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 several phenomenon 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=?
Io, ro
I= Io * ro/ 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 = p 1 * V1
Z2 = p 2 * 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 refracted when hits an interface
separating two media
Refraction depend on Snell’s law:
Notes:
1- When V2 is smaller than V1 so i1> i2, in this case
refraction will not take place, the wave will be
deflected.
2- When V2>V1, i2 will be greater than i1 , when i2=90 the
wave will travel along the interface and refraction will
take place,
So i2 is called critical angle.
i1
i2
Refraction not take place
Refraction not take place
Refraction will take place and i1 is called
critical angle =ic , i2 = 90, then sin 90 =1
So Sin ic = 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
Reflection Ghost
Near
Surface
Simple
Multiple Interformational
Long path Multiples
Short 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.