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

2. 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

3. Ray paths in 1 layer Note V 1 > V 0 required for head wave

4. V 1 < V 0 Waves in a 1 layer* model: V 1 < V 0 * one layer above a halfspace

5. The wavefield

6. V 1 > V 0 Waves in a 1 layer model: V 1 > V 0

7. Wavefield at 65 ms

8. ... at 110 ms

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

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

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