3D prestack seismic data migration method

Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science

Reexamination Certificate

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C702S018000

Reexamination Certificate

active

06574564

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of performing prestack migration of recorded seismic events for imaging a part of an underground zone.
The method according to the invention performs 3D prestack depth migration, for a given velocity model, for imaging the various geologic interfaces or heterogeneities of a part of the substrate.
2. Description of the Prior Art
Prestack migration is a conventional method of processing seismic data. The technique generally consists, knowing the value of a wavefield at known depth, on the surface for example, and a model of the distribution of the wave propagation velocities in the zone, in modelling the propagation of the source field and the retropropagation of the recorded reflection data and in seeking phase coherences between these two modelled fields.
There are three main prestack migation types:
shotpoint migration: the source field is the vibrating state generated by the shotpoint and the reflection data are the response of the subsurface to this source field;
plane wave migration, also called common illumination angle migration: the source field is the plane wave considered and the reflection data are the response of the subsurface to this source field;
offset migation: the source field is the one emitted by a shotpoint and the reflection data are the records obtained by the pickup(s) associated with this shotpoint having the offset considered; in such a migration, migration of the data associated with an offset requires as many wave propagation and retropropagation modellings as there are shotpoints and stacking of the results obtained for each shotpoint.
Examples of implementations of this type of techniques are for example described in:
Claerbout, J. F., 1985; Imaging the Earth's interior; Blackwell Publications,
Duquet, B., 1996; Amelioration de I'Imagerie Sismique de Structures Géologiques Complexes; thèse, Université Paris 13, or
Whitmore, N. D., Felinsky, W. F., Murphy, G. E. and Gray, S. H., 1993; The Application of Common Offset and Common Angle Pre-stack Depth Migration in the North sea, 55
th
Mtg., EAGE, Expanded abstract.
The main drawback of conventional implementations based on the Kirchhoff equation (or more elaborate versions of this technique, itself based on high-frequency asymptotic techniques) is that they are generally very costly in calculation time because of the volume of the data to be processed and of the results, especially when the velocity field varies laterally (which complicates the arrival time calculations required for implementing this method). For economy reasons, one is often led to limit the volumes of data (by decimation) and/or the amount of results produced (imaged volume of reduced size, rough sampling of the results).
FIG. 6
illustrates an example of a situation which occurs in marine seismic prospection where azimuth moveout correction is necessary. The seismic response of a subterranean formation to seismic waves generated by a seismic source (Si) (an airgun for example) are sensed by receivers (R1 . . . R
j
i
. . . Rn) in a seismic streamer
10
towed by a towing boat
12
. Because of a rough sea or currents, the streamer may drift with at least a part of the receivers being out of the route XX following the towing boat
12
. As a result of this drift, the azimuth of the directions between the seismic source Si position and at least some of the receivers (R
j
i
) position is not correct. Azimuth moveout correction have also been applied in many other situations in marine seismic prospection when for example several seismic streamers are towed by the towing boat, and also in seismic prospection on land.
In such a typical case, the well-known technique of azimuth moveout correction can be applied for correction of the data obtained from the drifted receivers R
j
i
, which are out of line with the towing boat route XX is used to process the data to convert the data to a form that the data would have been recorded if the receivers were in line with the boat route XX. See “Azimuth Moveout for 3-D Prestack Imaging” (pages 1-45) Biondo, Fomel and Chemingui, Jul. 30, 1997 which publication is incorporated herein by reference in its entirety. Azimuth moveout correction has also been applied in many other situations in marine prospection when for example several seismic streamers are towed by the towing boat and also in seismic prospection on land.
SUMMARY OF THE INVENTION
The method according to the invention performs migration of seismic data for imaging a part of an underground zone, the seismic data being obtained after a series of N
s
seismic reflection cycles comprising each successive emission of elementary wavefields defined each by association of a seismic signal W(t) and of a point of emission in a series of points of emission Si with 1≦i≦N
s
, reception, by seismic receivers placed in positions R
j
i
, of the seismic signals reflected by the zone in response to each of these wavefields, and recording of the various signals received by each seismic receiver a time-dependent seismic traces d
j
i
(t).
The invention, for a given velocity model, comprises the following steps:
a) applying an azimuth moveout correction to data representing the recorded seismic signal sensed by different seismic receivers located at the points of reception for having all directions between points of emission and points of reception collinear to a common direction.
b) defining a slowness vector p whose two components p
1
and p
2
can each assume a sequence of previously defined values;
c) defining, for a given slowness vector p and a given point of emission S
i
, a time lag function t
0
(p, i)
d) applying a time lag function t
0
(p, i) to each elementary wavefield (associated with point of emission S
i
) and forming a first surface composite wavefield by spatiotemporal superposition of the various elementary wavefields to which such a time lag is applied;
e) applying a time lag t
0
(p, i) to each seismic trace d
j
i
(t) marked by the pair (i,j) and forming a surface composite trace field by spatiotemporal superposition of the various seismic traces to which such a time lag is applied;
f) performing migration of the composite trace field using the composite wavefield as the wavefield, by modelling the propagation of the composite wavefield and the retropropagation of the composite trace field and by suitably combining the two composite fields thus modelled at any point of the zone to be imaged;
g) repeating steps c) to e) for all the values assumed by the components p
1
and p
2
of the vector p; and
h) for any set value of the second component p
2
of the vector p, stacking the result of these various combinations so as to obtain a migrated image associated with this set value of p
2
, thus performing prestack migration.
According to an embodiment, the results obtained can be stacked at g) for all the values assumed by parameter p
2
, thus performing post-migration stacking.
According to an embodiment, post-migration stacking can be achieved directly without step g).
The method can also comprise updating velocities by analysis of the deformations obtained when the second coordinate p
2
of vector p is varied.
According to an embodiment, a migrated image of a part of the zone to be imaged can be formed by using the wave conversion phenomenon, by definition of at least part of the velocity field in P waves and S waves (by applying previously for example preprocessing suited to the data so as to separate the various types of seismic events).
Steps a) to h) can be used for determining the gradient of a cost function involved in an inverse seismic problem.
It is also possible to replace a depth migration by a time migration.
The method of the invention has many advantages:
1) The invention performs migration at an attractive price (calculation cost) because of being independent of the volume of results calculated and of the number of seismic traces recorded, which is unlike conventional Kirchhoff type methods. Only the volume of the zon

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