Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2003-05-29
2004-08-31
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C703S010000
Reexamination Certificate
active
06785612
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of seismic data processing, and more particularly to a method for determining the vertical depth and the vertical P-wave and S-wave velocities to estimate anisotropic parameters via prestack depth migration of P-P and P-S seismic data in the presence of VTI anisotropy.
BACKGROUND OF THE INVENTION
A seismic survey begins with the sending of acoustic energy from a source such as an explosion, air gun, or seismic vibrator into the earth in the form of a non-polarized, omni-directional wavefield, and the recording by spaced-apart seismic sensors of acoustic energy which is reflected or refracted back from inhomogeneities or discontinuities in subsurface layers. Seismic energy propagates through the earth as compresional P-waves and shear S-waves. P-waves propagate in-line with the direction of travel of the source acoustic energy, and S-waves flow transversely and horizontally (“SH wave”) as well as vertically (“SV” wave) with respect to the direction of travel of the source acoustic energy. At any subsurface discontinuity, a P-wave may convert to an S-wave. If the conversion happens only once from an incident P-wave to a reflected S-wave, the converted wave is referred to as a P-S converted wave, C-mode converted wave, or C-wave.
Referring to
FIG. 1
, a thick uniform isotropic layer of thickness “z” is shown with an acoustic energy source “S” and a seismic detector “R” which are a distance of “x” apart. The distance “x” is referred to as the offset between the source “S” and the detector “R”. A compression or P-wave
11
originates from the source “S”, and is reflected from the bottom of the layer
10
at a horizontal distance of x/2 from the source “S”. The reflected acoustic energy wave is a P-wave
12
that is sensed and recorded by detector “R”, which may be a hydrophone or geophone. A further P-wave
13
originates from the source “S”, and is reflected from the bottom of layer
10
a horizontal distance of “s+x
c
” from the source “S”. P-wave
13
undergoes a conversion to a shear or S-wave
14
upon reflection from the bottom of the layer
10
. As before stated, since the conversion occurs only once, the converted wave may be referred to as a C-mode converted wave or C-wave.
Subsurface layers which are isotropic exhibit the same velocity of propagation of acoustic energy in all directions. Other subsurface layers are anisotropic in that the velocity of propagation of acoustic energy is azimuth dependent. Flat-lying polar anisotropic (“VTI”) subsurface layers, also referred to as transversely isotropic media with vertical symmetry, give rise to only one C-mode reflection.
Conventional seismic processing relies heavily on a stack (or average) of seismic traces from a common midpoint (“CMP”) gather to reduce coherent and incoherent noise in a seismic section. The stacking approach is generally satisfactory for single mode seismic data (P-wave, S-wave), but often fails when applied to converted mode (C-wave) data due to the asymmetrical travel paths. Data for a true common reflection point (“CRP”), which for C-wave reflections is a common conversion point gather (“CCP”), is required.
Reflection data of all types, whether P-wave or S-wave, must be corrected for irregular time delays. As reflection events are detected by seismic detectors increasingly distant from the source “S”, the arrival time of the reflected signals becomes increasingly long. Such a systematic shift to longer reflection times due to increasing source-detector offsets is generally referred to as normal moveout or NMO. It is well known that normal moveout causes errors in determining compressional and shear wave velocities. If such errors remain uncorrected, stacked amplitudes of seismic events will be misaligned, and the behavior of reflecting interfaces between subsurface layers will be misrepresented.
