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Seismic waves on a boundary: refraction method
Earth Physics EPSC 320 Autumn 2010
Seismic refraction method
Snell's law
sin(ip)/
1 = sin(i
s)/
1
= sin(rp)/
2
= sin(rs)/
2
= p, the ray parameter
Ray paths in 1 layer
Note V1 > V
0 required for head wave
V1 < V
0
Waves in a 1 layer* model: V1 < V
0
* one layer above a halfspace
The wavefield
V1 > V
0
Waves in a 1 layer model: V1 > V
0
Wavefield at 65 ms
... at 110 ms
... at 140 ms
Direct, reflected, refracted and 'head' waves
Snell's law in a 1-layer structure
Refractions and reflections ...a ray model
A seismic refraction survey
First and later 'arrivals'
A seismogram
The 'spread'
Travel-time curves
Dipping layer?
... modified travel-times
Down dip...
Up dip...
... earlier head wave
Two dipping layers...
... travel times
A 2-layer survey
The interpretation
http://www.epa.gov/oust/pubs/esa-ch3.pdf
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..
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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