Method for bispectral picking of anelliptical NMO correction parameters
View Patent ↗Method of determining the velocity V and anellipticity η parameters for processing seismic traces in a common midpoint (CMP) gather comprising:—a preliminary step to define a plurality of nodes (dtn, τ o ), for each node (dtn, τ o ) defined in the preliminary step, the following steps:—for static NMO correction of traces in the CMP gather as a function of the values of the said parameters dtn and τ o at the node considered, and calculation of the semblance function associated with the said NMO correction for the node considered; and—for each picked time t o , a step including determination of the maximum semblance node (dtn (t o ), τ o (t o )),—and a final step to convert the dtn (t o ) and τ o (t o ) parameters, so as to obtain the velocity t o ) and anellipticity η (t o ) laws.
1. Method of determining the velocity v and anellipticity η parameters for processing seismic traces obtained from seismic receivers in a common midpoint (CMP) gather including an anelliptic (Normal Move Out) NMO correction, comprising:
a preliminary step to define a plurality of nodes (dtn, τ 0 ) in a CMP gather of a series of seismic traces, at least a portion of said traces corresponding to signals reflected by geological interfaces of a subsurface zone, said nodes being indicative of parameters dtn and τ 0 , wherein dtn represent the NMO correction for the maximum offset and τ 0 represent the zero offset travel time in hyperbolic coordinates, said preliminary step being followed by
for each node (dtn, τ 0 ) defined in the preliminary step, the following steps:
performing static NMO correction of traces in the CMP gather as a function of the values of the said parameters dtn, τ 0 at the node considered, and
calculating the semblance function associated with the said NMO correction for the node considered; and
for each picked time t 0 , determining the maximum semblance node (dtn (t 0 ), τ 0 (t 0 )),
a step to convert the dtn (t 0 ) and τ 0 (t 0 ) parameters so as to obtain the velocity V (t 0 ) and anellipticity η (t 0 ) laws
and a step of processing the seismic traces in view of the obtained velocity V (t 0 ) and anellipticity η (t 0 ) laws, the output of said processing used to represent the seismic traces in one or more images of at least a portion of the geological interfaces of said subsurface zone.
2. Method according to claim 1 , wherein the nodes are defined during the preliminary step in an analysis volume (dtn, τ 0 , t 0 ) determined by minimum and maximum values respectively [dnt min , dtn max ] [τ 0min , τ 0max ] and [t 0min , t 0max ] of the dtn, τ 0 , t 0 parameters.
3. Method according to claim 2 , wherein, during the preliminary step, a corridor [dtn min (t 0 ), dtn max (t 0 )], [τ 0min (t 0 ), τ 0max (t 0 )] for changing dtn and τ 0 parameters is delimited inside the analysis volume as a function of plausible velocity V and anellipticity η values, the nodes (dtn, τ 0 ) defined for applying the NMO correction being then located along the corridor thus delimited.
4. Method according to claim 1 , further comprising, for each node (dtn, τ 0 ), a stacking step of the corrected seismic traces, following the semblance function calculation step.
5. Method according to claim 4 , wherein the stacking of corrected traces is done using only near offset traces.
6. Method according to claim 4 , further comprising for each picked time, and following the step for determining the maximum semblance node, a step of checking that values dtn and τ 0 of the maximum semblance node correspond to a stacking extreme value for the same values dtn and τ 0 .
7. Method according to claim 1 , further comprising a step of selecting and adjusting the pickings obtained, following the step implemented for determining the maximum semblance node (dtn (t 0 ), τ 0 (t 0 )) for each picked time t 0 , before the conversion step.
8. Method according to claim 7 , wherein the said step of selecting and adjusting the pickings comprises a step of only retaining pickings dtn and τ 0 for which time to the highest semblance pickings is greater than a predefined value.
9. Method according to claim 8 , wherein the said step of selecting and adjusting the pickings also comprises a step for adjusting the retained pickings dtn and τ 0 by parabolic interpolations using values about the said picked values.
10. Method according to claim 9 , wherein the said step of selecting and adjusting pickings also comprises a step of eliminating retained and adjusted pickings dtn and τ 0 when it is impossible to calculate the Dix interval velocities between the picking considered and higher semblance pickings.
11. Method according to claim 1 , wherein the processing applied to seismic traces is an NMO correction process implementing a static correction CORR NMO .
12. Method according to claim 11 , wherein, during the preliminary step, the NMO corrections CORR NMO are calculated for all nodes (dtn, τ 0 ) including in the analysis volume and all offsets of processed seismic traces.
13. Method according to claim 12 , wherein the NMO correction carried out for each node (dtn, τ 0 ), consists of applying NMO corrections CORR NMO calculated during the preliminary step.
14. Method according to claim 11 , wherein for a given (dtn, τ 0 ) pair, the static NMO correction CORR NMO of a seismic trace with offset x is carried out according to the following equation:
CORR
NMO
(
x
)
=
-
τ
0
+
τ
0
2
+
dtn
(
dtn
+
2
τ
0
)
x
max
2
x
2
in which X max represents the maximum offset in the CMP gather.
15. Method according to claim 14 , wherein, during the final conversion step, the parameters dtn (t 0 ) and (τ 0 ) are converted to the velocity law v (t 0 ) according to the following equation:
V
=
x
max
dtn
(
dtn
+
2
τ
0
)
t
0
τ
0
.
16. Method according to claim 14 , wherein, during the final conversion step, the parameter τ 0 (t 0 ) is converted to the anellipticity η (t 0 ) law according to
η
=
1
8
(
t
0
τ
0
-
1
)
.
