VẬT lý địa CHẤN 04 velocity

Important for :Conversion from traveltime to depth Check of results by modeling Imaging of the data migration Classification and Filtering of Signal and Noise Predictions of the Litholog

Important for :

Conversion from traveltime to depth

Check of results by modeling

Imaging of the data (migration)

Classification and Filtering of Signal and Noise Predictions of the Lithology

Aid for geological Interpretation

Seismic Velocities

Physical quantities to describe strain properties of isotropic medium

stress-• Bulk modulus k volume stress/strain

• Shear modulus µ shear stress/strain

• Poissons ratio σ transverse/longitudinal strain

• Young’s modulus E longitudinal stress/strain

Bulk modulus

V V

P k

/ κ

Poissons ratio

Poisson’s ratio varies from 0 to ½

Poisson’s ratio has the value ½ for fluids

µ) 2(3k

2µ

3k σ

+

−

=

-Young’s modulus

L+

µ 3k

9kµ E

+

=

µ = Shear modulus

ρ

2µ

λ ρ

3

4µ k

λ = Lame’s lambda constant µ

3

2 k

Ratio Vp and Vs depends on Poisson ratio:

p

s

V V

µ) 2(3k

2µ

3k σ

+

−

=

where

Seismic Velocity depending on rock properties

(Sheriff und Geldard, 1995)

Measurements of velocities

• Laboratory measurements using probes

• Borehole measurements

• Refraction seismics

• Analysis of reflection hyperbolas

• Vertical seismic profiling

Kearey and Brooks, 1991

0.2 - 1.0 1.5 - 2.0 1.0 - 2.5 1.5 - 2.5 3.5 - 4.0

2.0 - 6.0 2.0 - 2.5 4.0 - 4.5 5.5 - 6.0 2.0 - 6.0 2.0 - 2.5 3.0 - 4.0 5.0 - 5.5 2.5-6.5 4.5 - 5.0 4.5 - 6.5 2.0 - 3.5

5.5 - 6.0 6.5 - 7.0 7.5 - 8.5 5.5 - 6,5

0.3 1.4 - 1.5 3.4 1.3 - 1.4

6.1 5.8 6.6 3.6

Igneous / Metamorphic rocks

P-wave velocities vp for different material in (km/s)

Kearey and Brooks, 1991

n m

n

m I

τ

z

z t

i

i

n i

i av

v z

Velocities

tm : measured reflected ray traveltime

τm : one-way reflected ray traveltime only through mth layer

v v

1

1

2 2

τ τ

t1

t2 t

3

Measured traveltimes

Conversion from v rms in v int (interval velocities)

Dix’ Formula

n RMS

1

2 1 ,

2 , int

n n

n n

RMS n

n RMS

t t

t V

t

V V

1 ,n −

RMS V

n t

1

−

n t

Vrms is approximated by the stacking velocity that is obtained by NMO correction of a CMP measurement

(when maximum offset is small compared with reflector depth)

Anisotropy

Slow

Anisotropy(seismic): Variation of seismic velocity depending

on the direction in which it is measured