Reflection Coefficients
For a downward travelling P wave, for the most general case:
 
u 

x x z
 
u 

z z
x
Where the first term on the RHS is the P-wave
displacement component and the second term is the
shear-wave displacement component
Reflection Coefficients
and where both shear stress,
2
2
   
   (2


)
xz 2
xz x 2 z 2
1
2
 
and as well as normal stress is continuous across the boundary:
2
2
   
2
      2  (2

)
zz
2 xz
z
Reflection Coefficients
When all these conditions are met and for the
special case of normal incident conditions, we
have that Zoeppritz’s equations are:
 2V P  1V P
2
1
R


\
/
I 2  I1  2V P  1V P
P P
2
1
I 2  I1

T
\
P P
\
2 I1
I 2  I1

2 1V P
1
 2V P  1V P
2
1
On occasions these equations will not add up to
what you might expect…!
Reflection Coefficients
T
R
\
P P
/
I I
2 I1
 2 1
\
\
I 2  I1 I 2  I1
P P
I  I  2 I1
 2 1
I 2  I1
Reflection Coefficients
T
R
\
P P
/
I I
2 I1
 2 1
\
\
I 2  I1 I 2  I1
P P
I  I  2 I1
 2 1
I 2  I1
I I
 2 1
I 2  I1
Reflection Coefficients
T
R
\
P P
/
I I
2 I1
 2 1
\
\
I 2  I1 I 2  I1
P P
I  I  2 I1
 2 1
I 2  I1
I I
 2 1
I 2  I1
1
Reflection Coefficients
T
R
\
P P
/
I I
2 I1
 2 1
\
\
I 2  I1 I 2  I1
P P
I  I  2 I1
 2 1
I 2  I1
I I
 2 1
I 2  I1
1
T
R
\
P P
/
1
\
P P
\
Reflection Coefficients
T
R
\
P P
/
I I
2 I1
 2 1
\
\
I 2  I1 I 2  I1
P P
I  I  2 I1
 2 1
I 2  I1
I I
 2 1
I 2  I1
1
T
R
\
P P
/
1
\
P P
\
Reflection Coefficients
What happens when we have a complete reflection with a 180
degree phase shift, as we might have when a ray in water travels
upward toward a free surface and reflects completely at the
interface?
Reflection Coefficients
What happens when we have a complete reflection with a 180
degree phase shift, as we might have when a ray in water travels
upward toward a free surface and reflects completely at the
interface?
We know that in this case:
 1
R
\
P P
/
Reflection Coefficients
What happens when we have a complete reflection with a 180
degree phase shift, as we might have when a ray in water travels
upward toward a free surface and reflects completely at the
interface?
We know that in this case:
 1
R
\
P P
But,
T
R
\
P P
What must:
/
/
1
\
P P
\
?
T
\
P P
\
Reflection Coefficients
What happens when we have a complete reflection with a 180
degree phase shift, as we might have when a ray in water travels
upward toward a free surface and reflects completely at the
interface?
We know that in this case:
 1
R
\
P P
But,
T
R
\
P P
So,
/
/
1
\
P P
\
\
\
2
T
P P
Reflection Coefficients
Briefly, how to consider displacements at interfaces
using potentials, when mode conversion occurs:
Z-
layer 1
X+
layer 2
In layer 1, just above the boundary, at the
point of incidence:
 
P
u 
x  x

\
  \
S


z


 
P
P
P



x
z

\
/
\
S
/




Z+
Reflection Coefficients
Briefly, how to consider displacements at interfaces
using potentials, when mode conversion occurs:
Z-
layer 1
X+
layer 2
In layer 2, just below the boundary, at the
point of incidence:
u 
x

\
P P
z
\


\
P S
x
\
Z+
Reflection Coefficients
So, if we consider that (1) stresses as well as (2)
displacements are the same at the point of incidence
whether we are in the top or bottom layer the
following must hold true so that (3) Snell’s Law holds
true:
u   u  ,
x
x
 
u z u z


 zz   zz
     
zx
zx
Reflection Coefficients
We get the general case of all the different types of
reflection and transmission (refraction or not)
coefficients at all angles of incidence :
 sin  \
P

 cos  \
P

 sin 2 \
P

 cos 2 \
P

cos 
 R \ \
 P P
R \ /
 P S
 T \ \
 P P
T \ \
 P S

 
 

 
 
 







Reflection Coefficients
Variation of Amplitude with angle (“AVA”) for
the fluid-over-fluid case (NO SHEAR WAVES)
I 2 cos   I1 1 
R ( ) 
I 2 cos   I1 1 

 V 2 sin

V1



 V 2 sin

V1














2
2
(Liner, 2004; Eq.
3.29, p.68; ~Ikelle
and Amundsen,
2005, p. 94)
Reflection Coefficients
What occurs at and beyond the critical angle?
V
V
1
2
c

sin  c
sin 





2
 sin  1












V
V
1
2







Reflection Coefficients
FLUID-FLUID case
What occurs at the critical angle?
I 2 cos   I1 1 
R ( ) 
I 2 cos   I1 1 

 V 2 sin

V1



 V 2 sin

V1














2
2
(Liner, 2004; Eq. 3.29,
p.68; ~Ikelle and
Amundsen, 2005, p.94)
Reflection Coefficients
Reflection
Coefficients at
all angles: preand post-critical
Case:
Rho: 2.2 /1.8
V: 1800/2500
Matlab Code
NOTES: #1
Reflection Coefficients
At the critical angle, the
real portion of the RC goes
to 1. But, beyond it drops.
This does not mean that the
energy is dropping.
Remember that the RC is
complex and has two terms.
For an estimation of energy
you would need to look at
the square of the
amplitude. To calculate the
amplitude we include both
the imaginary and real
portions of the RC.
NOTES: #2
Reflection Coefficients
For the critical ray,
amplitude is maximum
(=1) at critical angle.
Post-critical angles
also have a maximum
amplitude because all
the energy is coming
back as a reflected
wave and no energy is
getting into the lower
layer
NOTES: #3
Reflection Coefficients
Post-critical angle
rays will experience a
phase shift, that is
the shape of the
signal will change.
Energy Coefficients
\
P P
E
For the energy coefficients at
normal incidence :
T
R
We saw that for reflection coefficients :
E
\ /
P P
/
1
\
P P
\
 R \ /
P P
 2V P
2
T \ /
\ \
P P 1V P
P P
1
Energy Coefficients
\
P P
E
For the energy coefficients at
normal incidence :
T
R
We saw that for reflection coefficients :
E
/
\ /
P P
1
\
P P
\
 R \ /
P P
 2V P
2
T \ /
\ \
P P 1V P
P P
1
The sum of the energy is expected to be conserved across the boundary
E
P
\
E
\ /
P P
E
\ /
P S
E
\ \
P P
E
\ \
P S
Amplitude versus Offset (AVO)
Zoeppritz’s equations can be simplied if we assume that the
following ratios are much smaller than 1:
VS
VP
VP
average
VS
average

 average
For example, the change in velocities across a boundary is very
small when compared to the average velocities across the
boundary; in other words when velocity variations occur in small
increments across boundaries… This is the ASSUMPTION
Amplitude versus Offset (AVO)
If the changes across boundaries are relatively small, then we can make a
lot of approximations to simplify the reflection and transmission
coefficients:
R
\ /
P P


2
2
V

VP

1 z  1
S


sin 2 

2
i
2 VP
 average
2 z
average V P


average