17. Method according to claim 15 , wherein parameter dtn is defined with respect to the velocity v and anellipticity η according to the following equation:
dtn
=
8
η
1
+
8
η
t
0
+
(
t
0
1
+
8
η
)
2
+
x
max
2
(
1
+
8
η
)
V
2
.
18. Method according to claim 16 , wherein parameter τ 0 is defined according to anellipticity η according to the following equation:
τ
0
=
t
0
1
+
8
η
.
19. Method according to claim 2 , further comprising, for each node (dtn, τ 0 ), a stacking step of the corrected seismic traces, following the semblance function calculation step.
20. Method according to claim 19 , wherein the stacking of corrected traces is done using only near offset traces.
21. Method according to claim 19 , further comprising for each picked time, and following the step for determining the maximum semblance node, a step of checking that values dtn and τ 0 of the maximum semblance node correspond to a stacking extreme value for the same values dtn and τ 0 .
22. Method according to claim 2 , further comprising a step of selecting and adjusting the pickings obtained, following the step implemented for determining the maximum semblance node (dtn (t 0 ), τ 0 (t 0 )) for each picked time t 0 , before the conversion step.
23. Method according to claim 22 , wherein the said step of selecting and adjusting the pickings comprises a step of only retaining pickings dtn and τ 0 for which time to the highest semblance pickings is greater than a predefined value.
24. Method according to claim 23 , wherein the said step of selecting and adjusting the pickings also comprises a step for adjusting the retained pickings dtn and τ 0 , by parabolic interpolations using values about the said picked values.
25. Method according to claim 24 , wherein the said step of selecting and adjusting pickings also comprises a step of eliminating retained and adjusted pickings dtn and τ 0 when it is impossible to calculate the Dix interval velocities between the picking considered and higher semblance pickings.
26. Method according to claim 2 , wherein the processing applied to seismic traces is an NMO correction process implementing a static correction CORR NMO .
27. Method according to claim 26 , wherein, during the preliminary step, the NMO corrections CORR NMO are calculated for all nodes (dtn, τ 0 ) including in the analysis volume and all offsets of processed seismic traces.
28. Method according to claim 26 , wherein the NMO correction carried out for each node (dtn, τ 0 ) consists of applying NMO corrections CORR NMO calculated during the preliminary step.
29. Method according to claim 26 , wherein for a given (dtn, τ 0 ) pair, the static NMO correction CORR NMO of a seismic trace with offset x is carried out according to the following equation:
CORR
NMO
(
x
)
=
-
τ
0
+
τ
0
2
+
dtn
(
dtn
+
2
τ
0
)
x
max
2
x
2
in which X max represents the maximum offset in the CMP gather.
30. Method according to claim 2 , wherein the processing applied to seismic traces is a PSTM migration using a static NMO correction CORR PSTM , and wherein, for a given pair (dtn and τ 0 ), the static NMO correction CORR PSTM is carried out according to the following equation:
CORR
PSTM
(
x
)
=
-
τ
0
+
τ
0
2
4
+
dtn
(
dtn
+
2
τ
0
)
(
x
-
x
+
h
)
2
x
max
2
+
τ
0
2
4
+
dtn
(
dtn
+
2
τ
0
)
(
x
-
x
-
h
)
2
x
max
2
where:
x m represents the coordinates of the midpoints,
x−x m represents the migration aperture PSTM,
h is the half source—receiver offset,
x max is the maximum offset and aperture of the migration.
31. Method according to claim 29 , wherein, during the final conversion step, the parameters dtn (t 0 ) and (τ 0 ) are converted to the velocity law v (t 0 ) according to the following equation:
V
=
x
max
dtn
(
dtn
+
2
τ
0
)
t
0
τ
0
.
32. Method according to claim 29 , wherein, during the final conversion step, the parameter τ 0 (t 0 ) is converted to the anellipticity η (t 0 ) law according to
η
=
1
8
(
t
0
τ
0
-
1
)
.
33. Method according to claim 31 , wherein parameter dtn is defined with respect to the velocity v and anellipticity η according to the following equation:
dtn
=
8
η
1
+
8
η
t
0
+
(
t
0
1
+
8
η
)
2
+
x
max
2
(
1
+
8
η
)
V
2
.
34. Method according to claim 32 , wherein parameter ρ 0 is defined according to anellipticity η according to the following equation:
τ
0
=
t
0
1
+
8
η
.
35. Method according to claim 3 , further comprising, for each node (dtn, τ 0 ), a stacking step of the corrected seismic traces, following the semblance function calculation step.
36. Method according to claim 35 , wherein the stacking of corrected traces is done using only near offset traces.
37. Method according to claim 35 , further comprising for each picked time, and following the step for determining the maximum semblance node, a step of checking that values dtn and τ 0 ; of the maximum semblance node correspond to a stacking extreme value for the same values dtn and τ 0 .
38. Method according to claim 3 , further comprising a step of selecting and adjusting the pickings obtained, following the step implemented for determining the maximum semblance node (dtn (t 0 ), τ 0 (t 0 )) for each picked time t 0 , before the conversion step.
39. Method according to claim 38 , wherein the said step of selecting and adjusting the pickings comprises a step of only retaining pickings dtn and τ 0 for which time to the highest semblance pickings is greater than a predefined value.
40. Method according to claim 39 , wherein the said step of selecting and adjusting the pickings also comprises a step for adjusting the retained pickings dtn and t 0 by parabolic interpolations using values about the said picked values.
41. Method according to claim 40 , wherein the said step of selecting and adjusting pickings also comprises a step of eliminating retained and adjusted pickings dtn and τ 0 when it is impossible to calculate the Dix interval velocities between the picking considered and higher semblance pickings.