To overcome the above maladies, methods for establishing and updating velocity models have become two important steps for multicomponent P-P and P-S prestack seismic depth imaging. Five anisotropic processing parameters are commonly used in multicomponent data processing: the vertical P-P velocity v
po
, the vertical P-S velocity v
so
, the vertical depth z
o
, and the parameters &dgr; and &sgr; as defined in the articles, “Weak elastic anisotropy”, by L. Thomsen, Geophysics, vol. 51, pp. 1954-1966 (October 1986) and “Parameter estimation for VTI media using PP and PS reflection data”, by Ilya Tsvankin and Vladimir Grechka, Proceedings of the 71
st
Annual International Meeting of the Society of Exploration Geophysicists (Copyright 2001). The anisotropic parameter &dgr; is defined by:
δ
=
(
C
13
+
C
44
)
2
-
(
C
33
-
C
44
)
2
2
C
33
(
C
33
-
C
44
)
,
where C
ii
are the components of the 6×6 symmetric elastic modulus matrix relating the stress components of a linearly elastic material to a linear combination of the strain components. The anisotropic parameter &sgr; is defined by:
σ
=
(
v
po
v
so
)
2
(
ϵ
-
δ
)
,
where the anisotropic parameter &egr; is defined by:
ϵ
=
C
11
-
C
33
2
C
33
.
The uncertainty caused by the presence of anisotropy in the estimation of vertical velocity and depth is addressed in Tsvankin and Grechka (2001). Unfortunately, no prior method is known which is successful in overcoming errors in determining such anisotropic processing parameters in the presence of a VTI anisotropic subsurface layer.
The present invention is directed to a method of depth consistent joint velocity inversion for substantially overcoming errors in estimating the anisotropic parameters v
so
, v
po
, &dgr;, &sgr;, and the vertical depth z
0
in a VTI anisotropic subsurface layer.
SUMMARY OF THE INVENTION
A process for updating velocity determinations for anisotropic P-P and P-S prestack depth migration in a transversely isotropic media with vertical symmetry (VTI).
In one aspect of the invention, upon corresponding reflections from P-P and P-S waves being identified, a depth consistent image gather is formed and a joint velocity inversion in depth is performed to estimate vertical depth z
o
, vertical P-wave velocity v
po
, and vertical S-wave velocity v
so
.
In a second aspect of the invention, an isotropic depth migration using the above estimates of v
so
, v
po
, and z
o
is performed by scanning focusing velocities from P-P depth consistent image gathers to determine an isotropic depth z
pp
.
In a third aspect of the invention, anisotropic parameters &dgr; and &sgr; are estimated based upon the above estimates of v
po
, v
so
, z
o
, and z
pp
.
In a fourth aspect of the invention, the above estimates of v
po
, v
so
, z
o
, and z
pp
, and the anisotropic parameters &dgr; and &sgr; are used to refine the estimates v
so
, v
po
, and z
o
.
In another aspect of the invention, VTI anisotropic parameters may be determined (whether or not well log data is available) to produce reliable P-P and P-S depth consistent imaging.
REFERENCES:
patent: 5343441 (1994-08-01), Alford
patent: 6128580 (2000-10-01), Thomsen
patent: 6351991 (2002-03-01), Sinha
Leon Thomsen, “Weak elastic anisotropy”, Geophysics, Oct. 1986, pp. 1954-1966, vol. 51, No. 10.
Ilya Tsvankin, Vladimir Grechka, “Parameter estimation for VTI media using PP and PS reflection data”, Society of Exploration Geophysicists International Exposition and 71stAnnual Meeting, Sep. 2001, pp. 857-860, vol. I.
Yaohui Zhang, Min Lou, Don Pham, “Velocity Update via Joint Velocity Inversion for Anisotropic Depth Migration”, Society of Exploration Geophysicists International Exposition and 72ndAnnual Meeting, Oct. 2002, pp. 141-144, vol. I.
Stuart William Fagin, “Model Based Depth Imaging ”, Society of Exploration Geophysicists, 1998, ISBN 0-931830-48-6.
Gerald H. .F. Gardner, “Migration of Seismic Data”, Society of Exploration Geophysicists, 1985, ISB 9-31830-35-4, Geophysics Reprint Series.
Yaohui Zhang, Long Don Pham, Fernando Neves, “Depth-Consistent P-P and P-S Seismic Image via Joint Velocity Inversion”, Society of Explor
Barlow John
PGS Americas, Inc.
Taylor Victor J.
Thigpen E. Eugene
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