Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Reexamination Certificate
2002-08-14
2004-05-04
Moskowitz, Nelson (Department: 3663)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
C367S046000, C367S073000, C702S018000
Reexamination Certificate
active
06731568
ABSTRACT:
The present invention relates to a seismic prospection method in which the converted waves are processed.
The general principle of seismic prospection consists in using a seismic source to create a disturbance in the subsoil and in using sensors to record seismic data generated by the disturbance so as to extract information therefrom about the geology of the subsoil, and in particular to detect the presence of hydrocarbons.
FIG. 1
shows a sound wave propagating in the subsoil from a source
1
. The sound wave in the example shown is a compression wave which is reflected in the subsoil to give rise to components comprising a compression reflected wave and a shear reflected wave.
Compression waves (so-called P-waves) vibrate in their direction of propagation, while shear waves (so-called S-waves) vibrate perpendicularly to their direction of propagation. The propagation speed of shear waves is slower than the propagation speed of compression waves, and knowledge of the speed fields of compression waves and of shear waves can be used to determine information about the subsoil. For example, the ratio between the speeds of the compression waves and of the shear waves can be used to determine the pressure coefficient of the rocks they have traveled through and can also be used as an indicator of hydrocarbon presence.
Conventionally, in order to invert seismic data, speed field models are used which depend on various parameters that are assumed to be invariant over a given range of source-receiver offsets and in a given three-dimensional acquisition zone, however these parameters can vary “slowly” in three dimensions, i.e. they can differ from one zone in three dimensions to another.
To invert seismic data corresponding to SS or PP reflections in the subsoil, parameters V
p
and V
s
are used which represent the apparent speeds of the compression waves and of the shear waves after dynamic correction (“normal move out” or “NMO”), and also parameters T
p
and T
s
which represent respectively the vertical travel times of the P-waves and of the S-waves. The parameters T
p
and V
p
suffice for PP speed analysis, while the parameters T
s
and V
s
suffice for SS speed analysis.
Converted speeds (PS reflections) are generally analyzed by using models in the time domain which make use of the parameters V
p
and V
s
, and also of a parameter V
c
where V
c
is such that:
T
c
·V
c
2
=T
p
·V
p
2
+T
s
·V
s
2
where
T
c
=T
s
+T
p
The models making use of those three parameters are effective with materials that are homogeneous and isotropic for S-waves and P-waves. However, in media that are vertically inhomogeneous or that are highly anisotropic, it has been shown that account needs to be taken of two other parameters, referred to in the literature as &ggr;
eff
and &ggr;
0
, where &ggr;
eff
=&ggr;
n
2
/&ggr;
0
with &ggr;
n
=V
p
/V
s
and &ggr;
0
=T
s
/T
p
.
The offset X between the reflection point and a source depends to the first order on the parameter &ggr;
eff
and to the second order on the parameter &ggr;
0
, and also on the quantity T
c
·V
c
2
.
In this respect, reference can advantageously be made to the following publication:
[1] L. Thomsen, 1998, “Converted-wave reflection seismology over anisotropic, inhomogeneous media”, 68th Annual Meeting, SEG Expanded Abstracts, pp. 2048-2051.
Nevertheless, in that publication, the parameter &ggr;
eff
is assumed to be known. Unfortunately, in practice and as a general rule, none of the above-mentioned parameters is known immediately.
An object of the invention is to provide a seismic processing method applicable to converted waves which is particularly reliable and independent of any prior knowledge of the parameters &ggr;
eff
and &ggr;
0
.
Proposals have recently been made to determine the lateral offset of the conversation point by using the lateral correlation between forward source-receiver offset images and backward source-receiver offset images, i.e. images obtained by inverting the positions of the sources and of the receivers.
In this respect, reference can be made to:
[2] P. Hermann, G. Michaud, P. Y. Granger, 1999, “Stacking mode-converted waves”, presented at the CSEG Conference, Calgary, May 1999.
