Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2000-02-22
2003-09-23
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C703S005000
Reexamination Certificate
active
06625544
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of controlling the quality of seismic data migration by using the generalized Radon transform (GRT).
BACKGROUND OF THE INVENTION
There exist numerous techniques that are used by geophysicists to find out about the geological layers constituting the subsoil. One of these techniques, known as a “well seismic surveying” is shown in FIG.
1
and consists in emitting sound signals at the surface of the ground (source S) and in using sensors Ci located in a well
10
to record the seismic waves as they are transmitted after reflection and/or refraction on the boundaries between the various geological layers. After the waves have been recorded by the sensors, they are filtered to determine which waves were reflected by reflectors situated beneath the sensors.
FIG. 2A
shows the signals as recorded prior to filtering and
FIG. 2B
shows the corresponding signals after filtering. A migration method applied to the filtered signals then makes it possible to obtain a migrated image which can be thought of as a vertical section plane through the subsoil containing the source(s) and the sensors in the well. As shown in FIG.
3
. the migrated image is constituted by a succession of vertical recordings referred to as “migrated seismic traces” constituted by signals of greater or lesser amplitude made up of positive and negative arches. The signals relating to the same reflector correlate from one trace to the next, so that the eye can see delineations which correspond to boundaries between geological layers.
The document “The generalized Radon transform, a breakthrough in seismic migration”, by M. Oristaglio, G Beylkin. and D. Miller (Seismics, The Technical Review. Volume 35, Number 3, 1987. pages 20 to 27) describes an example of the migration method which makes use of the generalized Radon transform (GRT). The GRT migration method constructs the migrated image point-by-point. As shown in
FIG. 4
, each image point I
1
or I
2
is considered as a point of velocity change in the medium that is capable of reflecting energy coming from the source S. The energy diffused by said point propagates in all directions and is finally detected by the sensors. The travel time of the seismic wave is calculated by dividing the sum of the distance ls from the source to said point plus the distance lc from the point to the sensor by the assumed propagation speed of the wave in the subsoil. This propagation speed is referred to as the velocity of the medium.
For each particular reflector point in the space of the migrated image it is possible to obtain a travel time curve in the data space: CI
1
, CI
2
; by calculating the seismic wave arrival times at each sensor as shown in FIG.
5
.
Inversely, each point P in the data space corresponds to the amplitude of the signal measured by a sensor at a given instant, which measured signal is due to a plurality of waves reflected by different reflector points all reaching the sensor at the same time. Thus, every point of the migrated image whose travel time curve passes through a given point P contributes to the amplitude of the signal at that point. The set of these reflecting points constitutes an isochronous curve which. as shown in
FIG. 4
, is elliptical in shape, with the source S and the sensor C being at the foci, assuming that the velocity in the subsoil is constant.
In application of the generalized Radon transform, each point of the migrated image is obtained by summing the contributions of data along the corresponding travel time curve, said contributions being weighted to take account of physical phenomena relating to wave propagation, for example loss of amplitude proportional to path length. To calculate path curves by ray tracing, an initial speed model is used which integrates a priori knowledge about the subsoil, such as the model shown in FIG.
6
.
The GRT migration algorithm thus has the following steps:
For each data trace T, for each point (x, z) of the migrated image:
i) the travel time ts from the soure S to the point (x, z) is calculated;
ii) the travel time tc from the sensor C to the point (x, z) is calculated;
iii) this gives the source-to-sensor travel time: tsc=ts+tc;
iv) the sample u of the trace T at time tsc is found;
v) the-weighting factor w relating to the source S, to the sensor C, and the point (x, z) is found;
vi) the value of an image table (x, z) at said point is calculated;
Image(
x, z
)=Image (
x, z
)+
u×w.
A variant of this GRT algorithm consists in a first step in calculating the table of travel time parameters t, the path length 1, and the direction of the ray at its ends d and a, and in a second step in performing the migration calculation. Thus, as shown in
FIG. 7
, for each source and for each sensor C, the travel times t(P, I
xy
) to go from said source or said sensor to all of the migrated image points I
xy
are calculated. The path lengths l(P, I
xy
) and the orientations of the ray at its ends a(P, I
xy
) and d (P, I
xy
) are calculated. In the second step. migration uses the table calculated during the first step.
For example, with filtered seismic signals as shown in
FIG. 8
, GRT migration enables a migrated image to be obtained of the kind shown in FIG.
9
.
Before using the results as shown in
FIG. 9
, the operator must verify that they are well-founded. It is necessary to perform quality control thereon. Traditionally. this has been based on more or less subjective criteria such as how the results correspond with a priori knowledge about the subsoil, or the continuity of the delineations representing the reflectors. It is thus very difficult to perform such quality control. particularly since the representation of the data before migration (
FIG. 8
) is very different from the representation of the data in the migrated image (FIG.
9
).
A method described in “Automatic association of kinematic information to pre-stack images”, by S. Geoltrain and E. Chovet (61st Annual International Meeting, Sot. Expl. Geophys., Expanded Abstracts, 1991, pages 890 to 892), for Kirchhoff migration. makes it possible to associate each point of the migrated image with a point in data space. Nevertheless, that method does not enable each point in the migrated image to be associated with all of the contributing points along the travel time curve. Nor does it make it possible to find the isochronous curve associated with a point in data space.
An object of the present invention is to solve those problems by proposing a quality control method for GRT migration of seismic data by interactive identification of events between the image obtained by migration and the seismic data prior to migration.
SUMMARY OF THE INVENTION
The present invention provides a method of controlling the quality of seismic data that has been migrated using the generalized Radon transform, the method serving to pass between a seismic data space and a migrated image space, in which method a parameter table is calculated giving, for a seismic wave going between a point of the image and a source or a sensor, its path length, its travel time, and the angles of incidence of the wave at the beginning and at the end of the path, the method being characterized in that correspondence is established between at least one zone of a first one of said two spaces and at least one zone of the second space by using said parameter table to fill in a correspondence table QCimage.
The parameter table is usually obtained by ray tracing, but it can also obtained by any other means making it possible to determine, for a seismic wave going between an image point and a source or a sensor, its path length, its travel time, and the angles of incidence of the wave at the beginning and end of the path.
The method of the invention makes it possible firstly to point to a zone in seismic data space and to project it into migrated image space by local migration relating to the pointed-to zone (“forward projection”). The method also makes it possible to point to a zone in migrated image space and
Cao Di
Laurent Christophe
Barlow John
Jeffery Brigitte
Nava Robin
Ryberg John
Schlumberger Technology Corporation
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