Anisotropic velocity analysis

Seismic Data Analysis

Series	Investigations in Geophysics
Author	Öz Yilmaz
DOI	http://dx.doi.org/10.1190/1.9781560801580
ISBN	ISBN 978-1-56080-094-1
Store	SEG Online Store

We shall confine the discussion on anisotropy primarily to the practical case of P-wave propagation in weakly anisotropic rocks. Consider the wavefront geometry shown in Figure 11.7-1. The P-wave phase velocity is given by

$\alpha (\theta )=\alpha _{0}(1+\delta \sin ^{2}\theta \cos ^{2}\theta +\varepsilon \sin ^{4}\theta ).$

(84)

Note that the P-wave velocity depends on the anisotropy parameters δ and ε, but not on the parameter γ. In fact, the SV-wave velocity also depends only on δ and ε, while the SH-wave velocity depends only on γ.

In the special case of vertical incidence, θ = 0, equation (84) gives the vertical P-wave velocity, α₀. In the special case of horizontal incidence, θ = 90 degrees, equation (84) gives

$\alpha _{h}=\alpha _{0}(1+\varepsilon ).$

(85)

Solving for ε, we obtain

$\varepsilon ={\frac {\alpha _{h}-\alpha _{0}}{\alpha _{0}}}.$

(86)

This equation provides a physical meaning for the Thomsen parameter ε. Specifically, the parameter ε indicates the degree of anisotropic behavior of the rock, measured as the fractional difference between vertical P-wave velocity α₀ and the horizontal P-wave velocity α_h. Since for most rocks ε > 0, note that the horizontal P-wave velocity is greater than the vertical P-wave velocity.

The normal-moveout velocity v_NMO(ϕ = 0), where ϕ is the dip angle, for a flat reflector in an anisotropic medium is given by Thomsen

$v_{NMO}(0)=\alpha _{0}{\sqrt {1+2\delta }}.$

(87)

In the special case of δ = 0, the moveout velocity is the same as the velocity of an isotropic medium represented by α₀ (normal moveout).

The P-wave traveltime equation for a flat reflector in an anisotropic medium is given by Tsvankin and Thomsen

$t^{2}=t_{0}^{2}+A_{2}x^{2}+{\frac {A_{4}x^{4}}{1+Ax^{2}}},$

(88)

where t is the two-way time from source to reflector to receiver, t₀ is the two-way zero-offset time, x is the source-receiver offset, and A₂ and A₄ are coefficients which are written below to acknowledge their complexity

$A_{2}={\frac {1}{\alpha _{0}^{2}(1+2\delta )}},$

(89a)

$A_{4}=-{\frac {2(\varepsilon -\delta )[1+2\delta (1-\beta _{0}^{2}/\alpha _{0}^{2})^{-1}]}{t_{0}^{2}\alpha _{0}^{4}(1+2\delta )^{4}}},$

(89b)

and

$A={\frac {A_{4}}{\alpha _{h}^{-2}-A_{2}}}.$

(89c)

Just for comparison, we write the traveltime equation (3-11) for a flat reflector in an isotropic medium using the current notation

$t^{2}=t_{0}^{2}+{\frac {x^{2}}{v_{NMO}^{2}}}.$

(90)

For isotropic velocity analysis using equation (90), we only need to scan one parameter — the velocity v_NMO (velocity analysis). For anisotropic velocity analysis using equation (88), note that we have to do a multiparameter scan involving α₀, α_h, β₀, δ, and ε — an impractical and unacceptable proposal.

As a way to circumvent this insurmountable problem of a multiparameter scan in velocity analysis, Alkhalifah and Tsvankin define a new effective anisotropy parameter

$\eta ={\frac {\varepsilon -\delta }{1+2\delta }}.$

(91)

By way of equations (85), (87), (91), and (89a,89b,89c), and neglecting the effect of β₀, equation (88) takes a form that is suitable for practical implementation

$t^{2}=t_{0}^{2}+{\frac {x^{2}}{v_{NMO}^{2}}}-{\frac {2\eta x^{4}}{v_{NMO}^{2}\left[t_{0}^{2}v_{NMO}^{2}+(1+2\eta )x^{2}\right]}},$

(92)

where v_NMO is the flat-reflector moveout velocity.

Equation (92) indicates that the traveltime for a reflector in an anisotropic medium follows a nonhyperbolic trajectory. By setting η = 0, equation (92) reduces to the isotropic case of equation (90). Note that the effect of anisotropy on reflection traveltimes is most significant at far offsets. The derivation of the anisotropic moveout equation (92) is based on a single flat layer. Nevertheless, in practice, it also is applicable to the case of a horizontally layered earth model with vertical transverse isotropy .

Figure 11.7-4a shows CMP raypaths associated with a flat reflector in a transversely isotropic medium with a velocity behavior as shown in Figure 11.7-2. Transverse isotropy causes fundamental departures from the CMP raypath geometry associated with an isotropic medium:

1. The zero-offset raypath is non-normal incident.
2. A single common reflection point does not exist; instead, anisotropy has given rise to reflection point dispersal even for a flat reflector.
3. The anisotropic moveout given by equation (92) in general is nonhyperbolic.
4. The NMO velocity measured from the slope of the t² − x² curve indicates that anisotropy makes the velocity offset dependent (Figure 11.7-4b).

