4. Reflection/transmisson coefficients Introduction R/T coefficient – reflectivity/transmissivity...
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Transcript of 4. Reflection/transmisson coefficients Introduction R/T coefficient – reflectivity/transmissivity...
4. Reflection/transmisson coefficients
• Introduction• R/T coefficient – reflectivity/transmissivity• Zoeppritz type equations• Critical angles/complex R/T• Weak-contrast approximation• Parametrization• Second order approximation• Reflection from single layer• Reflection from free surface
Introduction
• Reflection/transmission coefficient describes the effect of interface between two half spaces:
Solid, Liquid, VacuumThere are 5 different cases to be considered:- Solid-solid- Solid-liquid- Solid-vacuum- Liquid-liquid- Liquid-vacuum
Introduction
• There are two types of boundary conditions at the interface:
- Strains- Stress components
They have be continuous (boundary conditions)
or discontinuous (slip effect)
Introduction
• There are two types reflection/transmission problems:
- Amplitudes- Energy (energy flux), symmetries+geometrical
spreading
Reflection/transmission coefficients are frequency independent for pre-critical elastic reflections from flat smooth interface
R/T coefficient – reflectivity/transmissivity
2 ,
2 ,
2 ,Q2
,Q22
1
2
1
S
S
P
P
P
1 ,
1 ,
1 ,Q1
,Q1
P wave velocity
S wave velocity
density
Q P wavequality factor
Q S wavequality factor
1 2 1 2
1 2 1 2
Snell law :
sin sin sin sinp
RD
TD
TU
RU
I
I
zz
00
Figure 4.1. The R/T coefficients
Zoeppritz type equations assumptions
• Plane wave
• Isotropic elastic medium
• Plane interface
Zoeppritz type equations
21 2 1 2
1 2 1 2
1 2 21 1
1 2 2
cos cos cos cos
cos cos cos2
PP
PS
r b c F a d Hp D
r ab cd p D
2 2 2 22 2 1 1
2 2 2 22 2 1 1
2 2 2 22 2 1 1
2 22 2 1 1
1 2 1 2
1 2 2
2 1 2
2
a p p
b p p
c p p
d
(4.1)
1 2
1 2
1 2
1 2
1 2
1 2
2 1
2 1
2
cos cos
cos cos
cos cos
cos cos
E b c
F b c
G a d
H a d
D EF GHp
(4.2) (4.3)
Zoeppritz type equations
0 10 20 30 40 50 60 70 80 90-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1=2.0 km/s
2=2.2 km/s
1=0.9 km/s
2=1.2 km/s
1=2.1 g/cm3
2=2.2 g/cm3
Re(
r)
P-wave incident angle (), degrees
PP PS
Figure 4.2. The real part of Zoeppritz reflection coefficients
Critical angle
Pre-critical reflection Post-critical reflection
Zoeppritz type equations
0 10 20 30 40 50 60 70 80 90-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1=2.0 km/s
2=2.2 km/s
1=0.9 km/s
2=1.2 km/s
1=2.1 g/cm3
2=2.2 g/cm3
Im(r
)
P-wave incident angle (), degrees
PP PS
Figure 4.3. The imaginary part of Zoeppritz reflection coefficients
Post-critical reflection
Complex reflection coefficient
Re Im expr r i r r i
is the phase shift at interface
(4.4)
Zoeppritz type equations
0 10 20 30 40 50 60 70 80 90-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1=2.2 km/s
2=2.0 km/s
1=1.2 km/s
2=0.9 km/s
1=2.2 g/cm3
2=2.1 g/cm3
Re(
r)
P-wave incident angle (), degrees
PP PS
Figure 4.4. The real part of Zoeppritz reflection coefficients (index interchange)
Zoeppritz type equations
0 10 20 30 40 50 60 70 80 90-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1=2.0 km/s
2=2.0 km/s
1=0.9 km/s
2=1.2 km/s
1=2.1 g/cm3
2=2.2 g/cm3
Re(
r)
P-wave incident angle (), degrees
PP PS
Figure 4.5. The real part of Zoeppritz reflection coefficients (no post-critical)
Zoeppritz type equations
0 10 20 30 40 50 60 70 80 90-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1=2.0 km/s
2=2.01 km/s
1=0.9 km/s
2=0.91 km/s
1=2.1 g/cm3
2=2.11 g/cm3
Re(
r)
P-wave incident angle (), degrees
PP PS
Figure 4.6. The real part of Zoeppritz reflection coefficients (weak contrast)
Energy flux
1 2
1 1
1 1
cos
cos
energy amplitudePP PP
energy amplitudePS PS
r r
r r
(4.5)
Weak-contrast approximation
2 2 2 22
2 2 2 2 2 2
1 14 1 4
2cos 2
cos cos cos cos4 4 1 2 2
2cos
PP
PS
r p p
pr p p
Zoeppritz equation – 6 medium parameters.Weak-contrast approximation – 3 medium parameters.
All angles are real (pre-critical):
(4.6)
2 1
2 1
2m m m m
z m m m
Contrast in medium parameter m
(4.7)
What is now happened with index interchange?
