Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Patent
1998-03-17
1999-11-23
Oda, Christine K.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
367 52, 367 53, 367 59, G01U 136
Patent
active
059912375
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for determining migration velocities in seismic processing, as well as the accuracy on the said velocities.
The acquisition of knowledge regarding a medium employs interactive interpretation systems. Interactive interpretation is nowadays no longer limited to a two-dimensional (2D) representation whether it is performed on a physical medium such as paper or, displayed on a screen. Three-dimensional (3D) data display techniques have become widely generalized and lead to the production of a three-dimensional model of a part of the medium to be studied. Among the methods used to obtain the best possible model, depth migrations or time migrations are used, that is to say that a seismic event on a seismic section is reproduced in the model according to the depth at which it has been identified or according to the time after which it has been located.
One of the most difficult problems to solve in seismology for depth migration is that of obtaining a migration velocity field which is as representative as possible or as close as possible to that of the subsurface strata, since the propagation velocities of the acoustic waves can vary not only from one layer or stratum to another but also within a given stratum.
It is furthermore necessary to be able to estimate the uncertainty on the velocity values calculated or found. This is because it is using these velocity uncertainties, associated with the uncertainties on the arrival times, that a development geophysicist, for example, will be able to calculate the uncertainties on the volumes of rocks impregnated with hydrocarbons.
New techniques are used. They involve, for example, a method in which use is made of what are referred to as "image-gathers" or iso-X curves in order to converge to the correct velocity model. A method of this type does not make it possible to measure the uncertainty on the velocities which are obtained, because there is no concrete analytical criterion for measuring these uncertainties.
Another method consists of estimating an uncertainty which is conditional on the velocities assigned to a predefined model of the process. Although a process of this type gives good results in some applications, for example the ones described in the articles which have appeared in the publication "the Leading Edge" of October 1995, Vol. 14, No. 10, it is nevertheless still true that it does not make it possible to separate sufficiently the errors which are due to imperfections on the morphology of the model of the subsurface, from those which are due to an imperfection on the values of the velocities.
For the methods summarized above, the difficulty originates from the fact that, when changing the value of the local velocity of a zone when the interfaces or horizons are not plane and horizontal, the rays describing the propagation of the acoustic waves in this zone are displaced laterally at the same time as their propagation time changes, whereas the spatial measurement frame remains fixed. The result of this is that the law describing the deformation of the iso-X curves or image gathers as a function of a variation in the velocities is non-linear and non-reversible.
The image-gathers correspond to collections of traces which are sorted for a given X and originate from a constant-offset migration. These traces are generally ordered by increasing offset.
The iso-X curves originate from a shotpoint migration.
The object of the present invention is to propose a new method which overcomes the drawbacks connected with the methods of the prior art and which makes it possible to take account of the variations in the velocities of the environment.
One object of the method according to the invention is to obtain a velocity field which is very close to that of the environment.
Another object of the method according to the invention is to measure, or at the least acquire better knowledge of the uncertainties on the values of the velocities which are calculated.
To achieve these objects, the present invention provides a method fo
REFERENCES:
patent: 4888742 (1989-12-01), Beasley
patent: 4964089 (1990-10-01), Wang et al.
patent: 4992996 (1991-02-01), Wang et al.
patent: 5128899 (1992-07-01), Boyd et al.
G. Chavent, et al., "Determination of Background Velocities by Multiple Migration Fitting", Geophysics, vol. 60, No. 2, Mar. 1995-Apr. 1995, pp. 476-490.
P.D. Thore, et al., "The Three-Parameter Equation: An Efficient Tool To Enhance the Stack", Geophysics, vol. 59, No. 2, Feb. 1994, pp. 297-308.
Elf Exploration Production
Jolly Anthony
Oda Christine K.
LandOfFree
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