The invention provides a seismic prospection method in which a compression seismic wave is emitted into the subsoil and sensors are used to collect seismic data having at least a shear component, and in which the data corresponding to said shear component is processed to deduce information about the geology of the subsoil, the method being characterized in that an estimate of the ratio:
∫
Z
0
Z
v
p
·
ⅆ
l
/
∫
Z
0
Z
v
s
·
ⅆ
l
is determined where v
p
and v
s
are values for real local compression and shear speeds, where l is the depth coordinate in the subsoil, where Z is the value of this depth coordinate at the bottom surface of the last layer to be analyzed and where Z
0
is the value of this depth coordinate at the top surface of said layer or of a layer above it, and the seismic data is inverted in order to deduce the local values of compression and shear speed for said layer to be analyzed, by using a model in which this estimate is used for the invariant parameter &ggr;
eff
.
The invention advantageously further includes the following characteristics taken singly or in any technically feasible combination:
the parameter &ggr;
eff
is determined for various different possible values thereof by applying migration processing to the seismic data that corresponds to the shear component, and by determining the value for the parameter &ggr;
eff
at which the forward and backward seismic images are best correlated;
to vary the parameter &ggr;
eff
, the following notation is used:
v
p&agr;
=&agr;v
p0
and
v
s&bgr;
=&bgr;v
s0
where v
p0
and v
s0
are previously determined approximate values for v
p
and v
s
, and
both of the variables &agr; and &bgr; are varied;
the model uses as invariant parameters at least four of the following parameters: &ggr;
0
, &ggr;
eff
, T
p
, F
p
, T
c
, and F
c
where &ggr;
0
=T
s
/T
p
, &ggr;
eff
=F
p
/F
s
, T
c
=T
p
+T
s
, and where T
p
and T
s
represent the vertical travel times for the compression and shear waves respectively, where F
p
is such that (F
p
/T
p
)
1/2
represents a compression speed, and where F
c
is such that ((F
c
−F
p
)/T
s
)
1/2
represents a shear speed;
when the variables &agr; and &bgr; are varied, the parameters &ggr;
0
, &ggr;
eff
, T
p
, F
p
, and F
c
are replaced as follows:
&ggr;
0
′=&agr;/&bgr;*&ggr;
0
&ggr;
eff
′=&agr;/&bgr;*&ggr;
eff
T
p
′=T
p
*(1+&ggr;
0
)/(1+&ggr;
0
′)
F
p
′=F
p
*&agr;
2
*(1+&ggr;
0
)/(1+&ggr;
0
′)
F
c
′=F
c
*&agr;&bgr;*(1+&ggr;
0
)/(1+&ggr;
0
′)*(1+&ggr;
eff
′)/(1+&ggr;
eff
)
and migration is applied to the seismic data corresponding to these new parameters;
to vary the parameter &ggr;
eff
, &bgr; is set equal to 1/&agr;, and &agr; is varied;
after the parameter &ggr;eff has been determined, v
p
and v
s
are varied while keeping &ggr;
eff
constant, and the parameter F
c
is determined for which the alignment in the offset axis is at a maximum;
to vary v
p
and v
s
, the following notation is used:
v
p&agr;
=&agr;v
p0
and
v
s&agr;
=&agr;v
s1
where v
p1
and v
s1
are values determined for v
p
and v
s
in step 2, and the variable &agr; is varied;
after determining the parameter F
c
, the parameter T
p
and/or the parameter &ggr;
0 =T
s
/T
p
is/are determined;
the parameter T
p
is advantageously determined from the v
p
speed field determined by analyzing the compression component of the seismic data;
processing is subsequently performed to bring the S-speed and the P-speed models to a common depth; and
after processing to achieve a common depth, large-offset curvature processing is implemented by varying the anis
Audebert François
Granger Pierre-Yves
Compagnie Gererale de Geophysique
Duane Morris LLP
Moskowitz Nelson
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