recording as a function of the offset x of the receiver relatively to the midpoint. Making the conventional assumption of a homogeneous and isotropic subsurface, in plane and parallel layers, the reflections associated with each of the subsurface reflectors, observed on a common midpoint gather, are theoretically aligned along hyperbolas centered on the vertical to the midpoint and called time/distance curves. In order to build the stack of the recordings of each gather, it is necessary to apply a correction depending on time, called the normal-moveout correction, which is aimed to straighten the hyperbolas to bring them theoretically to the horizontal. Conventionally, the normal-moveout correction made is a correction based on the following equation: ##EQU1## where: x is the offset, the subsurface, and pair for offset x.
To make satisfactory NMO corrections, it is necessary to know the velocity distribution V(t) at each midpoint. To achieve this, velocity analyses are made at given locations on a limited number of common midpoint gathers. The results are then subjected to a double interpolation, in time for each of the analyses, each analysis only giving a maximum of some twenty velocity values associated with the same vertical, and in abscissa, between the analyses, said analyses being commonly performed only every 40 to 50 midpoints on the average, and an account of the fact that the analysis is made manually, thus implying relatively long analysis times and relatively high processing costs.
The conventional velocity analysis consists in applying constant velocities in succession to the common midpoint gathers, for the midpoints selected, to make the corresponding NMO corrections, and then in stacking the corrected traces for each of the velocities used and in manually retaining the velocities that lead to an energy peak of the stacked trace. The accuracy of the velocity field obtained by this process is insufficient for a large number of more sophisticated treatments applied to the prestack traces (recordings), for example migration, inversion, measurements of effects of variations in amplitude with offset (denoted by AVO for Amplitude Variation versus Offset), because these processes are distorted by the effects of the time and abscissa interpolations, by the inaccuracy of the velocity values selected, and by a signal distortion due to the NMO correction formula (1).
I
REFERENCES:
patent: 5157638 (1992-10-01), Loumos et al.
Geophysics, vol. 53, No. 2, Feb. 1988, pp. 143-157, De Bazelaire `Normal moveout revisited: inhomogenous media and curved interfaces`.
De Bazelaire Eric
Dunand Jean-Pierre
Elf Aquitaine Production
Lobo Ian J.
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