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

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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

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

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

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The wavefield

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

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Wavefield at 65 ms

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... at 110 ms

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

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

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A seismic refraction survey

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First and later 'arrivals'

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A seismogram

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The 'spread'

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Travel-time curves

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Dipping layer?

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... modified travel-times

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Down dip...

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Up dip...

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... earlier head wave

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Two dipping layers...

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... travel times

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A 2-layer survey

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

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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..

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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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