intro to seismic refraction, for weathering correction.

1. Seismic Refraction
Method
Overview
Prepared by
Dr. Amin Khalil

3. TYPES AND PROPERTIES OF SEISMIC
WAVES
• There are two types of elastic body wave in a solid:
– P-Waves: compression waves
– S-waves: shear waves
• P-waves are the faster and are usually the ones studied in
simple seismic methods.
• Other waves (surface waves) also exist but are much slower.
It is these waves that do the damage in earthquakes.
• We will focus our attention on P-waves from now on.

4. Compressional (“P”) Wave
Identical to sound wave – particle motion is parallel to propagation
direction.
Animation courtesy Larry Braile, Purdue University

5. Shear (“S”) Wave
Particle motion is perpendicular
to propagation direction.
Animation courtesy Larry Braile, Purdue University

6. Velocity of Seismic Waves
Depends on density elastic moduli


3
4


K
Vp


Vs
where K = bulk modulus,  = shear modulus, and
 = density.

7. Velocity of Seismic Waves
Bulk modulus = resistance to
compression = incompressibility
Shear modulus = resistance to
shear = rigidity
The less compressible a material is, the greater its p-wave
velocity, i.e., sound travels about four times faster in water
than in air. The more resistant a material is to shear, the
greater its shear wave velocity.

8. RIGIDITY
It a measure of how the medium resist the change in shape. Hence, rigidity in
fluids and fluid-like media is zero. This mean that no shear wave (S-waves) are
travelling in fluids.
This property help in the identification that the outer core is liquid like shell.

9. Surface waves in an elastic solid

11. Seismic waves at an interface
What happen when seismic waves encounters an
interface?!!!

12. We start by defining some important
phenomena:
1- Seismic wave propagation in a media is dependent on
elastic impedance Z. The elastic impedance Z is defined
by:
Z=  v
Where:
  density
v= seismic velocity

13. At an interface
Seismic waves exhibit number of actions named collectively as
energy partition at an interface. The seismic waves are reflected,
refracted and converted from P to S and from S to P. The
reflection is governed by the reflection coefficient which represent
the percentage of the energy that will be reflected.

14. Energy Partition

15. 21
12
zz
zz
R



REFLECTION Coefficient is defined by:
Where Z1 and Z2 are the impedances
for the first and second layer
respectivley.

18. Basic laws:
Snell Law
Reciprocity law

19. Snell’ law
This law control the refraction of seismic energy
at an interface:
2
1
2
1
)sin(
)sin(
V
V
i
i

Where i1 and i2 are the incident and refracted
angles and V1 and V2 are velocities of the first
layer and second layer respectively.

20. Primarily, refraction method depends on
the hypothesis that velocity increases with
depth. This is because refracted waves to
be recorded by an array of geophones on
the surface should be critically refracted,
i.e. The refraction angle be 90o , in this
case the refracted energy propagate
parallel to interface with the speed of the
faster second layer. This type of
propagation is called head waves. Head
waves itself acts as seismic rays incident
at the interface with 90o angle and
refraction back to the surface is then taking
place

21. source geophone
V1
V2
ic
ic
Ic is the angle of critical refraction and is given by:
Ic =sin-1 (V1 /V2 )

22. Snell’s Law
If V2>V1, then as i increases, r
increases faster

23. Principal of Reciprocity
• The travel time of seismic energy
between two points is independent
of the direction traveled, i.e.,
interchanging the source and the
geophone will not affect the seismic
travel time between the two.

24. END