Weak-contrast approximation
0,00 0,05 0,10 0,15 0,20 0,25 0,30-0,35
-0,30
-0,25
-0,20
-0,15
-0,10
Re rPP
0,00 0,05 0,10 0,15 0,20 0,25 0,30
-0,08
-0,06
-0,04
-0,02
0,00
Re rPS
0,00 0,05 0,10 0,15 0,20 0,25 0,30
0,02
0,04
0,06
0,08
0,10
Re rSS
Horizontal slowness [ms/m]0,00 0,05 0,10 0,15 0,20 0,25 0,30
0,000
0,005
0,010
0,015
0,020
0,025
0,030
Exact Weak-contrast
Re tPS
Horizontal slowness [ms/m]
Figure 4.7. Weak contrast model
Weak-contrast approximation
0,00 0,05 0,10 0,15 0,20 0,25-0,35
-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
Re rPP
0,00 0,05 0,10 0,15 0,20 0,25
-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
0,00
Re rPS
0,00 0,05 0,10 0,15 0,20 0,25-0,2
-0,1
0,0
0,1
0,2
0,3
0,4
Re rSS
Horizontal slowness [ms/m]0,00 0,05 0,10 0,15 0,20 0,25
-0,05
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
Exact Weak-contrast
Re tPS
Horizontal slowness [ms/m]
Figure 4.7. Strong contrast model
Parametrization
2 233 44
,
,
P SZ Z
K c c
1. Velocities2. Impedances3. Stiffness coefficients (elastic moduli)
33 44
33 44
,
1 1,
2 2
SP
P S
ZZ
Z Z
c c
c c
(4.9)
(4.8)
Parametrization
0,00 0,05 0,10 0,15 0,20 0,25-0,35
-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
Re rPP
0,00 0,05 0,10 0,15 0,20 0,25
-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
0,00
Re rPS
0,00 0,05 0,10 0,15 0,20 0,25-0,2
-0,1
0,0
0,1
0,2
0,3
0,4
Re rSS
Horizontal slowness [ms/m]0,00 0,05 0,10 0,15 0,20 0,25
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
Exact weak-contrast in ,, weak-contrast in Z,Z, weak-contrast in M,,
Re tPS
Horizontal slowness [ms/m]
Figure 4.8. Parametrization effect for the strong contrast model
Parametrization
10
2P
PPP
Zr
Z
(4.10)
Exact expression for normal incidence reflection coefficient
Second order approximation
12 2
D
11 12 12 11 222D
12 11 22 22 12
2 2 2
1g g f g f g g
21
g f g g g g f2
FG GF G F FG GFR G I F G
R
12 2 2 2
D
2 2 211 12 12 11 22
2D
2 2 212 11 22 12 22
2 2
1 11 g g f f g g g
2 21 1
f g g g 1 g g f2 2
G F F GT I F I F
T
Stovas&Ursin, 2002
(4.12)
(4.11)
Second order approximation
(1/2)
r(2)
DPSr(2)
DPP
t(2)
DPSt(2)
DPP
-f-f-f
f
f-f
g22
-g22
g12
-g12
-g11 -g
12-g
12-g
12
g12
-g11
-g11
g11
medium II
medium I
medium II
medium I
Figure 4.9. Interpretation of second order R/T
Second order approximation
0,00 0,05 0,10 0,15 0,20 0,25 0,30-0,25
-0,20
-0,15
-0,10
-0,05
RPP
0,00 0,05 0,10 0,15 0,20 0,25 0,30
-0,04
-0,02
0,00
Exact First order Second order
RPS
0,00 0,05 0,10 0,15 0,20 0,25 0,300,95
0,96
0,97
0,98
0,99
1,00
TPP
Horizontal slowness, ms/m0,00 0,05 0,10 0,15 0,20 0,25 0,30
0,00
0,01
0,02
0,03
0,04
TPS
Horizontal slowness, ms/m
Figure 4.10. Second order R/T for weak-contrast model
Second order approximation
0,00 0,05 0,10 0,15 0,20 0,25 0,30-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
0,00
RPP
0,00 0,05 0,10 0,15 0,20 0,25 0,30-0,16
-0,14
-0,12
-0,10
-0,08
-0,06
-0,04
-0,02
0,00
RPS
0,00 0,05 0,10 0,15 0,20 0,25 0,300,90
0,92
0,94
0,96
0,98
1,00
TPP
Horizontal slowness, ms/m0,00 0,05 0,10 0,15 0,20 0,25 0,30
0,00
0,05
0,10
0,15
0,20
0,25
Exact First order Second order
TPS
Horizontal slowness, ms/m
Figure 4.11. Second order R/T for strong-contrast model
Reflection from single layer
T21
T12
R23
R12
Model BModel A
Chalk
Black Shale
z3
z2
z1
Figure 4.12. Sketch of the models (Helle, Stovas & Carcione, 1999)
Reflection from single layer
0,0
0,2
0,4
0,6
10 %
Am
plit
ud
es
20 %
0 20 40 600,0
0,2
0,4
0,6
30 %
Incidence angle (degrees)0 20 40 60
40 %
10 m 25 m 50 m
Incidence angle (degrees)Figure 4.13. Reflection for model A (frequency 40 Hz)
Reflection from single layer
2 2 1
2 2 1
212 23 21
12 12 221 231
ik z z
ik z z
T R T eR R
R R e
1 ij ij ij jiR T R R
2 2 1
2 2 1
212 23
12 212 231
ik z z
ik z z
R R eR
R R e
0 10 20 30 40 50 60 70 80 90 100
-0,10
-0,05
0,00
0,05
0,10
z=10 m z=25 m z=50 m
Im(r
PP(0
))
Frequency, Hz
0 10 20 30 40 50 60 70 80 90 100
0,00
0,05
0,10
0,15
0,20
Re(
r PP(0
))
Frequency, Hz
(4.15)
(4.14)
(4.13)
Figure 4.13. Frequency dependent normal incidence reflection coefficient
Reflection from free surface
water
air
1R (4.16)
Source Receiver