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Nonhyperbolicity of reflection moveout in vertically heterogeneous
isotropic media has been extensively studied using the
Taylor series expansion in the powers of the offset
(Bolshykh, 1956; Taner and Koehler, 1969; Al-Chalabi, 1973). The most important property of
vertically heterogeneous media is that the ray parameter
does not change along
any given ray (Snell's law). This fact leads to the explicit parametric
relationships
where
![\begin{displaymath}
t_z = t(0) = 2 \int_{0}^{z} {{dz} \over {V_z(z)}}\;.
\end{displaymath}](img60.png) |
(21) |
Straightforward differentiation of parametric equations (19)
and (20) yields the first four coefficients of the Taylor
series expansion
![\begin{displaymath}
t^2(l) = a_0 + a_1 l^2 + a_2 l^4 + a_3 l^6 + \ldots
\end{displaymath}](img61.png) |
(22) |
in the vicinity of the vertical zero-offset ray. Series
(22) contains only even powers of the offset
because
of the reciprocity principle: the pure-mode reflection traveltime is an even
function of the offset. The Taylor series coefficients for the isotropic case
are defined as follows:
where
![$\displaystyle V_{rms}^2 = M_1 ,$](img70.png) |
|
|
(27) |
Equation (24) shows that, at small offsets, the reflection
moveout has a hyperbolic form with the normal-moveout velocity
equal to the root-mean-square velocity
. At large offsets,
however, the hyperbolic approximation is no longer accurate. Studying
the Taylor series expansion (22), Malovichko (1978)
introduced a three-parameter approximation for the reflection
traveltime in vertically heterogeneous isotropic media.
His equation has the form of a
shifted hyperbola (Castle, 1988; de Bazelaire, 1988):
![\begin{displaymath}
t(l) = \left(1 - {1 \over S}\right) t_0 +
{1 \over S} \sqrt{t_0^2 + S {l^2 \over V_n^2}}\;.
\end{displaymath}](img76.png) |
(30) |
If we set the zero-offset traveltime
equal to the vertical
traveltime
, the velocity
equal to
, and the parameter of heterogeneity
equal to
, equation
(30) guarantees the correct coefficients
,
, and
in the Taylor series (22). Note that the parameter
is related to the variance
of the squared velocity
distribution, as follows:
![\begin{displaymath}
\sigma^2 = M_2 - V_{rms}^4 = V_{rms}^4 (S_2 -1)\;.
\end{displaymath}](img84.png) |
(31) |
According to equation (31), this parameter is always greater
than unity (it equals 1 in homogeneous media). In many
practical cases, the value of
lies between
and
. We can
roughly estimate the accuracy of approximation (30) at
large offsets by comparing the fourth term of its Taylor series with
the fourth term of the exact traveltime expansion (22).
According to this estimate, the error of Malovichko's approximation is
![\begin{displaymath}
{{\Delta t^2(l)} \over t^2(0)} = {1 \over 8} (S_3-S_2^2)
\left({l \over {t_0 V_n}}\right)^6\;.
\end{displaymath}](img87.png) |
(32) |
As follows from the definition of the parameters
[equations (29)] and the Cauchy-Schwartz inequality, the expression
(32) is always nonnegative.
This means that the shifted-hyperbola approximation
tends to overestimate traveltimes at large offsets. As the offset
approaches infinity, the limit of this approximation is
![\begin{displaymath}
\lim_{l \rightarrow \infty} t^2(l) = {1 \over S} {l^2 \over V_n^2}\;.
\end{displaymath}](img89.png) |
(33) |
Equation (33) indicates that the effective horizontal
velocity for Malovichko's approximation (the slope of the shifted
hyperbola asymptote) differs from the normal-moveout velocity. One
can interpret this difference as evidence of some effective
depth-variant anisotropy. However, the anisotropy implied in
equation (30) differs from the true anisotropy in a
homogeneous transversely isotropic medium [see equation (1)].
To reveal this difference, let us substitute
the effective values
,
,
, and
into equation (30). After eliminating the
variables
and
, the result takes the form
![\begin{displaymath}
{1 \over {V_g(\psi)}} = {1 \over V_z} \left\{
\cos{\psi}\le...
...2} \sin^2{\psi} +
{V_n^4 \over V_x^4} \cos^2{\psi}}\right\}.
\end{displaymath}](img94.png) |
(34) |
If the anisotropy is induced by vertical heterogeneity,
.
Those inequalities follow from the definitions of
,
,
, and the Cauchy-Schwartz inequality. They reduce to equalities only
when velocity is constant. Linearizing expression
(34) with respect to Thomsen's anisotropic parameters
and
, we can transform it to the form analogous to
that of equation (5):
![\begin{displaymath}
V_g^2(\psi) = V_z^2 \left[ 1 + 2 \delta \sin^2{\psi} +
2 \eta (1 - \cos{\psi})^2\right]\;.
\end{displaymath}](img96.png) |
(35) |
Figure 3 illustrates the difference between the VTI
model and the effective anisotropy implied by the Malovichko approximation.
The differences are noticeable in both the shapes of the effective wavefronts
(Figure 3a) and the moveouts (Figure 3b).
|
---|
nmofrz
Figure 3. Comparison of the
wavefronts (a) and moveouts (b) in the VTI (solid) and vertically
inhomogeneous isotropic media (dashed). The values of the effective
vertical, horizontal, and NMO velocities are the same
in both media and correspond to Thomsen's parameters
and
.
|
---|
|
---|
In deriving equation (35), we have assumed the correspondence
![\begin{displaymath}
S = {V_x^2 \over V_n^2} =
{{1 + 2 \epsilon} \over {1 + 2 \delta}} \approx 1 + 2 \eta\;.
\end{displaymath}](img97.png) |
(36) |
We could also have chosen the value of the parameter of heterogeneity
that
matches the coefficient
given by equation (25) with
the corresponding term in the Taylor series (13).Then, the value of
is (Alkhalifah, 1997)
![\begin{displaymath}
S = 1 + 8 \eta\;.
\end{displaymath}](img98.png) |
(37) |
The difference between equations (36) and (37) is an
additional indicator of the fundamental difference between homogeneous
VTI and vertically heterogeneous isotropic media. The three-parameter
anisotropic approximation (12) can match the
reflection moveout in the isotropic model up to
the fourth-order term in the Taylor series expansion if the value of
is chosen in accordance with equation (37). We can
estimate the error of such an approximation with an equation analogous
to (32):
![\begin{displaymath}
{{\Delta t^2(l)} \over t^2(0)} = {1 \over 8} (S_3-2 + 3 S_2-2 S_2^2)
\left({l \over {t_0 V_n}}\right)^6\;.
\end{displaymath}](img99.png) |
(38) |
The difference between the error estimates (32) and
(38) is
![\begin{displaymath}
{{\Delta t^2(l)} \over t^2(0)} = {1 \over 8} (2 - S_2) (S_2-1)
\left({l \over {t_0 V_n}}\right)^6\;.
\end{displaymath}](img100.png) |
(39) |
For usual values of
, which range from
to
, the expression (39) is
positive. This means that the anisotropic approximation
(12) overestimates traveltimes in the isotropic
heterogeneous model even more than does the shifted hyperbola
(30) shown in Figure 3b.
Below, we examine which of the two approximations is more suitable when
the model includes both vertical heterogeneity and anisotropy.
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Next: Vertically heterogeneous VTI model
Up: VERTICAL HETEROGENEITY
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2013-03-03