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Seismic waves on a boundary: refraction method

Seismic waves on a boundary: refraction method. Earth Physics EPSC 320 Autumn 2010. Seismic refraction method. Snell's law sin(i p )/  1 = sin(i s )/  1 = sin(r p )/  2 = sin(r s )/  2 = p, the ray parameter. Ray paths in 1 layer.

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Seismic waves on a boundary: refraction method

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  1. Seismic waves on a boundary: refraction method Earth Physics EPSC 320 Autumn 2010

  2. Seismic refraction method Snell's law sin(ip)/1 = sin(is)/1 = sin(rp)/2 = sin(rs)/2 = p, the ray parameter

  3. Ray paths in 1 layer Note V1 > V0 required for head wave

  4. Waves in a 1 layer* model: V1 < V0 * one layer above a halfspace V1 < V0

  5. The wavefield

  6. Waves in a 1 layer model: V1 > V0 V1 > V0

  7. Wavefield at 65 ms

  8. ... at 110 ms

  9. ... at 140 ms Direct, reflected, refracted and 'head' waves

  10. Refractions and reflections ...a ray model Snell's law in a 1-layer structure

  11. A seismic refraction survey

  12. First and later 'arrivals'

  13. A seismogram

  14. The 'spread'

  15. Travel-time curves

  16. Dipping layer?

  17. ... modified travel-times

  18. Down dip...

  19. Up dip...

  20. ... earlier head wave

  21. Two dipping layers...

  22. ... travel times

  23. A 2-layer survey

  24. The interpretation http://www.epa.gov/oust/pubs/esa-ch3.pdf

  25. Global scaling As seismic velocities generally increase with depth, the P-waves and S-waves are refracted back to the surface. We can interpret the travel-time curves as an infinite number of infinitesmally thin layers in spherical shells..

  26. Reference Most of the nice graphical images used in this presentation are taken from the seismic noteset: http://galitzin.mines.edu/INTROGP/MISC/seisnotes.pdf

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