NMO in a horizontally stratified earth

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 now consider a medium composed of horizontal isovelocity layers (Figure 3.1-6). Each layer has a certain thickness that can be defined in terms of twoway zero-offset time. The layers have interval velocities (v₁, v₂, …, v_N), where N is the number of layers. Consider the raypath from source S to depth point D, back to receiver R, associated with offset x at midpoint location M. derived the traveltime equation for this path as

$t^{2}=C_{0}+C_{1}x^{2}+C_{2}x^{4}+C_{3}x^{6}+\cdots ,$

(3)

where ${{C}_{0}}={{t}_{0}}^{2},\ \ {{C}_{1}}={1}/{v_{rms}^{2}},$ and C₂, C₃, … are complicated functions that depend on layer thicknesses and interval velocities (Section C.1). The rms velocity v_rms down to the reflector on which depth point D is situated is defined as

$v_{rms}^{2}={\frac {1}{t_{0}}}\sum _{i=1}^{N}v_{i}^{2}\Delta \tau _{i},$

(4a)

where Δτ_i is the vertical two-way time through the ith layer and ${{t}_{0}}=\sum \nolimits _{i=1}^{N}{\Delta {{\tau }_{i}}.}$ By making the small-spread approximation (offset small compared to depth), the series in equation (3) can be truncated to obtain the familiar hyperbolic form

$t^{2}=t_{0}^{2}+{\frac {x^{2}}{v^{2}}},$

(1)

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

(4b)

When equations (1) and (4b) are compared, we see that the velocity required for NMO correction for a horizontally stratified medium is equal to the rms velocity, provided the small-spread approximation is made.

How much error is caused by dropping the higher order terms in equation (3)? Figure 3.1-7a shows a CMP gather based on the velocity model in Figure 3.1-8. Traveltimes to all four reflectors were computed by the raypath integral equations that exactly describe wave propagation in a horizontally layered earth model with a given interval velocity function. We now replace the layers above the second shallow event at t₀ = 0.8 s with a single layer with a velocity equal to the rms velocity down to this reflector — 2264 m/s. The resulting traveltime curve, computed using equation (4b), is shown in Figure 3.1-7b. This procedure is repeated for the deeper events at t₀ = 1.2 and 1.6 s as shown in Figures 3.1-7c and d. Note that the traveltime curves in Figures 3.1-7b, c, and d are perfect hyperbolas. How different are the traveltime curves in Figure 3.1-7a from these hyperbolas? After careful examination, note that the traveltimes are slightly different for the shallow events at t₀ = 0.8 and 1.2 s only at large offsets, particularly beyond 3 km. By dropping the higher order terms, we approximate the reflection times in a horizontally layered earth with a small-spread hyperbola.

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  Figure 3.1-6  A horizontally layered earth model geometry. The traveltime to the deepest reflector is described by equation (3). An approximate form of this equation used in practice is given by equation (4b).

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  Figure 3.1-7  (a) A synthetic CMP gather derived from the velocity function depicted in Figure 3.1-8; (b), (c), and (d) are CMP gathers derived from the rms velocities (indicated at the top of each gather) associated with the second, third, and fourth reflectors from the top. The traveltimes in (a) were derived using the raypath integral equations for a horizontally layered earth model.

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  Figure 3.1-8  A hypothetical velocity function used in generating the synthetic CMP gather in Figure 3.1-7a.

See also

• NMO for a flat reflector
• Fourth-order moveout
• NMO stretching
• NMO for a dipping reflector
• NMO for several layers with arbitrary dips
• Moveout velocity versus stacking velocity
• Exercises
• Topics in moveout and statics corrections

External links

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