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Seismic Waves-2

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  1. 1. Introduction to Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Seismic Waves (Continue)
  2. 2. Previous Lecture What is Seismic Wave? Waves in a Pound Sound Wave Analogy What is Wave? Wave Terminology Wavelength and Period Wave Speeds Sources of Seismic Waves Multiple Frequency Signals What is Elastic Behavior? Ductile Deformation Brittle Deformation Ductile/Brittle Behavior through the Lithosphere and Asthenosphere Elastic Constants Bulk Modules Shear Modules Poisson Ratio Typical Values of Elastic Constants for Selected Materials Body Waves Surface Waves
  3. 3. Particle Motions of Body Waves
  4. 4. 3D Components of Waves
  5. 5. 3D Components of P-wave 6.27
  6. 6. 3D Components of S-wave 6.35
  7. 7. Types of Surface Waves
  8. 8. Rayleigh Waves <ul><li>Rayleigh waves are the most complex wave, and they </li></ul><ul><li>are also the slowest. </li></ul><ul><li>They travel at speeds of 2 to 5 km/sec and vibrate </li></ul><ul><li>the ground in an elliptical pattern. </li></ul>
  9. 9. Where  are the Lamé coefficients and  is Seismic velocities P wave velocity α and S wave velocity β depend on physical properties of medium through which they travel: Question: How  and  depend on density  ?  = k - = 2   E 3 ( 1 +  ) ( 1 - 2  ) V =  = = k + ( )   + 2    p 4/3 V =  =   s
  10. 10. Elastic Coefficients and Seismic Velocities
  11. 11. <ul><li>A linear relationship between density and seismic velocity </li></ul><ul><li>where a and b are constants V = a ρ + b . </li></ul>Velocity and Density “Birch’s law” Crust and mantle rock observations 6km 18km 30km
  12. 12. Nafe-Drake Curve <ul><li>An important empirical relation exists between P </li></ul><ul><li>wave velocity and density. </li></ul>Cross-plotting velocity and density values of crustal rocks gives the Nafe-Drake curve after its discoverers. Only a few rocks such as salt (unusually low density) and sulphide ores (unusually high densities) lie off the curve.
  13. 13. Nafe-Drake Curve L=limestone; Q=quartz; Sh=shale; Ss=sandstone. Sediments and sedimentary rock Igneous and metamorphic rock Figure 3.10 of Lillie, 1999, modified from Birch, 1960
  14. 14. Factors affecting P-wave velocity <ul><li>Increases with </li></ul><ul><ul><li>mafic mineral content (Nafe-Drake curve) </li></ul></ul><ul><ul><li>pressure (modulus change > density change) </li></ul></ul><ul><li>Decreases with </li></ul><ul><ul><li>temperature (modulus change > density change) </li></ul></ul>
  15. 15. Factors affecting S-wave velocity <ul><li>Increases with </li></ul><ul><ul><li>mafic mineral content (Nafe-Drake curve) </li></ul></ul><ul><ul><li>with pressure (modulus change > density change) </li></ul></ul><ul><li>Decreases due to </li></ul><ul><ul><li>presence of fluid, e.g. porous sand or partial melt </li></ul></ul><ul><li>No S waves in </li></ul><ul><ul><li>fluids, e.g. water of molten rock . Velocity zero </li></ul></ul>
  16. 16. Velocity-Geology Grifts and King, 1981
  17. 17. Maximum amplitude of particle motion occurs along the 90 degree phase wave front. Other wave fronts correspond to positions where the wave goes from positive to negative amplitude (180 degree) and at the minimum amplitude (270). Amplitude Changes of Particle Motion
  18. 18. Raypaths thus bend (refract) as velocity changes. Seismic energy travels along trajectories perpendicular to wave fronts. Initial wavefronts for compressional (P),shear (S), and Rayleigh ( R ) waves. Changes in velocity cause segments of wave fronts to speed up or slow down, distorting the wave fronts from perfect spheres. Wave Fronts and Raypaths
  19. 19. Seismic waves radiating from a source to one receiver. Seismic trace recording ground motion by the receiver, as a function of the travel time from the source to the receiver. For controlled source studies (seismic refraction and reflection), the travel time is commonly plotted positive downward. Seismic Trace
  20. 20. <ul><li>Nafe-Drake Curves suggesting that compressional wave velocity and density are directly proportional . The below equation: </li></ul>Implies that P-wave velocity is inversely proportional to density, Explain the paradox. Homework
  21. 21. Questions?