propagation seismic waves w p waves .ppt

propagation seismic waves w p waves

1. Subject 4: Propagation of the seismic waves
Lecturer: Dr. Bakhtiar Q. Aziz
Objective: The student will get a detail idea about the propagation of the seismic
wave within the earth, and what are the different phenomenons subjected to the
propagated waves? such as Attenuation, Reflection, Refraction,…, . The effect of
each phenomenon on the waves will discuss to them.

Scientific contents
1- Attenuation of seismic wave. 5- Multiples of seismic waves.
2- Reflection of seismic wave. 6- Generation of wave face.
3- Refraction of seismic wave. 7- Change of velocity
4- Diffraction of seismic wave. 8- Frequency filtering of seismic wave
References
1. An introduction to applied and environmental geophysics, 1997, Reynolds, J. M.
2. Applied and environmental geophysics, 1999, Sharma,V.,P.
3. www.Geophysics.net
4. www.geology.wisc.edu/courses/g594/

2. Propagation of the seismic wave:
Seismic waves subjected to severalphenomenon when travel
through the earth
The most important are:
1-Attenuation
2-Reflection
3-Refraction
4-Diffraction
1-Attenuation
There are two types of attenuation:
a-Geometrical spreading: Take place due to traveling certain amount of distance
Example: Find attenuation of a wave after a distance (r) from the source
r
I=?
I
o, r
o
I= I
o* r
o/ r * e
-r
is absorption coefficient
5-Multiples
6-Generation of wave face
7-Change of velocity
8-Frequency filtering 

3. b-Intrinsic attenuation: loss of amplitude taken place due to dissipation of
energy into heat by friction
e
-œr
Io
Distance
2-Reflection:
It is take place when a seismic wave hits an interface separating two media of
different elastic Properties (or different acoustic impedance, z)
Acoustic impedance: define as the product of velocity with density
Higher frequencies attenuate over shorter distances due to their shorter wavelengths.
Therefore, high frequencies decay first leaving a low frequency signal remaining.
Note:
Z = p* V

4. Z1 = p1 * V1
Z2 = p2 * V2

5. Reflection Coefficient:It is a ratio of reflected wave amplitude (Ar) to
incidence wave amplitude (Ai)
R = Ar/ Ai = Z2 –Z1 / Z2 + Z1 = P2V2 –P1V1 /P2V2 + P1V1
Notes:
1-R is positive when Z2>Z1 and negative when Z1>Z2
2-R =+1 when Z1 = 0 and R = -1 when Z2 = 0
3-R is approach to unity in two cases:
a-When incidence angle = Critical incidence angle
b-Tangential (Grazing) incidence
(a)
(b)

6. 3-Refraction:Apart of seismic wave is refractedwhen hits an interface
separatingtwo media
Refraction depend onSnell’slaw:
Notes:
1-When V
2is smaller than V
1so i
1> i
2, in this case
refraction will not take place, the wave will be
deflected.
2-When V
2>V
1, i
2will be greater than i
1, when i
2=90 the
wave will travel along the interfaceand refraction will
take place,
So i
2is called critical angle.
i1
i2
Refraction not take place
Refraction not take place
Refraction will take placeandi
1is called
critical angle =i
c, i
2= 90, then sin 90 =1
So Sin i
c= V1 / V2

7. 4-Diffraction: it takes place when the seismic wave hits:
1-Irregularity
2-Abrupt discontinuity
3-Faults
In this case the irregular feature act as point source for radiating waves in all
directions.
Source
Surface
V1
V2
Faults
5-Multiple: They are signals undergone more than one reflection, and they are of
small energy
There are two types of multiples:
1-Short path multiples : They are almost arrived with useful signals and form a tail to
them, such as
A-Ghost
B-Near-surface multiples
C-Peg-leg Multiples

8. 2-Long path Multiples :They are arrived at a later time than primary
reflections, they appear as a separate signals.
Such as
A-Simple multiples.
B-Interformational multiples.
Primary
ReflectionGhost
Near
Surface
Simple
MultipleInterformational
Long path MultiplesShort path Multiples
Surface
Peg-Leg

9. 6-Generation of wave phase:
When P-wave hits an interface generate four types of the seismic waves:
Reflected P-Wave
Reflected S-Wave
Refracted P-Wave
Refracted S-Wave
Seismic Wave
Notes:
1-When the wave hits an interface vertically , does not generate other type of waves.
2-When the first medium is liquid, only three types will generate because S-wave does
not propagates through the liquid.