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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Reexamination Certificate

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06311133

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method allowing to perform prestack migration of recorded seismic events for imaging a part of an underground zone.
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 a 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 migration 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 migration: 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 implementation of this type of techniques are for example described in:
Claerbout, J. F., 1985; Imaging the Earth's interior; Blackwell Publications,
Duquet, B., 1996; Amélioration de l'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).
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 S
i
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 as time-dependent seismic traces d
j
i
(t).
The method according to the invention performs 3D prestack depth migration, for a given velocity model, for imaging the various geological interfaces or heterogeneities of a part of the subsurface.
The invention, for a given velocity model, comprises the following stages:
a) defining a slowness vector p whose two components p
1
and p
2
can each assume a sequence of previously defined values;
b) defining, for a given slowness vector p and a given point of emission S
i
, a time lag function t
0
(p, i);
c) applying 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;
d) 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;
e) 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;
f) repeating steps c) to e) for all the values assumed by the components p
1
and p
2
of the vector p, and
g) for any set value of the second component p
2
of the vector p, stacking the results 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 g) 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 perform 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 zone in which the waves are propagated has an effect on the calculation cost. Volume images are thus obtained by taking account of all the seismic traces at an advantageous cost. This method is considered to decrease by a factor of the order of several tens the calculation time required for 3D prestack migration.
2) The method is applied for velocity models of arbitrary complexity as long as the notion of prestack migration retains its meaning. It is applied without encountering any of the limitations specific to the high-frequency asymptotic techniques (geometric optics) commonly used for 3D prestack migrations.
The method can be implemented by means of conventional wave propagation and retropropagation modelling tools, described for example in the aforementioned book by Duquet B.
Application of the method obtains elementary migrated images associated with a given value of a parameter and the sum of these images (post-migration stack), in the depth domain as well as in the time domain.


REFERENCES:
patent: 4870580 (1989-09-01), Lang et al.
patent: 6058073 (2000-05-01), VerWest

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