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Seismic refraction method lecture 21

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intro to seismic refraction, for weathering correction.

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Seismic refraction method lecture 21

  1. 1. Seismic Refraction Method Overview Prepared by Dr. Amin Khalil
  2. 2. 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.
  3. 3. Compressional (“P”) Wave Identical to sound wave – particle motion is parallel to propagation direction. Animation courtesy Larry Braile, Purdue University
  4. 4. Shear (“S”) Wave Particle motion is perpendicular to propagation direction. Animation courtesy Larry Braile, Purdue University
  5. 5. Velocity of Seismic Waves Depends on density elastic moduli   3 4   K Vp   Vs where K = bulk modulus,  = shear modulus, and  = density.
  6. 6. 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.
  7. 7. 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.
  8. 8. Surface waves in an elastic solid
  9. 9. Seismic waves at an interface What happen when seismic waves encounters an interface?!!!
  10. 10. 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
  11. 11. 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.
  12. 12. Energy Partition
  13. 13. 21 12 zz zz R    REFLECTION Coefficient is defined by: Where Z1 and Z2 are the impedances for the first and second layer respectivley.
  14. 14. Basic laws: Snell Law Reciprocity law
  15. 15. 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.
  16. 16. 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
  17. 17. source geophone V1 V2 ic ic Ic is the angle of critical refraction and is given by: Ic =sin-1 (V1 /V2 )
  18. 18. Snell’s Law If V2>V1, then as i increases, r increases faster
  19. 19. 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.
  20. 20. END

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