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Velocity analysis

Common midpoint measurement(CMP CDP)

Velocity analysis

Normal move-out (NMO) correction

Artefacts that can occur: Stretching

Stacking

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horizontal Reflector

CMP = CDP

CMP = CDP

CMP CDP

CMP CDP

Dipping Reflector

CMP = common mid point

CDP = common depth point

Difference between CMP und CDP

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methods of velocity-analysis

x2

-t2

- analysis

Methods using NMO correction:

CVP - constant velocity panels

CVS - constant velocity stacks

velocity spectral analysis(semblance analysis)

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x2

-t2

- analysis

2

22

0

2

stackv

xtt +=

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Reflection hyperbola horizontal alignment Stacking

NMO-

(NMO = Normal Moveout)

Correction

Aim of Velocity analysis

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2NMO

20

2

22

0

2

tt

v

xtt

stack

+=

+=

tNMO= Moveout

+

+=2

0

0

2

0

02

111

tv

xt

tv

xtt

stackstack0

2

2

02 tv

xttt

stack

NMO=

Vstackapproximates Vrms for small offset

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Change of moveout with depth

Moveout decreases with depth

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(a) Reflection is not corrected

(b) Corrected with proper velocity

(c) Velocity is too low

(d) Velocity is too high Yilmaz, 1987

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Threshold stretch limits

50% 100%Yilmaz, 1987

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Stretching

0

2

2

0

2tv

xttt

stack

NMO=

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NMO correction and muting of a stretched zone

Yilmaz, 1987

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(a) NMO corrected gather

(b) Subtract gatherRight trace = right trace

Second trace on the right is the stack

of the two near traces of original gather

Leftmost trace is the stack of all traces

-> Dotted line indicates the mute zone

(c) Poor mute choice

Optimum mute selection

Yilmaz, 1987

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Constant velocity moveout corrections (1) (CVP)

Yilmaz, 1987

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Constant velocity moveout corrections (2) (CVP)

Yilmaz, 1987

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Constant velocity stacks of 24 CMP gathers (CVS)

Yilmaz, 1987

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Velocity spectrum

Mapping of the offset axis to the velocity axis

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Stacking velocity

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Influence of missing long offset traces on

velocity spectra

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Influence of missing long offset traces and

statics on velocity spectra

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Stacked Amplitudes

Normalized Stacked Amplitudes

Semblance

Methods to calculate the Velocity-Spectrum

=

=n

i

titws

1

,

=

=n

i

ti

t

t

w

sns

1

,

=t

i

ti

t

t

w

s

n2

,

2

t

1Semblance

tiw , value for i-th trace, time t

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Semblance-Analysis

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Results using wrong velocities

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Almost no

difference

Error for high velocities and large traveltimes

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Synthetic CMP gathers containing multiples

primariesWater-bottom

primary (W)

+multiples

+ = VM velocity multiples

VP velocity primaries

Yilmaz, 1987

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NMO correction using primary velocity function

Yilmaz, 1987

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CMP gathers with strong multiples

VM1= slow (water-bottom) multiples

VM2= fast (peg-leg) multiples

NMO corrected data

using primary velocitiesYilmaz, 1987

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CMP stack using former gathers

Yilmaz, 1987

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Factors affecting velocity estimates

Depth of the reflectors Move-out of the reflection

Spread length

S/N ratio

Static corrections

Dip of the reflector

Number of traces