Figure 11.7-5 shows a CMP gather and its velocity spectrum computed using the isotropic traveltime equation (90). Note that there are some overcorrected events between the two-way zero-offset times of 1.5-2 s and some undercorrected events above the two-way zero-offset time of 1.5 s. The undercorrected events are clearly multiples which produce distinct semblance peaks on the velocity spectrum. The overcorrected events result from one of the three possibilities — that the overcorrection is caused by the dip effect on moveout velocities, anisotropy, or a combination of the two phenomena. In Figure 11.7-5, a close-up display of the zone of interest between 1.5-2 s shows the overcorrected events with a distinct moveout behavior that is not quite the same as what a typical overcorrected event with hyperbolic moveout looks like. Specifically, we see that the events are nearly flat with no moveout within the near-offset range, while the moveout is like the end of a hockey-stick within the far-offset range.

Try flattening the events by experimenting with various velocity picks as shown in Figure 11.7-6. Note that, whatever the pick you make on the velocity spectrum, the hockey-stick events do not quite flatten; there is always some curvature left along the moveout-corrected event.

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  Figure 11.7-1  (a) Application of Huygens’ principle to anisotropic plane-wave propagation from an exploding reflector after . (b) The isotropic and anisotropic wavefront associated with an expanding P-wave after . See text for details.

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  Figure 11.7-2  Group velocity functions for P- and SH-waves in Green River shale (After .

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  Figure 11.7-3  Crossplot of P-wave anisotropy parameters, ε and δ, associated with various rock samples after .

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  Figure 11.7-4  (a) CMP raypaths associated with a flat reflector in a transversely isotropic medium; (b) the t² − x² plot that shows the offset-dependent behavior of the NMO velocity (After .

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  Figure 11.7-5  (a) An NMO-corrected CMP gather and (b) its velocity spectrum based on isotropic moveout equation (90); (c) and (d) are enlargements of (a) and (b), respectively.

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  Figure 11.7-6  (a) The CMP gather and (b) its velocity spectrum as in Figure (11.7-5c,d), except that the NMO correction has been applied using a different velocity function.

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  Figure 11.7-7  (a) The NMO-corrected CMP gather as in Figure 11.7-5a but with a different mute, and (b) its velocity spectrum; (c) and (d) are enlargements of (a) and (b), respectively.

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  Figure 11.7-8  (a) The NMO-corrected CMP gather as in Figure 11.7-5c and (b) its η-spectrum computed from the anisotropic moveout equation (92); (c) the same gather as in (a) after anisotropic moveout correction using the η function picked from the spectrum in (d).

Equation (92) indicates that, for anisotropic velocity analysis, we need to scan two parameters, v_NMO and η. Note that the parameter η is only present in the fourth-order moveout term that is significant at far offsets. This suggests a two-stage parameter scan as described below.

1. Perform hyperbolic velocity analysis using only the first two terms on the right-hand side of equation (92) and the near offsets where there is no hockey-stick effect. Figure 11.7-7 shows the same CMP gather as in Figure 11.7-5, but with far-offset traces muted, and the associated velocity spectrum. This first-stage analysis would give an estimate of the moveout velocity v_NMO as in equation (92).
2. Next, insert the estimated v_NMO function into equation (92) and compute the η spectrum as shown in Figure 11.7-8. After picking an η function in time, apply the fourth-order moveout correction given by equation 92 to the CMP gather.

Figures 11.7-9 and 11.7-10 show portions of a CMP stacked section with and without accounting for anisotropy in velocity analysis and moveout correction. The portion shown in Figure 11.7-9 is abundantly rich in diffractions and the portion shown in Figure 11.7-10 contains steeply dipping fault-plane reflections conflicting with gently dipping reflections. Panel (a) in both figures shows the full-fold stack and panel (b) shows the near-offset stack based on isotropic velocity analysis and moveout correction using the first two terms of equation (92). Note that the full-fold stack has been adversely affected by the hockey-stick effect on moveout at far offsets, while the near-offset stack has better preserved the diffractions and dipping events. Panel (c) of both Figures 11.7-9 and 11.7-10 shows the full-fold stack derived from anisotropic velocity analysis and moveout correction using both the second-order and fourth-order terms in equation (92). Note that the full-fold stacking based on anisotropic moveout correction has better preserved the diffractions and dipping events compared to the full-fold stacking based on isotropic moveout correction. These observations are more evident on the migrated sections shown in Figures 11.7-11 and 11.7-12.

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  Figure 11.7-9  Portions of (a) full-fold isotropic stack, (b) near-offset isotropic stack and (c) full-fold anisotropic stack.

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  Figure 11.7-10  Portions of (a) full-fold isotropic stack, (b) near-offset isotropic stack and (c) full-fold anisotropic stack.

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  Figure 11.7-11  Portions of (a) full-fold isotropic stack, (b) near-offset isotropic stack and (c) full-fold anisotropic stack as in Figure 11.7-9 after poststack time migration.

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  Figure 11.7-12  Portions of (a) full-fold isotropic stack, (b) near-offset isotropic stack and (c) full-fold anisotropic stack as in Figure 11.7-10 after poststack time migration.

See also

• Seismic anisotropy
• Anisotropic dip-moveout correction
• Anisotropic migration
• Effect of anisotropy on AVO
• Shear-wave splitting in anisotropic media

External links

find literature about Anisotropic velocity analysis